Antibody-mediated protection in mice with lethal intracerebral Cryptococcus neoformans infection

Disseminated Cryptococcus infection presenting as lytic skeletal lesions suggesting bony metastatic disease

A 76-year-old male diagnosed with sarcoidosis presented with atraumatic left anterior knee pain. Initial imaging of the left lower extremity revealed an eccentrically-based lytic lesion in the mid-distal femur with cortical erosion and an additional lytic lesion in the proximal tibia. Magnetic resonance imaging (MRI) demonstrated an aggressive lesion in the proximal tibia with surrounding marrow edema, cortical breach, and erosion into the distal patellar tendon. Given concern for metastatic bone lesions, a18-fluorodeoxyglucose positron emission tomography/computed tomography scan (FDG PET/CT) was performed which demonstrated concordant hypermetabolic lytic lesions at the left mid-distal femur and the left proximal tibia, as well as hypermetabolic diffuse lymphadenopathy. The patient was presumed to have metastatic lung cancer based on the presence of lung nodules. Due to concern for impending pathologic fracture, the patient underwent open biopsy with a plan for prophylactic fixation of both lesions. Intra-operatively, however, both lesions were found to contain pus, from which cultures ultimately grew Cryptococcus neoformans. This is a case of disseminated skeletal cryptococcosis masquerading as metastatic cancer in a patient without classic risk factors for disseminated cryptococcosis (defined as extrapulmonary evidence of infection). Classically, disseminated cryptococcosis is thought to occur in severely immunocompromised patients, such as those with human immunodeficiency virus (HIV) or organ transplant recipients. This case highlights the need to maintain a high index of suspicion in patients with underlying immunocompromising conditions, including less common conditions such as sarcoid, who present with bony lesions. This case report then discusses the diagnostic evaluation and treatment of disseminated skeletal cryptococcosis.

Structural basis of Blastomyces Endoglucanase-2 adjuvancy in anti-fungal and -viral immunity

The development of safe subunit vaccines requires adjuvants that augment immunogenicity of non-replicating protein-based antigens. Current vaccines against infectious diseases preferentially induce protective antibodies driven by adjuvants such as alum. However, the contribution of antibody to host defense is limited for certain classes of infectious diseases such as fungi, whereas animal studies and clinical observations implicate cellular immunity as an essential component of the resolution of fungal pathogens. Here, we decipher the structural bases of a newly identified glycoprotein ligand of Dectin-2 with potent adjuvancy, Blastomyces endoglucanase-2 (Bl-Eng2). We also pinpoint the developmental steps of antigen-specific CD4⁺ and CD8⁺ T responses augmented by Bl-Eng2 including expansion, differentiation and tissue residency. Dectin-2 ligation led to successful systemic and mucosal vaccination against invasive fungal infection and Influenza A infection, respectively. O-linked glycans on Bl-Eng2 applied at the skin and respiratory mucosa greatly augment vaccine subunit- induced protective immunity against lethal influenza and fungal pulmonary challenge.

Fungal disease remains a challenging clinical and public health problem in part because there is no commercial vaccine available. The lack of suitable adjuvants is a critical barrier to developing safe and effective vaccines against fungal pathogens. Current adjuvants such as alum preferentially induce antibody responses which may be limited in mediating protection against fungi. Clinical observations and animal studies implicate cellular immunity as the essential component for the resolution of fungal infections. We have recently discovered an adjuvant that augments cell mediated immune responses and vaccine induced protection against fungi. Here, we identified the structural and mechanistic requirements by which this newly discovered adjuvant induces cell mediated immunity against fungi. As a proof of principle we also demonstrate that the adjuvant drives cellular immune responses against viruses such as influenza. We anticipate that our adjuvant can be used for vaccination with safe subunit vaccines against many microbial pathogens including viruses, intracellular bacteria, fungi and parasites that require cell mediated immune responses.

Reassessing therapeutic antibodies for neglected and tropical diseases

In the past two decades there has been a significant expansion in the number of new therapeutic monoclonal antibodies (mAbs) that are approved by regulators. The discovery of these new medicines has been driven primarily by new approaches in inflammatory diseases and oncology, especially in immuno-oncology. Other recent successes have included new antibodies for use in viral diseases, including HIV. The perception of very high costs associated with mAbs has led to the assumption that they play no role in prophylaxis for diseases of poverty. However, improvements in antibody-expression yields and manufacturing processes indicate this is a cost-effective option for providing protection from many types of infection that should be revisited. Recent technology developments also indicate that several months of protection could be achieved with a single dose. Moreover, new methods in B cell sorting now enable the systematic identification of high-quality antibodies from humanized mice, or patients. This Review discusses the potential for passive immunization against schistosomiasis, fungal infections, dengue, and other neglected diseases.

Antifungal efficacy of amphotericin B encapsulated fibrin microsphere for treating Cryptococcus neoformans infection in Swiss albino mice

A natural and biocompatible fibrin microsphere is one of the most promising dual delivery vehicle as compared to other traditionally designed delivery modalities. It represents sustained delivery of encapsulated drug and is easily biodegradable in the blood circulation. In the present study, we evaluated the systemic augmentation of the antifungal activity of amphotericin B loaded in fibrin microsphere (AMB-fibrin microsphere) against cryptococcosis in Swiss albino mice. Mice infected with Cryptococcus neoformans were treated with 0.5mg/kg AMB-fibrin microsphere that was given alternately for 7 days. The antifungal potential of AMB-fibrin microsphere was assessed on the basis of reduction of cfu count in the systemic circulation and various vital organs of infected mice. The formulation was found to be highly effective in reducing intracellular pathogen from the experimental animals where fibrin microsphere significantly controlled the release of amphotericin B for longer time duration. The AMB-fibrin microsphere chemotherapy was significantly more effective than free amphotericin B in reducing the fungal burden and showed better survival efficacy (p<0.05). The current study demonstrating the use of novel amphotericin B loaded fibrin microsphere not only imparts protection to the encapsulated amphotericin B but also offers an effective strategy to decrease the drug associated toxicities.

Cryptococcus antigens and immune responses: implications for a vaccine

Cryptococcosis is a fungal disease primarily occurring in immunocompromised individuals, such as AIDS patients, and is associated with high morbidity and mortality. However, cryptococcosis can occur within immunocompetent populations as observed during an outbreak in Vancouver Island, British Columbia, Canada, the Pacific Northwest and other regions of the USA and in Mediterranean Europe. Mortality rates due to cryptococcosis have significantly declined in economically developed countries since the widespread implementation of highly active antiretroviral therapy. However, the incidence and mortality of this disease remains high in economically undeveloped areas in Africa and Asia where HIV infections are high and availability of HAART is limited. The continuing AIDS epidemic coupled with the increased usage of immunosuppressive drugs to prevent organ transplant rejection or to treat autoimmune diseases has resulted in an increase in individuals at risk for developing cryptococcosis. The purpose of this review is to discuss the need, challenges and potential for developing vaccines against cryptococcosis.

