Regulation of the expression of nitric oxide synthase and leishmanicidal activity by glycoconjugates of Leishmania lipophosphoglycan in murine macrophages

Leishmania major lipophosphoglycan modulates the phenotype and inhibits migration of murine Langerhans cells

Langerhans cells (LC), members of the dendritic cell family, play a central role in the initiation and regulation of the immune response against the protozoan parasite Leishmania major. LC take up antigens in the skin and transport them to the regional lymph nodes for presentation to T cells. However, it is not known whether LC functions are modulated by parasite antigens. In the present study, we examined the effect of a major parasite surface molecule, L. major lipophosphoglycan (LPG), on the maturation of LC and their migratory properties. The results show that exposure to LPG did not affect the expression of major histocompatibility complex (MHC) class II and B7, but induced an up-regulation of CD25, CD31 and vascular endothelial (VE)-cadherin expression and a down-regulation of Mac-1 expression, by LC. Importantly, LPG treatment inhibited the migratory activity of LC, as it reduced their efflux from skin explants and their migration in transwell cultures. These results suggest that Leishmania LPG impairs LC migration out of the skin and thus may modulate their immunostimulatory functions, which require LC translocation from skin to lymph nodes.

Role of host phosphotyrosine phosphatase SHP-1 in the development of murine leishmaniasis

Activation of host phosphotyrosine phosphatase SHP-1 by Leishmania and its subsequent impact on tyrosine phosphorylation-based signaling cascades were shown to represent an important mechanism whereby this pathogen may alter host cell functions. Herein, we report that Leishmania-induced macrophage SHP-1 activity is necessary for its survival within phagocytes through the attenuation of nitric oxide-dependent and -independent microbicidal mechanisms. In vivo, Leishmania major infection, which footpad inflammation is mostly undetectable in SHP-1-deficient viable motheaten mice, was accompanied by increased inducible nitric oxide synthase and activation of neutrophils. These enhanced cellular activities were paralleled by a marked activation of signaling events usually negatively regulated by SHP-1. Overall, this study firmly establishes that modulation of the signaling terminator SHP-1 by Leishmania is essential for its installment and propagation.

Generation of ceramide in murine macrophages infected with Leishmania donovani alters macrophage signaling events and aids intracellular parasitic survival

In the present study, we examined the involvement of intracellular ceramide in host pathogen interaction of BALB/c mouse peritoneal macrophages infected with the obligate intracellular protozoan, Leishmania donovani. Our findings indicate that the level of intracellular ceramide was enhanced as a result of the in vitro infection. While the elevated ceramide was largely due to de novo synthesis, activation of the sphingomyelinases was also observed. The enhanced ceramide was responsible for the downregulation of classical PKC activity, upregulation of calcium independent atypical PKC-zeta expression and activity of calcium independent PKC. Ceramide also impaired the phosphorylation of MAPK. Evidently, ceramide suppressed the generation of nitric oxide during leishmanial infection and also facilitated the survival of leishmanial parasites in the intramacrophageal milieu. These data present newer insight to the signaling events in leishmania-infected murine macrophages, which might offer ceramide as a new therapeutic target in the future.

Leishmania donovani lipophosphoglycan causes periphagosomal actin accumulation: correlation with impaired translocation of PKCalpha and defective phagosome maturation

Lipophosphoglycan (LPG) is the major surface glycoconjugate of Leishmania donovani promastigotes. The repeating disaccharide-phosphate units of LPG are crucial for promastigote survival inside macrophages and establishment of infection. LPG has a number of effects on the host cell, including inhibition of PKC activity, inhibition of nitric oxide production and altered expression of cytokines. LPG also inhibits phagosomal maturation, a process requiring depolymerization of periphagosomal F-actin. In the present study, we have characterized the dynamics of F-actin during the phagocytosis of L. donovani promastigotes in J774 macrophages. We observed that F-actin accumulated progressively around phagosomes containing wild-type L. donovani promastigotes during the first hour of phagocytosis. Using LPG-defective mutants and yeast particles coated with purified LPG, we obtained evidence that this effect could be attributed to the repeating units of LPG. LPG also disturbed cortical actin turnover during phagocytosis. The LPG-dependent accumulation of periphagosomal F-actin correlated with an impaired recruitment of the lysosomal marker LAMP1 and PKCalpha to the phagosome. Accumulation of periphagosomal F-actin during phagocytosis of L. donovani promastigotes may contribute to the inhibition of phagosomal maturation by physically preventing vesicular trafficking to and from the phagosome.

