Endometrial Toll-like Receptor 4 (TLR4) and Interleukin-8 (IL-8) Expression in Mares Resistant (RM) or Susceptible (SM) to Endometritis

Persistent uterine inflammation in the mare has been correlated to the expression of cytokines by the endometrium. Toll-like receptors (TLR) are host cell receptors for bacterial-associated molecules and play an important role in
modulating immune responses by activating antigen-presenting cells. However, the role of equine endometrial cell TLR4 during post-breeding endometritis has not been determined. The aims were to: characterise TLR4 mRNA expression in
the equine endometrium; establish changes in expression after insemination; determine differences in expression between mares resistant (RM) or susceptible (SM) to endometritis. Expression of the pro-inflammatory chemokine
IL-8 was measured to confirm the status of an inflamed uterus. Endometrial biopsies were collected from 6 RM and 9 SM during oestrus and 24 h after infusion of frozen/thawed spermatozoa and the mRNA expression of TLR4
and IL-8 was quantified by rt-PCR. The expression of TLR4 by SM tended to increase after insemination (4.9 ± 2.3-fold increase, p = 0.06) whereas expression did not differ after insemination of RM (0.61 ± 0.27-fold reduction, p > 0.05). IL-8 expression increased after insemination in SM but not in RM (3.11 ± 1.52-fold increase, p = 0.05; 1.2 ± 0.5-fold increase, p > 0.05, respectively). Endometrial TLR4 expression appeared to increase after insemination in SM. TLR4 may be important in the development of persistent inflammation in mares susceptible to endometritis (This study was funded by the RCVS Trust).