HS1,2 Enhancer Regulation of Germline ε and γ2b Promoters in Murine B Lymphocytes: Evidence for Specific Promoter-Enhancer Interactions

Abstract

During an immune response, activated B cells develop into high rate Ig-secreting plasma cells. They also switch from production of IgM to IgG, IgA, or IgE. This process requires a DNA recombination event, which is regulated at the transcriptional level by the production of isotype-specific, sterile germline (GL) transcripts. Induction of these transcripts is controlled by GL promoters and, possibly, by IgH 3′ enhancers. We investigated the interaction of the GL ε and γ2b promoters with the HS1,2 enhancer using transiently transfected mouse primary B cells and cell lines. The constructs used for the transfections contained a GL promoter upstream and HS1,2 downstream of a luciferase reporter gene. Both GL ε and γ2b promoters synergized strongly with the HS1,2 enhancer in activated primary B cells, a mature B cell line, and a plasma cell line. We show that the major activity of HS1,2 in activated primary B cells occurs within a 310-bp fragment that includes NF-κB, OCT, and NF of activated B cells (Ets/AP-1) sites. By mutating the consensus sequences for various transcription factors, we have determined which sites in HS1,2 are important for synergy with the GL ε and γ2b promoters. Our findings indicate that different sites in HS1,2 might selectively interact with the GL ε and γ2b promoters. We also provide evidence that B cell-specific activator protein is not an absolute suppressor of HS1,2 activity.