Impact of surfactant protein D, interleukin-5, and eosinophilia on Cryptococcosis

Use of 2D minilungs from human embryonic stem cells to study the interaction of Cryptococcus neoformans with the respiratory tract

Organoids can meet the needs between the use of cell culture and in vivo work, bringing together aspects of multicellular tissues, providing a more similar in vitro system for the study of various components, including host-interactions with pathogens and drug response. Organoids are structures that resemble organs in vivo, originating from pluripotent stem cells (PSCs) or adult stem cells (ASCs). There is great interest in deepening the understanding of the use of this technology to produce information about fungal infections and their treatments. This work aims the use 2D human lung organoid derived from human embryonic stem cells (hESCs), to investigate Cryptococcus neoformans-host interactions. C. neoformans is an opportunistic fungus acquired by inhalation that causes systemic mycosis mainly in immunocompromised individuals. Our work highlights the suitability of human minilungs for the study of C. neoformans infection (adhesion, invasion and replication), the interaction with the surfactant and induction of the host's alveolar pro-inflammatory response.

Development of a Heat-Killed fbp1 Mutant Strain as a Therapeutic Agent To Treat Invasive Cryptococcus Infection

In previous studies, we determined that the F-box protein Fbp1, a subunit of the SCF(Fbp1) E3 ligase in Cryptococcus neoformans, is essential for fungal pathogenesis. Heat-killed fbp1Δ cells (HK-fbp1) can confer vaccine-induced immunity against lethal challenge with clinically important invasive fungal pathogens, e.g., C. neoformans, C. gattii, and Aspergillus fumigatus. In this study, we found that either CD4+ T cells or CD8+ T cells were sufficient to confer protection against lethal challenge by C. neoformans in HK-fbp1-induced immunity. Given the potent effect of HK-fbp1 as a preventative vaccine, we further tested the potential efficacy of administering HK-fbp1 cells as a therapeutic agent for treating animals after infection. Remarkably, administration of HK-fbp1 provided robust host protection against preexisting C. neoformans infection. The mice infected with wild-type H99 cells and then treated with HK-fbp1 showed significant reduction of fungal burden in the infected lung and no dissemination of fungal cells to the brain and spleen. We find that early treatment is critical for the effective use of HK-fbp1 as a therapeutic agent. Immune analysis revealed that early treatment with HK-fbp1 cells elicited Th1-biased protective immune responses that help block fungal dissemination and promote better host protection. Our data thus suggest that HK-fbp1 is both an effective prophylactic vaccine candidate against C. neoformans infection in both immunocompetent and immunocompromised populations and a potential novel therapeutic strategy to treat early-stage cryptococcosis. IMPORTANCE Invasive fungal infections, e.g., cryptococcosis, are often life threatening and difficult to treat with very limited therapeutic options. There is no vaccine available in clinical use to prevent or treat fungal infections. Our previous studies demonstrated that heat-killed fbp1Δ cells (HK-fbp1) in Cryptococcus neoformans can be harnessed to confer protection against a challenge by the virulent parental strain, even in immunocompromised animals, such as ones lacking CD4+ T cells. In this study, we further determined that T cells are required for vaccine-induced protection against homologous challenge and that either CD4+ or CD8+ cells are sufficient. This finding is particularly important for the potential utility of this vaccine candidate in the context of HIV/AIDS-induced immune deficiency, the main risk factor for cryptococcosis in humans. Furthermore, in addition to the utility of HK-fbp1 as a prophylactic vaccine, we found that HK-fbp1 administration can inhibit disease dissemination when animals are treated at an early stage during Cryptococcus infection. Our findings could significantly expand the utility of HK-fbp1 not only as a prophylactic vaccine but also as a novel therapy against cryptococcosis. In all, our studies showed that the HK-fbp1 strain can be used both preventively and therapeutically to elicit robust host protection against cryptococcosis.

Comparative miRNA transcriptomics of macaques and mice reveals MYOC is an inhibitor for Cryptococcus neoformans invasion into the brain

