Occurrences, immunoglobulin classes, and biological activities of antibodies in normal human serum that are reactive with Cryptococcus neoformans glucuronoxylomannan

Aspartyl peptidase May1 induces host inflammatory response by altering cell wall composition in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans causes cryptococcal meningoencephalitis, a disease that kills more than 180,000 people annually. Contributing to its success as a fungal pathogen is its cell wall surrounded by a capsule. When the cryptococcal cell wall is compromised, exposed pathogen-associated molecular pattern molecules (PAMPs) could trigger host recognition and initiate attack against this fungus. Thus, cell wall composition and structure are tightly regulated. The cryptococcal cell wall is unusual in that chitosan, the acetylated form of chitin, is predominant over chitin and is essential for virulence. Recently, it was shown that acidic pH weakens the cell wall and increases exposure of PAMPs partly due to decreased chitosan levels. However, the molecular mechanism responsible for the cell wall remodeling in acidic pH is unknown. In this study, by screening for genes involved in cryptococcal tolerance to high levels of CO2, we serendipitously discovered that the aspartyl peptidase May1 contributes to cryptococcal sensitivity to high levels of CO2 due to acidification of unbuffered media. Overexpression of MAY1 increases the cryptococcal cell size and elevates PAMP exposure, causing a hyper-inflammatory response in the host while MAY1 deletion does the opposite. We discovered that May1 weakens the cell wall and reduces the chitosan level, partly due to its involvement in the degradation of Chs3, the sole chitin synthase that supplies chitin to be converted to chitosan. Consistently, overexpression of CHS3 largely rescues the phenotype of MAY1oe in acidic media. Collectively, we demonstrate that May1 remodels the cryptococcal cell wall in acidic pH by reducing chitosan levels through its influence on Chs3.

IMPORTANCE

The fungal cell wall is a dynamic structure, monitoring and responding to internal and external stimuli. It provides a formidable armor to the fungus. However, in a weakened state, the cell wall also triggers host immune attack when PAMPs, including glucan, chitin, and mannoproteins, are exposed. In this work, we found that the aspartyl peptidase May1 impairs the cell wall of Cryptococcus neoformans and increases the exposure of PAMPs in the acidic environment by reducing the chitosan level. Under acidic conditions, May1 is involved in the degradation of the chitin synthase Chs3, which supplies chitin to be deacetylated to chitosan. Consistently, the severe deficiency of chitosan in acidic pH can be rescued by overexpressing CHS3. These findings improve our understanding of cell wall remodeling and reveal a potential target to compromise the cell wall integrity in this important fungal pathogen.

Vaccination with a ZNF2oe Strain of Cryptococcus Provides Long-Lasting Protection against Cryptococcosis and Is Effective in Immunocompromised Hosts

Systemic cryptococcosis is fatal without treatment. Even with the current antifungal therapies, this disease kills 180,000 of 225,000 infected people annually. Exposure to the causative environmental fungus Cryptococcus neoformans is universal. Either reactivation of a latent infection or an acute infection after high exposure to cryptococcal cells can result in cryptococcosis. Currently, there is no vaccine to prevent cryptococcosis. Previously, we discovered that Znf2, a transcription factor that directs Cryptococcus yeast-to-hypha transition, profoundly affects cryptococcal interaction with the host. Overexpression of ZNF2 drives filamentous growth, attenuates cryptococcal virulence, and elicits protective host immune responses. Importantly, immunization with cryptococcal cells overexpressing ZNF2, in either live or heat-inactivated form, offers significant protection to the host from a subsequent challenge by the otherwise lethal clinical isolate H99. In this study, we found that the heat-inactivated ZNF2^oe vaccine offered long-lasting protection with no relapse upon challenge with the wild-type H99. Vaccination with heat-inactivated ZNF2^oe cells provides partial protection in hosts with preexisting asymptomatic cryptococcal infection. Importantly, once animals have been vaccinated with heat-inactivated or live short-lived ZNF2^oe cells, they are protected against cryptococcosis even when their CD4⁺ T cells are depleted at the time of fungal challenge. Remarkably, vaccination with live, short-lived ZNF2^oe cells in CD4-depleted hosts still provides strong protection to these hosts with preexisting immunodeficiency at the time of vaccination. This work raises hope for developing effective vaccines with long-lasting protection for individuals who are immunocompromised or could become immunocompromised later in life.

Association of Antibody Immunity With Cryptococcal Antigenemia and Mortality in a South African Cohort With Advanced Human Immunodeficiency Virus Disease

BACKGROUND

Asymptomatic cryptococcal antigenemia (positive blood cryptococcal antigen [CrAg]) is associated with increased mortality in individuals with human immunodeficiency virus (HIV) even after adjusting for CD4 count and despite receiving antifungal treatment. The association of antibody immunity with mortality in adults with HIV with cryptococcal antigenemia is unknown.

METHODS

Cryptococcal capsular glucuronoxylomannan (GXM)- and naturally occurring β-glucans (laminarin, curdlan)-binding antibodies were measured in blood samples of 197 South Africans with HIV who underwent CrAg screening and were followed up to 6 months. Associations between antibody titers, CrAg status, and all-cause mortality were sought using logistic and Cox regression, respectively.

RESULTS

Compared with CrAg-negative individuals (n = 130), CrAg-positive individuals (n = 67) had significantly higher IgG1 (median, 6672; interquartile range [IQR], 4696-10 414 vs 5343, 3808-7722 μg/mL; P = .007), IgG2 (1467, 813-2607 vs 1036, 519-2012 μg/mL; P = .01), and GXM-IgG (1:170, 61-412 vs 1:117, 47-176; P = .0009) and lower curdlan-IgG (1:47, 11-133 vs 1:93, 40-206; P = .01) titers. GXM-IgG was associated directly with cryptococcal antigenemia adjusted for CD4 count and antiretroviral therapy use (odds ratio, 1.64; 95% confidence interval [CI], 1.21 to 2.22). Among CrAg-positive individuals, GXM-IgG was inversely associated with mortality at 6 months adjusted for CD4 count and tuberculosis (hazard ratio, 0.50; 95% CI, .33 to .77).

CONCLUSIONS

The inverse association of GXM-IgG with mortality in CrAg-positive individuals suggests that GXM-IgG titer may have prognostic value in those individuals. Prospective longitudinal studies to investigate this hypothesis and identify mechanisms by which antibody may protect against mortality are warranted.

Host populations, challenges, and commercialization of cryptococcal vaccines

Vaccines are one of the most effective public health tools to prevent and manage infectious diseases. Since the first clinical use of vaccines in the late 18th century, many vaccines have been successfully developed to combat bacterial and viral infections, including the most recent Coronavirus Disease 2019 (COVID-19) pandemic. However, there remains no vaccine that is clinically available to treat or prevent invasive fungal diseases, including cryptococcal meningoencephalitis. This fungal disease is uniformly fatal without treatment and has a global mortality rate of over 70%. Despite a dire need for an effective cryptococcal vaccine, there are many scientific and economic challenges to overcome prior to making it a reality. Here, we discuss some of these challenges as well as steps that the community is taking for commercialization of effective cryptococcal vaccines.

