Inhibitor of Kappa B Epsilon (IκBε) Is a Non-Redundant Regulator of c-Rel-Dependent Gene Expression in Murine T and B Cells

Inhibitors of kappa B (I kappa Bs) -alpha, -beta and -epsilon effect selective regulation of specific nuclear factor of kappa B (NF-kappa B) dimers according to cell lineage, differentiation state or stimulus, in a manner that is not yet precisely defined. Lymphocyte antigen receptor ligation leads to degradation of all three I kappa Bs but activation only of subsets of NF-kappa B-dependent genes, including those regulated by c-Rel, such as anti-apoptotic CD40 and BAFF-R on B cells, and interleukin-2 (IL-2) in T cells. We report that pre-culture of a mouse T cell line with tumour necrosis factor-alpha (TNF) inhibits IL-2 gene expression at the level of transcription through suppressive effects on NF-kappa B, AP-1 and NFAT transcription factor expression and function. Selective upregulation of I kappa B epsilon and suppressed nuclear translocation of c-Rel were very marked in TNF-treated, compared to control cells, whether activated via T cell receptor (TCR) pathway or TNF receptor. I kappa B epsilon associated with newly synthesised c-Rel in activated cells and, in contrast to I kappa B alpha and -beta, showed enhanced association with p65/c-Rel in TNF-treated cells relative to controls. Studies in I kappa B epsilon-deficient mice revealed that basal nuclear expression and nuclear translocation of c-Rel at early time-points of receptor ligation were higher in I kappa B epsilon-/- T and B cells, compared to wild-type. I kappa B epsilon-/- mice exhibited increased lymph node cellularity and enhanced basal thymidine incorporation by lymphoid cells ex vivo. I kappa B epsilon-/- T cell blasts were primed for IL-2 expression, relative to wild-type. I kappa B epsilon-/- splenic B cells showed enhanced survival ex vivo, compared to wild-type, and survival correlated with basal expression of CD40 and induced expression of CD40 and BAFF-R. Enhanced basal nuclear translocation of c-Rel, and upregulation of BAFF-R and CD40 occurred despite increased I kappa B alpha expression in I kappa B epsilon-/- B cells. The data imply that regulation of these c-Rel-dependent lymphoid responses is a non-redundant function of I kappa B epsilon.