Is lipophosphoglycan a virulence factor? A surprising diversity between Leishmania species

Lipophosphoglycan is a prominent member of the phosphoglycan-containing surface glycoconjugates of Leishmania. Genetic tests enable confirmation of its role in parasite virulence and permit discrimination between the roles of lipophosphoglycan and related glycoconjugates. When two different lipophosphoglycan biosynthetic genes from Leishmania major were knocked out, there was a clear loss of virulence in several steps of the infectious cycle but, with Leishmania mexicana, no effect on virulence was found. This points to an unexpected diversity in the reliance of Leishmania species on virulence factors, a finding underscored by recent studies showing great diversity in the host response to Leishmania species.

Suppression of type 2 NO-synthase activity in macrophages by Candida albicans

Macrophages (Mphi) are important for the defence against experimental disseminated candidiasis. Nitric oxide (NO) generated by the inducible isoform of NO-synthase (iNOS or NOS2) is thought to contribute to candidacidal effector functions by activated Mphi. In vitro, however, Mphi cannot control the growth and hyphal formation of Candida (C.) albicans. Using mouse peritoneal exudate Mphi stimulated with IFN-gamma and LPS, we examined the effect of C. albicans on NO synthesis, NOS2 enzyme activity and macrophage survival. C. albicans effectively inhibited the production of NO via suppression of total NOS2 protein and enzyme activity. Hyphal formation of C. albicans and direct interaction with host cells was required for maximum inhibition of NO production, whereas non-filamentous C. albicans mutants released soluble products that effected only partial inhibition. Ultimately, Mphi underwent apoptotic cell death after infection with C. albicans wild-type strains capable of hyphal formation, indicated by loss of the mitochondrial membrane potential and onset of chromatin degradation. NO suppression and Mphi killing are potent activities of C. albicans that may augment virulence of C. albicans.

Interferon gamma induces enterocyte resistance against infection by the intracellular pathogen Cryptosporidium parvum

BACKGROUND & AIMS

Interferon (IFN)-gamma plays an important role in the immunologic control of infection by the protozoan enteropathogen Cryptosporidium parvum. We tested the hypothesis that IFN-gamma may directly inhibit infection of enterocytes by this pathogen.

METHODS

HT-29, Caco-2, and H4 human enterocyte cell lines were grown in monolayers and incubated with IFN-gamma before exposure with C. parvum. IFN-gamma receptor expression in the cell lines was determined by Western blot analysis.

RESULTS

IFN-gamma inhibited C. parvum infection of both HT-29 and Caco-2 cells but not H4 cells. Response to IFN-gamma was related to the expression of the IFN-gamma receptor in the respective cell lines. The effect of IFN-gamma was partially reversed by inhibition of the JAK/STAT signaling pathway. IFN-gamma mediated its action by at least 2 mechanisms: (1) inhibition of parasite invasion and (2) by modification of intracellular Fe(2+) concentration. No role for tryptophan starvation or nitric oxide synthase activity was found. TNF-alpha and IL-1beta also had anti-C. parvum activity but had no synergistic effect with IFN-gamma.

CONCLUSIONS

IFN-gamma directly induces enterocyte resistance against C. parvum infection; this observation may have important consequences for our understanding of the mucosal immune response to invasive pathogens.

Immunization with a recombinant stage-regulated surface protein from Leishmania donovani induces protection against visceral leishmaniasis

Vaccination against visceral leishmaniasis has received limited attention compared with cutaneous leishmaniasis, although the need for an effective vaccine against visceral leishmaniasis is pressing. In this study, we demonstrate for the first time that a recombinant stage-specific hydrophilic surface protein of Leishmania donovani, recombinant hydrophilic acylated surface protein B1 (HASPB1), is able to confer protection against experimental challenge. Protection induced by rHASPB1 does not require adjuvant and, unlike soluble Leishmania Ag + IL-12, extends to the control of parasite burden in the spleen, an organ in which parasites usually persist and are refractory to a broad range of immunological and chemotherapeutic interventions. Both immunohistochemistry (for IL-12p40) and enzyme-linked immunospot assay (for IL-12p70) indicate that immunization with rHASPB1 results in IL-12 production by dendritic cells, although an analysis of Ab isotype responses to rHASPB1 suggests that this response is not sufficient in magnitude to induce a polarized Th1 response. Although both vaccinated and control-infected mice have equivalent frequencies of rHASPB1-specific CD4(+) T cells producing IFN-gamma, vaccine-induced protection correlates with the presence of rHASPB1-specific, IFN-gamma-producing CD8(+) T cells. Thus, we have identified a novel vaccine candidate Ag for visceral leishmaniasis, which appears to operate via a mechanism similar to that previously associated with DNA vaccination.

