Human neutrophil-mediated fungistasis against Histoplasma capsulatum. Localization of fungistatic activity to the azurophil granules

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.

Neutrophil and Eosinophil DNA Extracellular Trap Formation: Lessons From Pathogenic Fungi

Fungal infections represent a worldwide health problem. Fungal pathogens are responsible for a variety of conditions, including superficial diseases, allergic pathologies and potentially lethal invasive infections. Neutrophils and eosinophils have been implicated as effector cells in several pathologies. Neutrophils are major effector cells involved in the control of fungal infections and exhibit a plethora of antifungal mechanisms, such as phagocytosis, reactive oxygen species production, degranulation, extracellular vesicle formation, and DNA extracellular trap (ET) release. Eosinophils are polymorphonuclear cells classically implicated as effector cells in the pathogenesis of allergic diseases and helminthic infections, although their roles as immunomodulatory players in both innate and adaptive immunity are currently recognized. Eosinophils are also endowed with antifungal activities and are abundantly found in allergic conditions associated with fungal colonization and sensitization. Neutrophils and eosinophils have been demonstrated to release their nuclear and mitochondrial DNA in response to many pathogens and pro-inflammatory stimuli. ETs have been implicated in the killing and control of many pathogens, as well as in promoting inflammation and tissue damage. The formation of ETs by neutrophils and eosinophils has been described in response to pathogenic fungi. Here, we provide an overview of the mechanisms involved in the release of neutrophil and eosinophil ETs in response to fungal pathogens. General implications for understanding the formation of ETs and the roles of ETs in fungal infections are discussed.

The Role of the Interleukin-17 Axis and Neutrophils in the Pathogenesis of Endemic and Systemic Mycoses

Systemic and endemic mycoses are considered life-threatening respiratory diseases which are caused by a group of dimorphic fungal pathogens belonging to the genera Histoplasma, Coccidioides, Blastomyces, Paracoccidioides, Talaromyces, and the newly described pathogen Emergomyces. T-cell mediated immunity, mainly T helper (Th)1 and Th17 responses, are essential for protection against these dimorphic fungi; thus, IL-17 production is associated with neutrophil and macrophage recruitment at the site of infection accompanied by chemokines and proinflammatory cytokines production, a mechanism that is mediated by some pattern recognition receptors (PRRs), including Dectin-1, Dectine-2, TLRs, Mannose receptor (MR), Galectin-3 and NLPR3, and the adaptor molecules caspase adaptor recruitment domain family member 9 (Card9), and myeloid differentiation factor 88 (MyD88). However, these PRRs play distinctly different roles for each pathogen. Furthermore, neutrophils have been confirmed as a source of IL-17, and different neutrophil subsets and neutrophil extracellular traps (NETs) have also been described as participating in the inflammatory process in these fungal infections. However, both the Th17/IL-17 axis and neutrophils appear to play different roles, being beneficial mediating fungal controls or detrimental promoting disease pathologies depending on the fungal agent. This review will focus on highlighting the role of the IL-17 axis and neutrophils in the main endemic and systemic mycoses: histoplasmosis, coccidioidomycosis, blastomycosis, and paracoccidioidomycosis.

Biodiverse Histoplasma Species Elicit Distinct Patterns of Pulmonary Inflammation following Sublethal Infection

Acute pulmonary histoplasmosis in healthy individuals comprises most of the disease burden caused by the fungal pathogen Histoplasma. Fungal pneumonia is frequently delayed in diagnosis and treatment due to a prolonged period of quiescence early during infection. In this study, we used a murine respiratory model of histoplasmosis to investigate how different Histoplasma species modulate lung inflammation throughout the complete course of infection. We propose that a relatively low, sublethal inoculum is ideal to model acute pulmonary histoplasmosis in humans, primarily due to the quiescent stage of fungal growth that occurs in the lungs of mice prior to the initiation of inflammation. Our results reveal the unique course of lung immunity associated with divergent species of Histoplasma and imply that the progression of clinical disease is considerably more heterogeneous than previously recognized.

