Immunoglobulin receptor signaling depends on the carboxyl terminus but not the heavy-chain class

To examine the isotypic and structural requirements involved in signaling through the immunoglobulin (Ig) receptor on B lymphocytes, we established a panel of T15 idiotype-positive transfectants that expressed wild-type IgM, wild-type IgD, or hybrid IgM molecules. Growth inhibition of the transfected lymphoma cells in response to anti-idiotype antibodies was used to measure signaling. Hybrid IgM molecules were constructed so that the membrane-spanning region of the mu heavy chain was replaced by that of delta, gamma 2b, or alpha heavy chains or that of the I-Ab class II (Ia) alpha chain. All transfectants that expressed IgM or hybrid IgM molecules with membrane-spanning regions from another Ig isotype underwent signaling in response to anti-idiotype antibodies, whereas the IgM-Ia hybrid transfectants did not. Transfectants that expressed wild-type IgD molecules also underwent signaling, although this response was particularly sensitive to serum concentrations. These results imply that signaling occurs in a similar manner through heavy-chain receptors of any isotype and suggest that conserved amino acid sequences in the transmembrane regions are important in this process.