Plasmacytomas with more than one immunoglobulin kappa mRNA: implications for allelic exclusion

Although only one allele of an immunoglobulin gene is thought to be expressed as a polypeptide by a given lymphocyte ("allelic exclusion"), three murine plasmacytomas were found to contain more than one kappa light chain mRNA species, as evidenced by the sequences of distinct kappa cDNA clones. Two different kappa cDNA sequences were cloned from BFPC 61 microsomal mRNA, two from MOPC 173, and three from S107. One cDNA sequence from each tumor matches the known secreted polypeptide, while the variant sequences differ in the variable (V) region. Hence fusion of a V kappa gene to a joining (J kappa) gene has occurred independently on separate homologous chromosomes and each allele is transcriptionally competent. The BFPC 61 variant sequence contained a normal V kappa sequence linked out of phase to J kappa 2; hence allelic exclusion in this line is accounted for by an error in DNA rearrangement. One S107 variant cDNA has an untranslatable sequence linked to J kappa--C kappa and may derive from a non-V kappa or pseudo-V kappa gene fused to J kappa. Another S107 variant cDNA, however, has a proper V kappa linked in phase to J kappa (albeit missing the first two germ-line J kappa codons) and the MOPC 173 variant sequence also contains a proper V kappa--J kappa join, although it does not encode a tryptophan residue common to all immunoglobulin chains. The presence of two potentially expressible kappa mRNAs in both S107 and MOPC 173 suggests that allelic exclusion does not hold in all lymphocytes, or that it sometimes reflects events subsequent to mRNA production, such as inability of certain kappa chains to assemble properly with the heavy chain. These observations are compatible with a stochastic model for allelic exclusion in which productive and nonproductive V--J recombination events occur at a certain frequency for each allele.