Phagocytosis of Histoplasma capsulatum yeasts and microconidia by human cultured macrophages and alveolar macrophages. Cellular cytoskeleton requirement for attachment and ingestion

An integrated toolkit for human microglia functional genomics

BACKGROUND

Microglia, the brain's resident immune cells, play vital roles in brain development, and disorders like Alzheimer's disease (AD). Human iPSC-derived microglia (iMG) provide a promising model to study these processes. However, existing iMG generation protocols face challenges, such as prolonged differentiation time, lack of detailed characterization, and limited gene function investigation via CRISPR-Cas9.

METHODS

Our integrated toolkit for in-vitro microglia functional genomics optimizes iPSC differentiation into iMG through a streamlined two-step, 20-day process, producing iMG with a normal karyotype. We confirmed the iMG's authenticity and quality through single-cell RNA sequencing, chromatin accessibility profiles (ATAC-Seq), proteomics and functional tests. The toolkit also incorporates a drug-dependent CRISPR-ON/OFF system for temporally controlled gene expression. Further, we facilitate the use of multi-omic data by providing online searchable platform that compares new iMG profiles to human primary microglia: https://sherlab.shinyapps.io/IPSC-derived-Microglia/ .

RESULTS

Our method generates iMG that closely align with human primary microglia in terms of transcriptomic, proteomic, and chromatin accessibility profiles. Functionally, these iMG exhibit Ca2 + transients, cytokine driven migration, immune responses to inflammatory signals, and active phagocytosis of CNS related substrates including synaptosomes, amyloid beta and myelin. Significantly, the toolkit facilitates repeated iMG harvesting, essential for large-scale experiments like CRISPR-Cas9 screens. The standalone ATAC-Seq profiles of our iMG closely resemble primary microglia, positioning them as ideal tools to study AD-associated single nucleotide variants (SNV) especially in the genome regulatory regions.

CONCLUSIONS

Our advanced two-step protocol rapidly and efficiently produces authentic iMG. With features like the CRISPR-ON/OFF system and a comprehensive multi-omic data platform, our toolkit equips researchers for robust microglial functional genomic studies. By facilitating detailed SNV investigation and offering a sustainable cell harvest mechanism, the toolkit heralds significant progress in neurodegenerative disease drug research and therapeutic advancement.

Pathogenicity & Virulence of Histoplasma capsulatum - a multifaceted organism adapted to intracellular environments

Histoplasmosis is a systemic mycosis caused by the thermally dimorphic fungus Histoplasma capsulatum. Although healthy individuals can develop histoplasmosis, the disease is particularly life-threatening in immunocompromised patients, with a wide range of clinical manifestations depending on the inoculum and virulence of the infecting strain. In this review, we discuss the established virulence factors and pathogenesis traits that make H. capsulatum highly adapted to a wide variety of hosts, including mammals. Understanding and integrating these mechanisms is a key step towards devising new preventative and therapeutic interventions.

Moonlighting proteins in medically relevant fungi

Moonlighting proteins represent an intriguing area of cell biology, due to their ability to perform two or more unrelated functions in one or many cellular compartments. These proteins have been described in all kingdoms of life and are usually constitutively expressed and conserved proteins with housekeeping functions. Although widely studied in pathogenic bacteria, the information about these proteins in pathogenic fungi is scarce, but there are some reports of their functions in the etiological agents of the main human mycoses, such as Candida spp., Paracoccidioides brasiliensis, Histoplasma capsulatum, Aspergillus fumigatus, Cryptococcus neoformans, and Sporothrix schenckii. In these fungi, most of the described moonlighting proteins are metabolic enzymes, such as enolase and glyceraldehyde-3-phosphate dehydrogenase; chaperones, transcription factors, and redox response proteins, such as peroxiredoxin and catalase, which moonlight at the cell surface and perform virulence-related processes, contributing to immune evasion, adhesions, invasion, and dissemination to host cells and tissues. All moonlighting proteins and their functions described in this review highlight the limited information about this biological aspect in pathogenic fungi, representing this a relevant opportunity area that will contribute to expanding our current knowledge of these organisms' pathogenesis.

Cbp1, a fungal virulence factor under positive selection, forms an effector complex that drives macrophage lysis

Intracellular pathogens secrete effectors to manipulate their host cells. Histoplasma capsulatum (Hc) is a fungal intracellular pathogen of humans that grows in a yeast form in the host. Hc yeasts are phagocytosed by macrophages, where fungal intracellular replication precedes macrophage lysis. The most abundant virulence factor secreted by Hc yeast cells is Calcium Binding Protein 1 (Cbp1), which is absolutely required for macrophage lysis. Here we take an evolutionary, structural, and cell biological approach to understand Cbp1 function. We find that Cbp1 is present only in the genomes of closely related dimorphic fungal species of the Ajellomycetaceae family that lead primarily intracellular lifestyles in their mammalian hosts (Histoplasma, Paracoccidioides, and Emergomyces), but not conserved in the extracellular fungal pathogen Blastomyces dermatitidis. We observe a high rate of fixation of non-synonymous substitutions in the Cbp1 coding sequences, indicating that Cbp1 is under positive selection. We determine the de novo structures of Hc H88 Cbp1 and the Paracoccidioides americana (Pb03) Cbp1, revealing a novel "binocular" fold consisting of a helical dimer arrangement wherein two helices from each monomer contribute to a four-helix bundle. In contrast to Pb03 Cbp1, we show that Emergomyces Cbp1 orthologs are unable to stimulate macrophage lysis when expressed in the Hc cbp1 mutant. Consistent with this result, we find that wild-type Emergomyces africanus yeast are able to grow within primary macrophages but are incapable of lysing them. Finally, we use subcellular fractionation of infected macrophages and indirect immunofluorescence to show that Cbp1 localizes to the macrophage cytosol during Hc infection, making this the first instance of a phagosomal human fungal pathogen directing an effector into the cytosol of the host cell. We additionally show that Cbp1 forms a complex with Yps-3, another known Hc virulence factor that accesses the cytosol. Taken together, these data imply that Cbp1 is a fungal virulence factor under positive selection that localizes to the cytosol to trigger host cell lysis.

Deletion of the Stress Response Gene DDR48 from Histoplasma capsulatum Increases Sensitivity to Oxidative Stress, Increases Susceptibility to Antifungals, and Decreases Fitness in Macrophages

The stress response gene DDR48 has been characterized in Saccharomyces cerevisiae and Candida albicans to be involved in combating various cellular stressors, from oxidative agents to antifungal compounds. Surprisingly, the biological function of DDR48 has yet to be identified, though it is likely an important part of the stress response. To gain insight into its function, we characterized DDR48 in the dimorphic fungal pathogen Histoplasma capsulatum. Transcriptional analyses showed preferential expression of DDR48 in the mycelial phase. Induction of DDR48 in Histoplasma yeasts developed after treatment with various cellular stress compounds. We generated a ddr48∆ deletion mutant to further characterize DDR48 function. Loss of DDR48 alters the transcriptional profile of the oxidative stress response and membrane synthesis pathways. Treatment with ROS or antifungal compounds reduced survival of ddr48∆ yeasts compared to controls, consistent with an aberrant cellular stress response. In addition, we infected RAW 264.7 macrophages with DDR48-expressing and ddr48∆ yeasts and observed a 50% decrease in recovery of ddr48∆ yeasts compared to wild-type yeasts. Loss of DDR48 function results in numerous negative effects in Histoplasma yeasts, highlighting its role as a key player in the global sensing and response to cellular stress by fungi.

Clearance of pegylated interferon by Kupffer cells limits NK cell activation and therapy response of patients with HBV infection

Pegylated interferon-α (PEG-IFN-α), where IFN-α is attached to polyethylene glycol (PEG), is an approved treatment for chronic hepatitis B virus (HBV) infection, a disease that causes liver-related morbidity and mortality in 257 million people worldwide. It is unknown why only a minority of patients respond to PEG-IFN-α. Using sequential blood samples and liver biopsies of patients with chronic HBV infection before, during, and after PEG-IFN-α treatment, we find that patients with early natural killer (NK) cell activation after PEG-IFN-α injection experienced greater liver inflammation, lysis of HBV-infected hepatocytes, and hepatitis B surface antigen (HBsAg) decline than those without. NK cell activation was associated with induction of interferon-stimulated genes and determined by PEG-IFN-α pharmacokinetics. Patients with delayed increases in PEG-IFN-α concentrations had greater amounts of PEG-specific immunoglobulin M (IgM) immune complexes in the blood and more PEG and IgM detected in the liver than patients with rapid increase in PEG-IFN-α concentration. This was associated with reduced NK cell activation. These results indicate that the immunomodulatory functions of PEG-IFN-α, particularly activation of NK cells, play a pivotal role in the response to treatment and further demonstrate that these functions are affected by PEG-IFN-α pharmacokinetics. Accelerated clearance of antibody-complexed pegylated drugs by Kupffer cells may be important beyond the field of HBV therapeutics. Thus, these findings may contribute to improving the efficacy of pegylated drugs that are now being developed for other chronic diseases and cancer.

