Arg-Gly-Asp (RGD) peptides alter hepatic killing of Candida albicans in the isolated perfused mouse liver model

Candida albicans-derived mannoproteins activate NF-κB in reporter cells expressing TLR4, MD2 and CD14

The ability of soluble C. albicans 20A (serotype A) mannoprotein (CMP) to serve as a ligand for toll-like receptor 4 (TLR4) and its co-receptors was examined using commercially available and stably-transfected HEK293 cells that express human TLR4, MD2 and CD14, but not MR. These TLR4 reporter cells also express an NF-κB-dependent, secreted embryonic alkaline phosphatase (SEAP) reporter gene. TLR4-reporter cells exhibited a dose-dependent SEAP response to both LPS and CMP, wherein peak activation was achieved after stimulation with 40–50 μg/mL of CMP. Incubation on polymyxin B resin had no effect on CMP’s ligand activity, but neutralized LPS-spiked controls. HEK293 Null cells lacking TLR4 and possessing the same SEAP reporter failed to respond to LPS or CMP, but produced SEAP when activated with TNFα. Reporter cell NF-κB responses were accompanied by transcription of IL-8, TNFα, and COX-2 genes. Celecoxib inhibited LPS-, CMP-, and TNFα-dependent NF-κB responses; whereas, indomethacin had limited effect on LPS and CMP responses. SEAP production in response to C. albicans A9 mnn4Δ mutant CMP, lacking phosphomannosylations on N-linked glycans, was significantly greater (p ≤ 0.005) than SEAP responses to CMP derived from parental A9 (both serotype B). These data confirm that engineered human cells expressing TLR4, MD2 and CD14 can respond to CMP with NF-κB activation and the response can be influenced by variations in CMP-mannosylation. Future characterizations of CMPs from other sources and their application in this model may provide further insight into variations observed with TLR4 dependent innate immune responses targeting different C. albicans strains.

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.

Recognition of endothelial cells byCandida albicans: role of complement-binding proteins

Candida albicans, a commensal of humans, can cause either mucosal or systemic infections. The virulence properties of the organism include cell-surface adhesins that recognize ligands of host cells. Hyphal forms of the organism possess a 60-kDa mannoprotein that recognizes a variety of host-cell ligands including the complement C3 conversion products, C3bi and C3d. In addition, a protein of similar molecular mass also binds to endothelial extracellular matrix proteins such as laminin and fibronectin. While the 60-kDa protein is associated with the cell surface of hyphal forms of the organism, a protein of 50 kDa with similar ligand-binding activities is associated with the plasma membrane of blastoconidia. This protein cross reacts with antibodies to the 60-kDa protein. Isolation of the gene(s) encoding these cell-surface proteins is underway using both a human B-lymphocyte CR2 gene fragment or oligonucleotides based upon peptide sequence to screen libraries of C. albicans. Mutants of the organism with reduced expression of either C3d or C3bi-binding activity have been isolated. These strains are less virulent and also less adherent in vitro. Studies are currently underway to define the contribution of these proteins to the virulence of the organism. Key words: adherence, complement receptor, mannoprotein, virulence, ligand recognition.

Biomaterial-associated infection with Candida albicans in mice

Candida yeasts are frequently isolated from patients with continuous ambulatory peritoneal dialysis peritonitis or other biomaterial-associated infections. The mouse model of candidal peritonitis was used to study the interaction of Candida cells with end-point attached heparinized polyethylene (H-PE) and with polymorphonuclear leukocytes (PMNs) or macrophages (M phi). Two Candida strains differing in cell surface hydrophobicity and in expression of fibronectin (Fn) binding were used for the study. Cells of both Candida strains adhered at higher numbers to H-PE surfaces preadsorbed with Fn or with human dialysis fluid (HDF) than to non-modified H-PE, supporting a role of Fn in mediating adhesion. C. albicans 4016 cells expressing low hydrophobicity and low binding of soluble Fn demonstrated stronger adhesion to PMNs than the more hydrophobic C. albicans 3248 yeasts, which express high binding of soluble Fn. However, C. albicans 4016 cells were more resistant to phagocytic killing and were hardly eradicated in intraperitoneally infected mice. The animals depleted in PMNs by treatment with CY were neither able to eradicate C. albicans 3248 (rapidly eliminated by normal mice) nor C. albicans 4016 yeasts (with a tendency to persist in the tissues of normal mice).

