FTY720 mediates apoptosis-independent lymphopenia in human renal allograft recipients: Different effects on CD62L+ and CCR5+ t lymphocytes

Background. The sphingolipid FTY720 (FTY), a novel immune modulator, induces lymphopenia and prevents allograft rejection. This study was designed to study the effect of FTY on lymphocyte subpopulations and apoptosis in stable renal allograft recipients. Methods. Stable renal allograft recipients received a single oral dose of 0.25 to 3.5 mg of FTY (n=13) or placebo (n=3). Whole blood was drawn immediately before and at 4, 8, 12, 24, 72, and 96 hr after administration. The number of lymphocyte subpopulations, with an emphasis on surface markers involved in lymphocyte migration, was analyzed by flow cytometry. Apoptotic lymphocytes were detected following Annexin V-FITC/PI staining. Lymphocyte mobility was investigated in a modified Boyden chamber. Results. FTY induced a transient lymphopenia by an apoptosis-independent mechanism. In vitro experiments with peripheral blood mononuclear cells (PBMC) confirmed that clinically relevant concentrations of FTY (0.1 μM) increased lymphocyte mobility, whereas only suprapharmacologic concentrations of FTY (10 μM) could induce apoptosis. FTY-treated patients had reversible changes in the composition of peripheral lymphocyte subpopulations. CD62L + T cells decreased to the greatest extent (-57%). In contrast, CCR5 + T-cell counts declined only marginally (-10%). In vitro, treatment of PBMC with FTY (1 mM-10 μM) did not induce changes in the expression of these surface markers. Conclusions. The data indicate that FTY mediates apoptosis-independent lymphopenia in human renal allograft recipients. FTY-induced lymphopenia preferentially affects CD62L + and CCR5 - T-lymphocyte subpopulations.