Antibodies against Candida: potential therapeutics?

Structural characterization of low molecular weight polysaccharide from Astragalus membranaceus and its immunologic enhancement in recombinant protein vaccine against systemic candidiasis

Structure and immunologic enhancement of low molecular weight polysaccharide (LMW-ASP) isolated from the root of Astragalus membranaceus (Fisch) Bge. Were detected in recombinant protein vaccine. Structure analysis of LMW-ASP revealed that LMW-ASP (Mw=5.6kDa) was an acid heteropolysaccharide, which consisted of Glc, Gal, Ara, Xyl and GalA in ratio of 10.0：1.3：1.7：1.0：0.9. Recombinant protein (rP-HSP90C) contained epitope C (LKVIRK) from heat shock protein 90 (HSP90) of Candida albicans was used as a vaccine. The results indicated that LMW-ASP significantly promoted specific antibody titers IgG, IgG1, IgG2b, and IL-2, IL-4, IL-10, IL-12 in sera of mice immunized with rP-HSP90C (p<0.05). It was also found LMW-ASP improved DTH response in HSP90C-injceted mice. More importantly, the mice immunized with rP-HSP90C/LMW-ASP had fewer CFU (colony forming unites) in the kidneys compared to the mice immunized with rP-HSP90C (p<0.05). Therefore, LMW-ASP could be exploited into the novel adjuvant to enhance the efficacy of recombinant protein vaccine.

Immunoproteomic profiling of Saccharomyces cerevisiae systemic infection in a murine model

Saccharomyces cerevisiae is considered a safe microorganism widely used as a dietary supplement. However, in the latest decades several cases of S. cerevisiae infections have been reported. Recent studies in a murine model of systemic infection have also revealed the virulence of some S. cerevisiae dietary strains. Here we use an immunoproteomic approach based on protein separation by 2D-PAGE followed by Western-blotting to compare the serological response against a virulent dietary and a non-virulent laboratory strains leading to the identification of highly different patterns of antigenic proteins. Thirty-six proteins that elicit a serological response in mice have been identified. Most of them are involved in stress responses and metabolic pathways. Their selectivity as putative biomarkers for S. cerevisiae infections was assessed by testing sera from S. cerevisiae-infected mice against Candida albicans and C. glabrata proteins. Some chaperones and metabolic proteins showed cross-reactivity. We also compare the S. cerevisiae immunodetected proteins with previously described C. albicans antigens. The results point to the stress-related proteins Ahp1, Yhb1 and Oye2, as well as the glutamine synthetase Gln1 and the oxysosterol binding protein Kes1 as putative candidates for being evaluated as biomarkers for diagnostic assays of S. cerevisiae infections.

BIOLOGICAL SIGNIFICANCE

S. cerevisiae can cause opportunistic infections, and therefore, a precise diagnosis of fungal infections is necessary. This immunoproteomic analysis of sera from a model murine infection with a virulent dietary S. cerevisiae strain has been shown to be a source of candidate proteins for being evaluated as biomarkers to develop assays for diagnosis of S. cerevisiae infections. To our knowledge, this is the first study devoted to the identification of S. cerevisiae immunogenic proteins and the results allowed the proposal of five antigens to be further investigated.

Protective immune responses against systemic candidiasis mediated by phage-displayed specific epitope of Candida albicans heat shock protein 90 in C57BL/6J mice

Specific epitope (DEPAGE) of Candida albicans heat shock protein 90 (SE-CA-HSP90) was successfully expressed on the surface of filamentous phage fd, fused to the major coat protein pVIII. Protective immune responses mediated by hybrid-phage expressing SE-CA-HSP90 in C57BL/6J mice were evaluated in this paper. The results showed that hybrid-phage particles induced the specific antibody response against SE-CA-HSP90, enhanced delayed-type hypersensitivity (DTH) response, natural killer (NK) cell activity and concanavalin A (ConA)-induced splenocyte proliferation. Furthermore, this study demonstrated that hybrid-phage-immunized mice had fewer colony forming unites (CFU) in the kidneys compared with wild-type phage-immunized mice and TE (1.0mM EDTA, 0.01M Tris-HCl, pH 8.0)-injected mice and showed statistically significant survival advantage over TE-injected group. In conclusion, the results suggest that the hybrid-phage displaying SE-CA-HSP90 may be considered as a potential candidate for producing vaccines against systemic candidiasis.

