SummarySurvival 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.

SummaryThe pathogenic fungus Candida albicans commonly causes mucosal surface infections. In immunocompromised patients, C. albicans may penetrate into deeper tissue, enter the bloodstream and disseminate within the host causing life-threatening systemic infections. In order to elucidate how C. albicans responds to the challenge of a blood environment, we analysed the transcription profile of C. albicans cells exposed to human blood using genomic arrays and a cDNA subtraction protocol. By combining data obtained with these two methods, we were able to identify unique sets of different fungal genes specifically expressed at different stages of this model that mimics bloodstream infections. By removing host cells and incubation in plasma, we were also able to identify several genes in which the expression level was significantly influenced by the presence of these cells. Differentially expressed genes included those that are involved in the general stress response, antioxidative response, glyoxylate cycle as well as putative virulence attributes. These data point to possible mechanisms by which C. albicans ensures survival in the hostile environment of the blood and how the fungus may escape the bloodstream as an essential step in its systemic dissemination.

␤-1,2-linked oligomannoside residues are present, associated with mannan and a glycolipid, the phospholipomannan, at the Candida albicans cell wall surface. ␤-1,2-linked oligomannoside residues act as adhesins for macrophages and stimulate these cells to undergo cytokine production. To characterize the macrophage receptor involved in the recognition of C. albicans ␤-1,2-oligomannoside we used the J774 mouse cell line, which is devoid of the receptor specific for ␣-linked mannose residues. A series of experiments based on affinity binding on either C. albicans yeast cells or ␤-1,2-oligomannoside-conjugated bovine serum albumin (BSA) and subsequent disclosure with biotinylated conjugated BSA repeatedly led to the detection of a 32-kDa macrophage protein. An antiserum specific for this 32-kDa protein inhibited C. albicans binding to macrophages and was used to immunoprecipitate the molecule. Two high-pressure liquid chromatography-purified peptides from the 32-kDa tryptic digest showed complete homology to galectin-3 (previously designated Mac-2 antigen), an endogenous lectin with pleiotropic functions which is expressed in a wide variety of cell types with which C. albicans interacts as a saprophyte or a parasite.The use of molecules derived from the yeast cell wall has led to the elucidation of both binding and signaling roles for lectin receptors present on macrophage membrane (48). Based on the use of mannan or zymosan from Saccharomyces cerevisiae, two macrophage surface proteins involved in carbohydrate recognition have been characterized. The macrophage mannose receptor (MMR) (47), a 175-kDa calcium-dependent lectin, has been described as a receptor specific for ␣-mannosides (49). A ␤-glucan receptor (5) first identified as a protein of 160 to 180 kDa (6) specific for zymosan (54) was subsequently shown to be made up of a ligand-binding 20-kDa subunit which was specific for a ␤-1,3-heptaglucoside (53). Based on its specificity for zymosan (43), the ␣M ␤ 2 -integrin CR3 (Mac-1, CD11b/CD18) was also shown to serve as a macrophage ␤-glucan receptor. The sugar specificity of CD11b/CD18 was examined using methylated monosaccharides, and a cation-independent lectin site was located C-terminal to the I domain of CD11b (55, 57). Both methylglucosides and methylmannosides, but not mannan, were recognized by CR3 (58), showing that in contrast to MMR, CR3 could bind a wide range of individual sugar but not the corresponding polymers.Both MMR and CR3 are involved in the phagocytosis of unopsonized, heat-killed S. cerevisiae yeasts by murine macrophages (18). Binding and phagocytosis of Candida albicans yeasts also involve the MMR (10, 32), and C. albicans stimulates macrophage secretory activities through its mannan and the binding of ␣-linked mannose (15, 37), although some of the activities are partly mediated by ␤-glucan (3). However, cumulative evidence (13,27,30,31) suggests that recognition of C. albicans by macrophages may also involve a sugar interaction independent of both ␣-linked mannose and ␤-linked glucan, ...

The human pathogenic yeast Candida albicans can cause an unusually broad range of infections reflecting a remarkable potential to adapt to various microniches within the human host. The exceptional adaptability of C. albicans is mediated by rapid alterations in gene expression in response to various environmental stimuli and this transcriptional flexibility can be monitored with tools such as microarrays. Using such technology it is possible to (1) capture a genome-wide portrait of the transcriptome that mirrors the environmental conditions, (2) identify known genes, signalling pathways and transcription factors involved in pathogenesis, (3) identify new patterns of gene expression and (4) identify previously uncharacterized genes that may be associated with infection. In this review, we describe the molecular dissection of three distinct stages of infections, covering both superficial and invasive disease, using in vitro, ex vivo and in vivo infection models and microarrays.