Pneumocystis murina Promotes Inflammasome Formation and NETosis during Pneumocystis Pneumonia

Pneumocystis jirovecii pneumonia (PjP) poses a serious risk to individuals with compromised immune systems, such as individuals with HIV/AIDS or undergoing immunosuppressive therapies for cancer or solid organ transplants. Severe PjP triggers excessive lung inflammation, resulting in lung function decline and consequential alveolar damage, potentially culminating in acute respiratory distress syndrome. Non-HIV patients face a 30-60%mortality rate, emphasizing the need for a deeper understanding of inflammatory responses in PjP. Prior research emphasized macrophages in Pneumocystis infections, neglecting neutrophils' role in tissue damage. Consequently, the overemphasis on macrophages led to an incomplete understanding of the role of neutrophils and inflammatory responses. In the current investigation, our RNAseq studies on a murine surrogate model of PjP revealed heightened activation of the NLRP3 inflammasome and NETosis cell death pathways in their lungs. Immunofluorescence staining confirmed Neutrophil Extracellular Trap (NET) presence in the lungs of the P. murina -infected mice, validating our findings. Moreover, isolated neutrophils exhibited NETosis when directly stimulated with P. murina . While isolated NETs did not compromise P. murina viability, our data highlight the potential role of neutrophils in promoting inflammation during P. murina pneumonia through NLRP3 inflammasome assembly and NETosis. These pathways, essential for inflammation and pathogen elimination, bear the risk of uncontrolled activation leading to excessive tissue damage and persistent inflammation. This pioneering study is the first to identify the formation of NETs and inflammasomes during Pneumocystis infection, paving the way for comprehensive investigations into treatments aimed at mitigating lung damage and augmenting survival rates for individuals with PjP.

Extracellular vesicles from Pneumocystis carinii-infected rats impair fungal viability but are dispensable for macrophage functions

Pneumocystis spp. are host obligate fungal pathogens that can cause severe pneumonia in mammals and rely heavily on their host for essential nutrients. The lack of a sustainable in vitro culture system poses challenges in understanding their metabolism, and the acquisition of essential nutrients from host lungs remains unexplored. Transmission electron micrographs show that extracellular vesicles (EVs) are found near Pneumocystis spp. within the lung. We hypothesized that EVs transport essential nutrients to the fungi during infection. To investigate this, EVs from P. carinii- and P. murina-infected rodents were biochemically and functionally characterized. These EVs contained host proteins involved in cellular, metabolic, and immune processes as well as proteins with homologs found in other fungal EV proteomes, indicating that Pneumocystis may release EVs. Notably, EV uptake by P. carinii indicated their potential involvement in nutrient acquisition and a possibility for using engineered EVs for efficient therapeutic delivery. However, EVs added to P. carinii in vitro did not show increased growth or viability, implying that additional nutrients or factors are necessary to support their metabolic requirements. Exposure of macrophages to EVs increased proinflammatory cytokine levels but did not affect macrophages' ability to kill or phagocytose P. carinii. These findings provide vital insights into P. carinii and host EV interactions, yet the mechanisms underlying P. carinii's survival in the lung remain uncertain. These studies are the first to isolate, characterize, and functionally assess EVs from Pneumocystis-infected rodents, promising to enhance our understanding of host-pathogen dynamics and therapeutic potential.IMPORTANCEPneumocystis spp. are fungal pathogens that can cause severe pneumonia in mammals, relying heavily on the host for essential nutrients. The absence of an in vitro culture system poses challenges in understanding their metabolism, and the acquisition of vital nutrients from host lungs remains unexplored. Extracellular vesicles (EVs) are found near Pneumocystis spp., and it is hypothesized that these vesicles transport nutrients to the pathogenic fungi. Pneumocystis proteins within the EVs showed homology to other fungal EV proteomes, suggesting that Pneumocystis spp. release EVs. While EVs did not significantly enhance P. carinii growth in vitro, P. carinii displayed active uptake of these vesicles. Moreover, EVs induced proinflammatory cytokine production in macrophages without compromising their ability to combat P. carinii. These findings provide valuable insights into EV dynamics during host-pathogen interactions in Pneumocystis pneumonia. However, the precise underlying mechanisms remain uncertain. This research also raises the potential for engineered EVs in therapeutic applications.