Cryptococcal meningoencephalitis (CM) is emerging as an infection in HIV/AIDS patients shifted from primarily ART­naive to ART-experienced individuals, as well as patients with COVID-19 and immunocompetent hosts. This fungal infection is mainly caused by the opportunistic human pathogen Cryptococcus neoformans. Brain or central nervous system (CNS) dissemination is the deadliest process for this disease; however, mechanisms underlying this process have yet to be elucidated. Moreover, illustrations of clinically relevant responses in cryptococcosis are currently limited due to the low availability of clinical samples. In this study, to explore the clinically relevant responses during C. neoformans infection, macaque and mouse infection models were employed and miRNA-mRNA transcriptomes were performed and combined, which revealed cytoskeleton, a major feature of HIV/AIDS patients, was a centric pathway regulated in both infection models. Notably, assays of clinical immune cells confirmed an enhanced macrophage “Trojan Horse” in patients with HIV/AIDS, which could be shut down by cytoskeleton inhibitors. Furthermore, myocilin, encoded by MYOC, was found to be a novel enhancer for the macrophage “Trojan Horse,” and an enhanced fungal burden was achieved in the brains of MYOC-transgenic mice. Taken together, the findings from this study reveal fundamental roles of the cytoskeleton and MYOC in fungal CNS dissemination, which not only helps to understand the high prevalence of CM in HIV/AIDS but also facilitates the development of novel therapeutics for meningoencephalitis caused by C. neoformans and other pathogenic microorganisms.

Pulmonary Fibrosis and Hypereosinophilia in TLR9-/- Mice Infected by Cryptococcus gattii

Cryptococcus gattii is a worldwide-distributed basidiomycetous yeast that can infect immunocompetent hosts. However, little is known about the mechanisms involved in the disease. The innate immune response is essential to the control of infections by microorganisms. Toll-like receptor 9 (TLR9) is an innate immune receptor, classically described as a non-methylated DNA recognizer and associated with bacteria, protozoa and opportunistic mycosis infection models. Previously, our group showed that TLR9^-/- mice were more susceptible to C. gattii after 21 days of infection. However, some questions about the innate immunity involving TLR9 response against C. gattii remain unknown. In order to investigate the systemic cryptococcal infection, we evaluated C57BL/6 mice and C57BL/6 TLR9^-/- after intratracheal infection with 10⁴C. gattii yeasts for 21 days. Our data evidenced that TLR9^-/- was more susceptible to C. gattii. TLR9^-/- mice had hypereosinophilia in pulmonary mixed cellular infiltrate, severe bronchiolitis and vasculitis and type 2 alveolar cell hyperplasia. In addition, TLR9^-/- mice developed severe pulmonary fibrosis and areas with strongly birefringent fibers. Together, our results corroborate the hypothesis that TLR9 is important to support the Th1/Th17 response against C. gattii infection in the murine experimental model.

Cryptococcus: History, Epidemiology and Immune Evasion

Cryptococcosis is a disease caused by the pathogenic fungi Cryptococcus neoformans and Cryptococcus gattii, both environmental fungi that cause severe pneumonia and may even lead to cryptococcal meningoencephalitis. Although C. neoformans affects more fragile individuals, such as immunocompromised hosts through opportunistic infections, C. gattii causes a serious indiscriminate primary infection in immunocompetent individuals. Typically seen in tropical and subtropical environments, C. gattii has increased its endemic area over recent years, largely due to climatic factors that favor contagion in warmer climates. It is important to point out that not only C. gattii, but the Cryptococcus species complex produces a polysaccharidic capsule with immunomodulatory properties, enabling the pathogenic species of Cryptococccus to subvert the host immune response during the establishment of cryptococcosis, facilitating its dissemination in the infected organism. C. gattii causes a more severe and difficult-to-treat infection, with few antifungals eliciting an effective response during chronic treatment. Much of the immunopathology of this cryptococcosis is still poorly understood, with most studies focusing on cryptococcosis caused by the species C. neoformans. C. gattii became more important in the epidemiological scenario with the outbreaks in the Pacific Northwest of the United States, which resulted in phylogenetic studies of the virulent variant responsible for the severe infection in the region. Since then, the study of cryptococcosis caused by C. gattii has helped researchers understand the immunopathological aspects of different variants of this pathogen.

Detection of Cytokines and Collectins in Bronchoalveolar Fluid Samples of Patients Infected with Histoplasma capsulatum and Pneumocystis jirovecii

Histoplasmosis and pneumocystosis co-infections have been reported mainly in immunocompromised humans and in wild animals. The immunological response to each fungal infection has been described primarily using animal models; however, the host response to concomitant infection is unknown. The present work aimed to evaluate the pulmonary immunological response of patients with pneumonia caused either by Histoplasma capsulatum, Pneumocystis jirovecii, or their co-infection. We analyzed the pulmonary collectin and cytokine patterns of 131 bronchoalveolar lavage samples, which included HIV and non-HIV patients infected with H. capsulatum, P. jirovecii, or both fungi, as well as healthy volunteers and HIV patients without the studied fungal infections. Our results showed an increased production of the surfactant protein-A (SP-A) in non-HIV patients with H. capsulatum infection, contrasting with HIV patients (p < 0.05). Significant differences in median values of SP-A, IL-1β, TNF-α, IFN-γ, IL-18, IL-17A, IL-33, IL-13, and CXCL8 were found among all the groups studied, suggesting that these cytokines play a role in the local inflammatory processes of histoplasmosis and pneumocystosis. Interestingly, non-HIV patients with co-infection and pneumocystosis alone showed lower levels of SP-A, IL-1β, TNF-α, IFN-γ, IL-18, IL-17A, and IL-23 than histoplasmosis patients, suggesting an immunomodulatory ability of P. jirovecii over H. capsulatum response.

