Azacytidine-induced reactivation of adenosine deaminase in a murine cytotoxic T cell line.

In C57BL/6 mice, cytotoxic T lymphocyte (CTL) lines have previously been found to exhibit low (less than 150 U/mg) or undetectable (less than 20 U/mg) adenosine deaminase (ADA) levels (Minkowski, Castellazzi and Buttin, J. Immunol. 1984. 133: 52) in contrast to what has been seen in T helper lines (1770 +/- 340 U/mg; Minkowski and Bandeira, Cell. Immunol. 1985. 95: 380). Treatment of one of these CTL ADA- lines with the demethylating agent 5-azacytidine gave rise to an ADA+ population. Subsequent cloning allowed the recovery of pure ADA+ clones showing a rather narrow range of activity with an average value of 2030 +/- 504 U/mg. The restored ADA+ activity is stable over several months of continuous growth. It is identical to the activity of C57BL/6 thymic cells or C57BL/6 T tumor lines regarding its sensitivity to ADA inhibitors 2-deoxycoformycin and erythro-9-(2-hydroxyl-3-nonyl)adenine, and its electrophoretic mobility under nondenaturing conditions (starch gel and isoelectric focusing). An ADA-specific, 1.4-kb mRNA is present in the reactivated clones but is undetectable in the CTL ADA- parental line. These results demonstrate that the ADA- phenotype is due to an extinction of the corresponding gene. They suggest that the extinction of the ADA gene which appears to be specific for CTL and to take place in vivo during T cell differentiation may result from increased methylation in or near the ADA gene. This extinction seems to affect specifically ADA since nucleoside phosphorylase, the enzyme which follows ADA in the purine salvage pathway, is present at equivalent levels in the ADA- and ADA+ CTL clones.