DISSECTING RESISTANCE TO IMMUNOTHERAPY IN MSS COLORECTAL CANCER

Patients with colorectal cancer (CRC) exhibiting DNA mismatch repair deficiency and microsatellite instability (MSI) often display favorable responses to immune checkpoint inhibitor (ICI) therapies, in contrast to mismatch repair proficient and microsatellite stable (MSS) tumors. While the high mutational burden and elevated neoantigen levels in MSI CRCs are recognized as key determinants of ICI sensitivity, our study delves into additional factors influencing the unresponsiveness of MSS CRCs to immune checkpoint blockade. To dissect these characteristics, we developed an experimental model for in vitro manipulation, establishing a controlled system using patient-derived organoids from CRC patients, ensuring a physiologically relevant representation. The initial part of the thesis focuses on optimizing organoid generation and culture. Subsequently, we built an experimental approach ensuring equal immune system recognition of MSS and MSI CRC tumors. To do it, we made use of a known matched TCR-antigen system in which MSS- and MSI- tumors present the same known antigen to T cells, engineered to express the corresponding known TCR. Employing this system, we unveil the active impairment of T-cell activation and cytotoxicity by MSS tumors. Based on our results, MSS tumors seem to be intrinsically unsensitive to the immune system activity. Exploring the unknown causes, we identify that soluble factors secreted by MSS cells hinder neoantigen-mediated recognition in MSI CRC cells. This comprehensive approach challenges the prevailing notion that mutation quantity alone is sufficient to overcome the intrinsic resistance of MSS tumors to immune checkpoint blockade.