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Anti-platelet factor 4 (PF4)/heparin immune complexes that cause heparin-induced thrombocytopenia (HIT) activate complement via the classical pathway. Previous studies have shown that the alternative pathway of complement substantially amplifies the classical pathway of complement activation through the C3b feedback cycle.These studies sought to examine the contributions of the alternative pathway to complement activation by HIT antibodies.Using IgG monoclonal (KKO) and/or patient-derived HIT antibodies, we compared the effects of classical pathway (BBK32 and C1-esterase inhibitor [C1-INH]), alternative pathway (anti-factor B [fB] or factor D [fD] inhibitor) or combined classical and alternative pathway inhibition (soluble complement receptor 1 [sCR1]) in whole blood or plasma.Classical pathway inhibitors BBK32 and C1-INH and the combined classical/alternative pathway inhibitor sCR1 prevented KKO/HIT immune complex-induced complement activation, including release of C3 and C5 activation products, binding of immune complexes to B cells, and neutrophil activation. The alternative pathway inhibitors fB and fD, however, did not affect complement activation by KKO/HIT immune complexes. Similarly, alternative pathway inhibition had no effect on complement activation by unrelated immune complexes consisting of anti-dinitrophenyl (DNP) antibody and the multivalent DNP--keyhole limpet hemocyanin antigen.Collectively, these findings suggest the alternative pathway contributes little in support of complement activation by HIT immune complexes. Additional in vitro and in vivo studies are required to examine if this property is shared by most IgG-containing immune complexes or if predominance of the classic pathway is limited to immune complexes composed of multivalent antigens.

Heparin-induced thrombocytopenia (HIT) is characterized by thrombocytopenia associated with a highly prothrombotic state due to the development of pathogenic antibodies that recognize human (h) platelet factor 4 (PF4) complexed with various polyanions. While non-heparin anticoagulants are the mainstay of care in HIT, subsequent bleeding may develop, and risk of new thromboembolic events remain. We had described a mouse IgGК2b antibody KKO that mimics the sentinel features of pathogenic HIT antibodies, including binding to the same neoepitope on hPF4:polyanion complexes. KKO, like HIT IgGs, both activates platelets through FcγRIIA and induces complement activation. We now asked whether Fc-modified KKO can be used as a novel therapeutic to prevent or treat HIT. Using the endoglycosidase EndoS, we created deglycosylated KKO (DGKKO). While DGKKO retained binding to PF4-polyanion complexes, it inhibited FcγRIIA-dependent activation of PF4-treated platelets triggered by unmodified KKO, 5B9 (another HIT-like monoclonal antibody), and IgGs isolated from HIT patients. DGKKO also decreased complement activation and deposition of C3c on platelets. Unlike the anticoagulant fondaparinux, injection of DGKKO into "HIT mice" lacking mouse PF4, but transgenic for hPF4 and FcγRIIA, prevented and reversed thrombocytopenia when injected before or after unmodified KKO, 5B9 or HIT IgG. DGKKO also reversed antibody-induced thrombus growth in HIT mice. In contrast, DGKKO was ineffective in preventing thrombosis by IgG from patients with the HIT-related anti-PF4 prothrombotic disorder, vaccine-induced immune thrombotic thrombocytopenia. Thus, DGKKO may represent a new class of therapeutics for targeted treatment of patients with HIT.

Although several members of protein disulfide isomerase (PDI) family support thrombosis, other PDI family members with the CXYC motif remain uninvestigated. ERp46 has 3 CGHC redox-active sites and a radically different molecular architecture than other PDIs. Expression of ERp46 on the platelet surface increased with thrombin stimulation. An anti-ERp46 antibody inhibited platelet aggregation, adenosine triphosphate (ATP) release, and αIIbβ3 activation. ERp46 protein potentiated αIIbβ3 activation, platelet aggregation, and ATP release, whereas inactive ERp46 inhibited these processes. ERp46 knockout mice had prolonged tail-bleeding times and decreased platelet accumulation in thrombosis models that was rescued by infusion of ERp46. ERp46-deficient platelets had decreased αIIbβ3 activation, platelet aggregation, ATP release, and P-selectin expression. The defects were reversed by wild-type ERp46 and partially reversed by ERp46 containing any of the 3 active sites. Platelet aggregation stimulated by an αIIbβ3-activating peptide was inhibited by the anti-ERp46 antibody and was decreased in ERp46-deficient platelets. ERp46 bound tightly to αIIbβ3 by surface plasmon resonance but poorly to platelets lacking αIIbβ3 and physically associated with αIIbβ3 upon platelet activation. ERp46 mediated clot retraction and platelet spreading. ERp46 more strongly reduced disulfide bonds in the β3 subunit than other PDIs and in contrast to PDI, generated thiols in β3 independently of fibrinogen. ERp46 cleaved the Cys473-Cys503 disulfide bond in β3, implicating a target for ERp46. Finally, ERp46-deficient platelets have decreased thiols in β3, implying that ERp46 cleaves disulfide bonds in platelets. In conclusion, ERp46 is critical for platelet function and thrombosis and facilitates αIIbβ3 activation by targeting disulfide bonds.

Neutrophil extracellular traps (NETs) are abundant in sepsis, and proposed NET-directed therapies in sepsis prevent their formation or accelerate degradation. Yet NETs are important for microbial entrapment, as NET digestion liberates pathogens and NET degradation products (NDPs) that deleteriously promote thrombosis and endothelial cell injury. We proposed an alternative strategy of NET-stabilization with the chemokine, platelet factor 4 (PF4, CXCL4), which we have shown enhances NET-mediated microbial entrapment. We now show that NET compaction by PF4 reduces their thrombogenicity. In vitro, we quantified plasma thrombin and fibrin generation by intact or degraded NETs and cell-free (cf) DNA fragments, and found that digested NETs and short DNA fragments were more thrombogenic than intact NETs and high molecular weight genomic DNA, respectively. PF4 reduced the thrombogenicity of digested NETs and DNA by interfering, in part, with contact pathway activation. In endothelial cell culture studies, short DNA fragments promoted von Willebrand factor release and tissue factor expression via a toll-like receptor 9-dependent mechanism. PF4 blocked these effects.Cxcl4-/-mice infused with cfDNA exhibited higher plasma thrombin anti-thrombin (TAT) levels compared to wild-type controls. Following challenge with bacterial lipopolysaccharide,Cxcl4-/-mice had similar elevations in plasma TAT and cfDNA, effects prevented by PF4 infusion. Thus, NET-stabilization by PF4 prevents the release of short fragments of cfDNA, limiting the activation of the contact coagulation pathway and reducing endothelial injury. These results support our hypothesis that NET-stabilization reduces pathologic sequelae in sepsis, an observation of potential clinical benefit.HIGHLIGHTSIn contrast to intact NETs, degraded NETs and cfDNA are prothrombotic and injure the endothelium.PF4 reduces the ability of degraded NETs and cfDNA to promote thrombosis and injure the endothelium.

Degenerative mitral valve (MV) regurgitation (MR) is a highly prevalent heart disease that requires surgery in severe cases. Here, we show that a decrease in the activity of the serotonin transporter (SERT) accelerates MV remodeling and progression to MR. Through studies of a population of patients with MR, we show that selective serotonin reuptake inhibitor (SSRI) use and SERT promoter polymorphism 5-HTTLPR LL genotype were associated with MV surgery at younger age. Functional characterization of 122 human MV samples, in conjunction with in vivo studies in SERT −/− mice and wild-type mice treated with the SSRI fluoxetine, showed that diminished SERT activity in MV interstitial cells (MVICs) contributed to the pathophysiology of MR through enhanced serotonin receptor (HTR) signaling. SERT activity was decreased in LL MVICs partially because of diminished membrane localization of SERT. In mice, fluoxetine treatment or SERT knockdown resulted in thickened MV leaflets. Similarly, silencing of SERT in normal human MVICs led to up-regulation of transforming growth factor β1 ( TGF β 1 ) and collagen ( COL1A1 ) in the presence of serotonin. In addition, treatment of MVICs with fluoxetine not only directly inhibited SERT activity but also decreased SERT expression and increased HTR2B expression. Fluoxetine treatment and LL genotype were also associated with increased COL1A1 expression in the presence of serotonin in MVICs, and these effects were attenuated by HTR2B inhibition. These results suggest that assessment of both 5-HTTLPR genotype and SERT-inhibiting treatments may be useful tools to risk-stratify patients with MV disease to estimate the likelihood of rapid disease progression.

