Noncovalent association of heavy and light chains of human immunoglobulins. IV. The roles of the CH1 and CL domains in idiotypic expression.

A rabbit anti-idiotypic antiserum was raised against a monoclonal human IgM kappa(Me) in order to analyze the possible modulation of idiotypic expression by Fab constant domains. IgM(Me) fragments, subunits, and domains were prepared by chemical and enzymatic cleavages. All molecular species were shown to have a well-defined secondary and tertiary structure by circular dichroism. Full recombination between domains and subunits was ascertained by difference spectroscopy. The expression of the idiotype on native and recombined fragments, domains, and subunits was quantitated in a competitive enzyme-linked immunosorbent assay (ELISA). Reduced and alkylated Fab, isolated H and L chains, purified Fv(Me), intact VH and VL domains and H-L, VH-L, VL-H, and VH-VL recombinants were compared on a molar basis to native Fab(Me) for idiotypic expression. VH-specific determinants were found, whereas the L chains were virtually devoid of idiotypic activity. Both the peptic FV(Me) fragment, which is composed of intact VH and VL domains, and the recombined VH-VL heterodimer were found to be fourfold less active for idiotype expression than native Fab(Me). However, full inhibition was achieved at high molar concentrations, suggesting that all the idiotopes present on Fab(Me) were expressed on FV(Me) but with a reduced antigenicity. Comparison of VH-L and VL-H hybrid molecules revealed that the presence of the C mu 1 domain was sufficient to restore full idiotypic expression as compared with native Fab(Me). These data support the hypothesis that the first constant domain of the mu heavy chain alters the quaternary interaction between the variable domains, and therefore modulates the expression of the idiotype through longitudinal interactions that are not affected by reduction of the inter-H-L chain disulfide bond.