Metabolites from apoptotic thymocytes inhibit thymopoiesis in adenosine deaminase-deficient fetal thymic organ cultures

Murine fetal thymic organ culture was used to investigate the mechanism by which adenosine deaminase (ADA) deficiency causes T-cell immunodeficiency. C57BL/6 fetal thymuses treated with the specific ADA inhibitor 2'-deoxycofo rmycin exhibited features of the human disease, including accumulation of A TP and inhibition of S-adenosylhomocysteine hydrolase enzyme activity. Alth ough T-cell receptor (TCR) V beta gene rearrangements and pre-TCR-alpha exp ression were normal in ADA-deficient cultures, the production of alpha beta TCR+ thymocytes was inhibited by 95%, and differentiation was blocked begi nning at the time of beta selection. In contrast, the production of gamma d elta TCR+ thymocytes was unaffected. Similar results were obtained using fe tal thymuses from ADA gene-targeted mice. Differentiation and proliferation were preserved by the introduction of a bcl-2 transgene or disruption of t he gene encoding apoptotic protease activating factor-1. The pan-caspase in hibitor carbobenzoxy-Val-Ala-Asp-fluoromethyl ketone also significantly les sened the effects of ADA deficiency and prevented the accumulation of dATP. Thus, ADA substrates accumulate and disrupt thymocyte development in ADA d eficiency. These substrates derive from thymocytes that undergo apoptosis a s a consequence of failing to pass developmental checkpoints, such as beta selection.