Inhibition of glutathione synthesis as a chemotherapeutic strategy for leishmaniasis

This study focuses on the use of buthionine sulphoximine (BSO), a gamma-glutamylcysteine synthetase inhibitor, on Leishmania donovani growth. The effect of BSO on amastigote multiplication within macrophages showed that 5 mM BSO decreased infectivity by about 50% and the mean number of amastigotes per 100 infected macrophages by 21%. The mechanism may be that BSO resulted in enhanced nitric oxide (NO) levels within macrophages, probably due to inhibition of GSH content since GSH (10 mm) given after BSO treatment led to a decrease in NO compared to macrophages treated with BSO alone which were preexposed to the Leishmania surface molecule lipophosphoglycan.

Lipophosphoglycan is not required for infection of macrophages or mice by Leishmania mexicana

Cell surface lipophosphoglycan (LPG) is commonly regarded as a multifunctional Leishmania virulence factor required for survival and development of these parasites in mammals. In this study, the LPG biosynthesis gene lpg1 was deleted in Leishmania mexicana by targeted gene replacement. The resulting mutants are deficient in LPG synthesis but still display on their surface and secrete phosphoglycan-modified molecules, most likely in the form of proteophosphoglycans, whose expression appears to be up-regulated. LPG-deficient L.mexicana promastigotes show no significant differences to LPG-expressing parasites with respect to attachment to, uptake into and multiplication inside macrophages. Moreover, in Balb/c and C57/BL6 mice, LPG-deficient L.mexicana clones are at least as virulent as the parental wild-type strain and lead to lethal disseminated disease. The results demonstrate that at least L. mexicana does not require LPG for experimental infections of macrophages or mice. Leishmania mexicana LPG is therefore not a virulence factor in the mammalian host.

Leishmania amazonensis infection does not inhibit systemic nitric oxide levels elicited by lipopolysaccharide in vivo

Leishmaniasis is a parasitic disease that leads to chronic inflammation. Macrophages, depending on their activation state, are either hosts or killers of the parasites. Downregulation of nitric oxide (NO) synthesis by the parasite infecting the macrophages has been proposed to be an important evading mechanism based on in vitro studies. We confirmed inhibition of NO release by macrophages infected with Leishmania amazonensis in vitro. To examine the role of the parasite in regulating NO production in vivo, we monitored systemic NO levels elicited by challenging naive and L. amazonensis-infected BALB/c mice with lipopolysaccharide (LPS). Animals were challenged after 1, 2, 6, and 9 wk of infection. NO production was monitored by electron paramagnetic resonance spectroscopy as the levels of hemoglobin nitrosyl complexes (HbNO) present in the animal's blood. No significant differences in HbNO levels were observed between LPS-treated naive and inoculated mice at any time during infection. To control for increased macrophage numbers in infected mice, naive mice were injected with a macrophage cell line before LPS challenge; this treatment did not increase produced NO levels. The results argue against a major role for the parasite in downregulating NO production in vivo.

Modulation of intramacrophage iron metabolism during microbial cell invasion

The mechanisms whereby vertebrate hosts withhold iron from microbial invaders, as well as the methods used in attempts by invaders to capture the growth-essential metal, differ between host extracellular and intracellular environments. This review focuses on procedures employed by host cells and by invaders, respectively, that alter intramacrophage iron metabolism.

