Protection by Anti-β-Glucan Antibodies Is Associated with Restricted β-1,3 Glucan Binding Specificity and Inhibition of Fungal Growth and Adherence

Abstract

Anti-β-glucan antibodies elicited by a laminarin-conjugate vaccine confer cross-protection to mice challenged with major fungal pathogens such as Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. To gain insights into protective β-glucan epitope(s) and protection mechanisms, we studied two anti-β-glucan monoclonal antibodies (mAb) with identical complementarity-determining regions but different isotypes (mAb 2G8, IgG2b and mAb 1E12, IgM). C. albicans, the most relevant fungal pathogen for humans, was used as a model.Both mAbs bound to fungal cell surface and to the β1,3-β1,6 glucan of the fungal cell wall skeleton, as shown by immunofluorescence, electron-microscopy and ELISA. They were also equally unable to opsonize fungal cells in a J774 macrophage phagocytosis and killing assay. However, only the IgG2b conferred substantial protection against mucosal and systemic candidiasis in passive vaccination experiments in rodents. Competition ELISA and microarray analyses using sequence-defined glucan oligosaccharides showed that the protective IgG2b selectively bound to β1,3-linked (laminarin-like) glucose sequences whereas the non-protective IgM bound to β1,6- and β1,4-linked glucose sequences in addition to β1,3-linked ones. Only the protective IgG2b recognized heterogeneous, polydisperse high molecular weight cell wall and secretory components of the fungus, two of which were identified as the GPI-anchored cell wall proteins Als3 and Hyr1. In addition, only the IgG2b inhibited in vitro two critical virulence attributes of the fungus, hyphal growth and adherence to human epithelial cells.Our study demonstrates that the isotype of anti-β-glucan antibodies may affect details of the β-glucan epitopes recognized, and this may be associated with a differing ability to inhibit virulence attributes of the fungus and confer protection in vivo. Our data also suggest that the anti-virulence properties of the IgG2b mAb may be linked to its capacity to recognize β-glucan epitope(s) on some cell wall components that exert critical functions in fungal cell wall structure and adherence to host cells.

Whole glucan particles as a vaccine against systemic coccidioidomycosis

“…Interestingly, only a minimal rise in antibody to b-glucan was found, suggestive that antibodies play a minimal role in protection (Clemons et al, 2014b). The latter is in opposition to the studies using laminarin conjugated to diphtheria toxoid, which suggested antibody directed against b-glucan was a major mechanism of protection against multiple fungal infections ( Torosantucci et al, 2005Torosantucci et al, , 2009). As we noted previously, induction of the various pro-inflammatory cytokines and chemokines is suggestive of a priming of an active innate immune response, whereas increases in IFNc and IL-17 suggest an induction of a cell-mediated immune response (Zelante et al, 2009;Clemons et al, 2014b;Whibley & Gaffen, 2014).…”

Immunoglobulin isotype influences affinity and specificity

“…However, in the past decade, a series of studies done with a family of V region identical isotype switched Abs to Cryptococcus neoformans polysaccharide using monovalent peptide mimetics provided strong evidence for C-mediated effects on V region function as manifested by isotype-related differences in specificity and affinity (9)(10)(11)(12). In addition, two other groups, including the report by Tudor et al (5), described additional examples in which Abs expressing identical V region sequences manifested altered specificity and/or affinity (13). Given that five independent groups have now reported that C region can affect V region affinity and/or specificity (5,7,8,10,13)…”

“…In addition, two other groups, including the report by Tudor et al (5), described additional examples in which Abs expressing identical V region sequences manifested altered specificity and/or affinity (13). Given that five independent groups have now reported that C region can affect V region affinity and/or specificity (5,7,8,10,13)…”

Fungal Vaccines and Immunotherapeutics

“…For example, when incubated with C. neoformans, 2G8(IgG2b) prevents capsule formation and reduces capsule size (Hole and Wormley 2012). 2G8 has also been found to be fungistatic against Candida albicans and prevent its attachment to human epithelial cells in vitro (Phan et al 2007;Torosantucci et al 2009;Dwivedi et al 2011). The efficacy of 2G8 in vivo has shown therapeutic promise in mouse models of pulmonary cryptococcosis, as well as invasive and vaginal candidiasis (Torosantucci et al 2009;Capodicasa et al 2011;Hole and Wormley 2012).…”