IκBα Nuclear Export Enables 4-1BB-Induced cRel Activation and IL-2 Production to Promote CD8 T Cell Immunity.

Optimal CD8 T cell immunity is orchestrated by signaling events initiated by TCR recognition of peptide Ag in concert with signals from molecules such as CD28 and 4-1BB. The molecular mechanisms underlying the temporal and spatial signaling dynamics in CD8 T cells remain incompletely understood. In this study, we show that stimulation of naive CD8 T cells with agonistic CD3 and CD28 Abs, mimicking TCR and costimulatory signals, coordinately induces 4-1BB and cRel to enable elevated cytosolic cRel:IκBα complex formation and subsequent 4-1BB-induced IκBα degradation, sustained cRel activation, heightened IL-2 production and T cell expansion. CD8 T cells harboring a mutated IκBα nuclear export sequence abnormally accumulate inactive cRel:IκBα complexes in the nucleus following stimulation with agonistic anti-CD3 and anti-CD28 Abs, rendering them resistant to 4-1BB induced signaling and a disrupted chain of events necessary for efficient T cell expansion. Consequently, CD8 T cells in mice poorly expand during viral infection, and this can be overcome by exogenous IL-2 administration. Consistent with cell-based data, adoptive transfer experiments demonstrated that the antiviral CD8 T cell defect in mice was cell intrinsic. Thus, these results reveal that IκBα, via its unique nuclear export function, enables, rather than inhibits 4-1BB-induced cRel activation and IL-2 production to facilitate optimal CD8 T cell immunity.