Expression of an epitope by surface glycoproteins of Candida albicans. Variability among species, strains and yeast cells of the genus Candida

Antibodies as Models and Tools to Decipher Candida albicans Pathogenic Development: Review about a Unique Monoclonal Antibody Reacting with Immunomodulatory Adhesins

Candidiasis, caused mainly by Candida albicans, a natural commensal of the human digestive tract and vagina, is the most common opportunistic fungal infection at the mucosal and systemic levels. Its high morbi-mortality rates have led to considerable research to identify the molecular mechanisms associated with the switch to pathogenic development and to diagnose this process as accurately as possible. Since the 1980s, the advent of monoclonal antibody (mAb) technology has led to significant progress in both interrelated fields. This linear review, intended to be didactic, was prompted by considering how, over several decades, a single mAb designated 5B2 contributed to the elucidation of the molecular mechanisms of pathogenesis based on β-1,2-linked oligomannoside expression in Candida species. These contributions starting from the structural identification of the minimal epitope as a di-mannoside from the β-1,2 series consisted then in the demonstration that it was shared by a large number of cell wall proteins differently anchored in the cell wall and the discovery of a cell wall glycoplipid shed by the yeast in contact of host cells, the phospholipomannan. Cytological analysis revealed an overall highly complex epitope expression at the cell surface concerning all growth phases and a patchy distribution resulting from the merging of cytoplasmic vesicles to plasmalema and further secretion through cell wall channels. On the host side, the mAb 5B2 led to identification of Galectin-3 as the human receptor dedicated to β-mannosides and signal transduction pathways leading to cytokine secretion directing host immune responses. Clinical applications concerned in vivo imaging of Candida infectious foci, direct examination of clinical samples and detection of circulating serum antigens that complement the Platelia Ag test for an increased sensitivity of diagnosis. Finally, the most interesting character of mAb 5B2 is probably its ability to reveal C. albicans pathogenic behaviour in reacting specifically with vaginal secretions from women infected versus colonized by this species as well as to display higher reactivity with strains isolated in pathogenic circumstances or even linked to an unfavourable prognosis for systemic candidiasis. Together with a detailed referenced description of these studies, the review provides a complementary reading frame by listing the wide range of technologies involving mAb 5B2 over time, evidencing a practical robustness and versatility unique so far in the Candida field. Finally, the basic and clinical perspectives opened up by these studies are briefly discussed with regard to prospects for future applications of mAb 5B2 in current research challenges.

Efficient diagnosis of vulvovaginal candidiasis by use of a new rapid immunochromatography test

The clinical symptoms of vulvovaginal candidiasis (VVC) are nonspecific, and misdiagnosis is common, leading to a delay in the initiation of antifungal treatment. We evaluated a new immunochromatography test (ICT), the CandiVagi assay (SR2B, Avrille, France), for the rapid diagnosis of VVC. This test, which employs an immunoglobulin M antibody directed against the beta-1,2-mannopyranosyl epitopes found in the yeast cell wall, was compared with direct microscopic examination and culture of vaginal swabs. Two-hundred five women were investigated, including 130 women with symptomatic vaginitis and 75 asymptomatic controls. Two vaginal swabs were obtained from each woman: one was used to prepare a wet mount and Gram-stained preparations for direct microscopic examination and was also cultured on Sabouraud dextrose agar for the isolation of Candida spp., and the second swab was used for ICT. The sensitivities of microscopic examination, culture, and ICT for the diagnosis of VVC were 61%, 100%, and 96.6%, respectively, while the specificities of the three methods were 100%, 82%, and 98.6%, respectively. ICT had a negative predictive value of 98.6%, a positive predictive value of 96.6%, and an efficiency of 98%. ICT provided a rapid result and a better compromise between sensitivity and specificity than conventional microscopy and culture for the diagnosis of VVC. This easy-to-perform diagnostic test will be useful to practitioners treating women with symptoms of vaginitis.

Synthetic analogues of beta-1,2 oligomannosides prevent intestinal colonization by the pathogenic yeast Candida albicans

The pathogenic yeast Candida albicans displays at its cell surface beta-1,2 oligomannosides (beta-1,2-Mans). In contrast to the ubiquitous alpha-Mans, beta-1,2-Mans bind to galectin-3, a major endogenous lectin expressed on epithelial cells. The specific role of beta-1,2-Mans in colonization of the gut by C. albicans was assessed in a mouse model. A selected virulent strain of C. albicans (expressing more beta-1,2-Man epitopes) induced more intense and sustained colonization than an avirulent strain (expressing less beta-1,2-Man epitopes). Synthetic (Sigma) beta-and alpha-linked tetramannosides with antigenicities that mimicked the antigenicities of C. albicans-derived oligomannosides were then constructed. Oral administration of Sigmabeta-1,2-Man (30 mg/kg of body weight) prior to inoculation with the virulent strain resulted in almost complete eradication of yeasts from stool samples, whereas administration of Sigmaalpha-Man at the same dose did not. As most cases of human systemic candidiasis are endogenous in origin, this first demonstration that a synthetic analogue of a yeast adhesin can prevent yeast colonization in the gut opens the possibility of new prophylactic strategies.