Human immune polymorphisms associated with the risk of cryptococcal disease

Cryptococcus neoformans is an opportunistic fungal pathogen that can cause lethal cryptococcal meningitis in immunocompromised individuals such as those with HIV/AIDS. In addition, cryptococcal infections occasionally arise in immunocompetent individuals or those with previously undiagnosed immunodeficiencies. The course of cryptococcosis is highly variable in both patient groups, and there is rapidly growing evidence that genetic polymorphisms may have a significant impact on the trajectory of disease. Here, we review what is currently known about the nature of these polymorphisms and their impact on host response to C. neoformans infection. Thus far, polymorphisms in Fc gamma receptors, mannose‐binding lectin, Dectin‐2, Toll‐like receptors and macrophage colony‐stimulating factor have been associated with susceptibility to cryptococcal disease. Notably, however, in some cases the impact of these polymorphisms depends on the genetic background of the population; for example, the FCGR3A 158 F/V polymorphism was associated with an increased risk of cryptococcal disease in both HIV‐positive and HIV‐negative white populations, but not in Han Chinese patients. In most cases, the precise mechanism by which the identified polymorphisms influence disease progression remains unclear, although impaired fungal recognition and phagocytosis by innate immune cells appears to play a major role. Finally, we highlight outstanding questions in the field and emphasize the need for future research to include more diverse populations in their genetic association studies.

Identification of Disease-Associated Cryptococcal Proteins Reactive With Serum IgG From Cryptococcal Meningitis Patients

Cryptococcus neoformans, an opportunistic fungal pathogen ubiquitously present in the environment, causes cryptococcal meningitis (CM) mainly in immunocompromised patients, such as AIDS patients. We aimed to identify disease-associated cryptococcal protein antigens targeted by the human humoral immune response. Therefore, we used sera from Colombian CM patients, with or without HIV infection, and from healthy individuals living in the same region. Serological analysis revealed increased titers of anti-cryptococcal IgG in HIV-negative CM patients, but not HIV-positive CM patients, compared to healthy controls. In contrast, titers of anti-cryptococcal IgM were not affected by CM. Furthermore, we detected pre-existing IgG and IgM antibodies even in sera from healthy individuals. The observed induction of anti-cryptococcal IgG but not IgM during CM was supported by analysis of sera from C. neoformans-infected mice. Stronger increase in IgG was found in wild type mice with high lung fungal burden compared to IL-4Rα-deficient mice showing low lung fungal burden. To identify the proteins targeted by human anti-cryptococcal IgG antibodies, we applied a quantitative 2D immunoproteome approach identifying cryptococcal protein spots preferentially recognized by sera from CM patients or healthy individuals followed by mass spectrometry analysis. Twenty-three cryptococcal proteins were recombinantly expressed and confirmed to be immunoreactive with human sera. Fourteen of them were newly described as immunoreactive proteins. Twelve proteins were classified as disease-associated antigens, based on significantly stronger immunoreactivity with sera from CM patients compared to healthy individuals. The proteins identified in our screen significantly expand the pool of cryptococcal proteins with potential for (i) development of novel anti-cryptococcal agents based on implications in cryptococcal virulence or survival, or (ii) development of an anti-cryptococcal vaccine, as several candidates lack homology to human proteins and are localized extracellularly. Furthermore, this study defines pre-existing anti-cryptococcal immunoreactivity in healthy individuals at a molecular level, identifying target antigens recognized by sera from healthy control persons.

A Glucuronoxylomannan Epitope Exhibits Serotype-Specific Accessibility and Redistributes towards the Capsule Surface during Titanization of the Fungal Pathogen Cryptococcus neoformans

Disseminated infections with the fungal species Cryptococcus neoformans or, less frequently, Cryptococcus gattii are an important cause of mortality in immunocompromised individuals. Central to the virulence of both species is an elaborate polysaccharide capsule that consists predominantly of glucuronoxylomannan (GXM). Due to its abundance, GXM is an ideal target for host antibodies, and several monoclonal antibodies (mAbs) have previously been derived using purified GXM or whole capsular preparations as antigens. In addition to their application in the diagnosis of cryptococcosis, anti-GXM mAbs are invaluable tools for studying capsule structure. In this study, we report the production and characterization of a novel anti-GXM mAb, Crp127, that unexpectedly reveals a role for GXM remodeling during the process of fungal titanization. We show that Crp127 recognizes a GXM epitope in an O-acetylation-dependent, but xylosylation-independent, manner. The epitope is differentially expressed by the four main serotypes of Cryptococcus neoformans and C. gattii, is heterogeneously expressed within clonal populations of C. gattii serotype B strains, and is typically confined to the central region of the enlarged capsule. Uniquely, however, this epitope redistributes to the capsular surface in titan cells, a recently characterized morphotype where haploid 5-μm cells convert to highly polyploid cells of >10 μm with distinct but poorly understood capsular characteristics. Titan cells are produced in the host lung and critical for successful infection. Crp127 therefore advances our understanding of cryptococcal morphological change and may hold significant potential as a tool to differentially identify cryptococcal strains and subtypes.

Host immunity to Cryptococcus neoformans

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

Analysis of asthma patients for cryptococcal seroreactivity in an urban German area

Cryptococcus neoformans is an opportunistic fungal pathogen that causes lung inflammation and meningoencephalitis in immunocompromised patients but is also able to asymptomatically infect immunocompetent individuals. C. neoformans is found ubiquitously especially in urban areas where it is spread by pigeons, and fungal exposure may predispose for asthma development already at an early age, as soon as confronted with pigeon droppings. In the study presented here, we investigated the presence of specific immunoglobulin G (IgG) against C. neoformans in sera from patients suffering from asthma in comparison to a healthy control cohort, accrued from the Leipzig Research Centre for Civilization Diseases (LIFE). For serological analysis we developed a flow cytometry (FACS) based assay specific for an acapsular strain of C. neoformans to comprehensively analyze different cryptococcal serotypes. Compared with the non-asthmatic cohort, asthmatics exhibited, as expected, an elevated level of total serum immunoglobulin E (IgE), whereas the IgG seroreactivity against C. neoformans was not significantly different among the two groups (P = .118). Nevertheless, there was a trend toward increased Cryptococcus-specific IgG antibodies in the serum of asthmatics. Additionally, in male asthmatics an increased IgG-mediated seroreactivity compared to female asthmatics was found. This points to a higher prevalence of subclinical C. neoformans infection in male asthmatics and may support the hypothesis of C. neoformans as a risk factor for the development of asthma in urban areas.

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.

