New Approaches to Define Molecular Mechanisms Underlying Dysregulation of Haemostasis

Defective haemostasis can contribute to development of a bleeding diathesis, whereas dysregulated coagulation increases the risk of thrombotic events. Acute or chronic inflammation contributes to haemostatic imbalance by promotion of pathological activation of blood coagulation, although the molecular basis for inflammation-driven thrombosis remains poorly defined.

In this thesis, I sought to identify novel biological parameters underlying phenotypic heterogeneity in people with severe haemophilia (PWH). Plasma from PWH was found to be more likely to have increased plasma fibrinolytic activity compared to plasma from a healthy age-matched cohort. This was found to arise due to significantly increased plasmin generation activity and/or increased resistance to thrombin-activatable fibrinolysis inhibitor (TAFI) activity in PWH plasma. In addition, a novel positive correlation was identified between increased plasma plasmin generation capacity and development of joint damage in PWH, whereas PWH body weight was inversely correlated with their plasmin generation capacity.

In the second part of this study, I showed that macrophage polarisation resulted in divergent effects on macrophage-mediated procoagulant and fibrinolytic activity. Macrophage procoagulant activity induced by exposure to bacterial (lipopolysaccharide, LPS) or viral (single-stranded RNA) mimetics was dependent upon a glycolytic shift in macrophages that could be reversed by chemical inhibitors of glycolysis, thus identifying a novel link between haemostatic macrophage activity and cellular metabolism.

Finally, I identified that b-glucan-induced myeloid cell trained immunity in vitro and in vivo lowered the threshold for induction of procoagulant and antifibrinolytic activity by secondary proinflammatory stimuli, and that this phenomenon was dependent upon b-glucan-induced metabolic and epigenetic modifications. Furthermore, I found that the protoporphyrin-IX component of free haem was a novel inducer of trained immunity in myeloid cells, which also promoted exaggerated procoagulant and antifibrinolytic activity upon exposure to subsequent pro-inflammatory stimuli.

Collectively, this study used novel assays to define new components of fibrinolytic pathway activity as mediators of phenotypic heterogeneity in PWH. In addition, I identified new molecular mechanisms and cellular pathways underlying myeloid cell procoagulant activity, that may contribute to the pathophysiology of thrombo-inflammatory disease.