Heparin-induced thrombocytopenia (HIT) is characterized by mild thrombocytopenia associated with a highly prothrombotic state due to the development of pathogenic antibodies that recognize human (h) platelet factor 4 (PF4) complexed with various polyanions. While non-heparin anticoagulants and intravenous immunoglobulin (IVIG) are the mainstay of care, bleeding may develop, and risk of new thromboembolic events remain. We had described a mouse IgGκ2b antibody KKO that mimics the sentinel features of pathogenic HIT antibodies, including binding to the same neoepitope on hPF4:polyanion complexes. KKO, like HIT IgGs, activates platelets through FcγRIIA and induces complement activation. We now asked whether Fc-modified KKO can be used as a novel therapeutic to prevent or treat HIT. Using the endoglycosidase EndoS, we created deglycosylated KKO (DGKKO). DGKKO bound to PF4-polyanion complexes, and blocked FcγRIIA-dependent activation of PF4 treated platelets by KKO, 5B9 (another HIT-like monoclonal antibody), and isolated IgGs from HIT patients. DGKKO also decreased complement activation and deposition of C3c on platelets. Injection of DGKKO into “HIT mice” lacking mouse PF4, but transgenic for hPF4 and FcγRIIA, prevented and reversed thrombocytopenia when injected before or after KKO, 5B9 or HIT IgG, respectively, in a microfluidic system. DGKKO reversed antibody-induced thrombus growth in HIT mice. In contrast, DGKKO was ineffective in preventing thrombosis by IgG from a patient with the HIT-related disorder, vaccine-induced immune thrombotic thrombocytopenia. Thus, DGKKO may represent a new class of therapeutics for targeted treatment of patients with HIT.Key PointsDeglycosylated (DG) KKO can reverse thrombocytopenia in a HIT murine model.DGKKO can prevent/reverse thrombosisin vitroand in a HIT murine model.

Patients with COVID-19 present with a wide variety of clinical manifestations. Thromboembolic events constitute a significant cause of morbidity and mortality in patients infected with SARS-CoV-2. Severe COVID-19 has been associated with hyperinflammation and pre-existing cardiovascular disease. Platelets are important mediators and sensors of inflammation and are directly affected by cardiovascular stressors. In this report, we found that platelets from severely ill, hospitalized COVID-19 patients exhibit higher basal levels of activation measured by P-selectin surface expression, and have a poor functional reserve upon in vitro stimulation. Correlating clinical features to the ability of plasma from COVID-19 patients to stimulate control platelets identified ferritin as a pivotal clinical marker associated with platelet hyperactivation. The COVID-19 plasma-mediated effect on control platelets was highest for patients that subsequently developed inpatient thrombotic events. Proteomic analysis of plasma from COVID-19 patients identified key mediators of inflammation and cardiovascular disease that positively correlated with in vitro platelet activation. Mechanistically, blocking the signaling of the FcγRIIa-Syk and C5a-C5aR pathways on platelets, using antibody-mediated neutralization, IgG depletion or the Syk inhibitor fostamatinib, reversed this hyperactivity driven by COVID-19 plasma and prevented platelet aggregation in endothelial microfluidic chamber conditions, thus identifying these potentially actionable pathways as central for platelet activation and/or vascular complications in COVID-19 patients. In conclusion, we reveal a key role of platelet-mediated immunothrombosis in COVID-19 and identify distinct, clinically relevant, targetable signaling pathways that mediate this effect. These studies have implications for the role of platelet hyperactivation in complications associated with SARS-CoV-2 infection., Cover illustration, One-sentence summary: The FcγRIIA and C5a-C5aR pathways mediate platelet hyperactivation in COVID-19

The 27th Congress of the International Society of Thrombosis and Haemostasis (ISTH) is an international conference held July 6-10, 2019, in Melbourne, the capital of the state of Victoria, Australia. The ISTH congress has previously been held every other year, with the Scientific and Standardization Committee (SSC) meeting held annually, until 2019 when it became one combined annual meeting of the ISTH and SSC. The conference covers clinical and basic aspects of hemostasis and thrombosis, and this year includes 5 Plenary lectures and >50 State of Art (SOA) lectures, presented by internationally recognized speakers, as well as numerous oral session and poster presentations selected from submitted abstracts, including many early career and reach the world support recipients. This SOA review article in RPTH contains concise Illustrated Review Articles or 'Capsules' consisting of short text, three references and a figure, with topics including stroke, cancer-associated thrombosis, hemophilia, coagulation, the interface between infection and inflammation, and in the experimental and discovery areas, megakaryocyte biology and platelet production, structure-function of key receptors and coagulation factors, and emerging new roles for thrombotic/hemostatic factors. Together, these articles highlight novel findings which will advance knowledge and with the potential to change clinical practice and improve outcomes. It is hoped that conference attendees and followers will enjoy utilizing the images for ongoing education and during the conference for live tweeting during sessions, to assist in the broadcasting and promotion of the science to those unable to attend, or who have chosen to attend a concurrent session. Use #IllustratedReview and #ISTH2019 on social media. ispartof: RESEARCH AND PRACTICE IN THROMBOSIS AND HAEMOSTASIS vol:3 issue:3 pages:431-497 ispartof: location:United States status: published

Thromboembolism complicates disorders caused by immunoglobulin G (IgG)–containing immune complexes (ICs), but the underlying mechanisms are incompletely understood. Prior evidence indicates that induction of tissue factor (TF) on monocytes, a pivotal step in the initiation, localization, and propagation of coagulation by ICs, is mediated through Fcγ receptor IIa (FcγRIIa); however, the involvement of other receptors has not been investigated in detail. The neonatal Fc receptor (FcRn) that mediates IgG and albumin recycling also participates in cellular responses to IgG-containing ICs. Here we asked whether FcRn is also involved in the induction of TF-dependent factor Xa (FXa) activity by IgG-containing ICs by THP-1 monocytic cells and human monocytes. Induction of FXa activity by ICs containing IgG antibodies to platelet factor 4 (PF4) involved in heparin-induced thrombocytopenia (HIT), β-2-glycoprotein-1 implicated in antiphospholipid syndrome, or red blood cells coated with anti-(α)-Rh(D) antibodies that mediate hemolysis in vivo was inhibited by a humanized monoclonal antibody (mAb) that blocks IgG binding to human FcRn. IgG-containing ICs that bind to FcγR and FcRn induced FXa activity, whereas IgG-containing ICs with an Fc engineered to be unable to engage FcRn did not. Infusion of an α-FcRn mAb prevented fibrin deposition after microvascular injury in a murine model of HIT in which human FcγRIIa was expressed as a transgene. These data implicate FcRn in TF-dependent FXa activity induced by soluble and cell-associated IgG-containing ICs. Antibodies to FcRn, now in clinical trials in warm autoimmune hemolytic anemia to lower IgG antibodies and IgG containing ICs may also reduce the risk of venous thromboembolism.

AimsHeart valve disease attributed to serotonin (5HT) has been observed with 5HT-secreting carcinoid tumors and in association with medications, such as the diet drug, Dexfenfluoramine, a serotonin transporter (SLC6A4) inhibitor and 5HT receptor (HTR) 2B agonist. HTR2B signaling upregulates TGFβ-1 resulting in increased production of extracellular matrix proteins. SLC6A4 internalizes 5HT, limiting HTR signaling. Selective 5HT reuptake inhibitors (SSRI), widely used antidepressants, target SLC6A4, thus enhancing HTR signaling. However, 5HT and SLC6A4 mechanisms have not been previously associated with degenerative mitral regurgitation (MR). The present studies investigated the hypothesis that both dysregulation of SLC6A4 and inhibition of SLC6A4 contribute to the pathophysiology of MR.Methods and ResultsHere we report SLC6A4 related studies of 225 patients with MR requiring surgery. A multivariate analysis showed that SSRI use in MR patients was associated with surgery at a younger age, indicating more rapidly progressive MR (p=0.0183); this was confirmed in a national dataset (pConclusionsDown regulation and inhibition of SLC6A4 influences MR through enhanced HTR signaling. SSRI may further influence MR through inhibition and down regulation of SLC6A4, upregulation of HTR2B, and increased platelet release of 5HT.Translational PerspectiveDegenerative mitral valve regurgitation (MR) affects millions, and there is no medical therapy for this disease. MR becomes progressively worse, and for severe MR, the only option is cardiac surgery. Serotonin (5HT) is best known as a neurotransmitter. However, 5HT secreting carcinoid tumors cause a cardiac valve disorder in many cases, and 5HT related medications, such as the diet drug Fenfluoramine, have been associated with the development of cardiac valve disease. The present paper presents evidence that diminished serotonin transporter (SLC6A4) expression and inhibition, lead to increased 5HT receptor signaling, contributing to the progression of MR.

Sepsis is characterized by multiorgan system dysfunction that occurs because of infection. It is associated with high morbidity and mortality and is in need of improved therapeutic interventions. Neutrophils play a crucial role in sepsis, releasing neutrophil extracellular traps (NETs) composed of DNA complexed with histones and toxic antimicrobial proteins that ensnare pathogens, but also damage host tissues. At presentation, patients often have a significant NET burden contributing to the multiorgan damage. Therefore, interventions that inhibit NET release would likely be ineffective at preventing NET-based injury. Treatments that enhance NET degradation may liberate captured bacteria and toxic NET degradation products (NDPs) and likely be of limited therapeutic benefit as well. We propose that interventions that stabilize NETs and sequester NDPs may be protective in sepsis. We showed that platelet factor 4 (PF4), a platelet-associated chemokine, binds and compacts NETs, increasing their resistance to DNase I. We now show that PF4 increases NET-mediated bacterial capture, reduces the release of NDPs, and improves outcome in murine models of sepsis. A monoclonal antibody KKO which binds to PF4-NET complexes, further enhances DNase resistance. However, the Fc portion of this antibody activates the immune response and increases thrombotic risk, negating any protective effects in sepsis. Therefore, we developed an Fc-modified KKO that does not induce these negative outcomes. Treatment with this antibody augmented the effects of PF4, decreasing NDP release and bacterial dissemination and increasing survival in murine sepsis models, supporting a novel NET-targeting approach to improve outcomes in sepsis.

