Human astrocytes inhibit Cryptococcus neoformans growth by a nitric oxide-mediated mechanism

IL-6 deficiency accelerates cerebral cryptococcosis and alters glial cell responses

Cryptococcus neoformans (Cn) is an opportunistic encapsulated fungal pathogen that causes life-threatening meningoencephalitis in immunosuppressed individuals. Since IL-6 is important for blood-brain barrier support and its deficiency has been shown to facilitate Cn brain invasion, we investigated the impact of IL-6 on systemic Cn infection in vivo, focusing on central nervous system (CNS) colonization and glial responses, specifically microglia and astrocytes. IL-6 knock-out (IL-6-/-) mice showed faster mortality than C57BL/6 (Wild-type) and IL-6-/- supplemented with recombinant IL-6 (rIL-6; 40 pg/g/day) mice. Despite showing early lung inflammation but no major histological differences in pulmonary cryptococcosis progression among the experimental groups, IL-6-/- mice had significantly higher blood and brain tissue fungal burden at 7-days post infection. Exposure of cryptococci to rIL-6 in vitro increased capsule growth. In addition, IL-6-/- brains were characterized by an increased dystrophic microglia number during Cn infection, which are associated with neurodegeneration and senescence. In contrast, the brains of IL-6-producing or -supplemented mice displayed high numbers of activated and phagocytic microglia, which are related to a stronger anti-cryptococcal response or tissue repair. Likewise, culture of rIL-6 with microglia-like cells promoted high fungal phagocytosis and killing, whereas IL-6 silencing in microglia decreased fungal phagocytosis. Lastly, astrogliosis was high and moderate in infected brains removed from Wild-type and IL-6-/- supplemented with rIL-6 animals, respectively, while minimal astrogliosis was observed in IL-6-/- tissue, highlighting the potential of astrocytes in containing and combating cryptococcal infection. Our findings suggest a critical role for IL-6 in Cn CNS dissemination, neurocryptococcosis development, and host defense.

Cryptococcus neoformans rapidly invades the murine brain by sequential breaching of airway and endothelial tissues barriers, followed by engulfment by microglia

Cryptococcus neoformans causes lethal meningitis and accounts for approximately 10%-15% of AIDS-associated deaths worldwide. There are major gaps in our understanding of how this fungus invades the mammalian brain. To investigate the dynamics of C. neoformans tissue invasion, we mapped fungal localization and host cell interactions in infected brain, lung, and upper airways using mouse models of systemic and airway infection. To enable this, we developed an in situ imaging pipeline capable of measuring large volumes of tissue while preserving anatomical and cellular information by combining thick tissue sections, tissue clarification, and confocal imaging. We confirm high fungal burden in mouse upper airway after nasal inoculation. Yeast in turbinates were frequently titan cells, with faster kinetics than reported in mouse lungs. Importantly, we observed one instance of fungal cells enmeshed in lamina propria of the upper airways, suggesting penetration of airway mucosa as a possible route of tissue invasion and dissemination to the bloodstream. We extend previous literature positing bloodstream dissemination of C. neoformans, by finding viable fungi in the bloodstream of mice a few days after intranasal infection. As early as 24 h post systemic infection, the majority of C. neoformans cells traversed the blood-brain barrier, and were engulfed or in close proximity to microglia. Our work presents a new method for investigating microbial invasion, establishes that C. neoformans can breach multiple tissue barriers within the first days of infection, and demonstrates microglia as the first cells responding to C. neoformans invasion of the brain.IMPORTANCECryptococcal meningitis causes 10%-15% of AIDS-associated deaths globally. Still, brain-specific immunity to cryptococci is a conundrum. By employing innovative imaging, this study reveals what occurs during the first days of infection in brain and in airways. We found that titan cells predominate in upper airways and that cryptococci breach the upper airway mucosa, which implies that, at least in mice, the upper airways are a site for fungal dissemination. This would signify that mucosal immunity of the upper airway needs to be better understood. Importantly, we also show that microglia, the brain-resident macrophages, are the first responders to infection, and microglia clusters are formed surrounding cryptococci. This study opens the field to detailed molecular investigations on airway immune response, how fungus traverses the blood-brain barrier, how microglia respond to infection, and ultimately how microglia monitor the blood-brain barrier to preserve brain function.

Cryptococcus neoformans rapidly invades the murine brain by sequential breaching of airway and endothelial tissues barriers, followed by engulfment by microglia

The fungus Cryptococcus neoformans causes lethal meningitis in humans with weakened immune systems and is estimated to account for 10-15% of AIDS-associated deaths worldwide. There are major gaps in our understanding of how this environmental fungus evades the immune system and invades the mammalian brain before the onset of overt symptoms. To investigate the dynamics of C. neoformans tissue invasion, we mapped early fungal localisation and host cell interactions at early times in infected brain, lung, and upper airways using mouse models of systemic and airway infection. To enable this, we developed an in situ imaging pipeline capable of measuring large volumes of tissue while preserving anatomical and cellular information by combining thick tissue sections, tissue clarification, and confocal imaging. Made possible by these techniques, we confirm high fungal burden in mouse upper airway turbinates after nasal inoculation. Surprisingly, most yeasts in turbinates were titan cells, indicating this microenvironment enables titan cell formation with faster kinetics than reported in mouse lungs. Importantly, we observed one instance of fungal cells enmeshed in lamina propria of upper airways, suggesting penetration of airway mucosa as a possible route of tissue invasion and dissemination to the bloodstream. We extend previous literature positing bloodstream dissemination of C. neoformans, via imaging C. neoformans within blood vessels of mouse lungs and finding viable fungi in the bloodstream of mice a few days after intranasal infection, suggesting that bloodstream access can occur via lung alveoli. In a model of systemic cryptococcosis, we show that as early as 24 h post infection, majority of C. neoformans cells traversed the blood-brain barrier, and are engulfed or in close proximity to microglia. Our work establishes that C. neoformans can breach multiple tissue barriers within the first days of infection. This work presents a new method for investigating cryptococcal invasion mechanisms and demonstrates microglia as the primary cells responding to C. neoformans invasion.

Phospholipase B Is Critical for Cryptococcus neoformans Survival in the Central Nervous System

Cryptococcus neoformans (Cn) is an opportunistic, encapsulated, yeast-like fungus that causes severe meningoencephalitis, especially in countries with high HIV prevalence. In addition to its well-known polysaccharide capsule, Cn has other virulence factors such as phospholipases, a heterogeneous group of enzymes that hydrolyze ester linkages in glycerophospholipids. Phospholipase B (PLB1) has been demonstrated to play a key role in Cn pathogenicity. In this study, we used a PLB1 mutant (plb1) and its reconstituted strain (Rec1) to assess the importance of this enzyme on Cn brain infection in vivo and in vitro. Mice infected with the plb1 strain survive significantly longer, have lower peripheral and central nervous system (CNS) fungal loads, and have fewer and smaller cryptococcomas or biofilm-like brain lesions compared to H99- and Rec1-infected animals. PLB1 causes extensive brain tissue damage and changes microglia morphology during cryptococcal disease, observations which can have important implications in patients with altered mental status or dementia as these manifestations are related to poorer survival outcomes. plb1 cryptococci are significantly more phagocytosed and killed by NR-9460 microglia-like cells. plb1 cells have altered capsular polysaccharide biophysical properties which impair their ability to stimulate glial cell responses or morphological changes. Here, we provide significant evidence demonstrating that Cn PLB1 is an important virulence factor for fungal colonization of and survival in the CNS as well as in the progression of cryptococcal meningoencephalitis. These findings may potentially help fill in a gap of knowledge in our understanding of cerebral cryptococcosis and provide novel research avenues in Cn pathogenesis. IMPORTANCE Cryptococcal meningoencephalitis (CME) is a serious disease caused by infection by the neurotropic fungal pathogen Cryptococcus neoformans. Due to the increasing number of cases in HIV-infected individuals, as well as the limited therapies available, investigation into potential targets for new therapeutics has become critical. Phospholipase B is an enzyme synthesized by Cn that confers virulence to the fungus through capsular enlargement, immunomodulation, and intracellular replication. In this study, we examined the properties of PLB1 by comparing infection of a Cn PLB1 mutant strain with both the wild-type and a PLB1-reconstituted strain. We show that PLB1 augments the survival and proliferation of the fungus in the CNS and strengthens virulence by modulating the immune response and enhancing specific biophysical properties of the fungus. PLB1 expression causes brain tissue damage and impacts glial cell functions, which may be responsible for the dementia observed in patients which may persist even after resolving from CME. The implications of PLB1 inhibition reveal its involvement in Cn infection and suggest that it may be a possible molecular target in the development of antifungal therapies. The results of this study support additional investigation into the mechanism of PLB1 to further understand the intricacies of cerebral Cn infection.

