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.
Antón
L C
LC
Department of Immunology, Fundación Jiménez Díaz, Madrid, Spain.
Alcolea
J M
JM
Sánchez-Corral
P
P
Marqués
G
G
Sánchez
A
A
Vivanco
F
F
D000936:Antigen-Antibody Complex / Q000378:metabolism*; D001665:Binding Sites; D003166:Complement Activating Enzymes / Q000378:metabolism; D003167:Complement Activation; D003172:Complement C1 / Q000378:metabolism; D015922:Complement C1q; D003176:Complement C3 / Q000378:metabolism*; D003170:Complement Pathway, Alternative; D006801:Humans; D019811:Hydroxylamine; D006898:Hydroxylamines / Q000494:pharmacology; D007074:Immunoglobulin G / Q000378:metabolism*
D000936:Antigen-Antibody Complex; D003172:Complement C1; D003176:Complement C3; D006898:Hydroxylamines; D007074:Immunoglobulin G; D019811:Hydroxylamine; D015922:Complement C1q; D003166:Complement Activating Enzymes