Inhibition of protein kinase C activity by the Leishmania donovani lipophosphoglycan

The glycoprotein gp63- a potential pan drug target for developing new antileishmanial agents

Leishmaniasis is a tropical parasitic disease caused by Leishmania spp. They cause several presentations of illness ranging from cutaneous leishmaniasis to visceral leishmaniasis. The current arsenal of drugs to treat leishmaniasis is limited, and drug resistance further impedes the problem. Therefore, it is necessary to revisit the available information to identify an alternative or new target for treatment. The glycoprotein 63 (gp63), is a potential anti-leishmanial target that plays a significant role in host-pathogen interaction and virulence. Many studies are ongoing to develop gp63 inhibitors or use it as a vaccine target. In this review, we will discuss the potential of gp63 as a drug target. This review summarises the studies focusing on gp63 as a drug target and its inhibitors identified using in silico approaches.

Leishmania donovani infection suppresses Allograft Inflammatory Factor-1 in monocytes and macrophages to inhibit inflammatory responses

Macrophages and monocytes are important for clearance of Leishmania infections. However, immune evasion tactics employed by the parasite results in suppressed inflammatory responses, marked by deficient macrophage functions and increased accumulation of monocytes. This results in an ineffective ability to clear parasite loads. Allograft Inflammatory Factor-1 (AIF1) is expressed in myeloid cells and serves to promote immune responses. However, AIF1 involvement in monocyte and macrophage functions during parasitic infections has not been explored. This study now shows that Leishmania donovani inhibits AIF1 expression in macrophages to block pro-inflammatory responses. Mice challenged with the parasite had markedly reduced AIF1 expression in splenic macrophages. Follow-up studies using in vitro approaches confirmed that L. donovani infection in macrophages suppresses AIF1 expression, which correlated with reduction in pro-inflammatory cytokine production and increased parasite load. Ectopic overexpression of AIF1 in macrophages provided protection from infection, marked by robust pro-inflammatory cytokine production and efficient pathogen clearance. Further investigations found that inhibiting AIF1 expression in bone marrow cells or monocytes impaired differentiation into functional macrophages. Collectively, results show that AIF1 is a critical regulatory component governing monocyte and macrophage immune functions and that L. donovani infection can suppress the gene as an immune evasion tactic.

Lipophosphoglycans from dermotropic Leishmania infantum are more pro-inflammatory than those from viscerotropic strains

Although Leishmania infantum is well-known as the aethiological agent of visceral leishmaniasis (VL), in some Central American countries it may cause atypical non-ulcerated cutaneous leishmaniasis (NUCL). However, the mechanisms favoring its establishment in the skin are still unknown. Lipophosphoglycan (LPG) is the major Leishmania multivirulence factor involved in parasite-host interaction. In the case of viscerotropic L. infantum, it causes an immunosuppression during the interaction with macrophages. Here, we investigated the biochemical and functional roles of LPGs from four dermotropic L. infantum strains from Honduras during in vitro interaction with murine macrophages. LPGs were extracted, purified and their repeat units analysed. They did not have side chains consisting of Gal(β1,4)Man(α1)-PO4 common to all LPGs. Peritoneal macrophages from BALB/c and C57BL/6 were exposed to LPG for nitric oxide (NO) and cytokine (TNF-α and, IL-6) production. LPGs from dermotropic strains from Honduras triggered higher NO and cytokine levels compared to those from viscerotropic strains. In conclusion, LPGs from dermotropic strains are devoid of side-chains and exhibit high pro-inflammatory activity.

Protein glycosylation inLeishmaniaspp

Protein glycosylation is a co- and post-translational modification that, inLeishmaniaparasites, plays key roles in vector–parasite–vertebrate host interaction.

Glycoconjugates of Gram-negative bacteria and parasitic protozoa - are they similar in orchestrating the innate immune response?

Innate immunity is an evolutionarily ancient form of host defense that serves to limit infection. The invading microorganisms are detected by the innate immune system through germline-encoded PRRs. Different classes of PRRs, including TLRs and cytoplasmic receptors, recognize distinct microbial components known collectively as PAMPs. Ligation of PAMPs with receptors triggers intracellular signaling cascades, activating defense mechanisms. Despite the fact that Gram-negative bacteria and parasitic protozoa are phylogenetically distant organisms, they express glycoconjugates, namely bacterial LPS and protozoan GPI-anchored glycolipids, which share many structural and functional similarities. By activating/deactivating MAPK signaling and NF-κB, these ligands trigger general pro-/anti-inflammatory responses depending on the related patterns. They also use conservative strategies to subvert cell-autonomous defense systems of specialized immune cells. Signals triggered by Gram-negative bacteria and parasitic protozoa can interfere with host homeostasis and, depending on the type of microorganism, lead to hypersensitivity or silencing of the immune response. Activation of professional immune cells, through a ligand which triggers the opposite effect (antagonist versus agonist) appears to be a promising solution to restoring the immune balance.

Immune regulation during chronic visceral leishmaniasis

Visceral leishmaniasis is a chronic parasitic disease associated with severe immune dysfunction. Treatment options are limited to relatively toxic drugs, and there is no vaccine for humans available. Hence, there is an urgent need to better understand immune responses following infection with Leishmania species by studying animal models of disease and clinical samples from patients. Here, we review recent discoveries in these areas and highlight shortcomings in our knowledge that need to be addressed if better treatment options are to be developed and effective vaccines designed.