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.

Surfactant Protein D Recognizes Multiple Fungal Ligands: A Key Step to Initiate and Intensify the Anti-fungal Host Defense

With limited therapeutic options and associated severe adverse effects, fungal infections are a serious threat to human health. Innate immune response mediated by pattern recognition proteins is integral to host defense against fungi. A soluble pattern recognition protein, Surfactant protein D (SP-D), plays an important role in immune surveillance to detect and eliminate human pathogens. SP-D exerts its immunomodulatory activity via direct interaction with several receptors on the epithelial cells lining the mucosal tracts, as well as on innate and adaptive immune cells. Being a C-type lectin, SP-D shows calcium- and sugar-dependent interactions with several glycosylated ligands present on fungal cell walls. The interactome includes cell wall polysaccharides such as 1,3-β-D-glucan, 1,6-β-D-glucan, Galactosaminogalactan Galactomannan, Glucuronoxylomannan, Mannoprotein 1, and glycosylated proteins such as gp45, gp55, major surface glycoprotein complex (gpA). Recently, binding of a recombinant fragment of human SP-D to melanin on the dormant conidia of Aspergillus fumigatus was demonstrated that was not inhibited by sugars, suggesting a likely protein-protein interaction. Interactions of the ligands on the fungal spores with the oligomeric forms of full-length SP-D resulted in formation of spore-aggregates, increased uptake by phagocytes and rapid clearance besides a direct fungicidal effect against C. albicans. Exogenous administration of SP-D showed significant therapeutic potential in murine models of allergic and invasive mycoses. Altered susceptibility of SP-D gene-deficient mice to various fungal infections emphasized relevance of SP-D as an important sentinel of anti-fungal immunity. Levels of SP-D in the serum or lung lavage were significantly altered in the murine models and patients of fungal infections and allergies. Here, we review the cell wall ligands of clinically relevant fungal pathogens and allergens that are recognized by SP-D and their impact on the host defense. Elucidation of the molecular interactions between innate immune humoral such as SP-D and fungal pathogens would facilitate the development of novel therapeutic interventions.

Basic principles of the virulence of Cryptococcus

Among fungal pathogens, Cryptococcus neoformans has gained great importance among the scientific community of several reasons. This fungus is the causative agent of cryptococcosis, a disease mainly associated to HIV immunosuppression and characterized by the appearance of meningoencephalitis. Cryptococcal meningitis is responsible for hundreds of thousands of deaths every year. Research of the pathogenesis and virulence mechanisms of this pathogen has focused on three main different areas: Adaptation to the host environment (nutrients, pH, and free radicals), mechanism of immune evasion (which include phenotypic variations and the ability to behave as a facultative intracellular pathogen), and production of virulence factors. Cryptococcus neoformans has two phenotypic characteristics, the capsule and synthesis of melanin that have a profound effect in the virulence of the yeast because they both have protective effects and induce host damage as virulence factors. Finally, the mechanisms that result in dissemination and brain invasion are also of key importance to understand cryptococcal disease. In this review, I will provide a brief overview of the main mechanisms that makes C. neoformans a pathogen in susceptible patients.

Abbreviations: RNS: reactive nitrogen species; BBB: brain blood barrier; GXM: glucuronoxylomannan; GXMGal: glucuronoxylomannogalactan

Pulmonary cryptococcosis: A review of pathobiology and clinical aspects

Pulmonary cryptococcosis is an important opportunistic invasive mycosis in immunocompromised patients, but it is also increasingly seen in immunocompetent patients. The main human pathogens are Cryptococcus neoformans and C. gattii, which have a worldwide distribution. In contrast to cryptococcal meningitis, pulmonary cryptococcosis is still underdiagnosed because of limitations in diagnostic tools. It can mimic lung cancer, pulmonary tuberculosis, bacterial pneumonia, and other pulmonary mycoses both clinically and radiologically. Pulmonary nodules are the most common radiological feature, but these are not specific to pulmonary cryptococcosis. The sensitivity of culture of respiratory samples for Cryptococcus is poor and a positive result may also reflect colonisation. Cryptococcal antigen (CrAg) with lateral flow device is a fast and sensitive test and widely used on serum and cerebrospinal fluid, but sera from patients with pulmonary cryptococcosis are rarely positive in the absence of disseminated disease. Detection of CrAg from respiratory specimens might assist the diagnosis of pulmonary cryptococcosis but there are very few data. Molecular detection techniques such as multiplex reverse transcription polymerase chain reaction (RT-PCR) could also provide better sensitivity but these still require validation for respiratory specimens. The first line of treatment for pulmonary cryptococcosis is fluconazole, or amphotericin B and flucytosine for those with central nervous system involvement. Pulmonary cryptococcosis worsens the prognosis of cryptococcal meningitis. In this review, we summarize the biological aspects of Cryptococcus and provide an update on the diagnosis and management of pulmonary cryptococcosis.

