Antonella Torosantucci

A novel glyco-conjugate vaccine against fungal pathogens.

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.

In Vitro and In Vivo Anticandidal Activity of Human Immunodeficiency Virus Protease Inhibitors

Highly active antiretroviral therapy that includes human immunodeficiency virus (HIV) aspartyl protease inhibitors (PIs) causes a decline in the incidence of some opportunistic infections in AIDS, and this decline is currently attributed to the restoration of specific immunity. The effect of two PIs (indinavir and ritonavir) on the enzymatic activity of a secretory aspartyl protease (Sap) of Candida albicans (a major agent of mucosal disease in HIV-infected subjects) and on growth and experimental pathogenicity of this fungus was evaluated. Both PIs strongly (>/=90%) and dose dependently (0.1-10 microM) inhibited Sap activity and production. They also significantly reduced Candida growth in a nitrogen-limited, Sap expression-dependent growth medium and exerted a therapeutic effect in an experimental model of vaginal candidiasis, with an efficacy comparable to that of fluconazole. Thus, besides the expected immunorestoration, patients receiving PI therapy may benefit from a direct anticandidal activity of these drugs.

Phagocytosis of Aspergillus fumigatus conidia by murine macrophages involves recognition by the dectin-1 beta-glucan receptor and Toll-like receptor 2

Aspergillus fumigatus is a fungal pathogen causing severe infections in immunocompromised patients. For clearance of inhaled conidia, an efficient response of the innate immune system is required. Macrophages represent the first line of defence and ingest and kill conidia. C‐type lectins represent a family of receptors, which recognize pathogen‐specific carbohydrates. One of them is β1–3 glucan, a major component of the fungal cell wall. Here we provide evidence that β1–3 glucan plays an important role for the elimination of A. fumigatus conidia. Laminarin, a soluble β1–3 glucan and antibodies to dectin‐1, a well known β1–3 glucan receptor, significantly inhibited conidial phagocytosis. On resting conidia low amounts of surface accessible β1–3 glucan were detected, whereas high amounts were found on small spores that appear early during germination and infection as well as on resting conidia of a pksP mutant strain. Swollen conidia also display larger quantities of β1–3 glucan, although in an irregular spotted pattern. Resting pksP mutant conidia and swollen wild‐type conidia are phagocytosed with high efficiency thereby confirming the relevance of β1–3 glucans for conidial phagocytosis. Additionally we found that TLR2 and the adaptor protein MyD88 are required for efficient conidial phagocytosis, suggesting a link between the TLR2‐mediated recognition of A. fumigatus and the phagocytic response.

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

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.

Role of protease inhibitors in preventing recurrent oral candidosis in patients with HIV infection: a prospective case-control study

This study was conducted to evaluate the efficacy of highly active anti-retroviral therapy (HAART) in preventing recurrence of oral candidosis (OC) associated with HIV. A prospective case-controlled observational study was performed in an inner-city university-hospital HIV/AIDS clinic. Ninety-three HIV-positive study subjects with a history of recurrent OC were divided into two groups: protease inhibitors (PI)-treated patients (group 1, n = 30) and non-PI-treated patients (group 2, n = 63). Study subjects were matched for sex, age, stage of HIV infection, and peripheral CD4+ T-cell counts. The non-PI-treated group was further subdivided into the following three subgroups: HIV-positive study subjects treated with reverse transcriptase inhibitors (RTI; groups 2a and 2c) and HIV-positive study subjects not treated with RTIs (group 2b). Group 2c met the same inclusion criteria as group 2a had but was matched 6 months after the beginning of the study. We also assessed in vitro peripheral blood mononuclear cells (PBMC) and their lymphoproliferative response, as well as cutaneous delayed-type hypersensitivity (DTH) response to Candida-associated antigens in a randomly selected sample of study subjects divided into those treated with PIs and those who were not. During a 1-year follow-up, OC was diagnosed in 2 (7%) PI-treated and 23 (36%) non-PI-treated patients (p<.001). In addition to comparing findings in group 1 with those in group 2c, OC was detected in 14 (50%) non-PI-treated patients compared with no HAART-treated study subjects (p<.001). Only 41% of PI-treated study subjects had positive lymphoproliferative response in PBMCs and none was positive in terms of DTH to Candida antigens (p = not significant versus non-PI-treated study subjects). While objectively demonstrating a beneficial effect of HAART in preventing recurrence of OC infections, our findings suggest this effect cannot be not fully accounted for by reconstitution of anti-Candida cell-mediated immunity, given that other mechanisms, even of a nonimmune nature, could have some effect.