Clinical and pathological characterization of Central Nervous System cryptococcosis in an experimental mouse model of stereotaxic intracerebral infection

Infection of the Central Nervous System (CNS) by the encapsulated fungus Cryptococcus neoformans can lead to high mortality meningitis, most commonly in immunocompromised patients. While the mechanisms by which the fungus crosses the blood-brain barrier to initiate infection in the CNS are well recognized, there are still substantial unanswered questions about the disease progression once the fungus is established in the brain. C. neoformans is characterized by a glucuronoxylomannan (GXM)-rich polysaccharide capsule which has been implicated in immune evasion, but its role during the host CNS infection needs further elucidation. Therefore, the present study aims to examine these key questions about the mechanisms underlying cryptococcal meningitis progression and the impact of fungal GXM release by using an intracerebral rodent infection model via stereotaxic surgery. After developing brain infection, we analyzed distinct brain regions and found that while fungal load and brain weight were comparable one-week post-infection, there were region-specific histopathological (with and without brain parenchyma involvement) and disease manifestations. Moreover, we also observed a region-specific correlation between GXM accumulation and glial cell recruitment. Furthermore, mortality was associated with the presence of subarachnoid hemorrhaging and GXM deposition in the meningeal blood vessels and meninges in all regions infected. Our results show that using the present infection model can facilitate clinical and neuropathological observations during the progression of neurocryptococcosis. Importantly, this mouse model can be used to further investigate disease progression as it develops in humans.

Cryptococcus neoformans Infection in the Central Nervous System: The Battle between Host and Pathogen

Cryptococcus neoformans (C. neoformans) is a pathogenic fungus with a global distribution. Humans become infected by inhaling the fungus from the environment, and the fungus initially colonizes the lungs. If the immune system fails to contain C. neoformans in the lungs, the fungus can disseminate to the blood and invade the central nervous system, resulting in fatal meningoencephalitis particularly in immunocompromised individuals including HIV/AIDS patients. Following brain invasion, C. neoformans will encounter host defenses involving resident as well as recruited immune cells in the brain. To overcome host defenses, C. neoformans possesses multiple virulence factors capable of modulating immune responses. The outcome of the interactions between the host and C. neoformans will determine the disease progression. In this review, we describe the current understanding of how C. neoformans migrates to the brain across the blood–brain barrier, and how the host immune system responds to the invading organism in the brain. We will also discuss the virulence factors that C. neoformans uses to modulate host immune responses.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

Fungal CNS Infections in Africa: The Neuroimmunology of Cryptococcal Meningitis

Cryptococcal meningitis (CM) is the leading cause of central nervous system (CNS) fungal infections in humans, with the majority of cases reported from the African continent. This is partly due to the high burden of HIV infection in the region and reduced access to standard-of-care including optimal sterilising antifungal drug treatments. As such, CM is responsible for 10-15% of all HIV-related mortality, with a large proportion being preventable. Immunity to the causative agent of CM, Cryptococcus neoformans, is only partially understood. IFNγ producing CD4⁺ T-cells are required for the activation of myeloid cells, especially macrophages, to enable fungal killing and clearance. However, macrophages may also act as a reservoir of the fungal yeast cells, shielding them from host immune detection thus promoting latent infection or persistent chronic inflammation. In this chapter, we review the epidemiology and pathogenesis of CNS fungal infections in Africa, with a major focus on CM, and the antifungal immune pathways operating to protect against C. neoformans infection. We also highlight the areas of research and policy that require prioritisation to help reduce the burden of CNS fungal diseases in Africa.

Reimagining the future of African brain health: Perspectives for basic research on the pathogenesis of cryptococcal meningitis

Cryptococcal meningitis is a fatal opportunistic infection of the brain and a leading cause of neurological damage and death in immunocompromised individuals. This neglected fungal disease of the brain is a huge burden on the health systems of developing countries, especially in Sub-Saharan Africa, where up to 25% of people living with HIV/AIDS succumb to it. Cryptococcal fungal cells have a predilection for the brain and they are capable of traversing the blood brain barrier and invade the brain where they cause infection, inflammation and a disruption of normal brain function. A robust host neuroimmune response is critical for pathogen clearance and survival, and a good understanding of the mechanisms underlying its development in the host is critical for the development of effective treatments. However, past basic research studies have been focussed on the characteristics of the fungus and its effect on the peripheral immune system; with little attention paid to how it interacts with brain immune cells. This mini review briefly discusses the paucity of basic research data on the neuroimmune response to cryptococcal infection, raises pertinent questions on how the brain cells respond to the fungal infection, and thereafter discusses models, techniques and advanced technologies that could be useful for carrying out high-throughput research on the pathogenesis of cryptococcal meningitis.

Cryptococcus neoformans-astrocyte interactions: effect on fungal blood brain barrier disruption, brain invasion, and meningitis progression

Cryptococcus neoformans is an opportunistic, neurotropic, and encapsulated fungus that causes life-threatening cryptococcal meningitis (CM), especially in regions of the world where AIDS is endemic. The polysaccharide capsule of C. neoformans is the fungus major virulent factor, being copiously released during infection and causing immunosuppressive defects in the host. Although the capsular material is commonly associated with reactive astrocytes in fatal CM, little is known about the molecular and cellular interactions among astroglia and C. neoformans. As astrocytes also make up the neurovascular unit at the blood-brain barrier (BBB), which C. neoformans must transverse to colonize the central nervous system and cause CM; these cells may play a significant regulatory role in the prevention and progression of infection. For example, astrocytes are implicated in neurological disease including the regulation of cerebral intracranial pressure, immune function, and water homeostasis. Hence, in this review, we provide a general overview of astroglia biology and discuss the current knowledge on C. neoformans-astrocyte interactions including their involvement in the development of CM. This "gliocentric view" of cerebral cryptococcosis suggests that therapeutic interventions particularly targeting at preserving the neuroprotective function of astrocytes may be used in preventing and managing C. neoformans BBB transmigration, brain invasion, colonization, and meningitis.

Infective capacity of Cryptococcus neoformans and Cryptococcus gattii in a human astrocytoma cell line

Pathogenesis of cryptococcosis in the central nervous system (CNS) is a topic of ongoing research, including the mechanisms by which this fungus invades and infects the brain. Astrocytes, the most common CNS cells, play a fundamental role in the local immune response. Astrocytes might participate in cryptococcosis either as a host or by responding to fungal antigens. To determine the infectivity of Cryptococcus neoformans var. grubii and Cryptococcus gattii in a human astrocytoma cell line and the induction of major histocompatibility complex (MHC) molecules. A glioblastoma cell line was infected with C. neoformans var. grubii and C. gattii blastoconidia labelled with FUN-1 fluorescent stain. The percentage of infection and expression of HLA class I and II molecules were determined by flow cytometry. The interactions between the fungi and cells were observed by fluorescence microscopy. There was no difference between C. neoformans var. grubii and C. gattii in the percentage infection, but C. neoformans var. grubii induced higher expression of HLA class II than C. gattii. More blastoconidia were recovered from C. neoformans-infected cells than from C. gattii infected cells. Cryptococcus neoformans var. grubii may have different virulence mechanisms that allow its survival in human glia-derived cells.

