Insights into the mechanisms of protective immunity against Cryptococcus neoformans infection using a mouse model of pulmonary cryptococcosis

Exploring immunotherapy to control human infectious diseases

Infectious diseases continue to pose significant challenges to global health, especially with the rise of antibiotic resistance and emerging pathogens. Traditional treatments, while effective, are often limited in the face of rapidly evolving pathogens. Immunotherapy, which harnesses and enhances the body's immune response, offers a promising alternative to conventional approaches for the treatment of infectious diseases. By employing use of monoclonal antibodies, vaccines, cytokine therapies, and immune checkpoint inhibitors, immunotherapy has demonstrated considerable potential in overcoming treatment resistance and improving patient outcomes. Key innovations, including the development of mRNA vaccines, use of immune modulators, adoptive cell transfer, and chimeric antigen receptor (CAR)-T cell therapy are paving the way for more targeted pathogen clearance. Further, combining immunotherapy with conventional antibiotic treatment has demonstrated effectiveness against drug-resistant strains, but this chapter explores the evolving field of immunotherapy for the treatment of bacterial, viral, fungal, and parasitic infections. The chapter also explores the recent breakthroughs and ongoing clinical trials in infectious disease immunotherapy.

Will the Real Immunogens Please Stand Up: Exploiting the Immunogenic Potential of Cryptococcal Cell Antigens in Fungal Vaccine Development

Cryptococcus neoformans is an opportunistic fungal pathogen that is a continuous global health concern, especially for immunocompromised populations. The World Health Organization recognized C. neoformans as one of four critical fungal pathogens, thus emphasizing the need for increased research efforts and clinical resource expansion. Currently, there are no fungal vaccines available for clinical use. Exciting new findings in cryptococcal vaccine development have identified whole cell-based and subunit-based vaccinations to help mitigate health risks and make commercialization attainable. Importantly, recent work has focused on how different cryptococcal cell-wall antigens modified in these vaccine candidates allow us to manipulate their immunogenicity to produce a desired long-term protective anti-fungal immune response. In this review, we discuss the different cryptococcal cell immunogens, namely the polysaccharide capsule, glucans, chitin/chitosan, mannoproteins, and extracellular vesicles, and their role in novel cryptococcal vaccination approaches. Additionally, we examine the immunological mechanisms responsible for protection in these vaccine candidates and the similar host response-stimulation pathways induced through different immunogen exposure.

Advances in Dendritic-Cell-Based Vaccines against Respiratory Fungal Infections

Ever since the discovery of dendritic cells by Ralph Steinman and Zanvil Cohn in 1973, it is increasingly evident that dendritic cells are integral for adaptive immune responses, and there is an undeniable focus on them for vaccines development. Fungal infections, often thought to be innocuous, are becoming significant threats due to an increased immunocompromised or immune-suppressed population and climate change. Further, the recent COVID-19 pandemic unraveled the wrath of fungal infections and devastating outcomes. Invasive fungal infections cause significant case fatality rates ranging from 20% to 90%. Regrettably, no licensed fungal vaccines exist, and there is an urgent need for preventive and therapeutic purposes. In this review, we discuss the ontogeny, subsets, tissue distribution, and functions of lung dendritic cells. In the latter part, we summarize and discuss the studies on the DC-based vaccines against pulmonary fungal infections. Finally, we highlight some emerging potential avenues that can be incorporated for DC-based vaccines against fungal infections.

Lack of an atypical PDR transporter generates an immunogenic Cryptococcus neoformans strain that drives a dysregulated and lethal immune response in murine lungs

Cryptococcus neoformans is an opportunistic fungal pathogen responsible for >150,000 deaths every year with a mortality rate as high as 81%. This high medical burden is due, in part, to an incomplete understanding of its pathogenesis. In a previous study, we identified a cryptococcal atypical pleiotropic drug resistance (PDR) transporter, PDR6, that regulated antifungal resistance and host interactions. Here, we follow-up on the role of PDR6 in cryptococcal virulence. In vivo, mice infected with the pdr6Δ strain display altered symptomatology and disease progression. Specifically, we observed a significant increase in the innate immune cell populations in the pdr6Δ-infected mice when compared to their WT-infected littermates. Furthermore, quantification of pulmonary cytokines/chemokines revealed a robust increase of pro-inflammatory cytokines in mice infected with the pdr6Δ mutant strain. Whereas antifungal treatment of pdr6Δ-infected animals did not affect survival, treatment with a corticosteroid significantly extended survival, highlighting the importance of a balanced/controlled host immune response. We determined that the hyper-inflammatory immune response occurs, in part, because the loss of the Pdr6 transporter indirectly alters the cryptococcal cell wall architecture and results in the increased exposure of chitin, β-glucan, and other cryptococcal-specific pathogen associated molecular patterns. Taken together, this study provides clinical insights regarding cryptococcal pathogenesis while also providing additional functions of PDR-type ATP-binding cassette (ABC) transporters in pathogenic fungi.

Preclinical Models for Cryptococcosis of the CNS and Their Characterization Using In Vivo Imaging Techniques

Infections caused by Cryptococcus neoformans and Cryptococcus gattii remain a challenge to our healthcare systems as they are still difficult to treat. In order to improve treatment success, in particular for infections that have disseminated to the central nervous system, a better understanding of the disease is needed, addressing questions like how it evolves from a pulmonary to a brain disease and how novel treatment approaches can be developed and validated. This requires not only clinical research and research on the microorganisms in a laboratory environment but also preclinical models in order to study cryptococci in the host. We provide an overview of available preclinical models, with particular emphasis on models of cryptococcosis in rodents. In order to further improve the characterization of rodent models, in particular the dynamic aspects of disease manifestation, development, and ultimate treatment, preclinical in vivo imaging methods are increasingly used, mainly in research for oncological, neurological, and cardiac diseases. In vivo imaging applications for fungal infections are rather sparse. A second aspect of this review is how research on models of cryptococcosis can benefit from in vivo imaging methods that not only provide information on morphology and tissue structure but also on function, metabolism, and cellular properties in a non-invasive way.

Immunization with a heat-killed prm1 deletion strain protects the host from Cryptococcus neoformans infection

Systemic infection with Cryptococcus neoformans, a dangerous and contagious pathogen found throughout the world, frequently results in lethal cryptococcal pneumonia and meningoencephalitis, and no effective treatments and vaccination of cryptococcosis are available. Here, we describe Prm1, a novel regulator of C. neoformans virulence. C. neoformans prm1Δ cells exhibit extreme sensitivity to various environmental stress conditions. Furthermore, prm1Δ cells show deficiencies in the biosynthesis of chitosan and mannoprotein, which in turn result in impairment of cell wall integrity. Treatment of mice with heat-killed prm1Δ cells was found to facilitate the host immunological defence against infection with wild-type C. neoformans. Further investigation demonstrated that prm1Δ cells strongly promote pulmonary production of interferon-γ, leading to activation of macrophage M1 differentiation and inhibition of M2 polarization. Therefore, our findings suggest that C. neoformans Prm1 may be a viable target for the development of anti-cryptococcosis medications and, cells lacking Prm1 represent a promising candidate for a vaccine.

Dendritic Cells: Multifunctional Roles in Host Defenses to Cryptococcus Infections

Fungal infections are an increasingly growing public health concern, and Cryptococcus is one of the most problematic fungal organisms causing substantial mortality and morbidity worldwide. Clinically, this high incidence of cryptococcosis is most commonly seen in immunocompromised patients, especially those who lack an adaptive T cell response, such as HIV/AIDS patients. However, patients with other underlying immunodeficiencies are also at an increased risk for cryptococcosis. The adaptive immune response, in particular the Th1/Th17 T-cell-mediated responses, to pulmonary Cryptococcus infections are required for host protection. Dendritic cells (DCs), encompassing multiple subsets identified to date, are recognized as the major professional antigen-presenting cell (APC) subset essential for the initiation and execution of T-cell immunity. Apart from their prominent role in orchestration of the adaptive arm of the immune defenses, DCs are fully armed cells from the innate immune system capable of the recognition, uptake, and killing of the fungal cells. Thus, DCs serve as a critical point for the endpoint outcomes of either fungal control or unrestrained fungal infection. Multiple studies have shown that DCs are required for anti-cryptococcal defense in the lungs. In addition, the role of DCs in Cryptococcus gattii infections is just starting to be elucidated. C. gattii has recently risen to prominence with multiple outbreaks in the US and Canada, demonstrating increased virulence in non-immunocompromised individuals. C. gattii infection fails to generate an inflammatory immune response or a protective Th1/Th17 T cell response, at least in part, through a lack of proper DC function. Here we summarize the multiple roles of DCs, including subsets of DCs in both mouse and human models, the roles of DCs during cryptococcal infection, and mechanisms by cryptococcal cells to attempt to undermine these host defenses.

