Inter-tumoral heterogeneity of CD3+ and CD8+ T-cell densities in the microenvironment of DNA mismatch repair-deficient colon cancers: implications for prognosis

Background: Colorectal cancers with deficient DNA mismatch repair (dMMR) are presumed to uniformly have dense lymphocytic infiltration that underlies their favorable prognosis and is critical to their responsiveness to immunotherapy, as compared to MMR-proficient (pMMR) tumors. We examined T-cell densities and their potential heterogeneity in a large cohort of dMMR tumors. Experimental Design: CD3⁺ and CD8⁺ T-cell densities were quantified at the invasive margin (IM) and tumor core (CT) in 561 stage III colon cancers (dMMR, n=278; pMMR, n=283) from a phase 3 adjuvant trial (N0147). Their association with overall survival (OS) was determined using multivariable Cox analysis. Results:While CD3⁺ and CD8⁺ T-cell densities in the tumor microenvironment were higher in dMMR vs pMMR tumors overall, inter-tumoral heterogeneity in densities between tumors was significantly higher by 30-88% among dMMR vs pMMR cancers (P+IM, CD3⁺CT, CD8⁺IM, CD8⁺CT]). A substantial proportion of dMMR tumors (26% to 35% depending on the T-cell subtype) exhibited T-cell densities as low as that in the bottom half of pMMR tumors. All four T-cell subtypes were prognostic in dMMR with CD3⁺IM being the most strongly prognostic. Low (vs high) CD3⁺IM was independently associated with poorer OS among dMMR (HR 4.76 [95% CI 1.43-15.87]; P=.0019) and pMMR tumors (P=.0103). Conclusions: Tumor-infiltrating T-cell densities exhibited greater inter-tumoral heterogeneity among dMMR than pMMR colon cancers, with CD3⁺IM providing robust stratification of both dMMR and pMMR tumors for prognosis. Potentially, lower T-cell densities among dMMR tumors may contribute to immunotherapy resistance.

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