Transgenes of the Mouse Immunoglobulin Heavy Chain Locus, Lacking Distal Elements in the 3' Regulatory Region, Are Impaired for Class Switch Recombination

The immunoglobulin heavy (H) chain class switch is mediated by a deletional recombination event between µ and γ, α, or ε constant region genes. This recombination event is upregulated during immune responses by a regulatory region that lies 3' of the constant region genes. We study switch recombination using a transgene of the entire murine H chain constant region locus. We isolated two lines of mice in which the H chain transgenes were truncated at their 3' ends. The truncation in both transgenic lines results in deletion of the 3'-most enhancer (HS4) and a region with insulator-like structure and activities. Even though both truncated transgenes express the µ H chain gene well, they undergo very low or undetectable switch recombination to transgenic γ and α constant region genes. For both transgenic lines, germline transcription of some H chain constant regions genes is severely impaired. However, the germline transcription of the γ1 and γ2a genes is at wild type levels for the transgenic line with the larger truncation, but at reduced levels for the transgenic line with the smaller truncation. The dramatic reduction in class switch recombination for all H chain genes and the varied reduction in germline transcription for some H chain genes could be caused by (i) insertion site effects or (ii) deletion of enhancer elements for class switch recombination and transcription, or (iii) a combination of both effects.