INTRODUCTION Thrombosis is a severe and frequent complication of heparin-induced thrombocytopenia (HIT). However, there is currently no knowledge of the effects of HIT-like antibodies on the resulting microstructure of the formed clot, despite such information being linked to thrombotic events. We evaluate the effect of the addition of pathogenic HIT-like antibodies to blood on the resulting microstructure of the formed clot. MATERIALS AND METHODS Pathogenic HIT-like antibodies (KKO) and control antibodies (RTO) were added to samples of whole blood containing Unfractionated Heparin and Platelet Factor 4. The formed clot microstructure was investigated by rheological measurements (fractal dimension; df) and scanning electron microscopy (SEM), and platelet activation was measured by flow cytometry. RESULTS AND CONCLUSIONS Our results revealed striking effects of KKO on clot microstructure. A significant difference in df was found between samples containing KKO (df = 1.80) versus RTO (df = 1.74; p

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder initiated by antibodies to platelet factor 4 (PF4)/heparin complexes. PF4 released from platelets binds to surface glycosaminoglycans on hematopoietic and vascular cells that are heterogenous in composition and differ in affinity for PF4. PF4 binds to monocytes with higher affinity than to platelets, and depletion of monocytes exacerbates thrombocytopenia in a murine HIT model. Here we show that the expression of PF4 on platelets and development of thrombocytopenia are modulated by the (re)distribution of PF4 among hematopoietic and endothelial cell surfaces. Binding of PF4 to platelets in whole blood in vitro varies inversely with the white cell count, likely because of the greater affinity of monocytes for PF4. In mice, monocyte depletion increased binding of PF4 to platelets by two- to three-fold. Induction of HIT in mice caused a transient >80-fold increase in binding of HIT antibody to monocytes vs 3.5-fold increase to platelets and rapid transient monocytopenia. Normalization of monocyte counts preceded the return in platelet counts. Exposure of blood to endothelial cells also depletes PF4 from platelet surfaces. These studies demonstrate a dynamic interchange of surface-bound PF4 among hematopoetic and vascular cells that may limit thrombocytopenia at the expense of promoting prothrombotic processes in HIT.

The questions of whether thrombopoiesis - the release of platelets from megakaryocytes - occurs both as megakaryocytes emerge from the intramedullar space or occurs as well in the pulmonary vascular bed remains unanswered. Studies by Lefrançais E, et al, (Nature, 2017) demonstrated by in situ microcopy that perhaps 50% of all platelet release in mice occurs from megakaryocytes released from the marrow and traveled to the lungs where they undergo thrombopoiesis over a 20- to 60-minute time-period. We examined whether CD34+-derived human megakaryocytes infused into immunocompromized NSG mice would also shed platelets in the lungs in a similar fashion. We differentiated CD34+-derived hematopoietic stem-progenitors for 12 days in culture using conditions previously described (Wang Y, et al., Blood 2015). We found that unlike platelet-like-particle (PLP) formation in in vitro cultures of CD34+ hematopoietic progenitor cell (HPC)-derived (CD34+) megakaryocytes, which undergo asynchronous shedding of the PLPs, that over 95% of infused CD34+ megakaryocytes shed their platelets within the first 40 minutes much as has been observed for endogenous murine megakaryocytes. The average number of cytoplasmic extensions per megakaryocytes was ~2.7, again very similar to what was seen with endogenous murine megakaryocytes. In contrast, CD34+ cells grown in culture into megakaryocytes for a shorter period of time of only 7 days, poorly shed any cytoplasmic fragments. We also studied human megakaryocytes grown from immortalized megakaryocyte progenitor cell lines (imMKCLs) from induced pluripotent stem cells (iPSCs) generated by the Eto laboratory and kindly provided by Dr. Koji Eto, Kyoto University). These cells were grown and differentiated into terminal megakaryocytes as described (Nakamura S, Cell Stem Cell, 2014) for 4 days in culture. These cells have been proposed to be useful for large-scale preparation of PLPs in vitro for clinical use in place of donor-derived platelets. The resultant infused human imMKCL-derived megakaryocytes also synchronously shed platelets, but only 50% of the infused cells shed their cytoplasm in contrast to >95% of CD34+ megakaryocytes. Moreover, cytoplasmic extensions were decreased to an average of ~1.1 per megakaryocyte. We had proposed that in vitro-generated megakaryocytes might be directly infused into patients in place of further manipulating the megakaryocytes to release functional platelets in vitro using a bioreactor. However, such megakaryocytes will likely be contaminated with a higher level of HPCs than anticipated from in vitro-prepared platelets, and concern exists that they may lead to unacceptable graft versus host complications. We, therefore, examined whether irradiating megakaryocytes as one strategy to eliminate this concern results in megakaryocytes that are still functional and found that megakaryocytes irradiated with up to 25 Gy retain platelet yield per infused megakaryocytes with the platelets having the same half-life. If irradiated and kept in culture, these megakaryocytes begin to shed platelets and undergo apoptosis notably by 24 hours. We also examined whether the pulmonary bed differs from other vascular beds, and infused CD34+ megakaryocytes both intravenously and intra-arterially in parallel studies and found that following intra-arterial infusion, megakaryocytes were mostly entrapped in various organs, but shed few platelets. Thus, our studies suggest that the pulmonary bed is unique for platelet shedding from entrapped megakaryocytes. Whether this is due to the structural organization of the pulmonary beds, its endothelial lining, its reverse exchange in oxygen, carbon dioxide and pH from other capillary beds or the mechanical forces of inhalation and exhalation that expand and contract the capillary cross-sectional area needs to be examined. Our studies show that infused human megakaryocytes synchronously release platelets over a 40-minute window and can do so even after being irradiated and that this occurs specifically in the lungs not only has potential clinical application, but also raises biological questions about what determines thrombopoiesis-readiness and what are the features of the pulmonary bed that allows this synchronous release. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Heparin induced thrombocytopenia (HIT) is an immunogenic prothrombotic disorder caused by antibodies that recognize human platelet factor 4 (PF4) complexed to polyanions. We had previously shown using chimeric constructs of hPF4 and mouse (m) PF4 and chimeras with the related chemokine, neutrophil-activating peptide-2 that there is a single antigenic locus on hPF4 in these complexes to which most HIT antibodies bind. KKO is a mouse monoclonal IgG2b k anti-hPF4/polyanion monoclonal antibody that mimics pathogenic antibodies that induce HIT and provokes thrombosis in a murine model of HIT. We previously established that specific hydrolysis of N-linked glycans in the Fc-region of KKO by endoglycosidase from Streptococcus pyogenes EndoS (Genovis) yields >97% deglycosylation on LC-MS/MS generating DGKKO. This modification has no significant effect on binding to PF4-heparin complexes as shown by ELISA and by dynamic light scattering, but abrogates FcgRIIA-mediated binding and platelet activation, and decreases complement activation as evaluated by flow cytometry. To examine if DGKKO reduces prothrombotic effects, we compared DGKKO with KKO in human microfluidic system lined with human umbilical vein cells (HUVECs) that are then photochemically injured and a murine model involving "HIT mice" (mice that express FcgRIIA and human PF4 and lack mouse PF4). Using the microfluidic system described above and infusing blood from healthy donors with added human PF4 (25 µg/ml) and KKO (50 µg/ml) or HIT IgG from three individuals with SRA-positive HIT (1mg/ml) resulted in increased platelet adherence to the injured endothelium (Figure 1). Addition of DGKKO (50 µg/ml) 15 minutes after addition of HIT antibodies eliminated platelet accumulation (Figure 1). In the HIT murine model, we found that intraperitoneal (IP, 200 µg/mice) or intravenous (IV, 20 µg/mice) DGKKO did not induce thrombocytopenia in HIT mice, but reversed the thrombocytopenia induced by either IP KKO (200 µg/mice) or HIT IgG (1 mg/mice) even when the DGKKO is given 6 hrs after HIT induction (Figure 2A). We used an intravital cremaster laser arteriole injury model in HIT mice to study the efficacy of DGKKO as an antithrombotic agent. We found that unlike KKO that enhances growth of established thrombi in these mice, DGKKO significantly reversed the size of the observed thrombi (Figure 2B). These studies suggest that DGKKO obstructs the HIT antigenic site recognized by HIT antibodies and leads to a reversal of thrombocytopenia and thrombus size. Additional studies are underway to examine if DGKKO can be used as a monotherapy or adjunctive therapy in the murine model of HIT thrombosis. Figure 1 Figure 1. Disclosures Cines: Rigel: Consultancy; Dova: Consultancy; Treeline: Consultancy; Arch Oncol: Consultancy; Jannsen: Consultancy; Taventa: Consultancy; Principia: Other: Data Safety Monitoring Board.

