Neutrophils are consistently associated with arterial thrombotic morbidity in human clinical studies but the causal basis for this association is unclear. We tested the hypothesis that neutrophils modulate platelet activation and
thrombus formation in vivo in a
cathepsin G-dependent manner. Neutrophils enhanced aggregation of human platelets in vitro in dose-dependent fashion and this effect was diminished by pharmacologic inhibition of
cathepsin G activity and knockdown of
cathepsin G expression. Tail bleeding time in the mouse was prolonged by a
cathepsin G inhibitor and in
cathepsin G knockout mice, and formation of neutrophil-platelet conjugates in blood that was shed from transected tails was reduced in the absence of
cathepsin G. Bleeding time was highly correlated with blood neutrophil count in wildtype but not
cathepsin G deficient mice. In the presence of elevated blood neutrophil counts, the anti-thrombotic effect of
cathepsin G inhibition was greater than that of
aspirin and additive to it when administered in combination. Both pharmacologic inhibition of
cathepsin G and its congenital absence prolonged the time for platelet
thrombus to form in
ferric chloride-injured mouse mesenteric arterioles. In a vaso-occlusive model of
ischemic stroke, inhibition of
cathepsin G and its congenital absence improved cerebral blood flow, reduced histologic
brain injury, and improved neurobehavioral outcome. These experiments demonstrate that neutrophil
cathepsin G is a physiologic modulator of platelet
thrombus formation in vivo and has potential as a target for novel anti-thrombotic
therapies.