Impact of Leishmania metalloprotease GP63 on macrophage signaling

The intramacrophage protozoan parasites of Leishmania genus have developed sophisticated ways to subvert the innate immune response permitting their infection and propagation within the macrophages of the mammalian host. Several Leishmania virulence factors have been identified and found to be of importance for the development of leishmaniasis. However, recent findings are now further reinforcing the critical role played by the zinc-metalloprotease GP63 as a virulence factor that greatly influence host cell signaling mechanisms and related functions. GP63 has been found to be involved not only in the cleavage and degradation of various kinases and transcription factors, but also to be the major molecule modulating host negative regulatory mechanisms involving for instance protein tyrosine phosphatases (PTPs). Those latter being well recognized for their pivotal role in the regulation of a great number of signaling pathways. In this review article, we are providing a complete overview about the role of Leishmania GP63 in the mechanisms underlying the subversion of macrophage signaling and functions.

Secreted virulence factors and immune evasion in visceral leishmaniasis

Evasion or subversion of host immune responses is a well-established paradigm in infection with visceralizing leishmania. In this review, we summarize current findings supporting a model in which leishmania target host regulatory molecules and pathways, such as the PTP SHP-1 and the PI3K/Akt signaling cascade, to prevent effective macrophage activation. Furthermore, we describe how virulence factors, secreted by leishmania, interfere with macrophage intracellular signaling. Finally, we discuss mechanisms of secretion and provide evidence that leishmania use a remarkably adept, exosome-based secretion mechanism to export and deliver effector molecules to host cells. In addition to representing a novel mechanism for trafficking of virulence factors across membranes, recent findings indicate that leishmania exosomes may have potential as vaccine candidates.

Host cell signalling and leishmania mechanisms of evasion

Leishmania parasites are able to secure their survival and propagation within their host by altering signalling pathways involved in the ability of macrophages to kill pathogens or to engage adaptive immune system. An important step in this immune evasion process is the activation of host protein tyrosine phosphatase SHP-1 by Leishmania. SHP-1 has been shown to directly inactivate JAK2 and Erk1/2 and to play a role in the negative regulation of several transcription factors involved in macrophage activation. These signalling alterations contribute to the inactivation of critical macrophage functions (e.g., Nitric oxide, IL-12, and TNF-α). Additionally, to interfere with IFN-γ receptor signalling, Leishmania also alters several LPS-mediated responses. Recent findings from our laboratory revealed a pivotal role for SHP-1 in the inhibition of TLR-induced macrophage activation through binding to and inactivating IL-1-receptor-associated kinase 1 (IRAK-1). Furthermore, we identified the binding site as an evolutionarily conserved ITIM-like motif, which we named kinase tyrosine-based inhibitory motif (KTIM). Collectively, a better understanding of the evasion mechanisms utilized by Leishmania parasite could help to develop more efficient antileishmanial therapies in the near future.

Leishmania mexicana lipophosphoglycan differentially regulates PKCalpha-induced oxidative burst in macrophages of BALB/c and C57BL/6 mice

Leishmania are protozoan parasites that infect macrophages and their survival is partially achieved through inhibition of the cellular oxidative burst by parasite lipophosphoglycan (LPG). PKCalpha is the predominant PKC isoenzyme required for macrophage oxidative burst, yet it is not known if different susceptibility of BALB/c and C57BL/6 mice to Leishmania mexicana could be related to PKCalpha. We analysed the effect of L. mexicana promastigotes and parasite LPG on expression of PKCalpha and on its activity in macrophages of both mouse strains. Our data show that expression of the isoenzyme was not altered either by LPG or by L. mexicana promastigotes. Yet LPG exerted opposing effects on PKCalpha activity of macrophages between both strains: in susceptible BALB/c cells, it inhibited PKCalpha activity, whereas in the more resistant strain it augmented enzymatic activity 2.8 times. In addition, LPG inhibited oxidative burst only in susceptible BALB/c macrophages and the degree of inhibition correlated with parasite survival. Promastigotes also inhibited PKCalpha activity and oxidative burst in macrophages of BALB/c mice, whereas in C57BL/6, they enhanced PKCalpha activity and oxidative burst inhibition was less severe. Our data indicate that control of PKCalpha-induced oxidative burst by L. mexicana LPG relates with its success to infect murine macrophages.

Functional paradox in host–pathogen interaction dictates the fate of parasites

The interactions between the protozoan parasite Leishmania and host macrophages are complex and involve several paradoxical functions that are meant for protection of the host but exploited by the parasite for its survival. The initial interaction of the parasite surface molecules with the host-cell receptors plays a major role in the final outcome of the disease state. While the interactions between macrophages and a virulent strain of Leishmania trigger a cascade of cell-signaling events leading to immunosuppression, the interaction with an avirulent strain triggers host-protective immune effector functions. Thus, an incisive study on Leishmania–macrophage interactions reveals functional paradoxes that highlight the concept of ‘relativity in parasite virulence’. Using Leishmania infection as a model, we propose that virulence of a pathogen and the resistance (or susceptibility) of a host to the pathogen are relative properties that equate to combinatorial functions of several sets of molecular processes.

Subversion of host cell signalling by the protozoan parasite Leishmania

The protozoa Leishmania spp. are obligate intracellular parasites that inhabit the macrophages of their host. Since macrophages are specialized for the identification and destruction of invading pathogens, both directly and by triggering an innate immune response, Leishmania have evolved a number of mechanisms for suppressing some critical macrophage activities. In this review, we discuss how various species of Leishmania distort the host macrophage's own signalling pathways to repress the expression of various cytokines and microbicidal molecules (nitric oxide and reactive oxygen species), and antigen presentation. In particular, we describe how MAP Kinase and JAK/STAT cascades are repressed, and intracellular Ca2+ and the activities of protein tyrosine phosphatases, in particular SHP-1, are elevated.

