Elsevier

Microbial Pathogenesis

Volume 19, Issue 2, August 1995, Pages 117-128
Microbial Pathogenesis

Regular Article
The pentameric structure of IgM is necessary to enhance opsonization of Bacteroides thetaiotaomicron and Bacteroides fragilis via the alternative complement pathway

https://doi.org/10.1006/mpat.1995.0051Get rights and content

Abstract

Studies were conducted to investigate the mechanisms by which natural IgM antibodies act together with the alternative complement pathway to promote opsonization and adherence of encapsulatedBacteroides thetaiotaomicronandBacteroides fragilisto polymorphonuclear leukocytes (PMN). A model system consisting of the six isolated proteins of the alternative pathway was used. A comparison of the opsonic effects of pentameric and monomeric forms of isolated normal IgM demonstrated that, although the monomeric form bound toBacteroidesas effectively as the pentameric form and promoted complement deposition to the same extent, it was unable to enhance alternative pathway-dependent opsonization and adherence ofBacteroidesto PMN. When opsonization was performed in two steps with pentameric IgM added either before or after alternative pathway components, a marked enhancement of adherence to PMN was observed only in the former case, suggesting IgM must act prior to complement to be effective. Electron microscopic studies demonstrated that, when added with complement, pentameric IgM, but not monomeric IgM, stabilized the bacterial capsule to the dehydration in dimethylformamide used for embedding in Lowicryl K4M. A strong correlation was observed between capsular stability and ability to be bound by PMN. The results suggest that pentameric IgM alters the structure of capsular components, perhaps through crosslinking, and this in turn facilitates interaction of C3bi and C3b with CR3 and CR1, their respective receptors on PMN.

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