Distinct isotype-specific factors for immunoglobulin class switch recombination

Immunoglobulin class switch recombination (CSR) occurs by a B cell specific, intrachromosomal deletion process between switch (S) regions. Despite insights into the process of CSR, little is known about the trans-acting factors mediating this recombinational event and their contributions to the specificity of CSR. A novel plasmid-based transient transfection assay has been developed that employs non-replicating switch substrates to analyze the trans-acting switch factors in B cell lines and mitogen activated splenic B cells. Recombination of the exogenous switch substrates is restricted to cells that support CSR at their endogenous loci. Analysis of plasmid CSR using four different substrates has provided evidence for a total of four distinct trans-acting switch activities for μ → γ3, μ → γ1, μ → α and μ → ϵ CSR. Furthermore, endogenous CSR is found to require coordinate expression of both isotype-specific germline transcripts and isotype matched switching activities, such that endogenous CSR is absent if either element is deficient. These results support a model in which isotype specific factors are either class specific recombinases or docking proteins with sequence specificity for S DNA and the ability to recruit the switch recombinase to the S loci.