Human HB4C5 hybridoma cells produce a lung cancer-specific IgM human monoclonal antibody (mAb). HB4C5 human mAb cross-reacts with Candida cytochrome c (Cyt c) and carboxypeptidase (Cpase). Concanavalin A (ConA)-resistant variants of HB4C5 cells loss the original light chain followed by expression of various new light chains at a high incidence (light chain shifting) (Tachibana et al., 1996). HTD8 cells, one of the ConA-resistant variant subclones of HB4C5 cells, undergo the active light chain shifting and produce various sublines, each of which stably secretes new mAb consisting of a new light chain and a HB4C5 heavy chain. The new mAb exhibits altered antigen binding ability from that of the original antibody. We could expect that HTD8 cells can be used as 'a light chain stem cell line' to improve antigen binding ability and specificity of established human mAbs. A BD9D12 IgG human mAb recognizes lung cancer cells and cross-reacts with cytokeratin 8. Introduction of the heavy chain gene of BD9D12 mAb into HTD8 cells resulted in establishment of various sublines which secreted various kinds of hybrid antibodies consisting of different light chains derived from HTD8 subclones which underwent light chain shifting and a common IgG heavy chain derived from BD9D12. These hybrid antibodies exhibited different or improved reactivities to Cyt, Cpase, cytokeratin 8 and various cancer cells from those of parental mAb, demonstrating that light chain shifting can be applied to improve the affinity and specificity of human mAb.
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