Association Between Endometrial Microbiome Composition and Local Immune Cell Distribution During the Window of Implantation

The aim of this study was to investigate the association between the endometrial microbiome and local immune cell composition during the implantation window. We conducted a single-center prospective observational study including endometrial samples from 58 women without endometrial pathology. All samples were obtained during the mid-secretory phase (day 5 post-ovulation). Endometrial stromal CD3+ T cells, CD4+ T helpers, CD56+ NK cells and CD68+ macrophages were identified by immunohistochemistry and quantified as a percentage of stromal cells using HALO image analysis software. Microbiome composition was assessed by 16S rRNA gene sequencing (V4–V5 region). Lactobacillus dominance (LD) was defined as the genus with the highest centered log-ratio value within each sample. Endometrial immune cell composition showed a median 2.28% CD3+ T cells of stromal cells, 1.75% CD56+ NK cells, 1.44% CD68+ macrophages and 0.29% CD4+ T helpers. Lactobacillus-dominated microbiota was identified in 51.7% (30/58) of samples. Correlation analysis showed that Lactobacillus-related taxa were negatively associated with NK/T ratios and positively with T helper-related ratios. LD samples exhibited reduced NK cell abundance (1.17% vs. 2.12%, p = 0.006, q = 0.023) and a lower NK/T ratio (0.52 vs. 1.05, p = 0.004, q = 0.023) compared to non-LD samples. This study provides evidence for a link between microbial composition and local immune regulation in the endometrium.