Unveil the transcriptional landscape at the Cryptococcus-host axis in mice and nonhuman primates

Pathogens and hosts require rapid modulation of virulence and defense mechanisms at the infection axis, but monitoring such modulations is challenging. In studying the human fungal pathogen Cryptococcus neoformans, mouse and rabbit infection models are often employed to shed light on the disease mechanisms but that may not be clinically relevant. In this study, we developed an animal infection model using the non-human primate cynomolgus monkey Macaca fascicularis. In addition, we systematically profiled and compared transcriptional responses between the infected mice and the cynomolgus monkey, using simultaneous or dual RNA next-generation sequencing. We demonstrated that there are shared but distinct transcriptional profiles between the two models following C. neoformans infection. Specifically, genes involved in immune and inflammatory responses are all upregulated. Osteoclastogenesis and insulin signaling are also significantly co-regulated in both models and disrupting an osteoclastogenesis-associated gene (OC-STAMP) or the insulin-signaling process significantly altered the host tolerance to C. neoformans. Moreover, C. neoformans was shown to activate metal sequestration, dampen the sugar metabolism, and control cell morphology during infection. Taking together, we described the development of a non-human primate model of cryptococcosis that allowed us to perform an in-depth analysis and comparison of transcriptome profiles during infections of two animal models and conceptually identify host genes important in disease responses. This study provides new insights in understanding fungal pathogenesis mechanisms that potentially facilitate the identification of novel drug targets for the treatment of cryptococcal infection.

The host-pathogen interaction is highly dynamic and tightly regulated, and yet is difficult to monitor. Traditional investigations provide valuable information for the understanding of pathogenic microbial biology but are time-consuming and often neglect the host immune responses. In addition, current animal models for studying pathogenic fungi are limited in mimicking the responses from humans. The development of a new Cryptococcus neoformans infection model using nonhuman primates and the utilization of simultaneous RNA sequencing analysis provide fast and clinically relevant research data allowing the identification of novel critical players from both the invading fungus and the host. The data from the current study would not only help to decipher disease mechanisms but also promote the discovery of novel antifungal drug targets.

Inflammatory monocytes are detrimental to the host immune response during acute infection with Cryptococcus neoformans

Cryptococcus neoformans is a leading cause of invasive fungal infections among immunocompromised patients. However, the cellular constituents of the innate immune response that promote clearance versus progression of infection upon respiratory acquisition of C. neoformans remain poorly defined. In this study, we found that during acute C. neoformans infection, CCR2⁺ Ly6C^hi inflammatory monocytes (IM) rapidly infiltrate the lungs and mediate fungal trafficking to lung-draining lymph nodes. Interestingly, this influx of IM is detrimental to the host, since ablating IM or impairing their recruitment to the lungs improves murine survival and reduces fungal proliferation and dissemination. Using a novel conditional gene deletion strategy, we determined that MHC class II expression by IM did not mediate their deleterious impact on the host. Furthermore, although ablation of IM reduced the number of lymphocytes, innate lymphoid cells, and eosinophils in the lungs, the effects of IM were not dependent on these cells. We ascertained that IM in the lungs upregulated transcripts associated with alternatively activated (M2) macrophages in response to C. neoformans, consistent with the model that IM assume a cellular phenotype that is permissive for fungal growth. We also determined that conditional knockout of the prototypical M2 marker arginase 1 in IM and deletion of the M2-associated transcription factor STAT6 were not sufficient to reverse the harmful effects of IM. Overall, our findings indicate that C. neoformans can subvert the fungicidal potential of IM to enable the progression of infection through a mechanism that is not dependent on lymphocyte priming, eosinophil recruitment, or downstream M2 macrophage polarization pathways. These results give us new insight into the plasticity of IM function during fungal infections and the level of control that C. neoformans can exert on host immune responses.

