Heterogeneity of CD34 and CD38 expression in acute B lymphoblastic leukemia cells is reversible and not hierarchically organized

Abstract

The existence and identification of leukemia-initiating cells in adult acute B lymphoblastic leukemia (B-ALL) remain controversial. We examined whether adult B-ALL is hierarchically organized into phenotypically distinct subpopulations of leukemogenic and non-leukemogenic cells or whether most B-ALL cells retain leukemogenic capacity, irrespective of their immunophenotype profiles. Our results suggest that adult B-ALL follows the stochastic stem cell model and that the expression of CD34 and CD38 in B-ALL is reversibly and not hierarchically organized.

Currently, the long-term survival of adult B-ALL patients is less than 50 % [1–4]. To improve the cure and survival rates of adults, there is an increasing need to understand the biology of B-ALL and to characterize the leukemia-initiating cells (LICs) in B-ALL if they exist [5, 6]. Primary B-ALL cells from 25 adult patients (Additional file 1: Table S1) were intravenously transplanted into groups of adult NSI mice [7–9] that had undergone preconditioning total body irradiation. Twelve of the 25 samples engrafted successfully (Additional file 2: Table S2). In the 12 cases of successful engraftment, the mice died or developed severe clinical signs suggestive of leukemia and requiring euthanasia (Additional file 3: Table S3). Consistent with primary xenografts, the human B-ALL cells that expressed CD19, CD34, CD38, and CD45 in serial transplanted NSI mice closely recapitulated the immunophenotypes of the original patient (Additional file 4: Figure S1, S2A). The morphology of leukemic cells in the peripheral blood, spleens, and bone marrow (BM) of xenografts resemble the original patient samples (Additional file 5: Figure S2B). The CD34 and CD38 expression profiles of engrafted B-ALL cells from transplanted NSI mice resemble the original patient samples (Additional file 5: Figure S2A and Additional file 6: Figure S3).

CD34 and CD38 molecules had been used as surface markers to distinguish LICs [10, 11]. To identify whether CD34 and CD38 can be used as LICs markers in B-ALL cells, we purified CD34⁺CD38⁻, CD34⁺CD38⁺, and CD34⁻CD38⁺ fractions from the xenografts of patients #1 and #3. We subsequently performed limited dilution transplantation of these subpopulations in NSI mice. The purities of the subpopulations were 97.3 % ± 0.89 (n = 12, Additional file 7: Figure S4). The xenotransplantation results showed that each fraction of B-ALL cells from xenografts of patients #1 and #3 was capable of engrafting in NSI mice (Additional file 3: Table S3). Each subpopulation from xenografts of patients individually reconstituted B-ALL that contained CD34⁺CD38⁻, CD34⁺CD38⁺, and CD34⁺CD38⁻ fractions in NSI mice (Fig. 1). Genome-wide expression profile analysis revealed that each population was clustered closely in patients #1 and #3 (Additional file 8: Figure S5). RNA-Seq results were further validated by measuring the messenger RNA (mRNA) levels of oncogenesis-related genes using quantitative RT-PCR (Additional file 9: Figure S6).

Fig. 1

Subpopulations of adult B-ALL cells reconstituted the leukemia in xenografts. Subpopulations of CD34⁺CD38⁻, CD34⁺CD38⁺, and CD34⁻CD38⁺ from xenografts of patients #1 and #3 were purified and injected into groups of NSI mice. a Representative FACS analysis of gated hCD45⁺ BM cells from NSI recipients that were transferred with different subpopulations of engrafted B-ALL cells from patient #1. b Representative FACS analysis of gated hCD45⁺ BM cells from NSI mice that were transferred with CD34⁺CD38⁺ and CD34⁻CD38⁺ fractions of engrafted B-ALL cells from patient #3

Next, we investigated whether expanded B-ALL cells in vitro still maintain original expression profiles of CD34 and CD38 and the LIC capacity. B-ALL cells from 11 of the 12 patient samples that successfully engrafted in NSI mice attached to OP9 cells and proliferated vigorously for at least 2 months (Additional file 10: Table S4). We then monitored the expression profiles of CD34 and CD38 in B-ALL cells in differential time. To our surprise, CD34⁺CD38⁻ and CD34⁺CD38⁺ subpopulations from patient #1 disappeared gradually in culture (Fig. 2a). Six weeks after co-culture with OP9 cells, all remaining leukemic cells were CD34⁻CD38⁺ (Additional file 10: Table S4). To investigate whether CD34⁻CD38⁺ B-ALL cells after culture were still capable of engrafting in mice, we further purified cultured CD34⁻CD38⁺ B-ALL cells from patients #1, #4, and #7 and injected them into groups of NSI mice. After 4 weeks transplantation, cultured CD34⁻CD38⁺ B-ALL cells from patient reconstituted B-ALL consisting of CD34⁺CD38⁻, CD34⁺CD38⁺, and CD34⁻CD38⁺subpopulations in mice (Fig. 2b and Additional file 11: Table S5). Whole exome-sequencing analysis [12] showed that B-ALL cells from co-culture and B-ALL cells from xenografts shared similar SNP profiles (Additional file 12: Figure S7). This result indicates B-ALL cells maintain stable genetic characteristics irrespective of phenotypes. Our results also showed that individual B-ALL cells successfully engrafted in 4 of the 70 hosts and repopulated original surface profiles (Additional file 13: Figure S8 and Additional file 14: Table S6, detailed  methodological information was included in Additional file 17: supplementary methods.).

Fig. 2

Cultured leukemic cells maintain the stem cell capacity. a Representative FACS analysis of CD34 and CD38 expression profiles in primary B-ALL cells from patient #1 in OP9 co-culture at indicated time points. b B-ALL cells from xenografts of patients #1, #4, and #7 were co-cultured with OP9 stromal cells. After 6 weeks, cultured B-ALL cells were subjected to FACS analysis. Then CD34⁻CD38⁺ populations were enriched from cultured B-ALL cells and were subsequently injected into groups of NSI mice for serial transplantations. Eight weeks after transplantation, BM cells from xenografts were subjected for FACS analysis. Representative FACS analysis of gated CD45⁺ cells from xenografts or co-cultures

In conclusion, our results demonstrate that leukemic blasts, irrespective of CD34 and CD38 expression, are able to engraft immunodeficient mice and reconstitute the original leukemia. Furthermore, we provide evidence that the heterogeneity of CD34 and CD38 expression in B-ALL obtained from patients reverses in different microenvironments. This phenotypic plasticity contrasts the cancer stem cell model, which largely attributes heterogeneity to irreversible epigenetic changes.