Structural and functional properties of mouse-human chimeric IgD.

A gene encoding mouse-human chimeric secreted IgD was constructed using the rearranged murine variable region specific for the hapten dansyl and the genomic gene sequences for the constant region of the heavy (H) chain of human IgD. When expressed with the dansyl-specific chimeric light (L) chain, chimeric IgD specific for the hapten dansyl was synthesized and secreted as an H2L2 molecule. The pathway of assembly was H + L----HL----H2L2. The chimeric IgD heavy chain contains three N-linked carbohydrate moieties; one of these appears to be added co-translationally, and the other two appear to be added post-translationally. In secreted chimeric IgD some of the N-linked carbohydrate remains in the high mannose form. The chimeric IgD heavy chain also contains O-linked carbohydrate, which is added at the time of secretion. Inhibition of N-linked glycosylation with tunicamycin halts assembly at the HL half-molecule stage and prevents secretion. Like natural human IgD, the chimeric IgD binds to and upregulates the IgD receptor (IgD-R) on human peripheral blood T cells, and it is equivalent to human myeloma IgD in the competitive inhibition of rosette formation between IgD-R-bearing cells and IgD-coated Ox-RBC, Cross-linking by dansyl-BSA is needed for the chimeric IgD in soluble form to cause IgD-R upregulation.