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D M Mosser

Publications and source records attributed to D M Mosser.

At least 37 records · Page 2Linked to original sources
PubMed1997

Leishmania, macrophages and complement: a tale of subversion and exploitation.

Leishmania are intracellular protozoan parasites which reside primarily, if not exclusively, in host mononuclear phagocytes. Several studies have demonstrated that infectious promastigotes rapidly and efficiently fix complement when they encounter serum components during their transmission to the mammalian host. Activation of the complement system by a microorganism can have 3 distinct biological effects. First, fixation of the terminal complement components can result in complement-mediated lysis. Second, fixation of the 3rd component of complement can lead to opsonization of the organism for uptake by phagocytic cells. Finally, the elaboration of the complement anaphylotoxins, C3a and C5a, can lead to inflammation. In the present chapter, we discuss the interaction of leishmania promastigotes with the complement system. We show that infectious promastigotes avoid the lytic effects of complement and resist fixation of the terminal complement components. At the same time, however, these organisms depend on fixation of opsonic complement to invade host mononuclear phagocytes efficiently. We discuss the mechanisms which allow metacyclic leishmania promastigotes to exploit the opsonic properties of complement and the receptors on macrophages involved in leishmania recognition. The role of complement mediated inflammatory processes in the host response to leishmania infection is an area which requires additional study.

Animals↗
PubMed1996

Immunological characterization of eristostatin and echistatin binding sites on alpha IIb beta 3 and alpha V beta 3 integrins.

Two disintegrins with a high degree of amino acid sequence similarity, echistatin and eristostatin, showed a low level of interaction with Chinese hamster ovary (CHO) cells, but they bound to CHO cells transfected with alpha IIb beta 3 genes (A5 cells) and to CHO cells transfected with alpha v beta 3 genes (VNRC3 cells) in a reversible and saturable manner. Scatchard analysis revealed that eristostatin bound to 816000 sites per A5 cell (Kd 28 nM) and to 200000 sites (Kd 14 nM) per VNRC3 cell respectively. However, VNRC3 cells did not bind to immobilized eristostatin. Echistatin bound to 495000 sites (Kd 53 nM) per A5 cell and to 443000 sites (Kd 20 nM) per VNRC3 cell. As determined by flow cytometry, radiobinding assay and adhesion studies, binding of both disintegrins to A5 cells and resting platelets and binding of echistatin to VNRC3 cells resulted in the expression of ligand-induced binding sites (LIBS) on the beta 3 subunit. Eristostatin inhibited, more strongly than echistatin, the binding of three monoclonal antibodies: OPG2 (RGD motif dependent), A2A9 (alpha IIb beta 3 complex dependent) and 7E3 (alpha IIb beta 3 and alpha v beta 3 complex dependent) to A5 cells, to resting and to activated platelets and to purified alpha IIb beta 3. Experiments in which echistatin and eristostatin were used alone or in combination to inhibit the binding of 7E3 and OPG2 antibodies to resting platelets suggested that these two disintegrins bind to different but overlapping sites on alpha IIb beta 3 integrin. Monoclonal antibody LM 609 and echistatin seemed to bind to different sites on alpha v beta 3 integrin. However, echistatin inhibited binding of 7E3 antibody to VNRC3 cells and to purified alpha v beta 3 suggesting that alpha v beta 3 and alpha IIb beta 3 might share the same epitope to which both echistatin and 7E3 bind. Eristostatin had no effect in these systems, providing further evidence that it binds to a different epitope on alpha v beta 3.

Animals↗
PubMed1996

Cooperation between CR1 (CD35) and CR3 (CD 11b/CD18) in the binding of complement-opsonized particles.

We analyzed the binding of sheep erythrocytes bearing C3b (EC3b) to cells transfected with human complement receptors. EC3b bound avidly to cells expressing CR1 but failed to bind to cells expressing CR3. In the presence of factor I, the binding of EC3b, to CR1 was transient. Primary monocytes and cotransfected cells expressing both CR1 and CR3 mediated a stable resetting of EC3b, even in the prolonged presence of factor I. This stable adhesion was dependent on the presence of CR3, because blocking CR3 with mAb resulted in the factor I-dependent release of erythrocytes from these cells. A model is proposed in which these two complement receptors cooperate in a unique manner. These results suggest that the stable adhesion of complement-opsonized particles to cells expressing CR1 and CR3 is actually a dynamic molecular process in which an important function of leukocyte CR1 is to generate the ligands for CR3.

