Role of interleukin-4 in resistance to Cryptococcus neoformans infection

Platelets and mast cells promote pathogenic eosinophil recruitment during invasive fungal infection via the 5-HIAA-GPR35 ligand-receptor system

Cryptococcus neoformans is the leading cause of fungal meningitis and is characterized by pathogenic eosinophil accumulation in the context of type-2 inflammation. The chemoattractant receptor GPR35 is expressed by granulocytes and promotes their migration to the inflammatory mediator 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite. Given the inflammatory nature of cryptococcal infection, we examined the role of GPR35 in the circuitry underlying cell recruitment to the lung. GPR35 deficiency dampened eosinophil recruitment and fungal growth, whereas overexpression promoted eosinophil homing to airways and fungal replication. Activated platelets and mast cells were the sources of GPR35 ligand activity and pharmacological inhibition of serotonin conversion to 5-HIAA, or genetic deficiency in 5-HIAA production by platelets and mast cells resulted in more efficient clearance of Cryptococcus. Thus, the 5-HIAA-GPR35 axis is an eosinophil chemoattractant receptor system that modulates the clearance of a lethal fungal pathogen, with implications for the use of serotonin metabolism inhibitors in the treatment of fungal infections.

Macrophage Mediated Immunomodulation During Cryptococcus Pulmonary Infection

Macrophages are key cellular components of innate immunity, acting as the first line of defense against pathogens to modulate homeostatic and inflammatory responses. They help clear pathogens and shape the T-cell response through the production of cytokines and chemokines. The facultative intracellular fungal pathogen Cryptococcus neoformans has developed a unique ability to interact with and manipulate host macrophages. These interactions dictate how Cryptococcus infection can remain latent or how dissemination within the host is achieved. In addition, differences in the activities of macrophages have been correlated with differential susceptibilities of hosts to Cryptococcus infection, highlighting the importance of macrophages in determining disease outcomes. There is now abundant information on the interaction between Cryptococcus and macrophages. In this review we discuss recent advances regarding macrophage origin, polarization, activation, and effector functions during Cryptococcus infection. The importance of these strategies in pathogenesis and the potential of immunotherapy for cryptococcosis treatment is also discussed.

Vomocytosis: Too Much Booze, Base, or Calcium?

Macrophages are well known for their phagocytic activity and their role in innate immune responses. Macrophages eat non-self particles, via a variety of mechanisms, and typically break down internalized cargo into small macromolecules. However, some pathogenic agents have the ability to evade this endosomal degradation through a nonlytic exocytosis process termed vomocytosis.

Macrophages are well known for their phagocytic activity and their role in innate immune responses. Macrophages eat non-self particles, via a variety of mechanisms, and typically break down internalized cargo into small macromolecules. However, some pathogenic agents have the ability to evade this endosomal degradation through a nonlytic exocytosis process termed vomocytosis. This phenomenon has been most often studied for Cryptococcus neoformans, a yeast that causes roughly 180,000 deaths per year, primarily in immunocompromised (e.g., human immunodeficiency virus [HIV]) patients. Existing dogma purports that vomocytosis involves distinctive cellular pathways and intracellular physicochemical cues in the host cell during phagosomal maturation. Moreover, it has been observed that the immunological state of the individual and macrophage phenotype affect vomocytosis outcomes. Here we compile the current knowledge on the factors (with respect to the phagocytic cell) that promote vomocytosis of C. neoformans from macrophages.

Cryptococcus neoformans and Cryptococcus gattii clinical isolates from Thailand display diverse phenotypic interactions with macrophages

Cryptococcus-macrophage interaction is crucial in the development of cryptococcocal diseases. C. neoformans and C. gattii are major pathogenic species that occupy different niches and cause different clinical manifestations. However, the differences of macrophage interaction among these species in affecting different disease outcomes and immune responses have not been clearly addressed. Here, we examined the macrophage uptake rates, intracellular loads and intracellular proliferation rates of C. neoformans and C. gattii clinical isolates from Thailand and analyzed the effect of those interactions on fungal burdens and host immune responses. C. neoformans isolates showed a higher phagocytosis rate but lower intracellular proliferation rate than C. gattii. Indeed, the high intracellular proliferation rate of C. gattii isolates did not influence the fungal burdens in lungs and brains of infected mice, whereas infection with high-uptake C. neoformans isolates resulted in significantly higher brain burdens that associated with reduced survival rate. Interestingly, alveolar macrophages of mice infected with high-uptake C. neoformans isolates showed distinct patterns of alternatively activated macrophage (M2) gene expressions with higher Arg1, Fizz1, Il13 and lower Nos2, Ifng, Il6, Tnfa, Mcp1, csf2 and Ip10 transcripts. Corresponding to this finding, infection with high-uptake C. neoformans resulted in enhanced arginase enzyme activity, elevated IL-4 and IL-13 and lowered IL-17 in the bronchoalveolar lavage. Thus, our data suggest that the macrophage interaction with C. neoformans and C. gattii may affect different disease outcomes and the high phagocytosis rates of C. neoformans influence the induction of type-2 immune responses that support fungal dissemination and disease progression.

Abbreviation: Arg1: Arginase 1; BAL: Bronchoalveolar lavage; CCL17: Chemokine (C-C motif) ligand 17; CNS: Central nervous system; CSF: Cerebrospinal fluid; Csf2: Colony-stimulating factor 2; Fizz1: Found in inflammatory zone 1; HIV: Human immunodeficiency virus; ICL: Intracellular cryptococcal load; Ifng: Interferon gamma; Ip10: IFN-g-inducible protein 10; IPR: Intracellular proliferation rate; Mcp1: Monocyte chemoattractant protein 1; Nos2: Nitric oxide synthase 2; PBS: Phosphate-Buffered Saline; Th: T helper cell; Tnfa: Tumor necrosis factor alpha.

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.

