A general theory for the precipitin reaction--based on Fc-mediated precipitation

Lupus-derived autoantibodies with dual autoactivity: anti-DNA and anti-Fc. II. Fine specificity of anti-self autoreactivity

The anti-immunoglobulin reactivity of two monoclonal, dual specific, autoantibodies, BV 17-45 and BV 04-01 was examined. The current study further defined the anti-immunoglobulin autoreactivity of these MoAbs to be Fc-specific. Both BV 17-45 and BV 04-01 bound their own Fc domains in addition to Fc regions of other MoAbs of similar isotype with varying levels of activity. The different anti-Fc reactivity patterns of BV 17-45 and BV 04-01 suggested that these MoAbs recognized distinct epitopes. Neither BV 17-45 nor BV 04-01 bound Fab fragments or single-chain antibody derivatives, which confirmed that the anti-immunoglobulin reactivity of these autoantibodies was Fc-specific. In addition, abrogation of anti-Fc reactivity was observed when affinity-labelled MoAbs were used as coating antigens in solid-phase ELISAs. These results implied that active-site ligand binding induced conformational changes which altered the Fc epitope(s) recognized by BV 17-45 and BV 04-01.

C3 binds covalently to the C gamma 3 domain of IgG immune aggregates during complement activation by the alternative pathway

Ovalbumin-antiovalbumin IgG immune aggregates were incubated with normal human serum in the presence of iodo[1-14C]acetamide, in conditions in which only the alternative pathway of complement was activated. The [14C]C3b-IgG covalent complexes formed were digested with pepsin, and analysed by SDS/polyacrylamide-gel electrophoresis and fluorography. Covalent complexes of [14C]C3-Fd and [14C]C3-pFc' were visualized, demonstrating that, during complement activation by the alternative pathway, C3 is covalently incorporated into the C gamma 3 domain of IgG, as well as into the Fd region. The C gamma 2 domain becomes protected from pepsin action by the bound C3b. All the covalent linkages between C3 and the IgG were sensitive to hydroxylamine. When [14C]C3-pFc' covalent complexes were treated with 1 M-NH2OH and loaded onto a Bio-Gel P-4 column, a radioactive peak of 3 kDa was obtained. The material released from [14C]C3-pFc' and [14C]C3-F(ab')2 complexes after treatment with 1 M-NH2OH was mixed and analysed in the Bio-Gel P-4 column. A similar radioactive peak of 3 kDa was obtained. When this peak, either from [14C]C3-pFc' alone or from the mixture of [14C]C3-F(ab')2 and [14C]C3-pFc', was fractionated by h.p.l.c., virtually the same radioactive peptide profile was obtained, indicating that very similar C3 peptides remained covalently bound to both regions (Fab and C gamma 3) of the antibody molecule. It is suggested that C3 bound to the C gamma 3 domain of IgG may interfere with the Fc-Fc interactions of immune aggregates and thus may be involved in several biological properties displayed by these complement-activating aggregates.