Toll-like receptor 4 mediates intracellular signaling without TNF-alpha release in response to Cryptococcus neoformans polysaccharide capsule

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.

Toll-like receptors (TLR2 and TLR4) recognize polysaccharides of Pseudallescheria boydii cell wall

Pseudallescheria boydii is an opportunistic fungus widespread in the environment, and has recently emerged as an agent of localized as well as disseminated infections in both immunocompromised and immunocompetent hosts. The host response to fungi is in part dependent on the activation of evolutionary conserved receptors including Toll-like receptors and phagocytic receptors. This review will discuss the isolation and structural characterization of α-glucans and rhamnomannans from P. boydii cell wall and their roles in the induction of innate immune response.

Capsular Material of Cryptococcus neoformans: Virulence and Much More

The capsule is generally considered one of the more powerful virulence factors of microorganisms, driving research in the field of microbial pathogenesis and in the development of vaccines. Cryptococcus neoformans is unique among the most common human fungal pathogens in that it possesses a complex polysaccharide capsule. This review focuses on the Cryptococcus neoformans capsule from the viewpoint of fungal pathogenesis, and the effective immune response target of the capsule's main component, glucuronoxylomannan.

Host defenses against cryptococcosis

The interaction of pathogenic Cryptococcus species with their various hosts is somewhat unique compared to other fungal pathogens such as Aspergillus fumigatus and Candida albicans. Cryptococcus shares an intimate association with host immune cells, leading to enhanced intracellular growth. Furthermore, unlike most other fungal pathogens, the signs and symptoms of cryptococcal disease are typically self-inflicted by the host during the host's attempt to clear this invader from sensitive organ systems such as the central nervous system. In this review, we will summarize the story of host-Cryptococcus interactions to date and explore strategies to exploit the current knowledge for treatment of cryptococcal infections.

Adaptive immunity to fungi

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

Fungal surface and innate immune recognition of filamentous fungi

The innate immune system performs specific detection of molecules from infectious agents through pattern recognition receptors. This recognition triggers inflammatory responses and activation of microbicidal mechanisms by leukocytes. Infections caused by filamentous fungi have increased in incidence and represent an important cause of mortality and morbidity especially in individuals with immunosuppression. This review will discuss the innate immune recognition of filamentous fungi molecules and its importance to infection control and disease.

Immunological variation between inbred laboratory mouse strains: points to consider in phenotyping genetically immunomodified mice

Inbred laboratory mouse strains are highly divergent in their immune response patterns as a result of genetic mutations and polymorphisms. The generation of genetically engineered mice (GEM) has, in the past, used embryonic stem (ES) cells for gene targeting from various 129 substrains followed by backcrossing into more fecund mouse strains. Although common inbred mice are considered "immune competent," many have variations in their immune system-some of which have been described-that may affect the phenotype. Recognition of these immune variations among commonly used inbred mouse strains is essential for the accurate interpretation of expected phenotypes or those that may arise unexpectedly. In GEM developed to study specific components of the immune system, accurate evaluation of immune responses must take into consideration not only the gene of interest but also how the background strain and microbial milieu contribute to the manifestation of findings in these mice. This article discusses points to consider regarding immunological differences between the common inbred laboratory mouse strains, particularly in their use as background strains in GEM.

The presence of 3-hydroxy oxylipins in pathogenic microbes

There is a sufficient body of work documenting the distribution of 3-hydroxy oxylipins in microbes. However, there is limited information on the role of these compounds in microbial pathogenesis. When derived from mammalian cells, these compounds regulate patho-biological processes, thus an understanding of 3-hydroxy oxylipin function and metabolism could prove important in shedding light on how these compounds mediate cellular pathology and physiology. This could present 3-hydroxy oxylipin biosynthetic pathways as targets for drug development. In this minireview, we interrogate the relevant yeast and bacterial 3-hydroxy oxylipin literature in order to appreciate how these compounds may influence the inflammatory response leading to disease development.

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

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

Inhibition of HIV entry by extracellular glucuronoxylomannan of Cryptococcus neoformans

Cryptococcosis, caused by Cryptococcus neoformans, is the most common opportunistic fungal disease in HIV/AIDS patients. The prognosis of AIDS patients with Cryptococcus infection is very poor. One of the major characteristics in cryptococcosis patients is the presence of high concentrations of the cryptococcal capsule polysaccharide (CCP) in the serum and cerebrospinal fluid. CCP enhances HIV replication in H9 T-cells, but the mechanism is unknown. In this study, we tested whether extracellular glucuronoxylomannan (GXM), a major component of CCP, enhances HIV entry using replication-incompetent HIV and a cell line which expresses a stable amount of CD4 and both of the HIV co-receptors. Extracellular GXM had no effect on cell-cell fusion however; viral entry surprisingly was inhibited by GXM. Hence, any enhancement of replication must be due to an effect that occurs post-entry.