Humoral immune response against soluble and fractionate antigens in experimental sporotrichosis

Sporotrichosis is a chronic granulomatous mycosis caused by the dimorphic fungus Sporothrix schenckii, which is widely distributed in nature, and presents a saprophytic mycelial form on plant debris and soil. The immunological mechanisms involved in the prevention and control of sporotrichosis are not yet fully understood. In this study, mice were studied after infection with Sporothrix schenckii. In the first week after infection, fungal loading increased and thence decreased drastically 14 days after infection. Analysis by immunoblotting showed that the sera of all mice tested had antibodies reacting only with a 70 kDa antigen, with predominance of IgG1 and IgG3. Taken together, our results show that antigens from S. schenckii induced a specific humoral response in infected mice.

Isolation and characterization of an immunogenic fragment of heat shock protein 60 from Trichophyton mentagrophytes. Isolierung und Charakterisierung eines immunogenen Fragmentes des Hitzeschockproteins 60 von Trichophyton mentagrophytes

Heat shock protein 60 (hsp60) were isolated from several fungal, protozoal and many bacterial pathogens and successfully used for protective vaccination in some infection models. This work focuses on the isolation of recombinant hsp60 from the dermatophyte, Trichophyton mentagrophytes as a potentially protective antigen in trichophytosis. With the help of a previously tested set of degenerated primers, it was used reverse transcriptase polymerase chain reaction (RT-PCR) for isolation of partial cDNA of the hsp60 T. mentagrophytes (labelled hsp60-TM814), which was cloned into cloning vector. The sequencing of hsp60-TM814 cDNA and global alignment confirmed homology of the hsp60-TM814 with other members of the hsp60 family. Hsp60-TM814 cDNA corresponds to the region encoding the immunoprotective fragment of the hsp60 from Histoplasma capsulatum, used successfully in mouse model of histoplasmosis. A recombinant fragment (r-hsp60-TM664), 220 amino acids in length, was prepared in a prokaryote expression system, and its identity confirmed by mass spectroscopy. High immunogenicity of r-hsp60-TM664 was proven after subcutaneous immunization of mice. Immunized mouse sera recognized r-hsp60-TM664 on Western blots as well as hsp60 from mouse liver lysate and lysate of Candida albicans.

Antibody response toCandida albicanscell wall antigens

The cell wall of Candida albicans is not only the structure where many essential biological functions reside but is also a significant source of candidal antigens. The major cell wall components that elicit a response from the host immune system are proteins and glycoproteins, the latter being predominantly mannoproteins. Both carbohydrate and protein moieties are able to trigger immune responses. Proteins and glycoproteins exposed at the most external layers of the wall structure are involved in several types of interactions of fungal cells with the exocellular environment. Thus, coating of fungal cells with host antibodies has the potential to profoundly influence the host-parasite interaction by affecting antibody-mediated functions such as opsonin-enhanced phagocytosis and blocking the binding activity of fungal adhesins to host ligands. In this review we examine various members of the protein and glycoprotein fraction of the C. albicans cell wall that elicit an antibody response in vivo. Some of the studies demonstrate that certain cell wall antigens and anti-cell wall antibodies may be the basis for developing specific and sensitive serologic tests for the diagnosis of candidiasis, particularly the disseminated form. In addition, recent studies have focused on the potential of antibodies against the cell wall protein determinants in protecting the host against infection. Hence, a better understanding of the humoral response triggered by the cell wall antigens of C. albicans may provide the basis for the development of (i) effective procedures for the serodiagnosis of disseminated candidiasis, and (ii) novel prophylactic (vaccination) and therapeutic strategies to control this type of infections.

