Diagnosis and treatment of transplantation-associated thrombotic microangiopathy: real progress or are we still waiting?

Neurological involvement in hematopoietic stem cell transplantation-associated thrombotic microangiopathy

Transplantation-associated thrombotic microangiopathy (TA-TMA) is a well-recognized serious complication of hematopoietic stem cell transplantation (HSCT). The understanding of TA-TMA pathophysiology has expanded in recent years. Dysregulation of the complement system is thought to cause endothelial injury and, consequently, microvascular thrombosis and tissue damage. TA-TMA can affect multiple organs, and each organ exhibits specific features of injury. Central nervous system (CNS) manifestations of TA-TMA include posterior reversible encephalopathy syndrome, seizures, and encephalopathy. The development of neurological dysfunction is associated with a significantly lower overall survival in patients with TA-TMA. However, there are currently no established histopathological or radiological criteria for the diagnosis of CNS TMA. Patients who receive total body irradiation (TBI), calcineurin inhibitors (CNI), and severe acute and chronic graft-versus-host disease (GVHD) are at a high risk of experiencing neurological complications related to TA-TMA and should be considered for directed TA-TMA therapy. However, the incidence and clinical manifestations of TA-TMA neurotoxicity remain unclear. Studies specifically examining the involvement of CNS in TMA syndromes are limited. In this review, we discuss clinical manifestations and imaging abnormalities in patients with nervous system involvement in TA-TMA. We summarize the mechanisms underlying TA-TMA and its neurological complications, including endothelial injury, evidence of complement activation, and treatment options for TA-TMA.

Development and implementation of evidence-based, nurse-leading early warning model and healthcare quality improvement project for transplant-associated thrombotic microangiopathy: a mixed-methods, before-and-after study

OBJECTIVE

The early identification and diagnosis of transplant-associated thrombotic microangiopathy (TA-TMA) are essential yet difficult in patients underwent hematopoietic stem cell transplantation (HSCT). To develop an evidence-based, nurse-leading early warning model for TA-TMA, and implement the healthcare quality review and improvement project.

METHODS

This study was a mixed-methods, before-and-after study. The early warning model was developed based on quality evidence from literature search. The healthcare quality review and improvement project mainly included baseline investigation of nurse, improvement action and effectiveness evaluation. The awareness and knowledge of early parameter of TA-TMA among nurses and the prognosis of patients underwent HSCT were compared before and after the improvement.

RESULTS

A total of 1 guideline, 1 evidence synthesis, 4 expert consensuses, 10 literature reviews, 2 diagnostic studies, and 9 case series were included in the best evidence. The early warning model including warning period, high-risk characteristics and early manifestation of TA-TMA was developed. The improvement action, including staff training and assessment, suspected TA-TMA identification and patient education, was implemented. The awareness and knowledge rate of early parameter of TA-TMA among nurses significantly improved after improvement action (100% vs. 26.7%, P < 0.001). The incidence of TA-TMA was similar among patients underwent HSCT before and after improvement action (2.8% vs. 1.2%, P = 0.643), while no fall event occurred after improvement action (0 vs. 1.2%, P < 0.001).

CONCLUSION

The evidence-based early warning model and healthcare quality improvement project could enhance the awareness and knowledge of TA-TMA among healthcare providers and might improve the prognosis of patients diagnosed with TA-TMA.

Transplant-associated thrombotic microangiopathy in pediatrics: incidence, risk factors, therapeutic options, and outcome based on data from a single center

Background

Transplant-associated thrombotic microangiopathy (TA-TMA) is a critical complication of hematopoietic stem cell transplantation. Awareness about TA-TMA has increased in recent years, resulting in the implementation of TA-TMA screening in most centers.

Methods

Retrospective analysis of children who underwent autologous or allogeneic hematopoietic stem cell transplantation at our center between January 2018 and December 2022 was conducted to evaluate the incidence, clinical features, and outcomes of TA-TMA following the administration of different therapeutic options.

Results

A total of 45 patients comprised the study cohort, of whom 10 developed TA-TMA with a cumulative incidence of 22% by 100 days after transplantation. Patients with and without TA-TMA in our cohort displayed an overall survival of 80% and 88%, respectively (p = 0.48), and a non-relapse mortality of 0% and 5.7%, respectively (p = 0.12), at 1 year after transplantation. Risk factors for TA-TMA development included allogeneic transplantation and total body irradiation-based conditioning regime. Among the 10 patients with TA-TMA, 7 did not meet the high-risk criteria described by Jodele and colleagues. Of these seven patients, two responded to calcineurin-inhibitor withdrawal without further therapy and five developed multiorgan dysfunction syndrome and were treated with anti-inflammatory steroids (prednisone), and all responded to therapy. The three patients with high-risk TA-TMA were treated with complement blockade or prednisone, and all responded to therapy.

Conclusion

TA-TMA is a multifactorial complication with high morbidity rates. Patients with high-risk TA-TMA may benefit from complement blockade using eculizumab. No consensus has been reached regarding therapy for patients who do not meet high-risk criteria. Our analysis showed that these patients may respond to anti-inflammatory treatment with prednisone.

Excessive cleavage of von Willebrand factor multimers by ADAMTS13 may predict the progression of transplant-associated thrombotic microangiopathy

Background

Transplant-associated thrombotic microangiopathy (TA-TMA) is a fatal complication of hematopoietic stem cell transplantation and is characterized by severe thrombocytopenia, hemolytic anemia, and organ dysfunction. In response to several possible triggers, dynamic multimetric change in von Willebrand factor (VWF) may contribute to inducing microthrombi in circulation in TA-TMA.

Objectives

By performing VWF multimer analysis and measuring VWF-degradation product (DP), we unraveled the relationship between multimeric changes in circulating VWF and the pathogenesis of TA-TMA.

Methods

This study analyzed 135 plasma samples from 14 patients who underwent allogeneic hematopoietic stem cell transplantation at a single institute. VWF-associated markers, namely VWF:antigen (VWF:Ag), VWF-DP/VWF:Ag ratio, VWF:ristocetin cofactor activity, VWF:ristocetin cofactor activity/VWF:Ag ratio, and ADAMTS13 activity, were analyzed in these samples collected every 7 days.

