Effect of cytokine interplay on macrophage polarization during chronic pulmonary infection with Cryptococcus neoformans

In vitro and in vivo effect of PD-1/PD-L1 blockade on microglia/macrophage activation and T cell subset balance in cryptococcal meningitis

This study aimed to investigate the PD-1/ PD-L1 signaling pathway and its effects the activation of microglia/macrophage and balancing T cell subsets in cryptococcal meningitis (CM). A total of 126 CM patients and 126 healthy individuals were recruited for the study. The CM patients were treated with amphotericin B (AmB). Seventy five C57BL/6 mice were grouped into the normal control, CM model, CM + AmB, sham, and CM + PD-1 antibodies (Ab) groups. CD4⁺ and CD8⁺ T cells as well as microglia/macrophages were analyzed by means of flow cytometry. Ionized calcium-binding adaptor molecule 1 (Ibal) expression was detected using western blotting and immunohistochemistry techniques. And the expression of Rab5 and Rab11 were detected using an immunofluorescence assay. Both PD-1 and PD-L1 mRNA and protein expression among the mice in the study were evaluated by qRT-PCR and western blotting methods. Compared to the CM model group, the CM + AmB and CM + PD-1 Ab groups exhibited increased levels of Th1 cytokines and chemokines expression, and reduced levels of Th2 cytokines expressions. Elevated cell purity and viability of CD4⁺ T cell were recorded as well as increases in microglia, however, there were reductions in the number of CD8⁺ T cells. Depleted expressions of Ibal, Rab5, and Rab11 as well as reduced mRNA expressions of PD-1 and PD-L1 in CD4⁺ , microglia, and macrophage cells. The findings suggested that suppression of the PD-1/PD-L1 signaling pathway restricts the proliferation of CM by down-regulating the expressions of Th2 cells and suppressing microglia and macrophage activation.

Adaptive Immunity to Cryptococcus neoformans Infections

The Cryptococcus neoformans/Cryptococcus gattii species complex is a group of fungal pathogens with different phenotypic and genotypic diversity that cause disease in immunocompromised patients as well as in healthy individuals. The immune response resulting from the interaction between Cryptococcus and the host immune system is a key determinant of the disease outcome. The species C. neoformans causes the majority of human infections, and therefore almost all immunological studies focused on C. neoformans infections. Thus, this review presents current understanding on the role of adaptive immunity during C. neoformans infections both in humans and in animal models of disease.

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

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

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

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.

Induction of Broad-Spectrum Protective Immunity against Disparate Cryptococcus Serotypes

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

Anti-PD-1 Antibody Treatment Promotes Clearance of Persistent Cryptococcal Lung Infection in Mice

Activation of immunomodulatory pathways in response to invasive fungi can impair clearance and promote persistent infections. The programmed cell death protein-1 (PD-1) signaling pathway inhibits immune effector responses against tumors, and immune checkpoint inhibitors that block this pathway are being increasingly used as cancer therapy. The objective of this study was to investigate whether this pathway contributes to persistent fungal infection and to determine whether anti-PD-1 Ab treatment improves fungal clearance. Studies were performed using C57BL/6 mice infected with a moderately virulent strain of Cryptococcus neoformans (52D), which resulted in prolonged elevations in fungal burden and histopathologic evidence of chronic lung inflammation. Persistent infection was associated with increased and sustained expression of PD-1 on lung lymphocytes, including a mixed population of CD4⁺ T cells. In parallel, expression of the PD-1 ligands, PD-1 ligands 1 and 2, was similarly upregulated on specific subsets of resident and recruited lung dendritic cells and macrophages. Treatment of persistently infected mice for 4 wk by repetitive administration of neutralizing anti-PD-1 Ab significantly improved pulmonary fungal clearance. Treatment was well tolerated without evidence of morbidity. Immunophenotyping revealed that anti-PD-1 Ab treatment did not alter immune effector cell numbers or myeloid cell activation. Treatment did reduce gene expression of IL-5 and IL-10 by lung leukocytes and promoted sustained upregulation of OX40 by Th1 and Th17 cells. Collectively, this study demonstrates that PD-1 signaling promotes persistent cryptococcal lung infection and identifies this pathway as a potential target for novel immune-based treatments of chronic fungal disease.

Exploitation of Scavenger Receptor, Macrophage Receptor with Collagenous Structure, by Cryptococcus neoformans Promotes Alternative Activation of Pulmonary Lymph Node CD11b+ Conventional Dendritic Cells and Non-Protective Th2 Bias

Macrophage receptor with collagenous structure (MARCO) contributes to fungal containment during the early/innate phase of cryptococcal infection; however, its role in adaptive antifungal immunity remains unknown. Using a murine model of cryptococcosis, we compared host adaptive immune responses in wild-type and MARCO^−/− mice throughout an extended time course post-infection. Unlike in early infection, MARCO deficiency resulted in improved pulmonary fungal clearance and diminished cryptococcal dissemination during the efferent phase. Improved fungal control in the absence of MARCO expression was associated with enhanced hallmarks of protective Th1-immunity, including higher frequency of pulmonary TNF-α-producing T cells, increased cryptococcal-antigen-triggered IFN-γ and TNF-α production by splenocytes, and enhanced expression of M1 polarization genes by pulmonary macrophages. Concurrently, we found lower frequencies of IL-5- and IL-13-producing T cells in the lungs, impaired production of IL-4 and IL-10 by cryptococcal antigen-pulsed splenocytes, and diminished serum IgE, which were hallmarks of profoundly suppressed efferent Th2 responses in MARCO-deficient mice compared to WT mice. Mechanistically, we found that MARCO expression facilitated early accumulation and alternative activation of CD11b⁺ conventional DC (cDC) in the lung-associated lymph nodes (LALNs), which contributed to the progressive shift of the immune response from Th1 toward Th2 at the priming site (LALNs) and local infection site (lungs) during the efferent phase of cryptococcal infection. Taken together, our study shows that MARCO can be exploited by the fungal pathogen to promote accumulation and alternative activation of CD11b⁺ cDC in the LALN, which in turn alters Th1/Th2 balance to promote fungal persistence and dissemination.

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.

