Expression patterns of TLR4 mRNA and protein in the equine endometrium during early pregnancy

Toll-like receptors (TLRs) are an important part of the innate immune system and play a role in activating the acquired immune system. TLR4 is the best characterised of the many TLRs; its signalling mechanisms are known to be mediated by many inflammatory pathways and by endogenous substances. The aim of the present study was to elucidate the expression profiles for TLR4 mRNA and protein and determine the cell-specific localisation of TLR4 in the equine endometrium during the oestrous cycle and early pregnancy. Endometrial biopsy samples were obtained from mares during early pregnancy (on day 14, P14, n = 4) and, for comparison, on day 14 of the oestrous cycle (late dioestrus, LD, n = 4) and when luteolysis was completed (after luteolysis, AL, n = 4). Total RNA was extracted and transcribed into cDNA, and the relative expression of TLR4 mRNA was quantified by qPCR. Immunohistochemistry was used to define the spatio-temporal localisation of TLR4 protein in the uterus. Expression of TLR4 mRNA was reduced (P < 0.05) on P14 compared to LD. The expression of TLR4 mRNA was similar for P14 and AL. TLR4 protein was localised mainly in the luminal and glandular epithelia, but was also present in endothelial cells of capillaries. Compared to P14, TLR4 protein was abundant in both luminal and glandular epithelia of uteri of mares in LD. These results suggest that downregulation of TLR4 in the endometrial epithelia in early pregnancy may play a role in the immune tolerance mechanisms that protect the conceptus (embryo/foetus and associated membranes).