Recombinant antibodies: a natural partner in combinatorial antifungal therapy

Monotherapy, in the form of amphotericin B or one of its liposomal derivatives, is the usual treatment for invasive fungal infections, due to lack of a safe, effective combination of antifungal drugs. Combination therapy is not necessarily beneficial-there may be mutual antagonism or indifference, increased toxicity or interference with concomitant medication. But the benefits of a well-tolerated, synergistic combination would be great-the enhanced efficacy would improve clinical outcome, reduce the need for prolonged courses of treatment and prevent the emergence of antifungal drug resistance. Antifungal antibodies would be a natural partner in a combinatorial approach to antifungal therapy. Analysis of the antibody response which occurs in patients with invasive candidiasis, being treated with amphotericin B, showed a close correlation between recovery and antibody to the immunodominant heat shock protein 90 (hsp90). The molecular chaperone hsp90 is essential for yeast viability. Mycograb is a human recombinant antibody to hsp90 which shows intrinsic antifungal activity and synergy with amphotericin B both in vitro and in vivo. It is now the subject of a multinational, double-blind, placebo-controlled trial, in patients with culture-confirmed invasive candidiasis on liposomal amphotericin B.

Immunoglobulins G could prevent adherence of Candida albicans to polystyrene and extracellular matrix components

Immunocompromised patients are at high risk of developing Candida infections. Although cell-mediated immunity is generally believed to play the main role in defence against fungi, antibodies could also be effective in immune defence by different mechanisms of action. The adherence capacity of four strains of Candida albicans to polystyrene and to some extracellular matrix components was investigated after incubation of the yeasts with non-specific and specific anti-C. albicans IgG. Experiments were carried out using a colorimetric method based upon the reduction of XTT tetrazolium (2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide) by mitochondrially active blastospores in the presence of menadione. Incubation of the yeasts with IgG, specific or not, caused a decrease in the capacity for adherence to the surfaces studied. There was no significant effect of the specificity of the tested antibodies on the reduction of adherence capacity. In conclusion, total IgG could play a role in blocking the binding of C. albicans to host and medical device surfaces. These results suggest that regular survey of levels of total IgG in patients suffering from severe hypogammaglobulinaemia could be of interest for the prevention of systemic candidiasis.

Increased resistance in BALB/c mice to reinfection with Candida albicans is due to immunoneutralization of a virulence-associated immunomodulatory protein

Here, it is shown that immunoneutralization of p43, a virulence-associated immunomodulatory protein secreted by Candida albicans, is responsible for immunoprotection against candidiasis after spontaneous healing of mice inoculated with 10(6) C. albicans blastoconidia. p43 is produced by the pathogenic Candida blastoconidia, and neither immunoprotection nor immunoneutralization can be elicited by priming the mice with attenuated or heat-killed C. albicans blastoconidia. The immunoprotection against systemic candidiasis was positively correlated with (i). serum levels of antibodies against p43 and (ii). a high ratio between antibodies against p43 and antibodies against C. albicans structural antigens. Immunoprotection against candidiasis can be induced in mice primed with heat-killed C. albicans, after treatment of the animals with anti-p43 antibodies. The data described here provide a biological explanation for active immunoprotection achieved after spontaneous healing of infectious diseases, namely in candidiasis.

New immunotherapeutic strategies to control vaginal candidiasis

The widespread occurrence of mucosal infections caused by Candida, in particular recurrent vulvovaginal candidiasis among fertile-age women, together with the paucity of safe candidacidal antimycotics, have prompted a great number of investigations into the immunotherapy of candidal vaginitis. This article will discuss three different experimental approaches demonstrated to be potentially transferable to human disease: (1) the use of antibodies against well-defined cell-surface adhesins or enzymes; (2) the generation of yeast killer-toxin-like candidacidal anti-idiotypic antibodies and their engineered molecular derivatives (e.g. single chains, peptides); and (3) the generation of therapeutic vaccines and immunomodulators.

Complement is essential for protection by an IgM and an IgG3 monoclonal antibody against experimental, hematogenously disseminated candidiasis