Extracellular Signal-Related Kinase (ERK) and p38 Mitogen-Activated Protein (MAP) Kinases Differentially Regulate the Lipopolysaccharide-Mediated Induction of Inducible Nitric Oxide Synthase and IL-12 in Macrophages: Leishmania Phosphoglycans Subvert Macrophage IL-12 Production by Targeting ERK MAP Kinase

Macrophage activation by cytokines or microbial products such as LPS results in the induction and release of several key immune effector molecules including NO and IL-12. These have been shown to play crucial roles in the development of immunity to intracellular pathogens such as Leishmania. The molecular mechanisms underlying the induction of these effector molecules are not fully understood. We now show that the extracellular signal-related kinase (ERK) and p38 mitogen-activated protein (MAP) kinases play differential roles in the regulation of LPS-stimulated inducible NO synthase and IL-12 gene expression. In macrophages, LPS stimulates the simultaneous activation of all three classes of MAP kinases, ERK, c-jun N-terminal kinase, and p38, albeit with differential activation kinetics. However, studies using inhibitors selective for ERK (PD98059) and p38 (SB203580) show that while p38 plays an essential role in the induction of inducible NO synthase, ERK MAP kinases play only a minor role in promoting NO generation. In contrast, while p38 promotes induction of IL-12 (p40) mRNA, ERK activation suppresses LPS-mediated IL-12 transcription. The biological relevance of these regulatory signals is demonstrated by our finding that Leishmania lipophosphoglycans, which promote parasite survival, act by stimulating ERK MAP kinase to inhibit macrophage IL-12 production. Thus, as ERK and p38 MAP kinases differentially regulate the induction of the macrophage effector molecules, inducible NO synthase and IL-12, these kinases are potential targets not only for the development of novel strategies to combat intracellular pathogens but also for therapeutic immunomodulation.

Molecular cloning and characterization of a novel repeat-containing Leishmania major gene, ppg1, that encodes a membrane-associated form of proteophosphoglycan with a putative glycosylphosphatidylinositol anchor

Leishmania parasites secrete a variety of proteins that are modified by phosphoglycan chains structurally similar to those of the cell surface glycolipid lipophosphoglycan. These proteins are collectively called proteophosphoglycans. We report here the cloning and sequencing of a novel Leishmania major proteophosphoglycan gene, ppg1. It encodes a large polypeptide of approximately 2300 amino acids. The N-terminal domain of approximately 70 kDa exhibits 11 imperfect amino acid repeats that show some homology to promastigote surface glycoproteins of the psa2/gp46 complex. The large central domain apparently consists exclusively of approximately 100 repetitive peptides of the sequence APSASSSSA(P/S)SSSSS(+/-S). Gene fusion experiments demonstrate that these peptide repeats are the targets of phosphoglycosylation in Leishmania and that they form extended filamentous structures reminiscent of mammalian mucins. The C-terminal domain contains a functional glycosylphosphatidylinositol anchor addition signal sequence, which confers cell surface localization to a normally secreted Leishmania acid phosphatase, when fused to its C terminus. Antibody binding studies show that the ppg1 gene product is phosphoglycosylated by phosphoglycan repeats and cap oligosaccharides. In contrast to previously characterized proteophosphoglycans, the ppg1 gene product is predominantly membrane-associated and it is expressed on the promastigote cell surface. Therefore this membrane-bound proteophosphoglycan may be important for direct host-parasite interactions.

Glycoconjugates in Leishmania infectivity

Leishmaniasis is a major health problem to humans and is caused by one of the world's major pathogens, the Leishmania parasite. These protozoa have the remarkable ability to avoid destruction in hostile environments they encounter throughout their life cycle. That Leishmania parasites have adapted to not only survive, but to proliferate largely is due to the protection conferred by unique glycoconjugates that are either on the parasites' cell surface or secreted. Most of these specialized molecules are members of a family of phosphoglycans while others are a family of glycosylinositol phospholipids. Together they have been implicated in a surprisingly large number of functions for the parasites throughout their life cycle and, therefore, are key players in their pathogenesis. This review summarizes the biological roles of these glycoconjugates and how they are believed to contribute to Leishmania survival in destructive surroundings.

