Plasma Kallikrein as a Forgotten Clotting Factor

Factor XII-driven coagulation traps bacterial infections

Blood coagulation is essential for stopping bleeding but also drives thromboembolic disorders. Factor XII (FXII)-triggered coagulation promotes thrombosis while being dispensable for hemostasis, making it a potential anticoagulant target. However, its physiological role remains unclear. Here, we demonstrate that FXII-driven coagulation enhances innate immunity by trapping pathogens and restricting bacterial infection in mice. Streptococcus pneumoniae infection was more severe in FXII-deficient (F12-/-) mice, with increased pulmonary bacterial burden, systemic spread, and mortality. Similarly, Staphylococcus aureus skin infections and systemic dissemination were exacerbated in F12-/- mice. Reconstitution with human FXII restored bacterial containment. Plasma kallikrein amplifies FXII activation, and its deficiency aggravated S. aureus skin infections, similarly to F12-/- mice. FXII deficiency impaired fibrin deposition in abscess walls, leading to leaky capsules and bacterial escape. Bacterial long-chain polyphosphate activated FXII, triggering fibrin formation. Deficiency in FXII substrate factor XI or FXII/factor XI co-deficiency similarly exacerbated S. aureus infection. The data reveal a protective role for FXII-driven coagulation in host defense, urging caution in developing therapeutic strategies targeting this pathway.

Factor XI and Atrial Fibrillation: A Mismatched Pairing?

Factor XI (FXI) is a liver-produced coagulation zymogen that evolutionarily originated from duplication of the gene encoding for prekallikrein. It circulates in complex with high-molecular-weight kininogen, and consists of two identical subunits that bind thrombin, FXIIa and FIX. Thus, the FXI molecule has features different from other coagulation factors. Pharmacological FXI blockade using small molecules, monoclonal antibodies and antisense oligonucleotides, has been developed, with a hypothesis of a bleeding-free, effective anticoagulation. Dose-finding Phase II trials were performed for thromboprophylaxis in orthopaedic surgery, non-valvular AF and as an add-on strategy to antiplatelet drugs in acute atherothrombosis (stroke or MI). None of those studies were powered for safety or efficacy, but rather, they were used to select the optimal dose for Phase III studies. Nevertheless, their limited results were often (over)interpreted as supporting the hypothesis of the first bleeding-free anticoagulation strategy. The failure of the Phase III OCEANIC-AF trial comparing the FXI inhibitor asundexian to the FXa inhibitor apixaban in AF obliged the scientific community to reconsider the bleeding-free hypothesis and the pathophysiology of FXI. Here, the molecular, disease-related and pharmacological features of FXI were analysed to provide possible explanation(s) and hypotheses for this (temporary) failure of FXI targeting. Specifically, the authors describe the peculiar features of the molecule in the coagulation cascade, the possible mechanisms for the bypassing of FXI activity, the clinical evidence related to FXI congenital deficiency, levels measured in pro-thrombotic settings, the pathophysiology of different thromboembolic disorders and the pharmacodynamics of FXI blockade in Phase I and II studies.

Plasma kallikrein supports FXII-independent thrombin generation in mouse whole blood

ABSTRACT

Plasma kallikrein (PKa) is an important activator of factor XII (FXII) of the contact pathway of coagulation. Several studies have shown that PKa also possesses procoagulant activity independent of FXII, likely through its ability to directly activate FIX. We evaluated the procoagulant activity of PKa using a mouse whole blood (WB) thrombin-generation (TG) assay. TG was measured in WB from PKa-deficient mice using contact pathway or extrinsic pathway triggers. PKa-deficient WB had significantly reduced contact pathway-initiated TG compared with that of wild-type controls and was comparable with that observed in FXII-deficient WB. PKa-deficient WB supported equivalent extrinsic pathway-initiated TG compared with wild-type controls. Consistent with the presence of FXII-independent functions of PKa, targeted blockade of PKa with either small molecule or antibody-based inhibitors significantly reduced contact pathway-initiated TG in FXII-deficient WB. Inhibition of activated FXII (FXIIa) using an antibody-based inhibitor significantly reduced TG in PKa-deficient WB, consistent with a PKa-independent function of FXIIa. Experiments using mice expressing low levels of tissue factor demonstrated that persistent TG present in PKa- and FXIIa-inhibited WB was driven primarily by endogenous tissue factor. Our work demonstrates that PKa contributes significantly to contact pathway-initiated TG in the complex milieu of mouse WB, and a component of this contribution occurs in an FXII-independent manner.

Red cell extracellular vesicles and coagulation activation pathways

PURPOSE OF REVIEW

Packed red blood cells (PRBCs) are the most commonly transfused blood products. Preparation of PRBCs requires blood collection from donors, processing, and storage prior to transfusion to recipients. Stored red blood cells (RBCs) undergo structural and metabolic changes collectively known as the storage lesion. RBC extracellular vesicles (sREVs) are released in PRBC units during storage, and are transfused along with intact RBCs into recipients. For several decades, extracellular vesicles have been the focus of intense research, leading to the discovery of a wide variety of endogenous biological properties that may impact numerous physiologic and/or pathologic pathways.

RECENT FINDINGS

This study reviews the characteristics of extracellular vesicles present in PRBC units and the impact of prestorage and pretransfusion processing, as well as storage conditions, on their generation. Importantly, we discuss recently described interactions of sREVs with coagulation pathways and related interplay with inflammatory pathways in vitro and in vivo using animal models.

SUMMARY

Extracellular vesicles present in stored PRBC units are capable of activating coagulation pathways. However, it remains unclear whether this affects clinical outcomes in recipients of PRBC units. Further understanding of these pathways and their relationship to any adverse outcomes may yield novel strategies to mitigate complications of blood transfusion.