The incidence of life-threatening, hematogenously disseminated candidiasis, which is predominantly caused by Candida albicans, parallels the use of modern medical procedures that adversely affect the immune system. Limited antifungal drug choices and emergence of drug-resistant C. albicans strains indicate the need for novel prevention and therapeutic strategies. We are developing vaccines and Abs that enhance resistance against experimental candidiasis. However, the prevalence of serum anti-Candida Abs in candidiasis patients has led to the misconception that Abs are not protective. To explain the apparent discrepancy between such clinical observations and our work, we compared functional activities of C. albicans-specific protective and nonprotective mAbs. Both kinds of Abs are agglutinins that fix complement and are specific for cell surface mannan, but the protective Abs recognize beta-mannan, and the nonprotective Ab is specific for alpha-mannan. By several indirect and direct measures, the protective mAbs more efficiently bind complement factor C3 to the yeast cell than do nonprotective Ab. We hypothesize that the C3 deposition causes preferential association of blood-borne fungi with host phagocytic cells that are capable of killing the fungus. We conclude from these results that the protective potential of Abs is dependent on epitope specificity, serum titer, and ability to rapidly and efficiently fix complement to the fungal surface. The mechanism of protection appears to be associated with enhanced phagocytosis and killing of the fungus.

Neutrophil Fc gamma RI as target for immunotherapy of invasive candidiasis

Invasive candidiasis represents a life-threatening disease for immunocompromised patients. This study focused on new immunotherapeutic approaches for systemic Candida albicans infections in a human FcgammaRI-transgenic mouse model. FcgammaRI (CD64) is a potent immunoactivating receptor on phagocytic and dendritic cells. In vivo targeting of C. albicans toward neutrophil-FcgammaRI by bispecific Abs and G-CSF effectively protected FcgammaRI-transgenic mice from lethal candidiasis. Nontransgenic mice were not protected, and treatment with bispecific Ab or G-CSF alone did not reduce mortality. Furthermore, infected FcgammaRI-transgenic mice developed high titers of anti-C. albicans IgG, and survival was extended on secondary infection without further treatment. These findings document the capacity of FcgammaRI to initiate potent anti-C. albicans immunity and support the development of FcgammaRI-directed immunotherapy of invasive fungal disease.

Identification of continuous B-cell epitopes on the protein moiety of the 58-kiloDalton cell wall mannoprotein of Candida albicans belonging to a family of immunodominant fungal antigens

The 58-kiloDalton mannoprotein (mp58) on the surface of Candida albicans is highly immunogenic, is expressed by all C. albicans isolates tested, and elicits strong antibody responses during candidiasis. It belongs to a family of immunodominant fungal antigens with representatives also in different species of Aspergillus. The amino acid sequence of the protein portion of mp58 as deduced from the DNA sequence of its encoding gene (FBP1/PRA1) was used to synthesize a complete set of overlapping dodecapeptides (overlap, 7; offset, 5) covalently attached to the surface of derivatized polyethylene pins. The pin-coupled peptides were used in a modified enzyme-linked immunosorbent assay (ELISA) to identify continuous epitopes recognized by a number of antiserum preparations containing anti-mp58 antibodies. This comprehensive epitope-scanning study revealed the presence of multiple immunoreactive continuous B-cell epitopes within the protein sequence. Regions of increased reactivity included both the amino and carboxy termini of the mature protein (encompassing amino acid residues 16 to 50 and 286 to 299, respectively) and four internal regions spanning amino acids at positions 66 to 92, 121 to 142, 148 to 192, and 211 to 232. Further delineation of epitopic regions and identification of the boundaries of the antigenic sites was performed upon ELISA testing with a second Pepset consisting of completely overlapping 8-mer peptides spanning these reactive regions in the protein moiety of mp58. The highly reactive epitopic region at the C terminus of the protein was further evaluated using both window net and replacement net analyses. A synthetic peptide corresponding to the last 10 amino acid residues at the C terminus of the protein was immunogenic when injected into mice after being coupled to a carrier protein. Moreover, antibodies in the resulting sera specifically recognized the homologous mp58 in ELISAs and immunoblot assays. Delineation of the antibody responses to mp58 could provide the basis for the development of novel immunity-based prophylactic, therapeutic, and diagnostic techniques for the management of candidiasis.