Leishmania species: models of intracellular parasitism

Leishmania species are obligate intracellular parasites of cells of the macrophage-dendritic cell lineage. Indeed, the ability to survive and multiply within macrophages is a feature of a surprising number of infectious agents of major importance to public health, including Mycobacterium tuberculosis, Mycobacterium leprae, Listeria monocytogenes, Salmonella typhimurium, Toxoplasma gondii and Trypanosoma cruzi. The relationship between such organisms and their host cells is particularly intriguing because, not only are macrophages capable of potent microbicidal activity, but in their antigen-presenting capacity they can orchestrate the developing immune response. Thus, to initiate a successful infection parasites must gain entry into macrophages, and also withstand or circumvent their killing and degradative functions. However, to sustain a chronic infection, parasites must also subvert macrophage-accessory-cell activities and ablate the development of protective immunity. The leishmanias produce a wide spectrum of disease in mice, and as such they have provided excellent models for studying problems associated with intracellular parasitism. In recent years, largely using these organisms, we have made enormous progress in elucidating the mechanisms by which successful intracellular infection occurs. Furthermore, characterization of immunological pathways that are responsible for resistance or susceptibility to Leishmania has given rise to the Th1/Th2 paradigm of cellular/humoral dominance of the immune response.

Costimulatory action of glycoinositolphospholipids from Trypanosoma cruzi: increased interleukin 2 secretion and induction of nuclear translocation of the nuclear factor of activated T cells 1

The effects of the glycoinositolphospholipids (GIPLs) fromTrypanosoma cruzi on T lymphocyte activation were investigated in a mouse T cell hybridoma (DO-11.10). Purified GIPLs from T. cruzi strains Y and G markedly increased IL-2 mRNA transcripts and IL-2 secretion induced by mitogenic anti-CD3 and anti-Thy1 mAbs. This costimulatory function was also revealed by the induction of IL-2 secretion after the simultaneous addition of the T. cruzi GIPLs and either the calcium ionophore A23187 or phorbol ester. The capacity of the GIPL molecule to induce an increase in cytoplasmic calcium levels was also demonstrated. After exposure of T cell hybridoma to GIPL, the nuclear transcription factor NFAT1 became partially dephosphorylated, and its nuclear localization was demonstrated both in the T cell hybridoma and in Balb/c CD3(+) cells. These results demonstrate that T. cruzi GIPL molecules are capable of signaling to T cells and therefore could be valuable tools for the study of T cell activation, besides playing a potential role in subverting the T lymphocyte immune response during T. cruzi infection.

The structure, biosynthesis and functions of glycosylphosphatidylinositol anchors, and the contributions of trypanosome research

The discovery of glycosylphosphatidylinositol (GPI) membrane anchors has had a significant impact on several areas of eukaryote cell biology. Studies of the African trypanosome, which expresses a dense surface coat of GPI-anchored variant surface glycoprotein, have played important roles in establishing the general structure of GPI membrane anchors and in delineating the pathway of GPI biosynthesis. The major cell-surface molecules of related parasites are also rich in GPI-anchored glycoproteins and/or GPI-related glycophospholipids, and differences in substrate specificity between enzymes of trypanosomal and mammalian GPI biosynthesis may have potential for the development of anti-parasite therapies. Apart from providing stable membrane anchorage, GPI anchors have been implicated in the sequestration of GPI-anchored proteins into specialised membrane microdomains, known as lipid rafts, and in signal transduction events.

Leishmania major proteophosphoglycan is expressed by amastigotes and has an immunomodulatory effect on macrophage function

Proteophosphoglycan (PPG) is a newly described mucin-like glycoprotein found on the surface of Leishmania major promastigotes and secreted in the culture supernatant. We show here that antigenically similar PPGs are present in several Leishmania species. PPG could also be detected on the surface of amastigotes and in small, parasite-free vesicles in infected macrophages. Because of the similarity of its carbohydrate chains to lipophosphoglycan, a parasite receptor for host macrophages, PPG was tested for binding to macrophages. PPG bound to macrophages and was internalized in a time-dependent manner. PPG inhibited the production of tumor necrosis factor-alpha and synergized with interferon-gamma to stimulate the production of nitric oxide by macrophages. PPG may contribute to the binding of Leishmania to host cells and may play a role in modulating the biology of the infected macrophage at the early stage of infection.

How do protozoan parasites survive inside macrophages?

