Binding of CC-chemokines to CCR5. A CHO-K1 cell line stably expressing human CCR5 and apoaequorin was established, and characterized by saturation-binding assay as expressing 2 pmoles receptor per mg protein. CC-chemokines were first tested for their ability to compete with [¹²⁵I]-MIP-1β binding at high concentration (200 nmol/L), and competition curves were further established for chemokines displaying CCR5-binding activity. Chemokines that did not compete significantly at 200 nmol/L included MIP-3, MIP-3β, MIP-4, HCC-1, HCC-2, HCC-3, MPIF-1, MPIF-2, TARC, TECK, SLC, LEC, MDC, and I-309. The results were analyzed by the Graphpad Prism software, using a single-site model, and the data were normalized for the nonspecific binding (0%) and the specific binding in the absence of competitor (100%). All points were run in triplicate (error bars: S.E.M.). The presented curves are representative of at least two independent experiments. Table 1 presents the averaged values from the various experiments.