Molecular mechanisms underlying synapsis of activation-induced deaminase (AID)-targeted S regions during class switch recombination (CSR) are poorly understood. By using chromosome conformation capture techniques, we found that in B cells, the Emicro and 3'Ealpha enhancers were in close spatial proximity, forming a unique chromosomal loop configuration. B cell activation led to recruitment of the germline transcript (GLT) promoters to the Emicro:3'Ealpha complex in a cytokine-dependent fashion. This structure facilitated S-S synapsis because Smicro was proximal to Emicro and a downstream S region was corecruited with the targeted GLT promoter to Emicro:3'Ealpha. We propose that GLT promoter association with the Emicro:3'Ealpha complex creates an architectural scaffolding that promotes S-S synapsis during CSR and that these interactions are stabilized by AID. Thus, the S-S synaptosome is formed as a result of the self-organizing transcription system that regulates GLT expression and may serve to guard against spurious chromosomal translocations.

Cells that undergo apoptosis in response to innate immune inflammation during accumulation of virally infected cells at sites of infection and suggest that E1B 19K-deleted, replicating adenoviral vectors might induce greater inflammatory responses to virally infected cells than E1B 19K-positive vectors, because of the net effect of their loss-of-function mutation. IMPORTANCE We observed that cells dying a nonapoptotic cell death induced by adenovirus infection repressed macrophage proinflammatory responses while cells dying by apoptosis induced by infection with an E1B 19K deletion mutant virus did not repress macro-

Mismatch repair (MMR) safeguards against genomic instability and is required for efficient Ig class switch recombination (CSR). Methyl CpG binding domain protein 4 (MBD4) binds to MutL homologue 1 (MLH1) and controls the post-transcriptional level of several MMR proteins, including MutS homologue 2 (MSH2). We show that in WT B cells activated for CSR, MBD4 is induced and interacts with MMR proteins, thereby implying a role for MBD4 in CSR. However, CSR is in the normal range in Mbd4 deficient mice deleted for exons 2–5 despite concomitant reduction of MSH2. We show by comparison in Msh2+/− B cells that a two-fold reduction of MSH2 and MBD4 proteins is correlated with impaired CSR. It is therefore surprising that CSR occurs at normal frequencies in the Mbd4 deficient B cells where MSH2 is reduced. We find that a variant Mbd4 transcript spanning exons 1,6–8 is expressed in Mbd4 deficient B cells. This transcript can be ectopically expressed and produces a truncated MBD4 peptide. Thus, the 3′ end of the Mbd4 locus is not silent in Mbd4 deficient B cells and may contribute to CSR. Our findings highlight a complex relationship between MBD4 and MMR proteins in B cells and a potential reconsideration of their role in CSR.