The role of seminal plasma in post-breeding uterine inflammation

The effect of seminal plasma on polymorphonuclear neutrophil (PMN) chemotaxis, PMN phagocytosis and complement-induced cytolysis was determined using blood plasma pooled from four horses and seminal plasma pooled from two stallions. To investigate chemotaxis, complement in blood plasma was activated with E. coli lipopolysaccharide in the presence of 0-50% seminal plasma diluted with a standardized volume of McCoy's medium and placed in a chemotactic chamber. Chemotaxis of blood derived equine PMNs toward the chemoattractants was determined after incubation at 37 degrees C for 45 min. To investigate phagocytosis, equine spermatozoa (5 x 10(7) ml(-1); n=5) from fertile stallions (n=2) were mixed with blood plasma and increasing concentrations of seminal plasma (50-90%). The seminal plasma was removed after opsonization at 37 degrees C for 30 min and 4 x 10(6) PMNs were added to the sperm cells and incubated at 37 degrees C for 30 min. The number of phagocytosed spermatozoa per 100 PMNs and the proportion of PMNs that had ingested at least one spermatozoon were counted under a microscope. Antibody-sensitized sheep red blood cells were used to investigate complement-induced cytolysis. The 50% haemolytic complement activity (CH50) was determined in blood plasma and in the presence of increasing concentrations of seminal plasma (3-40%). Haemolysis was quantified by spectrophotometry at 540 nm. A dose-dependent inhibition of chemotaxis (P < 0.001), phagocytosis (P < 0.001) and complement-induced cytolysis (P < 0.001) was observed when seminal plasma was included in the assays. These results indicate that seminal plasma may have a downregulatory role in post-breeding endometritis.