Fc gamma receptor 3A polymorphism and risk for HIV-associated cryptococcal disease

Cryptococcus neoformans is one of the most common causes of fungal disease in HIV-infected persons, but not all of those who are infected develop cryptococcal disease (CD). Although CD4⁺ T cell deficiency is a risk factor for HIV-associated CD, polymorphisms of phagocytic Fc gamma receptors (FCGRs) have been linked to CD risk in HIV-uninfected persons. To investigate associations between FCGR2A 131 H/R and FCGR3A 158 F/V polymorphisms and CD risk in HIV-infected persons, we performed PCR-based genotyping on banked samples from 164 men enrolled in the Multicenter AIDS Cohort Study (MACS): 55 who were HIV infected and developed CD and a matched control group of 54 who were HIV infected and 55 who were HIV uninfected. Using additive and allelic statistical models for analysis, the high-affinity FCGR3A 158V allele was significantly associated with CD status after adjusting for race/ethnicity (odds ratio [OR], 2.1; P = 0.005), as was the FCGR3A 158 VV homozygous genotype after adjusting for race/ethnicity, rate of CD4⁺ T cell decline, and nadir CD4⁺ T cell count (OR, 21; P = 0.005). No associations between CD and FCGR2A 131 H/R polymorphism were identified. In binding studies, human IgG (hIgG)-C. neoformans complexes exhibited more binding to CHO-K1 cells expressing FCGR3A 158V than to those expressing FCGR3A 158F, and in cytotoxicity assays, natural killer (NK) cells expressing FCGR3A 158V induced more C. neoformans-infected monocyte cytotoxicity than those expressing FCGR3A 158F. Together, these results show an association between the FCGR3A 158V allele and risk for HIV-associated CD and suggest that this polymorphism could promote C. neoformans pathogenesis via increased binding of C. neoformans immune complexes, resulting in increased phagocyte cargo and/or immune activation.

HIV-associated CD4⁺ T cell deficiency is a sine qua non for HIV-associated cryptococcal disease (CD), but not all patients with CD4⁺ T cell deficiency develop CD despite serological evidence of previous infection. At present, there are no biomarkers that predict HIV-associated CD risk. The goal of our study was to understand whether Fc gamma receptor (FCGR) polymorphisms that have been shown to portend CD risk in HIV-uninfected people are associated with CD risk in HIV-infected people. Such biomarkers could identify those who would benefit most from targeted prophylaxis and/or earlier treatment, particularly in sub-Saharan Africa, where there are nearly a million cases of HIV-associated CD annually. A biomarker of risk could also identify potential candidates for immunization, should there be a vaccine for Cryptococcus neoformans.

Lymphocyte transformation assay for C neoformans antigen is not reliable for detecting cellular impairment in patients with neurocryptococcosis

Background

Cryptococcus neoformans causes meningitis and disseminated infection in healthy individuals, but more commonly in hosts with defective immune responses. Cell-mediated immunity is an important component of the immune response to a great variety of infections, including yeast infections. We aimed to evaluate a specific lymphocyte transformation assay to Cryptococcus neoformans in order to identify immunodeficiency associated to neurocryptococcosis (NCC) as primary cause of the mycosis.

Methods

Healthy volunteers, poultry growers, and HIV-seronegative patients with neurocryptococcosis were tested for cellular immune response. Cryptococcal meningitis was diagnosed by India ink staining of cerebrospinal fluid and cryptococcal antigen test (Immunomycol-Inc, SP, Brazil). Isolated peripheral blood mononuclear cells were stimulated with C. neoformans antigen, C. albicans antigen, and pokeweed mitogen. The amount of ³H-thymidine incorporated was assessed, and the results were expressed as stimulation index (SI) and log SI, sensitivity, specificity, and cut-off value (receiver operating characteristics curve). We applied unpaired Student t tests to compare data and considered significant differences for p<0.05.

Results

The lymphotoxin alpha showed a low capacity with all the stimuli for classifying patients as responders and non-responders. Lymphotoxin alpha stimulated by heated-killed antigen from patients with neurocryptococcosis was not affected by TCD4+ cell count, and the intensity of response did not correlate with the clinical evolution of neurocryptococcosis.

Conclusion

Response to lymphocyte transformation assay should be analyzed based on a normal range and using more than one stimulator. The use of a cut-off value to classify patients with neurocryptococcosis is inadequate. Statistical analysis should be based on the log transformation of SI. A more purified antigen for evaluating specific response to C. neoformans is needed.

A purified capsular polysaccharide markedly inhibits inflammatory response during endotoxic shock

Capsular material of the opportunistic fungus Cryptococcus neoformans is composed mainly of a polysaccharide named glucuronoxylomannan (GXM). In this study, the effects of GXM were analyzed in an in vivo experimental system of lipopolysaccharide (LPS)-induced shock. Endotoxic shock was induced in mice by a single intraperitoneal injection of LPS from Escherichia coli. GXM treatment reduced the mortality of mice at early stages. Mice treated with LPS alone showed markedly increased plasma levels of tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), and IL-6, whereas mice that were also treated with GXM showed significantly lower plasma levels of these cytokines. This effect was related to a marked suppression of Akt and IκBα activation. Importantly, the inhibitory effect of GXM on proinflammatory cytokine secretion was reproduced by treatment with wortmannin, an inhibitor of the Akt transcription pathway. Our results indicate that GXM has a beneficial effect on endotoxic shock, resulting in a significant increase in the rate of survival by dampening the hyperinflammatory response.

Mesenteric cryptococcal granuloma in a dog caused by Cryptococcus neoformans var. grubii

Although cryptococcosis is usually associated with respiratory and neurologic signs in domestic species (such as sneeze, cough, nasal discharge, seizures, ataxia), clinical manifestations of the disease may be more subtle and nonspecific. A 3-year-old male castrated Boxer dog presented with a history of chronic vomiting, diarrhea, weight loss, and lethargy. At no time had respiratory or neurologic signs been noted by the owners or the primary care veterinarian. Palpation of an abdominal mass revealed an atypical lesion location: a large (16 × 9 × 7 cm) mass at the root of the mesentery. Diagnosis was achieved through cytology of this mass and a positive serologic Cryptococcus capsular antigen titer; polymerase chain reaction was utilized for speciation of the abdominal isolate as Cryptococcus neoformans variety grubii. The animal was euthanized due to poor prognosis. After necropsy and histopathologic analysis, the mesenteric mass and associated lymph nodes were identified as large fungal granulomas. This is a rare manifestation of cryptococcosis, involving several visceral organs, with no remaining evidence of the route of entry of the organism. As prompt diagnosis of mycotic illness is paramount to successful management, this case indicates that cryptococcal infection should be considered as a differential diagnosis in dogs with gastrointestinal signs and lymphadenopathy. The protean nature of cryptococcosis is discussed within the context of a brief review of emerging and unresolved issues in pathogenesis.

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.

Cryptococcal osteomyelitis of the scapula and rib in an immunocompetent patient

Cryptococcal osteomyelitis is extremely rare and almost always occurs in immunocompromised patients. We describe a case of osteomyelitis due to Cryptococcus neoformans involving both scapula and rib in an immunocompetent and previously healthy patient. The patient received treatment with amphotericin B deoxycholate and flucytosine for 4 weeks, followed by oral fluconazole 400 mg per day for 8 weeks and 200 mg per day for 9 months. The 12-month course of antifungal therapy resulted in complete clinical recovery and undetectable serum cryptococcal antigen. Cryptococcal osteomyelitis should be suspected in any immunocompetent patient with osteolytic lesions on radiological images.

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.