Heat Shock Protein 60, Insights to Its Importance in Histoplasma capsulatum: From Biofilm Formation to Host-Interaction

Heat shock proteins (Hsps) are among the most widely distributed and evolutionary conserved proteins, acting as essential regulators of diverse constitutive metabolic processes. The Hsp60 of the dimorphic fungal Histoplasma capsulatum is the major surface adhesin to mammalian macrophages and studies of antibody-mediated protection against H. capsulatum have provided insight into the complexity involving Hsp60. However, nothing is known about the role of Hsp60 regarding biofilms, a mechanism of virulence exhibited by H. capsulatum. Considering this, the present study aimed to investigate the influence of the Hsp60 on biofilm features of H. capsulatum. Also, the non-conventional model Galleria mellonella was used to verify the effect of this protein during in vivo interaction. The use of invertebrate models such as G. mellonella is highly proposed for the evaluation of pathogenesis, immune response, virulence mechanisms, and antimicrobial compounds. For that purpose, we used a monoclonal antibody (7B6) against Hsp60 and characterized the biofilm of two H. capsulatum strains by metabolic activity, biomass content, and images from scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). We also evaluated the survival rate of G. mellonella infected with both strains under blockage of Hsp60. The results showed that mAb 7B6 was effective to reduce the metabolic activity and biomass of both H. capsulatum strains. Furthermore, the biofilms of cells treated with the antibody were thinner as well as presented a lower amount of cells and extracellular polymeric matrix compared to its non-treated controls. The blockage of Hsp60 before fungal infection of G. mellonella larvae also resulted in a significant increase of the larvae survival compared to controls. Our results highlight for the first time the importance of the Hsp60 protein to the establishment of the H. capsulatum biofilms and the G. mellonella larvae infection. Interestingly, the results with Hsp60 mAb 7B6 in this invertebrate model suggest a pattern of fungus-host interaction different from those previously found in a murine model, which can be due to the different features between insect and mammalian immune cells such as the absence of Fc receptors in hemocytes. However further studies are needed to support this hypothesis

Living Within the Macrophage: Dimorphic Fungal Pathogen Intracellular Metabolism

Histoplasma and Paracoccidioides are related thermally dimorphic fungal pathogens that cause deadly mycoses (i.e., histoplasmosis and paracoccidioidomycosis, respectively) primarily in North, Central, and South America. Mammalian infection results from inhalation of conidia and their subsequent conversion into pathogenic yeasts. Macrophages in the lung are the first line of defense, but are generally unable to clear these fungi. Instead, Histoplasma and Paracoccidioides yeasts survive and proliferate within the phagosomal compartment of host macrophages. Growth within macrophages requires strategies for acquisition of sufficient nutrients (e.g., carbon, nitrogen, and essential trace elements and co-factors) from the nutrient-depleted phagosomal environment. We review the transcriptomic and recent functional genetic studies that are defining how these intracellular fungal pathogens tune their metabolism to the resources available in the macrophage phagosome. In addition, recent studies have shown that the nutritional state of the macrophage phagosome is not static, but changes upon activation of adaptive immune responses. Understanding the metabolic requirements of these dimorphic pathogens as they thrive within host cells can provide novel targets for therapeutic intervention.

How Environmental Fungi Cause a Range of Clinical Outcomes in Susceptible Hosts

Environmental fungi are globally ubiquitous and human exposure is near universal. However, relatively few fungal species are capable of infecting humans, and among fungi, few exposure events lead to severe systemic infections. Systemic infections have mortality rates of up to 90%, cost the US healthcare system $7.2 billion annually, and are typically associated with immunocompromised patients. Despite this reputation, exposure to environmental fungi results in a range of outcomes, from asymptomatic latent infections to severe systemic infection. Here we discuss different exposure outcomes for five major fungal pathogens: Aspergillus, Blastomyces, Coccidioides, Cryptococcus, and Histoplasma species. These fungi include a mold, a budding yeast, and thermal dimorphic fungi. All of these species must adapt to dramatically changing environments over the course of disease. These dynamic environments include the human lung, which is the first exposure site for these organisms. Fungi must defend themselves against host immune cells while germinating and growing, which risks further exposing microbe-associated molecular patterns to the host. We discuss immune evasion strategies during early infection, from disruption of host immune cells to major changes in fungal cell morphology.

In-Depth Characterization of Monocyte-Derived Macrophages using a Mass Cytometry-Based Phagocytosis Assay

Phagocytosis is a process in which target cells or particles are engulfed and taken up by other cells, typically professional phagocytes; this process is crucial in many physiological processes and disease states. The detection of targets for phagocytosis is directed by a complex repertoire of cell surface receptors. Pattern recognition receptors directly detect targets for binding and uptake, while opsonic and complement receptors detect objects coated by soluble factors. However, the importance of single and combinatorial surface marker expression across different phenotypes of professional phagocytes is not known. Here we developed a novel mass cytometry-based phagocytosis assay that enables the simultaneous detection of phagocytic events in combination with up to 40 other protein markers. We applied this assay to distinct monocyte derived macrophage (MDM) populations and found that prototypic M2-like MDMs phagocytose more E. coli than M1-like MDMs. Surface markers such as CD14, CD206, and CD163 rendered macrophages phagocytosis competent, but only CD209 directly correlated with the amount of particle uptake. Similarly, M2-like MDMs also phagocytosed more cancer cells than M1-like MDMs but, unlike M1-like MDMs, were insensitive to anti-CD47 opsonization. Our approach facilitates the simultaneous study of single-cell phenotypes, phagocytic activity, signaling and transcriptional events in complex cell mixtures.

Histoplasma Capsulatum: Mechanisms for Pathogenesis

Histoplasmosis, caused by the dimorphic environmental fungus Histoplasma capsulatum, is a major mycosis on the global stage. Acquisition of the fungus by mammalian hosts can be clinically silent or it can lead to life-threatening systemic disease, which can occur in immunologically intact or deficient hosts, albeit severe disease is more likely in the setting of compromised cellular immunity. H. capsulatum yeast cells are highly adapted to the mammalian host as they can effectively survive within intracellular niches in select phagocytic cells. Understanding the biological response by both the host and H. capsulatum will facilitate improved approaches to prevent and/or modify disease. This review presents our current understanding of the major pathogenic mechanisms involved in histoplasmosis.

Host membrane glycosphingolipids and lipid microdomains facilitate Histoplasma capsulatum internalisation by macrophages

Recognition and internalisation of intracellular pathogens by host cells is a multifactorial process, involving both stable and transient interactions. The plasticity of the host cell plasma membrane is fundamental in this infectious process. Here, the participation of macrophage lipid microdomains during adhesion and internalisation of the fungal pathogen Histoplasma capsulatum (Hc) was investigated. An increase in membrane lateral organisation, which is a characteristic of lipid microdomains, was observed during the first steps of Hc-macrophage interaction. Cholesterol enrichment in macrophage membranes around Hc contact regions and reduced levels of Hc-macrophage association after cholesterol removal also suggested the participation of lipid microdomains during Hc-macrophage interaction. Using optical tweezers to study cell-to-cell interactions, we showed that cholesterol depletion increased the time required for Hc adhesion. Additionally, fungal internalisation was significantly reduced under these conditions. Moreover, macrophages treated with the ceramide-glucosyltransferase inhibitor (P4r) and macrophages with altered ganglioside synthesis (from B4galnt1^-/- mice) showed a deficient ability to interact with Hc. Coincubation of oligo-GM1 and treatment with Cholera toxin Subunit B, which recognises the ganglioside GM1, also reduced Hc association. Although purified GM1 did not alter Hc binding, treatment with P4 significantly increased the time required for Hc binding to macrophages. The content of CD18 was displaced from lipid microdomains in B4galnt1^-/- macrophages. In addition, macrophages with reduced CD18 expression (CD18^low ) were associated with Hc at levels similar to wild-type cells. Finally, CD11b and CD18 colocalised with GM1 during Hc-macrophage interaction. Our results indicate that lipid rafts and particularly complex gangliosides that reside in lipid rafts stabilise Hc-macrophage adhesion and mediate efficient internalisation during histoplasmosis.