Histoplasma is an endemic dimorphic fungus that can cause disease in healthy and immunocompromised individuals after the transition of inhaled spores into the facultative intracellular yeast form. There is substantial diversity among Histoplasma species, but it is not clear how this heterogeneity impacts the progression of pathology and cellular immune responses during acute respiratory infection, which represents the vast majority of histoplasmosis disease burden. After inoculating mice intranasally with a sublethal inoculum, we characterized the immune response to Histoplasma capsulatum (strain G186A) and Histoplasma ohiense (strain G217B) using comprehensive flow cytometric and single-cell analyses. Within 8 days after inoculation, H. ohiense induced a significantly higher infiltration of neutrophils and inflammatory monocytes into the lung compared to H. capsulatum. Microscopic analysis of infected lung tissue revealed that although the total number of fungi was similar within inflamed lung lesions, we observed different species-dependent intracellular yeast distribution patterns. Inoculation with gfp-expressing strains indicated that H. ohiense, but not H. capsulatum, was associated primarily with alveolar macrophages early after infection. Interestingly, we observed a significant reduction in the total number of alveolar macrophages 12 to 16 days after H. ohiense, but not H. capsulatum infection, despite similar intracellular growth dynamics within AMJ2-C11 alveolar macrophages in vitro. Together, our data suggest that H. ohiense, but not H. capsulatum, preferentially interacts with alveolar macrophages early after infection, which may lead to a different course of inflammation and resolution despite similar rates of fungal clearance.

IMPORTANCE Acute pulmonary histoplasmosis in healthy individuals comprises most of the disease burden caused by the fungal pathogen Histoplasma. Fungal pneumonia is frequently delayed in diagnosis and treatment due to a prolonged period of quiescence early during infection. In this study, we used a murine respiratory model of histoplasmosis to investigate how different Histoplasma species modulate lung inflammation throughout the complete course of infection. We propose that a relatively low, sublethal inoculum is ideal to model acute pulmonary histoplasmosis in humans, primarily due to the quiescent stage of fungal growth that occurs in the lungs of mice prior to the initiation of inflammation. Our results reveal the unique course of lung immunity associated with divergent species of Histoplasma and imply that the progression of clinical disease is considerably more heterogeneous than previously recognized.

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.

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.

NADPH Oxidase Deficiency: A Multisystem Approach

The immune system is a complex system able to recognize a wide variety of host agents, through different biological processes. For example, controlled changes in the redox state are able to start different pathways in immune cells and are involved in the killing of microbes. The generation and release of ROS in the form of an “oxidative burst” represent the pivotal mechanism by which phagocytic cells are able to destroy pathogens. On the other hand, impaired oxidative balance is also implicated in the pathogenesis of inflammatory complications, which may affect the function of many body systems. NADPH oxidase (NOX) plays a pivotal role in the production of ROS, and the defect of its different subunits leads to the development of chronic granulomatous disease (CGD). The defect of the different NOX subunits in CGD affects different organs. In this context, this review will be focused on the description of the effect of NOX2 deficiency in different body systems. Moreover, we will also focus our attention on the novel insight in the pathogenesis of immunodeficiency and inflammation-related manifestations and on the protective role of NOX2 deficiency against the development of atherosclerosis.

Influence of Microbes on Neutrophil Life and Death

Neutrophils are the most abundant leukocyte in humans and they are among the first white cells recruited to infected tissues. These leukocytes are essential for the innate immune response to bacteria and fungi. Inasmuch as neutrophils produce or contain potent microbicides that can be toxic to the host, neutrophil turnover and homeostasis is a highly regulated process that prevents unintended host tissue damage. Indeed, constitutive neutrophil apoptosis and subsequent removal of these cells by mononuclear phagocytes is a primary means by which neutrophil homeostasis is maintained in healthy individuals. Processes that alter normal neutrophil turnover and removal of effete cells can lead to host tissue damage and disease. The interaction of neutrophils with microbes and molecules produced by microbes often alters neutrophil turnover. The ability of microbes to alter the fate of neutrophils is highly varied, can be microbe-specific, and ranges from prolonging the neutrophil lifespan to causing rapid neutrophil lysis after phagocytosis. Here we provide a brief overview of these processes and their associated impact on innate host defense.

Protective Effect of Galectin-1 during Histoplasma capsulatum Infection Is Associated with Prostaglandin E2 and Nitric Oxide Modulation