Paradoxical roles of alveolar macrophages in the host response to Cryptococcus neoformans

Cryptococcus neoformans (Cn) is a fungal pathogen that is a serious health threat to immunocompromised individuals. Upon environmental exposure, infectious fungal propagules are inhaled into the host's lungs. The anticryptococcal actions of alveolar macrophages (AM), the predominant host phagocyte of the innate immune system in the lungs, are fundamental in determining whether containment and clearance of the pathogen occurs by the development of an adapted immune response or whether infection is established and progresses to disease. However, the fungus is also capable of surviving the antimicrobial actions of AM and exploits these host phagocytes to establish infection and exacerbate disease. In addition, there is evidence suggesting that cryptococcosis may occur following reactivation of latent cryptococcal infection. Currently, the role of AM and the fungal factors contributing to latent cryptococcosis are unknown. This review examines the AM-Cn interaction and how it affects the development of pulmonary disease with a focus on host and pathogen factors enabling latency to occur.

A critical role for FcgammaRIIB in up-regulation of Fas ligand induced by a microbial polysaccharide

The microbial capsular polysaccharide glucuronoxylomannan (GXM) from the opportunistic fungus Cryptoccocus neoformans is able to alter the innate and adaptive immune response through multi-faceted mechanisms of immunosuppression. The ability of GXM to dampen the immune response involves the induction of T cell apoptosis, which is dependent on GXM-induced up-regulation of Fas ligand (FasL) on antigen-presenting cells. In this study we elucidate the mechanism exploited by GXM to induce up-regulation of FasL. We demonstrate that (i) the activation of FasL is dependent on GXM interaction with FcgammaRIIB (FcγRIIB); (ii) GXM induces activation of c-Jun NH(2) -terminal kinase (JNK) and p38 signal transduction pathways via FcγRIIB; (iii) this leads to downstream activation of c-Jun; (iv) JNK and p38 are simultaneously, but independently, activated; (v) FasL up-regulation occurs via JNK and p38 activation; and (vi) apoptosis occurs via FcγRIIB engagement with consequent JNK and p38 activation. Our results highlight a fast track to FasL up-regulation via FcγRIIB, and assign to this receptor a novel anti-inflammatory role that also accounts for induced peripheral tolerance. These results contribute to our understanding of the mechanism of immunosuppression that accompanies cryptococcosis.

The absence of serum IgM enhances the susceptibility of mice to pulmonary challenge with Cryptococcus neoformans

The importance of T cell-mediated immunity for resistance to the disease (cryptococcal disease) caused by Cryptococcus neoformans is incontrovertible, but whether Ab immunity also contributes to resistance remains uncertain. To investigate the role of IgM in resistance to C. neoformans, we compared the survival, fungal burden, lung and brain inflammatory responses, and lung phagocytic response of sIgM(-/-) mice, which lack secreted IgM, to that of IgM sufficient C57BL6x129Sv (heretofore, control) mice at different times after intranasal infection with C. neoformans (24067). sIgM(-/-) mice had higher mortality and higher blood and brain CFUs 28 d postinfection, but lung CFUs were comparable. Lungs of control mice manifested exuberant histiocytic inflammation with visible C. neoformans, findings that were not observed in sIgM(-/-) mice, whereas in brain sections, sIgM(-/-) mice had marked inflammation with visible C. neoformans that was not observed in control mice. Cytokine responses were significant for higher levels of lung IL-1beta and IL-12 24 h postinfection in control mice and higher levels of lung and brain IL-17 28 d postinfection in sIgM(-/-) mice. Alveolar macrophage phagocytosis was significantly higher for control than for sIgM(-/-) mice 24 h postinfection; however, phagocytic indices of sIgM(-/-) mice increased after reconstitution of sIgM(-/-) mice with polyclonal IgM. These data establish a previously unrecognized role for IgM in resistance to intranasal infection with C. neoformans in mice and suggest that the mechanism by which it mediates a host benefit is by augmenting Th1 polarization, macrophage recruitment and phagocytosis of C. neoformans.

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.

IgM(+) memory B cell expression predicts HIV-associated cryptococcosis status

BACKGROUND

The role of B cells in resistance to Cryptococcus neoformans disease (i.e., cryptococcosis) is unknown. Given evidence that IgM(+) memory B cells are required for immunity to other encapsulated pathogens, we hypothesized that these cells might contribute to resistance to cryptococcosis.

METHODS

We compared levels of IgM expression on memory B cells in 29 HIV-infected individuals who had a history of cryptococcosis (the HIV+CN+ group) with levels in 30 human immunodeficiency virus (HIV)-infected subjects who had no history of cryptococcosis (the HIV+CN- group) and 20 HIV-uninfected subjects who had no history of cryptococcosis (the HIV- group) (cohort 1). We also determined levels of IgM expression on memory B cells in banked samples obtained before cryptococcosis onset from 31 participants in the Multicenter AIDS Cohort Study, of whom 8 had HIV infection and subsequently developed cryptococcosis (the HIV+CN+ group), 8 had HIV infection and did not develop cryptococcosis (the HIV+CN- group), and 15 did not have HIV infection and did not develop cryptococcosis (the HIV- group) (cohort 2).

RESULTS

In cohort 1, the percentage of memory B cells that expressed IgM was lower among HIV+CN+ subjects, compared with HIV+CN- subjects (P < .01) and HIV- subjects (P < .05); expression of IgM on 50% of memory B cells was a significant predictor of C. neoformans disease status (odds ratio, 5.5; P = .03). In cohort 2, the percentage of memory B cells that expressed IgM was lower in HIV+CN+ subjects than in HIV+CN- subjects (P = .02) and HIV- subjects (P < .01); an IgM(+) memory B cell percentage of 38.5% was a significant predictor of future development of cryptococcosis (odds ratio, 14; P = .02).

CONCLUSIONS

These findings suggest that HIV-infected persons in whom the percentage of memory B cells that express IgM is decreased might be at greater risk for the development of cryptococcosis.

A microbial polysaccharide reduces the severity of rheumatoid arthritis by influencing Th17 differentiation and proinflammatory cytokines production

Rheumatoid arthritis (RA) is a chronic and debilitating autoimmune disease characterized by chronic joint inflammation with subsequent cartilage and bone destruction. RA is emerging as a model of IL-17-driven autoimmune inflammatory disease. IL-17 is a marker for Th17 cells, with its master regulator being the retinoic acid receptor-related orphan receptor (RORgammat) regulated by STAT3 signaling. Glucuronoxylomannan (GXM), a polysaccharide representing the main component of the capsular material of the opportunistic yeast Cryptococcus neoformans, exhibits potent immunosuppressive properties both in vitro and in vivo. The present study investigates the effects of GXM treatment on the progression of collagen-induced arthritis. GXM suppressed clinical signs of collagen-induced arthritis and blocked joint erosion progression. This effect was mediated by down-regulation of key cytokines involved in the pathogenesis of RA such as TNF-alpha and IL-1beta, and up-regulation of the inhibitory cytokine IL-10. Moreover, a reduction of IL-6 and TGF-beta, which inhibit Th17 differentiation with consequent decreased IL-17 production at the local and systemic level, was observed. The effect of GXM on Th17 differentiation mirrored the reduction in STAT3 activation and inhibition of RORgammat synthesis. Consequently, this work highlights the beneficial properties of an efficacious compound that could eventually be destined to the clinic.

