Antigenic differences in the surface mannoproteins of Candida albicans as revealed by monoclonal antibodies

Antifungals: need to search for a new molecular target

In the 1990s, drug resistance has become an important problem in a variety of infectious diseases including human immunodeficiency virus infection, tuberculosis, and other bacterial infections which have profound effects on human health. At the same time, there have been dramatic increase in the incidence of fungal infections, which are probably the result of alterations in immune status associated with the acquired immuno deficiency syndrome epidemic, cancer chemotherapy, and organ and bone marrow transplantation. The rise in the incidence of fungal infections has exacerbated the need for the next generation of antifungal agents, since many of the currently available drugs have undesirable side effects, are ineffective against new or reemerging fungi, or lead to the rapid development of the resistance. This review will focus on the pathogenic yeast Candida albicans, since a large body of work on the factors and mechanism associated with antifungal drug resistance in this organism is reported sufficiently. It will certainly elaborate the probable molecular targets for drug design, discovered to date.

Molecular organization of the cell wall of Candida albicans and its relation to pathogenicity

Candida albicans is one of the most important opportunistic pathogenic fungi. Weakening of the defense mechanisms of the host, and the ability of the microorganism to adapt to the environment prevailing in the host tissues, turn the fungus from a rather harmless saprophyte into an aggressive pathogen. The disease, candidiasis, ranges from light superficial infections to deep processes that endanger the life of the patient. In the establishment of the pathogenic process, the cell wall of C. albicans (as in other pathogenic fungi) plays an important role. It is the outer structure that protects the fungus from the host defense mechanisms and initiates the direct contact with the host cells by adhering to their surface. The wall also contains important antigens and other compounds that affect the homeostatic equilibrium of the host in favor of the parasite. In this review, we discuss our present knowledge of the structure of the cell wall of C. albicans, the synthesis of its different components, and the mechanisms involved in their organization to give rise to a coherent composite. Furthermore, special emphasis has been placed on two further aspects: how the composition and structure of C. albicans cell wall compare with those from other fungi, and establishing the role of some specific wall components in pathogenesis. From the data presented here, it becomes clear that the composition, structure and synthesis of the cell wall of C. albicans display both subtle and important differences with the wall of different saprophytic fungi, and that some of these differences are of utmost importance for its pathogenic behavior.

Differentiation of Candida albicans and Candida dubliniensis by using recombinant human antibody single-chain variable fragments specific for hyphae

To identify antigens specific for the filamentous form of Candida albicans, a combinatorial phage display library expressing human immunoglobulin heavy and light chain variable regions was used to select phage clones capable of binding to the surfaces of viable C. albicans filaments. Eight distinct phage clones that bound specifically to filament surface antigens not expressed on blastoconidia were identified. Single-chain antibody variable fragments (scFv) derived from two of these phage clones (scFv5 and scFv12) were characterized in detail. Filament-specific antigen expression was detected by an indirect immunofluorescence assay. ScFv5 reacted with C. dubliniensis filaments, while scFv12 did not. Neither scFv reacted with C. glabrata, C. parapsilosis, C. rugosa, C. tropicalis, or Saccharomyces cerevisiae grown under conditions that stimulated filament formation in C. albicans and C. dubliniensis. Epitope detection by the two scFv was sensitive to proteinase K treatment but not to periodate treatment, indicating that the cognate epitopes were composed of protein. The antigens reactive with scFv5 and scFv12 were extractable from the cell surface with Zymolyase, but not with sodium dodecyl sulfate (SDS) and 2-mercaptoethanol, and migrated as polydisperse, high-molecular-weight bands on SDS-polyacrylamide gel electrophoresis gels. The epitopes were detected on clinical specimens obtained from infants with thrush and urinary candidiasis without passage of the organisms on laboratory media, confirming epitope expression in human infection. The availability of a monoclonal immunologic reagent that recognizes filaments from both C. albicans and C. dubliniensis and another specific only to C. albicans adds to the repertoire of potential diagnostic reagents for differentiation between these closely related species.

Expression cloning of the Candida albicans CSA1 gene encoding a mycelial surface antigen by sorting of Saccharomyces cerevisiae transformants with monoclonal antibody-coated magnetic beads

