Differential expression of ecto-5' nucleotidase activity by functionally and phenotypically distinct subpopulations of human Leu-2⁺/T8⁺ lymphocytes

Subsets of Leu-2⁺/T8⁺ cytotoxic/suppressor T lymphocytes were isolated by using various methods of purification and were investigated for expression of ecto-5' nucleotidase (5'NT) enzyme activity by radiochemical, cytochemical, and ultrastructural techniques. By using both the radiochemical and the cytochemical methods, T4^-OKM1^- cells displayed higher 5'NT activity in comparison with the entire T4^- subpopulation. Analyses of the subpopulations of T4^- (and predominantly Leu-2⁺) cells defined by the Leu-15 or Lyt-1 (9.3) monoclonal antibodies demonstrated that T4^-Leu-15^- and T4^-Lyt-1⁺ cells displayed high 5'NT activity, whereas virtually no activity was present in T4^-Leu-15⁺ and T4^-Lyt-1^- cells. At the ultrastructural level, the 5'NT reaction product was detected on the plasma membrane of a proportion of nongranular Leu-2⁺/T8⁺ lymphocytes, but no activity was found on cells with a granular lymphocyte (GL) morphology. 5'NT activity was also analyzed in peripheral blood mononuclear cells from one patient with expanded numbers of GL and two patients with GL leukemia. The enzymatic activity was significantly lower in these patients than in normal controls. This study provides new cytochemical evidence demonstrating the heterogeneity of Leu-2⁺/T8⁺ cells, and indicates that the population with the suppressor phenotype and function (Leu-15⁺/Lyt-1^-, GL morphology) displayes low or absent 5'NT activity, whereas the population composed of cytotoxic cell precursors (Leu-15^-/Lyt-1⁺, nongranular morphology) has high 5'NT activity. Implications of these data for the interpretation of low 5'NT activity described in several immunodeficiency states and lymphoproliferative disorders are discussed.