The fungal pathogen Candida albicans has a multilayered cell wall composed of an outer layer of proteins glycosylated with N- or O-linked mannosyl residues and an inner skeletal layer of β-glucans and chitin. We demonstrate that cytokine production by human mononuclear cells or murine macrophages was markedly reduced when stimulated by C. albicans mutants defective in mannosylation. Recognition of mannosyl residues was mediated by mannose receptor binding to N-linked mannosyl residues and by TLR4 binding to O-linked mannosyl residues. Residual cytokine production was mediated by recognition of β-glucan by the dectin-1/TLR2 receptor complex. C. albicans mutants with a cell wall defective in mannosyl residues were less virulent in experimental disseminated candidiasis and elicited reduced cytokine production in vivo. We concluded that recognition of C. albicans by monocytes/macrophages is mediated by 3 recognition systems of differing importance, each of which senses specific layers of the C. albicans cell wall.

We have characterized CaNrg1 from Candida albicans , the major fungal pathogen in humans. CaNrg1 contains a zinc finger domain that is conserved in transcriptional regulators from fungi to humans. It is most closely related to ScNrg1, which represses transcription in a Tup1‐dependent fashion in Saccharomyces cerevisiae . Inactivation of CaNrg1 in C.albicans causes filamentous and invasive growth, derepresses hypha‐specific genes, increases sensitivity to some stresses and attenuates virulence. A tup1 mutant displays similar phenotypes. However, unlike tup1 cells, nrg1 cells can form normal hyphae, generate chlamydospores at normal rates and grow at 42°C. Transcript profiling of 2002 C.albicans genes reveals that CaNrg1 represses a subset of CaTup1‐regulated genes, which includes known hypha‐specific genes and other virulence factors. Most of these genes contain an Nrg1 response element (NRE) in their promoter. CaNrg1 interacts specifically with an NRE in vitro . Also, deletion of two NREs from the ALS8 promoter releases it from Nrg1‐mediated repression. Hence, CaNrg1 is a transcriptional repressor that appears to target CaTup1 to a distinct set of virulence‐related functions, including yeast–hypha morphogenesis.

Summary: Survival in blood and escape from blood vessels into tissues are essential steps for the yeast Candida albicans to cause systemic infections. To elucidate the influence of blood components on fungal growth, morphology and transcript profile during bloodstream infections, we exposed C. albicans to blood, blood fractions enriched in erythrocytes, polymorphonuclear or mononuclear leukocytes, blood depleted of neutrophils and plasma. C. albicans cells exposed to erythrocytes, mononuclear cells, plasma or blood lacking neutrophils were physiologically active and rapidly switched to filamentous growth. In contrast, the presence of neutrophils arrested C. albicans growth, enhanced the fungal response to overcome nitrogen and carbohydrate starvation, and induced the expression of a large number of genes involved in the oxidative stress response. In particular, SOD5, encoding a glycosylphosphatidylinositol (GPI)‐anchored superoxide dismutase localized on the cell surface of C. albicans, was strongly expressed in yeast cells that were associated with neutrophils. Mutants lacking key genes involved in oxidative stress, morphology or virulence had significantly reduced survival rates in blood and the neutrophil fraction, but remained viable for at least 1 h of incubation when exposed to erythrocytes, mononuclear cells, plasma or blood lacking neutrophils. These data suggest that C. albicans genes expressed in blood were predominantly induced in response to neutrophils, and that neutrophils play a key role during C. albicans bloodstream infections. However, C. albicans is equipped with several genes and transcriptional programmes, which may help the fungus to counteract the attack of neutrophils, to escape from the bloodstream and to cause systemic infections.

Discriminating between commensal and pathogenic states of opportunistic pathogens is critical for host mucosal defense and homeostasis. The opportunistic human fungal pathogen Candida albicans is also a constituent of the normal oral flora and grows either as yeasts or hyphae. We demonstrate that oral epithelial cells orchestrate an innate response to C. albicans via NF-κB and a biphasic MAPK response. Activation of NF-κB and the first MAPK phase, constituting c-Jun activation, is independent of morphology and due to fungal cell wall recognition. Activation of the second MAPK phase, constituting MKP1 and c-Fos activation, is dependent upon hypha formation and fungal burdens and correlates with proinflammatory responses. Such biphasic response may allow epithelial tissues to remain quiescent under low fungal burdens while responding specifically and strongly to damage-inducing hyphae when burdens increase. MAPK/MKP1/c-Fos activation may represent a "danger response" pathway that is critical for identifying and responding to the pathogenic switch of commensal microbes.

Recognition of microbial components by germ‐line encoded pattern recognition receptors (PRR) initiates immune responses to infectious agents. We and others have proposed that pairs or sets of PRR mediate host immunity. One such pair comprises the fungal β‐glucan receptor, Dectin‐1, which collaborates through an undefined mechanism with Toll‐like receptor 2 (TLR2) to induce optimal cytokine responses in macrophages. We show here that Dectin‐1 signaling through the spleen tyrosine kinase (Syk) pathway is required for this collaboration, which can also occur with TLR4, 5, 7 and 9. Deficiency of either Syk or the TLR adaptor MyD88 abolished collaborative responses, which include TNF, MIP‐1α and MIP‐2 production, and which are comparable to the previously described synergy between TLR2 and TLR4. Collaboration of the Syk and TLR/MyD88 pathways results in sustained degradation of the inhibitor of kB (IkB), enhancing NFkB nuclear translocation. These findings establish the first example of Syk‐ and MyD88‐coupled PRR collaboration, further supporting the concept that paired receptors collaborate to control infectious agents.

The survival of all microbes depends upon their ability to respond to environmental challenges. To establish infection, pathogens such as Candida albicans must mount effective stress responses to counter host defences while adapting to dynamic changes in nutrient status within host niches. Studies of C. albicans stress adaptation have generally been performed on glucose-grown cells, leaving the effects of alternative carbon sources upon stress resistance largely unexplored. We have shown that growth on alternative carbon sources, such as lactate, strongly influence the resistance of C. albicans to antifungal drugs, osmotic and cell wall stresses. Similar trends were observed in clinical isolates and other pathogenic Candida species. The increased stress resistance of C. albicans was not dependent on key stress (Hog1) and cell integrity (Mkc1) signalling pathways. Instead, increased stress resistance was promoted by major changes in the architecture and biophysical properties of the cell wall. Glucose- and lactate-grown cells displayed significant differences in cell wall mass, ultrastructure, elasticity and adhesion. Changes in carbon source also altered the virulence of C. albicans in models of systemic candidiasis and vaginitis, confirming the importance of alternative carbon sources within host niches during C. albicans infections.