Subversion mechanisms by which Leishmania parasites can escape the host immune response: a signaling point of view

The obligate intracellular parasite Leishmania must survive the antimicrobial activities of its host cell, the macrophage, and prevent activation of an effective immune response. In order to do this, it has developed numerous highly successful strategies for manipulating activities, including antigen presentation, nitric oxide and oxygen radical generation, and cytokine production. This is generally the result of interactions between Leishmania cell surface molecules, particularly gp63 and LPG, and less well identified macrophage surface receptors, causing the distortion of specific intracellular signaling cascades. We describe some of the signaling pathways and intermediates that are repressed in infected cells, including JAK/STAT, Ca(2+)-dependent protein kinase C (PKC) isoforms, and mitogen-activated protein kinases (especially ERK1/2), and proteasome-mediated transcription factor degradation. We also discuss protein tyrosine phosphatases (particularly SHP-1), intracellular Ca2+, Ca(2+)-independent PKC, ceramide, and the suppressors of cytokine signaling family of repressors, which are all reported to be activated following infection, and the role of parasite-secreted cysteine proteases.

Modulation of phagolysosome biogenesis by the lipophosphoglycan of Leishmania

Promastigotes of the protozoan parasite Leishmania are inoculated into the mammalian host by an infected sandfly and are phagocytosed by macrophages. There, they differentiate into amastigotes, which replicate in phagolysosomes. A family of glycoconjugates, the phosphoglycans (PGs), plays an important role in the ability of promastigotes to survive the potentially microbicidal consequences of phagocytosis. Lipophosphoglycan (LPG), an abundant promastigote surface glycolipid, has received considerable attention over the past several years. Of interest for this review, lipophosphoglycan confers upon Leishmania donovani promastigotes the ability to inhibit phagolysosome biogenesis. This inhibition correlates with an accumulation of periphagosomal F-actin, which may potentially form a physical barrier that prevents L. donovani promastigote-harboring phagosomes from interacting with late endosomes and lysosomes. Thus, similar to several other pathogens, Leishmania promastigotes hijack the host cell's cytoskeleton early during the infection process. Here, we review this phenomenon and discuss the potential underlying mechanisms.

Activation of p38 mitogen-activated protein kinase attenuates Leishmania donovani infection in macrophages

Leishmania-induced macrophage dysfunctions have been correlated with altered signaling events. In this work, we report that SB203580, a specific inhibitor of p38 mitogen-activated protein kinases (MAPK), increases Leishmania donovani survival in human peripheral blood mononuclear macrophages. Consistent with this finding, activation of p38 and c-jun N-terminal kinase (JNK) MAPK signaling pathways by anisomycin significantly reduced parasite survival within these cells. However, the majority of the effect was seen in a 50% reduction in the percentage of macrophages infected, with little effect on the highly infected macrophages. The observed effect was likely to be due to the p38 MAPK pathway since SB203580 was able to completely reverse the effect of anisomycin. These findings suggest that the previously reported p38 MAPK inhibition by Leishmania infection may be partially overcome by anisomycin. Similar effects were observed in pretreated macrophages or in treatment of infected macrophages. These results suggests that p38 MAPK activation may have a potential therapeutic value in the treatment of visceral leishmaniasis.

Functional aspects of the Leishmania donovani lipophosphoglycan during macrophage infection

The most abundant surface glycoconjugate of the Leishmania promastigotes is lipophosphoglycan, a glycosylphosphatidyl-inositol-anchored polymer of the repeating disaccharide-phosphate Gal(beta1,4)Manalpha1-PO4 unit. This complex molecule possesses properties that contribute to the ability of Leishmania to modulate macrophage signaling pathways during the initiation of infection.

Parasitic adaptive mechanisms in infection by leishmania

Leishmania are a resilient group of intracellular parasites that infect macrophages. The resultant complex of diseases, or leishmaniases, caused by the parasites affect over twelve million people worldwide. Leishmania have developed unique adaptive mechanisms to ensure their survival in the harsh environments faced throughout their life cycle. These parasites must not only contend with the hostile digestive conditions found within the sand fly vector, but they must also avoid destruction by the host immune system while in the bloodstream, before entering the macrophage. To do so, Leishmania express unique lipophosphoglycan (LPG) molecules and the metalloprotease gp63, among other proteins, on their cell surface. To enter the macrophage, Leishmania utilizes a variety of cellular receptors to mediate endocytosis. Once inside the macrophage, Leishmania is protected from phagolysosome degradation by a variety of adaptations to inhibit cellular defense mechanisms. These include the inhibition of phagosome-endosome fusion, hydrolytic enzymes, cell signaling pathways, nitric oxide production, and cytokine production. While other parasites can also infect macrophages, Leishmania is distinctive in that it not only relies on its own defenses to survive and reproduce within the macrophage phagolysosome, but Leishmania also manipulates the host immune response in order to protect itself and to gain entry into the cell. These unique adaptive mechanisms help promote Leishmania survival.