NLRX1 Facilitates Histoplasma capsulatum-Induced LC3-Associated Phagocytosis for Cytokine Production in Macrophages

LC3-associated phagocytosis (LAP) is an emerging non-canonical autophagy process that bridges signaling from pattern-recognition receptors (PRRs) to autophagic machinery. LAP formation results in incorporation of lipidated LC3 into phagosomal membrane (termed LAPosome). Increasing evidence reveals that LAP functions as an innate defense mechanism against fungal pathogens. However, the molecular mechanism involved and the consequence of LAP in regulating anti-fungal immune response remain largely unexplored. Here we show that Histoplasma capsulatum is taken into LAPosome upon phagocytosis by macrophages. Interaction of H. capsulatum with Dectin-1 activates Syk and triggers subsequent NADPH oxidase-mediated reactive oxygen species (ROS) response that is involved in LAP induction. Inhibiting LAP induction by silencing LC3α/β or treatment with ROS inhibitor impairs the activation of MAPKs-AP-1 pathway, thereby reduces macrophage proinflammatory cytokine response to H. capsulatum. Additionally, we unravel the importance of NLRX1 in fungus-induced LAP. NLRX1 facilitates LAP by interacting with TUFM which associates with autophagic proteins ATG5-ATG12 for LAPosome formation. Macrophages from Nlrx1 ^-/- mice or TUFM-silenced cells exhibit reduced LAP induction and LAP-mediated MAPKs-AP-1 activation for cytokine response to H. capsulatum. Furthermore, inhibiting ROS production in Nlrx1 ^-/- macrophages almost completely abolishes H. capsulatum-induced LC3 conversion, indicating that both Dectin-1/Syk/ROS-dependent pathway and NLRX1-TUFM complex-dependent pathway collaboratively contribute to LAP induction. Our findings reveal new pathways underlying LAP induction by H. capsulatum for macrophage cytokine response.

Flying under the radar: Histoplasma capsulatum avoidance of innate immune recognition

The dimorphic fungal pathogen Histoplasma capsulatum takes advantage of the innate immune system, utilizing host macrophages as a proliferative niche while largely avoiding stimulation of signaling host receptors. As a result, innate immune cells are unable to control H. capsulatum on their own. Not all host phagocytes respond to H. capsulatum in the same way, with neutrophils and dendritic cells playing important roles in impeding fungal growth and initiating a protective T_H1 response, respectively. Dendritic cells prime T-cell differentiation after internalization of yeasts via VLA-5 receptors and subsequent degradation of the yeasts. Dendritic cell-expressed TLR7 and TLR9 promote a type I interferon response for T_H1 polarization. In contrast to dendritic cells, macrophages provide a hospitable intracellular environment. H. capsulatum yeasts enter macrophages via binding to phagocytic receptors. Simultaneously, α-glucan masks immunostimulatory cell wall β-glucans and a secreted endoglucanase removes exposed β-glucans to minimize recognition of yeasts by Dectin-1. This review highlights how phagocytes interact with H. capsulatum yeasts and the mechanisms H. capsulatum uses to limit the innate immune response.

O-Mannosylation of Proteins Enables Histoplasma Yeast Survival at Mammalian Body Temperatures

The ability to grow at mammalian body temperatures is critical for pathogen infection of humans. For the thermally dimorphic fungal pathogen Histoplasma capsulatum, elevated temperature is required for differentiation of mycelia or conidia into yeast cells, a step critical for invasion and replication within phagocytic immune cells. Posttranslational glycosylation of extracellular proteins characterizes factors produced by the pathogenic yeast cells but not those of avirulent mycelia, correlating glycosylation with infection. Histoplasma yeast cells lacking the Pmt1 and Pmt2 protein mannosyltransferases, which catalyze O-linked mannosylation of proteins, are severely attenuated during infection of mammalian hosts. Cells lacking Pmt2 have altered surface characteristics that increase recognition of yeast cells by the macrophage mannose receptor and reduce recognition by the β-glucan receptor Dectin-1. Despite these changes, yeast cells lacking these factors still associate with and survive within phagocytes. Depletion of macrophages or neutrophils in vivo does not recover the virulence of the mutant yeast cells. We show that yeast cells lacking Pmt functions are more sensitive to thermal stress in vitro and consequently are unable to productively infect mice, even in the absence of fever. Treatment of mice with cyclophosphamide reduces the normal core body temperature of mice, and this decrease is sufficient to restore the infectivity of O-mannosylation-deficient yeast cells. These findings demonstrate that O-mannosylation of proteins increases the thermotolerance of Histoplasma yeast cells, which facilitates infection of mammalian hosts.IMPORTANCE For dimorphic fungal pathogens, mammalian body temperature can have contrasting roles. Mammalian body temperature induces differentiation of the fungal pathogen Histoplasma capsulatum into a pathogenic state characterized by infection of host phagocytes. On the other hand, elevated temperatures represent a significant barrier to infection by many microbes. By functionally characterizing cells lacking O-linked mannosylation enzymes, we show that protein mannosylation confers thermotolerance on H. capsulatum, enabling infection of mammalian hosts.

Fungal Strategies to Evade the Host Immune Recognition

The recognition of fungal cells by the host immune system is key during the establishment of a protective anti-fungal response. Even though the immune system has evolved a vast number of processes to control these organisms, they have developed strategies to fight back, avoiding the proper recognition by immune components and thus interfering with the host protective mechanisms. Therefore, the strategies to evade the immune system are as important as the virulence factors and attributes that damage the host tissues and cells. Here, we performed a thorough revision of the main fungal tactics to escape from the host immunosurveillance processes. These include the composition and organization of the cell wall, the fungal capsule, the formation of titan cells, biofilms, and asteroid bodies; the ability to undergo dimorphism; and the escape from nutritional immunity, extracellular traps, phagocytosis, and the action of humoral immune effectors.

Eng1 and Exg8 Are the Major β-Glucanases Secreted by the Fungal Pathogen Histoplasma capsulatum

Fungal cell walls contain β-glucan polysaccharides that stimulate immune responses when recognized by host immune cells. The fungal pathogen Histoplasma capsulatum minimizes detection of β-glucan by host cells through at least two mechanisms: concealment of β-glucans beneath α-glucans and enzymatic removal of any exposed β-glucan polysaccharides by the secreted glucanase Eng1. Histoplasma yeasts also secrete the putative glucanase Exg8, which may serve a similar role as Eng1 in removing exposed β-glucans from the yeast cell surface. Here, we characterize the enzymatic specificity of the Eng1 and Exg8 proteins and show that Exg8 is an exo-β1,3-glucanase and Eng1 is an endo-β1,3-glucanase. Together, Eng1 and Exg8 account for nearly all of the total secreted glucanase activity of Histoplasma yeasts. Both Eng1 and Exg8 proteins are secreted through a conventional secretion signal and are modified post-translationally by O-linked glycosylation. Both glucanases have near maximal activity at temperature and pH conditions experienced during infection of host cells, supporting roles in Histoplasma pathogenesis. Exg8 has a higher specific activity than Eng1 for β1,3-glucans; yet despite this, Exg8 does not reduce detection of yeasts by the host β-glucan receptor Dectin-1. Exg8 is largely dispensable for virulence in vivo, in contrast to Eng1. These results show that Histoplasma yeasts secrete two β1,3-glucanases and that Eng1 endoglucanase activity is the predominant factor responsible for removal of exposed cell wall β-glucans to minimize host detection of Histoplasma yeasts.