Cryptococcus neoformans is a fungus that is prevalent throughout the environment and can cause a fatal infection of the central nervous system when inhaled into the lungs by patients with impaired immune systems. Our understanding of the immune responses that either help clear C. neoformans from the lungs or permit development of disease remains limited. In this study, we used a mouse model of lethal C. neoformans infection to determine that inflammatory monocytes, immune cells that are often among the first responders to infections, actually facilitate the progression of infection rather than clearance. These findings establish a foundation for future work to target the immune response of inflammatory monocytes as a strategy to improve the outcomes of patients that develop C. neoformans infections.

Frequency-dependent airway hyperresponsiveness in a mouse model of emphysema and allergic inflammation

Asthma and chronic obstructive pulmonary disease (COPD), chronic airway inflammatory diseases characterized by airflow limitation, have different etiologies and pathophysiologies. Asthma–COPD Overlap (ACO) has recently been used for patients with mixed asthma and COPD. The pathophysiological mechanisms of ACO have not been clearly understood due to the lack of an appropriate murine model. To investigate its pathophysiology, we examined a murine model by allergen challenge in surfactant protein‐D (SP‐D)‐deficient mice that spontaneously developed pulmonary emphysema. SP‐D‐deficient mice were sensitized and challenged by ovalbumin (OVA). Lungs and bronchoalveolar lavage fluid (BALF) were collected for analysis, and static lung compliance and airway hyperresponsiveness (AHR) were measured 48 h after the last OVA challenge. In SP‐D‐deficient, naïve, or OVA‐challenged mice, the mean linear intercept and static lung compliance were increased compared with wild‐type (WT) mice. There was no significant difference in goblet cell hyperplasia and the gene expression of Mucin 5AC (MUC5AC) between SP‐D‐deficient and WT OVA‐challenged mice. In SP‐D‐deficient OVA‐challenged mice, airway hyperresponsiveness was significantly enhanced despite the lower eosinophil count and the concentration of interleukin (IL)‐5 and IL‐13 in BALF compared with WT OVA‐challenged mice at 120 ventilations per minute. When mice were ventilated at a lower ventilation frequency of 100 ventilations per minute, elevated airway hyperresponsiveness in SP‐D‐deficient OVA‐challenged mice was diminished. This model of emphysematous change with allergic airway inflammation raises the possibility that frequency‐dependent airway hyperresponsiveness may be involved in the pathophysiology of ACO.

The Interaction of Human Pathogenic Fungi With C-Type Lectin Receptors

Fungi, usually present as commensals, are a major cause of opportunistic infections in immunocompromised patients. Such infections, if not diagnosed or treated properly, can prove fatal. However, in most cases healthy individuals are able to avert the fungal attacks by mounting proper antifungal immune responses. Among the pattern recognition receptors (PRRs), C-type lectin receptors (CLRs) are the major players in antifungal immunity. CLRs can recognize carbohydrate ligands, such as β-glucans and mannans, which are mainly found on fungal cell surfaces. They induce proinflammatory immune reactions, including phagocytosis, oxidative burst, cytokine, and chemokine production from innate effector cells, as well as activation of adaptive immunity via Th17 responses. CLRs such as Dectin-1, Dectin-2, Mincle, mannose receptor (MR), and DC-SIGN can recognize many disease-causing fungi and also collaborate with each other as well as other PRRs in mounting a fungi-specific immune response. Mutations in these receptors affect the host response and have been linked to a higher risk in contracting fungal infections. This review focuses on how CLRs on various immune cells orchestrate the antifungal response and on the contribution of single nucleotide polymorphisms in these receptors toward the risk of developing such infections.

Cryptococcosis manifesting as isolated biliary infection: Case report and brief review of literature

BACKGROUND

Biliary cryptococci infection is rare, which is frequently diagnosed by exploratory laparotomy, preoperative diagnosis is difficult.

CASE PRESENTATION

A 14-year-old girl presented with intermittent jaundice for 6 years. She had no pruritus, anorexia, nausea or vomiting, fever, abdominal pain, or clay stools. Laboratory tests showed obstructive jaundice, eosinophilia, and increased IgE levels. The patient was ultimately diagnosed as Cryptococcal infection by bone marrow culture. The patient responded to antifungal therapy.

CONCLUSION

Unnecessary surgical intervention was avoided by an early and accurate diagnosis. Cryptococcosis infection of bile duct should be highly suspected, when the children with obstructive jaundice have eosinophilia and increased IgE levels.