The mycelial surface antigen recognized by monoclonal antibody (mAb) 4E1 has previously been shown to be present predominantly in the terminal third of the hyphal structures in Candida albicans. We report here the expression cloning of the corresponding gene (CSA1 ) by mAb 4E1-coated magnetic beads sorting of Saccharomyces cerevisiae transformants expressing a C. albicans genomic library. The strategy is both highly selective and highly sensitive and provides an additional genetic tool for the cloning and characterization of C. albicans genes encoding surface proteins. CSA1 is an intronless gene encoding a 1203-residue protein composed of repetitive motifs and domains. Northern analysis indicates that CSA1 is preferentially expressed during the mycelial growth phase, although a low level of CSA1 mRNA can be detected in the yeast form. As evidenced by indirect immunofluorescence microscopy with mAb 4E1, Csa1p is not randomly distributed over the surface of yeast cells, but localizes predominantly in the growing buds. This suggests that the distribution of Csa1p may be restricted to sites of cell surface elongation. Both heterozygous and homozygous C. albicans csa1Delta mutants are viable. Upon induction of mycelial growth, the number and size of hyphal structures derived from the mutants are similar to those observed in the parental wild-type strain. The physiological role of Csa1p has yet to be determined. However, the presence in Csa1p of repeated cysteine-rich hydrophobic domains with significant sequence similarity to motifs found in surface proteins (Ag2 and Pth11) from two distantly related fungal pathogens (Coccidioides immitis and Magnaporthe grisea respectively) suggests a common function in host interaction.

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.

Detection of antibodies to Candida albicans germ tubes during experimental infections by different Candida species

Identification and characterization of Candida albicans germ tube-specific antigens may be of relevance for the serodiagnosis of invasive candidiasis since they could be the basis for the development of new diagnostic tests. In this study, we have identified two antigens of 180 and >200 kDa in the cell wall of C. albicans germ tubes which are responsible for the induction of antibodies to C. albicans germ tubes. Antigens of similar molecular masses have been demonstrated in the cell walls of the Candida species C. stellatoidea, C. parapsilosis, C. guilliermondii, C. tropicalis, and C. krusei, but not C. glabrata. The kinetics of the antibody responses to C. albicans germ tubes were studied in rabbits infected with different Candida species. Although these antibodies were detected in rabbits infected with all Candida species except C. glabrata, the kinetics of the antibody responses to C. albicans germ tubes induced by the Candida species studied were different. Both the highest titer and the earliest response of antibodies to C. albicans germ tubes were observed in rabbits infected with either of the two serotypes of C. albicans used. However, the time needed to elicit the antibodies to C. albicans germ tubes can be reduced as the result of an anamnestic antibody response. The results presented in this study show that a test designed to detect antibodies against C. albicans germ tube antigens may be suitable for the diagnosis of infections caused by most of the medically important Candida species.

Cell wall and secreted proteins of Candida albicans: identification, function, and expression

The cell wall is essential to nearly every aspect of the biology and pathogenicity of Candida albicans. Although it was initially considered an almost inert cellular structure that protected the protoplast against osmotic offense, more recent studies have demonstrated that it is a dynamic organelle. The major components of the cell wall are glucan and chitin, which are associated with structural rigidity, and mannoproteins. The protein component, including both mannoprotein and nonmannoproteins, comprises some 40 or more moieties. Wall proteins may differ in their expression, secretion, or topological location within the wall structure. Proteins may be modified by glycosylation (primarily addition of mannose residues), phosphorylation, and ubiquitination. Among the secreted enzymes are those that are postulated to have substrates within the cell wall and those that find substrates in the extracellular environment. Cell wall proteins have been implicated in adhesion to host tissues and ligands. Fibrinogen, complement fragments, and several extracellular matrix components are among the host proteins bound by cell wall proteins. Proteins related to the hsp70 and hsp90 families of conserved stress proteins and some glycolytic enzyme proteins are also found in the cell wall, apparently as bona fide components. In addition, the expression of some proteins is associated with the morphological growth form of the fungus and may play a role in morphogenesis. Finally, surface mannoproteins are strong immunogens that trigger and modulate the host immune response during candidiasis.

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.

Cloning and characterization of PRA1, a gene encoding a novel pH-regulated antigen of Candida albicans

Candida albicans is an opportunistic fungal pathogen of humans. The cell wall of the organism defines the interface between the pathogen and host tissues and is likely to play an essential and pivotal role in the host-pathogen interaction. The components of the cell wall critical to this interaction are undefined. Immunoscreening of a lambda expression library with sera raised against mycelial cell walls of C. albicans was used to identify genes encoding cell surface proteins. One of the positive clones represented a candidal gene that was differentially expressed in response to changes in the pH of the culture medium. Maximal expression occurred at neutral pH, with no expression detected below pH 6.0. On the basis of the expression pattern, the corresponding gene was designated PRA1, for pH-regulated antigen. The protein predicted from the nucleotide sequence was 299 amino acids long with motifs characteristic of secreted glycoproteins. The predicted surface localization and N glycosylation of the protein were directly demonstrated by cell fractionation and immunoblot analysis. Deletion of the gene imparted a temperature-dependent defect in hypha formation, indicating a role in morphogenesis. The PRA1 protein was homologous to surface antigens of Aspergillus spp. which react with serum from aspergillosis patients, suggesting that the PRA1 protein may have a role in the host-parasite interaction during candidal infection.

