Abstract 3447: T cell antigen expression levels govern progression and therapy response in a novel model of colon cancer
Authors: Westcott Peter Maxwell Kienitz , Sacks Nathan J. , Smith Olivia , Schenkel Jason , Ely Zackery , Zhang Daniel , Beytagh Mary Clare , Hwang William , Eng George , Roper Jatin , Yilmaz Omer , Jacks Tyler 
Published: September 2020

Abstract
Genetically-engineered mouse models have greatly advanced our understanding of cancer, yet do not recapitulate the mutational complexity of human cancer, and likely lack neo-antigens capable of eliciting potent anti-tumor T cell responses. We developed a novel orthotopic organoid transplant model of colorectal cancer (CRC) harboring the strong T cell antigen SIINFEKL, and demonstrate the importance of antigen expression level in the anti-tumor T cell response. Although SIINFEKL low-expressing organoids (SIINLow) elicit an endogenous antigen-specific T cell response, the magnitude is substantially lower and kinetics delayed relative to SIINFEKL high-expressing organoids (SIINHi). Consistently, transplant of SIINHi results in rejection and T cell memory, while SIINLow results in tumor progression, terminally-exhausted T cells, and metastasis to the liver. We have shown that suboptimal T cell priming is the major factor underlying SIINLow tumor escape. Importantly, co-transplant of SIINHi and SIINLow organoids at distinct sites in the colon of the same animal results in complete rejection of both lines. In addition, co-transplant rescues the SIINLow tumor-infiltrating antigen-specific T cell response to a magnitude and quality comparable to that of SIINHi tumors. Single-cell RNA-sequencing of antigen-specific T cells from SIINHi and SIINLow tumors 8 days post-transplant revealed distinct clusters dominated by SIINLow-primed T cells, including a cluster enriched for immediate early response genes, Tox, and a number of immune checkpoints, indicative of early dysfunction. Collectively, our results establish the existence of a neo-antigen expression threshold at which T cell priming is limiting, resulting in attenuated magnitude and functionality of the T cell response, and tumor escape. To assess the therapeutic relevance of a poorly-expressed neo-antigen in CRC, we performed preclinical trials with immune checkpoint blockade and agonistic-CD40 (aCD40), which has been shown to potentiate T cell priming and response in poorly-immunogenic mouse and human pancreatic adenocarcinoma. While monotherapies showed only modest effects, the combination of checkpoint blockade and aCD40 resulted in an 80% response rate with a number of complete responses. In conclusion, antigen expression level is a critical determinant of T cell dysfunction, resulting from poor priming. Our results argue that targeting T cell priming may be a promising therapeutic strategy to invigorate anti-tumor immunity in human CRC, the majority of which remains refractory to immunotherapy.
Citation Format: Peter Maxwell Kienitz Westcott, Nathan J. Sacks, Olivia Smith, Jason Schenkel, Zackery Ely, Daniel Zhang, Mary Clare Beytagh, William Hwang, George Eng, Jatin Roper, Omer Yilmaz, Tyler Jacks. T cell antigen expression levels govern progression and therapy response in a novel model of colon cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3447.