Circular DNA is excised by immunoglobulin class switch recombination

We have purified extrachromosomal circular DNAs from adult mouse spleen cells, and cloned into a phage vector the BamHl fragments hybridizing with C μ and S γ1 probes. We obtained 52 S μ +S γ1 + clones by screening 1.4 million phage clones derived from spleen cells stimulated with bacterial lipopolysaccharide and interleukin 4. We have identified the breakpoints of six clones that contain S γ1 and S μ sequences fused in the 5′ to 3′ orientation. All these switch recombination sites were assigned to the central repetitive sequences of the S μ and S γ1 regions. Since the common S μ-S γ1 sequences at the recombination sites are at most 2 bases long, typical homologous recombination cannot account for their joining. These findings provide direct evidence that μ-γ1 class switching can occur by the looping out and excision of chromosomal DNA, with formation of a circle.