IL-2 receptors on circulating natural killer cells and T lymphocytes. Similarity in number and affinity but difference in transmission of the proliferation signal.

IL-2-binding sites expressed on purified circulating NK cells and high density T lymphocytes were enumerated in 125I-IL-2 binding assays and analyzed by autoradiography after chemical cross-linking of IL-2 to the cells. Quite similar profiles of IL-2R were exhibited by both types of cells consisting of the simultaneous expression of approximately 150 high affinity Tac+ receptors/cell (Kd congruent to 19 pM) and of approximately 540 intermediate affinity Tac- receptors/cell (Kd congruent to 800 pM) which appeared, in cross-linking experiments, to be the isolated 70-kDA protein (p70) subunit of the 55-kDa protein (p55)/p70 heterodimer. The high affinity receptors were distributed on less than 3% of the cells and could be eliminated by complement lysis with anti-p55 mAb. In these conditions, Scatchard analysis no longer revealed two classes of binding sites but only one class of binding sites with intermediate affinity. Although expressed in equal numbers on the surface of NK cells and resting T lymphocytes, the constitutive p70 chains seemed to transmit differently the proliferation signal after effective ligand interaction. Thus, NK cells proliferated strongly in the presence of only 260 pM IL-2, (40 U/ml), whereas resting T cells remained unresponsive to IL-2 concentrations able to saturate the existing p70 receptors (6.5 nM IL-2, 1000 U/ml IL-2) unless monocytes were present. The initiation of cell division seemed to involve the synthesis of p55 chains and the constitution of high affinity receptors as introducing anti-Tac antibody at the start of the cultures inhibited IL-2-induced proliferation. Tac mRNA transcripts accumulated rapidly in NK cells during a 18-h observation period whether anti-Tac antibody was present or not during IL-2 stimulation. In contrast a weak Tac mRNA induction was observed in resting T cells in the same conditions.