Role of host sphingosine kinase 1 in the lung response against Cryptococcosis

Sterylglucosides in Fungi

Sterylglucosides (SGs) are sterol conjugates widely distributed in nature. Although their universal presence in all living organisms suggests the importance of this kind of glycolipids, they are yet poorly understood. The glycosylation of sterols confers a more hydrophilic character, modifying biophysical properties of cell membranes and altering immunogenicity of the cells. In fungi, SGs regulate different cell pathways to help overcome oxygen and pH challenges, as well as help to accomplish cell recycling and other membrane functions. At the same time, the level of these lipids is highly controlled, especially in wild-type fungi. In addition, modulating SGs metabolism is becoming a novel tool for vaccine and antifungal development. In the present review, we bring together multiple observations to emphasize the underestimated importance of SGs for fungal cell functions.

An Immunogenic and Slow-Growing Cryptococcal Strain Induces a Chronic Granulomatous Infection in Murine Lungs

Many successful pathogens cause latent infections, remaining dormant within the host for years but retaining the ability to reactivate to cause symptomatic disease. The human opportunistic fungal pathogen Cryptococcus neoformans establishes latent pulmonary infections in immunocompetent individuals upon inhalation from the environment. These latent infections are frequently characterized by granulomas, or foci of chronic inflammation, that contain dormant and persistent cryptococcal cells. Immunosuppression can cause these granulomas to break down and release fungal cells that proliferate, disseminate, and eventually cause lethal cryptococcosis. This course of fungal latency and reactivation is understudied due to limited models, as chronic pulmonary granulomas do not typically form in mouse cryptococcal infections. A loss-of-function mutation in the Cryptococcus-specific MAR1 gene was previously described to alter cell surface remodeling in response to host signals. Here, we demonstrate that the mar1Δ mutant strain persists long term in a murine inhalation model of cryptococcosis, inducing a chronic pulmonary granulomatous response. We find that murine infections with the mar1Δ mutant strain are characterized by reduced fungal burden, likely due to the low growth rate of the mar1Δ mutant strain at physiological temperature, and an altered host immune response, likely due to inability of the mar1Δ mutant strain to properly employ virulence factors. We propose that this combination of features in the mar1Δ mutant strain collectively promotes the induction of a more chronic inflammatory response and enables long-term fungal persistence within these granulomatous regions.

PET Study of Sphingosine-1-phosphate Receptor 1 Expression in Response to S. aureus Infection

Sphingosine-1-phosphate receptor 1 (S1PR1) plays a crucial role in infectious diseases. Targeting S1PR1 provides protection against pathogens, such as influenza viruses. This study is aimed at investigating S1PR1 in response to bacterial infection by assessing S1PR1 expression in S. aureus-infected mice. A rodent local muscle bacterial infection model was developed by injecting S. aureus to the lower hind limb of Balb/c mice. The changes of S1PR1 expression in response to bacterial infection and blocking treatment were assessed using ex vivo biodistribution and in vivo positron emission tomography (PET) after intravenous injection of an S1PR1-specific radiotracer [18F]TZ4877. The specificity of [18F]TZ4877 was assessed using S1PR1-specific antagonist, NIBR-0213, and S1PR1-specific DsiRNA pretreated the animals. Immunohistochemical studies were performed to confirm the increase of S1PR1 expression in response to infection. Ex vivo biodistribution data showed that the uptake of [18F]TZ4877 was increased 30.6%, 54.3%, 74.3%, and 115.3% in the liver, kidney, pancreas, and thymus of the infected mice, respectively, compared to that in normal control mice, indicating that S1PR1 is involved in the early immune response to bacterial infection. NIBR-0213 or S1PR1-specific DsiRNA pretreatment reduced the tissue uptake of [18F]TZ4877, suggesting that uptake of [18F]TZ4877 is specific. Our PET/CT study data also confirmed that infected mice have increased [18F]TZ4877 uptake in several organs comparing to that in normal control mice. Particularly, compared to control mice, a 39% increase of [18F]TZ4877 uptake was observed in the infected muscle of S. aureus mice, indicating that S1PR1 expression was directly involved in the inflammatory response to infection. Overall, our study suggested that S1PR1 plays an important role in the early immune response to bacterial infection. The uptake of [18F]TZ4877 is tightly correlated with the S1R1 expression in response to S. aureus infection. PET with S1PR1-specific radiotracer [18F]TZ4877 could provide a noninvasive tool for detecting the early S1PR1 immune response to infectious diseases.

