Repositioning the Early Pathology of Type 1 Diabetes to the Extraislet Vasculature

Type 1 diabetes (T1D) is a prototypic T cell-mediated autoimmune disease. Because the islets of Langerhans are insulated from blood vessels by a double basement membrane and lack detectable lymphatic drainage, interactions between endocrine and circulating T cells are not permitted. Thus, we hypothesized that initiation and progression of anti-islet immunity required islet neolymphangiogenesis to allow T cell access to the islet. Combining microscopy and single cell approaches, the timing of this phenomenon in mice was situated between 5 and 8 wk of age when activated anti-insulin CD4 T cells became detectable in peripheral blood while peri-islet pathology developed. This "peri-insulitis," dominated by CD4 T cells, respected the islet basement membrane and was limited on the outside by lymphatic endothelial cells that gave it the attributes of a tertiary lymphoid structure. As in most tissues, lymphangiogenesis seemed to be secondary to local segmental endothelial inflammation at the collecting postcapillary venule. In addition to classic markers of inflammation such as CD29, V-CAM, and NOS, MHC class II molecules were expressed by nonhematopoietic cells in the same location both in mouse and human islets. This CD45- MHC class II+ cell population was capable of spontaneously presenting islet Ags to CD4 T cells. Altogether, these observations favor an alternative model for the initiation of T1D, outside of the islet, in which a vascular-associated cell appears to be an important MHC class II-expressing and -presenting cell.

Cardiac Myosin Promotes Thrombin Generation and Coagulation In Vitro and In Vivo

OBJECTIVE

Cardiac myosin (CM) is structurally similar to skeletal muscle myosin, which has procoagulant activity. Here, we evaluated CM's ex vivo, in vivo, and in vitro activities related to hemostasis and thrombosis. Approach and Results: Perfusion of fresh human blood over CM-coated surfaces caused thrombus formation and fibrin deposition. Addition of CM to blood passing over collagen-coated surfaces enhanced fibrin formation. In a murine ischemia/reperfusion injury model, exogenous CM, when administered intravenously, augmented myocardial infarction and troponin I release. In hemophilia A mice, intravenously administered CM reduced tail-cut-initiated bleeding. These data provide proof of concept for CM's in vivo procoagulant properties. In vitro studies clarified some mechanisms for CM's procoagulant properties. Thrombin generation assays showed that CM, like skeletal muscle myosin, enhanced thrombin generation in human platelet-rich and platelet-poor plasmas and also in mixtures of purified factors Xa, Va, and prothrombin. Binding studies showed that CM, like skeletal muscle myosin, directly binds factor Xa, supporting the concept that the CM surface is a site for prothrombinase assembly. In tPA (tissue-type plasminogen activator)-induced plasma clot lysis assays, CM was antifibrinolytic due to robust CM-dependent thrombin generation that enhanced activation of TAFI (thrombin activatable fibrinolysis inhibitor).

CONCLUSIONS

CM in vitro is procoagulant and prothrombotic. CM in vivo can augment myocardial damage and can be prohemostatic in the presence of bleeding. CM's procoagulant and antifibrinolytic activities likely involve, at least in part, its ability to bind factor Xa and enhance thrombin generation. Future work is needed to clarify CM's pathophysiology and its mechanistic influences on hemostasis or thrombosis.

Role of thrombomodulin expression on hematopoietic stem cells

BACKGROUND

Activation of protease-activated receptor 1 (PAR1) by either thrombin or activated protein C (aPC) differentially regulate the quiescence and bone marrow (BM) retention of hematopoietic stem cells (HSC). Murine HSC co-express THBD, PAR1, and endothelial protein C receptor (EPCR), suggesting that HSC sustain quiescence in a quasi-cell autonomous manner due to the binding of thrombin present in the microenvironment to THBD, activation of EPCR-bound protein C by the thrombin-THBD-complex, and subsequent activation of PAR1 by the aPC-EPCR complex.

OBJECTIVE

To determine the role of THBD expression on HSC for sustaining stem cell quiescence and BM retention under homeostatic conditions.

METHODS

Hematopoietic stem cell function was analyzed in mice with constitutive or temporally controlled complete THBD-deficiency by flow cytometry, functional assays, and single cell RNA profiling.

