The Importance of the Phagocytes' Innate Response in Resolution of the Infection Induced by a Low Virulent Candida albicans Mutant

Unravelling the Role of Candida albicans Prn1 in the Oxidative Stress Response through a Proteomics Approach

Candida albicans Prn1 is a protein with an unknown function similar to mammalian Pirin. It also has orthologues in other pathogenic fungi, but not in Saccharomyces cerevisiae. Prn1 highly increases its abundance in response to H2O2 treatment; thus, to study its involvement in the oxidative stress response, a C. albicans prn1∆ mutant and the corresponding wild-type strain SN250 have been studied. Under H2O2 treatment, Prn1 absence led to a higher level of reactive oxygen species (ROS) and a lower survival rate, with a higher percentage of death by apoptosis, confirming its relevant role in oxidative detoxication. The quantitative differential proteomics studies of both strains in the presence and absence of H2O2 indicated a lower increase in proteins with oxidoreductase activity after the treatment in the prn1∆ strain, as well as an increase in proteasome-activating proteins, corroborated by in vivo measurements of proteasome activity, with respect to the wild type. In addition, remarkable differences in the abundance of some transcription factors were observed between mutant and wild-type strains, e.g., Mnl1 or Nrg1, an Mnl1 antagonist. orf19.4850, a protein orthologue to S. cerevisiae Cub1, has shown its involvement in the response to H2O2 and in proteasome function when Prn1 is highly expressed in the wild type.

Endothelial nitric oxide synthase limits host immunity to control disseminated Candida albicans infections in mice

Three isoforms of nitric oxide synthase (NOS) occur in mammals. High levels of NO produced by NOS2/iNOS can protect against bacterial and parasitic infections, but the role of NOS in fungal innate immunity is less clear. Compared to wild type mice, Nos3-/- mice showed significantly higher survival of candidemia caused by Candida albicans SC5314. NOS3/eNOS is expressed by endothelial cells in the kidney, and colonization of this organ was decreased during the sub-acute stage of disseminated candidiasis. Nos3-/- mice more rapidly eliminated Candida from the renal cortex and exhibited more balanced local inflammatory reactions, with similar macrophage but less neutrophil infiltration than in infected wild type. Levels of the serum cytokines IL-9, IL-12, IL-17 and chemokines GM-CSF, MIP1α, and MIP1β were significantly elevated, and IL-15 was significantly lower in infected Nos3-/- mice. Spleens of infected Nos3-/- mice had significantly more Th2 and Th9 but not other CD4+ T cells compared with wild type. Inflammatory genes associated with leukocyte chemotaxis, IL-1 signaling, TLR signaling and Th1 and Th2 cell differentiation pathways were significantly overexpressed in infected Nos3-/- kidneys, with Nos2 being the most strongly induced. Conversely, the general NOS inhibitor NG-nitro-L-arginine methyl ester increased virulence in the mouse candidemia model, suggesting that iNOS contributes to the protective mechanism in infected Nos3-/- mice. By moderating neutrophil infiltration, the absence of eNOS may reduce the collateral damage to kidney cortex, and Th-9 CD4+ cells may enhance clearance of the infection. These data suggest that selective eNOS inhibition could mitigate candidemia by a combination of systemic and local responses that promote a more effective host immune response.

Graphene oxide nanosheets increase Candida albicans killing by pro-inflammatory and reparative peritoneal macrophages

Graphene oxide (GO) is a new nanomaterial with different potential biomedical applications due to its excellent physicochemical properties and ease of surface functionalization. Macrophages play key roles in the control of fungal infections preventing invasive candidiasis by both limiting the growth of the opportunistic fungal pathogen Candida albicans and activating other immune effector cells. In order to know if macrophages maintain their immunocompetence against this microorganism after GO uptake, we have evaluated the interactions at the interface of GO nanosheets, macrophages and Candida albicans. Poly (ethylene glycol-amine)-derivatized GO nanosheets labelled with fluorescein isothiocyanate (FITC-PEG-GO), were efficiently taken up by peritoneal macrophages inducing a significant increase of C. albicans phagocytosis by both pro-inflammatory macrophages (M1/stimulated with LPS/IFN-γ)  and reparative macrophages (M2/stimulated with IL-4). On the other hand, after FITC-PEG-GO treatment and C. albicans infection, the percentages of GO⁺ macrophages diminished when Candida uptake increased in every condition (macrophages with no stimuli, M1 and M2 macrophages), thus suggesting the exocytosis of this nanomaterial as a dynamic mechanism favoring fungal phagocytosis. For the first time, we have analyzed the effects of PEG-GO nanosheets on Candida albicans killing by unstimulated, M1 and M2 macrophages, evidencing that intracellular GO modulates the macrophage candidacidal activity in a multiplicity of infection (MOI) dependent manner. At MOI 1, the high intracellular GO levels increase the fungicidal activity of basal and stimulated macrophages. At MOI 5, as intracellular GO decreases, the previous pro-inflammatory or reparative stimulus predefines the killing ability of macrophages. In summary, GO treatment enhances classical M1 macrophage activation, important for pathogen eradication, and diminishes alternative activation of M2 macrophages, thus decreasing fungal persistence and avoiding chronic infectious diseases.

