Interaction with activated platelets results in adhesion, phagocytosis, and neutrophil MPO depletion. Purified neutrophils were incubated alone (without platelets) or challenged with resting or activated platelets at a platelet-to-neutrophil ratio of 20:1. (A) Platelet adhesion was evaluated by flow cytometry, assessing the CD61 platelet antigen in nonpermeabilized neutrophils. (B) Internalization was assessed by the CD61 expression after permeabilization of the neutrophil plasma membrane. (C) Neutrophils with adhering () or internalized platelets (■) were calculated. (D) Representative electron micrographs show that only neutrophils challenged with activated platelets were involved in phagocytosis (red arrow). (E) MPO content was assessed in parallel by flow cytometry: neutrophils degranulate after recognition of activated platelets. (F) Confocal microscopy allows the simultaneous assessment of intracellular platelets, revealed by the platelet GPIbα antigen (red), and MPO (green). Nuclei were counterstained with DAPI (blue). Neutrophils involved in recognition and clearance of activated platelets underwent MPO depletion. Data reported are from representative experiments (flow cytometry n = 16, confocal microscopy n = 10, electron microscopy n = 7).