Immunology of Cryptococcal Infections: Developing a Rational Approach to Patient Therapy

Cryptococcal meningoencephalitis is responsible for upwards of 15% of HIV-related deaths worldwide and is currently the most common cause of non-viral meningitis in the US, affecting both previously healthy and people with immune suppression caused by cancer chemotherapy, transplantation, and biologic therapies. Despite a continued 30-50% attributable mortality, recommended therapeutic strategies have remained largely unchanged since the 1950s. Recent murine models and human studies examining the role of the immune system in both susceptibility to the infection as well as host damage have begun to influence patient care decisions. The Damage Framework Response, originally proposed in 1999, was recently used to discuss dichotomous etiologies of host damage in cryptococcal disease. These include patients suffering microbiological damage with low host immunity (especially those immunosuppressed with HIV) and those having low (live) microbiological burden but high immune-mediated damage (HIV-related immune reconstitution syndrome and non-HIV-related postinfectious inflammatory response syndrome). Cryptococcal disease in previously healthy hosts, albeit rare, has been known for a long time. Immunophenotyping and dendritic cell-T cell signaling studies on cerebral spinal fluid of these rare patients reveal immune capacity for recognition and T-cell activation pathways including increased levels of HLA-DR and CD56. However, despite effective T-cell signals, brain biopsy and autopsy specimens demonstrated an M2 alternative macrophage polarization and poor phagocytosis of fungal cells. These studies expand the paradigm for cryptococcal disease susceptibility to include a prominent role for immune-mediated damage and suggest a need for careful individual consideration of immune activation during therapy of cryptococcal disease in diverse hosts.

Surfactant Protein D in Respiratory and Non-Respiratory Diseases

Surfactant protein D (SP-D) is a multimeric collectin that is involved in innate immune defense and expressed in pulmonary, as well as non-pulmonary, epithelia. SP-D exerts antimicrobial effects and dampens inflammation through direct microbial interactions and modulation of host cell responses via a series of cellular receptors. However, low protein concentrations, genetic variation, biochemical modification, and proteolytic breakdown can induce decomposition of multimeric SP-D into low-molecular weight forms, which may induce pro-inflammatory SP-D signaling. Multimeric SP-D can decompose into trimeric SP-D, and this process, and total SP-D levels, are partly determined by variation within the SP-D gene, SFTPD. SP-D has been implicated in the development of respiratory diseases including respiratory distress syndrome, bronchopulmonary dysplasia, allergic asthma, and chronic obstructive pulmonary disease. Disease-induced breakdown or modifications of SP-D facilitate its systemic leakage from the lung, and circulatory SP-D is a promising biomarker for lung injury. Moreover, studies in preclinical animal models have demonstrated that local pulmonary treatment with recombinant SP-D is beneficial in these diseases. In recent years, SP-D has been shown to exert antimicrobial and anti-inflammatory effects in various non-pulmonary organs and to have effects on lipid metabolism and pro-inflammatory effects in vessel walls, which enhance the risk of atherosclerosis. A common SFTPD polymorphism is associated with atherosclerosis and diabetes, and SP-D has been associated with metabolic disorders because of its effects in the endothelium and adipocytes and its obesity-dampening properties. This review summarizes and discusses the reported genetic associations of SP-D with disease and the clinical utility of circulating SP-D for respiratory disease prognosis. Moreover, basic research on the mechanistic links between SP-D and respiratory, cardiovascular, and metabolic diseases is summarized. Perspectives on the development of SP-D therapy are addressed.

RIPK3/Fas-Associated Death Domain Axis Regulates Pulmonary Immunopathology to Cryptococcal Infection Independent of Necroptosis

Fas-associated death domain (FADD) and receptor interacting protein kinase 3 (RIPK3) are multifunctional regulators of cell death and immune response. Using a mouse model of cryptococcal infection, the roles of FADD and RIPK3 in anti-cryptococcal defense were investigated. Deletion of RIPK3 alone led to increased inflammatory cytokine production in the Cryptococcus neoformans-infected lungs, but in combination with FADD deletion, it led to a robust Th1-biased response with M1-biased macrophage activation. Rather than being protective, these responses led to paradoxical C. neoformans expansion and rapid clinical deterioration in Ripk3^−/− and Ripk3^−/−Fadd^−/− mice. The increased mortality of Ripk3^−/− and even more accelerated mortality in Ripk3^−/−Fadd^−/− mice was attributed to profound pulmonary damage due to neutrophil-dominant infiltration with prominent upregulation of pro-inflammatory cytokines. This phenomenon was partially associated with selective alterations in the apoptotic frequency of some leukocyte subsets, such as eosinophils and neutrophils, in infected Ripk3^−/−Fadd^−/− mice. In conclusion, our study shows that RIPK3 in concert with FADD serve as physiological “brakes,” preventing the development of excessive inflammation and Th1 bias, which in turn contributes to pulmonary damage and defective fungal clearance. This novel link between the protective effect of FADD and RIPK3 in antifungal defense and sustenance of immune homeostasis may be important for the development of novel immunomodulatory therapies against invasive fungal infections.

