Utilization of a Histoplasma capsulatum zinc reporter reveals the complexities of fungal sensing of metal deprivation

Histoplasma capsulatum is a dimorphic fungal pathogen acquired via inhalation of soil-resident spores. Upon exposure to mammalian body temperatures, these fungal elements transform into yeasts that reside primarily within phagocytes. Macrophages (MΦ) provide a permissive environment for fungal replication until T cell-dependent immunity is engaged. MΦ activated by granulocyte macrophage colony stimulating factor (GM-CSF) induces metallothioneins (MTs) that bind zinc (Zn) and deprive yeast cells of labile Zn, thereby disabling fungal growth. Prior work demonstrated that the zinc transporter, ZRT2, was important for fungal survival in vivo. Hence, we constructed a yeast cell reporter strain that expresses green fluorescent protein (GFP) under control of the ZRT2 zinc-regulated promoter. This reporter accurately responds to a medium devoid of Zn. ZRT2 expression increased in GM-CSF, but not interferon-γ, stimulated MΦ. To examine the in vivo response, we infected mice with a reporter yeast strain and assessed ZRT2 expression at 0, 3, 7, and 14 days post-infection (dpi). ZRT2 expression minimally increased at 3 dpi and peaked at 7 dpi, corresponding with the onset of adaptive immunity. We discovered that the major MΦ populations that restrict Zn from the fungus are interstitial MΦ and exudate MΦ. Neutralizing GM-CSF blunted the control of infection but unexpectedly increased ZRT2 expression. This increase was dependent on another cytokine that activates MΦ to control H. capsulatum replication, M-CSF. These findings illustrate the reporter's ability to sense Zn in vitro and in vivo and correlate ZRT2 expression with GM-CSF and M-CSF activation of MΦ.IMPORTANCEPhagocytes use an arsenal of defenses to control the replication of Histoplasma yeasts, one of which is the limitation of trace metals. On the other hand, H. capsulatum combats metal restriction by upregulating metal importers such as the Zn importer ZRT2. This transporter contributes to H. capsulatum pathogenesis upon activation of adaptive immunity. We constructed a fluorescent ZRT2 transcriptional reporter to probe H. capsulatum Zn sensing during infection and exposed the role for M-CSF activation of macrophages when GM-CSF is absent. These data highlight the ways in which fungal pathogens sense metal deprivation in vivo and reveal the potential of metal-sensing reporters. The work adds a new dimension to study how intracellular pathogens sense and respond to the changing environments of the host.

Utilization of a Histoplasma capsulatum zinc reporter reveals the complexities of fungal sensing of metal deprivation

Histoplasma capsulatum is a dimorphic fungal pathogen acquired via inhalation of soil-resident spores. Upon exposure to mammalian body temperatures, these fungal elements transform into yeasts that reside primarily within phagocytes. Macrophages (MΦ) provide a permissive environment for fungal replication until T cell-dependent immunity is engaged. MΦ activated by granulocyte-MΦ colony stimulating factor (GM-CSF) induce metallothioneins (MTs) that bind zinc (Zn) and deprive yeast cells of labile Zn, thereby disabling fungal growth. Prior work demonstrated that the high affinity zinc importer, ZRT2, was important for fungal survival in vivo. Hence, we constructed a yeast cell reporter strain that expresses green fluorescent protein (GFP) under the control of this importer. This reporter accurately responds to medium devoid of Zn. ZRT2 expression increased (∼5-fold) in GM-CSF, but not interferon-γ, stimulated MΦ. To examine the in vivo response, we infected mice with reporter yeasts and assessed ZRT2 expression at 0-, 3-, 7-, and 14-days post-infection (dpi). ZRT2 expression minimally increased at 3-dpi and peaked on 7-dpi, corresponding with onset of adaptive immunity. We discovered that the major phagocyte populations that restrict Zn to the fungus are interstitial MΦ and exudate MΦ. Neutralizing GM-CSF blunted control of infection but unexpectedly increased ZRT2 expression. This increase was dependent on another cytokine that activates MΦ to control H. capsulatum replication, M-CSF. These findings illustrate the reporter's ability to sense Zn in vitro and in vivo and correlate ZRT2 activity with GM-CSF and M-CSF activation of MΦ.