Cryptococcus-Related Immune Reconstitution Inflammatory Syndrome(IRIS): Pathogenesis and Its Clinical Implications

This review provides an overview of Cryptococcus neoformans immunology and focuses on the pathogenesis of Cryptococcus-related paradoxical immune reconstitution inflammatory syndrome (IRIS). Cryptococcal IRIS has three phases: (1) before antiretroviral therapy (ART), with a paucity of cerebrospinal fluid (CSF) inflammation and defects in antigen clearance; (2) during initial ART immune recovery, with pro-inflammatory signaling by antigen-presenting cells without an effector response; and (3) at IRIS, a cytokine storm with a predominant type-1 helper T-cell (Th(1)) interferon-gamma (IFN-γ) response. Understanding IRIS pathogenesis allows for risk stratification and customization of HIV/AIDS care. In brief, persons at high IRIS risk may benefit from enhancing microbiologic clearance by use of adjunctive agents in combination with amphotericin, prolonging initial induction therapy, and/or increasing the initial consolidation antifungal therapy dose to at least 800 mg of fluconazole daily until the 2-week CSF culture is known to be sterile. Prophylactic anti-inflammatory therapies or undue delay of ART initiation in an attempt to prevent IRIS is unwarranted and may be dangerous.

Association of polymorphisms in TLR genes and in genes of the Toll-like receptor signaling pathway with cancer risk

Toll-like receptors (TLRs) constitute a family of receptors directly recognizing a wide spectrum of exogenous and endogenous ligands playing the key role in realization of innate and adaptive immune response, and participating in the processes of cell proliferation, survival, apoptosis, angiogenesis, tissue remodeling and repair. Polymorphisms in TLR genes may shift balance between pro- and anti-inflammatory cytokines, modulating the risk of infection, chronic inflammation and cancer. The short list of TLR polymorphisms perspective for oncogenomic investigations can include rs10008492, rs4833103, rs5743815, rs11466657, rs7696175 (TLR1-TLR6-TLR10 gene cluster); rs3804100, rs4696480, -196 - -174 del (Delta22), GT-microsatellite polymorphism (TLR2); 829A/C (TLR3); rs5743836, rs352140 (TLR9). The extended list can additionally include rs4833095 rs5743551, rs5743618 (TLR1); rs5743704, rs62323857, rs1219178642 (TLR2); rs5743305, rs3775291, rs121434431, rs5743316 (TLR3); rs5744168 (TLR5); rs179008 (TLR7); rs3764880, rs2407992 (TLR8); rs352139, rs187084, rs41308230, rs5743844 (TLR9); rs4129009 (TLR10). General reasons for discrepancies between studies are insufficiency of sample size, age/gender/BMI/ethnic/racial differences, differences in prevalence of infectious agent in case and control groups, differences in immune response caused by specific ligand, differences in stratification, methods of diagnostics of cancer or chronic inflammatory conditions, genotyping methods, and chance. Future well-designed studies on large samples should shed light on the significance of TLR polymorphisms for cancer prevention.

Vaccines for invasive fungal infections

Morbidity and mortality from invasive fungal infections remain unacceptably high despite availability of new antifungal agents, underscoring the need for more effective preventative strategies. Due to our enhanced understanding of the host defense and pathogenetic mechanisms that lead to invasive fungal infections, it should be feasible to develop vaccines targeting these infections. A common immunological theme across many vaccine candidates for invasive fungal infections has been the need to activate a cell-based, pro-inflammatory, Th1 or Th17 immune response to improve phagocytic killing of the fungi. Since neutralization of virulence factor functions has not been required for many active vaccines to function, the antigenic repertoire available for testing should not be limited to virulence factors. With expansion of our fundamental understanding of the immunology of fungal infections, the biggest barrier to development of fungal vaccines is the lack of available capital to translate discoveries made at the bench into biological agents used at the bedside. Continued education on the importance and feasibility of vaccination for such infections, combined with continued development of vaccine antigens and adjuvants, is necessary.

Ovarian cancer stem cells and inflammation

Epithelial ovarian cancer (EOC) is the fourth leading cause of cancer-related deaths in women in the United States and the leading cause of gynecologic cancer deaths. The major limiting factor in the treatment of ovarian cancer is recurrence and chemoresistance. Individuals who succumb to advanced-stage ovarian cancer inevitably become refractory to chemotherapy, resulting in disease progression and death. The source of recurrence and lack of response to chemotherapy is unknown. The focus of this review is to evaluate the question of recurrence and chemoresistance based on the concept of the cancer stem cells and inflammation.

Cracking the Toll-like receptor code in fungal infections

Innate control of fungal infection requires the specific recognition of invariant fungal molecular structures by a variety of innate immune receptors, including Toll-like receptors. In addition to the role in inducing protective immune responses, Toll-like receptor engagement may paradoxically favor fungal infections, by inducing inflammatory pathology and impairing antifungal immunity. Although the dissection of complex genetic traits modulating susceptibility to fungal infections is complex, the contribution of host genetics may hold the key to elucidating new risk factors for these severe, often fatal diseases. Understanding host-pathogen interactions at the innate immune interface will eventually lead to the development of new therapeutics and genetic markers in fungal infections.

NF-κB in immunobiology

NF-κB was first discovered and characterized 25 years ago as a key regulator of inducible gene expression in the immune system. Thus, it is not surprising that the clearest biological role of NF-κB is in the development and function of the immune system. Both innate and adaptive immune responses as well as the development and maintenance of the cells and tissues that comprise the immune system are, at multiple steps, under the control of the NF-κB family of transcription factors. Although this is a well-studied area of NF-κB research, new and significant findings continue to accumulate. This review will focus on these areas of recent progress while also providing a broad overview of the roles of NF-κB in mammalian immunobiology.

