Complement Biosensors Identify a Classical Pathway Stimulus in Complement-Mediated Thrombotic Microangiopathy.

Complement-mediated thrombotic microangiopathy or hemolytic uremic syndrome (CM-TMA/CM-HUS), previously identified as atypical hemolytic uremic syndrome, is a thrombotic microangiopathy characterized by germline variants or acquired antibodies to complement proteins and regulators. Building upon our prior experience with the modified Ham (mHam) assay for ex vivo diagnosis of complementopathies, we have developed an array of cell-based complement "biosensors' by selective removal of complement regulatory proteins (CD55 and CD59, CD46, or a combination thereof) in an autonomously bioluminescent HEK293 cell line. These biosensors can be used as a sensitive method for diagnosing CM-TMA and monitoring therapeutic complement blockade. Using specific complement pathway inhibitors, this model identifies IgM-driven classical pathway stimulus during both acute disease and in many patients during clinical remission. This provides a potential explanation for ~50% of CM-TMA patients who lack an alternative pathway "driving" variant and suggests at least a subset of CM-TMA is characterized by a breakdown of IgM immunologic tolerance.

Pomalidomide for Epistaxis in Hereditary Hemorrhagic Telangiectasia.

BACKGROUND: Hereditary hemorrhagic telangiectasia (HHT) is characterized by extensive telangiectasias and arteriovenous malformations. The primary clinical manifestation is epistaxis that results in iron-deficiency anemia and reduced health-related quality of life. METHODS: We conducted a randomized, placebo-controlled trial to evaluate the safety and efficacy of pomalidomide for the treatment of HHT. We randomly assigned patients, in a 2:1 ratio, to receive pomalidomide at a dose of 4 mg daily or matching placebo for 24 weeks. The primary outcome was the change from baseline through week 24 in the Epistaxis Severity Score (a validated bleeding score in HHT; range, 0 to 10, with higher scores indicating worse bleeding). A reduction of 0.71 points or more is considered clinically significant. A key secondary outcome was the HHT-specific quality-of-life score (range, 0 to 16, with higher scores indicating more limitations). RESULTS: The trial was closed to enrollment in June 2023 after a planned interim analysis met a prespecified threshold for efficacy. A total of 144 patients underwent randomization; 95 patients were assigned to receive pomalidomide and 49 to receive placebo. The baseline mean (±SD) Epistaxis Severity Score was 5.0±1.5, a finding consistent with moderate-to-severe epistaxis. At 24 weeks, the mean difference between the pomalidomide group and the placebo group in the change from baseline in the Epistaxis Severity Score was -0.94 points (95% confidence interval [CI], -1.57 to -0.31; P = 0.004). The mean difference in the changes in the HHT-specific quality-of-life score between the groups was -1.4 points (95% CI, -2.6 to -0.3). Adverse events that were more common in the pomalidomide group than in the placebo group included neutropenia, constipation, and rash. CONCLUSIONS: Among patients with HHT, pomalidomide treatment resulted in a significant, clinically relevant reduction in epistaxis severity. No unexpected safety signals were identified. (Funded by the National Heart, Lung, and Blood Institute; PATH-HHT Clinicaltrials.gov number, NCT03910244).

Catastrophic antiphospholipid syndrome: challenging case and importance of multidisciplinary evaluation and management.

Our case depicts a challenging diagnosis of catastrophic antiphospholipid syndrome in a young patient with a heterogenous presentation with extensive clinical course, a wide range of investigations, including multimodality imaging, and multidisciplinary expertise, to initiate prompt treatment addressing multiorgan thrombotic injury.

Complement Biosensors Identify a Classical Pathway Stimulus in Complement-Mediated Hemolytic Uremic Syndrome.

Complement-mediated hemolytic uremic syndrome (CM-HUS) is a thrombotic microangiopathy characterized by germline variants or acquired antibodies to complement proteins and regulators. Building upon our prior experience with the modified Ham (mHam) assay for ex vivo diagnosis of complementopathies, we have developed an array of cell-based complement "biosensors'' by selective removal of complement regulatory proteins (CD55 and CD59, CD46, or a combination thereof) in an autonomously bioluminescent HEK293 cell line. These biosensors can be used as a sensitive method for diagnosing CM-HUS and monitoring therapeutic complement blockade. Using specific complement pathway inhibitors, this model identifies IgM-driven classical pathway stimulus during both acute disease and in many patients during clinical remission. This provides a potential explanation for ~50% of CM-HUS patients who lack an alternative pathway "driving" variant and suggests at least a subset of CM-HUS is characterized by a breakdown of IgM immunologic tolerance. Key Points: CM-HUS has a CP stimulus driven by polyreactive IgM, addressing the mystery of why 40% of CM-HUS lack complement specific variantsComplement biosensors and the bioluminescent mHam can be used to aid in diagnosis of CM-HUS and monitor complement inhibitor therapy.

