Hyphae and yeasts of Candida albicans differentially regulate interleukin-12 production by human blood monocytes: inhibitory role of C. albicans germination

Cryptococcus neoformans suppresses the activation of bone marrow-derived dendritic cells stimulated with its own DNA, but not with DNA from other fungi

DNA from Cryptococcus neoformans activates bone marrow-derived dendritic cells (BM-DCs) in a TLR9-dependent manner. In this study, we examined the effect of the culture supernatants of C. neoformans on the activation of BM-DCs caused by its own DNA. C. neoformans supernatants suppressed IL-12p40, IL-6 production and CD40 expression by BM-DCs stimulated with its own DNA, but not with CpG-ODN and DNA from Candida albicans, Saccharomyces cerevisiae or Escherichia coli. In a confocal microscopic analysis, C. neoformans DNA was colocalized with LAMP-1, a late endosomal marker, and TLR9. The culture supernatants did not show any apparent suppression of these responses. In a luciferase reporter assay, C. neoformans supernatants inhibited NFκB activation caused by its own DNA. These inhibitory activities were attenuated by treatment with heat or trypsin. These results indicate that C. neoformans secrete certain proteinous molecules that suppress the activation of BM-DCs caused by its own DNA.

IL-12 and related cytokines: function and regulatory implications in Candida albicans infection

IL-12 is a cytokine with links to both innate and adaptive immunity systems. In mice, its deletion leads to acute susceptibility to oral infection with the yeast Candida albicans, whereas such mice are resistant to systemic disease. However, it is an essential component of the adaptive response that leads to the generation of Th1-type cytokine responses and protection against disseminated disease. This paper presents an overview of the role of IL-12 in models of systemic and mucosal infection and the possible relationships between them.

Characterization and partial purification of Candida albicans Secretory IL-12 Inhibitory Factor

Background

We have previously shown that supernatant from Candida albicans (CA) culture contains a Secretory Interleukin (IL)-12 Inhibitory Factor (CA-SIIF), which inhibits IL-12 production by human monocytes. However, the effect of CA-SIIF on secretion of other cytokines by monocytes is unknown, and detailed characterization of this factor has not been performed.

Results

In this study, we demonstrate that the IL-12 inhibitory activity of CA-SIIF was serum-independent, based on the reduction of IL-12 levels in monocytes stimulated under serum-independent conditions. The minimal inhibitory dose of CA-SIIF was found to be 200 μg/ml. Investigation of CA-SIIF's effect on macrophages IL-12 production in vitro and in vivo also showed that CA-SIIF inhibited IL-12 production by murine macrophages both in vitro (from 571 ± 24 pg/ml to 387 ± 87 pg/ml; P = 0.05) and in vivo (from 262 ± 6 pg/ml to 144 ± 30 pg/ml; P < 0.05). In addition to IL-12, cytokine array analysis revealed that CA-SIIF induced differential production of other cytokines also. In this regard, reduction in levels were observed for IL-8, IL-10, IL-13, monocyte chemoattractant protein (MCP)-1, MCP-2, macrophage inflammatory protein (MIP)-1, RANTES, etc. In contrast, levels of other chemokines e.g. MCP-4, MIF and MIP-3α (P < 0.05) were increased. We also found that CA-SIIF suppressed the maturation of human monocytes to dendritic cells (CD1a expression = 13 ± 3% vs 36 ± 2% of the control; P < 0.01). Next, to identify the biochemical nature of CA-SIIF, we separated this factor into a Concanavalin A (ConA)-binding glycoprotein fraction (CA-SIIF-GP) and a non-ConA-binding protein fraction (CA-SIIF-NGP) using ConA affinity chromatography. Both fractions were then tested for this inhibitory effect on human monocyte IL-12 production. CA-SIIF-GP produced a higher inhibitory effect on IL-12 production compared to CA-SIIF-NGP and CA-SIIF crude (P < 0.01), proving that CA-SIIF is a glycoprotein in nature.

