Figure 1.

A novel mechanism for plasmacytoid DCs (pDCs) to cross-prime antigen-specific CD8 T cells by transferring antigens to bystander DCs through exosomes. Upon pDC-targeted vaccination, pDCs achieve cross-priming by transferring targeted antigens to conventional DCs (cDCs). Antigen transfer from pDCs to cDCs is mediated through pDC-derived exosomes (pDCexos), which similarly require bystander cDCs to prime antigen-specific CD8 T cells. While both cDC1s (type 1 cDCs) and cDC2s (type 2 cDCs) acquire antigens from pDCs similarly, cDC1s play a non-redundant role in pDC-mediated cross-priming, especially effector differentiation of antigen-specific CD8 T cells. How antigens are transferred from pDCexos to bystander cDCs remains unclear, and future studies will be required to determine whether pDCexos carry and transfer MHCI–Ag complexes to cDCs or carry intact antigens to be processed and presented by cDCs as reported for protein-loaded DCexos [95].