Sepsis is a dysregulated response to infection leading to life-threating organ damage. Although it remains one of the most common causes of mortality worldwide, it lacks targeted treatments. Neutrophils play a crucial role in sepsis by releasing NETs, webs of DNA complexed with histones and antimicrobial proteins that capture pathogens and prevent bacterial dissemination. However, when NETs are degraded by circulating nucleases they release NET-degradation products (NDPs) including cell-free (cf) DNA, histones and myeloperoxidase, which trigger coagulation, induce complement activation, and cause oxidative tissue damage. We proposed a novel NET-directed therapy in sepsis, in which NETs are stabilized by the platelet chemokine PF4. Binding of PF4 enhances NET DNase-resistance, promotes NDP sequestration and increases bacterial capture, improving survival in murine sepsis. As NETs are considered prothrombotic, we were concerned that NET stabilization may increase the risk of clot formation. We therefore sought to determine the effect of PF4-NET stabilization on the thrombogenicity of NETs to learn if this strategy is safe for clinical application. To that end, we examined the effect of PF4 on the thrombotic potential of DNA and NET fragments at different states of nuclease digestion. High molecular weight (hmw) genomic DNA (hmwDNA, >50kbp) was isolated from human whole blood. hmwDNA was digested with restriction enzymes (EcoRI and AluI) for 15min to generate DNA fragments of ~4kbp and ~250bp, respectively. Neutrophils were also isolated from human blood and stimulated with 100 nM PMA to produce neutrophil-adherent NETs, which were cleaved from cell bodies by treatment with 4U/mL DNase I for 20 minutes, releasing NETs >50kbp (hmwNETs). Additional incubation of hmwNETs with DNase I yielded smaller NET fragments. We assessed in vitro activation of coagulation by DNA and NETs by measuring thrombin generation and fibrin formation in platelet-poor plasma using fluorogenic substrate and turbidity assays. Neutrophil-adherent NETs induced far less thrombin generation and fibrin formation in plasma than hmwDNA and hmwNETs. PF4 significantly increased lag time and reduced peak thrombin formation induced by both hmwDNA and hmwNETs. Binding of PF4 also delayed clot initiation time and reduced the rate of fibrin generation. Digestion of hmwDNA and hmwNETs to smaller fragments markedly enhanced thrombogenicity. We posited that shorter DNA fragments are more thrombogenic because they have a greater proportion of end-fragment DNA that exposes more single-stranded DNA. To test this hypothesis, we subjected hmwDNA and digested DNA to heat denaturation at 95°C and rapid cooling to generate single stranded DNA and found that this accelerated fibrin generation. Although the anti-thrombotic effect of PF4 was most pronounced with longer DNA and NET fragments, it continued to significantly reduce fibrin generation induced by shorter DNA fragments, perhaps by stabilizing the fragments to prevent exposure of single-stranded DNA. In conclusion, although prior studies have shown that NETs increase the risk of thrombosis in sepsis, we propose the counter-intuitive concept that PF4-stabilization decreases the risk of NET-mediated prothrombotic state by (1) inhibiting DNase cleavage of intact NETs and subsequent liberation of prothrombotic cfDNA from non-thrombogenic neutrophil-adherent NETs, and (2) preventing further digestion of circulating cfDNA into shorter and more prothrombotic fragments. Although NETs are a double-edged sword: capable of capturing pathogens but inducing host-tissue damage and thrombosis when degraded, treatment with PF4 tips the balance, limiting the capacity of NETs to induce fibrin generation and thrombosis, while enhancing their ability to fight infection by microbial entrapment. These studies add support to our hypothesis that PF4 stabilization of NETs is protective in sepsis and merits further investigation in translational studies. Disclosures No relevant conflicts of interest to declare.

Background The ChAdOx1 nCoV-19 vaccine has been shown to induce Vaccine-induced Immune Thrombotic Thrombocytopenia (VITT), a syndrome that shares clinical features with heparin-induced thrombocytopenia (HIT). The mechanism of thrombocytopenia and thrombosis in these disorders appears to be related to the development of pathologic anti-PF4/heparin antibodies, some of which could activate complement. Interestingly, we and others have found that complement activation is vital when both pediatric and adult patients have severe respiratory illness from SARS-CoV-2 virus (COVID-19) or in the post-infectious multisystem inflammatory syndrome in children (MIS-C). We hypothesized that patients with severe COVID-19 or MIS-C develop similar anti-PF4/heparin antibodies, which lead to endothelial complement activation that drive the inflammatory responses seen in these diseases. Methods Our cohort included 30 pediatric patients with positive SARS-CoV-2 RT-PCRs: 10 each of severe COVID-19 ("Severe", MIS-C, and mild/asymptomatic ("Mild") infection. Using ELISA, we evaluated the levels of antibodies to various platelet-related proteins including PF4, PF4-heparin, and NAP2; in addition, we examined the ability of plasma from each patient to activate complement. The antibody levels were compared to control samples including samples from adult patients with VITT and HIT. Statistical analyses with ANOVA were performed to evaluate differences. Results Patients with MIS-C have a significantly higher anti-PF4 antibody concentration (as measured by mean optical density [OD]) than patients with either mild/asymptomatic disease, or severe COVID-19: Severe 0.5 +/- 0.14; Mild 0.3 +/- 0.12; MIS-C 0.77 +/- 0.35, p=0.003 MIS-C vs. Mild); Similar results were seen for anti-PF4/heparin antibodies: Severe 0.4 +/- 0.14; Mild 0.35 +/- 0.12; MIS-C 0.64 +/- 0.3, p=0.003 MIS-C vs. Mild; p=0.034 MIS-C vs. Severe). These were similar to values obtained for the HIT sample (Figure). Conclusion Patients with MIS-C and severe COVID19 have significant detectable anti-PF4 and PF4/heparin antibodies in contrast to those patients with mild/asymptomatic disease. Our previous studies have shown that patients with MIS-C and COVID-19 have evidence of endovascular complement activation in the form of elevated soluble membrane attack complex (sC5-b9). We have also previously demonstrated that VITT anti-PF4 and anti-PF4/heparin antibodies activate complement and result in endothelial cell activation. These antibodies in pediatric SARS-CoV-2 infection may be involved in the development of more severe disease manifestations. Ongoing investigations will identify if this is due to endothelial complement activation and inflammatory responses that accompany severe disease. This is the first demonstration of the role of anti-PF4 and PF4/heparin antibodies in pediatric SARS-CoV-2. Figure 1 Figure 1. Disclosures Bassiri: Guidepoint Global: Consultancy; Kriya Therapeutics: Consultancy, Current holder of individual stocks in a privately-held company. Teachey: Janssen: Consultancy; NeoImmune Tech: Research Funding; Sobi: Consultancy; BEAM Therapeutics: Consultancy, Research Funding. Lambert: ClinGen, ISTH, ASH, GW University: Honoraria; Rigel: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Consultancy; Dova: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Shionogi: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Argenx: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Principia: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Astra Zeneca: Research Funding; Novartis: Consultancy, Honoraria, Research Funding; Sysmex: Research Funding; PDSA: Research Funding; Octapharma: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

VITT is an immune-based complication of adenoviral-based vaccines used to immunize against SARS_CoV2. The antibodies in VITT have been described as directed at the platelet-specific chemokine PF4 (CXCL4). While the clinical course and target chemokine in VITT has much in common with the better-known thrombocytopenic/prothrombotic disorder, heparin-induced thrombocytopenia (HIT), which involves antibodies directed against PF4 bound to the polyanion heparin, the specific loci where VITT and PF4/polyanion HIT antibodies bind appear to differ in studies using alanine-scanning mutations of PF4 (Nature, 2021. DOI: 10.1038/s41586-021-03744-4). The VITT antigenic site localizes to a heparin-binding domain. Unlike the dominant HIT locus, the VITT locus is conserved not only between human and mouse PF4, but also between PF4 and the related platelet-specific chemokine NAP2 (CXCL7). NAP2 is also expressed and stored in platelet alpha-granules and is present in equimolar concentrations to PF4. Unlike PF4, NAP2 avidly binds the chemokine receptor CXCR2 and strongly activates neutrophils. We now show that antibodies from patients who developed VITT after both AstraZeneca (AZ) or Johnson and Johnson (JJ) adenoviral vaccines, unlike HIT antibodies, recognize mouse PF4 (Figure 1A). More importantly, both AZ and JJ VITT antibodies bound NAP2, while none of the HIT antibodies tested bound PF4 or NAP2 in the absence of heparin (Figure 1A). These results are consistent with the alanine-scanning studies that distinguish the HIT and VITT binding sites. Dynamic light scattering (DLS) showed that NAP2 and PF4 bind to the adenoviral vectors, including Ad5 and the AZ vector ChAdOx5, which leads to expression of SARS_CoV2 spike protein. ChAdOx2 vaccine and CsCl 2-purified ChAdOx2 bound to both proteins, but form larger complexes with NAP2 than with PF4 even at lower concentrations of this chemokine (Figure 1C). Removal of anti-PF4 antibodies by hPF4-Sepharose abrogated PF4-dependent binding, but did not significantly reduce binding to NAP2 (not shown), indicating that VITT plasma contains discrete pools of anti-PF4 and anti-NAP2 antibodies that may have distinct functional properties. Sandwich ELISA (not shown) and Western blot analysis of purified VITT IgG demonstrates the presence of hPF4-IgG and NAP2-IgG immune complexes in purified patient's IgG (Figure 2A). Functional studies show that both PF4 and NAP2 can activate platelets in the presence of VITT antibodies. Anti-PF4-depleted VITT IgG fraction retains the ability to activate platelets in the presence of NAP2 (Figure 2B). Thus, unlike HIT, VITT appears to target a shared antigenic site on the related chemokines PF4 and NAP2. This raises the question as to whether NAP2, as one the most abundant platelet chemokines released from activated platelets, is involved in the initiation and propagation of the immunothrombotic response. Additional studies are needed to see whether NAP2, which can potently and specifically activate neutrophils via CXCLR2, contributes to the specific thromboinflammatory phenotype seen in VITT. We propose using FcgammaRIIA+ mice that concurrently express human PF4 and NAP2 and specific knockout of each chemokine, available in our group, to further understand the pathogenesis of VITT and its thrombocytopenic/ prothrombotic phenotype. Figure 1 Figure 1. Disclosures Padmanabhan: Veralox Therapeutics: Membership on an entity's Board of Directors or advisory committees. Cines: Dova: Consultancy; Rigel: Consultancy; Treeline: Consultancy; Arch Oncol: Consultancy; Jannsen: Consultancy; Taventa: Consultancy; Principia: Other: Data Safety Monitoring Board.

