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

Lysosomal control of dendritic cell function

Lysosomal compartments undergo extensive remodeling during dendritic cell (DC) activation to meet the dynamic functional requirements of DCs. Instead of being regarded as stationary and digestive organelles, recent studies have increasingly appreciated the versatile roles of lysosomes in regulating key aspects of DC biology. Lysosomes actively control DC motility by linking calcium efflux to the actomyosin contraction, while enhanced DC lysosomal membrane permeability contributes to the inflammasome activation. Besides, lysosomes provide a platform for the transduction of innate immune signaling and the intricate host-pathogen interplay. Lysosomes and lysosome-associated structures are also critically engaged in antigen presentation and cross-presentation processes, which are pivotal for the induction of antigen-specific adaptive immune response. Through the current review, we emphasize that lysosome targeting strategies serve as vital DC-based immunotherapies in fighting against tumor, infectious diseases, and autoinflammatory disorders.

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.

Antifungal activity of dendritic cell lysosomal proteins against Cryptococcus neoformans

Cryptococcal meningitis is a life-threatening disease among immune compromised individuals that is caused by the opportunistic fungal pathogen Cryptococcus neoformans. Previous studies have shown that the fungus is phagocytosed by dendritic cells (DCs) and trafficked to the lysosome where it is killed by both oxidative and non-oxidative mechanisms. While certain molecules from the lysosome are known to kill or inhibit the growth of C. neoformans, the lysosome is an organelle containing many different proteins and enzymes that are designed to degrade phagocytosed material. We hypothesized that multiple lysosomal components, including cysteine proteases and antimicrobial peptides, could inhibit the growth of C. neoformans. Our study identified the contents of the DC lysosome and examined the anti-cryptococcal properties of different proteins found within the lysosome. Results showed several DC lysosomal proteins affected the growth of C. neoformans in vitro. The proteins that killed or inhibited the fungus did so in a dose-dependent manner. Furthermore, the concentration of protein needed for cryptococcal inhibition was found to be non-cytotoxic to mammalian cells. These data show that many DC lysosomal proteins have antifungal activity and have potential as immune-based therapeutics.

Zymosan enhances in vitro phagocyte function and the immune response of mice infected with Paracoccidioides brasiliensis

Paracoccidioides brasiliensis is the major etiologic agent of Paracoccidioidomycosis (PCM), the most frequent human deep mycosis in Latin America. It is proposed that masking of β-glucan in P. brasiliensis cell wall is a critical virulence factor that contributes to the development of a chronic disease characterized by a long period of treatment, which is usually toxic. In this context, the search for immunomodulatory agents for therapeutic purposes is highly desirable. One strategy is to use pattern recognition receptors (PRRs) ligands to stimulate the immune response mediated by phagocytes. Here, we sought to evaluate if Zymosan, a β-glucan-containing ligand of the PRRs Dectin-1/TLR-2, would enhance phagocyte function and the immune response of mice challenged with P. brasiliensis. Dendritic cells (DCs) infected with P. brasiliensis and treated with Zymosan showed improved secretion of several proinflammatory cytokines and expression of maturation markers. In addition, when cocultured with splenic lymphocytes, these cells induced the production of a potential protective type 1 and 17 cytokine patterns. In macrophages, Zymosan ensued a significant fungicidal activity associated with nitric oxide production and phagolysosome acidification. Importantly, we observed a protective effect of Zymosan-primed DCs delivered intranasally in experimental pulmonary PCM. Overall, our findings support the potential use of β-glucan-containing compounds such as Zymosan as an alternative or complementary antifungal therapy.

LAY SUMMARY

We report for the first time that Paracoccidioides brasiliensis-infected phagocytes treated with Zymosan (cell wall extract from bakers' yeast) show enhanced cytokine production, maturation, and fungal killing. Also, Zymosan-primed phagocytes induce a protective immune response in infected mice.

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.

