Abstract 5261: The significance of CD200R1 on NK cells in the endometrial cancer tumor microenvironment

Abstract Endometrial cancer (EC) is the most common gynecologic cancer in the USA and is expected to surpass ovarian cancer in mortality. Advanced-stage EC has poor outcomes with high relapses (>35%) and low chemotherapy response (50%). Immune checkpoint blockade (ICB) therapies are often ineffective in EC, indicating a need for better understanding the tumor microenvironment (TME). In many cancers, cytotoxic NK cell (cNK) function is suppressed in the TME through dysregulated differentiation toward an immune-suppressive tissue-resident (trNK) state. Single cell RNA-sequencing (scRNA-seq) data on treatment naïve primary EC samples (n=6, unpublished; n=11; Regner MJ et al), revealed trNK populations expressing the inhibitory glycoprotein CD200 receptor 1 (CD200R1). CD200R1+ trNK cells correlated with high expression of CD200 (the corresponding ligand on EC). CD200 is known to promote NK cell-mediated immunotolerance in physiological settings such as pregnancy to prevent abortion and ectopic endometrial implantation (endometriosis). In cancers, CD200 mediates inhibition of CD200R1+NK cells and promotes an immunosuppressive TME through TGF-β. Given that TGF-β is known to drive cNK-to-trNK polarization, we investigated their patterns in the scRNA-seq data revealing correlation between CD200 expression and TGF-β. Immunohistochemical staining of EC tissue showed trNK cells were enriched within tumor glands compared to stroma (6 vs. 2 cells per 400× high-power field; n=6, p=0.046), suggesting an enrichment of non-cNK cells within EC glands. Hypothesis: CD200+ ECs promote a TME conducive to dysfunctional trNK phenotypes with impaired differentiation into effector cNK cells, enabling immune evasion and reduced ICB efficacy. To address this, we performed flow cytometry on EC primary samples (n=17) and confirmed CD200R1 expression on trNK compared to benign endometrium (n=8) (32% vs 5% respectively; p=0.006). CD200R1 expression on trNK correlated with low/undetectable levels of NK cell cytotoxic/maturity markers (granzymes, perforin, CD94, KIRs, CD16, and EOMES). Functionally, co-culturing CD200R1+ NK precursor cells with stromal feeder cells overexpressing CD200 impaired NK cell maturation (increased innate lymphoid cells and decreased cNK). Further, co-culturing immature NK cells with human stromal cells in the presence of TGF-β promoted a trNK phenotype with elevated CD200R1 expression. This trNK polarization was prevented by adding anti-TGF-β. Altogether, our findings suggest that a CD200/CD200R1 axis modulates NK cell differentiation and function within the EC TME, potentially downstream of TGF-β signaling. Ongoing studies seek to determine whether blocking this axis enhances ICB efficacy, and potentially support clinical trials using Samalizumab (humanized monoclonal anti-CD200 antibody). Citation Format: Youssef Youssef, Caprice D. Eisele, Micheal Ruesch, Courtney J. Riedinger, Paulina J. Haight, Ankur Sheel, Bethany L. Mundy-Bosse, Bradley L. Blaser, Casey M. Cosgrove, Aharon G. Freud. The significance of CD200R1 on NK cells in the endometrial cancer tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5261.