AMINO-ACID DIFFERENCES IN THE N-TERMINUS OF C(H)2 INFLUENCE THE RELATIVE ABILITIES OF IGG2 AND IGG3 TO ACTIVATE COMPLEMENT

The four human IgG isotypes are highly conserved in amino acid sequenc e, but show differential ability to activate complement (C'):IgG3 and IgG1 are very active, IgG2 is active under certain conditions, and IgG 4 is inactive. Although the second constant domain (C(H)2) is critical for C' activation, the individual amino acids that confer isotype-spe cific activity have not been identified. We have generated a series of mutants between IgG2 and IgG3, resulting in the exchange of the four N-terminal and six C-terminal polymorphic residues within C(H)2. Mutan ts containing the N-terminus of the C(H)2 of IgG3 were as effective as wildtype IgG3 in C1q binding, C1 activation and terminal complex (MAC ) formation, but had reduced ability to effect C'-mediated lysis. IgG2 and mutants containing the N-terminal portion of the C(H)2 of IgG2 we re reduced compared to IgG3 in activating C1, binding C1q and inducing assembly of the MAC, and were inactive in mediating lysis of target c ells. Thus, the amino acid sequence differences in the N-terminus of C (H)2 play a critical role in determining the relative abilities of IgG 2 and IgG3 to bind C1q and activate the C' cascade although additional residues of C(H)2 must be involved in mediating optimal target cells lysis. The sequence of the N-terminus of C(H)2 was less critical in de termining C4 and C3 binding. Characterization sf domain exchange mutan ts suggests that intermediate steps may be partly dependent on domains other than C(H)2. IgGs that do not direct target cell lysis neverthel ess activate intermediate steps in the pathway, which may contribute t o immune complex-associated disorders. (C) 1997 Elsevier Science Ltd. All rights reserved.