Immunological correlates of protection following vaccination with glucan particles containing Cryptococcus neoformans chitin deacetylases

Vaccination with glucan particles (GP) containing the Cryptococcus neoformans chitin deacetylases Cda1 and Cda2 protect mice against experimental cryptococcosis. Here, immunological correlates of vaccine-mediated protection were explored. Studies comparing knockout and wild-type mice demonstrated CD4⁺ T cells are crucial, while B cells and CD8⁺ T cells are dispensable. Protection was abolished following CD4⁺ T cell depletion during either vaccination or infection but was retained if CD4⁺ T cells were only partially depleted. Vaccination elicited systemic and durable antigen-specific immune responses in peripheral blood mononuclear cells (PBMCs), spleens, and lungs. Following vaccination and fungal challenge, robust T-helper (Th) 1 and Th17 responses were observed in the lungs. Protection was abrogated in mice congenitally deficient in interferon (IFN) γ, IFNγ receptor, interleukin (IL)-1β, IL-6, or IL-23. Thus, CD4⁺ T cells and specific proinflammatory cytokines are required for GP-vaccine-mediated protection. Importantly, retention of protection in the setting of partial CD4⁺ T depletion suggests a pathway for vaccinating at-risk immunocompromised individuals.

Protective interaction of human phagocytic APC subsets with Cryptococcus neoformans induces genes associated with metabolism and antigen presentation

Cryptococcal meningitis is the most common cause of meningitis among HIV/AIDS patients in sub-Saharan Africa, and worldwide causes over 223,000 cases leading to more than 181,000 annual deaths. Usually, the fungus gets inhaled into the lungs where the initial interactions occur with pulmonary phagocytes such as dendritic cells and macrophages. Following phagocytosis, the pathogen can be killed or can replicate intracellularly. Previous studies in mice showed that different subsets of these innate immune cells can either be antifungal or permissive for intracellular fungal growth. Our studies tested phagocytic antigen-presenting cell (APC) subsets from the human lung against C. neoformans. Human bronchoalveolar lavage was processed for phagocytic APCs and incubated with C. neoformans for two hours to analyze the initial interactions and fate of the fungus, living or killed. Results showed all subsets (3 macrophage and 3 dendritic cell subsets) interacted with the fungus, and both living and killed morphologies were discernable within the subsets using imaging flow cytometry. Single cell RNA-seq identified several different clusters of cells which more closely related to interactions with C. neoformans and its protective capacity against the pathogen rather than discrete cellular subsets. Differential gene expression analyses identified several changes in the innate immune cell's transcriptome as it kills the fungus including increases of TNF-α (TNF) and the switch to using fatty acid metabolism by upregulation of the gene FABP4. Also, increases of TNF-α correlated to cryptococcal interactions and uptake. Together, these analyses implicated signaling networks that regulate expression of many different genes - both metabolic and immune - as certain clusters of cells mount a protective response and kill the pathogen. Future studies will examine these genes and networks to understand the exact mechanism(s) these phagocytic APC subsets use to kill C. neoformans in order to develop immunotherapeutic strategies to combat this deadly disease.

Pulmonary Conventional Type 1 Langerin-Expressing Dendritic Cells Play a Role in Impairing Early Protective Immune Response against Cryptococcus neoformans Infection in Mice

Lung dendritic cells (DC) are powerful antigen-presenting cells constituted by various subpopulations that differ in terms of their function and origin and differentially regulate cell-mediated antifungal immunity. The lung is the primary target organ of Cryptococcus neoformans and C. gattii infections, which makes it essential in the establishment of the first line of anti-cryptococcal defense. However, the lung-specific dynamics and function of DC subsets are poorly understood in cryptococcosis. In this study, we provide evidence for the in vivo function of a conventional langerin-expressing DC1 dendritic cell (LangDC1) population during the first week of intratracheal C. neoformans infection in mice. By using conditional depletion of LangDC1 after diphtheria toxin treatment of LangDTREGFP mice, we demonstrate that these animals better control the fungal infection and produce type 1 and 17 cytokines in the context of a type 2 immune response, favoring a predominance of iNOS over arginase-1 expression by pulmonary cells. Our results suggest that LangDC1 cells play a role in impairing immune response for the clearance of C. neoformans in the early stage of pulmonary infection.

Hemolymph of triatomines presents fungistatic activity against Cryptococcus neoformans and improves macrophage function through MCP-I/TNF-α increase

BACKGROUND

Triatomines are blood-feeding arthropods belonging to the subfamily Triatominae (Hemiptera; Reduviidae), capable of producing immunomodulatory and water-soluble molecules in their hemolymph, such as antimicrobial peptides (AMPs). In this work, we evaluated the antifungal and immunomodulatory activity of the hemolymph of Meccus pallidipennis (MPH) and Rhodnius prolixus (RPH) against Cryptococcus neoformans.

METHODS

We assessed the activity of the hemolymph of both insects on fungal growth by a minimum inhibitory concentration (MIC) assay. Further, RAW 264.7 macrophages were cultivated with hemolymph and challenged with C. neoformans. Then, their phagocytic and killing activities were assessed. The cytokines MCP-1, IFN-γ, TNF-α, IL-10, IL-12, and IL-6 were measured in culture supernatants 4- and 48-hours post-infection.

RESULTS

Both hemolymph samples directly affected the growth rate of the fungus in a dose-dependent manner. Either MPH or RPH was capable of inhibiting fungal growth by at least 70%, using the lowest dilution (1:20). Treatment of RAW 264.7 macrophages with hemolymph of both insects was capable of increasing the production of MCP-I and TNF-α. In addition, when these cells were stimulated with hemolymph in the presence of C. neoformans, a 2- and a 4-fold increase in phagocytic rate was observed with MPH and RPH, respectively, when compared to untreated cells. For the macrophage killing activity, MPH decreased in approximately 30% the number of viable yeasts inside the cells compared to untreated control; however, treatment with RPH could not reduce the total number of viable yeasts. MPH was also capable of increasing MHC-II expression on macrophages. Regarding the cytokine production, MCP-I and TNF-α, were increased in the supernatant of macrophages treated with both hemolymphs, 4 and 48 hours after stimulation.

CONCLUSION

These results suggested that hemolymph of triatomines may represent a source of molecules capable of presenting antifungal and immunomodulatory activity in macrophages during fungal infection.

Cryptococcus neoformans Infection Induces IL-17 Production by Promoting STAT3 Phosphorylation in CD4+ T Cells

Cryptococcus neoformans infection in the central nervous system is a severe infectious disease with poor outcomes and high mortality. It has been estimated that there are 220,000 new cases each year. Over 90% of C. neoformans meningitis cases were diagnosed in AIDS patients with CD4⁺ T cell count <100 cells/μl; however, the mechanism of cryptococcal meningitis in patients with normal immune functions remains unclear. IL-17 is a pro-inflammatory cytokine and plays an important role in anti-fungal immunity. Here we report that significantly high levels of IL-17 were predominantly detected in the cerebrospinal fluid of patients with either AIDS- or non-AIDS-associated C. neoformans meningitis but not in patients with tuberculous meningitis or non-neurosyphilis. Antifungal therapy minimized the IL-17 level in the cerebrospinal fluid. An in vitro mechanistic study showed that C. neoformans stimulation of healthy peripheral blood mononuclear cells prompted IL-17 production, and CD4⁺ T cells were the predominant IL-17-producing cells. IL-17 production by C. neoformans stimulation was STAT3 signaling dependent. Inhibition of STAT3 phosphorylation attenuated the C. neoformans-mediated IL-17 expression. Our data highlighted the significance of CD4⁺ T cells in antifungal immunity and suggested IL-17 as a diagnostic biomarker of C. neoformans infection and STAT3 as a checkpoint for antifungal targeted therapies.