Histoplasma capsulatum is a dimorphic fungus that develops a yeast-like morphology in host's tissue, responsible for the pulmonary disease histoplasmosis. The recent increase in the incidence of histoplasmosis in immunocompromised patients highlights the need of understanding immunological controls of fungal infections. Here, we describe our discovery of the role of endogenous galectin-1 (Gal-1) in the immune pathophysiology of experimental histoplasmosis. All infected wild-type (WT) mice survived while only 1/3 of Lgals1^−/− mice genetically deficient in Gal-1 survived 30 days after infection. Although infected Lgals1^−/− mice had increased proinflammatory cytokines, nitric oxide (NO), and elevations in neutrophil pulmonary infiltration, they presented higher fungal load in lungs and spleen. Infected lung and infected macrophages from Lgals1^−/− mice exhibited elevated levels of prostaglandin E₂ (PGE₂, a prostanoid regulator of macrophage activation) and prostaglandin E synthase 2 (Ptgs2) mRNA. Gal-1 did not bind to cell surface of yeast phase of H. capsulatum, in vitro, suggesting that Gal-1 contributed to phagocytes response to infection rather than directly killing the yeast. The data provides the first demonstration of endogenous Gal-1 in the protective immune response against H. capsulatum associated with NO and PGE₂ as an important lipid mediator in the pathogenesis of histoplasmosis.

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.

Impairment of neutrophil migration to remote inflammatory site during lung histoplasmosis

Histoplasma capsulatum (Hc) induces a pulmonary disease in which leukotrienes promote activation and recruitment of effectors cells. It is also well-recognized that leukotriene B₄ (LTB₄) and platelet-activating factor (PAF) induce leukocyte recruitment to inflammatory sites. We investigated the impact of pulmonary Hc infection on PMN migration to a remote inflammatory site. Our results show that pulmonary Hc infection impairs LTB₄- or PAF-stimulated PMN recruitment to air pouch. Yet, remote inflammation did not modify PMN numbers in the bronchoalveolar lavage fluid (BALF) of Hc-infected mice. Interestingly, the concomitant administration of PAF and LTB₄ receptor antagonists inhibited PMN recruitment to both BALF and the remote site, demonstrating cooperation between both mediators. Along that line, our results show that PAF-elicited PMN chemotaxis was abrogated in 5-lipoxygenase-deficient animals. These results suggest caution in the indiscriminate use of anti-inflammatory drugs during infectious diseases.

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.

Celecoxib improves host defense through prostaglandin inhibition during Histoplasma capsulatum infection

Prostaglandins act as mediators of inflammation and, similar to cytokines, function as immune modulators during innate and adaptive immune responses. Therefore, using a pharmacological inhibitor, celecoxib, we investigated the role of prostaglandins in host defense against Histoplasma capsulatum infection in C57BL/6 mice. Our results showed that treatment with celecoxib inhibited cyclooxygenase 2, reduced the total fungal burden, and reduced the concentration of PGE₂, cytokines, lymphocytes, neutrophils, and mononuclear cells in the bronchoalveolar space and lung parenchyma. In addition, celecoxib treatment increased the synthesis of nitric oxide, IFN-γ, LTB₄, and the phagocytic capacity of alveolar macrophages. Moreover, celecoxib treatment increased the survival of mice after infection with a lethal inoculum of H. capsulatum. These results suggest that prostaglandins alter the host immune response and play an important role in the pathogenesis of histoplasmosis. Thus, the inhibition of prostaglandins could be a valuable immunomodulatory strategy and antifungal therapy for histoplasmosis treatment.

5-Lipoxygenase deficiency impairs innate and adaptive immune responses during fungal infection

5-lipoxygenase-derived products have been implicated in both the inhibition and promotion of chronic infection. Here, we sought to investigate the roles of endogenous 5-lipoxygenase products and exogenous leukotrienes during Histoplasma capsulatum infection in vivo and in vitro. 5-LO deficiency led to increased lung CFU, decreased nitric oxide production and a deficient primary immune response during active fungal infection. Moreover, H. capsulatum-infected 5-LO^−/− mice showed an intense influx of neutrophils and an impaired ability to generate and recruit effector T cells to the lung. The fungal susceptibility of 5-LO^−/− mice correlated with a lower rate of macrophage ingestion of IgG-H. capsulatum relative to WT macrophages. Conversely, exogenous LTB4 and LTC4 restored macrophage phagocytosis in 5-LO deficient mice. Our results demonstrate that leukotrienes are required to control chronic fungal infection by amplifying both the innate and adaptive immune response during histoplasmosis.

The role of cytokines and chemokines in Histoplasma capsulatum infection

Histoplasma capsulatum is a prevalent fungal pathogen in the United States, infecting approximately 500,000 individuals each year. Host protection requires an intact cell-mediated immune response. In this review, we will discuss how cytokines and chemokines influence protective immunity in H. capsulatum infection.

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.

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.