Extracellular Vesicles from Pneumocystis carinii -Infected Rats Impair Fungal Viability but are Dispensable for Macrophage Functions

Pneumocystis spp. are host obligate fungal pathogens that can cause severe pneumonia in mammals and rely heavily on their host for essential nutrients. The lack of a sustainable in vitro culture system poses challenges in understanding their metabolism and the acquisition of essential nutrients from host lungs remains unexplored. Transmission electron micrographs show Extracellular Vesicles (EVs) are found near Pneumocystis spp. within the lung. We hypothesized that EVs transport essential nutrients to the fungi during infection. To investigate this, EVs from P. carinii and P. murina infected rodents were biochemically and functionally characterized. These EVs contained host proteins involved in cellular, metabolic, and immune processes as well as proteins with homologs found in other fungal EV proteomes, indicating Pneumocystis may release EVs. Notably, EV uptake by P. carinii indicated their potential involvement in nutrient acquisition and indicate a possibility for using engineered EVs for efficient therapeutic delivery. However, EVs added to P. carinii in vitro , did not show increased growth or viability, implying that additional nutrients or factors are necessary to support their metabolic requirements. Exposure of macrophages to EVs increased proinflammatory cytokine levels, but did not affect macrophages' ability to kill or phagocytose P. carinii . These findings provide vital insights into P. carinii and host EV interactions, yet the mechanisms underlying P. carinii 's survival in the lung remain uncertain. These studies are the first to isolate, characterize, and functionally assess EVs from Pneumocystis -infected rodents, promising to enhance our understanding of host-pathogen dynamics and therapeutic potential.

Anidulafungin Treatment Blocks the Sexual Cycle of Pneumocystis murina and Prevents Growth and Survival without Rescue by an Alternative Mode of Replication

The proposed life cycle of fungi in the genus Pneumocystis has typically included both an asexual cycle via binary fission and a sexual cycle. Until recently, the strategy used for sexual replication was largely unknown, but genomic and functional assays now support a mode known as primary homothallism (self-fertilization). The question of whether an asexual cycle contributes to the growth of these fungi remains. Treatment of Pneumocystis pneumonia in immunosuppressed rodent models with the class of drugs known as echinocandins is challenging the historical concept of asexual replication. The echinocandins target 1,3-β-D-glucan (BG) synthesis resulting in death for most fungi. Because Pneumocystis species have both non-BG expressing life cycle stages (trophic forms) and BG-expressing asci, treatment with anidulafungin and caspofungin resulted in elimination of asci, with large numbers of non-BG expressing organisms remaining in the lungs. Transcriptional analyses of anidulafungin treated Pneumocystis murina-infected lungs indicated that these agents were blocking the sexual cycle. In the present study, we explored whether there was an asexual or alternative method of replication that could rescue P. murina survival and growth in the context of anidulafungin treatment. The effects of anidulafungin treatment on early events in the sexual cycle were investigated by RT-qPCR targeting specific mating genes, including mam2, map3, matMi, matPi, and matMc. Results from the in vivo and gene expression studies clearly indicated there was no rescue by an asexual cycle, supporting these fungi's reliance on the sexual cycle for survival and growth. Dysregulation of mating-associated genes showed that anidulafungin induced effects early in the mating process. IMPORTANCE The concept of a sexually obligate fungus is unique among human fungal pathogens. This reliance can be exploited for drug development and here we show a proof of principle for this unusual target. Most human fungal pathogens eschew the mammalian environment with its battery of immune responses. Pneumocystis appear to have evolved to survive in such an environment, perhaps by using sexual replication to help in DNA repair and to introduce genetic variation in its major surface antigen family because the lung is the primary environment of these pathogens. The concept of primary homothallism fits well into its chosen ecosystem, with ready mating partners expressing both mating type receptors, and a sexual cycle that can introduce beneficial genetic variation without the need for outbreeding.

