Role for platelet toll-like receptor 4 (TLR4) in the formation of Neutrophil Extracellular Traps (NETs) in sepsis (44.22)

Abstract It has been known for many years that LPS, neutrophils and platelets all participate in the pathogenesis of severe sepsis, but the inter-relationship between these players is completely unknown. Using flow chambers in vitro we suggest a novel innate immune response leading to enhanced trapping of bacteria in blood vessels. The mechanism involved platelet TLR4 detecting LPS in blood and inducing a unique response, specifically platelet binding to adherent neutrophils, but not platelet aggregation or P-selectin expression. Subsequently, the platelets stimulated very robust neutrophil activation leading to formation of NETs. These NETs retained their integrity under flow conditions and functioned to ensnare bacteria within the vasculature. Plasma from severely septic patients also induced TLR4-dependent platelet-neutrophil interactions leading to the production of NETs. We propose that this novel bacterial trapping mechanism would only occur under extreme conditions such as severe sepsis and platelet TLR4 (not leukocyte TLR4) functioned as the threshold switch for this innate immune response to occur. With the advent of antibiotics perhaps reducing the need for NET formation, we would propose that inhibiting platelet activation with TLR4 inhibitors may inhibit NET formation and reduce inadvertent tissue injury. This work was funded by: CIHR and AHFMR