Skip to main content

Next generation of immune checkpoint therapy in cancer: new developments and challenges


Immune checkpoints consist of inhibitory and stimulatory pathways that maintain self-tolerance and assist with immune response. In cancer, immune checkpoint pathways are often activated to inhibit the nascent anti-tumor immune response. Immune checkpoint therapies act by blocking or stimulating these pathways and enhance the body’s immunological activity against tumors. Cytotoxic T lymphocyte-associated molecule-4 (CTLA-4), programmed cell death receptor-1 (PD-1), and programmed cell death ligand-1(PD-L1) are the most widely studied and recognized inhibitory checkpoint pathways. Drugs blocking these pathways are currently utilized for a wide variety of malignancies and have demonstrated durable clinical activities in a subset of cancer patients. This approach is rapidly extending beyond CTLA-4 and PD-1/PD-L1. New inhibitory pathways are under investigation, and drugs blocking LAG-3, TIM-3, TIGIT, VISTA, or B7/H3 are being investigated. Furthermore, agonists of stimulatory checkpoint pathways such as OX40, ICOS, GITR, 4-1BB, CD40, or molecules targeting tumor microenvironment components like IDO or TLR are under investigation. In this article, we have provided a comprehensive review of immune checkpoint pathways involved in cancer immunotherapy, and discuss their mechanisms and the therapeutic interventions currently under investigation in phase I/II clinical trials. We also reviewed the limitations, toxicities, and challenges and outline the possible future research directions.


Tumor immune micro-environment encompasses a wide range of complex interactions between tumor cell, immune cells (antigen presenting cells, T cell, NK cell, B cell, etc.), and tumor stroma. Host immune response against tumor is a result of competition between inhibitory and stimulatory signals. Immune checkpoints are important immune regulators in maintaining immune homeostasis and preventing autoimmunity. These consist of both stimulatory and inhibitory pathways that are important for maintaining self-tolerance and regulating the type, magnitude, and duration of the immune response. Under normal circumstances, immune checkpoints allow the immune system to respond against infection and malignancy while protecting tissues from any harm that may derive from this action. However, the expression of some of these immune-checkpoint proteins by malignant cells dysregulates the antitumor immunity and favors the growth and expansion of cancer cells [1]. Figure 1 summarizes these molecules and their targets [1,2,3]. Immune checkpoint therapy for cancer encompasses strategies that target these regulatory pathways in order to enhance immunity activity against tumor cells [4, 5]. The most broadly studied checkpoints are the inhibitory pathways consisting of cytotoxic T lymphocyte-associated molecule-4 (CTLA-4), programmed cell death receptor-1 (PD-1), and programmed cell death ligand-1 (PD-L1). Ipilimumab [anti-CTLA-4 monoclonal antibody (mAb)] was the first immune checkpoint inhibitor (ICI) approved by FDA in 2011 [6]. Many biological agents that target these molecules are now broadly used in a variety of malignancies. Currently approved ICIs are only effective in a small fraction of patients and resistance after initial response is a common phenomenon. Nevertheless, new inhibitory and stimulatory pathways have emerged as potential targets for immune checkpoint therapy and immunotherapy is extending even beyond this approach [7, 8]. Novel immune checkpoint agents and combination therapies currently under investigation in phase I/II clinical trials are reviewed and discussed in this article.

Fig. 1

Immune interactions involving antigen presenting cells or tumor cells, T cells, and tumor microenvironment


We performed a PubMed search using the keywords and MeSH terms immunotherapy, immune checkpoint therapy, immune checkpoint inhibitors, immune checkpoint agonists, and immune checkpoint adjuvants. We also searched American Society of Clinical Oncology (ASCO) and American Association for Cancer Research (AACR) meeting abstracts, and from June 5, 2016, through January 30, 2018. We focused on phase I and phase II clinical trials of new agents in immune checkpoint therapy that were being used alone or in conjunction with other forms of immunotherapy. Data was collected from the trials reviewed with at least preliminary results published or presented before the date of the search. Exclusion criteria included phase III or later stage clinical trials, clinical trials focusing only on anti-CTLA-4 and anti-PD-1/PD-L1, trials focusing on pediatric population, and non-interventional trials. We have included 62 phase I and 23 phase II clinical trials in this review. Table 1 summarizes these agents and clinical trials.

Table 1 Summary of ongoing phase 1/2 clinical trials utilizing immune checkpoint therapy

Inhibitory pathways

Binding of CTLA-4 and PD-1/PD-L1 to cancer cell or tumor-microenvironmental ligands leads to T cell attenuation, which enables the tumor cells to avoid immune-mediated destruction [1]. Similarly, other inhibitory pathways have been identified and new blocking agents are being developed to induce immune reaction against malignant cells [4]. These inhibitory pathways can be classified as T cell associated and non-T cell associated, as follows.

T cell-associated inhibitory molecules

LAG-3 (CD223)

Lymphocyte activation gene-3 (LAG-3, CD223) is expressed by T cells and natural killer (NK) cells after major histocompatibility complex (MHC) class II ligation [9, 10]. Although its mechanism remains unclear, its modulation causes a negative regulatory effect over T cell function, preventing tissue damage and autoimmunity. LAG-3 and PD-1 are frequently co-expressed and upregulated on tumor-infiltrating lymphocytes (TILs) leading to immune exhaustion and tumor growth [11]. Thus, LAG-3 blockade not only improves anti-tumor immune responses but also potentiates other forms of immunotherapy given its different mechanism of action mainly mediated by impeding cell cycle progression [12,13,14]. Although simultaneous use with anti-PD-1 therapy is considered synergistic, it remains unclear whether other immune checkpoint inhibitory molecules in conjunction with anti-LAG-3 therapy will be as effective [15]. Furthermore, clinical benefits from combination come at the expense of increased incidence of autoimmune toxicities [1]. Currently two inhibitory approaches have been developed: a LAG-3-Ig fusion protein (IMP321, Immuntep®) and mAbs targeting LAG-3 [5].

IMP321, a soluble form of LAG-3, upregulates co-stimulatory molecules and increases interleukin (IL)-12 production to enhance tumor immune responses. Two phase I clinical trials using IMP321 in advanced renal cell carcinoma (RCC) and pancreatic adenocarcinoma showed an increase in tumor reactive T cells, but no meaningful objective response (OR) was observed [16, 17]. Another phase I clinical trial studied IMP321 in combination with paclitaxel in metastatic breast cancer (BC) and an objective response rate (ORR) of 50% was observed [18]. This promising result has prompted a phase IIb clinical trial that is currently recruiting patients with metastatic BC (NCT02614833).

Targeting LAG-3 with antagonistic mAbs interferes with the LAG-3 interaction between MCH II molecules expressed by tumor and/or immune cells, promoting tumor cell apoptosis [19]. A phase I clinical trial is recruiting melanoma patients to determine the safety of anti-LAG-3 (BMS-986016), with and without nivolumab (NCT01968109). Interim results show promising efficacy with an ORR of 16% and disease control rate (DCR) of 45% among patients who had progressed despite previous therapy with anti-PD-1/PD-L1. The safety profile is similar to nivolumab alone [20]. LAG525 is another anti-LAG-3 mAb being studied on a phase I/II clinical trial with metastatic solid malignancies (NCT02460224), and currently no data is available.


T cell immunoglobulin-3 (TIM-3) is a direct negative regulator of T cells and is expressed on NK cells and macrophages. TIM-3 indirectly promotes immunosuppression by inducing expansion of myeloid-derived suppressor cells (MDSCs). Its levels have been found to be particularly elevated on dysfunctional and exhausted T cells suggesting an important role in malignancy [21]. Presence of TIM-3+ T cells correlates with severity and poor prognosis in non-small cell lung carcinoma (NSCLC) and follicular lymphoma [11]. On the other hand, low levels of TIM-3 have been associated with autoimmune processes such as in diabetes or multiple sclerosis [22]. Similarly, the use of monoclonal antibodies to block TIM-3 causes an increase in T cell proliferation and cytokine production which may not only explain its antitumor activity but also its role in aggravating autoimmune diseases [22]. Furthermore, there has been concern with the use of these antibodies given that TIM-3 could act as an enhancer of CD8 T cells during certain acute infections including Listeria [23].

Modulation of this pathway occurs through multiple ligands including galectin-9, phosphatidyl serine, and CEACAM-1 [11]. These molecules play an important role in carcinogenesis, tumor survival, and even progression of different malignancies including melanoma, gastrointestinal, and lung cancer [24,25,26]. As opposed to other inhibitory pathways that interfere with cellular function, TIM-3 primarily exert its function by regulating cell apoptosis [27]. This could potentially explain its enhancing effects when used with other ICIs. However, the best complementary molecule to be used with TIM-3 remains unknown.

Currently, one anti-TIM-3 mAb (MBG453) is being investigated in phase I–II clinical trial in patients with advanced malignancies (NCT02608268). No clinical results are yet reported.


T cell immunoglobulin and ITIM domain (TIGIT) is part of the CD28 family-like receptors expressed by NK and T cells. It exerts direct immunosuppressive effects on these cells and indirectly increases the release of immunoregulatory cytokines (e.g., IL-10), decreases the production of interferon (IFN)-γ and IL-17, and prevents maturation of DCs [28, 29]. Two agonists, CD155 (poliovirus receptor-PVR) and CD112 (PVRL2, nectin-2), are expressed by immune cells, non-immune cells, and by tumor cells including melanoma [30]. Moreover, TILs often express high levels of TIGIT along with PD-1, TIM-3, and LAG-3, consistent with a dysfunctional phenotype [31].

Initial ex vivo and murine studies targeting dual blockade of TIGIT and either PD-1 or TIM-3 has shown a synergistic effect in immune cell proliferation, cytokine release, degranulation, and reversal of T cell exhaustion with subsequent tumor rejection and induction of protective memory responses [11, 32]. Importantly, the expression of TIGIT appears to be higher in the cells within tumor microenvironment than in those in the periphery, which would theoretically offer the advantage of a more targeted-directed therapy with less systemic autoimmune-like toxicities. Furthermore, TIGIT appears to exert its effects primarily by limiting cytokine competency and CD8 T cell function which would in theory explain its complementary effects when used with other forms of ICIs [27].

A phase I clinical trial is currently recruiting patients to evaluate the safety and efficacy of the anti-TIGIT mAb OMP-31M32 (NCT03119428). No results are yet available.


V-domain Ig suppressor of T cell activation (VISTA), also known as programmed death-1 homolog (PD-1H), is a unique molecule with dual activity. It behaves as a stimulatory ligand for antigen presenting cells (APCs) causing immune activation and as a negative ligand for T cells suppressing activation, proliferation, and cytokine production [33]. Le Mercier et al. demonstrated that its blockade improved TIL activation and enhanced tumor-specific T cell responses in the periphery despite the presence of high PD-L1 levels or the lack of expression of VISTA within tumor cells [34]. Therefore, both pathways are considered independent and simultaneous dual blockade of PD-1 and VISTA is often viewed as synergistic [35]. Interestingly, VISTA expression levels appear to vary among different tumors, often seen as a limitation given theoretical response heterogeneity. However, its blockade has proven to be effective even in the absence of detectable levels which offers the advantage of a broader clinical applicability but poses the challenge of finding specific biomarkers to predict response [35]. Additionally, this pathway is expressed mainly by TILs which, similar to TIGIT, allow it to be more tumor-specific and less toxic than other pathways.

