Down-regulation of mannose receptors on macrophages after infection with Leishmania donovani

Human Mannose Receptor 1 Attenuates HIV-1 Infectivity in a Virus Isolate-Specific Manner

Human mannose receptor 1 (hMRC1) is a transmembrane glycoprotein that belongs to the C-type lectin family and is expressed on the surface of most tissue macrophages. hMRC1 contributes to the binding and transmission of HIV-1 and is involved in the endocytic uptake of HIV-1 for subsequent antigen presentation. We previously reported that hMRC1 functions as an antiviral factor by inhibiting virus release through a BST-2-like mechanism. The inhibition of virus release was not virus isolate-specific and, surprisingly, was not Env-dependent. We now report on another hMRC1 antiviral function that affects the infectivity of viral particles. Unlike its effect on virus release, the inhibition of viral infectivity by hMRC1 was virus isolate-specific. An analysis of chimeric Env revealed that the Env V3 region was a critical determinant for the inhibitory effect of hMRC1. Of note, exogenously expressed hMRC1 was packaged into viral particles in an Env-independent manner. Co-immunoprecipitation studies revealed a strong interaction of the hMRC1-sensitive NL43 Env with hMRC1, while the hMRC1-insensitive Envs of AD8 and 49.5 isolates interacted poorly if at all with hMRC1. An analysis of a panel of Transmitted/Founder (T/F) viruses revealed that all of them were R5-tropic, and more than half of them were inhibited by hMRC1. The detailed mechanism of how hMRC1 inhibits viral infectivity remains to be investigated. However, the high-affinity binding of hMRC1 to Env may cause a conformational change around the Env V3 region or obstruct the Env V3 region and may make it inaccessible for subsequent interaction with the coreceptor during virus entry.

SILAC-based quantitative proteomics reveals pleiotropic, phenotypic modulation in primary murine macrophages infected with the protozoan pathogen Leishmania donovani

Leishmaniases are major vector-borne tropical diseases responsible for great human morbidity and mortality, caused by protozoan, trypanosomatid parasites of the genus Leishmania. In the mammalian host, parasites survive and multiply within mononuclear phagocytes, especially macrophages. However, the underlying mechanisms by which Leishmania spp. affect their host are not fully understood. Herein, proteomic alterations of primary, bone marrow-derived BALB/c macrophages are documented after 72 h of infection with Leishmania donovani insect-stage promastigotes, applying a SILAC-based, quantitative proteomics approach. The protocol was optimised by combining strong anion exchange and gel electrophoresis fractionation that displayed similar depth of analysis (combined total of 6189 mouse proteins). Our analyses revealed 86 differentially modulated proteins (35 showing increased and 51 decreased abundance) in response to Leishmania donovani infection. The proteomics results were validated by analysing the abundance of selected proteins. Intracellular Leishmania donovani infection led to changes in various host cell biological processes, including primary metabolism and catabolic process, with a significant enrichment in lysosomal organisation. Overall, our analysis establishes the first proteome of bona fide primary macrophages infected ex vivo with Leishmania donovani, revealing new mechanisms acting at the host/pathogen interface. SIGNIFICANCE: Little is known on proteome changes that occur in primary macrophages after Leishmania donovani infection. This study describes a SILAC-based quantitative proteomics approach to characterise changes of bone marrow-derived macrophages infected with L. donovani promastigotes for 72 h. With the application of SILAC and the use of SAX and GEL fractionation methods, we have tested new routes for proteome quantification of primary macrophages. The protocols developed here can be applicable to other diseases and pathologies. Moreover, this study sheds important new light on the "proteomic reprogramming" of infected macrophages in response to L. donovani promastigotes that affects primary metabolism, cellular catabolic processes, and lysosomal/vacuole organisation. Thus, our study reveals key molecules and processes that act at the host/pathogen interface that may inform on new immuno- or chemotherapeutic interventions to combat leishmaniasis.

Mannose Receptor 1 Restricts HIV Particle Release from Infected Macrophages

Human mannose receptor 1 (hMRC1) is expressed on the surface of most tissue macrophages, dendritic cells, and select lymphatic or liver endothelial cells. HMRC1 contributes to the binding of HIV-1 to monocyte-derived macrophages (MDMs) and is involved in the endocytic uptake of HIV-1 into these cells. Here, we identify hMRC1 as an antiviral factor that inhibits virus release through a bone marrow stromal antigen 2 (BST-2)-like mechanism. Virions produced in the presence of hMRC1 accumulated in clusters at the cell surface but were fully infectious. HIV-1 counteracted the effect by transcriptional silencing of hMRC1. The effect of hMRC1 was not virus isolate specific. Surprisingly, deletion of the Env protein, which is known to interact with hMRC1, did not relieve the hMRC1 antiviral activity, suggesting the involvement of additional cellular factor(s) in the process. Our data reveal an antiviral mechanism that is active in primary human macrophages and is counteracted by HIV-1 through downregulation of hMRC1.