The Long-Acting Echinocandin, Rezafungin, Prevents Pneumocystis Pneumonia and Eliminates Pneumocystis from the Lungs in Prophylaxis and Murine Treatment Models

Rezafungin is a novel echinocandin in Phase 3 development for prevention of invasive fungal disease caused by Candida spp., Aspergillus spp. and Pneumocystis jirovecii in blood and marrow transplantation patients. For such patients, standard antifungal prophylaxis currently comprises an azole for Candida and Aspergillus plus trimethoprim-sulfamethoxazole (TMP-SMX) for Pneumocystis pneumonia (PCP) despite drug-drug-interactions and intolerability that may limit their use, thus, alternatives are desirable. Rezafungin demonstrates a favorable safety profile and pharmacokinetic properties that allow for once-weekly dosing in addition, to antifungal activity against these predominant pathogens. Herein, the in vivo effects of rezafungin against Pneumocystis murina pneumonia were evaluated in immunosuppressed mouse models of prophylaxis and treatment using microscopy and qPCR assessments. In the prophylaxis model, immunosuppressed mice inoculated with P. murina were administered TMP-SMX (50/250 mg/kg 1×/week or 3×/week), caspofungin (5 mg/kg 3×/week), rezafungin (20 mg/kg, 1×/week or 3×/week; 5 mg/kg, 3×/week) intraperitoneally for 2, 4, 6 and 8 weeks, then immunosuppressed for an additional 6 weeks. Rezafungin administered for 4 weeks prevented P. murina from developing infection after rezafungin was discontinued. In the treatment model, immunosuppressed mice with P. murina pneumonia were treated with rezafungin 20 mg/kg 3×/week intraperitoneally for 2, 4, 6 and 8 weeks. Treatment with rezafungin for 8 weeks resulted in elimination of P. murina. Collectively, these studies showed that rezafungin could both prevent infection and eliminate P. murina from the lungs of mice. These findings support the obligate role of sexual reproduction for survival and growth of Pneumocystis spp. and warrant further investigation for treatment of P. jirovecii pneumonia in humans.

The Promise of Lung Organoids for Growth and Investigation of Pneumocystis Species

Pneumocystis species (spp.) are host-obligate fungal parasites that colonize and propagate almost exclusively in the alveolar lumen within the lungs of mammals where they can cause a lethal pneumonia. The emergence of this pneumonia in non-HIV infected persons caused by Pneumocystis jirovecii (PjP), illustrates the continued importance of and the need to understand its associated pathologies and to develop new therapies and preventative strategies. In the proposed life cycle, Pneumocystis spp. attach to alveolar type 1 epithelial cells (AEC1) and prevent gas exchange. This process among other mechanisms of Pneumocystis spp. pathogenesis is challenging to observe in real time due to the absence of a continuous ex vivo or in vitro culture system. The study presented here provides a proof-of-concept for the development of murine lung organoids that mimic the lung alveolar sacs expressing alveolar epithelial type 1 cells (AEC1) and alveolar type 2 epithelial cells (AEC2). Use of these 3-dimensional organoids should facilitate studies of a multitude of unanswered questions and serve as an improved means to screen new anti- PjP agents.

Efficacy of Rezafungin in Prophylactic Mouse Models of Invasive Candidiasis, Aspergillosis, and Pneumocystis Pneumonia

Antifungal prophylaxis is recommended to prevent invasive fungal disease caused by Candida spp., Aspergillus spp., and Pneumocystis jirovecii in patients at risk for opportunistic infections, such as allogeneic blood or marrow transplant recipients, patients with hematological disease undergoing chemotherapy, or patients on immunosuppressive therapies. Current approaches to antifungal prophylaxis require multiple agents to cover these key fungi. Rezafungin, a novel echinocandin designed for next-generation properties (e.g., greater stability and long-acting pharmacokinetics for once-weekly dosing), has demonstrated in vitro activity against Candida and Aspergillus spp. and efficacy against Pneumocystis spp. biofilms. Rezafungin was evaluated in in vivo studies of prophylactic efficacy using immunosuppressed mouse models of invasive candidiasis, aspergillosis, and Pneumocystis pneumonia. Rezafungin reduction of Candida CFU burden was generally greater with increasing drug concentrations (5, 10, or 20 mg/kg) and when rezafungin was administered closer to the time of fungal challenge (day -1, -3, or -5). Similarly, in the aspergillosis model, survival rates increased with drug concentrations and when rezafungin was administered closer to the time of fungal challenge. Against Pneumocystismurina, rezafungin significantly reduced trophic nuclei and asci counts at all doses tested. Rezafungin prevented infection at the two higher doses compared to vehicle and had comparable activity to the active control trimethoprim-sulfamethoxazole at human equivalent doses for prevention. These findings support phase 3 development of rezafungin and the potential for single-agent prophylaxis against invasive fungal disease caused by Candida spp., Aspergillus spp., and Pneumocystis jirovecii.

