Alloimmunized mice treated with FTY720 produce fewer alloantibodies than the control mice. BALB/C (H2^d) mice were injected intraperitoneally with FTY720 (0.1 mg/kg) 5 days, 2 days and 3 hours before each transfusion with 1 × 10⁸ H2^b platelets isolated from C57BL/6J mice or with vehicle control. (A) Representative images of spleen sections from mice treated with FTY720 (right panel) or vehicle (left panel) stained for IgM (gray) to detect marginal zone B cells, and MadCam (blue) to delineate the marginal sinus. The dashed lines indicate marginal zones. Scale bar, 10 μm. Original magnification x400. (B) The level of anti-H2^b IgG antibodies was evaluated weekly by flow cytometry in FTY720 (blue bars) and vehicle control (red bars) -treated BALB/C mice. Results are presented as the mean fluorescence intensity (MFI ± SEM) of Alexa-488 GaM IgG antibodies bound to H2^b platelets (ns, P ˃ .05; ∗P < .05; 1-way ANOVA; n = 12-14). (C) Alloimmunized BALB/C mice were injected with 2 × 10⁸ Oregon green CFDA-SE-labeled platelets from C57BL/6J mice. The kinetics of elimination of transfused platelets were deduced from the percentage of Oregon green-positive platelets among 10 000 RAM.1⁺ platelets in FTY720-treated (blue line) and control mice (red line) at each time point (± SEM; ∗∗P < .01; ∗P < .05; 2-way ANOVA; n = 12). ANOVA, analysis of variance; B Foll, B cell follicle; MZ, marginal zone; SEM, standard error of the mean.