Cloning and characterization of CSP37, a novel gene encoding a putative membrane protein of Candida albicans

In the course of an analysis of the functions and assembly of the cell wall of Candida albicans, we have cloned and characterized a gene, which we designated CSP37 (cell surface protein), encoding a 37-kDa polypeptide which is a membrane-associated protein. The gene was isolated by immunological screening of a DNA library constructed from mycelial cells with a polyclonal serum raised against cell walls of this morphology. Analysis of the nucleotide sequence of a corresponding genomic DNA fragment revealed a single open reading frame which encodes a predicted protein of 321 amino acids with no significant homology to others in the databases. Disruption of the CSP37 gene by the method described by Fonzi and Irwin (Genetics 134:717-728, 1993) eliminated expression of the Csp37 protein. The mutant strains showed no apparent defect in cell viability, growth, or cell wall assembly but displayed attenuated virulence in systemic infections induced in mice and reduced the ability to adhere to polystyrene.

The Candida albicans HYR1 gene, which is activated in response to hyphal development, belongs to a gene family encoding yeast cell wall proteins

A hyphally regulated gene (HYR1) from the dimorphic human pathogenic fungus Candida albicans was isolated and characterized. Northern (RNA) analyses showed that the HYR1 mRNA was induced specifically in response to hyphal development when morphogenesis was stimulated by serum addition and temperature elevation, increases in both culture pH and temperature, or N-acetylglucosamine addition. The HYR1 gene sequence revealed a 937-codon open reading frame capable of encoding a protein with an N-terminal signal sequence, a C-terminal glycosylphosphatidylinositol-anchoring domain, 17 potential N glycosylation sites, and a large domain rich in serine and threonine (51% of 230 residues). These features are observed in many yeast cell wall proteins, but no homologs are present in the databases. In addition, Hyr1p contained a second domain rich in glycine, serine, and asparagine (79% of 239 residues). The HYR1 locus in C. albicans CAI4 was disrupted by "Ura-blasting," but the resulting homozygous delta hyr1/delta hyr1 null mutant displayed no obvious morphological phenotype. The growth rates for yeast cells and hyphae and the kinetics of germ tube formation in the null mutant were unaffected. Aberrant expression of HYR1 in yeast cells, when an ADH1-HYR1 fusion was used, did not stimulate hyphal formation in C. albicans or pseudohyphal growth in Saccharomyces cerevisiae. HYR1 appears to encode a nonessential component of the hyphal cell wall.

Dynamic expression of cell-surface antigens probed with Candida albicans-specific monoclonal antibodies

IgG hybridomas were produced with preferentially reacted with cell-surface antigens of either yeast cells or hyphae of Candida albicans. Four mAbs were used in an immunostaining procedure to follow the expression dynamics of these antigens in media supplemented with glucose or galactose. Yeast cell growth was analysed during the lag phase, the early- and late-exponential phases and the stationary phase, and mycelium formation was analysed between 0.5 and 24 h induction at 37 degrees C. It appears that yeast cell-surface antigens 5C11 and 2E11 are expressed throughout all phases of yeast cell growth as well as on young hyphae after up to 1 h induction. Longer hyphae only faintly react with these two mAbs as they switch to hyphal cell-surface antigens 2G8 and 4E1 after 3 h induction. The reactivity to mAbs 2G8 and 4E1 was induced after a 3 h temperature shift and was confined to the terminal third of growing mycelia. Growth and hyphae induction in galactose prolonged the reactivity of young hyphae with the two anti-yeast-cell mAbs, whereas the expression of surface antigens 2G8 and 2E11 appeared delayed and desynchronized on hyphae. Whereas a similar reactivity was found with ten ATCC strains of C. albicans, four clinical isolates had a unique pattern of reactivity. Immunoblot analyses of DTT extracts of cell-surface constituents indicated that the antigens were proteinaceous in nature and showed that yeast-cell antigens 5C11 and 2E11 are detected in four bands between 68 and 104 kDa, whereas mycelial antigens 4E1 and 2G8 are detected in 117 kDa and 104 kDa bands found in mycelial but not in yeast-cell extracts. Present data support the concept of a dynamic balance in the expression of phase-specific antigens in C. albicans.