Toward identification of the genetic risk profile for cryptococcal disease in HIV-infected patients

Cryptococcus spp. are important fungal pathogens that represent a major cause of morbidity and mortality in both immunocompetent and immunodeficient patients. Although cryptococcal disease is one of the major causes of death in HIV-infected patients, especially in sub-Saharan Africa, not all patients at risk with low CD4 counts develop the disease. It has been thus hypothesized that host genetic variation may represent an important susceptibility risk factor for this infection. In their recent study in mBio, Rohatgi et al. [S. Rohatgi et al., mBio 4(5):e00573-13, 2013, doi:10.1128/mBio.00573-13] present an important piece of evidence to support this hypothesis, by demonstrating that the FCGR3A 158 F/V polymorphism has an important impact on susceptibility to cryptococcal disease in HIV-infected patients. The authors present both genetic evidence and immunological validation for the hypothesis that humoral immunity in general and FCGR3A-mediated uptake and antibody-dependent cellular cytotoxicity (ADCC) in particular play important roles in the pathogenesis of Cryptococcus infection. Their discovery that the 158V allele of this polymorphism can increase the risk of Cryptococcus infections up to 20-fold in homozygous individuals opens the possibility for risk stratification and personalized treatment of HIV-infected patients.

Epidemiology of cryptococcal meningitis in the US: 1997-2009

Cryptococcal meningitis (CM) causes significant morbidity and mortality globally; however, recent national trends have not been described. Incidence and trends for CM-associated hospitalizations in 18 states were estimated using the Agency for Healthcare and Research Quality (AHRQ) State Inpatient Databases (SID) datasets for 1997 through 2009. We identified 30,840 hospitalizations coded for CM, of which 21.6% were among HIV-uninfected patients. CM in-hospital mortality was significant (12.4% for women and 10.8% for men) with a total of 3,440 deaths over the study period. Co-morbidities of CM coded at increased frequency in HIV-uninfected CM hospitalized populations included hydrocephalus and acute/chronic renal failure as well as possible predispositions including transplantation, combined T and B cell defects, Cushing’s syndrome, liver disease and hypogammaglobulinemia. Median hospitalization costs were significant for CM and higher for HIV-uninfected patients (16,803.01 vs. 15,708.07; p<0.0001). Cryptococcal meningitis remains a disease with significant morbidity and mortality in the U.S. and the relative burden among persons without HIV infection is increasing.

Vaccine potential of cytosolic proteins loaded fibrin microspheres of Cryptococcus neoformans in BALB/c mice

Cryptococcosis is a leading mycological cause of mortality among immunologically compromised individuals. In order to develop an effective vaccine against Cryptococcus neoformans, the cytosolic proteins (Cp) of the pathogen have been used as an antigen in combination with different formulations. In the present study, we have demonstrated that Cp encapsulated poly-lactide co-glycolide (PLGA) microsphere further co-encapsulated into the biocompatible fibrin cross-linked plasma beads (Fib-PLGA-Cp) mediated cytosolic delivery elicited strong immune response in the BALB/c mice. In contrast, other formulations of Cp failed to impart significant level of protection. The immune response, involved with Fib-PLGA-Cp protection, appear to interact with the target cells by both endocytosis as well as membrane fusion mode, thus helping in the activation of both CD4(+) and CD8(+) T-cells. Analysis of cytokine profiles in immunized animals revealed that the protective response was associated with the Th1/Th2 polarization in favor of type-1 cytokine [interferons (IFN)-γ and interleukin (IL)-2] cells. Furthermore, vaccination with Fib-PLGA-Cp elicited high immunoglobulin (Ig) G(l) and IgG(2a) isotype response; successfully cleared fungal burden in vital organs and also increased the survival rate of immunized animals. Altogether the present study is a clear indicative of the possible use of fibrin microsphere-based targeted delivery of cytosolic proteins to induce protective immune responses against experimental murine cryptococcosis.

Strain-related differences in antibody-mediated changes in gene expression are associated with differences in capsule and location of binding

We recently established that antibody (Ab)-binding can induce gene expression changes in a serotype A strain (H99) of the pathogenic yeast, Cryptococcus neoformans. That study showed that monoclonal antibodies (mAbs) differing in epitope specificity and protective efficacy elicited differences in gene expression. Because many mAbs bind to serotypes A and D strains differently, we now investigate the binding of one mAb to two strains representing these serotypes. Cells of the serotype A strain H99 and the serotype D strain 24067 were incubated with near saturating concentrations of the IgG1 capsule-binding mAb 18B7 or MOPC, an irrelevant mAb matched control. Comparative immunofluorescence analysis of mAb 18B7 binding revealed that it bound closer to the cell wall in H99 than 24067, where it was associated with decreased or increased cell diameter, respectively. A comparison of encapsulated cell compressibility showed that strain 24067 was more compressible than that of strain H99. RNA was extracted and used for gene expression analysis using the C. neoformans JEC21 genomic microarray. After 1h incubation with mAb 18B7, there were just 2 gene expression changes observed with strain 24067 or strain JEC21, unlike the 43 seen with strain H99. After 4h incubation with mAb 18B7, there were 14 and 140 gene expression changes observed with strain 24067 and JEC21, respectively. Thus, C. neoformans strains differ both in the response and the time of response to mAb binding and these differences may reflect differences in the location of Ab binding, Ab-mediated changes in cell diameter and compressibility of the capsular polysaccharide.

Methylation of glycosylated sphingolipid modulates membrane lipid topography and pathogenicity of Cryptococcus neoformans

In previous studies we showed that the replication of Cryptococcus neoformans in the lung environment is controlled by the glucosylceramide (GlcCer) synthase gene (GCS1), which synthesizes the membrane sphingolipid GlcCer from the C9-methyl ceramide. Here, we studied the effect of the mutation of the sphingolipid C9 methyltransferase gene (SMT1), which adds a methyl group to position 9 of the sphingosine backbone of ceramide. The C. neoformans Δsmt1 mutant does not make C9-methyl ceramide and, thus, any methylated GlcCer. However, it accumulates demethylated ceramide and demethylated GlcCer. The Δsmt1 mutant loses more than 80% of its virulence compared with the wild type and the reconstituted strain. Interestingly, growth of C. neoformans Δsmt1 in the lung was decreased and C. neoformans cells were contained in lung granulomas, which significantly reduced the rate of their dissemination to the brain reducing the onset of meningoencephalitis. Thus, using fluorescent spectroscopy and atomic force microscopy we compared the wild type and Δsmt1 mutant and found that the altered membrane composition and GlcCer structure affects fungal membrane rigidity, suggesting that specific sphingolipid structures are required for proper fungal membrane organization and integrity. Therefore, we propose that the physical structure of the plasma membrane imparted by specific classes of sphingolipids represents a critical factor for the ability of the fungus to establish virulence.

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.

Cryptococcus neoformans and Cryptococcus gattii genes preferentially expressed during rat macrophage infection

Cryptococcus neoformans and Cryptococcus gattii are encapsulated yeast agents of cryptococcosis and facultative intracellular pathogens. The interaction of these yeasts with macrophages is essential for containing the infection. However, Cryptococcus spp. overcome this initial host defense barrier using a unique pathogenic strategy involving intracellular replication and cytoplasmic accumulation of polysaccharide-containing vesicles. Here, we employed representational difference analysis (RDA) to identify C. neoformans and C. gattii genes differentially expressed during intracellular growth in rat peritoneal macrophages. The upregulated transcripts of C. neoformans during macrophage interaction were related to ATP-binding cassette (ABC) transporters, intra-golgi transport, chaperone activity, ribosomal maintenance, NAD metabolism, histone methylation, stress response, and monosaccharide metabolism. In contrast, with C. gattii, upregulated genes were associated with cell growth, aerobic respiration, protein binding, microtubule nucleation, monosaccharides and nitrogen metabolism, inositol or phosphatidylinositol phosphatase activity, cellular signaling, and stress response. Our findings reveal new genes that may be necessary for the intracellular parasitism of C. neoformans and C. gattii.