Innate Immune Responses to Cryptococcus

Cryptococcus species are encapsulated fungi found in the environment that predominantly cause disease in immunocompromised hosts after inhalation into the lungs. Even with contemporary antifungal regimens, patients with cryptococcosis continue to have high morbidity and mortality rates. The development of more effective therapies may depend on our understanding of the cellular and molecular mechanisms by which the host promotes sterilizing immunity against the fungus. This review will highlight our current knowledge of how Cryptococcus, primarily the species C. neoformans, is sensed by the mammalian host and how subsequent signaling pathways direct the anti-cryptococcal response by effector cells of the innate immune system.

Surfactant proteins, SP-A and SP-D, in respiratory fungal infections: their role in the inflammatory response

Pulmonary surfactant is a complex fluid that comprises phospholipids and four proteins (SP-A, SP-B, SP-C, and SP-D) with different biological functions. SP-B, SP-C, and SP-D are essential for the lungs’ surface tension function and for the organization, stability and metabolism of lung parenchyma. SP-A and SP-D, which are also known as pulmonary collectins, have an important function in the host’s lung immune response; they act as opsonins for different pathogens via a C-terminal carbohydrate recognition domain and enhance the attachment to phagocytic cells or show their own microbicidal activity by increasing the cellular membrane permeability. Interactions between the pulmonary collectins and bacteria or viruses have been extensively studied, but this is not the same for fungal pathogens. SP-A and SP-D bind glucan and mannose residues from fungal cell wall, but there is still a lack of information on their binding to other fungal carbohydrate residues. In addition, both their relation with immune cells for the clearance of these pathogens and the role of surfactant proteins’ regulation during respiratory fungal infections remain unknown. Here we highlight the relevant findings associated with SP-A and SP-D in those respiratory mycoses where the fungal infective propagules reach the lungs by the airways.

DAP12 Inhibits Pulmonary Immune Responses to Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that is inhaled into the lungs and can lead to life-threatening meningoencephalitis in immunocompromised patients. Currently, the molecular mechanisms that regulate the mammalian immune response to respiratory cryptococcal challenge remain poorly defined. DAP12, a signaling adapter for multiple pattern recognition receptors in myeloid and natural killer (NK) cells, has been shown to play both activating and inhibitory roles during lung infections by different bacteria and fungi. In this study, we demonstrate that DAP12 plays an important inhibitory role in the immune response to C. neoformans Infectious outcomes in DAP12(-/-) mice, including survival and lung fungal burden, are significantly improved compared to those in C57BL/6 wild-type (WT) mice. We find that eosinophils and macrophages are decreased while NK cells are increased in the lungs of infected DAP12(-/-) mice. In contrast to WT NK cells, DAP12(-/-) NK cells are able to repress C. neoformans growth in vitro Additionally, DAP12(-/-) macrophages are more highly activated than WT macrophages, with increased production of tumor necrosis factor (TNF) and CCL5/RANTES and more efficient uptake and killing of C. neoformans These findings suggest that DAP12 acts as a brake on the pulmonary immune response to C. neoformans by promoting pulmonary eosinophilia and by inhibiting the activation and antifungal activities of effector cells, including NK cells and macrophages.

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.

Host immunity to Cryptococcus neoformans

Cryptococcosis is caused by the fungal genus Cryptococcus. Cryptococcosis, predominantly meningoencephalitis, emerged with the HIV pandemic, primarily afflicting HIV-infected patients with profound T-cell deficiency. Where in use, combination antiretroviral therapy has markedly reduced the incidence of and risk for disease, but cryptococcosis continues to afflict those without access to therapy, particularly in sub-Saharan Africa and Asia. However, cryptococcosis also occurs in solid organ transplant recipients and patients with other immunodeficiencies as well as those with no known immunodeficiency. This article reviews innate and adaptive immune responses to C. neoformans, with an emphasis on recent studies on the role of B cells, natural IgM and Fc gamma receptor polymorphisms in resistance to cryptococcosis.

Analysis of Immune Response Markers in Jorge Lobo's Disease Lesions Suggests the Occurrence of Mixed T Helper Responses with the Dominance of Regulatory T Cell Activity

