Although the inflammatory response is essential for protecting tissues from injury and infection, unrestrained inflammation can cause chronic inflammatory diseases such as arthritis, colitis and asthma. Physiological mechanisms that downregulate inflammation are poorly understood. Potent control might be achieved by regulating early stages in the inflammatory response, such as accumulation of neutrophils at the site of injury, where these cells release chemical mediators that promote inflammatory processes including plasma extravasation, bacteriocide and proteolysis. To access an inflammatory site, neutrophils must first adhere to the vascular endothelium in a process mediated in part by the leukocyte adhesion molecule L-selectin. This adhesion is prevented when L-selectin is shed from the neutrophil membrane. Although shedding of L-selectin is recognized as a potentially important mechanism for regulating neutrophils, its physiological function has not been demonstrated. Shedding of L-selectin may mediate endogenous downregulation of inflammation by limiting neutrophil accumulation at inflammatory sites. Here we show that activation of nociceptive neurons induces shedding of L-selectin from circulating neutrophils in vivo and that this shedding suppresses an ongoing inflammatory response by inhibiting neutrophil accumulation. These findings indicate a previously unknown mechanism for endogenous feedback control of inflammation. Failure of this mechanism could contribute to the etiology of chronic inflammatory disease.

This study examined the dependence of neurogenic and non-neurogenic synovial plasma extravasation on neutrophils. Perfusion of bradykinin into the knee joint produced both a rapid increase in the magnitude of plasma extravasation and a significant increase in number of neutrophils in the synovium. Both bradykinin-induced plasma extravasation and neutrophil accumulation were dependent on sympathetic post-ganglionic neuron terminals, since both were blocked in sympathectomized rats. Platelet activating factor, which produces plasma extravasation independent of sympathetic neurons, did not increase the number of neutrophils in the synovium. These findings support the suggestion that bradykinin acts on sympathetic nerve terminals in the knee leading to attraction of neutrophils, which promotes plasma extravasation.