Dectin-2 is a primary receptor for NLRP3 inflammasome activation in dendritic cell response to Histoplasma capsulatum

Inflammasome is an intracellular protein complex that serves as cytosolic pattern recognition receptor (PRR) to engage with pathogens and to process cytokines of the interleukin-1 (IL-1) family into bioactive molecules. It has been established that interleukin-1β (IL-1β) is important to host defense against Histoplasma capsulatum infection. However, the detailed mechanism of how H. capsulatum induces inflammasome activation leading to IL-1β production has not been studied. Here, we showed in dendritic cells (DCs) that H. capsulatum triggers caspase-1 activation and IL-1β production through NLRP3 inflammasome. By reciprocal blocking of Dectin-1 or Dectin-2 in single receptor-deficient DCs and cells from Clec4n-/-, Clec7a-/-, and Clec7a-/-Clec4n-/- mice, we discovered that while Dectin-2 operates as a primary receptor, Dectin-1 serves as a secondary one for NLRP3 inflammasome. In addition, both receptors trigger Syk-JNK signal pathway to activate signal 1 (pro-IL-1β synthesis) and signal 2 (activation of caspase-1). Results of pulmonary infection with H. capsulatum showed that CD103+ DCs are one of the major producers of IL-1β and Dectin-2 and Dectin-1 double deficiency abolishes their IL-1β response to the fungus. While K+ efflux and cathepsin B (but not ROS) function as signal 2, viable but not heat-killed H. capsulatum triggers profound lysosomal rupture leading to cathepsin B release. Interestingly, cathepsin B release is regulated by ERK/JNK downstream of Dectin-2 and Dectin-1. Our study demonstrates for the first time the unique roles of Dectin-2 and Dectin-1 in triggering Syk-JNK to activate signal 1 and 2 for H. capsulatum-induced NLRP3 inflammasome activation.

CD103+ Conventional Dendritic Cells Are Critical for TLR7/9-Dependent Host Defense against Histoplasma capsulatum, an Endemic Fungal Pathogen of Humans

Innate immune cells shape the host response to microbial pathogens. Here we elucidate critical differences in the molecular response of macrophages vs. dendritic cells (DCs) to Histoplasma capsulatum, an intracellular fungal pathogen of humans. It has long been known that macrophages are permissive for Histoplasma growth and succumb to infection, whereas DCs restrict fungal growth and survive infection. We used murine macrophages and DCs to identify host pathways that influence fungal proliferation and host-cell viability. Transcriptional profiling experiments revealed that DCs produced a strong Type I interferon (IFN-I) response to infection with Histoplasma yeasts. Toll-like receptors 7 and 9 (TLR7/9), which recognize nucleic acids, were required for IFN-I production and restriction of fungal growth in DCs, but mutation of TLR7/9 had no effect on the outcome of macrophage infection. Moreover, TLR7/9 were essential for the ability of infected DCs to elicit production of the critical cytokine IFNγ from primed CD4⁺ T cells in vitro, indicating the role of this pathway in T cell activation. In a mouse model of infection, TLR7/9 were required for optimal production of IFN-I and IFNγ, host survival, and restriction of cerebral fungal burden. These data demonstrate the critical role of this pathway in eliciting an appropriate adaptive immune response in the host. Finally, although other fungal pathogens have been shown to elicit IFN-I in mouse models, the specific host cell responsible for producing IFN-I has not been elucidated. We found that CD103⁺ conventional DCs were the major producer of IFN-I in the lungs of wild-type mice infected with Histoplasma. Mice deficient in this DC subtype displayed reduced IFN-I production in vivo. These data reveal a previously unknown role for CD103⁺ conventional DCs and uncover the pivotal function of these cells in modulating the host immune response to endemic fungi.

Innate immune cells such as macrophages and dendritic cells (DCs) are critical elements of the initial response to pathogens. Whereas both of these cell types utilize robust anti-microbial strategies to kill internalized microbes, intracellular pathogens have developed mechanisms to manipulate the host response and survive within host cells. In the case of the intracellular fungal pathogen Histoplasma capsulatum, the fungus proliferates within macrophages, resulting in host-cell lysis. In contrast, DCs are able to restrict Histoplasma growth. Here we discovered that the ability of DCs to produce Type I interferons (IFN-I) is critical to their capacity to restrict fungal proliferation and survive infection. IFN-I are cytokines that are elicited during a variety of viral, bacterial, and fungal infections. We performed in vivo and in vitro experiments to show that pattern recognition receptors TLR7 and TLR9 are critical for the IFN-I response and host survival in the mouse model of infection. Additionally we defined a specific DC subset (CD103⁺ conventional DCs) in the mouse lung that is responsible for the IFN-I response, revealing a previously unknown role for these cells. These data provide insight on the pivotal role of a specific host-response pathway at both a cellular and organismal level during infection with endemic fungi.

