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.

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.

Activation of murine resident peritoneal macrophages by a cell wall extract of Candida albicans

Ethylenediamine extraction of the cell walls of Candida albicans yields an extract which possesses antigenic activity, and like other whole cell and cell wall preparations of Candida, has been shown previously to possess immunomodulatory activity. We report here results which show that non-elicited, resident peritoneal macrophages treated with the ethylenediamine cell wall extract exhibit morphological characteristics of activated macrophages. Treated cells also manifest an increased respiratory burst response and secrete elevated levels of IL-6. The treatment with the extract appears to provide a co-priming signal to the resting macrophages, since co-stimulation with interferon-gamma results in an increased level of IL-6 and IL-1. Our results also show that extract-treated cells manifest increased responsiveness to a lipopolysaccharide as measured by the production of tumour necrosis factor-alpha. These results provide an additional basis for our understanding of the immunomodulatory activity of components of the Candida cell wall.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Treatment of murine macrophages with interferon-gamma inhibits their ability to bind leishmania promastigotes

The binding of leishmania promastigotes to macrophages pretreated with interferon-gamma (IFN-gamma) was compared to binding to untreated (resident) cells. IFN-gamma-treated macrophages bound fewer leishmania promastigotes than did untreated cells. The decreased binding was apparent over a wide dose range of parasite inocula when the assays were performed in the absence of exogenous complement. This decrease was specific to leishmania, since treated and untreated macrophages bound comparable amounts of immunoglobulin G- and complement-coated sheep red blood cells. Decreased parasite binding occurred early in the macrophage activation pathway. Pretreatment of macrophages with IFN-gamma for as little as 6 h, a time insufficient to induce other macrophage activation parameters, significantly reduced their ability to bind leishmania promastigotes. To determine the mechanism of this decreased phagocytosis by activated cells, macrophages were pretreated with specific inhibitors before the addition of leishmania. The binding of promastigotes to untreated (resident) macrophages was inhibited by approximately 50% by reagents that blocked either of two macrophage receptors, complement receptor type 3 (Mac-1) or a leishmania species-specific lectin-like receptor. Binding to IFN-gamma-treated macrophage populations, in contrast, was substantially inhibited only by antibody to Mac-1. Saccharides that were 50% inhibitory in the resident cell population, decreased binding by less than 10% in activated cells. The lack of saccharide inhibition by IFN-gamma-treated cells was also reflected in an inability of activated macrophages to bind to beads coated with purified leishmania lipophosphoglycan (LPG). These LPG-coated beads bound well to resident macrophages but poorly to activated cells. Thus, leishmania bind to macrophages by two distinct mechanisms, one that utilizes Mac-1 and a second mechanism that does not depend on complement and is saccharide inhibitable. These two binding mechanisms are distinct and differentially regulated in resident and activated cells.

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.

An assay to quantitate the binding of Rhodococcus equi to macrophages

A Rhodococcus equi radiobinding assay has been developed using organisms labeled with 3H-uracil. These labeled organisms resemble their unlabeled counterparts with respect to colony morphology, viability, and buoyant density. Bacteria routinely incorporate between 5 x 10(-3) and 5 x 10(-2) counts per minute per colony forming unit (cfu) which in this assay allows the detection of fewer than 0.2 cfu per macrophage. Once incorporated, greater than 90% of the label remains bacterial associated for at least 4 h postlabeling. The majority of the label is trichloroacetic acid precipitable, partitions into the aqueous phase following phenol/chloroform extraction and is ethanol precipitable. RNAse treatment of the ethanol precipitate abolishes label trichloroacetic acid precipitation. This radiolabeling technique has been used to quantitate the attachment of R. equi to both murine peritoneal and equine alveolar macrophages adherent to 13 mm glass coverslips. R. equi binding is dose dependent, saturable, and specific to macrophages. Further, binding is enhanced in the presence of fresh serum. Inhibition of radiolabeled bacterial binding can be obtained by competition with cold R. equi. This radiolabeled binding assay represents a crucial step in identifying the receptors on macrophages involved in the recognition of R. equi and may help to provide information on how macrophages recognize intracellular bacteria in general.