Cryptococcus neoformans Csn1201 Is Associated With Pulmonary Immune Responses and Disseminated Infection

Cryptococcus neoformans is a major etiological agent of fungal meningoencephalitis. The outcome of cryptococcosis depends on the complex interactions between the pathogenic fungus and host immunity. The understanding of how C. neoformans manipulates the host immune response through its pathogenic factors remains incomplete. In this study, we defined the roles of a previously uncharacterized protein, Csn1201, in cryptococcal fitness and host immunity. Use of both inhalational and intravenous mouse models demonstrated that the CSN1201 deletion significantly blocked the pulmonary infection and extrapulmonary dissemination of C. neoformans. The in vivo hypovirulent phenotype of the csn1201Δ mutant was attributed to a combination of multiple factors, including preferential dendritic cell accumulation, enhanced Th1 and Th17 immune responses, decreased intracellular survival inside macrophages, and attenuated blood–brain barrier transcytosis rather than exclusively to pathogenic fitness. The csn1201Δ mutant exhibited decreased tolerance to various stressors in vitro, along with reduced capsule production and enhanced cell wall thickness under host-relevant conditions, indicating that the CSN1201 deletion might promote the exposure of cell wall components and thus induce a protective immune response. Taken together, our results strongly support the importance of cryptococcal Csn1201 in pulmonary immune responses and disseminated infection.

Immunoprotection against Cryptococcosis Offered by Znf2 Depends on Capsule and the Hyphal Morphology

Systemic cryptococcosis is fatal without treatment. Globally, this disease kills 180,000 of the 225,000 infected people each year, even with the use of antifungal therapies. Currently, there is no vaccine to prevent cryptococcosis. Previously, we discovered that Znf2, a morphogenesis regulator that directs Cryptococcus yeast-to-hyphal transition, profoundly affects cryptococcal interaction with the host-overexpression of ZNF2 drives filamentous growth, attenuates cryptococcal virulence, and elicits protective host immune responses. Importantly, immunization with cryptococcal cells overexpressing ZNF2, either in live or heat-inactivated form, offers significant protection to the host from a subsequent challenge by the otherwise lethal wild-type H99 strain. We hypothesize that cellular components enriched in ZNF2^oe cells are immunoprotective. Here, we discovered that serum from protected animals vaccinated with inactivated ZNF2^oe cells recognizes cryptococcal antigens that reside within the capsule. Consistently, capsule is required for immunoprotection offered by ZNF2^oe cells. Interestingly, the serum from protective animals recognizes antigens in both wild-type yeast cells and ZNF2^oe cells, with higher abundance in the latter. Consequently, even the heat-inactivated wild-type cells become immunoprotective with an increased vaccination dose. We also found that disruption of a chromatin remodeling factor Brf1, which is important for initiation of filamentation by Znf2, reduces the antigen level in ZNF2^oe cells. Deletion of BRF1 drastically reduces the protective effect of ZNF2^oe cells in both live and heat-killed forms even though the ZNF2^oebrf1Δ strain itself is avirulent. Collectively, our findings underscore the importance of identifying the subset of cryptococcal surface factors that are beneficial in host protection. IMPORTANCE Cryptococcosis claims close to 200,000 lives annually. There is no vaccine clinically available for this fungal disease. Many avirulent mutant strains do not provide protection against cryptococcosis. We previously discovered that hyphal ZNF2^oe strains elicit protective host immune responses both in the live and heat-inactivated forms. Here we seek to understand the mechanism underlying the host protection provided by ZNF2^oe cells. We discovered increased accumulation of antigens located within the caspusle of ZNF2^oe cells and consequently the requirement of the capsule for ZNF2^oe strain-elicited host protection. Furthermore, genetically blocking the ability of ZNF2^oe cells to grow in the hyphal form significantly reduces antigen accumulation and impairs the ability of ZNF2^oe strain to provide host protection. Our findings highlight the importance of identifying the Znf2-regulated capsular surface factors that are fundamental in host protection.

Transcriptional Changes in Pulmonary Phagocyte Subsets Dictate the Outcome Following Interaction With The Fungal Pathogen Cryptococcus neoformans

With over 220,000 cases and 180,000 deaths annually, Cryptococcus neoformans is the most common cause of fungal meningitis and a leading cause of death in HIV/AIDS patients in Sub-Saharan Africa. Either C. neoformans can be killed by innate airway phagocytes, or it can survive intracellularly. Pulmonary murine macrophage and dendritic cell (DC) subsets have been identified in the naïve lung, and we hypothesize that each subset has different interactions with C. neoformans. For these studies, we purified murine pulmonary macrophage and DC subsets from naïve mice - alveolar macrophages, Ly6c^- and Ly6c⁺ monocyte-like macrophages, interstitial macrophages, CD11b⁺ and CD103⁺ DCs. With each subset, we examined cryptococcal association (binding/internalization), fungicidal activity, intracellular fungal morphology, cytokine secretion and transcriptional profiling in an ex vivo model using these pulmonary phagocyte subsets. Results showed that all subsets associate with C. neoformans, but only female Ly6c^- monocyte-like macrophages significantly inhibited growth, while male CD11b⁺ DCs significantly enhanced fungal growth. In addition, cytokine analysis revealed that some subsets from female mice produced increased amounts of cytokines compared to their counterparts in male mice following exposure to C. neoformans. In addition, although cells were analyzed ex vivo without the influence of the lung microenviroment, we did not find evidence of phagocyte polarization following incubation with C. neoformans. Imaging flow cytometry showed differing ratios of cryptococcal morphologies, c-shaped or budding, depending on phagocyte subset. RNA sequencing analysis revealed the up- and down-regulation of many genes, from immunological pathways (including differential regulation of MHC class I in the antigen processing pathway and the cell adhesion pathway) and pathways relating to relating to metabolic activity (genes in the Cytochrome P450 family, genes related to actin binding, calcium voltage channels, serine proteases, and phospholipases). Future studies gaining a more in-depth understanding on the functionality of individual genes and pathways specific to permissive and non-permissive pulmonary phagocytes will allow identification of key targets when developing therapeutic strategies to prevent cryptococcal meningitis.

Murine Inducible Nitric Oxide Synthase Expression Is Essential for Antifungal Defenses in Kidneys during Disseminated Cryptococcus deneoformans Infection

Disseminated cryptococcosis has a nearly 70% mortality, mostly attributed to CNS infection, with lesser-known effects on other organs. Immune protection against Cryptococcus relies on Th1 immunity with M1 polarization, rendering macrophages fungicidal. The importance of M1-upregulated inducible NO synthase (iNOS) has been documented in pulmonary anticryptococcal defenses, whereas its role in disseminated cryptococcosis remains controversial. Here we examined the effect of iNOS deletion in disseminated (i.v.) C. deneoformans 52D infection, comparing wild-type (C57BL/6J) and iNOS^-/- mice. iNOS^-/- mice had significantly reduced survival and nearly 100-fold increase of the kidney fungal burden, without increases in the lungs, spleen, or brain. Histology revealed extensive lesions and almost complete destruction of the kidney cortical area with a loss of kidney function. The lack of fungal control was not due to a failure to recruit immune cells because iNOS^-/- mice had increased kidney leukocytes. iNOS^-/- mice also showed no defect in T cell polarization. We conclude that iNOS is critically required for local anticryptococcal defenses in the kidneys, whereas it appears to be dispensable in other organs during disseminated infection. This study exemplifies a unique phenotype of local immune defenses in the kidneys and the organ-specific importance of a single fungicidal pathway.

Epitope-Based Immunoinformatic Approach on Heat Shock 70 kDa Protein Complex of Cryptococcus neoformans var. grubii

INTRODUCTION

Cryptococcosis is a ubiquitous opportunistic fungal disease caused by Cryptococcus neoformans var. grubii. It has high global morbidity and mortality among HIV patients and non-HIV carriers with 99% and 95%, respectively. Furthermore, the increasing prevalence of undesired toxicity profile of antifungal, multidrug-resistant organisms and the scarcity of FDA-authorized vaccines were the hallmark in the present days. This study was undertaken to design a reliable epitope-based peptide vaccine through targeting highly conserved immunodominant heat shock 70 kDa protein of Cryptococcus neoformans var. grubii that covers a considerable digit of the world population through implementing a computational vaccinology approach.

MATERIALS AND METHODS

A total of 38 sequences of Cryptococcus neoformans var. grubii's heat shock 70 kDa protein were retrieved from the NCBI protein database. Different prediction tools were used to analyze the aforementioned protein at the Immune Epitope Database (IEDB) to discriminate the most promising T-cell and B-cell epitopes. The proposed T-cell epitopes were subjected to the population coverage analysis tool to compute the global population's coverage. Finally, the T-cell projected epitopes were ranked based on their binding scores and modes using AutoDock Vina software. Results and Discussion. The epitopes (ANYVQASEK, QSEKPKNVNPVI, SEKPKNVNPVI, and EKPKNVNPVI) had shown very strong binding affinity and immunogenic properties to B-cell. (FTQLVAAYL, YVYDTRGKL) and (FFGGKVLNF, FINAQLVDV, and FDYALVQHF) exhibited a very strong binding affinity to MHC-I and MHC-II, respectively, with high population coverage for each, while FYRQGAFEL has shown promising results in terms of its binding profile to MHC-II and MHC-I alleles and good strength of binding when docked with HLA-C∗12:03. In addition, there is massive global population coverage in the three coverage modes. Accordingly, our in silico vaccine is expected to be the future epitope-based peptide vaccine against Cryptococcus neoformans var. grubii that covers a significant figure of the entire world citizens.

