Cyclosporin receptors on human lymphocytes

Utilizing (3H) cyclosporin C (3H CS-C), a dihydroderivative of cyclosporin A (CS-A), an assay for cyclosporin receptors on human peripheral blood lymphocytes was developed. The specific binding of (3H) CS-C was saturable, time-dependent, and reversible. A Kd of 1.2 x 10(-7) M and a maximum binding capacity (Bmax) of 7 pmol/10(6) cells was calculated from equilibrium binding studies. A Scatchard analysis confirmed a single population of high affinity binding sites and about 7 x 10(5) sites/cell. Kinetic analysis of specific binding at 37 degrees C yielded a pseudo-first order rate association constant of 1.1 x 10(6) M-1 min-1 and a zero order dissociation rate constant of 0.19 min-1. The Kd 1.7 x 10(-7) M calculated from kinetic studies agreed well with the value of 1.2 x 10(-7) M determined in equilibrium binding studies. Regarding the specificity of binding, (3H) CS-C binding to lymphocytes was inhibited with CS-A and CS-C; however, no inhibition with the biologically inert keto-CS-A was observed. Mitogens and various growth factors did not compete for the binding site when added simultaneously with the radiolabel. However, pretreatment with T cell mitogens such as PHA, Con A, or the OKT3 antibody reduced (3H) CS-C binding significantly. These results suggest that the binding site for CS-A is closely associated with the mitogenic receptor on lymphocytes.