Two molecules are being tested on phase I clinical trials: JNJ-61610588, a fully human mAb against VISTA, and CA-170, an oral inhibitor of both PD-L1/PD-L2 and VISTA. Both trials are currently recruiting (NCT02671955, NCT02812875).

B7-H3 (CD276)

B7 homolog 3 (B7-H3), also known as CD276, is a protein that belongs to the B7-CD28 pathway family and is widely expressed in different solid organs as well as immune cells including APCs, NKs, and B and T cells. It has an inhibitory function on T cell activation, proliferation, and cytokine production [36]. Furthermore, this pathway appears to promote cancer aggressiveness. Thus, blocking this agent would not only offer the advantage of enhancing innate immunological responses against malignancy but also would exert a direct effect over tumor behavior. B7-H3 expression is limited on healthy tissues but overexpression is common in multiple malignancies including melanoma, NSCLC, prostate, pancreatic, ovarian, and colorectal cancer (CRC) [36, 37]. Therefore, developing strategies to block this pathway would offer the advantage of exerting more localized effects over malignancies with less prominent systemic toxicities. Additionally, given its unique mechanism of action compared to other anticancer strategies, B7-H3 appears to have a synergistic effect when combined with chemotherapy or other ICIs [36].

Enoblituzumab (MGA271) is an engineered Fc humanized IgG1 monoclonal antibody against B7-H3 with potent anti-tumor activity. Interim results of an ongoing phase I clinical trial using MGA271 in melanoma, prostate cancer, and other solid tumors (NCT01391143) shows that it is overall well-tolerated without dose-limiting toxicities. Disease stabilization and objective responses ranging from 2 to 69% were noted across several tumor types [38]. Another phase I clinical trial is evaluating the use of enoblituzumab in combination with pembrolizumab (NCT02475213). Both studies are currently recruiting.

The use of dual affinity re-targeting (DART) proteins that bind both CD3 on T cells and B7-H3 on the target cell has been found to recruit T cells to the tumor site and promote tumor eradication [39]. MGD009 is a humanized DART protein that is being studied on a phase I clinical study in patients with B7-H3 expressing tumors including melanoma, NSCLC, mesothelioma, and urothelial cancers [40]. The trial is ongoing and recruiting patients (NCT02628535).

8H9 is an antibody against B7-H3 labeled with radioactive iodine (I-131) which, after internalization, promotes cancer cell death [36]. This drug has been tested on metastatic neuroblastoma in conjunction with radiation therapy and surgery [41]. The ongoing trial is assigning patients to be treated with either mAbs against B7-H3 or against GD-2 (NCT00089245). Preliminary results revealed that 17/21 patients studied were alive and free of disease after a median follow-up of 33 months [41]. 8H9 is also being studied on peritoneal cancers, gliomas, and advanced central nervous system malignancies (NCT01099644, NCT01502917, NCT00089245).

A2aR and CD73

The adenosine pathway encompasses specific adenosine receptors and enzymes that synthetize it. Adenosine A2a receptor (A2aR) is one of the most important factors in this pathway and is mainly activated by adenosine [1]. A2aR is expressed on immune cells, including T cells, APCs, NK cells, and on endothelial cells. Increased levels of adenosine in tumor microenvironment can promote formation of Treg cells and can dampen the immune response of multiple effectors including macrophages, NK, APCs, and neutrophils [42]. CD73, on the other hand, is widely expressed by most tissues and is thought to serve as an adhesion molecule for lymphocyte binding to the endothelium and to play an important role as a co-signal for T lymphocyte activation. However, it also widely expressed by malignant cells where it acts as an enzyme and promotes the formation of adenosine by the dephosphorylation of AMP, favoring tumor progression [43]. Not surprisingly, often these molecules are overexpressed in various malignancies and usually correlate with poor overall prognosis [44]. Given the multiple mechanisms that interact in this pathway and its importance in tumor microenvironment, different strategies to target both A2aR and CD73 have been developed. The main advantage of this approach is the potential use of combination strategies with other forms of therapy including chemotherapy or other ICIs. Furthermore, the use of combination strategies among the adenosine pathway is an additional possibility [44]. However, an area of concern with this approach is the blockade of adenosine synthetic enzymes which may favor the accumulation of ATP, a molecule that can play a pro-tumor role in the tumor microenvironment [44]. An additional limitation, as with other forms of immunotherapy, is the lack of clinical or biological markers that help with stratification of patients that are most likely to benefit from this form of therapy.

Blockade of A2aR on mice demonstrated increased proliferative capacity and function of T cells, as well as enhanced immunologic memory [42]. Preliminary results from a phase I clinical trial evaluating the oral adenosine A2aR antagonist CPI-444 alone and in combination with atezolizumab for advanced solid cancer showed that 42% of patients (10 of 24) who had been resistant to anti-PD-1/PD-L1 therapy, achieved disease control. Furthermore, grade 1 and 2 toxicities were the most common with only one case of grade 3 autoimmune hemolytic anemia [45]. This trial is ongoing and recruiting patients (NCT02655822).

MEDI9447 is a monoclonal antibody specific for CD73 that is being studied on a first-in-human clinical trial in patients with advanced solid tumors that have progressed or are refractory to standard therapy (NCT02503774). No preliminary results are yet available. Of note, CD73 could play a role in tumor angiogenesis; however, no studies have been designed yet to evaluate a possible synergistic effect of anti-CD73 and antiangiogenic therapy [46].


B and T cell lymphocyte attenuator (BTLA, CD272) is an inhibitory receptor that is structurally and functionally related to CTLA-4 and PD-1 and is expressed by the majority of lymphocytes. Ligation of BTLA by its ligand, herpes virus entry mediator (HVEM), blocks B and T cell activation, proliferation, and cytokine production [47]. Tumor cells exploit this pathway by either promoting the formation of dysfunctional T cells that persistently express BTLA and render them susceptible to inactivation, or by expressing HVEM, as it has been found with melanoma [47]. High levels of BTLA/HVEM on melanoma and gastric cancer patients correlate with poor prognosis [48, 49]. Thus, the BTLA-HVEM pathway is being considered as a new target for checkpoint blockade [48]. The main limitation with this form of therapy has been the complexity of the receptor-ligand system. Additionally, given a different mechanism of action compared to other forms of immunotherapy, combination with other molecules could be synergistic but also be associated with an increased risk of toxicity [47].

Non-T cell-associated inhibitory molecules


Transforming growth factor (TGF)-β is a cytokine that helps maintain tissue homeostasis by regulating cellular growth, differentiation, proliferation, and survival [50]. Although this pathway is able to control early-stage tumors by promoting cell cycle arrest and apoptosis, in advanced stages, it allows for tumor evasion by suppressing cytotoxic T cells and promotes cancer cell proliferation, invasion, and metastases, a functional switch known as the “TGF-β paradox” [51, 52]. Malignant cells achieve this switch through either the inactivation of their TGF-β receptors, or by selectively disabling the tumor-suppressive arm of this pathway, allowing cancer cells to use the TGF-β regulatory functions to their advantage by promoting immune tolerance [53]. In fact, tumors that produce high levels of TGF-β can shield themselves from immune surveillance [50]. Consistently, increased TGF-β expression by NSCLC, CRC, gastric, and prostate cancer has correlated with tumor progression and poor prognosis [50].

Many malignant cells have an abnormal TGF-β signaling pathway and blocking agents exert an indirect action mainly by acting over the cells within the tumor microenvironment [54]. This allows for potential combination with other forms of therapy including immune checkpoint targeting and chemotherapy. Some challenges to note with this approach include the lack of biomarkers that allow defining the microenvironment where these agents are most usefuland the potential risk of synchronous occult tumor growth by inhibiting the TGF-β suppressive action in early-stage cancers [54]. There are three methods for blocking the TGF-β pathway: blocking the ligand, ligand-receptor interaction, or the receptor tyrosine kinase activity. Trabedersen [AP12009], a synthetic antisense oligonucleotide that hybridizes with RNA sequences and blocks TGF-β translation, has been tested on patients with glioblastoma multiforme and anaplastic astrocytoma [55, 56]. It was also tested on advanced pancreatic cancer where OS improved by 9.9–11.8 months although no improvement on progression-free survival (PFS) was observed [57].

M7824 is a dual anti-PD-L1 monoclonal antibody fused with a soluble extracellular domain of TGF-β receptor II, which acts as a TGF-β trap. A phase I clinical trial is being conducted on patients with metastatic or locally advanced solid tumors using this novel chimeric molecule (NCT02517398). Preliminary results from a trial in 16 patients demonstrate an acceptable safety profile with no grade 4–5 adverse events. Preliminary assessments suggest clinical benefit with one patient demonstrating a CR, one with durable PR, one patient with a 25% reduction of target lesions after two doses, and two cases with prolonged stable disease (SD) [58].

Galusertinib (LY2157299), a blocker of the receptor’s tyrosine kinase activity was tested in a recent phase II clinical study, but failed to demonstrate improved OS compared to placebo [59]. This molecule is being studied on NSCLC, hepatocellular carcinoma (HCC), pancreatic cancer, and BC (NCT02423343, NCT02734160, and NCT02672475).


Killer immunoglobulin-like receptors (KIRs, CD158) are a family of transmembrane proteins that promote self-tolerance by dampening lymphocyte activation, cytotoxic activity, and cytokine release. They are expressed by NK cells and some T cells and assist with self-recognition of host cells through the binding of MHC-I. KIR aids in the identification and destruction of cells that have lost their MHC-I as with many tumor cells, a process termed “missing self” recognition [60]. Some malignancies, however, develop mechanisms to evade this pathway by either upregulating non-classical MHC-I molecules or by changing the tumor microenvironment properties rendering NK cells dysfunctional [61].

The use of monoclonal antibodies to manipulate the KIR pathway is an active area of investigation as interfering with MHC-I interactions can stimulate NK cells by mimicking the “missing self” response [62]. The main advantage of targeting KIR is activating mostly NK rather than T cells, which is a potentially synergistic antitumor approach by allowing T cell ligands be available for targeting with other forms of immunotherapy. However, given its importance in self-recognition, NK cell overactivation may lead to a proinflammatory state and increase the risk for autoimmune reactions [63]. Different molecules targeting KIR are under investigation. Lirilumab, a fully human monoclonal antibody that blocks KIR2DL1/2L3, is currently being studied in a phase I/II clinical trial with concurrent use of nivolumab and ipilimumab in patients with squamous cell carcinoma of the head and neck (NCT01714739). Preliminary results are promising, with an ORR of 24% and a DCR of 52%, and only 8% of patients stopping therapy due to adverse events [64].

KIR3DL2 is frequently expressed by cutaneous T cell lymphomas (CTCL) and has prognostic and diagnostic features within this population [65]. IPH4102 is a monoclonal antibody against kIR3DL2 which is currently being investigated in a phase I clinical trial in patients with relapsed or advanced CTCL (NCT02593045). Preliminary results reveal an ORR of 45%; 10 out of 22 patients with PR, 2 CR in skin, and 5 CR in blood. Six patients developed grade 3 or more severe adverse events [66].


The expression of Phosphoinositide 3-kinase gamma (PI3Kγ) by macrophages controls a critical switch towards immune suppression in presence of inflammation and cancer. Additionally, PI3Kγ seems to play a role in angiogenesis by affecting the function of tumor-associated macrophages, major producers of VEGF [67]. Thus, similar to TGF-β, blocking this pathway exerts an indirect antitumor effect by modifying the microenvironment, improving the immunological function against malignant cells, and affecting the tumor vasculature. Unfortunately, as with other forms of immunotherapy, blocking PI3K enzymes has been associated with multiple autoimmune-like toxicities, and therefore the use of lower doses in conjunction with other forms of immunotherapy is often used [67].

