Contributions of complement and immunoglobulin to neutrophil-mediated killing of enterococci

Enterococci have become a frequent causative agent in neonatal sepsis. The relative contributions of antibody and complement and their interactions in the neutrophil-mediated bacterial killing of 11 Enterococcus strains from neonates were investigated. Polymorphonuclear leukocytes (PMNL) from adult and term newborn infants were tested with normal human serum, adult hypogammaglobulinemic serum, and normal newborn serum in a neutrophil bactericidal assay. Neutrophil bactericidal activity for enterococci was not influenced by the serum source but was essentially ablated after heat inactivation of complement in all sera. No differences were observed in the killing capacity of healthy newborn versus adult PMNL regardless of serum source. Representative Enterococcus strains were then tested with agammaglobulinemic serum or C4-deficient serum, resulting in neutrophil bactericidal activities consistently exceeding 90%. A neutrophil bactericidal assay performed with normal rabbit serum and hyperimmune rabbit serum against enterococci showed that antibodies to enterococci enhanced neutrophil-mediated killing of this organism. Thus, neutrophil killing of enterococci appears to be mediated primarily by complement, with antibody playing a less essential but potentially important role. PMNL from adult and healthy term infants functioned with equal efficiency in the neutrophil killing of enterococci.