Differential host susceptibility to intracerebral infections with Candida albicans and Cryptococcus neoformans

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.

Pathogenic Fungal Infection in the Lung

Respiratory fungal infection is a severe clinical problem, especially in patients with compromised immune functions. Aspergillus, Cryptococcus, Pneumocystis, and endemic fungi are major pulmonary fungal pathogens that are able to result in life-threatening invasive diseases. Growing data being reported have indicated that multiple cells and molecules orchestrate the host's response to a fungal infection in the lung. Upon fungal challenge, innate myeloid cells including macrophages, dendritic cells (DC), and recruited neutrophils establish the first line of defense through the phagocytosis and secretion of cytokines. Natural killer cells control the fungal expansion in the lung via the direct and indirect killing of invading organisms. Adaptive immune cells including Th1 and Th17 cells confer anti-fungal activity by producing their signature cytokines, interferon-γ, and IL-17. In addition, lung epithelial cells (LEC) also participate in the resistance against fungal infection by internalization, inflammatory cytokine production, or antimicrobial peptide secretion. In the host cells mentioned above, various molecules with distinct functions modulate the immune defense signaling: Pattern recognition receptors (PRRs) such as dectin-1 expressed on the cell surface are involved in fungal recognition; adaptor proteins such as MyD88 and TRAF6 are required for transduction of signals to the nucleus for transcriptional regulation; inflammasomes also play crucial roles in the host's defense against a fungal infection in the lung. Furthermore, transcriptional factors modulate the transcriptions of a series of genes, especially those encoding cytokines and chemokines, which are predominant regulators in the infectious microenvironment, mediating the cellular and molecular immune responses against a fungal infection in the lung.

Kynurenine Pathway in Skin Cells: Implications for UV-Induced Skin Damage

The kynurenine pathway (KP) is the principle route of catabolism of the essential amino acid tryptophan, leading to the production of several neuroactive and immunoregulatory metabolites. Alterations in the KP have been implicated in various neuropsychiatric and neurodegenerative diseases, immunological disorders, and many other diseased states. Although the role of the KP in the skin has been evaluated in small niche fields, limited studies are available regarding the effect of acute ultra violet exposure and the induction of the KP in human skin-derived fibroblasts and keratinocytes. Since UV exposure can illicit an inflammatory component in skin cells, it is highly likely that the KP may be induced in these cells in response to UV exposure. It is also possible that some KP metabolites may act as pro-inflammatory and anti-inflammatory mediators, since the KP is important in immunomodulation.

Characterization of the kynurenine pathway in human neurons

The kynurenine pathway is a major route of L-tryptophan catabolism producing neuroactive metabolites implicated in neurodegeneration and immune tolerance. We characterized the kynurenine pathway in human neurons and the human SK-N-SH neuroblastoma cell line and found that the kynurenine pathway enzymes were variably expressed. Picolinic carboxylase was expressed only in primary and some adult neurons but not in SK-N-SH cells. Because of this difference, SK-N-SH cells were able to produce the excitotoxin quinolinic acid, whereas human neurons produced the neuroprotectant picolinic acid. The net result of kynurenine pathway induction in human neurons is therefore predicted to result in neuroprotection, immune regulation, and tumor inhibition, whereas in SK-N-SH cells, it may result in neurotoxicity, immune tolerance, and tumor promotion. This study represents the first comprehensive characterization of the kynurenine pathway in neurons and the first description of the involvement of the kynurenine pathway as a mechanism for controlling both tumor cell neurotoxicity and persistence.

Differential microbial clearance and immunoresponse of Balb/c (Nramp1 susceptible) and DBA2 (Nramp1 resistant) mice intracerebrally infected with Mycobacterium bovis BCG (BCG)

In mice, the gene encoding Nramp1 (natural resistance-associated protein 1) exists in two allelic forms, differing for a point mutation. According to Nramp1 genotype, extensive literature documents a clear-cut distinction of inbred strains in two non-overlapping groups that phenotypically express resistance (Nramp1r) and susceptibility (Nramp1s) to systemic infections. Here, we provide evidence that Nramp1r (DBA/2) and Nramp1s (Balb/c) mice differently handle intracerebral infection with Mycobacterium bovis BCG. Distinct trends of microbial clearance from the brain and also different patterns of local immune responses occur, thus arguing on the involvement of Nramp1 gene product on the accomplishment of cerebral anti-mycobacterial defenses.

