Organ-specific innate immune responses in a mouse model of invasive candidiasis

CSF1R inhibition by PLX5622 reduces pulmonary fungal infection by depleting MHCIIhi interstitial lung macrophages

PLX5622 is a small molecular inhibitor of the CSF1 receptor (CSF1R) and is widely used to deplete macrophages within the central nervous system (CNS). We investigated the impact of PLX5622 treatment in wild-type C57BL/6 mice and discovered that one-week treatment with PLX5622 was sufficient to deplete interstitial macrophages in the lung and brain-infiltrating Ly6Clow patrolling monocytes, in addition to CNS-resident macrophages. These cell types were previously indicated to act as infection reservoirs for the pathogenic fungus Cryptococcus neoformans. We found that PLX5622-treated mice had significantly reduced fungal lung infection and reduced extrapulmonary dissemination to the CNS but not to the spleen or liver. Fungal lung infection mapped to MHCIIhi interstitial lung macrophages, which underwent significant expansion during infection following monocyte replenishment and not local division. Although PLX5622 depleted CNS infiltrating patrolling monocytes, these cells did not accumulate in the fungal-infected CNS following pulmonary infection. In addition, Nr4a1-deficient mice, which lack patrolling monocytes, had similar control and dissemination of C. neoformans infection to wild-type controls. PLX5622 did not directly affect CD4 T-cell responses, or significantly affect production of antibody in the lung during infection. However, we found that mice lacking lymphocytes had reduced numbers of MHCIIhi interstitial macrophages in the lung, which correlated with reduced infection load. Accordingly, PLX5622 treatment did not alter fungal burdens in the lungs of lymphocyte-deficient mice. Our data demonstrate that PLX5622 may help reduce lung burden of pathogenic fungi that utilise CSF1R-dependent myeloid cells as infection reservoirs, an effect which is dependent on the presence of lymphocytes.

Elevated serum level of progranulin is associated with increased mortality in critically ill patients with candidemia

Candidemia is a severe disease with high mortality in both intensive care unit (ICU) and non-ICU settings. Considering that progranulin (PGRN) is a potential therapeutic target for the candidemia caused by Candida albicans, we determined the serum level of PGRN after candidemia and evaluated its association with mortality. A retrospective discovery cohort (62 patients) and a validation cohort (70 patients) were enrolled. Blood was collected on day of first blood culture positivity for C. albicans, and serum PGRN levels were then measured. In the discovery cohort, all serum PGRN studied were expressed at higher levels in candidemia patients than in bacteremia patients and healthy volunteers, non-survivors presented with significantly higher serum PGRN concentrations when compared with survivors. Serum PGRN concentration was associated with 30-day mortality and patients at a higher risk of death showed higher serum PGRN levels. These results were confirmed in the independent validation cohort. Interestingly, in vitro study demonstrated that macrophages, neutrophils and lymphocytes may be the major source of PGRN production after C. albicans infection instead of epithelial cells. Our findings highlight that serum PGRN appears as a biomarker in candidemia patients and as a promising tool for mortality risk stratification in managing candidemia.

Translational genetics identifies a phosphorylation switch in CARD9 required for innate inflammatory responses

Population genetics continues to identify genetic variants associated with diseases of the immune system and offers a unique opportunity to discover mechanisms of immune regulation. Multiple genetic variants linked to severe fungal infections and autoimmunity are associated with caspase recruitment domain-containing protein 9 (CARD9). We leverage the CARD9 R101C missense variant to uncover a biochemical mechanism of CARD9 activation essential for antifungal responses. We demonstrate that R101C disrupts a critical signaling switch whereby phosphorylation of S104 releases CARD9 from an autoinhibited state to promote inflammatory responses in myeloid cells. Furthermore, we show that CARD9 R101C exerts dynamic effects on the skin cellular contexture during fungal infection, corrupting inflammatory signaling and cell-cell communication circuits. Card9 R101C mice fail to control dermatophyte infection in the skin, resulting in high fungal burden, yet show minimal signs of inflammation. Together, we demonstrate how translational genetics reveals molecular and cellular mechanisms of innate immune regulation.

Invasive candidiasis

Invasive candidiasis is an important fungal disease caused by Candida albicans and, increasingly, non-albicans Candida pathogens. Invasive Candida infections originate most frequently from endogenous human reservoirs and are triggered by impaired host defences. Signs and symptoms of invasive candidiasis are non-specific; candidaemia is the most diagnosed manifestation, with disseminated candidiasis affecting single or multiple organs. Diagnosis poses many challenges, and conventional culture techniques are frequently supplemented by non-culture-based assays. The attributable mortality from candidaemia and disseminated infections is ~30%. Fluconazole resistance is a concern for Nakaseomyces glabratus, Candida parapsilosis, and Candida auris and less so in Candida tropicalis infection; acquired echinocandin resistance remains uncommon. The epidemiology of invasive candidiasis varies in different geographical areas and within various patient populations. Risk factors include intensive care unit stay, central venous catheter use, broad-spectrum antibiotics use, abdominal surgery and immune suppression. Early antifungal treatment and central venous catheter removal form the cornerstones to decrease mortality. The landscape of novel therapeutics is growing; however, the application of new drugs requires careful selection of eligible patients as the spectrum of activity is limited to a few fungal species. Unanswered questions and knowledge gaps define future research priorities and a personalized approach to diagnosis and treatment of invasive candidiasis is of paramount importance.

Evaluation of murine renal phagocyte-fungal interactions using intravital confocal microscopy and flow cytometry

Myeloid phagocytes are essential for antifungal host defense during systemic candidiasis. Here, we present a protocol for assessing phagocyte-fungal interactions in vivo in the kidney, the primary target organ of the murine systemic candidiasis model. We describe steps for intravital confocal microscopy and flow cytometry. We also detail a kidney tissue dissociation procedure to obtain highly pure functional phagocytes for utilization in downstream ex vivo fungal uptake and killing assays.

Activation of leukotriene B4 receptor 1 is a prerequisite for complement receptor 3-mediated antifungal responses of neutrophils

Invasive fungal infections are life-threatening, and neutrophils are vital cells of the innate immune system that defend against them. The role of LTA4H-LTB4-BLT1 axis in regulation of neutrophil responses to fungal infection remains poorly understood. Here, we demonstrated that the LTA4H-LTB4-BLT1 axis protects the host against Candida albicans and Aspergillus fumigatus, but not Cryptococcus neoformans infection, by regulating the antifungal activity of neutrophils. Our results show that deleting Lta4h or Blt1 substantially impairs the fungal-specific phagocytic capacity of neutrophils. Moreover, defective activation of the spleen tyrosine kinase (Syk) and extracellular signal-related kinase (ERK1/2) pathways in neutrophils accompanies this impairment. Mechanistically, BLT1 regulates CR3-mediated, β-1,3-glucan-induced neutrophil phagocytosis, while a physical interaction with CR3 with slight influence on its dynamics is observed. Our findings thus demonstrate that the LTA4H-LTB4-BLT1 axis is essential for the phagocytic function of neutrophils in host antifungal immune response against Candida albicans and Aspergillus fumigatus.

Hyphal-associated protein expression is crucial for Candida albicans-induced eicosanoid biosynthesis in immune cells

Candida albicans causes opportunistic infections ranging from mucosal mycoses to life-threatening systemic infections in immunocompromised patients. During C. albicans infection, leukotrienes and prostaglandins are formed from arachidonic acid by 5-lipoxygenase (5-LOX) and cyclooxygenases, respectively to amplify inflammatory conditions, but also to initiate macrophage infiltration to achieve tissue homeostasis. Since less is known about the cellular mechanisms triggering such lipid mediator biosynthesis, we investigated the eicosanoid formation in monocyte-derived M1 and M2 macrophages, neutrophils and HEK293 cells transfected with 5-LOX and 5-LOX-activating protein (FLAP) in response to C. albicans yeast or hyphae. Leukotriene biosynthesis was exclusively induced by hyphae in neutrophils and macrophages, whereas prostaglandin E2 was also formed in response to yeast cells by M1 macrophages. Eicosanoid biosynthesis was significantly higher in M1 compared to M2 macrophages. In HEK_5-LOX/FLAP cells only hyphae activated the essential 5-LOX translocation to the nuclear membrane. Using yeast-locked C. albicans mutants, we demonstrated that hyphal-associated protein expression is critical in eicosanoid formation. For neutrophils and HEK_5-LOX/FLAP cells, hyphal wall protein 1 was identified as the essential surface protein that stimulates leukotriene biosynthesis. In summary, our data suggest that hyphal-associated proteins of C. albicans are central triggers of eicosanoid biosynthesis in human phagocytes.

