Interleukin 2 functions through novel interleukin 2 binding molecules in T cells.

In this report, we examined whether novel interleukin 2 (IL-2) binding molecules (p70/75) are responsible for signal transduction and internalization of IL-2 in T cells by using a monoclonal antibody H-31 to Tac antigens. We found that H-31 inhibited the binding of IL-2 to Tac antigens but not novel IL-2 binding molecules. Scatchard plot analysis revealed that in the presence of H-31, intermediate affinity sites (Kd = 1 to 1.5 nM) were detectable and the number of them was similar to that of high affinity IL-2 receptor (IL-2R) (Kd = 10 to 15 pM) in the absence of H-31. Furthermore, the kinetics of endocytosis of IL-2 via p70/75 showed the same pattern as via high affinity IL-2R. Finally, high doses of IL-2 (100 to 10,000 U/ml) are required for the proliferation of T cells in the presence of H-31, whereas in the absence of H-31, physiologic doses of IL-2 (1 to 100 U/ml) induced the proliferation. These results taken together suggest that novel IL-2 binding molecules are related to signal transduction of IL-2 and that Tac antigens are essential for constructing of high affinity IL-2R, although Tac antigens may not be responsible for signal transduction.