Results

There were 2 patients with definite thrombotic microangiopathy (TMA) and 6 patients who presented with probable TMA that did not progress to definite TMA. Each plasma sample was classified into 3 groups: definite TMA, probable TMA, and non-TMA. VWF multimer analysis showed the absence of high-molecular-weight VWF multimers in probable TMA, whereas the appearance of unusually large VWF multimers was observed in definite TMA. The median value of the VWF-DP/VWF:Ag ratio in probable TMA was elevated to 4.17, suggesting that excessive cleavage of VWF multimers by VWF cleaving enzyme, ADAMTS13, resulted in the loss of high-molecular-weight VWF multimers.

Conclusion

During the transition from probable to definite TMA, drastic VWF multimer changes imply a switch from bleeding to thrombotic tendencies. Extensive VWF-DP and VWF multimer analyses provided novel insights.

A Fatal Case of Neuroblastoma Complicated by Posterior Reversible Encephalopathy with Rapidly Evolving Transplantation-Associated Thrombotic Microangiopathy

Background: Transplantation-associated thrombotic microangiopathy (TA-TMA) is a severe complication of hematopoietic stem cell transplantation and is sometimes fatal. Observations: A 4-year-old, male patient with stage M neuroblastoma (NBL) who had received an allogeneic bone marrow transplantation (BMT) from his sibling five months previously presented with rapidly progressive posterior reversible encephalopathy (PRES) complicated with TA-TMA. Although the patient was transferred to the pediatric intensive care unit, he died within one week after the onset of the latest symptoms. Conclusions: This is the first description of a fatal case of NBL complicated by PRES with rapidly evolving TA-TMA after an allogenic BMT.

Management of drug-induced neutropenia, thrombocytopenia, and anaemia after solid organ transplantation: A comprehensive review

WHAT IS KNOWN AND OBJECTIVE

Advances in the development of more effective immunosuppressive drugs have increased graft survival and drug induced adverse effects. Haematological complications including neutropenia, thrombocytopenia, and anaemia are common side effects that affect the grafts' and patients' outcomes. Several studies have stated the important role of various medications in haematological complications after transplantation. They have reported the incidence and different mechanisms of drug induced cytopenia, as well as an overview of possible treatment modalities. However, there is no comprehensive protocol for the management of these complications following transplantation. This narrative review was performed to develop a comprehensive practical approach for management of drug induced haematological complications following solid organ transplantation.

METHOD

PubMed, Embase, Cochrane library, Web of Science, and Google scholar databases were searched without time limitations until March, 2021. In addition, some valid drug information data bases (Uptodate and Micromedex) were searched for detailed information until October, 2021.

RESULTS AND DISCUSSION

Several immunosuppressive and antimicrobial medications may induce neutropenia, thrombocytopenia or anaemia following transplantation. Most of these agents cause dose-related cytopenia, which resolves with dose reduction or drug withdrawal. However, any change in medications may result in negative consequences such as severe infections, bleeding, cardiovascular complications, acute allograft rejection, and graft or patient loss. Thus, cautious evaluation of the patient's condition and the pharmacological properties of the culprit medication are required.

WHAT IS NEW AND CONCLUSION

Three algorithms are presented to guide healthcare providers in the stepwise management of drug-induced neutropenia, thrombocytopenia, and anaemia after solid organ transplantation.

Epidemiology and outcomes of pediatric transplant-associated thrombotic microangiopathy in Hong Kong

BACKGROUND

Transplant-associated thrombotic microangiopathy (TA-TMA) is an under-recognized yet potentially devastating complication of hematopoietic stem cell transplantation (HSCT) which had increased awareness in recent years. This report summarizes the demographics and outcomes of pediatric TA-TMA in Hong Kong.

METHODS

All patients aged below 18 years who underwent HSCT in the Hong Kong Children's Hospital and were diagnosed to have TA-TMA during the 2-year period from April 1, 2019 to March 31, 2021 were included.

RESULTS

A total of 73 transplants (51 allogeneic and 22 autologous) in 63 patients had been performed. Six patients (four males and two females) developed TA-TMA at a median duration of 2.5 months post-HSCT. The incidence rate was 9.52%. Of the six TA-TMA patients, five underwent allogenic one underwent autologous HSCT, respectively. Three of them were histologically proven. All four patients with cyclosporine had stopped the drug once TA-TMA was suspected. Median six doses of eculizumab were administered to five out of six patients. Three patients died (two due to fungal infection and one due to acute-on-chronic renal failure) within 3 months upon diagnosis of TA-TMA. Among three survivors, two stabilized with mild stage 2 chronic kidney disease (CKD) while the other suffered from stage 5 end-stage CKD requiring lifelong dialysis.

CONCLUSION

In conclusion, recognition and diagnosis of TA-TMA are challenging. Early recognition and prompt administration of complement blockage with eculizumab may be beneficial in selected cases. Further prospective research studies are recommended to improve the management and outcomes of TA-TMA.

N-Acetylcysteine as Prophylactic Therapy for Transplantation-Associated Thrombotic Microangiopathy: A Randomized, Placebo-Controlled Trial

Transplantation-associated thrombotic microangiopathy (TA-TMA) is a life-threatening complication for patients undergoing hematopoietic stem cell transplantation (HSCT). N-acetylcysteine (NAC) has recently been considered as a potential treatment for patients with thrombotic thrombocytopenic purpura. To assess the value of NAC for the prevention of TA-TMA, we conducted a prospective study at the First Affiliated Hospital of Soochow University. This open-label, randomized placebo-controlled trial included 160 patients who were scheduled for allogeneic HSCT. Participants were assigned at random 1:1 to either oral NAC (50 mg/kg/day from 9 days before HSCT to 30 days after HSCT) or placebo treatment. The primary outcome was the incidence of TA-TMA. Overall survival (OS) and event-free survival (EFS) were assessed in the NAC and placebo control groups. The incidence of TA-TMA was 9.1% (95% confidence interval [CI], 2% to 16.2%) in the NAC group, compared with 23% (95% CI, 13.2% to 32.8%) in the control group, with a rate ratio of .34 (95% CI, .123 to .911; P = .039). The median time to the onset of TA-TMA was 60 days (interquartile range [IQR], 42 to 129 days) in the NAC group and 36 days (IQR, 30.5 to 51 days) in the control group (P = .063). The 2-year OS rate was 75.4% (95% CI, 28.65% to 73.53%) in the NAC group and 63.0% (95% CI, 50.8% to 73.5%) in the control group, with a hazard ratio (HR) of .622 (95% CI, .334-1.155; P = .132). The EFS rate was 25.8% in the NAC patients and 8.1% in controls (HR, .254; 95% CI, .094 to .692; P = .024). The median time of EFS was 60 days in the NAC group and 38 days in controls. Our findings suggest that NAC may be a potential treatment to reduce the incidence of TA-TMA.

