CD8+ CD28− human T suppressor cells (Ts) act on APC, inhibiting their ability to elicit Th activation and proliferation. This effect is due to inhibition of the CD40 pathway which normally leads to CD80 and CD86 up-regulation. To determine whether Ts inhibit expression of B7 molecules by blocking transcription, we cloned and characterized the CD86 promoter. Mutational analysis revealed that Ts inhibit transcription driven by the CD86 promoter. The NF-κB binding site, at− 612 of the CD86 promoter, is essential for Th-induced transcription. In cultures containing Th and Ts, Ts inhibit Th-induced NF-κB activation in APC. Together, these findings indicate that Ts inhibition of NF-κB activation in APC is a means by which they regulate the activation and proliferation of Th.