Factors affecting phagocytosis of bacteria by neutrophils in the mare's uterus

Ten mares, 5 resistant and 5 susceptible to bacterial endometritis, were examined for differences pertaining to the efficiency of phagocytosis of bacteria by neutrophils in the uterus. An assay for chemiluminescence was used to evaluate various schemes of opsonization and relate them to phagocytic rate. 123I-labelled albumin was used to measure protein migration to the inoculated uterus. Neutrophil numbers moving to the uterine lumen were determined. Before inoculation resistant mares were found to have substances in their uterine secretions that opsonized bacteria, resulting in effective phagocytosis. In contrast, uterine contents of susceptible mares were not effective in opsonization. The addition of serum to uterine washings significantly enhanced opsonization of bacteria in both groups. In response to intrauterine challenge with pathogenic streptococci, no differences were observed between resistant and susceptible mares in the migration of serum protein or neutrophil numbers to the site of contamination. Following inoculation, the ability of the uterine contents to opsonize bacteria was similar between groups. Clinical observations after inoculation confirmed that mares were resistant or susceptible to endometritis.