The flagellum of Pseudomonas aeruginosa is required for resistance to clearance by surfactant protein A

Surfactant protein A (SP-A) is an important lung innate immune protein that kills microbial pathogens by opsonization and membrane permeabilization. We investigated the basis of SP-A-mediated pulmonary clearance of Pseudomonas aeruginosa using genetically-engineered SP-A mice and a library of signature-tagged P. aeruginosa mutants. A mutant with an insertion into flgE, the gene that encodes flagellar hook protein, was preferentially cleared by the SP-A^+/+ mice, but survived in the SP-A^−/− mice. Opsonization by SP-A did not play a role in flgE clearance. However, exposure to SP-A directly permeabilized and killed the flgE mutant, but not the wild-type parental strain. P. aeruginosa strains with mutation in other flagellar genes, as well as mucoid, nonmotile isolates from cystic fibrosis patients, were also permeabilized by SP-A. Provision of the wild-type fliC gene restored the resistance to SP-A-mediated membrane permeabilization in the fliC-deficient bacteria. In addition, non-mucoid, motile revertants of CF isolates reacquired resistance to SP-A-mediated membrane permeability. Resistance to SP-A was dependent on the presence of an intact flagellar structure, and independent of flagellar-dependent motility. We provide evidence that flagellar-deficient mutants harbor inadequate amounts of LPS required to resist membrane permeabilization by SP-A and cellular lysis by detergent targeting bacterial outer membranes. Thus, the flagellum of P. aeruginosa plays an indirect but important role resisting SP-A-mediated clearance and membrane permeabilization.

Gr-1+ cells play an essential role in an experimental model of disseminated histoplasmosis

Recent studies have shown the participation of Gr-1(+) cells in many types of infections; however, the role played by these cells in the immune response to fungal pathogens is controversial. In this study we determined whether Gr-1(+) cells are involved in the protective immune response in systemic Histoplasma capsulatum infection. Depletion of Gr-1(+) cells using the monoclonal antibody (MAb) RB6-8C5 increased histoplasmosis severity and inhibited the subsequent development of a protective immune response. In addition to the increased fungal burden in lungs and spleens, the Th1 response was found to be unbalanced in these mice and the suppression of the cellular immune response seemed to be associated with increased nitric oxide production. Taken together, these results indicate that Gr-1(+) cell depletion at the beginning of infection allows yeast multiplication and increases mice mortality. This study improves the understanding of the role of Gr-1(+) cells on the protective immunity in histoplasmosis.

Human dendritic cell activity against Histoplasma capsulatum is mediated via phagolysosomal fusion

Histoplasma capsulatum is a fungal pathogen that requires the induction of cell-mediated immunity (CMI) for host survival. We have demonstrated that human dendritic cells (DC) phagocytose H. capsulatum yeasts and, unlike human macrophages (Mø) that are permissive for intracellular growth, DC killed and degraded the fungus. In the present study, we sought to determine whether the mechanism(s) by which DC kill Histoplasma is via lysosomal hydrolases, via the production of toxic oxygen metabolites, or both. Phagosome-lysosome fusion (PL-fusion) was quantified by using fluorescein isothiocyanate-dextran and phase and fluorescence microscopy and by electron microscopy with horseradish peroxidase colloidal gold to label lysosomes. Unlike Mphi, Histoplasma-infected DC exhibited marked PL-fusion. The addition of suramin to Histoplasma-infected DC inhibited PL-fusion and DC fungicidal activity. Incubation of Histoplasma-infected DC at 18 degrees C also concomitantly reduced PL-fusion and decreased the capacity of DC to kill and degrade H. capsulatum yeasts. Further, culture of Histoplasma-infected DC in the presence of bafilomycin, an inhibitor of the vacuolar ATPase, did not block DC anti-Histoplasma activity, indicating that phagosome acidification was not required for lysosome enzyme activity. In contrast, culture of Histoplasma-infected DC in the presence of inhibitors of the respiratory burst or inhibitors of NO synthase had little to no effect on DC fungicidal activity. These data suggest that the major mechanism by which human DC mediate anti-Histoplasma activity is through the exposure of yeasts to DC lysosomal hydrolases. Thus, DC can override one of the strategies used by H. capsulatum yeasts to survive intracellularly within Mø.

