Tumour-infiltrating lymphocytes in colorectal cancer with microsatellite instability are activated and cytotoxic

Background: Patients with colorectal cancer that display high‐level microsatellite instability (MSI‐H) appear to have a better prognosis. This may be explained by the pronounced T cell infiltrate seen in MSI‐H tumours that is related to a specific antigen‐driven immune response. The nature of tumour‐infiltrating lymphocytes in colorectal cancers was investigated using quantitative real‐time polymerase chain reaction (PCR) and immunohistochemistry. Methods: Quantitative fluorescent hydrolysis probe‐based reverse transcriptase–PCR assays were used to detect levels of mRNA specifying T cell markers in fresh frozen colorectal tissue from MSI‐H tumours and those with little or no microsatellite instability (microsatellite stable (MSS) tumours). In addition, immunohistochemistry was performed on paraffin‐embedded sections to compare expression of the same T cell markers and the activation markers granzyme B and interleukin 2 receptor α‐subunit (IL‐2Rα) in MSI‐H and MSS tumours. Results: MSI‐H tumours contained higher ratios of CD8/CD3 mRNA copy numbers than MSS tumours (P = 0·016), confirming the cytotoxic nature of lymphocyte infiltrates in this subset of colorectal cancers. Furthermore, immunohistochemistry confirmed that MSI‐H tumours contained more infiltrating lymphocytes than MSS tumours, as shown by increased expression of CD3 (P = 0·003) and CD8 (P = 0·008). Consistent with other studies, the lymphocytes in MSI‐H tumours were activated as indicated by significantly higher granzyme B counts (P = 0·020) and a significantly higher level of expression of IL‐2Rα (P = 0·017). Conclusion: The results support the hypothesis that MSI‐H colorectal cancers may be more immunogenic than MSS tumours. Copyright © 2004 British Journal of Surgery Society Ltd. Published by John Wiley & Sons, Ltd. Possible explanation for better outcomes