Uterine NK Cell Polarization Associates with Chronic Endometritis and Predisposition to Recurrent Implantation Failure

Background: Disturbances in the endometrial immune microenvironment, particularly uterine natural killer (uNK) cell polarization, are linked to chronic endometritis (CE) and recurrent implantation failure (RIF). However, the underlying mechanisms and a lack of reliable biomarkers hinder effective clinical diagnosis and treatment. Methods: We integrated public single-cell RNA sequencing (scRNA-seq) datasets to characterize endometrial immune cell dynamics. Single-cell regulatory network inference identified key transcription factors (TFs) regulating uNK subtypes, and their functions were explored via pathway enrichment. A diagnostic model was subsequently developed and validated using bulk RNA-seq data from CE and RIF cohorts. Results: Our analysis identified two functionally distinct uNK subtypes: cytotoxic uNK2 cells regulated by TFs EOMES and ELF4, and uNK3 cells involved in platelet activation and tight junctions, driven by ELK4 and IRF1. The abundance of these TFs correlated with their respective uNK subtype proportions moderately (p< 0.001). Key marker genes-AFAP1L2, KLRC1, and SOCS1 for uNK2, and SAMD3 for uNK3-were identified and demonstrated altered expression patterns in samples from patients with CE and RIF. In a bulk RNA-seq dataset comprising 51 endometrial samples (18 CE and 33 normal), the ratio of uNK2/uNK3 signature expression was notably upregulated in CE samples. This finding was corroborated in an independent RIF dataset. The diagnostic model based on these markers demonstrated strong predictive power. For CE, the AUC for SDC1 was 0.48, the uNK2/uNK3 ratio was 0.675, and the logistic model reached 0.822. For RIF, the uNK2/uNK3 ratio had an AUC of 0.823, while the logistic model achieved 0.83. Conclusion: Our findings suggested that an imbalance in uNK cell polarization was a key feature of immune dysregulation in CE and RIF, with the uNK2/uNK3 signature ratio emerging as a potential biomarker. Extensive validation in prospective clinical cohorts and through functional experiments is essential to confirm the clinical utility and therapeutic potential of targeting these uNK subtypes to improve reproductive health.