Background: Recent studies show that ultra-large immune complexes consisting of IgG and platelet factor 4 and heparin (P+H) potently activate complement and facilitate complement dependent activation of cellular FcgRIIA (PMID 34189574). In whole blood assays using KKO, a monoclonal anti-PF4/heparin antibody, or antibodies from patients with heparin induced thrombocytopenia (HIT), we showed that classical pathway (CP) inhibition reduced immune complex-mediated complement activation (C3c and soluble C5b-9 generation), cell surface deposition of immune complexes and cellular activation. Aims: As previous studies suggest that the alternative pathway (AP) provides significant amplification (>80%) of the CP pathway, (PMID: 15544620) we compared the effects of AP, CP, and CP/AP inhibitors by KKO and HIT immune complexes in whole blood. Methods: Inhibitors of the CP (BBK32, a borrelia protein inhibitor to C1r), AP (anti-factor B antibody (α-fB), or Factor D (fD inhibitor or fD-INH) Alexion Pharmaceuticals, Boston, MA) or combined AP/CP (C1-esterase inhibitor, C1-INH, Berinert, CSL Behring; or soluble complement receptor 1, sCR1, Alexion) were tested in hemolytic assays of CP or AP to confirm pathway specificity. To examine effects of CP or AP inhibition on complement activation by immune complexes consisting of KKO or HIT IgG, whole blood was pre-incubated with CP, AP or CP/AP inhibitors prior to addition of P+H ± KKO/HIT IgG or isotype controls. WB was incubated for 45 minutes at 37ºC followed by addition of 10mM EDTA to quench further complement activation. Complement activation products (C3c and sC5b-9) and neutrophil degranulation (MMP9) markers were measured using commercial immunoassays. Effects of complement inhibitors on cellular deposition of immune complexes was examined by flow, using previously described methods (PMID 34189574) using fluorescently labeled anti-C3c antibody (Quidel, San Diego, CA) and anti-mouse or human IgG (Biolegend, San Diego, CA). Results: Consistent with prior publications (PMID: 26808924), BBK32 showed marked reduction CP, but not AP-dependent hemolytic assays. The converse was true of AP inhibitors: α-fB and fD-INH prevented AP-dependent, but not CP-dependent hemolysis (data not shown). C1-INH and sCR1 showed activity in both CP- and AP-dependent assays. The CP or CP/AP inhibitors showed potent inhibition of C3c and sC5b-9 generation by KKO and HIT immune complexes, while AP inhibitors had no effect (Figure A for KKO C3c generation; and Table 1 for KKO/HIT C3c generation; sC5b-9 data not shown). For a given CP or CP/AP inhibitor, the concentrations leading to 50% inhibition (IC 50) were generally comparable for KKO and HIT immune complexes for C3c (Figure A and Table 1) and sC5b-9 generation (data not shown), with potency as follows: C1-INH>>BBK32>sCR1 (Table 1). On the other hand, the AP inhibitors, α-fB and fD-INH, showed no inhibitory activity in C3c (Figure A and Table 1)/sC5b-9 (data not shown) generation by KKO or HIT ULICs. As our recent studies indicate that complement activation is critical to cell surface deposition of immune complexes and cellular activation via FcgRIIA, we examined effects of complement inhibitors on IC deposition on B-cells and MMP9 release from neutrophils. CP or CP/AP inhibitors, but not AP inhibitors, reduced cell surface binding of immune complexes (Figure B) as well as MMP9 release (Figure C and Table 1). Conclusion: Together, these studies demonstrate that the AP has a minimal role in supporting complement activation by KKO/HIT ULICs. Future studies should examine CP inhibition as a therapeutic strategy for modulating the cellular activating effects of HIT antibodies. To what extent these findings apply to other immune complexes and/or CP activators requires further study. Funding Agency: NIH HL151730; α-fB antibody, fD inhibitor and sCR1 was provided by Alexion Pharmaceuticals, Boston, MA. BBK32 was provided by Dr. Brandon Garcia, East Carolina University, Greenville, NC. Figure 1 Figure 1. Disclosures Cines: Dova: Consultancy; Rigel: Consultancy; Treeline: Consultancy; Arch Oncol: Consultancy; Jannsen: Consultancy; Taventa: Consultancy; Principia: Other: Data Safety Monitoring Board. OffLabel Disclosure: C1-esterase inhibitor off label for HIT

Heparin induced thrombocytopenia (HIT) is a prothrombotic disorder mediated by ultra-large immune complexes (ULICs) containing IgG antibodies bound to multivalent complexes of platelet factor 4 (PF4) and heparin (H). HIT ULICs activate cellular FcγIIA receptors that initiate diverse cellular effector functions including neutrophil degranulation and monocyte expression of tissue factor (TF). Previous studies have shown that HIT ULICs also potently activate complement through the classical pathway (Cines et al., 1980). Whether complement activation contributes to FcγRIIA-dependent prothrombotic pathways has not been addressed in detail. In studies that follow, we describe: 1) robust complement activation by HIT ULICs in plasma and whole blood (WB), 2) cell-surface deposition of complement and IgG triggered by HIT ULICs, 3) complement-dependent neutrophil degranulation and monocyte TF expression, 4) efficacy of proximal, but not terminal, pathway inhibition in regulating monocyte TF expression, and 5) deposition of complement in thrombi formed in "HIT mice" that generate ULICs containing KKO, a HIT-like monoclonal antibody (Arepally et al., 2000). Consistent with prior studies showing involvement of the classical pathway in HIT (Cines et al., 1980), we observed that binding of C1q induced marked enlargement of HIT ULICs in buffer assessed by dynamic light scattering as well as in plasma using confocal microscopy (data not shown). To assess complement activation by HIT ULICs, we incubated WB and plasma with PF4 (25 µg/mL) ± heparin (1 U/mL) in the presence of KKO (or isotype, "ISO"; 50 µg/mL) or HIT IgG (or control IgG, "CON"; 500 µg/mL) and measured C3c with a capture immunoassay as previously described (Khandelwal et al., 2018). KKO (Figure 1A) or HIT ULICs (n=3; HIT1-3, Figure 1B), showed robust generation of C3c in the presence of PF4/heparin, but not antigens alone or with control IgG (ISO/CON). Complement activation by HIT ULICs leads to downstream generation of C5a and formation of sC5b-9 (data not shown). Pre-incubation of plasma or WB with a variety of classical pathway inhibitors, including a C1r inhibitor derived from Borrelia burgdorferi (BBK 32), C1 esterase inhibitor (Berinert, CSL Behring) and anti-C1q antibody (α-C1q Ab; Annexon Biosciences) inhibited C3c generation by KKO ULICs (p Disclosures Arepally: Novartis: Consultancy; Alexion: Other; Annexon Biosciences: Consultancy, Other; Veralox Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Biokit: Consultancy, Patents & Royalties; Apotex: Consultancy, Research Funding.

