Binding and internalization of biotinylated interleukin-2 in human lymphocytes

The binding, internalization, and fate of interleukin-2 (IL-2) were studied in phytohemagglutinin (PHA)-activated human lymphocytes using biotinylated recombinant IL-2 (rIL-2). Streptavidin adsorbed to 18-nm colloidal gold beads (Au18-streptavidin) and streptavidin covalently bound to horseradish peroxidase (HRP-streptavidin) were used to follow the movement of biotinylated rIL-2 within cells over a 4-hour period. Results obtained from either probe were similar. Biotinylated rIL-2 was taken up in coated pits, transferred to a series of small uncoated vesicles and tubules in the peripheral cytoplasm of the cell, then concentrated and sequestered in uncoated vesicles, multivesicular bodies (MVB), and dense bodies (DB) in the peripheral and juxtanuclear cytoplasm of the cell. Occasionally, MVB containing Au18-streptavidin, or HRP-streptavidin, appear to have fused with the plasma membrane of the cell. No labeling of the Golgi cisternae, nuclear envelope, or nucleus was observed. Results from a competitive receptor binding assay and a cell proliferation assay indicate that both the affinity of rIL-2 for high affinity rIL-2 receptors and the proliferative activity of rIL-2 were negligibly affected by the biotinylation procedure. These studies suggest that in activated lymphocytes, IL-2 is bound to receptors on the cell surface, gathered in coated pits, internalized by receptor-mediated endocytosis, concentrated in the endosomal compartments, and delivered to lysosomes for degradation.