Differential effects of graphene oxide nanosheets on Candida albicans phagocytosis by murine peritoneal macrophages

Macrophages, as effector cells involved in the innate and adaptive immunity, play a key role in the response to nanomaterials as graphene oxide (GO) and in their cellular uptake. The interactions at the interface of GO nanosheets, macrophages and microbial pathogens need to be assessed to determine the possible impairment of the immune system induced by biomedical treatments with this nanomaterial. Here, we have evaluated by flow cytometry and confocal microscopy the ability of murine peritoneal macrophages to phagocytose the fungal pathogen Candida albicans, alive or heat-killed, after treatment with poly(ethylene glycol-amine)-derivatized GO nanosheets (PEG-GO). After GO treatment, differences in fungal phagocytosis were observed between macrophages that had taken up GO nanosheets (GO⁺ population) and those that had not (GO^- population). GO treatment increased the ingested alive yeasts in GO^- macrophages, whereas phagocytosis diminished in the GO⁺ population. Ingestion of heat-killed yeasts was slightly higher in both GO^- and GO⁺ populations when comparing with control macrophages. For the first time, we show that GO uptake by macrophages modulates its phagocytic capability, affecting differentially the subsequent ingestion of either alive or heat-killed yeasts. Enhanced ingestion of heat-killed yeast by GO-treated macrophages suggests a beneficial role of this nanomaterial for the clearance of dead microorganisms during infection.

Apoptosis of Candida albicans during the Interaction with Murine Macrophages: Proteomics and Cell-Death Marker Monitoring

Macrophages may induce fungal apoptosis to fight against C. albicans, as previously hypothesized by our group. To confirm this hypothesis, we analyzed proteins from C. albicans cells after 3 h of interaction with macrophages using two quantitative proteomic approaches. A total of 51 and 97 proteins were identified as differentially expressed by DIGE and iTRAQ, respectively. The proteins identified and quantified were different, with only seven in common, but classified in the same functional categories. The analyses of their functions indicated that an increase in the metabolism of amino acids and purine nucleotides were taking place, while the glycolysis and translation levels dropped after 3 h of interaction. Also, the response to oxidative stress and protein translation were reduced. In addition, seven substrates of metacaspase (Mca1) were identified (Cdc48, Fba1, Gpm1, Pmm1, Rct1, Ssb1, and Tal1) as decreased in abundance, plus 12 proteins previously described as related to apoptosis. Besides, the monitoring of apoptotic markers along 24 h of interaction (caspase-like activity, TUNEL assay, and the measurement of ROS and cell examination by transmission electron microscopy) revealed that apoptotic processes took place for 30% of the fungal cells, thus supporting the proteomic results and the hypothesis of macrophages killing C. albicans by apoptosis.

Stimulation of macrophages by immunobiotic Lactobacillus strains: influence beyond the intestinal tract

Lactobacillus rhamnosus CRL1505 (Lr1505), L. rhamnosus CRL1506 (Lr1506) and L. casei CRL431 (Lc431) are able to stimulate intestinal immunity, but only Lr1505 and Lc431 are able to stimulate immunity in the respiratory tract. With the aim of advancing the understanding of the immunological mechanisms involved in stimulation of distant mucosal sites, this study evaluated the effects of orally administered probiotics on the functions of alveolar and peritoneal macrophages. Compared to a control group, these three lactobacilli were able to significantly increase phagocytic and microbicidal activities of peritoneal macrophages. After intraperitoneal challenge with pathogenic Candida albicans, mice treated with immunobiotics had significantly lower pathogen counts in infected organs. Moreover, lactobacilli-treated mice had a stronger immune response against C. albicans. On the other hand, only Lc1505 and Lc431 were able to improve activity of and cytokine production by alveolar macrophages. Only in these two groups was there better resistance to respiratory challenge with C. albicans, which correlated with improved respiratory immune response. The results of this study suggest that consumption of some probiotic strains could be useful for improving resistance to infections in sites distant from the gut by increasing the activity of macrophages at those sites.

