The ability of the host to mount an appropriate immune response to aberrant cells is one factor that determines prognosis in
cancer patients. Naturally occurring regulatory T cells (T regs; CD4+ CD25+ cells) are key regulators of peripheral tolerance. It has been suggested that high levels of T regs are detrimental to the patient in some forms of
cancer, but the role of these
antigen-specific cells in individuals with
colorectal cancers with high levels of
microsatellite instability is unknown. Herein, we examined the ability of individuals with MSI-H or microsatellite stable
colon cancer to recruit lymphocytes to the
tumor site. Immunohistochemical staining was performed on archived
paraffin-embedded specimens from a total of 38 individuals with MSI-H (n = 25) or MSS (n = 13)
colon cancers to determine the proportion of CD3+, CD8+ and CD25+ cells infiltrating the
tumor site. Patients with MSI-H
colon cancers had increased percentages of CD8+ TILs (cytotoxic T cells) as compared to individuals with MSS
colon cancer (47.3 vs. 24.04% of the infiltrate CD8+, respectively). No differences in the levels of CD25+ T cells were observed between individuals with MSI-H
colon cancers and MSS
colon cancers (0.53 vs. 0.54% CD25+, respectively). Together, these data suggest that the survival advantage enjoyed by patients with MSI-H
colorectal cancer may, in part, be attributed to the increased cytolytic response, but not to an
antigen-specific immunosuppressive response in MSS patients.