Eosinophils subvert host resistance to an intracellular pathogen by instigating non-protective IL-4 in CCR2-/- mice

Eosinophils contribute to type II immune responses in helminth infections and allergic diseases; however, their influence on intracellular pathogens is less clear. We previously reported that CCR2-/- mice exposed to the intracellular fungal pathogen Histoplasma capsulatum exhibit dampened immunity caused by an early exaggerated interleukin (IL)-4 response. We sought to identify the cellular source promulgating IL-4 in infected mutant animals. Eosinophils were the principal instigators of non-protective IL-4 and depleting this granulocyte population improved fungal clearance in CCR2-/- animals. The deleterious impact of eosinophilia on mycosis was also recapitulated in transgenic animals overexpressing eosinophils. Mechanistic examination of IL-4 induction revealed that phagocytosis of H. capsulatum via the pattern recognition receptor complement receptor (CR) 3 triggered the heightened IL-4 response in murine eosinophils. This phenomenon was conserved in human eosinophils; exposure of cells to the fungal pathogen elicited a robust IL-4 response. Thus, our findings elucidate a detrimental attribute of eosinophil biology in fungal infections that could potentially trigger a collapse in host defenses by instigating type II immunity.

Sterol Regulatory Element Binding Protein (Srb1) Is Required for Hypoxic Adaptation and Virulence in the Dimorphic Fungus Histoplasma capsulatum

The Histoplasma capsulatum sterol regulatory element binding protein (SREBP), Srb1 is a member of the basic helix-loop-helix (bHLH), leucine zipper DNA binding protein family of transcription factors that possess a unique tyrosine (Y) residue instead of an arginine (R) residue in the bHLH region. We have determined that Srb1 message levels increase in a time dependent manner during growth under oxygen deprivation (hypoxia). To further understand the role of Srb1 during infection and hypoxia, we silenced the gene encoding Srb1 using RNA interference (RNAi); characterized the resulting phenotype, determined its response to hypoxia, and its ability to cause disease within an infected host. Silencing of Srb1 resulted in a strain of H. capsulatum that is incapable of surviving in vitro hypoxia. We found that without complete Srb1 expression, H. capsulatum is killed by murine macrophages and avirulent in mice given a lethal dose of yeasts. Additionally, silencing Srb1 inhibited the hypoxic upregulation of other known H. capsulatum hypoxia-responsive genes (HRG), and genes that encode ergosterol biosynthetic enzymes. Consistent with these regulatory functions, Srb1 silenced H. capsulatum cells were hypersensitive to the antifungal azole drug itraconazole. These data support the theory that the H. capsulatum SREBP is critical for hypoxic adaptation and is required for H. capsulatum virulence.

Inverse Correlation between IL-10 and HIF-1α in Macrophages Infected with Histoplasma capsulatum

Hypoxia-inducible factor (HIF)-1α is a transcription factor that regulates metabolic and immune response genes in the setting of low oxygen tension and inflammation. We investigated the function of HIF-1α in the host response to Histoplasma capsulatum because granulomas induced by this pathogenic fungus develop hypoxic microenvironments during the early adaptive immune response. In this study, we demonstrated that myeloid HIF-1α-deficient mice exhibited elevated fungal burden during the innate immune response (prior to 7 d postinfection) as well as decreased survival in response to a sublethal inoculum of H. capsulatum The absence of myeloid HIF-1α did not alter immune cell recruitment to the lungs of infected animals but was associated with an elevation of the anti-inflammatory cytokine IL-10. Treatment with mAb to IL-10 restored protective immunity to the mutant mice. Macrophages (Mϕs) constituted most IL-10-producing cells. Deletion of HIF-1α in neutrophils or dendritic cells did not alter fungal burden, thus implicating Mϕs as the pivotal cell in host resistance. HIF-1α was stabilized in Mϕs following infection. Increased activity of the transcription factor CREB in HIF-1α-deficient Mϕs drove IL-10 production in response to H. capsulatum IL-10 inhibited Mϕ control of fungal growth in response to the activating cytokine IFN-γ. Thus, we identified a critical function for Mϕ HIF-1α in tempering IL-10 production following infection. We established that transcriptional regulation of IL-10 by HIF-1α and CREB is critical for activation of Mϕs by IFN-γ and effective handling of H. capsulatum.

The Eng1 β-Glucanase Enhances Histoplasma Virulence by Reducing β-Glucan Exposure

The fungal pathogen Histoplasma capsulatum parasitizes host phagocytes. To avoid antimicrobial immune responses, Histoplasma yeasts must minimize their detection by host receptors while simultaneously interacting with the phagocyte. Pathogenic Histoplasma yeast cells, but not avirulent mycelial cells, secrete the Eng1 protein, which is a member of the glycosylhydrolase 81 (GH81) family. We show that Histoplasma Eng1 is a glucanase that hydrolyzes β-(1,3)-glycosyl linkages but is not required for Histoplasma growth in vitro or for cell separation. However, Histoplasma yeasts lacking Eng1 function have attenuated virulence in vivo, particularly during the cell-mediated immunity stage. Histoplasma yeasts deficient for Eng1 show increased exposure of cell wall β-glucans, which results in enhanced binding to the Dectin-1 β-glucan receptor. Consistent with this, Eng1-deficient yeasts trigger increased tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) cytokine production from macrophages and dendritic cells. While not responsible for large-scale cell wall structure and function, the secreted Eng1 reduces levels of exposed β-glucans at the yeast cell wall, thereby diminishing potential recognition by Dectin-1 and proinflammatory cytokine production by phagocytes. In α-glucan-producing Histoplasma strains, Eng1 acts in concert with α-glucan to minimize β-glucan exposure: α-glucan provides a masking function by covering the β-glucan-rich cell wall, while Eng1 removes any remaining exposed β-glucans. Thus, Histoplasma Eng1 has evolved a specialized pathogenesis function to remove exposed β-glucans, thereby enhancing the ability of yeasts to escape detection by host phagocytes.

IMPORTANCE

The success of Histoplasma capsulatum as an intracellular pathogen results, in part, from an ability to minimize its detection by receptors on phagocytic cells of the immune system. In this study, we showed that Histoplasma pathogenic yeast cells, but not avirulent mycelia, secrete a β-glucanase, Eng1, which reduces recognition of fungal cell wall β-glucans. We demonstrated that the Eng1 β-glucanase promotes Histoplasma virulence by reducing levels of surface-exposed β-glucans on yeast cells, thereby enabling Histoplasma yeasts to escape detection by the host β-glucan receptor, Dectin-1. As a consequence, phagocyte recognition of Histoplasma yeasts is reduced, leading to less proinflammatory cytokine production by phagocytes and less control of Histoplasma infection in vivo Thus, Histoplasma yeasts express two mechanisms to avoid phagocyte detection: masking of cell wall β-glucans by α-glucan and enzymatic removal of exposed β-glucans by the Eng1 β-glucanase.

Histoplasma capsulatum surmounts obstacles to intracellular pathogenesis

The fungal pathogen Histoplasma capsulatum causes respiratory and disseminated disease, even in immunocompetent hosts. In contrast to opportunistic pathogens, which are readily controlled by phagocytic cells, H. capsulatum yeasts are able to infect macrophages, survive antimicrobial defenses, and proliferate as an intracellular pathogen. In this review, we discuss some of the molecular mechanisms that enable H. capsulatum yeasts to overcome obstacles to intracellular pathogenesis. H. capsulatum yeasts gain refuge from extracellular obstacles such as antimicrobial lung surfactant proteins by engaging the β-integrin family of phagocytic receptors to promote entry into macrophages. In addition, H. capsulatum yeasts conceal immunostimulatory β-glucans to avoid triggering signaling receptors such as the β-glucan receptor Dectin-1. H. capsulatum yeasts counteract phagocyte-produced reactive oxygen species by expression of oxidative stress defense enzymes including an extracellular superoxide dismutase and an extracellular catalase. Within the phagosome, H. capsulatum yeasts block phagosome acidification, acquire essential metals such as iron and zinc, and utilize de novo biosynthesis pathways to overcome nutritional limitations. These mechanisms explain how H. capsulatum yeasts avoid and negate macrophage defense strategies and establish a hospitable intracellular niche, making H. capsulatum a successful intracellular pathogen of macrophages.