Effect of Leishmania donovani lipophosphoglycan on ornithine decarboxylase activity in macrophages

Lipophosphoglycan (LPG), a major surface molecule from Leishmania donovani, stimulated ornithine decarboxylase (ODC) activity in macrophages in a dose- and time-dependent manner. LPG stimulated the rapid increase in ODC activity within 30 min after exposure, suggesting that the interaction of LPG with its receptor stimulated a specific signal transduction pathway. However, LPG-induced ODC activity was a transient event because 3 hr after exposure to LPG, no stimulation of ODC activity was detectable. ODC activity appeared to be coupled to the activation of protein kinase C (PKC) in macrophages, as activators of PKC caused a rapid increase in the ODC activity. Macrophages pretreated with LPG for 1 hr became unresponsive to subsequent stimulation by the PKC activators 1-oleoyl-2-acetyl-glycerol and the calcium ionophore A23187. In contrast, the ability of macrophages to express ODC activity in response to the cyclic AMP analogue dibutyryl cyclic AMP was not impaired by LPG.

Inhibition of HIV-1-mediated syncytium formation and virus replication by the lipophosphoglycan from Leishmania donovani is due to an effect on early events in the virus life cycle

Previous findings have indicated that the major surface molecule of Leishmania, lipophosphoglycan (LPG), could abrogate HIV-1-induced syncytium formation and virus replication. In the present work, we were interested in characterizing this inhibitory process. Data from a new luciferase-based semiquantitative assay for syncytium formation, relying on the coincubation of a T-cell line containing an HIV-1 LTR-driven luciferase construct with a cell line chronically infected with HIV-1, confirmed that LPG was indeed a strong inhibitor of HIV-1-dependent syncytium formation and that this inhibition was dose-dependent. As determined by flow cytometric analyses, this inhibition was not apparently due to downregulation of CD4, CXCR4 or LFA-1, three distinct surface glycoproteins known to be important in HIV-1 mediated syncytium formation. Furthermore, LPG did not seem to affect signal transduction pathways in T cells as judged by measurement of HIV-1 LTR-driven reporter gene activity upon treatment with different stimuli. However, pretreatment of either of the cell lines used in the assay with LPG led to a significant decrease of virus-mediated syncytium formation, which was further accentuated when both cell lines were pretreated. LPG inhibition of HIV-1 replication was next assessed. When measuring either infection with luciferase-encoding recombinant HIV-1 particles or multinucleated giant cell formation following an acute virus infection, we again observed that LPG was efficient at blocking HIV-1 replication. Specific assays probing different steps of viral entry demonstrated that attachment was not hindered by LPG but that viral entry was modulated, suggesting that LPG targets a postbinding step. Hence, incorporation of LPG into a target cell membrane could influence its fluidity and diminish both the virus-cell and cell-to-cell fusion processes initiated by HIV-1.

Leishmania parasites and their ploys to disrupt macrophage activation

Leishmania are intracellular protozoan parasites of macrophages. At the cellular level, the disease leishmaniasis involves the invasion of tissue macrophages by the parasite, the avoidance of cellular killing mechanisms, and the subsequent intracellular replication of parasites, with the eventual spread of the organisms to adjacent macrophages. This paper describes the process by which Leishmania organisms invade macrophages, with an overview of some of the molecules involved in this process; the mechanisms available to macrophages that have the potential to restrict the growth of Leishmania within them; and the ways that Leishmania and Leishmania-derived molecules can modulate macrophage functions and circumvent leukocyte antimicrobial responses.

Specificity in signal transduction among glycosylphosphatidylinositols of Plasmodium falciparum, Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp

Glycosylphosphatidylinositols (GPIs) and related glycoconjugates of parasite origin have been shown to regulate both the innate and acquired immune systems of the host. This is achieved through the activation of novel GPI-dependent signalling pathways in macrophages, lymphocytes and other cell types. Parasite GPIs impart at least two distinct signals to host cells through the structurally distinct inositolphosphoglycan (IPG) and fatty acid domains. Binding of IPG to as yet uncharacterized cell surface receptor(s) leads to activation of src-family protein tyrosine kinases: depending upon structure, GPI-derived fatty acids can either activate or antagonize protein kinase C, and may enter the sphingomyelinase pathway. The degree of fatty acid saturation may also contribute to signalling activity. Thus, variation in structure of parasite GPIs imparts different properties of signal transduction upon this class of glycolipid. The divergent activities of GPIs from various protozoal taxa reflect global aspects of the host/parasite relationship, suggesting that GPI signalling is a central determinant of disease in malaria, leishmaniasis and both American and African trypanosomiases.

Cell biology of Leishmania

Leishmania are digenetic protozoa which inhabit two highly specific hosts, the sandfly, where they grow as motile flagellated promastigotes in the gut, and the mammalian macrophage, where they survive and grow intracellularly as non-flagellated amastigotes in the phagolysosome. Leishmaniasis is the outcome of an evolutionary 'arms race' between the host's immune system and the parasite's evasion mechanisms, which ensure survival and transmission in the population. The diverse spectrum of patterns and severity of disease reflect the varying contributions of parasite virulence factors and host responses, some of which act in a host protective manner while others exacerbate disease. This chapter describes the interaction of the Leishmania with their hosts, with emphasis on the molecules and mechanisms evolved by the parasites to avoid, subvert or exploit the environments in the sandfly and the macrophage, and to move from one to the other.

