Bovine S protein (vitronectin) increases phagocytosis of Streptococcus dysgalactiae

The effects of bovine S protein (vitronectin) on phagocytosis of Streptococcus dysgalactiae strains isolated from cattle with mastitis were investigated. Phagocytized streptococci were determined by a fluorometric microassay using glass adherent polymorphonuclear neutrophils (PMN). Preincubation of S. dysgalactiae with bovine S protein significantly increased their phagocytosis by PMN. Bovine S protein had no effect on phagocytic killing of non-S protein binding S. pyogenes cultures. Enzymatic digestion of the bovine S protein binding sites on S. dysgalactiae with pronase resulted in a significative reduction of the effects of S protein on phagocytosis. It could thus be concluded that in addition to its role as a promoter of cellular adhesion and complement inhibitor, bovine S protein may also influence the phagocytosis of S. dysgalactiae during inflammatory processes.