TNF-alpha is the critical mediator of the cyclic AMP-induced apoptosis of CD8(+)4(+) double-positive thymocytes

Apoptosis is one of the key regulatory mechanisms in tissue modeling and de velopment. In the thymus, 95-98% of all thymocytes die by apoptosis because they failed to express a TCR with an optimal affinity for the selecting in trathymic peptide-MHC complexes. We studied the possible role of two promin ent nerve growth factor (NGF-TNF) family member systems, Fas ligand (FasL)- Fas receptor (FasR) and TNF-alpha-TNFR, in apoptosis of murine CD8(+)4(+) d ouble-positive (DP) thymocytes induced via TCR-CD3- and cAMP-mediated signa ling. TCR-CD3 epsilon-mediated apoptosis of DP thymocytes was found not to be dependent on either of the two systems. The FasL-FasR system was also fo und to be dispensable for the cAMP-mediated apoptosis, By contrast, cAMP ag onists (dibutyryl-cAMP and forskolin) induced apoptosis via TNF-alpha, as e videnced by 1) the ability of anti-TNF-alpha mAbs to abrogate cAMP analogue -induced DP apoptosis in a dose-dependent manner; and 2) increased resistan ce of DP thymocytes from TNF-alpha(-/-) and TNFR I-/-II-/- animals to cAMP agonist-mediated apoptosis, cAMP agonists induced DP thymocyte death by a c ombination of two mechanisms: first, they induced selective up-regulation o f TNF-alpha production, and, second, they sensitized DP thymocytes to TNF-a lpha. The latter effect may be due to the down-regulation of TNFR-associate d factor 2 protein. These results identify TNF-alpha as the critical mediat or of cAMP-induced apoptosis in thymocytes and provide a molecular explanat ion for how the cAMP stimulators, including the sex steroids, may modulate T cell production output, as observed under physiological acid Pharmacologi cal conditions.