Infection and thrombotic risk: Role of TLR4 in platelet reactivity

Abstract

Infection increases the risk of thrombotic disease. Platelets contribute to defense against infection and to thrombosis. Gram negative bacteria produce lipopolysaccharides (LPS) that activate Toll-like-receptor-4 (TLR4) in the host and initiate inflammatory responses to infection. Platelets express TLR4, but whether or how LPS affects platelet functions that contribute to thrombotic disease is not understood. To determine if TLR4 signaling influences platelet functions that contribute to thrombotic disease, we studied responses of platelets from normal mice (C57BL/10SnJ; n=10) and mice that do not express TLR4 (C57BL/10ScN; dTLR4, n=10). Prior to LPS exposure, the number of circulating platelets, platelet RNA content and expression of P-selectin following stimulation with thrombin were diminished in mice that lack TLR4 compared to wild type mice, while platelet aggregation and secretion of ATP were unchanged. These results suggest that loss of TLR4 function affects platelet production and membrane binding characteristics of the circulating platelet pool. One week following a single intravenous injection of LPS (200μg/kg/iv), the time it takes for the circulating platelet pool to turnover, both wild-type and dTLR4 mice had similar numbers of circulating platelets, and thrombin-induced expression of P-selectin and platelet aggregation to collagen increased comparably in both groups of mice. These results indicate that TLR4 function is required for normal platelet homeostasis. Exposure to LPS stimulates a TLR4-independent shift of the platelet pool toward a reactive phenotype, which could contribute to increased thrombotic risk during or following infection.