Macrophage pyroptosis induced by Candida albicans

Candida albicans (C. albicans) is a prevalent opportunistic pathogen that causes mucocutaneous and systemic infections, particularly in immunocompromised individuals. Macrophages play a crucial role in eliminating C. albicans in local and bloodstream contexts, while also regulating antifungal immune responses. However, C. albicans can induce macrophage lysis through pyroptosis, a type of regulated cell death. This process can enable C. albicans to escape from immune cells and trigger the release of IL-1β and IL-18, which can impact both the host and the pathogen. Nevertheless, the mechanisms by which C. albicans triggers pyroptosis in macrophages and the key factors involved in this process remain unclear. In this review, we will explore various factors that may influence or trigger pyroptosis in macrophages induced by C. albicans, such as hypha, ergosterol, cell wall remodeling, and other virulence factors. We will also examine the possible immune response following macrophage pyroptosis.

TRIM26 alleviates fatal immunopathology by regulating inflammatory neutrophil infiltration during Candida infection

Fungal infections have emerged as a major concern among immunocompromised patients, causing approximately 2 million deaths each year worldwide. However, the regulatory mechanisms underlying antifungal immunity remain elusive and require further investigation. The E3 ligase Trim26 belongs to the tripartite motif (Trim) protein family, which is involved in various biological processes, including cell proliferation, antiviral innate immunity, and inflammatory responses. Herein, we report that Trim26 exerts protective antifungal immune functions after fungal infection. Trim26-deficient mice are more susceptible to fungemia than their wild-type counterparts. Mechanistically, Trim26 restricts inflammatory neutrophils infiltration and limits proinflammatory cytokine production, which can attenuate kidney fungal load and renal damage during Candida infection. Trim26-deficient neutrophils showed higher proinflammatory cytokine expression and impaired fungicidal activity. We further demonstrated that excessive neutrophils infiltration in the kidney was because of the increased production of chemokines CXCL1 and CXCL2, which are mainly synthesized in the macrophages or dendritic cells of Trim26-deficient mice after Candida albicans infections. Together, our study findings unraveled the vital role of Trim26 in regulating antifungal immunity through the regulation of inflammatory neutrophils infiltration and proinflammatory cytokine and chemokine expression during candidiasis.

The F1Fo-ATP synthase α subunit of Candida albicans induces inflammatory responses by controlling amino acid catabolism

Sepsis is a leading cause of fatality in invasive candidiasis. The magnitude of the inflammatory response is a determinant of sepsis outcomes, and inflammatory cytokine imbalances are central to the pathophysiological processes. We previously demonstrated that a Candida albicans F1Fo-ATP synthase α subunit deletion mutant was nonlethal to mice. Here, the potential effects of the F1Fo-ATP synthase α subunit on host inflammatory responses and the mechanism were studied. Compared with wild-type strain, the F1Fo-ATP synthase α subunit deletion mutant failed to induce inflammatory responses in Galleria mellonella and murine systemic candidiasis models and significantly decreased the mRNA levels of the proinflammatory cytokines IL-1β, IL-6 and increased those of the anti-inflammatory cytokine IL-4 in the kidney. During C. albicans-macrophage co-culture, the F1Fo-ATP synthase α subunit deletion mutant was trapped inside macrophages in yeast form, and its filamentation, a key factor in inducing inflammatory responses, was inhibited. In the macrophage-mimicking microenvironment, the F1Fo-ATP synthase α subunit deletion mutant blocked the cAMP/PKA pathway, the core filamentation-regulating pathway, because it failed to alkalinize environment by catabolizing amino acids, an important alternative carbon source inside macrophages. The mutant downregulated Put1 and Put2, two essential amino acid catabolic enzymes, possibly due to severely impaired oxidative phosphorylation. Our findings reveal that the C. albicans F1Fo-ATP synthase α subunit induces host inflammatory responses by controlling its own amino acid catabolism and it is significant to find drugs that inhibit F1Fo-ATP synthase α subunit activity to control the induction of host inflammatory responses.

Acquired Triazole Resistance Alters Pathogenicity-Associated Features in Candida auris in an Isolate-Dependent Manner

Fluconazole resistance is commonly encountered in Candida auris, and the yeast frequently displays resistance to other standard drugs, which severely limits the number of effective therapeutic agents against this emerging pathogen. In this study, we aimed to investigate the effect of acquired azole resistance on the viability, stress response, and virulence of this species. Fluconazole-, posaconazole-, and voriconazole- resistant strains were generated from two susceptible C. auris clinical isolates (0381, 0387) and compared under various conditions. Several evolved strains became pan-azole-resistant, as well as echinocandin-cross-resistant. While being pan-azole-resistant, the 0381-derived posaconazole-evolved strain colonized brain tissue more efficiently than any other strain, suggesting that fitness cost is not necessarily a consequence of resistance development in C. auris. All 0387-derived evolved strains carried a loss of function mutation (R160S) in BCY1, an inhibitor of the PKA pathway. Sequencing data also revealed that posaconazole treatment can result in ERG3 mutation in C. auris. Despite using the same mechanisms to generate the evolved strains, both genotype and phenotype analysis highlighted that the development of resistance was unique for each strain. Our data suggest that C. auris triazole resistance development is a highly complex process, initiated by several pleiotropic factors.

Proline catabolism is a key factor facilitating Candida albicans pathogenicity

Candida albicans, the primary etiology of human mycoses, is well-adapted to catabolize proline to obtain energy to initiate morphological switching (yeast to hyphal) and for growth. We report that put1-/- and put2-/- strains, carrying defective Proline UTilization genes, display remarkable proline sensitivity with put2-/- mutants being hypersensitive due to the accumulation of the toxic intermediate pyrroline-5-carboxylate (P5C), which inhibits mitochondrial respiration. The put1-/- and put2-/- mutations attenuate virulence in Drosophila and murine candidemia models and decrease survival in human neutrophils and whole blood. Using intravital 2-photon microscopy and label-free non-linear imaging, we visualized the initial stages of C. albicans cells infecting a kidney in real-time, directly deep in the tissue of a living mouse, and observed morphological switching of wildtype but not of put2-/- cells. Multiple members of the Candida species complex, including C. auris, are capable of using proline as a sole energy source. Our results indicate that a tailored proline metabolic network tuned to the mammalian host environment is a key feature of opportunistic fungal pathogens.

Toll-like receptor 4 and CD11b expressed on microglia coordinate eradication of Candida albicans cerebral mycosis

The fungal pathogen Candida albicans is linked to chronic brain diseases such as Alzheimer's disease (AD), but the molecular basis of brain anti-Candida immunity remains unknown. We show that C. albicans enters the mouse brain from the blood and induces two neuroimmune sensing mechanisms involving secreted aspartic proteinases (Saps) and candidalysin. Saps disrupt tight junction proteins of the blood-brain barrier (BBB) to permit fungal brain invasion. Saps also hydrolyze amyloid precursor protein (APP) into amyloid β (Aβ)-like peptides that bind to Toll-like receptor 4 (TLR4) and promote fungal killing in vitro while candidalysin engages the integrin CD11b (Mac-1) on microglia. Recognition of Aβ-like peptides and candidalysin promotes fungal clearance from the brain, and disruption of candidalysin recognition through CD11b markedly prolongs C. albicans cerebral mycosis. Thus, C. albicans is cleared from the brain through innate immune mechanisms involving Saps, Aβ, candidalysin, and CD11b.