Role of Protein Kinase C-α in the Control of Infection by Intracellular Pathogens in Macrophages

The protein kinase C (PKC) family regulates macrophage function involved in host defense against infection. In this study, we investigated the role of macrophage PKC-α in the uptake and subsequent fate of Leishmania donovani promastigotes and Legionella pneumophila infections. To this end, we used clones of the murine macrophage cell line RAW 264.7 overexpressing a dominant-negative (DN) mutant of PKC-α. While phagocytosis of L. donovani promastigotes was not affected by DN PKC-α overexpression, their intracellular survival was enhanced by 10- to 20-fold at 48 h postinfection. Intracellular survival of a L. donovani mutant defective in lipophosphoglycan repeating units synthesis, which normally is rapidly degraded in phagolysosomes, was enhanced by 100-fold at 48 h postinfection. However, IFN-γ-induced leishmanicidal activity was not affected by DN PKC-α overexpression. Similar to macrophages from genetically resistant C57BL/6 mice, control RAW 264.7 cells were not permissive for the intracellular replication of Legionella pneumophila. In contrast, DN PKC-α-overexpressing RAW 264.7 clones were phenotypically similar to macrophages from genetically susceptible A/J mice, as they allowed intracellular replication of L. pneumophila. Permissiveness to L. pneumophila was not the consequence of a general defect in the microbicidal capacities because killing of a temperature-sensitive mutant of Pseudomonas aeruginosa was normal in DN PKC-α-overexpressing RAW 264.7 clones. Collectively, these results support a role for PKC-α in the regulation of innate macrophage functions involved in the control of infection by intracellular parasites.

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.

Inhibition of host cell signal transduction by Leishmania: observations relevant to the selective impairment of IL-12 responses

Leishmania parasites are able to delay the onset of cell-mediated immunity by selectively impairing the ability of infected macrophages to produce interleukin (IL)-12. Leishmania infection arrests the JAK/STAT-mediated signal transduction involved in activation of the IL-12 p40 promoter; the phosphorylation defects may be initiated by ligation of the phagocyte receptors used by these organisms to gain entry into the host cell.

Down-regulation of MARCKS-related protein (MRP) in macrophages infected with Leishmania

Leishmania, a protozoan parasite of macrophages, has been shown to interfere with host cell signal transduction pathways including protein kinase C (PKC)-dependent signaling. Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP, MacMARCKS) are PKC substrates in diverse cell types. MARCKS and MRP are thought to regulate the actin network and thereby participate in cellular responses involving cytoskeletal rearrangement. Because MRP is a major PKC substrate in macrophages, we examined its expression in response to infection by Leishmania. Activation of murine macrophages by cytokines increased MRP expression as determined by Western blot analysis. Infection with Leishmania promastigotes at the time of activation or up to 48 h postactivation strongly decreased MRP levels. Leishmania-dependent MRP depletion was confirmed by [3H]myristate labeling and by immunofluorescence microscopy. All species or strains of Leishmania parasites tested, including lipophosphoglycan-deficient Leishmania major L119, decreased MRP levels. MRP depletion was not obtained with other phagocytic stimuli including zymosan, latex beads, or heat-killed Streptococcus mitis, a Gram-positive bacterium. Experiments with [3H]myristate labeled proteins revealed the appearance of lower molecular weight fragments in Leishmania-infected cells suggesting that MRP depletion may be due to proteolytic degradation.

Group B streptococci persist inside macrophages

Group B streptococci (GBS) are an important cause of neonatal sepsis, pneumonia and meningitis. In the early phase of infection, macrophages and polymorphonuclear cells (PMN) are the first immune cells that interact with GBS. In this in vitro study, to gain insight into GBS-macrophage interaction in the absence of type-specific antibodies, we examined the features of GBS survival in thioglycollate-elicited murine peritoneal macrophages and the effect of GBS on the protein kinase C (PKC)-dependent transduction pathway. Our results demonstrate that type Ia GBS, strain 090 (GBS-Ia) and type III GBS strain COH 31r/s (GBS-III), after in vitro phagocytosis survive and persist intracellularly in macrophages for up to 24 and 48 hr, respectively. However, macrophage activation by interferon-gamma (IFN-gamma) and lipopolysaccharide from Escherichia coli (LPS) caused a significant reduction in the time of intracellular persistence. Macrophage activation by IFN-gamma and LPS seems to be a multifactorial event involving multiple intracellular signal pathways also including PKC. Since PKC is one of the components in the signal network leading to macrophage activation and an important target for several intracellular micro-organisms, we wondered whether PKC could have a role in intracellular GBS survival. Both PKC depletion by treatment with phorbol 12-myristate 13-acetate (PMA) for 18 hr and PKC inhibition by Calphostin C rendered macrophages more permissive for the intracellular GBS survival. Furthermore, GBS-infected macrophages were unable to respond to PMA and LPS, activators of PKC, by inducing antimicrobial activity. The ability of GBS to impair PKC-dependent cell signalling was also demonstrated by the reduced c-fos gene expression in GBS-infected macrophages with respect to control macrophages, after LPS stimulation. In conclusion, our results indicate that GBS survive in macrophages and impairment of PKC signal transduction contributes to their intracellular survival.

Regulation of host cell function by glycosylphosphatidylinositols of the parasitic protozoa

Antigenic variation, antigenic drift, molecular mimicry, intracellular localization and sequestration in privileged sites are important mechanisms of immune evasion by infectious organisms. Added to this however is the phenomenon by which pathogens deliberately regulate host cell function by the production of glycolipids with agonistic or antagonistic signal transduction capacity. Such pro-active glycolipids are often pathogenicity factors, but they also serve as immunomodulators and immunosuppressants, and these activities may serve as mechanisms of immune evasion. Here we review glycosylphosphatidylinositols and related structures, a novel class of glycolipid common to eukaryotic parasites and their hosts, which recent studies suggest may play a role in immune evasion and immunosuppression by regulating host cell function via the activation or suppression of endogenous host signalling pathways.