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

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

Scavenger Receptor MARCO Orchestrates Early Defenses and Contributes to Fungal Containment during Cryptococcal Infection

The scavenger receptor macrophage receptor with collagenous structure (MARCO) promotes protective innate immunity against bacterial and parasitic infections; however, its role in host immunity against fungal pathogens, including the major human opportunistic fungal pathogen Cryptococcus neoformans, remains unknown. Using a mouse model of C. neoformans infection, we demonstrated that MARCO deficiency leads to impaired fungal control during the afferent phase of cryptococcal infection. Diminished fungal containment in MARCO^-/- mice was accompanied by impaired recruitment of Ly6C^high monocytes and monocyte-derived dendritic cells (moDC) and lower moDC costimulatory maturation. The reduced recruitment and activation of mononuclear phagocytes in MARCO^-/- mice was linked to diminished early expression of IFN-γ along with profound suppression of CCL2 and CCL7 chemokines, providing evidence for roles of MARCO in activation of the CCR2 axis during C. neoformans infection. Lastly, we found that MARCO was involved in C. neoformans phagocytosis by resident pulmonary macrophages and DC. We conclude that MARCO facilitates early interactions between C. neoformans and lung-resident cells and promotes the production of CCR2 ligands. In turn, this contributes to a more robust recruitment and activation of moDC that opposes rapid fungal expansion during the afferent phase of cryptococcal infection.

Dectin-3 Is Not Required for Protection against Cryptococcus neoformans Infection

C-type lectin receptors (CLRs) are diverse, trans-membrane proteins that function as pattern recognition receptors (PRRs) which are necessary for orchestrating immune responses against pathogens. CLRs have been shown to play a major role in recognition and protection against fungal pathogens. Dectin-3 (also known as MCL, Clecsf8, or Clec4d) is a myeloid cell-specific CLR that recognizes mycobacterial trehalose 6,6'-dimycolate (TDM) as well as α-mannans present in the cell wall of fungal pathogens. To date, a potential role for Dectin-3 in the mediation of protective immune responses against C. neoformans has yet to be determined. Consequently, we evaluated the impact of Dectin-3 deficiency on the development of protective immune responses against C. neoformans using an experimental murine model of pulmonary cryptococcosis. Dectin-3 deficiency did not lead to increased susceptibility of mice to experimental pulmonary C. neoformans infection. Also, no significant differences in pulmonary leukocyte recruitment and cytokine production were observed in Dectin-3 deficient mice compared to wild type infected mice. In addition, we observed no differences in uptake and anti-cryptococcal activity of Dectin-3 deficient dendritic cells and macrophages. Altogether, our studies show that Dectin-3 is dispensable for mediating protective immune responses against pulmonary C. neoformans infection.

Immunoregulation in Fungal Diseases

This review addresses specific regulatory mechanisms involved in the host immune response to fungal organisms. We focus on key cells and regulatory pathways involved in these responses, including a brief overview of their broader function preceding a discussion of their specific relevance to fungal disease. Important cell types discussed include dendritic cells and regulatory T cells, with a focus on specific studies relating to their effects on immune responses to fungi. We highlight the interleukin-10, programmed cell death 1, and cytotoxic T lymphocyte-associated protein 4 signaling pathways and emphasize interrelationships between these pathways and the regulatory functions of dendritic cells and regulatory T cells. Throughout our discussion, we identify selected studies best illustrating the role of these cells and pathways in response to specific fungal pathogens to provide a contextual understanding of the tightly-controlled network of regulatory mechanisms critical to determining the outcome of exposure to fungal pathogens. Lastly, we discuss two unique phenomena relating to immunoregulation, protective tolerance and immune reactivation inflammatory syndrome. These two clinically-relevant conditions provide perspective as to the range of immunoregulatory mechanisms active in response to fungi.

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

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

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

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

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

Utilizing Viral Nanoparticle/Dendron Hybrid Conjugates in Photodynamic Therapy for Dual Delivery to Macrophages and Cancer Cells

Photodynamic therapy (PDT) is a promising avenue for greater treatment efficacy of highly resistant and aggressive melanoma. Through photosensitizer attachment to nanoparticles, specificity of delivery can be conferred to further reduce potential side effects. While the main focus of PDT is the destruction of cancer cells, additional targeting of tumor-associated macrophages also present in the tumor microenvironment could further enhance treatment by eliminating their role in processes such as invasion, metastasis, and immunosuppression. In this study, we investigated PDT of macrophages and tumor cells through delivery using the natural noninfectious nanoparticle cowpea mosaic virus (CPMV), which has been shown to have specificity for the immunosuppressive subpopulation of macrophages and also targets cancer cells. We further explored conjugation of CPMV/dendron hybrids in order to improve the drug loading capacity of the nanocarrier. Overall, we demonstrated effective elimination of both macrophage and tumor cells at low micromolar concentrations of the photosensitizer when delivered with the CPMV bioconjugate, thereby potentially improving melanoma treatment.

Innate host defenses against Cryptococcus neoformans

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

Cryptococcus and Phagocytes: Complex Interactions that Influence Disease Outcome

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

Local GM-CSF-Dependent Differentiation and Activation of Pulmonary Dendritic Cells and Macrophages Protect against Progressive Cryptococcal Lung Infection in Mice

Patients with acquired deficiency in GM-CSF are susceptible to infections with Cryptococcus neoformans and other opportunistic fungi. We previously showed that GM-CSF protects against progressive fungal disease using a murine model of cryptococcal lung infection. To better understand the cellular and molecular mechanisms through which GM-CSF enhances antifungal host defenses, we investigated temporal and spatial relationships between myeloid and lymphoid immune responses in wild-type C57BL/6 mice capable of producing GM-CSF and GM-CSF-deficient mice infected with a moderately virulent encapsulated strain of C. neoformans (strain 52D). Our data demonstrate that GM-CSF deficiency led to a reduction in: 1) total lung leukocyte recruitment; 2) Th2 and Th17 responses; 3) total numbers of CD11b(+) dendritic cells (DC) and CD11b(-) and CD11b(+) macrophages (Mϕ); 4) DC and Mϕ activation; and 5) localization of DC and Mϕ to the microanatomic sites of alveolar infection. In contrast, GM-CSF deficiency resulted in increased accumulation of DC and Mϕ precursors, namely Ly-6C(high) monocytes, in the blood and lungs of infected mice. Collectively, these results show that GM-CSF promotes the local differentiation, accumulation, activation, and alveolar localization of lung DC and Mϕ in mice with cryptococcal lung infection. These findings identify GM-CSF as central to the protective immune response that prevents progressive fungal disease and thus shed new light on the increased susceptibility to these infections observed in patients with acquired GM-CSF deficiency.