Soluble Enolase 1 of Candida albicans and Aspergillus fumigatus Stimulates Human and Mouse B Cells and Monocytes

Because of the growing numbers of immunocompromised patients, the incidence of life-threatening fungal infections caused by Candida albicans and Aspergillus fumigatus is increasing. We have recently identified enolase 1 (Eno1) from A. fumigatus as an immune evasion protein. Eno1 is a fungal moonlighting protein that mediates adhesion and invasion of human cells and also immune evasion through complement inactivation. We now show that soluble Eno1 has immunostimulatory activity. We observed that Eno1 from both C. albicans and A. fumigatus directly binds to the surface of lymphocytes, preferentially human and mouse B cells. Functionally, Eno1 upregulated CD86 expression on B cells and induced proliferation. Although the receptor for fungal Eno1 on B lymphocytes is still unknown, the comparison of B cells from wild-type and MyD88-deficient mice showed that B cell activation by Eno1 required MyD88 signaling. With respect to infection biology, we noted that mouse B cells stimulated by Eno1 secreted IgM and IgG2b. These Igs bound C. albicans hyphae in vitro, suggesting that Eno1-induced Ab secretion might contribute to protection from invasive fungal disease in vivo. Eno1 also triggered the release of proinflammatory cytokines from monocytes, particularly IL-6, which is a potent activator of B cells. Together, our data shed new light on the role of secreted Eno1 in infections with C. albicans and A. fumigatus. Eno1 secretion by these pathogenic microbes appears to be a double-edged sword by supporting fungal pathogenicity while triggering (antifungal) immunity.

Isolation of Mouse Neutrophils

Neutrophils represent the first line of defense against bacterial and fungal pathogens. Indeed, patients with inherited or acquired qualitative and quantitative neutrophil defects are at high risk for developing bacterial and fungal infections and suffering adverse outcomes from these infections. Therefore, research aiming at defining the molecular factors that modulate neutrophil effector function under homeostatic conditions and during infection is essential for devising strategies to augment neutrophil function and improve the outcomes of infected individuals. This article describes reproducible density-gradient-centrifugation-based as well as positive and negative immunomagnetic selection protocols that can be applied in any laboratory to harvest large numbers of highly enriched and highly viable neutrophils from the bone marrow of mice. In another protocol, we also present a method that combines gentle enzymatic tissue digestion with a positive immunomagnetic selection technique or fluorescence-activated cell sorting (FACS) to harvest highly pure and highly viable preparations of neutrophils directly from mouse tissues such as the kidney, the liver, or the spleen. Mouse neutrophils isolated by these protocols can be used to examine several aspects of cellular function ex vivo, including pathogen binding, phagocytosis, and killing, neutrophil chemotaxis, oxidative burst, degranulation, and cytokine production, and for performing neutrophil adoptive transfer experiments. © 2023 Wiley Periodicals LLC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA. Basic Protocol 1: Isolation of Neutrophils from Mouse Bone Marrow Using Positive Immunomagnetic Separation Alternate Protocol 1: Purification of Neutrophils from Bone Marrow Using Negative Immunomagnetic Separation Alternate Protocol 2: Purification of Neutrophils from Bone Marrow Using Histopaque-Based Density Gradient Centrifugation Basic Protocol 2: Isolation of Neutrophils from Mouse Tissues Using Positive Immunomagnetic Separation Alternate Protocol 3: Isolation of Neutrophils from Mouse Tissues Using FACS.

Inflammatory ER stress responses dictate the immunopathogenic progression of systemic candidiasis

Recognition of pathogen-associated molecular patterns can trigger the inositol-requiring enzyme 1 α (IRE1α) arm of the endoplasmic reticulum (ER) stress response in innate immune cells. This process maintains ER homeostasis and also coordinates diverse immunomodulatory programs during bacterial and viral infections. However, the role of innate IRE1α signaling in response to fungal pathogens remains elusive. Here, we report that systemic infection with the human opportunistic fungal pathogen Candida albicans induced proinflammatory IRE1α hyperactivation in myeloid cells that led to fatal kidney immunopathology. Mechanistically, simultaneous activation of the TLR/IL-1R adaptor protein MyD88 and the C-type lectin receptor dectin-1 by C. albicans induced NADPH oxidase-driven generation of ROS, which caused ER stress and IRE1α-dependent overexpression of key inflammatory mediators such as IL-1β, IL-6, chemokine (C-C motif) ligand 5 (CCL5), prostaglandin E2 (PGE2), and TNF-α. Selective ablation of IRE1α in leukocytes, or treatment with an IRE1α pharmacological inhibitor, mitigated kidney inflammation and prolonged the survival of mice with systemic C. albicans infection. Therefore, controlling IRE1α hyperactivation may be useful for impeding the immunopathogenic progression of disseminated candidiasis.

Therapeutic Switching of Rafoxanide: a New Approach To Fighting Drug-Resistant Bacteria and Fungi

Control and management of life-threatening bacterial and fungal infections are a global health challenge. Despite advances in antimicrobial therapies, treatment failures for resistant bacterial and fungal infections continue to increase. We aimed to repurpose the anthelmintic drug rafoxanide for use with existing therapeutic drugs to increase the possibility of better managing infection and decrease treatment failures. For this purpose, we evaluated the antibacterial and antifungal potential of rafoxanide. Notably, 70% (70/100) of bacterial isolates showed multidrug resistance (MDR) patterns, with higher prevalence among human isolates (73.5% [50/68]) than animal ones (62.5% [20/32]). Moreover, 22 fungal isolates (88%) were MDR and were more prevalent among animal (88.9%) than human (87.5%) sources. We observed alarming MDR patterns among bacterial isolates, i.e., Klebsiella pneumoniae (75% [30/40; 8 animal and 22 human]) and Escherichia coli (66% [40/60; 12 animal and 28 human]), and fungal isolates, i.e., Candida albicans (86.7% [13/15; 4 animal and 9 human]) and Aspergillus fumigatus (90% [9/10; 4 animal and 5 human]), that were resistant to at least one agent in three or more different antimicrobial classes. Rafoxanide had antibacterial and antifungal activities, with minimal inhibitory concentration (MICs) ranging from 2 to 128 μg/mL. Rafoxanide at sub-MICs downregulated the mRNA expression of resistance genes, including E. coli and K. pneumoniae blaCTX-M-1, blaTEM-1, blaSHV, MOX, and DHA, C. albicans ERG11, and A. fumigatus cyp51A. We noted the improvement in the activity of β-lactam and antifungal drugs upon combination with rafoxanide. This was apparent in the reduction in the MICs of cefotaxime and fluconazole when these drugs were combined with sub-MIC levels of rafoxanide. There was obvious synergism between rafoxanide and cefotaxime against all E. coli and K. pneumoniae isolates (fractional inhibitory concentration index [FICI] values ≤ 0.5). Accordingly, there was a shift in the patterns of resistance of 16.7% of E. coli and 22.5% of K. pneumoniae isolates to cefotaxime and those of 63.2% of C. albicans and A. fumigatus isolates to fluconazole when the isolates were treated with sub-MICs of rafoxanide. These results were confirmed by in silico and mouse protection assays. Based on the in silico study, one possible explanation for how rafoxanide reduced bacterial resistance is through its inhibitory effects on bacterial and fungal histidine kinase enzymes. In short, rafoxanide exhibited promising results in overcoming bacterial and fungal drug resistance. IMPORTANCE The drug repurposing strategy is an alternative approach to reducing drug development timelines with low cost, especially during outbreaks of disease caused by drug-resistant pathogens. Rafoxanide can disrupt the abilities of bacterial and fungal cells to adapt to stress conditions. The coadministration of antibiotics with rafoxanide can prevent the failure of treatment of both resistant bacteria and fungi, as the resistant pathogens could be made sensitive upon treatment with rafoxanide. From our findings, we anticipate that pharmaceutical companies will be able to utilize new combinations against resistant pathogens.

