Alveolar macrophages (AMs) avidly bind and ingest inhaled environmental particles and bacteria. To identify the particle binding receptor(s) on human AMs, we used functional screening of anti-human AM hybridomas and isolated a mAb, PLK-1, which inhibits AM binding of unopsonized particles (e.g., TiO2, latex beads; 63 ± 5 and 67 ± 4% inhibition, respectively, measured by flow cytometry; n = 11) and unopsonized bacteria (∼84 and 41% inhibition of Escherichia coli and Staphylococcus aureus binding by mAb PLK-1, respectively). The PLK-1 Ag was identified as the human class A scavenger receptor (SR) MARCO (macrophage receptor with collagenous structure) by observing specific immunolabeling of COS cells transfected with human MARCO (but not SR-AI/II) cDNA and by immunoprecipitation by PLK-1 of a protein of appropriate molecular mass (∼70 kDa) from both normal human bronchoalveolar lavage cells (>90% AMs) and human MARCO-transfected COS cells. PLK-1 also specifically inhibited particle binding by COS cells, only after transfection with human MARCO cDNA. Immunostaining showed specific labeling of AMs within human lung tissue, bronchoalveolar lavage samples, as well as macrophages in other sites (e.g., lymph node and liver). Using COS transfectants with different truncated forms of MARCO, allowed epitope mapping for the PLK-1 Ab to MARCO domain V between amino acid residues 420 and 431. A panel of Abs to various SRs identified expression on AMs, but failed to inhibit TiO2 or S. aureus binding. The data support a dominant role for MARCO in the human AM defense against inhaled particles and pathogens.

The primary structure of human macrophage receptor with collagenous structure (MARCO) was determined from cDNA clones and shown to be highly similar to that of mouse (Elomaa, O., Kangas, M., Sahlberg, C., Tuukkanen, J., Sormunen, R., Liakka, A., Thesleff, I., Kraal, G., and Tryggvason, K.

Apoptotic cells are considered to be a major source for autoantigens in autoimmune diseases such as systemic lupus erythematosus (SLE). In agreement with this, defective clearance of apoptotic cells has been shown to increase disease susceptibility. Still, little is known about how apoptotic cell–derived self-antigens activate autoreactive B cells and where this takes place. In this study, we find that apoptotic cells are taken up by specific scavenger receptors expressed on macrophages in the splenic marginal zone and that mice deficient in these receptors have a lower threshold for autoantibody responses. Furthermore, antibodies against scavenger receptors are found before the onset of clinical symptoms in SLE-prone mice, and they are also found in diagnosed SLE patients. Our findings describe a novel mechanism where autoantibodies toward scavenger receptors can alter the response to apoptotic cells, affect tolerance, and thus promote disease progression. Because the autoantibodies can be detected before onset of disease in mice, they could have predictive value as early indicators of SLE.

MARCO is a trimeric class A scavenger receptor of macrophages and dendritic cells that recognizes polyanionic particles and pathogens. The distal, scavenger receptor cysteine-rich (SRCR) domain of the extracellular part of this receptor has been implicated in ligand binding. To provide a structural basis for understanding the ligand-binding mechanisms of MARCO, we have determined the crystal structure of the mouse MARCO SRCR domain. The recombinant SRCR domain purified as monomeric and dimeric forms, and their structures were determined at 1.78 and 1.77 Å resolution, respectively. The monomer has a compact globular fold with a twisted five-stranded antiparallel ␤-sheet and a long loop covering a single ␣-helix, whereas the dimer is formed via ␤-strand swapping of two monomers, thus containing a large eight-stranded ␤-sheet. Calculation of the surface electrostatic potential revealed that the ␤-sheet region with several arginines forms a basic cluster. Unexpectedly, an acidic cluster was found in the long loop region. In the monomer, the acidic cluster is involved in metal ion binding. Studies with cells expressing various SRCR domain mutants showed that all of the arginines of the basic cluster are involved in ligand binding, suggesting a cooperative binding mechanism. Ligand binding is also dependent on the acidic cluster and Ca 2؉ ions whose depletion appears to affect ligand binding at least by modulating the electrostatic potential or relative domain orientation. We propose that the SRCR domain dimerization can contribute to the recognition of large ligands by providing a means for the MARCO receptor oligomerization.MARCO belongs to a diverse group of scavenger receptors (SRs) 3 expressed by macrophages, dendritic cells, and certain endothelial cells (1). These germ line-encoded receptors, also known as pattern recognition receptors due to their ability to recognize conserved pathogen-associated molecular patterns, are considered as an important part of innate immunity, the evolutionarily conserved, first line host defense mechanism. In addition to pathogen-associated molecular patterns, a long list of SR ligands, often polyanionic in nature, includes pollution particles, polyribonucleotides, bacterial lipopolysaccharides, modified host molecules such as oxidized low density lipoprotein (LDL), and unmodified endogenous proteins (1, 2). The SRs are classified into several subgroups, of which class A SRs have primarily been associated with innate immunity. This class consists of five members: SR-A (SR-AI, -II, and -III/ SCARA1) (3, 4), MARCO (macrophage receptor with collagenous domain)/SCARA2 (5), CSR1 (cellular stress response 1) and CSR2/SCARA3 (6), SRCL (scavenger receptor with C-type lectin) I and II/SCARA4 (7, 8), and Tesr (testis-expressed scavenger receptor)/SCARA5 (class A scavenger receptor 5) (9, 10). All of these are trimeric type II membrane proteins with a similar predicted tertiary structure consisting of a short intracellular domain, a transmembrane domain, and a large extracellular domain with...

Nephrin is a signaling protein phosphorylated by Src family kinases.