Anton Ilich

Postpartum hemorrhage (PPH) remains the leading cause of pregnancy-related mortality worldwide. Regardless of the initiating cause, continued bleeding may progress to a systemic coagulopathy. This coagulopathy may be complicated further by profound fibrinolytic activation that progresses to systemic hyperfibrinolysis, a condition that we have termed acute obstetric coagulopathy (AOC). Patients with placental abruption or amniotic fluid embolism are among those at highest risk for AOC. In response to the unmet need for a rapid method to detect fibrinolytic activation in this scenario, we developed a novel assay that we have termed the fibrinolytic activity screening test (FAST). This assay measures endogenously generated plasmin activity in plasma within 5 minutes. Its high sensitivity for the detection of in vivo fibrinolytic activation was confirmed by strong correlation with elevated plasmin-antiplasmin (PAP) complex levels. We analyzed archived plasma samples from 33 women with PPH and 20 pregnant women just before elective cesarean delivery. Of the 33 patients with PPH, 12 had PAP complex levels >25 000 ng/mL, thereby meeting criteria for the diagnosis of AOC. Plasmin activity measured by the FAST assay differentiated AOC from non-AOC PPH (P = .0007) and from pregnant non-PPH control groups (P< .0001) and was strongly correlated with both PAP complex and D-dimer levels. Among patients with PPH for whom viscoelastic whole blood testing (ROTEM) was performed, none of the 18 without AOC or 9 of those with AOC had evidence of ROTEM-defined hyperfibrinolysis. The FAST assay is a rapid tool to detect activation of fibrinolysis associated with AOC in women after delivery.

Fibrinolysis is the process by which fibrin, the principal proteinaceous component of blood clots, is enzymatically degraded. A key event in this process is the formation of the tissue plasminogen activator (tPA)–plasminogen complex on partially degraded fibrin, which leads to the conversion of plasminogen to plasmin, which then proteolytically digests fibrin. This system is tightly regulated by several inhibitors and by the clot architecture. Many components of the fibrinolytic system have roles beyond fibrinolysis (e.g. in ageing, basal metabolism, inflammation and cancer), but these will not be addressed in detail in this brief narrative.

BACKGROUND: Patients with sickle cell disease (SCD) frequently receive RBC units stored near the end of their permissible storage duration. We aimed to determine whether RBC storage duration influences recipient hematological, metabolic, and clinical chemistry parameters. METHODS: In a randomized, prospective, double-blind trial, 24 adults with SCD receiving chronic transfusion therapy were assigned to receive three consecutive outpatient transfusions with RBCs stored for either ≤10 days (short-stored; n = 13) or ≥30 days (long-stored; n = 11). Blood samples were collected from transfused units and from recipients at predefined time points for metabolomics, cytokine, and clinical laboratory analyses. The primary outcomes included post-transfusion hemoglobin and RBC count increments, metabolic markers of oxidative stress, iron metabolism, inflammation, and renal function. RESULTS: Transfusion of short-stored RBCs was associated with significantly higher circulating 2,3-bisphosphoglycerate levels for up to 2 weeks after transfusion. Nadir RBC counts and hemoglobin A levels were higher in recipients of short-stored RBCs. In contrast, recipients of long-stored RBCs had higher transferrin saturation and plasma iron levels, elevated markers of oxidative stress and renal dysfunction, and increased proinflammatory cytokines and immunomodulatory metabolites. Metabolomics revealed storage age–dependent alterations in glycolysis, purine, and sphingolipid metabolism. Cytokine profiles and hematologic parameters corroborated the metabolic findings, indicating improved post-transfusion metabolic and inflammatory status with short-stored RBCs. CONCLUSION: Transfusion of short-stored RBCs yielded favorable metabolic and hematologic outcomes in adults with SCD, independent of immediate clinical endpoints. TRIAL REGISTRATION: ClinicalTrials.gov NCT03704922 FUNDING: National Heart, Lung, and Blood Institute (NHLBI), NIH (K23HL136787, R01HL148151, R01HL146442, and R01HL149714).