Potential of anti-Candida antibodies in immunoprophylaxis

The need for new options for the treatment of invasive candidiasis has fuelled the use of antibodies in combination with conventional antifungal therapy. After a long period of time in which antibodies were considered irrelevant in the resistance against invasive candidiasis, it was demonstrated that a number of antibodies or their engineered derivatives directed against Candida albicans cell-wall polysaccharides and glycopeptides, as well as against some protein epitopes, confer protection against invasive candidiasis. This has confirmed this approach as a new strategy for the prophylaxis of invasive candidiasis. Of particular interest is Mycograb, a human recombinant monoclonal antibody that inhibits heat shock protein 90, and has been administrated in combination with lipid-associated amphotericin B to patients with invasive candidiasis, and the fungicidal anti-beta-glucan antibodies induced by the glycoconjugate vaccine composed of a beta-glucan polysaccharide conjugated with the diphtheria toxoid CRM 197. However, despite the promising data obtained in vitro and in animal models, at present there is very little clinical experience on the use of antibodies in Candida immunoprophylaxis.

Pharmacotherapy of cryptococcosis

Cryptococcus neoformans is an opportunistic fungal pathogen that causes central nervous system and pulmonary disease among immunocompromised patients. Recent studies highlight the high incidence of cryptococcosis among patients in certain resource-poor areas, where there is also a high incidence of AIDS. Despite the availability of antifungal agents with anticryptococcal activity, the mortality and treatment-failure rates associated with cryptococcosis remain unacceptably high. This article reviews current treatment strategies and the basis for these strategies, as well as prospects for future approaches to the treatment of cryptococcosis.

Insights into the mechanisms of protective immunity against Cryptococcus neoformans infection using a mouse model of pulmonary cryptococcosis

Cryptococcus neoformans is an opportunistic fungal pathogen that causes life-threatening pneumonia and meningoencephalitis in immune compromised individuals. Previous studies have shown that immunization of BALB/c mice with an IFN-gamma-producing C. neoformans strain, H99gamma, results in complete protection against a second pulmonary challenge with an otherwise lethal cryptococcal strain. The current study evaluated local anamnestic cell-mediated immune responses against pulmonary cryptococcosis in mice immunized with C. neoformans strain H99gamma compared to mice immunized with heat-killed C. neoformans (HKC.n.). Mice immunized with C. neoformans strain H99gamma had significantly reduced pulmonary fungal burden post-secondary challenge compared to mice immunized with HKC.n. Protection against pulmonary cryptococcosis was associated with increased pulmonary granulomatous formation and leukocyte infiltration followed by a rapid resolution of pulmonary inflammation, which protected the lungs from severe allergic bronchopulmonary mycosis (ABPM)-pathology that developed in the lungs of mice immunized with HKC.n. Pulmonary challenge of interleukin (IL)-4 receptor, IL-12p40, IL-12p35, IFN-gamma, T cell and B cell deficient mice with C. neoformans strain H99gamma demonstrated a requirement for Th1-type T cell-mediated immunity, but not B cell-mediated immunity, for the induction of H99gamma-mediated protective immune responses against pulmonary C. neoformans infection. CD4(+) T cells, CD11c(+) cells, and Gr-1(+) cells were increased in both proportion and absolute number in protected mice. In addition, significantly increased production of Th1-type/pro-inflammatory cytokines and chemokines, and conversely, reduced Th2-type cytokine production was observed in the lungs of protected mice. Interestingly, protection was not associated with increased production of cytokines IFN-gamma or TNF-alpha in lungs of protected mice. In conclusion, immunization with C. neoformans strain H99gamma results in the development of protective anti-cryptococcal immune responses that may be measured and subsequently used in the development of immune-based therapies to combat pulmonary cryptococcosis.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

The relative susceptibility of mouse strains to pulmonary Cryptococcus neoformans infection is associated with pleiotropic differences in the immune response

CBA/J mice were highly susceptible to intratracheal (i.t.) Cryptococcus neoformans infection relative to BALB/c mice, while both strains were equally susceptible to intravenous (i.v.) infection. Increased susceptibility in i.t. infection was associated with higher brain CFU, lower serum immunoglobulin M (IgM) and IgG responses to glucuronoxylomannan (GXM), lack of IgE regulation during infection, and alveolar macrophage permissiveness to intracellular replication in vitro. In contrast, for BALB/c mice, relative resistance was associated with increased interleukin-12 (IL-12) and decreased IL-10 pulmonary levels. In CBA/J mice, relative susceptibility was associated with a decreased proportion of CD4+ and CD8+ T cells and an increase in macrophage percentage in pulmonary infiltrates. In contrast, no significant differences in these cytokines or cell recruitment were observed in the i.v. model, consistent with no differences in the survival rate. Passive antibody (Ab) protection experiments revealed a prozone effect in the BALB/c mice with i.v. infection, such that Ab efficacy decreased at higher doses. In the i.t. model using CBA/J mice, low Ab doses were disease enhancing and protection was observed only at high doses. Our results show (i) that differences in mouse strain susceptibility are a function of the infection model, (ii) that susceptibility to pulmonary infection was associated with macrophage permissiveness for intracellular replication, and (iii) that the efficacy of passive Ab in pulmonary infection is a function of dose and mouse strain. The results highlight significant differences in the pathogenesis of cryptococcal infection among inbred mice and associate their relative susceptibility with differences in numerous components of the innate and adaptive immune responses.

Characterization of two novel cryptococcal mannoproteins recognized by immune sera

Host defenses against the encapsulated yeast Cryptococcus neoformans involve both humoral and cell-mediated immunity. Mannoproteins (MPs) are a heterogeneous class of immunodominant glycoproteins which have been only incompletely characterized. In this study, we report on the molecular features of two novel MPs that are recognized by serum antibodies during cryptococcosis. After fractionation of extracellular cryptococcal products, MPs reacted more strongly than other components with sera from C. neoformans-infected AIDS patients. Further fractionation and Western blot analysis of MPs evidenced the presence of highly reactive bands with molecular masses of 250, 125, 115, and 84 kDa. The 115- and 84-kDa bands contained significant amounts of N-linked oligosaccharides, as shown by decreased molecular mass after peptide-N-glycosidase F treatment. N-terminal amino acid sequences of the two bands were used to search C. neoformans nucleotide databases. Homologous genomic sequences were used to synthesize DNA probes and isolate cDNA clones containing the full-length genes, which were designated MP84 and MP115. Both genes showed the presence of a serine/threonine-rich region, a potential site for heavy glycosylation. MP84 and MP115 showed homology with, respectively, polysaccharide deacetylases and carboxylesterases from other organisms. Recombinant, deglycosylated proteins expressed in Escherichia coli still reacted with sera from patients, albeit more weakly than natural MPs, indicating that at least some of the reactive epitopes were retained in the recombinant forms. In conclusion, we identified two novel MPs that are important targets of antibody responses during cryptococcosis. These data may be useful to devise alternative immunity-based strategies to control the disease.

