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Specific antibody promotes opsonization and PMN-mediated killing of phagocytosis-resistant Enterococcus faecium
R M Rakita1, V C Quan, K Jacques-Palaz
1Virginia Mason Medical Center. Seattle, WA 98111, USA. cidrmr@vmmc.org
Abstract:
Many clinical isolates of Enterococcus faecium are resistant to neutrophil (PMN)-mediated phagocytosis and killing in the presence of normal human serum. We have now examined the ability of specific polyclonal rabbit antibodies 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 organisms to PMNs and PMN-mediated killing. These effects were dramatically reduced by (a) adsorption of immune serum with E. faecium TX0016, but not by adsorption with a strain of E. faecium susceptible to phagocytosis, and (b) incubation of immune serum with carbohydrate purified from TX0016, but not by incubation with a surface protein extract from TX0016. IgG purified from immune serum was unable by itself to promote bacterial binding to PMNs. However, specific IgG was able to promote binding to PMNs and PMN-mediated killing 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 of complement was sufficient to promote IgG- and F(ab')(2)-mediated opsonization. PMN complement receptor type 3, but not complement receptor type 1, was involved in bacterial binding to PMNs induced by the combination of F(ab')(2) fragments and normal human serum. These results suggest that opsonization by antibodies potentially directed against bacterial carbohydrate, in conjunction with complement activation, has an important role in the host defense against phagocytosis-resistant E. faecium.
Insights
Specific antibodies targeting bacterial carbohydrates enhance the killing of phagocytosis-resistant Enterococcus faecium by neutrophils. This process involves complement activation and specific complement receptors, crucial for host defense against these resilient bacteria.
Area of Science:
- Immunology
- Microbiology
- Infectious Diseases
Background:
- Clinical isolates of Enterococcus faecium often resist killing by neutrophils (PMNs) and serum.
- Understanding mechanisms to overcome this resistance is vital for treating infections.
Purpose of the Study:
- To investigate the role of specific antibodies in promoting opsonization and killing of phagocytosis-resistant E. faecium.
- To elucidate the components of the immune response involved in this process.
Main Methods:
- Generated immune rabbit serum against a resistant E. faecium strain (TX0016).
- Tested serum's ability to promote PMN binding and killing of E. faecium.
- Used serum adsorption and purified bacterial components (carbohydrate, protein) to identify antibody targets.
- Investigated the role of IgG, complement, and complement receptors (CR3, CR1) in antibody-mediated opsonization.
Main Results:
- Immune rabbit serum significantly enhanced PMN-mediated binding and killing of phagocytosis-resistant E. faecium.
- This effect was dependent on antibodies targeting bacterial carbohydrate, not surface proteins.
- Specific IgG, along with normal human serum (complement source), promoted bacterial opsonization and killing.
- The alternative complement pathway and PMN complement receptor type 3 were involved.
Conclusions:
- Antibodies targeting bacterial carbohydrates are key for opsonizing phagocytosis-resistant E. faecium.
- Antibody-mediated opsonization, coupled with complement activation, is critical for host defense against these resistant bacteria.
- Complement receptor type 3 plays a role in antibody- and complement-mediated phagocytosis.