Infectious immunity in the central nervous system and brain function

Inflammation is emerging as a critical mechanism underlying neurological disorders of various etiologies, yet its role in altering brain function as a consequence of neuroinfectious disease remains unclear. Although acute alterations in mental status due to inflammation are a hallmark of central nervous system (CNS) infections with neurotropic pathogens, post-infectious neurologic dysfunction has traditionally been attributed to irreversible damage caused by the pathogens themselves. More recently, studies indicate that pathogen eradication within the CNS may require immune responses that interfere with neural cell function and communication without affecting their survival. In this Review we explore inflammatory processes underlying neurological impairments caused by CNS infection and discuss their potential links to established mechanisms of psychiatric and neurodegenerative diseases.

■ REVIEW : Glial NO: Normal and Pathological Roles

All nervous system cell types can be induced with cytokines or bacterial products to make nitric oxide, at least in culture. The signaling pathways invoked by inducers that result in transcriptional activation of the nitric oxide synthase gene are becoming clear, and modulators of this induction have been discovered. Much suggestive and, recently, more definitive evidence has accumulated for induction of nitric oxide synthase in glial cells in vivo associated with viral infection, as well as in animal models of trauma, ischemia, and autoimmunity. Whether nitric oxide from this source contributes to or limits the attendant conditions is not yet clear. The Neuroscientist 2:90-99, 1996

Fungal colonization of the brain: anatomopathological aspects of neurological cryptococcosis

Brain infection by the fungus Cryptococcus neoformans results in an estimated 500,000 human deaths per annum. Colonization of the central nervous system (CNS) by C. neoformans causes different clinical syndromes that involve interaction of a number of fungal components with distinct brain cells. In this manuscript, our literature review confirmed the notion that the Cryptococcus field is expanding rapidly, but also suggested that studies on neuropathogenesis still represent a small fraction of basic research activity in the field. We therefore discussed anatomical and physiological aspects of the brain during infection by C. neoformans, in addition to mechanisms by which brain resident cells interact with the fungus. This review suggests that multiple efforts are necessary to improve the knowledge on how C. neoformans affects brain cells, in order to enable the generation of new therapeutic tools in a near future.

The tools for virulence of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes almost half a million deaths each year. It is believed that most humans are infected with C. neoformans, possibly in a form that survives through latency in the lung and can reactivate to cause disease if the host becomes immunosuppressed. C. neoformans has a remarkably sophisticated intracellular survival capacities yet it is a free-living fungus with no requirement for mammalian virulence whatsoever. In this review, we discuss the tools that C. neoformans possesses to achieve survival, latency and virulence within its host. Some of these tools are mechanisms to withstand starvation and others aim to protect against microbicidal molecules produced by the immune system. Furthermore, we discuss how these tools were acquired through evolutionary pressures and perhaps accidental stochastic events, all of which combined to produce an organism with an unusual and unique intracellular pathogenic strategy.

IL-4 receptor-alpha-dependent control of Cryptococcus neoformans in the early phase of pulmonary infection

Cryptococcus neoformans is an opportunistic fungal pathogen that causes lung inflammation and meningoencephalitis in immunocompromised people. Previously we showed that mice succumb to intranasal infection by induction of pulmonary interleukin (IL)-4Rα-dependent type 2 immune responses, whereas IL-12-dependent type 1 responses confer resistance. In the experiments presented here, IL-4Rα⁻/⁻ mice unexpectedly show decreased fungal control early upon infection with C. neoformans, whereas wild-type mice are able to control fungal growth accompanied by enhanced macrophage and dendritic cell recruitment to the site of infection. Lower pulmonary recruitment of macrophages and dendritic cells in IL-4Rα⁻/⁻ mice is associated with reduced pulmonary expression of CCL2 and CCL20 chemokines. Moreover, IFN-γ and nitric oxide production are diminished in IL-4Rα⁻/⁻ mice compared to wild-type mice. To directly study the potential mechanism(s) responsible for reduced production of IFN-γ, conventional dendritic cells were stimulated with C. neoformans in the presence of IL-4 which results in increased IL-12 production and reduced IL-10 production. Together, a beneficial role of early IL-4Rα signaling is demonstrated in pulmonary cryptococcosis, which contrasts with the well-known IL-4Rα-mediated detrimental effects in the late phase.

Oxidative stress and reduced glutamine synthetase activity in the absence of inflammation in the cortex of mice with experimental allergic encephalomyelitis

Pathological changes occur in areas of CNS tissue remote from inflammatory lesions in multiple sclerosis (MS) and its animal model experimental allergic encephalomyelitis (EAE). To determine if oxidative stress is a significant contributor to this non-inflammatory pathology, cortex tissues from mice with clinical signs of EAE were examined for evidence of inflammation and oxidative stress. Histology and gene expression analysis showed little evidence of immune/inflammatory cell invasion but reductions in natural antioxidant levels and increased protein oxidation that paralleled disease severity. Two-dimensional oxyblots and mass-spectrometry-based protein fingerprinting identified glutamine synthetase (GS) as a particular target of oxidation. Oxidation of GS was associated with reductions in enzyme activity and increased glutamate/glutamine levels. The possibility that this may cause neurodegeneration through glutamate excitotoxicity is supported by evidence of increasing cortical Ca(2+) levels in cortex extracts from animals with greater disease severity. These findings indicate that oxidative stress occurs in brain areas that are not actively undergoing inflammation in EAE and that this can lead to a neurodegenerative process due to the susceptibility of GS to oxidative inactivation.

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

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

Gomesin, a peptide produced by the spiderAcanthoscurria gomesiana, is a potent anticryptococcal agent that acts in synergism with fluconazole

Gomesin is an 18-residue cysteine-rich antimicrobial peptide produced by hemocytes of the spider Acanthoscurria gomesiana. In the present study, the antifungal properties of gomesin against Cryptococcus neoformans, the etiologic agent of cryptococcosis, were evaluated. Gomesin bound to the cell surface of cryptococci, which resulted in cell death associated with membrane permeabilization. Antifungal concentrations of gomesin were not toxic for human brain cells. Supplementation of cryptococcal cultures with the peptide (1 microM) caused a decrease in capsule expression and rendered fungal cells more susceptible to killing by human brain phagocytes. The possible use of gomesin in combination with fluconazole, a standard antifungal drug, was also evaluated. In association with fluconazole, gomesin concentrations with low antimicrobial activity (0.1-1 microM) inhibited fungal growth and enhanced the antimicrobial activity of brain phagocytes. These results reveal the potential of gomesin to promote inhibition of cryptococcal growth directly or by enhancing the effectiveness of host defenses.

