Anti-OX40 antibody directly enhances the function of tumor-reactive CD8^þ T cells and synergizes with PI3Kb inhibition in PTEN loss melanoma

Purpose: OX40 agonist–based combinations are emerging as a novel avenue to improve the effectiveness of cancer immunotherapy. To better guide its clinical development, we characterized the role of the OX40 pathway in tumor-reactive immune cells. We also evaluated combining OX40 agonists with targeted therapy to combat resistance to cancer immunotherapy. Experimental Design: We utilized patient-derived tumor-infiltrating lymphocytes (TILs) and multiple preclinical models to determine the direct effect of anti-OX40 agonistic antibodies on tumor-reactive CD8^þ T cells. We also evaluated the antitumor activity of an anti-OX40 antibody plus PI3Kb inhibition in a transgenic murine melanoma model (Braf mutant, PTEN null), which spontaneously develops immunotherapy-resistant melanomas. Results: We observed elevated expression of OX40 in tumor-reactive CD8^þ TILs upon encountering tumors; activation of OX40 signaling enhanced their cytotoxic function. OX40 agonist antibody improved the antitumor activity of CD8^þ T cells and the generation of tumor-specific T-cell memory in vivo. Furthermore, combining anti-OX40 with GSK2636771, a PI3Kb-selective inhibitor, delayed tumor growth and extended the survival of mice with PTEN-null melanomas. This combination treatment did not increase the number of TILs, but it instead significantly enhanced proliferation of CD8^þ TILs and elevated the serum levels of CCL4, CXCL10, and IFNg, which are mainly produced by memory and/or effector T cells. Conclusions: These results highlight a critical role of OX40 activation in potentiating the effector function of tumor-reactive CD8^þ T cells and suggest further evaluation of OX40 agonist–based combinations in patients with immune-resistant tumors.