RESULTS

THBD was expressed in mouse, but not human, HSC, progenitors, and immature B cells. Expression in vascular endothelium was conserved in humans' BM. Mice with constitutive THBD deficiency had a normal peripheral blood profile, altered BM morphology, reduced numbers of progenitors and immature B cells, pronounced extramedullary hematopoiesis, increased HSC frequency, and marginally altered transcriptionally defined HSC stemness. Transplantation experiments indicated near normal engraftment and repopulating ability of THBD-deficient HSC. Transgenic aPC supplementation normalized BM histopathology and HSC abundance, and partially restored transcriptional stemness, but had no effect on B cell progenitors and extramedullary hematopoiesis. Temporally controlled THBD gene ablation in adult mice did not cause the above abnormalities.

CONCLUSION

THBD expression on HSPC has minor effects on homeostatic hematopoiesis in mice, and is not conserved in humans.

Mechanisms of vascular permeability and remodeling associated with hemarthrosis in factor VIII-deficient mice

BACKGROUND

Vascular remodeling associated with hemophilic arthropathy (HA) may contribute to bleed propagation, but the mechanisms remain poorly understood.

OBJECTIVES

To explore molecular mechanisms of HA and the effects of hemostasis correction on synovial vascular remodeling after joint injury in hypocoagulable mice.

METHODS

Factor VIII (FVIII)-deficient mice +/- FVIII treatment and hypocoagulable wild-type mice (Hypo BALB/c) were subjected to subpatellar puncture. Hypo BALB/c mice were treated with warfarin and anti-FVIII before injury, after which warfarin was continued for 2 weeks or reversed +/- continuous anti-FVIII until harvest. Synovial vascularity was analyzed at baseline and 2 to 4 weeks post injury by histology, musculoskeletal ultrasound with power Doppler (microvascular flow), and Evans blue extravasation (vascular permeability). Synovial gene expression and systemic markers of vascular collagen turnover were studied in FVIII-deficient mice by RNA sequencing and enzyme-linked immunosorbent assay.

RESULTS

Vascular changes occurred in FVIII-deficient and Hypo BALB/c mice after injury with minimal effect of hemostasis correction. Increased vascular permeability was only significant in FVIII-deficient mice, who exhibited more pronounced vascular remodeling than Hypo BALB/c mice despite similar bleed volumes. FVIII-deficient mice exhibited a strong transcriptional response in synovium that was only partially affected by FVIII treatment and involved genes relating to angiogenesis and extracellular matrix remodeling, with vascular collagen turnover markers detected systemically.

CONCLUSIONS

Intact hemostasis at the time of hemarthrosis and during healing are both critical to prevent vascular remodeling, which appears worse with severe and prolonged FVIII deficiency. Unbiased RNA sequencing revealed potential targets for intervention and biomarker development to improve management of HA.

TAFI deficiency causes maladaptive vascular remodeling after hemophilic joint bleeding

Excessive vascular remodeling is characteristic of hemophilic arthropathy (HA) and may contribute to joint bleeding and the progression of HA. Mechanisms for pathological vascular remodeling after hemophilic joint bleeding are unknown. In hemophilia, activation of thrombin-activatable fibrinolysis inhibitor (TAFI) is impaired, which contributes to joint bleeding and may also underlie the aberrant vascular remodeling. Here, hemophilia A (factor VIII-deficient; FVIII-deficient) mice or TAFI-deficient mice with transient (antibody-induced) hemophilia A were used to determine the role of FVIII and TAFI in vascular remodeling after joint bleeding. Excessive vascular remodeling and vessel enlargement persisted in FVIII-deficient and TAFI-deficient mice, but not in transient hemophilia WT mice, after similar joint bleeding. TAFI-overexpression in FVIII-deficient mice prevented abnormal vessel enlargement and vascular leakage. Age-related vascular changes were observed with FVIII or TAFI deficiency and correlated positively with bleeding severity after injury, supporting increased vascularity as a major contributor to joint bleeding. Antibody-mediated inhibition of uPA also prevented abnormal vascular remodeling, suggesting that TAFI's protective effects include inhibition of uPA-mediated plasminogen activation. In conclusion, the functional TAFI deficiency in hemophilia drives maladaptive vascular remodeling in the joints after bleeding. These mechanistic insights allow targeted development of potentially new strategies to normalize vascularity and control rebleeding in HA.