Candidiasis: From cutaneous to systemic, new perspectives of potential targets and therapeutic strategies

Candidiasis is an infection caused by fungi from a Candida species, most commonly Candida albicans. C. albicans is an opportunistic fungal pathogen typically residing on human skin and mucous membranes of the mouth, intestines or vagina. It can cause a wide variety of mucocutaneous barrier and systemic infections; and becomes a severe health problem in HIV/AIDS patients and in individuals who are immunocompromised following chemotherapy, treatment with immunosuppressive agents or after antibiotic-induced dysbiosis. However, the immune mechanism of host resistance to C. albicans infection is not fully understood, there are a limited number of therapeutic antifungal drugs for candidiasis, and these have disadvantages that limit their clinical application. Therefore, it is urgent to uncover the immune mechanisms of the host protecting against candidiasis and to develop new antifungal strategies. This review synthesizes current knowledge of host immune defense mechanisms from cutaneous candidiasis to invasive C. albicans infection and documents promising insights for treating candidiasis through inhibitors of potential antifungal target proteins.

Candida albicans causes brain regional invasion and necrosis, and activation of microglia during lethal neonatal neurocandidiasis

Neurocandidiasis is a fungal infection that primarily affects neonates, which is associated with 70% case fatality rates, while pediatric patients who survive infection often have long-term neurological sequelae, making it a clinical requirement to understand the pathogenesis of neonatal neurocandidiasis. Currently, the brain regions to Candida albicans invasion during the neonatal period are not characterized. In this study, 0-day-old mice were infected with C. albicans intravenously to determine dissemination and invasion into the brain at different times post-infection by fungal burden assay and histopathological analysis, additionally cellular death and microglial activation were evaluated by flow cytometry. The results evidenced the dissemination of C. albicans within the first hour of infection in the brain. The meninges were the initial site of invasion during the first 6 hours post infection and then filamentous structures into the brain parenchyma increases during infection, the anatomic regions most susceptible to invasion being the cerebral cortex, thalamus, hypothalamus, midbrain, pons, and medulla oblongata. Furthermore, C. albicans invasion of brain tissue results in cell necrosis and activation of microglia as a consequence of fungal invasion.

C5a-licensed phagocytes drive sterilizing immunity during systemic fungal infection

Systemic candidiasis is a common, high-mortality, nosocomial fungal infection. Unexpectedly, it has emerged as a complication of anti-complement C5-targeted monoclonal antibody treatment, indicating a critical niche for C5 in antifungal immunity. We identified transcription of complement system genes as the top biological pathway induced in candidemic patients and as predictive of candidemia. Mechanistically, C5a-C5aR1 promoted fungal clearance and host survival in a mouse model of systemic candidiasis by stimulating phagocyte effector function and ERK- and AKT-dependent survival in infected tissues. C5ar1 ablation rewired macrophage metabolism downstream of mTOR, promoting their apoptosis and enhancing mortality through kidney injury. Besides hepatocyte-derived C5, local C5 produced intrinsically by phagocytes provided a key substrate for antifungal protection. Lower serum C5a concentrations or a C5 polymorphism that decreases leukocyte C5 expression correlated independently with poor patient outcomes. Thus, local, phagocyte-derived C5 production licenses phagocyte antimicrobial function and confers innate protection during systemic fungal infection.

An experimental porcine model of invasive candidiasis

BACKGROUND

Invasive candidiasis (IC) is a severe and often fatal fungal infection that affects critically ill patients. The development of animal models that mimic human disease is essential for advancing our understanding of IC pathophysiology and testing experimental or novel treatments. We aimed to develop a large animal model of IC that could provide a much-needed addition to the widely used murine models.

RESULTS

A total of 25 pigs (including one control), aged between 9 and 12 weeks, with a median weight of 25.1 kg (IQR 24.1-26.2), were used to develop the porcine IC model. We present the setup, the results of the experiments, and the justification for the changes made to the model. The experiments were conducted in an intensive care setting, using clinically relevant anaesthesia, monitoring and interventions. The final model used corticosteroids, repeated Candida inoculation, and continuous endotoxin. The model consistently demonstrated quantifiable growth of Candida in blood and organs. The registered physiological data supported the development of the sepsis-induced circulatory distress observed in IC patients in the ICU.

CONCLUSIONS

Our proposed porcine model of IC offers a potential new tool in the research of IC.

What fungal CNS infections can teach us about neuroimmunology and CNS-specific immunity

Immunity to fungal infections of the central nervous system (CNS) is one of the most poorly understood subjects within the field of medical mycology. Yet, the majority of deaths from invasive fungal infections are caused by brain-tropic fungi. In recent years, there have been several significant discoveries in the regulation of neuroinflammation and the role of the immune system in tissue homeostasis within the CNS. In this review, I highlight five important advances in the neuroimmunology field over the last decade and discuss how we should capitalise on these discoveries to better understand the pathogenesis of fungal CNS infections. In addition, the latest insights into fungal invasion tactics, microglia-astrocyte crosstalk and regulation of antifungal adaptive immune responses are summarised in the context of our contemporary understanding of CNS-specific immunity.

Immunoglobulins at the interface of the gut mycobiota and anti-fungal immunity

The dynamic and complex community of microbes that colonizes the intestines is composed of bacteria, fungi, and viruses. At the mucosal surfaces, immunoglobulins play a key role in protection against bacterial and fungal pathogens, and their toxins. Secretory immunoglobulin A (sIgA) is the most abundantly produced antibody at the mucosal surfaces, while Immunoglobulin G (IgG) isotypes play a critical role in systemic protection. IgA and IgG antibodies with reactivity to commensal fungi play an important role in shaping the mycobiota and host antifungal immunity. In this article, we review the latest evidence that establishes a connection between commensal fungi and B cell-mediated antifungal immunity as an additional layer of protection against fungal infections and inflammation.

Pathogenesis and virulence of Candida albicans

Candida albicans is a commensal yeast fungus of the human oral, gastrointestinal, and genital mucosal surfaces, and skin. Antibiotic-induced dysbiosis, iatrogenic immunosuppression, and/or medical interventions that impair the integrity of the mucocutaneous barrier and/or perturb protective host defense mechanisms enable C. albicans to become an opportunistic pathogen and cause debilitating mucocutaneous disease and/or life-threatening systemic infections. In this review, we synthesize our current knowledge of the tissue-specific determinants of C. albicans pathogenicity and host immune defense mechanisms.

Deep tissue infection by an invasive human fungal pathogen requires lipid-based suppression of the IL-17 response

Candida albicans is the most common cause of fungal infection in humans. IL-17 is critical for defense against superficial fungal infections, but the role of this response in invasive disease is less understood. We show that C. albicans secretes a lipase, Lip2, that facilitates invasive disease via lipid-based suppression of the IL-17 response. Lip2 was identified as an essential virulence factor in a forward genetic screen in a mouse model of bloodstream infection. Murine infection with C. albicans strains lacking Lip2 display exaggerated IL-17 responses that lead to fungal clearance from solid organs and host survival. Both IL-17 signaling and lipase activity are required for Lip2-mediated suppression. Lip2 inhibits IL-17 production indirectly by suppressing IL-23 production by tissue-resident dendritic cells. The lipase hydrolysis product, palmitic acid, similarly suppresses dendritic cell activation in vitro. Thus, C. albicans suppresses antifungal IL-17 defense in solid organs by altering the tissue lipid milieu.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

IL-23 signaling prevents ferroptosis-driven renal immunopathology during candidiasis

During infection the host relies on pattern-recognition receptors to sense invading fungal pathogens to launch immune defense mechanisms. While fungal recognition and immune effector responses are organ and cell type specific, during disseminated candidiasis myeloid cells exacerbate collateral tissue damage. The β-glucan receptor ephrin type-A 2 receptor (EphA2) is required to initiate mucosal inflammatory responses during oral Candida infection. Here we report that EphA2 promotes renal immunopathology during disseminated candidiasis. EphA2 deficiency leads to reduced renal inflammation and injury. Comprehensive analyses reveal that EphA2 restrains IL-23 secretion from and migration of dendritic cells. IL-23 signaling prevents ferroptotic host cell death during infection to limit inflammation and immunopathology. Further, host cell ferroptosis limits antifungal effector functions via releasing the lipid peroxidation product 4-hydroxynonenal to induce various forms of cell death. Thus, we identify ferroptotic cell death as a critical pathway of Candida-mediated renal immunopathology that opens a new avenue to tackle Candida infection and inflammation.