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.

A preliminary study on the characteristics of Th1/Th2 immune response in cerebrospinal fluid of AIDS patients with cryptococcal meningitis

BACKGROUND

Cryptococcal Meningitis (CM) is a common opportunistic infection in the late stage of acquired immunodeficiency syndrome (AIDS). Despite the wide use of effective antiretroviral and antifungal therapy in AIDS patients, CM is still a major morbidity and mortality cause. Understanding the immune response in cryptococcal infection may help to improve the treatment strategies.

METHODS

We established a prospective cohort of twelve AIDS patients with CM (HIV + CM+) admitted to the hospital from 2019 to 2020. All patients were examined at the baseline, 2 weeks, and 4 weeks thereafter. The level of 19 cytokines in cerebrospinal fluid (CSF) were recorded to analyze the characteristics and dynamic changes of Th1/Th2 immune response. Meanwhile, six AIDS patients without CM (HIV + CM-) and seventeen healthy subjects (HIV-CM-) were included as control groups for CSF assessment.

RESULTS

The HIV+ CM+ group had higher CSF IFN-γ, TNF-α, IL-6, IL-7, IL-8, IL-10, IL-12 (P40), IL-15, IL-18, CCL2 levels but lower IL-4 when compared with the HIV-CM- group at baseline. And they also had a higher level of IL-12 (P40) and IL-17A compared with HIV + CM- patients. Except one patient dropped out of the study, eleven HIV + CM+ patients received induction antifungal therapy and regular CSF testing, and the mortality rate was 9.1% (1/11) and 18.2% (2/11) respectively at week 2 and week 4. Compared with baseline CSF cytokines, IL-2, IL-13, IL-17A, and VEGF-A decreased in week 2, and the VEGF-A levels further decreased in week 4. But there was no difference in the levels of all cytokines between survivors and the dead.

CONCLUSION

No evidence of Th1/Th2 imbalance was found in AIDS patients with CM. However, the CSF cytokine network may provide new clues for the treatment of AIDS patients with CM.

TRIAL REGISTRATION

This trial was prospectively registered in 2019.7.16. The registered number is ChiCTR1900024565 .

The role of Toll-like receptor 9 in a murine model of Cryptococcus gattii infection

Toll-like receptor 9 (TLR9) is crucial to the host immune response against fungi, such as Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, but its importance in Cryptococcus gattii infection is unknown. Our study aimed to understand the role of TLR9 during the course of experimental C. gattii infection in vivo, considering that the cryptococcal DNA interaction with the receptor could contribute to host immunity even in an extremely susceptible model. We inoculated C57BL/6 (WT) and TLR9 knock-out (TLR9^−/−) mice intratracheally with 10⁴C. gattii yeast cells. TLR9^−/− mice had a higher mortality rate compared to WT mice and more yeast cells that had abnormal size, known as titan cells, in the lungs. TLR9^−/− mice also had a greater number of CFUs in the spleen and brain than WT mice, in addition to having lower levels of IFN-γ and IL-17 in the lung. With these markers of aggressive cryptococcosis, we can state that TLR9^−/− mice are more susceptible to C. gattii, probably due to a mechanism associated with the decrease of a Th1 and Th17-type immune response that promotes the formation of titan cells in the lungs. Therefore, our results indicate the participation of TLR9 in murine resistance to C. gattii infection.

Treatment strategies for cryptococcal infection: challenges, advances and future outlook

Cryptococcus spp., in particular Cryptococcus neoformans and Cryptococcus gattii, have an enormous impact on human health worldwide. The global burden of cryptococcal meningitis is almost a quarter of a million cases and 181,000 deaths annually, with mortality rates of 100% if infections remain untreated. Despite these alarming statistics, treatment options for cryptococcosis remain limited, with only three major classes of drugs approved for clinical use. Exacerbating the public health burden is the fact that the only new class of antifungal drugs developed in decades, the echinocandins, displays negligible antifungal activity against Cryptococcus spp., and the efficacy of the remaining therapeutics is hampered by host toxicity and pathogen resistance. Here, we describe the current arsenal of antifungal agents and the treatment strategies employed to manage cryptococcal disease. We further elaborate on the recent advances in our understanding of the intrinsic and adaptive resistance mechanisms that are utilized by Cryptococcus spp. to evade therapeutic treatments. Finally, we review potential therapeutic strategies, including combination therapy, the targeting of virulence traits, impairing stress response pathways and modulating host immunity, to effectively treat infections caused by Cryptococcus spp. Overall, understanding of the mechanisms that regulate anti-cryptococcal drug resistance, coupled with advances in genomics technologies and high-throughput screening methodologies, will catalyse innovation and accelerate antifungal drug discovery.

Vaccines and Protective Immune Memory against Cryptococcosis

Cryptococcosis is a potentially lethal disease caused by fungal pathogens including Cryptococcus neoformans and Cryptococcus gattii species complex. These fungal pathogens live in the environment and are associated with certain tree species and bird droppings. This infectious disease is not contagious, and healthy individuals may contract cryptococcal infections by inhaling the airborne pathogens from the environment. Although cleaning a contaminated environment is a feasible approach to control environmental fungal pathogens, prophylactic immunization is also considered a promising method to regulate cryptococcal infections. We review the history of the development of cryptococcal vaccines, vaccine components, and the various forms of immune memory induced by cryptococcal vaccines.

Correlation between clinical outcome and tissue inflammatory response in kidney transplant recipients with cryptococcosis

Cryptococcosis is the second most common invasive fungal infection reported in renal transplant recipients. Tissue granulomatous inflammation is necessary to contain Cryptococcus infection. This study aims to analyze the granuloma patterns and in situ expression of regulatory T (Treg) immune response in tissue samples from 12 renal transplant recipients with cryptococcosis. Fungal isolates were molecularly identified as Cryptococcus neoformans species complex. A detailed characterization of granulomas in tissue samples from 12 kidney transplant recipients with cryptococcosis was described by checking six lung and six skin biopsies by conventional histology and for immunohistochemical detection of CD4 and Treg markers: forkhead box P3 (FoxP3), interleukin (IL)-10 and transforming-growth factor (TGF)-β. Granulomas were classified as compact, loose or mixed. Patients with mixed (n = 4) and compact (n = 3) granulomatous inflammation patterns were associated with a better prognosis and presented a higher number of CD4+FoxP3+T cells compared to the group of patients with loose granulomas. In counterpart, three out of five patients with loose granulomas died with cryptococcosis. We suggest that Treg may have a protective role in the tissue response to Cryptococcus infection given its association with compact and mixed granulomas in patients with better clinical outcomes.

Chemokine receptor CXCR3 is required for lethal brain pathology but not pathogen clearance during cryptococcal meningoencephalitis

Cryptococcal meningoencephalitis (CM) is the major cause of infection-related neurological death, typically seen in immunocompromised patients. However, T cell-driven inflammatory response has been increasingly implicated in lethal central nervous system (CNS) immunopathology in human patients and murine models. Here, we report marked up-regulation of the chemokine receptor CXCR3 axis in human patients and mice with CM. CXCR3 deletion in mice improves survival, diminishes neurological deficits, and limits neuronal damage without suppressing fungal clearance. CD4+ T cell accumulation and TH1 skewing are reduced in the CNS but not spleens of infected CXCR3-/- mice. Adoptive transfer of WT, but not CXCR3-/- CD4+ T cells, into CXCR3-/- mice phenocopies the pathology of infected WT mice. Collectively, we found that CXCR3+CD4+ T cells drive lethal CNS pathology but are not required for fungal clearance during CM. The CXCR3 pathway shows potential as a therapeutic target or for biomarker discovery to limit CNS inflammatory damages.

Pulmonary Macrophage and Dendritic Cell Responses to Cryptococcus neoformans

The fungal pathogen Cryptococcus neoformans can cause life-threatening infections in immune compromised individuals. This pathogen is typically acquired via inhalation, and enters the respiratory tract. Innate immune cells such as macrophages and dendritic cells (DCs) are the first host cells that encounter C. neoformans, and the interactions between Cryptococcus and innate immune cells play a critical role in the progression of disease. Cryptococcus possesses several virulence factors and evasion strategies to prevent its killing and destruction by pulmonary phagocytes, but these phagocytic cells can also contribute to anti-cryptococcal responses. This review will focus on the interactions between Cryptococcus and primary macrophages and dendritic cells (DCs), dealing specifically with the cryptococcal/pulmonary cell interface.

