Platelet-dependent pulmonary recruitment of neutrophils in abdominal sepsis

Abstract: Sepsis and subsequent multiple organ failure remain the major cause of mortality in intensive care units. Leukocyte-mediated tissue damage is a key feature in septic lung injury. Accumulating data suggest that platelets play a role in inflammation and tissue injury. However, the role of platelets in sepsis-induced leukocyte recruitment and lung edema formation in abdominal sepsis is not demonstrated yet. We hypothesized that platelets may play a significant role in pulmonary neutrophil recruitment and tissue damage in abdominal sepsis. For this purpose, we used the mice cecal ligation and puncture (CLP) model of abdominal sepsis. CLP causes significant pulmonary damage characterized by neutrophil infiltration, increased levels of CXC chemokines and increased edema formation in the lung. CLP also provoked Mac-1 expression on circulating neutrophils. Interestingly, depletion of platelets reduced CLP-induced lung damage, neutrophil recruitment in the bronchoalveolar space and edema formation as well as up-regulation of Mac-1 on neutrophils. However, blocking of platelet-neutrophil aggregates formation did not attenuate CLP-induced lung damage and neutrophil activation suggesting that platelets regulate sepsis-induced lung damage via up-regulation of Mac-1 in a contact independent manner. We also found that plasma levels of soluble CD40L was significantly increased in septic mice. Use of CD40L-deficient mice confirmed that platelet-derived CD40L is a pivotal mediator of neutrophil activation and recruitment in abdominal sepsis and this platelet mediated neutrophil activation was indirect and mediated via formation of MIP-2 and CXCR2 signaling. In addition, we observed a significant increase of soluble CD40L levels in septic patients. Interestingly, we found that inhibition of matrix MMPs reduced Mac-1 up-regulation on neutrophils and CXC chemokine formation in the septic lung injury. We also found that MMP-9 levels are significantly increased in septic mice but not MMP-2. In vitro studies revealed that activated platelets up-regulate surface expression of MMP-9 and that inhibition of MMP-9 decreased platelet shedding of CD40L. Use of MMP-9-deficient mice suggested that MMP-9 regulates platelet CD40L shedding in abdominal sepsis. Moreover, pulmonary infiltration of neutrophils as well as edema formation and lung injury were markedly decreased in septic animals lacking MMP-9. Plasma levels of MMP-9 were significantly increased in patients with septic shock compared to healthy controls. Taken together, platelets regulate neutrophil activation in abdominal sepsis via MMP-9-dependent shedding of platelet-derived CD40L. Thus, MMP-9 and CD40L may constitute novel and effective therapeutic targets in abdominal sepsis.