Platelet factor 4 efficiently reverses heparin anticoagulation in the rat without adverse effects of heparin-protamine complexes

BACKGROUND

It has been observed that the reversal of heparin anticoagulation in humans by protamine sulfate (PS) results in various adverse reactions including leukopenia, thrombocytopenia, activation of complement, increased vascular permeability, systemic hypotension, pulmonary vasoconstriction, and pulmonary edema. The purpose of this study was to compare the efficacy and effects of native platelet factor 4 (PF4) and recombinant platelet factor 4 (rPF4) with those of PS in heparin neutralization in vivo, using a rat model.

METHODS AND RESULTS

Sprague-Dawley rats were anesthetized with sodium pentobarbital, and the right femoral vein and carotid artery were cannulated. For determination of activated partial thromboplastin time, platelet count, white blood cell count, and complement titer, arterial blood samples were taken before and immediately after heparin (10 units/100 g) infusion and at several time points after the infusion of the neutralizing agent (PS, 0.1 mg/100 g; PF4, 0.5 mg/100 g). In separate groups of animals, mean arterial blood pressure was monitored throughout identical protocols and the lungs were prepared for histological examination. The anticoagulant activity of heparin was effectively reversed by all of the neutralizing agents (PS, PF4, and rPF4). Platelet count (48% of initial), white blood cell count (52% of initial), complement titer (60% of initial), and mean arterial pressure (20% decrease) decreased significantly in heparinized animals receiving PS but not in those receiving PF4 or rPF4. Lung interstitium appeared normal when heparin was followed by PF4; however, interstitial edema and hemorrhage were observed with heparin-PS.

CONCLUSIONS

These results suggest that PF4 efficiently reverses heparin anticoagulation in the rat without the adverse effects of heparin-protamine complexes. Therefore, rPF4 may be an appropriate substitute for PS in patients undergoing cardiovascular surgery and other procedures that require heparin anticoagulation.

Leishmania promastigotes require opsonic complement to bind to the human leukocyte integrin Mac-1 (CD11b/CD18)

Previous reports have suggested that Leishmania spp. interact with macrophages by binding to Mac-1 (CD1 1b/CD18), a member of the leukocyte integrin family. To better define this interaction, we tested the ability of leishmania promastigotes to bind to purified leukocyte integrins and to cloned integrins expressed in COS cells. We show that leishmania promastigotes bind to cellular or purified Mac-1 but not lymphocyte function-associated antigen-1 in a specific, dose-dependent manner that requires the presence of serum. Binding is inhibited with specific monoclonal antibodies to Mac-1. In the absence of complement opsonization, three different species of leishmania tested fail to bind directly to any of the three leukocyte integrins. We show that binding to Mac-1 requires the third component of complement (C3). Organisms incubated in heat-inactivated serum or serum that has been immunologically depleted of C3 fail to bind to Mac-1. Because the addition of purified C3 to C3-depleted serum restores leishmania binding to Mac-1, we suggest that parasites gain entry into macrophages by fixing complement and subverting a well-characterized adhesive interaction in the immune system between Mac-1 and iC3b.

Immunosuppression induced by attenuated Salmonella. Evidence for mediation by macrophage precursors

An attenuated aro A- strain of Salmonella typhimurium, SL3235, was previously shown to afford excellent protection in C3HeB/FeJ mice against challenge by virulent Salmonella and cross-protection against Listeria monocytogenes. In the present study, the immunologic status of immunized mice was evaluated by studying their ability to mount in vivo and in vitro plaque-forming cell responses to SRBC. SL3235-immunized mice exhibited marked suppression in their ability to generate anti-SRBC plaque-forming cell responses. Suppression was active and mediated by soluble factors as demonstrated by the capacity of immune cells to suppress the responses of normal cells in co-culture and across a membrane in transwell plates. Depletion of T cells from immune spleens did not alleviate the suppressive activity of the remaining cells. Depletion of splenic adherent cells resulted in partial alleviation of suppressive activity, demonstrating that mature macrophages are partly responsible for mediating the observed suppression. A sequential multi-step depletion procedure resulted in marked enrichment of a second suppressor cell population that was Mac1+, Thy-1.2-, surface Ig-, J11d-, non-adherent, non-phagocytic, and esterase negative. When cultured in vitro in the presence of L cell-conditioned medium, but not in the presence of Con A supernatant, these cells matured into typical macrophages within 72 h of culture. The cell population enriched for macrophage precursors (75%) retained the suppressive capacity of unfractionated splenocytes. Our data indicate that, in addition to the well-described involvement of mature macrophages in suppressing immune responses, bacterial infection may induce appearance of macrophage precursors that may also play an important regulatory role in the immune system.

