Monoclonal antibodies can affect complement deposition on the capsule of the pathogenic fungus Cryptococcus neoformans by both classical pathway activation and steric hindrance

Abstract

SummaryThe capsule of the human pathogenic fungus Cryptococcus neoformans presents the immune system with a formidable problem for phagocytosis. Capsulemediated activation of the alternative complement (C) pathway results in component 3 (particularly, C3) binding to the capsule near the cell wall surface. Hence, for cells with large capsule, C3 cannot interact with the complement receptor (CR) and is not opsonic. However, C activation in either immune serum or in the presence of monoclonal antibody (mAb) to capsular polysaccharide localizes C3 to the capsular edge. When C. neoformans cells were coated with both C and antibody (Ab) opsonins, Ab bound first and promoted C3 deposition at the edge of the capsule. The mechanism for the Ab-mediated change in C3 localization to the capsule edge involved both classical C pathway activation and steric hindrance preventing C3 penetration into the capsule. The change in C3 localization changed the mode of phagocytosis in macrophages, such that localizing C3 at the edge of the capsule allowed phagocytosis through C3-CR3 and C3-CR4 interactions, which did not occur in serum without Ab. These findings reveal a new mechanism of Ab action whereby Abs affect the location of C3 and its interaction with its receptor in macrophages depending on the immunoglobulin concentration.

Functions of Antibodies

“…Han et al found that protective IgM or IgG3 mAbs more efficiently bind C3 to the yeast cell than does a nonprotective mAb and that protection is likely ASMscience.org/MicrobiolSpectrumassociated with enhanced phagocytosis and killing (123). In C. neoformans, immune serum or an IgG1 mAb localize C3 at the edge of the organism's capsule, allowing phagocytosis through complement receptors (124). IgM also promotes complement deposition and PMN phagocytosis of C. neoformans (125).…”

Ab binding alters gene expression in Cryptococcus neoformans and directly modulates fungal metabolism

“…Interestingly, when nearsaturating concentrations of mAb 18B7 are used, there seemed to be a protective effect against amphotericin B. Since mAb binding crosslinks the capsule and reduces its permeability (22) and amphotericin B is a large molecule, saturating concentrations of the mAb may protect the cell from the effects of the antifungal drug by reducing accessibility.…”

Radial Mass Density, Charge, and Epitope Distribution in theCryptococcus neoformansCapsule

“…The antibody is most likely unable to reach the epitopes at inner regions due to the increased density of the fibers, since these inner epitopes became available for antibody binding only after 30 and 40 min of irradiation. Furthermore, antibody cross-linking of fibrils in the outer layers of the capsule may reduce penetration of subsequent molecules (67). This implies that for cells irradiated for less than 30 min, where the high-density region of the capsule was unexposed, the determined number of binding sites is actually a measure of the binding sites in the entire low-density capsule region.…”

“…It is noteworthy that the binding of the Ab at these concentrations to the capsule did not change the size of this structure, indicating that only gamma radiation was responsible for capsule size changes in our conditions. We did not use higher concentrations because they have been reported to deform the capsule (67). Therefore, exposure to gamma radiation results in a gradual release of the capsule which occurs at the capsule exterior, without affecting inner capsular regions.…”