Inherited deficiency of the enzyme purine nucleoside phosphorylase (PNP) results in selective and severe T lymphocyte depletion which is mediated by its substrate, 2'-deoxyguanosine. This observation provides a rationale for the use of PNP inhibitors as selective T cell immunosuppressive agents. We have studied the relative effects of the PNP inhibitor 8-aminoguanosine on the metabolism and growth of lymphoid cell lines of T and B cell origin. We have found that 2'-deoxyguanosine toxicity for T lymphoblasts is markedly potentiated by 8-aminoguanosine and is mediated by the accumulation of deoxyguanosine triphosphate. In contrast, the growth of T4+ mature T cell lines and B lymphoblast cell lines is inhibited by somewhat higher concentrations of 2'-deoxyguanosine (ID50 20 and 18 microM, respectively) in the presence of 8-aminoguanosine without an increase in deoxyguanosine triphosphate levels. Cytotoxicity correlates instead with a three- to fivefold increase in guanosine triphosphate (GTP) levels after 24 h. Accumulation of GTP and growth inhibition also result from exposure to guanosine, but not to guanine at equimolar concentrations. B lymphoblasts which are deficient in the purine salvage enzyme hypoxanthine guanine phosphoribosyltransferase are completely resistant to 2'-deoxyguanosine or guanosine concentrations up to 800 microM and do not demonstrate an increase in GTP levels. Growth inhibition and GTP accumulation are prevented by hypoxanthine or adenine, but not by 2'-deoxycytidine. 8-Aminoguanosine appears to effectively inhibit extracellular PNP activity; thus, it prolongs the extracellular half-life of 2'-deoxyguanosine and guanosine, but does not completely inhibit intracellular PNP activity in these lymphoid cells. As a result, 2'-deoxyguanosine and guanosine are phosphorolyzed and actively salvaged within the cell, accounting for the accumulation of GTP. Partial inhibition of PNP activity in vivo, therefore, may lead to nonselective cellular toxicity by a mechanism independent of dGTP accumulation.
Y Sidi, B S Mitchell