Anti-protamine (PRT)/heparin antibodies are a newly described class of heparin-dependent antibodies occurring in patients exposed to PRT and heparin during cardiac surgery. To understand the biologic significance of anti-PRT/heparin antibodies, we developed a murine monoclonal antibody (ADA) specific for PRT/heparin complexes and compared it to patient-derived anti-PRT/heparin antibodies, as well as comparing polyclonal and monoclonal antibodies with anti-platelet factor 4 (PF4)/heparin. Using monoclonal antibodies and polyclonal patient-derived antibodies, we show distinctive binding patterns of anti-PRT/heparin antibodies as compared with PF4/heparin antibodies. Whereas heparin-induced thrombocytopenia (HIT) antibody binding to PF4/heparin is inhibited by relatively low doses of heparin (0-1 U/mL), anti-PRT/heparin antibodies, including ADA, retain binding to PRT/heparin over a broad range of heparin concentrations (0-50 U/mL). Unlike PF4/heparin antibodies, which recognize PF4 complexed to purified or cell-associated glycosaminoglycans (GAGs), anti-PRT/heparin antibodies show variable binding to cell-associated GAGs. Further, binding of anti-PRT/heparin antibodies to PRT/dextran complexes correlates closely with the ability of antibodies to bind to cell-surface PRT. These findings suggest that antibody binding to PRT/dextran may identify a subset of clinically relevant anti-PRT/heparin antibodies that can bind to cell-surface GAGs. Together, these findings show important serologic differences between HIT and anti-PRT/heparin antibodies, which may account for the variability in disease expression of the two classes of heparin-dependent antibodies.

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder initiated by antibodies against complexes between human platelet factor 4 (hPF4) and heparin. A better understanding of the events that initiate the prothrombotic state may improve approaches to antithrombotic management. Here, we visualized thrombus formation in an in vivo murine model and an endothelialized microfluidic system that simulate the pathogenesis of HIT. hPF4 released from platelets predominantly bound to peri-injury endothelium and formed HIT antigenic complexes that were dissociated by heparin. In mice expressing both hPF4+ and human platelet IgG Fc receptor IIA (FcγRIIA), infusion of the HIT-like monoclonal antibody KKO increased fibrin and platelet deposition at sites of injury, followed immediately by antigen formation on proximate endothelial cells. After a few minutes, HIT antigen was detected within the thrombus itself at the interface between the platelet core and the surrounding shell. We observed similar results in the humanized, endothelialized microfluidic system. hPF4 and KKO selectively bound to photochemically injured endothelium at sites where surface glycocalyx was reduced. These studies support the concept that the perithrombus endothelium is the predominant site of HIT antigen assembly. This suggests that disrupting antigen formation along the endothelium or protecting the endothelium may provide a therapeutic opportunity to prevent thrombotic complications of HIT, while sparing systemic hemostatic pathways.

Systemic lupus erythematosus (SLE) is associated with a high risk of venous and arterial thrombosis, not necessarily associated with prothrombotic antiphospholipid antibodies (Abs). Alternatively, thrombosis may be due to an increased titer of anti-dsDNA Abs that presumably promote thrombosis via direct platelet activation. Here, we investigated effects of purified anti-dsDNA Abs from the blood of SLE patients, alone or in a complex with dsDNA, on isolated normal human platelets. We showed that anti-dsDNA Abs and anti-dsDNA Ab/dsDNA complexes induced strong platelet activation assessed by enhanced P-selectin expression and dramatic morphological and ultrastructural changes. Electron microscopy revealed a significantly higher percentage of platelets that lost their discoid shape, formed multiple filopodia and had a shrunken body when treated with anti-dsDNA Abs or anti-dsDNA Ab/dsDNA complexes compared with control samples. In addition, these platelets activated with anti-dsDNA Ab/dsDNA complexes typically contained a reduced number of secretory α-granules that grouped in the middle and often merged into a solid electron dense area. Many activated platelets released plasma membrane-derived microvesicles and/or fell apart into subcellular cytoplasmic fragments. Confocal microscopy revealed that platelets treated with anti-dsDNA Ab/dsDNA complex had a heterogeneous distribution of septin2 compared with the homogeneous distribution in control platelets. Structural perturbations were concomitant with mitochondrial depolarization and a decreased content of platelet ATP, indicating energetic exhaustion. Most of the biochemical and morphological changes in platelets induced by anti-dsDNA Abs and anti-dsDNA Ab/dsDNA complexes were prevented by pre-treatment with a monoclonal mAb against FcγRIIA. The aggregate of data indicates that anti-dsDNA Abs alone or in a complex with dsDNA strongly affect platelets via the FcγRIIA receptor. The immune activation of platelets with antinuclear Abs may comprise a prothrombotic mechanism underlying a high risk of thrombotic complications in patients with SLE.

Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy sometimes associated with thrombosis. The hallmark of HIT is antibodies to the heparin/platelet factor 4 (PF4) complex that cause thrombocytopenia and thrombosis through platelet activation. Despite the clinical importance, the molecular mechanisms and late consequences of immune platelet activation are not fully understood. Here, we studied immediate and delayed effects of the complexes formed by human PF4 and HIT-like monoclonal mouse anti-human-PF4/heparin IgG antibodies (named KKO) on isolated human platelets in vitro. Direct platelet-activating effect of the KKO/PF4 complexes was corroborated by the overexpression of phosphatidylserine (PS) and P-selectin on the platelet surface. The immune platelet activation was accompanied by a decrease of the mitochondrial transmembrane potential (ΔΨm), concurrent with a significant gradual reduction of the ATP content in platelets, indicating disruption of energy metabolism. A combination of PS expression and mitochondrial depolarization induced by the PF4-containing immune complexes observed in a substantial fraction of platelets was considered as a sign of ongoing platelet death, as opposed to a subpopulation of activated live platelets with PS on the plasma membrane but normal ΔΨm. Both activated and dying platelets treated with KKO/PF4 formed procoagulant extracellular microvesicles bearing PS on their surface. Scanning and transmission electron microscopy revealed dramatic morphological changes of KKO/PF4-treated platelets, including their fragmentation, another indicator of cell death. Most of the effects of KKO/PF4 were prevented by an anti-FcγRII monoclonal antibody IV.3. The adverse functional and structural changes in platelets induced by the KKO/PF4 complexes were associated with strong time-dependent activation of calpain, but only trace cleavage of caspase 3. The results indicate that the pathogenic PF4-containing HIT-like immune complexes induce direct prothrombotic platelet activation via FcγRIIA receptors followed by non-apoptotic calpain-dependent death of platelets, which can be an important mechanism of thrombocytopenia during HIT development.

Sepsis is characterized by multi-organ system dysfunction that occurs due to infection. It is associated with unacceptably high morbidity and mortality and in need of improved therapeutic intervention. Neutrophils play a crucial role in sepsis, releasing neutrophil extracellular traps (NETs) composed of DNA complexed with histones and toxic antimicrobial proteins that ensnare pathogens but also damage host tissues. At presentation, patients likely have a significant NET burden contributing to the multi-organ damage. Therefore, interventions that prevent NET release would likely be ineffective at preventing NET-based injury. Treatments that enhance NET degradation may liberate captured bacteria and toxic NET degradation products (NDPs) and therefore be of limited therapeutic benefit. We propose that interventions that stabilize NETs and sequester NDPs may be protective in sepsis. Platelet factor 4 (PF4, CXCL4) a platelet-associated chemokine, binds and compacts NETs, increasing their resistance to deoxyribonuclease I. A monoclonal antibody, KKO, which binds to PF4-NET complexes, further enhances this resistance. We now show that PF4 increases NET-mediated bacterial capture in vitro, reduces the release of NDPs, and improves outcome in murine models of sepsis. An Fc-modified KKO further enhances deoxyribonuclease resistance, decreases NDP release, and increases survival in these models, supporting a novel NET-targeting approach to improve outcomes in sepsis.

The mechanisms by which exposure to heparin initiates antibody responses in many, if not most, recipients are poorly understood. We recently demonstrated that antigenic platelet factor 4 (PF4)/heparin complexes activate complement in plasma and bind to B cells. Here, we describe how this process is initiated. We observed wide stable variation in complement activation when PF4/heparin was added to plasma of healthy donors, indicating a responder “phenotype” (high, intermediate, or low). Proteomic analysis of plasma from these healthy donors showed a strong correlation between complement activation and plasma immunoglobulin M (IgM) levels (r = 0.898; P < .005), but not other Ig isotypes. Complement activation response to PF4/heparin in plasma displaying the low donor phenotype was enhanced by adding pooled IgM from healthy donors, but not monoclonal IgM. Depletion of IgM from plasma abrogated C3c generation by PF4/heparin. The complement-activating features of IgM are likely mediated by nonimmune, or natural, IgM, as cord blood and a monoclonal polyreactive IgM generate C3c in the presence of PF4/heparin. IgM facilitates complement and antigen deposition on B cells in vitro and in patients receiving heparin. Anti-C1q antibody prevents IgM-mediated complement activation by PF4/heparin complexes, indicating classical pathway involvement. These studies demonstrate that variability in plasma IgM levels correlates with functional complement responses to PF4/heparin. Polyreactive IgM binds PF4/heparin, triggers activation of the classical complement pathway, and promotes antigen and complement deposition on B cells. These studies provide new insights into the evolution of the heparin-induced thrombocytopenia immune response and may provide a biomarker of risk.