Yeast Transcriptome and In Vivo Hypoxia Detection Reveals Histoplasma capsulatum Response to Low Oxygen Tension

Although there is growing understanding of the microenvironmental conditions fungal pathogens encounter as they colonize their host, nothing is known about Histoplasma capsulatum's response to hypoxia. Here we characterized hypoxia during murine histoplasmosis using an in vivo hypoxia detection agent, Hypoxyprobe-2 (HP-2); and analyzed H. capsulatum's transcriptional profile in response to in vitro hypoxia. Immunohistopathology and flow cytometry analyses revealed distinct regions of hypoxia during infection. Granuloma cells, enriched with macrophages and T-cells isolated from infected livers were 66-76% positive for HP-2, of which, 95% of macrophages and 55% of T-cells were hypoxic. Although inhibited, H. capsulatum was able to survive under in vitro hypoxic conditions (<1% O2), and restored growth when replaced in normoxia. Next-generation sequencing (RNA-seq) analysis after 24 hours of hypoxia demonstrated a significant increase in NIT50 (swirm domain DNA binding protein), a predicted ABC transporter (ABC), NADPH oxidoreductase (NADP/FAD), and guanine nucleotide exchange factor (RSP/GEF); and other genes with no known designated function. Computational transcription factor binding site analysis predicted human sterol regulatory element binding protein (SREBP) binding sites upstream of NIT50, ABC, NADP/FAD and RSP/GEF. Hypoxia resulted in a time-dependent increase in the H. capsulatum homolog of SREBP, here named Srb1. Srb1 peaked at 8 hours and returned to basal levels by 24 hours. Our findings demonstrate that H. capsulatum encounters and survives severe hypoxia during infection. Additionally, the hypoxic response may be regulated at the level of transcription, and these studies contribute to the understanding of hypoxic regulation and adaptation in H. capsulatum.

Profile of cytokines in the lungs of BALB/c mice after intra-nasal infection with Histoplasma capsulatum mycelial propagules

The host pulmonary response to the fungus Histoplasma capsulatum was evaluated, through the profile of cytokines detected by the MagPix magnetic beads platform in lung homogenates and by lung-granulomas formation, from mice intra-nasally infected with mycelial propagules (M-phase) of two virulent H. capsulatum strains, EH-46 and G-217B. Results highlight that mice lung inflammatory response depends on the H. capsulatum strain used, during the first step of the fungal infection. IL-1β and TNF-α increased their concentrations in mice infected with both strains. The highest levels of IL-6, IL-17, and IL-23 were found in EH-46-infected mice, whereas levels of IL-22 were variable at all post-infection times for both strains. Significant increases of IL-12, IFN-γ, IL-4, and IL-10 were associated to EH-46-infected mice. Histological lung findings from EH-46-infected mice revealed incipient and numerous well-developed granulomas, distributed in lung-lobes at the 14th and the 21st days after infection, according to cytokine profiles.

Dendritic cell interactions with Histoplasma and Paracoccidioides

Fungi are among the most common microbes encountered by humans. More than 100, 000 fungal species have been described in the environment to date, however only a few species cause disease in humans. Fungal infections are of particular importance to immunocompromised hosts in whom disease is often more severe, especially in those with impaired cell-mediated immunity such as individuals with HIV infection, hematologic malignancies, or those receiving TNF-α inhibitors. Nevertheless, environmental disturbances through natural processes or as a consequence of deforestation or construction can expose immunologically competent people to a large number of fungal spores resulting in asymptomatic acquisition to life-threatening disease. In recent decades, the significance of the innate immune system and more importantly the role of dendritic cells (DC) have been found to play a fundamental role in the resolution of fungal infections, such as in dimorphic fungi like Histoplasma and Paracoccidioides. In this review article the general role of DCs will be illustrated as the bridge between the innate and adaptive immune systems, as well as their specific interactions with these 2 dimorphic fungi.

Type II cytokines impair host defense against an intracellular fungal pathogen by amplifying macrophage generation of IL-33

Interleukin (IL)-4 subverts protective immunity to multiple intracellular pathogens, including the fungus Histoplasma capsulatum. Previously, we reported that H. capsulatum-challenged CCR2(-/-) mice manifest elevated pulmonary fungal burden owing to exaggerated IL-4. Paradoxical to our anticipation in IL-33 driving IL-4, we discovered that the latter prompted IL-33 in mutant mice. In infected CCR2(-/-) animals, amplified IL-33 succeeded the heightened IL-4 response and inhibition of IL-4 signaling decreased IL-33. Moreover, macrophages, but not epithelial cells or dendritic cells, from these mice expressed higher IL-33 in comparison with controls. Dissection of mechanisms that promulgated IL-33 revealed type-II cytokines and H. capsulatum synergistically elicited an IL-33 response in macrophages via signal transducer and activator of transcription factor 6/interferon-regulatory factor-4 and Dectin-1 pathways, respectively. Neutralizing IL-33 in CCR2(-/-) animals, but not controls, enhanced their resistance to histoplasmosis. Thus we describe a previously unrecognized role for IL-4 in instigating IL-33 in macrophages. Furthermore, in the presence of intracellular fungal pathogens, the type-II cytokine-driven IL-33 response impairs immunity.

Usefulness of the murine model to study the immune response against Histoplasma capsulatum infection

The present paper is an overview of the primary events that are associated with the histoplasmosis immune response in the murine model. Valuable data that have been recorded in the scientific literature have contributed to an improved understanding of the clinical course of this systemic mycosis, which is caused by the dimorphic fungus Histoplasma capsulatum. Data must be analyzed carefully, given that misinterpretation could be generated because most of the available information is based on experimental host-parasite interactions that used inappropriate proceedings, i.e., the non-natural route of infection with the parasitic and virulent fungal yeast-phase, which is not the usual infective phase of the etiological agent of this mycosis. Thus, due to their versatility, complexity, and similarities with humans, several murine models have played a fundamental role in exploring the host-parasite interaction during H. capsulatum infection.

Disseminated histoplasmosis in two juvenile raccoons (Procyon lotor) from a nonendemic region of the United States

Two 6-month-old raccoon kits, which had been rescued and fostered in preparation for return to the wild, became acutely ill and died 3 weeks before scheduled release. At necropsy, the kits had grossly enlarged livers and spleens, diffusely consolidated lungs, and generalized lymphadenopathy. Histologically, extensive infiltrates of macrophages containing yeast organisms were identified in lung, liver, kidney, spleen, lymph nodes, intestinal tissues, brain, adrenal gland, bone marrow, and thymus of both animals. Histiocytic inflammation with accompanying fibrosis was widespread, with necrotic foci evident in lungs, spleen, and intestinal sections. Fungal organisms were observed on sheep blood agar plates; however, repeated subcultures to fungal media designed to induce conidial structures for fungal identification were unsuccessful. Partial DNA sequencing of the 28S ribosomal RNA gene of the blood agar isolate identified 100% homology with Ajellomyces capsulatus (anamorphic name Histoplasma capsulatum). The kits were rescued and fostered in the San Francisco Bay area and it is likely that the exposure to H. capsulatum occurred in this area. Histoplasma sp. infection in wild mammal species is often used as an indication of spore contamination of a geographic region. Northern California is not known to be an endemic region for H. capsulatum, which is not a reportable disease in this state. The presence of severe, disseminated disease and the need for molecular identification associated with the isolate from a nonendemic region identified in the present report may indicate genetic adaptation and altered characteristics of this agent and may warrant further investigation.

Vitronectin inhibits efferocytosis through interactions with apoptotic cells as well as with macrophages

Effective removal of apoptotic cells, particularly apoptotic neutrophils, is essential for the successful resolution of acute inflammatory conditions. In these experiments, we found that whereas interaction between vitronectin and integrins diminished the ability of macrophages to ingest apoptotic cells, interaction between vitronectin with urokinase-type plasminogen activator receptor (uPAR) on the surface of apoptotic cells also had equally important inhibitory effects on efferocytosis. Preincubation of vitronectin with plasminogen activator inhibitor-1 eliminated its ability to inhibit phagocytosis of apoptotic cells. Similarly, incubation of apoptotic cells with soluble uPAR or Abs to uPAR significantly diminished efferocytosis. In the setting of LPS-induced ALI, enhanced efferocytosis and decreased numbers of neutrophils were found in bronchoalveolar lavage obtained from vitronectin-deficient (vtn(-/-)) mice compared with wild type (vtn(+/+)) mice. Furthermore, there was increased clearance of apoptotic vtn(-/-) as compared with vtn(+/+) neutrophils after introduction into the lungs of vtn(-/-) mice. Incubation of apoptotic vtn(-/-) neutrophils with purified vitronectin before intratracheal instillation decreased efferocytosis in vivo. These findings demonstrate that the inhibitory effects of vitronectin on efferocytosis involve interactions with both the engulfing phagocyte and the apoptotic target cell.