Identification of multiple cellular uptake pathways of polystyrene nanoparticles and factors affecting the uptake: relevance for drug delivery systems

Nanoparticles may address challenges by human diseases through improving diagnosis, vaccination and treatment. The uptake mechanism regulates the type of threat a particle poses on the host cells and how a cell responds to it. Hence, understanding the uptake mechanisms and cellular interactions of nanoparticles at the cellular and subcellular level is a prerequisite for their effective biomedical applications. The present study shows the uptake mechanisms of polystyrene nanoparticles and factors affecting their uptake in bone marrow-derived macrophages, 293T kidney epithelial cells and L929 fibroblasts. Labeling with the endocytic marker FM4-64 and transmission electron microscopy studies show that the nanoparticles were internalized rapidly via endocytosis and accumulated in intracellular vesicles. Soon after their internalizations, nanoparticles trafficked to organelles with acidic pH. Analysis of the ultrastructural morphology of the plasma membrane invaginations or extravasations provides clear evidence for the involvement of several uptake routes in parallel to internalize a given type of nanoparticles by mammalian cells, highlighting the complexity of the nanoparticle-cell interactions. Blocking the specific endocytic pathways by different pharmacological inhibitors shows similar outcomes. The potential to take up nanoparticles varies highly among different cell types in a particle sizes-, time- and energy-dependent manner. Furthermore, infection and the activation status of bone marrow-derived macrophages significantly affect the uptake potential of the cells, indicating the need to understand the diseases' pathogenesis to establish effective and rational drug-delivery systems. This study enhances our understanding of the application of nanotechnology in biomedical sciences.

Liposomal antibiotics for the treatment of infectious diseases

INTRODUCTION

Liposomal delivery systems have been utilized in developing effective therapeutics against cancer and targeting microorganisms in and out of host cells and within biofilm community. The most attractive feature of liposome-based drugs are enhancing therapeutic index of the new or existing drugs while minimizing their adverse effects.

AREAS COVERED

This communication provides an overview on several aspects of liposomal antibiotics including the most widely used preparation techniques for encapsulating different agents and the most important characteristic parameters applied for examining shape, size and stability of the spherical vesicles. In addition, the routes of administration, liposome-cell interactions and host parameters affecting the biodistribution of liposomes are highlighted.

EXPERT OPINION

Liposomes are safe and suitable for delivery of variety of molecules and drugs in biomedical research and medicine. They are known to improve the therapeutic index of encapsulated agents and reduce drug toxicity. Recent studies on liposomal formulation of chemotherapeutic and bioactive agents and their targeted delivery show liposomal antibiotics potential in the treatment of microbial infections.

Parasitic infections: a role for C-type lectins receptors

Antigen-presenting cells (APCs) sense the microenvironment through several types of receptors that recognize pathogen-associated molecular patterns. In particular, C-type lectins receptors (CLRs), which are expressed by distinct subsets of dendritic cells (DCs) and macrophages (MØs), recognize and internalize specific carbohydrate antigens in a Ca²⁺-dependent manner. The targeting of these receptors is becoming an efficient strategy for parasite recognition. However, relatively little is known about how CLRs are involved in both pathogen recognition and the internalization of parasites. The role of CLRs in parasite infections is an area of considerable interest because this research will impact our understanding of the initiation of innate immune responses, which influences the outcome of specific immune responses. This paper attempts to summarize our understanding of the effects of parasites' interactions with CLRs.

Eicosanoids in the innate immune response: TLR and non-TLR routes

The variable array of pattern receptor expression in different cells of the innate immune system explains the induction of distinct patterns of arachidonic acid (AA) metabolism. Peptidoglycan and mannan were strong stimuli in neutrophils, whereas the fungal extract zymosan was the most potent stimulus in monocyte-derived dendritic cells since it induced the production of PGE₂, PGD₂, and several cytokines including a robust IL-10 response. Zymosan activated κB-binding activity, but inhibition of NF-κB was associated with enhanced IL-10 production. In contrast, treatments acting on CREB (CRE binding protein), including PGE₂, showed a direct correlation between CREB activation and IL-10 production. Therefore, in dendritic cells zymosan induces il10 transcription by a CRE-dependent mechanism that involves autocrine secretion of PGE₂, thus unraveling a functional cooperation between eicosanoid production and cytokine production.

Macrophage and Leishmania: An Unacceptable Coexistence

Research over the past year has revealed several significant and interesting advances in the biology of macrophage, key cells responsible in body's host defense against invading pathogens and in immune responses. Perturbation of macrophage surface with different bacterial pathogens leads to activate general signal transduction pathways of macrophages, including activation of NADPH oxidase, nitric oxide synthase, and so on. However, in this review, the results of macrophage interactions only with Leishmania parasites, which harbors the host macrophages, are discussed. It appears that interference in transduction of regulatory signals during leishmanial invasion lead to an inadequate leishmanicidal response. In this connection, information concerning regulation of MHC molecules and other current events related to macrophage function after invasion by the parasites are also discussed.

