Submitted February 22, 2006
Accepted August 16, 2006
Regulation of 
germline transcription and switch region mutations by IgH locus 3' enhancers in transgenic mice
Jurga Laurencikiene, Vytas Tamosiunas, and Eva Severinson*
Dept of Cell and Molecular Biology, Karolinska Institutet, Stockholm, Sweden
Institute of Immunology at Vilnius University, Vilnius, Lithuania
* Corresponding author; email: eva.severinson{at}ki.se.
Germline (GL) transcription is regulated by specific promoters and Immunoglobulin heavy chain (IgH) 3' locus enhancers and is necessary for Ig class switch recombination (CSR). We have generated different transgenic lines containing the GL 
promoter, switch (S) region and constant (C) region with or without the HS3A-HS1,2 or HS3B-HS4 3' IgH enhancer pairs. The enhancerless construct was expressed in B cells activated by interleukin (IL)-4 and cluster of differentiation (CD)40, thus resembling regulation of the endogenous gene. Both enhancer containing transgenes efficiently increased expression in B cells and were strongly up-regulated by stimuli. In addition, S regions of the transgene containing HS3B-HS4 were mutated in activated, sorted B cells. Such mutations are known to precede CSR and are dependent on activation induced cytidine deaminase (AID). Our findings show that all elements necessary for recruitment of the recombination machinery are present in the transgene containing HS3 and HS4. These enhancers probably provide something more specific than mere increased accessibility of switch regions. We propose that transcription factors binding the enhancers help to target the recombination machinery to the switch regions.