Importance

Phagocytes use an arsenal of defenses to control replication of Histoplasma yeasts, one of which is limitation of trace metals. On the other hand, H. capsulatum combats metal restriction by upregulating metal importers such as the Zn importer ZRT2. This transporter contributes to H. capsulatum pathogenesis upon activation of adaptive immunity. We constructed a fluorescent ZRT2 reporter to probe H. capsulatum Zn sensing during infection and exposed a role for M-CSF activation of macrophages when GM-CSF is absent. These data highlight the ways in which fungal pathogens sense metal deprivation in vivo and reveal the potential of metal-sensing reporters. The work adds a new dimension to studying how intracellular pathogens sense and respond to the changing environments of the host.

A metabolic inhibitor arms macrophages to kill intracellular fungal pathogens by manipulating zinc homeostasis

Macrophages deploy numerous strategies to combat invasion by microbes. One tactic is to restrict acquisition of diverse nutrients, including trace metals, a process termed nutritional immunity. Intracellular pathogens adapt to a resource-poor environment by marshaling mechanisms to harvest nutrients. Carbon acquisition is crucial for pathogen survival; compounds that reduce availability are a potential strategy to control intracellular replication. Treatment of macrophages with the glucose analog 2-deoxy-D-glucose (2-DG) armed phagocytes to eliminate the intracellular fungal pathogen Histoplasma capsulatum in vitro and in vivo. Killing did not rely on altering access to carbon-containing molecules or changes in ATP, ER stress, or autophagy. Unexpectedly, 2-DG undermined import of exogenous zinc into macrophages, decreasing the quantity of cytosolic and phagosomal zinc. The fungus perished as a result of zinc starvation. This change in metal ingress was not ascribed to a defect in a single importer; rather, there was a collective impairment in transporter activity. This effect promoted the antifungal machinery of macrophages and expanded the complexity of 2-DG activities far beyond manipulating glycolysis. Mechanistic metabolic studies employing 2-DG will have to consider its effect on zinc transport. Our preclinical data support consideration of this agent as a possible adjunctive therapy for histoplasmosis.

1195. Influence of Histoplasma capsulatum Infection on Endothelin-1 mRNA Gene Expression in Bone Marrow Derived Macrophages

Background

Endothelin-1 (ET-1) is increasingly recognized as an immune modulator; it exerts a pro-inflammatory effect by increasing the release of cytokines like interferon gamma. ET-1 is secreted by a variety of cells such as macrophages, neurons and endothelial cells. Activation of the endothelin system has been implicated in the pathogenesis of sepsis caused by bacteria, viruses and even parasites. However, there are no published studies that have explored the role of ET-1 in Histoplasma capsulatum infection. Studying the role of ET-1 in histoplasmosis is important because understanding its role in the host defense mechanism may serve as the foundation for future discovery of novel therapeutic options.

Methods

Bone marrow cells were isolated from mice and set up for tissue culture. Bone marrow derived macrophages (BMDM) were harvested after 5-7 days of incubation, and infected with varying ratios (0.5,1 and 5) of yeasts to macrophages. RNA was extracted from the BMDM after 3, 6, 24 and 48 hours of infection. For comparison, RNA was also extracted from uninfected BMDM at the same time points. Real-time PCR (polymerase chain reaction) was performed on complementary DNA. ET-1 (Edn1) messenger RNA (mRNA) gene expression was quantified relative to the expression of the house keeping /endogenous control gene that encodes for beta-2 microglobulin (B2m).

Results

In BMDM infected with H. capsulatum there was upregulation of Edn1 after 3, 6 and 24 hours of infection. During this same time points, the expression of ET-1 mRNA in the uninfected BMDM remained constant. Expression of Edn1 was highest in the BMDM infected with 5x H. capsulatum after 3 and 6 hours of infection. After 24 hours, the expression of ET-1 mRNA decreased markedly in all concentrations of H. capsulatum. At 48 hours post-infection the Edn1 was downregulated in the 0.5,1 and 5-fold quantities of H. capsulatum across all time the time intervals.