Re-expression by Candida albicans germ tubes of antigens lost during subculture of blastospores

The effect of germ tube induction on the antigenic variability in C.albicans was studied in strains from blood cultures (Group I) and superficial candidiasis (Group II). When compared by immunoblotting with a rabbit antiserum, antigenic extracts from Group I strains grown as blastospores showed a higher reactivity than that of Group II strains. Major bands in Group I strains (45-47, 33, 30 kDa) were continuously expressed through the subcultures in vitro but, with the exception of the 45 kDa band, the reactivity of all of them decreased or disappeared after the tenth subculture in Group II strains. The induction of the germ tubes produced the re-expression of the antigens lost during subculture in the yeast form, the effect being very clear in Group II strains. The re-expression by C. albicans germ tubes of antigens lost during subculture of blastospores in vitro and the higher reactivity shown by Group I strains grown in mycelial phase should be taken into consideration when a test to detect anti-C. albicans antibodies is to be developed.

Common and form-specific cell wall antigens of Candida albicans as released by chemical and enzymatic treatments

In order to investigate the antigenic properties of the proteins and mannoproteins present in the cell surface of Candida albicans, and to identify individual antigenic moieties and their distribution, a number of polyclonal antisera were obtained by immunizing rabbits with chemical and enzymatic cell wall extracts obtained from intact cells from both growth forms (yeast and mycelium) of the fungus. Prior to injection, wall moieties present in the extracts were subjected to different treatments and/or purification procedures such as adsorption onto polystyrenelatex microbeads or electrophoretic separation. When used as probes in indirect immunofluorescence assays, the different antisera gave rise to different fluorescence patterns varying in intensity and topological localization of the reactivity in C. albicans cells. When the different antisera were used as probes in Western blots of wall proteinaceous materials solubilized from both blastospores and germ tubes, differences in reactivity and specificity were readily discernible, allowing to identify a number of common and form-specific cell wall components. Of special interest was the fact that one of the antisera raised, after adsorption onto heat-killed blastospores, specifically recognized medium to low molecular weight moieties present only in the cell wall extracts from germ tubes. When this antiserum was used as probe in immunofluorescence assays, reactivity was confined to the hyphal extensions. Together, these observations seem to indicate that the different antibody preparations described in this report could represent important tools in the study of different aspects of the cell wall biology in C. albicans, including the identification and study of the distribution of common and form-specific cell wall-bound antigens.

A comparative study on cell wall antigens and cell surface hydrophobicity in clinical isolates of Candida albicans

Characterization of common cell surface-bound antigens in Candida albicans strains, particularly those expressed in the walls of mycelial cells might be useful in the diagnosis of systemic candidiasis. Hence, antigenic similarities among wall proteins and mannoproteins from C. albicans clinical serotype A and B isolates, were studied using polyclonal (mPAbs) and monoclonal (MAb 4C12) antibodies raised against wall antigens from the mycelial form of a common C. albicans serotype A laboratory strain (ATCC 26555). Zymolyase digestion of walls isolated from cells of the different strains studied grown at 37 degrees C (germination conditions), released, in all cases, numerous protein and mannoprotein components larger than 100 kDa, along with a 33-34 kDa species. The pattern of major antigens exhibiting reactivity towards the mPAbs preparation was basically similar for all the serotype A and B isolates, though minor strain-specific bands were also observed. The immunodeterminant recognized by MAb 4C12 was found to be absent or present in very low amounts in C. albicans isolates other than the ATCC 26555 strain, yet high molecular weight species similar in size (e.g., 260 kDa) to the wall antigen against which MAb 4C12 was raised, were observed, particularly in wall digests from serotype A strains. Cell surface hydrophobicity, an apparently important virulence factor in C. albicans, of the cell population of each serotype B strain was lower than that of the corresponding serotype A counterparts, which is possibly due to the fact that the former strains exhibited a reduced ability to form mycelial filaments under the experimental conditions used.

Analysis of mannoproteins from blastoconidia and hyphae of Candida albicans with a common epitope recognized by anti-complement receptor type 2 antibodies

Mannoproteins of approximately 50 kDa from blastoconidia and 60 kDa from hyphae of Candida albicans reacted in Western blots (immunoblots) with either a polyclonal rabbit antiserum (CA-7) or a monoclonal antibody (CA-A) to the C. albicans C3d-binding protein (complement receptor type 2). The glycosylated nature of these proteins was demonstrated by their reactivity with concanavalin A and by selective labeling with the biotin-hydrazide reagent following periodate oxidation. Differences in the oligosaccharides of these proteins were observed in regard to their reactivity with lectin-peroxidase reagents and sensitivity to glycosidases such as N-glycanase or endoglycosidase F (but not endoglycosidase H). The 60-kDa mannoprotein reacted with wheat germ agglutinin, while the 50-kDa mannoprotein did not. Treatment of the 60-kDa mannoprotein with the glycosidases mentioned above resulted in its conversion into a species of 40 to 45 kDa. Enzyme treatment had no obvious effect on the electrophoretic mobility of the 50-kDa species from blastoconidia. Both the 50- and 60-kDa glycoproteins remained immunoreactive after treatment with the glycosidases. Reactivities of the two mannoproteins to neuraminidase also differed. Finally, the 50-kDa (blastoconidia) and the 60-kDa (hyphae) mannoproteins were purified by using ion-exchange chromatography and electroelution. The purified proteins differed in net charge, the 60-kDa species having a more acidic pI. Functional activity of the purified mannoproteins was demonstrated, as each inhibited the rosetting of antibody-sensitized sheep erythrocytes conjugated with iC3b or C3d by hyphae. Thus, an epitope(s) common to both a mycelial and blastoconidial mannoprotein is associated with a structurally different oligosaccharide for each growth form.

