Effect of fibronectin on IgA-mediated uptake of type III group B streptococci by phagocytes

Previous studies have shown that a type-specific IgA monoclonal antibody alone or in combination with fibronectin (Fn) enhances protective efficacy in two animal models of group B streptococcal infection. Toinvestigate the mechanisms by which IgA mediates protection, the effects ofFn on phagocytosisof group Bstreptococci(GBS)opsonized with a type III-specific IgA monoclonal antibody were examined. Specific IgA alone or in combination with Fn did not promote the phagocytosis of GBS by polymorphonuclear leukocytes (PMNL). Fibronectin also had no significanteffecton phagocytosisofIgA-opsonized GBSby monocytes. Specific IgAalone promoted phagocytosis of GBS by culture-derived macrophages in a dose-dependent fashion. Fibronectin enhanced macrophage uptake of the GBS opsonized in a suboptimal concentration of specific IgA (phagocytic index = 2.32±0.56 vs. 3.26 ±0.48with Fn; P<.05). These data suggest that protection against GBS in neonatal rats by a combination of Fn and specific IgA is mediated by macrophages rather than byPMNL or monocytes. Fibronectin may have a critical role in host defense at sites where IgA and macrophages predominate.