Fcγ receptor-mediated cross-linking codefines the immunostimulatory activity of anti-human CD96 antibodies

New strategies that augment T cell responses are required to broaden the therapeutic arsenal against cancer. CD96, TIGIT, and CD226 are receptors that bind to a communal ligand, CD155, and transduce either inhibitory or activating signals. The function of TIGIT and CD226 is established, whereas the role of CD96 remains ambiguous. Using a panel of engineered antibodies, we discovered that the T cell stimulatory activity of anti-CD96 antibodies requires antibody cross-linking and is potentiated by Fcγ receptors. Thus, soluble "Fc silent" anti-CD96 antibodies failed to stimulate human T cells, whereas the same antibodies were stimulatory after coating onto plastic surfaces. Remarkably, the activity of soluble anti-CD96 antibodies was reinstated by engineering the Fc domain to a human IgG1 isotype, and it was dependent on antibody trans-cross-linking by FcγRI. In contrast, neither human IgG2 nor variants with increased Fcγ receptor IIB binding possessed stimulatory activity. Anti-CD96 antibodies acted directly on T cells and augmented gene expression networks associated with T cell activation, leading to proliferation, cytokine secretion, and resistance to Treg suppression. Furthermore, CD96 expression correlated with survival in HPV+ head and neck squamous cell carcinoma, and its cross-linking activated tumor-infiltrating T cells, thus highlighting the potential of anti-CD96 antibodies in cancer immunotherapy.