Beta-1,2-mannosylation of Candida albicans mannoproteins and glycolipids differs with growth temperature and serotype

Increasing the growth temperature from 28 to 37 degrees C reduced the expression of beta-1,2-oligomannoside epitopes on mannoproteins of Candida albicans serotypes A and B. In contrast, beta-1,2-mannosylation of phospholipomannan (PLM) remained constant despite a slight decrease in the relative molecular weight (M(r)) of this compound. At all growth temperatures investigated, serotype A PLM displayed an M(r) and an antigenicity different from those of serotype B PLM when they were tested with a panel of monoclonal antibodies.

Switch of phenotype as an escape mechanism of the intruder

Phenotypic switching in Candida albicans is a reversible, high-frequency phenomenon that is readily detectable in a fungal population as changes in cell or colony morphology. Some putative attributes of virulence in C. albicans, including expression of cell wall glycoproteins, secretion of proteolytic enzymes and hypha formation have been associated with switching phenomena. C. albicans isolates from active infection tend to show a higher prevalence of phenotypic switching than those associated with commensalism. Moreover, some characteristics of azole resistance in C. albicans are compatible with a switch of phenotype. There is thus a preliminary basis of scientific evidence for a hypothesis that phenotypic switching may indeed serve as an attribute of virulence in at least one pathogenic fungus, facilitating invasion and escape from host defences.

Antibody response that protects against disseminated candidiasis

We previously showed that surface mannans of Candida albicans function as adhesins during yeast cell attachment to mouse splenic marginal zone macrophages. The mannan adhesin fraction was encapsulated into liposomes and used to vaccinate mice over a 5- to 6-week period. Circulating agglutinins specific for the fraction correlated with increased resistance to disseminated candidiasis. Antiserum from vaccinated animals protected naive BALB/cByJ mice against C. albicans serotype A and B strains and Candida tropicalis. Antiserum also protected SCID mice against disseminated disease. The serum protective ability was stable at 56 degrees C, but this ability was adsorbed by C. albicans cells. The antiserum was divided into three fractions after separation by high-performance liquid chromatography. One fraction contained all of the agglutinin activity and transferred resistance to naive mice. A second fraction also transferred resistance. Two monoclonal antibodies (MAbs) specific for candidal surface determinants were obtained. MAb B6.1 is specific for a mannan epitope in the adhesin fraction, and MAb B6 is specific for a different epitope in the fraction. Both MAbs are immunoglobulin M, and both strongly agglutinate candidal cells, but only MAb B6.1 protected both normal and SCID mice against disseminated candidiasis. In one experiment, 10 normal mice were given MAb B6.1 and challenged with yeast cells. Six mice survived the 67-day observation period; 4 of the survivors were cured as evidenced by the lack of CFU in the kidney and spleen. Our studies show that antibodies against certain cell surface antigens of C. albicans help the host resist disseminated candidiasis.

Mapping of Candida albicans oligomannosidic epitopes by using monoclonal antibodies

Six monoclonal antibodies (MAbs) from various laboratory sources (EB-CA1, EB-CA2, H5, AF1, C6, and 5B2), reacting with the polysaccharidic moieties of Candida albicans mannoproteins, were used for epitope mapping by an enzyme-linked immunosorbent assay (ELISA) with neoglycolipids and by Western blotting (immunoblotting) of a C. albicans germ tube extract. The ELISA involved neoglycolipids constructed from three families of oligomannosides released by sequential depolymerization of C. albicans phosphopeptidomannan by acid hydrolysis (NGLH), beta-elimination (NGLO), and acetolysis (NGLA). All of the MAbs exhibited low reactivities against NGLO. MAbs EB-CA1, EB-CA2, and H5 reacted mainly against NGLA, and MAbs C6 and AF1 recognized mainly NGLH, whereas MAb 5B2 reacted with both families of neoantigens. When this method was compared with Western blotting, strong reactivity to NGLA was associated with the presence of epitopes shared by high-molecular-weight mannoproteins, whereas strong reactivity to NGLH was associated with a reactivity to a family of 14- to 18-kDa antigens. The reactivity of MAb 5B2 was associated with both high-molecular-weight mannoproteins and the 14- to 18-kDa antigens. In relation to the present knowledge about the structure of the C. albicans phosphopeptidomannan oligomannosidic repertoire, these results provide preliminary data concerning the molecular basis of the recognition of mannopyranosyl sequences by MAbs and their distribution among C. albicans mannoproteins.

