Quantitative fluorescence analysis of cyclosporine binding to human leukocytes

The purpose of this investigation was to estimate the binding of cyclosporine at the single-cell level on human peripheral lymphocytes, and to test possible identity of the cyclosporine-binding site with a common receptor of T cell activation. A dansyl-coupled derivative (Dans cyclosporine) was used as a fluorescent probe. The histograms of unseparated, labeled peripheral leukocytes obtained by a fluorescence-activated cell sorter (FACS) showed that Dans cyclosporine stained all leukocytes--but two distinct populations could be separated based on the intensity of fluorescence. The more brightly labeled cells consisted mainly of granulocytes and monocytes, whereas the less-bright cells represented the lymphocyte compartment. Fluorescence microscopy revealed binding on the membrane for both cell populations; the label was, however, rapidly internalized in phagocytes. For both populations binding was saturable, time and temperature dependent, and reversible. Half-saturation occurred at approximately 5 X 10(-7) M (Kd). With respect to lymphocyte subpopulations, no difference of cellular fluorescence was found between unseparated lymphocytes and T cell subsets. In addition, mitogens such as concanavalin A, phytohemagglutinin, phorbol 12-myristate 13-acetate, or OKT3 antibody did not inhibit Dans cyclosporine binding. These results clearly indicate that cyclosporine binds to all peripheral blood lymphocytes, and no preferential binding on T cell subsets can be detected.