Abstract A16: Defining molecular mechanisms of resistance to tumor immunity

Recent success of immune checkpoint blockade solidifies the importance of the immune system in the defense against cancer. The clinical impact of the immune response is, however, very heterogeneous, with some patients achieving dramatic responses while others fail to respond. Known genomic correlates of response to immunotherapy are not perfectly predictive of clinical outcome, supporting the existence of unknown mechanisms of resistance to tumor immunity. I hypothesize somatic acquired mutations of individual tumors may account for heterogeneity in the spontaneous response to tumors and response to immunotherapy. I have undertaken a systematic in vivo screen to identify mechanisms of resistance to tumor immunity in order to discover new mechanisms of immune resistance, define a comprehensive set of therapeutic targets and provide biomarkers of sensitivity to immunotherapeutic strategies. Mouse tumor cell lines (MC38 colon carcinoma or B16 melanoma) were engineered to express a library of barcoded open reading frames (ORFs) mutagenized to encode known cancer-associated somatic mutations from the Pan Cancer analysis within The Cancer Genome Atlas (TCGA). These cell lines form tumors when implanted subcutaneously in immunocompetent animals. Tumor-bearing animals were then subjected to immunotherapy with either therapeutic vaccination or checkpoint blockade with anti-PD-1. Barcode relative representation was measured by next generation sequencing at the time of tumor implantation and at the time of tumor harvest post-immunotherapy. Barcoded mutant ORFs that confer immune resistance increased in representation under immune pressure in comparison to untreated or immunodeficient animals. A mutation in Phospho-Inositol 3 Kinase (PI3K), PIK3CA c.3140A>G, consistently increased in representation in both B16 and MC38 immunotherapy-treated tumors. This mutation encodes a constitutively active mutant catalytic domain of PI3K, PIK3CA H1047R. MC38 tumors homogenously expressing PIK3CA H1047R and implanted into wild type mice failed to respond to anti-PD-1 therapy, while tumors expressing a control gene or non-scoring mutation in PI3K regressed after treatment with anti-PD-1. Pharmacologic PI3K inhibition resensitized tumors to treatment with anti-PD-1. PD-1-treated PIK3CA H1047R tumors had fewer infiltrating CD8+ T cells as measured by immunohistochemistry and flow cytometry of tumor infiltrating lymphocytes. I conclude that PI3K has, in addition to its well-described oncogenic role, a role in tumor immune evasion. As such, activation of PI3K may be useful as a predictor of resistance to immunotherapy. Importantly, these findings also provide a rationale for therapeutic combination trials of immune checkpoint blockade and PI3K inhibition. Citation Format: Natalie B. Collins, Robert Manguso, Hans Pope, W. Nicholas Haining. Defining molecular mechanisms of resistance to tumor immunity. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr A16.