Mouse genetic background is a major determinant of isotype-related differences for antibody-mediated protective efficacy against Cryptococcus neoformans

The protective efficacy of mAbs to Cryptococcus neoformans glucuronoxylomannan depends on Ab isotype. Previous studies in A/JCr and C57BL/6J mice showed relative protective efficacy of IgG1, IgG2a >> IgG3. However, we now report that in C57BL/6J x 129/Sv mice, IgG3 is protective while IgG1 is not protective, with neither isotype being protective in 129/Sv mice. IgG1, IgG2a, and IgG3 had different effects on IFN-gamma expression in infected C57BL/6J x 129/Sv mice. IgG1-treated C57BL/6J x 129/Sv mice had significantly more pulmonary eosinophilia than IgG2a- and IgG3-treated C57BL/6J x 129/Sv mice. C. neoformans infection and Ab administration had different effects on FcgammaRI, FcgammaRII, and FcgammaRIII expression in C57BL/6J, 129/Sv, and C57BL/6J x 129/Sv mice. Our results indicate that the relative efficacy of Ab isotype function against C. neoformans is a function of the genetic background of the host and that IgG3-mediated protection in C57BL/6J x 129/Sv mice was associated with lower levels of IFN-gamma and fewer pulmonary eosinophils. The dependence of isotype efficacy on host genetics underscores a previously unsuspected complex relationship between the cellular and humoral arms of the adaptive immune response.

Phase I evaluation of the safety and pharmacokinetics of murine-derived anticryptococcal antibody 18B7 in subjects with treated cryptococcal meningitis

A promising approach to improving outcomes in patients with cryptococcal meningitis is to use adjunctive passive immunotherapy with a monoclonal antibody (MAb) directed against the capsular polysaccharide of Cryptococcus neoformans. This is the first application of MAb therapy for the treatment of a fungal disease in humans. We determined the safety and maximum tolerated dose of the murine anticryptococcal MAb 18B7 in a phase I dose-escalation study. The subjects were human immunodeficiency virus-infected patients who had been successfully treated for cryptococcal meningitis. Six dosing cohorts received MAb 18B7 at 0.01 to 2 mg/kg of body weight as a single infusion. Three patients each received 0.01, 0.05, 0.2, and 0.5 mg of MAb 18B7 per kg without significant adverse events. Four of the subjects who received the 1-mg/kg dose had mild study drug-associated toxicity, including transient nausea, vomiting, back pain, and urticarial rash. Two of the subjects who received 2 mg/kg developed drug-associated mild to moderate nausea, vomiting, chills, and myalgias. One of the subjects who received 2 mg/kg developed intracranial hypertension 10 weeks after MAb 18B7 administration. Serum cryptococcal antigen titers in the cohorts receiving doses of 1 and 2 mg/kg declined by a median of twofold at 1 week and a median of threefold at 2 weeks postinfusion, but the titers subsequently returned toward the baseline values by week 12. The half-life of MAb 18B7 in serum was approximately 53 h, while the MAb was undetectable in the cerebrospinal fluid of all patients. These data support the continued investigation of MAb 18B7 at a maximum single dose of 1.0 mg/kg.

Antibody-mediated protection against Cryptococcus neoformans pulmonary infection is dependent on B cells

The pathogenesis of pulmonary Cryptococcus neoformans infection and the efficacy of passive immunoglobulin G1 (IgG1) administration were investigated in B-cell-deficient and C57BL/6J mice. C57BL/6J mice lived longer than B-cell-deficient mice after both intratracheal and intravenous infections. Administration of IgG1 prior to infection prolonged the survival of C57BL/6J mice but had no effect on the survival or numbers of CFU in the lungs of B-cell-deficient mice. C. neoformans infection in B-cell-deficient mice resulted in significantly higher levels of gamma interferon (IFN-gamma), monocyte chemoattractant protein-1 (MCP-1), and macrophage inflammatory protein-1alpha (MIP-1alpha) than in C57BL/6J mice. IgG1 administration reduced IFN-gamma and MCP-1 levels in C57BL/6J mice but not in B-cell-deficient mice. In addition, compared to its effect in C57BL/6J mice, C. neoformans infection in FcRgammaIII-deficient, athymic, and SCID mice significantly increased IFN-gamma and MCP-1 levels. IgG1 administration was associated with reduced IFN-gamma levels in C57BL/6J mice but not in FcRgammaIII-deficient, athymic, and SCID mice. These observations suggest that IgG1-mediated protection in this system is a consequence of alterations in the inflammatory response that translate into less damage to the host without directly reducing the fungal burden. For hosts with impaired immunities, the ineffectiveness of passive antibody (Ab) may reflect an inability to down-regulate inflammation and avoid self-damage. The results indicate an important role for B cells in host defense against C. neoformans infection and demonstrate a surprising dependence of Ab-mediated protection on B cells in this system.

Effects of antifungal interventions on the outcome of experimental infections with phenotypic switch variants of Cryptococcus neoformans

In cryptococcal infection, phenotypic switching from a smooth to a mucoid variant can occur in vivo, producing variants with enhanced virulence that are subsequently selected and affect the outcome of infection. Here, we demonstrate that antifungal treatment of the chronically infected host can promote this phenomenon. Amphotericin B treatment reduces fungal burden less effectively in mucoid variant-infected than in smooth variant-infected mice. Consequently, amphotericin B treatment resulted in a more pronounced prolongation of survival in smooth variant-infected than in mucoid variant-infected mice (20 versus 42 days; P < 0.05). Administration of anticapsular monoclonal antibody mediated better protection in smooth variant-infected than in mucoid variant-infected mice, although a protective effect was not consistently observed at all doses. Most interestingly, both antifungal drug therapy and administration of anticapsular monoclonal antibody promoted the selection of mucoid variants in smooth variant-infected mice, a phenomenon manifested by a statistically higher percentage of mucoid colonies in smooth variant-infected mice than in nontreated control mice. This finding suggests that both chemotherapeutic and immunological antifungal interventions may promote the selection of the more virulent mucoid variant, which could affect the outcome of infection in chronically infected hosts.

Antibodies produced in response to Cryptococcus neoformans pulmonary infection in mice have characteristics of nonprotective antibodies

Murine cryptocococcal pulmonary infection elicited serum immunoglobulin M (IgM) and IgG to the capsular polysaccharide, but only IgG stained yeast cells in alveoli. Both isotypes produced punctuate immunofluorescence patterns on yeast cells like those of nonprotective antibodies. The difficulties involved in associating humoral immunity with protection in murine cryptocococcal infection could reflect nonprotective antibody responses.

