The influence of classical pathway components during alternative pathway--modulated immune complex aggregation: the role of C1 INH.
Alternative pathway (AP)-triggered reactions as well as classical pathway (CP)-mediated ones, were investigated turbidimetrically and/or immune electrophoretically, either in the presence or in the absence of in situ-generated immune complexes (ICs; tetanus toxoid/human anti-tetanus toxoid-IgG; ICs of equivalence) during the early stages of reaction. Monospecific Fab'- or Fab-fragments (rabbit) were used to block the complement function in normal human serum (NHS). C1q, functionally available following the addition of ethylene-glycol-bis-(beta-aminoethyl ether), N,N'-tetraacetic acid to NHS (EGTA-NHS), was found to increase the IC aggregation, thereby producing a biological surface upon which AP-dependent proteins were deposited. The functional inhibition of C1INH caused a C1s-mediated C3 conversion irrespective of the fact whether C1s was incorporated within macromolecular C1 (NHS) or dissociated from it (EGTA-NHS), thus, in the latter case inhibiting the AP-dependent portion of turbidity. It seemed probable that C3 conversion was effected by a fluid-phase CP C3 convertase. This process, normally counteracted by C1INH, worked more efficiently in EGTA-NHS than in NHS, indicating that the C1s-mediated reactions, initiated by presently unknown mechanisms, were less extensively regulated outside of the Ca2+-dependent C1 complex. The study demonstrates that in EGTA-NHS, too, where AP-triggered reactions have usually been investigated, sections of CP activation may play an important role, especially in situations where the function of C1INH is restricted.
D000936:Antigen-Antibody Complex; D003166:Complement Activating Enzymes / Q000276:immunology; D003167:Complement Activation; D003174:Complement C1 Inactivator Proteins / Q000276:immunology*; D003173:Complement C1s; D003176:Complement C3; D003170:Complement Pathway, Alternative; D003171:Complement Pathway, Classical; D006801:Humans; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D066298:In Vitro Techniques; D013745:Tetanus Toxoid
D000936:Antigen-Antibody Complex; D003174:Complement C1 Inactivator Proteins; D003176:Complement C3; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D013745:Tetanus Toxoid; D003166:Complement Activating Enzymes; D003173:Complement C1s