ABSTRACT: 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. Whereas the function of TIGIT and CD226 is established, 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 crosslinking and is potentiated by Fc-gamma 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 was dependent on antibody trans-crosslinking by Fc-γRI. In contrast, neither human IgG2 nor variants with increased Fc-γ 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 regulatory T-cell suppression. Furthermore, CD96 expression correlated with survival in HPV+ head and neck squamous cell carcinoma and its crosslinking activated tumor-infiltrating T cells, thus highlighting the potential of anti-CD96 antibodies in cancer immunotherapy. .