Comparative signature-tagged mutagenesis identifies Pseudomonas factors conferring resistance to the pulmonary collectin SP-A

The pulmonary collectin, surfactant protein A (SP-A), is a broad spectrum opsonin with microbicidal membrane permeabilization properties that plays a role in the innate immune response of the lung. However, the factors that govern SP-A's microbial specificity and the mechanisms by which it mediates membrane permeabilization and opsonization are not fully understood. In an effort to identify bacterial factors that confer susceptibility or resistance to SP-A, we used comparative signature-tagged mutagenesis to screen a library of 1,680 Pseudomonas aeruginosa mutants for evidence of differential pulmonary clearance in SP-A-sufficient (SP-A^+/+) and SP-A-deficient (SP-A^−/−) mice. Two SP-A-sensitive P. aeruginosa mutants harboring transposon insertions in genes required for salicylate biosynthesis (pch) and phosphoenolpyruvate-protein-phosphotransferase (ptsP) were recovered. The mutants were indistinguishable from the parental wild-type PA01 with regard to opsonization by SP-A, but they exhibited increased susceptibility to SP-A-mediated membrane permeabilization. These results suggest that bacterial gene functions that are required to maintain membrane integrity play crucial roles in resistance of P. aeruginosa to the permeabilizing effects of SP-A.

Everyday, normal breathing deposits numerous microorganisms on the surfactant membrane that lines the air-exchanging surfaces of the lung. Surfactant protein SP-A, a component of the surfactant membrane, helps to maintain the lung in a germ-free state by aggregating inhaled microorganisms and facilitating their ingestion by immune cells, and by increasing the permeability of their cell membranes. However, the bacterial pathogen Pseudomonas aeruginosa is resistant to SP-A-mediated membrane disruption.

Using a genetic tool called comparative signature-tagged mutagenesis, the authors have identified two P. aeruginosa genes, pch and ptsP, that are required to resist SP-A-mediated membrane permeabilization. Molecular analyses indicate that the pch gene encodes an enzyme that synthesizes salicylate, a compound utilized by bacteria to acquire essential metal ions. The ptsP gene encodes an enzyme called phosphoenolpyruvate-protein-phosphotransferase. The loss of salicylate and phosphoenolpyruvate-protein-phosphotransferase weakens the P. aeruginosa cell membrane, which allows SP-A to poke holes on the membrane and kill the bacteria.

This is the first known report of the roles played by salicylate and phosphoenolpyruvate-protein-phosphotransferase in maintenance of bacterial membrane, and consequently, protecting bacteria from killing by SP-A, through disruption of membrane integrity.

Apoptosis modulates protective immunity to the pathogenic fungus Histoplasma capsulatum

Pathogen-induced apoptosis of lymphocytes is associated with increased susceptibility to infection. In this study, we determined whether apoptosis influenced host resistance to the fungus Histoplasma capsulatum. The level of apoptotic leukocytes progressively increased in the lungs of naive and immune mice during the course of H. capsulatum infection. T cells constituted the dominant apoptotic population. Apoptosis was diminished in H. capsulatum-infected gld/gld and TNF-alpha-deficient mice; concomitantly, the fungal burden exceeded that of controls. Treatment of naive and H. capsulatum-immune mice with caspase inhibitors decreased apoptosis but markedly enhanced the severity of infection. Administration of a proapoptotic dose of suramin diminished the fungal burden. The increased burden in recipients of a caspase inhibitor was associated with elevations in IL-4 and IL-10 levels. In the absence of either of these cytokines, caspase inhibition suppressed apoptosis but did not increase the fungal burden. Thus, apoptosis is a critical element of protective immunity to H. capsulatum. Production of IL-4 and IL-10 is markedly elevated when apoptosis is inhibited, and the release of these cytokines exacerbates the severity of infection.

Surfactant protein A is a principal and oxidation-sensitive microbial permeabilizing factor in the alveolar lining fluid

We have reported that surfactant protein A kills some Gram-negative organisms by increasing membrane permeability. In this study, we investigated the physiologic importance of this activity and the effect of oxidative stress on the antimicrobial functions of SP-A in vitro and in vivo. Concentrated bronchoalveolar lavage fluids from SP-A+/+ mice increased the permeability of the Escherichia coli K12 cell membrane to a greater extent than lavage from SP-A-/- animals. Similarly, calcium-dependent surfactant-binding proteins of SP-A+/+ mice increased membrane permeability more than those from SP-A-/- mice and produced greater zonal killing of agar-embedded bacteria in a radial diffusion assay. Exposure of human SP-A to copper-initiated surfactant phospholipid peroxidation or to free radicals generated by human neutrophils in vitro increased the level of SP-A-associated carbonyl moieties and blocked the permeabilizing function of the protein. We also found that exposure of mice to 90% O2 for 4 days, sufficient to lead to consumption of glutathione, oxidation of protein thiols, and accumulation of airspace protein-associated carbonyl moieties, blocked the permeabilizing activity of lavage fluid from SP-A+/+ mice. We conclude that SP-A is a major microbial permeablizing factor in lavage fluid and that oxidative stress inhibits the antibacterial activity of SP-A by a mechanism that includes oxidative modification and functional inactivation of the protein.