Detection of IgM and IgG antibodies to Cryptococcus neoformans proteins in blood donors and HIV patients with active cryptococcosis

The serological responses to Cryptococcus neoformans proteins of blood donors and HIV patients with active cryptococcosis from a tropical region were investigated in this study. Exposure to C. neoformans, an organism ubiquitous in the environment, contributes to the antibody responses observed in the blood donors. IgG responses to cryptococcal proteins were stronger than IgM responses in most sera tested in this study. A 53-kDa cryptococcal protein fragment was identified as the most immunoreactive protein on the IgM immunoblots of both blood donors and patients. Overall, there was no obvious difference in IgG responses of patients when compared with blood donors. Some immunogenic protein fragments (27.5, 76, 78 and 91.5 kDa) were detected at least two times more frequently on IgM immunoblots of patients compared with those of blood donors. It is yet to be investigated whether the proteins identified in this study may have any potential to be used as biomarker for cryptococcosis.

Capsular polysaccharide induction of apoptosis by intrinsic and extrinsic mechanisms

A purified microbial capsular polysaccharide of Cryptococcus neoformans, glucuronoxylomannan (GXM), induces Fas ligand (FasL) upregulation on macrophages and, as a consequence, apoptosis of lymphocytes. The mechanisms that lead to lymphocyte apoptosis in both in vitro and in vivo systems were investigated by cytofluorimetric analysis and Western blotting experiments. Caspase 8 cleaves caspase 3 in two different pathways: directly as well as indirectly by activation of Bcl-2 interacting domain, which initiates caspase 9 cleavage. Therefore, the caspase 8 and caspase 9 pathways cooperate in an amplification loop for efficient cell death, and noteworthily we provide evidence that they are both activated in one single cell. Furthermore, both activation of GXM-mediated caspase 8 and apoptosis were also found in in vivo systems in an experimental model of murine candidiasis. Collectively, our data show that GXM-induced apoptosis involves, in a single cell, a cross-talk between extrinsic and intrinsic pathways. Such a finding offers opportunities for the therapeutic usage of this polysaccharide in appropriate clinical settings for taming T-cell responses.

Sex differences in the genetic architecture of susceptibility to Cryptococcus neoformans pulmonary infection

Cryptococcus neoformans is a major cause of fungal pneumonia, meningitis and disseminated disease in the immune compromised host. Here we have used a clinically relevant model to investigate the genetic determinants of susceptibility to progressive cryptococcal pneumonia in C57BL/6J and CBA/J inbred mice. At 5 weeks after infection, the lung fungal burden was over 1000-fold higher in C57BL/6J compared to CBA/J mice. A genome-wide scan performed on 210 male and 203 female (CBA/J x C57BL/6J) F2 progeny using lung colony-forming units as a quantitative trait revealed a sex difference in genetic architecture with three loci (designated Cnes1-Cnes3) associated with susceptibility to cryptococcal pneumonia. Single locus analysis identified significant loci on chromosomes 3 (Cnes1) and 17 (Cnes2) with logarithm of the odds (LOD) scores of 4.09 (P=0.0110) and 7.30 (P<0.0001) that explained 8.9 and 15.9% of the phenotypic variance, respectively, in female CBAB6F2 and one significant locus on chromosome 17 (Cnes3) with a LOD score of 4.04 (P=0.010) that explained 8.6% of the phenotypic variance in male CBAB6F2 mice. Genome-wide pair-wise analysis revealed significant quantitative trait locus interactions in both the female and male CBAB6F2 progeny that collectively explained 43.8 and 19.5% of phenotypic variance in each sex, respectively.

Antibody responses to Cryptococcus neoformans in Indian patients with cryptococcosis

An important element of the host response to cryptococcosis is humoral immunity. Specific antibody responses in patients with cryptococcosis however, have not been extensively studied. We analyzed the antibody responses of 22 Indian patients with cryptococcosis, including both HIV+ and HIV- individuals. Sera from 10 Indian patients with AIDS and without cryptococcosis were studied as controls. Antibody responses to cryptococcal proteins were detected by immunoblot, while antibodies to glucuronoxylomannan (GXM), the main component of the cryptococcal capsular polysaccharide were measured by ELISA. Our results indicate that cryptococcosis elicits antibodies to a specific pattern of cytoplasmic proteins. Further, we find that antibody responses to both cytoplasmic proteins and GXM are less robust in HIV+ patients when compared with HIV- patients.

Microbial immune suppression mediated by direct engagement of inhibitory Fc receptor

A microbial polysaccharide (glucuronoxylomannan (GXM)) exerts potent immunosuppression by direct engagement to immunoinhibitory receptor FcgammaRIIB. Activation of FcgammaRIIB by GXM leads to the recruitment and phosphorylation of SHIP that prevents IkappaBalpha activation. The FcgammaRIIB blockade inhibits GXM-induced IL-10 production and induces TNF-alpha secretion. GXM quenches LPS-induced TNF-alpha release via FcgammaRIIB. The addition of mAb to GXM reverses GXM-induced immunosuppression by shifting recognition from FcgammaRIIB to FcgammaRIIA. These findings indicate a novel mechanism by which microbial products can impair immune function through direct stimulation of an inhibitory receptor. Furthermore, our observations provide a new mechanism for the ability of specific Ab to reverse the immune inhibitory effects of certain microbial products.

Protective immunity against cryptococcus neoformans infection

Cryptococcus neoformans, the etiological agent of cryptococcosis, is an occasional opportunistic fungal pathogen of immune competent individuals. However, it is a relatively frequent cause of life-threatening meningoencephalitis and pulmonary infections in immunosuppressed hosts and is a leading mycological cause of morbidity and mortality among patients with AIDS in most parts of the world. The lack of an effective fungicidal regimen and the development of antifungal resistant strains suggest that continued investigation is necessary to devise immunotherapeutic strategies, drug targets and/or vaccines to combat C. neoformans infections. Until recently, cryptococcal virulence factors such as its polysaccharide capsule, macrophage parasitism, and its ability to induce an ineffective antibody mediated immune (AMI) response along with a non-protective type II (Th2) cell-mediated immune response have thwarted efforts to induce complete protective immunity against a lethal cryptococcal strain in murine models. The presence of C. neoformans antibodies in adult human serum suggests that immune competent individuals have difficulty resolving an early cryptococcal infection allowing for the establishment of a subclinical chronic infection. Recent studies have shown that pro-inflammatory cytokines, specifically interferon-g (IFN-gamma), associated with type I (Th1) cell-mediated immunity can successfully drive cell-mediated immune (CMI) responses to produce protective immunity to a second experimental C. neoformans infection in mice. This review will evaluate the intricacies of the host-cryptococcal interaction and discuss recent developments in C. neoformans research and the potential for human vaccines and/or drug therapies.

