The effects of IKKgamma zinc finger mutations on NF-kappaB signaling in T cells

Abstract

Mutations in the zinc finger of IKKgamma are associated with hypohidrotic ectodermal dysplasia-immune deficiency, a disease whose features include immunologic abnormalities. IL-2 is a cytokine secreted by helper T cells and it plays a key role in the immune response. Since IKKgamma is required for IL-2 induction, I hypothesized that mutations in the zinc finger of IKKgamma inhibit the induction of IL-2 in stimulated T cells. Also, since the exact role of the zinc finger of IKKgamma in NF-kappaB activation is not fully known, I wanted to examine the mutations within the background of an IKKgamma-null T cell line to gain insight into the roles of the IKKgamma zinc finger in particular and into the functions of IKKgamma in general. Using a human T cell line that was null for IKKgamma, cells stably expressing two of the mutations identified in patients, namely, D406V and C417R, were generated. Cells expressing the mutant forms of IKKgamma failed to induce IL-2 secretion following stimulation with PMA/ionomycin while the induction of IL-2 was restored in cells stably transfected with the wild type IKKgamma. The suppression of IL-2 induction occurred at the level of IL-2 transcription. The mutations blocked NFkappaB activation as evidenced by the loss of the ability to induce IkappaBalpha degradation, NFkappaB binding to DNA and the expression of a reporter gene. The suppression of IL-2 induction was solely due to the inhibition of NF-kappaB activation as the mutations did not impair the activation of AP-1 and NFAT. However, the mutations did not prevent the incorporation of IKKgamma into the IKK complex and, interestingly, the induced phosphorylation of IkappaBalpha at S32 and S36 and its subsequent ubiquitination were not affected. In conclusion, the data indicated that the failure of T cells to undergo activation in response to TCR stimuli may play a role in the pathophysiology of hypohidrotic ectodermal dysplasia-immune deficiency and also showed that IKKgamma has a role in the post-ubiquitination processing of IkappaBalpha.