Non-haematopoietic Sca-1+ Cells in the Retina of Adult Mice Express Functional TLR2

The mammal retina does not have the capacity to regenerate throughout life, although some stem and progenitor cells persist in the adult retina and might retain multipotentiality, as previously described in many tissues. In this work we demonstrate the presence of a small lineage- Sca-1+ cell population in the adult mouse retina which expresses functional TLR2 receptors as in vitro challenge with the pure TLR2 agonist Pam3CSK4 increases cell number and upregulates TLR2. Therefore, this population could be of interest in neuroregeneration studies to elucidate its role in these processes.

Systemic Candidiasis and TLR2 Agonist Exposure Impact the Antifungal Response of Hematopoietic Stem and Progenitor Cells

We have previously demonstrated that Candida albicans induces differentiation of hematopoietic stem and progenitor cells (HSPCs) toward the myeloid lineage both in vitro and in vivo in a TLR2- and Dectin-1-dependent manner, giving rise to functional macrophages. In this work, we used an ex vivo model to investigate the functional consequences for macrophages derived from HSPCs in vivo-exposed to Pam₃CSK₄ (a TLR2 agonist) or C. albicans infection. Short in vivo treatment of mice with Pam₃CSK₄ results in a tolerized phenotype of ex vivo HSPC-derived macrophages, whereas an extended Pam₃CSK₄ treatment confers a trained phenotype. Early during candidiasis, HSPCs give rise to macrophages trained in their response to Pam₃CSK₄ and with an increased fungicidal activity; however, as the infection progresses to higher fungal burden, HSPC-derived macrophages become tolerized, while their fungicidal capacity is maintained. These results demonstrate that memory-like innate immune responses, already described for monocytes and macrophages, also take place in HSPCs. Interestingly, extended Pam₃CSK₄ treatment leads to an expansion of spleen HSPCs and myeloid cells, and drastically reduces the fungal burden in the kidney and spleen during systemic C. albicans infection. This protection against tissue invasion is abrogated by immunodepletion of HSPCs, suggesting their protective role against infection in this model. In addition, HSPCs produce in vitro cytokines and chemokines in response to C. albicans and Pam₃CSK₄, and these secretomes are capable of inducing myeloid differentiation of HSPCs and modulating peritoneal macrophage cytokine responses. Taken together, these data assign an active role for HSPCs in sensing pathogens during infection and in contributing to host protection by diverse mechanisms.

Imiquimod inhibits growth and induces differentiation of myeloid leukemia cell lines

Background

The antitumoral effects of different Toll-like receptor (TLRs) agonists is mediated by activating immune responses to suppress tumors growth, although TLR ligands may also have a direct effect on tumoral cells. Given that TLR signaling induces hematopoietic cell differentiations this may serve as a novel differentiation therapeutic approach for AML.

Methods

We investigated the effects of agonists for the ten human TLRs on the proliferation, apoptosis, cell cycle and differentiation of ten different types of myeloid leukemia cell lines (HL-60, U-937, KG-1, KG-1a, K-562, Kasumi-1, EOL-1, NB4, MOLM-13 and HEL). Proliferation was measured using the CellTiter 96^® Aqueous One Solution Cell Proliferation Assay (Promega). Staining and analysis with a flow cytometer was used to identify cell cycle progression and apoptosis. Differentiation was measured by staining cells with the EuroFlow™ antibody panel for AML and analyzed by flow cytometry. FlowJo software was used to analyze the cytometric data. In all experiments, statistical significance was determined by a two-tailed t test.

Results

The activation of particular TLRs on some cell lines can induce growth inhibition and Imiquimod (a TLR 7 agonist) was the most effective agonist in all leukemic cell lines examined. Imiquimod was able to induce apoptosis, as well as to induce cell cycle alteration and upregulation of myeloid differentiation markers on some of the cell lines tested.

Conclusions

Our results, together with the known efficacy of Imiquimod against many tumor entities, suggest that Imiquimod can be a potential alternative therapy to AML. This drug has a direct cytotoxic effect on leukemic cells, has the potential to induce differentiation, and can also stimulate the activation of cellular immune responses anti-AML.

