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Simultaneous Blockade of Multiple Immune System Inhibitory Checkpoints Enhances Antitumor Activity Mediated by Interleukin-15 in a Murine Metastatic Colon Carcinoma Model

Purpose: Interleukin 15 (IL-15) is a promising cytokine for immunotherapy of cancer due to its ability to stimulate the immunity of natural killer, B, and T cells. Its effectiveness, however, may be limited by inhibitory checkpoints and pathways that can attenuate immune responses. Finding strategies to abrogate these negative regulators and enhance the efficacy of IL-15 is a critical challenge. Experimental Design: In a preclinical study, we evaluated IL-15 combined with antibodies to block the negative immune regulators cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed death ligand 1 (PD-L1) in a metastatic murine CT26 colon carcinoma model. Results: IL-15 treatment resulted in a significant prolongation of survival in mice with metastatic tumor. Administration of IL-15, however, also increased expression of PD-1 on the surface of CD8+ T cells including CD8+CD44high memory phenotype T cells. Moreover, IL-15 also increased the secretion of the immunosuppressive cytokine, IL-10. Combining IL-15 with anti-PD-L1 and anti-CTLA-4 (multiple immune checkpoint blockade) exhibited greater CTL killing and IFNγ secretion. Moreover, this combination resulted in a significant reduction in surface expression of PD-1 on CD8+ T cells, a decrease in IL-10 secretion, and led to significantly longer survival of tumor-bearing animals compared with mice treated with IL-15 alone or combined singularly with anti-PD-L1 or anti-CTLA-4. Conclusions: Combining the immune stimulatory properties of IL-15 with the simultaneous removal of 2 critical immune system inhibitory checkpoints, we showed enhancement of immune responses leading to increased antitumor activity. Clin Cancer Res; 16(24); 6019–28. ©2010 AACR.