Cross‐linking of surface IgM or IgD causes differential biological effects in spite of overlap in tyrosine (de)phosphorylation profile

Although displaying similar amounts of surface IgM and IgD, ECH 408‐1 cells only succumb to apoptosis after cross‐linking of IgM (not IgD), suggesting that different signaling pathways couple to both receptors. Immunoprecipitation studies revealed the presence of several proteins selectively associated with IgM and IgD, thus ruling out that the lack of inhibitory signaling mediated by IgD might be due to membrane expression in the absence of associated proteins belonging to the B cell receptor complex. ³²P metabolic labeling and immuno‐precipitation studies demonstrated that IgM and IgD are associated with phosphoproteins of 32–33 kDa in an isotype‐specific fashion. Kinetic analyses of tyrosine kinase activity showed that cross‐linking of surface IgM or IgD resulted in the rapid (1–3 min) phosphorylation of several protein substrates on tyrosine residues, followed by a dephosphorylation step. Isotype‐specific changes of the phosphorylation status specifically affected molecules in the 32–33 kDa range, i.e. IgM (not IgD) cross‐linking affected a ∼ 32‐kDa protein, whereas IgD (not IgM) cross‐linking induced phosphorylation of a protein exhibiting a slightly lower mobility (33 kDa). These results suggest that isotype‐specific immuno‐globulin‐associated molecules could be involved in the second messenger cascade leading to different biological effects upon IgM and IgD cross‐linking.