Immunosuppression, peripheral inflammation and invasive infection from endogenous gut microbiota activate retinal microglia in mouse models

Although its actual role in the progression of degenerative processes is not fully known, the persistent activated state of retinal microglia and the concurrent secretion of inflammatory mediators may contribute to neuronal death and permanent vision loss. Our objective was to determine whether non-ocular conditions (immunosuppression and peripheral inflammation) could lead to activation of retinal microglia. Mouse models of immunosuppression induced by cyclophosphamide and/or peripheral inflammation by chemically induced sublethal colitis in C57BL/6J mice were used. Retinal microglia morphology, spatial distribution and complexity, as well as MHCII and CD11b expression levels were determined by flow cytometry and confocal immunofluorescence analysis with anti-CD11b, anti-IBA1 and anti-MHCIIRT1B antibodies. Retinas of mice with double treatment showed changes in microglial morphology, spatial distribution and expression levels of CD11b and MHCII. These effects were higher than those observed with any treatment separately. In addition, we also observed in these mice: (i) translocation of endogenous bacteria from gut to liver, and (ii) upregulation of TLR2 expression in retinal microglia. Using a mouse model of immunosuppression and gut colonization by Candida albicans, translocation of fungal cells was confirmed to occur in wild type and, to a higher extent, in TLR2 KO mice, which are more susceptible to fungal invasion; interestingly microglial changes were also higher in TLR2 KO mice. Hence, non-ocular injuries (immunosuppression, peripheral inflammation and invasive infection from endogenous gut microbiota) can activate retinal microglia and therefore could affect the progression of neurodegenerative disorders and should be taken into account to improve therapeutic options.

High vancomycin MICs within the susceptible range in Staphylococcus aureus bacteraemia isolates are associated with increased cell wall thickness and reduced intracellular killing by human phagocytes

Vancomycin minimum inhibitory concentrations (MICs) at the upper end of the susceptible range for Staphylococcus aureus have been associated with poor clinical outcomes of bloodstream infections. We tested the hypothesis that high vancomycin MICs in S. aureus bacteraemia isolates are associated with increased cell wall thickness and suboptimal bacterial internalisation or lysis by human phagocytes. In total, 95 isolates were evaluated. Original vancomycin MICs were determined by Etest. The susceptibility of S. aureus isolates to killing by phagocytes was assessed in a human whole blood assay. Internalisation of bacterial cells by phagocytes was investigated by flow cytometry. Cell wall thickness was evaluated by transmission electron microscopy. Genotypic analysis of S. aureus isolates was performed using a DNA microarray system. Vancomycin MICs were significantly higher (P=0.006) in isolates that were killed suboptimally (killing index <60%) compared with those killed efficiently (killing index >70%) and tended to correlate inversely (P=0.08) with the killing indices. Isolates in both killing groups were internalised by human neutrophils and monocytes with comparable efficiency. The cell wall was significantly thicker (P=0.03) in isolates in the low killing group. No genotypic differences were found between the isolates in both killing groups. In summary, high vancomycin MICs in S. aureus bacteraemia isolates were associated with increased cell wall thickness and reduced intracellular killing by phagocytes.

TLR2, TLR4 and Dectin-1 signalling in hematopoietic stem and progenitor cells determines the antifungal phenotype of the macrophages they produce

TLRs represent an attractive target for the stimulation of myeloid cell production by HSPCs. We have previously demonstrated that HSPCs use TLR2 to sense Candida albicans in vivo and induce the production of macrophages. In this work, we used an in vitro model of HSPCs differentiation to investigate the functional consequences for macrophages of exposure of HSPCs to various PAMPs and C. albicans cells. Mouse HSPCs (Lin(-) cells) were cultured with M-CSF to induce macrophage differentiation, in the presence or absence of the following PRR agonists: Pam3CSK4 (TLR2 ligand), LPS (TLR4 ligand), depleted zymosan (which only activates Dectin-1), or C. albicans yeasts (which activate several PRRs, but principally TLR2 and Dectin-1). Our data show that these PAMPs differentially impact the anti-microbial function of the macrophages produced by the exposed HSPCs. Pure TLR2 and TLR4 ligands generate macrophages with a diminished ability to produce inflammatory cytokines. In contrast, HSPCs activation in response to C. albicans leads to the generation of macrophages that are better prepared to deal with the infection, as they produce higher amounts of inflammatory cytokines and have higher fungicidal capacity than control macrophages. Therefore, the tailored manipulation of the differentiation process may help to boost the innate immune response to infection.

Role of IFN-gamma in immune responses to Candida albicans infections

Candida albicans is the most frequent etiologic agent that causes opportunistic fungal infections called candidiasis, a disease whose systemic manifestation could prove fatal and whose incidence is increasing as a result of an expanding immunocompromised population. Here we review the role of interferon-gamma (IFN-gamma) in host protection against invasive candidiasis. This cytokine plays an essential role in both the innate and adaptive arms of the immune response to candidiasis. We focus on recent progress on host-pathogen interactions leading to the production of IFN-gamma by host cells. IFN-gamma is produced by CD4 Th1, CD8, gamma delta T, and natural killer (NK) cells, essentially in response to both IL-12 and/or IL-18; more recently, a subset of C. albicans -specific Th17 cells have been described to produce both IL-17 and IFN-gamma. IFN-gamma plays an important role in the regulation of the immune system as well as in the control of the infectious process, as it is required for optimal activation of phagocytes, collaborates in the generation of protective antibody response, and favors the development of a Th1 protective response.

