Mismatch repair–deficient (dMMR) endometrial tumors are often responsive to immune checkpoint inhibitors (ICI), yet recurrence and variable treatment responses remain significant clinical challenges. Characterization of the tumor microenvironment, including immune cell composition and spatial organization, may reveal predictors of recurrence and ICI responsiveness. We performed multiplex immunofluorescence imaging on 16 dMMR endometrial tumors using a 36-antibody panel. Cells were segmented, phenotyped by unsupervised clustering, and analyzed to quantify cell type proportions and spatial relationships among intratumoral, peritumoral, and whole-tissue cell populations across clinical groups. Non-recurrent tumors (n = 10) exhibited higher intratumoral CD8⁺ T-cell proportions, tumor cell enrichment around CD8+ T cells, CD8+/CD4+ ratios, and PD-1Low CD4⁺ T-cell proportions. In contrast, recurrent tumors (n = 6) showed higher CD4+ T cell proportions and endothelial cell enrichment surrounding CD8⁺ and PD-1⁺ CD8⁺ T cells. Among the recurrent tumors, compared to non-responders (n = 2), ICI responders (n = 4) had a higher proportion of PD-1+Ki67+ CD8+ T cells. Macrophage spatial organization also differed; non-responders displayed separate clusters of CD163⁺ macrophages and CD163⁻ macrophages, whereas responders demonstrated more dispersed macrophages co-localized with PD-1+ CD8⁺ T cells. Overall, these findings suggest that both immune cell composition and spatial arrangement are factors that contribute to recurrence and ICI response in dMMR endometrial cancer. Spatial profiling of the tumor microenvironment may provide biomarkers to guide patient stratification and precision immunotherapy strategies.

The authors have declared no competing interest.

This study did not receive any funding

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