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.

A hypercoagulable state, chronic inflammation, and increased risk of venous thrombosis and stroke are prominent features in patients with sickle cell disease (SCD). Coagulation factor XII (FXII) triggers activation of the contact system that is known to be involved in both thrombosis and inflammation, but not in physiological hemostasis. Therefore, we investigated whether FXII contributes to the prothrombotic and inflammatory complications associated with SCD. We found that when compared with healthy controls, patients with SCD exhibit increased circulating biomarkers of FXII activation that are associated with increased activation of the contact pathway. We also found that FXII, but not tissue factor, contributes to enhanced thrombin generation and systemic inflammation observed in sickle cell mice challenged with tumor necrosis factor α. In addition, FXII inhibition significantly reduced experimental venous thrombosis, congestion, and microvascular stasis in a mouse model of SCD. Moreover, inhibition of FXII attenuated brain damage and reduced neutrophil adhesion to the brain vasculature of sickle cell mice after ischemia/reperfusion induced by transient middle cerebral artery occlusion. Finally, we found higher FXII, urokinase plasminogen activator receptor, and αMβ2 integrin expression in neutrophils of patients with SCD compared with healthy controls. Our data indicate that targeting FXII effectively reduces experimental thromboinflammation and vascular complications in a mouse model of SCD, suggesting that FXII inhibition may provide a safe approach for interference with inflammation, thrombotic complications, and vaso-occlusion in patients with SCD.

The American Trial Using Tranexamic Acid (TXA) in Thrombocytopenia (A-TREAT, NCT02578901) demonstrated no superiority of TXA over placebo in preventing World Health Organization (WHO) grade 2 or higher bleeding in patients with severe thrombocytopenia requiring supportive platelet transfusion following myeloablative therapy for hematologic disorders. In this ancillary study, we sought to determine whether this clinical outcome could be explained on the basis of correlative assays of fibrinolysis. Plasma was collected from A-TREAT participants (n = 115) before the initiation of study drug (baseline) and when TXA was at steady-state trough concentration (follow-up). Global fibrinolysis was measured by 3 assays: euglobulin clot lysis time (ECLT), plasmin generation (PG), and tissue-type plasminogen activator (tPA)–challenged clot lysis time (tPA-CLT). TXA was quantified in follow-up samples by tandem mass spectrometry. Baseline samples did not demonstrate fibrinolytic activation by ECLT or tPA-CLT. Furthermore, neither ECLT nor levels of plasminogen activator inhibitor-1, tPA, plasminogen, alpha2-antiplasmin, or plasmin-antiplasmin complexes were associated with a greater risk of WHO grade 2+ bleeding. TXA trough concentrations were highly variable (range, 0.7-10 μg/mL) and did not correlate with bleeding severity, despite the fact that plasma TXA levels correlated strongly with pharmacodynamic assessments by PG (Spearman r, −0.78) and tPA-CLT (r, 0.74). We conclude that (1) no evidence of fibrinolytic activation was observed in these patients with thrombocytopenia, (2) trough TXA concentrations varied significantly between patients receiving the same dosing schedule, and (3) tPA-CLT and PG correlated well with TXA drug levels.