Targeting of mannosylated liposome incorporated benzyl derivative of Penicillium nigricans derived compound MT81 to reticuloendothelial systems for the treatment of visceral leishmaniasis

The antileishmanial property of a Benzyl derivative of a new antibiotic MT81 (Bz2MT81), isolated and purified from a fungal strain of Penicillium nigricans NRRL 917 was tested in free, liposome intercalated and mannose coated liposome intercalated forms in vivo against visceral leishmaniasis in hamsters. Mannose grafted liposome intercalated Bz2MT81 eliminated intracellular amastigotes of Leishmania donovani within splenic macrophages more efficiently than the liposome intercalated Bz2MT81 or free Bz2MT81. At a dose equivalent to 7.5 microg/Kg body weight when injected subcutaneously (s.c) in mannose grafted liposome intercalated form for 15 days in an interval of three days, the splenic parasitic load decreased to the extent of 79.1% of the total parasite present in infected control animals. Whereas, an identical amount (7.5 mug/Kg body weight) of Bz2MT81 in free or liposome intercalated form was found less effective in controlling the parasite in spleen (in free Bz2MT81 form, suppression of parasitic load is 49.8% and in liposome intercalated form, it is 55.1%). Both mannosylated liposomes and Bz2MT81 were noted non-toxic to the host peritoneal macrophages. Histological examinations of spleen and liver, kidney function tests (SGPT, alkaline phosphatase, creatinine and urea in blood plasma) showed that the toxicity of Bz2MT81 was reduced up to normal level when mannose grafted liposomal Bz2MT81 were administered.

Mannose receptor regulation of macrophage cell migration

The migration of macrophages through peripheral tissues is an essential step in the host response to infection, inflammation, and ischemia as well as in tumor progression and tissue repair. The mannose receptor (MR; CD206, previously known as the macrophage MR) is a 175-kDa type I transmembrane glycoprotein and is a member of a family of four recycling endocytic receptors, which share a common extracellular domain structure but distinct ligand-binding properties and cell type expression patterns. MR has been shown to bind and internalize carbohydrate and collagen ligands and more recently, to have a role in myoblast motility and muscle growth. Given that the related Endo180 (CD280) receptor has also been shown to have a promigratory role, we hypothesized that MR may be involved in regulating macrophage migration and/or chemotaxis. Contrary to expectation, bone marrow-derived macrophages (BMM) from MR-deficient mice showed an increase in random cell migration and no impairment in chemotactic response to a gradient of CSF-1. To investigate whether the related promigratory Endo180 receptor might compensate for lack of MR, mice with homozygous deletions in MR and Endo180 were generated. These animals showed no obvious phenotypic abnormality, and their BMM, like those from MR-deficient mice, retained an enhanced migratory behavior. As MR is down-regulated during macrophage activation, these findings have implications for the regulation of macrophage migration during different stages of pathogenesis.

Protection from experimental colitis by theaflavin-3,3'-digallate correlates with inhibition of IKK and NF-kappaB activation

BACKGROUND AND PURPOSE

Inflammatory bowel disease (IBD) is associated with activation of nuclear factor kappa B (NF-kappaB) involved in regulating the expression of inducible nitric oxide synthase (iNOS) and proinflammatory cytokine genes. As theaflavin-3,3'-digallate (TFDG), the most potent anti-oxidant polyphenol of black tea, down-regulates NF-kappaB activation, we investigated if TFDG is beneficial in colonic inflammation by suppressing iNOS and proinflammatory cytokines.

EXPERIMENTAL APPROACH

The in vivo efficacy of TFDG was assessed in mice with trinitrobenzene sulfonic acid (TNBS)-induced colitis. Both mRNA and protein levels of proinflammatory cytokines and iNOS were analyzed in colon tissue treated with or without TFDG. NF-kappaB activation was determined by electrophoretic mobility shift assay and levels of NF-kappaB inhibitory protein (IkappaBalpha) were analyzed by Western blotting.

KEY RESULTS

Oral administration of TFDG (5 mg kg(-1) daily i.g.) significantly improved TNBS-induced colitis associated with decreased mRNA and protein levels of TNF-alpha, IL-12, IFN-gamma and iNOS in colonic mucosa. DNA binding and Western blotting revealed increase in NF-kappaB activation and IkappaBalpha depletion in TNBS-treated mice from Day 2 through Day 8 with a maximum at Day 4, which resulted from increased phosphorylation of IkappaBalpha and higher activity of IkappaB kinase (IKK). Pretreatment with TFDG markedly inhibited TNBS-induced increases in nuclear localization of NF-kappaB, cytosolic IKK activity and preserved IkappaBalpha in colon tissue.

CONCLUSIONS AND IMPLICATIONS

TFDG exerts protective effects in experimental colitis and inhibits production of inflammatory mediators through a mechanism that, at least in part, involves inhibition of NF-kappaB activation.

