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

Patient-Derived Antibody Data Yields Development of Broadly Cross-Protective Monoclonal Antibody against ST258 Carbapenem-Resistant Klebsiella pneumoniae

The most pressing challenge for the development of anti-capsular antibodies is maximizing coverage against the heterogenous capsular polysaccharide (CPS) of carbapenem-resistant Klebsiella pneumoniae (CR-Kp). So far, only CR-Kp with wzi154 CPS has been successfully targeted by antibodies. Here, we present murine antibody 24D11, which was developed by vaccinating mice with purified wzi50-type CPS. Cross-reactivity and protective efficacy of MAb 24D11 were confirmed against CR-Kp that express the 3 most prevalent CPS types (wzi29, wzi154, wzi50) using both in vitro and in vivo infection models. 24D11 induced complement-mediated and independent opsonophagocytosis in macrophages as well as killing of all CR-Kp strains in whole blood cells derived from healthy donors. In a murine intratracheal infection model, 24D11 reduced lung burden and dissemination of CR-Kp strains when administered 4 h pre- or postinfection. The protective efficacy of 24D11 remained effective in neutropenic mice. This is the first antibody which exhibits cross-protective efficacy against clade 1 and 2 ST258 CR-Kp strains. It overcomes a major barrier to successfully target wzi29, a major CPS expressed by ST258 CR-Kp. The finding that 24D11 also exhibits potent protective efficacy against wzi154 CR-Kp strains highlights its high potential as a lead agent for the development of broadly active immunotherapy. IMPORTANCE Here, we present in vitro and in vivo data for the wzi50 CPS-specific monoclonal antibody MAb 24D11, demonstrating its cross-protective efficacy against three prominent win types (wzi29, wzi154, and wzi50) of the carbapenem-resistant clonal group CG258. In a murine pulmonary infection model, MAb 24D11 reduced bacterial lung burden and dissemination to other organs even if administered 4 h postinfection. Its protective efficacy was also observed in neutropenic mice, which highlights its potential value in clinical settings where oncology patients with CG258 infections may also be neutropenic.

What is the elephant in the room when considering new therapies for fungal diseases?

The global scenario of antimicrobial resistance is alarming, and the development of new drugs has not appeared to make substantial progress. The constraints on drug discovery are due to difficulties in finding new targets for therapy, the high cost of development, and the mismatch between the time of drug introduction in a clinic and microorganism adaptation to a drug. Policies to address neglected diseases miss the broad spectrum of mycosis. Society is not aware of the actual threat represented by fungi to human health, food security, and biodiversity. The evidence discussed here is critical for warning governments to establish effective surveillance policies for fungi.HIGHLIGHTSFungal diseases are ignored even among neglected disease classifications.There are few options to treat mycoses, which is an increasing concern regarding fungal resistance to drugs, as evidenced by the spread of Candida auris.Fungal diseases represent a real threat to human health and food security.Investment in research to investigate the potential of repurposing drugs already in use could obtain results in the short term.

Host Immune Defense upon Fungal Infections with Mucorales: Pathogen-Immune Cell Interactions as Drivers of Inflammatory Responses

During the last few decades, mucormycosis has emerged as one of the most common fungal infections, following candidiasis and aspergillosis. The fungal order responsible for causing mucormycosis is the Mucorales. The main hallmarks of this infection include the invasion of blood vessels, infarction, thrombosis, and tissue necrosis, which are exhibited at the latest stages of the infection. Therefore, the diagnosis is often delayed, and the rapid progression of the infection severely endangers the life of people suffering from diabetes mellitus, hematological malignancies, or organ transplantation. Given the fact that mortality rates for mucormycosis range from 40 to 80%, early diagnosis and novel therapeutic strategies are urgently needed to battle the infection. However, compared to other fungal infections, little is known about the host immune response against Mucorales and the influence of inflammatory processes on the resolution of the infection. Hence, in this review, we summarized our current understanding of the interplay among pro-inflammatory cytokines, chemokines, and the host-immune cells in response to mucoralean fungi, as well as their potential use for immunotherapies.

Infectious Complications of Biological and Small Molecule Targeted Immunomodulatory Therapies

The past 2 decades have seen a revolution in our approach to therapeutic immunosuppression. We have moved from relying on broadly active traditional medications, such as prednisolone or methotrexate, toward more specific agents that often target a single receptor, cytokine, or cell type, using monoclonal antibodies, fusion proteins, or targeted small molecules. This change has transformed the treatment of many conditions, including rheumatoid arthritis, cancers, asthma, and inflammatory bowel disease, but along with the benefits have come risks. Contrary to the hope that these more specific agents would have minimal and predictable infectious sequelae, infectious complications have emerged as a major stumbling block for many of these agents. Furthermore, the growing number and complexity of available biologic agents makes it difficult for clinicians to maintain current knowledge, and most review articles focus on a particular target disease or class of agent. In this article, we review the current state of knowledge about infectious complications of biologic and small molecule immunomodulatory agents, aiming to create a single resource relevant to a broad range of clinicians and researchers. For each of 19 classes of agent, we discuss the mechanism of action, the risk and types of infectious complications, and recommendations for prevention of infection.

Pathogen and host genetics underpinning cryptococcal disease

Cryptococcosis is a severe fungal disease causing 220,000 cases of cryptococcal meningitis yearly. The etiological agents of cryptococcosis are taxonomically grouped into at least two species complexes belonging to the genus Cryptococcus. All of these yeasts are environmentally ubiquitous fungi (often found in soil, leaves and decaying wood, tree hollows, and associated with bird feces especially pigeon guano). Infection in a range of animals including humans begins following inhalation of spores or aerosolized yeasts. Recent advances provide fundamental insights into the factors from both the pathogen and its hosts which influence pathogenesis and disease. The complex interactions leading to disease in mammalian hosts have also updated from the availability of better genomic tools and datasets. In this review, we discuss recent genetic research on Cryptococcus, covering the epidemiology, ecology, and evolution of Cryptococcus pathogenic species. We also discuss the insights into the host immune response obtained from the latest genetic modified host models as well as insights from monogenic disorders in humans. Finally we highlight outstanding questions that can be answered in the near future using bioinformatics and genomic tools.

