Cytotoxicity of activated platelets to autologous red blood cells

Summary Gel‐filtered human platelets exerted lytic activity on autologous red blood cells (RBC) when they were co‐incubated at 37°C with platelet‐activating agents, such as thrombin, collagen, ADP. LPS or PMA in the absence of plasma. Lysis of activated platelets themselves did not occur during the incubation period examined. Morphological observations showed that RBC exposed to thrombin‐activated platelets were fragmented and/or transformed into spherocytes. This haemolytic reaction by thrombin‐activated platelets did not occur at 4°C, or in the presence of agents which inhibited glycolysis or elevated intracellular levels of cAMP, indicating that energy‐dependent and cAMP‐regulated platelet metabolism was required for this reaction. When platelets and RBC were incubated in the same vessel, but were prevented from coming into direct cell to cell contact by means of a membrane barrier, their cytotoxicity was reduced but not eliminated completely. No cytotoxic activity against RBC was detected in platelet‐free supernatants obtained by centrifugation after activation of platelets with thrombin. On the contrary, activated and washed platelets retained the activity. These observations suggested that the cytotoxic activity was carried by some diffusible and easily inactivated factors, which were continuously produced and liberated from activated platelets. Cyclo‐oxygenase inhibitors inhibited the haemolytic activity of thrombin‐activated platelets, suggesting a role for some products of platelet‐cyclooxygenase pathway in platelet‐mediated haemolysis. These results provide the first evidence for a direct role of activated platelets in mediation of RBC‐damage in the absence of any plasma factors.