Defective TAFI activation in hemophilia A mice is a major contributor to joint bleeding

Joint bleeds are common in congenital hemophilia but rare in acquired hemophilia A (aHA) for reasons unknown. To identify key mechanisms responsible for joint-specific bleeding in congenital hemophilia, bleeding phenotypes after joint injury and tail transection were compared in aHA wild-type (WT) mice (receiving an anti-factor VIII [FVIII] antibody) and congenital HA (FVIII-/-) mice. Both aHA and FVIII-/- mice bled severely after tail transection, but consistent with clinical findings, joint bleeding was notably milder in aHA compared with FVIII-/- mice. Focus was directed to thrombin-activatable fibrinolysis inhibitor (TAFI) to determine its potentially protective effect on joint bleeding in aHA. Joint bleeding in TAFI-/- mice with anti-FVIII antibody was increased, compared with WT aHA mice, and became indistinguishable from joint bleeding in FVIII-/- mice. Measurements of circulating TAFI zymogen consumption after joint injury indicated severely defective TAFI activation in FVIII-/- mice in vivo, consistent with previous in vitro analyses in FVIII-deficient plasma. In contrast, notable TAFI activation was observed in aHA mice, suggesting that TAFI protected aHA joints against bleeding. Pharmacological inhibitors of fibrinolysis revealed that urokinase-type plasminogen activator (uPA)-induced fibrinolysis drove joint bleeding, whereas tissue-type plasminogen activator-mediated fibrinolysis contributed to tail bleeding. These data identify TAFI as an important modifier of hemophilic joint bleeding in aHA by inhibiting uPA-mediated fibrinolysis. Moreover, our data suggest that bleed protection by TAFI was absent in congenital FVIII-/- mice because of severely defective TAFI activation, underscoring the importance of clot protection in addition to clot formation when considering prohemostatic strategies for hemophilic joint bleeding.

Vascular Permeability and Remodelling Coincide with Inflammatory and Reparative Processes after Joint Bleeding in Factor VIII-Deficient Mice

Vascular remodelling is a prominent feature of haemophilic arthropathy (HA) that may underlie re-bleeding, yet the nature of vascular changes and underlying mechanisms remain largely unknown. Here, we aimed to characterize synovial vascular remodelling and vessel integrity after haemarthrosis, as well as temporal changes in inflammatory and tissue-reparative pathways. Thirty acutely painful joints in patients with haemophilia (PWH) were imaged by musculoskeletal ultrasound with Power Doppler (MSKUS/PD) to detect vascular abnormalities and bloody effusions. Nineteen out of 30 painful joint episodes in PWH were associated with haemarthrosis, and abnormal vascular perfusion was unique to bleeding joints. A model of induced haemarthrosis in factor VIII (FVIII)-deficient mice was used for histological assessment of vascular remodelling (α-smooth muscle actin [αSMA] expression), and monitoring of in vivo vascular perfusion and permeability by MSKUS/PD and albumin extravasation, respectively. Inflammatory (M1) and reparative (M2) macrophage markers were quantified in murine synovium over a 10-week time course by real-time polymerase chain reaction. The abnormal vascular perfusion observed in PWH was recapitulated in FVIII-deficient mice after induced haemarthrosis. Neovascularization and increased vessel permeability were apparent 2 weeks post-bleed in FVIII-deficient mice, after a transient elevation of inflammatory macrophage M1 markers. These vascular changes subsided by week 4, while vascular remodelling, evidenced by architectural changes and pronounced αSMA expression, persisted alongside a reparative macrophage M2 response. In conclusion, haemarthrosis leads to transient inflammation coupled with neovascularization and associated vascular permeability, while subsequent tissue repair mechanisms coincide with vascular remodelling. Together, these vascular changes may promote re-bleeding and HA progression.

Abstract 148: Cardiac Myosin Promotes Thrombin Generation and Attenuates Tissue Plasminogen Activator-induced Plasma Clot Lysis

Recently we discovered that skeletal muscle myosin, which is in the same family as cardiac myosin, exerts prothrombotic effects by binding factor Xa and enhancing prothrombin activation in the presence of factor Va. Thus, we tested the influence of cardiac myosin on thrombus formation and fibrinolysis. Studies of the effects of cardiac myosin on thrombogenesis ex vivo using fresh human flowing blood showed that perfusion of blood over cardiac myosin-coated surfaces at 300 s -1 shear rate caused extensive fibrin deposition. Addition of cardiac myosin to blood also promoted the thrombotic responses of human blood flowing over collagen-coated surfaces, evidence of myosin’s thrombogenicity. Further studies showed that cardiac myosin enhanced thrombin generation in whole blood, platelet rich plasma and platelet poor plasma, indicating that myosin promotes thrombin generation in plasma primarily independently of platelets or other blood cell components. In a purified system composed of factor Xa, factor Va, prothrombin and calcium ions, cardiac myosin greatly enhanced prothrombinase activity. Experiments using Gla-domainless factor Xa showed that the Gla domain of factor Xa was not required for cardiac myosin’s prothrombinase enhancement in contrast to phospholipid-enhanced prothrombinase activity which requires that Gla domain. In studies of tissue plasminogen activator (tPA)-induced plasma clot lysis, increasing concentrations of cardiac myosin attenuated tPA-mediated clot lysis. The ability of cardiac myosin to inhibit tPA-induced plasma clot lysis was ablated in the presence of the carboxypeptidase inhibitor from potatoes, an inhibitor of thrombin activatable fibrinolysis inhibitor (TAFI). Clot lysis assays using TAFI-deficient plasma confirmed the requirement for TAFI for the antifibrinolytic action of cardiac myosin. We hypothesize that cardiac myosin-dependent thrombin generation increases TAFI activation and subsequent inhibition of clot lysis. In summary, here we show that cardiac myosin is both procoagulant and anti-fibrinolytic due to its ability to bind factor Xa and strongly promote thrombin generation. This raises new questions about potential procoagulant functions for cardiac myosin in coronary health and disease.