Adherence of Candida albicans to human buccal epithelial cells: host-induced protein synthesis and signaling events

The synthesis of proteins by Candida albicans was studied following adherence of blastoconidia to human buccal epithelial cells (HBEC). Initially, labeling of HBEC, C. albicans, and HBEC-C. albicans with [35S]methionine was performed. After a 3-h incubation and prior to labeling with [35S]methionine, the cultures were treated with cycloheximide to prevent HBEC protein synthesis. The HBEC-C. albicans mixture as well as C. albicans and HBEC incubated separately were extracted with beta-mercaptoethanol (beta-ME). These extracts as well as the cell residue (solubilized by boiling with sodium dodecyl sulfate [SDS]) were examined by SDS-polyacrylamide gel electrophoresis and autoradiography. In comparison to cultures of C. albicans incubated without HBEC, proteins with molecular masses of approximately 52 to 56 kDa from beta-ME extracts and from SDS-solubilized cells were observed only from adhering cultures. In addition, unlabeled beta-ME extracts were electrotransferred to nitrocellulose and immunoblotted with antiphosphotyrosine antibodies to determine whether cell signaling events were occurring during adherence. Proteins with molecular masses of 54 and 60 kDa were recognized only in mixed cultures of C. albicans and HBEC. These data indicate that following adherence of C. albicans to HBEC, new Candida proteins are expressed. Further, these events are accompanied by the expression of signal proteins, presumably of Candida origin.

Vitronectin interacts with Candida albicans and augments organism attachment to the NR8383 macrophage cell line

Candida albicans is an increasingly important cause of mucocutaneous and bloodstream infections. The potential role of circulating adhesive glycoproteins such as vitronectin (Vn) in host defense against C. albicans is currently unknown. Accordingly, we investigated the binding of plasma-derived Vn with C. albicans strain 36082. Vn specifically bound to C. albicans in a concentration-dependent fashion. Higher affinity binding sites numbered 9.8 x 10(4) sites per organism, with a dissociation constant, Kd of 3.5 x 10(-7) M. Vn binding with C. albicans was significantly inhibited by heparin, suggesting interaction of the organism with Vn's glycosaminoglycan-binding region. To further determine which molecule(s) on the fungus interacted with Vn, C. albicans components were extracted, separated by SDS and blotted with radiolabeled Vn. These studies revealed that Vn binds to a 30 kDa molecule on C. albicans. Finally, we investigated the role of Vn in promoting interaction of C. albicans with phagocytic cells. Incubation of C. albicans in the presence of Vn significantly increased binding of the organism to cultured NR8383 macrophages compared to incubations performed in the absence of Vn. These data demonstrate that C. albicans interacts with the heparin-binding domain Vn and further suggest that Vn augments organism uptake by phagocytic cells.

Plasma and extracellular matrix proteins mediate in the fate of Candida albicans in the human host

The fungus, Candida albicans, causes trivial to life-threatening diseases in man when normal host defenses are compromised. The fungus appears to have evolved receptors (hereinafter referred to as adhesins) for human fluid phase glycoproteins such as fibronectin and immobilized basement membrane glycoproteins in order to establish and maintain a niche in the mucus-lined cavities of man. The hypothesis advanced is that the fate of the fungus may be determined by interactions with these same glycoproteins. For example, Candida may adhere to fibronectin on the surface of epithelial cells in order to maintain its residency in mucus-lined cavities, whereas when the fungus has escaped its normal niche and become bloodborne, yeast cells may be opsonized by fluid phase fibronectin and hence phagocytosed and killed more rapidly than uncoated fungi. On the other hand, bloodborne yeast cells may preferentially adhere to immobilized fibronectin exposed in the interstitial space or contained within fibrin-platelet aggregates. Adherence to immobilized proteins would enhance the ability of the fungus to establish a foothold in the human host outside its normal niche, avoid destruction by host phagocytic cells and hence establish a metastatic site of infection. This sequence of events, viz., adherence followed by growth may be similar to that which occurs in the metastasis of cancer cells. Many cancer cells employ receptors for basement membrane glycoproteins in order to effect movement from one area of the body to another. The adhesins of Candida may be analogous or perhaps homologous to the human integrin receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adhesins and ligands involved in the interaction of Candida spp. with epithelial and endothelial surfaces

Adhesion of candidal species to the epithelium of the gastrointestinal or genitourinary tract stands as a critical first step in the pathogenesis of candidal infection. After colonization and replication at mucosal surfaces, Candida albicans and other pathogenic species may penetrate the mucosal barrier, enter the vascular tree, and disseminate hematogenously. The consequences of this pathogenic cascade evoke considerable morbidity and mortality, especially among immunocompromised patients. Thus, interactions of C. albicans and other candidal species with epithelium and endothelium may lead to serious consequences for the human host. This review evaluates candidate candidal adhesions for epithelial and endothelial surfaces, with emphasis on the specificity of the interaction, the inhibitors that have been employed, and the ligands that have been identified on mammalian cells or matrices. Three types of interactions are described: protein-protein interactions, lectin-like interactions, and incompletely defined interactions in which the adhesive ligand is as yet unidentified. Special attention is given to the roles of integrin-like proteins. Differences in the mechanisms of candidal attachment to epithelium and endothelium are delineated. Last, on the basis of the available literature, avenues of potentially fruitful investigation are proposed.

