Membrane IgD

In sharp contrast to the low levels of serum IgD, both in humans and in mice, IgD is expressed on the membrane of the majority of B lymphocytes. In mice, membrane IgD appears around the end of the first week postpartum, and the percentage of B cells with membrane IgD then rapidly increases, more or less in parallel with the maturation of the T cell system of the animal. However, 8 chains that are not expressed on the membrane are apparently synthesized quite early in developing B cells. A completely-rearranged 8 gene causes allelic exclusion in transgenic mice.

Membrane IgD and IgM molecules, with the same VH and with the same light chain, and therefore with the same antigen-binding site, are expressed together on the majority of B lymphocytes. Since p and 8 chains do not pair, there are no hybrid p/8 molecules. The ratio between the two isotypes may vary between high 8/low p and the reverse; it may be rather stable in some cells, like the Lyl-B cells, that are low 8 as a rule, or it may be variable even in single clones of B cells, like some human F.pstein-Barr virus (EBV)-transformed cells where it has been found to vary between the two extremes in the same clone. The ratio of IgM to IgD on a given B lymphocyte must have functional consequences that are not as yet totally clear; it is established, however, that membrane IgD is reduced in memory B cells and that it disappears with the process of maturation to plasma cells.

After cross-linking with anti-8 antibodies, mem brane IgD is internalized by the cells, much in the same way as the other membrane immunoglobulin isotypes; however, in some B cell lines, differences in the intracytoplasmic location of the IgD/anti-IgD complexes versus the IgM/anti-IgM complexes have been observed; this difference may have relevance to the role of the two isotypes in the processing of antigens by B lymphoid cells.

Membrane IgD molecules are associated with a heterodimer consisting of two proteins of 35 kDa (IgD-ct) and 39 kDa (IgD-|3); the presence of the IgD-ct chain is necessary for the membrane expression ot IgD. Membrane IgM is also associated with a heterodimer formed by the same IgD-[3 chain discussed above and by an IgM-specific IgM-ot chain ot 34 kDa. Thus the two membrane immunoglobulin isotypes are associated with companion molecules which are related but different; these biochemical observations may go a long way toward the understanding of the function of the two classes of anti gen receptors that coexist on the membrane of B lymphocytes.

So far, however, no gross differences have been detected between the biochemical events that take place in B cells after cross-linking IgM and those seen after cross-linking IgD; therefore, the immediate signals given to the B cell by the two classes of antigen receptors seem to be equivalent.

However, a difference in the kinetics of phos-

phorylation of the protein tyrosine kinases after cross-linking of IgM and IgD with the same antigen specificity has been detected by Kim and Reth. In IgM-expressing cells phosphorylation reaches a maximum in 1 min and decreases after 60 min, whereas in IgD cells the phosphorylation of the protein kinases reaches a maximum in 60 min and persists longer than 240 min after exposure to antigen. This may be relevant in conditions where the B cells are competing for antigen, as in the germinal centers.

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