AbstractThe protein Z (PZ)-dependent plasma protease inhibitor (ZPI) is a glycoprotein that inhibits factor XIa and, in the presence of PZ, FXa. Recently, ZPI has been shown to be an acute-phase protein (APP). As usually APPs downregulate the harmful effects of inflammation, we tested whether ZPI could modulate the increase of cytokines observed in inflammatory states. We observed that recombinant human ZPI (rhZPI) significantly decreases the levels of interleukin (IL)-1, IL-6, and tumor necrosis factor- α (TNF-α) induced by lipopolysaccharide (LPS) in a whole blood model. This inhibitory effect was unaffected by the presence of PZ or heparin. A ZPI mutant within the reactive loop center ZPI (Y387A), lacking anticoagulant activity, still had an anti-inflammatory activity. Surprisingly, rhZPI did not inhibit the synthesis of IL-6 or TNF-α when purified monocytes were stimulated by LPS, whereas the inhibitory effect was evidenced when lymphocytes were added to monocytes. The requirement of lymphocytes could be due to the synthesis of CCL5 (RANTES), a chemokine mainly produced by activated lymphocytes which is induced by rhZPI, and which can reduce the production of proinflammatory cytokines in whole blood. Lastly, we observed that the intraperitoneal injection of rhZPI significantly decreased LPS-induced IL-6 and TNF-α production in mouse plasma.