The granuloma in cryptococcal disease

Although we have recognized cryptococcosis as a disease entity for well over 100 years, there are many details about its pathogenesis which remain unknown. A major barrier to better understanding is the very broad range of clinical and pathological forms cryptococcal infections can take. One such form has been historically called the cryptococcal granuloma, or the cryptococcoma. These words have been used to describe essentially any mass lesion associated with infection, due to their presumed similarity to the quintessential granuloma, the tubercle in tuberculosis. Although clear distinctions between tuberculosis and cryptococcal disease have been discovered, cellular and molecular studies still confirm some important parallels between these 2 diseases and what we now call granulomatous inflammation. In this review, we shall sketch out some of the history behind the term "granuloma" as it pertains to cryptococcal disease, explore our current understanding of the biology of granuloma formation, and try to place that understanding in the context of the myriad pathological presentations of this infection. Finally, we shall summarize the role of the granuloma in cryptococcal latency and present opportunities for future investigations.

FTY720 reactivates cryptococcal granulomas in mice through S1P receptor 3 on macrophages

FTY720 is a treatment for relapsing remitting multiple sclerosis (MS). It is an analog of sphingosine-1-phosphate (S1P) and targets S1P receptors 1, 3, 4, and 5. Recent reports indicate an association between long-term exposure to FTY720 and cases of cryptococcal infection. Here, we studied the effect of FTY720 and its derivative, BAF312, which only target S1P receptors 1 and 5, in a mouse model of cryptococcal infection. We found that treatment with FTY720, but not with BAF312, led to decreased survival and increased organ burden in mouse cryptococcal granulomas. Both FTY720 and BAF312 caused a profound CD4+ and CD8+ T cell depletion in blood and lungs but only treatment with FTY720 led to cryptococcal reactivation. Treatment with FTY720, but not with BAF312, was associated with disorganization of macrophages and with M2 polarization at the granuloma site. In a cell system, FTY720 decreased phagocytosis and production of reactive oxygen species by macrophages, a phenotype recapitulated in the S1pr3-/- knockout macrophages. Our results suggest that FTY720 reactivates cryptococcosis from the granuloma through a S1P receptor 3-mediated mechanism and support the rationale for development of more-specific receptor modulators for therapeutic use of MS.

Animal Models of Cryptococcus neoformans in Identifying Immune Parameters Associated With Primary Infection and Reactivation of Latent Infection

Cryptococcus species are environmental fungal pathogens and the causative agents of cryptococcosis. Infection occurs upon inhalation of infectious particles, which proliferate in the lung causing a primary infection. From this primary lung infection, fungal cells can eventually disseminate to other organs, particularly the brain, causing lethal meningoencephalitis. However, in most cases, the primary infection resolves with the formation of a lung granuloma. Upon severe immunodeficiency, dormant cryptococcal cells will start proliferating in the lung granuloma and eventually will disseminate to the brain. Many investigators have sought to study the protective host immune response to this pathogen in search of host parameters that keep the proliferation of cryptococcal cells under control. The majority of the work assimilates research carried out using the primary infection animal model, mainly because a reactivation model has been available only very recently. This review will focus on anti-cryptococcal immunity in both the primary and reactivation models. An understanding of the differences in host immunity between the primary and reactivation models will help to define the key host parameters that control the infections and are important for the research and development of new therapeutic and vaccine strategies against cryptococcosis.

