Unraveling the uterine fluid proteome of mares diagnosed with post-breeding and infectious endometritis

Endometritis is the leading cause of subfertility in mares and a significant challenge to equine reproduction. Unraveling uterine fluid proteome may promote advancements in the knowledge of endometritis etiopathogeneses and its diagnosis and therapeutic practices. Therefore, we aimed to characterize and compare the protein profile of the uterine fluid from healthy mares and animals diagnosed with endometritis. Mangalarga Marchador breed mares from Muriaé - Brazil were divided into control, infectious endometritis, and post-breeding endometritis groups (n = 8/ group). Uterine fluid was collected via low-volume lavage and subjected to protein identification and relative abundance counting. From the 549 proteins detected, 279 were in the uterine fluid of mares from the three experimental groups. Thirteen proteins expressed mostly in healthy mares were associated with endometrial remodeling and early embryonic development. Albumin and uteroglobin presented higher relative abundance in healthy mares and animals with infectious endometritis. Infectious endometritis exhibited proteins related to innate immune and inflammatory responses, including annexin and glutathione S-transferase, and the highest abundance of lipocalins. Fifty-five proteins detected in mares with post-breeding endometritis showed signaling pathways and biological processes related to the innate immune response. These animals also presented the highest abundance of PIGR proteins, which promote IgA transport from plasma into the endometrial mucosa. In conclusion, our results revealed distinct protein profiles from the uterine fluid of mares with infections and post-breeding endometritis. These findings provided valuable insights into the molecular alterations during the establishment and progression of endometritis, contributing to further identification of potential biomarkers.