The involvement of the macrophage mannose receptor in the innate immune response to infection with parasite Trichinella spiralis

The macrophage mannose receptor (MR) is a pattern recognition receptor of the innate immune system that binds to microbial structures bearing mannose, fucose and N-acetylglucosamine on their surface. The MR can mediate endocytosis and phagocytosis, as well as activation of macrophages and antigen presentation. Since Trichinella spiralis antigens are rich in oligomannose residues, we investigated whether a mannose-recognizing receptor, such as the MR, participated in the host-parasite interaction. The results show that the MR (either on the surface of macrophages or in the purified form) recognizes and binds components of T. spiralis muscle larvae. The presence of parasites provoked activation of peritoneal macrophages, which was indicated by down-regulation of MR expression, and the stimulation of NO secretion. In vitro stimulation of macrophages with T. spiralis components resulted in increased NO and IL-6 production. However, while the MR was partially involved in stimulation of NO production, it did not mediate IL-6 secretion.

Mannose-Containing Molecular Patterns Are Strong Inducers of Cyclooxygenase-2 Expression and Prostaglandin E2Production in Human Macrophages

The induction of cyclooxygenase-2 (COX-2) and the production of PGE(2) in response to pathogen-associated molecular patterns decorated with mannose moieties were studied in human monocytes and monocyte-derived macrophages (MDM). Saccharomyces cerevisiae mannan was a robust agonist, suggesting the involvement of the mannose receptor (MR). MR expression increased along the macrophage differentiation route, as judged from both its surface display assessed by flow cytometry and the ability of MDM to ingest mannosylated BSA. Treatment with mannose-BSA, a weak agonist of the MR containing a lower ratio of attached sugar compared with pure polysaccharides, before the addition of mannan inhibited COX-2 expression, whereas this was not observed when agonists other than mannan and zymosan were used. HeLa cells, which were found to express MR mRNA, showed a significant induction of COX-2 expression upon mannan challenge. Conversely, mannan did not induce COX-2 expression in HEK293 cells, which express the mRNA encoding Endo180, a parent receptor pertaining to the MR family, but not the MR itself. These data indicate that mannan is a strong inducer of COX-2 expression in human MDM, most likely by acting through the MR route. Because COX-2 products can be both proinflammatory and immunomodulatory, these results disclose a signaling route triggered by mannose-decorated pathogen-associated molecular patterns, which can be involved in both the response to pathogens and the maintenance of homeostasis.

The expression of mannose receptors in skin fibroblast and their involvement in Leishmania (L.) amazonensis invasion

Leishmania are protozoa that invade mononuclear phagocytes with the involvement of different ligand-receptor systems, including mannose receptors. Until now, scant data are available concerning the mechanisms that govern the infection of Leishmania in other host cell types such as fibroblasts. Our aim was to analyze the expression of mannose receptors in primary cultures of skin fibroblasts (SF) further characterizing their role during the invasion of promastigotes of Leishmania (L.) amazonensis. Both fluorescent, light, and electron microscopy assays revealed that SF have mannose receptors since they bound and internalized mannosylated ligands in addition to being positively labeled by fuc-BSA-FITC probes. d-mannose competition assays revealed the participation of mannose receptors during the parasite association with SF presenting upregulated receptor expression during the initial steps of the infection. After longer periods of Leishmania:fibroblasts contact, the modulation noted in the host mannose receptors was reverted concomitantly to the infection control, suggesting that the parasites were required for the alteration maintenance and providing evidences that the SF may display microbicidal mechanisms to control the Leishmania infection.

HIV-1 Nef mediates post-translational down-regulation and redistribution of the mannose receptor

Human immunodeficiency virus (HIV) has derived a variety of means to evade the host immune response. HIV-derived proteins, including Tat, Nef, and Env, have all been reported to decrease expression of host molecules such as CD4 and major histocompatibility complex I, which would assist in limiting viral replication. The mannose receptor (MR) on the surface of macrophages and dendritic cells (DC) has been proposed to function as an effective antigen-capture molecule, as well as a receptor for entering pathogens such as Mycobacterium tuberculosis and Pneumocystis carinii. Regulation of this receptor would therefore benefit HIV in removing an additional arm of the innate immune system. Previous work has shown that MR function is reduced in alveolar macrophages from HIV-infected patients and that surface MR levels are decreased by the HIV-derived protein Nef in DC. In addition, several laboratories have shown that CD4 is removed from the surface of T cells in a manner that might be applicable to decreased MR surface expression in macrophages. In the current study, we have investigated the role of Nef in removing MR from the cell surface. We have used a human macrophage cell line stably expressing the MR as well as human epithelial cells transiently expressing CD4 and a unique CD4/MR chimeric molecule constructed from the extracellular and transmembrane domains of CD4 and the cytoplasmic tail portion of the MR. We show that the MR is reduced on the cell surface by approximately 50% in the presence of Nef and that the MR cytoplasmic tail can confer susceptibility to Nef in the CD4/MR chimera. These data suggest that the MR is a potential intracellular target of Nef and that this regulation may represent a mechanism to further cripple the host innate immune system.

