Deoxyadenosine metabolism and cytotoxicity in cultured mouse T lymphoma cells: a model for immunodeficiency disease

Abstract The inherited absence of either adenosine deaminase (ADA) or purine nucleoside phosphorylase is associated with severe immunological impairment. We have developed a cell culture model using a mouse T cell lymphoma to simulate ADA deficiency and to study the relationship between purine salvage enzymes and immune function. 2′-deoxyadenosine triphosphate (deoxyATP) levels have been shown to be greatly elevated in erythrocytes of immunodeficient, ADA-deficient patients, suggesting that deoxyadenosine is the potentially toxic substrate in ADA deficiency. Using a potent ADA inhibitor, we have demonstrated that deoxyadenosine is growth-inhibitory and cytotoxic to S49 cells, and that deoxyATP accumulates in these cells. Cell variants, unable to transport or phosphorylate deoxyadenosine, are much less sensitive to deoxyadenosine, indicating that intracellular phosphorylation of deoxyadenosine is required for the lethal effects. We have partially reversed the cytotoxic effects of deoxyadenosine with deoxycytidine in wild-type cells, but we cannot show any reversal in cell lines lacking deoxycytidine kinase. Adenosine (ado) kinase-deficient cells are extremely resistant to deoxyadenosine in the presence of deoxycytidine. This deoxycytidine reversal of deoxyadenosine toxicity is consistent with an inhibition of ribonucleotide reductase by deoxyATP, and we have shown that incubation of S49 cells with deoxyadenosine markedly reduces intracellular levels of deoxyCTP, deoxyGTP and TTP. Kinetics data in wild-type cells and in cell variants are consistent with the presence of two deoxyadenosine-phosphorylating activities — one associated with ado kinase and another associated with deoxycytidine kinase. The S49 cells appear to be a valid model for the simulation of ADA deficiency in cell culture, and from our results, we can suggest administration of deoxycytidine as a pharmacological regimen to circumvent the clinicopathologic symptoms in ADA deficiency.