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

Effect of indoleamine dioxygenase-1 deficiency and kynurenine pathway inhibition on murine cerebral malaria

Cerebral malaria (CM) can be a fatal manifestation of Plasmodium falciparum infection. In this study, two different approaches were used to examine the role of indoleamine 2,3-dioxygenase-1 (IDO-1) and its metabolites in the development of murine CM. Mice genetically deficient in IDO-1 were not protected against CM, but partial protection was observed in C57BL/6 mice treated with Ro 61-8048, an inhibitor of kynurenine-3-hydroxylase. This protection was associated with suppressed levels of picolinic acid (PA) within the brain, but not with changes in the levels of kynurenic acid (KA) or quinolinic acid (QA). These data suggest that although IDO-1 is not directly involved in the pathogenesis of CM in C57BL/6 mice, the production of the kynurenine pathway metabolite PA may contribute to the development of murine CM.

Tissue expression and biochemical characterization of human 2-amino 3-carboxymuconate 6-semialdehyde decarboxylase, a key enzyme in tryptophan catabolism

2-amino 3-carboxymuconate 6-semialdehyde decarboxylase (ACMSD, EC 4.1.1.45) plays a key role in tryptophan catabolism. By diverting 2-amino 3-carboxymuconate semialdehyde from quinolinate production, the enzyme regulates NAD biosynthesis from the amino acid, directly affecting quinolinate and picolinate formation. ACMSD is therefore an attractive therapeutic target for treating disorders associated with increased levels of tryptophan metabolites. Through an isoform-specific real-time PCR assay, the constitutive expression of two alternatively spliced ACMSD transcripts (ACMSD I and II) has been examined in human brain, liver and kidney. Both transcripts are present in kidney and liver, with highest expression occurring in kidney. In brain, no ACMSD II expression is detected, and ACMSD I is present at very low levels. Cloning of the two cDNAs in yeast expression vectors and production of the recombinant proteins, revealed that only ACMSD I is endowed with enzymatic activity. After purification to homogeneity, this enzyme was found to be a monomer, with a broad pH optimum ranging from 6.5 to 8.0, a K(m) of 6.5 microM, and a k(cat) of 1.0 s(-1). ACMSD I is inhibited by quinolinic acid, picolinic acid and kynurenic acid, and it is activated slightly by Fe(2+) and Co(2+). Site-directed mutagenesis experiments confirmed the catalytic role of residues, conserved in all ACMSDs so far characterized, which in the bacterial enzyme participate directly in the metallocofactor binding. Even so, the properties of the human enzyme differ significantly from those reported for the bacterial counterpart, suggesting that the metallocofactor is buried deep within the protein and not as accessible as it is in bacterial ACMSD.

NF-kB activation and p38 phosphorilation in microglial cells infected with Leptospira or exposed to partially purified leptospiral lipoproteins

Recently, we have shown a differential susceptibility of non-pathogenic vs. pathogenic leptospires to phagocytosis and killing by microglial cells. Although all ingested to some extent, only the pathogenic strains survived intracellularly while the non-pathogenic ones were killed in a time-dependent manner. By the same infection model, here we demonstrate that microglial cells respond to Leptospira infection with a time- and dose-dependent induction of molecular signals (p38 phosphorilation and NF-kB activation) and the production of soluble factors (cytokines and nitric oxide). Such bio-molecular response is predominantly observed against the pathogenic Leptospira; the phenomenon is reproduced by leptospiral lipoproteins and, to a lower extent, by leptospiral-derived LPS. These data provide initial evidence that Leptospira affects microglial cell response in a different manner depending upon the virulence of the infecting strain; specific bacterial components happen to be involved in the induction of such pathogen-induced immune response.

Macrophage activating properties of the tryptophan catabolite picolinic acid

Recent studies have suggested a role for aminoacid catabolites as important regulators of macrophage (Mphi) activities. We reported previously that picolinic acid (PA), a tryptophan catabolite produced under inflammatory conditions and a costimulus with IFNgamma of Mphi effector functions, is a selective inducer of the Mphi inflammatory protein-1alpha (MIP-1alpha) and -1beta (MIPs), two CC-chemokines involved in the elicitation of the inflammatory reactions and in the development of the Th1 responses. In this study, we have investigated the effects of IFNgamma on PA-induced MIPs expression and secretion by mouse Mphi as well as the regulation of MIP-1alpha/beta receptor, CCR5, by both stimuli alone or in combination. We demonstrated that IFNgamma inhibited MIPs mRNA stimulation by PA in a dose-and time-dependent fashion, despite its ability to induce other CC- or CXC chemokines. MIPs mRNA down-regulation was associated with decreased intracellular chemokine expression and secretion and was dependent on both mRNA destabilization and gene transcription inhibition. Moreover, IFNgamma inhibitory effects were stimulus-specific because MIPs induction by PA was either unaffected or increased by the anti-inflammatory cytokines, IL-10 and IL-4, or the pro-inflammatory stimulus, LPS, respectively. In contrast, we found that IFNgamma increased CCR5 basal expression, whereas PA down-regulated both constitutive and IFNgamma-induced CCR5 mRNA and protein levels. These results demonstrate that IFNgamma and PA have reciprocal effects on the production of MIPs chemokines and the expression of their receptor. The concerted action of IFNgamma and PA on MIP-1alpha/beta chemokine/receptor system is likely to be of pathophysiological significance and to represent an important regulatory mechanism for leukocyte recruitment and distribution into damaged tissues during inflammatory responses.