An Anti-β-Glucan Monoclonal Antibody Inhibits Growth and Capsule Formation of Cryptococcus neoformans In Vitro and Exerts Therapeutic, Anticryptococcal Activity In Vivo

ABSTRACT In this study we tested the in vitro and in vivo anti-Cryptococcus neoformans activity of an antilaminarin (anti-β-glucan) monoclonal antibody (MAb 2G8) (immunoglobulin G2b) which was previously shown to inhibit the growth of β-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 β-glucan of encapsulated C. neoformans is accessible to antibodies which can exert remarkable anticryptococcal activities in vitro and in vivo.

Interplay between Protective and Inhibitory Antibodies Dictates the Outcome of Experimentally Disseminated Candidiasis in Recipients of a Candida albicans Vaccine

ABSTRACT Mice immunized with heat-inactivated, whole yeast-form cells (Y cells) of Candida 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 β-1,3 and β-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.

Antiretroviral Therapy with Protease Inhibitors Has an Early, Immune Reconstitution–Independent Beneficial Effect on Candida Virulence and Oral Candidiasis in Human Immunodeficiency Virus–Infected Subjects

Highly active antiretroviral therapies (HAARTs) that contain human immunodeficiency virus (HIV) protease inhibitors (PIs) or nonnucleoside reverse-transcriptase inhibitors (NNRTIs) were compared for their effect on secretory aspartyl proteinase (Sap), a virulence trait for mucosal candidiasis. In therapy-naive HIV-positive subjects, oral Sap was detected in 11, 6, 3, 0, and 0 of 15 subjects treated with PI-HAART and in 7, 7, 9, 6, and 5 of 15 subjects treated with NNRTI-HAART, on days 0, 14, 30, 90, and 180 of treatment, respectively. In another 30 subjects, Sap was detected in 0 and 7 of 15 subjects after 1 year of treatment with PI-HAART or NNRTI-HAART, respectively. The anti-Sap effect of PI-HAART was associated with clinical resolution of oral candidiasis but not with late and inconstant recovery of anticandidal cellular immunity. In all subjects, the 2 therapeutic regimens compared well in increasing CD4(+) cell count and abating viremia. Thus, PIs exert an early, immune reconstitution-independent effect on Candida virulence in the oral cavities of HIV-positive subjects.

Beta-glucan-CRM197 conjugates as candidates antifungal vaccines.

A laminarin-diphtheria toxoid (CRM197) conjugate vaccine conferred protection against fungal infections in mice. We have now generated novel beta-glucan-CRM197 vaccines, with either natural (Curd-CRM197) or synthetic linear (15mer-CRM197), or beta-(1,6)-branched (17mer-CRM197) beta-(1,3)-oligosaccharides, formulated with the human-acceptable adjuvant MF59. Curd-CRM197 and 15mer-CRM197 conjugates, which induced high titers of anti-beta-(1,3)-glucan IgG, but no antibodies against beta-(1,6)-glucan, conferred protection to mice lethally challenged with C. albicans. In contrast, the 17mer-CRM197 conjugate, which induced anti-beta-(1,6)-glucan antibodies in addition to the anti-beta-(1,3)-glucan IgG, was non-protective. These data provide some insights on beta-glucan epitope(s) mediating antifungal protection and open the way to develop a synthetic oligosaccharide vaccine against fungal diseases.

Differential chemokine response of human monocytes to yeast and hyphal forms of Candida albicans and its relation to the β-1,6 glucan of the fungal cell wall

Hyphae formation from yeast cells is a virulence trait enabling the human opportunistic pathogen Candida albicans to invade host tissues. Hyphal cells proved to be much less efficient than yeast cells in stimulating production of macrophage inflammatory protein‐1α (MIP‐1α), MIP‐1β, interleukin‐8 (IL‐8), and particularly, monocyte chemotactic protein‐1 (MCP‐1) by human monocyte. This different stimulation did not depend on the monocyte inability to ingest the hyphae nor did it imply hyphal resistance to the extracellular killing by the monocytes. Purified hyphal and yeast cell walls reproduced the differences shown by the intact cells, and chemical‐enzymatic dissection of cell wall components suggested that cell wall β‐1,6 rather than β‐1,3 glucan was the main chemokine inducer. Coherently, immunofluorescence studies with an anti β‐1,6 glucan serum showed that the surface expression of this polysaccharide was much lower on hyphae than on yeast cells. By minimizing chemokine induction, the formation of hyphal filaments might facilitate C. albicans escaping from host immunity.