Downstream class switching leads to IgE antibody production by B lymphocytes lacking IgM switch regions

Ig heavy chain (IgH) class-switch recombination (CSR) replaces the IgH Cμ constant region exons with one of several sets of downstream IgH constant region exons (e.g., Cγ, Ce, or Cα), which affects switching from IgM to another IgH class (e.g., IgG, IgE, or IgA). Activation-induced cytidine deaminase (AID) initiates CSR by promoting DNA double-strand breaks (DSBs) within switch (S) regions flanking the donor Cμ (Sμ) and a downstream acceptor CH (e.g., Sγ, Se, Sα) that are then joined to complete CSR. DSBs generated in Sμ frequently are joined within Sμ to form internal switch region deletions (ISD). AID-induced ISD and mutations have been considered rare in downstream S regions, suggesting that AID targeting to these S regions requires its prior recruitment to Sμ. We have now assayed for CSR and ISD in B cells lacking Sμ (Sμ−/− B cells). In Sμ−/− B cells activated for CSR to IgG1 and IgE, CSR to IgG1 was greatly reduced; but, surprisingly, CSR to IgE occurred at nearly normal levels. Moreover, normal B cells had substantial Sγ1 ISD and increased mutations in and near Sγ1, and levels of both were greatly increased in Sμ−/− B cells. Finally, Sμ−/− B cells underwent downstream CSR between Sγ1 and Se on alleles that lacked Sμ CSR to these sequences. Our findings show that AID targets downstream S regions independently of CSR with Sμ and implicate an alternative pathway for IgE class switching that involves generation and joining of DSBs within two different downstream S regions before Sμ joining.