Recurrent headache as the main symptom of acquired cerebral toxoplasmosis in nonhuman immunodeficiency virus-infected subjects with no lymphadenopathy: the parasite may be responsible for the neurogenic inflammation postulated as a cause of different types of headaches

Headache and/or migraine, a common problem in pediatrics and internal medicine, affect about 5% to 10% children and adolescents, and nearly 30% of middle-aged women. Headache is also one of the most common clinical manifestations of acquired Toxoplasma gondii infection of the central nervous system (CNS) in immunosuppressed subjects. We present 11 apparently nonhuman immunodeficiency virus-infected children aged 7 to 17 years (8 girls, 3 boys) and 1 adult woman with recurrent severe headaches in whom latent chronic CNS T. gondii infection not manifested by enlarged peripheral lymph nodes typical for toxoplasmosis, was found. In 7 patients, the mean serum IgG Toxoplasma antibodies concentration was 189 +/- 85 (SD) IU/mL (range 89 to 300 IU/mL), and in 5 other subjects, the indirect fluorescent antibody test titer ranged from 1:40 to 1:5120 IU/mL (n= <1:10 IU/mL). Some of the patients suffered also from atopic dermatitis (AD) and were exposed to cat and/or other pet allergens, associated with an increased IL-4 and decreased IFN-gamma production. These cytokine irregularities caused limited control of cerebral toxoplasmosis probably because IL-4 down-regulated both the production of IFN-gamma and its activity, and stimulated production of a low NO-producing population of monocytes, which allowed cysts rupture, increased parasite multiplication and finally reactivation of T. gondii infection. The immune studies performed in 4 subjects showed a decreased percentage of T lymphocytes, increased total number of lymphocytes B and serum IgM concentration, and impaired phagocytosis. In addition, few of them had also urinary tract diseases known to produce IL-6 that can mediate immunosuppressive functions, involving induction of the anti-inflammatory cytokine IL-10. These disturbances probably resulted from the host protective immune reactions associated with the chronic latent CNS T. gondii infection/inflammation. This is consistent with significantly lower enzyme indoleamine 2,3-dioxygenase (IDO) activity reported in atopic than in nonatopic individuals, and an important role that IDO and tryptophan degradation pathways plays in both, the host resistance to T. gondii infection and its reactivation. Analysis of literature information on the subjects with different types of headaches caused by foods, medications, and other substances, may suggest that their clinical symptoms and changes in laboratory data result at least in part from interference of these factors with dietary tryptophan biotransformation pathways. Several of these agents caused headache attacks through enhancing NO production via the conversion of arginine to citrulline and NO by the inducible nitric oxide synthase enzyme, which results in the high-output pathway of NO synthesis. This increased production of NO is, however, quickly down-regulated by NO itself because this biomolecule can directly inactivate NOS, may inhibit Ia expression on IFN-gamma-activated macrophages, which would limit antigen-presenting capability, and block T-cell proliferation, thus decreasing the antitoxoplasmatic activity. Moreover, NO inhibits IDO activity, thereby suppressing kynurenine formation, and at least one member of the kynurenine pathway, 3-hydroxyanthranilic acid, has been shown to inhibit NOS enzyme activity, the expression of NOS mRNA, and activation of the inflammatory transcription factor, nuclear factor-kB. In addition, the anti-inflammatory cytokines IL-4 and IL-10, TGF-beta, and a cytokine known as macrophage deactivating factor, have been shown to directly modulate NO production, sometimes expressing synergistic activity. On the other hand, IL-4 and TGF-beta can suppress IDO activity in some cells, for example human monocytes and fibroblasts, which is consistent with metabolic pathways controlled by IDO being a significant contributor to the proinflammatory system. Also, it seems that idiopathic intracranial hypertension, pseudotumor cerebri, and aseptic meningitis, induced by various factors, may result from their interference with IDO and inducible nitric oxide synthase activities, endogenous NO level, and cytokine irregularities which finally affect former T. gondii status 2mo in the brain. All these biochemical disturbances caused by the CNS T. gondii infection/inflammation may also be responsible for the relationship found between neurologic symptoms, such as headache, vertigo, and syncope observed in apparently immunocompetent children and adolescents, and physical and psychiatric symptoms in adulthood. We therefore believe that tests for T. gondii should be performed obligatorily in apparently immunocompetent patients with different types of headaches, even if they have no enlarged peripheral lymph nodes. This may help to avoid overlooking this treatable cause of the CNS disease, markedly reduce costs of hospitalization, diagnosis and treatment, and eventually prevent developing serious neurologic and psychiatric disorders.

Contemplating the murine test tube: lessons from natural killer cells andCryptococcus neoformans

Murine experimentation has provided many useful tools, including the ability to knockout or over-express genes and to perform experiments that are limited by ethical considerations. Over the past century, mice have imparted valuable insights into the biology of many systems, including human immunity. However, although there are many similarities between the immune response of humans and mice, there are also many differences; none is more prominent than when examining natural killer cell biology. These differences include tissue distribution, effector molecules, receptor repertoire, and cytokine responses, all of which have important implications when extrapolating the studies to the human immune responses to Cryptococcus neoformans.

Expression of IL-12 receptor by neurons

Interleukin (IL)-12, a key cytokine bridging innate and acquired immunity, is efficacious in enhancing recovery from experimental vesicular stomatitis virus (VSV) infection of the mouse central nervous system (CNS). This response is associated with the upregulation of neuronal nitric oxide synthase (NOS-1), independent of IFN-gamma and TNF-alpha. We hypothesized that neurons may respond directly IL-12. Our data are consistent with the expression of a functional IL-12 receptor (IL-12R) by neurons in culture and this receptor-ligand interaction results in the induction of an innate antiviral immune response. N18 cells, which did not express IL-12Rbeta2 were transfected with the IL-12Rbeta2 receptor gene; Koch's postulates were fulfilled, as clones derived from this transfection were reconstituted for IL-12 responsiveness.

Cytokine and inducible nitric oxide synthase mRNA expression during experimental murine cryptococcal meningoencephalitis

The immune events that take place in the central nervous system (CNS) during cryptococcal infection are incompletely understood. We used competitive reverse transcription-PCR to delineate the time course of the local expression of mRNAs encoding a variety of cytokines and inducible nitric oxide synthase (iNOS) during progressive murine cryptococcal meningoencephalitis and assessed the CNS inflammatory response using immunohistochemistry. Interleukin 18 (IL-18), transforming growth factor beta1, and IL-12p(40) mRNAs were constitutively expressed in the brains of infected and uninfected mice; IL-2 mRNA was not detected at any time. Increased levels of transcripts corresponding to IL-1 alpha, tumor necrosis factor alpha (TNF-alpha), and iNOS were detected as early as day 1 postinfection, with TNF-alpha rising by approximately 30-fold and iNOS increasing by approximately 5-fold by day 7. Each remained at these levels thereafter. IL-4, IL-6, and gamma interferon transcripts were detected on day 5, and IL-1 beta and IL-10 transcripts were detected beginning on day 7. Once detected, each remained at a relatively constant level through 28 days of infection. This cytokine profile does not suggest a polarized Th1 or Th2 response. Immunohistochemistry did not reveal inflammatory infiltrates before day 7, despite the presence of cryptococci. Intraparenchymal abscesses with inflammatory cells in their peripheries were found beginning on day 10. The infiltrates were comprised primarily of cells expressing CD4, CD8, or CD11b; low numbers of cells expressing CD45R/B220 were also present. The persistence of Cryptococcus observed in the CNS may result from an ineffective immune response, perhaps owing to an insufficient anticryptococcal effector function of endogenous glial cells resulting from competing pro- and anti-inflammatory cytokines. These data detail the immune response in the brain and could be important for the future design of specific immunomodulatory therapies for this important opportunistic infection.

