Localized CD47 blockade enhances immunotherapy for murine melanoma

Significance

CD47 is a broadly expressed membrane-associated innate immune regulator that acts as a ligand of signal regulatory protein alpha (SIRPα) on antigen-presenting cells to inhibit phagocytosis. In xenograft models, inhibitors of the CD47–SIRPα interaction selectively target tumor-expressed CD47 and improve antibody responses to tumors by enhancing antibody-dependent cellular phagocytosis. In syngeneic settings, however, broad expression of CD47 on cells of the hematopoietic lineage creates a formidable antigen sink and increases toxicity. We find that optimal synergy between anti-CD47 antibodies and several immune therapies, including anti–CTLA-4, requires near-complete blockade of CD47 in the tumor microenvironment. Thus, novel strategies to deliver localized CD47 blockade to tumors may be particularly valuable for immune therapy.

Abstract

CD47 is an antiphagocytic ligand broadly expressed on normal and malignant tissues that delivers an inhibitory signal through the receptor signal regulatory protein alpha (SIRPα). Inhibitors of the CD47–SIRPα interaction improve antitumor antibody responses by enhancing antibody-dependent cellular phagocytosis (ADCP) in xenograft models. Endogenous expression of CD47 on a variety of cell types, including erythrocytes, creates a formidable antigen sink that may limit the efficacy of CD47-targeting therapies. We generated a nanobody, A4, that blocks the CD47–SIRPα interaction. A4 synergizes with anti–PD-L1, but not anti-CTLA4, therapy in the syngeneic B16F10 melanoma model. Neither increased dosing nor half-life extension by fusion of A4 to IgG2a Fc (A4Fc) overcame the issue of an antigen sink or, in the case of A4Fc, systemic toxicity. Generation of a B16F10 cell line that secretes the A4 nanobody showed that an enhanced response to several immune therapies requires near-complete blockade of CD47 in the tumor microenvironment. Thus, strategies to localize CD47 blockade to tumors may be particularly valuable for immune therapy.