Arid5a Mediates an IL-17-Dependent Pathway That Drives Autoimmunity but Not Antifungal Host Defense

IL-17 contributes to the pathogenesis of certain autoimmune diseases, but conversely is essential for host defense against fungi. Ab-based biologic drugs that neutralize IL-17 are effective in autoimmunity but can be accompanied by adverse side effects. Candida albicans is a commensal fungus that is the primary causative agent of oropharyngeal and disseminated candidiasis. Defects in IL-17 signaling cause susceptibility to candidiasis in mice and humans. A key facet of IL-17 receptor signaling involves RNA-binding proteins, which orchestrate the fate of target mRNA transcripts. In tissue culture models we showed that the RNA-binding protein AT-rich interaction domain 5A (Arid5a) promotes the stability and/or translation of multiple IL-17-dependent mRNAs. Moreover, during oropharyngeal candidiasis, Arid5a is elevated within the oral mucosa in an IL-17-dependent manner. However, the contribution of Arid5a to IL-17-driven events in vivo is poorly defined. In this study, we used CRISPR-Cas9 to generate mice lacking Arid5a. Arid5a -/- mice were fully resistant to experimental autoimmune encephalomyelitis, an autoimmune setting in which IL-17 signaling drives pathology. Surprisingly, Arid5a -/- mice were resistant to oropharyngeal candidiasis and systemic candidiasis, similar to immunocompetent wild-type mice and contrasting with mice defective in IL-17 signaling. Therefore, Arid5a-dependent signals mediate pathology in autoimmunity and yet are not required for immunity to candidiasis, indicating that selective targeting of IL-17 signaling pathway components may be a viable strategy for development of therapeutics that spare IL-17-driven host defense.

Absence of Bacteria Permits Fungal Gut-To-Brain Translocation and Invasion in Germfree Mice but Ageing Alone Does Not Drive Pathobiont Expansion in Conventionally Raised Mice

Age-associated changes in the structure of the intestinal microbiome and in its interaction with the brain via the gut-brain axis are increasingly being implicated in neurological and neurodegenerative diseases. Intestinal microbial dysbiosis and translocation of microbes and microbial products including fungal species into the brain have been implicated in the development of dementias such as Alzheimer's disease. Using germ-free mice, we investigated if the fungal gut commensal, Candida albicans, an opportunistic pathogen in humans, can traverse the gastrointestinal barrier and disseminate to brain tissue and whether ageing impacts on the gut mycobiome as a pre-disposing factor in fungal brain infection. C. albicans was detected in different regions of the brain of colonised germ-free mice in both yeast and hyphal cell forms, often in close association with activated (Iba-1+) microglial cells. Using high-throughput ITS1 amplicon sequencing to characterise the faecal gut fungal composition of aged and young SPF mice, we identified several putative gut commensal fungal species with pathobiont potential although their abundance was not significantly different between young and aged mice. Collectively, these results suggest that although some fungal species can travel from the gut to brain where they can induce an inflammatory response, ageing alone is not correlated with significant changes in gut mycobiota composition which could predispose to these events. These results are consistent with a scenario in which significant disruptions to the gut microbiota or intestinal barrier, beyond those which occur with natural ageing, are required to allow fungal escape and brain infection.

Organ-specific host differential gene expression analysis in systemic candidiasis: A systems biology approach

Patients admitted to the hospital with coronavirus disease (COVID-19) are at risk for acquiring mycotic infections in particular Candidemia. Candida albicans (C. albicans) constitutes an important component of the human mycobiome and the most common cause of invasive fungal infections. Invasive yeast infections are gaining interest among the scientific community as a consequence of complications associated with severe COVID-19 infections. Early identification and surveillance for Candida infections is critical for decreasing the COVID-19 mortality. Our current study attempted to understand the molecular-level interactions between the human genes in different organs during systematic candidiasis. Our research findings have shed light on the molecular events that occur during Candidiasis in organs such as the kidney, liver, and spleen. The differentially expressed genes (up and down-regulated) in each organ will aid in designing organ-specific therapeutic protocols for systemic candidiasis. We observed organ-specific immune responses such as the development of the acute phase response in the liver; TGF-pathway and genes involved in lymphocyte activation, and leukocyte proliferation in the kidney. We have also observed that in the kidney, filament production, up-regulation of iron acquisition mechanisms, and metabolic adaptability are aided by the late initiation of innate defense mechanisms, which is likely related to the low number of resident immune cells and the sluggish recruitment of new effector cells. Our findings point to major pathways that play essential roles in specific organs during systemic candidiasis. The hub genes discovered in the study can be used to develop novel drugs for clinical management of Candidiasis.

Long-term antibiotic exposure promotes mortality after systemic fungal infection by driving lymphocyte dysfunction and systemic escape of commensal bacteria

Antibiotics are a modifiable iatrogenic risk factor for the most common human nosocomial fungal infection, invasive candidiasis, yet the underlying mechanisms remain elusive. We found that antibiotics enhanced the susceptibility to murine invasive candidiasis due to impaired lymphocyte-dependent IL-17A- and GM-CSF-mediated antifungal immunity within the gut. This led to non-inflammatory bacterial escape and systemic bacterial co-infection, which could be ameliorated by IL-17A or GM-CSF immunotherapy. Vancomycin alone similarly enhanced the susceptibility to invasive fungal infection and systemic bacterial co-infection. Mechanistically, vancomycin reduced the frequency of gut Th17 cells associated with impaired proliferation and RORγt expression. Vancomycin's effects on Th17 cells were indirect, manifesting only in vivo in the presence of dysbiosis. In humans, antibiotics were associated with an increased risk of invasive candidiasis and death after invasive candidiasis. Our work highlights the importance of antibiotic stewardship in protecting vulnerable patients from life-threatening infections and provides mechanistic insights into a controllable iatrogenic risk factor for invasive candidiasis.

In vitro and in vivo Characterization of Host–Pathogen Interactions of the L3881 Candida albicans Clinical Isolate

Candida albicans is a human commensal fungus and the etiologic agent of nosocomial infections in immunocompromised individuals. Candida spp. is the most studied human fungal pathogen, and the mechanisms by which this fungus can evade the immune system affecting immunosuppressed individuals have been extensively studied. Most of these studies focus on different species of Candida, and there is much to be understood in virulence variability among lineages, specifically different C. albicans clinical isolates. To better understand the main mechanisms of its virulence variability modulated in C. albicans clinical isolates, we characterized L3881 lineage, which has been previously classified as hypovirulent, and SC5314 lineage, a virulent wild-type control, by using both in vitro and in vivo assays. Our findings demonstrated that L3881 presented higher capacity to avoid macrophage phagocytosis and higher resistance to oxidative stress than the wild type. These characteristics prevented higher mortality rates for L3881 in the animal model of candidiasis. Conversely, L3881 has been able to induce an upregulation of pro-inflammatory mediators both in vitro and in vivo. These results indicated that in vitro and in vivo functional characterizations are necessary for determination of virulence in different clinical isolates due to its modulation in the host–pathogen interactions.

Invasive Candidiasis: Update and Current Challenges in the Management of This Mycosis in South America

Invasive candidiasis encompassing Candida bloodstream infections and deep-seated candidiasis can become a persistent health problem. These infections are caused by Candida species and have high morbidity and mortality rates. Species distribution, access to diagnosis, treatment and mortality are different around the world. The mortality rate is high in South America (30–70%), and Candida albicans is the most prevalent species in this region. However, a global epidemiological shift to non-albicans species has been observed. In this group, C. parapsilosis is the species most frequently detected, followed by C. tropicalis, and at a slower rate, C. glabrata, which has also increased, in addition to the emerging C. auris, resistance to several drugs. This article summarizes relevant aspects of candidemia pathogenesis, such as the mechanisms of fungal invasion, immune response, and the impact of genetic defects that increase host susceptibility to developing the infection. We also discuss relevant aspects of treatment and future challenges in South America.

