Multiple myeloma is an incurable plasma cell malignancy. Patients who fail conventional therapy are frequently treated with hematopoietic stem cell transplantation (HSCT), which results in reduced tumor burden, but the patients subsequently relapse from sites of chemotherapy-resistant disease. Using the 5T33 murine model of myeloma and a previously successful immunotherapy regimen consisting of autologous (syngeneic) HSCT and cell-based vaccine administration, we were unable to improve survival of myeloma-bearing mice. The 5T33 tumor line, similar to malignant plasma cells from myeloma patients, expresses high levels of programmed death receptor ligand-1 (PD-L1), which binds to the inhibitory receptor, PD-1. We observed that T cells from myeloma-bearing mice express high levels of PD-1, which has also been observed in patients with multiple myeloma. These PD-1 1 T cells were exhausted and produced IL-10. Based on these observations, we combined HSCT with whole-cell vaccination and PD-L1 blockade. Inhibition of the PD-1/PD-L1 pathway with HSCT and whole-cell vaccination increased the survival of myeloma-bearing mice from 0% to 40%. These data demonstrate a role for PD-L1 in suppressing immune responses to myeloma and suggest that blockade of this pathway may enhance immunotherapy for this disease.

Pancreatic cancer is characterized by an immune suppressive stromal reaction that creates a barrier to therapy. A murine transgenic pancreatic cancer cell line that recapitulates human disease was used to test whether a STimulator of Interferon Genes (STING) agonist could reignite immunologically inert pancreatic tumors. STING agonist treatment potently changed the tumor architecture, altered the immune profile, and increased the survival of tumor-bearing mice. Notably, STING agonist increased numbers and activity of cytotoxic T cells within tumors and decreased levels of suppressive regulatory T cells. Further, STING agonist treatment upregulated costimulatory molecule expression on cross-presenting dendritic cells and reprogrammed immune-suppressive macrophages into immune-activating subtypes. STING agonist promoted the coordinated and differential cytokine production by dendritic cells, macrophages, and pancreatic cancer cells. Cumulatively, these data demonstrate that pancreatic cancer progression is potently inhibited by STING agonist, which reignited immunologically cold pancreatic tumors to promote trafficking and activation of tumor-killing T cells.

Early phase clinical trials targeting the programmed death receptor-1/ligand-1 (PD-1/PD-L1) pathway to overcome tumor-mediated immunosuppression have reported promising results for a variety of cancers. This pathway appears to play an important role in the failure of immune reactivity to malignant plasma cells in multiple myeloma patients, as the tumor cells express relatively high levels of PD-L1 and T cells show increased PD-1 expression. In the current study, we demonstrate that PD-1/PD-L1 blockade with a PD-L1-specific antibody elicits rejection of a murine myeloma when combined with lymphodepleting irradiation. This particular combined approach by itself has not previously been shown to be efficacious in other tumor models. The anti-tumor effect of lymphodepletion/anti-PD-L1 therapy was most robust when tumor antigen-experienced T cells were present either through cell transfer or survival after non-myeloablative irradiation. In vivo depletion of CD4 or CD8 T cells completely eliminated anti-tumor efficacy of the lymphodepletion/anti-PD-L1 therapy, indicating that both T cell subsets are necessary for tumor rejection. Elimination of myeloma by T cells occurs relatively quickly as tumor cells in the bone marrow were nearly non-detectable by five days after the first anti-PD-L1 treatment, suggesting that anti-myeloma reactivity is primarily mediated by pre-activated T cells, rather than newly generated myeloma-reactive T cells. Anti-PD-L1 plus lymphodepletion failed to improve survival in two solid tumor models, but demonstrated significant efficacy in two hematologic malignancy models. In summary, our results support the clinical testing of lymphodepletion and PD-1/PD-L1 blockade as a novel approach for improving the survival of patients with multiple myeloma.