AMP-activated protein kinase: new regulation, new roles?

The hydrolysis of ATP drives virtually all of the energy-requiring processes in living cells. A prerequisite of living cells is that the concentration of ATP needs to be maintained at sufficiently high levels to sustain essential cellular functions. In eukaryotic cells, the AMPK (AMP-activated protein kinase) cascade is one of the systems that have evolved to ensure that energy homoeostasis is maintained. AMPK is activated in response to a fall in ATP, and recent studies have suggested that ADP plays an important role in regulating AMPK. Once activated, AMPK phosphorylates a broad range of downstream targets, resulting in the overall effect of increasing ATP-producing pathways whilst decreasing ATP-utilizing pathways. Disturbances in energy homoeostasis underlie a number of disease states in humans, e.g. Type 2 diabetes, obesity and cancer. Reflecting its key role in energy metabolism, AMPK has emerged as a potential therapeutic target. In the present review we examine the recent progress aimed at understanding the regulation of AMPK and discuss some of the latest developments that have emerged in key areas of human physiology where AMPK is thought to play an important role.

Extracellular superoxide dismutase protects Histoplasma yeast cells from host-derived oxidative stress

In order to establish infections within the mammalian host, pathogens must protect themselves against toxic reactive oxygen species produced by phagocytes of the immune system. The fungal pathogen Histoplasma capsulatum infects both neutrophils and macrophages but the mechanisms enabling Histoplasma yeasts to survive in these phagocytes have not been fully elucidated. We show that Histoplasma yeasts produce a superoxide dismutase (Sod3) and direct it to the extracellular environment via N-terminal and C-terminal signals which promote its secretion and association with the yeast cell surface. This localization permits Sod3 to protect yeasts specifically from exogenous superoxide whereas amelioration of endogenous reactive oxygen depends on intracellular dismutases such as Sod1. While infection of resting macrophages by Histoplasma does not stimulate the phagocyte oxidative burst, interaction with polymorphonuclear leukocytes (PMNs) and cytokine-activated macrophages triggers production of reactive oxygen species (ROS). Histoplasma yeasts producing Sod3 survive co-incubation with these phagocytes but yeasts lacking Sod3 are rapidly eliminated through oxidative killing similar to the effect of phagocytes on Candida albicans yeasts. The protection provided by Sod3 against host-derived ROS extends in vivo. Without Sod3, Histoplasma yeasts are attenuated in their ability to establish respiratory infections and are rapidly cleared with the onset of adaptive immunity. The virulence of Sod3-deficient yeasts is restored in murine hosts unable to produce superoxide due to loss of the NADPH-oxidase function. These results demonstrate that phagocyte-produced ROS contributes to the immune response to Histoplasma and that Sod3 facilitates Histoplasma pathogenesis by detoxifying host-derived reactive oxygen thereby enabling Histoplasma survival.

Histoplasma capsulatum is a fungal pathogen that is endemic to the Mississippi and Ohio River valleys. An estimated 200,000 infections occur annually in the United States. Histoplasma is adept at surviving within both neutrophils and macrophages, which normally kill fungal cells by producing reactive oxygen molecules that are toxic to microbes. In this study, we demonstrate the role of a superoxide dismutase enzyme (Sod3) produced by Histoplasma cells and we show that it enables Histoplasma to survive these reactive oxidative molecules produced by the host. We show that Histoplasma directs the Sod3 protein to the surface of yeast cells and into the extracellular environment, positioning it to destroy extracellular superoxide produced by neutrophils and macrophages. Our results highlight the importance of reactive oxygen produced by immune cells and define the mechanism by which Histoplasma survives these immune defenses and establishes infections in its host.

Adaptive immunity to fungi

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

AMP-activated protein kinase enhances the phagocytic ability of macrophages and neutrophils

Although AMPK plays well-established roles in the modulation of energy balance, recent studies have shown that AMPK activation has potent anti-inflammatory effects. In the present experiments, we examined the role of AMPK in phagocytosis. We found that ingestion of Escherichia coli or apoptotic cells by macrophages increased AMPK activity. AMPK activation increased the ability of neutrophils or macrophages to ingest bacteria (by 46 ± 7.8 or 85 ± 26%, respectively, compared to control, P<0.05) and the ability of macrophages to ingest apoptotic cells (by 21 ± 1.4%, P<0.05 compared to control). AMPK activation resulted in cytoskeletal reorganization, including enhanced formation of actin and microtubule networks. Activation of PAK1/2 and WAVE2, which are downstream effectors of Rac1, accompanied AMPK activation. AMPK activation also induced phosphorylation of CLIP-170, a protein that participates in microtubule synthesis. The increase in phagocytosis was reversible by the specific AMPK inhibitor compound C, siRNA to AMPKα1, Rac1 inhibitors, or agents that disrupt actin or microtubule networks. In vivo, AMPK activation resulted in enhanced phagocytosis of bacteria in the lungs by 75 ± 5% vs. control (P<0.05). These results demonstrate a novel function for AMPK in enhancing the phagocytic activity of neutrophils and macrophages.

Histoplasma capsulatum utilizes siderophores for intracellular iron acquisition in macrophages

Histoplasma capsulatum is a dimorphic fungal pathogen that survives and replicates within macrophages (MΦ). Studies in human and murine MΦ demonstrate that the intracellular growth of H. capsulatum yeasts is exquisitely sensitive to the availability of iron. As H. capsulatum produces hydroxamate siderophores, we sought to determine if siderophores were required for intracellular survival in MΦ, and in a murine model of pulmonary histoplasmosis. The expression of SID1 (coding for L-ornithine-N(5)-monooxygenase) was silenced by RNA interference (RNAi) in H. capsulatum strain G217B, and abolished by gene targeting in strain G186AR. G217B SID1-silenced yeasts grew normally in rich medium, did not synthesize siderophores, and were unable to grow on apotransferrin-chelated medium. Their intracellular growth in human and murine MΦ was significantly decreased compared to wild type (WT) yeasts, but growth was restored to WT levels by the addition of exogenous iron, or restoration of SID1 expression. Similar results were obtained with G186AR Δsid1 yeasts. Compared to WT yeasts, G217B SID1-silenced yeasts demonstrated in C57BL/6 mice significantly reduced growth in the lungs and spleens seven days after infection, and 40% of the mice given a normally lethal inoculum of G217B SID1-silenced yeasts survived. These experiments demonstrate that: (1) SID1 expression is required for siderophore biosynthesis by H. capsulatum strain G217B, (2) SID1 expression is required for optimum intracellular growth in MΦ, and (3) inhibition of SID1 expression in vivo reduces the virulence of H. capsulatum yeasts.

Dendritic cells restrict the transformation of Histoplasma capsulatum conidia into yeasts

Infections due to Histoplasma capsulatum occur as a result of the inhalation of airborne microconidia of the mold into the alveoli of the lungs. In this study we quantified the transformation over time of conidia into yeast-like cells within macrophages (MΦ) and dendritic cells (DC). Conidia from strain G217B which had been surface labeled with carboxy-fluorescein succinimidyl ester (CFSE), or conidia from strain G217B that expresses green fluorescent protein (GFP) only in the yeast phase, were used to infect MΦ and DC. At various time points, numbers of intracellular conidia or yeasts were quantified via phase-contrast and fluorescent microscopy. Transformation of conidia from non-GFP-expressing G217B also was quantified by their incorporation of ³H-leucine. In both human and murine MΦ, numerous yeast-like cells appeared by day 3 post-infection. The time course of conidia transformation into yeasts in culture medium was the same as in MΦ. However, transformation of conidia to yeasts was significantly restricted in human DC and murine lung DC. In DC, significant numbers of yeasts did not appear until 5 days post-infection. Further, MΦ monolayers were destroyed by day 6-7 post-infection, whereas DC monolayers remained intact throughout the study period. These data suggest that in vivo, conidia may transform into yeast-like cells efficiently whether or not they are phagocytosed by MΦ, but not when ingested by DC.

