Differential role of the transcription factor NF-kappaB in selection and survival of CD4+ and CD8+ thymocytes.

Inhibition of the transcription factor nuclear factor (NF)-kappaB activity leads to a reduction in numbers of CD8(+) single-positive (SP) thymocytes, suggesting a selective role for NF-kappaB in these cells. To further explore the role of NF-kappaB in SP thymocytes, we utilized transgenic models that allowed either inhibition or activation of NF-kappaB. We showed that activation of NF-kappaB played an important role in the selection of major histocompatibility complex (MHC) class I-restricted CD8(+) T cells. Surprisingly, NF-kappaB was not activated in positively selected CD4(+) thymocytes, and inhibition of NF-kappaB did not perturb positive or negative selection of CD4(+) cells. However, enforced activation of NF-kappaB via a constitutively active inhibitor of kappaB (IkappaB) kinase transgene led to a nearly complete deletion of CD4 cells by pushing positively selecting CD4(+) cells into negative selection. These studies therefore revealed a surprising difference of NF-kappaB activation in CD4(+) and CD8(+) thymocytes and suggested that NF-kappaB contributes to the establishment of thresholds of signaling that determine positive or negative selection of thymocytes.