To generate a vaccine to protect against a variety of human pathogenic fungi, we conjugated laminarin (Lam), a well-characterized but poorly immunogenic β-glucan preparation from the brown alga Laminaria digitata, with the diphtheria toxoid CRM197, a carrier protein used in some glyco-conjugate bacterial vaccines. This Lam-CRM conjugate proved to be immunogenic and protective as immunoprophylactic vaccine against both systemic and mucosal (vaginal) infections by Candida albicans. Protection probably was mediated by anti-β-glucan antibodies as demonstrated by passive transfer of protection to naive mice by the whole immune serum, the immune vaginal fluid, and the affinity-purified anti-β-glucan IgG fractions, as well as by administration of a β-glucan–directed IgG2b mAb. Passive protection was prevented by adsorption of antibodies on Candida cells or β-glucan particles before transfer. Anti-β-glucan antibodies bound to C. albicans hyphae and inhibited their growth in vitro in the absence of immune-effector cells. Remarkably, Lam-CRM–vaccinated mice also were protected from a lethal challenge with conidia of Aspergillus fumigatus, and their serum also bound to and markedly inhibited the growth of A. fumigatus hyphae. Thus, this novel conjugate vaccine can efficiently immunize and protect against two major fungal pathogens by mechanisms that may include direct antifungal properties of anti-β-glucan antibodies.

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.

In this study we tested the in vitro and in vivo anti-Cryptococcus neoformans activity of an antilaminarin (anti-␤-glucan) monoclonal antibody (MAb 2G8) (immunoglobulin G2b) which was previously shown to inhibit the growth of ␤-glucan-exposing Candida albicans cells. Here we show that MAb 2G8 binds to the cell wall of C. neoformans and inhibits its growth to an extent comparable to that observed for C. albicans. Binding and growth inhibition were detected almost equally for encapsulated and acapsular C. neoformans strains. In addition, at subinhibitory concentrations, MAb 2G8 reduced the capsule thickness without affecting protease or phospholipase production. Acapsular fungal cells, but not encapsulated fungal cells, were opsonized by the antibody and more efficiently phagocytosed and killed by human monocytes and by murine peritoneal macrophages. A single administration of MAb 2G8 resulted in a reduction in the fungal burden in the brains and livers of mice systemically infected with a highly virulent, encapsulated C. neoformans strain. This protective effect was also detected in neutropenic mice. Overall, these findings demonstrate that cell wall ␤-glucan of encapsulated C. neoformans is accessible to antibodies which can exert remarkable anticryptococcal activities in vitro and in vivo.Deep-seated mycoses are a severe clinical problem because of well-known diagnostic difficulties and the partial inability of antifungal drugs to eradicate the infections in immunocompromised hosts, often resulting in toxicity, drug resistance, and associated high costs of supportive treatment. As a consequence, the mortality rate for invasive fungal infections remains high, particularly in severely immunocompromised pa-

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To identify molecular targets of anticandidal cell-mediated immunity (CMI) in humans, a highly immunogenic mannoprotein fraction (MP-F2) of Candida albicans was studied. SDS-PAGE and gel-permeation chromatography separated MP-F2 into polydisperse mannoproteins of > 200-31.5 kDa. However, only a 65-kDa constituent specifically induced proliferation of human peripheral blood mononuclear cells (PBMC). Lymphoproliferation was accompanied by production of interleukin (IL)-1 beta, interferon-gamma, and IL-6 but not IL-4. MP-F2- and MP-65-induced PBMC proliferation was inhibited by an antagonist anti-T cell receptor antibody. Neither the purified protein derivative of Mycobacterium tuberculosis nor MP-65 activated naive lymphocytes from umbilical cord blood, although these cells proliferated extensively in response to both phytohemagglutinin and IL-2. These data strongly suggest that MP-65 is an immunodominant mannoprotein antigen that is ordinarily expressed as a target of anti-Candida CMI in healthy humans.

Mice immunized with heat-inactivated, whole yeast-form cells (Y cells) ofCandida albicans developed intense, specific humoral and cell-mediated immune responses. However, they were modestly protected against a lethal challenge by the fungus, and their sera did not confer passive protection upon nonimmunized animals. Surprisingly, this immune serum conferred an elevated degree of passive protection to normal and SCID mice when preadsorbed on whole C. albicans cells. After adsorption, no antibodies specific to mannoprotein (MP)-rich extracts or secretions were detected by indirect enzyme-linked immunosorbent assay and no serum reaction with the fungal cell surface was seen in immunofluorescence assays. However, this serum had totally preserved the level of other antibodies, in particular those reacting with ␤-1,3 and ␤-1,6 glucan (GG). The hypothesis that anti-GG antibodies contributed to the passive protection was suggested by the following circumstantial evidence: (i) mice immunized with C. albicans cells treated with dithiothreitol and protease (YDP cells), which exposed GG on their surfaces and generated anti-GG but not anti-MP antibodies, were substantially protected against a lethal fungus challenge; (ii) the sera, and their immunoglobulin fractions, of mice immunized with YDP cells transferred protection to nonimmune animals; and (iii) this passive protection was substantially abolished by preadsorption on GG but not on intact cells. Overall, our findings demonstrate that some anti-Candida antibodies can block the protective potential of immune serum, a potential to which anti-GG antibodies appear to contribute. Our observations may also help explain why subjects with elevated anti-Candida antibody titers, inclusive of anti-MP and anti-GG antibodies, remain nonetheless susceptible to invasive candidiasis.

Elucidation of the molecular immunity of glycoconjugate vaccines has focused on the carbohydrate moiety, herein the effect of the corresponding conjugation sites is studied.

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