B Cell Compartmentalization in Blood and Cerebrospinal Fluid of HIV-Infected Ugandans with Cryptococcal Meningitis

Activated B cells modulate infection by differentiating into pathogen-specific antibody-producing effector plasmablasts/plasma cells, memory cells, and immune regulatory B cells. In this context, the B cell phenotypes that infiltrate the central nervous system during human immunodeficiency virus (HIV) and cryptococcal meningitis coinfection are ill defined. We characterized clinical parameters, mortality, and B cell phenotypes in blood and cerebrospinal fluid (CSF) by flow cytometry in HIV-infected adults with cryptococcal (n = 31) and noncryptococcal (n = 12) meningitis and in heathy control subjects with neither infection (n = 10). Activation of circulating B cells (CD21low) was significantly higher in the blood of subjects with HIV infection than in that of healthy controls and greater yet in matched CSF B cells (P < 0.001). Among B cell subsets, elevated frequencies of memory and plasmablasts/plasma cells most clearly distinguished the CSF from blood compartments. With cryptococcal meningitis, lower frequencies of expression of the regulatory protein programmed death-1 (PD-1) on plasmablasts/plasma cells in blood (median, 7%) at presentation were associated with significantly decreased 28-day survival (29% [4/14 subjects]), whereas higher PD-1 expression (median, 46%) characterized subjects with higher survival (88% [14/16 subjects]). With HIV infection, B cell differentiation and regulatory markers are discrete elements of the circulating and CSF compartments with clinical implications for cryptococcal disease outcome, potentially due to their effects on the fungus and other local immune cells.

An inherent T cell deficit in healthy males to C. neoformans infection may begin to explain the sex susceptibility in incidence of cryptococcosis

Background

Cryptococcus neoformans, the causative agent of cryptococcosis, causes ~ 181,000 deaths annually, with males having a higher incidence of disease than females (7M:3F). The reason for this sex bias remains unclear. We hypothesized that this disparity was due to biological differences between the male and female immune response.

Methods

Peripheral blood mononuclear cells (PBMCs) from healthy donors were isolated and infected with C. neoformans ± exogenous testosterone or 17-β-estradiol. C. neoformans, B, T, and NK cell proliferation was quantified by flow cytometry. Cytokine analysis was conducted via protein array or ELISA. Serological testing was conducted to determine previous exposure to C. neoformans.

Results

C. neoformans proliferated more in male PBMCs. T cell percentages in both sexes were lower in infected versus uninfected cells. Male PBMCs had lower CD3⁺, CD4⁺, and CD8⁺ T cells percentages during infection compared to females. Cytokine profiles showed differences in uninfected male and female PBMCs, which subsided during infection. Only one donor was sero-negative for prior C. neoformans exposure. There was an effect of estrogen in one dataset.

Conclusions

These results suggest that males show an inherent deficit in T cell response during infection, which may contribute to the increased incidence of disease in males.

Electronic supplementary material

The online version of this article (10.1186/s13293-019-0258-2) contains supplementary material, which is available to authorized users.

Adaptive Immunity to Cryptococcus neoformans Infections

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

Naïve B cells reduce fungal dissemination in Cryptococcus neoformans infected Rag1-/- mice

IgM and B-1 cell deficient mice exhibit early C. neoformans dissemination from lungs to brain, but a definitive role for B cells in conferring resistance to C. neoformans dissemination has not been established. To address this question, we developed an intranasal (i.n.) C. neoformans infection model in B and T cell deficient Rag1^-/- mice and found they also exhibit earlier fungal dissemination and higher brain CFU than wild-type C57Bl/6 (wild-type) mice. To probe the effect of B cells on fungal dissemination, Rag1^-/- mice were given splenic (intravenously) or peritoneal (intraperitoneally) B cells from wild-type mice and infected i.n. with C. neoformans 7 d later. Mice that received B cells had lung histopathology resembling wild type mice 14 d post-infection, and B-1, not B-2 or T cells in their lungs, and serum and lung IgM and IgG 21 d post-infection. Lung CFU were comparable in wild-type, Rag1^-/-, and Rag1^-/- mice that received B cells 21 d post-infection, but brain CFU were significantly lower in mice that received B cells than Rag1^-/- mice that did not. To determine if natural antibody can promote immunity in our model, we measured alveolar macrophage phagocytosis of C. neoformans in Rag1^-/- mice treated with naive wild-type IgM-sufficient or sIgM^-/- IgM-deficient sera before infection. Compared to IgM-deficient sera, IgM-sufficient sera significantly increased phagocytosis. Our data establish B cells are able to reduce early C. neoformans dissemination in mice and suggest natural IgM may be a key mediator of early antifungal immunity in the lungs.