Antibody response to Cryptococcus neoformans capsular polysaccharide glucuronoxylomannan in patients after solid-organ transplantation

Cryptococcosis is an important complication of solid-organ transplantation, but the risk factors for disease are poorly understood. The goal of this study was to investigate whether specific or nonspecific serum immunoglobulin levels determined in samples obtained before and after solid-organ transplantation differed in patients who did or did not develop cryptococcosis after transplantation. We analyzed pretransplantation sera from 25 subjects, 15 who subsequently developed cryptococcosis and 10 who did not, and posttransplantation sera from 24 subjects, 13 who developed cryptococcosis and 11 who did not. All subjects received a tacrolimus-based immunosuppressive regimen. Total immunoglobulin levels were measured by immunodiffusion, and Cryptococcus neoformans capsular polysaccharide glucuronoxylomannan (GXM)-specific serum antibody levels were determined by enzyme-linked immunosorbent assays. The results showed that solid-organ transplantation had a significant effect on total immunoglobulin and GXM-reactive antibody levels. GXM-reactive antibody levels differed in subjects who did and did not develop cryptococcosis. In pretransplant serum samples, the levels of GXM-reactive immunoglobulin M (IgM) were significantly lower in subjects who developed cryptococcosis after transplantation than in those who did not. For posttransplant serum samples, the levels of GXM-reactive IgM and IgG were significantly higher among the subjects who developed cryptococcosis than among those who did not. These findings suggest that perturbations in the preexisting antibody or B-cell repertoire and/or related to treatment of rejection, transplantation, or immunosuppressive therapy could translate into an increased risk for transplant-associated cryptococcosis.

Towards a vaccine for Cryptococcus neoformans: principles and caveats

In the Damage-response framework of microbial pathogenesis, infectious diseases are one outcome of a host-microorganism interaction in a susceptible host. In cryptococcal disease, damage to the host is caused by Cryptococcus neoformans virulence determinants, the nature of the host response, or both. Further, the disease may be acute or reactivated from a latent state. Hence, a vaccine for C. neoformans would need to prevent disease resulting from either acute or reactivated infection. The evidence to support the development of a vaccine for C. neoformans that induces antibody-mediated immunity is discussed herein.

Differential localization of complement component 3 within the capsular matrix of Cryptococcus neoformans

The polysaccharide capsule of Cryptococcus neoformans is a powerful activator of the complement system. The goal of the present study was to assess serum and cellular variables that influence the sites for C3 binding within the capsular matrix. Confocal microscopy using fluorophore-labeled polyclonal anti-C3 and anticapsular monoclonal antibodies and rosetting of fluorescent microspheres coated with anti-C3 were used to identify sites of C3 binding relative to the capsular edge. The results showed that the source of serum was a major variable influencing localization of C3. C3 bound at or very near the capsular edge in the case of human serum. C3 deposition was further from the capsule edge with guinea pig and rat sera; in the case of mouse serum, there was no binding of C3 in the outer region of the capsule. Addition of human C3 to mouse serum led to deposition of the C3 at the capsular edge, indicating that distinct properties of mouse and human C3 account for the differential localization of C3. Finally, the density of the capsular matrix was an important variable in determining sites for C3 deposition. Yeast cells with a high concentration of polysaccharide near the capsule edge supported deposition of mouse C3 at or near the capsular edge, whereas cells with a low matrix density showed deposition well beneath the edge. Taken together, these results indicate that the spatial deposition of C3 within the capsular matrix is a complex process that is influenced by the serum source and the density of the capsular matrix.

Cryptococcus neoformans capsular polysaccharide component galactoxylomannan induces apoptosis of human T-cells through activation of caspase-8

The major virulence factor of Cryptococcus neoformans is its polysaccharide capsule composed of glucuronoxylomannan (GXM), galactoxylomannan (GalXM) and mannoproteins. A variety of immunomodulating activities have been described for GXM and mannoproteins but little is known about possible interactions of GalXM with the immune system. In the present article, we investigate the effect of purified soluble GalXM on human T lymphocytes. The results indicate that, GalXM (i) can affect selected immune responses; (ii) causes significant impairment of T cell proliferation and increases interferon-gamma and interleukin-10 production; and (iii) induces apoptosis of T lymphocytes through activation of caspase-8 that terminates with fragmentation of DNA. These results are the first to suggest a role for GalXM in C. neoformans virulence by demonstrating that it can target human T cells, and that it may impair the development of an effective specific T cell response.

Feasibility and prospects for a vaccine to prevent cryptococcosis

Cryptococcosis is a relatively common fungal disease caused by Cryptococcus neoformans that has high morbidity and mortality. Numerous studies have established the feasibility of enhancing host immunity to C neoformans in naive immunocompetent animal models by vaccination. Several antigens have been identified that appear to be suitable vaccine candidates. Induced immune responses can mediate protection through both humoral and cellular immunity. Hence, a vaccine against cryptococcosis in humans is probably feasible but there are significant obstacles to vaccine development that range from uncertainties about the pathogenesis of infection to economic considerations.

Cryptococcus neoformans-reactive and total immunoglobulin profiles of human immunodeficiency virus-infected and uninfected Ugandans

We determined total and Cryptococcus neoformans glucuronoxylomannan (GXM)-reactive antibody repertoires of human immunodeficiency virus (HIV)-infected and HIV-uninfected Ugandans in a retrospective, case-control study of participants in a randomized controlled trial of pneumococcal vaccination. The study included 192 adults: 48 who subsequently developed cryptococcal meningitis (CM); (HIV+ CM+); 2 individuals who matched them in CD4+ T-cell level, stage of HIV disease, and age but did not develop CM (HIV+ CM-); and 48 HIV-uninfected individuals. Total serum immunoglobulin concentrations and titers of immunoglobulin M (IgM), IgG, and IgA to GXM, pneumococcal polysaccharides, and antibodies expressing certain V(H)3 idiotypes were determined with banked sera obtained before the development of cryptococcosis for HIV+ CM+ subjects. The results showed that HIV-infected subjects had significantly lower levels of IgM to GXM but higher levels of total immunoglobulin and IgG and IgA to GXM than those of HIV-uninfected subjects. HIV-infected subjects with a history of pneumonia had higher levels, and those with a history of herpes zoster had lower levels of GXM-binding antibodies than subjects with no history of either disease. Minimal to no cross-reactivity was demonstrated between antibodies to GXM and polysaccharides in a pneumococcal vaccine. No significant differences between the antibody repertoires of HIV+ CM+ and HIV+ CM- subjects were identified, but among subjects without a history of pneumonia, there was a trend towards lower V(H)3-positive antibody levels among HIV+ CM+ than among HIV+ CM- subjects. Our findings demonstrate an association between previous infectious diseases and differences in the total and GXM-reactive antibody repertoires of HIV-infected subjects and suggest the question of whether certain microbes modulate subsequent antibody responses to GXM deserves further study.