Neisseria meningitidis capsular polysaccharides induce inflammatory responses via TLR2 and TLR4-MD-2

CPS are major virulence factors in infections caused by Neisseria meningitidis and form the basis for meningococcal serogroup designation and protective meningococcal vaccines. CPS polymers are anchored in the meningococcal outer membrane through a 1,2-diacylglycerol moiety, but the innate immunostimulatory activity of CPS is largely unexplored. Well-established human and murine macrophage cell lines and HEK/TLR stably transfected cells were stimulated with CPS, purified from an endotoxin-deficient meningococcal serogroup B NMB-lpxA mutant. CPS induced inflammatory responses via TLR2- and TLR4-MD-2. Meningococcal CPS induced a dose-dependent release of cytokines (TNF-α, IL-6, IL-8, and CXCL10) and NO from human and murine macrophages, respectively. CPS induced IL-8 release from HEK cells stably transfected with TLR2/6, TLR2, TLR2/CD14, and TLR4/MD-2/CD14 but not HEK cells alone. mAb to TLR2 but not an isotype control antibody blocked CPS-induced IL-8 release from HEK-TLR2/6-transfected cells. A significant reduction in TNF-α and IL-8 release was seen when THP-1- and HEK-TLR4/MD-2-CD14- but not HEK-TLR2- or HEK-TLR2/6-transfected cells were stimulated with CPS in the presence of Eritoran (E5564), a lipid A antagonist that binds to MD-2, and a similar reduction in NO and TNF-α release was also seen in RAW 264.7 cells in the presence of Eritoran. CD14 and LBP enhanced CPS bioactivity, and NF-κB was, as anticipated, the major signaling pathway. Thus, these data suggest that innate immune recognition of meningococcal CPS by macrophages can occur via TLR2- and TLR4-MD-2 pathways.

TLR4 recognizes Pseudallescheria boydii conidia and purified rhamnomannans

Pseudallescheria boydii (Scedosporium apiospermum) is a saprophytic fungus widespread in the environment, and has recently emerged as an agent of localized as well as disseminated infections, particularly mycetoma, in immunocompromised and immunocompetent hosts. We have previously shown that highly purified α-glucan from P. boydii activates macrophages through Toll-like receptor TLR2, however, the mechanism of P. boydii recognition by macrophage is largely unknown. In this work, we investigated the role of innate immune receptors in the recognition of P. boydii. Macrophages responded to P. boydii conidia and hyphae with secretion of proinflammatory cytokines. The activation of macrophages by P. boydii conidia required functional MyD88, TLR4, and CD14, whereas stimulation by hyphae was independent of TLR4 and TLR2 signaling. Removal of peptidorhamnomannans from P. boydii conidia abolished induction of cytokines by macrophages. A fraction highly enriched in rhamnomannans was obtained and characterized by NMR, high performance TLC, and GC-MS. Preparation of rhamnomannans derived from P. boydii triggered cytokine release by macrophages, as well as MAPKs phosphorylation and IκBα degradation. Cytokine release induced by P. boydii-derived rhamnomannans was dependent on TLR4 recognition and required the presence of non-reducing end units of rhamnose of the rhamnomannan, but not O-linked oligosaccharides from the peptidorhamnomannan. These results imply that TLR4 recognizes P. boydii conidia and this recognition is at least in part due to rhamnomannans expressed on the surface of P. boydii.

Immunogenetics of invasive aspergillosis

Invasive aspergillosis is one of the most important infections in hematopoietic stem cell transplant recipients, with an incidence rate of 5-15% and an associated mortality of 30-60%. It remains unclear why certain patients develop invasive aspergillosis while others, undergoing identical transplant regimen and similar post transplant immunosuppression, do not. Over the last decade, pattern recognition receptors such as Toll-like receptors (TLRs) and the C-type lectin receptors (CLRs) have emerged as critical components of the innate immune system. By detecting specific molecular patterns from invading microbes and initiating inflammatory and subsequent adaptive immune responses, pattern recognition receptors are strategically located at the molecular interface of hosts and pathogens. Polymorphisms in pattern recognition receptors and downstream signaling molecules have been associated with increased or decreased susceptibility to infections, suggesting that their detection may have an increasing impact on the treatment and prevention of infectious diseases in the coming years. Infectious risk stratification may be particularly relevant for patients with hematologic malignancies, because of the high prevalence and severity of infections in this population. This review summarizes the innate immune mechanisms involved in Aspergillus fumigatus detection and the role of host genetic polymorphisms in susceptibility to invasive aspergillosis.