Absence of the macrophage mannose receptor in mice does not increase susceptibility to Pneumocystis carinii infection in vivo

Host defense against the opportunistic pathogen Pneumocystis carinii requires functional interactions of many cell types. Alveolar macrophages are presumed to be a vital host cell in the clearance of P. carinii, and the mechanisms of this interaction have come under scrutiny. The macrophage mannose receptor is believed to play an important role as a receptor involved in the binding and phagocytosis of P. carinii. Although there is in vitro evidence for this interaction, the in vivo role of this receptor in P. carinii clearance in unclear. Using a mouse model in which the mannose receptor has been deleted, we found that the absence of this receptor is not sufficient to allow infection by P. carinii in otherwise immunocompetent mice. Furthermore, when mice were rendered susceptible to P. carinii by CD4(+) depletion, mannose receptor knockout mice (MR-KO) had pathogen loads equal to those of wild-type mice. However, the MR-KO mice exhibited a greater influx of phagocytes into the alveoli during infection. This was accompanied by increased pulmonary pathology in the MR-KO mice, as well as greater accumulation of glycoproteins in the alveoli (glycoproteins, including harmful hydrolytic enzymes, are normally cleared by the mannose receptor). We also found that the surface expression of the mannose receptor is not downregulated during P. carinii infection in wild-type mice. Our findings suggest that while the macrophage mannose receptor may be important in the recognition of P. carinii, in vivo, this mechanism may be redundant, and the absence of this receptor may be compensated for.

Intracellular delivery of drugs to macrophages

Toxic side effects which often complicate successful therapy in a number of diseases possibly arise due to the fact that at therapeutically effective concentrations the non-target cells in the body are also exposed to the cytotoxic effects of the drugs. Minimization of such adverse reactions might be feasible through drug delivery modalities that would reduce the uptake of the drugs by non-target cells and selectively deliver the drug only to the target cells (and/or intracellular sites) at relatively low extracellular concentrations. The current generic approach to site-specific drug delivery consists of attaching the therapeutic agent to a carrier recognized only by the cells where the pharmacological action is desired. Two types of recognition elements on the surface of target cells are being exploited for this purpose, viz., (i) antigens capable of generating specific, non-cross reactive antibodies, and (ii) receptors on the cell surface capable of efficient transport of the ligands. In general, incomplete specificity for the target cells and poor internalization of antibody-drug conjugates still limit the usefulness of antibodies for site-specific drug delivery applications necessitating exploration of alternatives. The alternate possibility is to exploit the exquisite cell type specificity and high efficiency of endocytosis of macromolecules mediated by specific receptors present on the surface of target cells for delivering drugs. A large number of infectious, metabolic, and neoplastic diseases are associated with macrophages leading to morbidities and mortalities to millions of people worldwide, thus an appropriate design of a drug delivery system to macrophages will be of tremendous help.

Mannose receptor contribution to Candida albicans phagocytosis by murine E-clone J774 macrophages

Mannoproteins, as the main constituents of the outer layer of yeast cell walls, are able to interact with phagocytic cells in an opsonin-independent manner through the mannose receptor (MR) and to induce yeast ingestion by the professional phagocytes. Moreover, the MR also mediates endocytosis of soluble ligands through clathrin-coated pits. Here, we studied some aspects of the interaction between the MR and Candida albicans using murine E-clone macrophages and the consequences on MR trafficking. Using a pull-down assay involving mixture E-clone macrophage detergent lysate with mannosylated Sepharose beads and glutaraldehyde-fixed, heat-killed (HK) C. albicans, we found that binding of solubilized MR to mannosylated particles occurred with characteristics similar to the receptor's cell-surface mannose-binding activity. We then demonstrated that MR expressed on E-clone macrophages contributed to phagocytosis of unopsonized, HK C. albicans and that yeast phagocytosis induced a decrease in MR endocytic activity without concomitant degradation of the receptor in the time lapse studied.

Methotrexate conjugate with branched polypeptide influences Leishmania donovani infection in vitro and in experimental animals

Methotrexate (MTX) has been coupled to various structurally related, polycationic (poly[Lys(DL-Ala(m))] (AK), poly[Lys(Ser(i)-DL-Ala(m))] (SAK), poly[Lys(DL-Ala(m)-Leu(i))] (ALK)), or amphoteric (poly[Lys(Glu(i)-DL-Ala(m))] (EAK)) synthetic branched polypeptides containing poly[L-Lys] backbone by the aid of BOP reagent. The average degree of MTX incorporation was found to be dependent on the charge properties of the polymer. Under the experimental conditions used, the molar substitution ratio achieved was higher for polycations (25%) than for the amphoteric polypeptide (10%). We have studied the effect of polycationic polypeptides on Leishmania donovani infection. Results demonstrated that MTX conjugates in which the drug is covalently attached to carrier have pronounced leishmanicid activity. In this communication we showed that (a) a branched polypeptide-methotrexate conjugate with a polycationic carrier (ALK) increases the effect of MTX against Leishmania donovani infection in mice; (b) the covalent bond between the carrier and methotrexate is essential for both in vivo and in vitro activity; and (c) the number of Leishmania donovani parasites in infected macrophages are markedly reduced in conjugate treated animals. In vitro observation might also indicate that the MTX conjugate exhibits an effect through an uptake by macrophages which is different from that of the free drug.