Production of pro-inflammatory cytokines during the early stages of experimentalParacoccidioides brasiliensisinfection

Pro-inflammatory cytokines play an important role in both recruitment and activation of leukocytes migrating into tissues in response to invading pathogens. In this study the production of pro-inflammatory cytokines, determined by ELISA assays, and the recruitment of leukocytes into the lungs of BALB/c mice infected with Paracoccidioides brasiliensis conidia were evaluated during the early stages of infection. The results showed that infected mice had a significant increase in leukocytes in the lung during the first 4 days with a peak at day 2 post-challenge; infiltrates were composed mainly of polymorphonuclear neutrophils (PMN). Pro-inflammatory cytokines such as tumour necrosis factor alpha (TNF-alpha), interleukin (IL) 6, IL-1beta and macrophage inflammatory protein (MIP) 2 were produced at elevated levels during the first 4 days post-challenge, but only in pulmonary samples and not in sera. Additionally, during the early stages of infection, overall weight loss was recorded in infected mice. These results suggest that pro-inflammatory cytokines could be responsible for the recruitment of leukocytes into the lung during the early stages of P. brasiliensis infection. In addition, both pro-inflammatory cytokine production and leukocyte recruitment may participate in the control of infection by influencing the organization of the immune response in the host exposed to P. brasiliensis conidia.

Histoplasma capsulatum inhibits apoptosis and Mac-1 expression in leucocytes

Histoplasma capsulatum is a fungus found intracellularly in neutrophils and peripheral blood mononuclear cells (PBMCs), suggesting that it is capable of evading damage and survives inside these cells. In this study, we report that neutrophils from H. capsulatum-infected mice, and human neutrophils and mononuclear cells exposed to H. capsulatum presented less apoptosis than those from noninfected animals or cells exposed to medium only. Moreover, cells harvested from infected animals are resistant to apoptosis induced by dexamethasone - a proapoptotic stimulant. We also show that neutrophils harvested from infected mice and PBMCs from humans exposed to the fungus had a greatly decreased Mac-1 expression. We conclude that H. capsulatum induces an antiapoptotic state on leucocytes, which correlates with decreased cell-surface Mac-1 expression. These facts may represent an escape mechanism for the fungus by delaying cell death and allowing the fungus to survive inside leucocytes.

Histoplasma capsulatum molecular genetics, pathogenesis, and responsiveness to its environment

Histoplasma capsulatum is a thermally dimorphic ascomycete that is a significant cause of respiratory and systemic disease in mammals including humans, especially immunocompromised individuals such as AIDS patients. As an environmental mold found in the soil, it is a successful member of a competitive polymicrobial ecosystem. Its host-adapted yeast form is a facultative intracellular pathogen of mammalian macrophages. H. capsulatum faces a variety of environmental changes during the course of infection and must survive under harsh conditions or modulate its microenvironment to achieve success as a pathogen. Histoplasmosis may be considered the fungal homolog of the bacterial infection tuberculosis, since both H. capsulatum and Mycobacterium tuberculosis exploit the macrophage as a host cell and can cause acute or persistent pulmonary and disseminated infection and reactivation disease. The identification and functional analysis of biologically or pathogenically important H. capsulatum genes have been greatly facilitated by the development of molecular genetic experimental capabilities in this organism. This review focuses on responsiveness of this fungus to its environment, including differential expression of genes and adaptive phenotypic traits.

Recombinant murine granulocyte-macrophage colony-stimulating factor modulates the course of pulmonary histoplasmosis in immunocompetent and immunodeficient mice

Several endogenous cytokines, including granulocyte-macrophage colony-stimulating factor (GM-CSF), are necessary for eliminating Histoplasma capsulatum from tissues. In this study, we explored the efficacy of recombinant murine GM-CSF in the treatment of pulmonary histoplasmosis. This cytokine significantly reduced fungal burden in a dose-dependent manner. Pretreatment did not consistently produce a better result than treatment started after infection. The biological effectiveness of GM-CSF was not associated with modulation of lung cytokine production or alteration in lung inflammation, but it directly activated a nonadherent lung cell population to exert anti-Histoplasma activity. GM-CSF improved survival of T-cell-depleted mice exposed to H. capsulatum. When combined with a suboptimal amount of amphotericin B, GM-CSF enhanced survival of normal or T-cell-depleted mice given a lethal challenge. These results suggest that this cytokine may be useful as an adjunctive treatment for histoplasmosis.