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.

Inducible expression of the long pentraxin PTX3 in the central nervous system

PTX3 is a prototypic long pentraxin consisting of a C terminal 203-amino acid pentraxin-like domain coupled with an N-terminal 178-amino acid unrelated portion. PTX3 is induced by primary proinflammatory signals in various cell types, most prominently macrophages and endothelial cells. Other long pentraxins, such as murine or rat neuronal pentraxin 1 (NP1) and human neuronal pentraxin 2 (NPTX2), are expressed in the central nervous system (CNS). The present study was designed to investigate whether PTX3 is expressed in the brain and to define the structures and cells involved. Intracerebroventricular (i.c.v.), but not i.v., injection of LPS induced high levels of PTX3 mRNA in the mouse brain. In contrast NP1 is constitutively expressed in the murine CNS and is not modulated by LPS administration. I.c.v. IL-1beta was also a potent inducer of PTX3 expression in the CNS, whereas TNFalpha was substantially less effective and IL-6 induced a barely detectable signal. Central administration of LPS and IL-1 induced PTX3 also in the periphery (heart), whereas the reverse did not occur. Expression of PTX3 was also observed in the brain of mice infected with Candida albicans (C. albicans) or Cryptococcus neoformans. (C. neoformans). The kinetics of PTX3 gene induction were consistently different between C. albicans- and C. neoformans-infected mice, according to the diverse outcome of the CNS immune reaction. In situ hybridization revealed that i.c.v. injection of LPS induced a strong PTX3 expression in presumptive glial cells, in the white matter (corpus callosum, fimbria) and meningeal pia mater as well as in dentate gyrus hilus and granule cells. No constitutive expression of PTX3 was detected. Central expression of PTX3 may amplify mechanisms of innate resistance and damage in the CNS. The possibility of a direct interaction of PTX3 with neuronal cells, as suggested for NPTX2, remains to be explored.

Inefficient T cell memory in the brain of mice infected with Candida albicans

We compared the contribution of T cell memory to the clearance of the fungus Candida albicans from the liver, kidneys and brain of Balb/c mice in a model of secondary systemic infection. In secondary infection, the fungi were more rapidly eliminated from the liver and kidneys than during primary infection. This was most pronounced in the liver where the fungi were eliminated at day 14 of infection. In contrast, in the brain, cultivable yeasts were still detectable 35 days after infection. Although both CD4(+) and CD8(+) cells could be detected in the brain with immunohistology, these cells appeared later in infection and in lower numbers than in the liver, and there were no significant differences in the numbers of T cells detected in the brain between primary and secondary infection. In contrast to the liver and the kidneys where an effect of T cells on the fungal load could be demonstrated, depletion of neither CD4(+) nor CD8(+) nor Thy-1.2(+) cells resulted in a significant increase of the amount of fungi in the brain above levels measured in secondarily infected mice treated with irrelevant antibodies. We conclude that the contribution of CD4(+) and CD8(+) cells to the clearance of C. albicans in secondary infection is organ-dependent and that T cell memory is inefficient in the brain.

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

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

Enhanced resistance to Cryptococcus neoformans infection induced by chloroquine in a murine model of meningoencephalitis

Although the pathogenesis of cerebral cryptococcosis is poorly understood, local immune cells, such as microglia and astrocytes, likely play a critical role in containing infection. Chloroquine (CQ) is a weak base that accumulates within acidic vacuoles and increases their pH. Consequently, proteolytic activity of lysosomal enzymes and intracellular iron release/availability are impaired, resulting in decreased availability of nutrients crucial to microorganism survival and growth in the host. We found that CQ enhances BV2 microglial-cell-mediated anticryptococcal activity in vitro. The phenomenon is (i) evident when both unopsonized and opsonized microorganisms are used and (ii) mimicked by NH4Cl, another weak base, and by bafilomycin A1, an inhibitor of vacuolar-type H+-ATPases. In vivo, intracerebral administration of CQ before lethal local challenge with Cryptococcus neoformans results in a significant augmentation of median survival time and a marked reduction of yeast growth in the brain and is associated with the enhancement of local interleukin 1beta (IL-1beta) and IL-6 mRNA transcripts. Overall, these results provide the first evidence that CQ enhances anticryptococcal host defenses.

