Analysis of human C8 with monoclonal antibodies: Characterization of a monoclonal antibody that recognizes free C8α-γ subunit

The eighth component of human C is essential for the formation of the membranolytic C attack complex. C8 has a unique structure in that two covalently linked chains, C8α and C8γ, are associated non-covalently with the third chain, C8β. In order to study the structure and assembly of the C8 molecule, a panel of mAb has been produced against the C component C8. Eight of these mAb had reactivity to the C8α-γ subunit, whereas four reacted with C8β. One of the C8α-γ mAb, C8A2, had specificity for an epitope on the C8α-chain and exhibited no cross-reactivity to any of the other terminal C components, including C8β. C8A2 inhibited the hemolytic activity of the C8α-γ subunit but had no effect on the activity of fluid phase whole C8 or C8 within membrane-bound C5b-8. Functional experiments suggest that C8A2 inhibits C8α-γ activity by interfering with its interaction with the C8β-chain. In an enzyme immunoassay using the C8A2 mAb, free C8α-γ subunit could be detected in both homozygous and heterozygous C8β-deficient serum. However, only low level binding was observed when homozygous C5- and C7-deficient sera were tested. Thus the mAb, C8A2, recognizes an epitope expressed on the C8α-γ subunit but not on intact C8 and can detect free C8α-γ in the presence of native C8.