Innate Immunity, Coagulation, and Thrombosis

In evolutionarily ancient animals such as insects and crustaceans, the host responses to physical injury and to invading pathogens can be mediated by the same mechanism of coagulum formation of the hemolymph. During vertebrate evolution hemostasis has emerged as an independent process primarily involved in the rapid repair of blood vessel injuries. The core processes of hemostasis are blood coagulation (resulting in fibrin formation) and platelet activation. Both processes can independently interact with inflammatory responses as apparent in a pathological context such as during development of disseminated intravascular coagulation (DIC). Moreover, extravascular fibrin formation can promote the trapping of pathogens and thereby help to contain infections. Nonetheless, the connections between fibrin formation, platelet activation and innate immunity are incompletely understood. We have recently shown that during early systemic infection with E. coli microvascular thrombi are formed which capture bacteria together with innate leukocytes. These thrombi are fibrin-rich and are in general observed in less than 10% of vessels with diameters < 25 µm. Their formation is not accompanied by marked activation of inflammation since the levels of pro-inflammatory markers are unchanged. Microvascular thrombosis is almost completely suppressed in mice deficient for the neutrophil serine proteases elastase and cathepsin G (NE/CG^-/-) which are major microbicidal effectors of neutrophils. In the microcirculation of NE/CG^-/- mice, microbes are mostly tissue-associated. In contrast, they are mostly present inside blood vessels in wild type mice. The results of experimental changes in microvascular fibrin formation show that intravascular blood coagulation is causally involved in the capturing of bacteria and of myeloid cells and, additionally, promotes the bacterial killing. Overall this suggests that microvascular thrombosis supports recognition, containment and elimination of bacteria without inducing noticeable damage to the host. It thus fulfills the criteria for a comprehensive intravascular process of innate immunity. This mechanism of intravascular immunity, which was termed "immunothrombosis," is supported by tissue factor (TF), the overall initiator of blood coagulation, and by factor XII, the starter protein of the contact pathway. In particular, extracellular nucleosomes (eNUC)/neutrophil extracellular traps (NETs) are indispensable for immunothrombosis. eNUC/NETs promote thrombosis by critically enhancing degradation of TFPI, the major antagonist of the coagulation start, via neutrophil elastase and by factor XII activation. Release of eNUC/NETs by neutrophils and induction of intravascular coagulation essentially require interaction of activated platelets with neutrophils. Interestingly, intravascular TF, factor XII, eNUC/NETs and innate leukocytes are almost completely dispensable for hemostasis. Furthermore, immunothrombosis in contrast to hemostasis develops in largely intact blood vessels. Together this indicates that thrombosis can be a physiological mechanism of innate immunity that is distinct from hemostasis. We have recently developed a new model for deep vein thrombosis (DVT) which closely reproduces the pathological changes in the vessel wall observed in most patients with DVT. Using this model, we show that intravascular TF, factor XII, eNUC/NETs, innate leukocytes and their interactions with platelets all critically promote DVT. Thus, DVT shares similar triggers (especially pathogens) and identical molecular and cellular mediators with immunothrombosis. In case of DIC, the connections to immunothrombosis are most likely similarly strong or even stronger. Finally, our results also show that mediators of immunothrombosis such as eNUC/NETs and neutrophil serine proteases are main triggers of arterial thrombosis. Hence, together with hemostasis, immunothrombosis likely constitutes the major biological template process for both (pathological) microvascular thrombosis and large vessel thrombosis.