Sterol-Response Pathways Mediate Alkaline Survival in Diverse Fungi

The ability for cells to maintain homeostasis in the presence of extracellular stress is essential for their survival. Stress adaptations are especially important for microbial pathogens to respond to rapidly changing conditions, such as those encountered during the transition from the environment to the infected host. Many fungal pathogens have acquired the ability to quickly adapt to changes in extracellular pH to promote their survival in the various microenvironments encountered during a host infection. For example, the fungus-specific Rim/Pal alkaline response pathway has been well characterized in many fungal pathogens, including Cryptococcus neoformans However, alternative mechanisms for sensing and responding to host pH have yet to be extensively studied. Recent observations from a genetic screen suggest that the C. neoformans sterol homeostasis pathway is required for growth at elevated pH. This work explores interactions among mechanisms of membrane homeostasis, alkaline pH tolerance, and Rim pathway activation. We find that the sterol homeostasis pathway is necessary for growth in an alkaline environment and that an elevated pH is sufficient to induce Sre1 activation. This pH-mediated activation of the Sre1 transcription factor is linked to the biosynthesis of ergosterol but is not dependent on Rim pathway signaling, suggesting that these two pathways are responding to alkaline pH independently. Furthermore, we discover that C. neoformans is more susceptible to membrane-targeting antifungals under alkaline conditions, highlighting the impact of microenvironmental pH on the treatment of invasive fungal infections. Together, these findings further connect membrane integrity and composition with the fungal pH response and pathogenesis.IMPORTANCE The work described here further elucidates how microorganisms sense and adapt to changes in their environment to establish infections in the human host. Specifically, we uncover a novel mechanism by which an opportunistic human fungal pathogen, Cryptococcus neoformans, responds to increases in extracellular pH in order to survive and thrive within the relatively alkaline environment of the human lung. This mechanism, which is intimately linked with fungal membrane sterol homeostasis, is independent of the previously well-studied alkaline response Rim pathway. Furthermore, this ergosterol-dependent alkaline pH response is present in Candida albicans, indicating that this mechanism spans diverse fungal species. These results are also relevant for novel antimicrobial drug development as we show that currently used ergosterol-targeting antifungals are more active in alkaline environments.

Sphingosine-1-Phosphate and Macrophage Biology-How the Sphinx Tames the Big Eater

The sphingolipid sphingosine-1-phosphate (S1P) is produced by sphingosine kinases to either signal through intracellular targets or to activate a family of specific G-protein-coupled receptors (S1PR). S1P levels are usually low in peripheral tissues compared to the vasculature, forming a gradient that mediates lymphocyte trafficking. However, S1P levels rise during inflammation in peripheral tissues, thereby affecting resident or recruited immune cells, including macrophages. As macrophages orchestrate initiation and resolution of inflammation, the sphingosine kinase/S1P/S1P-receptor axis emerges as an important determinant of macrophage function in the pathogenesis of inflammatory diseases such as cancer, atherosclerosis, and infection. In this review, we therefore summarize the current knowledge how S1P affects macrophage biology.

Sphingosine-1-phosphate signaling in Leishmania donovani infection in macrophages

BACKGROUND

Sphingosine-1-phosphate (S1P) is a crucial regulator of a wide array of cellular processes, such as apoptosis, cell proliferation, migration, and differentiation, but its role in Leishmania donovani infection is unknown.

METHODOLOGY/ PRINCIPAL FINDINGS

In the present study, we observed that L. donovani infection in THP-1 derived macrophages (TDM) leads to decrease in the expression of S1pr2 and S1pr3 at mRNA level. We further observed that Leishmania infection inhibits the phosphorylation of sphingosine kinase 1 (sphK1) in a time-dependent manner. Exogenous S1P supplementation decreases L. donovani induced ERK1/2 phosphorylation and increases p38 phosphorylation in TDM, resulting in a decrease in the intracellular parasite burden in a dose-dependent manner. On the other hand, sphK inhibition by DMS increases ERK1/2 phosphorylation leading to increased IL-10 and parasite load. To gain further insight, cytokines expression were checked in S1P supplemented TDM and we observed increase in IL-12, while decrease IL-10 expression at mRNA and protein levels. In addition, treatment of antagonist of S1PR2 and S1PR3 such as JTE-013 and CAY10444 respectively enhanced Leishmania-induced ERK1/2 phosphorylation and parasite load.

CONCLUSIONS

Our overall study not only reports the significant role of S1P signaling during L. donovani infection but also provides a novel platform for the development of new drugs against Leishmaniasis.

Sphingosine-1-phosphate receptors and innate immunity

Sphingosine-1-phosphate (S1P) is a signalling lipid that regulates many cellular processes in mammals. One well-studied role of S1P signalling is to modulate T-cell trafficking, which has a major impact on adaptive immunity. Compounds that target S1P signalling pathways are of interest for immune system modulation. Recent studies suggest that S1P signalling regulates many more cell types and processes than previously appreciated. This review will summarise current understanding of S1P signalling, focusing on recent novel findings in the roles of S1P receptors in innate immunity.