TLRs control hematopoiesis during infection

Recent research has shown that (i) Toll-like receptor (TLR) agonists drive hematopoietic stem and progenitor cells (HSPCs) to proliferate and differentiate along the myeloid lineage in vitro, and (ii) direct TLR-mediated stimulation of HSPCs also promotes macrophage differentiation in vivo following infection. These new insights demonstrate that TLR signaling in HSPCs, in addition to other TLR-dependent mechanisms, can contribute to HSPC expansion and myeloid differentiation after infection. Evidence is, therefore, mounting that direct TLR-induced programming of hematopoiesis plays a key role in host defense by rapidly replenishing the innate immune system with the cells needed to deal with pathogens.

Enterobacter and Klebsiella species isolated from fresh vegetables marketed in Valencia (Spain) and their clinically relevant resistances to chemotherapeutic agents

Occurrence of antibiotic-resistant pathogenic or commensal enterobacteria in marketed agricultural foodstuffs may contribute to their incorporation into the food chain and constitutes an additional food safety concern. In this work, we have determined the clinically relevant resistances to 11 common chemotherapeutic agents in Enterobacter and Klebsiella isolates from fresh vegetables from various sources (supermarkets and greengrocers' shops in Valencia, Spain). A total of 96 isolates were obtained from 160 vegetables analyzed (50% positive samples): 68 Enterobacter isolates (59 E. cloacae, two E. aerogenes, two E. cancerogenus, one E. gergoviae, and four E. sakazakii, currently Cronobacter spp.), and 28 Klebsiella isolates (19 K. oxytoca and 9 K. pneumoniae). Only seven isolates were susceptible to all agents tested, and no resistances to ceftazidime, ciprofloxacin, gentamicin, and chloramphenicol were detected. Most isolates were resistant to amoxicillin/clavulanic acid (74 [58 Enterobacter and 16 Klebsiella]) or to ampicillin (80 [55/25]). Other resistances were less frequent: nitrofurantoin (13 isolates [12/1]), tetracycline (6 [5/1]), co-trimoxazole (3 [3/0]), cefotaxime (1 [1/0]), and streptomycin (2 [1/1]). Multiresistant isolates to two (56 [41/15]), three (10 E. cloacae isolates), four (one E. cloacae and one K. pneumoniae isolate), and five (two E. cloacae isolates) chemotherapeutic agents were also detected. The presence of potential pathogens points to marketed fresh produce, which often is eaten raw, as a risk factor for consumer health. In addition, these results support the usefulness of these bacterial species as indicators of the spreading of antibiotic resistances into the environment, particularly in the food chain, and suggest their role as carriers of resistance determinants from farms to consumers, which may constitute an additional "silent" food safety concern. Therefore, there is a need to improve the hygienic quality of marketed fresh vegetables, from better methods to prevent contamination in the farms to the use of sanitizing practices at home.

Candida albicans stimulates in vivo differentiation of haematopoietic stem and progenitor cells towards macrophages by a TLR2-dependent signalling

Toll-like receptors (TLRs) are expressed by haematopoietic stem and progenitor cells (HSPCs), and may play a role in haematopoiesis in response to pathogens during infection. We have previously demonstrated that (i) inactivated yeasts of Candida albicans induce in vitro differentiation of HSPCs towards the myeloid lineage, and (ii) soluble TLR agonists induce in vivo their differentiation towards macrophages. In this work, using an in vivo model of HSPCs transplantation, we report for the first time that HSPCs sense C. albicans in vivo and subsequently are directed to produce macrophages by a TLR2-dependent signalling. Purified lineage-negative cells (Lin(-)) from bone marrow of C57BL/6 mice (CD45.2 alloantigen) were transplanted into B6Ly5.1 mice (CD45.1 alloantigen), which were then injected with viable or inactivated C. albicans yeasts. Transplanted cells were detected in the spleen and in the bone marrow of recipient mice, and they differentiate preferentially to macrophages, both in response to infection or in response to inactivated yeasts. The generation of macrophages was dependent on TLR2 but independent of TLR4, as transplanted Lin(-) cells from TLR2(-/-) mice did not give rise to macrophages, whereas Lin(-) cells from TLR4(-/-) mice generated macrophages similarly to control cells. Interestingly, the absence of TLR2, or in a minor extent TLR4, gives Lin(-) cells an advantage in transplantation assays, as increases the percentage of transplanted recovered cells. Our results indicatethat TLR-mediated recognition of C. albicans by HSPCs may help replace and/or increase cells that constitute the first line of defence against the fungus, and suggest that TLR-mediated signalling may lead to reprogramming early progenitors to rapidly replenishing the innate immune system and generate the most necessary mature cells to deal with the pathogen.

