Post-mating inflammatory responses of the uterus.

This review attempts to summarize the current knowledge on uterine inflammatory response after mating in horses, pigs and cattle. Post-mating endometritis has been extensively studied in horses as it has been considered to cause infertility. The inflammation is known to occur also in cattle, but it has not been investigated to a similar extent. There are a number of publications about mechanisms of post-mating uterine inflammation in pigs, which seem to resemble those in horses. The major focus of this review is the horse, but relevant literature is presented also on swine and cattle. Spermatozoa, seminal plasma and semen extenders play roles in the induction of inflammation. In addition, sperm numbers, concentration and viability, as well as the site of semen deposition may modulate the inflammatory response. Cytokines, polymorphonuclear leucocytes (PMN) and mononuclear cells represent the uterine inflammatory response to mating. Inflammation is the first line of defence against invasion and eliminates excess spermatozoa and bacteria. Semen deposition elicits a massive PMN invasion, followed by phagocytosis of sperm aided by the formation of neutrophil extracellular traps. Exposure of the female genital tract to semen is important also for endometrial receptivity and pre-implantation embryo development. Seminal plasma (SP) and inflammation elicit transient immune tolerance to antigens present in semen. SP contains immune-regulatory molecules that activate and control immune responses to antigens by stimulating expression of cytokines and growth factors and by initiating tissue remodelling. SP also regulates ovarian function. Effective elimination of excess sperm and inflammatory by-products and subsequent rapid return of the endometrium to the normal state is a prerequisite for pregnancy. Uterine backflow, driven by myometrial contractions and requiring a patent cervix, is an important physical tool in uterine drainage.