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CTLA-4 engagement inhibits TCR-dependent functions and CTLA-4(-/-) mice develop a lymphoproliferative disorder leading to early lethality. In vitro, ligation of CTLA-4 reduces TCR-mediated activation of NF-kappaB, a transcription factor implicated in promoting T cell survival and cytokine production. However, whether NF-kappaB inhibition downstream of CTLA-4 is necessary for down-regulation of T cell responses is not known. We hypothesized that signaling pathways that are antagonized when CTLA-4 is engaged should be augmented when CTLA-4 is absent and found thatspontaneous NF-kappaB activity was increased in T cells from CTLA-4(-/-) mice. To determine the importance of NF-kappaB inhibition upon CTLA-4 engagement in vivo, CTLA-4(-/-) mice were interbred with mice expressing a transdominant IkappaBalpha mutant under the control of the Lck promoter. The resulting mice had reduced spontaneous NF-kappaB activity in T cells,delayed mortality, and reduced leukocytic accumulation in spleen, lymph nodes, and exocrine pancreas as compared with CTLA-4(-/-) littermates. However, impaired NF-kappaB activation in T cells did not prevent the up-regulation of activation markers on T cells or the acquisition of effector cytokine production. Thus, impaired NF-kappaB activity in T cells prevents specific aspects of the CTLA-4(-/-) phenotype, suggesting that inhibition of NF-kappaB activation is one of the key biochemical events regulated by CTLA-4 ligation in vivo.