The Rise of Coccidioides: Forces Against the Dust Devil Unleashed

Coccidioidomycosis (Valley fever) is a fungal disease caused by the inhalation of Coccidioides posadasii or C. immitis. This neglected disease occurs in the desert areas of the western United States, most notably in California and Arizona, where infections continue to rise. Clinically, coccidioidomycosis ranges from asymptomatic to severe pulmonary disease and can disseminate to the brain, skin, bones, and elsewhere. New estimates suggest as many as 350,000 new cases of coccidioidomycosis occur in the United States each year. Thus, there is an urgent need for the development of a vaccine and new therapeutic drugs against Coccidioides infection. In this review, we discuss the battle against Coccidioides including the development of potential vaccines, the quest for new therapeutic drugs, and our current understanding of the protective host immune response to Coccidioides infection.

Induction of memory-like dendritic cell responses in vivo

Dendritic cells (DCs), a vital component of the innate immune system, are considered to lack antigen specificity and be devoid of immunological memory. Strategies that can induce memory-like responses from innate cells can be utilized to elicit protective immunity in immune deficient persons. Here we utilize an experimental immunization strategy to modulate DC inflammatory and memory-like responses against an opportunistic fungal pathogen that causes significant disease in immunocompromised individuals. Our results show that DCs isolated from protectively immunized mice exhibit enhanced transcriptional activation of interferon and immune signaling pathways. We also show long-term memory-like cytokine responses upon subsequent challenge with the fungal pathogen that are abrogated with inhibitors of specific histone modifications. Altogether, our study demonstrates that immunization strategies can be designed to elicit memory-like DC responses against infectious disease.

Wormley and colleagues present data showing that vaccine strategies can be devised to prime dendritic cells to respond in a memory-like fashion upon subsequent exposure to a pathogen.

Pathogenic Fungal Infection in the Lung

Respiratory fungal infection is a severe clinical problem, especially in patients with compromised immune functions. Aspergillus, Cryptococcus, Pneumocystis, and endemic fungi are major pulmonary fungal pathogens that are able to result in life-threatening invasive diseases. Growing data being reported have indicated that multiple cells and molecules orchestrate the host's response to a fungal infection in the lung. Upon fungal challenge, innate myeloid cells including macrophages, dendritic cells (DC), and recruited neutrophils establish the first line of defense through the phagocytosis and secretion of cytokines. Natural killer cells control the fungal expansion in the lung via the direct and indirect killing of invading organisms. Adaptive immune cells including Th1 and Th17 cells confer anti-fungal activity by producing their signature cytokines, interferon-γ, and IL-17. In addition, lung epithelial cells (LEC) also participate in the resistance against fungal infection by internalization, inflammatory cytokine production, or antimicrobial peptide secretion. In the host cells mentioned above, various molecules with distinct functions modulate the immune defense signaling: Pattern recognition receptors (PRRs) such as dectin-1 expressed on the cell surface are involved in fungal recognition; adaptor proteins such as MyD88 and TRAF6 are required for transduction of signals to the nucleus for transcriptional regulation; inflammasomes also play crucial roles in the host's defense against a fungal infection in the lung. Furthermore, transcriptional factors modulate the transcriptions of a series of genes, especially those encoding cytokines and chemokines, which are predominant regulators in the infectious microenvironment, mediating the cellular and molecular immune responses against a fungal infection in the lung.

Cryptococcus neoformans Glucuronoxylomannan and Sterylglucoside Are Required for Host Protection in an Animal Vaccination Model

The number of deaths from cryptococcal meningitis is around 180,000 per year. The disease is the second leading cause of mortality among individuals with AIDS. Antifungal treatment is costly and associated with adverse effects and resistance, evidencing the urgency of development of both therapeutic and prophylactic tools. Here we demonstrate the key roles of polysaccharide- and glycolipid-containing structures in a vaccination model to prevent cryptococcosis.

Cryptococcus neoformans is an encapsulated fungal pathogen that causes meningoencephalitis. There are no prophylactic tools for cryptococcosis. Previously, our group showed that a C. neoformans mutant lacking the gene encoding sterylglucosidase (Δsgl1) induced protection in both immunocompetent and immunocompromised murine models of cryptococcosis. Since sterylglucosidase catalyzes degradation of sterylglucosides (SGs), accumulation of this glycolipid could be responsible for protective immunity. In this study, we analyzed whether the activity of SGs is sufficient for the protective effect induced by the Δsgl1 strain. We observed that the accumulation of SGs impacted several properties of the main polysaccharide that composes the fungal capsule, glucuronoxylomannan (GXM). We therefore used genetic manipulation to delete the SGL1 gene in the acapsular mutant Δcap59 to generate a double mutant (strain Δcap59/Δsgl1) that was shown to be nonpathogenic and cleared from the lung of mice within 7 days post-intranasal infection. The inflammatory immune response triggered by the Δcap59/Δsgl1 mutant in the lung differed from the response seen with the other strains. The double mutant did not induce protection in a vaccination model, suggesting that SG-related protection requires the main capsular polysaccharide. Finally, GXM-containing extracellular vesicles (EVs) enriched in SGs delayed the acute lethality of Galleria mellonella against C. neoformans infection. These studies highlighted a key role for GXM and SGs in inducing protection against a secondary cryptococcal infection, and, since EVs notoriously contain GXM, these results suggest the potential use of Δsgl1 EVs as a vaccination strategy for cryptococcosis.

Lung-infiltrating T helper 17 cells as the major source of interleukin-17A production during pulmonary Cryptococcus neoformans infection

Background

IL-17A has emerged as a key player in the pathologies of inflammation, autoimmune disease, and immunity to microbes since its discovery two decades ago. In this study, we aim to elucidate the activity of IL-17A in the protection against Cryptococcus neoformans, an opportunistic fungus that causes fatal meningoencephalitis among AIDS patients. For this purpose, we examined if C. neoformans infection triggers IL-17A secretion in vivo using wildtype C57BL/6 mice. In addition, an enhanced green fluorescence protein (EGFP) reporter and a knockout (KO) mouse models were used to track the source of IL-17A secretion and explore the protective function of IL-17A, respectively.

Results

Our findings showed that in vivo model of C. neoformans infection demonstrated induction of abundant IL-17A secretion. By examining the lung bronchoalveolar lavage fluid (BALF), mediastinal lymph node (mLN) and spleen of the IL-17A–EGFP reporter mice, we showed that intranasal inoculation with C. neoformans promoted leukocytes lung infiltration. A large proportion (~ 50%) of the infiltrated CD4⁺ helper T cell population secreted EGFP, indicating vigorous T_H17 activity in the C. neoformans–infected lung. The infection study in IL-17A–KO mice, on the other hand, revealed that absence of IL-17A marginally boosted fungal burden in the lung and accelerated the mouse death.

Conclusion

Therefore, our data suggest that IL-17A is released predominantly from T_H17 cells in vivo, which plays a supporting role in the protective immunity against C. neoformans infection.

Electronic supplementary material

The online version of this article (10.1186/s12865-018-0269-5) contains supplementary material, which is available to authorized users.

Involvement of the capsular GalXM-induced IL-17 cytokine in the control of Cryptococcus neoformans infection

Cryptococcus neoformans is an opportunistic fungus that can cause lethal brain infections in immunosuppressed individuals. Infection usually occurs via the inhalation of a spore or desiccated yeast which can then disseminate from the lung to the brain and other tissues. Dissemination and disease is largely influence by the production of copious amounts of cryptococcal polysaccharides, both which are secreted to the extracellular environment or assembled into a thick capsule surrounding the cell body. There are two important polysaccharides: glucuronoxylomannan (GXM) and galactoxylomannan, also called as glucuronoxylomanogalactan (GXMGal or GalXM). Although GXM is more abundant, GalXM has a more potent modulatory effect. In the present study, we show that GalXM is a potent activator of murine dendritic cells, and when co-cultured with T cells, induces a Th17 cytokine response. We also demonstrated that treating mice with GalXM prior to infection with C. neoformans protects from infection, and this phenomenon is dependent on IL-6 and IL-17. These findings help us understand the immune biology of capsular polysaccharides in fungal pathogenesis.