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.

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.

Intracellular Pathogens within Alveolar Macrophages in a Patient with HIV Infection: Diagnostic Challenge

In HIV-infected individuals, macrophages, the key defense effector cells, manifest defective activity in their interactions with a wide variety of opportunistic pathogens, including fungi and protozoa. Understanding the morphological characteristics of intracellular opportunistic pathogens in addition to their pathogenesis is of critical importance to provide optimal therapy, thereby decreasing morbidity and mortality in HIV-infected patients. We herein present a case of disseminated histoplasmosis confused with disseminated visceral leishmaniasis in an HIV-infected individual from Guyana who developed intracellular organisms within alveolar macrophages.

NLRP3 inflammasome activation by Paracoccidioides brasiliensis

Paracoccidioides brasiliensis is the etiologic agent of paracoccidioidomycosis (PCM), the most prevalent systemic mycosis that is geographically confined to Latin America. The pro-inflammatory cytokine IL-1β that is mainly derived from the activation of the cytoplasmic multiprotein complex inflammasome is an essential host factor against opportunistic fungal infections; however, its role in infection with a primary fungal pathogen, such as P. brasiliensis, is not well understood. In this study, we found that murine bone marrow-derived dendritic cells responded to P. brasiliensis yeast cells infection by releasing IL-1β in a spleen tyrosine kinase (Syk), caspase-1 and NOD-like receptor (NLR) family member NLRP3 dependent manner. In addition, P. brasiliensis-induced NLRP3 inflammasome activation was dependent on potassium (K+) efflux, reactive oxygen species production, phagolysosomal acidification and cathepsin B release. Finally, using mice lacking the IL-1 receptor, we demonstrated that IL-1β signaling has an important role in killing P. brasiliensis by murine macrophages. Altogether, our results demonstrate that the NLRP3 inflammasome senses and responds to P. brasiliensis yeast cells infection and plays an important role in host defense against this fungus.

Paracoccidioidomycosis is a systemic disease that has an important mortality and morbidity impact in Latin America. It mainly affects rural workers of Argentina, Colombia, Venezuela and Brazil. Upon host infection, one of the most important aspects that contribute to the disease outcome is the initial interaction of the Paracoccidioides brasiliensis fungus with the phagocytic cells and the induction of the inflammatory process. Among several inflammatory mediators, the cytokine interleukin-1β is of pivotal importance in this complex process. Here, we demonstrate that P. brasiliensis is sensed by the NLRP3 inflammasome, a cytoplasmatic multiprotein complex that lead to the processing and secretion of IL-1β. In addition, we described the intracellular perturbations that may be associated with NLRP3 activation such as potassium efflux, production of reactive oxygen species, and lysosomal damage. Finally, our work provides evidence for the protective role of IL-1β during fungal infection of murine macrophages.

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.

Mechanisms of dendritic cell lysosomal killing of Cryptococcus

Cryptococcus neoformans is an opportunistic pulmonary fungal pathogen that disseminates to the CNS causing fatal meningitis in immunocompromised patients. Dendritic cells (DCs) phagocytose C. neoformans following inhalation. Following uptake, cryptococci translocate to the DC lysosomal compartment and are killed by oxidative and non-oxidative mechanisms. DC lysosomal extracts kill cryptococci in vitro; however, the means of antifungal activity remain unknown. Our studies determined non-oxidative antifungal activity by DC lysosomal extract. We examined DC lysosomal killing of cryptococcal strains, anti-fungal activity of purified lysosomal enzymes, and mechanisms of killing against C. neoformans. Results confirmed DC lysosome fungicidal activity against all cryptococcal serotypes. Purified lysosomal enzymes, specifically cathepsin B, inhibited cryptococcal growth. Interestingly, cathepsin B combined with its enzymatic inhibitors led to enhanced cryptococcal killing. Electron microscopy revealed structural changes and ruptured cryptococcal cell walls following treatment. Finally, additional studies demonstrated that osmotic lysis was responsible for cryptococcal death.

