Recent studies of the µ- and κ-chains of the first patient (GLI) with µHCD indicated that the observed defect was the result of the failure of assembly of the intact κ-chain to the µ-chain, which lacked the VH domain but had the CH1 Cys normally linked to the light chain. To explore the possibility that the VH region is necessary for the formation of the HL disulfide bond, in vitro studies were performed with GLI µ- and κ-chains and with the CH1 domain and κ-chain derived from an IgG3 myeloma protein, KUP, which yields separate VH, CH1, and κ-chains after papain digestion and reduction. The proteins were reduced and allowed to reoxidize, and the combination products were assessed by gel chromatography under dissociating conditions by SDS-PAGE and by immunoprecipitation techniques. The results suggest that, although in vitro covalent and noncovalent combinations are possible between intact light chains and their autologous heavy chains even in the absence of the VH domain, the efficiency is less than that when the intact Fd region is used. Hence, it seems likely that lack of VH alone is not sufficient to explain the failure of assembly observed in µHCD.
This work was supported in part by United States Public Health Service Research Grants AM 01431 and AM 02594, Training Grant AM 07176; Fellowship Program TW-2229, and Predoctoral Training Grant A-100439, all from the National Institutes of Health.