Influence of aging on murine neutrophil and macrophage function against Candida albicans

Previous work by our group showed that aged C57BL/6 mice develop an altered innate and adaptive immune response to Candida albicans and are more susceptible to systemic primary candidiasis. In this work, we used young (2-3 months old) and aged (18-20 months old) C57BL/6 mice to study in vitro the influence of aging on (1) the fungicidal activity of neutrophils and macrophages, (2) the production of cytokines by resident peritoneal macrophages in response to C. albicans, and (3) cell surface Toll-like receptor (TLR) 2 expression on resident peritoneal macrophages. Our results indicate that murine phagocytes have a fungicidal activity well preserved with aging. In vitro production of proinflammatory cytokines (IL-6, IL-1beta, and tumor necrosis factor-alpha and chemokines (MIP-2) by purified (CD11b(+)) peritoneal macrophages in response to yeasts and hyphae of C. albicans was significantly lower in aged mice as compared with young mice. However, the production of IL-10 by macrophages, in response to C. albicans, was similar in both young and aged animals. Moreover, baseline TLR2 surface expression level was lower on aged macrophages than on control macrophages. Taken together, these data indicate that the increased susceptibility to C. albicans disseminated infections in aged mice is correlated with defects in TLR2 expression and in cytokine production, but not with an impaired fungicidal activity.

Enhanced proinflammatory response to the Candida albicans gpi7 null mutant by murine cells

The Candida albicans gpi7/gpi7 null mutant strain (Deltagpi7), which is affected in glycosylphosphatidylinositol (GPI) anchor biosynthesis, showed a reduced virulence following systemic infection of C57BL/6 mice. In vitro production of TNF-alpha, IL-6 and IL-1beta by macrophages in response to Deltagpi7 cells was significantly increased as compared to control (wild type GPI7/GPI7 and revertant gpi7/GPI7) cells; this probably contributes to the enhanced recruitment of neutrophils to the peritoneal cavity in response to Deltagpi7 cells. Survival of knockout mice for Toll-like receptor (TLR) 2 and TLR4 following intravenous injection of Deltagpi7 cells showed no significant differences as compared to C57BL/6 mice. In vitro production of TNF-alpha by macrophages and neutrophil recruitment were significantly inhibited in TLR2-/- mice in response to control yeast strains. Interestingly both TNF-alpha production and neutrophil recruitment in response to Deltagpi7 were significantly increased in all three types of mice, with no differences among them, and laminarin failed to inhibit this increased production of TNF-alpha. These results indicate that the enhanced proinflammatory response to Deltagpi7 does not involve recognition through TLR2, TLR4 nor dectin-1. Therefore, complete GPI anchors confer surface properties that are involved in modulation of cytokine production by macrophages in response to C. albicans.

In vitroresponse toCandida albicansin cultures of whole human blood from young and aged donors

Invasive infections with opportunistic fungi, such as Candida albicans, have become an increasing problem in aged adults in recent years. This work investigates the influence of human ageing on C. albicans recognition by toll-like receptors (TLRs), essential components of the innate immune system, using a cohort of 96 young (15-42 years) and aged (>70 years) human volunteers. No significant differences between aged and young donors were observed on (1) cell surface TLR2, TLR6 and TLR4 expression on lymphocytes, monocytes and granulocytes, (2) production of cytokines [IL-8, IL-1beta, IL-6, IL-10, tumour necrosis factor (TNF)-alpha and IL-12p70] and prostaglandin E(2) (PGE(2)) by whole human blood in response to C. albicans and (3) fungicidal activity of whole blood. A statistically significant higher titre of natural anti-C. albicans antibodies was found in plasma of volunteers between 80 and 95 years old when compared with other age groups, probably as a consequence of the increased levels of serum Ig that has been described in elderly subjects. Therefore, the results indicate that the increased susceptibility to C. albicans infections in the elderly is not a consequence of defects in TLRs expression or signalling, nor of an impaired fungicidal activity of blood.