Cryptococcus neoformans capsular glucuronoxylomannan induces expression of fas ligand in macrophages

The major component of capsular material of Cryptococcus neoformans is glucuronoxylomannnan (GXM), a polysaccharide that exhibits potent immunosuppressive properties in vitro and in vivo. The results reported here show that 1) soluble purified GXM induces a prompt, long-lasting, and potent up-regulation of Fas ligand (FasL) on macrophages, 2) the up-regulation of FasL is related to induced synthesis and increased mobilization to the cellular surface, 3) this effect is largely mediated by interaction between GXM and TLR4, 4) FasL up-regulation occurs exclusively in GXM-loaded macrophages, 5) macrophages that show up-regulation of FasL induce apoptosis of activated T cells expressing Fas and Jurkat cells that constitutively express Fas, and 6) anti-Fas Abs rescue T cells from apoptosis induced by GXM. Collectively our results reveal novel aspects of the immunoregulatory properties of GXM and suggest that this nontoxic soluble compound could be used to dampen the immune response, to promote or accelerate the death receptor, and to fix FasL expression in a TLR/ligand-dependent manner. In the present study, we delineate potential new therapeutic applications for GXM that exploit death receptors as key molecular targets in regulating cell-mediated cytotoxicity, immune homeostasis, and the immunopathology of diseases.

Glucuronoxylomannan, the major capsular polysaccharide of Cryptococcus neoformans, inhibits the progression of group B streptococcal arthritis

Glucuronoxylomannan (GXM), the principal constituent of the Cryptococcus neoformans capsule, modulates the inflammatory response of human monocytes in vitro. Here we examine the efficacy of GXM as a novel anti-inflammatory compound for use against experimental septic arthritis. Arthritis was induced in mice by the intravenous injection of 8 x 10(6) CFU of type IV group B streptococcus (GBS). GXM was administered intravenously in different doses (50, 100, or 200 microg/mouse) 1 day before and 1 day after bacterial inoculation. GXM treatment markedly decreased the incidence and severity of articular lesions. Histological findings showed limited periarticular inflammation in the joints of GXM-treated mice, confirming the clinical observations. The amelioration of arthritis was associated with a significant reduction in the local production of interleukin-6 (IL-6), IL-1beta, macrophage inflammatory protein 1alpha (MIP-1alpha), and MIP-2 and an increase in systemic IL-10 levels. Moreover, peritoneal macrophages derived from GXM-treated mice and stimulated in vitro with heat-inactivated GBS showed a similar pattern of cytokine production. The present study provides evidence for the modulation of the inflammatory response by GXM in vivo and suggests a potential therapeutic use for this compound in pathologies involving inflammatory processes.

[Seroepidemiological survey for infections by fungi causing systemic mycoses in the Xacriabá Indian Reserve, Minas Gerais State, Brazil]

Xacriabá Indian Reserve is situated in the north of Minas Gerais State, Brazil, near the municipality of Manga and São Francisco River. The Indian population is miscegenated with caucasian and negroid people and have farming activities. Blood samples were collected from 180 inhabitants of the reserve (85 men and 95 women), 15 to 84 years old. Serum antibodies against antigens of Paracoccidioides brasiliensis, Histoplasma capsulatum, Cryptococcus neoformans, and Candida albicans were tested for by counterimmunoelectrophoresis method. Seropositivity was verified in 5%, 3.9%, 4.2%, and 6.7%, respectively of the 180 samples. Those seropositive for antigens of P. brasiliensis and H. capsulatum were predominantly women and had lower age and more elevated mean titers of antibodies than individuals whose sera reacted with antigens of C. neoformans and C. albicans. The results suggest the occurrence of paracoccidioidomycosis-infection and histoplasmosis-infection in the Xacriabá Indian Reserve, particularly in the first decades of life of the inhabitants.

Modulation of C5aR expression on human neutrophils by encapsulated and acapsular Cryptococcus neoformans

Cryptococcus neoformans and cryptococcal surface polysaccharides influenced C5aR expression on human polymorphonuclear neutrophils (PMN). Encapsulated and acapsular strains produced dramatically different effects. Treatment of PMN with acapsular cryptococci up-regulated C5aR expression; whereas treatment with encapsulated cells suppressed C5aR expression. Glucuronoxylomannan (GXM), the principal constituent of the cryptococcal capsule, was responsible for such inhibition. Increased C5aR expression following treatment with acapsular cryptococci was accompanied by increased binding of C5a to PMN, increased superoxide production in response to stimulation with C5a, and an increased chemotactic response to C5a. Conversely, decreased C5aR expression following treatment with encapsulated cryptococci or acapsular cryptococci that had been pretreated with GXM was accompanied by decreased binding of C5a to PMN and a decreased chemotactic response to C5a. Our results raise the possibility that the down-regulation of C5aR expression by encapsulated cryptococci might alter PMN function at the site of cryptococcal infection.

Interdependency of interleukin-10 and interleukin-12 in regulation of T-cell differentiation and effector function of monocytes in response to stimulation with Cryptococcus neoformans

We previously demonstrated that the principal component of capsular material of Cryptococcus neoformans, glucuronoxylomannan (GXM), induces interleukin-10 (IL-10) secretion from human monocytes. Here we report that encapsulation of the yeast with GXM is able to down-regulate interleukin-12 (IL-12) production by monocytes that would normally occur in the absence of encapsulation. This phenomenon appeared to be the result of inhibition of the phagocytic process by encapsulation with GXM as well as of negative signals such as IL-10 secretion produced by interaction of GXM with leukocytes. Decreased secretion of IL-12 correlated with decreased release of gamma interferon (IFN-gamma) from T cells, suggesting a role for encapsulation with GXM in hindering a T helper type 1 (Th1) response. This is supported by the ability of encapsulation with GXM to limit increased expression of B7-1 costimulatory molecules that otherwise might limit IL-10 secretion. Endogenous IL-10 played a critical role in modulatory activity associated with encapsulation with GXM. Blocking IL-10 with monoclonal antibody to IL-10 resulted in increased (i) IL-12 secretion, (ii) IFN-gamma release from T cells, and (iii) killing of C. neoformans by monocytes. These results suggest that encapsulation with GXM limits development of a protective Th1-type response, an inhibitory process in which IL-10 plays a critical role. Scavengers of GXM and/or IL-10 could be useful in a protective Th1-type response in patients with cryptococcosis.

Serologic evidence for Cryptococcus neoformans infection in early childhood

OBJECTIVE

Cryptococcus neoformans is an important cause of central nervous system infection in adults with acquired immunodeficiency syndrome (AIDS) but an unusual cause of disease in children with AIDS. The basis for this age-related difference in incidence is not known but may be caused by differences in exposure or immune response. The objective of this study was to determine whether the low prevalence of cryptococcal disease among children is related to a lack of exposure to C neoformans.

METHODS

Sera were obtained from 185 immunocompetent individuals ranging in age from 1 week to 21 years who were being evaluated in an urban emergency department. Sera were analyzed for antibodies to C neoformans and Candida albicans proteins by immunoblotting. Immunoblot patterns were compared with those obtained from sera of patients with cryptococcosis (n = 10) and workers in a laboratory devoted to the study of C neoformans. The specificity of our results was confirmed by several approaches, including antibody absorption and blocking studies. Sera were also analyzed for the presence of cryptococcal polysaccharide by both enzyme-linked immunosorbent assay and latex agglutination assays.