Mechanisms of Pulmonary Escape and Dissemination by Cryptococcus neoformans

Cryptococcus neoformans is a common environmental saprophyte and human fungal pathogen that primarily causes disease in immunocompromised individuals. Similar to many environmentally acquired human fungal pathogens, C. neoformans initiates infection in the lungs. However, the main driver of mortality is invasive cryptococcosis leading to fungal meningitis. After C. neoformans gains a foothold in the lungs, a critical early step in invasion is transversal of the respiratory epithelium. In this review, we summarize current knowledge relating to pulmonary escape. We focus on fungal factors that allow C. neoformans to disseminate from the lungs via intracellular and extracellular routes.

Sphingolipids as targets for treatment of fungal infections

Invasive fungal infections have significantly increased in the last few decades. Three classes of drugs are commonly used to treat these infections: polyenes, azoles and echinocandins. Unfortunately each of these drugs has drawbacks; polyenes are toxic, resistance against azoles is emerging and echinocandins have narrow spectrum of activity. Thus, the development of new antifungals is urgently needed. In this context, fungal sphingolipids have emerged as a potential target for new antifungals, because their biosynthesis in fungi is structurally different than in mammals. Besides, some fungal sphingolipids play an important role in the regulation of virulence in a variety of fungi. This review aims to highlight the diverse strategies that could be used to block the synthesis or/and function of fungal sphingolipids.

Sphingolipids as Regulators of the Phagocytic Response to Fungal Infections

Fungal infections pose a significant risk for the increasing population of individuals who are immunocompromised. Phagocytes play an important role in immune defense against fungal pathogens, but the interactions between host and fungi are still not well understood. Sphingolipids have been shown to play an important role in many cell functions, including the function of phagocytes. In this review, we discuss major findings that relate to the importance of sphingolipids in macrophage and neutrophil function and the role of macrophages and neutrophils in the most common types of fungal infections, as well as studies that have linked these three concepts to show the importance of sphingolipid signaling in immune response to fungal infections.

Potential Link between the Sphingosine-1-Phosphate (S1P) System and Defective Alveolar Macrophage Phagocytic Function in Chronic Obstructive Pulmonary Disease (COPD)

Introduction

We previously reported that alveolar macrophages from patients with chronic obstructive pulmonary disease (COPD) are defective in their ability to phagocytose apoptotic cells, with a similar defect in response to cigarette smoke. The exact mechanisms for this defect are unknown. Sphingolipids including ceramide, sphingosine and sphingosine-1-phosphate (S1P) are involved in diverse cellular processes and we hypothesised that a comprehensive analysis of this system in alveolar macrophages in COPD may help to delineate the reasons for defective phagocytic function.

Methods

We compared mRNA expression of sphingosine kinases (SPHK1/2), S1P receptors (S1PR1-5) and S1P-degrading enzymes (SGPP1, SGPP2, SGPL1) in bronchoalveolar lavage-derived alveolar macrophages from 10 healthy controls, 7 healthy smokers and 20 COPD patients (10 current- and 10 ex-smokers) using Real-Time PCR. Phagocytosis of apoptotic cells was investigated using flow cytometry. Functional associations were assessed between sphingosine signalling system components and alveolar macrophage phagocytic ability in COPD. To elucidate functional effects of increased S1PR5 on macrophage phagocytic ability, we performed the phagocytosis assay in the presence of varying concentrations of suramin, an antagonist of S1PR3 and S1PR5. The effects of cigarette smoking on the S1P system were investigated using a THP-1 macrophage cell line model.

Results

We found significant increases in SPHK1/2 (3.4- and 2.1-fold increases respectively), S1PR2 and 5 (4.3- and 14.6-fold increases respectively), and SGPL1 (4.5-fold increase) in COPD vs. controls. S1PR5 and SGPL1 expression was unaffected by smoking status, suggesting a COPD “disease effect” rather than smoke effect per se. Significant associations were noted between S1PR5 and both lung function and phagocytosis. Cigarette smoke extract significantly increased mRNA expression of SPHK1, SPHK2, S1PR2 and S1PR5 by THP-1 macrophages, confirming the results in patient-derived macrophages. Antagonising SIPR5 significantly improved phagocytosis.

Conclusion

Our results suggest a potential link between the S1P signalling system and defective macrophage phagocytic function in COPD and advise therapeutic targets.