Summary: Individuals with sickle cell disease (SCD) have persistently elevated thrombin generation that results in a state of systemic hypercoagulability. Antithrombin‐III (ATIII), an endogenous serine protease inhibitor, inhibits several enzymes in the coagulation cascade, including thrombin. Here, we utilize a biomimetic microfluidic device to model the morphology and adhesive properties of endothelial cells (ECs) activated by thrombin and examine the efficacy of ATIII in mitigating the adhesion of SCD patient‐derived red blood cells (RBCs) and EC retraction. Microfluidic devices were fabricated, seeded with ECs, and incubated under physiological shear stress. Cells were then activated with thrombin with or without an ATIII pretreatment. Blood samples from subjects with normal haemoglobin (HbAA) and subjects with homozygous SCD (HbSS) were used to examine RBC adhesion to ECs. Endothelial cell surface adhesion molecule expression and confluency in response to thrombin and ATIII treatments were also evaluated. We found that ATIII pretreatment of ECs reduced HbSS RBC adhesion to thrombin‐activated endothelium. Furthermore, ATIII mitigated cellular contraction and reduced surface expression of von Willebrand factor and vascular cell adhesion molecule‐1 (VCAM‐1) mediated by thrombin. Our findings suggest that, by attenuating thrombin‐mediated EC damage and RBC adhesion to endothelium, ATIII may alleviate the thromboinflammatory manifestations of SCD.

Background: Hypotensive transfusion reactions are adverse events typified by a sudden decrease in blood pressure that usually occurs within the first minutes after the initiation of a transfusion and resolves shortly after the transfusion is stopped. Due to current passive reporting practices, the incidence is likely underreported, but recent studies estimate an incidence of 1.3 cases per 10000 RBC units. The pathophysiology of these reactions are not fully understood. One hypothesis proposed is that increased bradykinin (BK), a nonapeptide released from cleavage of high molecular weight kininogen (HK), as seen with the use of negatively charged leukoreduction filters and the use of angiotensin-converting enzyme inhibitors, is a major contributor to the pathophysiology. We have recently demonstrated that red blood cell derived microvesicles (RBCMVs) from aging red blood cell (RBC) units are able to trigger thrombin generation via kallikrein activation - a predominant enzyme to cleave high molecular weight kininogen (Noubouossie, Blood, 2020). Thus, we hypothesize that the same RBCMVs would lead to bradykinin generation and might explain these hypotensive events. Objectives: To determine if RBC storage lesion-derived microvesicles are able to facilitate HK cleavage and BK release. Methods: RBCMVs were prepared from 4 recently expired RBC units (42 or 43 day old, AS-3 preserved, prestorage leukoreduced, all A+) via a series of centrifugations and washes. RBCMVs were quantified and characterized using nanoparticle tracking analysis. Obtained RBCMVs were first assessed for the capacity to initiate thrombin generation in microvesicle free human plasma via a substrate cleavage assay. Next, RBCMVs were added to a buffer reaction containing prekallikrein and HK, and kininogen cleavage was assessed via western blot. RBCMVs were also mixed with microvesicle-free human plasma and analyzed for evidence of kallikrein activation, cleavage of high molecular weight kininogen, and bradykinin production by ELISA. Cohn fractionation of plasma was used to enrich for BK. Results: RBCMVs were enumerated and concentrated to 7.5 ± 1.4 x 10 12 per mL (mean±SD size 160 ± 29µm). RBCMVs were able to initiate thrombin generation principally via contact pathway activation, independently of tissue factor. In a buffer system RBCMVs demonstrated activity to generate kallikrein with a sequential high molecular weight kininogen cleavage in a dose-dependent manner. Exclusion of kallikrein from the buffer system or addition of the small molecule inhibitor of kallikrein - ecallantide - halted cleavage of kininogen. A dose-dependent cleavage of high molecular weight kininogen indicated that RBCMVs could cause BK release in plasma; this was confirmed via an independent assay of Cohn -fractionated samples. Conclusions: Results of this current study demonstrate that RBCMVs are leading to high HK cleavage via kallikrein activation in vitro. We suspect that the same mechanisms could lead to BK generation in patients receiving older RBC units, possibly increasing the risk for hypotensive events from transfusion. Disclosures: Karafin: Westat, Inc.: Consultancy. Key: Sanofi: Consultancy; BioMarin: Honoraria, Other: Participation as a clinical trial investigator; Takeda: Research Funding; Grifols: Research Funding; Uniqure: Consultancy, Other: Participation as a clinical trial investigator.