Antibodies to keyhole limpet hemocyanin cross-react with an epitope on the polysaccharide capsule of Cryptococcus neoformans and other carbohydrates: implications for vaccine development

Cryptococcus neoformans causes a life-threatening meningoencephalitis in AIDS patients. Mice immunized with a glycoconjugate vaccine composed of the glucuronoxylomannan (GXM) component of the cryptococcal capsular polysaccharide conjugated to tetanus toxoid produce Abs that can be either protective or nonprotective. Because nonprotective Abs block the efficacy of protective Abs, an effective vaccine must focus the Ab response on a protective epitope. Mice immunized with peptide mimetics of GXM conjugated to keyhole limpet hemocyanin (KLH) with glutaraldehyde developed Abs to GXM. However, control peptides P315 and P24 conjugated to KLH also elicited Abs to GXM. GXM-binding Abs from mice immunized with P315-KLH were inhibited by KLH treated with glutaraldehyde (KLH-g), but not by P315. Furthermore, KLH-g inhibited binding of GXM by serum of mice immunized with GXM-TT, indicating that glutaraldehyde treatment of KLH reveals an epitope(s) that cross-reacts with GXM. Vaccination with KLH-g or unmodified KLH elicited Abs to GXM, but did not confer protection against C. neoformans, suggesting the cross-reactive epitope on KLH was not protective. This was supported by the finding that 4H3, a nonprotective mAb, cross-reacted strongly with KLH-g. Sera from mice immunized with either native KLH or KLH-g cross-reacted with several other carbohydrate Ags, many of which have been conjugated to KLH for vaccine development. This study illustrates how mAbs can be used to determine the efficacy of potential vaccines, in addition to describing the complexity of using KLH and glutaraldehyde in the development of vaccines to carbohydrate Ags.

High affinity mimotope of the polysaccharide capsule of Cryptococcus neoformans identified from an evolutionary phage peptide library

Cryptococcus neoformans causes a life-threatening meningoencephalitis in a significant percentage of AIDS patients. Mice immunized with a glycoconjugate vaccine composed of the glucuronoxylomannan (GXM) component of the cryptococcal capsular polysaccharide conjugated to tetanus toxoid (TT) produce Abs that, based on the epitope recognized, can be either protective or nonprotective. Since nonprotective Abs block the efficacy of protective Abs, we are interested in developing a vaccine that would focus the immune response specifically to protective epitopes. Previously, we screened a phage display library with 2H1, a protective anti-GXM mAb, and isolated PA1, a representative peptide that had a K(d) of 295 nM for 2H1. Mice immunized with PA1 conjugated to keyhole limpet hemocyanin developed high anti-peptide (1/13,000), but low anti-GXM (maximum, 1/200) titers. We now report our efforts to improve this vaccine by screening a sublibrary with six random amino acids added to either end of the PA1 motif to identify higher affinity peptides. P206.1, a peptide isolated from this sublibrary, had 80-fold higher affinity for 2H1 (K(d) = 3.7 nM) than PA1. P206.1 bound protective, but not nonprotective, anti-GXM Abs. Mice immunized with P206.1 conjugated to various carriers did not mount an Ab response to GXM despite developing high anti-peptide titers. However, mice primed with GXM-TT and boosted with P206.1-TT developed significant anti-GXM titers (maximum, 1/180,000). This latter immunization scheme focused the immune response on protective epitopes, since only 2-5% of these titers were directed against nonprotective de-O-acetylated GXM epitopes compared with 20-60% in animals primed and boosted with GXM-TT.

Cryptococcosis

Cryptococcus neoformans has risen to a worldwide highly recognizable major opportunistic pathogen with deadly consequences. It has become a model fungus to study a variety of paradigms in the host-fungus relationships. Genomic studies are advancing knowledge on its evolution and dissecting its virulence composite. Studies designed to understand host immunology to this fungus are leading to development of active and passive prevention and therapeutic strategies. This article collates and analyzes both new and old knowledge about the pathogen to help frame the meaning of human cryptococcosis as it starts to evolve in the new millennium.

Cryptococcal infection and Th1-Th2 cytokine balance

Cytokines have been recognized as key factors in determining host resistance to infectious pathogens. In particular, Th1-Th2 cytokine balance in hosts is profoundly associated with the outcome of infection caused by intracellular microbes. In a murine model of pulmonary and disseminated infection with Cryptococcus neoformans, an opportunistic fungal pathogen that frequently leads to fatal meningoencephalitis in severely immunocompromised hosts, expression of cytokine mRNA in the lungs from infected animals revealed Th2-dominant profiles, while administration of IL-12, which rescued mice from fatal infection, converted such balance toward Th1-dominant states in a drastic fashion. Thus, commitment of Th phenotypes critically determines host sensitivity to cryptococcal infection. In this review, we described how Th1-Th2 cytokine balance influences host protective responses to C. neoformans, and we identify the host and pathogen factors that regulate such balance.

Identification and cloning of a cryptococcal deacetylase that produces protective immune responses

Cell-mediated immunity plays a crucial role in host defenses against Cryptococcus (Filobasidiella) neoformans. Therefore, the identification of cryptococcal antigens capable of producing T-cell-mediated responses, such as delayed-type hypersensitivity (DTH) reactions, may be useful in the development of immune-based strategies to control cryptococcosis. In order to characterize DTH-producing antigens, culture supernatants from the unencapsulated Cap-67 strain were separated by anion-exchange chromatography. After further fractionation by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a purified protein with an apparent molecular mass of 25 kDa was found to produce DTH, as evidenced by increased footpad swelling in mice immunized with culture supernatants, relative to unimmunized mice. The 20-amino-acid N-terminal sequence of the 25-kDa protein was used to search data of the C. neoformans Genome Project. Based on the genomic DNA sequence, a DNA probe was used to screen a lambda cDNA library prepared from strain B3501. Clones were isolated containing the full-length gene (d25), which showed homology with a number of polysaccharide deacetylases from fungi and bacteria. The recombinant d25 protein expressed in Escherichia coli was similar to the natural one in DTH-producing activity. Moreover, immunization with either the natural or the recombinant protein prolonged survival and decreased fungal burden in mice challenged with the highly virulent C. neoformans strain H99. In conclusion, we have described the first cryptococcal gene whose product, a 25-kDa extracellular polysaccharide deacetylase, has been shown to induce protective immunity responses.

Antibody efficacy in murine pulmonary Cryptococcus neoformans infection: a role for nitric oxide

We investigated the pathogenesis of pulmonary Cryptococcus neoformans infection and passive Ab efficacy in mice deficient in inducible NO synthase (NOS2(-/-)) and the parental strain. Parental mice lived significantly longer than NOS2(-/-) mice after intratracheal infection, despite having a higher lung fungal burden. Administration of Ab reduced lung CFU in both NOS2(-/-) and parental mice, but prolonged survival and increased the inflammatory response only in parental mice. Ab administration was associated with increased serum nitrite and reduced polysaccharide levels in parental mice. Eosinophils were present in greater numbers in the lung of infected NOS2(-/-) mice than parental mice, irrespective of Ab administration. C. neoformans infection in NOS2(-/-) mice resulted in significantly higher levels of IFN-gamma, monocyte chemoattractant protein-1, and macrophage-inflammatory protein-1alpha than parental mice. Ab administration had different effects on infected NOS2(-/-) and parental mice with respect to IFN-gamma, monocoyte chemoattractant protein-1, and macrophage-inflammatory protein-1alpha levels. Ab administration increased lung levels of IFN-gamma in parental mice and reduced levels in NOS2(-/-) mice. The results indicate that NO is involved in the regulation of cytokine expression in response to cryptococcal pneumonia and is necessary for Ab efficacy against C. neoformans in mice. Our findings indicate a complex relationship between Ab efficacy against C. neoformans and cytokine expression, underscoring the interdependency of cellular and humoral defense mechanisms.

