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 C mu constant region exons with one of several sets of downstream IgH constant region exons (e.g., C gamma, C epsilon, or C alpha), 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 C mu (S mu) and a downstream acceptor C(H) (e.g., S gamma, S epsilon, S alpha) that are then joined to complete CSR. DSBs generated in S mu frequently are joined within S mu 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 S mu. We have now assayed for CSR and ISD in B cells lacking S mu (S mu(-/-) B cells). In S mu(-/-) 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 S gamma1 ISD and increased mutations in and near S gamma1, and levels of both were greatly increased in S mu(-/-) B cells. Finally, S mu(-/-) B cells underwent downstream CSR between S gamma1 and S epsilon on alleles that lacked S mu CSR to these sequences. Our findings show that AID targets downstream S regions independently of CSR with Smu and implicate an alternative pathway for IgE class switching that involves generation and joining of DSBs within two different downstream S regions before S mu joining.

Infection of human B cells with Epstein-Barr virus results in the expression of somatic hypermutation-inducing molecules and in the accrual of oncogene mutations.

Infection of human B cells with Epstein-Barr virus (EBV) was seen to result in activation-induced cytidine deaminase (AID) and polymerase-eta (pol-eta) gene expression. AID and pol-eta are cellular gene products that play central roles in the DNA-modifying processes involved in immunoglobulin gene class switch recombination and somatic hypermutation. Errors in these processes can result in oncogene mutation/translocation, thereby contributing to lymphomagenesis. It was seen that EBV infected, AID, and pol-eta expressing B cells accumulated mutations in cellular proto-oncogenes (BCL-6 and p53) that are known to be involved in the genesis of B cell lymphoma. The nature of the mutations seen in these oncogenes was consistent with the known activity of AID and pol-eta. These findings indicate that EBV induced AID and pol-eta expression, and that this was associated with oncogene mutation, providing a novel means by which EBV infection of B cells may contribute to lymphomagenesis.