Figure 1

Conclusion

Results from this study indicate that H. capsulatum infection induced an upregulation of the Edn-1 in BMDM. This may correlate with an increase in levels of ET-1 production by the BMDM in the face of H. capsulatum infection. These results provide a platform in which to examine the influence of ET-1 on the host response to this fungus.

Disclosures

All Authors: No reported disclosures

Vaccination with an alkaline extract of Histoplasma capsulatum packaged in glucan particles confers protective immunity in mice

Infection with the dimorphic fungus, Histoplasma capsulatum, occurs world-wide, but North and South America are regions of high endemicity. Interventions to mitigate exposure and consequent disease are limited to remediating a habitat harboring the fungus. The development of a vaccine to prevent infection or lessen its severity is an important advance in disease prevention. Accordingly, we prepared an alkaline extract from the yeast phase of Histoplasma and encased it in glucan particles that act as an adjuvant and delivery vehicle. Immunization of C57BL/6 mice with this encapsulated extract decreased the number of CFUs in lungs and spleens at days 7 and 14 following intranasal infection. Moreover, this vaccine conferred protection against a lethal challenge with the fungus. Cytokine assessment in lungs at a time when the CFUs were similar between controls and vaccinated groups revealed increased quantities of interferon-γ and interleukin-17 in vaccine recipients. This finding was supported by increased generation of both Th1 and Th17 cells in lungs and draining lymph nodes of vaccinated mice compared to controls. Neutralization of interferon-γ or interleukin-17 blunted the effectiveness of vaccination. To identify the proteins comprising this extract, liquid chromatography tandem mass spectrometry was performed. Thus, an H. capsulatum alkaline extract packaged in glucan particles confers protection in an interferon-γ and interleukin-17-dependent manner. Discovery of a single protein or a few proteins in this admixture that mediate protective immunity would represent significant progress in efforts to prevent histoplasmosis.

Deciphering the pathways of death of Histoplasma capsulatum-infected macrophages: implications for the immunopathogenesis of early infection

Apoptosis of leukocytes is known to strongly influence the immunopathogenesis of infection. In this study, we dissected the death pathways of murine macrophages (MΦs) infected with the intracellular pathogen Histoplasma capsulatum. Yeast cells caused apoptosis of MΦs at a wide range of multiplicity of infection, but smaller inocula resulted in delayed detection of apoptosis. Upon infection, caspases 3 and 1 were activated, and both contributed to cell death; however, only the former was involved in apoptosis. The principal driving force for apoptosis involved the extrinsic pathway via engagement of TNFR1 by TNF-α. Infected MΦs produced IL-10 that dampened apoptosis. The chronology of TNF-α and IL-10 release differed in vitro. The former was detected by 2 h postinfection, and the latter was not detected until 8 h postinfection. In vivo, the lungs of TNFR1(-/-) mice infected for 1 d contained fewer apoptotic MΦs than wild-type mice, whereas the lungs of IL-10(-/-) mice exhibited more. Blockade of apoptosis by a pan-caspase inhibitor or by simvastatin sharply reduced the release of TNF-α but enhanced IL-10. However, these treatments did not modify the fungal burden in vitro over 72 h. Thus, suppressing cell death modulated cytokine release but did not alter the fungal burden. These findings provide a framework for the early pathogenesis of histoplasmosis in which yeast cell invasion of lung MΦs engenders apoptosis, triggered in part in an autocrine TNF-α-dependent manner, followed by release of IL-10 that likely prevents apoptosis of newly infected neighboring phagocytes.