IPI-549 is an oral selective inhibitor of PI3Kγ being studied on a phase I clinical trial as monotherapy or in combination with nivolumab in patients with melanoma, NSCLC, or head and neck cancer (NCT02637531). Preliminary results demonstrate no dose-limiting toxicities and only mild adverse events including nausea and fatigue. Importantly, 12 out of 15 patients have demonstrated durable clinical benefit, and 50% of patients have been able to remain on treatment ≥ 16 weeks [68].


CD47, also known as integrin-associated protein, is a molecule that exerts its action through the signal regulatory protein alpha (SIRPα). It is ubiquitously expressed by healthy cells to help with autologous recognition and avoid inappropriate phagocytosis [69]. Solid tumors (e.g., bladder and BC) and hematologic cancers (e.g., acute myeloid leukemia and non-Hodgkin’s lymphoma) overexpress CD47 causing an inhibitory effect over macrophages and other myeloid cells and high levels of CD47 correlate with poor prognosis [69]. The blockade of the CD47/ SIRPα axis results in an increased macrophage recruitment and antitumor activity through phagocytosis and cytokines secretion. However, the use of this pathway has pointed certain limitations mainly derived from the diffuse expression of CD47. First, a potential “antigen sink” effect where high doses may be required to achieve an appropriate therapeutic blockade [70]. Second, there is an increased risk for “on-target” systemic toxicities over healthy cells that express CD47. Until now, therapy has been overall well tolerated and anemia has been the most common adverse event [70]. Hu5F9-G4, a humanized monoclonal antibody targeting CD47, is being studied on a phase 1 clinical trial in patients with solid tumors (NCT02216409). In preliminary results, it showed acceptable tolerability and SD in 2 out of 16 patients for 16 and 8 months, respectively [71]. Another phase I/II clinical trial using this molecule in combination with rituximab in patients with relapsed or refractory B cell non-Hodgkin’s lymphoma is still recruiting patients (NCT02953509).

TTI-621 (SIRPαFc) is a fully recombinant fusion protein consisting of a CD47 binding domain linked to the Fc region of IgG1 to block the CD47 “do not eat me” signal and engage macrophage Fcγ receptors to enhance phagocytosis and antitumor activity [72]. A phase I clinical trial using TTI-621 in patients with relapsed or refractory percutaneously accessible solid tumors and mycosis fungoides is currently recruiting patients (NCT02890368).

Co-stimulatory pathways

As opposed to inhibitory pathways that attenuate the immune system, co-stimulatory molecules augment immunological responses against malignant cells. Malignant cells inhibit these pathways to promote tumorigenesis [5].


OX40 (CD134) is a member of the TNF receptor super family, highly expressed by activated CD4, CD8 T cells, and Tregs, and in a lesser degree by neutrophils and NK cells. This molecule, along with its ligand, OX40L, plays a pivotal role in activation, potentiation, proliferation, and survival of T cells and modulation of NK cell function [73]. Furthermore, this molecule inhibits the suppressive activity of Tregs by directly interfering with their function and proliferation, and indirectly antagonizing their inhibitory byproducts (e.g., TGFβ) [74]. Importantly, when tumor antigens are recognized by TILs, its expression of OX40 increases, and not surprisingly, the amount of OX40-expressing TILs correlates with improved prognosis in certain populations [75].

The use of mAbs to activate OX40 has been a strategy used to increase the antitumor activity by the immune system. Of note, these antibodies have been associated with depletion of TILs through an antibody-dependent cell cytotoxicity. NK cells recognize the antibodies bound to antigens over cell surfaces and kill these cells [76]. However, this only occurs in the presence of NKs within the tumor, which varies depending on the host and type of malignancy. Another limitation is a potential activation of peripheral lymphocytes rather than TILs when therapy is given systemically. Thus, its intratumoral administration has been proposed as a way to minimize systemic toxicity [76]. Despite its limitations, use of these antibodies has demonstrated tumor regression in several preclinical models, although often are used in conjunction with other forms of immunotherapy [75]. 9B12 is a murine IgG monoclonal agonistic antibody against OX40 that was studied in a phase I clinical trial in 30 patients with metastatic solid malignancies [77]. Although no patients achieved PR, SD was achieved in 6 patients. Adverse events were overall tolerable and limited to grades 1 and 2 except for transient lymphopenia which was found to be grade 3 or more in 7 patients [77].

MOXR 0916 is a humanized IgG agonistic monoclonal OX40-specific antibody that is currently being tested in combination with atezolizumab in patients with advanced solid malignancies (NCT02410512). Preliminary results show no dose-limiting toxicities but efficacy results are not yet available [78]. PF-04518600 (PF-8600) is an IgG2 humanized agonistic monoclonal antibody of OX40 that is undergoing a first-in-human trial (NCT02315066). Preliminary results in patients with selected advances solid tumors including melanoma and NSCLC revealed no dose-limiting toxicities, and 4 out of 9 patients demonstrated SD [79].

MEDI6383, MEDI0562, MEDI6469, INCAGN01949, and GSK3174998 are other agonistic monoclonal antibodies that are part of different phase I clinical trials for which no preliminary results are yet available (NCT02221960, NCT02528357, NCT02923349, NCT02705482).


Glucocorticoid-induced TNF receptor family-related protein (GITR) is a co-stimulatory cell surface receptor that is constitutively expressed by T cells and NK cells, and expression increases markedly following T cell activation. Its ligand, GITRL, is mainly expressed by APCs and endothelial cells and appears to have a role in upregulating the immune system, leukocyte adhesion, and migration [80]. The expression of GITR by TILs in the tumor microenvironment has been found to be higher than levels expressed by peripheral lymphocytes, indicating local T cell activation [80]. Agonizing agents of this pathway have been considered as a way to increase the immune antitumor activity, although the clinical utility of such agents depends on the presence of T cells in the tumor and the subset of TILs which may vary among different malignancy [81]. Thus, selection of patients who will derive the most benefit from this therapy is still unclear. Immune-related adverse events should also be considered. Preclinical data suggests that GITR therapy appears to be better tolerated than anti-CTLA4 agents [81].

GITR modulation in the preclinical models has shown promising antitumor activity via significant increase in effector T cells and decrease in Tregs [80]. TRX-518, an aglycosylated human mAb that agonizes GITR, is currently undergoing phase I clinical study in various solid malignancies (NCT01239134). Preliminary results demonstrate an acceptable safety profile without dose-limiting toxicities and SD in 10% of study patients (4 out of 40 patients) [82]. BMS-986156 is another anti-GITR antibody that being studied in a phase I clinical trial alone or in combination with nivolumab in patients with advanced solid tumors (NCT02598960). Preliminary results showed no dose-limiting toxicities, though no efficacy results were reported [83]. AMG 228, an agonistic IgG1 monoclonal antibody of GITR, was also recently studied in a first-in-human clinical trial in 30 patients with refractory CRC, head and neck squamous cell carcinoma, urothelial carcinoma, and melanoma [84]. None of the patients demonstrated OR, and no dose-limiting toxicities were identified. Up to 90% of patients (27/30) experienced adverse events consisting of electrolyte imbalances, anemia, and fever [84].

Other similarly agents including MEDI1873, MK-4166, INCAGN01876, and GWN323 are also being studied in multiple solid and hematologic malignancies (NCT02583165, NCT02132754, NCT02697591, NCT03126110, NCT02740270).


Inducible co-stimulator (ICOS), a specific T cell co-stimulatory molecule of the CD28/CTLA-4 family mainly expressed by CD4 T cells, is a co-stimulator of proliferation and cytokine production by these cells [85]. Its levels are upregulated in activated T lymphocytes, especially after the use of anti-CTLA4 therapies, and its expression is considered a biomarker to indicate that anti-CTLA4 agents are binding its target [86]. Increased ICOS expression on circulating T cells after ipilimumab administration has been associated with improved clinical outcomes [87]. Interestingly, ICOS appears to be a less potent pathway compared to other forms of immunotherapy mainly because of a predominant CD4 expression. However, its use with other approaches, particularly CTLA4 blockade, can lead to a potent synergistic effect as a result of an increase in the expression of ICOS after anti-CTLA4 therapy [85].

Some molecules have been developed and are being investigated. JTX-2011 is an agonistic monoclonal antibody of ICOS that is currently being tested in a phase I/II clinical trial alone and in combination with nivolumab in patients with advanced solid malignancies including endometrial, breast, lung, pancreatic, and CRC (ICONIC Trial—NCT02904226). Preliminary results showed no dose-limiting toxicities, although efficacy is not reported [88]. Similarly, GSK3359609 is a humanized IgG4 monoclonal agonistic antibody of ICOS that is undergoing clinical investigation in a phase I clinical trial, alone or in combination with pembrolizumab in patients with advanced solid tumors (INDUCE-1 trial - NCT02723955). Finally, MEDI-570, an agonist monoclonal IgG1 antibody directed against ICOS is also being studied in a phase I clinical trial in patients with Non-Hodgkin lymphomas (NCT02520791).


4-1BB (CD137) is an inducible co-stimulatory receptor expressed by T cells, NK cells, and APCs. Once expressed, it binds its ligand (4-1BBL) and triggers subsequent immune cell proliferation and activation, particularly of T and NK cells [89]. The activation of NK cells leads to an increased antibody-dependent cell-mediated toxicity. Thus, the use of anti-41BB agonists not only increases immune-mediated antitumor activity but is also considered an ideal agent to use in combination with other monoclonal antibodies such as rituximab and trastuzumab [89]. Of note, the use of 4-1BB antibodies in conjunction with other ICIs may lead to an important antitumor response with potential increased toxicity. In fact, given the diffuse expression of 4-1BB, there is a notorious risk for “on-target” systemic adverse events [89].

These antibodies have been expanded to clinical studies after demonstrating potent anti-cancer efficacy in murine models [90]. Utomilumab (PF-05082566), a fully human mAb that stimulates 4-1BB, has been studied in a phase I clinical trial in combination with pembrolizumab in patients with advanced solid tumors [91]. No dose-limiting toxicities were reported and 6 out of 23 patients had either CR or PR. This drug is currently being studied in multiple phase I clinical trials: alone or in various combinations with rituximab (NCT01307267), mogamulizumab (NCT02444793), an experimental OX40 agonist (NCT02315066), and avelumab (NCT02554812).

Urelumab is another agonist antibody of 4-1BB that has been studied in various clinical trials in patients with advanced solid tumors. A safety analysis from these trials concluded that this agent can occasionally cause significant transaminitis when high doses are used [92]. Currently, this medication is being evaluated in combination with nivolumab in a phase I/II clinical trial in patients with solid tumors and B cell non-Hodgkin’s lymphoma (NCT02253992). Preliminary results showed that 6/60 of the patients with lymphoma treated with urelumab monotherapy achieved a PR (n = 3) or CR (n = 3), 9/86 patients who received combination therapy achieved PR although none of the patients with NSCLC or diffuse large B cell lymphoma had reported response. Of note, at least 3% of patients developed grade 3–4 transaminitis, and 7% of the 123 enrolled patients developed serious adverse events leading to discontinuation in 5% of study patients [93]. Another phase I clinical trial evaluating urelumab in combination with rituximab is being conducted in patients with metastatic solid tumors and refractory NHL (NCT01471210). No results have been yet published.

