Excision products of immunoglobulin gene rearrangements

We have isolated circular DNAs from splenocytes of euthymic and athymic mice, and prepared the DNA libraries of 1.5 X 10(6) clones. Hundreds of clones homologous to immunoglobulin (Ig) heavy chain segments (DSP2 and DQ52-JH) or light chain segments (J kappa and J lambda) have been identified. Southern hybridization predicted that three of 12 euthymic mouse clones homologous to DQ52-JH and four of 10 athymic mouse clones homologous to DSP2 contained reciprocal recombination products of D-J joining. Some of these clones were characterized by sequencing: two clones contained the precise excision product of the recombination of a DSP2 segment with either JH2 or JH3 segment; two clones showed imprecise ligation of the DSP2-JH2 coding joint and precise ligation of the DFL16.1-JH3 reciprocal joint in the same molecule. They seem to represent a replacement of the pre-existing DSP2-JH2 rearrangement by joining an upstream DFL16 segment to a downstream JH3 segment. The presence in extrachromosomal DNA of a reciprocal recombination product of DH-JH joining is consistent with the view that immunoglobulin genes, like T cell receptor (TCR) genes, can be rearranged in B cell lineage by the looping-out and excision of chromosomal DNA.