Animals↗
PubMed1996

Exploitation of the complement system by Leishmania promastigotes.

Complement has long been acknowledged as an important component of host defense to extracellular pathogens. David Mosser and Andrew Brittingham here describe how Leishmania spp, intracellular pathogens of mononuclear phagocytes, exploit the opsonic and chemotactic effects of complement to initiate infection in the mammalian host.

Journal Article↗
PubMed1996

Rhodococcus equi: an emerging opportunistic pathogen.

Rhodococcus equi is emerging as a cause of pneumonia in immunocompromised people, especially those with AIDS. Like mycobacteria, R. equi is phagocytosed by alveolar macrophages and replicates within them. Recent work is beginning to elucidate the cell and molecular biology of this opportunistic pathogen and the host immune response to it.

Actinomycetales Infections↗
PubMed1996

Effect of amplification of the Cap b locus on complement-mediated bacteriolysis and opsonization of type b Haemophilus influenzae.

Amplification of the Cap b locus of Haemophilus influenzae occurs frequently in clinical isolates and has been proposed to be a mechanism by which this organism evades host defense. To determine if amplification of this locus affected complement fixation, in vitro studies to determine complement-mediated bacteriolysis and complement-mediated opsonization of an isogenic set of organisms containing 2, 3, and 4 copies of the Cap b locus were performed. Organisms containing 4 copies of the Cap b locus were significantly more resistant to antibody-dependent, classical complement pathway-directed bacteriolysis than were organisms containing 2 copies. Organisms containing 3 copies of this locus exhibited intermediate susceptibility to lysis. Complement-mediated opsonization of these organisms was assessed by determining the degree of binding of bacteria to murine or human macrophages or to nonphagocytic cells transfected with the genes for human Mac-1, the complement receptor type 3. In all three assay systems, organisms containing 4 copies of the Cap b locus bound less well than did organisms containing 2 copies of this locus. Consistent with their decreased susceptibility to lysis and opsonization, organisms with 4 copies of the Cap b locus fixed less C3 than did organisms containing 2 copies. These data demonstrate that amplification of the Cap b locus is associated with decreased susceptibility to complement-mediated lysis and decreased complement-mediated opsonization and suggest that amplification is used by these pathogens to increase their resistance to complement-dependent host defense mechanisms [correction of mecanisms].

Adult↗
PubMed1996

Leishmania major-human macrophage interactions: cooperation between Mac-1 (CD11b/CD18) and complement receptor type 1 (CD35) in promastigote adhesion.

It has been suggested that the developmental maturation of Leishmania major promastigotes can affect their interaction with human complement receptors. To study this, we measured the adhesion of metacyclic and logarithmic-phase L. major promastigotes to complement receptors expressed on primary macrophages, to recombinant receptors expressed on transfected cells, or to purified complement receptors in a cell-free system. We demonstrate that complement-opsonized promastigotes can bind to both Mac-1 and complement receptor type 1 (CR1) and that the transition of promastigotes from the noninfectious logarithmic phase of growth to the infectious metacyclic stage does not affect this interaction. Furthermore, we show that Mac-1 and CR1 can cooperate to mediate the efficient adhesion of complement-opsonized metacyclic promastigotes to cells expressing both receptors. On human monocyte-derived macrophages, Mac-1 appears to make a quantitatively greater contribution to this adhesion than does CR1, since blocking macrophage Mac-1 diminishes metacyclic promastigote adhesion to a greater extent than does blocking CR1. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic decrease in complement-dependent promastigote adhesion, relative to normal monocytes. The predominance of Mac-1 in these interactions is due, at least in part, to the factor I cofactor activity of CR1, which facilitates the conversion of C3b to iC3b. The stable adhesion of complement-opsonized metacyclic promastigotes to Mac-1 is a prerequisite for phagocytosis by human monocyte-derived macrophages. Blocking Mac-1 on macrophages abrogates the majority of the complement-dependent phagocytosis of promastigotes, whereas blocking CR1 has no detectable effect on phagocytosis. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic reduction in promastigote phagocytosis relative to normal bovine monocytes. We conclude, therefore, that the two complement receptors, Mac-1 and CR1, can cooperate to mediate the initial complement-dependent adhesion of metacyclic promastigotes to human monocyte-derived macrophages and that Mac-1 is the predominant complement receptor responsible for the phagocytosis of complement-opsonized metacyclic promastigotes.