Activation pattern of neutrophils from blood of elderly individuals with Candida-related denture stomatitis

We have identified impaired neutrophils in elderly individuals which could be involved with Candida-related denture stomatitis (DS), an oral infection predominantly caused by Candida albicans, affecting especially elderly individuals using dental prosthesis. However, specific mechanisms performed by neutrophil contributing to the susceptibility of the elderly to DS are not fully understood. This study evaluated activation features of blood neutrophils from elderly and young individuals with DS. Blood neutrophils cultured with C. albicans from elderly subjects secreted decreased levels of CXCL8. However, C. albicans challenged-neutrophils from DS patients produced high IL-4 and IL-10, and low GM-CSF levels, regardless of age. Additional elastase activity of neutrophils from both elderly groups was detected after incubation with C. albicans, but only neutrophils from elderly DS demonstrated high myeloperoxidase activity. Therefore, DS patients have affected neutrophils, and the advance of age intensifies these damages. In summary, individuals with Candida-related denture stomatitis presented variation in the neutrophil phenotype and activation. Such alterations were more intense in neutrophils from infected elderly individuals.

Fungi sensing environmental stress

Cells need to adapt to the external environment in order to survive. Signal transduction pathways are crucial mechanisms that allow cells to sense and respond to extracellular stimuli. Among the signal transduction pathways, we point out the cascades mediated by mitogen-activated protein kinases (MAPKs). The MAPKs are conserved from yeast to human and play relevant roles in the physiology of the cell. In pathogenic fungi these MAPK pathways control virulence factors. This review describes the MAPK cascades described in Candida albicans, the most frequently isolated fungus, from fungal systemic infections among individuals in developed countries.

Candida albicans actively modulates intracellular membrane trafficking in mouse macrophage phagosomes

The intracellular trafficking/survival strategies of the opportunistic human pathogen Candida albicans are poorly understood. Here we investigated the infection of RAW264.7 macrophages with a virulent wild-type (WT) filamentous C. albicans strain and a hyphal signalling-defective mutant (efg1Delta/cph1Delta). A comparative analysis of the acquisition by phagosomes of actin, and of early/late endocytic organelles markers of the different fungal strains was performed and related to Candida's survival inside macrophages. Our results show that both fungal strains have evolved a similar mechanism to subvert the 'lysosomal' system, as seen by the inhibition of the phagosome fusion with compartments enriched in the lysobisphosphatidic acid and the vATPase, and thereby the acquisition of a low pH from the outset of infection. Besides, the virulent WT strain displayed additional specific survival strategies to prevent its targeting to compartmentsdisplaying late endosomal/lysosomal features, such as induction of active recycling out of phagosomes of the lysosomal membrane protein LAMP-1, the lysosomal protease cathepsin D and preinternalized colloidal gold. Finally, both virulent and efg1Delta/cph1Delta mutant fungal strains actively suppressed the production of macrophage nitric oxide (NO), although their cell wall extracts were potent inducers of NO.

Integrated Proteomics and Genomics Strategies Bring New Insight into Candida albicans Response upon Macrophage Interaction

The interaction of Candida albicans with macrophages is considered a crucial step in the development of an adequate immune response in systemic candidiasis. An in vitro model of phagocytosis that includes a differential staining procedure to discriminate between internalized and non-internalized yeast was developed. Upon optimization of a protocol to obtain an enriched population of ingested yeasts, a thorough genomics and proteomics analysis was carried out on these cells. Both proteins and mRNA were obtained from the same sample and analyzed in parallel. The combination of two-dimensional PAGE with MS revealed a total of 132 differentially expressed yeast protein species upon macrophage interaction. Among these species, 67 unique proteins were identified. This is the first time that a proteomics approach has been used to study C. albicans-macrophage interaction. We provide evidence of a rapid protein response of the fungus to adapt to the new environment inside the phagosome by changing the expression of proteins belonging to different pathways. The clear down-regulation of the carbon-compound metabolism, plus the up-regulation of lipid, fatty acid, glyoxylate, and tricarboxylic acid cycles, indicates that yeast shifts to a starvation mode. There is an important activation of the degradation and detoxification protein machinery. The complementary genomics approach led to the detection of specific pathways related to the virulence of Candida. Network analyses allowed us to generate a hypothetical model of Candida cell death after macrophage interaction, highlighting the interconnection between actin cytoskeleton, mitochondria, and autophagy in the regulation of apoptosis. In conclusion, the combination of genomics, proteomics, and network analyses is a powerful strategy to better understand the complex host-pathogen interactions.