Summary

Serpins maintain haemostasis through regulation of serine proteinasesin the thrombotic and thrombolytic pathways.Virusesencode serpins that can alter thrombotic and thrombolytic responsesproducing, in some cases, disseminated intravascularcoagulation (DIC). However, it has not been precisely definedhow viral serpins induce these profound responses.The rabbitmyxoma viral serpin, Serp-1 inhibits urokinase- and tissue-typeplasminogen activators (uPA and tPA), plasmin and factor Xa invitro and exhibits remarkable anti-inflammatory activity in variousanimal models.The effects of Serp-1 on activation of humanplatelets, endothelial cells, monocytes and T cells that mediatethrombosis and innate immune responses were thereforeexamined.We found that Serp-1 attenuated platelet and mononuclearcell adhesion to fibronectin and collagen. Serp-1 similarlyinhibited monocyte migration into the peritoneum. Serp-1 inhibition of monocyte migration was lost in uPA receptor(uPAR) deficient mice. Serp-1 bound to the plasma membranesurface and altered uPA activation of endothelial cells (p=0.001),thrombin activation of platelets (p=0.021) and phorbol ester activationof endothelial (p=0.047), monocyte (p=0.011) and JurkatT cells (p=0.012) as measured by intracellular calcium.Modulation of cellular activation was confirmed by membranefluidity analysis. Microarray analysis of Serp-1 treated endothelialcells revealed alterations in Inositol 1,4,5-triphosphate receptortype II (ITPR2) a calcium-regulating gene.This study demonstratesthe unique capacity of a viral serpin, Serp-1 to modifyadhesion, activation, gene expression and calcium homeostasisin a wide range of cells that regulate coagulation and inflammation.Endothelial cells potentially represent a pivotal regulatorypoint for Serp-1 anti-inflammatory activity.