Differential regulation of S1P receptor expression on activation. T cells were either left intact (nonactivated) or were activated with PMA plus ionomycin or anti-CD3 plus anti-CD28 for 3 days. These cells were then labeled with antibodies to S1P₁, S1P₂, S1P₃, S1P₄, or S1P₅. Background controls in the presence of rabbit IgG (for S1P₁, S1P₄, and S1P₅) or mouse IgG (for S1P₂ and S1P₃) and FITC-conjugated secondary antibodies are shown in black. Numbers indicate the percentage of positive cells in one experiment. In 3 to 4 experiments performed, the percentage of positive cells for S1P₁, S1P₂, S1P₃, S1P₄, and S1P₅ ranges between 83% and 98%, less than 0.1%, 1% and 15%, 25% and 50%, and 5% and 25% for nonactivated cells; 52% to 83%, less than 0.1%, 15% and 40%, 20% and 30%, and 23% and 41% for PMA plus ionomycin-treated cells; and 5% and 70%, less than 0.1%, 15% and 30%, 20% and 25%, and 12% and 26% for anti-CD3 plus anti-CD28 activated cells, respectively. White histograms represent the binding of the specific antibodies.