Phagocytes as central players in the defence against invasive fungal infection

Fungal pathogens cause severe and life-threatening infections worldwide. The majority of invasive infections occurs in immunocompromised patients and is based on acquired as well as congenital defects of innate and adaptive immune responses. In many cases, these defects affect phagocyte functions. Consequently, professional phagocytes - mainly monocytes, macrophages, dendritic cells and polymorphonuclear neutrophilic granulocytes - have been shown to act as central players in initiating and modulating antifungal immune responses as well as elimination of fungal pathogens. In this review we will summarize our current understanding on the role of these professional phagocytes in invasive fungal infection to emphasize two important aspects. (i) Analyses on the interaction between fungi and phagocytes have contributed to significant new insights into phagocyte biology. Important examples for this include the identification of pattern recognition receptors for β-glucan, a major cell wall component of many fungal pathogens, as well as the identification of genetic polymorphisms that determine individual host responses towards invading fungi. (ii) At the same time it was shown that fungal pathogens have evolved sophisticated mechanisms to counteract the attack of professional phagocytes. These mechanisms range from complete mechanical destruction of phagocytes to exquisite adaptation of some fungi to the hostile intracellular environment, enabling them to grow and replicate inside professional phagocytes.

Contribution of IL-1RI Signaling to Protection against Cryptococcus neoformans 52D in a Mouse Model of Infection

Interleukin-1 alpha (IL-1α) and interleukin-1 beta (IL-1β) are pro-inflammatory cytokines that are induced after Cryptococcus neoformans infection and activate the interleukin-1 receptor type I (IL-1RI). To establish the role of IL-1RI signaling in protection against cryptococcal infection, we analyzed wild-type (WT) and IL-1RI-deficient (IL-1RI^−/−) mice on the BALB/c background. IL-1RI^−/− mice had significantly reduced survival compared to WT mice after intratracheal challenge with C. neoformans 52D. Microbiological analysis showed a significant increase in the lung and brain fungal burden of IL-1RI^−/− compared to WT mice beginning at weeks 1 and 4 postinfection, respectively. Histopathology showed that IL-1RI^−/− mice exhibit greater airway epithelial mucus secretion and prominent eosinophilic crystals that were absent in WT mice. Susceptibility of IL-1RI^−/− mice was associated with significant induction of a Th2-biased immune response characterized by pulmonary eosinophilia, M2 macrophage polarization, and recruitment of CD4⁺ IL-13⁺ T cells. Expression of pro-inflammatory [IL-1α, IL-1β, TNFα, and monocyte chemoattractant protein 1 (MCP-1)], Th1-associated (IFNγ), and Th17-associated (IL-17A) cytokines was significantly reduced in IL-1RI^−/− lungs compared to WT. WT mice also had higher expression of KC/CXCL1 and sustained neutrophil recruitment to the lung; however, antibody-mediated depletion of these cells showed that they were dispensable for lung fungal clearance. In conclusion, our data indicate that IL-1RI signaling is required to activate a complex series of innate and adaptive immune responses that collectively enhance host defense and survival after C. neoformans 52D infection in BALB/c mice.

Exploration of Antifungal and Immunomodulatory Potentials of a Furanone Derivative to Rescue Disseminated Cryptococosis in Mice

Cryptococcus neoformans infection is quite complex with both host-pathogen interaction and host immune profile determining disease progress and therapeutic outcome. Hence in the present study, the potential utility of (E)-5-benzylidenedihydrofuran-2(3 H)-one (compound-6) was explored as an effective anticryptococcal compound with immunomodulatory potential. The efficacy of compound-6 in pulmonary cryptococosis model using H99 strain was investigated. The effective dose was found to provide 100% survival, with a significant reduction of yeast burden in lungs and brain. The biodistribution analysis provided evidence for the presence of higher concentration of compound-6 in major organs including lungs and brain. In addition, compound-6 treated mice had significantly higher expression of IL-6, IL-4 and IFN-γ in lung and brain. Similarly, elevated expression of TNF-α, IL-β1 and IL-12 were observed in lungs, suggesting the protective host response against C. neoformans. The reduction and clearance of fungal load in systemic organs and mouse survival are notable results to confirm the ability of compound-6 to treat cryptococcosis. In conclusion, the low molecular weight (174 Da), lipophilicity, its ability to cross blood brain barrier, and facilitating modulation of cytokine expression are the added advantages of compound-6 to combat against disseminated cryptococosis.

Disruption of Early Tumor Necrosis Factor Alpha Signaling Prevents Classical Activation of Dendritic Cells in Lung-Associated Lymph Nodes and Development of Protective Immunity against Cryptococcal Infection

Anti-tumor necrosis factor alpha (anti-TNF-α) therapies have been increasingly used to treat inflammatory diseases and are associated with increased risk of invasive fungal infections, including Cryptococcus neoformans infection. Using a mouse model of cryptococcal infection, we investigated the mechanism by which disruption of early TNF-α signaling results in the development of nonprotective immunity against C. neoformans We found that transient depletion of TNF-α inhibited pulmonary fungal clearance and enhanced extrapulmonary dissemination of C. neoformans during the adaptive phase of the immune response. Higher fungal burdens in TNF-α-depleted mice were accompanied by markedly impaired Th1 and Th17 responses in the infected lungs. Furthermore, early TNF-α depletion also resulted in disrupted transcriptional initiation of the Th17 polarization program and subsequent upregulation of Th1 genes in CD4(+) T cells in the lung-associated lymph nodes (LALN) of C. neoformans-infected mice. These defects in LALN T cell responses were preceded by a dramatic shift from a classical toward an alternative activation of dendritic cells (DC) in the LALN of TNF-α-depleted mice. Taken together, our results indicate that early TNF-α signaling is required for optimal DC activation, and the initial Th17 response followed by Th1 transcriptional prepolarization of T cells in the LALN, which further drives the development of protective immunity against cryptococcal infection in the lungs. Thus, administration of anti-TNF-α may introduce a particularly greater risk for newly acquired fungal infections that require generation of protective Th1/Th17 responses for their containment and clearance.