Characterization of Candida albicans cell wall antigens with monoclonal antibodies

The antigenic composition of Candida albicans is very complex. In order to study the antigenic relationship between blastoconidia and germ tubes of C. albicans, we produced several monoclonal antibodies and analyzed their reactivity against cell wall antigens either in intact cells or in cells treated with dithiothreitol. Overall, four types of reactivity were found. Monoclonal antibodies 3D9 and 15C9 stained the germ tubes only when tested by indirect immunofluorescence. However, they showed a different reactivity by immunoblotting. Monoclonal antibody 3D9 reacted with antigens with molecular masses of > 200 and 180 kDa specifically expressed in the germ tube. Monoclonal antibody 15C9 reacted with antigens of 87, 50, and 34 kDa present in the germ tube extract and with antigens of 92, 50, 34, and 32 kDa present in the blastoconidium extract. The reactivity of blastoconidia treated for different times with dithiothreitol with these monoclonal antibodies was also studied by enzyme-linked immunosorbent assay. The reactivity of monoclonal antibody 3D9 did not significantly change during the cell wall extraction. However, the reactivity of monoclonal antibody 15C9 was increased for blastoconidia extracted for 60 min and decreased markedly for blastocondia extracted for 120 min. Monoclonal antibody G3B was nonreactive by indirect immunofluoresence but reacted with antigens of 47 and 38 kDa present in the germ tube extract and with an antigen of 47 kDa present in the blastoconidium extract. Monoclonal antibody B9E stained both morphological phases by indirect immunofluorescence. By immunoblotting, it reacted with antigens of > 70 kDa present in the germ tube extract and with antigens of > 63, 56, 47, and 38 kDa present in the blastoconidium extract. Based on the results presented in this study, four types of antigens are described. Type I antigens are expressed on the outermost layers of the germ tube cell wall only. Type II antigens are expressed both on the germ tube cell wall surface and within the blastoconidium cell wall. Type III antigens are found within the cell wall of both blastoconidia and germ tubes. Type IV antigens are expressed on both the blastoconidium and germ tube surface. Two types more can be hypothesized for antigens expressed on the blastoconidium cell surface and within the germ tube cell wall (type V) and for those expressed on the blastoconidium surface only (type VI).

Cloning and characterization of ECE1, a gene expressed in association with cell elongation of the dimorphic pathogen Candida albicans

The gene ECE1 (extent of cell elongation 1) was isolated by differential hybridization screening of a Candida albicans cDNA library by using probes derived from populations of yeast cells or hyphae. Expression of this gene was not detected when C. albicans grew as a budding yeast cell but was observed within 30 min after cells had been induced to form hyphae. In all strains tested, regardless of the induction signal, ECE1 expression correlated with the extent of cell elongation. The genomic version of ECE1 was cloned and sequenced. The deduced 271-amino-acid polypeptide consisted of eight tandem repeats of a degenerate 34-amino-acid sequence which contained no discernible homology with other known sequences. An ECE1 null mutant displayed no morphological alterations, which demonstrated that ECE1 is not essential for cell elongation or hypha formation despite the strict morphological association of its expression.

Characterization of mutant strains of Candida albicans deficient in expression of a surface determinant

Monoclonal antibody (MAb) 17E4 reacts with a surface carbohydrate determinant and agglutinates cells of Candida albicans. Using this MAb, we have isolated N-methyl-N'-nitro-N-nitrosoguanidine-induced nonagglutinating mutants. Eleven of these were characterized for the presence and expression of the surface antigen recognized by MAb 17E4 by immunoblot analysis of whole cells and by fluorescence flow cytometry. Soluble cell wall extracts from five mutant strains were negative by immunoblot analysis. The reactivities of the strains with several other MAbs and commercial antisera (Candida Check; Iatron Laboratories, Tokyo, Japan) which also recognize carbohydrate determinants were examined by immunoblot analysis of whole cells. Mutant strains showed no or reduced expression of the MAb 17E4 antigen and could be placed into at least two distinct phenotypic classes. Recognition by the other MAbs tested showed a similar pattern, while recognition by the commercial antisera was unchanged in the mutant strains. 1H and 13C nuclear magnetic resonance spectral analysis of mannan prepared from the wild type and nonexpressing mutant-strain 4A showed that the spectra from the mutant strain were simpler than those of the wild type. Most of the beta-1,2 linkages and all of the C-1 phosphate linkages were absent in the 4A mannan spectra, which suggested that the mutant mannan lacked the phosphate-bound beta-1,2-linked mannooligosaccharides. The effect of the surface defect on the ability of mutant strain 4A to adhere and to invade host tissue was examined in two murine models. In ex vivo binding assays, strain 4A showed reduced binding to the marginal zone and increased binding to the white pulp of splenic tissue, decreased binding to kidney tissue, and no change in binding to liver tissue compared with the wild type. In vivo, no difference was observed in translocation of the wild type or strain 4A to liver following immuno-compromising treatment of infant mice which had been challenged with either strain by the oral-intragastric route.