Sphingosine-1-phosphate signaling: unraveling its role as a drug target against infectious diseases

Sphingosine-1-phosphate (S1P) signaling is reported in variety of cell types, including immune, endothelial and cancerous cells. It is emerging as a crucial regulator of cellular processes, such as apoptosis, cell proliferation, migration, differentiation and so on. This signaling pathway is initiated by the intracellular production and secretion of S1P through a cascade of enzymatic reactions. Binding of S1P to different S1P receptors (S1PRs) activates different downstream signaling pathways that regulate the cellular functions differentially depending upon the cell type. An accumulating body of evidence suggests that S1P metabolism and signaling is often impaired during infectious diseases; thus, its manipulation might be helpful in the treatment of such diseases. In this review, we summarize recent advances in our understanding of the S1P signaling pathway and its candidature as a novel drug target against infectious diseases.

The Granuloma Response Controlling Cryptococcosis in Mice Depends on the Sphingosine Kinase 1-Sphingosine 1-Phosphate Pathway

Cryptococcus neoformans is a fungal pathogen that causes pulmonary infections, which may progress into life-threatening meningitis. In commonly used mouse models of C. neoformans infections, fungal cells are not contained in the lungs, resulting in dissemination to the brain. We have previously reported the generation of an engineered C. neoformans strain (C. neoformans Δgcs1) which can be contained in lung granulomas in the mouse model and have shown that granuloma formation is dependent upon the enzyme sphingosine kinase 1 (SK1) and its product, sphingosine 1-phosphate (S1P). In this study, we have used four mouse models, CBA/J and C57BL6/J (both immunocompetent), Tgε26 (an isogenic strain of strain CBA/J lacking T and NK cells), and SK(-/-) (an isogenic strain of strain C57BL6/J lacking SK1), to investigate how the granulomatous response and SK1-S1P pathway are interrelated during C. neoformans infections. S1P and monocyte chemotactic protein-1 (MCP-1) levels were significantly elevated in the bronchoalveolar lavage fluid of all mice infected with C. neoformans Δgcs1 but not in mice infected with the C. neoformans wild type. SK1(-/-) mice did not show elevated levels of S1P or MCP-1. Primary neutrophils isolated from SK1(-/-) mice showed impaired antifungal activity that could be restored by the addition of extracellular S1P. In addition, high levels of tumor necrosis factor alpha were found in the mice infected with C. neoformans Δgcs1 in comparison to the levels found in mice infected with the C. neoformans wild type, and their levels were also dependent on the SK1-S1P pathway. Taken together, these results suggest that the SK1-S1P pathway promotes host defense against C. neoformans infections by regulating cytokine levels, promoting extracellular killing by phagocytes, and generating a granulomatous response.

Molecules at the interface of Cryptococcus and the host that determine disease susceptibility

Cryptococcus neoformans and Cryptococcus gattii, the predominant etiological agents of cryptococcosis, are fungal pathogens that cause disease ranging from a mild pneumonia to life-threatening infections of the central nervous system (CNS). Resolution or exacerbation of Cryptococcus infection is determined following complex interactions of several host and pathogen derived factors. Alternatively, interactions between the host and pathogen may end in an impasse resulting in the establishment of a sub-clinical Cryptococcus infection. The current review addresses the delicate interaction between the host and Cryptococcus-derived molecules that determine resistance or susceptibility to infection. An emphasis will be placed on data highlighted at the recent 9th International Conference on Cryptococcus and Cryptococcosis (ICCC).

Francisella tularensis LVS induction of prostaglandin biosynthesis by infected macrophages requires specific host phospholipases and lipid phosphatases