Protection against candidiasis by an immunoglobulin G3 (IgG3) monoclonal antibody specific for the same mannotriose as an IgM protective antibody

We previously reported that a liposome-mannan vaccine (L-mann) of Candida albicans induces production of mouse antibodies that protect against disseminated candidiasis and vaginal infection. Immunoglobulin M (IgM) monoclonal antibody (MAb) B6.1, specific for a C. albicans cell surface beta-1,2-mannotriose, protects mice against both infections. Another IgM MAb, termed B6, which is specific for a different cell surface mannan epitope, does not protect against disseminated candidiasis. The B6.1 epitope is displayed homogeneously over the entire cell surface, compared to a patchy distribution of the B6 epitope. To determine if protection is restricted to an IgM class of antibody, we tested an IgG antibody. MAb C3.1 was obtained from L-mann-immunized mice. By results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, capture enzyme-linked immunosorbent assay, and immunodiffusion tests, MAb C3.1 is an IgG3 isotype. By epitope inhibition assays, we determined that MAb C3.1 is specific for same mannotriose as MAb B6. 1. As expected by the results of the inhibition assays, immunofluorescence microscopy showed that the C3.1 epitope is distributed on the yeast cell surface in a pattern identical to that of the B6.1 epitope. Kidney CFU and mean survival times of infected mice pretreated with MAb C3.1 indicated that the antibody enhanced resistance of mice against disseminated candidiasis. Mice in pseudoestrus that were given MAb C3.1 prior to vaginal infection developed fewer vaginal Candida CFU than control animals that received buffered saline instead of the antibody. The finding that an IgG3 antibody is protective is consistent with our hypothesis that epitope specificity and complement activation are related to the ability of an antibody to protect against candidiasis.

The Hsp90 of Candida albicans can confer Hsp90 functions in Saccharomyces cerevisiae: a potential model for the processes that generate immunogenic fragments of this molecular chaperone in C. albicans infections

During infections with a number of important eukaryotic pathogens the Hsp90 molecular chaperone of the pathogen is recognized as an immunodominant antigen by the host immune system. Yeast molecular genetics should allow study of the extent of sequence variation within conserved immunodominant epitopes on pathogen Hsp90s that is compatible with essential Hsp90 functions, as well as the processes that generate antigenic subfragments of these Hsp90s. The Hsp90 of the fungal pathogen Candida albicans was shown in this study to provide both essential and nonessential (pheromone signalling and mammalian steroid receptor activation) Hsp90 functions in Saccharomyces cerevisiae cells. Much of the C. albicans Hsp90 expressed in respiratory S. cerevisiae cells was shown to undergo a partial degradation in vivo, a degradation that closely resembles that of the native Hsp82 (one isoform of the homologous Hsp90) in S. cerevisiae. Allowing for the differences in the length of the charged linker region between the N- and C-terminal domains of C. albicans Hsp90 and S. cerevisiae Hsp82, these two proteins expressed in S. cerevisiae appear to give the same major degradation products. These Hsp90 fragments are similar to the products of incomplete Hsp90 degradation found in C. albicans cultures.

Low levels of antigenic variability in fluconazole-susceptible and -resistant Candida albicans isolates from human immunodeficiency virus-infected patients with oropharyngeal candidiasis

Three serial isolates of Candida albicans were obtained by direct swab or by oral saline rinses from each of five human immunodeficiency virus-infected patients with recurrent oropharyngeal candidiasis. Genotyping techniques confirmed the presence of a persistent strain in multiple episodes from the same patient, which was different from the strains isolated from other patients. Fluconazole susceptibility was determined by both an agar dilution method and the National Committee for Clinical Laboratory Standards macrobroth procedure. In four of these patients the strains developed fluconazole resistance, and in one patient the strain remained susceptible. The different isolates were propagated as yeast cells on a synthetic medium, and their cell wall proteinaceous components were extracted by treatment with beta-mercaptoethanol. Protein and mannoprotein components present in the extracts were analyzed by electrophoresis, immunoblotting, and lectin-blotting techniques. The analysis showed a similar composition, with only minor qualitative and quantitative differences in the polypeptidic and antigenic patterns associated with the cell wall extracts from serial isolates from the same patient, as well as those from different strains isolated from different patients. Use of monospecific antibodies generated against two immunodominant antigens during candidiasis (enolase and the 58-kDa fibrinogen-binding mannoprotein) demonstrated their expression in all isolates tested. Overall, the antigenic makeup of C. albicans strains remained constant during the course of infection and was not affected by development of fluconazole resistance. In contrast to previous reports, the low degree of antigenic variability observed in this study may be due to the fact that the isolates were obtained from a highly homogeneous population of patients and to the uniformity in techniques used for the isolation, storage, and culture of the different strains, as well as extraction methodologies.