Potent antifungal effects of a new derivative of partricin A in a murine model of cerebral cryptococcosis

A new member of the polyene family, N-dimethylaminoacetyl-partricin A 2-dimethylaminoethylamide diaspartate (SPA), was investigated and was found to be more effective than amphotericin B (i) in vivo by enhancing mouse resistance to cryptococcal meningoencephalitis and (ii) in vitro by potentiating the anticryptococcal activity of murine microglial cells.

Effects of pentoxifylline or dexamethasone in combination with amphotericin B in experimental murine cerebral cryptococcosis: evidence of neuroexcitatory pathogenic mechanisms

In a murine model of intracerebral infection by Cryptococcus neoformans the therapeutic effects of pentoxifylline or dexamethasone were studied alone and in combination with amphotericin B. Assessed parameters were mean survival time, brain histopathology index, amounts of glutamate and gamma-aminobutyric acid in the brain, and yeast CFU per brain. Survival increased significantly in mice treated with dexamethasone, amphotericin B, amphotericin B plus dexamethasone, and amphotericin B plus pentoxifylline; the latter had significantly longer survival than other treated groups. Indices of histopathological damage were similar in all treated groups. In infected untreated mice, the amounts of glutamate in the brain were decreased, presumably by depletion. In mice treated with amphotericin B plus dexamethasone, glutamate levels returned to the range of control mice. No differences in the amounts of gamma-aminobutyric acid were found between control and treatment groups. Brain fungal counts were significantly lower in mice treated with amphotericin B, amphotericin B plus dexamethasone, and amphotericin B plus pentoxifylline than in untreated animals. In this model, pentoxifylline in combination with amphotericin B improved survival, decreasing the fungal burden, and has potential as adjuvant therapy in cerebral cryptococcosis.

Biomolecular events involved in the establishment of brain anticandidal resistance

Using a murine model, we have demonstrated the establishment of cerebral resistance to local lethal challenge with Candida albicans strain CA-6, by previous intracerebral (i.c.) infection with the low-virulent strain PCA-2. Here we show that i.c. infection with PCA-2 is effective in drastically reducing brain colonization following secondary infection with CA-6. As assessed by colony forming unit assay and histopathological analysis, microbial counts are impaired, granuloma formation and hyphal growth are also reduced in brains of PCA-2- and CA-6-infected mice with respect to CA-6-challenged mice. Furthermore, using PCR studies, we found that, while PCA-2 (i.e. healing infection) induces transient cytokine gene expression in the mouse brain, CA-6 lethal challenge results in long-lasting (until mouse death) high levels of all cytokine gene transcripts assessed. Finally brains from mice that will resist CA-6 challenge, because of previous infection with PCA-2, also exhibit a transient induction of all cytokine genes. Only IL-1 beta remains highly expressed at all time- points tested. Overall, these results provide evidence that healing and non-healing C. albicans i.c. infections differ in the immune reaction(s) locally evoked, at least in terms of cytokine gene expression, strongly suggesting cytokine involvement in the establishment of brain anticandidal resistance.

Adhesion of Candida albicans to brain tissue of Macaca mulata in an ex vivo assay

An ex vivo adhesion assay was used to examine adhesion of Candida albicans yeast cells to brain tissue of the primate Macaca mulata. Tissues from frontal lobes and striatum (caudate, putamen, and portions of the globus pallidus) were used in the assay. Yeast cells adhered to gray matter at about six times the level of adhesion to white matter. The fungus was able to bind to different cell types within the cortex, basal ganglia, and white matter. Binding to neurons, small neurons or glia, endothelial cells, and neuropil was observed.