Conidia but not yeast cells of the fungal pathogen Histoplasma capsulatum trigger a type I interferon innate immune response in murine macrophages

Histoplasma capsulatum is the most common cause of fungal respiratory infections and can lead to progressive disseminated infections, particularly in immunocompromised patients. Infection occurs upon inhalation of the aerosolized spores, known as conidia. Once inside the host, conidia are phagocytosed by alveolar macrophages. The conidia subsequently germinate and produce a budding yeast-like form that colonizes host macrophages and can disseminate throughout host organs and tissues. Even though conidia are the predominant infectious particle for H. capsulatum and are the first cell type encountered by the host during infection, very little is known at a molecular level about conidia or about their interaction with cells of the host immune system. We examined the interaction between conidia and host cells in a murine bone-marrow-derived macrophage model of infection. We used whole-genome expression profiling and quantitative reverse transcription-PCR (qRT-PCR) to monitor the macrophage signaling pathways that are modulated during infection with conidia. Our analysis revealed that type I interferon (IFN)-responsive genes and the beta type I IFN (IFN-beta) were induced in macrophages during infection with H. capsulatum conidia but not H. capsulatum yeast cells. Further analysis revealed that the type I IFN signature induced in macrophages in response to conidia is independent of Toll-like receptor (TLR) signaling and the cytosolic RNA sensor MAVS but is dependent on the transcription factor interferon regulatory factor 3 (IRF3). Interestingly, H. capsulatum growth was restricted in mice lacking the type I IFN receptor, indicating that an intact host type I IFN response is required for full virulence of H. capsulatum in mice.

Distinct roles of complement receptor 3, Dectin-1, and sialic acids in murine macrophage interaction with Histoplasma yeast

The yeast cells of dimorphic fungal pathogen Histoplasma reside primarily within the macrophages of an infected host; the interaction between the yeast and macrophage has a profound impact on host defense against the fungus. We used blocking antibodies and saccharides to identify the receptors that participate in the phagocytosis of and the cytokine response to Histoplasma. The phagocytosis and cytokine response results show that sialic acids on the macrophages were involved in the interaction between macrophages and Histoplasma. CR3, although not the only receptor involved, was responsible for phagocytosis and cytokine response. It is unclear which receptors other than CR3 are responsible for phagocytosis, but we did rule out the participation of TLR2, TLR4, MR, DC-SIGN/SIGNR1, FcgammaR, VLA-5, and Dectin-1. Even though Dectin-1 did not participate in phagocytosis, it collaborated with CR3 in the cytokine response to Histoplasma, suggesting that in the presence of phagocytic receptors, Histoplasma triggers cytokine signals through Dectin-1. Moreover, macrophage phagocytosis of and cytokine response to Histoplasma are Syk kinase-dependent. Our study delineated the distinct roles of CR3, Dectin-1, and sialic acids in the interaction with Histoplasma and suggested that multiple receptor use might be important to host defense against Histoplasma.

Engagement of Penicillium marneffei conidia with multiple pattern recognition receptors on human monocytes

P. marneffei is a thermal dimorphic fungus which causes penicilliosis, an opportunistic infection in immunocompromised patients in South and Southeast Asia. Little is known about the innate immune response to P. marneffei infection. Therefore, the initial response of macrophages to P. marneffei conidia was evaluated by us. Adhesion between monocytes from healthy humans and fungal conidia was examined and found to be specifically inhibited by MAbs against PRR, such as MR, (TLR)1, TLR2, TLR4, TLR6, CD14, CD11a, CD11b, and CD18. To study the consequences of these interactions, cytokines were also examined by ELISA. Binding of P. marneffei conidia to monocytes was significantly inhibited, in a dose-dependent manner, by MAbs against MR, TLR1, TLR2, TLR4, TLR6, CD14, CD11b and CD18. When monocytes were co-cultured with the conidia, there was an increase in the amount of surface CD40 and CD86 expression, together with TNF-alpha and IL-1beta production, compared to unstimulated controls. In assays containing anti-TLR4 or anti-CD14 antibody, reduction in the amount of TNF-alpha released by monocytes stimulated with P. marneffei conidia was detected. In addition, it was found that production of TNF-alpha and IL-1beta from adherent peripheral blood monocytes was partially impaired when heat-inactivated autologous serum, in place of untreated autologous serum, was added to the assay. These results demonstrate that various PRR on human monocytes participate in the initial recognition of P. marneffei conidia, and the engagement of PRR could partly initiate proinflammatory cytokine production.

Monoclonal antibodies to heat shock protein 60 alter the pathogenesis of Histoplasma capsulatum

Heat shock proteins with molecular masses of approximately 60 kDa (Hsp60) are widely distributed in nature and are highly conserved immunogenic molecules that can function as molecular chaperones and enhance cellular survival under physiological stress conditions. The fungus Histoplasma capsulatum displays an Hsp60 on its cell surface that is a key target of the cellular immune response during histoplasmosis, and immunization with this protein is protective. However, the role of humoral responses to Hsp60 has not been fully elucidated. We generated immunoglobulin G (IgG) isotype monoclonal antibodies (MAbs) to H. capsulatum Hsp60. IgG1 and IgG2a MAbs significantly prolonged the survival of mice infected with H. capsulatum. An IgG2b MAb was not protective. The protective MAbs reduced intracellular fungal survival and increased phagolysosomal fusion of macrophages in vitro. Histological examination of infected mice showed that protective MAbs reduced the fungal burden and organ damage. Organs of infected animals treated with protective MAbs had significantly increased levels of interleukin-2 (IL-2), IL-12, and tumor necrosis factor alpha and decreased levels of IL-4 and IL-10. Hence, IgG1 and IgG2a MAbs to Hsp60 can modify H. capsulatum pathogenesis in part by altering the intracellular fate of the fungus and inducing the production of Th1-associated cytokines.

Histoplasma capsulatum cyclophilin A mediates attachment to dendritic cell VLA-5

Histoplasma capsulatum (Hc) is a pathogenic fungus that replicates in macrophages (Mphi). In dendritic cells (DC), Hc is killed and fungal Ags are processed and presented to T cells. DC recognize Hc yeasts via the VLA-5 receptor, whereas Mphi recognize yeasts via CD18. To identify ligand(s) on Hc recognized by DC, VLA-5 was used to probe a Far Western blot of a yeast freeze/thaw extract (F/TE) that inhibited Hc binding to DC. VLA-5 recognized a 20-kDa protein, identified as cyclophilin A (CypA), and CypA was present on the surface of Hc yeasts. rCypA inhibited the attachment of Hc to DC, but not to Mphi. Silencing of Hc CypA by RNA interference reduced yeast binding to DC by 65-85%, but had no effect on binding to Mphi. However, F/TE from CypA-silenced yeasts still inhibited binding of wild-type Hc to DC, and F/TE from wild-type yeasts depleted of CypA also inhibited yeast binding to DC. rCypA did not further inhibit the binding of CypA-silenced yeasts to DC. Polystyrene beads coated with rCypA or fibronectin bound to DC and Mphi and to Chinese hamster ovary cells transfected with VLA-5. Binding of rCypA-coated beads, but not fibronectin-coated beads, was inhibited by rCypA. These data demonstrate that CypA serves as a ligand for DC VLA-5, that binding of CypA to VLA-5 is at a site different from FN, and that there is at least one other ligand on the surface of Hc yeasts that mediates binding of Hc to DC.

The Histoplasma capsulatum vacuolar ATPase is required for iron homeostasis, intracellular replication in macrophages and virulence in a murine model of histoplasmosis

Histoplasma capsulatum is a dimorphic fungal pathogen that survives and replicates within macrophages (Mphi). To identify specific genes required for intracellular survival, we utilized Agrobacterium tumefaciens-mediated mutagenesis, and screened for H. capsulatum insertional mutants that were unable to survive in human Mphi. One colony was identified that had an insertion within VMA1, the catalytic subunit A of the vacuolar ATPase (V-ATPase). The vma1 mutant (vma1::HPH) grew normally on iron-replete medium, but not on iron-deficient media. On iron-deficient medium, the growth of the vma1 mutant was restored in the presence of wild-type (WT) H. capsulatum yeasts, or the hydroxamate siderophore, rhodotorulic acid. However, the inability to replicate within Mphi was only partially restored by the addition of exogenous iron. The vma1::HPH mutant also did not grow as a mold at 28 degrees C. Complementation of the mutant (vma/VMA1) restored its ability to replicate in Mphi, grow on iron-poor medium and grow as a mold at 28 degrees C. The vma1::HPH mutant was avirulent in a mouse model of histoplasmosis, whereas the vma1/VMA1 strain was as pathogenic as WT yeasts. These studies demonstrate the importance of V-ATPase function in the pathogenicity of H. capsulatum, in iron homeostasis and in fungal dimorphism.