Transcription of the gene for a pepsinogen, PEP1, is regulated by white-opaque switching in Candida albicans

Cells of Candida albicans WO-1 spontaneously switch between a white and opaque CFU, and this phase transition involves a dramatic change in cellular phenotype. By using a differential hybridization screen, an opaque-specific cDNA, Op1a, which represents the transcript of a gene regulated by switching, has been isolated. The gene for Op1a is transcribed by opaque but not by white cells. The nucleotide sequence of the Op1a cDNA reveals over 99% base homology with an acid protease gene of C. albicans, and the predicted amino acid sequence demonstrates that the product of this gene is a member of the family of pepsinogens, which possess a hydrophobic leader sequence for secretion and two catalytic aspartate domains. Southern blots of both genomic DNA digested with 14 different endonucleases and electrophoretically separated chromosomes were probed with the Op1a cDNA. No polymorphisms were detected in either case between white and opaque cells, suggesting that no genomic reorganization occurs in the proximity of the gene during the white-opaque transition. Although transcription of Op1a correlates with the high levels of extracellular protease activity in opaque cell cultures and the absence of activity in white cell cultures, stimulation of extracellular protease activity by addition of serum albumin is not accompanied by Op1a transcription in cultures of WO-1 white cells or cultures of two additional clinical isolates of C. albicans, suggesting that expression of one or more other protease genes is stimulated in these cases. The results demonstrate that transcription of the Op1a gene is under the rigid control of switching in strain WO-1.

Transcription of the gene for a pepsinogen, PEP1, is regulated by white-opaque switching in Candida albicans

Cells of Candida albicans WO-1 spontaneously switch between a white and opaque CFU, and this phase transition involves a dramatic change in cellular phenotype. By using a differential hybridization screen, an opaque-specific cDNA, Op1a, which represents the transcript of a gene regulated by switching, has been isolated. The gene for Op1a is transcribed by opaque but not by white cells. The nucleotide sequence of the Op1a cDNA reveals over 99% base homology with an acid protease gene of C. albicans, and the predicted amino acid sequence demonstrates that the product of this gene is a member of the family of pepsinogens, which possess a hydrophobic leader sequence for secretion and two catalytic aspartate domains. Southern blots of both genomic DNA digested with 14 different endonucleases and electrophoretically separated chromosomes were probed with the Op1a cDNA. No polymorphisms were detected in either case between white and opaque cells, suggesting that no genomic reorganization occurs in the proximity of the gene during the white-opaque transition. Although transcription of Op1a correlates with the high levels of extracellular protease activity in opaque cell cultures and the absence of activity in white cell cultures, stimulation of extracellular protease activity by addition of serum albumin is not accompanied by Op1a transcription in cultures of WO-1 white cells or cultures of two additional clinical isolates of C. albicans, suggesting that expression of one or more other protease genes is stimulated in these cases. The results demonstrate that transcription of the Op1a gene is under the rigid control of switching in strain WO-1.

Cytological immunodetection of yeast glycoprotein secretion

Expression of antigenic epitopes shared by secreted yeast glycoproteins was studied using specific immunological probes. Application of cytological and ultrastructural methods of immunodetection, employing monoclonal antibodies, permitted us to localize these glycoproteins in the cytoplasm, through the cell wall and at the yeast cell surface. Importance of glycosylation-secretion relationships were evaluated in the secretion process of these molecules. The cell wall crossing and the cell surface distribution of antigenic glycoproteins was described in immunoelectron microscopy and immunofluorescence. Some preferential secretion "ways" were suspected through the yeast cell wall leading to an heterogenous distribution of cell surface glycoproteins destined to be excreted into the medium. Antigenic variability of cell wall glycoproteins expression was discussed in relation with the glycoprotein secretion.

High-frequency switching in Candida albicans

Most strains of Candida albicans are capable of switching frequently and reversibly between a number of phenotypes distinguishable by colony morphology. A number of different switching systems have been defined according to the limited set of phenotypes in each switching repertoire, and each strain appears to possess a single system. Switching can affect many aspects of cellular physiology and morphology and appears to be a second level of phenotypic variability superimposed upon the bud-hypha transition. The most dramatic switching system so far identified is the "white-opaque transition." This system dramatizes the extraordinary effects switching can have on the budding cell phenotype, including the synthesis of opaque-specific antigens, the expression of white-specific and opaque-specific genes, and the genesis of unique cell wall structures. Switching has been demonstrated to occur at sites of infection and between episodes of recurrent vaginitis, and it may function to generate variability in commensal and infecting populations for adaptive reasons. Although the molecular mechanisms involved in the switch event are not understood, recent approaches to its elucidation are discussed and an epigenetic mechanism is proposed.

