THE B-CELL ANTIGEN RECEPTOR OF CLASS IGD INDUCES A STRONGER AND MORE PROLONGED PROTEIN-TYROSINE PHOSPHORYLATION THAN THAT OF CLASS IGM

Most mature B lymphocytes coexpress two classes of antigen receptor, i mmunoglobulin (Ig)M and IgD. The differences in the signal transductio n from the two receptors are still a matter of controversy. We have an alyzed B cell lines expressing IgM or IgD antigen receptors with the s ame antigen specificity. Cross-linking of these receptors with either antigen, or class-specific antibodies, results in the activation of pr otein tyrosine kinases and the phosphorylation of the same substrate p roteins. The kinetic and the intensity of phosphorylation, however, wa s quite different between the two receptors when they were cross-linke d by antigen. In membrane IgM-expressing cells, the substrate phosphor ylation reached a maximum after 1 minute and diminished after 60 minut es whereas, in the membrane IgD-expressing cells, the substrate phosph orylation increased further over time, reached its maximum at 60 minut es, and persisted longer than 240 minutes after exposure to antigen. A s a result, the intensity of protein tyrosine phosphorylation induced by cross-linking of membrane IgD was stronger than that induced by mem brane IgM. Studies of chimeric receptors demonstrate that only the mem brane-proximal C domain and/or the transmembrane part of membrane-boun d IgD molecule is required for the long-lasting substrate phosphorylat ion. Together, these data suggest that the signal emission from the tw o receptors is controlled differently.