Efficient nontoxic delivery of PD-L1 and PD-L2 siRNA into dendritic cell vaccines using the cationic lipid SAINT-18.

: Dendritic cell (DC)-based vaccination is an appealing strategy to boost graft-versus-tumor immunity after allogeneic stem cell transplantation (allo-SCT), and thereby prevent or counteract tumor recurrence. By exploiting minor histocompatibility antigens (MiHA) presented on hematopoietic cells, donor CD8 T-cell immunity can be selectively targeted to patient's hematological tumor cells without the risk of inducing graft-versus-host disease. Previously, we demonstrated that silencing RNA (siRNA) of programmed death-ligand 1 (PD-L1) and PD-L2 on DCs markedly augments the expansion and function of MiHA-specific CD8 T cells. However, previously applied methods based on electroporation or lipid nanoparticles were either incompatible with target antigen mRNA delivery or required complex manufacturing compliant to Good Manufacturing Practice. Here, we investigated whether transfection using lipoplexes composed of PD-L1 and PD-L2 siRNAs plus SAINT-18:DOPE (ie, SAINT-RED) is an effective and feasible clinical-grade method in DC vaccine manufacturing. We observed that a single siRNA/SAINT-RED transfection resulted in efficient and long-term knockdown of the PD-1 ligands without affecting DC maturation or viability. Furthermore, we demonstrated that SAINT-RED can be heat sterilized without loss of function, facilitating its use in aseptic DC vaccine production. Finally, we showed that the established transfection method can be combined with target antigen mRNA or peptide loading to efficiently stimulate MiHA-specific T-cell expansion and cytokine production. Together, these findings indicate that the developed PD-L siRNA/SAINT-RED transfection protocol in combination with MiHA mRNA or peptide loading can be applied in the generation of clinical-grade DC vaccines to boost antitumor immunity after allo-SCT.