Conclusion

CA-SIIF is a glycoprotein which exhibits serum-independent inhibition of IL-12 production from monocytes in vitro and in vivo, and also modulates differentiation of monocytes into dendritic cells. These results suggest important role for CA-SIIF in interactions of C. albicans with the host immune system.

Monocyte responses to Candida albicans are enhanced by antibody in cooperation with antibody-independent pathogen recognition

Although most individuals are colonized with Candida albicans, only patients with insufficient or nonfunctional phagocytes develop life-threatening C. albicans disease. Because recognition of bacterial pathogens through phagocyte receptors for IgG (FcgammaR) is known to augment phagocyte responses, we postulated that antibody opsonization would enhance monocyte damage to C. albicans and subsequent tumor necrosis factor-alpha (TNF-alpha) production. After exposure to the human monocytic cell line THP-1, opsonized yeast showed an 89% decrease in metabolic activity, compared with 40% for unopsonized yeast (P<0.05). Culture supernatants contained 1316 pg mL(-1) of TNF-alpha after monocytes were exposed to opsonized yeast vs. 341 pg mL(-1) for unopsonized yeast (P=0.003). Similar results were obtained using peripheral blood mononuclear cells. Antibody opsonization of C. albicans germ tubes enhanced TNF-alpha production but did not affect organism damage. Antibody-dependent and antibody-independent factors were found to act synergistically to increase TNF-alpha production. ERK activation was important for both antibody-dependent and antibody-independent stimulation of TNF-alpha production, but not for monocyte-mediated organism damage. These data suggest that FcgammaR cooperates positively with antibody-independent recognition mechanisms in what may be a novel link between innate and adaptive immunity to C. albicans.

Host defence against disseminated Candida albicans infection and implications for antifungal immunotherapy

The different manifestations of Candida albicans infection are dictated by an underlying defect in the immune response of the host. Protective immunity to disseminated candidiasis, the manifestation of C. albicans infection discussed in this review, has traditionally been ascribed to innate immunity with emphasis on the role of granulocytes. Lately, however, immunological studies have learned that host defence against disseminated candidiasis is based on a complex interplay between innate and cell-mediated immunity. Despite the availability of new antifungal agents, mortality associated with disseminated C. albicans infection remains high. Immunotherapy that augments host defence is an important strategic option in the battle against disseminated candidiasis. Here, the authors review the chronological events in the pathogenesis of disseminated candidiasis that aid in predicting the impact of existing immunotherapy and the development of future immunomodulating strategies.

TLR2, but not TLR4, triggers cytokine production by murine cells in response to Candida albicans yeasts and hyphae

Toll-like receptors (TLRs) function as sensors for infection that induce the activation of the immune responses. Recent studies have demonstrated a crucial involvement of TLRs in the recognition of fungal pathogens such as Candida albicans. Although both TLR2 and TLR4 have been implicated in the host interaction with C. albicans, their specific role during infection has not been unequivocally established, as conflicting results have been reported. In this review, we summarize and discuss our own and others' key findings about the specific role of TLR2 and TLR4 in murine resistance to candidiasis, and in triggering cytokine secretion by murine cells in response to C. albicans yeasts and hyphae.

Toll-like receptor 4 defective mice carrying point or null mutations do not show increased susceptibility to Candida albicans in a model of hematogenously disseminated infection

We have studied the role of TLR4 in murine defenses against Candida albicans in two TLR4-defective mouse strains: C3H/HeJ mice which have defective TLR4 signaling, and TLR4-/- knockout mice. Both TLR4-defective mice strains experimentally infected with virulent C. albicans cells showed no significant difference in survival as compared with their respective controls. Recruitment of neutrophils to the peritoneal cavity of i.p. infected mice was not affected in TLR4-/-animals, but significantly enhanced in C3H/HeJ mice, compared with their control mice. In vitro production of TNF-alpha by macrophages from both types of TLR4-defective mice, in response to yeasts and hyphae of C. albicans, was not diminished as compared with production by macrophages from wild-type mice. In vitro production of TNF-alpha by yeast-stimulated splenocytes from mice intravenously infected with the low-virulence C. albicans PCA2 strain was not affected in TLR4-defective mice, but the TNF-alpha production in response to hyphae was higher in TLR4-defective than in control animals; the production of IFN-gamma by these splenocytes was similar to controls, as well as the frequency of IFN-gamma-producing CD4+T lymphocytes, indicating that TLR4-defective mice are capable of mounting a Th1 adaptive immune response. Our data indicate that TLR4 is dispensable for murine immune resistance to C. albicans.