The Efficacy of Humanized Antibody against the Sporothrix Antigen, gp70, in Promoting Phagocytosis and Reducing Disease Burden

Sporotrichosis is a subcutaneous mycosis distributed worldwide and is frequently reported in countries with tropical climates, as Latin America countries. We previously demonstrated that mice with sporotrichosis produce specific antibodies against a 70-kDa fungal protein, indicating that specific antibodies against this molecule may help to control the sporotrichosis. IgG1 monoclonal antibody was generated, and called mAbP6E7, in mice against a 70-kDa glycoprotein (gp70) of S. schenckii. The mAbP6E7 showed prophylactic and therapeutic activity against sporotrichosis. However, this antibody has a murine origin, and this can generate an immune response when administered to humans, precluding its use for a prolonged time. For its possible use in the treatment of human sporotrichosis, we humanized the mAbP6E7 by genetic engineering. Once expressed, the humanized antibodies had good stability and were able to bind to the 70-kDa cell wall antigens of Sporothrix schenckii and S. brasiliensis. The humanized P6E7 were able to opsonize S. schenckii yeasts, thus increasing the phagocytic index in human monocyte-derived macrophages. The treatment with humanized P6E7 decreased fungal burden in vivo. These data suggest that humanized P6E7 may have a therapeutic role in sporotrichosis.

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.

Genetic and pathological characteristics of Cryptococcus gattii and Cryptococcus neoformans var. neoformans from meningoencephalitis in autochthonous goats and mouflons, Sardinia, Italy

In this study, we examined in Sardinia the brain of 555 autochthonous sheep, 50 goats, and 4 mouflons which were found affected by neurological signs. We found 6 goats and one mouflon with meningoencephalitis caused by Cryptococcus sp. There was no evidence of cryptococcal infections in any of the examined sheep. MLST genotyping on Cryptococcus sp. isolates identified Cryptococcus gatti genotype AFLP4/VGI and Cryptococcus neoformans var. neoformans genotype AFLP2/VNIV. Phylogenetically, all Cryptococcus gattii isolates fell within the autochthonous animal, human and environmental Mediterranean isolate cluster, forming a distinct branch along with environmental strains from Alicante, in the southern Mediterranean coast of Spain.

Adaptive immunity to fungi

Life-threatening fungal infections have risen sharply in recent years, owing to the advances and intensity of medical care that may blunt immunity in patients. This emerging crisis has created the growing need to clarify immune defense mechanisms against fungi with the ultimate goal of therapeutic intervention. We describe recent insights in understanding the mammalian immune defenses that are deployed against pathogenic fungi. We focus on adaptive immunity to the major medically important fungi and emphasize three elements that coordinate the response: (1) dendritic cells and subsets that are mobilized against fungi in various anatomical compartments; (2) fungal molecular patterns and their corresponding receptors that signal responses and shape the differentiation of T-cell subsets and B cells; and, ultimately (3) the effector and regulatory mechanisms that eliminate these invaders while constraining collateral damage to vital tissue. These insights create a foundation for the development of new, immune-based strategies for prevention or enhanced clearance of systemic fungal diseases.

Therapeutic monoclonal antibody for sporotrichosis

Sporotrichosis is a chronic subcutaneous mycosis that affects both humans and animals worldwide. This subcutaneous mycosis had been attributed to a single etiological agent, Sporothrix schenckii. S. schenckii exhibits considerable genetic variability, and recently, it was suggested that this taxon consists of a complex of species. Sporotrichosis is caused by traumatic inoculation of the fungus, which is a ubiquitous environmental saprophyte that can be isolated from soil and plant debris. The infection is limited to cutaneous forms, but recently, more severe clinical forms of this mycosis have been described, especially among immunocompromised individuals. The immunological mechanisms involved in the prevention and control of sporotrichosis are not well understood. Some studies suggest that cell-mediated immunity plays an important role in protecting the host against S. schenckii. In contrast, the role of the humoral immune response in protection against this fungus has not been studied in detail. In a previous study, we showed that antigens secreted by S. schenckii induced a specific humoral response in infected animals, primarily against a 70-kDa molecule, indicating a possible role of specific antibodies against this molecule in infection control. In another study by our group, we produced a mAb against a 70-kDa glycoprotein of S. schenckii to better understand the effect of the passive immunization of mice infected with S. schenckii. The results showed a significant reduction in the number of CFUs in various mice organs when the mAb was injected before or during S. schenckii infection. Similar results were observed when T-cell-deficient mice were used. The drugs of choice in the treatment of sporotrichosis require long periods, and relapses are frequently observed, primarily in immunocompromised patients. The strong protection induced by the mAb against a 70-kDa glycoprotein makes it a strong candidate as a therapeutic vaccine against sporotrichosis.

Molecular characterization of the early B cell response to pulmonary Cryptococcus neoformans infection

The role of B cells in host defense against fungi has been difficult to establish. We quantified and determined the molecular derivation of B-1a, B-1b, and B-2 B cell populations in C57BL/6 mice after pulmonary infection with Cryptococcus neoformans. Total B-1 and B-2 cell numbers increased in lungs and peritoneal cavity as early as day 1 postinfection, but lacked signs of clonal expansion. Labeled capsular (24067) and acapsular (Cap67) C. neoformans strains were used to identify C. neoformans-binding B cell subsets by flow cytometry. Peritoneal cavity B-1a B cells exhibited the most acapsular and capsular C. neoformans binding in C. neoformans-infected mice, and C. neoformans-selected B-1 B cells secreted laminarin- and C. neoformans-binding IgM. Single-cell PCR-based sequence analysis of B-1a, B-1b, and B-2 cell IgH V region H chain (V(H)) genes revealed increased usage of V(H)11 and V(H)12, respectively, in acapsular and capsular C. neoformans-selected B-1a cells. Germline V(H) segments were used, with capsular C. neoformans-selected cells having less junctional diversity than acapsular C. neoformans-selected cells. Further studies in B-1 B cell-depleted mice showed that these mice had higher brain and lung fungal burdens and less alveolar macrophage phagocytosis of C. neoformans than did control and B-1a B cell-reconstituted mice. Taken together, these results establish a mechanistic role for B-1 B cells in the innate B cell response to pulmonary infection with C. neoformans and reveal that IgM-producing B-1a cells, which express germline V(H) genes, bind C. neoformans and contribute to early fungal clearance. Thus, B-1a B cells provide a first line of defense during pulmonary C. neoformans infection in mice.