Distinct roles for Dectin-1 and TLR4 in the pathogenesis of Aspergillus fumigatus keratitis

Aspergillus species are a major worldwide cause of corneal ulcers, resulting in visual impairment and blindness in immunocompetent individuals. To enhance our understanding of the pathogenesis of Aspergillus keratitis, we developed a murine model in which red fluorescent protein (RFP)-expressing A. fumigatus (Af293.1RFP) conidia are injected into the corneal stroma, and disease progression and fungal survival are tracked over time. Using Mafia mice in which c-fms expressing macrophages and dendritic cells can be induced to undergo apoptosis, we demonstrated that the presence of resident corneal macrophages is essential for production of IL-1beta and CXCL1/KC, and for recruitment of neutrophils and mononuclear cells into the corneal stroma. We found that beta-glucan was highly expressed on germinating conidia and hyphae in the cornea stroma, and that both Dectin-1 and phospho-Syk were up-regulated in infected corneas. Additionally, we show that infected Dectin-1(-/-) corneas have impaired IL-1beta and CXCL1/KC production, resulting in diminished cellular infiltration and fungal clearance compared with control mice, especially during infection with clinical isolates expressing high beta-glucan. In contrast to Dectin 1(-/-) mice, cellular infiltration into infected TLR2(-/-), TLR4(-/-), and MD-2(-/-) mice corneas was unimpaired, indicating no role for these receptors in cell recruitment; however, fungal killing was significantly reduced in TLR4(-/-) mice, but not TLR2(-/-) or MD-2(-/-) mice. We also found that TRIF(-/-) and TIRAP(-/-) mice exhibited no fungal-killing defects, but that MyD88(-/-) and IL-1R1(-/-) mice were unable to regulate fungal growth. In conclusion, these data are consistent with a model in which beta-glucan on A.fumigatus germinating conidia activates Dectin-1 on corneal macrophages to produce IL-1beta, and CXCL1, which together with IL-1R1/MyD88-dependent activation, results in recruitment of neutrophils to the corneal stroma and TLR4-dependent fungal killing.

TLR9 signaling is required for generation of the adaptive immune protection in Cryptococcus neoformans-infected lungs

To determine whether TLR9 signaling contributes to the development of the adaptive immune response to cryptococcal infection, wild-type (TLR9+/+) and TLR9 knockout (TLR9-/-) BALB/c mice were infected intratracheally with 10(4) C. neoformans 52D. We evaluated 1) organ microbial burdens, 2) pulmonary leukocyte recruitment, 3) pulmonary and systemic cytokine induction, and 4) macrophage activation profiles. TLR9 deletion did not affect pulmonary growth during the innate phase, but profoundly impaired pulmonary clearance during the adaptive phase of the immune response (a 1000-fold difference at week 6). The impaired clearance in TLR9-/- mice was associated with: 1) significantly reduced CD4(+), CD8+ T cell, and CD19+ B cell recruitment into the lungs; 2) defects in Th polarization indicated by altered cytokine responses in the lungs, lymphonodes, and spleen; and 3) diminished macrophage accumulation and altered activation profile, including robust up-regulation of Arg1 and FIZZ1 (indicators of alternative activation) and diminished induction of inducible nitric oxide synthase (an indicator of classical activation). Histological analysis revealed defects in granuloma formation and increased numbers of intracellular yeast residing within macrophages in the lungs of TLR9-/- mice. We conclude that TLR9 signaling plays an important role in the development of robust protective immunity, proper recruitment and function of effector cells (lymphocytes and macrophages), and, ultimately, effective cryptococcal clearance from the infected lungs.

Susceptibility of TLR4-defective C3H/HeJ mice to Coccidioides posadasii infection

Coccidioides posadasii is one of the two fungal pathogens that cause coccidioidomycosis. The inhalation of air-borne arthroconidia leads to the formation of endospore-forming spherules in the lungs and pulmonary infection. In severe condition, the endospores are disseminated to other non-pulmonary organs in the body. The Toll-like receptors (TLR) expressed by a number of immune and non-immune cells can significantly impact the host defense and susceptibility to C. posadasii infection. In this study, we infected TLR4-defective C3H/HeJ mice with a sublethal dose of C. posadasii and studied fungal dissemination, mortality and humoral response. We also measured IL-12 cytokine secreted by C. posadasii-infected dendritic cells. We found that the C3H/HeJ mice were equally susceptible to C. posadasii as compared to C3H/OuJ mice which have intact TLR4. No significant changes were observed in pulmonary fungal load, survival and humoral response. The blockade of TLR4 did not affect C. posadasii-induced IL-12 secretion. However, the fungal counts were 10 times less in spleens of C3H/HeJ mice as compared to C3H/OuJ mice (P<0.05). Our results suggest that the TLR4 may not be involved in inducing protective host defense against C. posadasii, but it appears to be critical for fungal dissemination.

Immunomodulatory effects of serotype B glucuronoxylomannan from Cryptococcus gattii correlate with polysaccharide diameter

