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PubMed1982

The binding and activation of the Clr-Cls subunit of the first component of human complement.

The value of the functional affinity constant between 125I-labelled Clq and the Clr-Cls tetramer (when free in solution) in the formation of Cl was found to be 3.6 X 10(7) M-1. When Clq was bound to activating immune complexes, the value of K was about 10-fold higher before initiation of activation and there was a further two to three-fold rise as activation proceeded. The addition of an excess of unlabelled Clq increased the rate of activation of 125I-labelled Cl, suggesting an interaction between Clr-Cls and two neighbouring Clq molecules. It is suggested that the tetramer Clr-Cls may bind bivalently to Clq when free in solution, but on binding to activating complexes, one of the Clr-Cls binding sites is detached from Clq and becomes bound to a site on the complex. The resultant spatial rearrangement of the Clr molecules within the tetramer may be optimal for autocatalytic activation of Clr.

Antigen-Antibody Complex↗
PubMed1985

Hyaluronic acid-complement interactions--II. Role of divalent cations and gelatin.

Native hyaluronic acid (HA) is reported to be a weak anticomplementary agent. However, the normal buffer systems used for complement tests incorporate gelatin, Ca2+ and Mg2+, which may bind to HA, influence its conformation and interfere with its anticomplementary reactions with complement components such as Cl. In this study, metal ions (Ca2+ and Mg2+), gelatin and fibronectin appeared to react with native HA preparations and block their anticomplementary effects on Cl. In previous studies, we obtained evidence for a relationship between reversible heat-induced HA conformational changes and a subsequent reversible increase in anticomplementary activity. The anticomplementary activity of heat-treated HA preparations was also reduced by gelatin.

Binding Sites↗
PubMed1985

Mouse monoclonal antibodies at the red cell surface--II. Effect of hapten density on complement fixation and activation.

Complement (C)-dependent hemolytic dose-response curves of anti-TNP IgG2b and IgM monoclonal antibodies as a function of TNP density were analyzed: sheep red cells coupled with TNP served as targets. Under conditions when equal numbers of either IgG2b or IgM anti-TNP antibodies were taken up by cells with various TNP densities, both antibodies showed optimal activity at a hapten density of approximately 10(6) TNP/E with regard to C-mediated lysis. These results were similar to those obtained with polyclonal antibodies. The effectiveness of monoclonal antibodies in utilizing guinea pig or mouse C was also investigated. IgM anti-TNP monoclonal antibodies lysed E-TNP in the presence of guinea pig C, but failed to produce lysis in the presence of mouse C. Two monoclonal IgG1 (anti-TNP and anti-SRBC) and an IgG2a anti-TNP antibody failed to produce hemolysis in the presence of guinea pig and mouse C. IgG2b monoclonal antibodies, whether directed against TNP or Forssman antigen, activated guinea pig and, to a lesser extent, mouse C. Finally, monoclonal anti-TNP IgG3 antibodies exhibited a low but measurable hemolytic activity with guinea pig C (10-20 times below that of IgG2b).

Animals↗
PubMed1986

Human IgM antibodies do not activate guinea-pig complement after interaction with soluble antigen.

Activation of guinea-pig complement by human IgM antibodies after interaction with a particulate antigen is well established. Human IgM antibodies directed against meningococcal group-C capsular polysaccharide, however, were not able to fix guinea-pig complement in a classical complement fixation test with soluble antigen. In the same test, human complement was readily activated by these antibodies. This inability of human IgM antibodies to activate guinea-pig complement after interaction with soluble antigen was confirmed in other antigen systems. In contrast, efficient activation of both human and guinea-pig complement was found with the same antigen in a particulate form.

Animals↗
PubMed1988

Enhancement of the binding of C1q to immune complexes by polyethylene glycol.

The binding of 125I-labelled human C1q to insoluble rabbit IgG:ovalbumin immune complexes was enhanced by polyethylene glycol (PEG, Mr 8 x 10(3)) in the concn range 0-2.5% (w/v). C1q with native immunoglobulin bindings sites rendered inactive by diethylpyrocarbonate treatment did not bind to immune complexes in the presence of PEG. The ionic strength dependence of the binding was independent of the presence of PEG. There was a linear relationship between the logarithm of the apparent affinity constant of the C1q:immune complex interaction and PEG concn.

Animals↗
PubMed1988

Guinea pig macrophages synthesize a low molecular weight form of C1q with affinity for the C1r2C1s2-complex but which does not bind to Fc in immunoglobulin aggregates.

Biosynthetically labelled C1q secreted by guinea pig peritoneal macrophages was analysed by sedimentation through sucrose gradients followed by SDS-PAGE. In addition to the haemolytically active C1q of mol. wt 460,000 Da a low mol. wt (LMW) form of C1q was identified which had no detectable affinity for Fc of aggregated immunoglobulin, but which retained the ability to associate with the C1r2s2-complex. This LMW-C1q was covalently associated with two additional polypeptides of mol. wt 46 and 50 kDa.

Animals↗