Susceptibility to progressive Cryptococcus neoformans pulmonary infection is regulated by loci on mouse chromosomes 1 and 9

Genetic factors that regulate the pathogenesis of pneumonia caused by the fungus Cryptococcus neoformans are poorly understood. Through a phenotypic strain survey we observed that inbred C3H/HeN mice develop a significantly greater lung fungal burden than mice of the resistant CBA/J strain 4 weeks following intratracheal infection with C. neoformans ATCC 24067. The aim of the present study was to characterize the inflammatory response of C3H/HeN mice following C. neoformans pulmonary infection and to identify genetic loci that regulate host defense. Following cryptococcal infection, C3H/HeN mice demonstrated a Th2 immune response with heightened airway and tissue eosinophilia, goblet cell metaplasia, and significantly higher lung interleukin-5 (IL-5) and IL-13 protein expression relative to CBA/J mice. Conversely, CBA/J mice exhibited greater airway and tissue neutrophilia that was associated with significantly higher pulmonary expression of gamma interferon, CXCL10, and IL-17 proteins than C3H/HeN mice. Using the fungal burden at 4 weeks postinfection as a phenotype, genome-wide quantitative trait locus (QTL) analysis among 435 segregating (C3H/HeN × CBA/J)F2 (C3HCBAF2) hybrids identified two significant QTLs on chromosomes 1 (Cnes4) and 9 (Cnes5) that control susceptibility to cryptococcal pneumonia in an additive manner. Susceptible C3H/HeN mice carry a resistance allele at Cnes4 and a susceptibility allele at Cnes5. These studies reveal additional genetic complexity of the host response to C. neoformans that is associated with divergent patterns of pulmonary inflammation.

Protective immunity against pulmonary cryptococcosis is associated with STAT1-mediated classical macrophage activation

Experimental pulmonary Cryptococcus neoformans infection in BALB/c mice is associated with polarized Th2-type cytokine production, alternative macrophage activation, and severe bronchopneumonia. In contrast, pulmonary infection with a C. neoformans strain that secretes IFN-γ, H99γ, elicits Th1-type cytokine production and classical macrophage activation. Additionally, mice infected with H99γ resolve the acute infection and are subsequently protected against challenge with wild-type C. neoformans. The present study characterizes macrophage activation during the protective response to wild-type C. neoformans in mice previously immunized with H99γ. We observed increased pulmonary Th1-type cytokine production in lung homogenates and classical macrophage activation as evidenced by enhanced expression of inducible NO synthase in the lungs of H99γ-immunized mice compared with mice given a nonprotective immunization with heat-killed C. neoformans (HKCn). Furthermore, macrophages isolated from H99γ-immunized mice on day 7 postchallenge and cultured in vitro were fungistatic against C. neoformans, whereas cryptococcal growth was uncontrolled within macrophages from HKCn-immunized mice. Th2-type cytokine production and induction of alternatively activated macrophages were also observed in lungs of HKCn-immunized mice during rechallenge. Gene expression arrays showed that classical macrophage activation during challenge infection in H99γ-immunized mice was associated with induction of the transcription factor STAT1 and its downstream targets IFN regulatory factor-1, suppressor of cytokine signaling-1, CXCL9, and CXCL10. These studies demonstrate that protective responses to C. neoformans challenge in immunized mice include classical macrophage activation and enhanced macrophage fungistasis of C. neoformans yeasts. Finally, the classical activation phenotype of protective anticryptococcal macrophages is likely mediated via STAT1 signal transduction pathways.

Immunoglobulins in defense, pathogenesis, and therapy of fungal diseases

Only two decades ago antibodies to fungi were thought to have little or no role in protection against fungal diseases. However, subsequent research has provided convincing evidence that certain antibodies can modify the course of fungal infection to the benefit or detriment of the host. Hybridoma technology was the breakthrough that enabled the characterization of antibodies to fungi, illuminating some of the requirements for antibody efficacy. As discussed in this review, fungal-specific antibodies mediate protection through direct actions on fungal cells and through classical mechanisms such as phagocytosis and complement activation. Although mechanisms of antibody-mediated protection are often species-specific, numerous fungal antigens can be targeted to generate vaccines and therapeutic immunoglobulins. Furthermore, the study of antibody function against medically important fungi has provided fresh immunological insights into the complexity of humoral immunity that are likely to apply to other pathogens.

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.

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.

Functional analysis of host factors that mediate the intracellular lifestyle of Cryptococcus neoformans