During infections with intracellular microbes, macrophages have two roles. On the one hand, they are important effector cells for the control and killing of intracellular bacteria and protozoan parasites by oxidative and non-oxidative mechanisms. On the other hand, macrophages may also serve as long-term host cells that facilitate the replication and survival of the pathogens, for example, by protecting them against toxic components of the extracellular milieu. In this review, Christian Bogdan and Martin Röllinghoff summarize some of the more recently discovered mechanisms by which intracellular protozoan parasites, such as Leishmania spp, Trypanosoma cruzi and Toxoplasma gondii, manage to exploit macrophages as safe target cells.

Regulation of macrophage IL-12 synthesis by Leishmania phosphoglycans

It is now generally accepted that IFN-gamma, secreted by Th1 cells, is the most potent cytokine leading to macrophage activation and host resistance against infection with the intracellular protozoan parasite Leishmania. It is also established that IL-12 is a critical cytokine involved in the differentiation and expansion of Th1 cells. Therefore, the ability of Leishmania parasites to actively suppress IL-12 production by host macrophages may be an important strategy for parasite survival. Here we report that a major parasite cell surface molecule, phosphoglycan (PG), of Leishmania could selectively inhibit the synthesis of IL-12(p40, p70) by activated murine macrophages. Furthermore, synthetic PG (sPG) was able to inhibit IL-12 release in a dose-dependent manner. Inhibition was dependent on the galactose(beta1-4)mannose(alpha1)-PO4 repeating units and not the glycophosphoinositol lipid anchor of lipophosphoglycan. At the concentration used, sPG had no effect on the release of TNF-alpha or IL-6 in activated macrophages. The inhibition of IL-12(p40) production was at the transcriptional level, but was not mediated through NF kappaB inhibition. These data demonstrate that PG may be an important molecule for the establishment and survival of the parasite in permissive hosts.

Proapoptotic Activity of a Trypanosoma cruzi Ceramide-Containing Glycolipid Turned on in Host Macrophages by IFN-γ

The effects of glycoinositolphospholipid (GIPL), from the pathogenic protozoan Trypanosoma cruzi, and its isolated glycan and lipid (dihydroceramide) components, were investigated in J774 cells and primary macrophages. Isolated GIPL ceramide, but not intact GIPL or its glycan, induced intense fluid phase endocytosis when added exogenously. In the presence of the cytokine IFN-γ, GIPL ceramide induced marked apoptosis in J774 cells and macrophages, independent of nitric oxide secretion. When cells were preincubated with the GIPL-derived glycan chain, addition of intact GIPL induced macrophage apoptosis in the presence of IFN-γ. Synthetic C2-dihydroceramide also induced apoptosis in the presence of IFN-γ. Induction of apoptosis in T. cruzi-infected macrophages by GIPL ceramide plus IFN-γ led to increased parasite release compared with IFN-γ treatment alone. Viable parasites released comprised both infective trypomastigote and spheromastigote forms. These results identify a novel pathway by which T. cruzi glycosylphosphatidylinositol family molecules affect host macrophages, with implications for the infectious process.

Stage-specific proteophosphoglycan from Leishmania mexicana amastigotes. Structural characterization of novel mono-, di-, and triphosphorylated phosphodiester-linked oligosaccharides

Intracellular amastigotes of the protozoan parasite Leishmania mexicana secrete a macromolecular proteophosphoglycan (aPPG) into the phagolysosome of their host cell, the mammalian macrophage. The structures of aPPG glycans were analyzed by a combination of high pH anion exchange high pressure liquid chromatography, gas chromatography-mass spectrometry, enzymatic digestions, electrospray-mass spectrometry as well as 1H and 31P NMR spectroscopy. Some glycans are identical to oligosaccharides known from Leishmania mexicana promastigote lipophosphoglycan and secreted acid phosphatase. However, the majority of the aPPG glycans represent amastigote stage-specific and novel structures. These include neutral glycans ([Glcbeta1-3]1-2Galbeta1-4Man, Galbeta1-3Galbeta1-4Man, Galbeta1-3Glcbeta1-3Galbeta1-4Man), several monophosphorylated glycans containing the conserved phosphodisaccharide backbone (R-3-[PO4-6-Gal]beta1-4Man) but carrying stage-specific modifications (R = Galbeta1-, [Glcbeta1-3]1-2Glcbeta1-), and monophosphorylated aPPG tri- and tetrasaccharides that are uniquely phosphorylated on the terminal hexose (PO4-6-Glcbeta1-3Galbeta1-4Man, PO4-6-Glcbeta1-3Glcbeta1-3Galbeta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3Galbeta1-4Man). In addition aPPG contains highly unusual di- and triphosphorylated glycans whose major species are PO4-6-Glcbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-+ ++4Man, PO4-6-Glcbeta1-3[PO4-6-Glc]beta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3[PO4-6-Glc]beta1-3Glcbeta1-3[PO4-6-Gal]beta1 -4Man, and PO4-6-Glcbeta1-3[PO4-6-Glc]beta1-3Glcbeta1-3[PO4-6-Gal]beta1 -4Man. These glycans are linked together by the conserved phosphodiester R-Manalpha1-PO4-6-Gal-R or the novel phosphodiester R-Manalpha1-PO4-6-Glc-R and are connected to Ser(P) of the protein backbone most likely via the linkage R-Manalpha1-PO4-Ser. The variety of stage-specific glycan structures in Leishmania mexicana aPPG suggests the presence of developmentally regulated amastigote glycosyltransferases which may be potential anti-parasite drug targets.