Histoplasma capsulatum manifests preferential invasion of phagocytic subpopulations in murine lungs

Numerous in vitro studies have demonstrated that Histoplasma capsulatum is engulfed by the diverse populations of phagocytic cells including monocytes/macrophages (Mphi), immature dendritic cells (DC), and neutrophils. The in vivo distribution of H. capsulatum has yet to be examined following an intrapulmonary challenge. To accomplish this goal, we engineered GFP into two genetically dissimilar strains of H. capsulatum, G217B and 186R. C57BL/6 mice were infected with each of these strains, and we analyzed the distribution of this fungus in the three major phagocytic populations on successive days. Yeast cells were found in all three populations of cells from Days 1 through 7. Proportionally, DC dominated at Day 1, whereas the majority of yeast cells was detected in neutrophils thereafter. Yeast cells were present in inflammatory and resident Mphi on Day 3, but on Day 7, they were chiefly in inflammatory Mphi. Yeast cells were predominantly in a CD11c(+intermediate/high), F4/80(-), CD11b(+), Ly-6C(+), CD205(+) DC population. Neutralization of TNF-alpha or IFN-gamma produced a significant redistribution of yeast cells. These results reveal the complex nature of intracellular residence of this fungus. Moreover, the findings demonstrate that there is a skewing in the subpopulations of cells that are infected, especially DC.

Histoplasma capsulatum pathogenesis: making a lifestyle switch

The dimorphism of Histoplasma reflects a developmental switch in morphology and lifestyle that is necessary for virulence. The dimorphism regulating kinase DRK1 and the Histoplasma WOR1 homolog RYP1 mediate the thermally induced transition to the pathogenic yeast-phase program. The genes expressed as part of this regulon influence the host-pathogen interaction to favor Histoplasma virulence. While surface localized HSP60 supports yeast attachment to host macrophages, yeast alpha-glucan polysaccharides conceal immunostimulatory cell wall beta-glucans from detection by macrophage receptors. Intramacrophage growth of yeast cells is facilitated by CBP a secreted, protease-resistant calcium-binding protein tailored to function within the phagolysosomal environment. In some Histoplasma strains, YPS3 promotes dissemination of yeast from pulmonary infection sites. The Histoplasma yeast-phase program includes additional cell surface and extracellular molecules that potentially function in further aspects of Histoplasma virulence.

TNF-alpha antagonism generates a population of antigen-specific CD4+CD25+ T cells that inhibit protective immunity in murine histoplasmosis

In both humans and mice, treatment with TNF-alpha antagonists is associated with serious infectious complications including disseminated histoplasmosis. The mechanisms by which inhibition of endogenous TNF-alpha alter protective immunity remain obscure. Herein, we tested the possibility that neutralization of this cytokine triggered the emergence of T cells that dampen immunity. The lungs of mice given mAb to TNF-alpha contained a higher proportion and number of CD4+CD25+ cells than controls. This elevation was not observed in IFN-gamma- or GM-CSF-deficient mice or in those given a high inoculum. Phenotypic analysis revealed that these cells lacked many of the characteristics of natural regulatory T cells, including Foxp3. CD4+CD25+ cells from TNF-alpha-neutralized mice suppressed Ag-specific, but not nonspecific, responses in vitro. Elimination of CD25+ cells in vivo restored protective immunity in mice given mAb to TNF-alpha and adoptive transfer of CD4+CD25+ cells inhibited immunity. In vitro and in vivo, the suppressive effect was reversed by mAb to IL-10. Thus, neutralization of TNF-alpha is associated with the induction of a population of regulatory T cells that alter protective immunity in an Ag-specific manner to Histoplasma capsulatum.

Tumor necrosis factor-alpha antagonism by the murine tumor necrosis factor-alpha receptor 2-Fc fusion protein exacerbates histoplasmosis in mice

Treatment of some inflammatory conditions with tumor necrosis factor-alpha (TNF-alpha) antagonists is efficacious, but such treatments are associated with infections with intracellular pathogens, including Histoplasma capsulatum. We explored protective immunity to H. capsulatum in mice given a fusion protein consisting of TNF-alpha receptor 2 (TNFR2) bound to the Fc portion of mouse IgG1. Intraperitoneal administration of this inhibitor exacerbated primary or secondary pulmonary infection at dosages ranging from 1 to 5 mg/kg. All mice with primary infection given the inhibitor succumbed to infection within 10-21 days of treatment. In secondary histoplasmosis, mice receiving 1, but not 5, mg/kg survived treatment. Fungal burden was increased even if treatment with the inhibitor was initiated after the onset of infection. The inflammatory response of the lungs of mice given the inhibitor did not differ from that of mice given control vehicle. Susceptibility was not associated with major alterations in cytokines known to protect or exacerbate infection. However, expression of nitric oxide synthase 2 (NOS2) was depressed early in primary infection. These results demonstrate that antagonism of endogenous TNF-alpha by this fusion protein modulates susceptibility. Impaired immunity is not a result of altered cytokine responses or changes in the inflammation and may not be demonstrable in other murine strains.

Advances in combating fungal diseases: vaccines on the threshold

The dramatic increase in fungal diseases in recent years can be attributed to the increased aggressiveness of medical therapy and other human activities. Immunosuppressed patients are at risk of contracting fungal diseases in healthcare settings and from natural environments. Increased prescribing of antifungals has led to the emergence of resistant fungi, resulting in treatment challenges. These concerns, together with the elucidation of the mechanisms of protective immunity against fungal diseases, have renewed interest in the development of vaccines against the mycoses. Most research has used murine models of human disease and, as we review in this article, the knowledge gained from these studies has advanced to the point where the development of vaccines targeting human fungal pathogens is now a realistic and achievable goal.

Defining Virulence Genes in the Dimorphic Fungi

Most dimorphic fungal pathogens cause respiratory disease in mammals and must therefore possess virulence mechanisms to combat and overcome host pulmonary defenses. Over the past decade, advances in genetic tools have made it possible to investigate the basis of dimorphic fungal pathogenesis at the molecular level. Gene disruptions and RNA interference have now formally demonstrated the involvement of six virulence factors: CBP, alpha-(1,3)-glucan, BAD1, SOWgp, Mep1, and urease. Additional candidate virulence-associated genes have been identified on the premise that factors necessary for pathogenicity are associated specifically with the parasitic form. This principle continues to form the foundation for genomics-based analyses to further augment the list. Thus, the stage is set and the tools are in place for the next phase of medical mycology research: defining the virulence-associated factors underlying the success of dimorphic fungal pathogens.

Human macrophages do not require phagosome acidification to mediate fungistatic/fungicidal activity against Histoplasma capsulatum

Histoplasma capsulatum (Hc) is a facultative intracellular fungus that modulates the intraphagosomal environment to survive within macrophages (Mphi). In the present study, we sought to quantify the intraphagosomal pH under conditions in which Hc yeasts replicated or were killed. Human Mphi that had ingested both viable and heat-killed or fixed yeasts maintained an intraphagosomal pH of approximately 6.4-6.5 over a period of several hours. These results were obtained using a fluorescent ratio technique and by electron microscopy using the 3-(2,4-dinitroanilo)-3'-amino-N-methyldipropylamine reagent. Mphi that had ingested Saccharomyces cerevisae, a nonpathogenic yeast that is rapidly killed and degraded by Mphi, also maintained an intraphagosomal pH of approximately 6.5 over a period of several hours. Stimulation of human Mphi fungicidal activity by coculture with chloroquine or by adherence to type 1 collagen matrices was not reversed by bafilomycin, an inhibitor of the vacuolar ATPase. Human Mphi cultured in the presence of bafilomycin also completely degraded heat-killed Hc yeasts, whereas mouse peritoneal Mphi digestion of yeasts was completely reversed in the presence of bafilomycin. However, bafilomycin did not inhibit mouse Mphi fungistatic activity induced by IFN-gamma. Thus, human Mphi do not require phagosomal acidification to kill and degrade Hc yeasts, whereas mouse Mphi do require acidification for fungicidal but not fungistatic activity.