Cryptococcus neoformans (Cn), the major causative agent of human fungal meningoencephalitis, replicates within phagolysosomes of infected host cells. Despite more than a half-century of investigation into host-Cn interactions, host factors that mediate infection by this fungal pathogen remain obscure. Here, we describe the development of a system that employs Drosophila S2 cells and RNA interference (RNAi) to define and characterize Cn host factors. The system recapitulated salient aspects of fungal interactions with mammalian cells, including phagocytosis, intracellular trafficking, replication, cell-to-cell spread and escape of the pathogen from host cells. Fifty-seven evolutionarily conserved host factors were identified using this system, including 29 factors that had not been previously implicated in mediating fungal pathogenesis. Subsequent analysis indicated that Cn exploits host actin cytoskeletal elements, cell surface signaling molecules, and vesicle-mediated transport proteins to establish a replicative niche. Several host molecules known to be associated with autophagy (Atg), including Atg2, Atg5, Atg9 and Pi3K59F (a class III PI3-kinase) were also uncovered in our screen. Small interfering RNA (siRNA) mediated depletion of these autophagy proteins in murine RAW264.7 macrophages demonstrated their requirement during Cn infection, thereby validating findings obtained using the Drosophila S2 cell system. Immunofluorescence confocal microscopy analyses demonstrated that Atg5, LC3, Atg9a were recruited to the vicinity of Cn containing vacuoles (CnCvs) in the early stages of Cn infection. Pharmacological inhibition of autophagy and/or PI3-kinase activity further demonstrated a requirement for autophagy associated host proteins in supporting infection of mammalian cells by Cn. Finally, systematic trafficking studies indicated that CnCVs associated with Atg proteins, including Atg5, Atg9a and LC3, during trafficking to a terminal intracellular compartment that was decorated with the lysosomal markers LAMP-1 and cathepsin D. Our findings validate the utility of the Drosophila S2 cell system as a functional genomic platform for identifying and characterizing host factors that mediate fungal intracellular replication. Our results also support a model in which host Atg proteins mediate Cn intracellular trafficking and replication.

B cell-deficient mice display markedly enhanced resistance to the intracellular bacterium Brucella abortus

BACKGROUND

Brucella species are facultative intracellular bacteria that cause lifelong infections in humans and livestock.

METHODS

Here we evaluated the contribution of B cells in control of murine brucellosis in the more susceptible BALB/c and the more resistant C57BL/6 mice by infecting B cell-deficient mice.

RESULTS

Strikingly, in the absence of B cells in both C57BL/6 and BALB/c mice, 99% and 99.5% of the infection found in wild type mice was cleared, respectively. This augmented clearance was not reversed in either strain by passive transfer of immune serum. In C57BL/6 mice, the clearance of infection coincided with an increase in interferon γ (IFN-γ)-producing CD4 and CD8 T cells and a reduction in interleukin 10 (IL-10)-producing cells. In BALB/c mice, this clearance was IFN-γ-dependent, as B cell/IFN-γ dual knockout mice were unable to clear the infection, and was inversely related to the levels of transforming growth factor β (TGF-β). Furthermore, B cells were found to produce TGF-β and IL-10 during early stages of infection in BALB/c wild-type and C57BL/6 wild-type mice, respectively.

CONCLUSIONS

Thus, we demonstrate that the establishment of the high plateau phase of infection is dependent on non-antibody-mediated B cell effector mechanisms, including B regulatory functions, during murine brucellosis.

New insights on the pathogenesis of invasive Cryptococcus neoformans infection

Disseminated cryptococcosis begins with infection of the lungs via inhalation. This is followed by escape from the lungs and entry into the bloodstream allowing dissemination to the brain and central nervous system. We discuss the steps involved in dissemination and the host and microbial factors that influence each step. For the host, containment in the lung is accomplished with a combination of cell-mediated and antibody responses. Dissemination occurs when these systems fail and/or when phagocytic cells that fail to kill the yeast instead act as a niche for replication. One of the main microbial factors affecting dissemination is the polysaccharide capsule, a major virulence factor that promotes dissemination at every step. Secreted enzymes are important, including laccase and phospholipase B, which promote escape from the lungs, and urease, which contributes to crossing the blood-brain barrier. Lastly, a number of regulatory factors contribute, especially to growth of Cryptococcus neoformans in the brain.

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.

Aggregation of Streptococcus pneumoniae by a pneumococcal capsular polysaccharide-specific human monoclonal IgM correlates with antibody efficacy in vivo

Acquired antibody immunity to Streptococcus pneumoniae (pneumococcus) has been linked to serotype (ST)-specific opsonic antibodies to the relevant pneumococcal capsular polysaccharide (PPS) that mediate protection by enhancing the bactericidal effect of host phagocytes. Despite the well-recognized role of opsonic IgG in host defense against pneumococcus, PPS-specific monoclonal antibodies (MAbs) that mediate protection against lethal challenge with ST3 pneumococcus in mice but do not promote phagocytic killing in vitro (nonopsonic antibodies) have been described. In this study, we sought to determine the biological activity of one such MAb, A7 (a human PPS3-specific IgM), and the mechanism by which it mediates protection. In vitro studies demonstrated that coincubation of A7 with ST3 in the absence of phagocytes or a complement source resulted in a reduction in CFU on blood agar plates that was largely reversible by sonication. A chromogenic cellular proliferation assay demonstrated that A7 did not affect replication of ST3 in liquid culture. The ability of A7 to induce aggregation of ST3 was confirmed by fluorescence microscopy and flow cytometry: A7 induced aggregation of ST3, and in the presence of a complement source, A7 promoted deposition of complement component 3 (C3) on aggregated bacteria in a dose-dependent fashion. Similarly, administration of preincubated mixtures of A7 and ST3 intraperitoneally to mice protected them from the lethality of ST3 in a dose-dependent fashion. These findings suggest that A7-mediated aggregation enhances resistance to ST3, most likely by enhancing C3 deposition on the ST3 capsule, thereby promoting host antipneumococcal activity in vivo.