A machine learning approach to predict mortality due to immune-mediated thrombotic thrombocytopenic purpura.

Background: Mortality due to immune-mediated thrombotic thrombocytopenic purpura (iTTP) remains significant. Predicting mortality risk may potentially help individualize treatment. The French Thrombotic Microangiopathy (TMA) Reference Score has not been externally validated in the United States. Recent advances in machine learning technology can help analyze large numbers of variables with complex interactions for the development of prediction models. Objectives: To validate the French TMA Reference Score in the United States Thrombotic Microangiopathy (USTMA) iTTP database and subsequently develop a novel mortality prediction tool, the USTMA TTP Mortality Index. Methods: We analyzed variables available at the time of initial presentation, including demographics, symptoms, and laboratory findings. We developed our model using gradient boosting machine, a machine learning ensemble method based on classification trees, implemented in the R package gbm. Results: In our cohort (n = 419), the French score predicted mortality with an area under the receiver operating characteristic curve of 0.63 (95% CI: 0.50-0.77), sensitivity of 0.35, and specificity of 0.84. Our gradient boosting machine model selected 8 variables to predict acute mortality with a cross-validated area under the receiver operating characteristic curve of 0.77 (95% CI: 0.71-0.82). The 2 cutoffs corresponded to sensitivities of 0.64 and 0.50 and specificities of 0.76 and 0.87, respectively. Conclusion: The USTMA Mortality Index was acceptable for predicting mortality due to acute iTTP in the USTMA registry, but not sensitive enough to rule out death. Identifying patients at high risk of iTTP-related mortality may help individualize care and ultimately improve iTTP survival outcomes. Further studies are needed to provide external validation. Our model is one of many recent examples where machine learning models may show promise in clinical prediction tools in healthcare.

The 2022 review of the 2019 American Society of Hematology guidelines on immune thrombocytopenia.

ABSTRACT: The 2019 American Society of Hematology (ASH) guidelines for immune thrombocytopenia (ITP) included recommendations on the management of adults (recommendations 1-9) and children (recommendations 10-21) with primary ITP . We describe here the results of a review of the 2019 guidelines by a working group of experts requested by ASH to inform decision-making about the need for and timing of a guideline revision. An updated Medline and Embase search applied the same search terms as in the 2019 ASH guidelines, limited to systematic reviews and clinical trials, from May 2017 to July 2022. There were 193 studies identified, 102 underwent abstract reviews, and 54 full reviews. Each study was assessed based on relevance to the previous recommendation with regard to the population, prioritized outcomes, new outcomes, and study design. Reviewers assessed if the data would change the strength or the directionality of the existing recommendation or merit development of a new recommendation. Based on this review, the ASH Committee on Quality endorsed a focused update on second-line management for adults with ITP. In addition, there will be continued annual monitoring and reviewing of the 2019 ASH guidelines on ITP in full to evaluate when there is sufficient new evidence to warrant additional revisions.

Dysregulated platelet function in patients with postacute sequelae of COVID-19.

