Human B lymphocytes and thymocytes but not peripheral blood mononuclear cells accumulate high dATP levels in conditions simulating ADA deficiency

Abstract

Inherited adenosine deaminase (ADA) deficiencey is associated with a lymphospecific cytotoxicity affecting both dividing and non-dividing cells. The metabolic basis for this was investigated using different cell types and the potentially toxic metabolite 2′-deoxyadenosine (dAR) in short-term experiments under physiological conditions simulating ADA deficiency (1 mM P i 8.7 μM dAR). In the uncultured cells, [8- 14C] dAR alone was metabolized almost completely only by thymocytes and tonsil-derived B-lymphocytes. The greater percentage of counts (>75%) were in the medium (deoxyinosine, hypoxanthine). Cellular counts were predominantly in adenine nucleotides, and to a lesser extent guanine nucleotides. Interestingly, both thymocytes and tonsil-derived B-lymphocytes, and a partially ADA deficient B lymphoblast line, accumulated detectable amounts of dATP even in the absence of ADA inhibition. Peripheral blood lymphocytes (PBMs) did not, and showed little dAR metabolism. In experiments simulating ADA deficiency varying amounts of 2′-deoxycoformycin (2′dCF) were needed to completely inhibit ADA (20–60 μM), with thymocytes requiring the highest amount. ADA inhibited thymocytes and tonsillar B-lymphocytes accumulated very high dATP levels, which were sustained to an equal extent by both over a 60-min period; PBMs accumulated the lowest values. Results in cultured cells reflected findings in previous studies. Some counts were also found in ATP by a route excluding ADA or PNP. These results again question the hypothesis that B-cells are more resistant than T-cells to the toxic effects of dAR because of an inability to accumulate and sustain elevated dATP levels and underline the lack of comparability between enzyme activity in intact as distinct from lysed cells. They cast doubt on the validity of cultured cells as a model for ADA deficiency and suggest the observed toxicity in some instances might result from altered ATP or GTP pools through inadequate ADA inhibition. They indicate that combined immunodeficiency in ADA deficiency could relate to an equal sensitivity of B-cells and T-cell precursors to the toxic effects of dATP accumulation.