Role of the murine 3′ enhancers in IGH expression and class switching

The murine IgH 3′ regulatory region contains four enhancers-hs3A, hs1,2, hs3B and hs4. Various studies have suggested a role for these enhancers in regulating IgH expression and class switching. Here we assess the role of hs3A and hs1,2 in these processes by exploiting a naturally occurring deletion of these enhancers from the expressed allele of the F1 pre-B cell line, 70Z/3. Comparable mu expression in 70Z/3 and 18-81 (which has an intact 3′ region) indicated that hs3A and hs1,2 are not essential for mu expression at the pre-B cell stage. To assess the role of hs3A and hs1,2 in IgH function at the plasma cell stage, we fused 70Z/3 with the NSO plasma cell line. EMSA analysis of transcription factors known to modulate the 3′ enhancers confirmed the NSO-like plasma cell milieu in the 70Z-NSO hybrid, consistent with the activation of 3′ enhancers. The level of mu protein in a 70Z/3-NSO fusion hybridoma was substantially elevated relative to its pre-B parent and almost comparable to that in several mu-producing spleen cell hybridomas. Furthermore, ELISPOT assays showed that the 70Z-NSO hybrid underwent spontaneous class switching at a frequency comparable to most hybridomas. These results indicate that hs3A and hs1,2 are not essential for high levels of IgH expression or spontaneous class switching in a plasma cell line, suggesting that these processes depend either exclusively on hs3B and/or hs4, and/or there is a partial redundancy of 3′ enhancer function.