title: Microbe-specific C3b deposition in the horseshoe crab complement system in a C2/factor B-dependent or -independent manner.
abstract: Complement C3 plays an essential role in the opsonization of pathogens in the mammalian complement system, whereas the molecular mechanism underlying C3 activation in invertebrates remains unknown. To understand the molecular mechanism of C3b deposition on microbes, we characterized two types of C2/factor B homologs (designated TtC2/Bf-1 and TtC2/Bf-2) identified from the horseshoe crab Tachypleus tridentatus. Although the domain architectures of TtC2/Bf-1 and TtC2/Bf-2 were identical to those of mammalian homologs, they contained five-repeated and seven-repeated complement control protein domains at their N-terminal regions, respectively. TtC2/Bf-1 and TtC2/Bf-2 were synthesized and glycosylated in hemocytes and secreted to hemolymph plasma, which existed in a complex with C3 (TtC3), and their activation by microbes was absolutely Mg(2+)-dependent. Flow cytometric analysis revealed that TtC3b deposition was Mg(2+)-dependent on Gram-positive bacteria or fungi, but not on Gram-negative bacteria. Moreover, this analysis demonstrated that Ca(2+)-dependent lectins (C-reactive protein-1 and tachylectin-5A) were required for TtC3b deposition on Gram-positive bacteria, and that a Ca(2+)-independent lectin (Tachypleus plasma lectin-1) was definitely indispensable for TtC3b deposition on fungi. In contrast, a horseshoe crab lipopolysaccharide-sensitive protease factor C was necessary and sufficient to deposit TtC3b on Gram-negative bacteria. We conclude that plasma lectins and factor C play key roles in microbe-specific TtC3b deposition in a C2/factor B-dependent or -independent manner.
authors.affiliation: Graduate School of Systems Life Sciences, Kyushu University, Fukuoka, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Tagawa
authors.forename: Toyoki
authors.initials: T
authors.lastname: Yoshihara
authors.forename: Toshio
authors.initials: T
authors.lastname: Shibata
authors.forename: Kazuki
authors.initials: K
authors.lastname: Kitazaki
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Endo
authors.forename: Teizo
authors.initials: T
authors.lastname: Fujita
authors.forename: Takumi
authors.initials: T
authors.lastname: Koshiba
authors.forename: Shun-ichiro
authors.initials: S
authors.lastname: Kawabata
mesh_terms: D000818:Animals; D003001:Cloning, Molecular; D003167:Complement Activation / Q000276:immunology; D003175:Complement C2 / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D003179:Complement C3b / Q000276:immunology* / Q000378:metabolism; D011415:Complement Factor B / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D018076:DNA, Complementary; D051858:Forkhead Transcription Factors / Q000276:immunology / Q000378:metabolism; D005658:Fungi / Q000276:immunology; D006094:Gram-Positive Bacteria / Q000276:immunology; D006737:Horseshoe Crabs / Q000276:immunology* / Q000382:microbiology; D008274:Magnesium / Q000378:metabolism; D008954:Models, Biological; D011485:Protein Binding / Q000276:immunology; D017434:Protein Structure, Tertiary
chemical_list: D003175:Complement C2; D018076:DNA, Complementary; D051858:Forkhead Transcription Factors; D003179:Complement C3b; D011415:Complement Factor B; D008274:Magnesium