title: Inhibition of the hemolytic activity of the first component of complement C1 by an Escherichia coli C1q binding protein.
abstract: Molecular mimicry is a well established mechanism via which bacteria protect themselves from complement-mediated killing. We have previously demonstrated that a number of human cells express receptors for C1q (C1qR) and that the soluble form of this receptor inhibits activation of the classical pathway of complement. We now investigated whether Escherichia coli possesses a C1qR-like protein that protects these bacteria from complement-mediated injury. By FACS analysis it was shown that approximately 60% of the bacteria bound C1q directly in the absence of Abs. With ELISA we confirmed that the bacterial cell envelope was able to bind C1q in a dose-dependent fashion. We isolated a cell envelope associated C1q binding protein (C1qBP) by C1q affinity chromatography, then by anion exchange chromatography and gel filtration chromatography. On SDS-PAGE, the m.w. of C1qBP appeared to be 57 kDa and 51 kDa under reducing and nonreducing conditions, respectively. It was demonstrated that C1qBP specifically binds C1q and inhibits the hemolytic activity of C1q in both a dose- and time-dependent fashion. The binding of C1qBP to C1q is inhibited by C1q itself and also by the collagen-like stalks and the globular heads of C1q. In this respect, bacterial C1qBP is different from human C1qR because the binding of C1q to C1qR is only inhibited by the collagen-like stalks of C1q and not by the globular heads of C1q. C1qBP, when bound to C1q, prevents the assembly with C1r and C1s to form a functional C1 complex. The occurrence of C1qBP is not limited to certain E. coli strains, but is also found on Staphylococcus aureus, Citrobacter freundii, and Pseudomonas aeruginosa. Also, the binding of 125(I)-labeled C1q to these bacteria is specific because the binding of C1q to these bacteria is inhibitable with isolated soluble C1qBP. These findings provide evidence for the existence of a C1qR-like protein on bacteria that might protect them from complement-mediated damage.
authors.affiliation: Department of Nephrology, University Hospital, Leiden, The Netherlands.
authors.forename: R H
authors.initials: RH
authors.lastname: van den Berg
authors.forename: M C
authors.initials: MC
authors.lastname: Faber-Krol
authors.forename: J A
authors.initials: JA
authors.lastname: van de Klundert
authors.forename: L A
authors.initials: LA
authors.lastname: van Es
authors.forename: M R
authors.initials: MR
authors.lastname: Daha
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D001426:Bacterial Proteins / Q000378:metabolism / Q000494:pharmacology; D002352:Carrier Proteins; D003172:Complement C1 / Q000378:metabolism*; D003174:Complement C1 Inactivator Proteins / Q000378:metabolism / Q000494:pharmacology*; D015923:Complement C1r / Q000378:metabolism; D003173:Complement C1s / Q000378:metabolism; D003171:Complement Pathway, Classical; D004926:Escherichia coli / Q000276:immunology / Q000472:pathogenicity; D006461:Hemolysis / Q000187:drug effects; D006801:Humans; D066298:In Vitro Techniques; D018716:Molecular Mimicry; D011506:Proteins / Q000378:metabolism / Q000494:pharmacology*; D011817:Rabbits
chemical_list: D000911:Antibodies, Monoclonal; D001426:Bacterial Proteins; D002352:Carrier Proteins; D003172:Complement C1; D003174:Complement C1 Inactivator Proteins; D011506:Proteins; C007105:c1q-binding proteins; D015923:Complement C1r; D003173:Complement C1s