Transbilayer inhibition of protein kinase C by the lipophosphoglycan from Leishmania donovani

Lipophosphoglycan (LPG), the predominant molecule on the surface of the parasite Leishmania donovani, has previously been shown to be a potent inhibitor of protein kinase C (PKC) isolated from rat brain. The mechanism by which LPG inhibits PKC was further investigated in this study. LPG was found to inhibit the PKC alpha-catalyzed phosphorylation of histone in assays using large unilamellar vesicles composed of 1-palmitoyl, 2-oleoyl phosphatidylserine and 1-palmitoyl, 2-oleoyl phosphatidylcholine either with or without 1% 1,2 diolein added. The results also indicated that while PKC binding to sucrose-loaded vesicles was not substantially reduced in the presence of LPG at concentrations of 1-2%, the activity of membrane-bound PKC was inhibited by 70%. This inhibition of the membrane-bound form of PKC is not a consequence of reduced substrate availability to the membrane. However, Km shifted from approximately 31 +/- 4 microM to 105 +/- 26 microM in the presence of 5% LPG. LPG caused PKC to bind to membranes without inducing a conformational change as revealed by the lack of an increased susceptibility to trypsin. An LPG fragment containing only one repeating disaccharide unit was not as effective as the entire LPG molecule or of larger fragments in inhibiting the membrane-bound form of the enzyme. The shorter fragments were also less potent in raising the bilayer to hexagonal phase transition temperature of a model membrane. LPG is also able to inhibit the membrane-bound form of PKC alpha from the inner monolayer of large unilamellar vesicles, the opposite monolayer to which the enzyme binds in our assay. Inhibition is likely a result of alterations in the physical properties of the membrane. To our knowledge, this is the first example of a membrane additive that can inhibit the membrane-bound form of PKC in the presence of other lipid cofactors.

Antioxidant defense mechanisms in parasitic protozoa

Many of the parasitic protozoa, such as Entamoeba histolytica, Giardia, Trypanosoma, Leishmania, and Plasmodium, are considered to be anaerobes because they can be grown in vitro only under conditions of reduced oxygen tension. However, these parasitic protozoa have been found to be aerotolerant or microaerophilic, and also to consume oxygen to a certain extent. Furthermore, these organisms are highly susceptible to exogenous reactive oxygen species, such as hydrogen peroxide. They must, therefore, detoxify both oxygen and free radical products of enzymatic reactions. However, they lack some or all of the usual antioxidant defense mechanisms present in aerobic or other aerotolerant cells, such as catalase, superoxide dismutase, reduced glutathione, and the glutathione-recycling enzymes glutathione peroxidase and glutathione reductase. Instead, they possess alternative mechanisms for detoxification similar to those known to exist in certain prokaryotes. Although the functional aspects of these alternative mechanisms are yet to be understood completely, they could provide new insights into the biochemical peculiarities of these enigmatic pathogens.

Inhibition of Mononuclear Cell Procoagulant Activity by Lipophosphoglycan of Leishmania donovani

BACKGROUND: Since fibrin formation is an expression of the response of the host to parasite spread, the lipophosphoglycan (LPG) of Leishmania donovani and its carbohydrate fragment (PG) were examined for their capacity to inhibit procoagulant activity (PCA) production by human mononuclear cells stimulated with Escherichia coli endotoxin in vitro. METHODS: the putative inhibitory effect of LPG and its PG fragment was evaluated on the basis of their in vitro capacity to prolong significantly the time required for coagulation induced by endotoxin-stimulated mononuclear cells. RESULTS: LPG exhibited the most inhibitory activity, whereas the carbohydrate domain was not effective. These results are in agreement with the notion that LPG (but not PG) has an inhibitory effect on protein kinase C activity which plays a key role in the production of PCA by human monocytes. CONCLUSIONS: From a pathophysiological point of view, these data suggest the possibility that Leishmania avoids fibrin entrapment in the host through this inhibitory mechanism.

Suppression of chemiluminescence of eastern oyster (Crassostrea virginica) hemocytes by the protozoan parasite Perkinsus marinus

Experiments were conducted to determine the ability of the protistan parasite, Perkinsus marinus, to inhibit chemiluminescence of hemocytes from the eastern oyster, Crassostrea virginica. Luminol-enhanced chemiluminescence (CL) was used to measure the production of reactive oxygen intermediates (ROI) generated by oyster hemocytes using zymosan as a stimulant. To determine whether P. marinus suppresses ROI evoked from zymosan-stimulated hemocytes, live or heat killed P. marinus in filtered estuarine water (YRW) (salinity = 20 ppt) were added to (1) zymosan-stimulated hemocytes after CL reached its peak, or (2) hemocytes at the same time as zymosan, and reduction of CL responses were recorded. In both tests, controls received only estuarine water. Live P. marinus meronts significantly suppressed ROI production by zymosan-stimulated hemocytes. The suppression of ROI production was dose dependent. Suppression of ROI production from zymosan-stimulated hemocytes by heat killed P. marinus was significantly less than by live P. marinus. Similarly, CL of hemocytes was reduced, though not significantly when hemocytes were exposed to YRW preincubated with P. marinus. When P. marinus meronts were used as a stimulant, no CL response was elicited. Results of this study suggest that P. marinus cells are able to suppress ROI release from oyster hemocytes, thus evading this component of the host's defense.