Francisella tularensis induces the synthesis of prostaglandin E(2) (PGE(2)) by infected macrophages to alter host immune responses, thus providing a survival advantage to the bacterium. We previously demonstrated that PGE(2) synthesis by F. tularensis-infected macrophages requires cytosolic phospholipase A2 (cPLA(2)), cyclooxygenase 2 (COX-2), and microsomal prostaglandin E synthase 1 (mPGES1). During inducible PGE(2) synthesis, cPLA(2) hydrolyzes arachidonic acid (AA) from cellular phospholipids to be converted to PGE(2). However, in F. tularensis-infected macrophages we observed a temporal disconnect between Ser505-cPLA(2) phosphorylation (a marker of activation) and PGE(2) synthesis. These results suggested to us that cPLA(2) is not responsible for the liberation of AA to be converted into PGE(2) by F. tularensis-infected macrophages. Utilizing small-molecule inhibitors, we demonstrated that phospholipase D and diacylglycerol lipase were required for providing AA for PGE(2) biosynthesis. cPLA(2), on the other hand, was required for macrophage cytokine responses to F. tularensis. We also demonstrated for the first time that lipin-1 and PAP2a contribute to macrophage inflammation in response to F. tularensis. Our results identify both an alternative pathway for inducible PGE(2) synthesis and a role for lipid-modifying enzymes in the regulation of macrophage inflammatory function.

Sphingosine kinase 1 in viral infections

Sphingosine kinase 1 (SphK1) is an enzyme that phosphorylates the lipid sphingosine to generate sphingosine-1-phosphate (S1P). S1P can act intracellularly as a signaling molecule and extracellularly as a receptor ligand. The SphK1/S1P axis has well-described roles in cell signaling, the cell death/survival decision, the production of a pro-inflammatory response, immunomodulation, and control of vascular integrity. Agents targeting the SphK1/S1P axis are being actively developed as therapeutics for cancer and immunological and inflammatory disorders. Control of cell death/survival and pro-inflammatory immune responses is central to the pathology of infectious disease, and we can capitalize on the knowledge provided by investigations of SphK1/S1P in cancer and immunology to assess its application to selected human infections. We have herein reviewed the growing literature relating viral infections to changes in SphK1 and S1P. SphK1 activity is reportedly increased following human cytomegalovirus and respiratory syncytial virus infections, and elevated SphK1 enhances influenza virus infection. In contrast, SphK1 activity is reduced in bovine viral diarrhea virus and dengue virus infections. Sphingosine analogs that modulate S1P receptors have proven useful in animal models in alleviating influenza virus infection but have shown no benefit in simian human immunodeficiency virus and lymphocytic choriomeningitis virus infections. We have rationalized a role for SphK1/S1P in dengue virus, chikungunya virus, and Ross River virus infections, on the basis of the biology and the pathology of these diseases. The increasing number of effective SphK1 and S1P modulating agents currently in development makes it timely to investigate these roles with the potential for developing modulators of SphK1 and S1P for novel anti-viral therapies.

Paradoxical roles of alveolar macrophages in the host response to Cryptococcus neoformans

Cryptococcus neoformans (Cn) is a fungal pathogen that is a serious health threat to immunocompromised individuals. Upon environmental exposure, infectious fungal propagules are inhaled into the host's lungs. The anticryptococcal actions of alveolar macrophages (AM), the predominant host phagocyte of the innate immune system in the lungs, are fundamental in determining whether containment and clearance of the pathogen occurs by the development of an adapted immune response or whether infection is established and progresses to disease. However, the fungus is also capable of surviving the antimicrobial actions of AM and exploits these host phagocytes to establish infection and exacerbate disease. In addition, there is evidence suggesting that cryptococcosis may occur following reactivation of latent cryptococcal infection. Currently, the role of AM and the fungal factors contributing to latent cryptococcosis are unknown. This review examines the AM-Cn interaction and how it affects the development of pulmonary disease with a focus on host and pathogen factors enabling latency to occur.

Contributions of the MyD88-dependent receptors IL-18R, IL-1R, and TLR9 to host defenses following pulmonary challenge with Cryptococcus neoformans

Signaling via the adapter protein, MyD88, is important in the host defense against Cryptococcus neoformans infection. While certain Toll-like receptors (TLRs) can enhance the clearance of Cryptococcus, the contributions of MyD88-dependent, TLR-independent pathways have not been fully investigated. We examined the roles of IL-1R and IL-18R in vivo by challenging C57BL/6 mice with a lethal strain of Cryptococcus. We found that the absence of IL-18R, but not IL-1R, causes a shift in the survival curve following pulmonary delivery of a virulent strain of C. neoformans (H99). Specifically, IL-18R-deficient mice have significantly shorter median survival times compared to wild-type mice following infection. Cytokine analysis of lung homogenates revealed that deficiency of IL-IR, IL-18R, or MyD88 is associated with diminished lung levels of IL-1β. In order to compare these findings with those related to TLR-deficiency, we studied the effects of TLR9-deficiency and found that deficiency of TLR9 also affects the survival curve of mice following challenge with C. neoformans. Yet the lungs from infected TLR9-deficient mice have robust levels of IL-1β. In summary, we found that multiple signaling components can contribute the MyD88-dependent host responses to cryptococcal infection in vivo and each drives distinct pulmonary responses.