IMPORTANCE

Increased susceptibility to invasive fungal infections in patients on anti-TNF-α therapies underlines the need for understanding the cellular effects of TNF-α signaling in promoting protective immunity to fungal pathogens. Here, we demonstrate that early TNF-α signaling is required for classical activation and accumulation of DC in LALN of C. neoformans-infected mice. Subsequent transcriptional initiation of Th17 followed by Th1 programming in LALN results in pulmonary accumulation of gamma interferon- and interleukin-17A-producing T cells and effective fungal clearance. All of these crucial steps are severely impaired in mice that undergo anti-TNF-α treatment, consistent with their inability to clear C. neoformans This study identified critical interactions between cells of the innate immune system (DC), the emerging T cell responses, and cytokine networks with a central role for TNF-α which orchestrate the development of the immune protection against cryptococcal infection. This information will be important in aiding development and understanding the potential side effects of immunotherapies.

Interleukin 4 Deficiency Reverses Development of Secondary Pseudomonas aeruginosa Pneumonia During Sepsis-Associated Immunosuppression

BACKGROUND

Interleukin 4 (IL-4) is an important cytokine that may modulate development of secondary bacterial pneumonia during sepsis-induced immunosuppression.

METHODS

We established an experimental model of cecal ligation and puncture (CLP)-induced sublethal polymicrobial sepsis followed by secondary Pseudomonas aeruginosa pulmonary infection,

RESULTS

 IL-4-deficient mice that underwent CLP were resistant to secondary pulmonary P. aeruginosa infection. As compared to wild-type mice, IL-4 knockout (KO) mice displayed improved survival and better bacterial clearance. Furthermore, IL-4 KO mice exhibited enhanced lung inflammation, neutrophil recruitment to airspaces, and elevated pulmonary cytokine production, with significantly increased tumor necrosis factor α (TNF-α) production. Neutralization of TNF-α could reverse the enhanced protection against secondary P. aeruginosa pneumonia in septic IL-4 KO mice, indicating that the resistance of septic IL-4 KO mice to secondary bacterial pneumonia was partially mediated by TNF-α. In addition, IL-4 priming displayed marked impairment of the ability of alveolar macrophages to phagocytose and kill P. aeruginosa in vitro, and this defect was associated with decreased activation of Akt, JNK, p38MAPK, and ERK intracellular signaling pathways by IL-4. Finally, neutralization of IL-4 in septic mice could improve survival and clearance of bacteria from the lungs of septic mice infected with P. aeruginosa.

CONCLUSIONS

Our findings provide new insight for immunopathologic mechanisms of sepsis-induced secondary bacterial pneumonia.

Dectin-2 deficiency promotes Th2 response and mucin production in the lungs after pulmonary infection with Cryptococcus neoformans

Dectin-2 is a C-type lectin receptor that recognizes high mannose polysaccharides. Cryptococcus neoformans, a yeast-form fungal pathogen, is rich in polysaccharides in its cell wall and capsule. In the present study, we analyzed the role of Dectin-2 in the host defense against C. neoformans infection. In Dectin-2 gene-disrupted (knockout) (Dectin-2KO) mice, the clearance of this fungus and the inflammatory response, as shown by histological analysis and accumulation of leukocytes in infected lungs, were comparable to those in wild-type (WT) mice. The production of type 2 helper T (Th2) cytokines in lungs was higher in Dectin-2KO mice than in WT mice after infection, whereas there was no difference in the levels of production of Th1, Th17, and proinflammatory cytokines between these mice. Mucin production was significantly increased in Dectin-2KO mice, and this increase was reversed by administration of anti-interleukin 4 (IL-4) monoclonal antibody (MAb). The levels of expression of β1-defensin, cathelicidin, surfactant protein A (Sp-A), and Sp-D in infected lungs were comparable between these mice. In in vitro experiments, IL-12p40 and tumor necrosis factor alpha (TNF-α) production and expression of CD86 and major histocompatibility complex (MHC) class II by bone marrow-derived dendritic cells and alveolar macrophages were completely abrogated in Dectin-2KO mice. Finally, the disrupted lysates of C. neoformans, but not of whole yeast cells, activated Dectin-2-triggered signaling in an assay with nuclear factor of activated T cells (NFAT)-green fluorescent protein (GFP) reporter cells expressing this receptor. These results suggest that Dectin-2 may oppose the Th2 response and IL-4-dependent mucin production in the lungs after infection with C. neoformans, and it may not be required for the production of Th1, Th17, and proinflammatory cytokines or for clearance of this fungal pathogen.

CD4(+) FoxP3(+) regulatory T cells suppress fatal T helper 2 cell immunity during pulmonary fungal infection

The opportunistic fungal pathogen Cryptococcus neoformans causes lung inflammation and fatal meningitis in immunocompromised patients. Regulatory T (Treg) cells play an important role in controlling immunity and homeostasis. However, their functional role during fungal infection is largely unknown. In this study, we investigated the role of Treg cells during experimental murine pulmonary C. neoformans infection. We show that the number of CD4(+) FoxP3(+) Treg cells in the lung increases significantly within the first 4 weeks after intranasal infection of BALB/c wild-type mice. To define the function of Treg cells we used DEREG mice allowing selective depletion of CD4(+) FoxP3(+) Treg cells by application of diphtheria toxin. In Treg cell-depleted mice, stronger pulmonary allergic inflammation with enhanced mucus production and pronounced eosinophilia, increased IgE production, and elevated fungal lung burden were found. This was accompanied by higher frequencies of GATA-3(+) T helper (Th) 2 cells with elevated capacity to produce interleukin (IL)-4, IL-5, and IL-13. In contrast, only a mild increase in the Th1-associated immune response unrelated to the fungal infection was observed. In conclusion, the data demonstrate that during fungal infection pulmonary Treg cells are induced and preferentially suppress Th2 cells thereby mediating enhanced fungal control.