BACKGROUND: Postacute sequelae of COVID-19 (PASC), also referred to as "Long COVID", sometimes follows COVID-19, a disease caused by SARS-CoV-2. Although SARS-CoV-2 is well known to promote a prothrombotic state, less is known about the thrombosis risk in PASC. Our objective was to evaluate platelet function and thrombotic potential in patients following recovery from SARS-CoV-2, but with clear symptoms of patients with PASC. METHODS: patients with PASC and matched healthy controls were enrolled in the study on average 15 months after documented SARS-CoV-2 infection. Platelet activation was evaluated by light transmission aggregometry (LTA) and flow cytometry in response to platelet surface receptor agonists. Thrombosis in platelet-deplete plasma was evaluated by Factor Xa activity. A microfluidics system assessed thrombosis in whole blood under shear stress conditions. RESULTS: A mild increase in platelet aggregation in patients with PASC through the thromboxane receptor was observed, and platelet activation through the glycoprotein VI (GPVI) receptor was decreased in patients with PASC compared to age- and sex-matched healthy controls. Thrombosis under shear conditions as well as Factor Xa activity were reduced in patients with PASC. Plasma from patients with PASC was an extremely potent activator of washed, healthy platelets - a phenomenon not observed when stimulating healthy platelets after incubation with plasma from healthy individuals. CONCLUSIONS: patients with PASC show dysregulated responses in platelets and coagulation in plasma, likely caused by a circulating molecule that promotes thrombosis. A hitherto undescribed protective response appears to exist in patients with PASC to counterbalance ongoing thrombosis that is common to SARS-CoV-2 infection.

High molecular weight kininogen interactions with the homologs prekallikrein and factor XI: importance to surface-induced coagulation.

BACKGROUND: In plasma, high molecular weight kininogen (HK) is either free or bound to prekallikrein (PK) or factor (F) XI (FXI). During contact activation, HK is thought to anchor PK and FXI to surfaces, facilitating their conversion to the proteases plasma kallikrein and FXIa. Mice lacking HK have normal hemostasis but are resistant to injury-induced arterial thrombosis. OBJECTIVES: To identify amino acids on the HK-D6 domain involved in PK and FXI binding and study the importance of the HK-PK and HK-FXI interactions to coagulation. METHODS: Twenty-four HK variants with alanine replacements spanning residues 542-613 were tested in PK/FXI binding and activated partial thromboplastin time clotting assays. Surface-induced FXI and PK activation in plasma were studied in the presence or absence of HK. Kng1-/- mice lacking HK were supplemented with human or murine HK and tested in an arterial thrombosis model. RESULTS: Overlapping binding sites for PK and FXI were identified in the HK-D6 domain. HK variants with defects only in FXI binding corrected the activated partial thromboplastin time of HK-deficient plasma poorly compared to a variant defective only in PK-binding. In plasma, HK deficiency appeared to have a greater deleterious effect on FXI activation than PK activation. Human HK corrected the defect in arterial thrombus formation in HK-deficient mice poorly due to a specific defect in binding to mouse FXI. CONCLUSION: Clinical observations indicate FXI is required for hemostasis, while HK is not. Yet, the HK-FXI interaction is required for contact activation-induced clotting in vitro and in vivo suggesting an important role in thrombosis and perhaps other FXI-related activities.

Dysregulated Platelet Function in Patients with Post-Acute Sequelae of COVID-19.

Background: Post-acute sequelae of COVID-19 (PASC), also referred as Long-COVID, sometimes follows COVID-19, a disease caused by SARS-CoV-2. While SARS-CoV-2 is well-known to promote a prothrombotic state, less is known about the thrombosis risk in PASC. Aim: Our objective was to evaluate the platelet function and thrombotic potential in patients following recovery from SARS-CoV-2 with clear symptoms of PASC. Methods: PASC patients and matched healthy controls were enrolled in the study on average 15 months after documented SARS-CoV-2 infection. Platelet activation was evaluated by Light Transmission Aggregometry (LTA) and flow cytometry in response to platelet surface receptor agonists. Thrombosis in platelet-deplete plasma was evaluated by Factor Xa activity. A microfluidics system assessed thrombosis in whole blood under shear stress conditions. Results: A mild increase in platelet aggregation in PASC patients through the thromboxane receptor was observed and platelet activation through the glycoprotein VI (GPVI) receptor was decreased in PASC patients compared to age- and sex-matched healthy controls. Thrombosis under shear conditions as well as Factor Xa activity were reduced in PASC patients. Plasma from PASC patients was an extremely potent activator of washed, healthy platelets - a phenomenon not observed when stimulating healthy platelets after incubation with plasma from healthy individuals. Conclusions: PASC patients show dysregulated responses in platelets and coagulation in plasma, likely caused by a circulating molecule that promotes thrombosis. A hitherto undescribed protective response appears to exists in PASC patients to counterbalance ongoing thrombosis that is common to SARS-CoV-2 infection.

How to diagnose and manage antiphospholipid syndrome.

