Interactions between Candida albicans and the human extracellular matrix component tenascin-C

Fungal recognition by mammalian fibrinogen-related proteins

Fungi are ubiquitous eukaryotic micro-organisms present in virtually all environmental habitats. Although rarely pathogenic to the healthy population, many fungal species are capable of causing human disease in immunocompromised individuals. Thus, fungal infections remain a significant cause of morbidity and mortality, with rising prevalence accompanying the worldwide increase in immunosuppression-based therapies. Therefore, better understanding of the mutual interactions between the protective host mechanisms and the invading fungi remains of critical importance. The innate immune system constitutes the first line of defence against exogenous insults. The innate antifungal immunity is mediated through recognition of specific pathogen-associated molecular patterns (PAMPs) by a broad panel of host pattern recognition receptors (PRRs), responsible for mounting adequate protective responses. In this review, we describe fungal PAMPs as well as a selection of PRRs able to recognize them. We focus on the members of the fibrinogen-related domain (FReD) protein family that have been shown to recognize fungi-derived molecules: ficolins, fibrinogen C domain containing 1 (FIBCD1) and tenascin-C. We describe their structure, their binding targets and their established as well as putative biological functions related to fungal recognition and immunity.

Candida albicans cell wall proteins

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

A proteomic-based approach for the identification ofCandida albicans protein components present in a subunit vaccine that protects against disseminated candidiasis

Candidiasis has become a prevalent infection in different types of immunocompromised patients. The cell wall of Candida albicans plays important functions during the host-fungus interactions. Cell wall (surface) proteins of C. albicans are major elicitors of host immune responses during candidiasis, and represent candidates for vaccine development. Groups of mice were vaccinated subcutaneously with a beta-mercaptoethanol (beta-ME) extract from C. albicans containing cell wall proteins. Vaccinated mice were then infected with a lethal dose of C. albicans. Increased survival and decreased fungal burden were observed in vaccinated mice as compared to a control group, and 75% of vaccinated mice with the beta-ME extract survived this otherwise lethal infection. We used a proteomic approach (2-DE followed by immunoblotting) to demonstrate a complex polypeptidic pattern associated with the beta-ME extract used in the vaccine formulation and to detect immunogenic components recognized by antibodies in immune sera from vaccinated animals. Reactive protein spots were identified by MALDI-TOF-MS and searches in genomic databases. As a conclusion, vaccination strategies using C. albicans cell wall proteins induce protective responses. These antigens can be identified by proteomic approaches and may be used as components of subcellular vaccines against candidiasis.

Interaction of pathogenic fungi with host cells: Molecular and cellular approaches

This review provides an overview of several molecular and cellular approaches that are likely to supply insights into the host-fungus interaction. Fungi present intra- and/or extracellular host-parasite interfaces, the parasitism phenomenon being dependent on complementary surface molecules. The entry of the pathogen into the host cell is initiated by the fungus adhering to the cell surface, which generates an uptake signal that may induce its cytoplasmatic internalization. Furthermore, microbial pathogens use a variety of their surface molecules to bind to host extracellular matrix (ECM) components to establish an effective infection. On the other hand, integrins mediate the tight adhesion of cells to the ECM at sites referred to as focal adhesions and also play a role in cell signaling. The phosphorylation process is an important mechanism of cell signaling and regulation; it has been implicated recently in defense strategies against a variety of pathogens that alter host-signaling pathways in order to facilitate their invasion and survival within host cells. The study of signal transduction pathways in virulent fungi is especially important in view of their putative role in the regulation of pathogenicity. This review discusses fungal adherence, changes in cytoskeletal organization and signal transduction in relation to host-fungus interaction.

Immunofluorescence and flow cytometry analysis of fibronectin and laminin binding to Sporothrix schenckii yeast cells and conidia

The adherence of Sporothrix schenckii yeast cells to several extracellular matrix (ECM) components has already been demonstrated, but the mechanisms of these interactions remained to be defined. In indirect immunofluorescence assays with polyclonal antibodies directed towards the ECM proteins, both hyphae and yeast cells of S. schenckii exhibited the ability to bind laminin and fibronectin. Flow cytometry confirmed the binding of these proteins, and revealed a significant greater binding capability for the yeast cells than for the conidia. Fibronectin and laminin binding was dose-dependent and specific. In addition, competition experiments with synthetic peptides mimicking the adhesive sequences of these proteins, or with cell wall fractions and carbohydrates constitutive of their sugar chains, were performed in order to specify the peptide or carbohydrate motifs involved in the recognition process. A 50% reduction was noticed in fibronectin binding in the presence of the synthetic peptide RGD, and a 38% reduction in laminin binding with the peptide YIGSR. Some carbohydrate-containing fractions of the yeast cell wall also inhibited the binding of fibronectin, but had no significant effect on laminin binding. Together, these results suggest the presence at the yeast surface of distinct receptors for laminin and fibronectin.

