Function of uterine and blood-derived polymorphonuclear neutrophils in mares susceptible and resistant to chronic uterine infection: Phagocytosis and chemotaxis

In vitro phagocytosis and chemotaxis of uterine and blood-derived polymorphonuclear neutrophils (PMNs) were compared in mares with different resistance to chronic uterine infection (CUI). Both the primary in vitro function of PMNs and the role of uterine environmental factors on PMN function were investigated. The uteri of mares susceptible to (n = 6) and resistant to CUI (n = 5) were inoculated with 5 x 10⁶ Streptococcus zooepidemicus when the mares were in estrus. Uterine secretions in addition to uterine and blood-derived PMNs were sampled at 5 and 24 h later. During a subsequent estrus, bacterial inoculation of the uterus was repeated, and samples were removed from the mares 12 and 36 h later. Neither the phagocytic nor the chemotactic capacity of PMNs changed over time in any of the groups. However, chemoattractive properties of uterine secretions declined over time in both resistant (p < 0.0007) and susceptible mares (p < 0.01). Significantly higher phagocytosis (p < 0.03) and chemotaxis (p < 0.05) by uterine derived PMNs were found in the susceptible mares compared to resistant mares when a standardized opsonin (pooled plasma) was used. However, uterine secretions from susceptible mares demonstrated a poorer opsonizing capacity (p < 0.00002) but were more chemoattractant (p < 0.004) than secretions from resistant mares. When opsonins and chemoattractants were provided by plasma, no differences were detected in phagocytosis between blood-derived and uterine PMNs. In contrast, chemotaxis of uterine PMNs were superior to blood-derived PMNs in both resistant (p < 0.007) and susceptible mares (p < 0.0001) under these conditions. Furthermore, when yeast particles were opsonized with uterine secretions, phagocytosis as well as chemotaxis by uterine PMNs was superior to that by blood-derived PMNs in both resistant mares (p < 0.0002 for phagocytosis; p < 0.00005 for chemotaxis) and susceptible mares (p < 0.04 for phagocytosis; p < 0.00005 for chemotaxis). Uterine secretions demonstrated remarkably lower opsonizing and chemoattractant properties when compared to pooled or autologous plasma (p < 0.00005) but were superior to the negative control (p < 0.001). It was concluded that phagocytosis but not chemotaxis of uterine PMNs is impaired in mares susceptible to CUI. Since uterine PMNs in susceptible mares were demonstrated to be fully functional if given an optimal environment, the cause of the observed dysfunction is believed to be the result of a negative effect from uterine secretions on phagocytosis in susceptible mares.