Enzymes that counteract nitrosative stress promote fungal virulence

Enzymes that protect cells from reactive oxygen species (superoxide dismutase, catalase, peroxidase) have well-established roles in mammalian biology and microbial pathogenesis. Two recently identified enzymes detoxify nitric oxide (NO)-related molecules; flavohemoglobin denitrosylase consumes NO, and S-nitrosoglutathione (GSNO) reductase metabolizes GSNO. Although both enzymes protect microorganisms from nitrosative challenge in vitro, their relevance has not been established in physiological contexts. Here we studied their biological functions in Cryptococcus neoformans, an established human fungal pathogen that replicates in macrophages and whose growth in vitro and in infected animals is controlled by NO bioactivity. We show that both flavohemoglobin denitrosylase and GSNO reductase contribute to C. neoformans pathogenesis. FHB1 and GNO1 mutations abolished NO- and GSNO-consuming activity, respectively. Growth of fhb1 mutant cells was inhibited by nitrosative challenge, whereas that of gno1 mutants was not. fhb1 mutants showed attenuated virulence in a murine model, and virulence was restored in iNOS(-/-) animals. Survival of the fhb1 mutant was also reduced in activated macrophages and restored to wild-type by inhibition of NOS activity. Combining mutations in flavohemoglobin and GSNO reductase, or flavohemoglobin and superoxide dismutase, further attenuated virulence. These studies illustrate that fungal pathogens elaborate enzymatic defenses against nitrosative stress mounted by the host.

Synthetic cannabinoid WIN55,212-2 inhibits generation of inflammatory mediators by IL-1?-stimulated human astrocytes

Activated glial cells have been implicated in the neuropathogenesis of many infectious and inflammatory diseases of the brain. A number of inflammatory mediators have been proposed to play a role in glial cell-related brain damage; e.g., free radicals such as nitric oxide (NO), cytokines, and chemokines. Our laboratory has been interested in the effect of psychoactive drugs and their derivatives on the production of these mediators. Cannabinoids have been shown to possess immunomodulatory as well as psychoactive properties. We previously have shown that interleukin (IL)-1beta-stimulated human astrocytes, but not microglia, produce NO. In this study, we investigated the effects of the synthetic cannabinoid WIN55,212-2 on the production of several key inflammatory mediators by human fetal astrocytes activated by IL-1beta. Expression of the cannabinoid receptors CB1 and CB2 was detected on human astrocytes. WIN55,212-2 (10(-5) M) potently inhibited inducible NO synthase (iNOS) and corresponding NO production by IL-1beta-stimulated astrocytes. The CB1 and CB2 receptor-specific antagonists SR141716A and SR144528, respectively, partially blocked this suppressive effect. In addition, treatment of astrocytes with WIN55,212-2 downregulated in a concentration-dependent manner IL-1beta-induced tumor necrosis factor (TNF)-alpha release. Treatment with WIN55,212-2 also inhibited production of the chemokines CXCL10, CCL2 and CCL5 by IL-1beta-activated astrocytes. These findings indicate that WIN55,212-2 inhibits the production of inflammatory mediators by IL-1beta-stimulated human astrocytes and suggest that comparable agents may have therapeutic potential for the management of brain inflammation.

Role of IDO activation in anti-microbial defense in human native astrocytes

The most serious complication of human toxoplasmosis is the development of toxoplasmic encephalitis. It is well established that in the brain Toxoplasma gondii is able to replicate in microglial cells, astrocytes and neurons, and that all three cell types can harbor toxoplasma cysts. The role of astrocytes in the defense against toxoplasma is not clear. The most prominent effector-mechanisms against toxoplasma are the induction of the inducible form of the nitric oxide synthase (iNOS), and the induction of indoleamine 2,3-dioxygenase (IDO). In this paper we show that interferon (IFN)-gamma-activated, native human astrocytes express IDO activity, as shown by the detection of IDO mRNA using RT-PCR, detection of enzyme expression with IDO-specific monoclonal antibodies in Western blots, as well as by direct measurement of enzyme activity in the activated cells. IFN-gamma-mediated IDO activity in human astrocytes inhibits the growth of Toxoplasma gondii and of group B streptococci. Furthermore, we show for the first time that IFN-gamma induced IDO activity is also effective in inhibiting the growth of Herpes Simplex Virus in astrocyte cultures. In addition, iNOS expression was detectable by RT-PCR in all batches of astrocytes tested when stimulated with a cytokine cocktail of IFN-gamma, TNF-alpha, IL-1 and LPS. Furthermore, we found that the amount of nitric oxide produced by astrocytes is not sufficient to inhibit either toxoplasmal or bacterial growth. Co-activation of iNOS and IDO on the other hand, results in an inhibition of IDO activity in astrocytes.

Neuroinflammatory role of prostaglandins during experimental meningitis: evidence suggestive of an in vivo relationship between nitric oxide and prostaglandins

Nitric oxide (NO) and prostaglandins are inflammatory mediators produced during meningitis. The purpose of the present study was to pharmacologically inhibit cyclooxygenase-2 (COX-2) and inducible NO synthase (iNOS) to 1) explore the prostaglandin contribution to blood-cerebrospinal fluid barrier permeability alterations and 2) elucidate the in vivo concentration relationship between prostaglandin E2 (PGE2) and NO during experimental meningitis. Intracisternal injection of lipopolysaccharides (LPSs, 200 microg) induced neuroinflammation. Rats were dosed with nimesulide (COX-2 inhibitor), aminoguanidine (iNOS inhibitor), or vehicle. Evans blue was used to assess blood-cerebrospinal fluid barrier permeability. Meningeal NO and cerebrospinal fluid PGE2 were assayed using conventional methods. (Results are expressed as mean +/- S.E.M. of 5-9 rats/group.) Nimesulide failed to prevent blood-cerebrospinal fluid barrier disruption [cerebrospinal fluid Evans blue (micrograms per milliliter): control, 0.22 +/- 0.22*; LPS, 11.58 +/- 0.66; LPS + nimesulide, 10.58 +/- 0.86; *p < 0.05; ANOVA]. Although nimesulide decreased PGE2 (picograms per microliter; p < 0.01) in LPS + nimesulide rats (13.9 +/- 1.96) versus LPS + vehicle (73.8 +/- 12.4), meningeal NO production (picomoles/30 min/10(6) cells; p < 0.01) increased unexpectedly in LPS + nimesulide rats (439 +/- 47) versus LPS + vehicle rats (211 +/- 31). In contrast, aminoguanidine inhibited meningeal NO (picomoles/30 min/10(6) cells; p < 0.005) in LPS + aminoguanidine (111 +/- 20) versus LPS (337 +/- 48) but had no effects (p > 0.05) on PGE2. The in vivo relationship between PGE2 and NO was mathematically described by a biphasic, bell-shaped curve (r2 = 0.42; n = 27 rats; p < 0.0001). Based on these results, inhibition of prostaglandin synthesis not only fails to prevent blood-cerebrospinal fluid barrier disruption during neuroinflammation and but also promotes increased meningeal NO production. The in vivo concentration relationship between PGE2 and NO is biphasic, suggesting that inhibition of COX-2 alone may promote NO toxicity through enhanced NO synthesis.

The role of IFN-gamma in immune responses to viral infections of the central nervous system

Interferon (IFN)-gamma, is not only a marker of T(H)1 CD4, CD8 and natural killer (NK) cells, it is also a critical antiviral mediator which is central to the elimination of viruses from the CNS. In this review, we describe IFN-gamma, its receptor, signal transduction from receptor engagement, and antiviral downstream mediators. We demonstrate that although neurons are post-mitotic and non-renewing, they respond to IFN-gamma in a fashion similar to peripheral fibroblasts or lymphocytes. We have illustrated this review with details about studies on the role(s) of IFN-gamma in the pathogenesis of measles virus (MV), herpes simplex virus (HSV) type 1, and vesicular stomatitis virus (VSV) infections of the CNS. For VSV infection, IFN-gamma signals through Jaks 1 and 2 and STAT1 to activate (interferon regulatory factor) IRF-1; although viral protein synthesis is inhibited, PKR is not a critical mediator in the antiviral response to VSV in murine neurons. In contrast, induction of nitric oxide synthase (NOS) type 1 and its production of nitric oxide is essential in the elimination of viruses from neurons.