Adhesiveness↗
PubMed1995

Role of the Leishmania surface protease gp63 in complement fixation, cell adhesion, and resistance to complement-mediated lysis.

The Leishmania surface protease gp63 has been identified as a parasite virulence factor. To better define the role of gp63 in Leishmania infectivity, the interaction of recombinant gp63 with complement and complement receptors was examined. On Leishmania, gp63 was not necessary for complement fixation. Complement activation occurred on transfected organisms expressing varying amounts of gp63 and on organisms expressing a mutant form of gp63 devoid of proteolytic activity. However, organisms expressing wild-type gp63 on their surface fixed only small amounts of the terminal complement components and were dramatically more resistant to lysis by complement than were those lacking functional gp63. Organisms expressing wild-type gp63 more rapidly converted C3b on their surface to a form that exhibited the neoantigen of iC3b and interacted avidly with cells expressing Mac-1, the receptor for iC3b. Complement fixation by transfected mammalian cells expressing recombinant Leishmania gp63 on their surface was also examined. The presence of Leishmania gp63 on the surface of these cells converted them into efficient activators of complement. Cells expressing gp63 on their surface fixed complement and bound avidly to the human complement receptors. The proteolytic activity of this molecule was not necessary for complement activation or adhesion to complement receptors. Thus, gp63 may contribute to parasite virulence by exerting a novel type of control over complement fixation. Organisms expressing gp63 can exploit the opsonic properties of complement while avoiding its lytic effects.

Animals↗
PubMed1994

High-molecular-weight surface-exposed proteins of Haemophilus influenzae mediate binding to macrophages.

The molecular basis for direct bacteria-macrophage interactions that distinguishes nontypeable (NT) Haemophilus influenzae from type b organisms is not known. Because of similarities between filamentous hemagglutinin (FHA) adhesin of Bordetella pertussis and high-molecular-weight (HMW) proteins commonly expressed by NT H. influenzae, the role that HMW proteins play in determining NT H. influenzae-macrophage interactions was assessed. In tests with genetically engineered organisms, HMW protein-expressing bacteria bound significantly better than isogenic HMW protein-deficient bacteria to macrophages. HMW protein-dependent binding to macrophages is trypsin-sensitive, is independent of divalent cations, does not occur via the leukocyte integrin CD11b/CD18, and is not affected by galactose-containing carbohydrates. Organisms bound via HMW proteins remain largely extracellular and viable. Like FHA of Bordetella organisms, HMW proteins mediate binding of NT H. influenzae to macrophages. However, unlike the interaction determined by FHA, this interaction is characteristically one of adhesion and requires additional serum opsonization for efficient killing of bacteria by macrophages.

Animals↗
PubMed1994

Survival and replication of Rhodococcus equi in macrophages.

Rhodococcus equi is a facultative intracellular bacterium of macrophages that can cause serious pneumonia in both young horses and immunocompromised people. Essential to understanding rhodococcus pathogenesis is a quantitative documentation of the intracellular events that follow macrophage phagocytosis of the organism. By using a bacterial immunofluorescence staining assay, we verified the intracellular survival and replicative potential of R. equi in both murine peritoneal macrophages and equine alveolar macrophages in vitro. Following an initial lag period of 6 to 12 h, the intracellular numbers of R. equi begin to rise, often reaching macrophage-compromising levels by 48 h. A quantitative determination of bacterial growth by a novel image analysis cytometry technique confirmed our fluorescence microscopic results. By 48 h postinfection, bacterial numbers had increased by more than fivefold, and the majority of infected macrophages in the monolayer contained 10 or more bacteria per cell. The intracellular organisms were viable, as evidenced by the ability to incorporate radiolabeled uracil. The use of these techniques has identified differences in the in vitro replicative capacities of a virulent strain and an avirulent strain of R. equi. A clinical isolate of R. equi expressing a 17-kDa virulence-associated plasmid-encoded antigen was able to survive and replicate within macrophages, whereas an avirulent, non-plasmid-containing strain replicated poorly. These results suggest that plasmid-encoded bacterial virulence factors may contribute to the ability of R. equi to replicate within its host cell, the macrophage.

Animals↗
PubMed1994

Production of a hemolytic factor by Candida albicans.

Candida albicans exhibits hemolytic activity when grown on glucose-enriched blood agar. This activity is present on intact organisms, and it is secreted into the culture medium. Hemoglobin released from lysed erythrocytes can restore the transferrin-inhibited growth of C. albicans. We conclude that C. albicans expresses a hemolytic factor which allows it to acquire iron from host erythrocytes.

