Fig. 2

DEXs stimulate T cells via direct and indirect antigen presentation processes. MHC I and MHC II molecules and peptides on the surface of DEXs can be directly presented to T cells, thereby activating T cells. The costimulatory molecules on the surface of DEXs aid this process. Indirect DEX-to-T cell stimulation via bystander DCs is a far more efficient pathway. Two possible mechanisms have been observed in the indirect presentation process. One may be called reprocessing. In this process, the DEX-MHC antigens are captured and reprocessed by APCs and act as the APC-MHC antigens. The other process, known as cross-dressing, is still debated. DEX peptide-MHC complexes attach to mature APC surfaces, which provide the required costimulatory molecules that are absent in the DEXs, and thus can be recognized by T cells directly without the need of APC reprocessing. Abbreviations: DCs dendritic cell, DEXs dendritic cell-derived exosomes, Hsp70 heat shock protein 70, LPS lipopolysaccharide, MHC major histocompatibility complex, pMHC peptide-MHC, iCAMs intercellular cell adhesion molecules