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.

The serum mannose-binding protein and the macrophage mannose receptor are pattern recognition molecules that link innate and adaptive immunity.

The innate immune system evolved to protect the host in the early phases of an infectious challenge. The soluble mannose binding protein, and the cell surface mannose receptor are two key pattern recognition molecules of innate immunity. The ligand binding specificity of these molecules enables them to differentiate 'self' from 'non-self'. These pattern recognition capabilities are coupled to effector functions, which enable them to interact with other molecules of the immune system. In this way, these pattern recognition molecules are able to serve as a link between the innate and adaptive immune systems.

Murine macrophage scavenger receptor: in vivo expression and function as receptor for macrophage adhesion in lymphoid and non-lymphoid organs.

Macrophage scavenger receptors are trimeric integral membrane glycoproteins which have been implicated in various macrophage functions including uptake of oxidized lipoprotein and the serum-dependent, divalent cation-independent adhesion of macrophages to tissue culture-treated plastic. In this study we have used a recently defined monoclonal antibody (2F8) which recognizes murine macrophage scavenger receptor, to explore its expression in lymphoid and non-lymphoid organs of the normal adult. Scavenger receptor was detected in the red pulp and marginal zone of normal adult mouse spleen, medulla of the thymus and subcapsular region of lymph nodes. Kupffer cells in the liver, alveolar macrophages in the lung and lamina propria macrophages in the gut all reacted with 2F8 monoclonal antibody. The antigen was not detected on any non-macrophage cells, with the exception of sinusoidal endothelial cells in the liver. In the spleen, lymph node and liver, scavenger receptor antigen expression was associated specifically with phagocytic cells which had taken up colloidal carbon. To examine macrophage adhesion in a context relevant to the interactions occurring within lymphoid and non-lymphoid organs, and the contribution of macrophage scavenger receptor to this adhesion, we designed an assay of macrophage adhesion to frozen tissue sections. Adhesion to most tissues was high and uniform in the absence of any chelating agents. The chelation of Ca2+ and Mg2+ revealed specific patterns of macrophage adhesion in lymphoid and non-lymphoid organs which was completely inhibited by 2F8. The ability of this antibody to block the EDTA-resistant adhesion correlated with tissue expression of the antigen in some tissues. Unlike adhesion to tissue culture-treated plastic, macrophage scavenger receptor-dependent adhesion of macrophages to frozen tissue sections did not exhibit an absolute requirement for exogenous fetal bovine serum indicating the presence of an endogenous ligand for scavenger receptor within the tissues. We propose that macrophage scavenger receptor is a candidate homing or retention molecule for macrophage localization within ligand-rich tissues.

Murine M phi scavenger receptor: adhesion function and expression.

In the absence of divalent cations M phi lose their spread morphology but remain adherent to tissue culture treated plastic. We have exploited this property of M phi adhesion in vitro, to isolate a rat monoclonal 2F8, which totally inhibits the divalent cation-independent adhesion of M phi to tissue culture treated plastic and is partly responsible for the trypsin-resistant adhesion of M phi to the same substratum. Immunoprecipitation from macrophages and stably transfected Chinese Hamster Ovary Cells revealed that the antigen recognised by monoclonal antibody 2F8 is identical to the murine macrophage scavenger receptor. Macrophage scavenger receptors are trimeric integral membrane glycoproteins which have been implicated in various macrophage functions including uptake of oxidised lipoprotein and the formation of foam cells in atherosclerotic lesions. We have used monoclonal antibody 2F8 to explore the expression of murine macrophage scavenger receptor in lymphoid and non-lymphoid organs of the normal adult mouse and to examine the contribution of macrophage scavenger receptor to macrophage adhesion within tissues.

Cytokine and growth factor regulation of macrophage scavenger receptor expression and function.

Regulation of macrophage scavenger receptor (MSR) activity may be an important determinant of the extent of atherogenesis and the efficacy of host defense. The effect of M-CSF on this pathway was studied using a recently developed monoclonal antibody to murine MSR. M-CSF markedly and selectively increased MSR synthesis in murine macrophages (M phi); post-translationally the receptor appeared more stable and shifted to a predominantly surface distribution. Functionally M-CSF enhanced modified lipoprotein uptake and increased divalent cation-independent adhesion in vitro. These results suggest a plausible mechanism whereby M-CSF production in the atheromatous plaque microenvironment could promote the recruitment and retention of mononuclear phagocytes and subsequent foam cell formation. In addition, the Th1 cytokine (gamma-interferon) and Th2 cytokine (interleukin-4) had differential effects on MSR glycosylation in vitro suggesting a further possible regulatory role by these lymphokines on macrophage MSR function.

Murine erythroleukemia (MEL) cells bear ligands for the sialoadhesin and erythroblast receptor macrophage hemagglutinins.

While many studies of erythropoiesis have focused on interactions between developing cells and the extracellular matrix, scant attention has been paid to the role of erythroid-macrophage interactions in this process. Murine erythroleukemia (MEL) cell lines have been used extensively in these studies to define erythroid adhesion events. In this study, we compared the adhesion to both fibronectin and macrophages of MEL cells and primary fetal liver erythroblasts. The avidity of MEL cells for purified mouse fibronectin was greater than that of the primary cells. In addition, we have shown that MEL cells carry ligands/counter-receptors for sialoadhesin (Sn) and the erythroblast receptor (EbR), two hemagglutinins expressed by hematopoietic stromal macrophages. These two macrophage-binding activities were independent of erythroid fibronectin-binding activity. Proteolysis of MEL cell surface receptors was necessary to reveal these activities, with this process providing a means for using MEL cells as ligand cells in studies of macrophage hemagglutinins. While erythroid adhesion to fibronectin and components of the extracellular matrix may be of importance in early erythropoiesis, hemagglutinin-expressing stromal macrophages provide the appropriate microenvironmental niche for the terminal stages of erythropoiesis preceding the release of enucleated cells into the circulation.

Macrophage-colony-stimulating factor selectively enhances macrophage scavenger receptor expression and function.

Regulation of macrophage scavenger receptor (MSR) activity may be an important determinant of the extent of atherogenesis. The effect of macrophage-colony-stimulating factor (M-CSF) on this pathway was studied using a recently developed monoclonal antibody to murine MSR. M-CSF markedly and selectively increased MSR synthesis in murine macrophages: posttranslationally, the receptor appeared more stable and shifted to a predominantly surface distribution. Functionally, M-CSF enhanced modified lipoprotein uptake and increased divalent cation-independent adhesion in vitro. These results suggest a plausible mechanism whereby M-CSF production in the atheromatous plaque microenvironment could promote the recruitment and retention of mononuclear phagocytes and subsequent foam cell formation.