Response of Leishmania chagasi promastigotes to oxidant stress

At the onset of infection, Leishmania promastigotes are phagocytized by mammalian macrophages. They must survive despite exposure to toxic oxidants such as hydrogen peroxide (H2O2) and superoxide (.O2-) generated during phagocytosis. We investigated the effects of these oxidants on Leishmania chagasi promastigotes and promastigote mechanisms for oxidant resistance. According to spin trapping and electron paramagnetic resonance spectrometry, .O2- could be generated by exposure of promastigotes to the redox-cycling compound menadione. Incubation in either menadione or H2O2 caused a concentration-dependent loss of promastigote viability. However, incubation in sublethal concentrations of H2O2 or menadione caused a stress response in promastigotes. This oxidant-induced response was associated with an increase in the amount of heat shock protein hsp70. Induction of a stress response by exposure of promastigotes either to heat shock or to sublethal oxidants (H2O2 or menadione) caused promastigotes to become more resistant to H2O2 toxicity. Sublethal menadione also caused promastigotes to become more virulent in a BALB/c mouse model of leishmaniasis. We previously correlated H2O2 cytotoxicity for promastigotes with the formation of hydroxyl radical (.OH) from H2O2. However, according to electron paramagnetic resonance spectrometry, the increase in H2O2 resistance after exposure to sublethal oxidants was not associated with diminished generation (i.e., scavenging) of .OH. These data suggest that there is a cross-protective stress response that occurs after exposure of L. chagasi promastigotes to heat shock or to sublethal H2O2 or .O2-, exposures that also occur during natural infection. This response results in increased resistance to H2O2 toxicity and increased virulence for a mammalian host.

Solution structure and dynamics of a glycoinositol phospholipid (GIPL-6) from Leishmania major

By use of a combination of 1H nuclear Overhauser effect measurements, restrained molecular dynamics simulations, and 13C spin-lattice relaxation time measurements, the solution behavior of the glycan moiety of a complex glycoinositol phospholipid termed GIPL-6, from the protozoan parasite Leishmania major has been determined. The glycan moiety of GIPL-6 has the following structure, which is characterized by the presence of an internal beta-galactofuranose residue: [formula: see text] The glycan does not adopt a single conformation in solution, due to significant torsional variations about the two phosphodiester linkages and certain glycosidic linkages in the molecule. The presence of the internal galactofuranose residue results in an average solution conformation of the oligosaccharide, which resembles a "hairpin," with the galactofuranose residue at the apex.

Cytokines in the treatment of leishmaniasis: from studies of immunopathology to patient therapy

The genus Leishmania, an obligate intramacrophage parasite, causes a wide spectrum of clinical diseases. It is worldwide in distribution and causes 20 million new cases annually with an at risk population of approximately 1.5 billion persons. The most severe forms are associated with high morbidity, mortality and relapses with conventional therapy. The therapeutic issues and responses to standard and alternative therapies are reviewed. Recent developments in molecular biology and immunology methods employed in the study of leishmaniasis have defined an intricate interaction of the parasite with host immune system. Perturbation of the host immune responses may be part of the survival mechanisms of Leishmania. In murine model, the finding of T helper cells that differ by their panel of cytokines has allowed a more precise definition of immunopathogenesis of leishmaniasis. Preliminary data from leishmaniasis patients lend support to this concept of altered immunomodulation. Furthermore, the data from leishmaniasis patients lend support to this concept of altered enhancement of therapeutic response by interferon-gamma has provided a new approach for treatment of patients using recombinant cytokines and for the study of the disease. Current research for early diagnosis, alternative therapies and need for vaccines are reviewed in the context of the immunopathology of leishmaniasis.

Impairment of the human phagocyte oxidative responses caused by Leishmania lipophosphoglycan (LPG): in vitro studies

Lipophosphoglycan (LPG), a surface glycoconjugate of Leishmania promastigotes, has been reported as playing an active role in protecting the parasite within phagolysosomes, by an impairment of monocyte oxidative responses. In this study the effect of LPG on the oxidative burst of human peripheral monocytes, eosinophils and neutrophils was evaluated. Our results demonstrated that either superoxide anion (O2-) or hydrogen peroxide (H2O2) release by LPG-pretreated cells was diminished, emphasizing the ability of this glycoconjugate to impair the oxidative activity of all phagocytes.

The lipophosphoglycan of Leishmania and macrophage protein kinase C

Lipophosphoglycan (LPG), the major cellsurface glycoconjugate of Leishmania promastigotes, is on essential virulence determinant. One feature of LPG resides in its strong inhibitory effect on the activity of purified protein kinase C (PKC). In this article, Albert Descoteaux and Salvatore Turco briefly review the evidence that LPG effectively inhibits PKC activity in the macrophage, and discuss the implication of such inhibition on Leishmania intramacrophoge survival.