Mechanisms of adherence of Candida albicans to cultured human epidermal keratinocytes

We established an in vitro adherence model with primarily cultured human keratinocytes as target cells which allows for the investigation of the molecular mechanisms that are responsible for Candida albicans host cell attachment in the initiation of cutaneous candidosis. The extent of C. albicans binding to cultured human keratinocytes was dependent on the yeast inoculum size and the incubation temperature. Heat and paraform-aldehyde treatment of yeasts completely abolished the binding activity of C. albicans. Of the different Candida species tested, C. albicans was by far the most adhesive species. C. albicans adherence was blocked by the acid protease inhibitor pepstatin A and the metabolic inhibitor sodium azide. The latter, however, was much less effective when yeasts were preincubated, suggesting that sodium azide was mainly acting on the keratinocytes. The extracellular matrix protein fibronectin was slightly inhibitory, whereas the fibronectin-derived peptides RGD and RGDS were not able to prevent attachment. PepTite-2000, another RGD-containing synthetic peptide, reduced C. albicans adherence by a margin of 25% (P < 0.005). CDPGYIGSR-NH2, which is a synthetic adhesive peptide derived from the laminin B chain, was much more efficient in its inhibitory activity than the RGD peptides and reduced C. albicans adherence to cultured human keratinocytes up to 76% (P < 0.001). Laminin itself and the synthetic pentapeptide YIGSR were less active. A dose-dependent reduction in adherence was also observed with collagen type III. Additionally, saccharides were tested for their potential to inhibit C. albicans attachment to keratinocytes. The most potent competitive saccharide inhibitors of C. albicans adherence to human keratinocytes were the amino sugars D-(+)-glucosamine and D-(+)-galactosamine with one isolate of C. albicans (4918) and D-(+)-glucosamine and alpha-D-(+)-fucose with another C. albicans isolate (Sp-1). Collectively, our data suggest the existence of multiple molecular mechanisms such as protein-protein, lectin-carbohydrate, and yeast-yeast coaggregational interactions that are responsible for optimal C. albicans attachment to cultured human keratinocytes.

Nonculture methods for diagnosis of disseminated candidiasis

Two of the nonculture approaches to the diagnosis of DC, enzymatic-fluorometric determination of serum D-arabinitol and detection of marker antigens in antigenemia (enolase and CWMP), have been commercialized and have shown promise in limited clinical trials. These approaches are not new but are the culmination of efforts made over 10 or more years. Clearly, further fine-tuning of both metabolite and antigen detection is needed to simplify the methods and to improve their sensitivity and specificity so that they will be valuable in guiding clinical treatment decisions. An alternative approach, detection of DC by DNA amplification methods such as PCR, is a special case of a compelling technology and one that is capable of standardization across microbial genera. The availability of simplified PCR diagnostic methods for DC remains a tantalizing prospect. Nevertheless, the development of methods to release DNA from very small numbers of Candida organisms in the blood in a form that is sufficiently free of inhibitors of PCR will require further intensive effort. The maturation of these converging laboratory approaches to nonculture diagnosis of DC leads to more optimism about the eventual use of these methods in clinical laboratories.

Distinct mechanisms of epithelial adhesion for Candida albicans and Candida tropicalis. Identification of the participating ligands and development of inhibitory peptides

The yeast Candida albicans is the leading cause of disseminated fungal infection in neonates, immunocompromised hosts, diabetics, and postoperative patients; Candida tropicalis is the second most frequent isolate. Because the integrin analogue in C. albicans shares antigenic, structural, and functional homologies with the beta 2-integrin subunits alpha M and alpha X, we investigated the role of integrin analogues in epithelial adhesion of C. albicans and C. tropicalis. On flow cytometry with the monoclonal antibody (mAb) OKM1, surface fluorescence was highest for C. albicans and significantly reduced for C. tropicalis (P < 0.001). However, adhesion to the human epithelial cell line HeLa S3 did not differ for these two candidal species: specific adhesion was highest for C. albicans at 44.0 +/- 1.8%, and only slightly lower for C. tropicalis at 38.8 +/- 3.6% (P = NS). The disparity between expression of the integrin analogue and epithelial adhesion suggested distinct mechanisms for this process in C. albicans versus C. tropicalis. Preincubation of C. albicans with anti-alpha M mAbs, with purified iC3b (the RGD ligand for the integrin analogue), or with 9-15-mer RGD peptides from iC3b all inhibited epithelial adhesion significantly (P < 0.001-0.04). Purified fibronectin or fibronectin-RGD peptides failed to block C. albicans adhesion. In contrast, epithelial adhesion of C. tropicalis was significantly inhibited by purified fibronectin and its RGD peptides (P < or = 0.021), but not by iC3b nor the iC3b-RGD peptides. Both iC3b and fibronectin were identified on the surface of epithelial cells after growth in serum-free medium. A polyclonal antibody to C3 inhibited C. albicans adhesion while a control antibody to fibronectin was ineffective; the converse was true for C. tropicalis. These results indicate that the pathogenic yeasts C. albicans and C. tropicalis recognize distinct RGD ligands present at the surface of the epithelial cell and that these interactions can be differentially inhibited by defined RGD peptides containing appropriate flanking sequences.