Jorge Lobo's disease (JLD) is a chronic infection that affects the skin and subcutaneous tissues. Its etiologic agent is the fungus Lacazia loboi. Lesions are classified as localized, multifocal, or disseminated, depending on their location. Early diagnosis and the surgical removal of lesions are the best therapeutic options currently available for JLD. The few studies that evaluate the immunological response of JLD patients show a predominance of Th2 response, as well as a high frequency of TGF-β and IL-10 positive cells in the lesions; however, the overall immunological status of the lesions in terms of their T cell phenotype has yet to be determined. Therefore, the objective of this study was to evaluate the pattern of Th1, Th2, Th17 and regulatory T cell (Treg) markers mRNA in JLD patients by means of real-time PCR. Biopsies of JLD lesions (N = 102) were classified according to their clinical and histopathological features and then analyzed using real-time PCR in order to determine the expression levels of TGF-β1, FoxP3, CTLA4, IKZF2, IL-10, T-bet, IFN-γ, GATA3, IL-4, IL-5, IL-13, IL-33, RORC, IL-17A, IL-17F, and IL-22 and to compare these levels to those of healthy control skin (N = 12). The results showed an increased expression of FoxP3, CTLA4, TGF-β1, IL-10, T-bet, IL-17F, and IL-17A in lesions, while GATA3 and IL-4 levels were found to be lower in diseased skin than in the control group. When the clinical forms were compared, TGF-β1 was found to be highly expressed in patients with a single localized lesion while IL-5 and IL-17A levels were higher in patients with multiple/disseminated lesions. These results demonstrate the occurrence of mixed T helper responses and suggest the dominance of regulatory T cell activity, which could inhibit Th-dependent protective responses to intracellular fungi such as L. loboi. Therefore, Tregs may play a key role in JLD pathogenesis.

Type I IFN Induction via Poly-ICLC Protects Mice against Cryptococcosis

Cryptococcus neoformans is the most common cause of fungal meningoencephalitis in AIDS patients. Depletion of CD4 cells, such as occurs during advanced AIDS, is known to be a critical risk factor for developing cryptococcosis. However, the role of HIV-induced innate inflammation in susceptibility to cryptococcosis has not been evaluated. Thus, we sought to determine the role of Type I IFN induction in host defense against cryptococci by treatment of C. neoformans (H99) infected mice with poly-ICLC (pICLC), a dsRNA virus mimic. Unexpectedly, pICLC treatment greatly extended survival of infected mice and reduced fungal burdens in the brain. Protection from cryptococcosis by pICLC-induced Type I IFN was mediated by MDA5 rather than TLR3. PICLC treatment induced a large, rapid and sustained influx of neutrophils and Ly6Chigh monocytes into the lung while suppressing the development of eosinophilia. The pICLC-mediated protection against H99 was CD4 T cell dependent and analysis of CD4 T cell polyfunctionality showed a reduction in IL-5 producing CD4 T cells, marginal increases in Th1 cells and dramatic increases in RORγt+ Th17 cells in pICLC treated mice. Moreover, the protective effect of pICLC against H99 was diminished in IFNγ KO mice and by IL-17A neutralization with blocking mAbs. Furthermore, pICLC treatment also significantly extended survival of C. gattii infected mice with reduced fungal loads in the lungs. These data demonstrate that induction of type I IFN dramatically improves host resistance against the etiologic agents of cryptococcosis by beneficial alterations in both innate and adaptive immune responses.

Eosinophil-associated lung diseases. A cry for surfactant proteins A and D help?

Surfactant proteins (SP)-A and SP-D (SP-A/-D) play important roles in numerous eosinophil-dominated diseases, including asthma, allergic bronchopulmonary aspergillosis, and allergic rhinitis. In these settings, SP-A/-D have been shown to modulate eosinophil chemotaxis, inhibit eosinophil mediator release, and mediate macrophage clearance of apoptotic eosinophils. Dysregulation of SP-A/-D function in eosinophil-dominated diseases is also not uncommon. Alterations in serum SP-A/-D levels are associated with disease severity in allergic rhinitis and chronic obstructive pulmonary disease. Furthermore, oligimerization of SP-A/-D, necessary for their proper function, can be perturbed by reactive nitrogen species, which are increased in eosinophilic disease. In this review, we highlight the associations of eosinophilic lung diseases with SP-A and SP-D levels and functions.

Immunity to Cryptococcus neoformans and C. gattii during cryptococcosis

The vast majority of infection with cryptococcal species occurs with Cryptococcus neoformans in the severely immunocompromised. A significant exception to this is the infections of those with apparently normal immune systems by Cryptococcus gattii. Susceptibility to cryptococcosis can be broadly categorised as a defect in adaptive immune responses, especially in T cell immunity. However, innate immune cells such as macrophages play a key role and are likely the primary effector cell in the killing and ultimate clearance of cryptococcal infection. In this review we discuss the current state of our understanding of how the immune system responds to cryptococcal infection in health and disease, with reference to the work communicated at the 9th International Conference on Cryptococcus and Cryptococcosis (ICCC9). We have focussed on cell mediated responses, particularly early in infection, but with the aim of presenting a broad overview of our understanding of immunity to cryptococcal infection, highlighting some recent advances and offering some perspectives on future directions.