Early vascular toxicity after pediatric allogeneic hematopoietic stem cell transplantation

Treatment-related mortality and morbidity remain a challenge in hematopoietic stem cell transplantation (HSCT). In this retrospective, single-center study, we analyzed endothelial damage as a potential, common denominator and mechanism for the adverse effects. We evaluated the prevalence of key vascular complications and graft-versus-host disease among 122 pediatric patients with an allogeneic HSCT between 2001 and 2013. The spectrum and frequency of acute adverse events emerging ≤100 days post transplant were graded according to the CTCAE 4.03 and analyzed. We identified a total of 19/122 (15.6%) patients with vascular complications, fulfilling the criteria of capillary leak syndrome, veno-occlusive disease/sinusoidal obstruction syndrome or thrombotic microangiopathy. The patients had a poorer overall survival (77% versus 26%, p < 0.001). Nearly one half (56/122, 45.9%) had at least one, severe (grade 3 or 4) adverse event. Patients with vascular complications had more often edema/effusions (p = 0.023), thrombocytopenia (p = 0.001), gastrointestinal bleeding (p < 0.001), acute kidney injury (p < 0.001), ascites (p < 0.001) or bilirubin increase (p = 0.027). These endotheliopathy-related adverse events appeared early post HSCT, varied in their clinical phenotype and predicted a poor outcome. An unrelated donor but not previous exposure to leukemia or irradiation-based conditioning was identified as a risk factor for vascular complications and endotheliopathy.

Rapid Improvement in Jaundice Using Transdermal Isosorbide Tape as a Nitric Oxide Donor in Two Adult Patients with Transplantation-associated Microangiopathy Related to Graft-versus-host Disease

Two adult patients with acute leukemia developed transplantation-associated microangiopathy (TAM) related to graft-versus-host disease (GVHD). Both patients were resistant to standard therapy for TAM and GVHD, which led to markedly elevated serum total bilirubin levels of 47.5 and 10.6 mg/dL, respectively. Transdermal isosorbide tape as a nitric oxide donor was applied to Patients 1 and 2 on post-transplantation days 60 and 66, respectively, which rapidly improved their jaundice after 1 day. This is the first report to describe the efficacy of transdermal isosorbide tape for adult patients with jaundice associated with TAM related to GVHD.

Treatment outcome and efficacy of therapeutic plasma exchange for transplant-associated thrombotic microangiopathy in a large real-world cohort study

The efficacy and outcome of therapeutic plasma exchange (TPE) for transplant-associated thrombotic microangiopathy (TA-TMA) remain controversial. We therefore sought to evaluate the outcome and efficacy of TPE in patients with TA-TMA and to identify TA-TMA patients who would benefit from TPE management. Eighty-two patients with TA-TMA were treated with TPE. We reported a response rate of 52% and overall survival rates of 20% and 15% at 100 days and 1 year after TA-TMA, respectively, in TPE-treated patients, with a significantly lower survival in gastrointestinal (GI) bleeding patients (5% vs. 41% in non-GI bleeding patients, P = 0.003). Multivariate analysis revealed that patients with GI bleeding, grade III-IV aGVHD, severe anemia, and a lower cumulative volume of TPE were less likely to respond to TPE. GI bleeding, a lower initial volume of TPE, and elevated total bilirubin were independently associated with 100-day mortality. The leading causes of death were infection, active TA-TMA, and MODS. The results of this large cohort of real-world practice indicate that the efficacy and outcome of TPE for TA-TMA patients without GI bleeding are encouraging, and a higher volume of TPE is warranted to achieve favorable outcomes.

Endothelial Dysfunction after Hematopoietic Stem Cell Transplantation: A Review Based on Physiopathology

Endothelial dysfunction (ED) is frequently encountered in transplant medicine. ED is an argument of high complexity, and its understanding requires a wide spectrum of knowledge based on many fields of basic sciences such as molecular biology, immunology, and pathology. After hematopoietic stem cell transplantation (HSCT), ED participates in the pathogenesis of various complications such as sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD), graft-versus-host disease (GVHD), transplant-associated thrombotic microangiopathy (TA-TMA), idiopathic pneumonia syndrome (IPS), capillary leak syndrome (CLS), and engraftment syndrome (ES). In the first part of the present manuscript, we briefly review some biological aspects of factors involved in ED: adhesion molecules, cytokines, Toll-like receptors, complement, angiopoietin-1, angiopoietin-2, thrombomodulin, high-mobility group B-1 protein, nitric oxide, glycocalyx, coagulation cascade. In the second part, we review the abnormalities of these factors found in the ED complications associated with HSCT. In the third part, a review of agents used in the treatment of ED after HSCT is presented.

Case Report: Successful Treatment With Anti-C5 Monoclonal Antibody in a Japanese Adolescent Who Developed Thrombotic Microangiopathy After Autologous Bone Marrow Transplantation for Malignant Lymphoma

BACKGROUND

Transplant-associated thrombotic microangiopathy (TA-TMA) is a serious complication of bone marrow transplantation (BMT). Recently, abnormalities in the complement system have been identified in the pathogenesis of TA-TMA, and there are series of reports stating that anti-C5 monoclonal antibody (eculizumab) is effective in patients with high levels of the membrane attack complex (C5b-9).

CASE PRESENTATION

A 12-year-old boy underwent autologous BMT after receiving high-dose chemotherapy for malignant lymphoma. The patient was engrafted on day 19 after transplantation; however, hemolytic anemia and non-immune thrombocytopenia persisted, and haptoglobin decreased on day 46. Moreover, on day 83, the patient developed pulmonary hemorrhage, hypertension, severe proteinuria, hematuria, and acute kidney injury (AKI). Pulmonary bleeding stopped with daily platelet transfusion and hemostatic agents, but reappeared on day 100. Based on the presence of destruction of red blood cells, elevated lactate dehydrogenase levels, negative direct and indirect Coombs tests, normal ADAMTS13 levels, hemolytic anemia, non-immune thrombocytopenia, and AKI, the patient was diagnosed with systemic TA-TMA and we initiated plasma exchange (PE) and continuous hemodialysis for AKI. High C5b-9 levels were identified at the start of the series of PE, therefore we decided to administer eculizumab. After three courses of eculizumab, no pulmonary hemorrhage was observed, and anemia, thrombocytopenia, renal dysfunction, hematuria, and proteinuria all tended to improve. Three years after transplantation, the patient is alive and does not require eculizumab.