A Novel Encochleated Formulation Improves Atovaquone Activity in a Murine Model of Pneumocystis Pneumonia

Although atovaquone is effective in treating and preventing Pneumocystis pneumonia (PCP), its use is limited by nonlinear absorption and adverse events. The current study was undertaken to examine the activity of encochleated atovaquone (eATQ), a novel lipid-crystal nanoparticle formulation, in a mouse model of PCP. eATQ 100-200 mg was superior to commercially available atovaquone at 14 days in decreasing total Pneumocystis nuclei and asci. eATQ plus anidulafungin reduced nuclei significantly better than commercial atovaquone plus anidulafungin. eATQ is a novel formulation of atovaquone that warrants further evaluation for treatment and prevention of PCP.

A quantitative systems pharmacology (QSP) model for Pneumocystis treatment in mice

BACKGROUND

The yeast-like fungi Pneumocystis, resides in lung alveoli and can cause a lethal infection known as Pneumocystis pneumonia (PCP) in hosts with impaired immune systems. Current therapies for PCP, such as trimethoprim-sulfamethoxazole (TMP-SMX), suffer from significant treatment failures and a multitude of serious side effects. Novel therapeutic approaches (i.e. newly developed drugs or novel combinations of available drugs) are needed to treat this potentially lethal opportunistic infection. Quantitative Systems Pharmacological (QSP) models promise to aid in the development of novel therapies by integrating available pharmacokinetic (PK) and pharmacodynamic (PD) knowledge to predict the effects of new treatment regimens.

RESULTS

In this work, we constructed and independently validated PK modules of a number of drugs with available pharmacokinetic data. Characterized by simple structures and well constrained parameters, these PK modules could serve as a convenient tool to summarize and predict pharmacokinetic profiles. With the currently accepted hypotheses on the life stages of Pneumocystis, we also constructed a PD module to describe the proliferation, transformation, and death of Pneumocystis. By integrating the PK module and the PD module, the QSP model was constrained with observed levels of asci and trophic forms following treatments with multiple drugs. Furthermore, the temporal dynamics of the QSP model were validated with corresponding data.

CONCLUSIONS

We developed and validated a QSP model that integrates available data and promises to facilitate the design of future therapies against PCP.

Identification of a New Class of Antifungals Targeting the Synthesis of Fungal Sphingolipids

Recent estimates suggest that >300 million people are afflicted by serious fungal infections worldwide. Current antifungal drugs are static and toxic and/or have a narrow spectrum of activity. Thus, there is an urgent need for the development of new antifungal drugs. The fungal sphingolipid glucosylceramide (GlcCer) is critical in promoting virulence of a variety of human-pathogenic fungi. In this study, we screened a synthetic drug library for compounds that target the synthesis of fungal, but not mammalian, GlcCer and found two compounds [N'-(3-bromo-4-hydroxybenzylidene)-2-methylbenzohydrazide (BHBM) and its derivative, 3-bromo-N'-(3-bromo-4-hydroxybenzylidene) benzohydrazide (D0)] that were highly effective in vitro and in vivo against several pathogenic fungi. BHBM and D0 were well tolerated in animals and are highly synergistic or additive to current antifungals. BHBM and D0 significantly affected fungal cell morphology and resulted in the accumulation of intracellular vesicles. Deep-sequencing analysis of drug-resistant mutants revealed that four protein products, encoded by genes APL5, COS111, MKK1, and STE2, which are involved in vesicular transport and cell cycle progression, are targeted by BHBM.

