Abstract 136: T-cell clonotype convergence in colorectal cancer driven by tumor-specific neoantigens

Abstract

Abstract A cornerstone of the biological process which mediates response to immunotherapy in cancer patients is the expression of neoantigens resulting from somatic mutations in cancer cells. Little attention has been paid so far to the neoantigen's immune cell counterpart, the T-cell receptor (TCR). In this study, we aimed at characterizing the tumor infiltrating T-cell repertoire in colorectal cancer (CRC), and to evaluate evidence for neoantigen driven T-cell expansion within the tumor microenvironment. To characterize the T-cell repertoire, we used the Ion AmpliSeq Immune Repertoire Assay Plus with an Ion S5 XL system. This assay works with as little as 10ng of starting total RNA, allows 400bp read length, and offers complete characterization of the CDR1, CDR2 and CDR3 regions. A cohort of 20 CRC cases was used, including microsatellite instability (MSI) positive and MSI-negative cases. Our results show a higher T-cell richness in distal CRC samples compared with proximal samples (p=0.04), and lower T-cell richness in M1 as compared with M0 cases (p=0.0001). No association was observed with MSI status, inflammatory infiltrate, tumor grade or lymph node metastasis. We observed that the T-cell repertoire in CRC samples is highly convergent compared to the peripheral blood lymphocyte repertoire of healthy donors (p<0.0001). In CRC samples we observed evidence of selection of TCR clonotypes coding for the same CDR3 aminoacid sequence, suggesting neoantigen-driven TCR convergence. TCR clone overlap was observed only between pairs of samples (4 pairs of primary tumor and metastasis samples available) from the same patient, and not between samples from different patients, suggesting high tumor specificity of T-cell clonotype selection. A validation cohort of CRC cases, including additional pairs of primary tumor and metastasis samples, is currently being analyzed to validate our findings. In this study we provide evidence of T-cell clonotype selection in CRC driven by tumor-specific neoantigens. The specificity of this process is further reinforced by the T-cell clonotype overlap within patients. It remains to be seen whether it is possible to map convergent T-cell clonotypes to its respective tumor neoantigens. Citation Format: Jose C. Machado, Joana Reis, Margarida Fernandes, Rafaela Silva, Luis Cirnes, Fatima Carneiro, Jose L. Costa. T-cell clonotype convergence in colorectal cancer driven by tumor-specific neoantigens [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 136.