According to a recent comparison between urelumab and utomilumab, the former seems to exert a more marked agonistic activity on the receptor [94].


Binding of CD27, a member of the TNF receptor family, with its ligand CD70, results in a potent signal to activate and differentiate T cells into effector and memory cells, and to boost B cells [95]. Despite its wide spectrum of action, this pathway has not demonstrated to be particularly effective in overcoming the immunosuppressive features of the tumor microenvironment. Thus, CD27 is considered most useful as combination rather than monotherapy. Furthermore, its use with other blocking agents like anti-CTLA-4 or anti-PD-1/PD-L1 may not only be synergistic but also associated with less autoimmune toxicities [96]. When used as monotherapy, CD27 agonist has been well tolerated and only minor adverse events are reported. An important aspect in this pathway is the identification of CD27 phenotype on tumor, as cancers that express this molecule could achieve a more favorable outcome [96].

The use of CD27-CD70 agonist agents has been evaluated in various preclinical settings and is being studied in multiple clinical trials. ARGX-110 is an agonistic anti-CD70 monoclonal antibody that has been studied in a phase I clinical trial in patients with T cell lymphoma [97]. Of note, 2 out of 9 patients had a reduction of malignant clones of > 90%, one patient achieved radiological PR, and 2 patients reached PR in the skin. Currently one phase I clinical trial is recruiting patients with advanced malignancies (NCT01813539). BMS-936561 (MDX-1203) is another fully human monoclonal agonistic CD70-specific antibody that was studied in RCC and B cell lymphoma [98]. Results demonstrated disease stabilization in 69% of treated individuals. Varlilumab, a monoclonal agonistic antibody against CD27, is currently under investigation in a phase I clinical trial with simultaneous use of nivolumab in patients with advanced solid tumors (NCT02335918). Preliminary results showed a notable increase of TILs in post-treatment biopsies [99]. Currently this molecule is being studied in other phase I and II clinical trials in patients with gliomas, melanomas, RCC, and other solid tumors (NCT02924038, NCT02302339, NCT02386111, NCT02543645).


CD40 is a member of the TNF receptor family expressed by APCs and B cells whereas its ligand, CD154, is expressed by activated T cells. Interaction between CD40-CD154 stimulates cytokines secretion of B cells with subsequent T cell activation and tumor cell death [100]. Despite its potential synergy with other forms of anticancer therapy, the use of CD40 agonists has also been associated with particular toxicities including cytokine release syndrome, thromboembolic events, and tumor angiogenesis. It is probably related to the expression of CD40 by platelets and endothelial cells [101]. The main challenges that remain with this particular form of therapy include the identification of appropriate combinations and patient population that would benefit from these agents. As of now, eight mAbs have entered clinical trials: CP-870893, APX005M, ADC-1013, lucatumumab, Chi Lob 7/4, dacetuzumab, SEA-CD40, and RO7009789. Some of these were recently reviewed [102, 103]. Others are still under investigation (NCT02482168, NCT03165994, NCT02706353, NCT03123783, NCT02829099, NCT02588443, NCT02760797, NCT02665416, NCT02304393).

Other potential pathways


Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-degrading enzyme that converts tryptophan to kynurenines. Kynurenines promote the differentiation and activity of Treg and decrease the amount and activity of CD8 T cells leading to an immunosuppressed environment only worsened by the high levels of PD-1/PD-L1 concurrently present in this milieu [104]. IDO has been found overexpressed in various tumor cell types including melanoma, chronic lymphocytic leukemia, ovarian, CRC, and more recently in sarcomas [104, 105]. Furthermore, high levels of IDO not only correlate with poor outcomes in some malignancies but may also be involved in drug resistance to chemotherapeutic agents [106]. Though their ability to counterbalance the immunosuppressive tumor microenvironment is promising, treatment with IDO inhibitors has also raised specific concerns. First, IDO is induced by inflammatory molecules such as IFNγ. Therefore, the lack of inflammation in the tumor microenvironment may be associated with a suboptimal response to anti-IDO agents [106]. Second, IDO and other similar enzymes are also expressed by healthy tissue, and its inhibition may lead to cross-reaction side effects. Regardless, IDO inhibitors remain a great area of interest among immune checkpoint therapy and different molecules are under investigation.

BMS-986205 is a once-daily, selective, and potent oral IDO1 inhibitor that is currently undergoing in a phase I clinical trial with concomitant use of nivolumab (NCT02658890). All reported toxicities have been grades 1–2 except for three cases of grade 3 hepatitis, rash, and hypophosphatemia. No efficacy was reported [107].

Indoximod is another IDO inhibitor that is being studied in phase II clinical trials in melanoma (NCT02073123), pancreatic cancer (NCT02077881), and castrate-resistant prostate cancer (CRPC) (NCT01560923). Results seem promising. ORR was 52% in patients with melanoma in whom indoximod was given with either ipilimumab, nivolumab, or pembrolizumab [108]. Patients with pancreatic cancer had an ORR of 37% with concomitant use of indoximod, gemcitabine, and nab-paclitaxel [109]. With indoximod, median PFS has increased from 4.1 to 10.3 months in metastatic CRPC compared to placebo [110].

Finally, epacadostat is another oral agent that blocks IDO pathway and is undergoing investigation in phase I/II clinical trials evaluating multiple malignancies (NCT02327078, NCT02178722). Preliminary results have demonstrated an ORR ranging from 75% in melanoma to 4% in CRC. Its use seems to be safe with pembrolizumab. Although no dose-limiting toxicities have been identified, up to 3% of patients have discontinued therapy due to adverse events [111, 112]. In another completed phase I clinical trial with 52 patients who had advanced solid tumors (INCB024360), treatment with epacadostat demonstrated overall well tolerable adverse reactions except for 1/52 grade 3 pneumonitis and 1/52 grade 3 fatigue. No OR was reported, but 7/52 patients achieved SD greater than 16 weeks [113].


Toll-like receptors (TLRs) are considered critical in the recognition of pathogens and control of the immune response. However, their role in tumorigenesis is far more complex. Some TLRs, like TLR4, may promote cancer progression by either favoring inflammation in the tumor microenvironment or inducing Tregs or PD-L1. Other TLRs like TLR7/8 and TLR9, induce antitumor responses by promoting a “danger signal” within the tumor microenvironment and activating the immune system against malignant cells [114]. The use of agents to manipulate these TLRs pathways seem to not only promote an immune response against malignancy but also induce autophagy and apoptosis of cancer cells [115]. There are certain important aspects to note with TLR therapy. First, its non-specific capability of inducing not only cytotoxic T cells but also immunosuppressive cells within the tumor microenvironment leads to an overall attenuated tumoricidal effect [116]. Second, an appropriate combination partner and identification of patients that would benefit the most of these agents remains unclear. It has been established that concomitant use of these molecules with other forms of antitumor therapy including radiation and chemotherapy appears to offer stronger anticancer responses than either therapy alone [117]. These combinations, unfortunately, may also be associated with an increased frequency of toxicities and autoimmune reactions. Despite these challenges, multiple agents are being evaluated in different clinical trials. MEDI9197 is a dual agonist of TLR7/8 that is currently under phase I clinical test in combination with durvalumab and radiation therapy in metastatic or locally advanced solid malignancies (NCT02556463). Preliminary results demonstrate that the agent is overall safe with only mild adverse events. No efficacy data has been yet reported [118]. PG545 (pixatimod, pINN) is an agonist of TLR9/IL-12 that was tested in a phase I clinical trial in patients with advance solid tumors (NCT02042781). Results revealed that 3 out of 23 patients developed dose-limiting toxicities, and the disease control rate of 38% [119].

Polyinosinic-polycytidylic acid polylysine carboxymethylcellulose (poly-ICLC) is a potent TLR3 agonist that has been recently studied in combination with radiation in a phase I clinical trial in patients with HCC not eligible for surgery [120]. Intratumoral injection of this agent was found to be overall safe with mostly grade I or II adverse events. A PFS of 66% at 6 months and 28% at 24 months, OS of 69% after 1 year and 38% after 2 years were demonstrated [120].


IL-2 mediates its immune-enhancing effect through either a low-affinity dimeric and/or a high-affinity trimeric IL-2 receptor (IL-2R). The dimeric IL-2R consists of CD122 (also known as IL-2Rβ) and CD132 (also known as ϒc), whereas the trimeric IL-2R comprises an additional component, the CD25 (also known as IL-2Rα) which increases the affinity for its ligand [121].

IL-2 has been part of cancer treatment for many decades and is considered the first immunotherapy proven to be effective in human cancer in 1984 [121]. However, IL-2 has had certain limitations including a dual role enhancing both T cells and Tregs favoring immunosuppression, and a short life span with subsequent high doses requirements and potential severe toxicities including pulmonary edema, hypotension, and vascular leak syndrome [122]. In need of better strategies, IL-2R agonists have been developed to potentiate and prolong IL-2 antitumor effects allowing for lower doses and decreased toxicities [123]. Furthermore, IL-2R agonists could also enhance other forms of immunotherapy without the associated toxicity provided by IL-2.

NKTR-214, an engineered cytokine that specifically stimulates through CD122 (IL-2Rβ), is being tested in solid tumors including melanoma, NSCLC, and BC (NCT02869295, NCT02983045). Studies using both NKTR-214 and nivolumab showed no dose-limiting toxicities. One patient had a mixed radiographic response with a 40% decrease in LDH, and another patient had an unconfirmed CR after only 6 weeks of treatment [124]. Another trial showed no dose-limiting toxicities, a tumor shrinkage ranging from 10 to 30% in 6 out of 26 patients (23%) and an increase of T cells and NK cells within the tumor microenvironment in 100% of patients [125].

Arginase inhibitors

Arginine is an important amino acid for T cell activation and proliferation. High levels of arginase are produced by malignant cells and MDSCs leading to depletion of arginine and a subsequent immunosuppressive tumor microenvironment [126]. The use of arginase inhibitors could allow overcoming the immunosuppressive effects of the tumor microenvironment and achieve a better antitumor control with the use of other immune checkpoint inhibitors or radiation therapy. Furthermore, the blockade of arginase may also have direct antitumor effects by decreasing the availability of substances that favor tumor growth [127]. Finally, given a higher expression of arginine among the tumor microenvironment than that in plasma, the use of these molecules could be associated with a more specific and less toxic effect than other forms of immunotherapy.

CB-1158 is a selective arginase inhibitor being studied in a phase I clinical trial alone or in combination with nivolumab in patients with metastatic solid tumors (NCT02903914). Preliminary results show that the drug is well tolerated with no dose-limiting toxicities, > 90% of arginase inhibition, and up to a fourfold increase in plasma arginine levels [128].

Oncolytic peptides

Lactoferrin-derived lytic peptide LTX-315 is a cytotoxic chemotherapeutic peptide that permeabilizes mithocondrial membrane and triggers caspase-independent necrosis [129]. This agent modifies the tumor microenvironment by decreasing immunosuppressive cells and increasing T cells [130]. Intratumoral injection of this agent leads to tumor antigen release, with subsequent increase of TIL activity. This form of administration makes it an attractive way to limit systemic toxicities, but it also limits its applicability to more localized malignancies. Another important aspect of LTX-315 is the substantial increase of CTLA-4 expression following its administration. This suggests that this form of therapy may be particularly useful when used in conjunction with anti-CLTA-4 agents [131].

A phase I clinical trial using this molecule as monotherapy or in combination with ipilimumab or pembrolizumab is being conducted in patients with metastatic solid tumors, particularly melanoma and BC (NCT01986426). Preliminary results showed that 2/28 patients achieved a CR, 5 patients had a decreased of > 50% of tumor size, and 8 patients achieved SD [132].