Animals↗
PubMed1994

High-molecular-weight proteins of nontypeable Haemophilus influenzae mediate bacterial adhesion to cellular proteoglycans.

A family of high-molecular-weight (HMW) surface-exposed proteins of nontypeable Haemophilus influenzae (NT H. influenzae) mediated adherence of these organisms to human epithelium. To better understand the molecular basis for this adherence, the role of glycosaminoglycans (GAGs), substances commonly expressed on cell surfaces, was examined. Bacterial adherence to cells with specific deficiencies in GAG biosynthesis was measured. HMW protein-dependent bacterial adherence to normal cells was significantly greater than adherence to cells deficient in sulfated GAGs or to cells deficient in heparan sulfate but overexpressing chondroitin sulfate. Cells expressing undersulfated heparan sulfate exhibited intermediate levels of bacterial adherence. The addition of exogenous dextran sulfate or heparin inhibited over 70% of the adherence of NT H. influenzae to normal cells, whereas hyaluronic acid and chondroitin sulfate tested at the same concentration (100 micrograms/ml) inhibited bacterial adherence by less than 11%. Treatment of cells with heparinase significantly reduced bacterial adherence. Following electrophoretic separation, HMW proteins were shown to bind directly to radiolabeled heparin. These results indicate that HMW protein-dependent adherence of NT H. influenzae is mediated by cellular sulfated GAGs and that heparan sulfate may be the predominant GAG involved in this process. However, the decreased adherence of bacteria to cells expressing undersulfated heparan sulfate and the inhibition of bacterial adherence by the addition of exogenous dextran sulfate suggest that bacterial adhesion to mammalian cells is likely to be influenced by a variety of factors, including the degree of sulfation and the specificity of the carbohydrate moieties contained in the cellular proteoglycans.

Animals↗
PubMed1994

Receptors on phagocytic cells involved in microbial recognition.

There are two general concepts that we hope to have stressed concerning the recognition of microbes by phagocytic cells. The first is the concept of receptor redundancy and receptor cooperatively. Multiple receptors on leukocytes often participate in a given microbial recognition event. This concept can be illustrated by leishmania promastigotes that utilize both mannose receptors and Mac-1 to bind efficiently to macrophages. Likewise, macrophages use receptors for both IgG and complement to phagocytize encapsulated bacteria. This cooperativity between multiple receptors often changes the apparent affinity of the receptors for their ligand. Consequently the physiology of these receptors is altered. Fibronectin ligation, for example, results in the internalization of C3b-coated particles by the CR1. The second concept concerns the transduction of specific cellular signals following receptor ligation. Often, the receptor to which a microbe binds orchestrates many of the subsequent intracellular events during phagocytosis by transducing specific cellular signals. Some receptors, for example the mannose and Fc gamma-receptors, are particularly well suited to direct particles to phagolysosomes and trigger a respiratory burst, whereas other receptors, for example the CR1, may not. Indeed, from this perspective, one can view the immune response as being designed to target microbes preferentially to those receptors on phagocytic cells capable of making the appropriate cellular responses. In the case of leishmania, phagocytosis mediated by the Fc gamma receptors leads to parasite killing even by resident macrophages, while complement-mediated phagocytosis leads to parasite survival.

Amino Acid Sequence↗
PubMed1993

Leishmania-macrophage interactions: multiple receptors, multiple ligands and diverse cellular responses.

Although a great deal of progress has been made over the last several years in understanding the interactions of leishmania with mammalian cells, much work remains. The consensus from many of these studies is that promastigotes utilize multiple receptors to bind to macrophages. Ongoing studies involving the use of both purified and molecularly cloned receptors and ligands should eventually provide a more detailed understanding of the mechanisms by which promastigotes infect macrophages. At this time, the mechanism(s) involved in the interaction of amastigotes with mammalian cells remains somewhat enigmatic. Since amastigotes are responsible for the cell to cell spread of leishmania, gaining a better understanding of amastigote-macrophage interactions represents an important goal of future leishmania research.

Animals↗
PubMed1993

A heparin-binding activity on Leishmania amastigotes which mediates adhesion to cellular proteoglycans.