Role of sphingosine-1-phosphate (S1P) and S1P receptor 2 in the phagocytosis of Cryptococcus neoformans by alveolar macrophages

The pathogenic fungus Cryptococcus neoformans is a major cause of morbidity and mortality in immunocompromised individuals. Infection of the human host occurs through inhalation of infectious propagules following environmental exposure. In the lung, C. neoformans can reside in the extracellular environment of the alveolar spaces or, upon phagocytosis, it can survive and grow intracellularly within alveolar macrophages (AMs). In previous studies, we found that sphingosine kinase 1 (SK1) influenced the intracellular residency of C. neoformans within AMs. Therefore, with this study we aimed to examine the role of the SK1 lipid product, sphingosine-1-phosphate (S1P), in the AMs-C. neoformans interaction. It was found that extracellular S1P enhances the phagocytosis of C. neoformans by AMs. Using both genetic and pharmacological approaches we further show that extracellular S1P exerts its effect on the phagocytosis of C. neoformans by AMs through S1P receptor 2 (S1P2). Interestingly, loss of S1P2 caused a dramatic decrease in the mRNA levels of Fcγ receptors I (FcγRI), -II and -III. In conclusion, our data suggest that extracellular S1P increases antibody-mediated phagocytosis through S1P2 by regulating the expression of the phagocytic Fcγ receptors.

Surface localization of glucosylceramide during Cryptococcus neoformans infection allows targeting as a potential antifungal

Cryptococcus neoformans (Cn) is a significant human pathogen that, despite current treatments, continues to have a high morbidity rate especially in sub-Saharan Africa. The need for more tolerable and specific therapies has been clearly shown. In the search for novel drug targets, the gene for glucosylceramide synthase (GCS1) was deleted in Cn, resulting in a strain (Δgcs1) that does not produce glucosylceramide (GlcCer) and is avirulent in mouse models of infection. To understand the biology behind the connection between virulence and GlcCer, the production and localization of GlcCer must be characterized in conditions that are prohibitive to the growth of Δgcs1 (neutral pH and high CO(2)). These prohibitive conditions are physiologically similar to those found in the extracellular spaces of the lung during infection. Here, using immunofluorescence, we have shown that GlcCer localization to the cell surface is significantly increased during growth in these conditions and during infection. We further seek to exploit this localization by treatment with Cerezyme (Cz), a recombinant enzyme that metabolizes GlcCer, as a potential treatment for Cn. Cz treatment was found to reduce the amount of GlcCer in vitro, in cultures, and in Cn cells inhabiting the mouse lung. Treatment with Cz induced a membrane integrity defect in wild type Cn cells similar to Δgcs1. Cz treatment also reduced the in vitro growth of Cn in a dose and condition dependent manner. Finally, Cz treatment was shown to have a protective effect on survival in mice infected with Cn. Taken together, these studies have established the legitimacy of targeting the GlcCer and other related sphingolipid systems in the development of novel therapeutics.

Biochemical systems analysis of signaling pathways to understand fungal pathogenicity

Over the past decade, researchers have recognized the need to study biological systems as integrated systems. While the reductionist approaches of the past century have made remarkable advances of our understanding of life, the next phase of understanding comes from systems-level investigations. Additionally, biology has become a data-intensive field of research. The introduction of high throughput sequencing, microarrays, high throughput proteomics, metabolomics, and now lipidomics are producing significantly more data than can be interpreted using existing methods. The field of systems biology brings together methods from computer science, modeling, statistics, engineering, and biology to explore the volumes of data now being produced and to develop mathematical representations of metabolic, signaling, and gene regulatory systems. Advances in these methods are allowing biologists to develop new insights into the complexities of life, to predict cellular responses and treatment outcomes, and to effectively plan experiments that extend our understanding. In this chapter, we are providing the basic steps of developing and analyzing a small S-system model of a biochemical pathway related to sphingolipid metabolism in the regulation of virulence of the human fungal microbial pathogen Cryptococcus neoformans (Cn).