Successful therapy of lethal murine visceral leishmaniasis with cystatin involves up-regulation of nitric oxide and a favorable T cell response

The virulence of Leishmania donovani in mammals depends at least in part on cysteine proteases because they play a key role in CD4(+) T cell differentiation. A 6-fold increase in NO production was observed with 0.5 microM chicken cystatin, a natural cysteine protease inhibitor, in IFN-gamma-activated macrophages. In a 45-day BALB/c mouse model of visceral leishmaniasis, complete elimination of spleen parasite burden was achieved by cystatin in synergistic activation with a suboptimal dose of IFN-gamma. In contrast to the case with promastigotes, cystatin and IFN-gamma inhibited the growth of amastigotes in macrophages. Although in vitro cystatin treatment of macrophages did not induce any NO generation, significantly enhanced amounts of NO were generated by macrophages of cystatin-treated animals. Their splenocytes secreted soluble factors required for the induction of NO biosynthesis, and the increased NO production was paralleled by a concomitant increase in antileishmanial activity. Moreover, splenocyte supernatants treated with anti-IFN-gamma or anti-TNF-alpha Abs suppressed inducible NO generation, whereas i.v. administration of these anticytokine Abs along with combined therapy reversed protection against infection. mRNA expression and flow cytometric analysis of infected spleen cells suggested that cystatin and IFN-gamma treatment, in addition to greatly reducing parasite numbers, resulted in reduced levels of IL-4 but increased levels of IL-12 and inducible NO synthase. Not only was this treatment curative when administered 15 days postinfection, but it also imparted resistance to reinfection. These studies provide a promising alternative for protection against leishmaniasis with a switch of CD4(+) differentiation from Th2 to Th1, indicative of long-term resistance.

Pneumocystis carinii enhances soluble mannose receptor production by macrophages

Phagocytosis of extracellular organisms in the alveolar spaces of the lungs represents the first-line of host defense against pulmonary pathogens. Disruption of this process is likely to interfere with the generation of appropriate specific immune responses, and lead to a delayed or inefficient clearance of the pathogen. Pneumocystis carinii, an opportunistic pathogen in immunodeficient individuals, is cleared from the lung by alveolar macrophages. In the absence of specific anti-Pneumocystis antibodies, phagocytosis is dependent on the non-opsonic macrophage mannose receptor (MR). Recent studies have demonstrated that alveolar macrophage MR activity is downregulated in individuals infected with HIV, and that functional MR is shed from the macrophage cell surface. Here we report that P. carinii enhances the formation of soluble MR by macrophages in vitro. Soluble MR was detected in cell-free alveolar fluid from humans infected with HIV and/or P. carinii, but not in alveolar fluid from healthy controls. Soluble MR was found in association with extracellular clumps of P. carinii in the lungs of mice with P. carinii pneumonia, and was associated with P. carinii organisms purified from these mice. When purified P. carinii organisms were incubated with soluble MR-containing supernatants, they were phagocytosed less readily by alveolar macrophages than were control organisms. Our results suggest that P. carinii organisms enhance the shedding of MR from the surface of alveolar macrophages, and that the resultant soluble MR binds to intra-alveolar organisms, thereby interfering with their non-opsonic uptake via the macrophage cell surface MR.

A cardiomyocyte mannose receptor system is involved in Trypanosoma cruzi invasion and is down-modulated after infection

Mannosyl binding sites were detected "in vitro" on cardiomyocytes (CM) surface using horseradish peroxidase (HRP) as the ligand. Binding assays revealed a specific recognition system, which was time- and concentration-dependent. The binding required physiological pH and was inhibited by EDTA and trypsin treatments. HRP binding was reduced by pre-incubations with low concentrations of D-mannose. Ultrastructural analysis of the endocytic process was followed using HRP coupled to colloidal gold particles (HRP-Au). The tracer was found within caveolae characterizing early steps of the receptor-mediated endocytosis. The addition of 10 mM D-mannose to the interaction medium blocked Trypanosoma cruzi uptake by CM. The labeling of CM with a subsaturating concentration of HRP-Au before their infection showed, by ultrastructural studies, that its association with trypomastigote forms occurred frequently near to HRP-gold particles that could also be seen to comprise the parasitophorous vacuole. After infection of CM with T. cruzi, a considerable reduction on HRP binding was noticed. Binding was almost completely restored by treating the infected cultures with the trypanocidal drug Nifurtimox. Our "in vitro" findings suggest that cardiomyocyte's mannose receptors localized at the sarcolemma mediates T. cruzi recognition and can be down-modulated by parasite infection.