Immunosuppression induced by attenuated Salmonella. Evidence for mediation by macrophage precursors

An attenuated aro A- strain of Salmonella typhimurium, SL3235, was previously shown to afford excellent protection in C3HeB/FeJ mice against challenge by virulent Salmonella and cross-protection against Listeria monocytogenes. In the present study, the immunologic status of immunized mice was evaluated by studying their ability to mount in vivo and in vitro plaque-forming cell responses to SRBC. SL3235-immunized mice exhibited marked suppression in their ability to generate anti-SRBC plaque-forming cell responses. Suppression was active and mediated by soluble factors as demonstrated by the capacity of immune cells to suppress the responses of normal cells in co-culture and across a membrane in transwell plates. Depletion of T cells from immune spleens did not alleviate the suppressive activity of the remaining cells. Depletion of splenic adherent cells resulted in partial alleviation of suppressive activity, demonstrating that mature macrophages are partly responsible for mediating the observed suppression. A sequential multi-step depletion procedure resulted in marked enrichment of a second suppressor cell population that was Mac1+, Thy-1.2-, surface Ig-, J11d-, non-adherent, non-phagocytic, and esterase negative. When cultured in vitro in the presence of L cell-conditioned medium, but not in the presence of Con A supernatant, these cells matured into typical macrophages within 72 h of culture. The cell population enriched for macrophage precursors (75%) retained the suppressive capacity of unfractionated splenocytes. Our data indicate that, in addition to the well-described involvement of mature macrophages in suppressing immune responses, bacterial infection may induce appearance of macrophage precursors that may also play an important regulatory role in the immune system.

An assay to quantitate the binding of Leishmania amastigotes to macrophages

A leishmania amastigote radiobinding assay has been developed using organisms labeled with tritiated uracil. These labeled amastigotes resemble freshly isolated unlabeled amastigotes in metabolic activity, buoyant density, morphology, viability and their ability to transform into promastigotes. Organisms routinely incorporate between 5 x 10(-3) and 3 x 10(-2) cpm per amastigote, which allows the detection of as little as 1 x 10(4) amastigotes per assay well. This radiolabeling technique has been used to quantitate the attachment of amastigotes to macrophages adherent to either 13 mm coverslips or to 96 well plates. It can also be used to screen monoclonal antibodies to macrophage surface proteins involved in amastigote binding. Once incorporated, the label remains amastigote associated, even after intact organisms have been internalized by macrophages. It remains parasite associated until the organisms have been degraded by macrophages, at which time label is released into the supernatant. Thus, a small adaptation of the binding assay can be used to compare the intracellular survival of amastigotes in macrophages following various experimental manipulations. This amastigote radiolabeling assay, therefore, represents an important step toward determining the receptors on macrophages involved in amastigote recognition and can also be used to study the degradation of intracellular pathogens by macrophages.

Role of complement in mouse macrophage binding of Haemophilus influenzae type b

Previous in vivo studies demonstrated that clearance of encapsulated Haemophilus influenzae from blood is associated with the deposition of C3 on these bacteria and is independent of the later complement components (C5-C9). Since clearance of encapsulated bacteria is determined by phagocytosis of bacteria by fixed tissue macrophages, we studied the interaction of H. influenzae type b with macrophages in vitro. Organisms bound to macrophages in the presence of nonimmune serum. Binding was not evident in heat-treated serum or in serum from complement depleted animals and was inhibited by F(ab')2 fragments of antibody to C3 and by blockade of the macrophage complement receptor type 3. The majority of organisms bound in the presence of complement alone remained extracellular. Antibody in the form of convalescent serum or an IgG1 monoclonal to type b capsule did not increase the total number of organisms associated with macrophages, but did increase the number of organisms ingested. Furthermore, complement enhanced antibody-mediated ingestion. This in vitro study demonstrates that complement largely mediates binding of H. influenzae to macrophages. This binding may be critical in determining the early clearance of these bacteria from blood and may be an important mechanism of defense in the nonimmune, as well as the immune host.