Evidence that mannans of Candida albicans are responsible for adherence of yeast forms to spleen and lymph node tissue

We have described a unique binding system between Candida albicans yeast-form cells and the marginal zone of mouse spleen (16). The chemical nature of the fungal adhesin(s) involved in this binding phenomenon was examined. A fraction obtained by 2-mercaptoethanol extraction (2-ME extract) of fungal cells caused a dose-response inhibition of yeast cell adherence to splenic marginal zone sites and also to subcapsular and medullary sinuses of mouse popliteal lymph nodes. Latex beads coated with the 2-ME extract showed a pattern of spleen and lymph node tissue binding identical to that observed with yeast cells. The extracted adhesins retained their binding activity in vivo. When 0.5 mg of the 2-ME extract was given intravenously to mice, spleen tissue removed up to 3 h later showed over 80% inhibition of yeast cell binding to the spleen marginal zone, and over 50% inhibition was retained for at least 24 h. The adhesins bound to a concanavalin A affinity column and were eluted by 0.5 M alpha-methyl-D-mannopyranoside, and the eluted adhesins were designated Fr.II. Fr.II was further fractioned by DEAE-Sephacel ion-exchange column chromatography, and one especially active and abundant fraction was designated Fr.IIa. The adhesin moiety appeared to be carbohydrate, because the activity of Fr.IIa was destroyed by 20 mM sodium periodate or by 5 U of alpha-mannosidase, but boiling (30 min) or proteinase K (100 micrograms/ml) treatments had no effect. Chemically, whereas the 2-ME extract contained significant amounts of protein and mannose, Fr.IIa consisted of over 98% mannose and less than 0.5% protein. These data strongly suggest that the mannan portion within a mannoprotein is responsible for the binding of yeast cells to splenic marginal zone and to subcapsular and medullary sinuses of mouse lymph node tissue.

Mechanism of fibronectin enhancement of group B streptococcal phagocytosis by human neutrophils and culture-derived macrophages

In previous studies, we reported that fibronectin (FN) markedly enhances phagocytic uptake of antibody-coated group B streptococci (GBS) by human polymorphonuclear leukocytes. Furthermore, administration of FN along with a GBS type-specific monoclonal or polyclonal antibody to infected neonatal rats significantly enhances survival. In this study, we have examined the molecular mechanism of this enhancement through phagocyte receptors which recognize the Arg-Gly-Asp (RGD) peptide sequences contained within the FN molecule. Incubation of human polymorphonuclear leukocytes or culture-derived macrophages on coverslips coated with GRGDSP but not GRGESP markedly enhanced uptake of immunoglobulin G-coated GBS. The enhancing effect of the RGD-containing peptides was blocked by monoclonal antibodies B6H12 (directed against the integrin-associated protein) and 7G2 (directed against the beta 3-integrin receptor for RGD). These data suggest that FN enhancement of antibody-coated GBS uptake is mediated by the critical RGD sequence. Furthermore, this active peptide sequence may have an important role in immunotherapy of bacterial infections, especially in patients with decreased plasma FN concentrations.

Effect of an arginine-glycine-aspartic acid-containing peptide on hematogenous candidal infections in rabbits

The adherence of Candida albicans yeast cells to the subendothelial extracellular matrix, fibronectin, laminin, and type I and IV collagen was tested. Fibronectin (10(-7) M) and a peptide, PepTite-2000 (Telios Pharmaceuticals Inc., San Diego, Calif.), containing the sequence arginine-glycine-aspartic acid (RGD) inhibited Candida adherence to these targets by greater than 90%. When C. albicans was perfused over ex vivo rabbit aortic endothelium, there was no significant difference in the amount of adherence in the presence or absence of the RGD-containing peptide. However, the RGD-containing peptide reduced the number of Candida organisms present in liver, brain, heart, and kidneys (P less than 0.05) of rabbits 4 h after intravenous inoculation of 5 x 10(7) C. albicans yeast cells. The peptide also reduced the number of macroscopic Candida abscesses in the kidneys of rabbits 72 h after intravenous inoculation of 10(7) C. albicans yeast cells (P less than 0.05). Inhibition of Candida adherence in vitro and in vivo may occur because the peptide blocks a fungal receptor that is necessary for adherence.