REGULATION OF 3'-IGH ENHANCERS BY A COMMON SET OF FACTORS, INCLUDING KAPPA-B-BINDING PROTEINS

Targeted disruption of the p50 subunit of NF-kappa B resulted in isoty pe class switch defects resembling those observed in mice in which the downstream IgH enhancer 3'alpha E(hs1,2) was deleted. We postulated t hat kappa B binding proteins may regulate class switching by interacti ng with 3'alpha E(hs1,2) or with other IgH 3' enhancers with which 3'a lpha E(hs1,2) synergizes, kappa B binding sites were identified in 3'a lpha E(hs1,2) and 3'alpha-hs4, the distal 3' IgH enhancer. A kappa B b inding site within 3'alpha E(hs1,2) contributes to at least half the a ctivity of the enhancer in plasma cells, while the same kappa B bindin g site participates in the complex repression of the enhancer in B cel ls. In the case of 3'alpha-hs4, a kappa B binding complex activates th e enhancer in pre-B, B cells and plasma cells. Additional binding site s within 3'alpha-hs4 for factors known to regulate 3'alpha E(hs1,2), i ncluding Oct-1 and BSAP, were identified, and their contribution to 3' alpha-hs3 regulation during B cell development was assessed. Oct-1 pos itively regulates the enhancer in pre-B and B cells, while BSAP is a r epressor in pre-B cells and an activator at the B cell stage. These st udies identify kappa B binding proteins as key modulators of 3'alpha E (hs1,2) and 3'alpha-hs4, and suggest coregulation of the two enhancers by a common set of factors.