Immune checkpoint inhibitors result in impressive clinical responses but optimal results will require combination with each other and other therapies. This raises fundamental questions about mechanisms of non-redundancy and resistance. Here, we report major tumor regressions in a subset of patients with metastatic melanoma treated with an anti-CTLA4 antibody (anti-CTLA4) and radiation (RT) on a phase one clinical trial and reproduced this effect in mouse models. Although combined treatment improved responses in irradiated and unirradiated tumors, resistance was common. Computational analysis of genome-wide and immune profiles of mice revealed resistance was due to T cell exhaustion driven by adaptive resistance and prolonged interferon-gamma exposure, resulting in STAT1-mediated upregulation of PD-L1 on melanoma cells and tumor macrophages. Accordingly, optimal response in melanoma and other cancer types requires RT, anti-CTLA4, and anti-PD-L1/PD-1. Anti-CTLA4 predominantly inhibits T regulatory cells, and macrophage depletion and/or PD-L1 blockade reverses T cell exhaustion. RT promotes the infiltration of intratumoral antigen-specific CD8 T cells and enhances the diversity of the T cell receptor (TCR) repertoire. RT with dual checkpoint blockade shapes the TCR repertoire of the expanded peripheral clones in a manner consistent with antigen-driven selection. Similar to results from mice, patients on our clinical trial with tumors showing high PD-L1 did not respond to RT + anti-CTLA4, demonstrated persistent T cell exhaustion, and rapidly progressed. In contrast, patients with low PD-L1 on melanoma cells or macrophages had markedly improved survival, with the best survival observed among those patients with low PD-L1 on both cell types. Thus, our results suggest that 1) RT can enhance response to anti-CTLA4 when the TCR and/or antigen repertoire are sub-optimal, 2) upregulation of PD-L1 through STAT1-mediated adaptive resistance mechanisms inhibits response to anti-CTLA4-based therapy unless PD-L1/PD-1 is blocked, and 3) the combination of RT, anti-CTLA4, and anti-PD-L1 promotes response and immunity through distinct mechanisms. Finally, although PD-L1 was a dominant resistance mechanism in our models, PD-L1-independent resistance mechanisms were also evident. The next generation of clinical trials based on these findings are underway.

Citation Format: Christina Twyman-Saint Victor, Andrew Rech, Joseph Benci, Amit Maity, Ramesh Rengan, Kristen Pauken, Erietta Stelekati, Bihui Xu, Hannah Dada, Pamela Odorizzi, Ramin Herati, Ravi Amaravadi, Lynn Schuchter, Hemant Ishwaran, Rosemarie Mick, Daniel Pryma, Xiaowei Xu, Michael Feldman, Tara Gangadhar, Steve Hahn, John Wherry, Robert Vonderheide, Andy Minn. Mechanisms of tumor response and resistance to radiation and dual checkpoint blockade in mice and patients. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr PR05.