Innate immune responses against the fungal pathogen Candida auris

Candida auris is a multidrug-resistant human fungal pathogen responsible for nosocomial outbreaks worldwide. Although considerable progress has increased our understanding of the biological and clinical aspects of C. auris, its interaction with the host immune system is only now beginning to be investigated in-depth. Here, we compare the innate immune responses induced by C. auris BJCA001 and Candida albicans SC5314 in vitro and in vivo. Our results indicate that C. auris BJCA001 appears to be less immunoinflammatory than C. albicans SC5314, and this differential response correlates with structural features of the cell wall.

Candida auris is a multidrug-resistant human fungal pathogen responsible for nosocomial outbreaks worldwide. Here, the authors identify differential innate immune responses induced by C. auris and Candida albicans in vitro and in vivo, which correlate with structural features of the cell wall.

Efficacy of Cochleated Amphotericin B in Mouse and Human Mucocutaneous Candidiasis

Candida albicans causes debilitating, often azole-resistant, infections in patients with chronic mucocutaneous candidiasis (CMC). Amphotericin B (AMB) resistance is rare, but AMB use is limited by parenteral administration and nephrotoxicity. In this study, we evaluated cochleated AMB (CAMB), a new oral AMB formulation, in mouse models of oropharyngeal candidiasis (OPC) and vulvovaginal candidiasis (VVC) and in patients with azole-resistant CMC. OPC and VVC were modeled in Act1^-/- mice, and mucosal tissue fungal burden was assessed after once-daily treatment with CAMB, vehicle, or AMB-deoxycholate (AMB-d). Four patients with azole-resistant CMC enrolled in a phase 2 CAMB dose-escalation study. The primary endpoint was clinical improvement at 2 weeks followed by optional extension for long-term CMC suppression to assess safety and efficacy. CAMB-treated mice had significantly reduced tongue and vaginal fungal burdens compared to vehicle-treated mice and exhibited comparable fungal burden reduction relative to AMB-d-treated mice. All CAMB-treated patients reached clinical efficacy by 2 weeks, three at 400 mg twice daily and one at 200 mg twice-daily dosing. All patients continued to the extension phase, with three having sustained clinical improvement of OPC and esophageal candidiasis (EC) for up to 60 months. One patient had a relapse of esophageal symptoms at week 24 and was withdrawn from further study. Clinical responses were not seen for onychomycosis or VVC. CAMB was safe and well-tolerated, without any evidence of nephrotoxicity. In summary, oral CAMB reduced tongue and vaginal fungal burdens during murine candidiasis. A proof-of-concept clinical trial in human CMC showed efficacy with good tolerability and safety. This study has been registered at ClinicalTrials.gov under identifier NCT02629419.

Collaborative Action of Microglia and Astrocytes Mediates Neutrophil Recruitment to the CNS to Defend against Escherichia coli K1 Infection

Escherichia coli K1 is a leading cause of neonatal bacterial meningitis. Recruitment of neutrophils to the central nervous system (CNS) via local immune response plays a critical role in defense against E. coli K1 infection; however, the mechanism underlying this recruitment remains unclear. In this study, we report that microglia and astrocytes are activated in response to stimulation by E. coli K1 and/or E. coli K1-derived outer membrane vesicles (OMVs) and work collaboratively to drive neutrophil recruitment to the CNS. Microglial activation results in the release of the pro-inflammatory cytokine TNF-α, which activates astrocytes, resulting in the production of CXCL1, a chemokine critical for recruiting neutrophils. Mice lacking either microglia or TNF-α exhibit impaired production of CXCL1, impaired neutrophil recruitment, and an increased CNS bacterial burden. C-X-C chemokine receptor 2 (CXCR2)-expressing neutrophils primarily respond to CXCL1 released by astrocytes. This study provides further insights into how immune responses drive neutrophil recruitment to the brain to combat E. coli K1 infection. In addition, we show that direct recognition of E. coli K1 by microglia is prevented by the K1 capsule. This study also reveals that OMVs are sufficient to induce microglial activation.

Systemic Infection by Non-albicans Candida Species Affects the Development of a Murine Model of Multiple Sclerosis

Candidiasis may affect the central nervous system (CNS), and although Candida albicans is predominant, non-albicans Candida species can also be associated with CNS infections. Some studies have suggested that Candida infections could increase the odds of multiple sclerosis (MS) development. In this context, we investigated whether systemic infection by non-albicans Candida species would affect, clinically or immunologically, the severity of experimental autoimmune encephalomyelitis (EAE), which is an animal model used to study MS. For this, a strain of C. glabrata, C. krusei, and C. parapsilosis was selected and characterized using different in vitro and in vivo models. In these analysis, all the strains exhibited the ability to form biofilms, produce proteolytic enzymes, and cause systemic infections in Galleria mellonella, with C. glabrata being the most virulent species. Next, C57BL/6 mice were infected with strains of C. glabrata, C. krusei, or C. parapsilosis, and 3 days later were immunized with myelin oligodendrocyte glycoprotein to develop EAE. Mice from EAE groups previously infected with C. glabrata and C. krusei developed more severe and more prevalent paralysis, while mice from the EAE group infected with C. parapsilosis developed a disease comparable to non-infected EAE mice. Disease aggravation by C. glabrata and C. krusei strains was concomitant to increased IL-17 and IFN-γ production by splenic cells stimulated with fungi-derived antigens and with increased percentage of T lymphocytes and myeloid cells in the CNS. Analysis of interaction with BV-2 microglial cell line also revealed differences among these strains, in which C. krusei was the strongest activator of microglia concerning the expression of MHC II and CD40 and pro-inflammatory cytokine production. Altogether, these results indicated that the three non-albicans Candida strains were similarly able to reach the CNS but distinct in terms of their effect over EAE development. Whereas C. glabrata and C. Krusei aggravated the development of EAE, C. parapsilosis did not affect its severity. Disease worsening was partially associated to virulence factors in C. glabrata and to a strong activation of microglia in C. krusei infection. In conclusion, systemic infections by non-albicans Candida strains exerted influence on the experimental autoimmune encephalomyelitis in both immunological and clinical aspects, emphasizing their possible relevance in MS development.

PD-L1 Antibody Pharmacokinetics and Tumor Targeting in Mouse Models for Infectious Diseases

Background

Programmed death-ligand 1 (PD-L1) regulates immune homeostasis by promoting T-cell exhaustion. It is involved in chronic infections and tumor progression. Nuclear imaging using radiolabeled anti-PD-L1 antibodies can monitor PD-L1 tissue expression and antibody distribution. However, physiological PD-L1 can cause rapid antibody clearance from blood at imaging doses. Therefore, we hypothesized that inflammatory responses, which can induce PD-L1 expression, affect anti-PD-L1 antibody distribution. Here, we investigated the effects of three different infectious stimuli on the pharmacokinetics and tumor targeting of radiolabeled anti-PD-L1 antibodies in tumor-bearing mice.

Materials/Methods

Anti-mouse-PD-L1 and isotype control antibodies were labelled with indium-111 ([¹¹¹In]In-DTPA-anti-mPD-L1 and [¹¹¹In]In-DTPA-IgG2a, respectively). We evaluated the effect of inflammatory responses on the pharmacokinetics of [¹¹¹In]In-DTPA-anti-mPD-L1 in RenCa tumor-bearing BALB/c mice in three conditions: lipopolysaccharide (LPS), local Staphylococcus aureus, and heat-killed Candida albicans. After intravenous injection of 30 or 100 µg of [¹¹¹In]In-DTPA-anti-mPD-L1 or [¹¹¹In]In-DTPA-IgG2a, blood samples were collected 1, 4, and 24 h p.i. followed by microSPECT/CT and ex vivo biodistribution analyses. PD-L1 expression, neutrophil, and macrophage infiltration in relevant tissues were evaluated immunohistochemically.