Antiphospholipid antibodies (aPL) are autoimmune antibodies directed toward phospholipids or phospholipid-protein complexes, particularly those containing ß2-glycoprotein I (ß2GPI). Persistently positive aPL accompanied by arterial or venous thrombosis, or recurrent pregnancy loss, constitutes the antiphospholipid syndrome (APS). Several types of aPL with different specificities have been defined and may be detected in the clinical lab, including lupus anticoagulants (detected using clotting assays) and anticardiolipin, anti-ß2GPI and anti-prothrombin/phosphatidylserine antibodies (detected by ELISA); each of the last 3 aPL may be either IgG, IgM, or IgA, though IgA antibodies are not included in criteria for APS. Due to the relative rarity of APS and the heterogeneity of aPL, thrombosis risk stratification is challenging, and randomized clinical trials for thrombosis treatment and prevention have been limited. This lack of high-quality data has made the clinical management of APS difficult, and existing guidelines are few and could not possibly cover many of the scenarios encountered in managing patients with APS. In this review, we present 3 patients with aPL and/or APS who highlight treatment dilemmas, and we discuss background information that may help guide clinical judgment in developing individualized treatment plans for patients with these enigmatic antibodies.

Factor XII promotes the thromboinflammatory response in a rat model of venoarterial extracorporeal membrane oxygenation.

BACKGROUND: Factor XII (FXII) is a multifunctional protease capable of activating thrombotic and inflammatory pathways. FXII has been linked to thrombosis in extracorporeal membrane oxygenation (ECMO), but the role of FXII in ECMO-induced inflammatory complications has not been studied. We used novel gene-targeted FXII- deficient rats to evaluate the role of FXII in ECMO-induced thromboinflammation. METHODS: FXII-deficient (FXII-/-) Sprague-Dawley rats were generated using CRISPR/Cas9. A minimally invasive venoarterial (VA) ECMO model was used to compare wild-type (WT) and FXII-/- rats in 2 separate experimental cohorts: rats placed on ECMO without pharmacologic anticoagulation and rats anticoagulated with argatroban. Rats were maintained on ECMO for 1 hour or until circuit failure occurred. Comparisons were made with unchallenged rats and rats that underwent a sham surgical procedure without ECMO. RESULTS: FXII-/- rats were maintained on ECMO without pharmacologic anticoagulation with low resistance throughout the 1-hour experiment. In contrast, WT rats placed on ECMO without anticoagulation developed thrombotic circuit failure within 10 minutes. Argatroban provided a means to maintain WT and FXII-/- rats on ECMO for the 1-hour time frame without thrombotic complications. Analyses of these rats demonstrated that ECMO resulted in increased neutrophil migration into the liver that was significantly blunted by FXII deficiency. ECMO also resulted in increases in high molecular weight kininogen cleavage and complement activation that were abrogated by genetic deletion of FXII. CONCLUSIONS: FXII initiates hemostatic system activation and key inflammatory sequelae in ECMO, suggesting that therapies targeting FXII could limit both thromboembolism and inopportune inflammatory complications in this setting.

Protection of ß2GPI Deficient Mice from Thrombosis Reflects a Defect in PAR3-facilitated Platelet Activation.