The structure, biosynthesis and function of glycosylated phosphatidylinositols in the parasitic protozoa and higher eukaryotes

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Leishmania major and Leishmania donovani: effect of LPG-containing and LPG-deficient strains on monocyte chemotaxis and chemiluminescence

Lipophosphoglycan (LPG) is a major glycolipid present on the membrane of Leishmania promastigotes and amastigotes. We have previously shown that preincubation of peripheral blood monocytes with purified LPG inhibits IL-1 production, chemotactic locomotion, and luminol-dependent chemiluminescence (LDCL). In the present study we tested the effect of LPG present on live parasites on monocyte activity. For this purpose, we used two mutant strains deficient in LPG and two LPG-containing strains. One pair was Leishmania major and the other Leishmania donovani. Monocytes in suspension were infected with the different parasite strains and tested for chemotactic locomotion and LDCL at different times between 1 and 72 hr after infection. In parallel, the percentage of infected monocytes was measured in stained cytospin preparations. The results obtained showed that at 1 hr of incubation only the LPG-containing strains inhibited chemotaxis, while the mutant strains showed a normal response. From 4 hr of incubation onwards, the mutant strains also inhibited monocyte chemotactic locomotion. LDCL was only slightly inhibited by the LPG-containing strains after 1 hr, because of a high level of spontaneous stimulation, probably due to phagocytosis. At 24 and 72 hr all strains inhibited LDCL. These results suggest that LPG is responsible for early inhibition of macrophage activity, but that other factors are responsible for inhibition at later stages of in vitro infection. The model described here might represent a useful tool to further analyze the mechanisms involved in immune evasion of Leishmania parasites.

Potencies of protein kinase C inhibitors are dependent on the activators used to stimulate the enzyme

The aim was to examine systematically the potencies of protein kinase C inhibitors as a function of the kinase activator. Protein kinase C is activated by at least four stimulators: calcium plus phosphatidylserine (Ca/PS), phorbol 12-myristate 13-acetate plus PS (PS/PMA), arachidonic acid plus calcium (Ca/AA) and the synthetic peptide activator PCK530-558. With histone or GS1-12 as substrates, protein kinase C was maximally activated by Ca/PS, or to maxima of 62%, 89% or 82% with PS/PMA, Ca/AA or PKC530-558, respectively. One group of inhibitors, including H-7 and staurosporine, were equipotent, regardless of the activator. All other inhibitors showed variable selectivity, dependent upon the activator. A second group of inhibitors, including sphingosine and lipophosphoglycan, were eight or 200 times more potent for inhibition of PS/PMA-stimulated activity (relative to Ca/PS) and a third group, including retinal and palmitoylcarnitine, were 14 or 262 times more potent towards Ca/PS-stimulated activity. A final group (rhodamine 6G) was nine times more potent when Ca/AA was the activator. Similar results were obtained using the endogenous substrates dephosphin or MARCKS in synaptosol. Phosphorylation of MARCKS was stimulated by PS/PMA or Ca/PS, while phosphorylation of dephosphin was stimulated only by Ca/PS. The phosphorylation of either by Ca/PS-activated kinase was nine times more potently inhibited by palmitoylcarnitine, while phosphorylation of MARCKS by PS/PMA-activated kinase was 10 times more potently inhibited by sphingosine. H-7 inhibited both at similar concentrations. A model encompasses these differences in potency if the inhibitors are divided into four groups (A-D) according to their competitive inhibition with the appropriate activator or at the active site. The non-selective inhibitors interact at the active sites of protein kinase C (group A). The compounds which preferentially inhibit PS/PMA-activated kinase (sphingosine and lipophosphoglycan) are competitive inhibitors of PMA and 1,2-diacylglycerol (group B), those selective for Ca/PS-activated kinase (palmitoylcarnitine and retinal) are competitive with PS (group C) and those selective for Ca-AA activation (rhodamine 6G) are likely to be competitive with fatty acid (group D). Therefore, the effectiveness of protein kinase C inhibitors is dependent upon the activator employed.

Inhibition of murine macrophage protein kinase C activity by nonviable Histoplasma capsulatum

Ingestion of Histoplasma capsulatum yeast cells inhibits the oxidative burst response of murine macrophages (M phi's). Since protein kinase C (PKC) is believed to be an integral part of the signal transduction pathway involved in the production of reactive oxygen intermediates, we investigated the relationship between PKC activity and oxidative burst inhibition in H. capsulatum-containing murine peritoneal M phi's. An inhibitory effect on both basal and phorbol myristate acetate-mobilized membrane PKC activities was observed in M phi's which had ingested H. capsulatum but not latex spheres. These results suggest that one way in which H. capsulatum may disrupt the oxidative burst is through a PCK-dependent mechanism.

Leishmaniasis epidemiology: all down to the DNA

Application of quantitative methods to the study of leishmaniasis epidemiology has allowed Dye (1992) to pinpoint important biological parameters which, if they could be accurately measured in the field, would contribute most to our knowledge of the spread of disease and key targets for control. Three areas in which laboratory-based research could impact most on leishmaniasis epidemiology were highlighted by Dye (1992): (i) the development of accurate diagnostic tools which can distinguish between current and past infection; (ii) to determine the underlying molecular/genetic basis to virulence polymorphisms in the parasite and study these in the context of field epidemiological studies; and (iii) to provide the molecular tools to measure genetic variation in resistance to infection in humans and in reservoir hosts of disease. This paper describes current progress in attaining these goals, highlighting first the work on isolation and field application of genomic and kDNA probes for species-specific diagnosis, and the development of PCR-based assays which can be performed under field conditions. At a more preliminary stage, studies are described in which variability in the major molecular determinants of virulence (lipophosphoglycan, GP63, and members of the HSP70 family of stress proteins) identified through studies of laboratory models of infection, is being measured in primary field isolates of Leishmania peruviana. To complete the picture, current progress in identifying and cloning the genes which control host resistance to leishmanial infection is described, along with field studies of multicase families of human disease in which linkage analysis using marker genes from the chromosomal regions bearing these genes can be used to find evidence for their role in determining disease phenotypes in man. The projected view from these studies is that the future of leishmaniasis epidemiology will be all down to the DNA.