Glucuronoxylomannan (GXM), the major capsular component in the Cryptococcus complex, interacts with the immune system in multiple ways, which include the activation of Toll-like receptors (TLRs) and the modulation of nitric oxide (NO) production by phagocytes. In this study, we analyzed several structural parameters of GXM samples from Cryptococcus neoformans (serotypes A and D) and Cryptococcus gattii (serotypes B and C) and correlated them with the production of NO by phagocytes and the activation of TLRs. GXM fractions were differentially recognized by TLR2/TLR1 (TLR2/1) and TLR2/6 heterodimers expressed on TLR-transfected HEK293A cells. Higher NF-kappaB luciferase reporter activity induced by GXM was observed in cells expressing TLR2/1 than in cells transfected with TLR2/6 constructs. A serotype B GXM from C. gattii was the most effective polysaccharide fraction activating the TLR-mediated response. This serotype B polysaccharide, which was also highly efficient at eliciting the production of NO by macrophages, was similar to the other GXM samples in monosaccharide composition, zeta potential, and electrophoretic mobility. However, immunofluorescence with four different monoclonal antibodies and dynamic light-scattering analysis revealed that the serotype B GXM showed particularities in serological reactivity and had the smallest effective diameter among the GXM samples analyzed in this study. Fractionation of additional serotype B GXMs, followed by exposure of these fractions to macrophages, revealed a correlation between NO production and reduced effective diameters. Our results demonstrate a great functional diversity in GXM samples from different isolates and establish their abilities to differentially activate cellular responses. We propose that serological properties as well as physical chemical parameters, such as the diameter of polysaccharide molecules, may potentially influence the inflammatory response against Cryptococcus spp. and may contribute to the differences in granulomatous inflammation between cryptococcal species.

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.

Innate immune function of the neisserial porins and the relationship to vaccine adjuvant activity

Neisseria meningitidis is a Gram-negative pathogenic bacteria responsible for bacterial meningitis and septicemia. Porins are the most represented outer membrane proteins in the pathogenic Neisseria species, functioning as pores for the exchange of ions, and are characterized by a trimeric beta-barrel structure. Neisserial porins have been shown to act as adjuvants in the immune response via activation of B cells and other antigen-presenting cells. Their effect on the immune response is mediated by upregulation of the costimulatory molecule B7-2 (CD86) on the surface of antigen-presenting cells, an effect that is dependent on Toll-like receptor (TLR)2 and MyD88, through a cascade of signal transduction events mediated by direct binding of the porin to the TLR2-TLR1 heterodimer. This article summarizes work carried out investigating the mechanisms of the immune stimulating capacity of the neisserial porins (specifically meningococcal PorB), emphasizing cellular events involved in antigen-presenting cell activation and induction of expression of cell surface molecules involved in the immune response.

Immune restoration diseases reflect diverse immunopathological mechanisms

Up to one in four patients infected with human immunodeficiency virus type 1 and given antiretroviral therapy (ART) experiences inflammatory or cellular proliferative disease associated with a preexisting opportunistic infection, which may be subclinical. These immune restoration diseases (IRD) appear to result from the restoration of immunocompetence. IRD associated with intracellular pathogens are characterized by cellular immune responses and/or granulomatous inflammation. Mycobacterial and cryptococcal IRD are attributed to a pathological overproduction of Th1 cytokines. Clinicopathological characteristics of IRD associated with viral infections suggest different pathogenic mechanisms. For example, IRD associated with varicella-zoster virus or JC polyomavirus infection correlate with a CD8 T-cell response in the central nervous system. Exacerbations or de novo presentations of hepatitis associated with hepatitis C virus (HCV) infection following ART may also reflect restoration of pathogen-specific immune responses as titers of HCV-reactive antibodies rise in parallel with liver enzymes and plasma markers of T-cell activation. Correlations between immunological parameters assessed in longitudinal sample sets and clinical presentations are required to illuminate the diverse immunological scenarios described collectively as IRD. Here we present salient clinical features and review progress toward understanding their pathogeneses.

Response of macrophage Toll-like receptor 4 to a Sporothrix schenckii lipid extract during experimental sporotrichosis

Toll-like receptors have been implicated in the recognition of various pathogens, including bacteria, viruses, protozoa and fungi. However, no information is available about Toll-like receptor 4 (TLR4) participation in Sporothrix schenckii recognition and the consequent triggering of the immune response to this fungal pathogen. Following activation of TLRs by ligands of microbial origin, several responses are provoked, including reactions in immune cells that may lead them to produce signalling factors that trigger inflammation. The present study was designed to elucidate the role of TLR4 during the host response to S. schenckii. TLR4-deficient (C3H/HeJ) and control mice (C3H/HePas) were infected with S. schenckii yeast cells and immune response was assessed over 10 weeks by assaying production of pro-inflammatory mediator (nitric oxide and tumour necrosis factor-alpha) and anti-inflammatory cytokine (interleukin-10) by peritoneal macrophages and their correlation with apoptosis in peritoneal exudate cells. We found that both pro-inflammatory and anti-inflammatory mediators are reduced in TLR4-deficient mice, suggesting the involvement of this receptor in the recognition of this infectious agent. Translocation into the nucleus of nuclear transcription factor, nuclear factor-kappaB, was also evaluated and showed higher levels in TLR-4 normal mice, consistent with the results found for cytokine production. We are showing here, for the first time, the involvement of TLR4 in S. schenckii recognition. Taken together, our results demonstrate that the activation of peritoneal macrophages in response to S. schenckii lipid extracts has different responses in these two mouse strains which differ in TLR4 expression, suggesting an important role for TLR4 in governing the functions of macrophages in this fungal infection.