[Up-to-date findings in the host defence mechanism to cryptococcus infection]

Cryptococcus neoformans is a medically important opportunistic fungal pathogen with a polysaccharide capsule surrounding the yeast-like cells. In hosts with impaired cell-mediated immunity such as AIDS, uncontrolled infection causes life-threatening meningoencephalitis. In immunocompetent individuals, the host immune response usually limits the growth of the fungal pathogen at the primary infected site, where it may persist, without completely eradicated, in a latent state because of its ability to escape from killing by macrophages. Th1 response in adaptive immunity is essential for the host defense to cryptococcal infection, in which interferon (IFN)-γ polarizes innate macrophages into fungicidal M1 macrophages. Recently, we found that caspase recruitment domain family member (CARD9), an adaptor protein in a signal transduction triggered by C-type lectin receptors, plays a key role in the early production of IFN-γ at the site of infection by recruiting NK cells and CD4(+) and CD8(+) memory-phenotype T cells. We also found that IL-4 produced by Th2 cells stimulates broncoepithelial cells to secrete mucin, which may lead to promotion in the mucociliary clearance of C. neoformans. Here, we summarize the up-to-date findings in the host defense mechanism to this infection with focusing on our recent data.

The Effects of Paracoccidioides brasiliensis Infection on GM-CSF- and M-CSF-Induced Mouse Bone Marrow-Derived Macrophage from Resistant and Susceptible Mice Strains

Considering the importance of macrophages as the first line of defense against fungal infection and the different roles played by the two M1- and M2-like polarized macrophages, we decided to evaluate the effects of Paracoccidioides brasiliensis infection on GM-CSF- and M-CSF-induced bone marrow-derived macrophages (BMM) from the A/J and B10.A mouse strains, an established model of resistance/susceptibility to PCM, respectively. Upon differentiation, the generated GM- or M-BMMs were characterized by morphological analyses, gene expression profiles, and cytokines production. Our main results demonstrate that GM-BMMs derived from A/J and B.10 produced high levels of pro- and anti-inflammatory cytokines that may contribute to generate an unbalanced early immune response. In accordance with the literature, the B10.A susceptible mice lineage has an innate tendency to polarize into M1-like phenotype, whereas the opposite phenotype occurs in A/J resistance mice. In this context, our data support that susceptibility and resistance are strongly correlated with M1 and M2 polarization, respectively.

Cryptococcus neoformans Infection in Mice Lacking Type I Interferon Signaling Leads to Increased Fungal Clearance and IL-4-Dependent Mucin Production in the Lungs

Type I interferons (IFNs) are secreted by many cell types upon stimulation via pattern recognition receptors and bind to IFN-α/β receptor (IFNAR), which is composed of IFNAR1 and IFNAR2. Although type I IFNs are well known as anti-viral cytokines, limited information is available on their role during fungal infection. In the present study, we addressed this issue by examining the effect of IFNAR1 defects on the host defense response to Cryptococcus neoformans. In IFNAR1KO mice, the number of live colonies was lower and the host immune response mediated not only by Th1 but also by Th2 and Th17-related cytokines was more accelerated in the infected lungs than in WT mice. In addition, mucin production by bronchoepithelial cells and expression of MUC5AC, a major core protein of mucin in the lungs, were significantly higher in IFNAR1KO mice than in WT mice. This increase in mucin and MUC5AC production was significantly inhibited by treatment with neutralizing anti-IL-4 mAb. In contrast, administration of recombinant IFN-αA/D significantly suppressed the production of IL-4, but not of IFN-γ and IL-17A, in the lungs of WT mice after cryptococcal infection. These results indicate that defects of IFNAR1 led to improved clearance of infection with C. neoformans and enhanced synthesis of IFN-γ and the IL-4-dependent production of mucin. They also suggest that type I IFNs may be involved in the negative regulation of early host defense to this infection.

The Cnes2 locus on mouse chromosome 17 regulates host defense against cryptococcal infection through pleiotropic effects on host immunity

The genetic basis of natural susceptibility to progressive Cryptococcus neoformans infection is not well understood. Using C57BL/6 and CBA/J inbred mice, we previously identified three chromosomal regions associated with C. neoformans susceptibility (Cnes1, Cnes2, and Cnes3). To validate and characterize the role of Cnes2 during the host response, we constructed a congenic strain on the C57BL/6 background (B6.CBA-Cnes2). Phenotypic analysis of B6.CBA-Cnes2 mice 35 days after C. neoformans infection showed a significant reduction of fungal burden in the lungs and spleen with higher pulmonary expression of gamma interferon (IFN-γ) and interleukin-12 (IL-12), lower expression of IL-4, IL-5, and IL-13, and an absence of airway epithelial mucus production compared to that in C57BL/6 mice. Multiparameter flow cytometry of infected lungs also showed a significantly higher number of neutrophils, exudate macrophages, CD11b(+) dendritic cells, and CD4(+) cells in B6.CBA-Cnes2 than in C57BL/6 mice. The activation state of recruited macrophages and dendritic cells was also significantly increased in B6.CBA-Cnes2 mice. Taken together, these findings demonstrate that the Cnes2 interval is a potent regulator of host defense, immune responsiveness, and differential Th1/Th2 polarization following C. neoformans infection.

STAT1 signaling within macrophages is required for antifungal activity against Cryptococcus neoformans

Cryptococcus neoformans, the predominant etiological agent of cryptococcosis, is an opportunistic fungal pathogen that primarily affects AIDS patients and patients undergoing immunosuppressive therapy. In immunocompromised individuals, C. neoformans can lead to life-threatening meningoencephalitis. Studies using a virulent strain of C. neoformans engineered to produce gamma interferon (IFN-γ), denoted H99γ, demonstrated that protection against pulmonary C. neoformans infection is associated with the generation of a T helper 1 (Th1)-type immune response and signal transducer and activator of transcription 1 (STAT1)-mediated classical (M1) macrophage activation. However, the critical mechanism by which M1 macrophages mediate their anti-C. neoformans activity remains unknown. The current studies demonstrate that infection with C. neoformans strain H99γ in mice with macrophage-specific STAT1 ablation resulted in severely increased inflammation of the pulmonary tissue, a dysregulated Th1/Th2-type immune response, increased fungal burden, deficient M1 macrophage activation, and loss of protection. STAT1-deficient macrophages produced significantly less nitric oxide (NO) than STAT1-sufficient macrophages, correlating with an inability to control intracellular cryptococcal proliferation, even in the presence of reactive oxygen species (ROS). Furthermore, macrophages from inducible nitric oxide synthase knockout mice, which had intact ROS production, were deficient in anticryptococcal activity. These data indicate that STAT1 activation within macrophages is required for M1 macrophage activation and anti-C. neoformans activity via the production of NO.