Both viable and killed Candida albicans cells induce in vitro production of TNF-alpha and IFN-gamma in murine cells through a TLR2-dependent signalling

The in vitro production of TNF-alpha and IFN-gamma in response to Candida albicans was investigated in wild type, TLR2-/- and TLR4-/- murine cells. TLR2-/- resident peritoneal macrophages showed a strong impairment of TNF-alpha production in response to viable and non-viable (heat-killed, antimycotic-treated and formaldehyde-fixed) yeasts and hyphae (germ tube-bearing cells) of the high virulence C. albicans ATCC 26555 strain, as compared with macrophages from wild-type and TLR4-/- mice. The in vitro production of IFN-gamma was investigated in murine splenocytes obtained three days after intravenous injection with the low virulence, non-germinative C. albicans PCA2 strain, and again, TLR2-/- splenocytes showed a strong impairment of the in vitro production of IFN-gamma in response to non-viable (heat-killed, antimycotic-treated and formaldehyde-fixed) C. albicans ATCC 26555 yeasts, as compared with splenocytes of TLR4-/- and wild type mice. These results indicate that the TLR2-mediated recognition of C. albicans leading to a proinflammatory Th1 host response appears to be well conserved in killed C. albicans cells, regardless of the inactivating treatment employed.

Impaired immune response to Candida albicans in aged mice

The prevalence of opportunistic fungal infections has increased dramatically among the aged population in recent years. This work investigated the effect of ageing on murine defences against Candida albicans. Aged C57BL/6 mice that were experimentally infected intravenously had a significantly impaired survival and a higher tissue fungal burden compared with young mice. In vitro production of tumour necrosis factor (TNF)-alpha by macrophages from aged mice in response to yeast cells and hyphae of C. albicans was significantly lower than production by macrophages from young mice. In vitro production of cytokines, such as TNF-alpha and gamma interferon (IFN-gamma), by antigen-stimulated splenocytes from mice intravenously infected with C. albicans cells was also diminished in old mice. This decrease in production of T helper 1 cytokines in old mice correlated with a diminished frequency of IFN-gamma-producing CD4+ T lymphocytes, although the ability to develop an acquired resistance upon vaccination (primary sublethal infection) of mice with the low-virulence PCA2 strain was not affected in aged mice. The diversity of antigens recognized by C. albicans-specific antibodies in sera from infected aged mice was clearly diminished when compared with that from infected young mice. Taken together, these data show that aged mice develop an altered innate and adaptive immune response to C. albicans and are more susceptible to systemic primary candidiasis.

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.

Evaluation of the usefulness of anti-glyceraldehyde-3-phosphate dehydrogenase antibodies as a treatment for invasive candidiasis in a murine model

We have evaluated the effect of antibodies against the Candida albicans glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a potential immunotherapeutic treatment for acute invasive candidiasis in a murine model of infection. Three different approaches were assayed: (i) active immunization of mice using recombinant His-tagged GAPDH, (ii) treatment of fungal yeast cells with anti-GAPDH antibodies prior to infection, and (iii) passive transfer of polyclonal anti-GAPDH antibodies. Results showed that all three approaches, although tending to show a slight beneficial effect in some instances, fail to have a relevant and statistically significant effect on the infection course, determined by survival curves and fungal burden in kidneys. This suggests that the cell wall-associated GAPDH of C. albicans, despite its potential role in virulence, does not appear to be a suitable target protein for the development of immunotherapeutic strategies against candidiasis, although further studies may be required to confirm this observation.

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.

Killed Candida albicans yeasts and hyphae inhibit gamma interferon release by murine natural killer cells

Killed yeasts and hyphae of Candida albicans inhibit gamma interferon secretion by highly purified murine NK cells in response to the Toll-like receptor ligands lipopolysaccharide and zymosan. This effect, which is also observed in the presence of NK-activating cytokines (interleukin-2 [IL-2], IL-12, and IL-15), may represent a novel mechanism of immune evasion that contributes to the virulence of C. albicans.

Toll-like receptor 2 mediates prostaglandin E(2) production in murine peritoneal macrophages and splenocytes in response to Candida albicans

The involvement of Toll-like receptor 2 (TLR2) and TLR4 in triggering signal transduction pathways leading to prostaglandin E(2) (PGE(2)) production in response to Candida albicans has been studied in cells from wild-type, TLR2-/- and TLR4-/- knockout mice. In vitro PGE(2) production by macrophages challenged with zymosan, yeast or hypha cells was strongly inhibited in TLR2-deficient cells, but not in TLR4-/- cells, as compared to macrophages from wild-type mice. PGE(2) production was dependent on de novo cyclooxygenase-2 (Cox2) synthesis, since unchallenged cells failed to produce PGE(2) and specific Cox2 inhibition during challenge totally blocked PGE(2) production. Similar results were obtained following in vitro challenge of splenocytes from mice intravenously infected with the low-virulent C. albicans PCA2 strain. This indicates that TLR2 is the major receptor that mediates PGE(2) production in response to C. albicans, probably by upregulating Cox2 expression.