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.

Hydrophobic surface protein masking by the opportunistic fungal pathogen Candida albicans

Ultrastructural and biochemical analyses of hydrophobic and hydrophilic yeast cell surface proteins of Candida albicans were performed. Hydrophobic and hydrophilic yeast cells were obtained by growth at 23 and 37 degrees C, respectively. In addition, hydrophilic yeast cells were converted to surface hydrophobicity by treatment with tunicamycin and dithiothreitol. When freeze-etched cells were examined, the temperature-induced hydrophilic cells had long (0.198 micron), compact, evenly distributed fibrils while temperature-induced hydrophobic cells had short (0.085 micron), blunt fibrils. Hydrophobic microsphere attachment to the hydrophobic cells occurred at the basement of and within the short fibril layer. Dithiothreitol-induced hydrophobic cells had the long fibrils removed; tunicamycin-induced hydrophobic cells retained some of the long fibrils, but the fibrils were less compact and more aggregated than the untreated controls. These results suggest that the long fibrils prevent hydrophobic microsphere attachment to the hydrophobic area of the cell surface. This was confirmed by assessing the hydrophobic avidity of hydrophobic yeast cell populations differing in fibril density and arrangement. 125I-labelled surface proteins from hydrophobic and hydrophilic cells were compared after separation by hydrophobic interaction chromatography-high-performance liquid chromatography and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The yeast cell populations had hydrophilic proteins of similar molecular masses (greater than 200 kDa), but the hydrophilic cells possessed at least two additional proteins (ca. 63 and 69 to 71 kDa). Hydrophobic surface proteins appeared to be similar. However, the amount of total radiolabelled hydrophobic proteins was approximately 10-fold higher for the hydrophobic cells than for the hydrophilic cells. This result agrees with the ultrastructural observations which showed that yeast cell surface hydrophobic proteins are masked by hydrophilic high-molecular-mass surface fibrils. Taken together, the data indicate that yeast cell hydrophobicity is not determined by differences in surface hydrophobic proteins but by the presence of hydrophilic, surface fibrils.

Characterization of cell wall proteins of yeast and hydrophobic mycelial cells of Candida albicans

Cell surface hydrophobicity (CSH) of blastoconidia and blastoconidia bearing germ tubes of Candida albicans ATCC 26555 was monitored by assessing attachment of polystyrene microspheres to the cell surface, and we found that mature hyphae were significantly hydrophobic. Treatment of intact cells with low concentrations of beta-glucanase (Zymolyase 20T) or proteases abolished or significantly reduced attachment of latex beads to hyphae. This effect paralleled an obvious reduction in CSH of the entire cell population, as measured by an aqueous-hydrocarbon biphasic partitioning assay. Analysis of the cell wall material released by Zymolyase and adsorbed on polystyrene microspheres indicated that germ tube-specific cell wall proteins and mannoproteins with apparent molecular masses of 20 to 67 kDa may be responsible for the hydrophobicity of hyphae. Zymolyase released from blastoconidia cell walls a different set of proteins and mannoproteins that were able to adsorb to polystyrene microbeads. Such molecular species might in turn be responsible for the CSH exhibited by blastoconidium populations as determined by the biphasic partitioning assay, although these probably hydrophobic components can be masked on the surface of blastoconidia, as the latter had no or very few latex microspheres attached to their surfaces. Treatment of cells of both C. albicans morphologies with 2-mercaptoethanol released qualitatively distinct species of polystyrene-adsorbed proteins and mannoproteins from yeast and mycelial cells. These observations suggested that hydrophobic proteins and mannoproteins that could be associated with CSH are bound to the cell wall structure through diverse types of linkages.

