Contribution of antibody to neutrophil-mediated killing of Enterococcus faecalis

Late-onset septicemia due to Enterococcus faecalis is common among very low-birth weight neonates. These infants have low concentrations of placentally derived IgG and developmentally low levels of complement. The aim of the present study was to determine the contribution of antibody to in vitro neutrophil-mediated phagocytosis of E. faecalis. Antibody alone, as contained in an adult serum pool heated to inactivate complement, promoted only a modest reduction in the initial bacterial inoculum (50 +/- 12%) for 6 of 10 E. faecalis bacterial strains tested and allowed growth of the other four strains. In the presence of complement, NHS promoted greater than or equal to 90% reduction in the initial bacterial inoculum of two representative strains at serum concentrations as low as 0.5%. Hypogammaglobulinemic serum supported similar activity only at concentrations above 5%. Purification of IgG and IgM fractions from NHS revealed that IgM had the higher specific activity to promote phagocytic activity. Absorption to remove specific antibody significantly reduced bactericidal activity by normal human serum, complement-deficient sera, and hypogammaglobulinemic serum. Reconstitution of hypogammaglobulinemic serum with antibody as contained in 1% healed normal human serum or in immune globulin for intravenous use (1200 mg/dl) restored phagocytic activity. Thus, E. faecalis-specific antibody enhances PMN-mediated killing of this organism. Adjunctive therapy with intravenous immunoglobulin could augment the host response to enterococcal infections in infancy.