Expression of Toll-like receptors 2, 4 and 6 in equine endometrial epithelial cells: A comparative in situ and in vitro study

- Equine endometrial epithelial cells mediate innate immune defenses.
- In situ and in vitro these cells are immunopositive for TLRs 2, 4 and 6.
- In situ and in vitro a cytoplasmic and/or nuclear immunoreaction is detected.
- For each TLR, quantitative differences exist between individual mares.
- The applied cell culture is a suitable in vitro model for functional studies.

Endometrial epithelial cells form a luminal barrier and are exposed to pathogens and non-infectious antigens. Toll-like receptors (TLRs) mediate pathogen defenses and tissue homeostasis, but are also involved in the pathogenesis of inflammatory and fibrosing alterations. Endometrial diseases are important causes of subfertility in mares. The pathogenesis of some types of persistent inflammation and periglandular fibrosis (endometrosis) is unknown. The aim of this study was to compare by in situ and in vitro immunohistochemistry the expression of TLRs 2, 4 and 6 in equine endometrial epithelial cells. An epithelial immunostaining for TLRs 2, 4 and 6 was detected in 76%, 57% and 90% of tissue sections, respectively. Positive cells lined the luminal surface, glandular ducts, mid glands and/or basal glands. An immunoreaction for TLRs 2, 4 and 6 was observed in 100%, 33% and 94% of cell cultures, respectively. The immunosignal was located in the cytoplasm and/or nucleus of endometrial epithelial cells under in situ and in vitro conditions. Results indicate a complex regulation of the epithelial expression of TLR 2, 4 and 6 proteins. The examined cell culture has to be regarded as suitable in vitro model. This study provides the basis for comparative investigations into the impact of different stimuli on the cellular expression of TLRs 2, 4 and 6. These will assist to find out if TLRs are involved in the pathogenesis of endometrial diseases and may help to understand as to why some mares develop persistent endometritis.