Heparin-induced thrombocytopenia is a prothrombotic disorder caused by antibodies to platelet factor 4 (PF4)/heparin complexes. The mechanism that incites such prevalent anti-PF4/heparin antibody production in more than 50% of patients exposed to heparin in some clinical settings is poorly understood. To investigate early events associated with antigen exposure, we first examined the interaction of PF4/heparin complexes with cells circulating in whole blood. In healthy donors, PF4/heparin complexes bind preferentially to B cells (>90% of B cells bind to PF4/heparin in vitro) relative to neutrophils, monocytes, or T cells. Binding of PF4 to B cells is heparin dependent, and PF4/heparin complexes are found on circulating B cells from some, but not all, patients receiving heparin. Given the high proportion of B cells that bind PF4/heparin, we investigated complement as a mechanism for noncognate antigen recognition. Complement is activated by PF4/heparin complexes, co-localizes with antigen on B cells from healthy donors, and is present on antigen-positive B cells in patients receiving heparin. Binding of PF4/heparin complexes to B cells is mediated through the interaction between complement and complement receptor 2 (CR2 [CD21]). To the best of our knowledge, these are the first studies to demonstrate complement activation by PF4/heparin complexes, opsonization of PF4/heparin to B cells via CD21, and the presence of complement activation fragments on circulating B cells in some patients receiving heparin. Given the critical contribution of complement to humoral immunity, our observations provide new mechanistic insights into the immunogenicity of PF4/heparin complexes.

SummaryHeparin-induced thrombocytopenia (HIT) is an autoimmune disorder characterised by thrombocytopenia and thrombosis. The mechanisms leading to platelet destruction are complex and the thrombotic complications of HIT appear to be due to multiple different intravascular targets. The dual binding of HIT antibodies to platelet surface PF4/GAG complexes and to FcγRIIA likely leads to both platelet clearance and to their direct activation. Monocytes and endothelial cells bind PF4 with higher avidity than platelets and are more resistant to competitive removal of surface-bound PF4 in the presence of heparin. Binding of HIT antibodies to PF4/glycosaminoglycan complexes on the surface on these cells leads to their activation and increased procoagulant activity. Binding of higher levels of PF4 released from activated platelets to the endothelium may lead to changes of the anticoagulant properties of the glycocalyx and target the endothelial cells for HIT antibodies. Pathogenic antibodies bound to endothelial cells further promote prothrombotic conditions by a mechanism that is independent of FcγR activation, yet not completely understood. A more detailed understanding of the role of monocytes and endothelium may identify new targets for intervention to mitigate the risk of thrombosis with less impact on systemic haemostasis than current approaches to treatment for this serious disorder.

Platelet factor 4 (PF4) has been recently shown to inhibit infection by a broad range of human immunodeficiency virus type 1 (HIV-1) isolates in vitro. We found that the inhibitory effects of PF4 are limited to a defined concentration range where PF4 exists largely in a monomeric state. Under these conditions, PF4 bound the HIV-1 envelope protein and inhibited HIV-1 attachment to the cell surface. However, as concentrations increased to the point where PF4 exists largely in tetrameric or higher-order forms, viral infection in vitro was enhanced. Enhancement could be inhibited by mutations in PF4 that shift the oligomeric equilibrium toward the monomeric state, or by using soluble glycosaminoglycans (GAGs) to which tetrameric PF4 avidly binds. We conclude that at physiologically relevant concentrations, oligomeric PF4 enhances infection by HIV-1 by interacting with the viral envelope protein as well as cell surface GAGs, enhancing virus attachment to the cell surface. This effect was not specific to HIV-1, as enhancement was seen with some but not all other viruses tested. The biphasic effects of PF4 on HIV-1 infection suggest that native PF4 will not be a useful antiviral agent and that PF4 could contribute to the hematologic abnormalities commonly seen in HIV-infected individuals by enhancing virus infection in the bone marrow.

Heparin-induced thrombocytopenia (HIT) is characterized by a high incidence of thrombosis, unlike other antibody-mediated causes of thrombocytopenia. We have shown that monocytes complexed with surface-bound platelet factor 4 (PF4) activated by HIT antibodies contribute to the prothrombotic state in vivo, but the mechanism by which this occurs and the relationship to the requirement for platelet activation via fragment crystallizable (Fc)γRIIA is uncertain. Using a microfluidic model and human or murine blood, we confirmed that activation of monocytes contributes to the prothrombotic state in HIT and showed that HIT antibodies bind to monocyte FcγRIIA, which activates spleen tyrosine kinase and leads to the generation of tissue factor (TF) and thrombin. The combination of direct platelet activation by HIT immune complexes through FcγRIIA and transactivation by monocyte-derived thrombin markedly increases Annexin V and factor Xa binding to platelets, consistent with the formation of procoagulant coated platelets. These data provide a model of HIT wherein a combination of direct FcγRIIA-mediated platelet activation and monocyte-derived thrombin contributes to thrombosis in HIT and identifies potential new targets for lessening this risk.

Protein disulfide isomerase (PDI) has two distinct CGHC redox-active sites; however, the contribution of these sites during different physiologic reactions, including thrombosis, is unknown. Here, we evaluated the role of PDI and redox-active sites of PDI in thrombosis by generating mice with blood cells and vessel wall cells lacking PDI (Mx1-Cre Pdifl/fl mice) and transgenic mice harboring PDI that lacks a functional C-terminal CGHC motif [PDI(ss-oo) mice]. Both mouse models showed decreased fibrin deposition and platelet accumulation in laser-induced cremaster arteriole injury, and PDI(ss-oo) mice had attenuated platelet accumulation in FeCl3-induced mesenteric arterial injury. These defects were rescued by infusion of recombinant PDI containing only a functional C-terminal CGHC motif [PDI(oo-ss)]. PDI infusion restored fibrin formation, but not platelet accumulation, in eptifibatide-treated wild-type mice, suggesting a direct role of PDI in coagulation. In vitro aggregation of platelets from PDI(ss-oo) mice and PDI-null platelets was reduced; however, this defect was rescued by recombinant PDI(oo-ss). In human platelets, recombinant PDI(ss-oo) inhibited aggregation, while recombinant PDI(oo-ss) potentiated aggregation. Platelet secretion assays demonstrated that the C-terminal CGHC motif of PDI is important for P-selectin expression and ATP secretion through a non-αIIbβ3 substrate. In summary, our results indicate that the C-terminal CGHC motif of PDI is important for platelet function and coagulation.

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction characterized by thrombocytopenia and a high risk for venous or arterial thrombosis. HIT is caused by antibodies that recognize complexes of platelet factor 4 and heparin. The pathogenic mechanisms of this condition are not fully understood. In this study, we used flow cytometry, fluorimetry, and Western blot analysis to study the direct effects of pathogenic immune complexes containing platelet factor 4 on human platelets isolated by gel-filtration. HIT-like pathogenic immune complexes initially caused pronounced activation of platelets detected by an increased expression of phosphatidylserine and P-selectin. This activation was mediated either directly through the Fc&gamma, RIIA receptors or indirectly via protease-activated receptor 1 (PAR1) receptors due to thrombin generated on or near the surface of activated platelets. The immune activation was later followed by the biochemical signs of cell death, such as mitochondrial membrane depolarization, up-regulation of Bax, down-regulation of Bcl-XL, and moderate activation of procaspase 3 and increased calpain activity. The results show that platelet activation under the action of HIT-like immune complexes is accompanied by their death through complex apoptotic and calpain-dependent non-apoptotic pathways that may underlie the low platelet count in HIT.

Despite improvements in cancer early detection and treatment, metastatic breast cancer remains deadly. Current therapeutic approaches have very limited efficacy in patients with triple negative breast cancer. Among the many mechanisms associated that contribute to cancer progression, signaling through the CXCL12-CXCR4 is an essential step in cancer cell migration. We previously demonstrated the formation of CXCL12-CXCL4 heterodimers (Carlson et al., 2013). Here, we investigated whether CXCL12-CXCL4 heterodimers alter tumor cell migration. CXCL12 alone dose-dependently promoted the MDA-MB 231 cell migration (p .05), which could be prevented by blocking the CXCR4 receptor. The addition of CXCL4 inhibited the CXCL12-induced cell migration (p .05). Using NMR spectroscopy, we identified the CXCL4-CXCL12 binding interface. Moreover, we generated a CXCL4-derived peptide homolog of the binding interface that mimicked the activity of native CXCL4 protein. These results confirm the formation of CXCL12-CXCL4 heterodimers and their inhibitory effects on the migration of breast tumors cells. These findings suggest that specific peptides mimicking heterodimerization of CXCL12 might prevent breast cancer cell migration.

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder mediated by complexes between platelet factor 4 (PF4) and heparin or other polyanions, but the risk of thrombosis extends beyond exposure to heparin implicating other PF4 partners. We recently reported that peri-thrombus endothelium is targeted by HIT antibodies, but the binding site(s) has not been identified. We now show that PF4 binds at multiple discrete sites along the surface of extended strings of von Willebrand factor (VWF) released from the endothelium following photochemical injury in an endothelialized microfluidic system under flow. The HIT-like monoclonal antibody KKO and HIT patient antibodies recognize PF4-VWF complexes, promoting platelet adhesion and enlargement of thrombi within the microfluidic channels. Platelet adhesion to the PF4-VWF-HIT antibody complexes is inhibited by antibodies that block FcγRIIA or the glycoprotein Ib-IX complex on platelets. Disruption of PF4-VWF-HIT antibody complexes by drugs that prevent or block VWF oligomerization attenuate thrombus formation in a murine model of HIT. Together, these studies demonstrate assembly of HIT immune complexes along VWF strings released by injured endothelium that might propagate the risk of thrombosis in HIT. Disruption of PF4-VWF complex formation may provide a new therapeutic approach to HIT.