Adherence and receptor relationships of Candida albicans

The cell surface of Candida albicans is composed of a variety of polysaccharides such as glucan, chitin, and mannan. The first two components primarily provide structure, while the mannan, often covalently linked to protein, constitutes the major antigen of the organism. Mannoproteins also have enzymatic activity (acid protease) and ligand-receptor functions. The complement receptors of C. albicans appear to be mannoproteins that are required for the adherence of the organism to endothelial cells. This is certainly true of the CR3-like protein of C. albicans. Proof that the CR3 is the Candida receptor for endothelial cells is derived from two observations. First, mutants lacking CR3 activity are less adherent in vitro and, in fact, less virulent. Second, the ligand recognized by the CR3 receptor (C3bi) as well as anti-CR3 antibodies blocks adherence of the organism to endothelial cells. The CR2 of C. albicans appears to promote the adherence of the organism to plastic substrates. Unlike the CR2 of mammalian cells, the Candida CR2 recognizes ligands containing the RGD sequence of amino acids in addition to the C3d ligand, which does not contain the RGD sequence. There is uncertainty as to whether the Candida CR2 and CR3 are, in fact, different proteins. A mannoprotein has also been described as the adhesin for epithelial cells. In this case, the receptor has a lectinlike activity and recognizes fucose- or glucosamine-containing glycoproteins of epithelial cells, depending on the strain of C. albicans. The oligosaccharide component of the receptor is probably not involved in ligand recognition and may serve to stabilize the receptor. However, the oligosaccharide factor 6 epitope of mannan may also provide adhesin activity in the recognition of epithelial cells. Mannoproteins can be extracted from cells by a number of reagents. Zymolyase, for instance, tends to remove structural mannoproteins, which contain relatively little protein and are linked to glucan. Reagents such as dithiothreitol, on the other hand, tend to extract mannoproteins containing higher amounts of protein that appear to have receptor function. The mannoproteins of C. albicans are dynamically expressed and may be growth phase and growth form specific.

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.

Fungal cell adhesion molecules in Candida albicans

Adherence of Candida albicans to host tissues is considered a crucial step in the pathogenesis of candidiasis. Using in vitro assays, it was demonstrated that the yeast-mycelium transition was an important phenomenon in the acquisition of adhesive properties. Proteins with MWs of 60, 68, 200 and greater than 200 kDa seemed to be involved in germ tube adherence to plastic surfaces. Likewise, recent investigations have revealed that C. albicans expresses on its surface receptors which interact with a wide variety of host proteins, particularly some extracellular matrix components like fibronectin, laminin and collagen. Plasmatic components, such as fibrinogen, iC3b and C3d, have also been proposed as mediators of adherence of C. albicans. Thus, by their reaction with laminin, fibrinogen and C3d, the mannoproteins of 68 and 60 kDa demonstrated multiple biological activities. Proteins of similar MWs were detected as C3d and iC3b receptors, the latter showing similarities with the neutrophil CR3. Based upon the antigenic, structural and functional homologies between the candidal receptors and mammalian integrins, it was postulated that these fungal cell adhesion molecules (F-CAM) are members of the integrin family. Interactions with host proteins and molecular mimicry of mammalian adhesion molecules may be a fertile area for further research.

Fab fragments from a monoclonal antibody against a germ tube mannoprotein block the yeast-to-mycelium transition in Candida albicans

Fab fragments prepared from the immunoglobulin G monoclonal antibody (MAb) 4C12, which reacts with a determinant expressed on the hyphal extension of germ tubes of Candida albicans, inhibited germ tube formation, but intact MAb 4C12 did not. Indirect immunofluorescence showed a punctate binding pattern on cells incubated with Fab fragments but a confluent binding on cells incubated with intact MAb 4C12.

Candida albicans strain delineation

Candida albicans is a major opportunistic pathogen causing a wide spectrum of disease in human beings. Methods for strain delineation of this species to assess or predict virulence or to conduct epidemiologic or pathogenetic investigations have been developed. Although factors associated with virulence have been identified, there is no rapid system to quantitate them in a clinical laboratory. Therefore, many typing methods are based on variable phenotypic characteristics within this species including morphotyping, serotyping, antibiogram, resistogram typing, biotyping, biotyping based on commercial carbon assimilation patterns, enzyme profiles, sensitivity to yeast killer toxins, and typing based on protein variability. Phenotypically defined strains generally do not correlate with the pathogenic potential of a strain with the exception of morphotyping. However, these methods can be useful in epidemiologic investigations; for example, they have revealed that most individuals harbor one strain and that infections are frequently due to an endogenous strain. Problems with these methods usually relate to their discriminatory power. When this is maximized, reproducibility (especially between laboratories) suffers. Recently, methods based on differences in DNA structure (genotyping) for strain delineation have been developed, including electrophoretic karyotyping and restriction enzyme fragment length polymorphisms. The development of a computer-assisted data bank and analysis for these genotypic strain delineators will open investigations into the pathogenesis of this infection and permit epidemiologic studies previously not possible with this important human pathogen.

