<i>TLR‐2</i> and <i>TLR‐4</i> mRNA expression in different grades of histopathological lesions of equine endometrium from follicular phase

Increased synthesis and deposition of collagen (COL) in the extracellular matrix (ECM) of equine endometrium contributes to endometrosis. Toll-like receptors (TLRs) are transmembrane receptors involved in the innate immune response, recognized for their role in antigen recognition and previously associated with equine endometritis. The TLRs not only recognize pathogen-associated molecular patterns but also regulate inflammations, fibrosis and cancer. The aim of this study was to explore the relationship between TLR expression at different stages of Kenney and Doig's (K-D) grading and COL1 expression during the follicular phase of the oestrous cycle. Forty samples of endometrial tissues were collected post-mortem from mares on the follicular phase of the oestrous cycle (10 samples of each K-D category). Relative mRNA transcription of TLR-2, TLR-4 and COL1A2 genes was assessed using qPCR, and COL1 protein expression by Western blot analysis. The COL1A2 transcription increased in category IIB when compared to categories I, IIA and III endometria (p < .01). The relative protein abundance of COL1 showed no significant differences between endometrial categories (p > .05). As for the TLRs mRNA transcription, TLR-2/-4 transcripts increased in IIA when compared to the other K-D endometria categories (p < .05). Our findings suggest that TLRs may be involved in the initiation of the endometrial inflammatory response. Additional studies are needed to explore TLRs' potential role as diagnostic markers for monitoring inflammation progression and fibrosis development, as well as their involvement in the mechanisms underlying fibrotic pathways.