Monoclonal antibodies targeting CTLA-4 are now a central part of the standard of care for metastatic melanoma, and have shown efficacy against diverse tumor types. However, the extent to which direct blockade of CTLA-4 contributes to the efficacy of these antibodies remains incompletely understood. Mice lacking FcγRIV fail to respond to αCTLA-4 therapy, and maintain robust tumor infiltrating Treg populations. These findings suggest that anti-CTLA-4 antibody therapy enhances antitumor responses through Treg depletion, though a requirement for FcγR in other aspects of the antitumor response has not been excluded, and the importance of CTLA-4 blockade alone has not be directly assessed. We have developed an alpaca-derived single domain antibody (H11) that binds CTLA-4 with high affinity, occluding the B7 binding motif as determined by x-ray crystallography. We are able to use this single domain antibody in 18F immuno-PET to produce a comprehensive image of CTLA-4 expression in a tumor-bearing live mouse, confirming penetration of H11 into the tumor microenvironment. Therapeutically, we find that H11 potently blocks CTLA-4 co-inhibition in vitro, but fails to elicit protective immunity in vivo. Therapeutic activity is restored when H11 is attached to the Fc region of murine IgG2a, but not when half-life alone is increased through H11 dimerization or conjugation to PEG, confirming a central role for Fc-dependent function in αCTLA-4 responses, as well as the minimal efficacy of CTLA-4 blockade alone. We further find that the efficacy of αCTLA-4 therapy can be enhanced through blockade of CD47, consistent with an important role for Treg depletion by myeloid cells in the activity of this therapy. These findings have implications for the development of combinations immune therapies.
Citation Format: Michael Dougan, Jessica Ingram, Olga Blomberg, Mohammad Rashidian, Edmund Keliher, Ralph Weissleder, Steven Almo, Hidde Ploegh. Single domain antibodies targeting CTLA-4 demonstrate a critical role for Fc in the antitumor response [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B021.
- ©2016 American Association for Cancer Research.