Prostaglandin E2 blockade enhances the pulmonary anti-Cryptococcus neoformans immune reaction via the induction of TLR-4

The present study aimed to explore whether the inhibition of prostaglandin E2 enhances pulmonary anti-Cryptococcus neoformans immunity. Lung colony forming unit (CFU) assays demonstrated that the cryptococcal infection was dramatically depressed in mice given EP2 and EP4 or single EP antagonist treatment compared to the untreated wild type mice (p<0.05), leading to the increased survival of the infected mice by 8-9 days or 2-4 days, respectively. RT-PCR and flow cytometry assays showed that the expression of IFN-γ, IL-17, IL-22 in M1 macrophages and IL-10 in M2 macrophages increased significantly at 1 week post-infection in mice with either EP2 or EP4 blockade (p<0.05). The polarization of alveolar macrophages showed that, at 1 week post infection, the alveolar macrophages in untreated wild type mice, TLR4(-/-) mice and TLR4(-/-) mice with EP2 and EP4 blockade were strongly M2 polarized, whereas the alveolar macrophages in wild type mice with EP2 and EP4 blockade were M1 polarized. In conclusion, the blockade of EP2 and EP4 promotes mouse survival after cryptococcus infection by promoting the production of cytokines via TLR4, as well as the enhanced M1 polarization of alveolar macrophages.

Paradoxical Immune Responses in Non-HIV Cryptococcal Meningitis

The fungus Cryptococcus is a major cause of meningoencephalitis in HIV-infected as well as HIV-uninfected individuals with mortalities in developed countries of 20% and 30%, respectively. In HIV-related disease, defects in T-cell immunity are paramount, whereas there is little understanding of mechanisms of susceptibility in non-HIV related disease, especially that occurring in previously healthy adults. The present description is the first detailed immunological study of non-HIV-infected patients including those with severe central nervous system (s-CNS) disease to 1) identify mechanisms of susceptibility as well as 2) understand mechanisms underlying severe disease. Despite the expectation that, as in HIV, T-cell immunity would be deficient in such patients, cerebrospinal fluid (CSF) immunophenotyping, T-cell activation studies, soluble cytokine mapping and tissue cellular phenotyping demonstrated that patients with s-CNS disease had effective microbiological control, but displayed strong intrathecal expansion and activation of cells of both the innate and adaptive immunity including HLA-DR+ CD4+ and CD8+ cells and NK cells. These expanded CSF T cells were enriched for cryptococcal-antigen specific CD4+ cells and expressed high levels of IFN-γ as well as a lack of elevated CSF levels of typical T-cell specific Th2 cytokines -- IL-4 and IL-13. This inflammatory response was accompanied by elevated levels of CSF NFL, a marker of axonal damage, consistent with ongoing neurological damage. However, while tissue macrophage recruitment to the site of infection was intact, polarization studies of brain biopsy and autopsy specimens demonstrated an M2 macrophage polarization and poor phagocytosis of fungal cells. These studies thus expand the paradigm for cryptococcal disease susceptibility to include a prominent role for macrophage activation defects and suggest a spectrum of disease whereby severe neurological disease is characterized by immune-mediated host cell damage.

Cryptococcus is an important cause of fungal meningitis with significant mortality globally. Susceptibility to the fungus in humans has been related to T-lymphocyte defects in HIV-infected individuals, but little is known about possible immune defects in non HIV-infected patients including previously healthy individuals. This latter group also has some of the worst response rates to therapy with almost a third dying in the United States, despite available therapy. Here we conducted the first detailed immunological analysis of non-HIV apparently immunocompetent individuals with active cryptococcal disease. In contrast to HIV-infected individuals, these studies identified a highly activated antigen-presenting dendritic cell population within CSF, accompanied by a highly active T-lymphocyte population with potentially damaging inflammatory cytokine responses. Furthermore, elevated levels of CSF neurofilament light chains (NFL), a marker of axonal damage in severe central nervous system infections suggest a dysfunctional role to this acute inflammatory state. Paradoxically, CSF macrophage proportions were reduced in patients with severe disease and biopsy and autopsy samples identified alternatively activated tissue macrophage populations that failed to appropriately phagocytose fungal cells. Our study thus provides new insights into the susceptibility to human cryptococcal disease and identifies a paradoxically active T-lymphocyte response that may be amenable to adjunctive immunomodulation to improve treatment outcomes in this high-mortality disease.

Cryptococcal heat shock protein 70 homolog Ssa1 contributes to pulmonary expansion of Cryptococcus neoformans during the afferent phase of the immune response by promoting macrophage M2 polarization

Numerous virulence factors expressed by Cryptococcus neoformans modulate host defenses by promoting nonprotective Th2-biased adaptive immune responses. Prior studies demonstrate that the heat shock protein 70 homolog, Ssa1, significantly contributes to serotype D C. neoformans virulence through the induction of laccase, a Th2-skewing and CNS tropic factor. In the present study, we sought to determine whether Ssa1 modulates host defenses in mice infected with a highly virulent serotype A strain of C. neoformans (H99). To investigate this, we assessed pulmonary fungal growth, CNS dissemination, and survival in mice infected with either H99, an SSA1-deleted H99 strain (Δssa1), and a complement strain with restored SSA1 expression (Δssa1::SSA1). Mice infected with the Δssa1 strain displayed substantial reductions in lung fungal burden during the innate phase (days 3 and 7) of the host response, whereas less pronounced reductions were observed during the adaptive phase (day 14) and mouse survival increased only by 5 d. Surprisingly, laccase activity assays revealed that Δssa1 was not laccase deficient, demonstrating that H99 does not require Ssa1 for laccase expression, which explains the CNS tropism we still observed in the Ssa1-deficient strain. Lastly, our immunophenotyping studies showed that Ssa1 directly promotes early M2 skewing of lung mononuclear phagocytes during the innate phase, but not the adaptive phase, of the immune response. We conclude that Ssa1's virulence mechanism in H99 is distinct and laccase-independent. Ssa1 directly interferes with early macrophage polarization, limiting innate control of C. neoformans, but ultimately has no effect on cryptococcal control by adaptive immunity.

