Specific antibody promotes opsonization and PMN-mediated killing of phagocytosis-resistant Enterococcus faecium

Many clinical isolates of Enterococcus faecium are resistant to neutrophil (PMN)-mediated phagocytosis and killing in the presence of normal human ser um. We have now examined the ability of specific polyclonal rabbit antibodi es to promote opsonization and killing of phagocytosis-resistant E. faecium . Immune rabbit serum generated against formalin-killed E. faecium TX0016,: a phagocytosis-resistant strain, markedly promoted binding of TX0016 organ isms to PMNs and PMN-mediated killing. These effects were dramatically redu ced by (a) adsorption of immune serum with E. faecium TX0016, but not by ad sorption with a strain of E. faecium susceptible to phagocytosis, and (b) i ncubation of immune serum with carbohydrate purified from TX0016, but not b y incubation with a surface protein extract from TX0016. IgG purified from immune serum was unable by itself to promote bacterial binding to PMNs. How ever, specific IgG was able to promote binding to PMNs and PMN-mediated kil ling in the presence of normal human serum as a complement source, as were F(ab')(2) and Fab fragments produced from it, and the alternative pathway o f complement was sufficient to promote IgG- and F(ab')(2)-mediated opsoniza tion. PMN complement receptor type 3, but not complement receptor type 1, w as involved in bacterial binding to PMNs induced by the combination of F(ab ')(2) fragments and normal human serum. These results suggest that opsoniza tion by antibodies potentially directed against bacterial carbohydrate, in conjunction with complement activation, has an important role in the host d efense against phagocytosis-resistant E. faecium. (C) 2000 Federation of Eu ropean Microbiological Societies. Published by Elsevier Science B.V. All ri ghts reserved.