Intratumoral density of regulatory T cells is a predictor of host immune response and chemotherapy response in colorectal cancer.

Regulatory T cells (Tregs) are a subset of CD4+ T lymphocytes known to dampen the host immune response against cancer cells. Within the tumor microenvironment, Tregs are potent facilitators of immune tolerance, and a higher proportion of Tregs compared to cytotoxic T cells predicts a worse outcome in most solid tumors. We studied the association between Treg density, and cancer biology and clinical outcome in colorectal cancer (CRC). We used xCell to estimate intratumoral Tregs in total of 898 CRC patients in the Cancer Genome Atlas (TCGA) and GCE39582 cohorts. High-Treg CRCs enriched immune response-related gene sets; inflammatory response, IFN-γ and IFN-α response, IL2/IL6 signaling, and allograft rejection, and had significantly high infiltration of CD8, CD4, M1 and M2 macrophage, and dendritic cells in both cohorts. While high-Treg CRCs enriched multiple pro-cancer signaling pathways compared to low-Treg CRCs, such as Epithelial Mesenchymal Transition, K-ras, Hypoxia, TGF-β, TNF-α, and angiogenesis, Treg infiltration was surprisingly associated with earlier CRC stage in TCGA. Notably, in two separate cohorts a higher proportion of Tregs predicted an improved response to chemotherapy. In the GSE28702 cohort, metastatic CRCs with more Tregs showed a significantly better response to mFOLFOX6 versus low-Treg CRC metastases (88.9% response vs. 16.7%, P<0.001). In the GSE72970 cohort, high-Treg CRCs were found to have a 68.8% response to FOLFOX/FOLFIRI without bevacizumab, compared to 44% response in the low-Treg CRCs. Additionally, high-Treg CRCs were associated with increased expression of immune checkpoint molecules PD-L1/PD-L2, CTLA4, TIGIT and BTLA, implying susceptibility to immunotherapy. We also found that CRCs with higher proportions of Tregs were associated with lower amounts of three microorganisms in the tumor: Lachnoclostridium, flavivirus, and Ornithobacterium. In conclusion, we show that amount of Treg in the tumor is a predictor of host immune response and chemotherapy response in CRC. AJCR