Antagonistic effect of picolinic acid and interferon-gamma on macrophage inflammatory protein-1alpha/beta production

The L-tryptophan catabolite, picolinic acid (PA), is an activator of macrophage effector functions and an inducer of macrophage inflammatory protein-1alpha (MIP-1alpha) and -1beta (MIPs). We have investigated the regulation of PA-induced MIPs production in mouse macrophages. We demonstrated a dose- and time-dependent downregulation of MIPs mRNA by the Th1 cytokine, IFN-gamma, that was associated with inhibition of intracellular chemokine production and secretion. This effect was IFN-gamma-specific because MIPs induction was unaffected by the Th2 cytokines, IL-10 and IL-4, or the proinflammatory stimulus, LPS. Moreover, MIPs downregulation by IFN-gamma was dependent on both mRNA destabilization and gene transcription inhibition. These results demonstrate that MIP-1alpha/beta production by macrophages is a tightly regulated process resulting from the interaction between inhibitory stimuli derived from the immune system and stimulatory signals of non-immunologic origin. The antagonistic effect of PA and IFN-gamma on MIPs production may be important for the regulation of the inflammatory responses in vivo.

New high-performance liquid chromatographic method for the detection of picolinic acid in biological fluids

A HPLC method is described to quantify picolinic acid in milk, blood serum and tissue culture supernatant. The method requires very little sample preparation because acid precipitation allows total recovery of picolinic acid. High specificity and sensitivity were obtained using ion-pair chromatography on a C18 reversed-phase column with tetrabutylammonium hydrogen sulfate as ion pairing reagent. We describe the conditions for the automated testing of multiple samples and for the detection of L-tryptophan and L-kynurenine together with picolinic acid. This system will be utilized to elucidate the relationship between picolinic acid production and human disease. Furthermore, we provide the first evidence of picolinic acid in human blood serum.

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.

The tryptophan catabolite picolinic acid selectively induces the chemokines macrophage inflammatory protein-1 alpha and -1 beta in macrophages

We previously found that the tryptophan catabolite picolinic acid (PA) is a costimulus for the activation of macrophage effector functions. In this study, we have investigated the ability of PA to modulate the expression of chemokines in macrophages. We demonstrate that PA is a potent activator of the inflammatory chemokines MIP (macrophage inflammatory protein)-1 alpha and MIP-1 beta (MIPs) mRNA expression in mouse macrophages in a dose- and time-dependent fashion and through a de novo protein synthesis-dependent process. The induction by PA occurred within 3 h of treatment and reached a peak in 12 h. The stimulatory effects of PA were selective for MIPs because other chemokines, including monocyte chemoattractant protein-1, RANTES, IFN-gamma-inducible protein-10, MIP-2, and macrophage-derived chemokine, were not induced under the same experimental conditions and were not an epiphenomenon of macrophage activation because IFN-gamma did not affect MIPs expression. Induction of both MIP-1 alpha and MIP-1 beta by PA was associated with transcriptional activation and mRNA stabilization, suggesting a dual molecular mechanism of control. Iron chelation could be involved in MIPs induction by PA because iron sulfate inhibited the process and the iron-chelating agent, desferrioxamine, induced MIPs expression. We propose the existence of a new pathway leading to inflammation initiated by tryptophan catabolism that can communicate with the immune system through the production of PA, followed by secretion of chemokines by macrophages. These results establish the importance of PA as an activator of macrophage proinflammatory functions, providing the first evidence that this molecule can be biologically active without the need for a costimulatory agent.

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.

Macrophages in resistance to candidiasis

Candida albicans, an increasingly common opportunistic pathogenic fungus, frequently causes disease in immunodeficient but not immunocompetent hosts. Clarifying the role of the phagocytic cells that participate in resistance to candidiasis not only is basic to understanding how the host copes with this dimorphic pathogen but also will expedite the development of innovative prophylactic and therapeutic approaches for treating the multiple clinical presentations that candidiasis encompasses. In this review, we present evidence that a diverse population of mononuclear phagocytes, in different states of activation and differentiation and from a variety of host species, can phagocytize C. albicans blastoconidia via an array of opsonic and nonopsonic mechanisms and can kill C. albicans blastoconidia and hyphae by means of oxygen-dependent and -independent mechanisms. Reactive nitrogen intermediates should now be added to the well-established candidacidal reactive oxygen intermediates of macrophages. Furthermore, what were thought to be two independent pathways, i.e., nitric oxide and superoxide anion, have now been shown to combine to form a potent macrophage candidacidal molecule, peroxynitrite. In contrast to monocytes and neutrophils, which are important in resistance to early stages of C. albicans infections, more differentiated macrophages activated by cytokines such as gamma interferon participate in the acquired resistance of hosts with C. albicans-specific, cell-mediated immunity. Evidence presented in this review demonstrates that mononuclear phagocytes, in some instances in the absence of other professional phagocytes such as neutrophils, play an import role in resistance to systemic and mucosal candidiasis.

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.

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.

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.