ABSTRACT
Acute interdigital phlegmon (AIP) is a commonly occurring anaerobic bacterial infection in cattle. This study examined in vitro the interaction of bovine polymorphonuclear granulocytic neutrophils (PMN) from blood with bacterial species involved in AIP. Polymorphonuclear neutrophils were purified from whole bovine blood, exposed to one of the three putative etiologic agents of AIP and comparatively assessed for phagocytosis using light microscopy. Fusobacterium necrophorum and Prevotella intermedia were effectively phagocytosed by PMN, but Porphyromonas levii was phagocytosed significantly less effectively by PMN. The effect of high titre anti-P. levii bovine serum on antibody-mediated phagocytosis by PMN was also evaluated. High titre serum increased the efficiency of phagocytosis of P. levii by bovine PMN. This was independent of heat labile complement factors. Antibodies specific for P. levii were assessed for protease activity capable of cleaving bovine immunoglobulins (IgG, IgG1, IgG2, and IgM). Partially purified supernatant from broth cultures of P. levii were incubated with biotinylated immunoglobulins (Igs). Samples were taken from times 0 to 72 h and examined using SDS-PAGE followed by Western blot analysis. Streptavidin-alkaline phosphatase and NBT-BCIP were used to visualize the Igs for heavy and light chains as well as lower molecular weight fragments of these glycoproteins. Porphyromonas levii produced an immunoglobulin protease which readily cleaved bovine IgG into fragments, but did not act against IgM. Specifically, the enzyme may be a significant virulence factor as it may act to neutralize the antibodies demonstrated necessary for effective PMN-mediated phagocytosis.