Cutting Edge: Mice Defective in Fas Are Highly Susceptible to Leishmania major Infection Despite Elevated IL-12 Synthesis, Strong Th1 Responses, and Enhanced Nitric Oxide Production

MRL/MP-lpr/lpr (MRL/lpr) mice have a single mutation (lpr) of the fasapoptosis gene. The mutant mice developed significantly smaller lesions than the wild-type mice at the earlier stage of infection with the intracellular protozoan parasite Leishmania major. However, while all the wild-type mice achieved complete lesion resolution, the disease in the mutant mice progressed inexorably. The mutant mice had more IL-12 and nitrite/nitrate in the serum than wild-type mice following infection. Lymphoid cells from infected MRL/lpr mice produced more IFN-γ but less IL-4 and IL-5 than cells from MRL-+/+ mice. Peritoneal macrophages from the mutant mice also produced more IL-12 and NO after stimulation with LPS. Thus, Fas expression is essential for resistance against leishmaniasis, and Fas-mediated apoptosis may form an integral part of the Th1-mediated microbicidal function.

Trypanosoma cruzi: Tc52 Released Protein-Induced Increased Expression of Nitric Oxide Synthase and Nitric Oxide Production by Macrophages

Trypanosoma cruzi target molecules that might regulate the host immune responses have not yet been fully identified. In the present study, we demonstrate that the parasite-released molecule (Tc52) was able to synergize with IFN-γ to stimulate nitric oxide production by macrophages. This synergistic effect was also observed at the level of inducible nitric oxide synthase gene expression. Furthermore, Tc52 was also shown to induce gene expression for IL-1α, IL-12, and IL-10. Moreover, the combination of Tc52 and IFN-γ down-regulates IL-1α and IL-10 gene expression, but not IL-12. Isotype profiles and Tc52 or anti-CD3-induced T cell proliferation were also analyzed, indicating that active immunization with Tc52 partially relieves the immunosuppression observed during the acute phase of the disease. Moreover, under conditions of experimental infection, the Tc52 appears immunologically silent, failing to elicit Ab response and lymphocyte proliferation during the initial acute phase infection. Following active immunization, Tc52 was capable of stimulating T cell proliferation and Ab production with a predominance of IgG1, IgG2a, IgG2b, IgG3, and to a lesser extent IgA. Taken together, these results demonstrate that T. cruzi Tc52-released Ag could be involved in the immunoregulatory processes. The immune response against Tc52 that appears late in the T. cruzi infection may play a role in the modulation of its biological function(s).

In vivo formation of electron paramagnetic resonance-detectable nitric oxide and of nitrotyrosine is not impaired during murine leishmaniasis