Improved survival of mice deficient in secretory immunoglobulin M following systemic infection with Cryptococcus neoformans

Cryptococcus neoformans causes severe, and often fatal, disease (cryptococcosis) in immunocompromised patients, particularly in those with HIV/AIDS. Although resistance to cryptococcosis requires intact T-cell immunity, a possible role for antibody/B cells in protection against natural disease has not been definitively established. Previous studies of the antibody response to the C. neoformans capsular polysaccharide glucuronoxylomannan (GXM) have demonstrated that patients who are at increased risk for cryptococcosis have lower serum levels of GXM-reactive IgM than those who are not at risk, leading to the hypothesis that IgM might contribute to resistance to cryptococcosis. To determine the influence of IgM on susceptibility to systemic cryptococcosis in a murine model, we compared the survival of mice deficient in serum IgM (secretory IgM deficient [sIgM(-/-)]) and C57BL/6 x 129Sv (control) mice after intraperitoneal infection with C. neoformans strain 24067 and analyzed the splenic B- and T-cell subsets by flow cytometry and the serum and splenic cytokine/chemokine and serum antibody profiles of each mouse strain. The results showed that sIgM(-/-) mice survived significantly longer than control mice when challenged with 10(5) CFU of C. neoformans 24067. Naïve sIgM(-/-) mice had higher levels of B-1 (CD5(+)) B cells, proinflammatory mediators (interleukin-6 [IL-6], IL-1beta, MIP-1beta, tumor necrosis factor alpha [TNF-alpha], and gamma interferon [IFN-gamma]), and anti-inflammatory mediators (IL-10 and IL-13) and significantly higher titers of GXM-specific IgG2a 3 weeks postinfection. In addition, CD5(+) splenocytes from both mouse strains had fungicidal activity against C. neoformans. Taken together, these results suggest that the inflammatory milieu in sIgM(-/-) mice might confer enhanced resistance to systemic cryptococcosis, stemming in part from the antifungal activity of B-1 B cells.

B cells are essential for moderating the inflammatory response and controlling bacterial multiplication in a mouse model of vaccination against Chlamydophila abortus infection

The use of inactivated vaccines associated with suitable adjuvants has been demonstrated to confer a good level of protection against Chlamydophila abortus. However, the basis of the immune protective response induced by these vaccines has been poorly studied. B cells act as an immune regulatory population during primary infection by C. abortus. Thus, it was considered of interest to study the role of B cells in an infection after immunization with a killed vaccine. For this, C57BL/6 and B-cell-deficient mice were immunized with a killed vaccine against C. abortus using QS-21 as the adjuvant. After challenge, the course of infection was established by analysis of morbidity, C. abortus burden in the liver, and histopathological changes. The immune response induced was studied by real-time PCR techniques. Experiments involving transfer of immune serum from vaccinated or previously infected mice were also carried out. The lack of B cells reduced the protection conferred by the QS-21 adjuvant vaccine. Vaccinated B-cell-deficient mice showed a 1,000-fold-greater bacterial burden in the liver than their wild-type counterparts. Obvious differences existed in the liver, where a severe neutrophilic reaction and extended areas of necrosis were observed with vaccinated B-cell-deficient mice. An analysis of the immune response pointed to a significant increase in inflammatory cytokines and chemokines and the deficient production of transforming growth factor beta. The transfer of antibodies restored the level of protection. This study demonstrates that B cells play a crucial role in controlling C. abortus multiplication and prevent an exacerbated inflammatory response.

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.

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

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

The PD-1/PD-L costimulatory pathway critically affects host resistance to the pathogenic fungus Histoplasma capsulatum

The PD-1 costimulatory receptor inhibits T cell receptor signaling upon interacting with its ligands PD-L1 and PD-L2. The PD-1/PD-L pathway is critical in maintaining self-tolerance. In this study, we examined the role of PD-1 in a mouse model of acute infection with Histoplasma capsulatum, a major human pathogenic fungus. In a lethal model of histoplasmosis, all PD-1-deficient mice survived infection, whereas the wild-type mice died with disseminated disease. PD-L expression on macrophages and splenocytes was up-regulated during infection, and macrophages from infected mice inhibited in vitro T cell activation. Of interest, antibody blocking of PD-1 significantly increased survival of lethally infected wild-type mice. Thus, our studies extend the role of the PD-1/PD-L pathway in regulating antimicrobial immunity to fungal pathogens. The results show that the PD-1/PD-L pathway has a key role in the regulation of antifungal immunity, and suggest that manipulation of this pathway represents a strategy of immunotherapy for histoplasmosis.

Cryptococcus neoformans induces IL-8 secretion and CXCL1 expression by human bronchial epithelial cells

Background

Cryptococcus neoformans (C. neoformans) is a globally distributed fungal pathogen with the potential to cause serious disease, particularly among immune compromised hosts. Exposure to this organism is believed to occur by inhalation and may result in pneumonia and/or disseminated infection of the brain as well as other organs. Little is known about the role of airway epithelial cells in cryptococcal recognition or their ability to induce an inflammatory response.

Methods

Immortalized BEAS-2B bronchial epithelial cells and primary normal human bronchial epithelium (NHBE) were stimulated in vitro with encapsulated or acapsular C. neoformans cultivated at room temperature or 37°C. Activation of bronchial epithelial cells was characterized by analysis of inflammatory cytokine and chemokine expression, transcription factor activation, fungal-host cell association, and host cell damage.

Results

Viable C. neoformans is a strong activator of BEAS-2B cells, resulting in the production of the neutrophil chemokine Interleukin (IL)-8 in a time- and dose-dependent manner. IL-8 production was observed only in response to acapsular C. neoformans that was grown at 37°C. C. neoformans was also able to induce the expression of the chemokine CXCL1 and the transcription factor CAAT/enhancer-binding protein beta (CEBP/β) in BEAS-2B cells. NHBE was highly responsive to stimulation with C. neoformans; in addition to transcriptional up regulation of CXCL1, these primary cells exhibited the greatest IL-8 secretion and cell damage in response to stimulation with an acapsular strain of C. neoformans.

Conclusion

This study demonstrates that human bronchial epithelial cells mediate an acute inflammatory response to C. neoformans and are susceptible to damage by this fungal pathogen. The presence of capsular polysaccharide and in vitro fungal culture conditions modulate the host inflammatory response to C. neoformans. Human bronchial epithelial cells are likely to contribute to the initial stages of pulmonary host defense in vivo.

