Intratumoural CD8+ CXCR5+ follicular cytotoxic T cells have prognostic value and are associated with CD19+ CD38+ B cells and tertiary lymphoid structures in colorectal cancer.

Background: Colorectal cancer (CRC) is the most common digestive cancer in the world. Microsatellite stability (MSS) and microsatellite instability (MSI-high) are important molecular subtypes of CRC closely related to tumor occurrence and progression and immunotherapy efficacy. The presence of CD8⁺ CXCR5⁺ follicular cytotoxic T (T_FC) cells is strongly associated with autoimmune disease and CD8⁺ effector function. However, the roles of T_FC cells in MSI-high CRC and MSS CRC are unclear. Here, we aimed to explore the characteristics of T_FC cells in CRC and compare their biological functions between MSI-high and MSS CRC. Methods: We explored the expression of T_FC cell in tumor tissues and peripheral blood in our clinical cohort and public datasets. By combining single-cell RNA sequencing (scRNA-seq) and bulk RNA sequencing, we explored the potential function of T_FC cells and developed a prediction model for CRC. We also compared the biological functions of these cells between MSS and MSI-high CRC and used flow cytometry and coculture experiments to explore their potential regulatory functions. Results: T_FC cell markers are downregulated in tumor tissues and patient peripheral blood vs. controls. The prediction model for CRC performed well in the training and validation cohorts (KM plot p < 0.001). MSS CRC patients exhibit enrichment of genes related to the cell cycle (MKI67) and T cell activation (CD38 and HLA-DR) and decreased enrichment of immune checkpoint markers (PD1, TIM3, and LAG3). The expression of T_FC cell-related genes is positively correlated with that of CD8⁺IFN-γ⁺-related genes and closely related to that of TLS-related genes in MSS CRC. The proportion of T_FC cells is positively correlated with that of CD19⁺CD38⁺ B cells in MSS CRC. Conclusions: The prognostic prediction model has good predictive value. In MSS CRC, T_FC cells function mostly in T cell activation and the cell cycle and have low expression of immune checkpoint molecules, which may influence the effectiveness of ICB therapy. T_FC cells may regulate antitumor function by regulating CD19⁺ CD38⁺ B cells and TLSs. [ABSTRACT FROM AUTHOR]