Beta-1,2-linked oligomannosides from Candida albicans act as signals for tumor necrosis factor alpha production

Different cell wall components from Candida albicans have been shown to stimulate murine macrophages for tumor necrosis factor alpha (TNF-alpha) secretion. All of these molecules contain beta-1,2-oligomannosides. In order to examine their role in TNF-alpha production, acid-labile oligosaccharides, released from C. albicans VW32 cell wall phosphopeptidomannan by mild acid hydrolysis, and previously shown to correspond to homopolymers of beta-1,2-linked mannopyranosyl units, were separated by gel filtration chromatography according to their degree of polymerization. Murine macrophages incubated with purified oligomannosides (M2 to M8) released TNF-alpha to an extent which was dependent on, although not directly correlated with, the length of the mannosyl chain. Slight activity was observed with M4 and M5; M6 and M7 had virtually no effect, whereas M8 was associated with strong TNF-alpha release. This effect of M8 was dose dependent and was not altered by polymyxin B, known to interfere with lipopolysaccharide-induced TNF-alpha production. These results suggest that stimulation of TNF-alpha release by C. albicans glycoconjugates containing beta-1,2-linked oligomannosides may be due, at least in part, to the presence of these components.

Differential susceptibility of yeast and hyphal forms of Candida albicans to proteolytic activity of macrophages

The dimorphic transition of Candida albicans from the yeast (Y-Candida) to the hyphal (H-Candida) form is a complex event whose relevance in fungal pathogenicity is still poorly understood. Using a cloned macrophage (M phi) cell line (ANA-1), we have previously shown that a M phi can discriminate between the two fungal forms, eliciting different secretory responses. In the present study, we investigated the susceptibility of Y-Candida and H-Candida to M phi proteolytic activity. In particular, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (immunoblot) techniques were employed to analyze the patterns of lyticase proteinaceous extracts from cell walls of Y-Candida and H-Candida which had been unexposed or exposed to ANA-1 M phis for 3 h. Silver staining allowed detection of a complex protein pattern in both forms of C. albicans, qualitatively and quantitatively differing from each other, mainly at molecular masses below 106 kDa. Western blot staining with anti-C. albicans mannan antibodies and convalescent-phase sera of mice previously infected systemically or intracerebrally with C. albicans showed that, after contact with M phis, Y-Candida but not H-Candida proteinaceous cell wall components are profoundly modified, with substantial reduction and/or disappearance of many bands. Our experimental approach provides initial insights into the differential susceptibility of Y-Candida and H-Candida to the proteolytic activity of M phis.

Biomolecular events involved in anticryptococcal resistance in the brain

We have recently shown that intracerebral (i.c.) administration of heat-killed Cryptococcus neoformans (HCN) enhances mouse resistance to a subsequent local challenge with lethal doses of viable yeast cells. Here we show that i.c. administration of HCN is also effective in significantly delaying brain colonization of mice intravenously infected with viable C. neoformans. PCR analysis revealed that interleukin 6 (IL-6) and IL-1 beta gene expression occurs in brain of HCN-treated mice but not in brains of saline-treated controls. In contrast, no differences are observed in terms of tumor necrosis factor alpha and IL-1 alpha gene transcripts, which are slightly and highly detectable, respectively, in saline-treated mice and which remain such also following HCN treatment. Furthermore, i.c. administration of exogenous IL-6 or IL-1 beta, but not tumor necrosis factor alpha, before local challenge with viable C. neoformans results in significantly reduced microbial counts in the brain and blood and in increased mouse survival. Taken together, these observations provide initial evidence that brain anticryptococcal resistance involves elicitation of a local cytokine response, involving primarily IL-6 and IL-1 beta.

L3T4(CD4)-, Lyt-2(CD8)- and Mac-1(CD11b)-phenotypic leukocytes in murine cryptococcal meningoencephalitis

An immunohistological study of L3T4(CD4)+ and LYT-2(CD8)+ lymphocytes, Mac-1(CD11b)+ monocytes and granulocytes in experimental murine cryptococcal meningoencephalitis was conducted. To assess the concomitant inflammatory reaction in an extracerebral site, livers were examined in parallel. Mice were infected i.v. with Cryptococcus neoformans, group A/D, and organs were examined immunohistologically for CD4-, CD8- and monocyte- and granulocyte-specific CD11b-phenotypic leukocytes over a period of 60 days. Intracerebrally, agglomerations of cryptococci formed pseudocysts that were surrounded by CD4+ and CD8+ lymphocytes at the end of the second week post-infection, followed by the invasion of monocytes and granulocytes into the lesions. After the fourth week post-infection, most of the invaded lesions were transformed into glious scars. Meningitis was usually marked and showed a homogenous distribution of CD4-, CD8- and CD11b-phenotypic cells, with a predominance of monocytes and CD4+ lymphocytes. Inflammatory infiltrates in the liver were found already 4 days post-infection. CD4+ lymphocytes and monocytes were distributed homogeneously in the infiltrates, with a lower number of CD8+ lymphocytes being located rather in the periphery of the infiltrates. Comparing leukocyte kinetics in brain and liver, an important observation was the delayed immigration of immune cells at the intracerebral cryptococcal lesions as compared with the liver, and the different migration patterns of T-lymphocyte subgroups and macrophages. These results suggest that there are differential leukocyte migration patterns in the liver and brain following disseminated cryptococcosis. The immunological aspects of the observed leukocyte kinetics are discussed.