Pathogen recognition and inflammatory signaling in innate immune defenses

The innate immune system constitutes the first line of defense against invading microbial pathogens and relies on a large family of pattern recognition receptors (PRRs), which detect distinct evolutionarily conserved structures on pathogens, termed pathogen-associated molecular patterns (PAMPs). Among the PRRs, the Toll-like receptors have been studied most extensively. Upon PAMP engagement, PRRs trigger intracellular signaling cascades ultimately culminating in the expression of a variety of proinflammatory molecules, which together orchestrate the early host response to infection, and also is a prerequisite for the subsequent activation and shaping of adaptive immunity. In order to avoid immunopathology, this system is tightly regulated by a number of endogenous molecules that limit the magnitude and duration of the inflammatory response. Moreover, pathogenic microbes have developed sophisticated molecular strategies to subvert host defenses by interfering with molecules involved in inflammatory signaling. This review presents current knowledge on pathogen recognition through different families of PRRs and the increasingly complex signaling pathways responsible for activation of an inflammatory and antimicrobial response. Moreover, medical implications are discussed, including the role of PRRs in primary immunodeficiencies and in the pathogenesis of infectious and autoimmune diseases, as well as the possibilities for translation into clinical and therapeutic applications.

Differential activation of peritoneal cells by subcutaneous treatment of rats with cryptococcal antigens

Previous studies in our laboratory have shown that the subcutaneous pretreatment of rats with heat-killed cells (HKC) of Cryptococcus neoformans emulsified in complete Freund adjuvant (CFA) promotes protective immunity against an intraperitoneal challenge with C. neoformans. In contrast, subcutaneous treatment with the capsular polysaccharide (PSC) emulsified in CFA exacerbates the cryptococcal infection. The purpose of this study was to analyze the mechanisms involved in these phenomena. Adherent peritoneal cells from rats treated with HKC-CFA showed upregulated ED2, CD80, and CD86 expression; an increase in the level of production of anticryptococcal metabolites; and the enhanced production of interleukin-12 (IL-12) in comparison with the findings for cells from rats treated with CFA-phosphate-buffered saline (PBS). Adherent peritoneal cells from rats treated with PSC-CFA, however, also presented upregulated ED2, CD80, and CD86 expression compared to the level of expression for peritoneal cells from controls, but these cells showed an increase in arginase activity and decreased levels of production of IL-12 and tumor necrosis factor (TNF) compared with the activity and levels of production by peritoneal cells from CFA-PBS-treated rats. In addition, treatment with HKC-CFA resulted in a rise in the phagocytic and anticryptococcal activities of adherent peritoneal cells compared to those for control rats. However, adherent peritoneal cells from rats treated with PSC-CFA presented a reduction in anticryptococcal activity in comparison with that for cells from animals treated with CFA-PBS. These results show the differential activation between adherent peritoneal cells from HKC-CFA- and PSC-CFA-treated rats, with this differential activation at the primary site of infection possibly being responsible, at least in part, for the phenomena of protection and exacerbation observed in our model.

Engagement of Penicillium marneffei conidia with multiple pattern recognition receptors on human monocytes

P. marneffei is a thermal dimorphic fungus which causes penicilliosis, an opportunistic infection in immunocompromised patients in South and Southeast Asia. Little is known about the innate immune response to P. marneffei infection. Therefore, the initial response of macrophages to P. marneffei conidia was evaluated by us. Adhesion between monocytes from healthy humans and fungal conidia was examined and found to be specifically inhibited by MAbs against PRR, such as MR, (TLR)1, TLR2, TLR4, TLR6, CD14, CD11a, CD11b, and CD18. To study the consequences of these interactions, cytokines were also examined by ELISA. Binding of P. marneffei conidia to monocytes was significantly inhibited, in a dose-dependent manner, by MAbs against MR, TLR1, TLR2, TLR4, TLR6, CD14, CD11b and CD18. When monocytes were co-cultured with the conidia, there was an increase in the amount of surface CD40 and CD86 expression, together with TNF-alpha and IL-1beta production, compared to unstimulated controls. In assays containing anti-TLR4 or anti-CD14 antibody, reduction in the amount of TNF-alpha released by monocytes stimulated with P. marneffei conidia was detected. In addition, it was found that production of TNF-alpha and IL-1beta from adherent peripheral blood monocytes was partially impaired when heat-inactivated autologous serum, in place of untreated autologous serum, was added to the assay. These results demonstrate that various PRR on human monocytes participate in the initial recognition of P. marneffei conidia, and the engagement of PRR could partly initiate proinflammatory cytokine production.

In contrast to anti-tumor activity, YT cell and primary NK cell cytotoxicity for Cryptococcus neoformans bypasses LFA-1

NK cell cytotoxicity requires two positive signals for killing of tumors. Activation receptors induce polarization of the microtubule organization center and degranulation, while leukocyte function-associated antigen (LFA)-1 is required for conjugate formation and actin polymerization and under some circumstances may be sufficient for NK cell cytotoxicity. Although the receptor for direct killing of fungi is not known, CD18, the beta2 chain of LFA-1, binds components of the capsule and cell wall of the opportunistic pathogen Cryptococcus neoformans, namely the polysaccharides glucoronoxylomannan and galactoxylomannan. Herein, we also demonstrate that LFA-1 was concentrated in regions of the NK cell surface interacting with C. neoformans. Consequently, there was compelling evidence to hypothesize that NK cells would also use LFA-1 to recognize and kill C. neoformans. Using a combination of NK cell lines that did or did not express LFA-1 or by using a CD18-specific functional blocking antibody, we confirm that NK cell anti-tumor activity is critically dependent upon the expression of LFA-1. Duplicating the events of tumor cytotoxicity, NK cells form conjugates with cryptococcal targets, rearrange the cell cytoskeleton to develop an NK immunologic synapse and release perforin-containing granules; however, each of these events occurred independently of LFA-1. Furthermore, NK cell-mediated killing of C. neoformans was detectable in both NK cells pre-treated with CD18-blocking antibodies and in NK cells lacking cell surface LFA-1 expression. These results demonstrate that in the absence of LFA-1 expression, NK cells are fully capable of recognizing a target (C. neoformans) and retain all of the events required for cytotoxicity.

