Lipopolysaccharide‐dependent transactivation of the temporally regulated immunoglobulin heavy chain 3′ enhancer

To execute different biological functions, the expression pattern of immunoglobulin heavy chain genes (IgH) is altered during B lymphocyte differentiation. Early in B cell differentiation, it is assumed that the heavy chain promoter and the intragenic enhancer (Eμ) ensure VDJ recombination. This leads to the expression of the immunoglobulin receptor on the cell surface. An additional strong enhancer in the far 3′ end of the IgH locus has, however, prompted a re-evaluation of the regulation of immunoglobulin gene expression. To define the temporal and spatial regulation of the IgH 3′ enhancer, transgenic mice harboring an enhancerdependent reporter gene construct were generated. Here we demonstrate that IgH 3′ enhancer activity is largely restricted to activated immunocompetent B cells. Furthermore, the enhancer can be transactivated following mitogen stimulation with lipopolysaccharide and 12-O-tetradecanylphorbol 13-acetate. We propose a model whereby 3′ enhancer activation is linked to the activation of resting immunocompetant B cells. The implications of the enhancer being active in late B lymphocyte differentiation, when heavy chain class switching occurs, are discussed.