OBJECTIVE To define the role of CD40L in abdominal sepsis. BACKGROUND Platelets promote sepsis-induced pulmonary recruitment of neutrophils. However, the identity of the platelet-derived molecule regulating neutrophil infiltration is not known. The hypothesis of the present study was that platelet-derived CD40L might be responsible for platelet-mediated activation and accumulation of neutrophils in sepsis. METHODS Wild-type C57BL/6 mice and CD40L gene-deficient mice were exposed to cecal ligation and puncture (CLP). Lung edema, bronchoalveolar neutrophils, CD40L and macrophage inflammatory protein-2 (MIP-2) plasma levels, myeloperoxidase activity and Mac-1 expression were determined up to 24 hours after CLP induction. For platelet depletion was an anti-GP1balpha antibody administered before CLP. RESULTS Plasma levels of soluble CD40L increased and surface expression of CD40L on platelets decreased in CLP mice. Platelet depletion reduced CLP-induced CD40L levels by 90%. CLP-provoked Mac-1 expression on neutrophils was abolished in CD40L-deficient mice. Interestingly, CLP-induced edema and myeloperoxidase activity in the lung as well as neutrophil infiltration in the broncoalveolar space were markedly reduced in mice lacking CD40L. In vitro experiments showed that CD40L was not capable of directly increasing Mac-1 levels on neutrophils. Instead, CLP-induced plasma levels of MIP-2 were significantly reduced in CD40L-deficient mice and inhibition of the MIP-2 receptor (CXCR2) decreased Mac-1 expression on neutrophils in septic animals. CONCLUSIONS CD40L derived from platelets is a potent activator of neutrophils and mediates sepsis-induced neutrophil recruitment and lung edema. The neutrophil activating mechanism of CD40L is indirect and mediated via MIP-2 formation and CXCR2 signaling. Targeting CD40L may be an effective approach to limit pulmonary damage in abdominal sepsis.