Somatic hypermutation of a lambda 2 transgene under the control of the lambda enhancer or the heavy chain intron enhancer.

Previous experiments have suggested an important role for the Ig enhancers and transcription in targeting somatic hypermutation. To determine whether the requirement of the enhancers is Ig chain specific, we analyzed two lambda 2 light chain transgenes under the control of different enhancers, either the lambda 2-4 enhancer or the heavy chain intron enhancer. The transgenes were amplified and cloned from B220+PNAhigh B cells from either Peyer's patches (PP) or SRBC-immunized spleen. The lambda 2 transgene under the control of the lambda 2-4 enhancer underwent mutation in both the PP and splenic B cell populations, but at a frequency lower than endogenous light chain genes. This requirement for the lambda 2-4 enhancer could be replaced by the heavy chain intron enhancer. Interestingly, the heavy chain intron enhancer-driven lambda 2 construct showed evidence of statistically significant mutation in the PP B cell population, but not the spleen-derived population. This difference in somatic hypermutation suggests that a lower mutation frequency can be more readily detected in B220+PNAhigh B cells isolated from the PP. The ability of the heavy chain intron enhancer to replace the lambda 2-4 enhancer shows that the requirement for the Ig enhancers in somatic hypermutation is not Ig locus specific. Given that these Ig enhancers have very few elements in common, our results further suggest that the Ig enhancers are primarily important in the timing of transcription in the mutating B cell population.