Intracellular pathway of interleukin 2 following receptor-mediated endocytosis

Electron microscope autoradiography was used to examine the intracellular pathway of radioiodinated interleukin 2 [( 125I]IL2) following its receptor-mediated endocytosis in CTLL cells. Direct measurement of the kinetics of endocytosis showed that 60-70% of surface-bound [125I]IL2 was internalized after 1 h at 37 degrees C. [125I]IL2 was observed to enter cells through invagination of coated pits at the cell surface and to concentrate into lysosomal multivesicular bodies as early as 5-10 min after endocytosis. These results provide ultrastructural information on the intracellular pathway of IL2 and on its probable site of degradation within the cell.