Enhancing the anti-leukemia immunity of leukemia-derived exosome-based vaccine by downregulation of PD-L1 expression

Cell-released nanovesicles can induce anti-leukemia immunity. Leukemia cell-derived exosomes (LEXs) are promising anti-tumor vaccine components for cancer immunotherapy. Nonetheless, LEX-based vaccines show modest potency in vivo, likely due to the presence of immunosuppressive PD-L1 proteins in the exosomes. We hypothesized that targeting exosomal PD-L1 could optimize LEX-based vaccines. To test this hypothesis, we compared the capacity of exosomes derived from PD-L1-silenced leukemia cells (LEXPD-L1si) and non-modified exosomes to induce anti-leukemia immunity.Lentivirus-mediated PD-L1 shRNA was used to downregulate PD-L1 expression in parental leukemia cells and LEXs. LEXPD-L1si were characterized by electron microscopy, western blotting, and flow cytometry, and their anti-leukemia immune effects were tested on immune cells and in animal models.In the present study, lentivirus-mediated PD-L1 shRNA successfully downregulated PD-L1 expression in parental leukemia cells and in LEXs. LEXPD-L1si induced better DC maturation and subsequently enhanced T-cell activation, as compared with non-modified LEXs. Consistently, immunization with LEXPD-L1si induced greater T-cell proliferation and Th1 cytokine release. LEXPD-L1si was a more potent inducer of antigen-specific cytotoxic lymphocyte (CTL) response. Finally, we vaccinated DBA/2 mice with exosome formulations to test their ability to induce both protective and therapeutic anti-tumor CTL responses in vivo. Vaccination with LEXPD-L1si strongly inhibited tumor growth and prolonged survival. Downregulation of exosomal PD-L1 expression in LEXs effectively induce more potent anti-leukemia immunity. Therefore our strategy for optimizing LEX-based vaccine has a potential application in leukemia immunotherapy.