Differential cytokine production and Toll-like receptor signaling pathways by Candida albicans blastoconidia and hyphae

Toll-like receptors (TLR) are crucial for an efficient antifungal defense. We investigated the differential recognition of blastoconidia and hyphae of Candida albicans by TLRs. In contrast to Candida blastoconidia, which stimulated large amounts of gamma interferon (IFN-gamma), the tissue-invasive Candida hyphae did not stimulate any IFN-gamma by human peripheral blood mononuclear cells (PBMC) or murine splenic lymphocytes. After stimulation with blastoconidia, the production of IFN-gamma was TLR4 dependent, as shown by the significantly decreased IFN-gamma production in anti-TLR4-treated PBMC and in splenic lymphocytes from TLR4-defective ScCr mice. In addition, peritoneal macrophages from ScCr mice produced less tumor necrosis factor alpha (TNF-alpha) than macrophages of control mice did when stimulated with Candida blastoconidia, but not with hyphae, indicating that TLR4-mediated signals are lost during hyphal germination. In contrast, macrophages from TLR2 knockout mice had a decreased production of TNF-alpha in response to both Candida blastoconidia and hyphae. Candida hyphae stimulated production of interleukin-10 through TLR2-dependent mechanisms. In conclusion, TLR4 mediates proinflammatory cytokine induction after Candida stimulation, whereas Candida recognition by TLR2 leads mainly to anti-inflammatory cytokine release. TLR4-mediated proinflammatory signals are lost during germination of Candida blastoconidia into hyphae. Phenotypic switching during germination may be an important escape mechanism of C. albicans, resulting in counteracting host defense.

Toll-like receptor-2 is essential in murine defenses against Candida albicans infections

In this work, we studied the role of toll-like receptor-2 (TLR2) in murine defenses against Candida albicans. TLR2-deficient mice experimentally infected intraperitoneally (i.p.) or intravenously (i.v.) in vivo had very significant impaired survival compared with that of control mice. In vitro production of TNF-alpha and macrophage inhibitory protein-2 (MIP-2) by macrophages from TLR2-/- mice in response to yeasts and hyphae of C. albicans were significantly lower (80% and 40%, respectively; P <0.05) than production by macrophages from wild-type mice. This impaired production of TNF-alpha and MIP-2 probably contributed to the 41% decreased recruitment of neutrophils to the peritoneal cavity of i.p. infected TLR2-/- mice. In contrast, in vitro phagocytosis of yeasts and production of reactive oxygen intermediates (ROI) were not affected in macrophages from TLR2-/- animals. Our data indicate that TLR2 plays a major role in the response of macrophages to C. albicans, triggering cytokine and chemokine expression, and it is essential for in vivo protection against infection.

Granulocytes govern the transcriptional response, morphology and proliferation of Candida albicans in human blood

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.

Toll-like receptor 2 is dispensable for acquired host immune resistance to Candida albicans in a murine model of disseminated candidiasis

Previous work by our group showed that Toll-like receptor 2 (TLR2) is essential for activation of innate immunity, playing a major role in the response of macrophages to Candida albicans, triggering cytokine and chemokine expression, and therefore TLR2 -/- mice are more susceptible to systemic primary candidiasis. In this work, we used a murine model of systemic C. albicans infection, in which resistance to reinfection with virulent wild-type cells is induced by prior exposure of mice to a low-virulence agerminative strain of C. albicans (primary sublethal infection), to study the influence of TLR2 gene deletion on (i) the ability to develop an acquired resistance upon vaccination; (ii) the development of the acquired humoral response; and (iii) the production of Th1 cytokines IFN-gamma, IL-12 and TNF-alpha. Our results indicate that, although TLR2 -/- mice have a very impaired production of Th1 cytokines compared with control mice, they are equally capable of mounting a specific humoral response to the fungus and developing a vaccine-induced resistance.