The effect of a chimeric human/murine anti-cocaine monoclonal antibody on cocaine self-administration in rats

The predominantly human sequence anti-cocaine monoclonal antibody (mAb), 2E2, has high affinity and specificity for cocaine and antagonizes cocaine distribution to the brain in mice. To determine whether 2E2 can alter the self-administration of cocaine in rats, both cocaine-induced reinstatement (priming) of self-administration, and the rates of cocaine consumption were assessed during daily sessions. After self-administration training, the rats' cocaine priming threshold values were stable over a 2-week baseline period. Furthermore, the rates of cocaine consumption at unit doses of 0.3 and 3.0 micromol/kg were steady within sessions and stable between sessions. Then, 2E2 (120 mg/kg i.v.) or an equivalent dose of nonspecific human polyclonal IgG (control) was infused and daily sessions continued. 2E2 produced an initial, approximately 3-fold, increase in the cocaine priming threshold that declined toward baseline values over the subsequent 3 weeks, with an effect t((1/2)) of approximately 4 days. In contrast to the substantial increase in the cocaine priming threshold, 2E2 produced only modest dose-dependent increases (42 and 18%) in the cocaine consumption rates, and these also gradually declined toward baseline values. There was no significant effect of the control IgG on the priming threshold or rates of consumption of cocaine. After infusion, antibody blood concentrations declined over time, and a two-compartment pharmacokinetic model generated values for the distribution and elimination half-lives of 0.5 and 11.6 days for 2E2 and 0.4 and 6.0 days for control IgG. 2E2 had a long-lasting effect on cocaine-induced priming, which may predict its efficacy as an immunotherapy for cocaine abuse.

A chimeric human/murine anticocaine monoclonal antibody inhibits the distribution of cocaine to the brain in mice

The predominantly human sequence, high-affinity anticocaine monoclonal antibody (mAb) 2E2 was cleared slowly from mouse blood by a first-order process with an elimination t(1/2) of 8.1 days. Infused 2E2 also produced a dramatic dose-dependent increase in plasma cocaine concentrations and a concomitant decrease in the brain cocaine concentrations produced by an i.v. injection of cocaine HCl (0.56 mg/kg). At the highest dose of 2E2 tested (3:1, mAb/drug), cocaine was not detectable in the brain. Pharmacokinetic studies showed that the normal disappearance of cocaine from plasma was described by a two-compartment pharmacokinetic model with distribution t(1/2alpha) and terminal elimination t(1/2beta) values of 1.9 and 26.1 min, respectively. In the presence of an equimolar dose of mAb 2E2, there was a 26-fold increase in the area under the plasma cocaine concentration-time curve (AUC) relative to the AUC in the absence of 2E2. Consequently, 2E2 decreased the volume of distribution of cocaine from 6.0 to 0.20 l/kg, which approximated that of 2E2 (0.28 l/kg). However, cocaine was still rapidly cleared from plasma, and its elimination was now described by a single-compartment model with an elimination t(1/2) of 17 min. Importantly, 2E2 also produced a 4.5-fold (78%) decrease in the cocaine AUC in the brain. Therefore, the effect of 2E2 on plasma and brain cocaine concentrations was predominantly caused by a change in the distribution of cocaine with negligible effects on its rate of clearance. These data support the concept of immunotherapy for drug abuse.

The self-administration of WIN 35,428 and cocaine: comparisons of satiety threshold and elimination half-life in rats

Rats that self-administered cocaine at unit doses between 0.75 and 12 micromol/kg with mean inter-injection intervals between approximately 2 and 18 min also reliably self-administered the cocaine analogue WIN 35,428 (beta-CFT; (-)-3 beta-(4-fluorophenyl)tropane-2 beta-carboxylic acid methyl ester) at unit doses between 0.1 and 1.6 micromol/kg with mean intervals between 10 and 116 min. The long inter-injection intervals of WIN 35,428 necessitated sessions of more than 12 h. The inter-injection intervals were regular and proportional to the unit dose, consistent with the satiety threshold model. Analysis of the mean intervals as a function of unit doses generated values for the mean satiety threshold of cocaine and WIN 35,428 of 6.10 and 0.87 micromol/kg, respectively. The mean t(1/2) for cocaine and WIN 35,428 were 11.1 and 69.4 min, respectively. The approximately 43-fold lower rate of consumption of WIN 35,428 relative to cocaine was a product of the seven-fold greater pharmacodynamic potency and the six-fold greater pharmacokinetic potency.