IL-4 receptor-alpha-dependent control of Cryptococcus neoformans in the early phase of pulmonary infection

Cryptococcus neoformans is an opportunistic fungal pathogen that causes lung inflammation and meningoencephalitis in immunocompromised people. Previously we showed that mice succumb to intranasal infection by induction of pulmonary interleukin (IL)-4Rα-dependent type 2 immune responses, whereas IL-12-dependent type 1 responses confer resistance. In the experiments presented here, IL-4Rα⁻/⁻ mice unexpectedly show decreased fungal control early upon infection with C. neoformans, whereas wild-type mice are able to control fungal growth accompanied by enhanced macrophage and dendritic cell recruitment to the site of infection. Lower pulmonary recruitment of macrophages and dendritic cells in IL-4Rα⁻/⁻ mice is associated with reduced pulmonary expression of CCL2 and CCL20 chemokines. Moreover, IFN-γ and nitric oxide production are diminished in IL-4Rα⁻/⁻ mice compared to wild-type mice. To directly study the potential mechanism(s) responsible for reduced production of IFN-γ, conventional dendritic cells were stimulated with C. neoformans in the presence of IL-4 which results in increased IL-12 production and reduced IL-10 production. Together, a beneficial role of early IL-4Rα signaling is demonstrated in pulmonary cryptococcosis, which contrasts with the well-known IL-4Rα-mediated detrimental effects in the late phase.

Defect of CARD9 leads to impaired accumulation of gamma interferon-producing memory phenotype T cells in lungs and increased susceptibility to pulmonary infection with Cryptococcus neoformans

Caspase recruitment domain-containing protein 9 (CARD9) is an adaptor molecule signal that is critical for NF-κB activation and is triggered through C-type lectin receptors (CLRs), which are pattern recognition receptors that recognize carbohydrate structures. Previous studies have reported that Cryptococcus neoformans, a fungal pathogen that causes meningoencephalitis in AIDS patients, is recognized through some CLRs, such as mannose receptors or DC-SIGN. However, the role of CARD9 in the host defense against cryptococcal infection remains to be elucidated. In the present study, we analyzed the role of CARD9 in the host defense against pulmonary infection with C. neoformans. CARD9 gene-disrupted (knockout [KO]) mice were highly susceptible to this infection, as shown by the reduced fungal clearance in the infected lungs of CARD9 KO mice, compared to that in wild-type (WT) mice. Gamma interferon (IFN-γ) production was strongly reduced in CARD9 KO mice during the innate-immunity phase of infection. Reduced IFN-γ synthesis was due to impaired accumulation of NK and memory phenotype T cells, which are major sources of IFN-γ innate-immunity-phase production; a reduction in the accumulation of these cells was correlated with reduced CCL4, CCL5, CXCL9, and CXCL10 synthesis. However, differentiation of Th17 cells, but not of Th1 cells, was impaired at the adaptive-immunity phase in CARD9 KO mice compared to WT mice, although there was no significant difference in the infection susceptibility between interleukin 17A (IL-17A) KO and WT mice. These results suggest that CARD9 KO mice are susceptible to C. neoformans infection probably due to the reduced accumulation of IFN-γ-expressing NK and memory phenotype T cells at the early stage of infection.

Interleukin-17A enhances host defense against cryptococcal lung infection through effects mediated by leukocyte recruitment, activation, and gamma interferon production

Infection of C57BL/6 mice with the moderately virulent Cryptococcus neoformans strain 52D models the complex adaptive immune response observed in HIV-negative patients with persistent fungal lung infections. In this model, Th1 and Th2 responses evolve over time, yet the contribution of interleukin-17A (IL-17A) to antifungal host defense is unknown. In this study, we show that fungal lung infection promoted an increase in Th17 T cells that persisted to 8 weeks postinfection. Our comparison of fungal lung infection in wild-type mice and IL-17A-deficient mice (IL-17A(-/-) mice; C57BL/6 genetic background) demonstrated that late fungal clearance was impaired in the absence of IL-17A. This finding was associated with reduced intracellular containment of the organism within lung macrophages and deficits in the accumulation of total lung leukocytes, including specific reductions in CD11c+ CD11b+ myeloid cells (dendritic cells and exudate macrophages), B cells, and CD8+ T cells, and a nonsignificant trend in the reduction of lung neutrophils. Although IL-17A did not alter the total number of CD4 T cells, decreases in the total number of CD4 T cells and CD8 T cells expressing gamma interferon (IFN-γ) were observed in IL-17A(-/-) mice. Lastly, expression of major histocompatibility complex class II (MHC-II) and the costimulatory molecules CD80 and CD86 on CD11c+ CD11b+ myeloid cells was diminished in IL-17A(-/-) mice. Collectively, these data indicate that IL-17A enhances host defenses against a moderately virulent strain of C. neoformans through effects on leukocyte recruitment, IFN-γ production by CD4 and CD8 T cells, and the activation of lung myeloid cells.

Macrophage Cryptococcus interactions: an update

Cryptococcus species are fungal pathogens that are a leading cause of mortality. Initial inoculation is through the pulmonary route and, if disseminated, results in severe invasive infection including meningoencephalitis. Macrophages are the dominant phagocytic cell that interacts with Cryptococcus. Emerging theories suggest that Cryptococcus microevolution in macrophages is linked to survival and virulence within the host. In addition, Cryptococcus elaborates virulence factors as well as usurps host machinery to establish macrophage activation states that are permissive to intracellular survival and replication. In this review, we provide an update of the recent findings pertaining to macrophage interaction with Cryptococcus and focus on new avenues for biomedical research.

Scavenger receptor A modulates the immune response to pulmonary Cryptococcus neoformans infection

Scavenger receptors represent an important class of pattern recognition receptors shown to mediate both beneficial and detrimental roles in host defense against microbial pathogens. The role of the major macrophage scavenger receptor, scavenger receptor A (SRA), in the immune response against the pathogenic fungus, Cryptococcus neoformans, is unknown. To evaluate the role of SRA in anticryptococcal host defenses, SRA(+/+) mice and SRA(-/-) mice were infected intratracheally with C. neoformans. Results show that infection of SRA(-/-) mice resulted in a reduction in the pulmonary fungal burden at the efferent phase (3 wk) compared with SRA(+/+) mice. Improved fungal clearance in SRA(-/-) mice was associated with decreased accumulation of eosinophils and greater accumulation of CD4(+) T cells and CD11b(+) dendritic cells. Additional parameters were consistent with enhanced anticryptococcal immunity in the infected SRA(-/-) mice: 1) increased expression of the costimulatory molecules CD80 and CD86 by lung APCs, 2) decreased expression of Th2 cytokines (IL-4 and IL-13) and IL-10 in lung leukocytes and in cryptococcal Ag-pulsed splenocytes, 3) diminished IgE production in sera, and 4) increased hallmarks of classical pulmonary macrophage activation. These effects were preceded by increased expression of early pro-Th1 genes in pulmonary lymph nodes at the afferent phase (1 wk). Collectively, our data show that SRA can be exploited by C. neoformans to interfere with the early events of the afferent responses that support Th1 immune polarization. This results in amplification of Th2 arm of the immune response and subsequently impaired adaptive control of C. neoformans in the infected lungs.