Cerebrospinal fluid cytokine profiles predict risk of early mortality and immune reconstitution inflammatory syndrome in HIV-associated cryptococcal meningitis

Understanding the host immune response during cryptococcal meningitis (CM) is of critical importance for the development of immunomodulatory therapies. We profiled the cerebrospinal fluid (CSF) immune-response in ninety patients with HIV-associated CM, and examined associations between immune phenotype and clinical outcome. CSF cytokine, chemokine, and macrophage activation marker concentrations were assayed at disease presentation, and associations between these parameters and microbiological and clinical outcomes were examined using principal component analysis (PCA). PCA demonstrated a co-correlated CSF cytokine and chemokine response consisting primarily of Th1, Th2, and Th17-type cytokines. The presence of this CSF cytokine response was associated with evidence of increased macrophage activation, more rapid clearance of Cryptococci from CSF, and survival at 2 weeks. The key components of this protective immune-response were interleukin (IL)-6 and interferon-γ, IL-4, IL-10 and IL-17 levels also made a modest positive contribution to the PC1 score. A second component of co-correlated chemokines was identified by PCA, consisting primarily of monocyte chemotactic protein-1 (MCP-1) and macrophage inflammatory protein-1α (MIP-1α). High CSF chemokine concentrations were associated with low peripheral CD4 cell counts and CSF lymphocyte counts and were predictive of immune reconstitution inflammatory syndrome (IRIS). In conclusion CSF cytokine and chemokine profiles predict risk of early mortality and IRIS in HIV-associated CM. We speculate that the presence of even minimal Cryptococcus-specific Th1-type CD4+ T-cell responses lead to increased recruitment of circulating lymphocytes and monocytes into the central nervous system (CNS), more effective activation of CNS macrophages and microglial cells, and faster organism clearance; while high CNS chemokine levels may predispose to over recruitment or inappropriate recruitment of immune cells to the CNS and IRIS following peripheral immune reconstitution with ART. These results provide a rational basis for future studies of immune modulation in CM, and demonstrate the potential of baseline immune profiling to identify CM patients most at risk of mortality and subsequent IRIS.

Cryptococcal meningitis is a severe opportunistic infection, estimated to kill several hundred thousand HIV-infected individuals each year. One of the factors contributing to this high death toll is the inadequacy of antifungal treatments. As few novel antifungal drugs are being developed, several groups have started to investigate the potential of immune modulation, with treatments designed to change the patient’s immune response to infection. However, our understanding of the immune response to cryptococcal infection in HIV-infected patients, and how these responses impact on clinical outcomes, is limited. In this study, we took advantage of the fact that we can sample cerebrospinal fluid (CSF) from the site of the infection in patients when they develop cryptococcal meningitis. We undertook a detailed analysis measuring levels of immune response parameters in the CSF of these patients, and demonstrated that there were several distinct components of the immune response. Variations in these responses were associated with both the rate at which patients cleared their infection during treatment, and with mortality. Our results provide a basis for the development of future immunomodulatory therapies, and may allow identification of patients most at risk of dying, enabling more intensive treatments to be given to those at highest risk.

Macrophage mitochondrial and stress response to ingestion of Cryptococcus neoformans

Human infection with Cryptococcus neoformans, a common fungal pathogen, follows deposition of yeast spores in the lung alveoli. The subsequent host-pathogen interaction can result in eradication, latency, or extrapulmonary dissemination. Successful control of C. neoformans infection is dependent on host macrophages, but macrophages display little ability to kill C. neoformans in vitro. Recently, we reported that ingestion of C. neoformans by mouse macrophages induces early cell cycle progression followed by mitotic arrest, an event that almost certainly reflects host cell damage. The goal of the present work was to understand macrophage pathways affected by C. neoformans toxicity. Infection of macrophages by C. neoformans was associated with alterations in protein translation rate and activation of several stress pathways, such as hypoxia-inducing factor-1-α, receptor-interacting protein 1, and apoptosis-inducing factor. Concomitantly we observed mitochondrial depolarization in infected macrophages, an observation that was replicated in vivo. We also observed differences in the stress pathways activated, depending on macrophage cell type, consistent with the nonspecific nature of C. neoformans virulence known to infect phylogenetically distant hosts. Our results indicate that C. neoformans infection impairs multiple host cellular functions and undermines the health of these critical phagocytic cells, which can potentially interfere with their ability to clear this fungal pathogen.

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.

Protective phenotypes of club cells and alveolar macrophages are favored as part of endotoxin-mediated prevention of asthma

Atopic asthma is a chronic allergic disease that involves T-helper type 2 (Th2)-inflammation and airway remodeling. Bronchiolar club cells (CC) and alveolar macrophages (AM) are sentinel cells of airway barrier against inhaled injuries, where allergy induces mucous metaplasia of CC and the alternative activation of AM, which compromise host defense mechanisms and amplify Th2-inflammation. As there is evidence that high levels of environmental endotoxin modulates asthma, the goal of this study was to evaluate if the activation of local host defenses by Lipopolysaccharide (LPS) previous to allergy development can contribute to preserving CC and AM protective phenotypes. Endotoxin stimulus before allergen exposition reduced hallmarks of allergic inflammation including eosinophil influx, Interleukin-4 and airway hyperreactivity, while the T-helper type 1 related cytokines IL-12 and Interferon-γ were enhanced. This response was accompanied by the preservation of the normal CC phenotype and the anti-allergic proteins Club Cell Secretory Protein (CCSP) and Surfactant-D, thereby leading to lower levels of CC metaplasia and preventing the increase of the pro-Th2 cytokine Thymic stromal lymphopoietin. In addition, classically activated alveolar macrophages expressing nitric oxide were promoted over the alternatively activated ones that expressed arginase-1. We verified that LPS induced a long-term overexpression of CCSP and the innate immune markers Toll-like receptor 4, and Tumor Necrosis Factor-α, changes that were preserved in spite of the allergen challenge. These results demonstrate that LPS pre-exposition modifies the local bronchioalveolar microenvironment by inducing natural anti-allergic mechanisms while reducing local factors that drive Th2 type responses, thus modulating allergic inflammation.

TH1/TH2 paradigm extended: macrophage polarization as an unappreciated pathogen-driven escape mechanism?