Background: Antibodies to ß2-glycoprotein I (ß2GPI) cause thrombosis in antiphospholipid syndrome, however the role of ß2GPI itself in regulation of coagulation pathways in vivo is not well understood. Methods: We developed ß2GPI-deficient mice (Apoh -/- ) by deleting exon 2 and 3 of Apoh using CRISPR/Cas9 and compared the propensity of wild-type (WT) and Apoh -/- mice to develop thrombosis using rose bengal and FeCl 3 -induced carotid thrombosis, laser-induced cremaster arteriolar injury, and inferior vena cava (IVC) stasis models. We also compared tail bleeding times and assessed platelet activation in WT and Apoh -/- mice in the absence and presence of exogenous ß2GPI. Results: Compared to WT littermates, Apoh -/- mice demonstrated a prolonged time to occlusion of the carotid artery after exposure to rose bengal or FeCl 3 , and reduced platelet and fibrin accumulation in cremasteric arterioles after laser injury. Similarly, significantly smaller thrombi were retrieved from the IVC of Apoh -/- mice 48 hours after IVC occlusion. The activated partial thromboplastin time (aPTT) and prothrombin time, as well as aPTT reagent- and tissue factor-induced thrombin generation times using plasma from Apoh -/- and WT mice revealed no differences. However, we observed significant prolongation of tail bleeding in Apoh -/- mice, and reduced P-selectin expression and binding of fibrinogen to the activated α2bß3 integrin on platelets from these mice after stimulation with low thrombin concentrations; these changes were reversed by exogenous ß2GPI. An antibody to PAR3 blocked thrombin-induced activation of WT, but not Apoh -/- platelets, as well as the ability of ß2GPI to restore the activation response of Apoh -/- platelets to thrombin. ß2GPI deficiency did not affect platelet activation by a PAR4-activator peptide, or ADP. Conclusions: In mice, ß2GPI may mediate procoagulant activity by enhancing the ability of PAR3 to present thrombin to PAR4, promoting platelet activation at low thrombin concentrations. Key Points: ß2GPI deficient mice are protected from experimental arterial, venous, and microvascular thrombosis.ß2GPI deficient mice display prolonged tail bleeding times and reduced PAR3-facilitated platelet activation by low concentrations of thrombin.

C1 inhibitor deficiency enhances contact pathway-mediated activation of coagulation and venous thrombosis.

C1 inhibitor (C1INH) is a multifunctional serine protease inhibitor that functions as a major negative regulator of several biological pathways, including the contact pathway of blood coagulation. In humans, congenital C1INH deficiency results in a rare episodic bradykinin-mediated swelling disorder called hereditary angioedema (HAE). Patients with C1INH deficiency-associated HAE (C1INH-HAE) have increased circulating markers of activation of coagulation. Furthermore, we recently reported that patients with C1INH-HAE had a moderate but significant increased risk of venous thromboembolism. To further investigate the impact of C1INH deficiency on activation of coagulation and thrombosis, we conducted studies using patient samples and mouse models. Plasmas from patients with C1INH-HAE had significantly increased contact pathway-mediated thrombin generation. C1INH-deficient mice, which have been used as a model of C1INH-HAE, had significantly increased baseline circulating levels of prothrombin fragment 1+2 and thrombin-antithrombin complexes. In addition, whole blood from C1INH-deficient mice supported significantly increased contact pathway-mediated thrombin generation. Importantly, C1INH-deficient mice exhibited significantly enhanced venous, but not arterial, thrombus formation. Furthermore, purified human C1INH normalized contact pathway-mediated thrombin generation and venous thrombosis in C1INH-deficient mice. These findings highlight a key role for endogenous C1INH as a negative regulator of contact pathway-mediated coagulation in humans and mice. Further, this work identifies endogenous C1INH as an important negative regulator of venous thrombus formation in mice, complementing the phenotype associated with C1INH-HAE.

Anti-HK antibody inhibits the plasma contact system by blocking prekallikrein and factor XI activation in vivo.

A dysregulated plasma contact system is involved in various pathological conditions, such as hereditary angioedema, Alzheimer disease, and sepsis. We previously showed that the 3E8 anti-high molecular weight kininogen (anti-HK) antibody blocks HK cleavage and bradykinin generation in human plasma ex vivo. Here, we show that 3E8 prevented not only HK cleavage but also factor XI (FXI) and prekallikrein (PK) activation by blocking their binding to HK in mouse plasma in vivo. 3E8 also inhibited contact system-induced bradykinin generation in vivo. Interestingly, FXII activation was also inhibited, likely because of the ability of 3E8 to block the positive feedback activation of FXII by kallikrein (PKa). In human plasma, 3E8 also blocked PK and FXI binding to HK and inhibited both thrombotic (FXI activation) and inflammatory pathways (PK activation and HK cleavage) of the plasma contact system activation ex vivo. Moreover, 3E8 blocked PKa binding to HK and dose-dependently inhibited PKa cleavage of HK. Our results reveal a novel strategy to inhibit contact system activation in vivo, which may provide an effective method to treat human diseases involving contact system dysregulation.

Emerging Therapies in Antiphospholipid Syndrome.

