Lynch syndrome is rarely associated with rectal cancer (RC) and thus, metachronous RC has been scarcely investigated. This study aimed to analyze the mucosal immune microenvironment in sporadic and metachronous RC. We analyzed the mucosal immune microenvironment in the 25 metachronous RCs present in the IMMUNOREACT 1 and 2 multicentre observational studies (624 patients). A panel of immune markers was retrospectively investigated at immunohistochemistry: CD3, CD4, CD8, CD8b, Tbet, FoxP3, PD-L1, MSH6, and PMS2 and CD80. Single-cell suspensions were subjected to flow-cytometry to determine the proportion of epithelial cells (pan-cytokeratin) acting as antigen-presenting cells (expressing CD80, CD86, HLA-ABC) and the proportion of activated CD8 + T cells (CD8 + positive for CD28, CD38), inhibitory T cells (CD3 + CTLA-4+) of activated CD4 + T helper cells (CD4 + CD25+) and activated T regulatory cells (CD4 + CD25 + FoxP3+). No mismatch repair gene deficiencies were observed in the patients. The previous history of colorectal adenoma was significantly more frequent in metachronous RC. In healthy epithelial cells, HLA-ABC expression was significantly higher in patients with metachronous RC. In therapy-naïve metachronous RC patients, a significantly lower level of circulating lymphocytes and CD3 + T-cell infiltration in the healthy mucosa surrounding the RC was observed compared to patients with non-metachronous cancer. Our study supports the hypothesis that metachronous RC can occur in a cancerization field in patients with weak systemic and local immune systems. The peculiar site of RC makes the mismatch-repair genes deficiency in metachronous cancer onset less relevant.

IMMUNOREACT 9 metachronous rectal cancers have high HLA-ABC expression on healthy epithelium but a lower infiltration of CD3+ T cells than primary lesions