INSIGHTS INTO THYMIC PURINE METABOLISM AND ADENOSINE-DEAMINASE DEFICIENCY REVEALED BY TRANSGENIC MICE OVEREXPRESSING ECTO-5'-NUCLEOTIDASE (CD73)

The adenosine producing enzyme ecto-5'-nucleotidase (5'-NT) is not nor mally expressed during thymocyte development until the medullary stage . To determine whether earlier expression would lead to adenosine accu mulation and/or be deleterious for thymocyte maturation, thymic purine metabolism, and T cell differentiation were studied in lck(NT) transg enic mice overexpressing 5'-NT in cortical thymocytes under the contro l of the lck proximal promoter. In spite of a 100-fold elevation in th ymic 5'-NT activity, transgenic adenosine levels were unchanged and T cell immunity was normal. Inosine, the product of adenosine deaminatio n, was elevated more than twofold, however, indicating that adenosine deaminase (ADA) can prevent the accumulation of adenosine, even with a dramatic increase in 5'-NT activity, and demonstrating the availabili ty of 5'-NT substrates in the thymus for the first time. Thymic adenos ine concentrations of mice treated with the ADA inhibitor 2'-deoxycofo rmycin (dCF) were elevated over 30-fold, suggesting that high ADA acti vity, rather than an absence of 5'-NT, is mainly responsible for low t hymic adenosine levels. The adenosine concentrations in dCF-treated mi ce are sufficient to cause adenosine receptor-mediated thymocyte apopt osis in vitro, suggesting that adenosine accumulation could play a rol e in ADA-deficient severe combined immunodeficiency.