DISCUSSION

Eculizumab is a humanized monoclonal antibody that binds complement protein C5, preventing cleavage C5 and the formation of C5b-9. In this case, TA-TMA could not be controlled with PE alone. We therefore decided to use eculizumab relatively early based on the high C5b-9 level and could resolve the momentum of TA-TMA.

CONCLUSION

In previous reports, TA-TMA typically occurred in early post-allogeneic BMT of patients with lymphoma or in post-autologous BMT of patients with neuroblastoma and was treated with eculizumab. We here reported that eculizumab could be successful treatment for TA-TMA in post-autologous BMT of patient with lymphoma.

Acute GVHD, BK hemorrhagic cystitis and age are risk factors for transplant-associated thrombotic microangiopathy in adults

Clinically actionable thrombotic microangiopathy (CA-TMA) occurring in the absence of aGVHD occurs earlier than day 100.

aGVHD is a key and consistent risk factor for late-onset CA-TMA.

Hematopoietic cell transplantation–associated thrombotic microangiopathy (TMA) is a complication associated with higher nonrelapse mortality (NRM) in patients who undergo allogeneic transplant (HCT). Current classification criteria are not generally agreed on or validated, and the presence of confounding factors after transplant contribute to underdiagnosis or delayed diagnosis of TMA. We studied risk factors, incidence, and biomarkers of TMA in 119 adult allogeneic HCT recipients. Twenty-seven patients developed a clinically actionable phenotype of TMA (CA-TMA) and the incidence of CA-TMA was 22% by day 180. Among the 27 patients who developed CA-TMA, 10 developed it before the onset of acute graft-versus-host disease (aGVHD), and 17 patients developed it after the onset of aGVHD. We report for the first time that age >50 years, BK hemorrhagic cystitis, and other viral infections (CMV, HHV-6, or adenovirus) are risk factors for adult CA-TMA. Even after adjustment for aGVHD, CA-TMA was independently associated with significantly higher NRM. These data illustrate relationships between CA-TMA and aGVHD, describe new risk factors for CA-TMA and emphasizes the need to develop validated set of criteria for timely diagnosis.

Management of thrombotic microangiopathy after hematopoietic cell transplantation: A position statement of ThREG (Turkish Hematology Research and Education Group)

Hematopoietic stem cell transplantation associated-thrombotic microangiopathy (TA-HCT) is one of the early complications of endothelial origin in the course of HCT. Endothelial damage to the microvascular structure and the platelet-rich microthrombi, which are formed as a result of accompanying complement activation, constitute the main pathological conditions resulting TA-TMA. Early diagnosis and management are of utmost importance to prevent multi-organ failure and death. This review summarizes the current understanding of TA-TMA regarding pathogenesis, definition, differential diagnosis, risk factors, surveillance for early diagnosis and management.

A prognostic model (BATAP) with external validation for patients with transplant-associated thrombotic microangiopathy

Transplant-associated thrombotic microangiopathy (TA-TMA) is a potentially life-threatening complication following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Information on markers for early prognostication remains limited, and no predictive tools for TA-TMA are available. We attempt to develop and validate a prognostic model for TA-TMA. A total of 507 patients who developed TA-TMA following allo-HSCT were retrospectively identified and separated into a derivation cohort and a validation cohort according to the time of transplantation to perform external temporal validation. Patient age (OR 2.371, 95% CI 1.264-4.445), anemia (OR 2.836, 95% CI 1.566-5.138), severe thrombocytopenia (OR 3.871, 95% CI 2.156-6.950), elevated total bilirubin (OR 2.716, 95% CI 1.489-4.955) and proteinuria (OR 2.289, 95% CI 1.257-4.168) were identified as independent prognostic factors for the 6-month outcome of TA-TMA. A risk score model termed BATAP (Bilirubin, Age, Thrombocytopenia, Anemia, Proteinuria) was then constructed according to the regression coefficients. The validated c-statistics were 0.816 (95% CI 0.766-0.867) and 0.756 (95% CI 0.696-0.817) in the internal and external validation, respectively. Calibration plots indicated that the model-predicted probabilities correlated well with the actual observed frequencies. This predictive model may facilitate the prognostication of TA-TMA and contribute to the early identification of high-risk patients.

Risk factors and appropriate therapeutic strategies for thrombotic microangiopathy after allogeneic HSCT

Transplant-associated thrombotic microangiopathy (TA-TMA) is a fatal complication of allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, so far, no large cohort study determined the risk factors and the most effective therapeutic strategies for TA-TMA. Thus, the present study aimed to clarify these clinical aspects based on a large multicenter cohort. This retrospective cohort study was performed by the Kyoto Stem Cell Transplantation Group (KSCTG). A total of 2425 patients were enrolled from 14 institutions. All patients were aged ≥16 years, presented with hematological diseases, and received allo-HSCT after the year 2000. TA-TMA was observed in 121 patients (5.0%) on day 35 (median) and was clearly correlated with inferior overall survival (OS) (hazard ratio [HR], 4.93). Pre- and post-HSCT statistically significant risk factors identified by multivariate analyses included poorer performance status (HR, 1.69), HLA mismatch (HR, 2.17), acute graft-versus-host disease (aGVHD; grades 3-4) (HR, 4.02), Aspergillus infection (HR, 2.29), and veno-occlusive disease/sinusoidal obstruction syndrome (VOD/SOS; HR, 4.47). The response rate and OS significantly better with the continuation or careful reduction of calcineurin inhibitors (CNI) than the conventional treatment strategy of switching from CNI to corticosteroids (response rate, 64.7% vs 20.0%). In summary, we identified the risk factors and the most appropriate therapeutic strategies for TA-TMA. The described treatment strategy could improve the outcomes of patients with TA-TMA in the future.

