Platelets are deposited on the surface of leukocytes after anti-CD44 or anti–Gr-1 treatment. (A) Surface topography of adherent leukocytes in (top left) IM7-treated and (bottom left) anti–Gr-1–treated WT mice after in vivo staining with R6G suggests deposition of particles, which proved to be platelets as indicated by (middle panel) platelet-specific CD41-PE mAb staining in vivo. The right panel shows colocalization of CD41 (platelets, red) and Gr-1 (granulocytes, green) fluorescence within adherent cells in the blood vessel of an IM7-treated mouse, indicating preferential deposition of platelets on granulocytes. Arrows indicate the direction of blood flow in the synovial venules. (B) Treatment of WT mice with IM7 or anti–Gr-1 mAb results in a significant reduction of platelet counts in the circulation 1 hour after treatment (0 minutes, dark blue bars; 60 minutes, light blue bars; n = 8 mice/group; *P < .05).