The endometrial microbiome influences local innate immune activity, fertility and pregnancy. Microbial metabolites at other mucosal surfaces, such as the gut, act as important modulators of immune and barrier function. Here we examine innate immune gene expression and local bacterial microbial populations in uterine tissue from infertile women, and explore how their metabolites might influence endometrial cell function.
Endometrial tissue samples were taken from 29 nulliparous women with unexplained infertility, for transcriptomic and microbiome analysis prior to ART. Transcriptomic analysis revealed increased expression of the IL17 pathway in tissue from women who did not become pregnant after ART. 16S sequencing of endometrial microbiomes revealed lower abundance of Lactobacillus spp. and significantly higher abundance of pathogenic species such as Prevotella spp. and Corynebacterium spp in women who failed ART. Endometrial microbiota from women who had positive ART outcomes showed significantly lower diversity indexes. Negative ART outcomes were associated with higher levels of receptivity and decidualisation marker expression; these were found to correlate positively with the diversity indexes from microbiome analysis: SPP1 showed positive correlation with Shannon diversity index (R2=0.589 and p-value 0.0269) and with Simpson diversity index (R2=0.5235 and p-value 0.0425). Counts per million expression of ITGAV, another marker for epithelial receptivity, correlated negatively with the beta-diversity index from the total cohort (R2=0.229 and p-value 0.048). Using in vitro models of implantation, butyrate, but not lactate or acetate, activated defence mechanisms in cultured endometrial epithelial cells by inducing expression of antimicrobial peptides and inflammation markers. Butyrate exposure also impaired barrier integrity of endometrial epithelial cell monolayers, and increased markers of epithelial receptivity and stromal decidualisation. In summary, the more diverse endometrial microbiome in women who fail to get pregnant after Assisted Reproductive Technology (ART) may produce metabolites that compromise local innate immune activity and implantation.