Breakdown of C 3 after Complement Activation

The reaction mechanisms of C3 are central to both the classical and alternative pathways of complement activation and have in consequence been extensively studied (see 1 and 2 for reviews). The activation of C3 by cleavage to C3b is brought about by the C3-converting enzymes of the classical and alternative pathways, as well as by a number ofnoncomplement enzymes. C3b is subsequently inactivated and prevented from further participation in the C3b feedback cycle and in the hemolytic reaction by cleavage to C3bi by factor I, which splits C3b when the latter is combined with factor H. C3bi is the C3 conversion product that appears in serum in vitro after complement activation by either pathway, and it is fairly clear that the C3 product, which was known as "beta 1A" in much of the earlier complement literature, is C3bi and not C3c, the fragment with which beta 1A has usually been identified (3). C3bi does, however, undergo further breakdown both in serum and when cell bound. It was originally shown by Lachmann and Mi.iller-Eberhard (4) that the conglutinable form of C3 bound on cells (cell-bound C3bi) could be cleaved by 1/~g/ml of trypsin to lose its conglutinability. This was accompanied by the elution of C3c from the cell, C3d remaining bound. In serum, too, C3 gave rise, on aging, to two fragments, one of which was described as beta 1A and the other, a fast-migrating fragment, as alpha 2D (5). Alpha 2D was generally equated with C3d (3). As will be pointed out in this paper, this identification is not wholly correct. Antigenic analysis with polyclonal antisera to C3 had shown the existence of distinct antigenic specificities in C3a, C3c, and C3d, as well as antigenic determinants found only in native C3 (5, 6). Studies with monoclonal anti-C3 antibodies (7) showed one that reacted with C3c, one with C3d, and one that showed an unusual pattern of reactivity that has now been found to react with the fragment here called C3g. By the use of the monoclonal antibodies, the breakdown of C3bi both on cells and in plasma has been reinvestigated, and the results of this investigation are presented herein.