Abstract Background: The therapeutic efficacy of dendritic cell (DC)-based immunotherapy may be potentiated in combination with other anticancer therapies that enhance DC function by modulating immune responses and the tumor microenvironment. In this study, we investigated the efficacy of DC vaccination in combination with lenalidomide and programmed death (PD)-1 blockade in a model of murine myeloma. Materials &Methods: MOPC-315 cell lines were injected subcutaneously to establish myeloma-bearing mice and the following five test groups were established: PBS control, DCs, DCs + lenalidomide, DCs + PD-1 blockade, and DCs + lenalidomide + PD-1 blockade. On day 0, mice were injected subcutaneously in the right flank with 5 × 105 MOPC-315 cells in a volume of 0.1 mL. After tumor growth, lenalidomide (0.5 mg/kg/day) was administrated orally once a day for 25 days with a 3-day break after the first 11-day dosing period. Each dose of DCs (1 × 106/mouse) was injected subcutaneously into the left flank of BALB/c mice in a volume of 0.1 mL PBS on days 11, 15, 25, and 29; anti-PD-1 (250 µg/mouse) was injected intraperitoneally in a 0.1 mL volume on the same days as DC vaccination. Results: This study showed that DC vaccination combined to the lenalidomide and PD-1 blockade regiment further inhibited MM tumor growth, consequently prolonging the survival of tumor-bearing mice. These effects were associated with a significant increase in IFN-γ-secreting splenocytes against MOPC-315 and YAC-1 cells, as well as the significantly increased the number of effector CD4+ T cells, CD8+ T cells, effector memory T cells, effector NK cells, and M1 macrophages while effectively discouraging suppressor cells, such as myeloid-derived suppressor cells (MDSCs), M2 macrophages, and regulatory T cells (Tregs) in the systemic immune compartment. These findings evidence the induction of systemic immune response potentially being able to eradicate disseminated diseases. In this study, DCs combined with lenalidomide and PD-1 blockade also heightened the anti-myeloma cell mediate immunity by inducing the Th1 polarization, as evidenced by the high-level production of IFN-γ in the spleen, and by suppressing Th2 immune responses, as evidenced by the low-level production of IL-10 and TGF-β in the spleen and tumor site. Tregs, MDSCs, and M2 macrophages are major elements molding the potent immunosuppressive environment in tumor tissues. The inhibition of Treg, MDSC, and M2 macrophage accumulation in the spleen should further contribute to effective anti-myeloma cell mediate immunity in the systemic immune compartment by reciprocally activating DCs or cytotoxic T lymphocytes. Conclusion: This study suggests that lenalidomide plus PD-1 blockade treatment synergistically enhances the efficacy of DC vaccination in a murine myeloma model by inhibiting the generation of immunosuppressive cells and the Th2 immune response and enhancing effector cells and the Th1 immune responses. We hereby propose a framework for a more efficacious DC-based vaccination strategy against MM with the combination of immunomodulatory drug lenalidomide and anti-PD-1 antibody. Figure. Figure. Disclosures No relevant conflicts of interest to declare.

Abstract Dendritic cell (DC)-based vaccines are one of promising tools in patients with multiple myeloma (MM). However, the source of tumor antigens is still challenging in this field. Here, we evaluated the feasibility of tumor-associated antigens (TAAs) to be loaded onto DCs for improving the efficacy of immunotherapy against MM. Quantitative PCR was performed to evaluate the expression of TAAs from malignant plasma cells isolated from bone marrow mononuclear cells of MM patients. Based on quantitative PCR analysis, 4 cancer testis antigens (SSX2, SSX4, MAGE-A3 and MAGE-C2), hTERT, BCMA, and CD38 were selected to make CTP-fused recombinant TAAs. Then, we made cytoplasm transduction peptide (CTP)-fused recombinant proteins (CTP-SSX2, CTP-SSX4, CTP-MAGEA3, CTP-MAGEC2, CTP-hTERT, CTP-BCMA, and CTP-CD38) for the efficient delivery of TAAs to DCs. After determination of optimal protein dose for loading onto DCs, several functional studies were performed by the DCs. The CTP-fused recombinant TAAs did not deteriorate the function of DCs in terms of phenotype expressions and cytokine productions, including a Th1-polarizing capacity of naive T cells. DCs loaded with each CTP-fused recombinant TAA had successfully generated a myeloma-specific cytotoxic T lymphocyte (CTL) to kill myeloma cell lines and primary myeloma cells. This study showed that CTP-fused recombinant myeloma-associated proteins were a useful tumor antigen source to be loaded onto DCs for generation of myeloma-specific CTLs in vitro . Therefore, CTP-fused recombinant TAAs will become a feasible tumor antigen for the clinical application of DC-based cancer immunotherapy in the field of MM. Disclosures No relevant conflicts of interest to declare.