title: The class A macrophage scavenger receptor type I (SR-AI) recognizes complement iC3b and mediates NF-κB activation.
abstract: The macrophage scavenger receptor SR-AI binds to host tissue debris to perform clearance and it binds to bacteria for phagocytosis. In addition, SR-AI modulates macrophage activation through cell signaling. However, investigation of SR-AI signaling on macrophages is complicated due to its promiscuous ligand specificity that overlaps with other macrophage receptors. Therefore, we expressed SR-AI on HEK 293T cells to investigate its ligand binding and signaling. On 293Tcells, SR-AI could respond to E. coli DH5α, leading to NF-κB activation and IL-8 production. However, this requires E. coli DH5α to be sensitized by fresh serum that is treated with heat-inactivation or complement C3 depletion. Anti-C3 antibody inhibits the binding of SR-AI to serum-sensitized DH5α and blocks DH5α stimulation of SR-AI signaling. Further analysis showed that SR-AI can directly bind to purified iC3b but not C3 or C3b. By mutagenesis, The SRCR domain of SR-AI was found to be essential in SR-AI binding to serum-sensitized DH5α. These results revealed a novel property of SR-AI as a complement receptor for iC3b-opsonized bacteria that can elicit cell signaling.
authors.affiliation: Department of Microbiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117597, Singapore.
authors.forename: Jason W K
authors.initials: JW
authors.lastname: Goh
authors.forename: Yen Seah
authors.initials: YS
authors.lastname: Tan
authors.forename: Alister W
authors.initials: AW
authors.lastname: Dodds
authors.forename: Kenneth B M
authors.initials: KB
authors.lastname: Reid
authors.forename: Jinhua
authors.initials: J
authors.lastname: Lu
mesh_terms: D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D003179:Complement C3b / Q000378:metabolism*; D004926:Escherichia coli / Q000276:immunology; D057809:HEK293 Cells; D006801:Humans; D008262:Macrophage Activation; D008969:Molecular Sequence Data; D016296:Mutagenesis; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism*; D010587:Phagocytosis; D000068103:Serine-Arginine Splicing Factors; D015398:Signal Transduction
chemical_list: D002352:Carrier Proteins; D016328:NF-kappa B; C503566:SCAF1 protein, human; D000068103:Serine-Arginine Splicing Factors; D003179:Complement C3b