Characterization of mannose receptor-dependent phagocytosis mediated by Mycobacterium tuberculosis lipoarabinomannan

The macrophage mannose receptor (MR) along with complement receptors mediates phagocytosis of the M. tuberculosis virulent strains Erdman and H37Rv. We have determined that the terminal mannosyl units of the M. tuberculosis surface lipoglycan, lipoarabinomannan (LAM), from the Erdman strain serve as ligands for the MR. The biology of the MR (receptor binding and trafficking) in response to phagocytic stimuli is not well characterized. This study analyzes the MR-dependent phagocytosis mediated by Erdman LAM presented on a 1-micron-diameter phagocytic particle. Erdman LAM microspheres exhibited a time- and dose-dependent rapid increase in attachment and internalization by human monocyte-derived macrophages (MDMs). In contrast, internalization of LAM microspheres by monocytes was minimal. Microsphere internalization by MDMs was visualized and quantitated by immunofluorescence and confocal and electron microscopy and resembled conventional phagocytosis. Phagocytosis of LAM microspheres by MDMs was energy, cytoskeleton, and calcium dependent and was mannan inhibitable. Trypsin treatment of MDMs at 37 degrees C, which depleted surface and recycling intracellular pools of the MR, reduced the subsequent attachment of LAM microspheres. Trypsin treatment at 4 degrees C allowed for subsequent recovery of LAM microsphere phagocytosis at 37 degrees C by recycled MRs. Pretreatment of MDMs with cycloheximide influenced LAM microsphere phagocytosis to only a small extent, indicating that MR-dependent phagocytosis of the microspheres was occurring primarily by preformed recycled receptors. This study characterizes the requirements for macrophage phagocytosis of a LAM-coated particle mediated by the MR. This model will be useful in further characterization of the intracellular pathway taken by phagocytic particles coated with different LAM types in macrophages following ingestion.

Ingestion of Candida albicans down-regulates mannose receptor expression on rat macrophages

The frequency of infection and death due to various Candida species has increased steadily during the past decade, with mucocutaneous candidal infections as a common problem in the immunocompromised host. Mononuclear phagocytes are important in phagocytosis of this organism. In areas where there are low levels of opsonins, the macrophage-specific mannose receptor plays a dominant role in mediating Candida albicans ingestion. Following receptor-mediated infection, the host macrophage produces inflammatory cytokines and mediators that lead to ultimate killing of the invading Candida. Infection of macrophages by pathogens often leads to altered function that might effect their subsequent host defense properties. For example, function of both the complement receptor type 3 and the mannose receptor are down-regulated following exposure to pathogens or pathogen-derived products. In the current study, we have examined the down-regulation of mannose receptor expression following Candida infection and have investigated possible mechanisms that might be involved. Mannose receptor activity was decreased following 24 h postinfection with Candida. Both tumor necrosis factor and nitric oxide were produced during the infection, and inhibition of the these mediators partially blocked the effect on the receptor. Infection with Candida also inhibited the ability of dexamethasone to up-regulate mannose receptor expression. Finally, mannose receptor protein turnover was accelerated in Candida-infected macrophages. We conclude that Candida down-regulates one of the receptors involved in its internalization through a combination of production of modulatory molecules and enhanced receptor degradation. These results support the hypothesis that pathogens that infect macrophages have the ability to alter the phagocytic pathways available for subsequent host defense.

Leishmania donovani attachment stimulates PKC-mediated oxidative events in bone marrow-derived macrophages

We investigated activation signaling events in bone marrow-derived macrophages after infection with Leishmania donovani, an intracellular parasite of macrophages. Leishmania donovani infection caused a general suppression of activation parameters like O2- and NO production. However, conditions which allow parasite attachment and prevent entry resulted in triggering of O2- and NO production and stimulation of O2 consumption. Optimal NO and O2- production occurred when bone marrow-derived macrophages and Leishmania ratio was 1:100. The activation signal for O2- production was initiated 15 min after parasite attachment, whereas augmentation of NO production started 6 h after attachment Activation of O2- and NO generation by L. donovani attachment was inhibited by staurosporine as well as by prolonged treatment of phorbol myristate acetate suggesting a protein kinase C-dependent mechanism. Translocation studies showed that protein kinase C activity in cell membrane fraction rapidly and transiently increased following parasite attachment. No such protein kinase C translocation event occurred in L. donovani infected bone marrow-derived macrophages. Phorbol myristate acetate was found to stimulate membrane translocation of protein kinase C in parasite attached cells whereas it was impaired in infected cells. However, both attachment and infection induced a similar shift of phorbol receptors from cytosolic to membrane fraction indicating that in infected cells the translocation of protein kinase C protein was not impaired but the activity of the membrane associated enzyme was somehow inhibited. These results suggest that although internalization of intracellular parasites like L. donovani caused inhibition of nitrite and superoxide release, mere attachment on macrophage surface resulted in an activation of protein kinase C-mediated downstream oxidative events.

