Mechanisms of bacterial opsonization by immune globulin intravenous: Correlation of complement consumption with opsonic activity and protective efficacy

Tostudy the mechanismof bacterialopsonization byimmuneglobulinintravenous(IGIV) complement consumption and polymorphonuclear leukocyte(PMNL) membrane receptor (FcRlo, CRI, and CR3)-mediated phagocytosisof Staphylococcus epidermidis, Klebsiella pneumoniae, and groups A and B streptococci wereexamined. IGIV alone did not consumecomplementand showedno opsonic activitybyitself for these organisms. When these bacteria werepreopsonized in IGIV, significant amounts of complement wereconsumed (44%-94%) and the uptake and killing of bacteria occurred. The in vitro opsonic activityof IGIV for these organismswassignificantlycorrelated with the amount of complement consumed bythe IGIV-opsonized bacteria (r =.85, P<.05). The in vivoprotective efficacy of IGIV also appeared to be directly associated with its ability to activate and consume complement (r = 1.0, P <.001). Antibodies to FcRlo (Leu II) markedly inhibited phagocytosisof bacteria opsonized in IGIV but not that of bacteria opsonized in specific IgM. Both CR1 and CR3 receptors on PMNLs were involved in uptake, but the contribution of each is different with different organisms.