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 encapsulated Bacteroides thetaiotaomicron and Bacteroides fragilis to 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 to Bacteroides as effectively as the pentameric form and promoted
complement deposition to the same extent, it was unable to enhance alternative pathway-dependent opsonization and adherence of Bacteroides to 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 is in turn facilitates interaction of
C3bi and C3b with CR3 and CR1, their respective receptors on PMN.