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.

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.

Myeloid differentiation factor 88 (MyD88) is required for murine resistance to Candida albicans and is critically involved in Candida -induced production of cytokines

We have studied the role of myeloid differentiation factor 88 (MyD88), the universal Toll-like receptor (TLR) adaptor protein, in murine defenses against Candida albicans. MyD88-deficient mice, experimentally infected in vivo, had a very significant impaired survival, and a higher tissue fungal burden when compared with control mice. The recruitment of neutrophils to the site of infection was also significantly diminished in MyD88-\- mice. In vitro production of proinflammatory cytokines such as TNF-alpha, IFN-gamma and IL-12p70, by antigen-stimulated splenocytes from mice intravenously infected with the low-virulence C. albicans PCA2 strain, could not be detected in MyD88-\- mice. This default of production of Th1 cytokines in MyD88-deficient mice correlated with a greatly diminished frequency of IFN-gamma-producing CD4 + T lymphocytes. Also, the frequency of IFN-gamma-producing CD8 + T lymphocytes was lower in MyD88-\- mice than in control mice. Although C. albicans-specific antibody titers in PCA2-infected mice appeared more quickly in MyD88-\- mice than in control mice, the MyD88-\- group was not able to maintain the Candida-specific IgM nor IgG titers at the third week of infection. The complexity of antigens recognized by sera from MyD88-\- mice was quite similar to that from infected control mice. Taken together, these data show that MyD88-\- mice are extremely susceptible to C. albicans infections, suggesting that MyD88-dependent signaling pathways are essential for both the innate and adaptive immune responses to C. albicans.

Candida and Candidiasis: The Cell Wall as a Potential Molecular Target for Antifungal Therapy

The fungal species Candida albicans is an opportunistic pathogen, which causes serious infections in humans, particularly in immunocompromised patients. Depending on the underlying host defect, C. albicans causes a variety of infections, ranging from superficial mucocutaneous candidiasis to life-threatening disseminated infections. Both the limited spectrum of antifungal drugs currently in clinical use and the emergence of resistances make necessary the development of new effective antifungal drugs with minimal side effects; however, such a research is limited by the small number of specific target sites identified to date. The cell wall is a fungal specific dynamic structure essential to almost every aspect of the biology and pathogenicity of C. albicans. Its structure confers physical protection and shape to fungal cells, and as the most external part of the fungus, the cell wall mediates the interaction with the host, including adhesion to host tissues and modulation of the host anti-Candida immune response. Consequently, the fungal cell wall can be considered as a suitable target for development of new antifungal compounds. Therefore two distinct types of potential cell wall-related targets can be envisaged, according to their mode of action in inhibiting infection: (i) inhibition of cell wall biogenesis, which may impair cell wall integrity and thus cell viability, and (ii) modification of host-fungus interactions by inhibiting or blocking putative virulence factors, which may impair host colonization and progress of the infectious process. Antibodies specific to cell wall antigens may protect against infection by a variety of mechanisms and may evolve into save antifungal agents.

Starvation and temperature upshift cause an increase in the enzymatically active cell wall-associated glyceraldehyde-3-phosphate dehydrogenase protein in yeast

The cell wall-associated glyceraldehyde-3-phosphate dehydrogenase (cwGAPDH) activity in Saccharomyces cerevisiae increases (two- to 10-fold, depending on the strain) in response to starvation and temperature upshift. Assays using transformants carrying pTDH, a yeast centromer derivative plasmid containing the Candida albicans TDH3 gene (encoding GAPDH) fused in frame with the yeast SUC2-coding region for internal invertase, showed that starvation and/or temperature upshift result in a similar increase in both cwGAPDH and cell wall-associated invertase activities. In addition, this incorporation of GAPDH protein into the cell wall in response to stress does not require (i) de novo protein synthesis, indicating that preexisting cytosolic enzyme is incorporated into the cell wall, (ii) nor the participation of the ubiquitin yeast stress response system, as no differences were observed between wild-type and polyubiquitin-depleted (Deltaubi4) strains.

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.

Glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme present in the periplasm of Aeromonas hydrophila

This is the first report describing the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), as a protein associated with the cell envelope of a gram-negative bacterium (Aeromonas hydrophila). Dose-dependent GAPDH activity was detected in whole bacterial cells from exponentially growing cultures, indicating that an active form of GAPDH is located outside the plasma membrane. This activity represents roughly 10-20% of total cell activity, and it is not reduced by pretreatment of the cells with trypsin. Assays with soluble GAPDH indicate that the activity measured in intact cells does not originate by rebinding to intact cells of cytosolic enzyme released following cell lysis. GAPDH activity levels detected in intact cells varied during the growth phase. The relationship between GAPDH activity and cell culture density was not linear, showing this activity as a major peak in the late-logarithmic phase (A600 = 1.1-1.3), and a decrease when cells entered the stationary phase. The late exponential growing cells showed a GAPDH activity 3 to 4-fold higher than early growing or stationary cells. No activity was detected in culture supernatants. Enzymatic and Western-immunoblotting analysis of subcellular fractions (cytosol, whole and outer membranes, and periplasm) showed that GAPDH is located in the cytosol, as expected, and also in the periplasm. These results place the periplasmic GAPDH of A. hydrophila into the family of multifunctional microbial cell wall-associated GAPDHs which retain their catalytic activity.

Depletion of polyubiquitin encoded by the UBI4 gene confers pleiotropic phenotype to Candida albicans cells

We have studied the roles of polyubiquitin in Candida albicans physiology. Heterologous expression of the C. albicans polyubiquitin (UBI4) gene in a ubi4 Saccharomyces cerevisiae strain suppressed the mutant phenotype (hypersensitivity to heat shock). A heterozygous strain UBI4/Deltaubi4::hisG, obtained following the ura-blaster procedure, was used to construct a conditional mutant using a pCaDis derivative plasmid. By serendipity we isolated the UBI4 conditional mutant as well as a UBI4 mutant containing a non-functional MET3 promoter. Depletion of polyubiquitin conferred pleiotropic effects to mutant cells: (i) a limited increased sensitivity to mild heat shock; (ii) increased formation of colony morphology variants; and (iii) induction of hyphal and pseudohypal development. These results indicate that polyubiquitin in C. albicans is involved in the negative control of switching, as well as in maintaining the yeast cell morphology, probably by silencing mechanisms triggering the hyphal and pseudohyphal development in the absence of environmental inducers.

Candida albicans TDH3 gene promotes secretion of internal invertase when expressed in Saccharomyces cerevisiae as a glyceraldehyde-3-phosphate dehydrogenase-invertase fusion protein

We have checked the ability of the Candida albicans GAPDH polypeptide, which lacks a conventional N-terminal signal peptide, to reach the cell wall in Saccharomyces cerevisiae by using an intracellular form of the yeast invertase as a reporter protein. A hybrid TDH3-SUC2 gene containing the C. albicans TDH3 promoter sequences and a coding region encoding a fusion protein formed by the C. albicans GAPDH polypeptide, fused at its C-terminus with the yeast internal invertase, was constructed in a centromer derivative plasmid and transformed into a Suc(-) S. cerevisiae strain. Transformants displayed invertase activity measured in intact whole cells, and were able to grow on sucrose as the sole fermentable carbon source. Northern blot analysis with both TDH3 and SUC2 probes detected a single mRNA species of the expected size (about 2.7 kb), and Western immunoblot analysis of cell-free extracts, using a monoclonal antibody (mAb49) against a C. albicans GAPDH epitope, showed the presence of a 90 kDa polypeptide corresponding to the GAPDH-invertase fusion protein. This indicates that the TDH3 gene is able to direct part of the encoded gene product to the cell wall, and that any putative motifs for this targeting should be within the GAPDH amino acid sequence. Further analysis, using the same approach, of a panel of seven N- and C-terminal GAPDH truncates revealed that the region required for the cell wall targeting is located within the N-terminal half of the protein.

Candida albicans UBI3 and UBI4 promoter regions confer differential regulation of invertase production to saccharomyces cerevisiae cells in response to stress

Candida albicans ubiquitin genes UBI3 and UBI4 encode a ubiquitin-hybrid protein involved in ribosome biogenesis and polyubiquitin, respectively. In this work we show that UBI3 and UBI4 promoter regions confer differential expression consistent with the function of their encoded gene products. Hybrid genes were constructed containing the SUC2 coding region under the control of UBI3 or UBI4 promoters in the yeast vector pLC7. Invertase production in Saccharomyces cerevisiae transformants was differentially regulated: the UBI4 promoter was induced by stress conditions (thermal upshift and/or starvation) whereas the UBI3 promoter conferred constitutive invertase production in growing yeast cells. These results indicate that the UBI4 promoter is regulated by stress-response signaling pathways, whereas the UBI3 promoter is controlled according to the requirement for protein synthesis to support cell growth.