Monoclonal antibody-gold silver staining dot assay for the detection of antigenaemia in candidosis

A monoclonal antibody (Mab), designated 5B2, reacting with cell wall mannoproteins of Candida albicans has been purified and coupled to colloidal gold. The ability of the gold conjugated Mab to detect C. albicans antigens in serum during infection has been assessed in a dot immunobinding assay involving an immunogold silver enhancement procedure (GSS). A double blind study was made with sera from 140 guinea pigs, including 40 control animals and 100 infected intravenously with C. albicans. Sera from infected animals were collected either 2 days (4 animals), 15 days (9 animals), or 21 days (87 animals) after inoculation. The overall sensitivity, specificity and positive predictive value of the test were 89, 95, and 98%, respectively. However, differences were encountered in the ratio of positive tests in relation to duration of the infection. Mab-GSS dot immunobinding has also been applied, together with Mab co-counterimmunoelectrophoresis on successive sera from 2 patients who recovered from clinically and mycologically proven episodes of systemic candidosis. It was demonstrated that both patients synthesized antibodies against glycoproteins sharing the 5B2 epitope, which was initially present transiently in their sera.

Biochemical and antigenic characterization of mannoprotein constituents released from yeast and mycelial forms of Candida albicans

Yeast or mycelial cultures of Candida albicans released comparable amounts of Concanavalin A-reactive mannoprotein material after 24-h of growth, and in both cases this material showed a qualitatively similar SDS-PAGE pattern, with predominantly polydisperse constituents of high molecular mass. The two secretion mixtures also showed similar reactivity by ELISA with serum from a subject with high titre anti-Candida antibodies, as well as with an anti-Candida hyperimmune antiserum raised in rabbits. Both secreted extracts were separated by ion-exchange chromatography into two major fractions (designated F1 and F2), each containing mannoprotein antigens recognized by rabbit and human sera, although the immunoreactivity of the two fractions from the two growth forms was not uniform. The mannoproteins released from mycelial cultures, in particular those present in the F1 fraction, were poorly reactive or not reactive at all in ELISA with a monoclonal antibody (mAbAF1) which strongly recognized the material released from yeast cultures. Immunoblots of the more acidic, more antigenic F2 fractions with mAbAF1 and polyclonal anti-Candida antisera demonstrated that the monoclonal antibody did not recognize several mannoprotein molecules which were recognized by the polyclonal antibodies, in particular a 45-47 kDa component present only in the secreted extract from mycelium. A quantitative ELISA-inhibition method showed that the rate of release of mannoprotein antigen during growth in the yeast form was either constant (as assayed with polyclonal antibodies) or fluctuated without any definite trend (as seen with mAbAF1). On the other hand, cultures of mycelial cells exhibited an early (90 min) peak of antigen release, followed by either a decrease to a rate corresponding to that of yeast cells (with polyclonal antibodies) or a total lack of secretion (with mAbAF1). This modulation in the secretion of mAbAF1 reactive molecules was temporarily associated with germ tube emergence-elongation, and was not observed in an agerminative mutant of C. albicans grown under germination permissive conditions. These results highlight the dynamic aspects of the secretion of specific mannoprotein epitopes released from C. albicans during hyphal growth, and the direct relationship between this release and the dynamic expression of the same epitopes on the cell surface demonstrated previously.

Immunoreactivity of neoglycolipids constructed from oligomannosidic residues of the Candida albicans cell wall

To establish a model to study the immunoreactivity of oligosaccharidic structures from the Candida albicans cell wall, we attempted to construct neoglycolipids with these residues by using oligomannosides released after mild acid hydrolysis of the phosphopeptidomannans isolated from yeast forms. From a mixture of manno-oligosaccharides ranging from mannobiose to mannononaose, the structure of a quantitatively major component (mannotriose) was determined to be Man (beta 1-2) Man (beta 1-2) Man alpha by 1H nuclear magnetic resonance analysis. After coupling of the pool of oligosaccharides to a lipid (4-hexadecylaniline), the synthesized molecules were injected into mice and rats. Antibody responses were detected on enzyme-linked immunosorbent assay plates coated with either phosphopeptidomannans or neoglycolipids. The hybrid molecules exhibited both immunogenicity and antigenicity. The kinetics of antibody responses as well as immunofluorescence patterns observed on whole C. albicans cells strongly mimicked results from the immunization of animals with natural antigens. Construction of neoglycolipids could therefore provide an interesting approach to the study of specific oligosaccharides of C. albicans and their recognition by the host immune system.