Integrated immunophenotypic and ProMisE molecular profiling as predictors of immune checkpoint inhibitor response in recurrent endometrial cancer

Background: Immune checkpoint inhibitors (ICIs) are an important treatment option for recurrent endometrial cancer (EC); however, responses vary widely. Although the ProMisE molecular classification provides prognostic and predictive insights, it does not fully explain the heterogeneity of ICI outcomes. Further characterization of the tumor immune microenvironment (TIME) is required to refine biomarker development. Methods: We retrospectively analyzed 72 patients with recurrent EC treated with ICIs. Immune markers, including CD8, CD68, CD163, CD47, CD276, PD-L1, and HLA class I, were quantified using digital pathology. Associations between immune markers, ICI response, and progression-free survival (PFS) were evaluated overall and within ProMisE molecular subtypes. In an exploratory supplementary analysis, Foxp3 immunostaining was performed in 51 cases with additional FFPE material available. Results: Responders exhibited a more immunologically active TIME, characterized by increased CD8⁺ T-cell infiltration in the central tumor (CT), whereas non-responders showed enriched CD68⁺ tumor-associated macrophages at the invasive margin (IM) and higher CD47 expression. Distinct immune landscapes were observed across ProMisE subtypes. Mismatch repair-deficient (MMRd) tumors demonstrated higher intratumoral CD8+ infiltration, whereas p53-abnormal (p53abn) tumors showed a more immunosuppressive profile, including elevated CD163+ macrophage density and CD47 expression. Subtype-specific analyses revealed that PFS was associated with different immune determinants, including CD8+ T cell infiltration in MMRd, CD47 expression level in no specific molecular profile (NSMP), and CD68.IM in p53abn. In the Forkhead box P3 (Foxp3) subset, the CD8/Foxp3 ratio showed a stronger association with ICI response and PFS than Foxp3 alone. Conclusions: Tumor immune phenotypes provide clinically relevant information complementary to ProMisE classification. Rather than indicating a single universal determinant, our results support a model in which cytotoxic T-cell infiltration and immunosuppressive pathways jointly shape ICI outcome in recurrent EC. Integrating ProMisE classification with immunological profiling may improve patient stratification of ICI benefits.