Membrane microdomain components of Histoplasma capsulatum yeast forms, and their role in alveolar macrophage infectivity

Analysis of membrane lipids of Histoplasma capsulatum showed that ~40% of fungal ergosterol is present in membrane microdomain fractions resistant to treatment with non-ionic detergent at 4°C. Specific proteins were also enriched in these fractions, particularly Pma1p a yeast microdomain protein marker (a plasma membrane proton ATPase), a 30kDa laminin-binding protein, and a 50kDa protein recognized by anti-α5-integrin antibody. To better understand the role of ergosterol-dependent microdomains in fungal biology and pathogenicity, H. capsulatum yeast forms were treated with a sterol chelator, methyl-beta-cyclodextrin (mβCD). Removal of ergosterol by mβCD incubation led to disorganization of ergosterol-enriched microdomains containing Pma1p and the 30kDa protein, resulting in displacement of these proteins from detergent-insoluble to -soluble fractions in sucrose density gradient ultracentrifugation. mβCD treatment did not displace/remove the 50kDa α5-integrin-like protein nor had effect on the organization of glycosphingolipids present in the detergent-resistant fractions. Ergosterol-enriched membrane microdomains were also shown to be important for infectivity of alveolar macrophages; after treatment of yeasts with mβCD, macrophage infectivity was reduced by 45%. These findings suggest the existence of two populations of detergent-resistant membrane microdomains in H. capsulatum yeast forms: (i) ergosterol-independent microdomains rich in integrin-like proteins and glycosphingolipids, possibly involved in signal transduction; (ii) ergosterol-enriched microdomains containing Pma1p and the 30kDa laminin-binding protein; ergosterol and/or the 30kDa protein may be involved in macrophage infectivity.

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.

Interaction of pathogenic yeasts with phagocytes: survival, persistence and escape

Pathogenic yeasts, either from the environment or the normal flora, have to face phagocytic cells that constitute the first line of defence during infection. In order to evade or counteract attack by phagocytes, pathogenic yeasts have acquired a repertoire of strategies to survive, colonize and infect the host. In this review we focus on the interaction of yeasts, such as Candida, Histoplasma or Cryptococcus species, with macrophages or neutrophils. We discuss strategies used by these fungi to prevent phagocytosis or to counteract phagocytic activities. We go on to describe the strategies that permit intracellular survival within phagocytes and that may eventually lead to damage of and escape from the phagocyte.

Antigen-presenting dendritic cells rescue CD4-depleted CCR2-/- mice from lethal Histoplasma capsulatum infection

Excessive production of interleukin-4 impairs clearance of the fungal pathogen Histoplasma capsulatum in mice lacking the chemokine receptor CCR2. An increase in the interleukin-4 level is associated with decreased recruitment of dendritic cells to lungs; therefore, we investigated the possibility that these cells influence interleukin-4 production. Adoptive transfer of wild-type or CCR2(-/-) bone marrow-derived dendritic cells loaded with heat-killed yeast cells to infected CCR2(-/-) mice suppressed interleukin-4 transcription. Surprisingly, transfer of cells did not reduce the fungal burden despite the fact that it limited interleukin-4 transcription. Yeast cell-loaded bone marrow-derived dendritic cell-mediated regulation of interleukin-4 transcription was dependent on major histocompatibility complex II antigen presentation to CD4(+) T cells. We previously showed that CD4(+) T cells were a source of interleukin-4 in infected CCR2(-/-) mice, but their contribution to the TH2 phenotype was unclear. Here we demonstrated that these cells were functionally important since elimination of them prior to infection, but not elimination of them at the time of infection, reduced the interleukin-4 level in infected CCR2(-/-) mice. However, the fungal burden was reduced only in CD4-depleted CCR2(-/-) mice that received yeast cell-loaded bone marrow-derived dendritic cells. Taken together, the data indicate that generation of excess interleukin-4 in lungs of H. capsulatum-infected CCR2(-/-) mice is at least partially a consequence of decreased recruitment of dendritic cells capable of antigen presentation. Furthermore, CD4(+) T cells had a deleterious impact on immunity in infected CCR2(-/-) mice.