Two-dimensional gel electrophoresis as analytical tool for identifying Candida albicans immunogenic proteins

This paper reports the usefulness of two-dimensional gel electrophoresis followed by Western blotting with sera from patients with systemic candidiasis in the identification of the major Candida albicans antigens. In order to have different patterns of protein expression and subcellular localization, three types of protein preparations were obtained: cytoplasmic extracts, protoplast lysates and proteins secreted by protoplasts regenerating their cell wall. These proteins were separated by high-resolution two-dimensional electrophoresis using an immobilized pH gradient. Western blotting with sera from patients with systemic candidiasis allowed the detection of more than 18 immunoreactive proteins. Some of these proteins had different isoforms. All sera reacted with at least three C. albicans proteins and the most reactive serum detected up to eleven proteins. Some of these antigens, e.g., enolase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), have been identified on the 2-D map. The most reactive proteins were enolase and a 34 kDa protein in the acidic part of the gel (pI 4-4.4) that was only detected in regenerating protoplast-secreted proteins. The identification of all these antigens would be useful for the development of diagnostic strategies.

Heat shock proteins in immune reactions

The review concerns heat shock proteins and their significance in immune reactions. It focuses on problems of physiological and pathological interactions in etiology and duration of autoimmune diseases and infection processes, especially fungal infections. New trends are described in exploitation of heat shock proteins for preparation of specific protective vaccines.

Development of the primer set for the detection of the hsp60 gene in Trichophyton mentagrophytes cDNA

Three sequences of hsp60 from Saccharomyces cerevisiae, Schizosaccharomyces pombe and Histoplasma capsulatum were compared. Local multiple alignment of these sequences allowed the selection of two oligonucleotides suitable as primers for the polymerase chain reaction. This primer set was used for the amplification of a part of the hsp60 gene from cDNA of Trichophyton mentagrophytes and S. cerevisiae. Similar fragments detected in both PCR's imply the possible future use of the developed primer set for the detection of the hsp60 gene in other fungal species.

A 70-kilodalton recombinant heat shock protein of Candida albicans is highly immunogenic and enhances systemic murine candidiasis

The 70-kDa recombinant Candida albicans heat shock protein (CaHsp70) and its 21-kDa C-terminal and 28-kDa N-terminal fragments (CaHsp70-Cter and CaHsp70-Nter, respectively) were studied for their immunogenicity, including proinflammatory cytokine induction in vitro and in vivo, and protection in a murine model of hematogenous candidiasis. The whole protein and its two fragments were strong inducers of both antibody (Ab; immunoglobulin G1 [IgG1] and IgG2b were the prevalent isotypes) and cell-mediated immunity (CMI) responses in mice. CaHsp70 preparations were also recognized as CMI targets by peripheral blood mononuclear cells of healthy human subjects. Inoculation of CaHsp70 preparations into immunized mice induced rapid production of interleukin-6 (IL-6) and tumor necrosis factor alpha, peaking at 2 to 5 h and declining within 24 h. CaHsp70 and CaHsp70-Cter also induced gamma interferon (IFN-gamma), IL-12, and IL-10 but not IL-4 production by CD4+ lymphocytes cocultured with splenic accessory cells from nonimmunized mice. In particular, the production of IFN-gamma was equal if not superior to that induced in the same cells by whole, heat-inactivated fungal cells or the mitogenic lectin concanavalin A. In immunized mice, however, IL-4 but not IL-12 was produced in addition to IFN-gamma upon in vitro stimulation of CD4+ cells with CaHsp70 and CaHsp70-Cter. These animals showed a decreased median survival time compared to nonimmunized mice, and their mortality was strictly associated with organ invasion by fungal hyphae. Their enhanced susceptibility was attributable to the immunization state, as it did not occur in congenitally athymic nude mice, which were unable to raise either Ab or CMI responses to CaHsp70 preparations. Together, our data demonstrate the elevated immunogenicity of CaHsp70, with which, however, no protection against but rather some enhancement of Candida infection seemed to occur in the mouse model used.

