Endometrial microbiota in maternal-fetal tolerance: An in vitro study of extravillous trophoblasts and uNK cells

Accumulating clinical studies have reported the significance of endometrial microbiota during pregnancy, especially in assisted reproductive technology. Although there are discussions on the relationship between endometrial microbiota and immunological tolerance at the maternal-fetal interface, the data supporting these processes is still insufficient. We, therefore, investigated the effect of endometrial microbiota culture supernatants on maternal-fetal tolerance in vitro, focusing on extravillous trophoblasts (EVT) and uterine natural killer (uNK) cells, which constitute approximately 70% of lymphocytes in human uterine decidua in early pregnancy. The cells were treated with the culture supernatants of Lactobacillus crispatus, Lactobacillus iners, Gardnerella vaginalis, Klebsiella pneumoniae subsp. pneumoniae, and Enterobacter cloaca. Cell killing assay showed that the bacterial culture supernatant of L. crispatus significantly enhanced the cell killing ability of NK-92MI cells against Sw.71 cells., whereas that of K. pneumoniae decreased the cell killing rate. Western blotting showed that the L. crispatus supernatant significantly decreased the expression of leukocyte immunoglobulin-like receptor B1 (LILRB1) on NK-92MI cells. The culture supernatant of K. pneumoniae significantly increased human leukocyte antigen (HLA)-C, HLA-G, and programmed death-ligand 1 (PD-L1) expressions on Sw.71 cells, and promoted the secretion of sPD-L1 from the cells. The L. iners culture supernatant significantly decreased PD-L1 expression in Sw.71 cells. The present results suggest that endometrial microbiota may modulate EVT and uNK cell interaction and affect maternal-fetal tolerance during implantation and placentation. To improve infertility treatments and elucidate the pathophysiology of pregnancy complications, further clarification of the function of the endometrial microbiota during pregnancy is warranted.