Chaperonin 60 unfolds its secrets of cellular communication

The cell biology of the chaperonins (Cpns) has been intensively studied over the past 25 years. These ubiquitous and essential molecules assist proteins to fold into their native state and function to protect proteins from denaturation after stress. The structure of the most widely studied Cpn60, Escherichia coli GroEL, has been solved and its mechanism of protein folding action largely established. But in the last decade, evidence has accumulated to suggest that the Cpn60s have functions in addition to intracellular protein folding, particularly the ability to act as intercellular signals with a wide variety of biological effects. Cpn60 has the ability to stimulate cells to produce proinflammatory cytokines and other proteins involved in immunity and inflammation and may, therefore, provide a link between innate and adaptive immunity. Cpn60s are also thought to be pathogenic factors in a wide range of diseases and have recently been reported to be present in the circulation of normal subjects and those with heart disease. An interesting facet of these proteins is the finding that in spite of significant sequence conservation, individual Cpn60 proteins can express very different biological activities. This review discusses the work to date, which has revealed the cell-cell signaling actions of Cpn60 proteins.

Antibodies to alpha5beta1 and alpha(v)beta3 integrins react with Candida albicans alcohol dehydrogenase

It has been hypothesized that Candida albicans possesses integrin-like receptors on its cell surface. This is because C. albicans binds numerous fluid-phase extracellular matrix (ECM) proteins on its cell surface and adheres to the same ECM proteins when immobilized. In addition, numerous antibodies to human integrins (receptors for ECM proteins) bind to the fungal cell surface and in so doing inhibit the binding of the respective proteins. To demonstrate the presence of such a cell surface integrin, a cDNA library of C. albicans yeast cells was screened with polyclonal antiserum to the human fibronectin receptor (alpha5beta1 integrin). Clones isolated by this screening technique also reacted specifically to antiserum against the human vitronectin receptor (alpha(v)beta3 integrin). DNA sequence analysis of the cloned insert predicted a 350 aa protein (37 kDa). This predicted protein showed 75% homology at the nucleotide sequence level to alcohol dehydrogenase (ADH) of Saccharomyces cerevisiae. In vitro transcription/translation of the cloned inserts yielded a 37 kDa protein that was immunoprecipitated with antibodies to the alpha5beta1 and alpha(v)beta3 integrins and an antibody to a C. albicans fibronectin receptor. These antibodies and an mAb to the human vitronectin receptor demonstrated an antigen of -37 kDa present in the cell-wall preparations of C. albicans and in spent growth medium. All four antibodies reacted with authentic ADH. The possible significance of these results in relation to C. albicans adherence is discussed.

Involvement of fungal cell wall components in adhesion of Sporothrix schenckii to human fibronectin

Systemic sporotrichosis is an emerging infection potentially fatal for immunocompromised patients. Adhesion to extracellular matrix proteins is thought to play a crucial role in invasive fungal diseases. Here we report studies of the adhesion of Sporothrix schenckii to the extracellular protein fibronectin (Fn). Both yeast cells and conidia of S. schenckii were able to adhere to Fn as detected by enzyme-linked immunosorbent binding assays. Adhesion of yeast cells to Fn is dose dependent and saturable. S. schenckii adheres equally well to 40-kDa and 120-kDa Fn proteolytic fragments. While adhesion to Fn was increased by Ca(2+), inhibition assays demonstrated that it was not RGD dependent. A carbohydrate-containing cell wall neutral fraction blocked up to 30% of the observed adherence for the yeast cells. The biochemical nature of this fraction suggests the participation of cell surface glycoconjugates in binding by their carbohydrate or peptide moieties. These results provide new data concerning S. schenckii adhesion mechanisms, which could be important in host-fungus interactions and the establishment of sporotrichosis.