Recent studies have provided evidence for a dual role of nitric oxide (NO) during murine leishmaniasis. To explore this problem, we monitored the formation of NO and its derived oxidants during the course of Leishmania amazonensis infection in tissues of susceptible (BALB/c) and relatively resistant (C57BL/6) mice. NO production was detected directly by low-temperature electron paramagnetic resonance spectra of animal tissues. Both mouse strains presented detectable levels of hemoglobin nitrosyl (HbNO) complexes and of heme nitrosyl and iron-dithiol-dinitrosyl complexes in the blood and footpad lesions, respectively. Estimation of the nitrosyl complex levels demonstrated that most of the NO is synthesized in the footpad lesions. In agreement, immunohistochemical analysis of the lesions demonstrated the presence of nitrotyrosine in proteins of macrophage vacuoles and parasites. Since macrophages lack myeloperoxidase, peroxynitrite is likely to be the nitrating NO metabolite produced during the infection. The levels of HbNO complexes in the blood reflected changes occurring during the infection such as those in parasite burden and lesion size. The maximum levels of HbNO complexes detected in the blood of susceptible mice were higher than those of C57BL/6 mice but occurred at late stages of infection and were accompanied by the presence of bacteria in the cutaneous lesions. The results indicate that the local production of NO is an important mechanism for the elimination of parasites if it occurs before the parasite burden becomes too high. From then on, elevated production of NO and derived oxidants aggravates the inflammatory process with the occurrence of a hypoxic environment that may favor secondary infections.

The surface glycoconjugates of trypanosomatid parasites

Insect-transmitted protozoan parasites of the order Kinetoplastida, suborder Trypanosomatina, include Trypanosoma brucei (aetiological agent of African sleeping sickness), Trypanosoma cruzi (aetiological agent of Chagas' disease in South and Central America) and Leishmania spp. (aetiological agents of a variety of diseases throughout the tropics and sub-tropics). The structures of the most abundant cell-surface molecules of these organisms is reviewed and correlated with the different modes of parasitism of the three groups of parasites. The major surface molecules are all glycosylphosphatidylinositol (GPI)-anchored glycoproteins, such as the variant surface glycoproteins of T. brucei and the surface mucins of T. cruzi, or complex glycophospholipids, such as the lipophosphoglycans and glycoinositolphospholipids of the leishmanias. Significantly, all of the aforementioned structures share a motif of Man alpha 1-4GlcN alpha 1-6-myo-inositol-1-HPO4-lipid and can therefore be considered to be members of a GPI superfamily.

Cytokines and nitric oxide as effector molecules against parasitic infections

Nitric oxide (NO) derived from L-arginine by the catalytic action of inducible NO synthase (iNOS) plays an important role in killing parasites. Many cell types express high levels of iNOS when activated by a number of immunological stimuli which include interferon-gamma (IFN-gamma), tumour necrosis factor alpha, and lipopolysaccharide. IFN-gamma is typically produced by the Th1 subject of CD4+ T cells, whose differentiation depends on interleukin-12 (IL-12) produced by macrophages. Mice with a disrupted iNOS gene were highly susceptible to Leishmania major infection compared with similarly infected control wild-type mice. The mutant mice developed significantly higher levels of TH1-cell response compared with the control mice, suggesting that NO is likely to be the effector molecule in the immunological control of this and other intracellular parasitic infections. To ensure their survival, the Leishmania parasites have evolved effective means to inhibit NO synthesis. The highly conserved major surface glycolipids, glycoinositol-phospholipids and lipophosphoglycan (LPG), of Leishmania are potent inhibitors of NO synthesis. Furthermore, LPG can also inhibit IL-12 synthesis, thereby indirectly blocking the induction of iNOS. The evolutionary and therapeutic implications of these findings are discussed.

Inhibition of phagolysosomal biogenesis by the Leishmania lipophosphoglycan

Whereas amastigotes of the protozoan parasite Leishmania proliferate inside acidic phagolysosomal vacuoles of the macrophage, vacuoles induced by Leishmania donovani promastigotes during initiation of infection are poorly characterized. Here, evidence is presented that interaction of these parasitophorous vacuoles with endocytic organelles is very limited. In contrast, vacuoles formed around L. donovani mutants lacking the cell surface lipophosphoglycan (LPG) fuse extensively with endosomes and lysosomes. The role of LPG repeating units in the inhibition of phagosome-endosome fusion was demonstrated using two different approaches. First, genetic complementation of the LPG-defective C3PO mutant restored its ability to inhibit phagosome-endosome fusion to a degree similar to that of wild-type promastigotes. Second, opsonization of C3PO mutant cells with purified L. donovani LPG also conferred to this mutant the ability to inhibit phagosome-endosome fusion. Inasmuch as LPG is essential for infecting macrophages, these results suggest that inhibition of phagolysosomal biogenesis by LPG repeating units represents an intramacrophage survival strategy used by promastigotes to establish infection.