Influence of endogenous pro-inflammatory cytokines on neutrophil-mediated damage ofCandida albicanspseudohyphae, quantified in a modified tetrazolium dye assay

For quantitative assessment of polymorphonuclear granulocyte (PMN)-mediated pseudohyphal damage, an improved tetrazolium (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide; XTT) dye assay was developed. The modified assay proved to be a reliable indicator of viable pseudohyphal inoculum sizes. In addition, the influence of various endogenous pro-inflammatory cytokines on the capacity of PMN to damage Candida albicans pseudohyphae was investigated. PMN obtained from mice in which the genes encoding for tumor necrosis factor-alpha/lymphotoxin-alpha (TNF/LT), interferon-gamma (IFNgamma), interleukin (IL)-1alpha, or IL-1beta were disrupted, showed a significantly reduced pseudohyphal damage capacity in comparison with control PMN. The reduction amounted 25% for TNF-/- LT-/-, 11% for IFNgamma-/-, 21% for IL-1alpha-/-, and 34% for IL-1alpha-/-beta-/- PMN. In contrast, deficiency of IL-12 or IL-18 did not result in a diminished capacity to damage pseudohyphae and the capacity of PMN to damage Candida pseudohyphae was even slightly increased by 10% in IL-18-/- mice. These data suggest that endogenous pro-inflammatory cytokines are able to modulate antihyphal activity of PMN, the main effector cells against disseminated candidiasis by virtue of their capacity to kill both Candida blastoconidia and pseudohyphae.

Inhibition of monocytic interleukin-12 production by Candida albicans via selective activation of ERK mitogen-activated protein kinase

Our previous data demonstrated that live Candida albicans inhibits interleukin-12 (IL-12) production by human monocytes. Here we explored whether C. albicans inhibits IL-12 via a released factor and whether the inhibition is mediated via mitogen-activated protein kinase (MAPK) regulation. Supernatant fluids were obtained from cultured C. albicans (SC5314) as well as cultured Saccharomyces cerevisiae after 20 h of incubation. At 2 h postincubation of monocytes with heat-killed C. albicans (HKCA) (2:1) to stimulate IL-12, concentrated fungal supernatant fluids were added and incubated for an additional 20 h. The present data show that, unlike supernatant fluids obtained from S. cerevisiae, the C. albicans supernatant fluids significantly suppressed IL-12 production induced by HKCA. This suggested that the inhibition is Candida specific. A preliminary biochemical analysis revealed that this secretory IL-12 inhibitory factor is glycoprotein in nature. The inhibitory activity had no effect on the phagocytosis of yeasts. Supernatant fluids from C. albicans markedly induced the phosphorylation of ERK44/42 MAPK, but not p38 and SAPK, 1 min after they were added to monocytes. To test if the induction of ERK44/42 MAPK was central to the IL-12 inhibition, we used gamma interferon (IFN-gamma) (1 ng/ml) plus lipopolysaccharide (LPS) (100 ng/ml) to stimulate IL-12 production by monocytes. The inhibition of ERK MAPK by the specific inhibitor PD 98059 significantly reduced phospho-ERK44/42 MAPK levels induced by C. albicans supernatant fluids in the IFN-gamma-plus-LPS-driven monocytes. Concomitantly, PD 98059 reversed the IL-12 inhibitory activity of the C. albicans supernatant (P < 0.01). These data indicate that C. albicans can inhibit IL-12 production by secreting an ERK44/42 MAPK-stimulating factor and thus can attenuate effective immune responses.