The classical view of the Th1/Th2 paradigm posits that the pathogen nature, infectious cycle, and persistence represent key parameters controlling the choice of effector mechanisms operating during an immune response. Thus, efficient Th1 responses are triggered by replicating intracellular pathogens, while Th2 responses would control helminth infection and promote tissue repair during the resolution phase of an infectious event. However, this vision does not account for a growing body of data describing how pathogens exploit the polarization of the host immune response to their own benefit. Recently, the study of macrophages has illustrated a novel aspect of this arm race between pathogens and the immune system, and the central role of macrophages in homeostasis, repair and defense of all tissues is now fully appreciated. Like T lymphocytes, macrophages differentiate into distinct effectors including classically (M1) and alternatively (M2) activated macrophages. Interestingly, in addition to represent immune effectors, M1/M2 cells have been shown to represent potential reservoir cells to a wide range of intracellular pathogens. Subversion of macrophage cell metabolism by microbes appears as a recently uncovered immune escape strategy. Upon infection, several microbial agents have been shown to activate host metabolic pathways leading to the production of nutrients necessary to their long-term persistence in host. The purpose of this review is to summarize and discuss the strategies employed by pathogens to manipulate macrophage differentiation, and in particular their basic cell metabolism, to favor their own growth while avoiding immune control.

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

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

Early or late IL-10 blockade enhances Th1 and Th17 effector responses and promotes fungal clearance in mice with cryptococcal lung infection

The potent immunoregulatory properties of IL-10 can counteract protective immune responses and, thereby, promote persistent infections, as evidenced by studies of cryptococcal lung infection in IL-10-deficient mice. To further investigate how IL-10 impairs fungal clearance, the current study used an established murine model of C57BL/6J mice infected with Cryptococcus neoformans strain 52D. Our results demonstrate that fungal persistence is associated with an early and sustained expression of IL-10 by lung leukocytes. To examine whether IL-10-mediated immune modulation occurs during the early or late phase of infection, assessments of fungal burden and immunophenotyping were performed on mice treated with anti-IL-10R-blocking Ab at 3, 6, and 9 d postinfection (dpi) (early phase) or at 15, 18, and 21 dpi (late phase). We found that both early and late IL-10 blockade significantly improved fungal clearance within the lung compared with isotype control treatment when assessed 35 dpi. Immunophenotyping identified that IL-10 blockade enhanced several critical effector mechanisms, including increased accumulation of CD4(+) T cells and B cells, but not CD8(+) T cells; specific increases in the total numbers of Th1 and Th17 cells; and increased accumulation and activation of CD11b(+) dendritic cells and exudate macrophages. Importantly, IL-10 blockade effectively abrogated dissemination of C. neoformans to the brain. Collectively, this study identifies early and late cellular and molecular mechanisms through which IL-10 impairs fungal clearance and highlights the therapeutic potential of IL-10 blockade in the treatment of fungal lung infections.

STAT1 signaling is essential for protection against Cryptococcus neoformans infection in mice

Nonprotective immune responses to highly virulent Cryptococcus neoformans strains, such as H99, are associated with Th2-type cytokine production, alternatively activated macrophages, and inability of the host to clear the fungus. In contrast, experimental studies show that protective immune responses against cryptococcosis are associated with Th1-type cytokine production and classical macrophage activation. The protective response induced during C. neoformans strain H99γ (C. neoformans strain H99 engineered to produce murine IFN-γ) infection correlates with enhanced phosphorylation of the transcription factor STAT1 in macrophages; however, the role of STAT1 in protective immunity to C. neoformans is unknown. The current studies examined the effect of STAT1 deletion in murine models of protective immunity to C. neoformans. Survival and fungal burden were evaluated in wild-type and STAT1 knockout (KO) mice infected with either strain H99γ or C. neoformans strain 52D (unmodified clinical isolate). Both strains H99γ and 52D were rapidly cleared from the lungs, did not disseminate to the CNS, or cause mortality in the wild-type mice. Conversely, STAT1 KO mice infected with H99γ or 52D had significantly increased pulmonary fungal burden, CNS dissemination, and 90-100% mortality. STAT1 deletion resulted in a shift from Th1 to Th2 cytokine bias, pronounced lung inflammation, and defective classical macrophage activation. Pulmonary macrophages from STAT1 KO mice exhibited defects in NO production correlating with inefficient inhibition of fungal proliferation. These studies demonstrate that STAT1 signaling is essential not only for regulation of immune polarization but also for the classical activation of macrophages that occurs during protective anticryptococcal immune responses.

Classical versus alternative macrophage activation: the Ying and the Yang in host defense against pulmonary fungal infections

Macrophages are innate immune cells that possess unique abilities to polarize toward different phenotypes. Classically activated macrophages are known to have major roles in host defense against various microbial pathogens, including fungi, while alternatively activated macrophages are instrumental in immune-regulation and wound healing. Macrophages in the lungs are often the first responders to pulmonary fungal pathogens, and the macrophage polarization state has the potential to be a deciding factor in disease progression or resolution. This review discusses the distinct macrophage polarization states and their roles during pulmonary fungal infection. We focus primarily on Cryptococcus neoformans and Pneumocystis model systems as disease resolution of these two opportunistic fungal pathogens is linked to classically or alternatively activated macrophages, respectively. Further research considering macrophage polarization states that result in anti-fungal activity has the potential to provide a novel approach for the treatment of fungal infections.

Development of optical probes for in vivo imaging of polarized macrophages during foreign body reactions

Plasticity of macrophage (MΦ) phenotypes exist in a spectrum from classically activated (M1) cells, to alternatively activated (M2) cells, contributing to both the normal healing of tissues and the pathogenesis of implant failure. Here, folate- and mannose-based optical probes were fabricated to simultaneously determine the degree of MΦ polarization. In vitro tests show the ability of these probes to specifically target M1 and M2 cells. In an in vivo murine model, they were able to distinguish between the M1-dominated inflammatory response to infection and the M2-dominated regenerative response to particle implants. Finally, the probes were used to assess the inflammatory/regenerative properties of biomaterial implants. Our results show that these probes can be used to monitor and quantify the dynamic processes of MΦ polarization and their role in cellular responses in real time.

Macrophage activation state determines the response to rhinovirus infection in a mouse model of allergic asthma

Background

The mechanisms by which viruses cause asthma exacerbations are not precisely known. Previously, we showed that, in ovalbumin (OVA)-sensitized and -challenged mice with allergic airway inflammation, rhinovirus (RV) infection increases type 2 cytokine production from alternatively-activated (M2) airway macrophages, enhancing eosinophilic inflammation and airways hyperresponsiveness. In this paper, we tested the hypothesis that IL-4 signaling determines the state of macrophage activation and pattern of RV-induced exacerbation in mice with allergic airways disease.