Human neutrophil-mediated nonoxidative antifungal activity against Cryptococcus neoformans

It has long been appreciated that polymorphonuclear leukocytes (PMN) kill Cryptococcus neoformans, at least in part via generation of fungicidal oxidants. The aim of this study was to examine the contribution of nonoxidative mechanisms to the inhibition and killing of C. neoformans. Treatment of human PMN with inhibitors and scavengers of respiratory burst oxidants only partially reversed anticryptococcal activity, suggesting that both oxidative and nonoxidative mechanisms were operative. To define the mediators of nonoxidative anticryptococcal activity, PMN were fractionated into cytoplasmic, primary (azurophil) granule, and secondary (specific) granule fractions. Incubation of C. neoformans with these fractions for 18 h resulted in percent inhibition of growth of 67.4 +/- 3.4, 84.6 +/- 4.4, and 29.2 +/- 10.5 (mean +/- standard error, n = 3), respectively. Anticryptococcal activity of the cytoplasmic fraction was abrogated by zinc and depletion of calprotectin. Antifungal activity of the primary granules was significantly reduced by pronase treatment, boiling, high ionic strength, and magnesium but not calcium. Fractionation of the primary granules by reverse phase high-pressure liquid chromatography on a C(4) column over an acetonitrile gradient revealed multiple peaks with anticryptococcal activity. Of these, peaks 1 and 6 had substantial fungistatic and fungicidal activity. Peak 1 was identified by acid-urea polyacrylamide gel electrophoresis (PAGE) and mass spectroscopy as human neutrophil proteins (defensins) 1 to 3. Analysis of peak 6 by sodium dodecyl sulfate-PAGE revealed multiple bands. Thus, human PMN have nonoxidative anticryptococcal activity residing principally in their cytoplasmic and primary granule fractions. Calprotectin mediates the cytoplasmic activity, whereas multiple proteins, including defensins, are responsible for activity of the primary granules.

Identification of constituents of human neutrophil azurophil granules that mediate fungistasis against Histoplasma capsulatum

Previously we demonstrated that human neutrophils mediate potent and long-lasting fungistasis against Histoplasma capsulatum yeasts and that all of the fungistatic activity resides in the azurophil granules. In the present study, specific azurophil granule constituents with fungistatic activity were identified by incubation with H. capsulatum yeasts for 24 h and by quantifying the subsequent growth of yeasts via the incorporation of [(3)H]leucine. Human neutrophil defensins HNP-1, HNP-2, and HNP-3 inhibited the growth of H. capsulatum yeasts in a concentration-dependent manner with maximum inhibition at 8 microg/ml. At a concentration of 4 microg/ml, all possible paired combinations of defensins exhibited additive fungistatic activity against H. capsulatum yeasts. Cathepsin G and bactericidal-permeability-increasing protein (BPI) also mediated fungistasis against H. capsulatum in a concentration-dependent manner. The fungistatic activities of combinations of cathepsin G and BPI were additive, as were those of combinations of cathepsin G or BPI with HNP-1, HNP-2, and HNP-3. Lysozyme and elastase exhibited modest antifungal activity, and azurocidin and proteinase 3 exhibited no significant fungistasis against H. capsulatum yeasts. Thus, defensins, cathepsin G, and BPI are the major anti-H. capsulatum effector molecules in the azurophil granules of human neutrophils.

Immune response to early and late Histoplasma capsulatum infections

Knowledge of the host response to the intracellular pathogenic fungus Histoplasma capsulatum has increased dramatically. Information has accumulated regarding the cellular and molecular determinants that lead to resolution of both primary and secondary infection. The significance of cytokines and other endogenous soluble mediators to the protective immune response have been analyzed. Moreover, work concerning the relative importance of T cell subsets to protective immunity has been initiated.

Macrophages in host defense against Histoplasma capsulatum

Macrophages function in both innate and cell-mediated immunity in host defense against pathogenic fungi. They initially serve as a protected environment in which the primary fungal pathogen Histoplasma capsulatum multiplies and disseminates from the lung to other organs. Upon induction of cell-mediated immunity, cytokines activate macrophages to destroy the yeasts and thus remove them from the host.