Transplant-associated thrombotic microangiopathy: theoretical considerations and a practical approach to an unrefined diagnosis

Transplant-associated thrombotic microangiopathy (TA-TMA) is an increasingly recognized complication of hematopoietic stem cell transplant (HSCT) with high morbidity and mortality. The triad of endothelial cell activation, complement dysregulation, and microvascular hemolytic anemia has the potential to cause end organ dysfunction, multiple organ dysfunction syndrome and death, but clinical features mimic other disorders following HSCT, delaying diagnosis. Recent advances have implicated complement as a major contributor and the therapeutic potential of complement inhibition has been explored. Eculizumab has emerged as an effective therapy and narsoplimab (OMS721) has been granted priority review by the FDA. Large studies performed mostly in pediatric patients suggest that earlier recognition and treatment may lead to improved outcomes. Here we present a clinically focused summary of recently published literature and propose a diagnostic and treatment algorithm.

Secondary failure of platelet recovery in patients treated with high-dose thiotepa and busulfan followed by autologous stem cell transplantation

Autologous stem cell transplantation (ASCT) with high-dose thiotepa and busulfan is a treatment option for patients with central nervous system (CNS) lymphoma. We report here the occurrence of secondary failure of platelet recovery (SFPR) in three out of 24 patients who received high-dose thiotepa and busulfan followed by ASCT. Although there was no obvious abnormality in the primary platelet engraftment as well as the recovery of other blood cells, they developed SFPR with a median time to onset of day 38, and the platelets gradually recovered over several months with steroid therapy. During the same period, there was no development of SFPR among 50 patients who received ASCT with a conditioning regimen of MEAM (ranimustine, etoposide, cytarabine, and melphalan) or high-dose melphalan. However, one of the two patients who received a conditioning regimen of busulfan and melphalan developed SFPR, suggesting that the use of a busulfan-based conditioning regimen may be one of the risk factors for SFPR. It is important to be aware of this possible adverse effect of ASCT with high-dose thiotepa and busulfan to ensure timely diagnosis and prevention of subsequent serious complications.

A critical role of the Gas6-Mer axis in endothelial dysfunction contributing to TA-TMA associated with GVHD

Endothelial dysfunction in the early phases of hematopoietic stem cell transplantation (HSCT) contributes to a common pathology between transplant-associated thrombotic microangiopathy (TA-TMA) and graft-versus-host disease (GVHD), which are serious complications of HSCT. Growth arrest-specific (Gas) 6 structurally belongs to the family of plasma vitamin K-dependent proteins working as a cofactor for activated protein C, and has growth factor-like properties through its interaction with receptor tyrosine kinases of the TAM family: Tyro3, Axl, and Mer. Serum Gas6 levels were significantly increased in HSCT patients with grade II to IV acute GVHD (aGVHD), and Gas6 and Mer expression levels were upregulated in aGVHD lesions of the large intestine and skin. The increased serum Gas6 levels were also correlated with elevated lactate dehydrogenase, d-dimer, and plasmin inhibitor complex values in HSCT patients with aGVHD. In human umbilical vein endothelial cells (ECs), exogenous Gas6 or the exposure of sera isolated from patients with grade III aGVHD to ECs induced the downregulation of thrombomodulin and the upregulation of PAI-1, as well as the upregulation of intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, which were inhibited by UNC2250, a selective Mer tyrosine kinase inhibitor. In mouse HSCT models, we observed hepatic GVHD with hepatocellular apoptosis, necrosis, and fibrosis, as well as TA-TMA, which is characterized pathologically by thrombosis formation in the microvasculature of the liver and kidney. Of note, intravenous administration of UNC2250 markedly suppressed GVHD and TA-TMA in these mouse HSCT models. Our findings suggest that the Gas6-Mer axis is a promising target for TA-TMA after GVHD.

New insights into risk factors for transplant-associated thrombotic microangiopathy in pediatric HSCT

This study aimed to identify a risk profile for development of transplant-associated thrombotic microangiopathy (TA-TMA) in children undergoing hematopoietic stem cell transplantation (HSCT). Between 2013 and 2016, 439 children underwent 474 HSCTs at 2 supraregional United Kingdom centers. At a median of 153 days post-HSCT, TA-TMA occurred among 25 of 441 evaluable cases (5.6%) with no evidence of center variation. Sex, underlying disease, intensity of the conditioning, total body irradiation-based conditioning, the use of calcineurin inhibitors, venoocclusive disease, and viral reactivation did not influence the development of TA-TMA. Donor type: matched sibling donor/matched family donor vs matched unrelated donor vs mismatched unrelated donor/haplo-HSCT, showed a trend toward the development of TA-TMA in 1.8% vs 6.1% vs 8.3%, respectively. Presence of active comorbidity was associated with an increased risk for TA-TMA; 13% vs 3.7% in the absence of comorbidity. The risk of TA-TMA was threefold higher among patients who received >1 transplant. TA-TMA rates were significantly higher among patients with acute graft-versus-host disease (aGVHD) grades III to IV vs aGVHD grade 0 to II. On multivariate analysis, the presence of active comorbidity, >1 transplant, aGVHD grade III to IV were risk factors for TA-TMA (odds ratio [OR]: 5.1, 5.2, and 26.9; respectively), whereas the use of cyclosporine A/tacrolimus-based GVHD prophylaxis was not a risk factor for TA-TMA (OR: 0.3). Active comorbidity, subsequent transplant, and aGVHD grades III to IV were significant risk factors for TA-TMA. TA-TMA might represent a form of a vascular GVHD, and therefore, continuing control of aGVHD is important to prevent worsening of TA-TMA associated with GVHD.

Endothelial cell function and endothelial-related disorders following haematopoietic cell transplantation

Use of haematopoietic cell transplantation (HCT) in the treatment of haematologic and neoplastic diseases may lead to life-threatening complications that cause substantial morbidity and mortality if untreated. In addition to patient- and disease-related factors, toxicity associated with HCT puts patients at risk for complications that share a similar pathophysiology involving endothelial cells (ECs). Normally, the endothelium plays a role in maintaining homeostasis, including regulation of coagulation, vascular tone, permeability and inflammatory processes. When activated, ECs acquire cellular features that may lead to phenotypic changes that induce procoagulant, pro-inflammatory and pro-apoptotic mediators leading to EC dysfunction and damage. Elevated levels of coagulation factors, cytokines and adhesion molecules are indicative of endothelial dysfunction, and endothelial damage may lead to clinical signs and symptoms of pathological post-HCT conditions, including veno-occlusive disease/sinusoidal obstruction syndrome, graft-versus-host disease, transplant-associated thrombotic microangiopathy and idiopathic pneumonia syndrome/diffuse alveolar haemorrhage. The endothelium represents a rational target for preventing and treating HCT complications arising from EC dysfunction and damage. Additionally, markers of endothelial damage may be useful in improving diagnosis of HCT-related complications and monitoring treatment effect. Continued research to effectively manage EC activation, injury and dysfunction may be important in improving patient outcomes after HCT.