Isolation and purification of antigenic components of Cryptococcus

The encapsulated fungal pathogens Cryptococcus neoformans and Cryptococcus gattii are significant agents of life-threatening infections, particularly in persons with suppressed cell-mediated immunity. This chapter provides detailed methodology for the purification of two of the major antigen fractions of C. neoformans: glucuronoxylomannan (GXM) and mannoprotein (MP). GXM is the primary component of the polysaccharide capsule, which is the major cryptococcal virulence factor. In contrast, MPs have been identified as key antigens that stimulate T-cell responses. Purification of GXM and MP should assist investigators studying the antigenic, biochemical, and virulence properties of Cryptococcus species.

Klebsiella pneumoniae increases the levels of Toll-like receptors 2 and 4 in human airway epithelial cells

Airway epithelial cells act as the first barrier against pathogens. These cells recognize conserved structural motifs expressed by microbial pathogens via Toll-like receptors (TLRs) expressed on the surface. In contrast to the level of expression in lymphoid cells, the level of expression of TLR2 and TLR4 in airway epithelial cells is low under physiological conditions. Here we explored whether Klebsiella pneumoniae upregulates the expression of TLRs in human airway epithelial cells. We found that the expression of TLR2 and TLR4 by A549 cells and human primary airway cells was upregulated upon infection with K. pneumoniae. The increased expression of TLRs resulted in enhancement of the cellular response upon stimulation with Pam3CSK4 and lipopolysaccharide, which are TLR2 and TLR4 agonists, respectively. Klebsiella-dependent upregulation of TLR expression occurred via a positive IkappaBalpha-dependent NF-kappaBeta pathway and via negative p38 and p44/42 mitogen-activated protein kinase-dependent pathways. We showed that Klebsiella-induced TLR2 and TLR4 upregulation was dependent on TLR activation. An isogenic capsule polysaccharide (CPS) mutant did not increase TLR2 and TLR4 expression. Purified CPS upregulated TLR2 and TLR4 expression, and polymyxin B did not abrogate CPS-induced TLR upregulation. Although no proteins were detected in the CPS preparation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and colloidal gold staining, we could not rule out the possibility that traces of protein in our CPS preparation could have been responsible, at least in part, for the TLR upregulation.

Toll-like receptor 4 polymorphisms and aspergillosis in stem-cell transplantation

BACKGROUND

Toll-like receptors (TLRs) are essential components of the immune response to fungal pathogens. We examined the role of TLR polymorphisms in conferring a risk of invasive aspergillosis among recipients of allogeneic hematopoietic-cell transplants.

METHODS

We analyzed 20 single-nucleotide polymorphisms (SNPs) in the toll-like receptor 2 gene (TLR2), the toll-like receptor 3 gene (TLR3), the toll-like receptor 4 gene (TLR4), and the toll-like receptor 9 gene (TLR9) in a cohort of 336 recipients of hematopoietic-cell transplants and their unrelated donors. The risk of invasive aspergillosis was assessed with the use of multivariate Cox regression analysis. The analysis was replicated in a validation study involving 103 case patients and 263 matched controls who received hematopoietic-cell transplants from related and unrelated donors.

RESULTS

In the discovery study, two donor TLR4 haplotypes (S3 and S4) increased the risk of invasive aspergillosis (adjusted hazard ratio for S3, 2.20; 95% confidence interval [CI], 1.14 to 4.25; P=0.02; adjusted hazard ratio for S4, 6.16; 95% CI, 1.97 to 19.26; P=0.002). The haplotype S4 was present in carriers of two SNPs in strong linkage disequilibrium (1063 A/G [D299G] and 1363 C/T [T399I]) that influence TLR4 function. In the validation study, donor haplotype S4 also increased the risk of invasive aspergillosis (adjusted odds ratio, 2.49; 95% CI, 1.15 to 5.41; P=0.02); the association was present in unrelated recipients of hematopoietic-cell transplants (odds ratio, 5.00; 95% CI, 1.04 to 24.01; P=0.04) but not in related recipients (odds ratio, 2.29; 95% CI, 0.93 to 5.68; P=0.07). In the discovery study, seropositivity for cytomegalovirus (CMV) in donors or recipients, donor positivity for S4, or both, as compared with negative results for CMV and S4, were associated with an increase in the 3-year probability of invasive aspergillosis (12% vs. 1%, P=0.02) and death that was not related to relapse (35% vs. 22%, P=0.02).

CONCLUSIONS

This study suggests an association between the donor TLR4 haplotype S4 and the risk of invasive aspergillosis among recipients of hematopoietic-cell transplants from unrelated donors.