Interaction of murine macrophage-membrane proteins with components of the pathogenic fungus Histoplasma capsulatum

The interaction of macrophage-membrane proteins and histoplasmin, a crude antigen of the pathogenic fungus Histoplasma capsulatum, was studied using murine peritoneal macrophages. Membrane proteins were purified via membrane attachment to polycationic beads and solubilized in Tris-HCl/SDS/DTT/glycerol for protein extraction; afterwards they were adsorbed or not with H. capsulatum yeast or lectin binding-enriched by affinity chromatography. Membrane proteins and histoplasmin interactions were detected by ELISA and immunoblotting assays using anti-H. capsulatum human or mouse serum and biotinylated goat anti-human or anti-mouse IgG/streptavidin-peroxidase system to reveal the interaction. Results indicate that macrophage-membrane proteins and histoplasmin components interact in a dose-dependent reaction, and adsorption of macrophage-membrane proteins by yeast cells induces a critical decrease in the interaction. Macrophage-membrane glycoproteins with terminal D-galactosyl residues, purified by chromatography with Abrus precatorius lectin, bound to histoplasmin; and two bands of 68kD and 180kD of transferred membrane protein samples interacted with histoplasmin components, as revealed by immunoblot assays. Specificity for beta-galactoside residues on the macrophage-membrane was confirmed by galactose inhibition of the interaction between macrophage-membrane proteins and histoplasmin components, in competitive ELISA using sugars, as well as by enzymatic cleavage of the galactoside residues.

Factors Involved in Regulating Primary and Secondary Immunity to Infection with Histoplasma capsulatum: TNF-α Plays a Critical Role in Maintaining Secondary Immunity in the Absence of IFN-γ

Primary infection to Histoplasma capsulatum often results in a self-limited upper respiratory infection in humans; however, in immunocompromised hosts, disseminated infection can occur through reactivation of a previous infection. Since disseminated histoplasmosis has emerged as a difficult clinical entity to treat in individuals infected with HIV, it was of interest to study the mechanisms involved in maintaining an effective memory immune response. It has been previously shown in a murine model of disseminated histoplasmosis that IL-12, IFN-γ, and TNF-α were important factors in mediating primary protection. To study whether these and other factors were involved in maintaining a protective immune response following secondary infection, normal C57BL/6 mice were first infected with a sublethal dose of H. capsulatum (1 × 105) and then reinfected 3 wk later with a lethal dose of H. capsulatum (6 × 105). Under these conditions, all mice developed an effective immune response with sterilizing immunity. Moreover, normal C57BL/6 mice treated with neutralizing Abs against either IL-12, TNF-α, or IFN-γ, depleted of neutrophils or treated with aminoguanidine at the time of reinfection, maintained an effective immune response. The ability of animals to survive a secondary infection in the absence of IFN-γ was verified by showing that IFN-γ−/− mice previously immunized with H. capsulatum and treated with amphotericin B at the time of primary infection had prolonged survival following reinfection with a normally lethal dose. It was further shown that enhancement of TNF-α production in IFN-γ−/− mice was the major mechanism by which these mice were effective in controlling secondary infection.

Antifungal mechanisms of activated murine bronchoalveolar or peritoneal macrophages for Histoplasma capsulatum

The first line of defence against natural infection by Histoplasma capsulatum (Hc) consists of bronchoalveolar macrophages (BAM) and an early inflammatory response in the lungs. Little is known about the interaction of BAM and Hc, consequently we studied murine BAM in vitro to assess their role in the pulmonary defence in histoplasmosis. A short-term 3-h assay was used to measure fungicidal activity of control BAM and interferon-gamma (IFN-gamma) plus lipopolysaccharide (LPS)-activated BAM. Fungistatic activity of BAM was determined with a 24-h assay. A method devised for measuring colony-forming units (CFU) of non-ingested non-adherent and adherent ingested yeast cells of Hc in BAM cocultures was used. Activated BAM killed Hc (reduced inoculum CFU by 25 +/- 12%; n = 4). The fungicidal activity of BAM was abrogated by 0.2 mM NG-monomethyl-L-arginine (NMMA) or catalase but not by superoxide dismutase. In fungistatic assays activated BAM inhibited multiplication of Hc by 61 +/- 4% (n = 3) compared with cocultures with control BAM. However, Hc multiplied 100% more in control BAM cocultures than in medium alone. Data indicated that this was due to advantages that Hc has in the intracellular environment. Only NMMA inhibited fungistatic activity of activated BAM. In experiments with peritoneal macrophages (PM), results similar to those with BAM were obtained. In conclusion, activated BAM and PM kill yeast cells of Hc by a mechanism dependent on hydrogen peroxide and products of the nitric oxide synthase (NOS) pathway, whereas fungistasis depends only on products of the NOS pathway.