IFN-γ immune priming of macrophages in vivo induces prolonged STAT1 binding and protection against Cryptococcus neoformans

Development of vaccines against opportunistic infections is difficult as patients most at risk of developing disease are deficient in aspects of the adaptive immune system. Here, we utilized an experimental immunization strategy to induce innate memory in macrophages in vivo. Unlike current trained immunity models, we present an innate memory-like phenotype in macrophages that is maintained for at least 70 days post-immunization and results in complete protection against secondary challenge in the absence of adaptive immune cells. RNA-seq analysis of in vivo IFN-γ primed macrophages revealed a rapid up-regulation of IFN-γ and STAT1 signaling pathways following secondary challenge. The enhanced cytokine recall responses appeared to be pathogen-specific, dependent on changes in histone methylation and acetylation, and correlated with increased STAT1 binding to promoter regions of genes associated with protective anti-fungal immunity. Thus, we demonstrate an alternative mechanism to induce macrophage innate memory in vivo that facilitates pathogen-specific vaccine-mediated immune responses.

Fungal infections are a significant global health problem that can affect anyone, however, individuals with a weakened immune system are most at risk. Cryptococcus neoformans infections can progress to meningitis in immune compromised individuals accounting for nearly 220,000 new cases annually, resulting in 181,000 deaths. Vaccine strategies tend to target CD4⁺ T cells for the generation of protective memory responses. However, immune compromised individuals have decreased numbers of these adaptive cells, providing a challenge for anti-fungal vaccine design. Here, we define a cellular mechanism by which macrophages, an innate cell population, generate protective immune responses against C. neoformans following initial exposure to a C. neoformans strain that secretes IFN-γ. We determined that the macrophages primed in vivo have heightened proinflammatory cytokine responses upon secondary exposure to C. neoformans in a manner that is mTOR-independent, yet dependent on histone modification dynamics. We show that IFN-γ primed macrophages can maintain STAT1 binding to the promoter regions of key proinflammatory genes long after the initial exposure. Remarkably, our studies show long-lived, cryptococcal-specific protective immunity in vivo. The results presented herein demonstrate that innate cell populations, namely macrophages, can be utilized as vaccine targets to protect against cryptococcal infections in immune compromised populations.

Autocrine IL-10 Signaling Promotes Dendritic Cell Type-2 Activation and Persistence of Murine Cryptococcal Lung Infection

The substantial morbidity and mortality caused by invasive fungal pathogens, including Cryptococcus neoformans, necessitates increased understanding of protective immune responses against these infections. Our previous work using murine models of cryptococcal lung infection demonstrated that dendritic cells (DCs) orchestrate critical transitions from innate to adaptive immunity and that IL-10 signaling blockade improves fungal clearance. To further understand interrelationships among IL-10 production, fungal clearance, and the effect of IL-10 on lung DCs, we performed a comparative temporal analysis of cryptococcal lung infection in wild type C57BL/6J mice (designated IL-10^+/+) and IL-10^-/- mice inoculated intratracheally with C. neoformans (strain 52D). Early and sustained IL-10 production by lung leukocytes was associated with persistent infection in IL-10^+/+ mice, whereas fungal clearance was improved in IL-10^-/- mice during the late adaptive phase of infection. Numbers of monocyte-derived DCs, T cells, and alveolar and exudate macrophages were increased in lungs of IL-10^-/- versus IL-10^+/+ mice concurrent with evidence of enhanced DC type-1, Th1/Th17 CD4 cell, and classical macrophage activation. Bone marrow-derived DCs stimulated with cryptococcal mannoproteins, a component of the fungal capsule, upregulated expression of IL-10 and IL-10R, which promoted DC type-2 activation in an autocrine manner. Thus, our findings implicate fungus-triggered autocrine IL-10 signaling and DC type-2 activation as important contributors to the development of nonprotective immune effector responses, which characterize persistent cryptococcal lung infection. Collectively, this study informs and strengthens the rationale for IL-10 signaling blockade as a novel treatment for fungal infections.

A Call to Arms: Quest for a Cryptococcal Vaccine

Cryptococcosis remains a significant cause of morbidity and mortality world-wide, particularly among AIDS patients. Yet, to date, there are no licensed vaccines clinically available to treat or prevent cryptococcosis. In this review, we provide a rationale to support continued investment in Cryptococcus vaccine research, potential challenges that must be overcome along the way, and a literature review of the current progress underway towards developing a vaccine to prevent cryptococcosis.

Variability in innate host immune responses to cryptococcosis

Cryptococcosis is an invasive fungal disease caused by Cryptococcus neoformans and the closely related species C. gattii. The severe form of the disease, cryptococcal meningitis (CM), is rapidly fatal without treatment. Although typically a disease of immunocompromised (especially HIV-positive) individuals, there is growing awareness of cryptococcal disease amongst non-immunocompromised patients. Whilst substantial progress has been made in understanding the pathogenicity of C. neoformans in HIV patients, prospective data on cryptococcosis outside the context of HIV remains lacking. Below we review how innate immune responses vary between hosts depending on immunological status, and discuss risk factors and predictors of disease outcome in different groups.

Interactions of Cryptococcus with Dendritic Cells

The fungal pathogens Cryptococcus neoformans and Cryptococcus gattii can cause life-threatening infections in immune compromised and immune competent hosts. These pathogens enter the host via inhalation, and respiratory tract innate immune cells such as dendritic cells (DCs) are one of the first host cells they encounter. The interactions between Cryptococcus and innate immune cells play a critical role in the progression of disease in the host. This review will focus specifically on the interactions between Cryptococcus and dendritic cells (DCs), including recognition/processing by DCs, effects of immune mediators on DC recruitment and activity, and the potential for DC vaccination against cryptococcosis.

Innate Immunity against Cryptococcus, from Recognition to Elimination

Cryptococcus species, the etiological agents of cryptococcosis, are encapsulated fungal yeasts that predominantly cause disease in immunocompromised individuals, and are responsible for 15% of AIDS-related deaths worldwide. Exposure follows the inhalation of the yeast into the lung alveoli, making it incumbent upon the pattern recognition receptors (PRRs) of pulmonary phagocytes to recognize highly conserved pathogen-associated molecular patterns (PAMPS) of fungi. The main challenges impeding the ability of pulmonary phagocytes to effectively recognize Cryptococcus include the presence of the yeast's large polysaccharide capsule, as well as other cryptococcal virulence factors that mask fungal PAMPs and help Cryptococcus evade detection and subsequent activation of the immune system. This review will highlight key phagocyte cell populations and the arsenal of PRRs present on these cells, such as the Toll-like receptors (TLRs), C-type lectin receptors, NOD-like receptors (NLRs), and soluble receptors. Additionally, we will highlight critical cryptococcal PAMPs involved in the recognition of Cryptococcus. The question remains as to which PRR-ligand interaction is necessary for the recognition, phagocytosis, and subsequent killing of Cryptococcus.

The F-Box Protein Fbp1 Shapes the Immunogenic Potential of Cryptococcus neoformans

Cryptococcus neoformans is the main etiologic agent of cryptococcal meningitis and causes a significant number of deadly infections per year. Although it is well appreciated that host immune responses are crucial for defense against cryptococcosis, our understanding of factors that control the development of effective immunity to this fungus remains incomplete. In previous studies, we identified the F-box protein Fbp1 as a novel determinant of C. neoformans virulence. In this study, we found that the hypovirulence of the fbp1Δ mutant is linked to the development of a robust host immune response. Infection with the fbp1Δ mutant induces a rapid influx of CCR2⁺ monocytes and their differentiation into monocyte-derived dendritic cells (mo-DCs). Depletion of CCR2⁺ monocytes and their derivative mo-DCs resulted in impaired activation of a protective inflammatory response and the rapid death of mice infected with the fbp1Δ mutant. Mice lacking B and T cells also developed fungal meningitis and succumbed to infection with the fbp1Δ mutant, demonstrating that adaptive immune responses to the fbp1Δ mutant help to maintain the long-term survival of the host. Adaptive immune responses to the fbp1Δ mutant were characterized by enhanced differentiation of Th1 and Th17 CD4⁺ T cells together with diminished Th2 responses compared to the H99 parental strain. Importantly, we found that the enhanced immunogenicity of fbp1Δ mutant yeast cells can be harnessed to confer protection against a subsequent infection with the virulent H99 parental strain. Altogether, our findings suggest that Fbp1 functions as a novel virulence factor that shapes the immunogenicity of C. neoformansIMPORTANCECryptococcus neoformans is the most common cause of deadly fungal meningitis, with over 270,000 infections per year. Immune responses are critically required for the prevention of cryptococcosis, and patients with impaired immunity and low CD4⁺ T cell numbers are at high risk of developing these deadly infections. Although it is well appreciated that the development of protective immunity is shaped by the interactions of the host immune system with fungal cells, our understanding of fungal products that influence this process remains poor. In this study, we found that the activity of F-box protein 1 (Fbp1) in highly virulent C. neoformans clinical strain H99 shapes its immunogenicity and thus affects the development of protective immune responses in the host. The identification of this new mechanism of virulence may facilitate the future development of therapeutic interventions aimed at boosting antifungal host immunity.