Antibody-mediated protection in murine Cryptococcus neoformans infection is associated with pleotrophic effects on cytokine and leukocyte responses

Cryptococcus neoformans, an encapsulated yeast, is a common cause of life-threatening meningoencephalitis in immunosuppressed patients. We previously observed that administration of a monoclonal antibody (MAb) to the capsular polysaccharide to mice with pulmonary infection prolonged survival and enhanced granulomatous inflammation without reducing lung CFU. To understand the mechanism of MAb action, we studied leukocyte recruitment and cytokine profiles in lungs of A/JCr mice. B lymphocytes were the predominant cell type in lung infiltrates, comprising 15 to 30% of the leukocytes. Despite alterations in histological appearance, fluorescence-activated cell sorter analysis revealed no significant difference in total numbers of lung leukocytes in MAb-treated mice and controls. Differences in the immune response to C. neoformans between MAb-treated mice and controls included (i) an increase in the percentage of granulocytes among lung leukocytes on day 14, (ii) higher macrophage surface expression of CD86 on day 28, (iii) larger amounts of IL-10 in lung homogenates at day 7, (iv) a trend toward smaller amounts of gamma interferon mRNA and protein on day 7, and (v) a smaller increase in the levels of interleukin-4 mRNA and protein on day 7. Hence, the immune responses to C. neoformans infection in the presence and absence of specific antibody were qualitatively similar, and antibody administration was associated with several subtle quantitative differences in immune response parameters that could translate into enhanced survival. MAb may function partly by down-regulating the inflammatory response and reducing host damage. Our findings demonstrate unexpected complexity in the interaction between specific MAb and other components of the host immune response.

An immunohistochemical method that differentiates Cryptococcus neoformans varieties and serotypes in formalin-fixed paraffin-embedded tissues

An immunohistochemical method for determining the variety of Cryptococcus neoformans in formalin-fixed paraffin-embedded tissues was developed using mAbs 471, 302 and CRND8. The method was validated primarily using veterinary patients for which both formalin-fixed lesions and a cultured isolate were available. L-Canavanine glycine bromothymol blue (CGB) agar and the 'Crypto-Check' kit were used to determine the variety and serotype, respectively, of cultured isolates. Immunohistochemistry accurately predicted the C. neoformans variety in all tissue specimens. The CGB agar method of determining C. neoformans variety gave the same result as immunohistochemistry for 30/31 specimens. For the single discordant isolate, the serotype, random amplification of polymorphic DNA profile, microscopic and colony morphology all supported the immunohistochemical staining pattern in suggesting C. neoformans var. gattii; however, the CGB agar result was at variance. Of the C. neoformans var. neoformans cases, immunohistochemistry was congruent with variety for 13/13 cases and with serotyping for 10/13 cases. The three discordant cases were classified as having some serotype D reactivity by immunohistochemistry, but were considered to be serotype A using the Crypto-Check kit. This new method should prove a valuable epidemiological tool in studies of cryptococcosis, especially in the veterinary setting where archival tissue specimens may exist but corresponding mycological data is typically absent. The versatility of this method will expand in the future as other monoclonal antibodies with different specificities are developed.

Both Th1 and Th2 cytokines affect the ability of monoclonal antibodies to protect mice against Cryptococcus neoformans

Variable-region-identical mouse immunoglobulin G1 (IgG1), IgG2b, and IgG2a monoclonal antibodies to the capsular polysaccharide of Cryptococcus neoformans prolong the lives of mice infected with this fungus, while IgG3 is either not protective or enhances infection. CD4+ T cells are required for IgG1-mediated protection, and CD8+ T cells are required for IgG3-mediated enhancement. Gamma interferon is required for both effects. These findings revealed that T cells and cytokines play a role in the modulation of cryptococcal infection by antibodies and suggested that it was important to more fully define the cytokine requirements of each of the antibody isotypes. We therefore investigated the efficacy of passively administered variable-region-identical IgG1, IgG2a, IgG2b, and IgG3 monoclonal antibodies against intravenous infection with C. neoformans in mice genetically deficient in interleukin-12 (IL-12), IL-6, IL-4, or IL-10, as well as in the parental C57BL/6J strain. The relative inherent susceptibilities of these mouse strains to C. neoformans were as follows: IL-12(-/-) > IL-6(-/-) > C57BL/6J approximately IL-4(-/-) >> IL-10(-/-). This is consistent with the notion that a Th1 response is necessary for natural immunity against cryptococcal infection. However, none of the IgG isotypes prolonged survival in IL-12(-/-), IL-6(-/-), or IL-4(-/-) mice, and all isotypes significantly enhanced infection in IL-10(-/-) mice. These results indicate that passive antibody-mediated protection against C. neoformans requires both Th1- and Th2-associated cytokines and reveal the complexity of the mechanisms through which antibodies modulate infection with this organism.

Immunoglobulin M efficacy against Cryptococcus neoformans: mechanism, dose dependence, and prozone-like effects in passive protection experiments

The IgM mAbs 12A1 and 13F1 are protective and nonprotective, respectively, against lethal Cryptococcus neoformans infection in mice. To better understand the variables that contribute to IgM efficacy against C. neoformans, we studied the effects of inoculum size, route of infection, and Ab dose for each of these mAbs. mAb 13F1 did not prolong survival under any condition studied. mAb 12A1 prolonged survival after the administration of certain Ab doses after i.p. infection with defined inocula and promoted phagocytosis, agglutination, and the formation of inflammatory cell rings around yeast cells in vivo. Large Ab doses of mAb 12A1 resulted in either no protection or enhanced infection, consistent with a prozone-like effect. Investigation of this phenomenon revealed that the fungal cell was protected against microbicidal nitrogen-derived oxidants when large amounts of Ab were bound to the C. neoformans capsule. mAb 12A1 was opsonic in vitro for peritoneal, but not splenic or alveolar macrophages. In summary, our results indicate that IgM efficacy against C. neoformans is a function of the route of infection, inoculum, and Ab dose and is associated with its ability to promote opsonization, agglutination, and phagocytic ring formation in vivo. The occurrence of the prozone-like phenomenon implies that high Ab titers are not necessarily beneficial in assuring protection against certain pathogens and that caution should be exercised in using high Ab titer as a measure for vaccine efficacy.

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.

Human antibodies against a purified glucosylceramide from Cryptococcus neoformans inhibit cell budding and fungal growth

A major ceramide monohexoside (CMH) was purified from lipidic extracts of Cryptococcus neoformans. This molecule was analyzed by high-performance thin-layer chromatography (HPTLC), gas chromatography coupled with mass spectrometry, and fast atom bombardment-mass spectrometry. The cryptococcal CMH is a beta-glucosylceramide, with the carbohydrate residue attached to 9-methyl-4,8-sphingadienine in amidic linkage to 2-hydroxyoctadecanoic acid. Sera from patients with cryptococcosis and a few other mycoses reacted with the cryptococcal CMH. Specific antibodies were purified from patients' sera by immunoadsorption on the purified glycolipid followed by protein G affinity chromatography. The purified antibodies to CMH (mainly immunoglobulin G1) bound to different strains and serological types of C. neoformans, as shown by flow cytofluorimetry and immunofluorescence labeling. Transmission electron microscopy of yeasts labeled with immunogold-antibodies to CMH and immunostaining of isolated cell wall lipid extracts separated by HPTLC showed that the cryptococcal CMH predominantly localizes to the fungal cell wall. Confocal microscopy revealed that the beta-glucosylceramide accumulates mostly at the budding sites of dividing cells with a more disperse distribution at the cell surface of nondividing cells. The increased density of sphingolipid molecules seems to correlate with thickening of the cell wall, hence with its biosynthesis. The addition of human antibodies to CMH to cryptococcal cultures of both acapsular and encapsulated strains of C. neoformans inhibited cell budding and cell growth. This process was complement-independent and reversible upon removal of the antibodies. The present data suggest that the cryptococcal beta-glucosylceramide is a fungal antigen that plays a role on the cell wall synthesis and yeast budding and that antibodies raised against this component are inhibitory in vitro.