The Candida albicans UBI3 gene encoding a hybrid ubiquitin fusion protein involved in ribosome biogenesis is essential for growth

We have constructed a conditional null mutant Candida albicans strain for the UBI3 gene which encodes a ubiquitin fusion protein involved in ribosome biogenesis. A one-step gene disruption procedure, using the plasmid pCaDis, was designed to place the second copy of the UBI3 gene under the control of the tightly regulated MET3 promoter in a C. albicans heterozygous strain (UBI3/Deltaubi3::hisG), previously isolated in the first step of the ura-blaster protocol. Analysis of the conditional null mutant in repressing and inducing conditions indicates that UBI3 is an essential gene whose expression is required for growth of C. albicans.

The Candida albicans cell wall-associated glyceraldehyde-3-phosphate dehydrogenase activity increases in response to starvation and temperature upshift

We have determined the effect of environmental factors (mild thermal upshift and starvation) on the Candida albicans cell wall-associated glyceraldehyde-3-phosphate dehydrogenase (cwGAPDH) activity. Temperature upshift (from 28 to 37 degrees C) and/ or starvation (at 28 or 37 degrees C in water) of exponentially growing yeast cells caused an increase in cwGAPDH activity (3 to 5-, and 7 to 8-fold, respectively). This increase in activity did not correlate with an increase in the amount of cwGAPDH protein present, as determined by flow cytometry, immunoelectron microscopy and Western-blotting. These results indicate that thermal upshift and starvation cause an activation of the cwGAPDH in C. albicans cells.

The glyceraldehyde-3-phosphate dehydrogenase polypeptides encoded by the Saccharomyces cerevisiae TDH1, TDH2 and TDH3 genes are also cell wall proteins

The authors show that the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) of Saccharomyces cerevisiae, previously thought to be restricted to the cell interior, is also present in the cell wall. GAPDH activity, proportional to cell number and time of incubation, was detected in intact wild-type yeast cells. Intact cells of yeast strains containing insertion mutations in each of the three structural TDH genes (tdh1, tdh2 and tdh3) and double mutants (tdh1 tdh2 and tdh1 tdh3) also displayed a cell-wall-associated GAPDH activity, in the range of parental wild-type cells, although with significant differences among strains. A cell wall location of GAPDH was further confirmed in wild-type and tdh mutants by indirect immunofluorescence and flow cytometry analysis with a polyclonal antibody against S. cerevisiae GAPDH. By immunoelectron microscopy, the GAPDH protein was detected at the outer surface of the cell wall of wild-type cells, as well as in the cytoplasm. Western immunoblot analysis of cell wall extracts and cytosol showed that Tdh2 and Tdh3 polypeptides are present in the cell wall, as well as in the cytosol, of exponentially growing cells. Tdh1 is only detected in stationary-phase cells, again in both cytosol and cell wall extracts. The results incorporate the GAPDH of S. cerevisiae, encoded by TDH1-3, into the newly emerging family of multifunctional cell-wall-associated GAPDHs which retain their catalytic activity.

Molecular cloning and characterization of the Candida albicans UBI3 gene coding for a ubiquitin-hybrid protein

Using a polyubiquitin cDNA as a probe, we have isolated a clone (pPR3, a pEMBLYe23 derivative plasmid) containing the Candida albicans UBI3 gene coding for a fusion protein. This protein is formed by one ubiquitin subunit fused, at its C-terminus, to an unrelated peptide which is similar to the ribosomal protein encoded by the 3' tail of the Saccharomyces cerevisiae UBI3 gene. Southern blot analysis of chromosomal DNA probed with the 3' non-ubiquitin tail of UBI3 indicated that only one homologous gene is present in the C. albicans genome. Heterelogous expression of pPR3 in a S. cerevisiae ubi3 mutant strain complements the mutant phenotype (slow growth) conferred by the ubi3 defect; this provides direct evidence indicating that the clone contains the C. albicans UBI3 gene Northern blot analysis showed that UBI3 gene is expressed in yeast and germ-tube cells of C. albicans, although the UBI3 mRNA levels in starved yeast cells are below the detection limit; UBI3 mRNA drops to undetectable levels on shifting the temperature of growing yeast cells from 28 degrees C to 42 degrees C. When Northern blot analysis was performed using a specific probe for the polyubiquitin (UBI4) gene, no drop in the mRNA levels was detected following thermal upshift or in starved cells. These results indicate that stress conditions (starvation or thermal upshift) negatively regulate UBI3 expression (transcriptional arrest and/or enhanced mRNA decay), and suggest that UBI4 gene provides ubiquitin during the stress response. In addition, we failed to obtain C. albicans UBI3 null mutant cells by sequential disruption of both alleles using the hisG::URA3::hisG ('ura-blaster') cassette, suggesting that null mutants cells may be unable to grow on selective media after transformation.