Homologous chromosome recombination generating immunoglobulin allotype and isotype switch variants.

We investigated whether spontaneous isotype switching in monoclonal antibody-producing hybridomas always occurs with genes on the same chromosome. Spleen cells of (BAB/ 25 X AKR/J) F1 mice, immunized with dansyl-keyhole limpet hemocyanin (DNS-KLH), were hybridized with NS-1 to generate hybridomas producing monoclonal anti-DNS antibodies of either the b or d haplotype of the BAB/25 or AKR/J parent, respectively. We selected isotype switch variants of such hybridomas using the fluorescence-activated cell sorter (FACS). Although in most cases the allotypic haplotype expressed by the parent and switch-variant hybridomas are the same, in one family of variants we noted a switch in haplotype along with the switch in isotype. This was noted in the selection of IgG2a switch variants from an IgG1 switch variant originally derived from an IgG3-producing parent. Biochemical and molecular studies confirm that the allotype switch variant expresses the same heavy-chain variable region gene complex as its parent hybridomas. As such, the allotype switch represents an example of spontaneous mitotic recombination between immunoglobulin heavy-chain genes, generating a single actively transcribed gene from loci previously positioned on different chromosomes.