Adhesive interaction between peripheral blood mononuclear cells and activated platelets in the presence of anti‐human leukocyte antigen Class I alloantibody causes production of IL‐1β and IL‐8

Background:  Activated platelets form heterogeneous aggregates of platelets and monocytes, which are involved in a variety of inflammatory disorders. Some anti‐human leukocyte antigen (HLA) Class I antibodies have been shown to activate platelets. Materials and Methods:  Human leukocyte antigen‐A2‐positive or HLA‐A2‐negative platelets were incubated with HLA‐A2‐negative peripheral blood mononuclear cells (PBMNCs) in the presence of anti‐HLA‐A2 serum at 37°C. The binding of platelets to monocytes was analysed by flow cytometry. The levels of IL‐1 β and IL‐8 in the culture supernatant were determined by ELISA. Results:  Anti‐HLA‐A2 serum increased the formation of aggregates between monocytes and HLA‐A2‐positive platelets, but not HLA‐A2‐negative platelets, in a dose‐dependent manner. Antiserum also increased the number of platelets bound to monocytes in a time‐dependent manner. The addition of anti‐P‐selectin glycoprotein ligand (PSGL‐1) mAb almost completely inhibited the formation of platelet–monocyte aggregates as well as the number of platelets bound to monocytes. When HLA‐A2‐positive or HLA‐A2‐negative platelets were incubated with HLA‐A2‐negative PBMNCs in the presence of anti‐HLA‐A2, the level of IL‐1β and IL‐8 in the supernatant of coculture was significantly higher in HLA‐A2‐positive platelets than in HLA‐A2‐negative platelets. The addition of anti‐PSGL‐1 mAb partially but significantly inhibited the production of both IL‐1β and IL‐8. Conclusions:  The activation of platelets with anti‐HLA Class I alloantibody caused the formation of platelet–monocyte aggregates, followed by the production of IL‐1β and IL‐8, in a cognate antigen–antibody manner. The adhesive interaction of P‐selectin and PSGL‐1 at least partially contributed to these phenomena.