Distinct roles of eicosanoids in the immune response to viral encephalitis: or why you should take NSAIDS

Prostaglandins (PGs) and leukotrienes (LTs) are important proinflammatory mediators. They are both derived from arachidonic acid (AA). Cyclooxygenase (COX), the key enzyme in transforming AA into PGs, has two isoforms: COX-1 is constitutively expressed, and COX-2, is inducible. Lipoxygenase (5-LO) is the key enzyme for LT production. PGs and LTs have been intensively studied. Release of these molecules is associated with mucus secretion, redness, pain, fever and other inflammatory manifestations. Both PGs and LTs are involved in host defense against various pathogens. In addition to mediating inflammatory symptoms, PGs might suppress some innate immune factors, including nitric oxide (NO) production. PGs also suppress a TH1 response. LTs have pathologic potential, especially in asthma. LTs also have been found to have positive roles in host defense, either against virus or bacteria. Finally, PGs and LTs might regulate the production of each other, possibly at the level of substrate competition by their enzymes. Because they are clinically important molecules, a further understanding of the roles that PGs and LTs played in host defense will have great impact on therapeutic research.

Revisiting an old antimicrobial drug: amphotericin B induces interleukin-1-converting enzyme as the main factor for inducible nitric-oxide synthase expression in activated endothelia

We have investigated the impact of the widely used antifungal agent Amphotericin B (AmB) on cytokine activated aortic endothelial cells (AEC) and their inflammatory response as monitored by cytokine and inducible nitric-oxide synthase (iNOS) expression as well as high-output nitric oxide synthesis. Because both blood-borne infections and systemically administered drugs will first encounter vessel lining endothelial cells, this cell type represents an important participant in innate immune reactions against xenobiotics. Culturing cytokine-activated AEC in the presence of 1.25 microg/ml AmB, a concentration equivalent to serum levels during patient treatment, we find increases in iNOS promoter activity up to 120%, in iNOS mRNA or protein expressions by factors of up to 3.5 +/- 1.1, and in iNOS activity of up to 180% compared with cells with cytokines only. In parallel, a strong increase in endothelial interleukin (IL)-1beta-converting enzyme (ICE) and IL-1beta expression and activity was observed. Specific inhibition of ICE activity or IL-1beta functionality significantly reduces expression and activity of the iNOS to control values. Because ICE activity is essential for the endogenous synthesis of active IL-1beta, ICE overexpression represents the key signal in the AmB-induced and IL-1beta-mediated effects on iNOS activity. In summary, in endothelial cells, AmB strongly augments cytokine-induced iNOS expression and activity by increasing the expression and activity of the ICE. This adjuvant activity for augmented endogenous cytokine processing adds to the efficacy of the antimycotic activity of AmB. Furthermore, our data underline the relevance of the endothelial iNOS as a potent effector of the innate immune system.

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.

Amphotericin B potentiates the activation of inducible nitric oxide synthase and causes nitric oxide-dependent mitochondrial dysfunction in cytokine-treated rodent astrocytes

Because the neurotoxic effects of the antifungal drug amphotericin B (AMB) closely resemble those ascribed to the highly reactive gaseous free radical nitric oxide (NO), we investigated the effect of AMB on NO production in rodent astrocytes. AMB caused a dose-dependent increase of NO generation in interferon-gamma (IFN-gamma)-stimulated rat and mouse astrocytes, as well as in IFN-gamma + tumor necrosis factor-alpha (TNF-alpha)-activated rat astrocytoma cell line C6. Treatment of rat astrocytes with AMB markedly potentiated IFN-gamma-triggered expression of mRNA for iNOS, but not for its transcription factor IRF-1. The activation of transcription factor NF-kappaB was apparently required for AMB-induced iNOS mRNA expression, as the latter was abolished by NF-kappaB inhibitors: pyrrolidine dithiocarbamate and MG132. AMB-mediated enhancement of astrocyte NO production was partly dependent on endogenous IL-1, as shown by partial inhibition of AMB effect with IL-1 receptor antagonist. IFN-gamma + AMB treatment led to reduction of astrocyte mitochondrial respiration (measured by MTT assay) that has been completely reverted by selective iNOS inhibitor aminoguanidine. AMB toxicity toward IFN-gamma-stimulated astrocytes was dependent on both AMB and NO action, since AMB and NO-releasing substance SNP synergized in inducing astrocyte mitochondrial dysfunction. These results suggest that the enhancement of cytokine-induced iNOS activation in astrocytes and the subsequent release of high amounts of NO might be at least partly responsible for AMB neurotoxicity.

Expression of functional formyl peptide receptors by human astrocytoma cell lines

Activation of astrocytes is important in the pathogenesis of a variety of diseases in the central nervous system, such as infection and neurodegeneration. We found that the bacterial chemotactic peptide, N-formyl-methionyl-leucyl-phenylalanine (fMLF) induced potent migration and Ca(2+) mobilization in human astrocytoma cell lines. The effect of fMLF was pertussis toxin-sensitive, suggesting the involvement of seven transmembrane, G protein-coupled receptor(s) for fMLF. Scatchard analyses revealed that astrocytoma cell lines express both high- and low-affinity binding sites for [3H]fMLF. RT-PCR confirmed the expression of transcripts of fMLF receptors, the high-affinity FPR and the low-affinity FPRL1 by these cells. Both fMLF and F peptide, a synthetic peptide domain of HIV-1 envelope protein which specifically activates FPRL1, increased secretion of IL-6 by astrocytoma cells. Our study demonstrates for the first time that FPR and FPRL1 expressed by astrocytoma cell lines are functional, and suggests a molecular basis for the involvement of these receptors in host defense in the brain.

NSAID treatment suppresses VSV propagation in mouse CNS

Cyclooxygenase (COX) is the key enzyme in the conversion of arachidonic acid to prostaglandins. COX has two isoforms: COX-1, the constitutively expressed form, and COX-2, the inducible form. Prostaglandins are mediators of many critical physiological and inflammatory responses, but little is known about their roles during a viral infection in the central nervous system (CNS). We used non-selective inhibitors of COX, aspirin and indomethacin, and a selective antagonist of COX-2, celecoxib, to study the role of prostaglandins in Vesicular Stomatitis Virus (VSV) induced encephalitis. We found that the inhibition of COX antagonizes VSV propagation both in vitro and in vivo. In addition, aspirin and celecoxib both prevented the disruption of the blood brain barrier in VSV-infected mice. In vitro experiments showed that the effect of COX inhibition was at least partially mediated by increased production of Nitric Oxide (NO), a molecule known to inhibit VSV replication. When NO production was inhibited by N(omega)-nitro-L-methyl-arginine-ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, the difference in viral titer between aspirin (or celecoxib)-treated and the control cells was abolished. VSV-infected mice treated with celecoxib expressed more NOS-1 and produced more NO in their CNS compared to the controls. Our data suggest that the product(s) of COX have antagonistic effect(s) on NO production in the mouse CNS.