Serologic response to cell wall mannoproteins and proteins of Candida albicans

The cell wall of Candida albicans not only is the structure in which many biological functions essential for the fungal cells reside but also is a significant source of candidal antigens. The major cell wall components that elicit a response from the host immune system are proteins and glycoproteins, the latter being predominantly mannoproteins. Both the carbohydrate and protein moieties are able to trigger immune responses. Although cell-mediated immunity is often considered to be the most important line of defense against candidiasis, cell wall protein and glycoprotein components also elicit a potent humoral response from the host that may include some protective antibodies. Proteins and glycoproteins exposed at the most external layers of the wall structure are involved in several types of interactions of fungal cells with the exocellular environment. Thus, coating of fungal cells with host antibodies has the potential to influence profoundly the host-parasite interaction by affecting antibody-mediated functions such as opsonin-enhanced phagocytosis and blocking the binding activity of fungal adhesins for host ligands. In this review, the various members of the protein and glycoprotein fraction of the C. albicans cell wall that elicit an antibody response in vivo are examined. Although a number of proteins have been shown to stimulate an antibody response, for some of these species the response is not universal. On the other hand, some of the studies demonstrate that certain cell wall antigens and anti-cell wall antibodies may be the basis for developing specific and sensitive serologic tests for the diagnosis of candidasis, particularly the disseminated form. In addition, recent studies have focused on the potential for antibodies to cell wall protein determinants to protect the host against infection. Hence, a better understanding of the humoral response to cell wall antigens of C. albicans may provide the basis for the development of (i) effective procedures for the serodiagnosis of disseminated candidiasis and (ii) novel prophylactic (vaccination) and therapeutic strategies for the management of this type of infection.

Biochemical characterization of Candida albicans epitopes that can elicit protective and nonprotective antibodies

We previously reported that the immunoglobulin M (IgM) monoclonal antibody (MAb) B6.1 protects mice against disseminated candidiasis, whereas the IgM MAb B6 does not. Both MAbs are specific for an adhesin fraction isolated from the cell surface of Candida albicans, but their epitope specificities differ. In the present study, we examined the surface locations of both epitopes and obtained structural information regarding the B6.1 epitope. Immunofluorescence confocal microscopic analysis of C. albicans yeast forms showed that epitope B6.1 is displayed rather homogeneously over the entire cell surface, whereas epitope B6 appears to have a patchy distribution. Both antibodies were essentially nonreactive with the surfaces of mycelial forms of the fungus, indicating that neither epitope is expressed on the surfaces of these forms. For isolation of the B6.1 epitope, the adhesin fraction consisting of cell surface phosphomannan was subjected to mildly acidic (10 mM HCl) hydrolysis and was fractionated into acid-labile and acid-stable portions by size exclusion chromatography. Antibody blocking experiments showed that the B6.1 epitope is an acid-labile moiety of the phosphomannan and that the B6 epitope is located in the acid-stable fraction. The B6 epitope appeared to be mannan because it was stable to heat (boiling) and protease treatments but was destroyed by alpha-mannosidase digestion. The B6.1 epitope eluted from the size exclusion column in two fractions. Mass spectroscopic analyses showed that one fraction contained material with the size of a mannotriose and that the other was a mixture of mannotriose- and mannotetraose-size substances. Dose response inhibition tests of the fractions indicated that the B6.1 epitope is associated with the mannotriose. Nuclear magnetic resonance (NMR) spectroscopic analysis of the epitope yielded data consistent with a beta-(1-->2)-linked mannotriose. The fine structure of the B6 epitope is under investigation. Information derived from these investigations will be useful both in understanding protective versus nonprotective antibody responses to C. albicans and in improving anti-Candida vaccine formulations.

The T cell response against fungal infections

A variety of pathological conditions, including impaired immune function, is believed to underlie host susceptibility to fungal infections and to determine both the severity and the characteristic of the associated pathology. Although the redundancy and the interdependence of antifungal responses may not favor the proper dissection and appreciation of individual effector mechanisms, the T helper type 1/type 2 paradigm of acquired immunity to fungi is proving essential for a better understanding of the host response from a regulatory perspective. The recent understanding of the importance of the different T helper cell subsets in fungal infections and the increasing appreciation of the reciprocal regulation between the innate, humoral, and adaptive immune systems in the development of optimal antimicrobial immunity have offered us new clues which may lead to an understanding of T cell dependent immunity to fungi.