An extrachromosomal switch recombination substrate reveals kinetics and substrate requirements of switch recombination in primary murine B cells

Ig class switch recombination occurs in a lymphocytes upon activation, and is targeted to distinct switch (S) regions by cytokine-mediated induction o f switch transcripts spanning the entire S region and the adjacent constant region gene segments. Using a novel type of switch recombination substrate , constructed according to the intron-exon structure of the IgH locus, but with heterologous elements, we here have tested the structural requirements for targeting and the kinetics of switch recombination in activated primar y murine B cells. When transfected at various times after activation, up to 10% of the transfected B cells perform recombination of the substrate with in 12 h. Switch recombination in primary B cells is restricted to the first 72 h after onset of activation, then rapidly decreases to background level s, as obtained in plasmacytoma cells or with substrates carrying no S regio n sequences. In terms of structural requirements, switch recombination is t argeted to any transcription unit that contains an intronic S region and de pends on processing of the primary transcript by splicing.