The phenotype of the Cryptococcus-specific CD4+ memory T-cell response is associated with disease severity and outcome in HIV-associated cryptococcal meningitis

Background. Correlates of immune protection in patients with human immunodeficiency virus (HIV)–associated cryptococcal meningitis are poorly defined. A clearer understanding of these immune responses is essential to inform rational development of immunotherapies.

Methods. Cryptococcal-specific peripheral CD4⁺ T-cell responses were measured in 44 patients with HIV-associated cryptococcal meningitis at baseline and during follow-up. Responses were assessed following ex vivo cryptococcal mannoprotein stimulation, using 13-color flow-cytometry. The relationships between cryptococcal-specific CD4⁺ T-cell responses, clinical parameters at presentation, and outcome were investigated.

Results. Cryptococcal-specific CD4⁺ T-cell responses were characterized by the production of macrophage inflammatory protein 1α, interferon γ (IFN-γ), and tumor necrosis factor α (TNF-α). Conversely, minimal interleukin 4 and interleukin 17 production was detected. Patients surviving to 2 weeks had significantly different functional CD4⁺ T-cell responses as compared to those who died. Patients with a response predominantly consisting of IFN-γ or TNF-α production had a 2-week mortality of 0% (0/20), compared with 25% (6/24) in those without this response (P = .025). Such patients also had lower fungal burdens (10 400 vs 390 000 colony-forming units/mL; P < .001), higher cerebrospinal fluid lymphocyte counts (122 vs 8 cells/μL; P < .001), and a trend toward faster rates of clearance of infection.

Conclusions. The phenotype of the peripheral CD4⁺ T-cell response to Cryptococcus was associated with disease severity and outcome in HIV-associated cryptococcal meningitis. IFN-γ/TNF-α–predominant responses were associated with survival.

Immune modulation mediated by cryptococcal laccase promotes pulmonary growth and brain dissemination of virulent Cryptococcus neoformans in mice

C. neoformans is a leading cause of fatal mycosis linked to CNS dissemination. Laccase, encoded by the LAC1 gene, is an important virulence factor implicated in brain dissemination yet little is known about the mechanism(s) accounting for this observation. Here, we investigated whether the presence or absence of laccase altered the local immune response in the lungs by comparing infections with the highly virulent strain, H99 (which expresses laccase) and mutant strain of H99 deficient in laccase (lac1Δ) in a mouse model of pulmonary infection. We found that LAC1 gene deletion decreased the pulmonary fungal burden and abolished CNS dissemination at weeks 2 and 3. Furthermore, LAC1 deletion lead to: 1) diminished pulmonary eosinophilia; 2) increased accumulation of CD4+ and CD8+ T cells; 3) increased Th1 and Th17 cytokines yet decreased Th2 cytokines; and 4) lung macrophage shifting of the lung macrophage phenotype from M2- towards M1-type activation. Next, we used adoptively transferred CD4+ T cells isolated from pulmonary lymph nodes of mice infected with either lac1Δ or H99 to evaluate the role of laccase-induced immunomodulation on CNS dissemination. We found that in comparison to PBS treated mice, adoptively transferred CD4+ T cells isolated from lac1Δ-infected mice decreased CNS dissemination, while those isolated from H99-infected mice increased CNS dissemination. Collectively, our findings reveal that immune modulation away from Th1/Th17 responses and towards Th2 responses represents a novel mechanism through which laccase can contribute to cryptococcal virulence. Furthermore, our data support the hypothesis that laccase-induced changes in polarization of CD4+ T cells contribute to CNS dissemination.

Virulence factors identified by Cryptococcus neoformans mutant screen differentially modulate lung immune responses and brain dissemination

Deletions of cryptococcal PIK1, RUB1, and ENA1 genes independently rendered defects in yeast survival in human CSF and within macrophages. We evaluated virulence potential of these genes by comparing wild-type Cryptococcus neoformans strain H99 with deletant and complement strains in a BALB/c mouse model of pulmonary infection. Survival of infected mice; pulmonary cryptococcal growth and pathology; immunological parameters; dissemination kinetics; and CNS pathology were examined. Deletion of each PIK1, RUB1, and ENA1 differentially reduced pulmonary growth and dissemination rates of C. neoformans and extended mice survival. Furthermore, pik1Δ induced similar pathologies to H99, however, with significantly delayed onset; rub1Δ was more efficiently contained within pulmonary macrophages and was further delayed in causing CNS dissemination/pathology; whereas ena1Δ was progressively eliminated from the lungs and did not induce pathological lesions or disseminate into the CNS. The diminished virulence of mutant strains was associated with differential modulation of pulmonary immune responses, including changes in leukocyte subsets, cytokine responses, and macrophage activation status. Compared to H99 infection, mutants induced more hallmarks of a protective Th1 immune response, rather than Th2, and more classical, rather than alternative, macrophage activation. The magnitude of immunological effects precisely corresponded to the level of virulence displayed by each strain. Thus, cryptococcal PIK1, RUB1, and ENA1 differentially contribute to cryptococcal virulence, in correlation with their differential capacity to modulate immune responses.