BackgroundMultiple myeloma is characterized by the presence of transformed neoplastic plasma cells in the bone marrow and is generally considered to be an incurable disease. Successful treatments will likely require multi-faceted approaches incorporating conventional drug therapies, immunotherapy and other novel treatments. Our lab previously showed that a combination of transient lymphodepletion (sublethal whole body irradiation) and PD-1/PD-L1 blockade generated anti-myeloma T cell reactivity capable of eliminating established disease. We hypothesized that blocking a combination of checkpoint receptors in the context of low-dose, lymphodepleting whole body radiation would boost anti-tumor immunity.MethodsTo test our central hypothesis, we utilized a 5T33 murine multiple myeloma model. Myeloma-bearing mice were treated with a low dose of whole body irradiation and combinations of blocking antibodies to PD-L1, LAG-3, TIM-3, CD48 (the ligand for 2B4) and CTLA4.ResultsTemporal phenotypic analysis of bone marrow from myeloma-bearing mice demonstrated that elevated percentages of PD-1, 2B4, LAG-3 and TIM-3 proteins were expressed on T cells. When PD-L1 blockade was combined with blocking antibodies to LAG-3, TIM-3 or CTLA4, synergistic or additive increases in survival were observed (survival rates improved from ~30% to >80%). The increased survival rates correlated with increased frequencies of tumor-reactive CD8 and CD4 T cells. When stimulated in vitro with myeloma cells, CD8 T cells from treated mice produced elevated levels proinflammatory cytokines. Cytokines were spontaneously released from CD4 T cells isolated from mice treated with PD-L1 plus CTLA4 blocking antibodies.ConclusionsThese data indicate that blocking PD-1/PD-L1 interactions in conjunction with other immune checkpoint proteins provides synergistic anti-tumor efficacy following lymphodepletive doses of whole body irradiation. This strategy is a promising combination strategy for myeloma and other hematologic malignancies.Electronic supplementary materialThe online version of this article (doi:10.1186/s40425-014-0043-z) contains supplementary material, which is available to authorized users.

Evidence that CD4CD25 regulatory T (Treg) cells play a role in the progression of cancer continues to mount. There is a great deal of interest as to whether transient elimination or functional inhibition of these cells can improve the efficacy of immunotherapy for cancer. Our goals in this study were to test whether treatment of mice with anti-CD25 monoclonal antibody (mAb) (PC61) could induce rejection of a murine neuroblastoma, whether anti-CD25 treatment could increase tumor immunity when administered just before cell-based vaccination, and to learn how anti-CD25 treatment influences the vaccine-induced antitumor response. Treatment of mice with anti-CD25 mAb induced rejection of the mouse neuroblastoma, Neuro-2a, as 90% of anti-CD25-treated mice survived challenge with a lethal dose of tumor cells. In vivo anti-CD25 mAb treatment before the first of 2 weekly vaccines significantly improved the survival of tumor-vaccinated/challenged mice (75% vs. 33% survival), whereas antibody treatment before each of the 2 vaccines did not, suggesting that excessive treatment with anti-CD25 mAb interferes with activated antitumor effector cells. A detailed phenotypic analysis of tissues from anti-CD25-treated mice indicated that the antibody partially depletes CD4Foxp3 Treg cells (25% to 40%) in A/J mice, and that the antibody may inhibit the remaining cells by inducing loss of CD25 expression and blocking CD25 molecules, partially confirming recent data from other investigators. Importantly, we found that in vivo anti-CD25 mAb treatment significantly decreased the contribution of asialo GM1 cells in the antitumor response. As we did not see a direct effect of anti-CD25 mAb on in vitro assays of immune cell function in spleen cells from treated animals, this indicates that inhibition of Treg cells amplifies the immune response in vivo in a manner that bypasses the requirement for innate immune activation, potentially mediated by natural killer cells, and allows for protective CD4 and CD8 cells to expand directly in response to cell-based vaccines.