The alternative complement pathway activation product Ba as a marker for transplant-associated thrombotic microangiopathy

BACKGROUND

Transplant-associated thrombotic microangiopathy (TA-TMA) occurs after hematopoietic stem cell transplantation (HSCT) and is characterized by microvascular thrombosis and end-organ injury particularly of the kidneys. TA-TMA is challenging to diagnose and treat, which can lead to long-term complications and death in patients with severe disease. Studies have shown that genetic abnormalities of the alternative complement pathway (AP) are associated with TA-TMA. We hypothesized that patients with TA-TMA may generate elevated levels of the AP activation product, Ba, compared with HSCT patients without TA-TMA.

PROCEDURE

We longitudinally measured plasma levels of complement activation products C3a, Ba, and C5a in 14 HSCT patients: 7 with TA-TMA and 7 without TA-TMA. We assessed renal function by calculating estimated glomerular filtration rate (eGFR) and correlated the extent of AP activation with renal dysfunction in both patient populations.

RESULTS

The median days from HSCT to study enrollment were 154 (39-237) in the TA-TMA group and 84 (39-253) in the HSCT group without TA-TMA. Median Ba levels (ng/mL) at enrollment were 1096.9 (826.5-1562.0) in the TA-TMA group and 725.7 (494.7-818.9) in the HSCT group without TA-TMA (P = 0.007). Over the study duration, Ba levels inversely correlated with eGFR. There were no differences in C3a, C5a, or sC5b9 levels between the two populations at any measured interval.

CONCLUSIONS

We conclude in this preliminary study that Ba protein may serve as a marker for TA-TMA, and furthermore, that components generated in the early phase of AP activation may be involved in the pathogenesis of renal endothelial injury in TA-TMA.

Acute kidney injury in the patient with cancer

Dramatic advances in the care of patients with cancer have led to significant improvement in outcomes and survival. However, renal manifestations of the underlying cancer as well as the effects of anti-neoplastic therapies leave patients with significant morbidity and chronic kidney disease risks. The most common renal manifestations associated with cancer include acute kidney injury (AKI) in the setting of multiple myeloma, tumor lysis syndrome, post-hematopoietic stem cell therapy, and AKI associated with chemotherapy. Knowledge of specific risk factors, modification of risk and careful attention to rapid AKI diagnosis are critical for improving outcomes.

Kidney Injury in Murine Models of Hematopoietic Stem Cell Transplantation

Acute graft-versus-host disease (GVHD) affects different organs, including the skin, liver, and gastrointestinal tract. Although kidneys are not among the organs commonly known to be the target of acute GVHD, kidney damage is frequently reported after allogeneic hematopoietic stem cell transplantation (allo-HSCT). We have studied the effect of bone marrow transplantation (BMT) on the kidneys in different murine models of GVHD. We found that glomerular damage in the kidneys is a common pathological finding in mice after BMT. The histopathological features of glomeruli damage included mesengiolysis, mesangial proliferation and edema, subendothelial and endothelial thickening, splitting of capillary walls in glomeruli, narrowing and collapsing of capillary lumens, fibrinoid necrosis of afferent arterioles, intimal hyperplasia, and microthrombi. These pathological features are similar to those detected in kidneys of patients with thrombotic microangiopathy (TMA) after allo-HSCT. We previously showed that activation of the complement system plays a role in GVHD-induced tissue injury in mice. Here we report the presence of complement activation products in the kidney specimens of mice after BMT. We also report that complement deficiency reduced the extent and severity of post-BMT glomerular damage in mice. We conclude that BMT in mice is associated with glomerular injury and tubulointerstitial nephritis, and that kidney damage is at least partially mediated by activation of the complement system.

A Heterozygous CFHR3-CFHR1 Gene Deletion in a Pediatric Patient With Transplant-associated Thrombotic Microangiopathy Who was Treated With Eculizumab

Complement system dysregulation, such as complement Factor H (CFH) autoantibodies and deletions in CFH-related (CFHR) genes 3 and 1, might cause transplant-associated thrombotic microangiopathy (TA-TMA). The use of eculizumab, a terminal complement inhibitor, could be a targeted therapy for TA-TMA. We report a 1-year-old girl who developed TA-TMA, just after autologous peripheral blood stem cell transplantation in neuroblastoma therapy. Eculizumab improved TA-TMA. Investigation for the complement alternative pathway showed a heterozygous CFHR3-CFHR1 gene deletion, which is involved in complement activation. The patient might develop TA-TMA as a result of complement regulatory gene mutation.

Emerging Concepts in Hematopoietic Stem Cell Transplantation-Associated Renal Thrombotic Microangiopathy and Prospects for New Treatments

Thrombotic microangiopathy associated with hematopoietic stem cell transplantation (HSCT-TMA) is a well-recognized complication of HSCT that has a high risk for death. Even in patients who survive, HSCT-TMA is associated with long-term morbidity and chronic organ injury. HSCT-TMA is a multisystem disease that often affects the kidneys. Renal manifestations of HSCT-TMA include reduced glomerular filtration rate, proteinuria, and hypertension. Understanding of the pathophysiology of HSCT-TMA has expanded in the last decade. Endothelial injury plays a major role. Recent studies also suggest involvement of complement activation. HSCT-TMA has also been considered by some to be an endothelial variant of graft-versus-host disease. Understanding the pathophysiology of HSCT-TMA and its association with activation of the complement system may aid in developing novel therapeutic options. In this review, we summarize current knowledge focusing on epidemiology and prognosis, evidence of complement activation, and endothelial injury; the possible link to graft-versus-host disease; and treatment options for HSCT-TMA.

Eculizumab for transplant-associated thrombotic microangiopathy in adult allogeneic stem cell transplant recipients

OBJECTIVE

To evaluate response rates and survival in adults with transplant-associated thrombotic microangiopathy (TA-TMA) after allogeneic hematopoietic stem cell transplantation (HSCT) who were treated with eculizumab (ECU).

