Platelets kill bacteria by bridging innate and adaptive immunity via platelet factor 4 and FcγRIIA

Background Activated platelets release the chemokine platelet factor 4 (PF4) stored in their granules. PF4 binds to polyanions (P) on bacteria, undergoes a conformational change and exposes neoepitopes. These neoepitopes induce production of anti-PF4/P antibodies. As PF4 binds to a variety of bacteria, anti-PF4/P IgG can bind and opsonize several bacterial species. Objective Here we investigated whether platelets are able to kill bacteria directly after recognizing anti-PF4/P IgG opsonized bacteria in the presence of PF4 via their FcRIIA. Methods Using platelet-bacteria suspension co-culture experiments and micropatterns with immobilized viable bacteria, in combination with pharmacological inhibitors and human anti- PF4/P IgG we analyzed the role of platelet-mediated killing of bacteria. Results In the presence of PF4, human anti-PF4/P IgG and platelets, E. coli killing (> 50%) with colony forming units (CFU mL(-1)) 0.71 x 10(4) 0.19 was observed compared with controls incubated only with anti-PF4/P IgG (CFU mL(-1) 3.4 x 10(4) +/- 0.38). Blocking of platelet FcRIIA using mAb IV.3 (CFU mL(-1) 2.5 x 10(4) +/- 0.45), or integrin IIb3 (CFU mL(-1) 2.26 x 10(4) +/- 0.31), or disruption of cytoskeletal functions (CFU mL(-1) 2.7 x 10(4) +/- 0.4) markedly reduced E. coli killing by this mechanism. Our observation of E. coli killing by platelets on micropatterned arrays is compatible with the model that platelets kill bacteria by covering them, actively concentrating them into the area under their granulomere and then releasing antimicrobial substances of platelet -granules site directed towards bacteria. Conclusion These findings collectively indicate that by bridging of innate and adaptive immune mechanisms, platelets and anti-PF4/polyanion antibodies cooperate in an antibacterial host response.