The intracellular amastigote form of leishmania is responsible for the cell-to-cell spread of leishmania infection in the mammalian host. In this report, we identify a high-affinity, heparin-binding activity on the surface of the amastigote form of leishmania. Amastigotes of Leishmania amazonensis bound approximately 120,000 molecules of heparin per cell, with a Kd of 8.8 x 10(-8) M. This heparin-binding activity mediates the adhesion of amastigotes to mammalian cells via heparan sulfate proteoglycans, which are expressed on the surface of mammalian cells. Amastigotes bound efficiently to a variety of adherent cells which express cell-surface proteoglycans. Unlike wild-type CHO cells, which bound amastigotes avidly, CHO cells with genetic deficiencies in heparan sulfate proteoglycan biosynthesis or cells treated with heparitinase failed to bind amastigotes even at high parasite-input dosages. Cells which express normal levels of undersulfated heparan bound amastigotes nearly as efficiently as did wild-type cells. The adhesion of amastigotes to wild-type nonmyeloid cells was almost completely inhibited by the addition of micromolar amounts of soluble heparin or heparan sulfate but not by the addition of other sulfated polysaccharides.l Binding of amastigotes to macrophages, however, was inhibited by only 60% after pretreatment of amastigotes with heparin, suggesting that macrophages have an additional mechanism for recognizing amastigotes. These results suggest that leishmania amastigotes express a high-affinity, heparin-binding activity on their surface which can interact with heparan sulfate proteoglycans on mammalian cells. This interaction may represent an important first step in the invasion of host cells by amastigotes.

Animals↗
PubMed1993

The intracellular bacterium Rhodococcus equi requires Mac-1 to bind to mammalian cells.

Rhodococcus equi is a facultative intracellular bacterium of macrophages that causes disease in immunocompromised individuals, particularly those with AIDS. In this report, we demonstrate that R. equi binding to mammalian cells requires complement and is mediated primarily by the leukocyte complement receptor, Mac-1. Bacteria bind to macrophages poorly unless exogenous complement is added to the incubation medium. The addition of fresh nonimmune serum, which contains no detectable antibodies to R. equi, greatly enhances bacterial binding to macrophages, whereas heat inactivation of this serum or immunological depletion of C3 from the serum reduces binding to levels only slightly higher than those of binding under serum-free conditions. Human serum depleted of C2 or C4 is fully opsonic, indicating that complement activation and fixation occur by the alternative pathway. The serum-dependent binding of rhodococci to macrophages is mediated primarily by the macrophage complement receptor type 3, Mac-1 (CD11b/CD18). Bacteria do not bind to fibroblastoid or epithelial cells that lack this receptor. Most of the bacterial binding to macrophages is inhibited by a monoclonal antibody to Mac-1 but is unaffected by a monoclonal antibody to complement receptor type 1. Furthermore, opsonized, but not unopsonized, bacteria bind to purified Mac-1 immobilized on plastic. In addition, in the presence of opsonic complement, rhodococci bind efficiently to fibroblastoid cells transfected with cloned Mac-1 but relatively poorly to cells transfected with the complement receptor type 1. Hence, R. equi fixes complement by activating the alternative complement pathway, and this fixation is a requirement for bacterial adhesion and invasion. Furthermore, complement fixation defines rhodococcal host cell tropism, since R. equi binds specifically and exclusively to cells expressing Mac-1.

Animals↗
PubMed1992

A role for complement receptor-like molecules in iron acquisition by Candida albicans.

Candida albicans, an opportunistic fungal pathogen of humans, is dependent upon iron for growth. Consequently, human serum inhibits C. albicans growth due to the presence of high affinity iron-binding proteins that sequester serum iron, making it unavailable for use by the organism. We report that in the inhibitory environment of human serum, the growth of C. albicans can be restored by the addition of exogenous hemoglobin or heme, but not by protoporphyrin IX, the heme precursor that does not contain iron. We further report that C. albicans can utilize cell surface proteins that are homologues of the mammalian complement receptors (CR) to rosette complement-coated red blood cells (RBC) and obtain RBC-derived iron for growth. The ability of Candida to acquire RBC-derived iron under these conditions is dependent upon Candida-RBC rosetting mediated by CR-like molecules. Unopsonized RBC do not support Candida growth in serum, and restoration of Candida growth in serum by complement-opsonized RBC is inhibited by monoclonal antibodies to the human CR type 3 (CR3). In addition, activation of the human alternative pathway of complement by Candida leads to "bystander" deposition of C3 fragments on the surface of autologous, unopsonized RBC, generating the ligands necessary for Candida-RBC rosetting. These results suggest that C. albicans has evolved a unique strategy for acquiring iron from the host, which exploits the host complement system, and which may contribute to the pathogenic potential of the organism.

Animals↗