Animal models: an important tool in mycology

Animal models of fungal infections are, and will remain, a key tool in the advancement of the medical mycology. Many different types of animal models of fungal infection have been developed, with murine models the most frequently used, for studies of pathogenesis, virulence, immunology, diagnosis, and therapy. The ability to control numerous variables in performing the model allows us to mimic human disease states and quantitatively monitor the course of the disease. However, no single model can answer all questions and different animal species or different routes of infection can show somewhat different results. Thus, the choice of which animal model to use must be made carefully, addressing issues of the type of human disease to mimic, the parameters to follow and collection of the appropriate data to answer those questions being asked. This review addresses a variety of uses for animal models in medical mycology. It focuses on the most clinically important diseases affecting humans and cites various examples of the different types of studies that have been performed. Overall, animal models of fungal infection will continue to be valuable tools in addressing questions concerning fungal infections and contribute to our deeper understanding of how these infections occur, progress and can be controlled and eliminated.

Pathogenic role of B cells and antibodies in murine Leishmania amazonensis infection

Leishmania amazonensis infection, occurring predominantly in Central and South America, can manifest itself in several forms, including those of cutaneous and diffuse cutaneous leishmaniasis. The outcome of L. amazonensis infection depends largely on host immune responses to the parasites. While CD4+ T cell activation is a prerequisite for pathogenesis in L. amazonensis-infected mice, the roles of B cells and their antibody production are unclear. In this study, we provide evidence suggesting that B cells and antibodies are involved in disease pathogenesis. We documented a correlation between B cell activation and lesion progress in immunocompetent mice. In the absence of functional B cells and antibodies, JhD mice showed a delayed onset of disease and developed small lesions. Histological examination of these mice revealed a significant reduction in CD4+ and CD8+ T cells, but not in MAC1+ macrophages, at the infection site. In contrast to the wild-type mice that showed typical tissue necrosis, L. amazonensis-infected JhD mice showed no or minimal signs of necrotic foci. A marked reduction in CD4+ T cell proliferation and cytokine (IFN-gamma and IL-10) production in infected JhD mice suggested an involvement of B cells and antibodies in the priming of parasite-specific T cells. This notion was further supported by the observations that adoptive transfer of B cells or antibodies could restore CD4+ T cell activation and migration in infected JhD mice. Moreover, antibody coating of parasites could stimulate dendritic cells to produce high levels of cytokines and increase their ability to prime nai ve CD4+ T cells. Since CD4+ T cells are crucial to disease pathogenesis, this study suggests that B cells and their antibody production enhanced L. amazonensis infection, partially by promoting T cell priming and cellular migration to the infection site.

Monoclonal antibody to fungal glucosylceramide protects mice against lethal Cryptococcus neoformans infection

Glucosylceramides (GlcCer) are involved in the regulation of Cryptococcus neoformans virulence. In the present study, we demonstrate that passive immunization with a monoclonal antibody to GlcCer significantly reduces host inflammation and prolongs the survival of mice lethally infected with C. neoformans, revealing a potential therapeutic strategy to control cryptococcosis.

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.

In vivo role of dendritic cells in a murine model of pulmonary cryptococcosis

Dendritic cells (DC) have been shown to phagocytose and kill Cryptococcus neoformans in vitro and are believed to be important for inducing protective immunity against this organism. Exposure to C. neoformans occurs mainly by inhalation, and in this study we examined the in vivo interactions of C. neoformans with DC in the lung. Fluorescently labeled live C. neoformans and heat-killed C. neoformans were administered intranasally to C57BL/6 mice. At specific times postinoculation, mice were sacrificed, and lungs were removed. Single-cell suspensions of lung cells were prepared, stained, and analyzed by microscopy and flow cytometry. Within 2 h postinoculation, fluorescently labeled C. neoformans had been internalized by DC, macrophages, and neutrophils in the mouse lung. Additionally, lung DC from mice infected for 7 days showed increased expression of the maturation markers CD80, CD86, and major histocompatibility complex class II. Finally, ex vivo incubation of lung DC from infected mice with Cryptococcus-specific T cells resulted in increased interleukin-2 production compared to the production by DC from naïve mice, suggesting that there was antigen-specific T-cell activation. This study demonstrated that DC in the lung are capable of phagocytosing Cryptococcus in vivo and presenting antigen to C. neoformans-specific T cells ex vivo, suggesting that these cells have roles in innate and adaptive pulmonary defenses against cryptococcosis.

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.

Essential role for humoral immunity during Ehrlichia infection in immunocompetent mice

Although cellular immunity is essential for host defense during intracellular bacterial infections, humoral immunity can also play a significant role in host defense during infection by some intracellular bacteria, including the ehrlichiae. Antibodies can protect susceptible SCID mice from fatal Ehrlichia chaffeensis infection, an observation that has been hypothesized to involve the opsonization of bacteria released from host cells. To determine whether humoral immunity plays an essential role during ehrlichia infection in immunocompetent mice, we utilized a murine model of fatal monocytotropic ehrlichiosis caused by Ixodes ovatus ehrlichia. Mice lacking either B cells or FcgammaRI were unable to resolve a low-dose (sublethal) I. ovatus ehrlichia infection, which suggested that humoral immunity is essential for resistance. Polyclonal sera generated in I. ovatus ehrlichia-infected mice recognized a conserved ehrlichia outer membrane protein and, when administered to infected mice, caused a significant decrease in bacterial infection. Mice experimentally depleted of complement, or deficient for complement receptors 1 and 2, were also susceptible to sublethal I. ovatus ehrlichia infection, as were mice that lacked the phox91 subunit of NADPH oxidase. The data are consistent with a mechanism whereby bacteria released from infected cells are lysed directly by complement or undergo antibody-mediated FcgammaR-dependent phagocytosis and subsequent exposure to reactive oxygen intermediates. The findings suggest mechanisms whereby antibodies contribute to immunity against intracellular bacteria in immunocompetent mice.