RESULTS

Sera from children 1.1 to 2 years old demonstrated minimal reactivity to C neoformans proteins. In contrast, the majority of sera from children >2 years old recognized many (>/=6) C neoformans proteins. For children between 2.1 and 5 years old, 56% of sera (n = 25) reacted with many proteins, whereas for children >5 years old (n = 120), 70% of samples reacted with many proteins. Reactivity was decreased by absorbing sera with C neoformans extracts or by preincubating blots with sera from experimentally infected but not from control rats. Reactivity to C neoformans proteins did not correlate with reactivity to C albicans proteins, which was common in sera from children between the ages of 1.1 and 2 years. Cryptococcal polysaccharide was detected at a titer of 1:16 (~10 ng/mL) in the sera of 1 child, a 5.6-year-old boy who presented to the emergency department with vomiting.

CONCLUSIONS

Our findings provide both indirect and direct evidence of C neoformans infection in immunocompetent children. Our results indicate that C neoformans infects a majority of children living in the Bronx after 2 years old. These results are consistent with several observations: the ubiquitous nature of C neoformans in the environment, including its association with pigeon excreta; the large number of pigeons in urban areas; and the increased likelihood of environmental exposure for children once they have learned to walk. The signs and symptoms associated with C neoformans infection in immunocompetent children remained to be determined. Primary pulmonary cryptococcosis may be asymptomatic or produce symptoms confused with viral infections and, therefore, not recognized as a fungal infection. Our results suggest that the low incidence of symptomatic cryptococcal disease in children with AIDS is not a result of lack of exposure to C neoformans. These findings have important implications for C neoformans pathogenesis and the development of vaccine strategies.

Molecular basis for immunoglobulin M specificity to epitopes in Cryptococcus neoformans polysaccharide that elicit protective and nonprotective antibodies

The protective efficacy of antibodies (Abs) to Cryptococcus neoformans glucuronoxylomannan (GXM) is dependent on Ab fine specificity. Two clonally related immunoglobulin M monoclonal Abs (MAbs) (12A1 and 13F1) differ in fine specificity and protective efficacy, presumably due to variable (V)-region sequence differences resulting from somatic mutations. MAb 12A1 is protective and produces annular immunofluorescence (IF) on serotype D C. neoformans, while MAb 13F1 is not protective and produces punctate IF. To determine the Ab molecular determinants responsible for the IF pattern, site-directed mutagenesis of the MAb 12A1 heavy-chain V region (V(H)) was followed by serological and functional studies of the various mutants. Changing two selected amino acids in the 12A1 V(H) binding cavity to the corresponding residues in the 13F1 V(H) altered the IF pattern from annular to punctate, reduced opsonic efficacy, and abolished recognition by an anti-idiotypic Ab. Analysis of the binding of the various mutants to peptide mimetics revealed that different amino acids were responsible for GXM binding and peptide specificity. The results suggest that V-region motifs associated with annular binding and opsonic activity may be predictive of Ab efficacy against C. neoformans. This has important implications for immunotherapy and vaccine design that are reinforced by the finding that GXM and peptide reactivities are determined by different amino acid residues.

Capsular reactions of Cryptococcus neoformans with polyspecific and oligospecific polyclonal anticapsular antibodies

Monoclonal antibodies to the capsular polysaccharide of Cryptococcus neoformans produce distinct capsular reactions and have biological activities that are determined by serotype specificity. In the present study, polyclonal rabbit anticapsular antibodies were cross-absorbed to produce serotype specificities similar to those of monoclonal antibodies. The results showed that polyclonal and monoclonal antibodies with similar serotype specificities have similar capsular reactions and biological activities.

Immunoregulation by capsular components of Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated yeast that is pathogenic for humans. The capsule is a major virulence factor composed mainly of glucuronoxylomannan (GXM) and two minor constituents, galactoxylomannan, and mannoprotein (MP). A hallmark of disseminated cryptococcosis is the presence of high concentrations of GXM in body fluids of infected hosts. GXM provides a critical negative signal for T cell activation and neutrophil migration at the site of the inflammatory process. There is also strong evidence that MP promotes critical events associated with protective responses such as delayed type hypersensitivity and presumably a T helper type 1 response. The contrasting roles of GXM and MP in regulation of the immune response to C. neoformans offer a promising template for a successful approach to intervention, by scavenging GXM to attenuate its negative signals, while preserving the positive effects of MP.

Hipogamaglobulinemia como fator de risco para infecção por Cryptococcus neoformans: a propósito de dois casos

Dois pacientes HIV-soronegativos com doença criptocócica refratária à terapia antifúngica convencional foram submetidos a estudo do sistema imune. Hipogamaglobulinemia foi evidenciada em ambos e associada à alteração funcional da imunidade mediada por células. Hipogamaglobulinemia é considerada como um possível fator predisponente para infecção criptocócica. A importância dos anticorpos no controle da infecção por Cryptococcus neoformans é discutida.

A novel ELISA for determination of polysaccharide specific immunoglobulins

A novel ELISA using Nunc CovaLink microtiter plates has been developed for the determination of polysaccharide specific antibodies in mice sera. Glucuronoxylomannan (GXM), a major capsular polysaccharide of Cryptococcus neoformans serotype B, was immobilised on CovaLink plates by covalent linkage of CNBr activated hydroxyl groups of the sugar and free bnd NH(2) groups of the plates. The binding characteristics of GXM to CovaLink and to conventional polystyrene ELISA plate (Costar) were compared. The differences were observed in quality of standard curve with anti-GXM MoAb 2H1 (R(2) values were 0.9468 and 0.9872 for Costar and CovaLink plates, respectively) and in absorbance values of sera of immunised mice which were 2.5 times higher on CovaLink than on Costar plates. Negative control was low and having the same value on both the plates. Using the novel ELISA we have analysed the influence of immunomodulatory peptidoglycan monomer on humoral immune response to GXM in mice. The treatment with the conjugate of the immunomodulator and GXM resulted in the increase of GXM-specific IgGs in mice sera. Finally, the method has been successfully modified for the determination of dextran-specific antibodies in mice sera, indicating that the described procedure could be applied for other polysaccharides that have &z.sbnd;OH groups available for CNBr activation.

An opsonizing monoclonal antibody that recognizes a noncapsular epitope expressed on Cryptococcus neoformans

A mouse hybridoma secreting a monoclonal antibody (MAb) that bound a noncapsular epitope expressed on C. neoformans was developed by immunizing BALB/c mice with formalin-killed serotype A yeasts. The hybridoma, designated CSFi, secreted an immunoglobulin G2b MAb that reacted with all C. neoformans serotypes tested, including the acapsular mutant ATCC 52817 (Cap67). Postsectioned immune electron microscopy revealed extensive binding of the MAb to the cell walls of both encapsulated and acapsular yeasts. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis of secreted antigens recovered from concentrated culture supernatants from both encapsulated and acapsular strains was conducted. The results showed that this MAb bound predominantly to antigens with molecular masses of approximately 75 and 100 kDa. A competitive enzyme-linked immunosorbent assay was used to demonstrate that the MAb was not cross-reactive with purified glucuronoxylomannan derived from either serotypes A or D. Experiments conducted with mouse peritoneal phagocytes and the mouse phagocyte-like cell line, J774A.1, demonstrated that the CSFi MAb opsonized the yeasts and increased their adherence to both types of phagocytic cells. We conclude, therefore, that antibodies directed at noncapsular epitopes can serve as opsonins and may have a role in modulating cryptococcal infection.