To study the interaction between C4b-binding protein (C4BP) and Riemerella anatipestifer (RA), we cloned duck C4BPα, conducted prokaryotic expression and prepared the polyclonal antibody by immunizing mice. Then indirect immunofluorescence assay and dot blotting hybridization assay were used to verify the interaction between C4BP and RA. The full length of duck C4BPα nucleotide sequence was 1 230 bp, with the highest similarity to chicken C4BPα (82.1%). Phylogenetic tree analysis showed that duck C4BPα and chicken C4BPα were on the same phylogenetic tree branch and the genetic evolution relationship between them was the closest. C4BPα was efficiently expressed in Escherichia coli BL21 (DE3). The recombinant proteins existed in intracellular soluble form. The titer of polyclonal antibody was more than 1:10 000 and polyclonal antibodies could specifically recognize the recombinant proteins. The results of indirect immunofluorescence assay and dot blot hybridization assay showed that RA could interact with duck C4BP. The results provide a basis to further reveal the pathogenesis of RA.
Animals ; Cloning, Molecular ; Complement C4b-Binding Protein ; chemistry ; genetics ; metabolism ; Ducks ; classification ; genetics ; microbiology ; Gene Expression Regulation ; Mice ; Phylogeny ; Riemerella ; metabolism

Proteins of the complement system are known to interact with many charged substances. We recently characterized binding of C1q and factor H to immobilized and liposomal anionic phospholipids. Factor H inhibited C1q binding to anionic phospholipids, suggesting a role for factor H in regulating activation of the complement classical pathway by anionic phospholipids. To extend this finding, we examined interactions of C1q and factor H with lipid A, a well-characterized activator of the classical pathway. We report that C1q and factor H both bind to immobilized lipid A, lipid A liposomes and intact Escherichia coli TG1. Factor H competes with C1q for binding to these targets. Furthermore, increasing the factor H: C1q molar ratio in serum diminished C4b fixation, indicating that factor H diminishes classical pathway activation. The recombinant forms of the Cterminal, globular heads of C1q A, B and C chains bound to lipid A and E. coli in a manner qualitatively similar to native C1q, confirming that C1q interacts with these targets via its globular head region. These observations reinforce our proposal that factor H has an additional complement regulatory role of down-regulating classical pathway activation in response to certain targets. This is distinct from its role as an alternative pathway down-regulator. We suggest that under physiological conditions, factor H may serve as a downregulator of bacterially-driven inflammatory responses, thereby fine-tuning and balancing the inflammatory response in infections with Gram-negative bacteria.
Binding, Competitive ; immunology ; Complement Activation ; immunology ; Complement C1q ; chemistry ; immunology ; metabolism ; Complement C4b ; analysis ; Complement Factor H ; chemistry ; immunology ; metabolism ; Complement Pathway, Classical ; immunology ; Escherichia coli ; immunology ; metabolism ; Humans ; Iodine Radioisotopes ; Isotope Labeling ; Lipid A ; immunology ; metabolism ; Liposomes ; immunology ; metabolism ; Protein Binding ; immunology ; Recombinant Proteins ; chemistry ; immunology ; metabolism ; Substrate Specificity