IL-10 inhibits secretion of proinflammatory cytokines (e.g., IFNγ, TNFα, IL-1β, IL-6) and also inhibits the expression of MHC molecules and costimulatory molecules at several levels, leading to inhibition of T cell function [133]. Recently, IL-10 was also found to play some antitumor role by inducing the activation and proliferation of CD8. CD8 cells expressing IL-10 has been associated with a favorable prognosis in patients with lung cancer [134]. However, similar to other interleukins like IL-2, its effects are pleotropic and this raises concern for potential systemic toxicity. Other unresolved issues similar to IL-2 therapy include determining the patient population that could benefit the most from this form of therapy and the most appropriate therapeutic combinations [135]. In this regard, both PD-1 and IL-10 receptors are upregulated in TILs and therefore the combined use of these molecules is reasonable [136]. AM0010 is a PEGylated recombinant human IL-10 that is currently being studied in combination with pembrolizumab in melanoma patients in a phase I clinical trial (NCT02009449). Preliminary results revealed that 11 out of 25 recruited patients developed grade 3 or 4 adverse events including fatigue, thrombocytopenia, and anemia. Although no objective tumor response was seen, DCR was 45% [137].

Limitations and challenges of immune checkpoint therapy

Although immune checkpoint therapy has been a great advancement in cancer treatment, several challenges such as immune-associated toxicity, treatment resistance, and clinical benefit limited to only a fraction of patients remain unresolved.

Immune checkpoint therapies are often associated with a set of toxicities known as immune-related adverse events, a form of autoimmune-like reactions resulting from an increased activity of the immune system. These toxicities can manifest as generalized symptoms including fatigue or fever, or can produce organ-specific damage leading to rash, colitis, pneumonitis, and adrenal or thyroid insufficiency, among many others [138,139,140]. Thus, using immune checkpoint therapy mandates a comprehensive understanding of these adverse events from clinicians as a way to prevent, recognize, and appropriately treat each specific reaction. Most adverse events are resolved with interruption of treatment and short course of steroids. Serious pneumonitis and colitis refractory of steroids may require use of biological agents, like infliximab [141,142,143,144].

Despite durable response rates observed with immune checkpoint therapy, the majority of patients do not benefit from the treatment (primary resistance), and some responders develop cancer progression after initial response (acquired resistance) [145]. Even within the same patient, heterogeneous responses have been observed in different metastatic lesions. Both tumor intrinsic and micro-environmental extrinsic factors contribute to this resistance. Tumor intrinsic mechanisms for resistance include the absence of tumor antigen, loss or downregulation of MHC, alteration of antigen presenting machinery such as beta-2 microglobulin mutation, alteration of pathways that prevent immune cell infiltration or function (mitogen-activated protein kinase, PI3K, WNT/b-catenin, Interferon-gamma pathways), and escape mutations in IFN signaling [145, 146]. Resistance can also be derived from extrinsic factors from tumor microenvironment. Regulatory T cells (Treg), MDSCs, M2 macrophages, and other inhibitory immune checkpoints may all contribute to inhibition of anti-tumor immune responses [147]. Understanding these mechanisms will assist with the process of designing new strategies to overcome resistance and provide the rationale for combination of different forms of immunotherapy [145, 147].

Clinical responses to checkpoint immunotherapy are variable. The identification of biomarkers to predict response and treatment-mediated toxicity remains an important unresolved issue. A number of biomarkers have been found promising. For example, immunohistochemical determination of PD-L1 expression, high mutational load, selective CD8+ T cell infiltration, and distribution at tumor invasive margins correlate with clinical response to anti-PD-1/PD-L1 treatment [148, 149]. A study demonstrated that the presence of epithelial-mesenchymal transition correlates with a distinct tumor microenvironment in lung cancer consisting of elevated inflammatory signals and multiple immune checkpoints [150]. Specific genes involved in chromatin remodeling may also serve as markers of response. As an example, the loss of function of the PBRM1 gene encoding for the chromatin remodeling complex SWI/SNF was recently found to correlate with response to anti-PD-1 therapy in patients with clear cell RCC [151]. A recently developed model using malignancy-specific neoantigens appears to predict tumor response to ICI therapy in patients with melanoma and lung cancer receiving anti-CTLA-4 and anti-PD-1 therapy, respectively [152]. Furthermore, this model may also be useful to identify acquired resistance to therapy.

Lastly, immunotherapy is expensive and the cost per-quality life-year gained can be prohibitive in many developing countries, limiting its access to the eligible patients.


Significant advances have been made in cancer immunotherapy in the last decade. Immune checkpoint therapy, particularly anti-CTLA4, anti-PD-1, or anti-PD-L1 antibodies, has revolutionized oncology care and quickly has become the standard of care in multiple malignancies. Immunotherapy targeting immune checkpoints is often better tolerated than traditional chemotherapy and durable responses are frequently seen. However, the clinical benefit has been limited to a subset of cancer patients. Furthermore, some who initially respond to treatment often relapse due to cancer resistance. Expanding clinical benefit to the majority of patients and preventing cancer resistance requires a better understanding of the mechanisms that lead to an effective anti-tumor response. The discovery of new immune inhibitory, stimulatory pathways, and rational combination strategies as discussed in this article will likely shed the light to the next step towards improvement of cancer immunotherapy.



Adenosine A2a receptor


American Association for Cancer Research


Antigen presenting cells


American Society of Clinical Oncology


B7 homolog 3


Breast cancer


B and T cell lymphocyte attenuator


Colorectal cancer


Castrate-resistant prostate cancer


Cutaneous T cell lymphomas


Cytotoxic T lymphocyte-associated molecule-4


Dual affinity re-targeting


Disease control rate


Hepatocellular carcinoma


Herpes virus entry mediator


Radioactive iodine


Immune checkpoint inhibitors


Inducible co-stimulator


Indoleamine 2,3-dioxygenase






IL-2 receptor


Killer immunoglobulin-like receptors


Lymphocyte activation gene-3


Myeloid-derived suppressor cells


Major Histocompatibility Complex


Natural killer


Non-small cell lung carcinoma


Objective responses


Objective response rate


Programmed cell death receptor-1


Programmed death-1 homolog


Programmed cell death ligand-1


Progression-free survival


Phosphoinositide 3-kinase gamma


Polyinosinic-polycytidylic acid polylysine carboxymethylcellulose


Renal cell carcinoma


Stable disease


Transforming growth factor-β


T cell immunoglobulin and ITIM domain


Tumor-infiltrating lymphocytes


T cell immunoglobulin-3


Toll-like receptors


V-domain Ig suppressor of T cell activation


  1. 1.

    Pardoll DM. The blockade of immune checkpoints in cancer immunotherapy. Nat Rev Cancer. 2012;12:252–64.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  2. 2.

    Drake CG, Lipson EJ, Brahmer JR. Breathing new life into immunotherapy: review of melanoma, lung and kidney cancer. Nat Rev Clin Oncol. 2014;11:24–37.

    CAS  PubMed  Article  Google Scholar 

  3. 3.

    Chen L, Flies DB. Molecular mechanisms of T cell co-stimulation and co-inhibition. Nat Rev Immunol. 2013;13:227–42.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  4. 4.

    Sharma P, Allison JP. The future of immune checkpoint therapy. Science. 2015;348:56–61.

    CAS  PubMed  Article  Google Scholar 

  5. 5.

    Sharma P, Allison JP. Immune checkpoint targeting in cancer therapy: toward combination strategies with curative potential. Cell. 2015;161:205–14.

    CAS  PubMed  Article  Google Scholar 

  6. 6.

    Ledford H. Melanoma drug wins US approval. Nature. 2011;471:561.

    CAS  PubMed  Article  Google Scholar 

  7. 7.

    Granier C, De Guillebon E, Blanc C, et al. Mechanisms of action and rationale for the use of checkpoint inhibitors in cancer. Esmo Open. 2017;2

  8. 8.

    Marin-Acevedo JA, Soyano AE, Dholaria B, et al. Cancer immunotherapy beyond immune checkpoint inhibitors. J Hematol Oncol. 2018;11

  9. 9.

    Workman CJ, Dugger KJ, Vignali DA. Cutting edge: molecular analysis of the negative regulatory function of lymphocyte activation gene-3. J Immunol. 2002;169:5392–5.

    CAS  PubMed  Article  Google Scholar 

  10. 10.

    Dholaria B, Hammond W, Shreders A, Lou Y. Emerging therapeutic agents for lung cancer. J Hematol Oncol. 2016;9:138.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  11. 11.

    Anderson AC, Joller N, Kuchroo VK. Lag-3, Tim-3, and TIGIT: co-inhibitory receptors with specialized functions in immune regulation. Immunity. 2016;44:989–1004.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  12. 12.

    He Y, Rivard CJ, Rozeboom L, et al. Lymphocyte-activation gene-3, an important immune checkpoint in cancer. Cancer Sci. 2016;107:1193–7.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  13. 13.

    Woo SR, Turnis ME, Goldberg MV, et al. Immune inhibitory molecules LAG-3 and PD-1 synergistically regulate T-cell function to promote tumoral immune escape. Cancer Res. 2012;72:917–27.

    CAS  PubMed  Article  Google Scholar 

  14. 14.

    Goldberg MV, Drake CG. LAG-3 in cancer immunotherapy. Curr Top Microbiol Immunol. 2011;344:269–78.

    CAS  PubMed  PubMed Central  Google Scholar 

  15. 15.

    Andrews LP, Marciscano AE, Drake CG, Vignali DAA. LAG3 (CD223) as a cancer immunotherapy target. Immunol Rev. 2017;276:80–96.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  16. 16.

    Brignone C, Escudier B, Grygar C, et al. A phase I pharmacokinetic and biological correlative study of IMP321, a novel MHC class II agonist, in patients with advanced renal cell carcinoma. Clin Cancer Res. 2009;15:6225–31.

    CAS  PubMed  Article  Google Scholar 

  17. 17.

    Wang-Gillam A, Plambeck-Suess S, Goedegebuure P, et al. A phase I study of IMP321 and gemcitabine as the front-line therapy in patients with advanced pancreatic adenocarcinoma. Investig New Drugs. 2013;31:707–13.

    CAS  Article  Google Scholar 

  18. 18.

    Brignone C, Gutierrez M, Mefti F, et al. First-line chemoimmunotherapy in metastatic breast carcinoma: combination of paclitaxel and IMP321 (LAG-3Ig) enhances immune responses and antitumor activity. J Transl Med. 2010;8:71.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  19. 19.

    Nguyen LT, Ohashi PS. Clinical blockade of PD1 and LAG3—potential mechanisms of action. Nat Rev Immunol. 2015;15:45–56.

    CAS  PubMed  Article  Google Scholar 

  20. 20.

    Ascierto PA, Melero I, Bhatia S et al. Initial efficacy of anti-lymphocyte activation gene-3 (anti–LAG-3; BMS-986016) in combination with nivolumab (nivo) in pts with melanoma (MEL) previously treated with anti–PD-1/PD-L1 therapy. J Clin Oncol 2017; 35: 9520–9520.

  21. 21.

    Sakuishi K, Ngiow SF, Sullivan JM, et al. TIM3+FOXP3+ regulatory T cells are tissue-specific promoters of T-cell dysfunction in cancer. Oncoimmunology. 2013;2:e23849.