IMPORTANCE

Fungal infections are a significant cause of morbidity and mortality worldwide. Current antifungal drugs suffer from various drawbacks, including toxicity, drug resistance, and narrow spectrum of activity. In this study, we have demonstrated that pharmaceutical inhibition of fungal glucosylceramide presents a new opportunity to treat cryptococcosis and various other fungal infections. In addition to being effective against pathogenic fungi, the compounds discovered in this study were well tolerated by animals and additive to current antifungals. These findings suggest that these drugs might pave the way for the development of a new class of antifungals.

In vitro selection and in vivo efficacy of piperazine- and alkanediamide-linked bisbenzamidines against Pneumocystis pneumonia in mice

Bisbenzamidines, such as pentamidine isethionate, are aromatic dicationic compounds that are active against Pneumocystis and other microbes but are oftentimes toxic to the host. To identify potential anti-Pneumocystis agents, we synthesized bisbenzamidine derivatives in which the parent compound pentamidine was modified by a 1,4-piperazinediyl, alkanediamide, or 1,3-phenylenediamide moiety as the central linker. Several of the compounds were more active against P. carinii and less toxic than pentamidine in cytotoxicity assays. For this study, we evaluated nine bisbenzamidine derivatives representing a range of in vitro activities, from highly active to inactive, for the treatment of pneumocystosis in an immunosuppressed mouse model. Six of these in vitro-active compounds, 01, 02, 04, 06, 100, and 101, exhibited marked efficacies against infection at a dose of 10 mg/kg of body weight, and four compounds, 01, 04, 100, and 101, showed significant increases in survival versus that of untreated infected control mice. Compound 100 was highly efficacious against the infection at 20 mg/kg and 40 mg/kg, with > 1,000-fold reductions in burden, and resulted in improved survival curves versus those for pentamidine-treated mice (at the same doses). All six bisbenzamidine compounds that exhibited high in vitro activity significantly decreased the infection in vivo; two compounds, 12 and 102, with marked to moderate in vitro activities had slight or no activity in vivo, while compound 31 was inactive in vitro and was also inactive in vivo. Thus, the selection of highly active compounds from in vitro cytotoxicity assays was predictive of activity in the mouse model of Pneumocystis pneumonia. We conclude that a number of these bisbenzamidine compounds, especially compound 100, may show promise as new anti-Pneumocystis drugs.

Efficient resolution of Pneumocystis murina infection in surfactant protein A-deficient mice following withdrawal of corticosteroid-induced immunosuppression

Following withdrawal of immunosuppression, surfactant protein A (SP-A)-deficient and wild-type mice cleared Pneumocystis murina infection in a similar manner, but exhibited significant differences in lymphocyte populations, interleukin (IL)-6 levels and chemokine expression levels. A higher percentage of lymphocytes were detected in lung lavage fluid from SP-A-deficient mice, but more CD4+ T cells were isolated from lung tissue of wild-type mice. Higher concentrations of IL-6 were detected in lavage fluid and enhanced expression of lymphotactin and RANTES were detected in the lungs of wild-type mice. Equal levels of surfactant protein D were detected in SP-A-deficient and wild-type mice and no differences were detected in markers of lung injury between the two strains of mice. Thus, SP-A does not enhance organism clearance, but does modulate the host immune response during resolution of P. murina infection.

Resolution of Pneumocystis murina infection following withdrawal of corticosteroid induced immunosuppression

The host response during resolution of Pneumocystis murina infection following withdrawal of Dexamethasone (Dex) induced immunosuppression was analyzed. Mice were inoculated with P. murina and treated with Dex for 4 weeks. Treatment was stopped and mice were sacrificed at d1, d7, and d14. Control mice were treated in the same manner, but were inoculated with nonviable P. murina. P. murina was actively cleared from the lungs following withdrawal of Dex treatment. No P. murina was detected in control mice. Significantly more neutrophils, lymphocytes, macrophages, and eosinophils were recovered from the lungs of mice that had been infected with P. murina than from control mice at d7, but only neutrophils remained significantly elevated at d14. Significantly more CD4+ and CD8+ T cells were purified from the lungs of mice that had been infected with P. murina mice at d7 and d14. Cytokine levels were measured in lung lavage fluid by ELISA. TNF-alpha, IFN-gamma, IL-1, and IL-6 levels were higher in mice that had been inoculated with P. murina at all three time points. TNF-alpha and IL-1 levels did not change significantly following withdrawal of Dex treatment. Low levels of IL 6 were detected at d1, but increased significantly by d7 and d14. IFN-gamma levels peaked at d14. Chemokine message levels were measured in lung tissue by ribonuclease protection assay. MIP-1beta and IP-10 message increased between d1 and d7 and then decreased by d14. RANTES message levels increased from d1 to d7 and remained elevated at d14. Withdrawal of Dex induced immunosuppression from P. murina infected mice resulted in activation of many arms of the host response that lead to resolution of the infection.

