Role of Growth Arrest-Specific Gene 6 Product (Gas6) in Severe Sepsis.

Gas6 binding to its cognate receptor tyrosine kinases (Tyro3, Axl and Mer) down-regulates the activation state of macrophages and thereby their production of proinflammatory cytokines induced by various stimuli. Mer activation inhibits TNF-α production by macrophages and alleviates endotoxic shock in mice. Gas6 receptors are cleaved in the extracellular domain to generate soluble receptors. Soluble receptors function as decoy receptors, thereby inhibiting the Gas6 interaction with cell-associated receptors. The aim of the study was to determine whether Gas6 pathway plays a role in sepsis in human and mice. In the first part of the study, we measured plasma levels of Gas6 and its soluble receptors sTyro3 and sAxl in 13 healthy subjects, 29 patients with severe sepsis, and 18 patients with acute non-infectious inflammatory diseases. Gas6 and sAxl concentrations were higher in septic patients than in healthy subjects or in patients with non-infectious inflammatory disease (P≤ 0.0001 for Gas6 and <0.004 for sAxl) and correlated with C-reactive protein levels (P=0.0019 for Gas6 and =0.0037 for sAxl). The sensitivity and specificity of Gas6 levels to predict fatal outcome was 83% and 88%. In the second part of the study, we investigated whether Gas6 affects cytokine production and outcome in experimental models of endotoxemia and peritonitis in Gas6+/+ and Gas6−/ − mice. Circulating levels of Gas6 after LPS 50mg/kg i.p. peaked at 1 h. (66±3ng/ml), were still elevated 4 h. later (47±1ng/ml) and returned approximately to baseline levels 8 h. after the LPS challenge (34±1ng/ml); P<0.001 for all time points versus baseline (23±1ng/ml). Cytokine (TNF-α and IL-6) production was higher in Gas6−/ − than Gas6+/+ mice after LPS 50mg/kg (TNF-α at 1 h.: 10±1ng/ml in Gas6−/ − versus 4.5±0.5ng/ml in Gas6+/+ mice, P<0.001; IL-6 at 4h.: 77±4ng/ml in Gas6+/+ versus 56±6ng/ml in Gas6−/ − mice, P=0.02). Similar data were obtained with Tyro3−/ − and Axl−/ − mice. Mortality induced by LPS 25mg/kg was 25% in Gas6+/+ versus 87% in Gas6−/ − mice (P=0.0023). In peritonitis models (cecal ligation and puncture, and i.p. injection of E. coli), plasma levels of Gas6 increased two-fold above baseline and remained elevated at least 24h. Increases of Gas6 levels were proportional to the importance of the size of the E. coli inoculum. In vitro experiments revealed that Gas6−/ − LPS-stimulated macrophages produced more TNF-α than Gas6+/+ macrophages. In conclusion, in septic patients, plasma levels of Gas6 and its cognate receptors were elevated and associated with fatal outcome. In mice, Gas6 plasma levels raised in experimental endotoxemia and sepsis models, and correlated also with sepsis severity. Thus, Gas6 and its receptors hold promise as early sepsis markers and outcome predictors, and could constitute therapeutic targets for new immunomodulating drugs.