Results

In 30 µg of [¹¹¹In]In-DTPA-anti-mPD-L1 injected tumor-bearing mice the LPS-challenge significantly increased lymphoid organ uptake compared with vehicle controls (spleen: 49.9 ± 4.4%ID/g versus 21.2 ± 6.9%ID/g, p < 0.001), resulting in lower blood levels (3.6 ± 1.6%ID/g versus 11.5 ± 7.2%ID/g; p < 0.01) and reduced tumor targeting (8.1 ± 4.5%ID/g versus 25.2 ± 5.2%ID/g, p < 0.001). Local S. aureus infections showed high PD-L1⁺ neutrophil influx resulting in significantly increased [¹¹¹In]In-DTPA-anti-mPD-L1 uptake in affected muscles (8.6 ± 2.6%ID/g versus 1.7 ± 0.8%ID/g, p < 0.001). Heat-killed Candida albicans (Hk-C. albicans) challenge did not affect pharmacokinetics. Increasing [¹¹¹In]In-DTPA-anti-mPD-L1 dose to 100 µg normalized blood clearance and tumor uptake in LPS-challenged mice, although lymphoid organ uptake remained higher. Infectious stimuli did not affect [¹¹¹In]In-DTPA-IgG2a pharmacokinetics.

Conclusions

This study shows that anti-PD-L1 antibody pharmacokinetics and tumor targeting can be significantly altered by severe inflammatory responses, which can be compensated for by increasing the tracer dose. This has implications for developing clinical PD-L1 imaging protocols in onco-immunology. We further demonstrate that radiolabeled anti-PD-L1 antibodies can be used to evaluate PD-L1 expression changes in a range of infectious diseases. This supports the exploration of using these techniques to assess hosts' responses to infectious stimuli.

TLR Signaling Rescues Fungicidal Activity in Syk-Deficient Neutrophils

An impaired neutrophil response to pathogenic fungi puts patients at risk for fungal infections with a high risk of morbidity and mortality. Acquired neutrophil dysfunction in the setting of iatrogenic immune modulators can include the inhibition of critical kinases such as spleen tyrosine kinase (Syk). In this study, we used an established system of conditionally immortalized mouse neutrophil progenitors to investigate the ability to augment Syk-deficient neutrophil function against Candida albicans with TLR agonist signaling. LPS, a known immunomodulatory molecule derived from Gram-negative bacteria, was capable of rescuing effector functions of Syk-deficient neutrophils, which are known to have poor fungicidal activity against Candida species. LPS priming of Syk-deficient mouse neutrophils demonstrates partial rescue of fungicidal activity, including phagocytosis, degranulation, and neutrophil swarming, but not reactive oxygen species production against C. albicans, in part due to c-Fos activation. Similarly, LPS priming of human neutrophils rescues fungicidal activity in the presence of pharmacologic inhibition of Syk and Bruton's tyrosine kinase (Btk), both critical kinases in the innate immune response to fungi. In vivo, neutropenic mice were reconstituted with wild-type or Syk-deficient neutrophils and challenged i.p. with C. albicans. In this model, LPS improved wild-type neutrophil homing to the fungal challenge, although Syk-deficient neutrophils did not persist in vivo, speaking to its crucial role on in vivo persistence. Taken together, we identify TLR signaling as an alternate activation pathway capable of partially restoring neutrophil effector function against Candida in a Syk-independent manner.

Transfusable neutrophil progenitors as cellular therapy for the prevention of invasive fungal infections

The use of mature neutrophil (granulocyte) transfusions for the treatment of neutropenic patients with invasive fungal infections (IFIs) has been the focus of multiple clinical trials. Despite these efforts, the transfusion of mature neutrophils has resulted in limited clinical benefit, likely owing to problems of insufficient numbers and the very short lifespan of these donor cells. In this report, we employed a system of conditionally immortalized murine neutrophil progenitors that are capable of continuous expansion, allowing for the generation of unlimited numbers of homogenous granulocyte-macrophage progenitors (GMPs). These GMPs were assayed in vivo to demonstrate their effect on survival in 2 models of IFI: candidemia and pulmonary aspergillosis. Mature neutrophils derived from GMPs executed all cardinal functions of neutrophils. Transfused GMPs homed to the bone marrow and spleen, where they completed normal differentiation to mature neutrophils. These neutrophils were capable of homing and extravasation in response to inflammatory stimuli using a sterile peritoneal challenge model. Furthermore, conditionally immortalized GMP transfusions significantly improved survival in models of candidemia and pulmonary aspergillosis. These data confirm the therapeutic benefit of prophylactic GMP transfusions in the setting of neutropenia and encourage development of progenitor cellular therapies for the management of fungal disease in high-risk patients.

Control of β-glucan exposure by the endo-1,3-glucanase Eng1 in Candida albicans modulates virulence

Candida albicans is a major opportunistic pathogen of humans. It can grow as morphologically distinct yeast, pseudohyphae and hyphae, and the ability to switch reversibly among different forms is critical for its virulence. The relationship between morphogenesis and innate immune recognition is not quite clear. Dectin-1 is a major C-type lectin receptor that recognizes β-glucan in the fungal cell wall. C. albicans β-glucan is usually masked by the outer mannan layer of the cell wall. Whether and how β-glucan masking is differentially regulated during hyphal morphogenesis is not fully understood. Here we show that the endo-1,3-glucanase Eng1 is differentially expressed in yeast, and together with Yeast Wall Protein 1 (Ywp1), regulates β-glucan exposure and Dectin-1-dependent immune activation of macrophage by yeast cells. ENG1 deletion results in enhanced Dectin-1 binding at the septa of yeast cells; while eng1 ywp1 yeast cells show strong overall Dectin-1 binding similar to hyphae of wild-type and eng1 mutants. Correlatively, hyphae of wild-type and eng1 induced similar levels of cytokines in macrophage. ENG1 expression and Eng1-mediated β-glucan trimming are also regulated by antifungal drugs, lactate and N-acetylglucosamine. Deletion of ENG1 modulates virulence in the mouse model of hematogenously disseminated candidiasis in a Dectin-1-dependent manner. The eng1 mutant exhibited attenuated lethality in male mice, but enhanced lethality in female mice, which was associated with a stronger renal immune response and lower fungal burden. Thus, Eng1-regulated β-glucan exposure in yeast cells modulates the balance between immune protection and immunopathogenesis during disseminated candidiasis.

Effects of two formulations containing Phyllanthus emblica and Tinospora cordifolia with and without Ocimum sanctum in immunocompromised mice

Background

Current pandemic has led us to explore the role of traditional system of medicine to look for formulations that enhance immunity.

Objective

The obejctive of this experimental study was to evaluate the immunomodulatory effects of two formulations, Tinospora cordifolia (Tc) and Phyllanthus emblica (Pe) with and without coating of Ocimum sanctum (Os).

Materials and methods

After obtaining Institutional Animal Ethics Committee approval, present experimental study was conducted to evaluate the immunomodulatory effects of plant drug formulations against infection induced in mice subjected to major surgical stress. Hemisplenectomy was selected to induce major stress and the procedure for hemisplenectomy was standardized. A model of secondary fungal infection after hemisplenectomy was established followed by the treatment of mice with plant drugs and controls. They were subjected to hemisplenectomy or sham operation and 10⁵ C. albicans were injected intravenously. The therapy continued for next 14 days. Kidneys were isolated to estimate fungal load. Fungal load of the kidneys was estimated on post-operative Day 15.

Results

The test formulations Tc + Pe and Tc + Pe + Os showed significant reduction in the fungal burden of kidneys as compared to hemisplenectomized control group. However, Tc alone exerted better degree of protection as compared to Tc + Pe and Tc + Pe + Os.

Conclusion

The formulations, Tc + Pe and Tc + Pe + Os that were developed on the basis of theoretical concepts were not found to be superior to Tc. Though the individual ingredients have been shown to possess immunnostimulant activities, in combination, Pe and Os blunted the effects of Tc. The basis for this drug interaction needs further exploration. Thus, the current experimental study validates immunomodulatory role of Tc. However, the addition of Pe with bhavana of Os does not lead to any augmentation of immunomodulatory activity of Tc. This study also underlines the need to generate data on Ayurveda formulations to understand the rationality of the multi-ingredient Ayurvedic formulations.