    PubMed  PubMed Central  Article  Google Scholar 

  22. 22.

    Du W, Yang M, Turner A, et al. TIM-3 as a Target for Cancer Immunotherapy and Mechanisms of Action. Int J Mol Sci. 2017;18:645.

    PubMed Central  Article  Google Scholar 

  23. 23.

    Gorman JV, Starbeck-Miller G, Pham NL, et al. Tim-3 directly enhances CD8 T cell responses to acute Listeria monocytogenes infection. J Immunol. 2014;192:3133–42.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  24. 24.

    Ebrahim AH, Alalawi Z, Mirandola L, et al. Galectins in cancer: carcinogenesis, diagnosis and therapy. Ann Transl Med. 2014;2:88.

    PubMed  PubMed Central  Google Scholar 

  25. 25.

    Fiori V, Magnani M, Cianfriglia M. The expression and modulation of CEACAM1 and tumor cell transformation. Ann Ist Super Sanita. 2012;48:161–71.

    CAS  PubMed  Article  Google Scholar 

  26. 26.

    Ohue Y, Kurose K, Nishio Y et al. Abstract A101: role of TIM-3/Galectin-9 pathway in lung cancer. Cancer Immunol Res 2016; 4: A101-A101.

  27. 27.

    Zhu C, Anderson AC, Schubart A, et al. The Tim-3 ligand galectin-9 negatively regulates T helper type 1 immunity. Nat Immunol. 2005;6:1245–52.

    CAS  PubMed  Article  Google Scholar 

  28. 28.

    Yu X, Harden K, Gonzalez LC, et al. The surface protein TIGIT suppresses T cell activation by promoting the generation of mature immunoregulatory dendritic cells. Nat Immunol. 2009;10:48–57.

    CAS  PubMed  Article  Google Scholar 

  29. 29.

    Stanietsky N, Simic H, Arapovic J, et al. The interaction of TIGIT with PVR and PVRL2 inhibits human NK cell cytotoxicity. Proc Natl Acad Sci U S A. 2009;106:17858–63.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  30. 30.

    Casado JG, Pawelec G, Morgado S, et al. Expression of adhesion molecules and ligands for activating and costimulatory receptors involved in cell-mediated cytotoxicity in a large panel of human melanoma cell lines. Cancer Immunol Immunother. 2009;58:1517–26.

    CAS  PubMed  Article  Google Scholar 

  31. 31.

    Johnston RJ, Yu X, Grogan JL. The checkpoint inhibitor TIGIT limits antitumor and antiviral CD8+ T cell responses. Oncoimmunology. 2015;4:e1036214.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  32. 32.

    Chauvin JM, Pagliano O, Fourcade J, et al. TIGIT and PD-1 impair tumor antigen-specific CD8(+) T cells in melanoma patients. J Clin Invest. 2015;125:2046–58.

    PubMed  PubMed Central  Article  Google Scholar 

  33. 33.

    Lines JL, Pantazi E, Mak J, et al. VISTA is an immune checkpoint molecule for human T cells. Cancer Res. 2014;74:1924–32.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  34. 34.

    Le Mercier I, Chen W, Lines JL, et al. VISTA regulates the development of protective antitumor immunity. Cancer Res. 2014;74:1933–44.

    CAS  PubMed  Article  Google Scholar 

  35. 35.

    Lines JL, Sempere LF, Broughton T, et al. VISTA is a novel broad-spectrum negative checkpoint regulator for cancer immunotherapy. Cancer Immunol Res. 2014;2:510–7.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  36. 36.

    Picarda E, Ohaegbulam KC, Zang XX. Molecular pathways: targeting B7-H3 (CD276) for human cancer immunotherapy. Clin Cancer Res. 2016;22:3425–31.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  37. 37.

    Castellanos JR, Purvis IJ, Labak CM, et al. B7-H3 role in the immune landscape of cancer. American Journal of Clinical and Experimental Immunology. 2017;6:66–75.

    PubMed  PubMed Central  Google Scholar 

  38. 38.

    Powderly J, Cote G, Flaherty K et al. Interim results of an ongoing phase I, dose escalation study of MGA271 (Fc-optimized humanized anti-B7-H3 monoclonal antibody) in patients with refractory B7-H3-expressing neoplasms or neoplasms whose vasculature expresses B7-H3. J Immunotherapy Cancer 2015; 3: O8-O8.

  39. 39.

    Weidle UH, Kontermann RE, Brinkmann U. Tumor-antigen-binding bispecific antibodies for cancer treatment. Semin Oncol. 2014;41:653–60.

    CAS  PubMed  Article  Google Scholar 

  40. 40.

    Tolcher AW, Alley EW, Chichili G et al. Phase 1, first-in-human, open label, dose escalation ctudy of MGD009, a humanized B7-H3 x CD3 dual-affinity re-targeting (DART) protein in patients with B7-H3-expressing neoplasms or B7-H3 expressing tumor vasculature. J Clin Oncol 2016; 34: TPS3105-TPS3105.

  41. 41.

    Kramer K, Kushner BH, Modak S, et al. Compartmental intrathecal radioimmunotherapy: results for treatment for metastatic CNS neuroblastoma. J Neuro-Oncol. 2010;97:409–18.

    Article  Google Scholar 

  42. 42.

    Leone RD, Lo YC, Powell JD. A2aR antagonists: next generation checkpoint blockade for cancer immunotherapy. Comput Struct Biotechnol J. 2015;13:265–72.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  43. 43.

    Zhang B. CD73: a novel target for cancer immunotherapy. Cancer Res. 2010;70:6407–11.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  44. 44.

    Vijayan D, Young A, Teng MWL, Smyth MJ. Targeting immunosuppressive adenosine in cancer (vol 17, pg 709, 2017). Nat Rev Cancer 2017; 17: 724–724.

  45. 45.

    Emens L, Powderly J, Fong L, et al. Abstract CT119: CPI-444, an oral adenosine A2a receptor (A2aR) antagonist, demonstrates clinical activity in patients with advanced solid tumors. Cancer Res. 2017;77:CT119.

  46. 46.

    Antonioli L, Novitskiy SV, Sachsenmeier KF, et al. Switching off CD73: a way to boost the activity of conventional and targeted antineoplastic therapies. Drug Discov Today. 2017;22:1686–96.

    CAS  PubMed  Article  Google Scholar 

  47. 47.

    Paulos CM, June CH. Putting the brakes on BTLA in T cell-mediated cancer immunotherapy. J Clin Invest. 2010;120:76–80.

    CAS  PubMed  Article  Google Scholar 

  48. 48.

    Malissen N, Macagno N, Granjeaud S et al. HVEM: a novel cosignaling molecule of major interest in melanoma. J Clin Oncol 2017; 35: e14591-e14591.

  49. 49.

    Lan X, Li S, Gao H, et al. Increased BTLA and HVEM in gastric cancer are associated with progression and poor prognosis. Onco Targets Ther. 2017;10:919–26.

    PubMed  PubMed Central  Article  Google Scholar 

  50. 50.

    Massague J. TGF beta in cancer. Cell. 2008;134:215–30.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  51. 51.

    Wendt MK, Tian MZ, Schiemann WP. Deconstructing the mechanisms and consequences of TGF-beta-induced EMT during cancer progression. Cell Tissue Res. 2012;347:85–101.

    CAS  PubMed  Article  Google Scholar 

  52. 52.

    Thomas DA, Massagué J. TGF-β directly targets cytotoxic T cell functions during tumor evasion of immune surveillance. Cancer Cell. 2005;8:369–80.

    CAS  PubMed  Article  Google Scholar 

  53. 53.

    Padua D, Massague J. Roles of TGF beta in metastasis. Cell Res. 2009;19:89–102.

    CAS  PubMed  Article  Google Scholar 

  54. 54.

    Neuzillet C, Tijeras-Raballand A, Cohen R, et al. Targeting the TGFbeta pathway for cancer therapy. Pharmacol Ther. 2015;147:22–31.

    CAS  PubMed  Article  Google Scholar 

  55. 55.

    Smith AL, Robin TP, Ford HL. Molecular pathways: targeting the TGF-beta pathway for cancer therapy. Clin Cancer Res. 2012;18:4514–21.

    CAS  PubMed  Article  Google Scholar 

  56. 56.

    Bogdahn U, Hau P, Stockhammer G, et al. Targeted therapy for high-grade glioma with the TGF-beta2 inhibitor trabedersen: results of a randomized and controlled phase IIb study. Neuro-Oncology. 2011;13:132–42.

    CAS  PubMed  Article  Google Scholar 

  57. 57.

    Hwang L, Ng K, Wang W, Trieu VN. OT-101: an anti-TGF-beta-2 antisense-primed tumors to subsequent chemotherapies. J Clin Oncol 2016; 34: e15727-e15727.

  58. 58.

    Gulley JL, Heery CR, Schlom J et al. Preliminary results from a phase 1 trial of M7824 (MSB0011359C), a bifunctional fusion protein targeting PD-L1 and TGF-β, in advanced solid tumors. J Clin Oncol 2017; 35: 3006–3006.

  59. 59.

    Brandes AA, Carpentier AF, Kesari S, et al. A phase II randomized study of galunisertib monotherapy or galunisertib plus lomustine compared with lomustine monotherapy in patients with recurrent glioblastoma. Neuro-Oncology. 2016;18:1146–56.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  60. 60.

    Long EO, Barber DF, Burshtyn DN, et al. Inhibition of natural killer cell activation signals by killer cell immunoglobulin-like receptors (CD158). Immunol Rev. 2001;181:223–33.

    CAS  PubMed  Article  Google Scholar 

  61. 61.

    Dahlberg CI, Sarhan D, Chrobok M, et al. Natural killer cell-based therapies targeting cancer: possible strategies to gain and sustain anti-tumor activity. Front Immunol. 2015;6:605.

    PubMed  PubMed Central  Article  Google Scholar 

  62. 62.

    Muntasell A, Ochoa MC, Cordeiro L, et al. Targeting NK-cell checkpoints for cancer immunotherapy. Curr Opin Immunol. 2017;45:73–81.

    CAS  PubMed  Article  Google Scholar 

  63. 63.

    Benson DM, Caligiuri MA. Killer immunoglobulin-like receptors and tumor immunity. Cancer Immunology Research. 2014;2:99–104.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  64. 64.

    Leichner R, Kang H, Haddad R, et al. Preliminary efficacy from a phase I/II study of the natural killer cell–targeted antibody lirilumab in combination with nivolumab in squamous cell carcinoma of the head and neck. Journal for Immunotherapy of Cancer. 2016;4

  65. 65.

    Schmitt C, Marie-Cardine A, Bensussan A. Therapeutic antibodies to KIR3DL2 and other target antigens on cutaneous T-cell lymphomas. Front Immunol. 2017;8

  66. 66.

    Bagot M, Porcu P, Ram-Wolff C, et al. Phase I study of IPH4102, anti-KIR3DL2 Mab, in relapsed/refractory cutaneous T-cell lymphomas (CTCL): dose-escalation safety, biomarker and clinical activity results. Hematol Oncol. 2017;35:48–9.

    Article  Google Scholar 

  67. 67.

    Gyori D, Chessa T, Hawkins PT, Stephens LR. Class (I) phosphoinositide 3-kinases in the tumor microenvironment. Cancers. 2017;9:24.

    PubMed Central  Article  Google Scholar 

  68. 68.