Autophagy supports Candida glabrata survival during phagocytosis

The opportunistic human fungal pathogen Candida glabrata is confronted with phagocytic cells of the host defence system. Survival of internalized cells is thought to contribute to successful dissemination. We investigated the reaction of engulfed C. glabrata cells using fluorescent protein fusions of the transcription factors CgYap1 and CgMig1 and catalase CgCta1. The expression level and peroxisomal localization of catalase was used to monitor the metabolic and stress status of internalized C. glabrata cells. These reporters revealed that the phagocytosed C. glabrata cells were exposed to transient oxidative stress and starved for carbon source. Cells trapped within macrophages increased their peroxisome numbers indicating a metabolic switch. Prolonged phagocytosis caused a pexophagy-mediated decline in peroxisome numbers. Autophagy, and in particular pexophagy, contributed to survival of C. glabrata during engulfment. Mutants lacking CgATG11 or CgATG17, genes required for pexophagy and non-selective autophagy, respectively, displayed reduced survival rates. Furthermore, both CgAtg11 and CgAtg17 contribute to survival, since the double mutant was highly sensitive to engulfment. Inhibition of peroxisome formation by deletion of CgPEX3 partially restored viability of CgATG11 deletion mutants during engulfment. This suggests that peroxisome formation and maintenance might sequester resources required for optimal survival. Mobilization of intracellular resources via autophagy is an important virulence factor that supports the viability of C. glabrata in the phagosomal compartment of infected innate immune cells.

Mechanism involved in phagocytosis and killing of Listeria monocytogenes by Acanthamoeba polyphaga

Intra-cellular pathogen, Listeria monocytogenes, is capable of invasion and survival within mammalian cells. However, Acanthamoeba polyphaga trophozoites phagocytose and rapidly degrade Listeria cells. In order to provide more information on amoeba phagocytosis and killing mechanisms, this study used several inhibitor agents known to affect the phagocytosis and killing of bacteria by eukaryotes. Amoebae were pre-treated with mannose, cytochalasin D, wortmannin, suramin, ammonium chloride, bafilomycin A and monensin followed by co-culture with bacteria. Phagocytosis and killing of bacterial cells by amoeba trophozoites was assessed using plate counting methods and microscopy. The data presented indicates that actin polymerisation and cytoskeletal rearrangement are involved in phagocytosis of L. monocytogenes cells by A. polyphaga trophozoites. Further, both phagosomal acidification and phagosome-lysosome fusion are involved in killing and degradation of L. monocytogenes cells by A. polyphaga. However, the mannose-binding protein receptor does not play an important role in uptake of bacteria by amoeba trophozoites. In conclusion, this data reveals the similar principles of molecular mechanisms used by different types of eukaryotes in uptake and killing of bacteria.

The CCL7-CCL2-CCR2 axis regulates IL-4 production in lungs and fungal immunity

Expression of the chemokine receptor CCR2 can be detrimental or beneficial for infection resolution. Herein, we examined whether CCR2 was requisite for control of infection by the dimorphic fungus Histoplasma capsulatum. H. capsulatum-infected CCR2(-/-) mice manifested defects in inflammatory cell recruitment, increased IL-4, and progressive infection. Increased IL-4 in CCR2(-/-) mice primarily contributed to decreased host resistance as demonstrated by the ability of IL-4-neutralized CCR2(-/-) mice to resolve infection without altering inflammatory cell recruitment. Surprisingly, numerous alveolar macrophages and dendritic cells contributed to IL-4 production in CCR2(-/-) mice. IL-4-mediated impairment of immunity in CCR2(-/-) mice was associated with increased arginase-1 and YM1 transcription and increased transferrin receptor expression by phagocytic cells. Immunity in mice lacking the CCR2 ligand CCL2 was not impaired despite decreased inflammatory cell recruitment. Neutralization of the CCR2 ligand CCL7 in CCL2(-/-) mice, but not wild type, resulted in increased IL-4 and fungal burden. Thus, CCL7 in combination with CCL2 limits IL-4 generation and exerts control of host resistance. Furthermore, increased phagocyte-derived IL-4 in CCR2(-/-) mice is associated with the presence of alternatively activated phagocytic cells.

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.

Cryptococcus neoformans enters the endolysosomal pathway of dendritic cells and is killed by lysosomal components

