The antigen receptor on B lymphocytes is composed of membrane immunoglobulin sheathed by an alpha/beta heterodimer. This structure is in several respects analogous to the antigen receptor on T cells except that, in the case of the T cell but not the B cell receptor, several receptor-associated proteins have been described which may modulate the effects of antigen interaction (e.g. CD4, CD8, CD2 and CD5). To screen for specific associations with the B cell antigen receptor that might be of only low stoichiometry, we have exploited the sensitivity of in vitro kinase assays. We show that the B cell antigen receptor associates with CD22. The association is specific and stable, but Western blotting reveals it to be of low stoichiometry (0.2 to 2% of membrane immunoglobulin is CD22 associated). The CD22/antigen receptor association was demonstrated with multiple isotypes (IgM, IgD and IgG) and was evident both in Burkitt lymphoma lines and in tonsil cells.Whilst the significance of the association is unknown, it is notable that CD22 is a B cell-specific adhesion molecule which we find contains within its cytoplasmic domain a sequence bearing high homology to the ''Reth motif' implicated in signal transduction. Indeed, CD22 becomes tyrosine phosphorylated less than one minute after antigen-receptor cross-linking. Thus, it is tempting to speculate that interactions involving CD22 assist in the antigen-mediated triggering of B cell activation.
