CHARACTERIZATION OF THE INTERLEUKIN-2 RECEPTORS (IL-2R) EXPRESSED ON HUMAN NATURAL-KILLER-CELLS ACTIVATED INVIVO BY IL-2 - ASSOCIATION OF THE P64 IL-2R GAMMA-CHAIN WITH THE IL-2R BETA-CHAIN IN FUNCTIONAL INTERMEDIATE-AFFINITY IL-2R

Interleukin 2 (IL-2) receptors expressed on the surface of activated T cells and natural killer (NK) cells exhibit a variety of affinity states depending on their subunit composition. Low-affinity binding is associated with a 55-kD alpha-chain, intermediate-affinity binding with a 70-75-kD beta-chain, and high-affinity binding with a bimolecular complex of the alpha and beta-subunits. In a previous study of the IL-2 receptors expressed on NK cells obtained from cancer patients after in vivo IL-2 therapy, we documented a discrepancy between the level of beta-chain and the level of intermediate-affinity IL-2 binding sites expressed on the cell surface. Based on this result, we postulated that formation of intermediate-affinity receptor sites required a component in addition to the beta-chain, and that this component was present at limiting levels in the patient NK cells. In the present study we have examined the structure of the intermediate-affinity receptor complex using monoclonal antibodies that recognize the beta-chain, but that do not interfere with its ability to bind IL-2. Evidence is presented establishing the physical association of a novel protein of 64 kD with the beta-chain in intermediate-affinity IL-2 binding sites. This molecule, termed IL-2R gamma-chain, coprecipitated with beta-chains prepared from cells that had been incubated with IL-2, but was undetectable in immunoprecipitates prepared in the absence of IL-2. Examination of gamma-chain expression in post-IL-2 therapy NK cells, where only low levels of intermediate-affinity IL-2 binding were detectable, revealed that the gamma-chain was associated with, on average, only 10-12% of the beta-chains expressed on such cells. This contrasted with approximately equal levels of beta and gamma-chain expression on YT cells, a cell line that has both high levels of cell surface beta-chain expression and high levels of IL-2 binding. Thus, the ratio of gamma-chain to beta-chain present in the immunoprecipitates roughly correlated with the proportion of beta-chain involved in intermediate-affinity receptor sites. This result suggests that the 64-kD gamma-chain is the component responsible for regulating the affinity of IL-2 association with the beta-subunit. By further defining the structural components necessary for IL-2 receptor formation, these studies provide additional insight into mechanisms whereby lymphocytes might regulate their responsiveness to IL-2.