Biological correlates of capsular (quellung) reactions of Cryptococcus neoformans

The capsular swelling or quellung reaction was reported almost 100 years ago and described the effect of Abs on the appearance of microbial capsules. Despite widespread use to assess Ab binding to capsules, relatively little is known as to the mechanism of this effect or its biological consequences. The fungus Cryptococcus neoformans is an attractive system to study capsule reactions because it has a large polysaccharide capsule that is readily visible by light microscopy. When viewed by differential interference contrast microscopy, binding of mAb to C. neoformans cells produced two distinct capsular reactions that depended on the Ab epitope specificity and the yeast serotype. In the first pattern, termed "rim," the capsule appears transparent with a highly refractive outer edge. In the second pattern, termed "puffy," the capsule appears opaque and lacks a highly refractive outer rim. mAbs that bind with a rim pattern suppress the overall rate of C3 deposition on the yeast via the classical and alternative complement pathways. In contrast, mAbs that bind with a puffy pattern do not affect C3 deposition. Protective and nonprotective IgM mAbs produce rim and puffy patterns, respectively. These results indicate that: 1) capsule reactions are a consequence of Ab-induced changes in capsular refractive index; 2) the type of capsule reaction depends on the Ab specificity; and 3) Ab-induced changes in refractive index correlate with biological activities important for host defense against C. neoformans. Our results provide the first evidence associating distinct capsule reaction patterns with Ab biological activity.

Molecular and idiotypic analyses of the antibody response to Cryptococcus neoformans glucuronoxylomannan-protein conjugate vaccine in autoimmune and nonautoimmune mice

The antibody response to Cryptococcus neoformans capsular glucuronoxylomannan (GXM) in BALB/c mice frequently expresses the 2H1 idiotype (Id) and is restricted in variable gene usage. This study examined the immunogenicity of GXM-protein conjugates, V (variable)-region usage, and 2H1 Id expression in seven mouse strains: BALB/c, C57BL/6, A/J, C3H, NZB, NZW, and (NZB x NZW)F(1) (NZB/W). All mouse strains responded to vaccination with GXM conjugated to tetanus toxoid (TT), the relative magnitude of the antibody response being BALB/c approximately C3H > C57BL/6 approximately NZB approximately NZW approximately NZB/W > A/J. Analysis of serum antibody responses to GXM with polyclonal and monoclonal antibodies to the 2H1 Id revealed significant inter- and intrastrain differences in idiotype expression. Thirteen monoclonal antibodies (MAbs) (two immunoglobulin M [IgM], three IgG3, one IgG1, three IgG2a, two IgG2b, and two IgA) to GXM were generated from one NZB/W mouse and one C3H/He mouse. The MAbs from the NZB/W mouse were all 2H1 Id positive (Id(+)) and structurally similar to those previously generated in BALB/c mice, including the usage of a V(H) from the 7183 family and Vkappa5.1. Administration of both 2H1 Id(+) and Id(-) MAbs from NZB/W and C3H/H3 mice prolonged survival in a mouse model of cryptococcosis. Our results demonstrate (i) that V-region restriction as indicated by the 2H1 Id is a feature of both primary and secondary responses of several mouse strains; and (ii) that there is conservation of V-region usage and length of the third complementarity-determining region in antibodies from three mouse strains. The results suggest that V-region restriction is a result of antibody structural requirements necessary for binding an immunodominant antigen in GXM.

Granulocyte-macrophage colony-stimulating factor and fluconazole enhance anti-cryptococcal activity of monocytes from AIDS patients

OBJECTIVE

To investigate the effect of human recombinant granulocyte-macrophage colony-stimulating factor (hrGM-CSF) and fluconazole on anti-cryptococcal activity of monocytes from AIDS patients and normal subjects.

DESIGN

The effect of GM-CSF and fluconazole on fungistatic and fungicidal activity of monocytes was studied in an in vitro system.

METHODS

Monocytes were treated in vitro with hrGM-CSF and fluconazole or either agent alone for 24 or 48 h, and fungistatic and fungicidal activity was evaluated in a colony-forming unit inhibition assay. CD11b/CD18 expression in monocytes was measured by flow cytometry analysis. Superoxide anion generation by peripheral blood monocytes was measured in the presence of pre-opsonized zymosan.

RESULTS

Defective antifungal capacity of monocytes from AIDS patients was observed. GM-CSF treatment of monocytes from AIDS patients increased fungistatic activity, and the combination of hrGM-CSF and fluconazole resulted in fungicidal activity. The mechanisms involved in the GM-CSF-mediated effect appeared to be mediated by (i) enhancement of phagocytic activity, (ii) increase of superoxide anion generation, and (iii) upregulation of CD11b/CD18 expression on the monocyte surface.

CONCLUSIONS

Our data highlight the effect of GM-CSF on anti-cryptococcal activity of human monocytes and show a synergistic effect of GM-CSF with fluconazole, suggesting a new therapeutic strategy in the treatment of cryptococcosis.

Immunogenicity and protective efficacy of the WI-1 adhesin of Blastomyces dermatitidis

People infected with Blastomyces dermatitidis develop strong immunity to the yeast surface adhesin WI-1, including antibody responses to the adhesive domain, a 25-amino-acid repeat, and cellular responses to the N terminus. We studied the immunogenicity of WI-1 and the ability of anti-WI-1 immune responses to protect against lethal pulmonary infection in mice. WI-1 immunization, given in Freund's adjuvant subcutaneously in two doses 2 weeks apart, evoked delayed hypersensitivity responses in a concentration-dependent manner. Immunized mice also had anti-WI-1 antibody responses, with titers reaching an endpoint dilution of approximately 1:800,000. Anti-WI-1 immunoglobulin G (IgG) antibody subclasses were IgG1 > IgG2b > IgG2a > IgG3, indicating a mixed T helper 1 and T helper 2 immune response. In protection experiments, WI-1 immunization significantly prolonged the survival of C57BL/6 and BALB/c mice compared to controls following intranasal administration of a lethal dose of B. dermatitidis yeasts (Kaplan-Meier survival curve P values of 0.027 to 0.0002) and also protected a proportion of the animals from death due to progressive pulmonary blastomycosis. Taken together, our results suggest that administration of WI-1 raises antibody and cell-mediated immune responses, which enhance resistance against pulmonary infection with B. dermatitidis. Mechanisms of vaccine-induced resistance require further investigation.