    Tolcher A, Hong D, Sullivan R et al. Abstract CT089: IPI-549-01—a phase 1/1b, first-in-human study of IPI-549, a PI3K-γ inhibitor, as monotherapy and in combination with nivolumab in patients with advanced solid tumors. Cancer Res 2017; 77: CT089-CT089.

  69. 69.

    Liu XJ, Kwon H, Li ZH, Fu YX. Is CD47 an innate immune checkpoint for tumor evasion? J Hematol Oncol. 2017;10

  70. 70.

    Weiskopf K. Cancer immunotherapy targeting the CD47/SIRP alpha axis. Eur J Cancer. 2017;76:100–9.

    CAS  PubMed  Article  Google Scholar 

  71. 71.

    Sikic BI, Narayanan S, Colevas AD et al. A first-in-human, first-in-class phase I trial of the anti-CD47 antibody Hu5F9-G4 in patients with advanced cancers. J Clin Oncol 2016; 34: 3019–3019.

  72. 72.

    Thompson JA, Akilov O, Querfeld C et al. A phase 1 dose-escalation trial of intratumoral TTI-621, a novel immune checkpoint inhibitor targeting CD47, in subjects with relapsed or refractory percutaneously-accessible solid tumors and mycosis fungoides. J Clin Oncol 2017; 35: TPS3101-TPS3101.

  73. 73.

    Willoughby J, Griffiths J, Tews I, Cragg MS. OX40: structure and function—what questions remain? Mol Immunol. 2017;83:13–22.

    CAS  PubMed  Article  Google Scholar 

  74. 74.

    Aspeslagh S, Postel-Vinay S, Rusakiewicz S, et al. Rationale for anti-OX40 cancer immunotherapy. Eur J Cancer. 2016;52:50–66.

    CAS  PubMed  Article  Google Scholar 

  75. 75.

    Linch SN, McNamara MJ, Redmond WL. OX40 agonists and combination immunotherapy: putting the pedal to the metal. Front Oncol. 2015;5

  76. 76.

    Turner JG, Rakhmilevich AL, Burdelya L, et al. Anti-CD40 antibody induces antitumor and antimetastatic effects: the role of NK cells. J Immunol. 2001;166:89–94.

    CAS  PubMed  Article  Google Scholar 

  77. 77.

    Curti BD, Kovacsovics-Bankowski M, Morris N, et al. OX40 is a potent immune stimulating target in late stage cancer patients. Cancer Res. 2013;73:7189–98.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  78. 78.

    Infante JR, Hansen AR, Pishvaian MJ et al. A phase Ib dose escalation study of the OX40 agonist MOXR0916 and the PD-L1 inhibitor atezolizumab in patients with advanced solid tumors. J Clin Oncol 2016; 34: 101–101.

  79. 79.

    Hamid O, Thompson JA, Diab A et al. First in human (FIH) study of an OX40 agonist monoclonal antibody (mAb) PF-04518600 (PF-8600) in adult patients (pts) with select advanced solid tumors: preliminary safety and pharmacokinetic (PK)/pharmacodynamic results. J Clin Oncol 2016; 34: 3079–3079.

  80. 80.

    Dempke WCM, Fenchel K, Uciechowski P, Dale SP. Second- and third-generation drugs for immuno-oncology treatment-The more the better? Eur J Cancer. 2017;74:55–72.

    CAS  PubMed  Article  Google Scholar 

  81. 81.

    Knee DA, Hewes B, Brogdon JL. Rationale for anti-GITR cancer immunotherapy. Eur J Cancer. 2016;67:1–10.

    CAS  PubMed  Article  Google Scholar 

  82. 82.

    Koon HB, Shepard DR, Merghoub T et al. First-in-human phase 1 single-dose study of TRX-518, an anti-human glucocorticoid-induced tumor necrosis factor receptor (GITR) monoclonal antibody in adults with advanced solid tumors. J Clin Oncol 2016; 34: 3017–3017.

  83. 83.

    Siu LL, Steeghs N, Meniawy T et al. Preliminary results of a phase I/IIa study of BMS-986156 (glucocorticoid-induced tumor necrosis factor receptor–related gene [GITR] agonist), alone and in combination with nivolumab in pts with advanced solid tumors. J Clin Oncol 2017; 35: 104–104.

  84. 84.

    Tran B, Carvajal RD, Marabelle A et al. Dose escalation results from a first-in-human, phase 1 study of the glucocorticoid-induced TNF receptor-related protein (GITR) agonist AMG 228 in patients (pts) with advanced solid tumors. J Clin Oncol 2017; 35: 2521–2521.

  85. 85.

    Sanmamed MF, Pastor F, Rodriguez A, et al. Agonists of co-stimulation in cancer immunotherapy directed against CD137, OX40, GITR, CD27, CD28, and ICOS. Semin Oncol. 2015;42:640–55.

    CAS  PubMed  Article  Google Scholar 

  86. 86.

    Fan XZ, Quezada SA, Sepulveda MA, et al. Engagement of the ICOS pathway markedly enhances efficacy of CTLA-4 blockade in cancer immunotherapy. J Exp Med. 2014;211:715–25.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  87. 87.

    Harvey C, Elpek K, Duong E, et al. Efficacy of anti-ICOS agonist monoclonal antibodies in preclinical tumor models provides a rationale for clinical development as cancer immunotherapeutics. Journal for ImmunoTherapy of Cancer. 2015;3:O9.

    PubMed Central  Article  Google Scholar 

  88. 88.

    Burris HA, Callahan MK, Tolcher AW et al. Phase 1 safety of ICOS agonist antibody JTX-2011 alone and with nivolumab (nivo) in advanced solid tumors; predicted vs observed pharmacokinetics (PK) in ICONIC. J Clin Oncol 2017; 35: 3033–3033.

  89. 89.

    Chester C, Ambulkar S, Kohrt HE. 4-1BB agonism: adding the accelerator to cancer immunotherapy. Cancer Immunol Immunother. 2016;65:1243–8.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  90. 90.

    Takeda K, Kojima Y, Uno T, et al. Combination therapy of established tumors by antibodies targeting immune activating and suppressing molecules. J Immunol. 2010;184:5493–501.

    CAS  PubMed  Article  Google Scholar 

  91. 91.

    Tolcher AW, Sznol M, Hu-Lieskovan S et al. Phase Ib study of PF-05082566 in combination with pembrolizumab in patients with advanced solid tumors. J Clin Oncol 2016; 34: 3002–3002.

  92. 92.

    Segal NH, Logan TF, Hodi FS, et al. Results from an integrated safety analysis of urelumab, an agonist anti-CD137 monoclonal antibody. Clin Cancer Res. 2017;23:1929–36.

    CAS  PubMed  Article  Google Scholar 

  93. 93.

    Massarelli E, Segal N, Ribrag V. Clinical safety and efficacy assessment of the CD137 agonist urelumab alone and in combination with nivolumab in patients with hematologic and solid tumor malignancies. J Immunother Cancer. 2016;4:O7.

    Article  Google Scholar 

  94. 94.

    Perez-Ruiz E, Etxeberria I, Rodriguez-Ruiz ME, Melero I. Anti-CD137 and PD-1/PD-L1 antibodies en route toward clinical synergy. Clin Cancer Res. 2017;23:5326–8.

    CAS  PubMed  Article  Google Scholar 

  95. 95.

    Denoeud J, Moser M. Role of CD27/CD70 pathway of activation in immunity and tolerance. J Leukoc Biol. 2011;89:195–203.

    CAS  PubMed  Article  Google Scholar 

  96. 96.

    van de Ven K, Borst J. Targeting the T-cell co-stimulatory CD27/CD70 pathway in cancer immunotherapy: rationale and potential. Immunotherapy. 2015;7:655–67.

    CAS  PubMed  Article  Google Scholar 

  97. 97.

    Michot J-M, Maerevoet M, Aftimos PG et al. Clinical response observed in a phase I study in T cell lymphoma patients treated with anti-CD70 SIMPLE antibody ARGX-110. J Clin Oncol 2016; 34: 7556–7556.

  98. 98.

    Owonikoko TK, Hussain A, Stadler WM, et al. First-in-human multicenter phase I study of BMS-936561 (MDX-1203), an antibody-drug conjugate targeting CD70. Cancer Chemother Pharmacol. 2016;77:155–62.

    CAS  PubMed  Article  Google Scholar 

  99. 99.

    Sanborn RE, Pishvain MJ, Callahan MK et al. Abstract CT023: phase I results from the combination of an immune-activating anti-CD27 antibody (varlilumab) in combination with PD-1 blockade (nivolumab): activation across multiple immune pathways without untoward immune-related adverse events. Cancer Res 2016; 76: CT023-CT023.

  100. 100.

    Vonderheide RH. Prospect of targeting the CD40 pathway for cancer therapy. Clin Cancer Res. 2007;13:1083–8.

    CAS  PubMed  Article  Google Scholar 

  101. 101.

    Vonderheide RH, Glennie MJ. Agonistic CD40 antibodies and cancer therapy. Clin Cancer Res. 2013;19:1035–43.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  102. 102.

    Dempke WCM, Fenchel K, Uciechowski P, Dale SP. Second- and third-generation drugs for immuno-oncology treatment-the more the better? Eur J Cancer. 2017;74:55–72.

    CAS  PubMed  Article  Google Scholar 

  103. 103.

    Cabo M, Offringa R, Zitvogel L, et al. Trial watch: immunostimulatory monoclonal antibodies for oncological indications. Oncoimmunology. 2017;6:e1371896.

    PubMed  Article  Google Scholar 

  104. 104.

    Moon YW, Hajjar J, Hwu P, Naing A. Targeting the indoleamine 2,3-dioxygenase pathway in cancer. J Immunother Cancer. 2015;3:51.

    PubMed  PubMed Central  Article  Google Scholar 

  105. 105.

    Toulmonde M, Penel N, Adam J, et al. Use of pd-1 targeting, macrophage infiltration, and ido pathway activation in sarcomas: a phase 2 clinical trial. JAMA Oncology. 2017;

  106. 106.

    Bilir C, Sarisozen C. Indoleamine 2,3-dioxygenase (IDO): only an enzyme or a checkpoint controller? Journal of Oncological Sciences. 2017;3:52–6.

    Article  Google Scholar 

  107. 107.

    Siu LL, Gelmon K, Chu Q, et al. Abstract CT116: BMS-986205, an optimized indoleamine 2,3-dioxygenase 1 (IDO1) inhibitor, is well tolerated with potent pharmacodynamic (PD) activity, alone and in combination with nivolumab (nivo) in advanced cancers in a phase 1/2a trial. Cancer Res. 2017;77:CT116.

    Article  Google Scholar 

  108. 108.

    Zakharia Y, Drabick JJ, Khleif S et al. Updates on phase 1b/2 trial of the indoleamine 2,3-dioxygenase pathway (IDO) inhibitor indoximod plus checkpoint inhibitors for the treatment of unresectable stage 3 or 4 melanoma. J Clin Oncol 2016; 34: 3075–3075.

  109. 109.

    Bahary N, Garrido-Laguna I, Cinar P et al. Phase 2 trial of the indoleamine 2,3-dioxygenase pathway (IDO) inhibitor indoximod plus gemcitabine/nab-paclitaxel for the treatment of metastatic pancreas cancer: interim analysis. J Clin Oncol 2016; 34: 3020–3020.

  110. 110.

    Jha GG, Gupta S, Tagawa ST et al. A phase II randomized, double-blind study of sipuleucel-T followed by IDO pathway inhibitor, indoximod, or placebo in the treatment of patients with metastatic castration resistant prostate cancer (mCRPC). J Clin Oncol 2017; 35: 3066–3066.

