Monotherapeutically nonactive CTLA-4 blockade results in greatly enhanced antitumor effects when combined with tumor-targeted superantigens in a B16 melanoma model

摘要

Immunotherapy aiming to block immune suppression with anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) monoclonal antibodies is a recently clinically established strategy to enhance immune driven antitumor activities. To be successful, this approach depends on the existence of a suppressed immune response against the tumor that can be released by the treatment or alternatively needs to be combined with an immune-enhancing therapy. A tumor-targeted superantigen (TTS) fusion protein utilizes the strong T-cell activating property of bacterial superantigens in concert with the tumor cell binding capacity in antitumor Fab-fragments. Our purpose was to investigate the feasibility of combining anti-CTLA-4 with TTS therapy against the poorly immunogenic B16 mouse melanoma tumor transfected with the human tumor-associated antigen EpCAM recognized by the C215 monoclonal antibody. B16-EpCAM tumors growing in the lung were completely insensitive to anti-CTLA-4 monotherapy. C215Fab-SEA treatment of the B16-EpCAM tumors induced strong infiltration and targeting of both CD4 +and CD8 +T cells. In parallel, Foxp3 +CTLA-4 high regulatory T cells accumulated in the tumors. Combining activation with C215Fab-SEA and anti-CTLA-4 showed greatly enhanced antitumor effects and prolonged long-term survival. In parallel, when the expansion of regulatory T cells was inhibited, the number of specific effector T cells was enhanced and the cytotoxic T-lymphocyte activity was significantly improved. Collectively, these data emphasize the potential of combining cancer treatment using anti-CTLA-4 monoclonal antibodies with T-cell activation and directed killing by TTS therapy.