Differing requirement for inducible nitric oxide synthase activity in clearance of primary and secondary Cryptococcus neoformans infection

The role of nitric oxide in resistance to cryptococcal infection was investigated. Mice deficient in inducible nitric oxide synthase (INOS) did not survive a primary intratracheal infection as did INOS-replete control mice. Despite adequate recruitment of host cells and generation of interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha at the site of infection, INOS-deficient mice failed to clear yeast from their lungs by five weeks of infection, in contrast to wild-type mice. INOS-deficient mice also had higher yeast brain burdens than did control mice after a primary intracerebral infection. Therefore, generation of nitric oxide is required for resistance to primary cryptococcal infection. However, INOS-deficient mice vaccinated subcutaneously and rechallenged intravenously had lung and brain yeast burdens equivalent to those of vaccinated controls, and therefore expressed effective acquired immunity to Cryptococcus neoformans. Cells harvested from infected INOS-deficient mice by bronchoalveolar lavage acted as anti-cryptococcal effectors in vitro at an effector:target ratio of 100:1, provided IFN-gamma was present, but did not inhibit yeast proliferation at a 10:1 effector:target ratio as cells from wild-type mice did. Therefore, INOS activity is important for anti-cryptococcal function of effectors of immunity during the primary response, but not for the generation or expression of secondary immunity to C. neoformans.

Establishment of protective immunity against cerebral cryptococcosis by means of an avirulent, non melanogenic Cryptococcus neoformans strain

The opportunistic fungal pathogen, Cryptococcus neoformans, shows a marked predilection for the central nervous system (CNS). This can be partially explained by its ability to synthesize melanin starting from the catecholamines, highly concentrated at the CNS level. Two cryptococcal strains, the avirulent non-melanogenic strain Sb26 and the virulent melanogenic revertant strain Sb26Rev, were used in a murine model of intracerebral (i.c.) infection, in order to evaluate their virulence and immunomodulating properties at the cerebral level. We found that, unlike Sb26Rev, Sb26 i.c. infection was never lethal regardless of the challenging dose. Sb26Rev infection resulted in massive CNS tissue damage, associated with little or no cytokine response, as established by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). Differently, Sb26 infection failed to alter CNS structure, while inducing IL-12 p40, TNF-alpha, IL-1beta, IFN-gamma and iNOS specific-gene expression as well as IL-12, TNF-alpha and IL-1beta cytokine production. Interestingly, all Sb26 infected mice survived a subsequent lethal challenge with Sb26Rev. The phenomenon was associated with enhanced IL-12, TNF-alpha and IL-1beta production and was strictly specific, as shown by heterologous challenges and delayed type of hypersensitivity assay. Overall, we provide evidence that protective immunity against cerebral cryptococcosis is established by means of an avirulent strain of C. neoformans.

IFN-gamma activated indoleamine 2,3-dioxygenase activity in human cells is an antiparasitic and an antibacterial effector mechanism

In nearly all human cells IFN-gamma stimulation leads to an activation of indoleamine 2,3-dioxygenase (IDO) activity, which is responsible for anti-toxoplasma and anti-chlamydia effects. We have recently shown that IDO activation is also a defense mechanism against extracellular beta-hemolytic streptococci groups A, B, C and G in human glioblastoma cells, fibroblasts and macrophages. Similar effects were also seen with enterococci and in approximately 65% of staphylococci tested, including multiresistant strains of both species. In addition, we have found that IDO activity is differentially regulated in different cells. For example we have found that TNF-alpha enhances IFN-gamma induced IDO activity and antimicrobial effect in human glioblastoma cells whereas both IFN-gamma mediated effects were blocked by TNF-alpha as well as by IL-1 in a human uroepithelial cell line. We were able to show that the IL-1 and TNF-alpha mediated inhibition of IFN-gamma-induced IDO activity in uroepithelial cells is due to stimulation of inducible nitric oxide synthase. In human astrocytoma cells, IL-1 and TNF-alpha did not inhibit IDO activity and in concordance with this finding these cells did not show a detectable nitric oxide production.

Nitric oxide participation in the fungicidal mechanism of gamma interferon-activated murine macrophages against Paracoccidioides brasiliensis conidia

Paracoccidioidomycosis, a systemic mycosis restricted to Latin America and produced by the dimorphic fungus Paracoccidioides brasiliensis, is probably acquired by inhalation of conidia produced by the mycelial form. The macrophage (Mphi) represents the major cell defense against this pathogen; when activated with gamma interferon (IFN-gamma), murine Mphis kill the fungus by an oxygen-independent mechanism. Our goal was to determine the role of nitric oxide in the fungicidal effect of Mphis on P. brasiliensis conidia. The results revealed that IFN-gamma-activated murine Mphis inhibited the conidium-to-yeast transformation process in a dose-dependent manner; maximal inhibition was observed in Mphis activated with 50 U/ml and incubated for 96 h at 37 degrees C. When Mphis were activated with 150 to 200 U of cytokine per ml, the number of CFU was 70% lower than in nonactivated controls, indicating that there was a fungicidal effect. The inhibitory effect was reversed by the addition of anti-IFN-gamma monoclonal antibodies. Activation by IFN-gamma also enhanced Mphi nitric oxide production, as revealed by increasing NO(2) values (8 +/- 3 microM in nonactivated Mphis versus 43 +/- 13 microM in activated Mphis). The neutralization of IFN-gamma also reversed nitric oxide production at basal levels (8 +/- 5 microM). Additionally, we found that there was a significant inverse correlation (r = -0.8975) between NO(2)(-) concentration and transformation of P. brasiliensis conidia. Additionally, treatment with any of the three different nitric oxide inhibitors used (arginase, N(G)-monomethyl-L-arginine, and aminoguanidine), reverted the inhibition of the transformation process with 40 to 70% of intracellular yeast and significantly reduced nitric oxide production. These results show that IFN-gamma-activated murine Mphis kill P. brasiliensis conidia through the L-arginine-nitric oxide pathway.

Cryptococcus neoformans neutralizes macrophage and astrocyte derived nitric oxide without interfering with inducible nitric oxide synthase induction or catalytic activity - possible involvement of nitric oxide consumption

The effect of Cryptococcus neoformans on the accumulation of nitrite, an indicator of nitric oxide (NO) synthesis, was investigated in cytokine (interferon-gamma [IFN-gamma] and interleukin [IL]-1)-stimulated cultures of rat peritoneal macrophages and C6 astrocytoma cells. Cytokine-induced nitrite generation in cultures of both cell types was inhibited in a dose-dependent manner by live C. neoformans, but not by heat-killed cryptococcal cells or conditioned medium from yeast cultures. C. neoformans-mediated reduction of nitrite formation coincided with impairment of NO-dependent macrophage tumoricidal activity. Cytokine-triggered induction of inducible NO synthase (iNOS) was unaffected in C6 cells, and only marginally reduced in macrophages. When cells were pretreated with cytokines for 24 h to induce iNOS, and any further induction was prevented by inhibition of protein synthesis, C. neoformans was still able to reduce nitrite accumulation in cultures of both cell types. Finally, live C. neoformans, but not heat-killed yeast cells or yeast culture supernatant, significantly reduced nitrite production in a culture solution of NO-releasing compound S-nitrosoglutathione (GSNO). Thus, it appears that cryptococcal reduction of nitrite formation in macrophage and C6 cultures was caused by the consumption of NO by some yeast molecule, rather than by the inhibition of cellular NO synthesis.

Cytokine profiles of AIDS patients are similar to those of mice with disseminated Cryptococcus neoformans infection

Cryptococcosis is an hematogenously disseminated meningoencephalitis during which the relationship between the disease severity and the immune response remains unclear. We thus analyzed, by enzyme-linked immunosorbent assay, proinflammatory (tumor necrosis factor alpha [TNF-alpha] and interleukin-6 [IL-6]) and anti-inflammatory (IL-10) cytokine levels in plasma at the time of diagnosis in 51 AIDS patients with culture-proven cryptococcosis. We used a murine model to determine the correlation between cytokine levels and fungal burden in blood and tissues and the kinetics of the immune response and of the formation of cerebral lesions. In AIDS patients, plasma TNF-alpha and IL-10, but not IL-6, levels were significantly higher in the case of fungemia or disseminated infection than in their absence, whereas the presence of meningitis had no influence on these levels. In mice, none of these cytokines were detected within the first day after inoculation. Later on, TNF-alpha and IL-10, but not IL-6, levels in plasma correlated significantly with the fungal burden in the blood and spleen but not the brain. In the brain, cytokine levels were low compared to those in other compartments, and tissue lesions and a degree of infection similar to those observed in humans were seen, further suggesting the relevance of this experimental model. Thus, AIDS patients with cryptococcosis produce an immune response that reflects the dissemination but not the meningeal involvement. This murine model of disseminated cryptococcosis can be used to investigate the pathophysiology of cryptococcosis and new therapeutic approaches.

