ALLELIC EXCLUSION OF MEMBRANE BUT NOT SECRETED IMMUNOGLOBULIN IN A MATURE B-CELL LINE

Although many B lymphocytes contain two completely rearranged immunoglobulin (Ig) heavy (H) chain genes, only one of the alleles specifies a protein product. This phenomenon is termed allelic exclusion and is controlled in part by the membrane portion of Ig H chains. We have identified a mature B cell line derived from murine bone marrow that expresses two H chain proteins, gamma-3 and gamma-2b. Proteins of appropriate size for the membrane and secreted forms of both H chains are produced and both IgG3 and IgG2b are secreted from the cells. However, only IgG3 is expressed on the membrane. Detailed restriction mapping of the H chain genes indicates that both are rearranged, one containing a V(H)J558 gene linked to the gamma-3 constant region and the other a V(H)S107 gene linked to the gamma-2b constant region. Primer extension sequencing of the RNA reveals that the gamma-2b RNA is transcribed from the allele containing V(H)S107 sequences while the gamma-3 RNA is transcribed from the allele containing V(H)J558 sequences. Thus, this mature B cell line secretes two distinct antibody molecules but is allelically excluded at the level of surface Ig expression. This cell line provides a model system to study factors, in addition to aberrant rearrangement, that influence allelic exclusion.