Experimental systemic infection withCryptococcus neoformansvar.grubiiandCryptococcusgattiiin normal and immunodeficient mice

Cryptococcus neoformans (Cn) var. grubii or Cryptococcus neoformans var. neoformans infection is usually associated with immunocompromised hosts, whereas Cryptococcusgattii more frequently causes disease in immunocompetent hosts. We examined the effects of immunodeficiency and glucocorticoid-induced immunosuppression on systemic murine infection induced by i.v. inoculation with these pathogens. SCID and immunocompetent BALB/c and C57BL/6 mice were infected with <or=107 yeast of Cn var. grubii or C. gattii; immunosuppressed BALB/c mice were infected with <or=106 yeast. Mortality was inoculum size-dependent in each model system, for both organisms. Following infection with 106 CFU of either Cn var. grubii or C. gattii immunocompetent BALB/c mice survived longer than immunosuppressed mice (P<0.0001 in both cases); no differences were found using lower inocula. SCID mice infected with Cn var. grubii or C. gattii died sooner than BALB/c mice (P<0.0013, all comparisons). Unexpectedly, BALB/c mice infected with C. gattii developed external lesions. Immunocompetent mice developed rectal prolapse more frequently whereas immunosuppressed mice developed more frequent skin lesions, predominantly on the tail. The fungal burden was especially high in rectum, skin and lung tissues. Histologic examination showed extensive infection of the rectum and skin and pneumonitis. Determination of CFU from various organs of immunocompetent BALB/c mice infected i.v. with 105 CFU of C. gattii or Cn var. grubii showed significant temporal increase of burdens of Cn var. grubii in brain and liver (P<0.003); other organs showed decreasing fungal burden. C.gattii was recovered only from liver and lungs, no CFU were detected in the other organs. As opposed to epidemiologic observations, our results demonstrate no predilection by C. gattii for infection of immunocompetent over immunosuppressed hosts; immunosuppression increased the risk of severe cryptococcosis by both varieties, especially at high inocula. This is the first report of C. gattii inducing experimental cutaneous and intestinal mucosal infection; Cn var. grubii did not affect these tissues, indicating differences in tissue tropism of these pathogens.

A Reappraisal of Humoral Immunity Based on Mechanisms of Antibody‐Mediated Protection Against Intracellular Pathogens

Sometime in the mid to late twentieth century the study of antibody-mediated immunity (AMI) entered the doldrums, as many immunologists believed that the function of AMI was well understood, and was no longer deserving of intensive investigation. However, beginning in the 1990s studies using monoclonal antibodies (mAbs) revealed new functions for antibodies, including direct antimicrobial effects and their ability to modify host inflammatory and cellular responses. Furthermore, the demonstration that mAbs to several intracellular bacterial and fungal pathogens were protective issued a serious challenge to the paradigm that host defense against such microbes was strictly governed by cell-mediated immunity (CMI). Hence, a new view of AMI is emerging. This view is based on the concept that a major function of antibody (Ab) is to amplify or subdue the inflammatory response to a microbe. In this regard, the "damage-response framework" of microbial pathogenesis provides a new conceptual viewpoint for understanding mechanisms of AMI. According to this view, the ability of an Ab to affect the outcome of a host-microbe interaction is a function of its capacity to modify the damage ensuing from such an interaction. In fact, it is increasingly apparent that the efficacy of an Ab cannot be defined either by immunoglobulin or epitope characteristics alone, but rather by a complex function of Ab variables, such as specificity, isotype, and amount, host variables, such as genetic background and immune status, and microbial variables, such as inoculum, mechanisms of avoiding host immune surveillance and pathogenic strategy. Consequently, far from being understood, recent findings in AMI imply a system with unfathomable complexity and the field is poised for a long overdue renaissance.

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.

Role of IFN-gamma in regulating T2 immunity and the development of alternatively activated macrophages during allergic bronchopulmonary mycosis

Pulmonary Cryptococcus neoformans infection of C57BL/6 mice is an established model of a chronic pulmonary fungal infection accompanied by an "allergic" response (T2) to the infection, i.e., a model of an allergic bronchopulmonary mycosis. Our objective was to determine whether IFN-gamma plays a role in regulating the pulmonary T2 immune response in C. neoformans-infected C57BL/6 mice. Long-term pulmonary fungistasis was lost in IFN-gamma knockout (KO) mice, resulting in an increased pulmonary burden of fungi at wk 3. IFN-gamma was required for the early influx of leukocytes into the lungs but was not required later in the infection. By wk 3, eosinophil and macrophage numbers were elevated in the absence of IFN-gamma. The inducible NO synthase to arginase ratio was lower in the lungs of IFN-gamma KO mice and the macrophages had increased numbers of intracellular cryptococci and YM1 crystals, indicative of alternatively activated macrophages in these mice. There was evidence of pulmonary fibrosis in both wild-type and IFN-gamma KO mice by 5 wk postinfection. IFN-gamma production was not required for the development of T2 cytokine (IL-4, IL-5, IL-13) producing cells in the lungs and lung-associated lymph nodes or induction of an IgE response. At a number of time points, T2 cytokine production was enhanced in IFN-gamma KO mice. Thus, in the absence of IFN-gamma, C57BL/6 mice develop an augmented allergic response to C. neoformans, including enhanced generation of alternatively activated macrophages, which is accompanied by a switch from a chronic to a progressive pulmonary cryptococcal infection.