Pattern of cytokine gene expression in brains of mice protected by picolinic acid against lethal intracerebral infection with Candida albicans

Recently, we demonstrated that intracerebral (i.c.) administration of picolinic acid (PLA) confers protection against a lethal local challenge with the opportunistic pathogen Candida albicans. By histopathological studies, we show here that mice receiving PLA treatment survive challenge and no evidence of fungal invasion is found within the brain compartment. In contrast, PLA-untreated mice succumb to infection within 7-10 days and show massive brain colonization with extensive granulomatous reaction. By PCR analysis, we show that, unlike naive brains, PLA-treated brains show transient activation of TNF alpha, IL-1 beta and IL-6 genes. C. albicans infection results in high levels of all cytokine transcripts, the phenomenon being long-lasting in PLA-untreated brains, while gradually declining in PLA-treated brains. The only exception is IL-1 beta, whose levels remain high at the latest time-points tested, also in PLA-treated brains. Finally, IL-1 alpha, constitutively detectable in naive brains, is slightly enhanced by C. albicans challenge, regardless of prior treatment. These findings, together with the knowledge that PLA is a potent co-stimulus for macrophages, suggest the involvement of cytokine circuits, likely of macrophage origin, in anti-Candida resistance established by PLA at the cerebral level.

Cytokine treatment of central nervous system infection: efficacy of interleukin-12 alone and synergy with conventional antifungal therapy in experimental cryptococcosis

Cell-mediated immune responses appear to be critical in the outcome of cryptococcosis. Interleukin-12 (IL-12) was studied for its potential use as a therapeutic agent because of its stimulation of natural killer cells and gamma interferon production by stimulated T cells and natural killer cells. Gamma interferon-activated macrophages are important in host resistance against cryptococcosis. In two separate studies, male BALB/c mice were infected intravenously with Cryptococcus neoformans. In the first study, mice received either no treatment, 5.0 mg of fluconazole alone per kg of body weight per day (by gavage twice daily), or IL-12 subcutaneously at 0.01, 0.1, or 1.0 microgram/day once daily (low-dose study) alone or in combination with 5.0 mg of fluconazole per kg/day. In a second study (high dose), the dosages of IL-12 used were 1.0, 2.5, or 5.0 micrograms/day. Therapy was given for 10 consecutive days, and the number of CFU of C. neoformans remaining in various organs was quantitated 1 or 2 days after administration of the last dose. In the low-dose study, IL-12 at 0.1 or 1.0 microgram reduced the level of brain infection by approximately 10-fold (P < 0.05) and IL-12 at 1.0 or 0.1 microgram/day enhanced the efficacy of fluconazole. In liver, both the efficacy of IL-12 alone (0.01 or 0.1 microgram; P < 0.05) and enhancement of the efficacy of fluconazole (P < 0.05) were seen. No efficacy of IL-12 was seen in spleens or lungs, although spleen weights increased fourfold in mice given 1.0 microgram of IL-12 per day. In the high-dose study, all IL-12 doses alone again reduced the levels of brain infection (5- to 8-fold; P < 0.05) when the two were given in combination. No overt toxicities were observed at any dose, and overall, 1.0 microgram of IL-12 per day was found to be the optimal dosage for reducing infection in the brain. To our knowledge, this is the first demonstration of the efficacy of cytokine therapy in systemic and particularly brain infections with C. neoformans. The stimulation of cell-mediated immunity represents a new approach to therapy and can enhance suboptimal antimicrobial chemotherapy. IL-12 should be considered for further study and for clinical trials. These studies suggest that other opportunistic central nervous system pathogens should also be investigated.