On and Off: Epigenetic Regulation of C. albicans Morphological Switches

The human fungal pathogen Candida albicans is a dimorphic opportunistic pathogen that colonises most of the human population without creating any harm. However, this fungus can also cause life-threatening infections in immunocompromised individuals. The ability to successfully colonise different host niches is critical for establishing infections and pathogenesis. C. albicans can live and divide in various morphological forms critical for its survival in the host. Indeed, C. albicans can grow as both yeast and hyphae and can form biofilms containing hyphae. The transcriptional regulatory network governing the switching between these different forms is complex but well understood. In contrast, non-DNA based epigenetic modulation is emerging as a crucial but still poorly studied regulatory mechanism of morphological transition. This review explores our current understanding of chromatin-mediated epigenetic regulation of the yeast to hyphae switch and biofilm formation. We highlight how modification of chromatin structure and non-coding RNAs contribute to these morphological transitions.

Trained Innate Immunity Induced by Vaccination with Low-Virulence Candida Species Mediates Protection against Several Forms of Fungal Sepsis via Ly6G+ Gr-1+ Leukocytes

We recently discovered a novel form of trained innate immunity (TII) induced by low-virulence Candida species (i.e., Candida dubliniensis) that protects against lethal fungal/bacterial infection. Mice vaccinated by intraperitoneal (i.p.) inoculation are protected against lethal sepsis following Candida albicans/Staphylococcus aureus (Ca/Sa) intra-abdominal infection (IAI) or Ca bloodstream infection (BSI). The protection against IAI is mediated by long-lived Gr-1⁺ leukocytes as putative myeloid-derived suppressor cells (MDSCs) and not by prototypical trained macrophages. This study aimed to determine if a similar TII mechanism (Gr-1⁺ cell-mediated suppression of sepsis) is protective against BSI and whether this TII can also be induced following intravenous (i.v.) vaccination. For this, mice were vaccinated with low-virulence Candida strains (i.p. or i.v.), followed by lethal challenge (Ca/Sa i.p. or Ca i.v.) 14 days later, and observed for sepsis (hypothermia, sepsis scoring, and serum cytokines), organ fungal burden, and mortality. Similar parameters were monitored following depletion of macrophages or Gr-1⁺ leukocytes during lethal challenge. The results showed that mice vaccinated i.p. or i.v. were protected against lethal Ca/Sa IAI or Ca BSI. In all cases, protection was mediated by Ly6G⁺ Gr-1⁺ putative granulocytic MDSCs (G-MDSCs), with no role for macrophages, and correlated with reduced sepsis parameters. Protection also correlated with reduced fungal burden in spleen and brain but not liver or kidney. These results suggest that Ly6G⁺ G-MDSC-mediated TII is induced by either the i.p. and i.v. route of inoculation and protects against IAI or BSI forms of systemic candidiasis, with survival correlating with amelioration of sepsis and reduced organ-specific fungal burden. IMPORTANCE Trained innate immunity (TII) is induced following immunization with live attenuated microbes and represents a clinically important strategy to enhance innate defenses. TII was initially demonstrated following intravenous inoculation with low-virulence Candida albicans, with protection against a subsequent lethal C. albicans intravenous bloodstream infection (BSI) mediated by monocytes with enhanced cytokine responses. We expanded this by describing a novel form of TII induced by intraperitoneal inoculation with low-virulence Candida that protects against lethal sepsis induced by polymicrobial intra-abdominal infection (IAI) via Gr-1⁺ leukocytes as putative myeloid-derived suppressor cells (MDSCs). In this study, we addressed these two scenarios and confirmed an exclusive role for Ly6G⁺ Gr-1⁺ leukocytes in mediating TII against either IAI or BSI via either route of inoculation, with protection associated with suppression of sepsis. These studies highlight the previously unrecognized importance of Ly6G⁺ MDSCs as central mediators of a novel form of TII termed trained tolerogenic immunity.

Murine Inducible Nitric Oxide Synthase Expression Is Essential for Antifungal Defenses in Kidneys during Disseminated Cryptococcus deneoformans Infection

Disseminated cryptococcosis has a nearly 70% mortality, mostly attributed to CNS infection, with lesser-known effects on other organs. Immune protection against Cryptococcus relies on Th1 immunity with M1 polarization, rendering macrophages fungicidal. The importance of M1-upregulated inducible NO synthase (iNOS) has been documented in pulmonary anticryptococcal defenses, whereas its role in disseminated cryptococcosis remains controversial. Here we examined the effect of iNOS deletion in disseminated (i.v.) C. deneoformans 52D infection, comparing wild-type (C57BL/6J) and iNOS^-/- mice. iNOS^-/- mice had significantly reduced survival and nearly 100-fold increase of the kidney fungal burden, without increases in the lungs, spleen, or brain. Histology revealed extensive lesions and almost complete destruction of the kidney cortical area with a loss of kidney function. The lack of fungal control was not due to a failure to recruit immune cells because iNOS^-/- mice had increased kidney leukocytes. iNOS^-/- mice also showed no defect in T cell polarization. We conclude that iNOS is critically required for local anticryptococcal defenses in the kidneys, whereas it appears to be dispensable in other organs during disseminated infection. This study exemplifies a unique phenotype of local immune defenses in the kidneys and the organ-specific importance of a single fungicidal pathway.

The Role of HMGB1 in Invasive Candida albicans Infection

INTRODUCTION

High mobility group box 1 (HMGB1) is an important "late" inflammatory mediator in bacterial sepsis. Ethyl pyruvate (EP), an inhibitor of HMGB1, can prevent bacterial sepsis by decreasing HMGB1 levels. However, the role of HMGB1 in fungal sepsis is still unclear. Therefore, we investigated the role of HMGB1 and EP in invasive C. albicans infection.

METHODS

We measured serum HMGB1 levels in patients with sepsis with C. albicans infection and without fungal infection, and control subjects. We collected clinical indices to estimate correlations between HMGB1 levels and disease severity. Furthermore, we experimentally stimulated mice with C. albicans and C. albicans + EP. Then, we examined HMGB1 levels from serum and tissue, investigated serum levels of tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6), determined pathological changes in tissues, and assessed mortality.

RESULTS

Serum HMGB1 levels in patients with severe sepsis with C. albicans infection were elevated. Increased HMGB1 levels were correlated with procalcitonin (PCT), C-reactive protein (CRP), 1,3-β-D-Glucan (BDG) and C. albicans sepsis severity. HMGB1 levels in serum and tissues were significantly increased within 7 days after mice were infected with C. albicans. The administration of EP inhibited HMGB1 levels, decreased tissue damage, increased survival rates and inhibited the release of TNF-α and IL-6.

CONCLUSIONS

HMGB1 levels were significantly increased in invasive C. albicans infections. EP prevented C. albicans lethality by decreasing HMGB1 expression and release. HMGB1 may provide an effective diagnostic and therapeutic target for invasive C. albicans infections.

Candida tropicalis Systemic Infection Redirects Leukocyte Infiltration to the Kidneys Attenuating Encephalomyelitis

Environmental factors, including infections, are strongly associated with the pathogenesis of multiple sclerosis (MS), which is an autoimmune and demyelinating disease of the central nervous system (CNS). Although classically associated with bacterial and viral agents, fungal species have also been suspected to affect the course of the disease. Candida tropicalis is an opportunistic fungus that affects immunocompromised individuals and is also able to spread to vital organs. As C. tropicalis has been increasingly isolated from systemic infections, we aimed to evaluate the effect of this fungus on experimental autoimmune encephalomyelitis (EAE), a murine model to study MS. For this, EAE was induced in female C57BL/6 mice 3 days after infection with 10⁶ viable C. tropicalis yeasts. The infection decreased EAE prevalence and severity, confirmed by the less inflammatory infiltrate and less demyelization in the lumbar spinal cord. Despite this, C. tropicalis infection associated with EAE results in the death of some animals and increased urea and creatinine serum levels. The kidneys of EAE-infected mice showed higher fungal load associated with increased leukocyte infiltration (CD45⁺ cells) and higher expression of T-box transcription factor (Tbx21) and forkhead box P3 (Foxp3). Altogether, our results demonstrate that although C. tropicalis infection reduces the prevalence and severity of EAE, partially due to the sequestration of leukocytes by the inflamed renal tissue, this effect is associated with a poor disease outcome.