Infectious endometritis is a common pathology in many species, including horses, and can severely affect fertility. Efficiency of the uterine immune response is influenced by ovarian hormone levels throughout the oestrous cycle, but little is known about the molecular mechanisms underlying this phenomenon.
The objectives of this study were to characterise the changes in expression of immune genes in the equine endometrium caused by the introduction of Escherichia coli (E. coli)and to identify the transcriptional impact of ovarian hormone levels on these gene expression changes.
Thus, endometrial biopsies were collected from five horses before and at several time points after the inoculation of E. coli once in oestrus (follicle >35 mm in diameter, presence of uterine oedema) and once in dioestrus (5 days after ovulation). Absence of inflammatory signs was confirmed between treatments. Transcription in biopsies taken before and 3 hours after inoculation with bacteria was analysed using high-throughput RNA sequencing (RNA-Seq). Guided by these results, genes involved in the uterine immune response were further analysed at additional time points using quantitative polymerase chain reactions (qPCR) to quantify their expression levels until 3 days post infection.
By 3 hours after the introduction of bacteria, almost 2500 and 1500 genes were expressed at significantly higher levels compared to pre inoculation levels in oestrus and dioestrus, respectively. These included pathogen recognition receptors, particularly toll-like receptors TLR2 and 4 and NOD-like receptor NLRC5, genes for chemokines, including CCL 2, CXCL 9, 10 and 11 and those for antimicrobial peptides, including secretory leukocyte protease inhibitor, lipocalin 2, lysozyme and equine β-defensin 1. Further studies will characterize these genes at later time points post inoculation.
In-depth analysis of the uterine innate immune response will help to improve fertility in horses, but potentially also in other domestic animal species and humans.