Cryptococcus neoformans is an opportunistic fungal pathogen that primarily causes disease in immunocompromised individuals. Dendritic cells (DCs) can phagocytose C. neoformans, present cryptococcal antigen, and kill C. neoformans. However, early events following C. neoformans phagocytosis by DCs are not well defined. We hypothesized that C. neoformans traffics to the endosome and the lysosome following phagocytosis by DCs and is eventually killed in the lysosome. Murine bone marrow-derived DCs (BMDCs) or human monocyte-derived DCs (HDCs) were incubated with live, encapsulated C. neoformans yeast cells and opsonizing antibody. Following incubation, DCs were intracellularly stained with antibodies against EEA1 (endosome) and LAMP-1 (late endosome/lysosome). As assessed by confocal microscopy, C. neoformans trafficked to endosomal compartments of DCs within 10 min and to lysosomal compartments within 30 min postincubation. For HDCs, the studies were repeated using complement-sufficient autologous plasma for the opsonization of C. neoformans. These data showed results similar to those for antibody opsonization, with C. neoformans localized to endosomes within 20 min and to lysosomes within 60 min postincubation. Additionally, the results of live real-time imaging studies demonstrated that C. neoformans entered lysosomal compartments within 20 min following the initiation of phagocytosis. The results of scanning and transmission electron microscopy demonstrated conventional zipper phagocytosis of C. neoformans by DCs. Finally, lysosomal extracts were purified from BMDCs and incubated with C. neoformans to determine their potential to kill C. neoformans. The extracts killed C. neoformans in a dose-dependent manner. This study shows that C. neoformans enters into endosomal and lysosomal pathways following DC phagocytosis and can be killed by lysosomal components.

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.

A monoclonal antibody to Histoplasma capsulatum alters the intracellular fate of the fungus in murine macrophages

Monoclonal antibodies (MAbs) to a cell surface histone on Histoplasma capsulatum modify murine infection and decrease the growth of H. capsulatum within macrophages. Without the MAbs, H. capsulatum survives within macrophages by modifying the intraphagosomal environment. In the present study, we aimed to analyze the affects of a MAb on macrophage phagosomes. Using transmission electron and fluorescence microscopy, we showed that phagosome activation and maturation are significantly greater when H. capsulatum yeast are opsonized with MAb. The MAb reduced the ability of the organism to regulate the phagosomal pH. Additionally, increased antigen processing and reduced negative costimulation occur in macrophages that phagocytose yeast cells opsonized with MAb, resulting in more-efficient T-cell activation. The MAb alters the intracellular fate of H. capsulatum by affecting the ability of the fungus to regulate the milieu of the phagosome.

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.

A vesicle surface tyrosine kinase regulates phagosome maturation

Phagocytosis is crucial for host defense against microbial pathogens and for obtaining nutrients in Dictyostelium discoideum. Phagocytosed particles are delivered via a complex route from phagosomes to lysosomes for degradation, but the molecular mechanisms involved in the phagosome maturation process are not well understood. Here, we identify a novel vesicle-associated receptor tyrosine kinase-like protein, VSK3, in D. discoideum. We demonstrate how VSK3 is involved in phagosome maturation. VSK3 resides on the membrane of late endosomes/lysosomes with its C-terminal kinase domain facing the cytoplasm. Inactivation of VSK3 by gene disruption reduces the rate of phagocytosis in cells, which is rescued by re-expression of VSK3. We found that the in vivo function of VSK3 depends on the presence of the kinase domain and vesicle localization. Furthermore, VSK3 is not essential for engulfment, but instead, is required for the fusion of phagosomes with late endosomes/lysosomes. Our findings suggest that localized tyrosine kinase signaling on the surface of endosome/lysosomes represents a control mechanism for phagosome maturation.

Expression of hygromycin phosphotransferase alters virulence of Histoplasma capsulatum

The Escherichia coli hygromycin phosphotransferase (hph) gene, which confers hygromycin resistance, is commonly used as a dominant selectable marker in genetically modified bacteria, fungi, plants, insects, and mammalian cells. Expression of the hph gene has rarely been reported to induce effects other than those expected. Hygromycin B is the most common dominant selectable marker used in the molecular manipulation of Histoplasma capsulatum in the generation of knockout strains of H. capsulatum or as a marker in mutant strains. hph-expressing organisms appear to have no defect in long-term in vitro growth and survival and have been successfully used to exploit host-parasite interaction in short-term cell culture systems and animal experiments. We introduced the hph gene as a selectable marker together with the gene encoding green fluorescent protein into wild-type strains of H. capsulatum. Infection of mice with hph-expressing H. capsulatum yeast cells at sublethal doses resulted in lethality. The lethality was not attributable to the site of integration of the hph construct into the genomes or to the method of integration and was not H. capsulatum strain related. Death of mice was not caused by altered cytokine profiles or an overwhelming fungal burden. The lethality was dependent on the kinase activity of hygromycin phosphotransferase. These results should raise awareness of the potential detrimental effects of the hph gene.

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.