Enhanced innate immune responsiveness to pulmonary Cryptococcus neoformans infection is associated with resistance to progressive infection

Genetically regulated mechanisms of host defense against Cryptococcus neoformans infection are not well understood. In this study, pulmonary infection with the moderately virulent C. neoformans strain 24067 was used to compare the host resistance phenotype of C57BL/6J with that of inbred mouse strain SJL/J. At 7 days or later after infection, C57BL/6J mice exhibited a significantly greater fungal burden in the lungs than SJL/J mice. Characterization of the pulmonary innate immune response at 3 h after cryptococcal infection revealed that resistant SJL/J mice exhibited significantly higher neutrophilia, with elevated levels of inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) and keratinocyte-derived chemokine (KC)/CXCL1 in the airways, as well as increased whole-lung mRNA expression of chemokines KC/CXCL1, MIP-1alpha/CCL3, MIP-1beta/CCL4, MIP-2/CXCL2, and MCP-1/CCL2 and cytokines interleukin 1beta (IL-1beta) and IL-1Ra. At 7 and 14 days after infection, SJL/J mice maintained significantly higher levels of TNF-alpha and KC/CXCL1 in the airways and exhibited a Th1 response characterized by elevated levels of lung gamma interferon (IFN-gamma) and IL-12/IL-23p40, while C57BL/6J mice exhibited Th2 immunity as defined by eosinophilia and IL-4 production. Alveolar and resident peritoneal macrophages from SJL/J mice also secreted significantly greater amounts of TNF-alpha and KC/CXCL1 following in vitro stimulation with C. neoformans. Intracellular signaling analysis demonstrated that TNF-alpha and KC/CXCL1 production was regulated by NF-kappaB and phosphatidylinositol 3 kinase in both strains; however, SJL/J macrophages exhibited heightened and prolonged activation in response to C. neoformans infection compared to that of C57BL/6J. Taken together, these data demonstrate that an enhanced innate immune response against pulmonary C. neoformans infection in SJL/J mice is associated with natural resistance to progressive infection.

Expression of toll-like receptors in the Fusarium solani infected cornea

PURPOSE

To investigate the expression of toll-like receptors (TLRs) in the human cornea with Fusarium solani (F. solani) keratitis.

METHODS

Five human corneas with F. solani keratitis and 5 healthy human corneas were evaluated for TLR1-10 mRNA by reverse transcribed polymerase chain reaction (RT-PCR) and relative real-time PCR. The proteins of TLR2, 4, and 9 were also compared by Western blot. The mean times between these corneas were collected, and the onset of treatment to surgery was 34.4 +/- 12.4 days, ranging from 15 to 47 days. The data were analyzed with SPSS15.0.

RESULTS

TLR1-10 mRNA was expressed in both healthy and F. solani infected human corneas. The expression of TLR2, 4, 6, and 9 mRNA in the F. solani infected human corneas were upregulated. The Western blot showed the protein expression of TLR2, 4, and 9 was also upregulated in the corneas with F. solani keratitis than that of the healthy corneas.

CONCLUSIONS

TLRs are expressed diversely in the F. solani infected human cornea, and TLR2, 4, and 9 may be implicated in the pathogenesis of F. solani infection in the cornea.

IFN-alpha/beta signaling is required for polarization of cytokine responses toward a protective type 1 pattern during experimental cryptococcosis

The antiviral activities of type I IFNs have long been established. However, comparatively little is known of their role in defenses against nonviral pathogens. We examined here the effects of type I IFNs on host resistance against the model pathogenic yeast Cryptococcus neoformans. After intratracheal or i.v. challenge with this fungus, most mice lacking either the IFN-alpha/beta receptor (IFN-alpha/betaR) or IFN-beta died from unrestrained pneumonia and encephalitis, while all wild-type controls survived. The pulmonary immune response of IFN-alpha/betaR-/- mice was characterized by increased expression of IL-4, IL-13, and IL-10, decreased expression of TNF-alpha, IFN-gamma, inducible NO synthetase, and CXCL10, and similar levels of IL-12 mRNA, compared with wild-type controls. Histopathological analysis showed eosinophilic infiltrates in the lungs of IFN-alpha/betaR-/- mice, although this change was less extensive than that observed in similarly infected IFN-gammaR-deficient animals. Type I IFN responses could not be detected in the lung after intratracheal challenge. However, small, but statistically significant, elevations in IFN-beta levels were measured in the supernatants of bone marrow-derived macrophages or dendritic cells infected with C. neoformans. Our data demonstrate that type I IFN signaling is required for polarization of cytokine responses toward a protective type I pattern during cryptococcal infection.

Host-microbe interactions: innate pattern recognition of fungal pathogens

The recognition of fungi is mediated by germline pattern recognition receptors (PRRs) such as Toll-like receptors and lectin receptors that interact with conserved structures of the microorganisms, the pathogen-associated molecular patterns (PAMPs). Subsequently, PRRs activate intracellular signals that collaborate for the efficient activation of the host defense. The specificity of these responses is achieved through the activation of a particular mosaic of PRRs, that is determined by the available fungal PAMPs and the innate immune cells involved. This will determine a divergence of the final type of reaction, and in this way the innate host defense has the capability to deliver tailored responses to each pathogen.