|Detection of MARCO in Mouse Splenocytes by Flow Cytometry. Mouse splenocytes were stained with Rat Anti-Mouse MARCO Fluorescein-conjugated Monoclonal Antibody (Catalog # FAB2956F, filled histogram) or isotype control antibody (Catalog # IC005F, open histogram). View our protocol for Staining Membrane-associated Proteins.|
MARCO (macrophage receptor with collagenous structure), also known as SCARA2, is an 80 kDa type II transmembrane glycoprotein that belongs to the class A scavenger receptor family (1). Mouse MARCO consists of a 48 amino acid (aa) cytoplasmic domain, a 21 aa transmembrane segment, and a 449 aa extracellular domain (ECD) that includes a stalk region, a collagen-like region, and one SRCR domain (2). Within the ECD, mouse MARCO shares 69% and 86% aa sequence identity with human and rat MARCO, respectively. It shares 18%‑28% aa sequence identity with other mouse class A scavenger receptors CL-P1, SCARA3, SCARA5, and SR-A1/MSR. MARCO is constitutively expressed on the surface of splenic and lymph node macrophages (2, 3). Its expression is induced on Kupffer cells and alveolar macrophages by microbial infection, chemical irritants, and Th1 polarizing factors (3‑5). MARCO binds LPS, lipoteichoic acid, and other determinants on Gram positive and Gram negative bacteria (2, 6‑8). It also binds modified LDL, CpG oligonucleotides, UGRP1, silica, and TiO2 (2, 9‑11). MARCO is required for the organization of the splenic marginal zone and the interaction of local macrophages and B cells (12, 13). The SRCR domain mediates binding of MARCO to its various ligands (3, 12), while the collagen-like region mediates assembly into a disulfide-linked trimeric molecule (2, 7). MARCO ligation induces, but is not required for the production of IL-12, NO, or TNF-alpha by macrophages (5, 6, 9). MARCO knockout mice show a reduced clearance of bacterial infections, reduced mast cell mediated silicosis, increased pulmonary inflammation, and increased sensitivity to ozone induced lung damage (4, 9, 14‑16).