Inhibition of Thrombin-Activatable Fibrinolysis Inhibitor and Plasminogen Activator Inhibitor-1 Reduces Ischemic Brain Damage in Mice

BACKGROUND AND PURPOSE

Cerebral ischemia and reperfusion is associated with activation of the coagulation cascade and fibrin deposition in cerebral microvessels. Both thrombin-activatable fibrinolysis inhibitor (TAFI) and plasminogen activator inhibitor-1 (PAI-1) attenuate fibrinolysis and are therefore attractive targets for the treatment of ischemic stroke.

METHODS

TAFI and PAI-1 were inhibited by monoclonal antibodies in a mouse model of transient middle cerebral artery occlusion. Twenty-four hours after stroke, mice were neurologically scored, cerebral thrombotic burden was assessed, and brain infarct sizes were calculated.

RESULTS

Inhibition of TAFI or PAI-1 significantly decreased cerebral infarct sizes by 50% 24 hours after stroke. This reduction in cerebral damage was associated with a significant decrease in fibrin(ogen) deposition in the ischemic brain. Concurrently, functional recovery of the animals was improved. Interestingly, combined targeting of TAFI and PAI-1 using low, and by themselves inactive, doses of antibodies improved cerebral blood flow and reduced cerebral fibrin(ogen) deposition and infarct sizes by 50%. When dual treatment was delayed to 1 hour after the start of reperfusion, it still reduced brain injury; however, this was not statistically significant.

CONCLUSIONS

Targeting of PAI-1 and TAFI is protective in an ischemic stroke model by attenuating fibrin(ogen) deposition, thereby improving reperfusion. Combined inhibition has a co-operative effect that could become useful in ischemic stroke therapy.

Advances and challenges in hemophilic arthropathy

Hemophilic arthropathy is a form of joint disease that develops secondary to joint bleeding and presents with synovial hypertrophy, cartilage and bony destruction. The arthropathy can develop despite clotting factor replacement and is especially disabling in the aging population. Pathobiological tissue changes are triggered by release of hemoglobin and iron deposition in the joint, but the sequence of events and the molecular mechanisms resulting in joint deterioration are incompletely understood. Treatment options other than clotting factor replacement are limited. Improvements in the treatment of hemophilia necessitate a better understanding of the processes that lead to this disabling condition and better diagnostic tools. Towards that end, studies of the molecular mechanisms leading to the arthropathy, as well as the development of sensitive imaging techniques and biomarkers are needed. These will pave the way to identify the cause of acute pain such as joint bleeding or synovitis, detect early, potentially reversible structural changes, and predict progression of disease. This review describes current imaging techniques and the development of high resolution musculoskeletal ultrasound with power Doppler to afford point-of-care diagnosis and management, the potential utility of diagnostic biomarkers, and summarizes our current knowledge of the pathobiology of hemophilic arthropathy.

Novel or expanding current targets in fibrinolysis

Globally the leading cause of long-term disability and mortality stems from cardiovascular diseases, which creates an enormous economic burden. Currently available treatments for intravascular thrombosis consist of a large repertoire of antithrombotic agents targeting coagulation and platelet function. However, the only agents available to enhance fibrinolysis are recombinant or modified forms of plasminogen activators. Their clinical use is limited by low efficacy, life-threatening side-effects (primarily caused by the high systemic dose required) and the inapplicability for prophylactic use. This review provides an update on the latest advances in targeting the antifibrinolytic proteins, plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor, and will highlight novel therapeutic avenues to enhance fibrinolysis.