Multiple myeloma (MM) is the second-most-common hematologic malignancy, and develops from clonal malignant plasma cells within bone marrow. Despite tremendous improvements in therapeutic strategies (e.g. stem cell transplantation, immune-modulatory drugs (IMiDs), proteasome inhibitors, and, more recently, immunotherapy), which have led to improved responses to treatment and overall survival, most patients eventually relapse. We have previously shown that the immunization with tumor antigen-loaded dendritic cells (DCs) and pomalidomide/dexamethasone synergistically potentiates the enhancing the antitumor immunity in a myeloma mouse model. In the present study, we investigated whether a DC-based vaccine combined with pomalidomide and PD-L1 blockade has a synergistic effect in a murine MM model. MOPC-315 cell lines were injected subcutaneously to establish MM-bearing mice. Four test groups were used to mimic the clinical protocol: (1) PBS control, (2) DCs + pomalidomide/dexamethasone, (3) pomalidomide/dexamethasone + PD-L1 blockade, and (4) DCs + pomalidomide/dexamethasone + PD-L1 blockade. After treatment, preclinical response and in vitro immunological responses were evaluated. The study was designed to closely mimic the clinical MM treatment protocol and clearly demonstrated that combination treatment with DCs + pomalidomide with dexamethasone + PD-L1 blockade more strongly inhibited MM tumor growth. Consequently, the mice treated with DCs + pomalidomide with dexamethasone + PD-L1 blockade displayed markedly induced tumor regression and significantly prolonged survival, as well as very strong anti-myeloma CTL responses and increased numbers of effector cells (such as CD4+ T cells, CD8+ T cells, memory T cells, NK cells and M1 macrophages) associated with antitumor effects. This treatment also effectively decreased the proportions of suppressor cells, including MDSCs, Tregs and M2 macrophages, in the spleen and tumor microenvironment of treated mice. Tregs, MDSCs and M2 macrophages play crucial roles in immunosuppression and tolerance, which are mediated by tumor-secreted cytokines. The inhibition of Tregs, MDSC and M2 macrophage accumulation may enhance systemic cell-mediated immunity through the activation of DCs or CTLs. Importantly, treatment with pomalidomide with dexamethasone + PD-L1 blockade led to decreased expression of PD-L1 and CTLA-4 in treated mice, which further induced effector cell infiltration of the tumor microenvironment. Moreover, DCs + pomalidomide with dexamethasone + PD-L1 blockade induced the activation of cell-mediated immunity by increasing Th1-specific immune responses, as evidenced by the increased production of IFN-γ and a decrease in the regulatory-specific immune response, as evidenced by the decreased production of TGF-β, IL-10 and VEGF in the spleen and tumor microenvironment. These findings show that inducing the systemic immune response represent a means of treating myeloma. Immunotherapy clearly represents a revolution in cancer care, and promising responses have been shown to various treatments, particularly immune checkpoint inhibitors, IMiDs, DCs and CAR T cells. However, not all patients are responsive to current immunotherapies, and among those patients who do respond, the effects are not always long-lasting. Thus, combination approaches are a cornerstone of cancer therapy for improving patient outcomes in MM. This study demonstrated that the combination of DC vaccination + pomalidomide with dexamethasone + PD-L1 blockade synergistically enhances myeloma immune responses to inhibit tumor growth, restores and enhances host immune effector cells, and reduces the generation of immune suppressor cells in MM. This study provides a framework for developing and understanding the role of immunotherapeutic modalities employing DCs, pomalidomide and PD-L1 blockade to inhibit tumor growth and restore immune function in myeloma-bearing mice.
No relevant conflicts of interest to declare.
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