IL-10 receptor expression on B-CLL cells. (A) MNCs (5 × 10⁶) from a B-CLL patient, containing 97% CD19/CD5 positive B-CLL cells, were incubated with increasing amounts of radiolabeled IL-10 with or without a 1,000-fold excess of unlabeled IL-10. Points shown are averages of duplicates, with a standard deviation of less than 15%. The specific binding was determined as the difference between the total CPM and the CPM in the presence of 1,000-fold excess unlabeled IL-10. Maximal binding occurs at 1.5 to 2.5 nmol/L. (B) Scatchard analysis of equilibrium binding data from the same patient sample as shown in (A). The kd derived is 306 pmol/L and the Bmax is 4.8 pmol/L, giving an estimate of 116 IL-10 receptors per cell. These data are representative of three such experiments from three different B-CLL patients.