Sepsis is a dysregulated response to infection that leads to life-threatening organ damage. This process induces the release of neutrophil extracellular traps (NETs), webs of negatively-charged cell-free DNA (cfDNA) complexed with positively-charged histones that capture pathogens but also damage host tissue. We believe that upon presentation, most patients with sepsis have already released a large amount of NETs so that preventing NET release (NETosis), or accelerating their lysis with DNase, may be ineffective or harmful. Murine studies of NET lysis in sepsis have met with mixed results raising the concern that this strategy may lead to the release of captured bacteria and harmful NET degradation products (NDPs) such as cell free (cf) DNA and histones. We propose NET stabilization as an alternative therapy in sepsis. In a microfluidic system, we have shown that NETs bound the platelet chemokine platelet factor 4 (PF4, CXCL4), a highly-positively charged protein that aggregates polyanionic molecules including DNA. As a result of this aggregating effect, PF4 physically compacts NETs without inducing histone release. These compact PF4-NET complexes are resistant to DNase lysis. Using a microfluidic system, in which channels coated with NETs were infused with labeled Staphylococcus aureus particles, we now show that PF4 compaction of NETs markedly enhances their ability to capture bacteria. Likely, PF4, which is known to bind to the bacterial cell wall, cross aggregates pathogens to NET DNA. Thus, without added PF4, little bacteria is entrapped in the microfluidic chamber (Fig. 1A & 1B) and those captured are readily released by the infusion of DNase I (Fig. 1C & 1D). In contrast, when PF4 is added, microbial entrapment is markedly enhanced (Fig. 1A & 1B) and persists even with the infusion of DNase (Fig. 1C & 1D). When human (h) PF4 binds to polyanions it changes conformation, revealing HIT antigenic sites that bind the HIT-like monoclonal antibody KKO. Bound KKO further enhances PF4-NET complex resistance to DNase I in the microfluidic chamber, preventing the release of NDPs and entrapped bacteria upon exposure to DNases. These in vitro observations led us to hypothesize that infused PF4 and/or KKO may serve as a targeted therapeutic in sepsis. However, unmodified KKO stimulates leukocytes and induces a prothrombotic state. Indeed, infused KKO accelerates mortality in murine models of sepsis in mice that express hPF4 (hPF4+) (Fig. 2C). We, therefore, modified KKO using an IgG-specific endoglycosidase to create deglycosylated KKO (DG-KKO) that retains the ability to bind to PF4-NET complexes, but has little capacity to interact with hematopoietic cell Fc receptors or activate complement. DG-KKO binding also protects hPF4-NET complexes from DNase lysis and does not interfere with bacterial capture (Fig. 1B). In a murine lipopolysaccharide (LPS) endotoxemia model, DG-KKO infusion prevented thrombocytopenia, decreased the release of NDPs, and enhanced survival exclusively in hPF4+ mice (not shown). Although treatment with LPS recapitulates many aspects of sepsis, it relies on treatment with a bacterial toxin rather than live pathogens. Therefore, to determine if hPF4 and/or DG-KKO mitigate bacterial-induced sepsis, we repeated our studies using cecal ligation and puncture (CLP) to induce polymicrobial sepsis. Infused hPF4, but more so, infused DG-KKO decreased the severity of thrombocytopenia, reduced NDP release, and improved survival (Fig. 2A-2C). These murine studies demonstrate that PF4 released from activated platelets stabilizes NETs, making them resistant to DNase lysis and enhancing their ability to capture bacteria. Infused PF4 and DG-KKO further enhance NET stability, decrease the release of NDPs, and enhance bacterial entrapment, leading to better outcomes. These studies provide mechanistic insights into how NETs contribute to end-organ damage in sepsis and offer a targeted and novel therapeutic that minimizes their harmful effects, while enhancing their protective properties. Disclosures Arepally: Biokit: Patents & Royalties; Apotex Pharmaceuticals: Consultancy; Veralox Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Heparin-induced thrombocytopenia (HIT) is an immune-mediated thrombocytopenic disorder associated with a severe prothrombotic state. We investigated whether neutrophils and neutrophil extracellular traps (NETs) contribute to the development of thrombosis in HIT. Using an endothelialized microfluidic system and a murine passive immunization model, we show that HIT induction leads to increased neutrophil adherence to venous endothelium. In HIT mice, endothelial adherence is enhanced immediately downstream of nascent venous thrombi, after which neutrophils undergo retrograde migration via a CXCR2-dependent mechanism to accumulate into the thrombi. Using a microfluidic system, we found that PF4 binds to NETs, leading them to become compact and DNase resistant. PF4-NET complexes selectively bind HIT antibodies, which further protect them from nuclease digestion. In HIT mice, inhibition of NET formation through Padi4 gene disruption or DNase treatment limited venous thrombus size. PAD4 inactivation did affect arterial thrombi or severity of thrombocytopenia in HIT. Thus, neutrophil activation contributes to the development of venous thrombosis in HIT by enhancing neutrophil-endothelial adhesion and neutrophil clot infiltration, where incorporated PF4-NET-HIT antibody complexes lead to thrombosis propagation. Inhibition of neutrophil endothelial adhesion, prevention of neutrophil chemokine-dependent recruitment of neutrophils to thrombi, or suppression of NET release should be explored as strategies to prevent venous thrombosis in HIT.

Summary Background ERp57 is required for platelet function; however, whether ERp57 contributes to fibrin generation is unknown. Methods and Results Using an inhibitory anti-ERp57 antibody (mAb1), Pf4-Cre/ERp57fl/fl mice, Tie2-Cre/ERp57fl/fl mice, and mutants of ERp57, we analyzed the function of ERp57 in laser-induced thrombosis. Fibrin deposition was decreased in Pf4-Cre/ERp57fl/fl mice, consistent with a role for platelet ERp57 in fibrin generation. Fibrin deposition was further decreased with infusion of mAb1 and in Tie2-Cre/ERp57fl/fl mice, consistent with endothelial cells also contributing to fibrin deposition. Infusion of eptibifatide inhibited platelet and fibrin deposition, confirming a role for platelets in fibrin deposition. Infusion of recombinant ERp57 corrected the defect in fibrin deposition but not platelet accumulation, suggesting a direct effect of ERp57 on coagulation. mAb1 inhibited thrombin generation in vitro, consistent with a requirement for ERp57 in coagulation. Platelet accumulation was decreased to similar extents in Pf4-Cre/ERp57fl/fl mice, Tie2-Cre/ERp57fl/fl mice and normal mice infused with mAb1. Infusion of completely inactivated ERp57 or ERp57 with a non-functional second active site inhibited fibrin deposition and platelet accumulation, indicating that the isomerase activity of the second active site is required for these processes. Conclusion ERp57 regulates thrombosis via multiple targets.

Several CGHC motif-containing disulfide isomerases support thrombosis. We here report that endoplasmic reticulum protein 72 (ERp72), with 3 CGHC redox-active sites (ao, a, and a'), supports thrombosis. We generated a new conditional knockout mouse model and found that Tie2-Cre/ERp72fl/fl mice with blood and endothelial cells lacking ERp72 had prolonged tail bleeding times and decreased platelet accumulation in laser-induced cremaster arteriole injury and FeCl3-induced mesenteric arterial injury. Fibrin deposition was decreased in the laser injury model. Both platelet and fibrin accumulation defects were fully rescued by infusion of recombinant ERp72 containing functional a and a' CGHC motifs (ERp72(oo-ss-ss)). Infusion of ERp72 containing inactivated a and a' CGHC motifs (ERp72(ss-oo-oo)) inhibited platelet accumulation and fibrin deposition in wild-type mice. Infusion of ERp72(oo-ss-ss) into β3-null mice increased fibrin deposition in the absence of platelets. ERp72-null platelets had defective aggregation, JON/A binding, P-selectin expression, and adenosine triphosphate (ATP) secretion. The aggregation and ATP secretion defects were fully rescued by ERp72(oo-ss-ss) but partially rescued by ERp72(ss-oo-ss) and ERp72(ss-ss-oo). Aggregation and ATP secretion of human platelets was potentiated by ERp72(oo-ss-ss) but inhibited by ERp72(ss-oo-ss) and ERp72(ss-ss-oo). These data suggest that both the a and a' active sites are required for platelet function. ERp72 bound poorly to β3-null mouse platelets, and the addition of ERp72(oo-ss-ss) to human platelets generated thiols in αIIbβ3, suggesting a direct interaction of ERp72 with αIIbβ3. Defective aggregation of ERp72-null platelets was recovered by ERp72, but not other thiol isomerases. In summary, ERp72 plays a critical role in platelet function and coagulation through the a and a' CGHC motifs.