CD4+ T Cells Orchestrate Lethal Immune Pathology despite Fungal Clearance during Cryptococcus neoformans Meningoencephalitis

Cryptococcus neoformans is a major fungal pathogen that disseminates to the central nervous system (CNS) to cause fatal meningoencephalitis, but little is known about immune responses within this immune-privileged site. CD4⁺ T cells have demonstrated roles in anticryptococcal defenses, but increasing evidence suggests that they may contribute to clinical deterioration and pathology in both HIV-positive (HIV+) and non-HIV patients who develop immune reconstitution inflammatory syndrome (IRIS) and post-infectious inflammatory response syndrome (PIIRS), respectively. Here we report a novel murine model of cryptococcal meningoencephalitis and a potential damaging role of T cells in disseminated cryptococcal CNS infection. In this model, fungal burdens plateaued in the infected brain by day 7 postinfection, but activation of microglia and accumulation of CD45^hi leukocytes was significantly delayed relative to fungal growth and did not peak until day 21. The inflammatory leukocyte infiltrate consisted predominantly of gamma interferon (IFN-γ)-producing CD4⁺ T cells, conventionally believed to promote fungal clearance and recovery. However, more than 50% of mice succumbed to infection and neurological dysfunction between days 21 and 35 despite a 100-fold reduction in fungal burdens. Depletion of CD4⁺ cells significantly impaired IFN-γ production, CD8⁺ T cell and myeloid cell accumulation, and fungal clearance from the CNS but prevented the development of clinical symptoms and mortality. These findings conclusively demonstrate that although CD4⁺ T cells are necessary to control fungal growth, they can also promote significant immunopathology and mortality during CNS infection. The results from this model may provide important guidance for development and use of anti-inflammatory therapies to minimize CNS injury in patients with severe cryptococcal infections.

CNS infection with the fungal pathogen Cryptococcus neoformans often results in debilitating brain injury and has a high mortality rate despite antifungal treatment. Treatment is complicated by the fact that immune responses needed to eliminate infection are also thought to drive CNS damage in a subset of both HIV+ and non-HIV patients. Thus, physicians need to balance efforts to enhance patients’ immune responses and promote microbiological control with anti-inflammatory therapy to protect the CNS. Here we report a novel model of cryptococcal meningoencephalitis demonstrating that fungal growth within the CNS does not immediately cause symptomatic disease. Rather, accumulation of antifungal immune cells critically mediates CNS injury and mortality. This model demonstrates that antifungal immune responses in the CNS can cause detrimental pathology and addresses the urgent need for animal models to investigate the specific cellular and molecular mechanisms underlying cryptococcal disease in order to better treat patients with CNS infections.

RNA Interference Screening Reveals Host CaMK4 as a Regulator of Cryptococcal Uptake and Pathogenesis

Cryptococcus neoformans, the causative agent of cryptococcosis, is an opportunistic fungal pathogen that kills over 200,000 individuals annually. This yeast may grow freely in body fluids, but it also flourishes within host cells. Despite extensive research on cryptococcal pathogenesis, host genes involved in the initial engulfment of fungi and subsequent stages of infection are woefully understudied. To address this issue, we combined short interfering RNA silencing and a high-throughput imaging assay to identify host regulators that specifically influence cryptococcal uptake. Of 868 phosphatase and kinase genes assayed, we discovered 79 whose silencing significantly affected cryptococcal engulfment. For 25 of these, the effects were fungus specific, as opposed to general alterations in phagocytosis. Four members of this group significantly and specifically altered cryptococcal uptake; one of them encoded CaMK4, a calcium/calmodulin-dependent protein kinase. Pharmacological inhibition of CaMK4 recapitulated the observed defects in phagocytosis. Furthermore, mice deficient in CaMK4 showed increased survival compared to wild-type mice upon infection with C. neoformans This increase in survival correlated with decreased expression of pattern recognition receptors on host phagocytes known to recognize C. neoformans Altogether, we have identified a kinase that is involved in C. neoformans internalization by host cells and in host resistance to this deadly infection.

Induction of Broad-Spectrum Protective Immunity against Disparate Cryptococcus Serotypes

Cryptococcosis is a fungal disease caused by multiple Cryptococcus serotypes; particularly C. neoformans (serotypes A and D) and C. gattii (serotypes B and C). To date, there is no clinically available vaccine to prevent cryptococcosis. Mice given an experimental pulmonary vaccination with a C. neoformans serotype A strain engineered to produce interferon-γ, denoted H99γ, are protected against a subsequent otherwise lethal experimental infection with C. neoformans serotype A. Thus, we determined the efficacy of immunization with C. neoformans strain H99γ to elicit broad-spectrum protection in BALB/c mice against multiple disparate Cryptococcus serotypes. We observed significantly increased survival rates and significantly decreased pulmonary fungal burden in H99γ immunized mice challenged with Cryptococcus serotypes A, B, or D compared to heat-killed H99γ (HKH99γ) immunized mice. Results indicated that prolonged protection against Cryptococcus serotypes B or D in H99γ immunized mice was CD4⁺ T cell dependent and associated with the induction of predominantly Th1-type cytokine responses. Interestingly, immunization with H99γ did not elicit greater protection against challenge with the Cryptococcus serotype C tested either due to low overall virulence of this strain or enhanced capacity of this strain to evade host immunity. Altogether, these studies provide “proof-of-concept” for the development of a cryptococcal vaccine that provides cross-protection against multiple disparate serotypes of Cryptococcus.

T Cell-Restricted Notch Signaling Contributes to Pulmonary Th1 and Th2 Immunity during Cryptococcus neoformans Infection

Cryptococcus neoformans is a ubiquitous, opportunistic fungal pathogen but the cell signaling pathways that drive T cell responses regulating antifungal immunity are incompletely understood. Notch is a key signaling pathway regulating T cell development, and differentiation and functional responses of mature T cells in the periphery. The targeting of Notch signaling within T cells has been proposed as a potential treatment for alloimmune and autoimmune disorders, but it is unknown whether disturbances to T cell immunity may render these patients vulnerable to fungal infections. To elucidate the role of Notch signaling during fungal infections, we infected mice expressing the pan-Notch inhibitor dominant negative mastermind-like within mature T cells with C. neoformans Inhibition of T cell-restricted Notch signaling increased fungal burdens in the lungs and CNS, diminished pulmonary leukocyte recruitment, and simultaneously impaired Th1 and Th2 responses. Pulmonary leukocyte cultures from T cell Notch-deprived mice produced less IFN-γ, IL-5, and IL-13 than wild-type cells. This correlated with lower frequencies of IFN-γ-, IL-5-, and IL-13-producing CD4⁺ T cells, reduced expression of Th1 and Th2 associated transcription factors, Tbet and GATA3, and reduced production of IFN-γ by CD8⁺ T cells. In contrast, Th17 responses were largely unaffected by Notch signaling. The changes in T cell responses corresponded with impaired macrophage activation and reduced leukocyte accumulation, leading to diminished fungal control. These results identify Notch signaling as a previously unappreciated regulator of Th1 and Th2 immunity and an important element of antifungal defenses against cryptococcal infection and CNS dissemination.

Trojan Horse Transit Contributes to Blood-Brain Barrier Crossing of a Eukaryotic Pathogen

The blood-brain barrier (BBB) protects the central nervous system (CNS) by restricting the passage of molecules and microorganisms. Despite this barrier, however, the fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that is estimated to kill over 600,000 people annually. Cryptococcal infection begins in the lung, and experimental evidence suggests that host phagocytes play a role in subsequent dissemination, although this role remains ill defined. Additionally, the disparate experimental approaches that have been used to probe various potential routes of BBB transit make it impossible to assess their relative contributions, confounding any integrated understanding of cryptococcal brain entry. Here we used an in vitro model BBB to show that a “Trojan horse” mechanism contributes significantly to fungal barrier crossing and that host factors regulate this process independently of free fungal transit. We also, for the first time, directly imaged C. neoformans-containing phagocytes crossing the BBB, showing that they do so via transendothelial pores. Finally, we found that Trojan horse crossing enables CNS entry of fungal mutants that cannot otherwise traverse the BBB, and we demonstrate additional intercellular interactions that may contribute to brain entry. Our work elucidates the mechanism of cryptococcal brain invasion and offers approaches to study other neuropathogens.

The fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that kills hundreds of thousands of people each year. One route that has been proposed for this brain entry is a Trojan horse mechanism, whereby the fungus crosses the blood-brain barrier (BBB) as a passenger inside host phagocytes. Although indirect experimental evidence supports this intriguing mechanism, it has never been directly visualized. Here we directly image Trojan horse transit and show that it is regulated independently of free fungal entry, contributes to cryptococcal BBB crossing, and allows mutant fungi that cannot enter alone to invade the brain.

Are Th17 Cells Playing a Role in Immunity to Dermatophytosis?

Despite their superficial localization in the skin, pathogenic dermatophytes can induce a complex but still misunderstood immune response in their hosts. The cell-mediated immunity (CMI) is correlated with both clinical recovery and protection against reinfection, and CD4+ T lymphocytes have been recognized as a crucial component of the immune defense against dermatophytes. Before the discovery of the Th17 pathway, CMI was considered to be only dependent of Th1 cells, and thus most studies on the immunology of dermatophytosis have focused on the Th1 pathway. Nevertheless, the fine comparative analysis of available scientific data on immunology of dermatophytosis in one hand and on the Th17 pathway mechanisms involved in opportunistic mucosal fungal infections in the other hand reveals that some key elements of the Th17 pathway can be activated by dermatophytes. Stimulation of the Th17 pathway could occur through the activation of some C-type lectin-like receptors and inflammasome in antigen-presenting cells. The Th17 cells could go back to the affected skin and by the production of signature cytokines could induce the effector mechanisms like the recruitment of polymorphonuclear neutrophils and the synthesis of antimicrobial peptides. In conclusion, besides the Th1 pathway, which is important to the immune response against dermatophytes, there are also growing evidences for the involvement of the Th17 pathway.

Antifungal Activity of Plasmacytoid Dendritic Cells against Cryptococcus neoformans In Vitro Requires Expression of Dectin-3 (CLEC4D) and Reactive Oxygen Species

Conventional dendritic cells (cDCs) are critical for protection against pulmonary infection with the opportunistic fungal pathogen Cryptococcus neoformans; however, the role of plasmacytoid dendritic cells (pDCs) is unknown. We show for the first time that murine pDCs have direct activity against C. neoformans via reactive oxygen species (ROS), a mechanism different from that employed to control Aspergillus fumigatus infections. The anticryptococcal activity of murine pDCs is independent of opsonization but appears to require the C-type lectin receptor Dectin-3, a receptor not previously evaluated during cryptococcal infections. Human pDCs can also inhibit cryptococcal growth by a mechanism similar to that of murine pDCs. Experimental pulmonary infection of mice with a C. neoformans strain that induces protective immunity demonstrated that recruitment of pDCs to the lungs is CXCR3 dependent. Taken together, our results show that pDCs inhibit C. neoformans growth in vitro via the production of ROS and that Dectin-3 is required for optimal growth-inhibitory activity.

Induction of Protective Immunity to Cryptococcal Infection in Mice by a Heat-Killed, Chitosan-Deficient Strain of Cryptococcus neoformans

Cryptococcus neoformans is a major opportunistic fungal pathogen that causes fatal meningoencephalitis in immunocompromised individuals and is responsible for a large proportion of AIDS-related deaths. The fungal cell wall is an essential organelle which undergoes constant modification during various stages of growth and is critical for fungal pathogenesis. One critical component of the fungal cell wall is chitin, which in C. neoformans is predominantly deacetylated to chitosan. We previously reported that three chitin deacetylase (CDA) genes have to be deleted to generate a chitosan-deficient C. neoformans strain. This cda1Δ2Δ3Δ strain was avirulent in mice, as it was rapidly cleared from the lungs of infected mice. Here, we report that clearance of the cda1Δ2Δ3Δ strain was associated with sharply spiked concentrations of proinflammatory molecules that are known to be critical mediators of the orchestration of a protective Th1-type adaptive immune response. This was followed by the selective enrichment of the Th1-type T cell population in the cda1Δ2Δ3Δ strain-infected mouse lung. Importantly, this response resulted in the development of robust protective immunity to a subsequent lethal challenge with a virulent wild-type C. neoformans strain. Moreover, protective immunity was also induced in mice vaccinated with heat-killed cda1Δ2Δ3Δ cells and was effective in multiple mouse strains. The results presented here provide a strong framework to develop the cda1Δ2Δ3Δ strain as a potential vaccine candidate for C. neoformans infection.

The most commonly used anticryptococcal therapies include amphotericin B, 5-fluorocytosine, and fluconazole alone or in combination. Major drawbacks of these treatment options are their limited efficacy, poor availability in limited resource areas, and potential toxicity. The development of antifungal vaccines and immune-based therapeutic interventions is promising and an attractive alternative to chemotherapeutics. Currently, there are no fungal vaccines in clinical use. This is the first report of a C. neoformans deletion strain with an avirulent phenotype in mice exhibiting protective immunity when used as a vaccine after heat inactivation, although other strains that overexpress fungal or murine proteins have recently been shown to induce a protective response. The data presented here demonstrate the potential for developing the avirulent cda1Δ2Δ3Δ strain into a vaccine-based therapy to treat C. neoformans infection.

Glucosylceramide Administration as a Vaccination Strategy in Mouse Models of Cryptococcosis

Cryptococcus neoformans is an opportunistic fungal pathogen and the causative agent of the disease cryptococcosis. Cryptococcosis is initiated as a pulmonary infection and in conditions of immune deficiency disseminates to the blood stream and central nervous system, resulting in life-threatening meningoencephalitis. A number of studies have focused on the development of a vaccine against Cryptococcus, primarily utilizing protein-conjugated components of the Cryptococcus polysaccharide capsule as antigen. However, there is currently no vaccine against Cryptococcus in the clinic. Previous studies have shown that the glycosphingolipid, glucosylceramide (GlcCer), is a virulence factor in C. neoformans and antibodies against this lipid inhibit fungal growth and cell division. In the present study, we have investigated the possibility of using GlcCer as a therapeutic agent against C. neoformans infections in mouse models of cryptococcosis. GlcCer purified from a non-pathogenic fungus, Candida utilis, was administered intraperitoneally, prior to infecting mice with a lethal dose of C. neoformans. GlcCer administration prevented the dissemination of C. neoformans from the lungs to the brain and led to 60% mouse survival. GlcCer administration did not cause hepatic injury and elicited an anti-GlcCer antibody response, which was observed independent of the route of administration and the strains of mouse. Taken together, our results suggest that fungal GlcCer can protect mice against lethal doses of C. neoformans infection and can provide a viable vaccination strategy against Cryptococcus.

Innate host defenses against Cryptococcus neoformans

Cryptococcus neoformans, the predominant etiological agent of cryptococcosis, can cause life-threatening infections of the central nervous system in immunocompromised and immunocompetent individuals. Cryptococcal meningoencephalitis is the most common disseminated fungal infection in AIDS patients, and remains the third most common invasive fungal infection among organ transplant recipients. The administration of highly active antiretroviral therapy (HAART) has resulted in a decrease in the number of cases of AIDS-related cryptococcosis in developed countries, but in developing countries where HAART is not readily available, Cryptococcus is still a major concern. Therefore, there is an urgent need for the development of novel therapies and/or vaccines to combat cryptococcosis. Understanding the protective immune responses against Cryptococcus is critical for development of vaccines and immunotherapies to combat cryptococcosis. Consequently, this review focuses on our current knowledge of protective immune responses to C. neoformans, with an emphasis on innate immune responses.

Cryptococcus and Phagocytes: Complex Interactions that Influence Disease Outcome

Cryptococcus neoformans and C. gattii are fungal pathogens that cause life-threatening disease. These fungi commonly enter their host via inhalation into the lungs where they encounter resident phagocytes, including macrophages and dendritic cells, whose response has a pronounced impact on the outcome of disease. Cryptococcus has complex interactions with the resident and infiltrating innate immune cells that, ideally, result in destruction of the yeast. These phagocytic cells have pattern recognition receptors that allow recognition of specific cryptococcal cell wall and capsule components. However, Cryptococcus possesses several virulence factors including a polysaccharide capsule, melanin production and secretion of various enzymes that aid in evasion of the immune system or enhance its ability to thrive within the phagocyte. This review focuses on the intricate interactions between the cryptococci and innate phagocytic cells including discussion of manipulation and evasion strategies used by Cryptococcus, anti-cryptococcal responses by the phagocytes and approaches for targeting phagocytes for the development of novel immunotherapeutics.