The murine IL 2 receptor. I. Monoclonal antibodies that define distinct functional epitopes on activated T cells and react with activated B cells

The properties of three distinct rat monoclonal antibodies, designated 3C7, 7D4, and 2E4, to the murine IL 2 receptor have been compared in binding, biochemical, and functional assays. 3C7 appears to define an epitope near or identical to the IL 2-binding site of the receptor, because 3C7 inhibited the binding of radiolabeled IL 2 to CTL-L cells and because unlabeled IL 2 inhibited the binding of FITC-3C7 to CTL-L cells. 7D4 and 2E4 had no effect on IL 2 binding. Competitive antibody-binding studies confirmed that the epitope seen by 3C7 was distinct from the epitope(s) seen by 7D4 and 2E4. Sequential immunoprecipitation studies demonstrated that all three antibodies were reactive with the same molecular species, and that each precipitated identical components of 20,000 to 25,000 daltons, 50,000 to 60,000 daltons, and 100,000 to 120,000 daltons from the surface of CTL-L. FACS studies demonstrated a quantitatively and qualitatively identical cell distribution for the antigen defined by each antibody. They failed to stain more than 95% of resting lymphocytes, but were strongly reactive with Con A T blasts and substantially less reactive with LPS B blasts. Unlabeled IL 2 was also able to inhibit the binding of FITC-3C7 to LPS B cell blasts, suggesting the presence of IL 2-binding sites on activated B cells. Each antibody inhibited IL 2-driven proliferation of T2 or CTL-L cells. 3C7 and 7D4 were more potent inhibitors of proliferation than was 2E4, and the combined use of 3C7 and 7D4 resulted in greater levels of inhibition of proliferation than that shown from the use of either antibody alone. Collectively, the results support the hypothesis that these antibodies detect two distinct functional regions of the IL 2 receptor.