Laccase protects Cryptococcus neoformans from antifungal activity of alveolar macrophages

While laccase of Cryptococcus neoformans is implicated in the virulence of the organism, our recent studies showing absence of melanin in the infected mouse brain has led us to a search for alternative roles for laccase in cryptococcosis. We investigated the role of laccase in protection of C. neoformans against murine alveolar macrophage (AM)-mediated antifungal activity by using a pair of congenic laccase-positive (2E-TUC) and laccase-deficient (2E-TU) strains. The laccase-positive cells with laccase derepression were more resistant to the antifungal activity of AM than a laccase-deficient strain ([28.9 +/- 1.2]% versus [40.2 +/- 2.6]% killing). Addition of L-dopa to Cryptococcus to produce melanin in a laccase-positive strain resulted in a slight increase in protection of C. neoformans from the antifungal activity of macrophages ([25.4 +/- 3.4]% versus [28.9 +/- 1.2]% killing). Recombinant cryptococcal laccase exhibited iron oxidase activity in converting Fe(II) to Fe(III). Moreover, recombinant laccase inhibited killing of C. neoformans by hydroxyl radicals catalyzed by iron in a cell-free system. Addition of the hydroxyl radical scavenger mannitol or dimethyl sulfoxide to AMs prior to the introduction of cryptococcal cells decreased killing of both strains and reduced the difference in susceptibility between the laccase-positive and laccase-deficient strains. Furthermore, laccase-mediated protection from AM killing was inhibited by the addition of Fe(II), presumably by overcoming the effects of the iron oxidase activity of cryptococcal laccase. These results suggest that the iron oxidase activity of laccase may protect C. neoformans from macrophages by oxidation of phagosomal iron to Fe(III) with a resultant decrease in hydroxyl radical formation.

Intracellular CXCR4 signaling, neuronal apoptosis and neuropathogenic mechanisms of HIV-1-associated dementia

The mechanism(s) by which HIV-1 affects neural injury in HIV-1-associated dementia (HAD) remains unknown. To ascertain the role that cellular and viral macrophage products play in HAD neurotoxicity, we explored one potential route for neuronal demise, CXCR4. CXCR4, expressed on lymphocytes and neurons, is both a part of neural development and a co-receptor for HIV-1. Its ligand, stromal cell-derived factor-1alpha (SDF-1alpha), affects neuronal viability. GTP binding protein (G-protein) linked signaling after neuronal exposure to SDF-1alpha, virus-infected monocyte-derived macrophage (MDM) secretory products, and virus was determined. In both human and rat neurons, CXCR4 was expressed at high levels. SDF-1alpha/beta was detected predominantly in astrocytes and at low levels in MDM. SDF-1beta/beta was expressed in HAD brain tissue and upregulated in astrocytes exposed to virus infected and/or immune activated MDM conditioned media (fluids). HIV-1-infected MDM secretions, virus and SDF-1beta induced a G inhibitory (Gi) protein-linked decrease in cyclic AMP (cAMP) and increase inositol 1,4, 5-trisphosphate (IP3) and intracellular calcium. Such effects were partially blocked by antibodies to CXCR4 or removal of virus from MDM fluids. Changes in G-protein-coupled signaling correlated, but were not directly linked, to increased neuronal synaptic transmission, Caspase 3 activation and apoptosis. These data, taken together, suggest that CXCR4-mediated signal transduction may be a potential mechanism for neuronal dysfunction during HAD.

Mechanisms of cytokine-mediated inhibition of viral replication

In this report, the role of nitric oxide synthase (NOS) and IL-12 administration in inhibition of vesicular stomatitis virus (VSV) from infected neuroblastoma cells was examined. We previously have shown that cytokine treatment of cells results in the induction of NOS-1, and this is associated with a 2 log inhibition of VSV production. We performed these studies to examine the mechanism by which viral replication is suppressed. Neuroblastoma cells (NB41A3) were treated with either IL-12 or medium and subsequently infected with VSV. Viral protein and mRNA were isolated from these cells, and their levels were measured by Western or Northern blots, respectively. mRNA levels were decreased modestly, but viral proteins were decreased substantially in cells pretreated with IL-12, suggesting that the inhibitory effect of NO is working at the translational level. Cytokine treatment of cells was not associated with oxidative stress. The viral proteins also were nitrosylated. These data suggest that the mechanism of NO inhibition of viral replication occurs through translational interference and posttranslational modifications of viral components.

Neuronal expression of NOS-1 is required for host recovery from viral encephalitis

The role of nitric oxide synthase (NOS) in host defense and clearance of vesicular stomatitis virus (VSV) from the central nervous system (CNS) was examined. NOS-1, NOS-2, and NOS-3 knockout mice were infected with VSV and were treated with either IL-12 or medium. IL-12 treatment resulted in substantially decreased VSV titers in wildtype and NOS-3 knockout mice, but had a marginal effect in the NOS-1 and NOS-2 knockout mice. NOS-1 expression in neurons was associated with survival from VSV infection. The data indicate that the enzyme activity is local, since NOS-2 expression in microglia and inflammatory macrophages and NOS-3 expression in astrocytes, endothelial cells, and ependymal cells did not compensate.

Human defenses against Cryptococcus neoformans: an update

Cryptococcus neoformans var. neoformans is an opportunistic fungal pathogen, especially in AIDS patients, and is found world-wide. On the other hand, Cryptococcus neoformans var. gatti (CN-g) is restricted to an association with two species of Eucalyptus trees. Alveolar macrophages (AM) constitute the first line of defense to Cryptococcus neoformans and offers some resistance. The inflammatory response to Cryptococcus neoformans with an influx of neutrophils and monocytes affords a second line of defense. Secretion of proinflammatory monokines by human AM is now being defined. The inflammatory phagocytes are efficient in killing Cryptococcus neoformans and offer strong resistance. T and B cell responses to infection, a third line of defense, results in production of lymphokines (IFNg, etc.) and specific antibodies. Enhancement of lymphocyte responses by IL-12 and IL-18 to Cryptococcus neoformans infection appears to be critical. Susceptibility of AIDS patients to Cryptococcus neoformans is associated with low CD4+ T cell counts and likely reduced efficacy of the second line of defense.

Inflammatory pathogenesis in Alzheimer's disease: biological mechanisms and cognitive sequeli

Experimental evidence from molecular biology, biochemistry, epidemiology and behavioral research support the conclusion that brain inflammation contributes to the pathogenesis of Alzheimer's disease and other types of human dementias. Aspects of neuroimmunology relating to the pathogenesis of Alzheimer's disease are briefly reviewed. The effects of brain inflammation, mediated through cytokines and other secretory products of activated glial cells, on neurotransmission (specifically, nitric oxide, glutamate, and acetylcholine), amyloidogenesis, proteolysis, and oxidative stress are discussed within the context of the pathogenesis of learning and memory dysfunction in Alzheimer's disease. Alzheimer's disease is proposed to be an etiologically heterogeneous syndrome with the common elements of amyloid deposition and inflammatory neuronal damage.