Tumor-infiltrating lymphocytes in colorectal carcinoma and their association with microsatellite instability and clinicopathological parameters

Background: Tumor-infiltrating lymphocytes (TILs) are an important part of the tumor histomorphology and reflect the host immune response against colorectal carcinoma (CRC). Microsatellite instability (MSI) is known to enhance tumor immunogenicity and is frequently associated with increased lymphocytic infiltration. Objective: The objective is to evaluate TILs in CRC and analyze their association with MSI and various clinicopathological parameters. Materials and Methods: Sixty-two cases of CRC diagnosed between January 2023 and December 2025 were included in this cohort and observational study. Hematoxylin- and eosin-stained slides were reviewed to assess intratumoral and peritumoral TILs. Intratumoral lymphocytes were categorized according to the College of American Pathologists guidelines as mild to moderate (≤3 intraepithelial lymphocytes/high-power field [HPF]) or marked (>3/HPF). Peritumoral lymphocytic infiltration was defined by the presence of two or more lymphoid aggregates at the invasive tumor front. Immunohistochemistry for mismatch repair proteins MLH1, PMS2, MSH2, and MSH6 was done to evaluate the MSI status. The Chi-square test was used for the statistical analysis. Results: There were 36 males and 26 females. Mild-to-moderate intratumoral lymphocytic infiltration was observed in 32 cases (51.6%), marked infiltration in 30 cases (48.4%), and peritumoral lymphoid aggregates in 21 cases (33.9%). Nineteen cases (30.6%) demonstrated MSI, including 15 high MSI (MSI-H) and 4 low MSI tumors. A statistically significant association was observed between intratumoral and peritumoral lymphocytic infiltration and MSI-H status. Conclusion: TILs in CRC shows a significant association with MSI-H tumors. Evaluation of TILs on routine histopathological sections may serve as a useful adjunct in identifying tumors with MSI and may have prognostic and therapeutic implications.