Isolation and characterization of Candida albicans morphological mutants derepressed for the formation of filamentous hypha-type structures

Several Candida albicans morphological mutants were obtained by a procedure based on a combined treatment with nitrous acid plus UV irradiation and a double-enrichment step to increase the proportion of mutants growing as long filamentous structures. Altered cell morphogenesis in these mutants correlated with an altered colonial phenotype. Two of these mutants, C. albicans NEL102 and NEL103, were selected and characterized. Mutant blastoconidia initiated budding but eventually gave rise to filamentous hypha-type formations. These filaments were long and septate, and they branched very regularly at positions near septa. Calcofluor white (which is known to bind chitin-rich areas) stained septa, branching zones, and filament tips very intensely, as observed under the fluorescence microscope. Wild-type hybrids were obtained by fusing protoplasts of strain NEL102 with B14, another morphological mutant previously described as being permanently pseudomycelial, indicating that genetic determinants responsible for the two altered phenotypes are different. The mutants characterized in this work seemed to sequentially express the morphogenic characteristics of C. albicans, from blastoconidia to hyphae, in the absence of any inducer. Further characterization of these strains could be relevant to gain understanding of the genetic control of dimorphism in this species.

Caveats in the investigation of form-specific molecules of Candida albicans

Numerous reports purporting the existence of form-specific antigens of Candida albicans have been published, but it is generally unclear whether antigenic variability is an acceptable alternative interpretation. In this study, we used indirect immunofluorescence and immunogold electron microscopy to determine the distribution and form specificities of two antigens during yeast and hyphal growth in several defined and complex media. The results confirmed that antigen expression varies with length of incubation, nutrition, and serotype and indicate that the form specificities of antigens may be misinterpreted when conclusions are based exclusively on indirect immunofluorescence and extraction procedures. We therefore suggest that investigations be designed to include serotype A and B isolates grown in both complex and chemically defined media and that agglutination, immunofluorescence, or enzyme-linked immunosorbent assays on whole cells or cell extracts be used as presumptive tests. Confirmation of form-specific antigens should be done by appropriate immunoelectron microscopic evaluation.

Serological techniques for diagnosis of fungal infection

This review summarizes recent developments in the serodiagnosis of candidiasis, aspergillosis, cryptococcosis, histoplasmosis, blastomycosis, coccidioidomycosis, mucormycosis and sporotrichosis. A number of studies have substantiated the presence of circulating antigens in invasive candidiasis, invasive aspergillosis, disseminated histoplasmosis and coccidioidomycosis, and immunoassays for antigen detection provide moderate sensitivity but high specificity for disease. Improved detection may result mainly from repeated serum or concentrated urine samplings rather than from the development of more sensitive immunoassays. Immunoblot analysis of the serological response is a useful tool for the identification of immunogenic fungal components that elicit a specific antibody response in invasive disease. This method, and others, have been successfully applied to the study of the immune response to several fungi, including Candida, Aspergillus and Rhizopus.

Identification of wall-specific antigens synthesized during germ tube formation by Candida albicans

Walls of the two cellular forms (blastoconidia and mycelia) of Candida albicans ATCC 26555 were obtained from cells metabolically labeled (6-h pulse) with 14C-protein hydrolysate and [3H]threonine. Walls were purified by thorough washings with buffered and sodium dodecyl sulfate solutions and digested with Zymolyase 20T. The enzymatic treatment released four major high-molecular-weight mannoproteins (HMWM), with apparent molecular masses of 650, 500, 340, and 200 kilodaltons (HMWM-650, HMWM-500, HMWM-340, and HMWM-200, respectively), from yeast cells, whereas two high-molecular-mass mannoproteins (HMWM-260 and HMWM-180) were solubilized from mycelial cells. Some additional minor low-molecular-weight species were also detected in the enzymatic digests of walls from both types of cell. Single and dual pulse-chase experiments indicated that the HMWM-260 and HMWM-180 species reflect de novo synthesis of new proteins specific for the mycelia and do not represent a topological rearrangement of blastoconidium wall components. Monoclonal antibodies were raised against the HMWM-260 species (quantitatively the predominant component in the mycelial walls), and polyclonal rabbit antibodies were obtained against yeast or mycelial cell walls. Anti-mycelial cell wall polyclonal antibodies were adsorbed to whole killed blastoconidia to remove antibodies against common blastoconidium and mycelial wall antigens. Titration by enzyme-linked immunosorbent assay revealed that the monoclonal antibodies could recognize an epitope of the protein moiety of the HMWM-260 mannoprotein. Immunoblotting and immunofluorescence techniques using these monoclonal and polyclonal antibodies confirmed that the HMWM-260 and HMWM-180 species are specific components of the envelope of the mycelial cell walls.