Methods

Eight week-old wild type or IL-4 receptor knockout (IL-4R KO) mice were sensitized and challenged with OVA and inoculated with RV1B or sham HeLa cell lysate.

Results

In contrast to OVA-treated wild-type mice with both neutrophilic and eosinophilic airway inflammation, OVA-treated IL-4R KO mice showed increased neutrophilic inflammation with few eosinophils in the airways. Like wild-type mice, IL-4R KO mice showed OVA-induced airway hyperreactivity which was further exacerbated by RV. There was a shift in lung cytokines from a type 2-predominant response to a type 1 response, including production of IL-12p40 and TNF-α. IL-17A was also increased. RV infection of OVA-treated IL-4R KO mice further increased neutrophilic inflammation. Bronchoalveolar macrophages showed an M1 polarization pattern and ex vivo RV infection increased macrophage production of TNF-α, IFN-γ and IL-12p40. Finally, lung cells from OVA-treated IL-4R KO mice showed reduced CD206+ CD301+ M2 macrophages, decreased IL-13 and increased TNF-α and IL-17A production by F4/80+, CD11b+ macrophages.

Conclusions

OVA-treated IL-4R KO mice show neutrophilic airway inflammation constituting a model of allergic, type 1 cytokine-driven neutrophilic asthma. In the absence of IL-4/IL-13 signaling, RV infection of OVA-treated mice increased type 1 cytokine and IL-17A production from conventionally-activated macrophages, augmenting neutrophilic rather than eosinophilic inflammation. In mice with allergic airways inflammation, IL-4R signaling determines macrophage activation state and the response to subsequent RV infection.

Elucidating the immunological function of the Cryptococcus neoformans capsule

The encapsulated fungal pathogen Cryptococcus neoformans represents a significant agent of life-threatening infections in immunocompromised subjects. A unique characteristic of Cryptococcus species is the presence of a polysaccharide capsule, which is essential for virulence and endows Cryptococcus with potent immunoregulatory properties. This review provides an overview of the immunological properties of the principal components of C. neoformans capsule.

PI3K/Akt signaling pathway modulates influenza virus induced mouse alveolar macrophage polarization to M1/M2b

Macrophages polarized to M1 (pro-inflammation) or M2 (anti-inflammation) phenotypes in response to environmental signals. In this study, we examined the polarization of alveolar macrophage (AM), following induction by different influenza virus strains (ST169 (H1N1), ST602 (H3N2) and HKG9 (H9N2)). Macrophages from other tissues or cell line exert alternative responding pattern, and AM is necessary for investigating the respiratory system. AM polarized toward the M1 phenotype after 4 hours of infection by all three virus strains, and AM to presented M2b phenotype after 8 hours induction, and immunosuppressive phenotype after 24 hours of induction. Protein expression assay showed similar results as the gene expression analysis for phenotype verification. The ELISA assay showed that TNF-α secretion was up-regulated after 4 and 8 hours of infection by influenza viruses, and it returned to basal levels after 24 hours of infection. IL-10 expression was elevated after 8 and 24 hours of infection. Immunofluorescence showed that iNOS expression was up-regulated but not Arg1 expression. Influenza virus notably increased phospho-Akt but not phospho-Erk1/2 or phospho-p38, and the AM polarization pattern have been changed by LY294002 (PI3K inhibitor). In conclusion, our results demonstrate the dynamic polarization of AM induced by influenza viruses, and suggested that PI3K/Akt signaling pathway modulates AM polarization to M1/M2b.

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 polarization in response to wear particles in vitro

Total joint replacement is a highly successful surgical procedure for treatment of patients with disabling arthritis and joint dysfunction. However, over time, with high levels of activity and usage of the joint, implant wear particles are generated from the articulating surfaces. These wear particles can lead to activation of an inflammatory reaction, and subsequent bone resorption around the implant (periprosthetic osteolysis). Cells of the monocyte/macrophage lineage orchestrate this chronic inflammatory response, which is dominated by a pro-inflammatory (M1) macrophage phenotype rather than an anti-inflammatory pro-tissue healing (M2) macrophage phenotype. While it has been shown that interleukin-4 (IL-4) selectively polarizes macrophages towards an M2 anti-inflammatory phenotype which promotes bone healing, rather than inflammation, little is known about the time course in which this occurs or conditions in which repolarization through IL-4 is most effective. The goal of this work was to study the time course of murine macrophage polarization and cytokine release in response to challenge with combinations of polymethyl methacrylate (PMMA) particles, lipopolysaccharide (LPS) and IL-4 in vitro. Treatment of particle-challenged monocyte/macrophages with IL-4 led to an initial suppression of pro-inflammatory cytokines and inducible nitric oxide synthase (iNOS) production and subsequent polarization into an M2 anti-inflammatory phenotype. This result was optimized when IL-4 was delivered before PMMA particle challenge, to an M1 phenotype rather than to uncommitted (M0) macrophages. The effects of this polarization were sustained over a 5-day time course. Polarization of M1 macrophages into an M2 phenotype may be a strategy to mitigate wear particle associated periprosthetic osteolysis.

The intracellular life of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen with worldwide distribution. Serological studies of human populations show a high prevalence of human infection, which rarely progresses to disease in immunocompetent hosts. However, decreased host immunity places individuals at high risk for cryptococcal disease. The disease can result from acute infection or reactivation of latent infection, in which yeasts within granulomas and host macrophages emerge to cause disease. In this review, we summarize what is known about the cellular recognition, ingestion, and killing of C. neoformans and discuss the unique and remarkable features of its intracellular life, including the proposed mechanisms for fungal persistence and killing in phagocytic cells.

Macrophage M1/M2 polarization dynamically adapts to changes in cytokine microenvironments in Cryptococcus neoformans infection

The outcome of cryptococcal pneumonia correlates with local macrophage polarization status, as M1 and M2 polarization marks protective and nonprotective responses, respectively. Overall, pulmonary macrophage polarization status changes over time during a cryptococcal infection. This could have been caused by repolarization of individual macrophages or by a replacement of M2-polarized cells by new M1-polarized cells. To explore the ability of macrophages to change between polarization states, we conducted a series of experiments using in vitro macrophages. Coculture of macrophages with Cryptococcus neoformans resulted in development of a weak M1-like phenotype, with modestly increased inducible nitric oxide synthase (iNOS) but lacking interleukin 6 (IL-6) induction. The C. neoformans-induced M1-like polarization state was plastic, as macrophages stimulated first with C. neoformans and then with gamma interferon (IFN-γ) or IL-4 expressed mRNA polarization patterns similar to those stimulated with cytokines alone. To further evaluate macrophage polarization plasticity, cytokine stimulatory conditions were established which fully polarized macrophages. IFN-γ and IL-4 stimulation differentially induced complete M1 and M2 polarization, defined by differential expression of marker mRNA panels, surface marker expression, and tumor necrosis factor alpha (TNF-α) protein production. Switching IFN-γ- to IL-4-stimulating conditions, and vice versa, resulted in uniform changes in profiles of polarization marker genes consistent with the most recent cytokine environment. Furthermore, the ability of sequentially stimulated macrophages to inhibit C. neoformans reflected the most recent polarizing condition, independent of previous polarization. Collectively, these data indicate that M1/M2 macrophage polarization phenotypes are highly plastic to external signals, and interventions which therapeutically repolarize macrophages could be beneficial for treatment of cryptococcosis.