METHODS

Patients were identified retrospectively and data collected through HSCT and pharmacy databases.

RESULTS

Ten patients with TA-TMA after allogeneic HSCT were treated with ECU between 2013 and 2016. TA-TMA was diagnosed at a median of 93 days post-HSCT. Organ-specific injury was documented in all ten patients at time of TA-TMA diagnosis, the most common being renal dysfunction (90%). Acute GVHD (70%) and active infection (80%) were common at time of diagnosis. The median time to ECU initiation from TA-TMA diagnosis was 4 days. Seven patients received ECU as first-line therapy in combination with other treatment modalities, while three patients were treated with ECU as second-line therapy. ECU was well tolerated with the exception of one case of severe skin rash leading to discontinuation. ECU achieved an overall hematologic response rate of 70% and an overall survival rate of 60%. One patient achieved a complete response with corresponding organ recovery.

CONCLUSION

Early initiation of ECU may not alter the disease process enough to restore organ function, but it may prolong survival.

Acute kidney injury and electrolyte disorders in the critically ill patient with cancer

PURPOSE OF REVIEW

Patients with cancer increasingly make up a significant proportion of patients receiving care in the intensive care unit (ICU). Acute kidney injury and cancer-associated electrolyte disorders are encountered in many of these patients and can significantly impact both short-term and long-term outcomes.

RECENT FINDINGS

Advances in chemotherapeutic regimens as well as in our understanding of cancer-associated kidney disease highlight the need for specialized knowledge of the unique causes and therapies required in this subset of critically ill patients. This is especially the case as targeted cancer therapies may have off-target effects that need to be recognized in a timely manner.

SUMMARY

This review outlines key knowledge areas for critical care physicians and nephrologists caring for patients with cancer and associated kidney issues such as acute kidney injury and electrolyte disorders. Specifically, understanding kidney-specific effects of new chemotherapeutic approaches is outlined, and provides an up-to-date compendium of these effects.

Brain microvascular endothelial cells exhibit lower activation of the alternative complement pathway than glomerular microvascular endothelial cells

Atypical hemolytic uremic syndrome (aHUS) and bone marrow transplantation-associated thrombotic microangiopathy (TA-TMA) are associated with excessive activation of the alternative complement pathway (AP) and with severe renal, but rarely cerebral, microvascular damage. Here, we compared AP activation and regulation in human glomerular and brain microvascular endothelial cells (GMVECs and BMVECs, respectively) unstimulated or stimulated by the proinflammatory cytokine, tumor necrosis factor (TNF). Compared with GMVECs and under both experimental conditions, BMVECs had increased gene expression of the AP-related genes C3, CFB, and C5 and decreased expression of CFD This was associated with increased expression in BMVECs (relative to GMVECs) of the genes for surface and soluble regulatory molecules (CD46, THBD, CD55, CFI, and CFH) suppressing formation of the AP C3 and C5 convertases. Of note, unlike GMVECs, BMVECs generated extremely low levels of C3a and C5a and displayed decreased activation of the AP (as measured by a lower percentage of Ba generation than GMVECs). Moreover, BMVECs exhibited increased function of CD141, mediating activation of the natural anticoagulant protein C, compared with GMVECs. We also found that the C3a receptor (C3aR) is present on both cell types and that TNF greatly increases C3AR1 expression in GMVECs, but only slightly in BMVECs. Higher AP activation and C3a generation in GMVECs than in BMVECs, coupled with an increase in C3aR production in TNF-stimulated GMVECs, provides a possible explanation for the predominance of renal damage, and the absence of cerebral injury, in individuals with episodes of aHUS and TA-TMA.

Thrombotic microangiopathy following haematopoietic stem cell transplant

Thrombotic microangiopathy is a potentially lethal complication of haematopoietic stem cell (bone marrow) transplantation. The pathophysiology is incompletely understood, although endothelial damage appears to be central. Platelet activation, neutrophil extracellular traps and complement activation appear to play key roles. Diagnosis may be difficult and universally accepted diagnostic criteria are not available. Treatment remains controversial. In some cases, withdrawal of calcineurin inhibitors is adequate. Rituximab and defibrotide also appear to have been used successfully. In severe cases, complement inhibitors such as eculizumab may play a valuable role. Further research is required to define the pathophysiology and determine both robust diagnostic criteria and the optimal treatment.

Successful Treatment of Transplant Associated Thrombotic Microangiopathy (TA-TMA) with Low Dose Defibrotide

Transplant associated microangiopathy (TA-TMA) is a potentially serious complication of stem cell transplantation. Though stopping calcineurin/mTOR inhibitor is the first step in managing TA-TMA, this is not always adequate. The pathophysiology of TA-TMA is different from microangiopathy seen in other settings. Many drugs have been used in TA-TMA with modest responses. Defibrotide has been explored in TA-TMA in the past with good results. However, its availability is erratic and cost of therapy very high. Hence its routine use in low middle income country (LMIC) is financially demanding. We report the use of low dose defibrotide safely and successfully in this case series. This is pertinent more to LMIC's and warrants prospective evaluation.

Everolimus for pediatric patients with acute graft-versus-host disease after hematopoietic stem cell transplantation: A pilot study

Acute graft-versus-host disease (aGVHD) is a significant cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). Due to the poor prognosis for patients not responding to first-line steroids treatment, improvements in aGVHD therapy are needed. Everolimus is a promising candidate that combines immunosuppressive properties with anti-neoplastic effects. Here, we retrospectively reviewed the efficacy of everolimus with steroids as primary treatment in 13 patients with grade II to grade IV aGVHD after HSCT. Among them, 12 (92.3%) had complete response to everolimus with steroids without additional immunosuppressive agents. The median duration of therapy was 76 days (range 20-110). Asymptomatic hypertriglyceridemia was the most common therapy complication (69.2%), but treatment interruption was not needed. Thrombotic microangiopathy was rare (7.7%), but can be quickly solved by stopping everolimus and cyclosporine treatment. Other toxicities were manageable. Two patients developed chronic GVHD (15.4%), limited in one and extensive in the other. The overall survival was 76.9% with a median follow-up of 3.4 years after HSCT (range 0.7-5.7). Accordingly, everolimus with steroids were feasible for patients with aGVHD after HSCT as primary treatment. Further large-scale studies are required.