The third component of complement (C3) is responsible for the intracellular survival of Leishmania major

Leishmania are obligate intracellular parasites of mononuclear phagocytes. We and others have shown that the promastigote form of all species of leishmania activates complement from non-immune serum and that this activation can result in parasite lysis. This work, as well as earlier in vivo studies, suggested that complement is an important component of host defence against leishmaniasis. We now present evidence that parasite complement fixation, in addition to increasing parasite phagocytosis, is required for the intracellular survival of leishmania in macrophages. We specifically show a strong correlation between parasite C3 fixation and intracellular survival. We attribute this survival, in part, to a decrease in the magnitude of the macrophage respiratory burst which is triggered by complement-coated, as opposed to uncoated, parasites.

Leishmania promastigotes are recognized by the macrophage receptor for advanced glycosylation endproducts

In this paper we demonstrate the involvement of the macrophage receptor for advanced glycosylation endproducts (AGE) in the phagocytosis of Leishmania major promastigotes. Blocking of this receptor with the ligand, AGE-BSA, leads to a 50% decrease in phagocytosis relative to controls, and a comparable decrease in the respiratory burst. The inhibition of phagocytosis by AGE-BSA was specific to leishmania. The binding of zymosan or C3bi-RBC and the phagocytosis of IgG-RBC or latex beads was not affected by the presence of AGE-BSA. Blocking of both the AGE receptor and CR3 decreases leishmania binding by nearly 90%, and reduces the respiratory burst by 80%, indicating that the two receptors account for the bulk of L. tropica promastigote recognition and uptake by the macrophage.

Leishmania species: mechanisms of complement activation by five strains of promastigotes

The interaction of fresh serum with promastigotes of Leishmania major, L. donovani, L. mexicana mexicana, L. mexicana amazonensis, and L. braziliensis guyanensis results in lysis of all strains tested with either fresh human or guinea pig serum at 37 C for 30 min. Lysis does not occur in the cold and requires divalent cations and complement that is active hemolytically. Serum deficient in the eighth component of complement is not lytic. Lysis of L. major, L. mexicana, and L. braziliensis proceeds fully in human serum containing EGTA/Mg2+ or in guinea pig serum deficient in the fourth complement component. These species consume only small amounts of C4 from human serum and do not require calcium to optimally bind C3. The data indicate that all are activators of the alternative complement pathway and that the classical pathway is not required for the lysis of these organisms. Promastigotes of L. donovani, in contrast, activate the classical pathway. The presence of calcium is required for both optimal C3 binding and parasite lysis, and L. donovani promastigotes consume C4 when incubated in human serum. In high concentrations, human serum agglutinates all tested Leishmania spp. The agglutinating factor does not require divalent cations, is heat stable, and works at 4 C, suggesting that it is an antibody. This "naturally occurring" antibody cross reacts with all Leishmania spp. and agglutinates them. The adsorption of serum with any Leishmania species or with beads that are Protein A coated, removes the agglutinogen. This factor causes a slight enhancement in alternative pathway activation by L. major and mediates the classical activation by L. donovani. In adsorbed serum, L. donovani promastigotes only weakly activate the alternative complement pathway. Increased concentrations of adsorbed serum are therefore necessary for lysis to proceed. The titer can be partially restored by the addition of heat inactivated serum. Using purified components of the classical cascade, we are unable to visualize surface bound C3 on L. donovani promastigotes unless heat inactivated serum is also present. We conclude that all Leishmania spp. promastigotes are susceptible to lysis by normal serum independent of antibody. The presence of small amounts of naturally occurring antibody in human serum enhances the susceptibility of L. donovani promastigotes to lysis by activating the classical complement pathway.