Monoclonal Antibodies Reveal Additional Epitopes of Serotype D Cryptococcus neoformans Capsular Glucuronoxylomannan that Elicit Protective Antibodies

Epitope specificity and isotype influence mAb efficacy against Cryptococcus neoformans; however, the relative contribution of each attribute is poorly understood. To date, only mAbs that recognize two epitopes of capsular glucuronoxylomannan (GXM), defined by the IgG1 mAbs 2H1 and E1, consistently mediate protection against C. neoformans. The role of epitope specificity was further examined using six additional IgG1 mAbs and serotype D C. neoformans ATCC 24067. mAbs 3C2, 439, and 471 recognize the 2H1 epitope, whereas mAbs 339, 1255, and 302 recognize two separate epitopes. mAbs 3C2, 439, and 471 competed for GXM with the IgA mAb 18G9, a 2H1 mAb family member, whereas mAbs 302, 339, and 1255 did not. Each mAb bound GXM similarly, as determined by agglutination, direct Ag binding, Ag inhibition, and indirect capsular immunofluorescence assays. mAb apparent affinity constants for GXM ranged from 5 to 26 × 107 M−1 with mAb 1255 &gt; 3C2 &gt; 339 &gt; 439 &gt; 471 &gt; 302. Each mAb significantly prolonged survival (p &lt; 0.05); the average survival times of control and mice passively immunized with mAbs 3C2, 302, 339, 439, 471, and 1255 were 10.8, 36.6, 33, 25.5, 24.9, 17, and 22.6 days, respectively. Although each mAb enhanced J774.16 cell fungicidal activity, differences were observed in the ability of each mAb to facilitate attachment and ingestion of cryptococci. These results indicate 1) two additional epitope specificities associated with mAb efficacy, 2) differences in opsonic and protective efficacy for IgG1 anti-GXM mAbs, 3) an association between affinity and protective efficacy, and 4) additional support for association between an annular indirect capsular immunofluorescence pattern and mAb efficacy.

Characterization of a murine monoclonal antibody to Cryptococcus neoformans polysaccharide that is a candidate for human therapeutic studies

The murine monoclonal antibody (MAb) 18B7 [immunoglobulin G1(kappa)] is in preclinical development for treatment of Cryptococcus neoformans infections. In anticipation of its use in humans, we defined the serological and biological properties of MAb 18B7 in detail. Structural comparison to the related protective MAb 2H1 revealed conservation of the antigen binding site despite several amino acid differences. MAb 18B7 was shown by immunofluorescence and agglutination studies to bind to all four serotypes of C. neoformans, opsonize C. neoformans serotypes A and D, enhance human and mouse effector cell antifungal activity, and activate the complement pathway leading to deposition of complement component 3 (C3) on the cryptococcal capsule. Administration of MAb 18B7 to mice led to rapid clearance of serum cryptococcal antigen and deposition in the liver and spleen. Immunohistochemical studies revealed that MAb 18B7 bound to capsular glucuronoxylomannan in infected mouse tissues. No reactivity of MAb 18B7 with normal human, rat, or mouse tissues was detected. The results show that both the variable and constant regions of MAb 18B7 are biologically functional and support the use of this MAb in human therapeutic trials.

Bivalency is required for anticapsular monoclonal antibodies to optimally suppress activation of the alternative complement pathway by the Cryptococcus neoformans capsule

Encapsulated cells of Cryptococcus neoformans are potent activators of the alternative complement pathway. Previous studies found that monoclonal antibodies (MAbs) specific for the major capsular polysaccharide, termed glucuronoxylomannan (GXM), can markedly suppress the ability of the capsule to accumulate C3 from normal human serum via the alternative pathway. The present study examined the abilities of F(ab)2 and Fab fragments of three MAbs (MAbs 439, 3C2, and 471) to mediate the suppressive effect. The results showed that F(ab)2 fragments of all three MAbs suppressed activation and binding of C3 via the alternative pathway in a manner similar to that of intact antibodies. In contrast, Fab fragments of MAb 439 and MAb 3C2 showed no suppressive activity, and Fab fragments of MAb 471 were markedly reduced in suppressive activity. Indeed, there was an earlier accumulation of C3 on encapsulated cryptococci in the presence of the Fab fragments. Study of subclass switch families of MAb 439 and MAb 471 found that MAbs of an immunoglobulin G (IgG) subclass with increased flexibility in the hinge region (IgG2b) had less suppressive activity than MAbs of IgG subclasses with less flexibility (IgG1 or IgG2a). Taken together, these results indicate that cross-linking of the capsular matrix is an essential component in suppression of the alternative complement pathway by anti-GXM MAbs.

Characterization of anticapsular monoclonal antibodies that regulate activation of the complement system by the Cryptococcus neoformans capsule

Incubation of the encapsulated yeast Cryptococcus neoformans in human serum leads to alternative pathway-mediated deposition of C3 fragments in the capsule. We examined the ability of monoclonal antibodies (MAbs) specific for different epitopes of the major capsular polysaccharide to alter the kinetics for classical and alternative pathway-mediated deposition of C3 onto a serotype A strain. We studied MAbs reactive with capsular serotypes A, B, C, and D (MAb group II); serotypes A, B, and D (MAb group III); and serotypes A and D (MAb group IV). The MAb groupings are based on antibody variable region usage which determines the antibody molecular structure. When both the classical and alternative pathways were operative, group II MAbs induced early classical pathway-mediated binding of C3 but reduced the overall rate of C3 accumulation and the amount of bound C3. Group III MAbs closely mimicked the effects of group II MAbs but exhibited reduced support of early classical pathway-facilitated accumulation of C3. Depending on the antibody isotype, group IV MAbs slightly or markedly enhanced early binding of C3 but had no effect on either the rate of C3 accumulation or the amount of bound C3. When the classical pathway was blocked, group II and III MAbs markedly suppressed C3 binding that normally would have occurred via the alternative pathway. In contrast, MAbs of group IV had no effect on alternative pathway-mediated C3 binding. These results indicate that anticapsular antibodies with different epitope specificities may have distinct regulatory effects on activation and binding of C3.

A low virulent strain of Candida albicans enhances brain anticryptococcal defenses: characterization of the local immune reaction by RT-PCR and histochemical analysis

Here we studied the involvement of PCA-2, a low-virulent strain of Candida albicans known to act as a potent stimulating agent in the development of cryptococcal meningoencephalitis. To this purpose, mice received saline or PCA-2 intracerebrally 7 days before lethal local challenge with Cryptococcus neoformans. We found that, following C. neoformans challenge, PCA-2-treated but not saline-treated mice exhibited (a) delayed brain colonization, (b) enhanced median survival times, (c) massive local immune reaction consisting of abundant astrocytes, microglial and inflammatory cells, and (d) a peculiar trend of cytokine gene expression, including high steady-state levels of interleukin (IL)-1 beta and tumor necrosis factor alpha transcripts, fluctuating levels of interferon gamma and inducible nitric oxide synthase mRNA and lately detectable IL-6 gene expression. PCA-2-mediated immunostimulating properties were partially impaired by aminoguanidine or pentoxifylline treatment, further strengthening the conclusion that soluble mediators, including proinflammatory cytokines and nitric oxide, are important defense elements against cryptococcal meningoencephalitis.