Immunotherapy of Cryptococcus infections

Despite appropriate antifungal treatment, the management of cryptococcal disease remains challenging, especially in immunocompromised patients, such as human immunodeficiency virus-infected individuals and solid organ transplant recipients. During the past two decades, our knowledge of host immune responses against Cryptococcus spp. has been greatly advanced, and the role of immunomodulation in augmenting the response to infection has been investigated. In particular, the role of 'protective' Th1 (tumour necrosis factor-α, interferon (IFN)-γ, interleukin (IL)-12, and IL-18) and Th17 (IL-23 and IL-17) and 'non-protective' Th2 (IL-4, IL-10, and IL-13) cytokines has been extensively studied in vitro and in animal models of cryptococcal infection. Immunomodulation with monoclonal antibodies against the capsular polysaccharide glucuronoxylomannan, glucosylceramides, melanin and β-glucan and, lately, with radioimmunotherapy has also yielded promising results in animal models. As a balance between sufficiently protective Th1 responses and excessive inflammation is important for optimal outcome, the effect of immunotherapy may range from beneficial to deleterious, depending on factors related to the host, the infecting organism, and the immunomodulatory regimen. Clinical evidence supporting immunomodulation in patients with cryptococcal infection remains too limited to allow firm recommendations. Limited human data suggest a role for IFN-γ. Identification of surrogate markers characterizing patients' immunological status could possibly suggest candidate patients for immunotherapy and the type of immunomodulation to be administered.

Cryptococcus neoformans suppresses the activation of bone marrow-derived dendritic cells stimulated with its own DNA, but not with DNA from other fungi

DNA from Cryptococcus neoformans activates bone marrow-derived dendritic cells (BM-DCs) in a TLR9-dependent manner. In this study, we examined the effect of the culture supernatants of C. neoformans on the activation of BM-DCs caused by its own DNA. C. neoformans supernatants suppressed IL-12p40, IL-6 production and CD40 expression by BM-DCs stimulated with its own DNA, but not with CpG-ODN and DNA from Candida albicans, Saccharomyces cerevisiae or Escherichia coli. In a confocal microscopic analysis, C. neoformans DNA was colocalized with LAMP-1, a late endosomal marker, and TLR9. The culture supernatants did not show any apparent suppression of these responses. In a luciferase reporter assay, C. neoformans supernatants inhibited NFκB activation caused by its own DNA. These inhibitory activities were attenuated by treatment with heat or trypsin. These results indicate that C. neoformans secrete certain proteinous molecules that suppress the activation of BM-DCs caused by its own DNA.

Cryptococcal interactions with the host immune system

Opportunistic pathogens have become of increasing medical importance over the last decade due to the AIDS pandemic. Not only is cryptococcosis the fourth-most-common fatal infectious disease in sub-Saharan Africa, but also Cryptococcus is an emerging pathogen of immunocompetent individuals. The interaction between Cryptococcus and the host's immune system is a major determinant for the outcome of disease. Despite initial infection in early childhood with Cryptococcus neoformans and frequent exposure to C. neoformans within the environment, immunocompetent individuals are generally able to contain the fungus or maintain the yeast in a latent state. However, immune deficiencies lead to disseminating infections that are uniformly fatal without rapid clinical intervention. This review will discuss the innate and adaptive immune responses to Cryptococcus and cryptococcal strategies to evade the host's defense mechanisms. It will also address the importance of these strategies in pathogenesis and the potential of immunotherapy in cryptococcosis treatment.

Pretransplant cryptococcosis and outcome after liver transplantation

The posttransplant outcomes and optimal management of patients with end-stage liver disease who develop cryptococcosis prior to transplantation have not been defined. We discuss these issues in the context of successful liver transplantation and pretransplant cryptococcal disease. Our report suggests that liver transplantation may be cautiously considered under the umbrella of fluconazole therapy in patients with end-stage liver disease and pretransplant cryptococcosis, provided that disease control is achieved with adequate treatment before transplantation.

Administration of IL-23 engages innate and adaptive immune mechanisms during fungal infection

IL-23 is a key cytokine in promotion of chronic inflammation. Here, we address if its pro-inflammatory potential can be harnessed to protect against chronic cryptococcosis. Mice were infected with Cryptococcus neoformans and treated with recombinant IL-23. Administration of IL-23 led to prolonged survival and reduced fungal burden but was inferior to IL-12 treatment. Independent of endogenous IL-23/IL-12, IL-23 treatment induced an altered cytokine profile accompanied by marked changes in composition of the inflammatory infiltrate characterized by T cell and dendritic cell recruitment. Although IL-23 induced hallmarks of the T(h)17 pathway, also non-T cells produced IL-17A and IL-22. IL-23 treatment of T-cell-deficient mice resulted in increased IL-17A and IL-22 production and modulation of the cellular response at the site of infection with elevated expression of CD86 on macrophages. Our data show that IL-23 treatment induces innate and adaptive tissue inflammation with limited impact on resistance to chronic cryptococcosis.

Robust Th1 and Th17 immunity supports pulmonary clearance but cannot prevent systemic dissemination of highly virulent Cryptococcus neoformans H99

The present study dissected the role of a Th2 bias in pathogenesis of Cryptococcus neoformans H99 infection by comparing inhalational H99 infections in wild-type BALB/c and IL-4/IL-13 double knockout mice. H99-infected wild-type mice showed all major hallmarks of Th2 but not Th1/Th17 immunity in the lungs and lung-associated lymph nodes. In contrast, the IL-4/13(-/-) mice developed robust hallmarks of Th1 and Th17 but not Th2 polarization. The IL-4/IL-13 deletion prevented pulmonary eosinophilia, goblet cell metaplasia in the airways and resulted in elevated serum IgE, and a switch from alternative to classical activation of macrophages. The development of a robust Th1/Th17 response and classical activation of macrophages resulted in significant containment of H99 in the lungs of IL-4/13(-/-) mice compared with unopposed growth of H99 in the lungs of wild-type mice. However, IL-4/13(-/-) mice showed only 1-week longer survival compared with wild-type mice. The comparison of brain and spleen cryptococcal loads at weeks 2, 3, and 4 postinfection revealed that the systemic dissemination in IL-4/13(-/-) mice occurred with an approximate 1-week delay but subsequently progressed with similar rate as in the wild-type mice. Furthermore, wild-type and IL-4/13(-/-) mice developed equivalently severe meningitis/encephalitis at the time of death. These data indicate that the Th2 immune bias is a crucial mechanism for pulmonary virulence of H99, whereas other mechanisms are largely responsible for its central nervous system tropism and systemic dissemination.