The antiphospholipid syndrome (APS) is the most common cause of acquired immune-mediated thrombophilia. This syndrome is broadly defined by the presence of arterial or venous thrombosis, or pregnancy morbidity, in the presence of high levels of antiphospholipid antibodies. Despite recognition of this disorder more than 50 years ago, a fundamental unifying pathogenesis has not been determined. Due to this, mechanism-based therapies for APS are not available, and current management following thrombotic events suggests anticoagulation of indeterminate duration, or for obstetric complications, heparin/low molecular weight heparin and aspirin. However, APS is an autoimmune disorder, and several approaches focused on modulating the immune response or its effectors have been employed. Those which have been most extensively studied include hydroxychloroquine, rituximab and eculizumab, an inhibitor of complement C5. In this report, we review in depth, and critique, key clinical studies of these agents. Since all of these studies are small, our conclusions are qualified. However, it appears that hydroxychloroquine may enhance the anticoagulant efficacy of vitamin K antagonists in APS patients, and that rituximab may ameliorate some of the "non-criteria" manifestations of APS. The catastrophic antiphospholipid syndrome (CAPS) is associated with diffuse thrombosis, multi-organ dysfunction, and ∼30% mortality. A high incidence of complement regulatory gene mutations, and compelling data concerning the efficacy of eculizumab in CAPS, suggests an important role for complement in this disorder. However, additional work is needed to clarify the role of complement in non-catastrophic APS, though emerging data suggests that complement inhibition may be effective in preventing thrombosis in these patients as well.

Old is new again: emergence of thromboembolic complications in cancer patients on immunotherapy.

Cancer immunotherapy has emerged as one of the most important new treatments for cancer in many years, moving rapidly to front-line therapy for several cancers. Cancer immunotherapy is based on treatment with immune checkpoint inhibitors (ICI), which are monoclonal antibodies directed toward immunoregulatory proteins including PD-1, PD-L1 and CTLA-4. ICI inhibit interactions between these proteins and their ligands, disabling physiologic immune regulatory networks and enhancing anti-tumor immunity. However, since the immune response cannot be directed specifically to the tumor, ICI are associated with immune-related adverse events (irAEs) resulting from immune-mediated attack of normal tissues. We and others have reported a high incidence of thrombosis in patients treated with ICI, which may approach 20%. Given the rapidly increasing use of ICIs, it is clear that ICI-Associated Thrombosis (IAT) is a major emerging clinical problem. However, there is a remarkable knowledge gap concerning mechanisms of IAT. IAT may be a composite irAE resulting from activation of blood and vascular cells, leading to thromboinflammation. Cancer itself is an inflammatory disorder, and inducing further inflammation through ICI administration may stimulate procoagulant activity by multiple cell types. Moreover, some blood and vascular cells express ICI target proteins. Here, we review the results of several studies describing the clinical manifestations of IAT, as well as our recent studies demonstrating that elevated levels of myeloid derived suppressor cells and inflammatory cytokines may serve as biomarkers of IAT. It is hoped that the concepts reviewed here may stimulate further research into this important clinical problem.

Race, rituximab, and relapse in TTP.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is characterized by recurring episodes of thrombotic microangiopathy, causing ischemic organ impairment. Black patients are overrepresented in iTTP cohorts in the United States, but racial disparities in iTTP outcome and response to therapy have not been studied. Using the United States Thrombotic Microangiopathies Consortium iTTP Registry, we evaluated the impact of race on mortality and relapse-free survival (RFS) in confirmed iTTP in the United States from 1995 to 2020. We separately examined the impact of rituximab therapy and presentation with newly diagnosed (de novo) or relapsed iTTP on RFS by race. A total of 645 participants with 1308 iTTP episodes were available for analysis. Acute iTTP mortality did not differ by race. When all episodes of iTTP were included, Black race was associated with shorter RFS (hazard ratio [HR], 1.60; 95% CI, 1.16-2.21); the addition of rituximab to corticosteroids improved RFS in White (HR, 0.37; 95% CI, 0.18-0.73) but not Black patients (HR, 0.96; 95% CI, 0.71-1.31). In de novo iTTP, rituximab delayed relapse, but Black patients had shorter RFS than White patients, regardless of treatment. In relapsed iTTP, rituximab significantly improved RFS in White but not Black patients. Race affects overall relapse risk and response to rituximab in iTTP. Black patients may require closer monitoring, earlier retreatment, and alternative immunosuppression after rituximab treatment. How race, racism, and social determinants of health contribute to the disparity in relapse risk in iTTP deserves further study.