APL cells bind activated platelets through CD44. (A) Representative examples of leukemic cells (NB4 cells, upper; APL cells, the next row) interacting with resting/activated platelets. Activated platelet-bound cells were assessed by myeloperoxidase (MPO) (leukemic cells) and CD62P (activated platelets) staining. (B) The number of leukemia cell–platelet complexes from the data in panel A. (C) NB4 and APL cells (green) were mixed with resting and activated platelets (red) with/without anti–P-selectin antibody or CD44 knockdown (CD44 KD), respectively. The overlap indicates leukemic cell adherence to platelets (arrowheads). Scale bar, 20 µm. (D) A total of 1 × 10⁴ leukemic cells, with or without surrounding fibrinogen (Fbg), were mixed with isolated platelets (2 × 10⁶) for 45 minutes. The formation of FXa was measured as described in “Methods.” (E) APL cells coated with Fbg at various concentrations were mixed with isolated platelets, and the formation of FXa was measured. (F) Representative images of platelets with Fbg treatment. PS exposure was shown by lactadherin staining. Scale bar, 10 µm. (G) Platelets were treated with Fbg or Fbg-coated APL/NB4 cells, and PS exposure was measured by using flow cytometry. **P < .01, *P < .05, #P < .05 vs the data in the remaining platelet group in panel B. *P < .05 in panels D and G. Ctrl, control.