Plasma levels of complement activation fragments C3b and sC5b-9 significantly increased in patients with thrombotic microangiopathy after allogeneic stem cell transplantation

Transplantation-associated thrombotic microangiopathy (TA-TMA) is an uncommon but severe complication in patients undergoing allogeneic stem cell transplantation (allo-SCT). However, the mechanism is unclear. From 2011 to 2014, 20 patients with TA-TMA, 20 patients without, and 54 patients with various other complications, including veno occlusive disease (VOD), graft-versus-host disease (GVHD), and infection, were recruited in the study. Plasma vWF antigen (vWFAg), vWF activity (vWFAc), and ADAMTS13 activity were determined in these patients by ELISAs and FRETS-vWF73 assay, respectively. Plasma C3b, sC5b-9, and CH50 were also determined by ELISAs. Plasma levels of C3b were significantly increased in patients with either TA-TMA (p < 0.0001) or GVHD (p < 0.01). Plasma sC5b-9 and CH50 levels in patients with TA-TMA were also significantly increased (p < 0.001). Plasma ADAMTS13 activity was lower in patients with VOD, but normal with other complications. Both plasma vWFAg and vWFAc levels were not elevated in patients with TA-TMA or VOD compared with those of other groups. Complement activation likely via an alternative pathway (increased C3b, sC5b-9, and CH50) may play a role in the pathogenesis of TA-TMA. ADAMTS13 activity is reduced in VOD, but the ADAMTS13/vWF axis appears to be unaffected in patients with TA-TMA.

Central nervous system complications after allogeneic hematopoietic stem cell transplantation

Allogenic hematopoietic stem cell transplant (alloSCT) is a potentially curative modality of treatment for patients with hematological malignancies. However, CNS complications following transplant pose a risk to survival of the patients. Early recognition and management of these complications are crucial to reduce morbidity and mortality of patients following transplant. Early CNS complications associated with alloSCT are infection, cerebrovascular events, chemotherapy and radiation-induced toxicities while late complications include post-transplant lymphoproliferative disorder, CNS relapse of underlying malignancy and viral and fungal infections. Development of graft-versus-host disease can further increase the risk of CNS complications and outcomes after alloSCT. Strategies aimed to reduce the risk of CNS complications and early management may ameliorate the morbidity and mortality in transplant recipients.

In vitro evidence of complement activation in transplantation-associated thrombotic microangiopathy

Transplantation-associated thrombotic microangiopathy is associated with complement activation in vitro.This data further supports the use of eculizumab for the treatment of patients with TA-TMA.

Severe transplant-associated thrombotic microangiopathy in patients with hemoglobinopathies

Incidence and severity of transplant-associated thrombotic microangiopathy (TA-TMA) in patients with hemoglobinopathies receiving hematopoietic cell transplant is unknown. We report the outcomes for two patients with TA-TMA who received eculizumab. A 2.5-year-old male with sickle cell disease developed TA-TMA-associated pericardial tamponade, severe hypertension, and acute kidney injury 2 months after transplant. A 7-year-old female with β-thalassemia major developed TA-TMA-related acute kidney injury, severe hypertension, and seizures at 6 months after transplant. Both patients progressed to chronic kidney disease (CKD). In patients with hemoglobinopathies, preexisting endothelial dysfunction may place them at a greater risk for TA-TMA and subsequent CKD.

A Complete Histologic Approach to Gastrointestinal Biopsy From Hematopoietic Stem Cell Transplant Patients With Evidence of Transplant-Associated Gastrointestinal Thrombotic Microangiopathy

CONTEXT

- Transplant-associated thrombotic microangiopathy is a serious complication of hematopoietic stem cell transplant that may progress to multi-organ dysfunction. Transplant-associated thrombotic microangiopathy may involve the intestinal vasculature (intestinal transplant-associated thrombotic microangiopathy [iTMA]), causing patients to experience debilitating symptoms of ischemic colitis, including disproportionately severe abdominal pain and gastrointestinal bleeding, requiring heavy narcotic use and frequent transfusion support. Pathophysiology remains poorly investigated but may include endothelial damage mediated by inflammatory markers and the complement system. Endoscopy of hematopoietic stem cell transplant patients often produces biopsy samples, in which mucosal lamina propria capillaries are sufficient for an evaluation of iTMA features.

OBJECTIVE

- To provide a detailed review of histologic features of iTMA.

DATA SOURCES

- We conducted a systematic review of studies assessing histologic features of iTMA. Studies were identified by PubMed search and included a cohort study performed by our group.

CONCLUSIONS

- The histologic hallmark of iTMA is endothelial cell injury that leads to hemorrhage and thrombosis of the capillaries. Histologic features include endothelial cell swelling, endothelial cell separation, perivascular mucosal hemorrhage, intraluminal schistocytes, intraluminal fibrin, intraluminal microthrombi, loss of glands, and total denudation of mucosa. Identification of features consistent with iTMA has immediate implications for clinical management that could potentially improve outcome and survival.

Transplant-associated thrombotic microangiopathy is an endothelial complication associated with refractoriness of acute GvHD

There is increasing evidence that endothelial dysfunction is involved in refractoriness of acute GvHD (aGvHD). Here we investigated the hypothesis that another endothelial complication, transplant-associated thrombotic microangiopathy (TMA), contributes to the pathogenesis of aGvHD refractoriness. TMA was retrospectively assessed in 771 patients after allogeneic stem cell transplantation (alloSCT). Incidences of TMA and refractory aGvHD were correlated with biomarkers of endothelial damage obtained before alloSCT for patients receiving or not receiving statin-based endothelial prophylaxis (SEP). Diagnostic criteria for TMA and refractory aGvHD were met by 41 (5.3%) and 76 (10%) patients, respectively. TMA was overrepresented in patients with refractory aGvHD (45.0 vs 2.3% in all other patients, P<0.001). TMA independently increased mortality. Elevated pretransplant suppressor of tumorigenicity-2 and nitrates along with high-risk variants of the thrombomodulin gene were associated with increased risk of TMA. In contrast, SEP abolished the unfavorable outcome predicted by pretransplant biomarkers on TMA risk. Patients on SEP had a significantly lower risk of TMA (P=0.001) and refractory aGvHD (P=0.055) in a multivariate multistate model. Our data provide evidence that TMA contributes to the pathogenesis of aGvHD refractoriness. Patients with an increased TMA risk can be identified pretransplant and may benefit from pharmacological endothelium protection.