PARs mediate the interplay between coagulation and inflammation
The discovery of the PARs in 1991 represented a watershed moment in our understanding of the mechanism by which coagulation proteinases directly influence cell function. PARs are seven transmembrane domain G-protein-coupled receptors that have unique mechanisms of activation, which involves limited proteolysis of their amino-terminal exodomains to unmask a tethered ligand. Four PARs (PAR1–4) are differentially expressed on lung-resident cells, including lung epithelial cells, endothelial cells and fibroblasts, as well as on recruited monocytes and neutrophils following tissue injury. Thrombin is a major activator of PAR1 and PAR3, whereas factor Xa and the potent TF-Xa-VIIa ternary complex activate PAR1 and PAR2 [80]. PAR1 can also be activated by the EPCR­-APC complex [81], plasmin [82] and matrix metalloprotease-1 [83], whereas trypsin and tryptase can activate PAR2 [80].
Activation of PAR1 induces the expression of a host of inflammatory cytokines, chemokines and growth factors that influence inflammatory cell trafficking, leukocyte activation and endothelial permeability [84, 85]. However, PAR1 downstream signalling is highly context dependent and influenced by both the nature and the extracellular concentration of activating proteinases [86]. For example, in vitro studies using exogenously added proteinases have shown that thrombin increases vascular permeability via PAR1 activation and coupled signalling of the S1P3 receptor [87]. Conversely, APC inhibits thrombin-mediated vascular hyper-permeability via PAR1 activation and signalling via the S1P1 receptor [86, 88]. During infection the deleterious thrombin­-PAR1 signalling responses appear to predominate over protective APC-PAR1 signalling responses because endogenous APC levels are depleted and APC has 500 times lower affinity and efficiency for cleaving PAR1 compared to thrombin [89, 90]. The differential effects of PAR-1 activation by APC or thrombin are also explained by differential cleavage of the PAR1 N-terminus. Thrombin cleaves PAR1 at Arg41, whereas APC cleaves PAR1 at Arg41 and Arg46, with preferential cleavage at the latter site mediating the cytoprotective effects of APC [91, 92]. Hence, in the presence of increased thrombin generation, as often seen in severe pneumonia, the beneficial effects of low levels of endogenous APC may be overcome by the barrier-disruptive effects of thrombin-induced PAR1 cleavage.
In preclinical studies, survival from sepsis appears to be related to the time of activation of PAR1 [93]; mice treated early with a PAR1 antagonist were protected against thrombocytopaenia and had reduced thrombin levels and improved survival [93]. S. pneumoniae pneumonia in PAR1-knockout mice exhibited evidence of reduced lung injury and neutrophil recruitment without any effect on bacterial clearance [94], whereas the PAR1 antagonist vorapaxar reduced neutrophilic inflammation; TNF, IL-1β, C-C motif chemokine ligand 2 (CCL2) and CCL7 levels; coagulation activation; and vascular permeability (alveolar leak) without adversely affecting bacterial clearance. Murine studies of viral infection, including influenza [95, 96], suggest PAR1 is required for host control of virus load initially, but if viral replication is left unrestricted PAR1 promotes inflammation and increases mortality. These preclinical findings support the rationale that blockade of PAR1 may be beneficial in attenuating pathogen-induced hyper-inflammatory responses and in maintaining integrity of the alveolar endothelial barrier, but that the benefits of this treatment may differ with the timing of treatment and the causative pathogen.