Surfactant protein A limits Pneumocystis murina infection in immunosuppressed C3H/HeN mice and modulates host response during infection

The development of Pneumocystis murina pneumonia and host response were characterized over time and at different levels of infection in corticosteroid immunosuppressed surfactant protein A (SP-A) knockout and wild-type (WT) mice. Infection increased over time in both strains of mice; however, significantly more cyst forms were detected in the knockout mice at intermediate and late stages of infection. In mice with heavy infections, TNF-alpha and IFN-gamma protein concentrations were significantly higher in pulmonary lavage fluid from knockout mice. There was a significant positive correlation between TNF-alpha and IFN-gamma concentrations and the level of infection in knockout mice, but not in WT mice. No significant differences were detected in IL-1 levels between the two strains of mice at any of the time points or at any level of infection. At heavier infection levels, significantly more MIP-2 protein was detected in the lungs of knockout mice, but a significant positive correlation between MIP-2 concentrations and the infection level was detected in both groups of mice. At the intermediate stage of infection, a significantly higher percentage of neutrophils was detected in the lungs of knockout mice than in WT mice. There was no difference in SP-D levels between WT and KO mice with identical levels of infection. These data support a protective role for SP-A in host defense against Pneumocystis and suggest that the effects of SP-A on the host response vary based on the intensity of the infection.

Use of terbinafine in mouse and rat models of Pneumocystis carinii pneumonia

Terbinafine, an allylamine used to treat onychomycosis, has been reported to be active against rat Pneumocystis carinii in vitro and in vivo. By contrast, our in vitro data showed that the 50% inhibitory concentration of terbinafine against rat P. carinii is 3.7 microg/ml, a level that cannot be clinically achieved in serum. In the present study, terbinafine administered orally at doses of 20 to 400 mg/kg/day and 50 to 250 mg/kg/day was ineffective therapy for mouse and rat models of pneumocystosis, respectively. These results emphasize the complexities of P. carinii drug testing and the need for caution before considering studies in humans.

Anti-human immunodeficiency virus drugs are ineffective against Pneumocystis carinii in vitro and in vivo

Human immunodeficiency virus (HIV) protease inhibitors (PIs) recently have been reported to be active against Pneumocystis carinii in cell culture. Twelve anti-HIV drugs were analyzed for their effects against rat P. carinii by an ATP cytotoxicity assay. Indinavir and saquinavir exhibited slight anti-P. carinii activity at concentrations above those that can be clinically achieved in serum; other PIs and nucleoside and nonnucleoside reverse-transcriptase inhibitors were inactive against the organism. Anti-HIV drugs, alone or in combination, did not materially reduce the organism count in the treatment of P. carinii pneumonia in immunosuppressed mice. Thus, anti-HIV drugs have little or no activity against P. carinii in these in vitro and in vivo systems. Caution should be used when interpreting reports of the susceptibility of P. carinii to anti-HIV drugs on the basis of in vitro testing only.

In vitro and in vivo effects of quinupristin-dalfopristin against Pneumocystis carinii

Quinupristin-dalfopristin (Q-D), which is active against bacteria and Toxoplasma gondii, was examined for its activity against Pneumocystis carinii. After 72 h of incubation with rat P. carinii in an ATP cytotoxicity assay, the 50% inhibitory concentration of Q-D was 10.6 microg/ml, a level that can be achieved in serum with high-dose administration. Q-D administered intraperitoneally at doses of 50 to 200 mg per kg of body weight per day in the treatment and 100 mg/kg/day three times per week in the prophylaxis of pneumocystosis in immunosuppressed mice reduced the organism burden up to 15- and 302-fold, respectively. We conclude that Q-D has activity against P. carinii in vitro and in vivo.