LAAE-14, a new anti-inflammatory drug, increases the survival ofCandida albicans-inoculated mice

LAAE-14, a lipidic acid-amido ether derivative, has been recently described as a new anti-inflammatory drug. We have studied the effect of treatment with this compound on the susceptibility of mice to in vivo experimental Candida albicans infection. ICR mice orally treated with LAAE-14 (25 mg kg(-1)) and experimentally intravenously infected showed a significantly increased survival as compared to control mice. In vitro, the compound did not inhibit the growth of C. albicans yeast cells or the yeast-to-hyphal transition. The in vitro production of prostaglandin E2 by peritoneal macrophages in response to the yeasts and hyphae of C. albicans was significantly decreased upon treatment with LAAE-14, in a dose-dependent manner. Thus, reduced prostaglandin production during fungal infection could be an important factor in controlling fungal colonisation and infection.

The interaction of human dendritic cells with yeast and germ-tube forms ofCandida albicansleads to efficient fungal processing, dendritic cell maturation, and acquisition of a Th1 response-promoting function

T helper cell type 1 (Th1) cell-mediated immunity plays a critical role in protection against the opportunistic pathogen Candida albicans. Virulence of the fungus is closely associated with its ability to form germ-tubes (GT), the early phase of the dimorphic transition from the commensal yeast (Y) to the more invasive hyphal (H) form. In this study, we examined the functional outcome of the interaction of Y or GT forms with human dendritic cells (DCs), professional antigen-presenting cells, which are pivotal for initiation and modulation of T cell responses. DCs phagocytosed and killed Y and GT cells with a comparable efficiency, becoming able to trigger strong proliferative responses by Candida-specific, autologous T cell clones. Both fungal forms induced DC maturation, as indicated by up-regulation of CD83, CD80, CD86, CD40, and major histocompatibility complex classes I and II surface antigens. Chemokine receptors were also modulated in Candida-DCs, which showed increased CCR7/CXCR4 and decreased CCR5 expression. Y- and GT-activated DCs differed in the pattern of cytokine expression. In particular, GT cells, in common with fully differentiated H cells, induced significantly more elevated levels of interleukin (IL)-10 than Y cells. Nevertheless, Y-, GT-, or H-pulsed DCs secreted comparable amounts of IL-12p70. In addition, irrespective of the fungal form triggering DC activation, Candida-DCs acquired the ability to prime naive T lymphocytes with a defined Th1 phenotype. Overall, our findings highlight the induction of substantially similar functional patterns in human DCs encountering the different forms of growth of C. albicans, both seemingly activating the Th1-type immunity which is characteristic of the healthy human subjects, naturally immunized and protected against the fungus.

Adaptation of Candida albicans to the host environment: the role of morphogenesis in virulence and survival in mammalian hosts

Although morphological flexibility could be a key contributor to fungal virulence, no molecular data has unambiguously established fungal morphogenesis as a virulence factor for Candida albicans, nor can specific forms of Candida be regarded as absolutely indicative of saprophytism or infection at a given site on the host. The fitness of the fungus in vivo probably reflects its adaptation to the variety of microenvironments in which this opportunist must survive.

Linking fungal morphogenesis with virulence

Pathogenic fungi have become an increasingly common cause of systemic disease in healthy people and those with impaired immune systems. Although a vast number of fungal species inhabit our planet, just a small number are pathogens, and one feature that links many of them is the ability to differentiate morphologically from mould to yeast, or yeast to mould. Morphological differentiation between yeast and mould forms has commanded attention for its putative impact on the pathogenesis of invasive fungal infections. This review explores the current body of evidence linking fungal morphogenesis and virulence. The topics addressed cover work on phase-locked fungal cells, expression of phase-specific virulence traits and modulation of host responses by fungal morphotypes. The effect of morphological differentiation on fungal interaction with host cells, immune modulation and the net consequence on pathogenesis of disease in animal model systems are considered. The evidence argues strongly that morphological differentiation plays a vital role in the pathogenesis of fungal infection, suggesting that factors associated with this conversion process represent promising therapeutic targets.