Cytokine signaling regulates the outcome of intracellular macrophage parasitism by Cryptococcus neoformans

The pathogenic yeast Cryptococcus neoformans and C. gattii commonly cause severe infections of the central nervous system in patients with impaired immunity but also increasingly in immunocompetent individuals. Cryptococcus is phagocytosed by macrophages but can then survive and proliferate within the phagosomes of these infected host cells. Moreover, Cryptococcus is able to escape into the extracellular environment via a recently discovered nonlytic mechanism (termed expulsion or extrusion). Although it is well established that the host's cytokine profile dramatically affects the outcome of cryptococcal disease, the molecular basis for this effect is unclear. Here, we report a systematic analysis of the influence of Th1, Th2, and Th17 cytokines on the outcome of the interaction between macrophages and cryptococci. We show that Th1 and Th17 cytokines activate, whereas Th2 cytokines inhibit, anticryptococcal functions. Intracellular yeast proliferation was significantly lower after treatment with the Th1 cytokines gamma interferon and tumor necrosis factor alpha and the Th17 cytokine interleukin-17 (IL-17). Interestingly, however, the Th2 cytokines IL-4 and IL-13 significantly increased intracellular yeast proliferation while reducing the occurrence of pathogen expulsion. These results help explain the observed poor prognosis associated with the Th2 cytokine profile (e.g., in human immunodeficiency virus-infected patients).

Virulence in Cryptococcus species

Cryptococcus neoformans and Cryptococcus gattii are the cause of life-threatening meningoencephalitis in immunocompromised and immunocompetent individuals respectively. The increasing incidence of cryptococcal infection as a result of the AIDS epidemic, the recent emergence of a hypervirulent cryptococcal strain in Canada and the fact that mortality from cryptococcal disease remains high have stimulated intensive research into this organism. Here we outline recent advances in our understanding of C. neoformans and C. gattii, including intraspecific complexity, virulence factors, and key signaling pathways. We discuss the molecular basis of cryptococcal virulence and the interaction between these pathogens and the host immune system. Finally, we discuss future challenges in the study and treatment of cryptococcosis.

Cryptococcal urease promotes the accumulation of immature dendritic cells and a non-protective T2 immune response within the lung

Urease, a major virulence factor for Cryptococcus neoformans, promotes lethal meningitis/encephalitis in mice. The effect of urease within the lung, the primary site of most invasive fungal infections, is unknown. An established model of murine infection that utilizes either urease-producing (wt and ure1::URE1) or urease-deficient (ure1) strains (H99) of C. neoformans was used to characterize fungal clearance and the resultant immune response evoked by these strains within the lung. Results indicate that mice infected with urease-producing strains of C. neoformans demonstrate a 100-fold increase in fungal burden beginning 2 weeks post-infection (as compared with mice infected with urease-deficient organisms). Infection with urease-producing C. neoformans was associated with a highly polarized T2 immune response as evidenced by increases in the following: 1) pulmonary eosinophils, 2) serum IgE levels, 3) T2 cytokines (interleukin-4, -13, and -4 to interferon-gamma ratio), and 4) alternatively activated macrophages. Furthermore, the percentage and total numbers of immature dendritic cells within the lung-associated lymph nodes was markedly increased in mice infected with urease-producing C. neoformans. Collectively, these data define cryptococcal urease as a pulmonary virulence factor that promotes immature dendritic cell accumulation and a potent, yet non-protective, T2 immune response. These findings provide new insights into mechanisms by which microbial factors contribute to the immunopathology associated with invasive fungal disease.

IL-4/IL-13-dependent alternative activation of macrophages but not microglial cells is associated with uncontrolled cerebral cryptococcosis

Both interleukin (IL)-4- and IL-13-dependent Th2-mediated immune mechanisms exacerbate murine Cryptococcus neoformans-induced bronchopulmonary disease. To study the roles of IL-4 and IL-13 in cerebral cryptococcosis, IL-4 receptor alpha-deficient (IL-4Ralpha(-/-)), IL-4-deficient (IL-4(-/-)), IL-13-deficient (IL-13(-/-)), IL-13 transgenic (IL-13(T/+)), and wild-type mice were infected intranasally. IL-13(T/+) mice displayed a higher fungal brain burden than wild-type mice, whereas the brain burdens of IL-4Ralpha(-/-), IL-4(-/-), and IL-13(-/-) mice were significantly lower as compared with wild-type mice. On infection, 68% of wild-type mice and 88% of IL-13-overexpressing IL-13(T/+) mice developed significant cerebral lesions. In contrast, only a few IL-4Ralpha(-/-), IL-4(-/-), and IL-13(-/-) mice had small lesions in their brains. Furthermore, IL-13(T/+) mice harbored large pseudocystic lesions in the central nervous system parenchyma, bordered by voluminous foamy alternatively activated macrophages (aaMphs) that contained intracellular cryptococci, without significant microglial activation. In wild-type mice, aaMphs tightly bordered pseudocystic lesions as well, and these mice, in addition, showed microglial cell activation. Interestingly, in resistant IL-4(-/-), IL-13(-/-), and IL-4Ralpha(-/-) mice, no aaMphs were discernible. Microglial cells of all mouse genotypes neither internalized cryptococci nor expressed markers of alternative activation, although they displayed similar IL-4Ralpha expression levels as macrophages. These data provide the first evidence of the development of aaMphs in a central nervous system infectious disease model, pointing to distinct roles of macrophages versus microglial cells in the central nervous system immune response against C. neoformans.