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.

Is T-helper type 2 shift schizophrenia-specific? Primary results from a comparison of related psychiatric disorders and healthy controls

AIM

An imbalance between T-helper type 1 (Th1) and type 2 (Th2) cytokines has been implicated in schizophrenia, although empirical evidence is rare. The aim of this study was to examine if a Th1/Th2 imbalance occurs in schizophrenia and schizophrenia-related disorder.

METHODS

Twenty-six subjects with schizophrenia, 26 subjects with schizophrenia-related disorders, and 26 healthy controls were recruited. The Human Th1/Th2 Cytokine Cytometric Bead Array Kit-II was utilized to assess serum Th1/Th2 cytokines and ratios simultaneously. MANOVA was used to detect differences among the three diagnostic groups in distinct Th1/Th2 cytokines/ratios. Pearson/Spearman correlations were used to examine the relationships between distinct Th1/Th2 cytokines/ratios and clinical/psychopathological data in schizophrenia.

RESULTS

Interferon (IFN)-γ/interleukin (IL)-4, IFN-γ/IL-10, IL-2/IL-4, and tumor necrosis factor (TNF)-α/IL-4 ratios were significantly decreased in schizophrenia, but not in schizophrenia-related disorders compared to healthy controls. IFN-γ/IL-4 and IFN-γ/IL-10 in schizophrenia subjects positively correlated with age, but not in schizophrenia-related disorder subjects or in healthy controls.

CONCLUSION

A clear Th2 shift was observed in schizophrenia, but not in schizophrenia-related disorders. The Th2 shift in schizophrenia appeared to be an aberrant developmental phenomenon.

Abrogation of IL-4 receptor-α-dependent alternatively activated macrophages is sufficient to confer resistance against pulmonary cryptococcosis despite an ongoing T(h)2 response

In the murine model of pulmonary infection with Cryptococcus neoformans, IL-4 receptor α (IL-4Rα)-dependent polyfunctional T(h)2 cells induce disease progression associated with alternative activation of lung macrophages. To characterize the effector role of IL-4Rα-dependent alternatively activated macrophages (aaMph), we intra-nasally infected mice with genetically ablated IL-4Rα expression on macrophages (LysM(Cre)IL-4Rα(-/lox) mice) and IL-4Rα(-/lox) littermates. LysM(Cre)IL-4Rα(-/lox) mice were significantly more resistant to pulmonary cryptococcosis with higher survival rates and lower lung burden than non-deficient heterozygous littermates. Infected LysM(Cre)IL-4Rα(-/lox) mice had reduced but detectable numbers of aaMph expressing arginase-1, chitinase-like enzyme (YM1) and CD206. Similar pulmonary expression of inducible nitric oxide synthase was found in LysM(Cre)IL-4Rα(-/lox) and IL-4Rα(-/lox) control mice, but macrophages from LysM(Cre)IL-4Rα(-/lox) mice showed a higher potential to produce nitric oxide. In contrast to the differences in the macrophage phenotype, pulmonary T(h)2 responses were similar in infected LysM(Cre)IL-4Rα(-/lox) and IL-4Rα(-/lox) mice with each mouse strain harboring polyfunctional T(h)2 cells. Consistently, type 2 pulmonary allergic inflammation associated with eosinophil recruitment and epithelial mucus production was present in lungs of both LysM(Cre)IL-4Rα(-/lox) and IL-4Rα(-/lox) mice. Our results demonstrate that, despite residual IL-4Rα-independent alternative macrophage activation and ongoing T(h)2-dependent allergic inflammation, abrogation of IL-4Rα-dependent aaMph is sufficient to confer resistance in pulmonary cryptococcosis. This is even evident on a relatively resistant heterozygous IL-4Rα(+/-) background indicating a key contribution of macrophage IL-4Rα expression to susceptibility in allergic bronchopulmonary mycosis.

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.

Cryptococcus neoformans growth and protection from innate immunity are dependent on expression of a virulence-associated DEAD-box protein, Vad1

The fungus Cryptococcus neoformans has emerged as a major cause of meningoencephalitis worldwide. Host response to the fungus involves both innate and adaptive immunity, but fungal genes that modulate these processes are poorly understood. Previous studies demonstrated attenuated virulence of a mutant of a virulence-associated DEAD-box protein (VAD1) in mice, despite normal growth at host temperatures, suggesting modulation of the immune response. In the present study, the Δvad1 mutant demonstrated progressive clearance from lung and was unable to induce pathological lesions or to cause extrapulmonary disease, despite retaining its ability to grow in mouse serum and a J774.16 macrophage cell line. Pulmonary clearance occurred with a minimal cellular infiltrate, marked by reduced CD4 cells, CD11b(+) Ly6C(high) monocytes, and F4/80(+) macrophages, but the mutant strain retained recruitment of CD8 cells, compared to infections with wild-type fungi. Adaptive cytokine responses were reduced, including Th1, Th2, and Th17 cytokines; however, early gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) responses were retained while nonprotective interleukin 4 (IL-4) and IL-5 were diminished. Furthermore, the Δvad1 mutant was controlled in lungs despite CD4/CD8 cell depletion. These data, along with improved phagocytosis by macrophages and increases in early/innate IL-1α, IFN-γ, and chemokines elicited in the lungs within 3 days of infection with the Δvad1 mutant, indicate that VAD1 expression reduces innate recognition of C. neoformans, rendering the yeast resistant to elimination by the innate mechanisms of host defense. Thus, our studies define a novel role of the cryptococcal Vad1 protein as a central regulator of cryptococcal virulence and illustrate that Vad1 promotes microbe resistance to innate host defenses.