Separation of different forms of the fourth component of human complement by fast protein liquid chromatography

The complement regulator C4b-binding protein (C4BP) interacts with both the C4c and C4dg subfragments of the parent C4b ligand: evidence for synergy in C4BP subsite binding

C4b-binding protein (C4BP) is a multimeric serum protein that is a potent regulator of the classical and lectin complement pathways. The binding site for C4b has been localized to complement control protein (CCP) domains 1-3 of the C4BP alpha-chain and, in particular, to a cluster of positively charged amino acids predicted to be at the interface between CCP 1 and CCP 2. To determine the regions of C4b contributing to C4BP binding, we have examined via surface plasmon resonance technology the binding of the C4c and C4dg subfragments of C4b to C4BP. At half-physiologic ionic strength, specific and saturable binding was observed for both C4c and C4dg. C4c exhibited much greater ionic strength sensitivity in its binding than did C4dg. Analysis of the effect on binding of the subfragments to various C4b-binding-defective C4BP mutants, together with cross-competition experiments, suggests that the subsites in C4BP for C4c and C4dg are adjacent, but distinct. Additionally, we observed synergy in subsite filling such that the presence of C4dg enhanced the extent of C4c binding over its basal level, and vice versa. The enhanced binding of C4c in the presence of C4dg was not due to an increase in affinity but rather reflected a 2-3-fold increase in the number of sites capable of binding C4c. This suggests the existence of a conformational equilibrium between high- and low-affinity states in the C4c binding subsite within each C4BP subunit, an equilibrium which is shifted in favor of the high-affinity state by the filling of the C4dg subsite.

The C4A and C4B isotypic forms of human complement fragment C4b have the same intrinsic affinity for complement receptor 1 (CR1/CD35)

Several previous reports concluded that the C4b fragment of human C4A (C4Ab) binds with higher affinity to CR1 than does C4Bb. Because the isotypic residues, (1101)PCPVLD and (1101)LSPVIH in C4A and C4B, respectively, are located within the C4d region, one may have expected a direct binding contribution of C4d to the interaction with CR1. However, using surface plasmon resonance as our analytical tool, with soluble rCR1 immobilized on the biosensor chip, we failed to detect significant binding of C4d of either isotype. By contrast, binding of C4c was readily detectable. C4A and C4B, purified from plasma lacking one of the isotypes, were Cs converted to C4Ab and C4Bb. Spontaneously formed disulfide-linked dimers were separated from monomers and higher oligomers by sequential chromatographic steps. The binding sensorgrams of C4Ab and C4Bb monomers as analytes reached steady state plateaus, and these equilibrium data yielded essentially superimposable saturation curves that were well fit by a one-site binding model. Although a two-site model was required to fit the equilibrium-binding data for the dimeric forms of C4b, once again there was little difference in the K(D) values obtained for each isotype. Independent verification of our surface plasmon resonance studies came from ELISA-based inhibition experiments in which monomers of C4Ab and C4Bb were equipotent in inhibiting the binding of soluble CR1 to plate-bound C4b. Although divergent from previous reports, our results are consistent with recent C4Ad structural data that raised serious doubts about there being a conformational basis for the previously reported isotypic differences in the C4b-CR1 interaction.

Interaction of Vaccinia Virus Complement Control Protein with Human Complement Proteins: Factor I-Mediated Degradation of C3b to iC3b1 Inactivates the Alternative Complement Pathway

Vaccinia virus complement control protein (VCP) is a virulence determinant of vaccinia virus that helps protect the virus from the complement attack of the host. To characterize the interaction of VCP with C3 and C4 and understand the mechanism by which VCP inactivates complement, we have expressed VCP in a yeast expression system and compared the biologic activity of the purified protein to that of human factor H and complement receptor 1 (CR1). Recombinant VCP bound to C3 and the proteolytically cleaved form of C3 (C3b), but not to the 135,300-m.w. fragment of C3 generated using elastase (C3c) and the 35,000-m.w. fragment of C3 generated using elastase (C3d) and inhibited both the classical and alternative pathways of complement activation. Although rVCP was less effective at inhibiting the alternative pathway than factor H or CR1, it was more effective than factor H at inhibiting the classical pathway. Unlike factor H, rVCP was unable discriminate between alternative pathway-mediated lysis of rabbit and sheep E. A comparison of the cofactor activity in factor I-mediated cleavage of C3b suggested that in contrast to factor H and CR1, which displayed cofactor activity for the three sites, rVCP displayed cofactor activity primarily for the first site, leading to generation of C3b cleaved by factor I between Arg1281-Ser1282 (iC3b1). Its cofactor activity for C4b cleavages was similar to that of soluble complement receptor type 1. Purification and functional analysis of iC3b1 showed that it was unable to interact with factor B to form the alternative pathway C3 convertase, C3b,Bb. These results suggest that the interaction of VCP with C3 is different from that of factor H and CR1 and that VCP-supported first cleavage of C3b by factor I is sufficient to render C3b nonfunctional.

Preparation and properties of a therapeutic inter-alpha-trypsin inhibitor concentrate from human plasma

Inter-alpha-trypsin inhibitor (ITI) is a serine protease inhibitor found in human plasma. Its antiprotease activity is due to bikunin which is effective in various types of experimental shock and pancreatitis. Therefore ITI, which releases bikunin by proteolytic cleavage, could be of therapeutic interest. A method for the large-scale isolation of ITI from human plasma is described. ITI was purified from the prothrombin complex concentrate (PCC) by diethylaminoethyl-Sepharose fast-flow chromatography followed by a chromatographic step on immobilized heparin designed to remove C4, factor X and protein C. With this procedure, which was performed under mild conditions, a homogeneous preparation of native ITI was obtained, as demonstrated by electrophoretic and chromatographic analyses. ITI maintained its biological activity, as exhibited by its specific antitryptic activity of 420 +/- 65 IU/g. In order to decrease or eliminate the risk of transmission of viral disease due to lipid-enveloped viruses, the process incorporated a solvent-detergent treatment. Animal studies on the final product revealed no adverse side-effects in terms of toxicity, thrombogenicity or hypotension. This preparation appears suitable for therapeutic evaluation in animal experimental models.

Application of a monoclonal antibody against a neoepitope on activated C4 in an ELISA for the quantification of complement activation via the classical pathway

In order to study the activation of the complement system via the classical pathway we have attempted to raise antibodies specific for C4 activation products. Of 20 mouse monoclonal antibodies (mAbs) obtained, one appeared to react with an activation dependent epitope exposed on the activation products C4b, C4bi, C4c (C4b/c) as well as on iC4, but not on native C4. Using this antibody as a capture antibody and polyclonal biotinylated antibodies against C4 as detecting antibodies we developed an ELISA for the quantification of C4b/c in biological fluids. The lower limit of detection was approximately 0.025 nmol C4b/c per litre. Mean C4b/c levels in plasma samples collected from healthy volunteers in tubes containing 10 mM EDTA and 0.05% (w/v) polybrene, final concentrations, appeared to be 30 nmol/l. The potential of the ELISA procedure for evaluating complement activation in clinical samples was demonstrated.