A gene-dosage effect for interleukin-4 receptor alpha-chain expression has an impact on Th2-mediated allergic inflammation during bronchopulmonary mycosis

Interleukin (IL)-4 and IL-13 are key factors in the pathogenesis of bronchopulmonary mycosis induced in mice by infection with Cryptococcus neoformans. Both cytokines use the IL-4 receptor alpha-chain (IL-4Ralpha). In this study, we investigated the role played by IL-4Ralpha expression in susceptibility to pulmonary C. neoformans infection. IL-4Ralpha(-/-) mice were extremely resistant. To characterize the effect of IL-4Ralpha expression level on disease outcome, we generated IL-4Ralpha(+/-) first-generation (F1) mice. IL-4Ralpha(+/-) mice showed intermediate levels of IL-4Ralpha expression, in contrast to higher levels in wild-type mice and no expression in IL-4Ralpha(-/-) mice, indicating biallelic expression of the gene for IL-4Ralpha (Il4ra). Concomitant with intermediate IL-4Ralpha expression, F1 mice showed intermediate susceptibility associated with altered Th2/Th17 cytokine production, decreased immunoglobulin E levels, and reduced allergic inflammation. This indicates a gene-dosage effect of IL-4Ralpha expression on susceptibility to bronchopulmonary mycosis. These data provide the basis for novel therapies antagonizing IL-4Ralpha in Th2-related pulmonary infection and possibly also in asthma.

IL-13 induces disease-promoting type 2 cytokines, alternatively activated macrophages and allergic inflammation during pulmonary infection of mice with Cryptococcus neoformans

In the murine model of Cryptococcus neoformans infection Th1 (IL-12/IFN-gamma) and Th17 (IL-23/IL-17) responses are associated with protection, whereas an IL-4-dependent Th2 response exacerbates disease. To investigate the role of the Th2 cytokine IL-13 during pulmonary infection with C. neoformans, IL-13-overexpressing transgenic (IL-13Tg(+)), IL-13-deficient (IL-13(-/-)), and wild-type (WT) mice were infected intranasally. Susceptibility to C. neoformans infection was found when IL-13 was induced in WT mice or overproduced in IL-13Tg(+) mice. Infected IL-13Tg(+) mice had a reduced survival time and higher pulmonary fungal load as compared with WT mice. In contrast, infected IL-13(-/-) mice were resistant and 89% of these mice survived the entire period of the experiment. Ag-specific production of IL-13 by susceptible WT and IL-13Tg(+) mice was associated with a significant type 2 cytokine shift but only minor changes in IFN-gamma production. Consistent with enhanced type 2 cytokine production, high levels of serum IgE and low ratios of serum IgG2a/IgG1 were detected in susceptible WT and IL-13Tg(+) mice. Interestingly, expression of IL-13 by susceptible WT and IL-13Tg(+) mice was associated with reduced IL-17 production. IL-13 was found to induce formation of alternatively activated macrophages expressing arginase-1, macrophage mannose receptor (CD206), and YM1. In addition, IL-13 production led to lung eosinophilia, goblet cell metaplasia and elevated mucus production, and enhanced airway hyperreactivity. This indicates that IL-13 contributes to fatal allergic inflammation during C. neoformans infection.

Immune response and immunotherapy to Cryptococcus infections

Cryptococcus neoformans is a ubiquitous fungus that can cause lifethreatening infections during immunosuppressive states such as acquired immunodeficiency syndrome (AIDS) and after bone marrow transplantation (BMT). Infected individuals normally succumb to meningitis and meningoencephalitis caused by dissemination of C. neoformans to the brain. In this review, we analyze the current understanding of the interaction between host immune response and C. neoformans as well as the current state of immunotherapeutic strategies for treating cryptococcosis.

Interleukin-4-dependent innate collaboration between iNKT cells and B-1 B cells controls adaptative contact sensitivity

We showed that hepatic Valpha14+ invariant natural killer T (iNKT) cells, via their rapid interleukin (IL)-4 production, activate B-1 cells to initiate contact sensitivity (CS). This innate collaboration was absent in IL-4(-/-) and signal transducer and activator of transcription (STAT)-6(-/-) mice and was inhibited by anti-IL-4 treatment. These mice have defective CS because they fail to locally recruit the sensitized effector T cells of acquired immunity. Their CS is reconstituted by transfer of downstream-acting 1-day immune B-1 cells from wild-type mice. Responses were not reconstituted with B-1 cells from IL-4 receptor-alpha(-/-) or STAT-6(-/-) mice, nor by IL-4 treatment of B cell-deficient mice at immunization. Finally, IL-4 was preferentially and transiently produced by hepatic iNKT cells within 7 min after sensitization to mediate collaboration between innate-like iNKT cells and the B-1 B cells that participate in the recruitment of effector T cells in vivo.

Involvement of CD14, toll-like receptors 2 and 4, and MyD88 in the host response to the fungal pathogen Cryptococcus neoformans in vivo

The major capsular polysaccharide of Cryptococcus neoformans, glucuronoxylomannan (GXM), is recognized by Toll-like receptor 2 (TLR2), TLR4, and CD14. In these studies, mice deficient in CD14, TLR2, TLR4, and the TLR-associated adaptor protein, MyD88, were utilized to investigate the contribution of TLRs and CD14 to in vivo host defenses against C. neoformans. MyD88(-/-) mice had significantly reduced survival compared with wild-type C57BL/6 mice after intranasal (i.n.) and intravenous (i.v.) infection with live C. neoformans. CD14(-/-) mice had reduced survival when infected i.v., while TLR2(-/-) mice died significantly earlier after i.n. infection. Mortality was similar comparing TLR4 mutant C3H/HeJ mice and control C3H/HeOuJ mice following i.v. or i.n. challenge with C. neoformans. The course of pulmonary cryptococcosis was studied in more detail in the CD14(-/-), TLR2(-/-), and MyD88(-/-) mice. MyD88(-/-) mice infected i.n. had higher numbers of CFU in the lungs as well as higher GXM levels in the sera and lungs 7 days after infection than wild-type mice did. Surprisingly, there were no major differences in the levels of tumor necrosis factor alpha, interleukin-4 (IL-4), IL-10, IL-12p70, or gamma interferon in the lungs of C. neoformans-infected knockout mice compared with wild-type mice. Histopathologic analysis of the lungs on day 7 postinfection revealed minimal inflammation in all mouse groups. These studies demonstrate a major role for MyD88 and relatively minor roles for CD14 and TLR2 in the response to cryptococcal infection, with the decreased survival of MyD88(-/-) mice correlating with increased numbers of lung CFU and serum and lung GXM levels.