Receptor-mediated endocytosis of fucosylated neoglycoprotein by macrophages

The characteristics of the recognition system involved in the receptor mediated endocytosis of the neoglycoprotein, fucose-human serum albumin (HSA) were studied. It was found that (i) fucose-HSA showed strong affinity binding and uptake by various macrophages; (ii) binding was specific for L-fucose and D-mannose; (iii) binding was found to be inhibited by oxidant like H2O2 and swainsonine whereas it was elevated by dexamethasone; (iv) clearance of 125I-fucose-HSA was rapid and strongly inhibited by unlabelled fucose-HSA. Greater than 70% of fucose-HSA was found in liver and more than 60% of this was found in liver lysosomes; (v) uptake of fucose-HSA was thirty-fold more efficient in liver macrophages (Kupffer cells) than in hepatocytes; (vi) moreover, mannose-HSA and ovalbumin which are potent inhibitors of mannose/N-acetylglucosamine receptors inhibited clearance and uptake of fucose-HSA by liver as well as by isolated Kupffer cells suggesting the involvement of both fucose and mannose receptors or a single type of receptor having greater affinity for fucose-HSA than for mannose-HSA. These results emphasize the important role of fucose-terminated glycoproteins in site-specific drug targeting.

Altered cell signaling and mononuclear phagocyte deactivation during intracellular infection

Given the critical antimicrobial properties of mononuclear phagocytes, an important concern in cell biology and immunology has been to understand how intracellular microbes are able to establish states of chronic infection within these cells. Recent studies indicate that mononuclear phagocytes become functionally deactivated during intracellular infection. Here, Neil Reiner considers the experimental evidence to indicate that this is a frequent event that may be accounted for by induced defects in the signaling pathways required to bring cells to an activated state.

Expression and characterization of a parasite-specific antigen on macrophages after infection with Leishmania donovani

A rabbit polyclonal antibody to crude soluble antigen of Leishmania donovani promastigotes recognized a determinant expressed on the surface membrane of mouse peritoneal macrophages and human monocyte derived macrophages infected in vitro. The determinant was recognized on infected macrophage surface only when F(ab')2 fragments of anti-leishmanial antiserum was employed in immunofluorescence. F(ab')2 fragments of human patient sera also could recognize the determinant. The expression of this antigen was not stage-specific for the parasite. Immunochemical analyses revealed this antigen to be of 51 kDa protein. Specific leaching of membrane proteins by trypsin showed three bands of expressed antigens of 26, 11 and 10 kDa, which in all likelihood might be arising from the 51 kDa antigen. The antigen was not expressed until 12 h of post infection, reached a maximum level at 24 h and thereafter attained a steady state level as studied upto 96 h of post infection. This type of antigen might have a great potential in immunodiagnostics and site-specific drug targeting.

Targeted delivery of superoxide dismutase to macrophages via mannose receptor-mediated mechanism

Human recombinant superoxide dismutase (SOD) was modified into a mannosylated form (Man-SOD), and its cellular uptake and inhibitory effect on superoxide anion release were studied in vitro, using cultured mouse peritoneal macrophages. [111In]Man-SOD was taken up by the macrophages to a great extent, whereas no significant uptake was observed for native and galactosylated SOD. The uptake of Man-SOD was inhibited significantly at a low temperature and by the presence of mannan, mannose and colchicine, demonstrating the targeted delivery of Man-SOD via mannose receptor-mediated endocytosis. Man-SOD exhibited a superior inhibitory effect on superoxide anion release from inflammatory macrophages stimulated by phorbol-myristate acetate. The present study suggested the potential of Man-SOD as a therapeutic agent for the inflammatory disease mediated by superoxide anions generated by macrophages.

Comparison of receptors required for entry of Leishmania major amastigotes into macrophages

We investigated the mechanisms of entry of amastigotes of Leishmania major from two different sources into macrophages by comparing their use of the Fc receptor (FcR), complement receptor type 3 (CR3), and mannose-fucose receptor (MFR). Amastigotes were obtained from BALB/c mice and SCID mice. FcR involvement was examined by opsonizing L. major with parasite-specific immunoglobulin G (IgG). Antiparasite IgG did not alter the uptake of amastigotes from BALB/c mice since these amastigotes had antibody bound to their surface: IgG1 was the most predominant antibody, followed by IgG2b, IgM, and IgG2a. However, opsonization with antiparasite IgG enhanced the entry of amastigotes that lacked antibody on their surface, namely, amastigotes obtained from SCID mice or from macrophages infected in vitro. These results indicate that the FcR is important for amastigote entry into macrophages. Down-modulation of FcRs onto immune complexes, however, did not reduce the entry of amastigotes containing surface-bound IgG into macrophages. Monoclonal antibodies against the CR3 inhibited the entry of amastigotes from either BALB/c or SCID mice into J774A.1 macrophage-like cells. Simultaneous blocking of FcR and CR3 further increased the inhibition of phagocytosis. Treatment of macrophages with soluble mannan or down-modulating the MFR onto mannan-coated coverslips had no effect on the entry of amastigotes from BALB/c or SCID mice. Thus, the MFR does not appear to be used by amastigotes of L. major. We show that ingestion of amastigotes appears to occur primarily through the FcR and CR3; however, additional receptors may also participate in the uptake of amastigotes.