  111. 111.

    Hamid O, Bauer TM, Spira AI et al. Safety of epacadostat 100 mg bid plus pembrolizumab 200 mg Q3W in advanced solid tumors: phase 2 data from ECHO-202/KEYNOTE-037. J Clin Oncol 2017; 35: 3012–3012.

  112. 112.

    Perez RP, Riese MJ, Lewis KD et al. Epacadostat plus nivolumab in patients with advanced solid tumors: preliminary phase I/II results of ECHO-204. J Clin Oncol 2017; 35: 3003–3003.

  113. 113.

    Beatty GL, Dwyer PJ, Clark J, et al. First-in-human phase I study of the oral inhibitor of indoleamine 2,3-dioxygenase-1 epacadostat (INCB024360) in patients with advanced solid malignancies. Clin Cancer Res. 2017;23:3269.

    CAS  PubMed  Article  Google Scholar 

  114. 114.

    Lu H. TLR agonists for cancer immunotherapy: tipping the balance between the immune stimulatory and inhibitory effects. Front Immunol. 2014;5:83.

    PubMed  PubMed Central  Google Scholar 

  115. 115.

    Shi M, Chen X, Ye K, et al. Application potential of toll-like receptors in cancer immunotherapy: systematic review. Medicine. 2016;95:e3951.

    PubMed  PubMed Central  Article  Google Scholar 

  116. 116.

    Dowling JK, Mansell A. Toll-like receptors: the swiss army knife of immunity and vaccine development. Clinical & Translational Immunology. 2016;5

  117. 117.

    Li K, Qu S, Chen X, et al. Promising targets for cancer immunotherapy: TLRs, RLRs, and STING-mediated innate immune pathways. Int J Mol Sci. 2017;18:404.

    PubMed Central  Article  Google Scholar 

  118. 118.

    Gupta S, Grilley-Olson J, Hong D, et al. Abstract CT091: Safety and pharmacodynamic activity of MEDI9197, a TLR 7/8 agonist, administered intratumorally in subjects with solid tumors. Cancer Res. 2017;77:CT091.

  119. 119.

    Dredge K, Brennan T, Brown MP et al. An open-label, multi-center phase I study of the safety and tolerability of the novel immunomodulatory agent PG545 in subjects with advanced solid tumors. J Clin Oncol 2017; 35: 3083–3083.

  120. 120.

    de la Torre AN, Contractor S, Castaneda I, et al. A phase I trial using local regional treatment, nonlethal irradiation, intratumoral and systemic polyinosinic-polycytidylic acid polylysine carboxymethylcellulose to treat liver cancer: in search of the abscopal effect. Journal of Hepatocellular Carcinoma. 2017;4

  121. 121.

    Tomala J, Kovar M. IL-2/anti-IL-2 mAb immunocomplexes: a renascence of IL-2 in cancer immunotherapy? Oncoimmunology. 2016;5

  122. 122.

    Jiang T, Zhou C, Ren S. Role of IL-2 in cancer immunotherapy. Oncoimmunology. 2016;5:e1163462.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  123. 123.

    Su EW, Moore CJ, Suriano S et al. IL-2Rα mediates temporal regulation of IL-2 signaling and enhances immunotherapy. Sci Transl Med 2015; 7: 311ra170-311ra170.

  124. 124.

    Diab A, Tannir NM, Bernatchez C et al. A phase 1/2 study of a novel IL-2 cytokine, NKTR-214, and nivolumab in patients with select locally advanced or metastatic solid tumors. J Clin Oncol 2017; 35: e14040-e14040.

  125. 125.

    Bernatchez C, Haymaker CL, Hurwitz ME et al. Effect of a novel IL-2 cytokine immune agonist (NKTR-214) on proliferating CD8+T cells and PD-1 expression on immune cells in the tumor microenvironment in patients with prior checkpoint therapy. J Clin Oncol 2017; 35: 2545–2545.

  126. 126.

    Ananieva E. Targeting amino acid metabolism in cancer growth and anti-tumor immune response. World J Biol Chem. 2015;6:281–9.

    PubMed  PubMed Central  Article  Google Scholar 

  127. 127.

    Timosenko E, Hadjinicolaou AV, Cerundolo V. Modulation of cancer-specific immune responses by amino acid degrading enzymes. Immunotherapy. 2017;9:83–97.

    CAS  PubMed  Article  Google Scholar 

  128. 128.

    Papadopoulos KP, Tsai FY-C, Bauer TM et al. CX-1158-101: a first-in-human phase 1 study of CB-1158, a small molecule inhibitor of arginase, as monotherapy and in combination with an anti-PD-1 checkpoint inhibitor in patients (pts) with solid tumors. J Clin Oncol 2017; 35: 3005–3005.

  129. 129.

    Zhou H, Forveille S, Sauvat A, et al. The oncolytic peptide LTX-315 triggers immunogenic cell death. Cell Death Dis. 2016;7

  130. 130.

    Sveinbjornsson B, Camilio KA, Haug BE, Rekdal O. LTX-315: a first-in-class oncolytic peptide that reprograms the tumor microenvironment. Future Med Chem. 2017;9:1339–44.

    CAS  PubMed  Article  Google Scholar 

  131. 131.

    Yamazaki T, Pitt JM, Vétizou M, et al. The oncolytic peptide LTX-315 overcomes resistance of cancers to immunotherapy with CTLA4 checkpoint blockade. Cell Death Differ. 2016;23:1004–15.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  132. 132.

    Spicer JF, Baurain J-F, Awada A et al. LTX-315, an oncolytic peptide, to convert immunogenically ‘cold’ tumors to ‘hot’ in patients with advanced or metastatic tumours: results from an ongoing phase I study. J Clin Oncol 2017; 35: 3085–3085.

  133. 133.

    Mittal SK, Cho KJ, Ishido S, Roche PA. Interleukin 10 (IL-10)-mediated immunosuppression: MARCH-I INDUCTION REGULATES ANTIGEN PRESENTATION BY MACROPHAGES BUT NOT DENDRITIC CELLS. J Biol Chem. 2015;290:27158–67.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  134. 134.

    Miotto D, Lo Cascio N, Stendardo M, et al. CD8+ T cells expressing IL-10 are associated with a favourable prognosis in lung cancer. Lung Cancer. 2010;69:355–60.

    CAS  PubMed  Article  Google Scholar 

  135. 135.

    Zhang H, Wang Y, Hwang ES, He YW. Interleukin-10: an immune-activating cytokine in cancer immunotherapy. J Clin Oncol. 2016;34:3576.

    CAS  PubMed  Article  Google Scholar 

  136. 136.

    Sun ZJ, Fourcade J, Pagliano O, et al. IL10 and PD-1 cooperate to limit the activity of tumor-specific CD8(+) T cells. Cancer Res. 2015;75:1635–44.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  137. 137.

    Naing A, Wong DJL, Infante JR et al. PEGylated human IL-10 (AM0010) in combination with pembrolizumab in anti-PD1 and CTLA-4 refractory melanoma. J Clin Oncol 2017; 35: 3084–3084.

  138. 138.

    Naidoo J, Page DB, Li BT, et al. Toxicities of the anti-PD-1 and anti-PD-L1 immune checkpoint antibodies. Ann Oncol. 2015;26:2375–91.

    CAS  PubMed  Google Scholar 

  139. 139.

    Michot JM, Pruvost R, Mateus C, et al. Fever reaction and haemophagocytic syndrome induced by immune checkpoint inhibitors. Ann Oncol. 2018;29:518–20.

    PubMed  Article  Google Scholar 

  140. 140.

    Picchi H, Mateus C, Chouaid C, et al. Infectious complications associated with the use of immune checkpoint inhibitors in oncology: reactivation of tuberculosis after anti PD-1 treatment. Clin Microbiol Infect. 2017;

  141. 141.

    Champiat S, Lambotte O, Barreau E, et al. Management of immune checkpoint blockade dysimmune toxicities: a collaborative position paper. Ann Oncol. 2016;27:559–74.

    CAS  PubMed  Article  Google Scholar 

  142. 142.

    Dine J, Gordon R, Shames Y, et al. Immune checkpoint inhibitors: an innovation in immunotherapy for the treatment and management of patients with cancer. Asia-Pacific Journal of Oncology Nursing. 2017;4:127–35.

    PubMed  PubMed Central  Article  Google Scholar 

  143. 143.

    Linardou H, Gogas H. Toxicity management of immunotherapy for patients with metastatic melanoma. Annals of Translational Medicine. 2016;4

  144. 144.

    Gupta A, De Felice KM, Loftus EV Jr, Khanna S. Systematic review: colitis associated with anti-CTLA-4 therapy. Aliment Pharmacol Ther. 2015;42:406–17.

  145. 145.

    Sharma P, Hu-Lieskovan S, Wargo JA, Ribas A. Primary, adaptive, and acquired resistance to cancer immunotherapy. Cell. 2017;168:707–23.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  146. 146.

    Zaretsky JM, Garcia-Diaz A, Shin DS, et al. Mutations associated with acquired resistance to PD-1 blockade in melanoma. N Engl J Med. 2016;375:819–29.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  147. 147.

    Jenkins RW, Barbie DA, Flaherty KT. Mechanisms of resistance to immune checkpoint inhibitors. Br J Cancer. 2018;118:9–16.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  148. 148.

    Taube JM, Anders RA, Young GD, et al. Colocalization of inflammatory response with B7-h1 expression in human melanocytic lesions supports an adaptive resistance mechanism of immune escape. Sci Transl Med. 2012;4:127ra137.

    Article  CAS  Google Scholar 

  149. 149.

    Tumeh PC, Harview CL, Yearley JH, et al. PD-1 blockade induces responses by inhibiting adaptive immune resistance. Nature. 2014;515:568–71.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  150. 150.

    Lou Y, Diao L, Cuentas ER, et al. Epithelial-mesenchymal transition is associated with a distinct tumor microenvironment including elevation of inflammatory signals and multiple immune checkpoints in lung adenocarcinoma. Clin Cancer Res. 2016;22:3630–42.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  151. 151.

    Miao D, Margolis CA, Gao W, et al. Genomic correlates of response to immune checkpoint therapies in clear cell renal cell carcinoma. Science. 2018;359:801–6.

    CAS  PubMed  Article  Google Scholar 

  152. 152.

    Luksza M, Riaz N, Makarov V, et al. A neoantigen fitness model predicts tumour response to checkpoint blockade immunotherapy. Nature. 2017;551:517.

    CAS  PubMed  Google Scholar 

Download references


Not applicable.


Not applicable.

Availability of data and materials

All data generated or analyzed in this study are included in this published article.

Author information




JMA drafted the manuscript; YL designed and supervised the study, as well as edited the manuscript; BD, AS, SC, and KK reviewed and edited the text. All authors reviewed and approved the final version of the manuscript.

Corresponding author

Correspondence to Yanyan Lou.

Ethics declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( applies to the data made available in this article, unless otherwise stated.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Marin-Acevedo, J.A., Dholaria, B., Soyano, A.E. et al. Next generation of immune checkpoint therapy in cancer: new developments and challenges. J Hematol Oncol 11, 39 (2018).

Download citation


  • Cancer
  • Immunotherapy
  • Tumor microenvironment
  • Immune evasion
  • Cytotoxic T lymphocytes
  • Immunotherapy
  • Immune checkpoint therapy
  • Co-stimulatory pathways
  • Inhibitory pathways
  • Tumor microenvironment