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

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.

Thrombotic Microangiopathy After Hematopoietic Stem Cell and Solid Organ Transplantation: A Review for Intensive Care Physicians

Intensive care physicians may assume the primary care of patients with transplant-associated thrombotic microangiopathy (TA-TMA), an uncommon but potentially critical complication of hematopoietic stem cell transplants (HSCTs) and solid organ transplants. TA-TMA can have a dramatic presentation with multiple organ dysfunction syndrome (MODS) associated with high morbidity and mortality. The typical presenting clinical features are hemolytic anemia, thrombocytopenia, refractory hypertension, proteinuria and worsening renal failure. Intestinal involvement, with abdominal pain, nausea and vomiting, gastrointestinal bleeding, and ascites are also common. Cardiopulmonary involvement may develop from various causes including pulmonary arteriolar hypertension, pleural and pericardial effusions, and diffuse alveolar hemorrhage. Due to other often concurrent complications after HSCT, early diagnosis and effective management of TA-TMA may be challenging. Close collaboration between ICU and transplant physicians, along with other relevant specialists, is needed to best manage these patients. There are currently no approved therapies for the treatment of TA-TMA. Plasma exchange and rituximab are not recommended unless circulating factor H (CFH) antibodies or thrombotic thrombocytopenic purpura (TTP; ADAMTS activity < 10%) are diagnosed or highly suspected. The role of the complement pathway activation in the pathophysiology of TA-TMA has led to the successful use of targeted complement inhibitors, such as eculizumab. However, the relatively larger studies using eculizumab have been mostly conducted in the pediatric population with limited data on the adult population. This review is focused on the role of intensive care physicians to emphasize the clinical approach to patients with suspected TA-TMA and to discuss diagnosis and treatment strategies.

External quality assurance program for diagnostic complement laboratories: evaluation of the results of the past seven years

Introduction

The complement external quality assurance (EQA) program was first organized in 2010 by a group of researchers working in diagnostic complement laboratories. Starting in 2016, INSTAND e.V., a German, non-profit interdisciplinary scientific medical society dedicated to providing expert EQA programs for medical laboratories, started organizing the EQAs for complement diagnostic laboratories together with the same group of experienced scientists and doctors who also work as EQA experts. The aim of the current work is to provide descriptive analysis of the past seven years' complement EQA results and evaluate timeline changes in proficiency testing.

Methods

Each year, in March and October, blinded samples (normal, pathological) were sent to the participating diagnostic laboratories, where complement parameters were evaluated exactly as in daily routine samples. Since no reference method/target values exist for these parameters, and participants used different units for measurement, the reported results were compared to the stable mean (Algorithm A) of the participants using the same method/measurement units. A reported result was qualified as "passed" if it fell into the 30-50% evaluation/target range around the mean of reported results (depending on the given parameter).

Results

While the number of participating laboratories has increased in the past years (from around 120 to 347), the number of complement laboratories providing multiple determinations remained mostly unchanged (around 30 worldwide). C3, C4, C1-inhibitor antigen and activity determinations provided the best proficiency results, with >90% passing quotas in the past years, independent of the applied method. Determination of the functional activity of the three activation pathways was good in general, but results showed large variance, especially with the pathological samples. Complement factor C1q and regulators FH and FI are determined by only a few laboratories, with variable outcomes (in general in the 85-90% pass range). Activation products sC5b-9 and Bb were determined in 30 and 10 laboratories, respectively, with typical passing quotas in the 70-90% range, without a clear tendency over the past years.

Conclusion

With these accumulated data from the past seven years, it is now possible to assess sample-, method-, and evaluation related aspects to further improve proficiency testing and protocolize diagnostic complement determinations.

MASP2 inhibition by narsoplimab suppresses endotheliopathies characteristic of transplant-associated thrombotic microangiopathy: in vitro and ex vivo evidence

Transplant-associated thrombotic microangiopathy (TA-TMA) is an endotheliopathy complicating up to 30% of allogeneic hematopoietic stem cell transplants (alloHSCT). Positive feedback loops among complement, pro-inflammatory, pro-apoptotic, and coagulation cascade likely assume dominant roles at different disease stages. We hypothesized that mannose-binding lectin-associated serine protease 2 (MASP2), principal activator of the lectin complement system, is involved in the microvascular endothelial cell (MVEC) injury characteristic of TA-TMA through pathways that are susceptible to suppression by anti-MASP2 monoclonal antibody narsoplimab. Pre-treatment plasmas from 8 of 9 TA-TMA patients achieving a complete TMA response in a narsoplimab clinical trial activated caspase 8, the initial step in apoptotic injury, in human MVEC. This was reduced to control levels following narsoplimab treatment in 7 of the 8 subjects. Plasmas from 8 individuals in an observational TA-TMA study, but not 8 alloHSCT subjects without TMA, similarly activated caspase 8, which was blocked in vitro by narsoplimab. mRNA sequencing of MVEC exposed to TA-TMA or control plasmas with and without narsoplimab suggested potential mechanisms of action. The top 40 narsoplimab-affected transcripts included upregulation of SerpinB2, which blocks apoptosis by inactivating procaspase 3; CHAC1, which inhibits apoptosis in association with mitigation of oxidative stress responses; and pro-angiogenesis proteins TM4SF18, ASPM, and ESM1. Narsoplimab also suppressed transcripts encoding pro-apoptotic and pro-inflammatory proteins ZNF521, IL1R1, Fibulin-5, aggrecan, SLC14A1, and LOX1, and TMEM204, which disrupts vascular integrity. Our data suggest benefits to narsoplimab use in high-risk TA-TMA and provide a potential mechanistic basis for the clinical efficacy of narsoplimab in this disorder.

Onconephrology and Thrombotic Microangiopathy: Looking Beyond the Horizon

Thrombotic microangiopathies (TMAs) represent a complex interaction of endothelial and podocyte biology, nephron physiology, complement genetics, and oncologic therapies with host immunology. The complexity of various factors, such as molecular causes, genetic expressions, and immune system mimicking, along with incomplete penetrance, make it difficult to find a straightforward solution. As a result, there may be variations in diagnosis, study, and treatment approaches, and achieving a consensus can be challenging. Here, we review the molecular biology, pharmacology, immunology, molecular genetics, and pathology of the various TMA syndromes in the setting of cancer. Controversies in etiology, nomenclature, and points requiring further clinical, translational, and bench research are discussed. Complement-mediated TMAs, chemotherapy drug-mediated TMAs, TMAs in monoclonal gammopathy, and other TMAs central to onconephrology practice are reviewed in detail. In addition, established and emerging therapies within the US Food and Drug Administration pipeline subsequently are discussed. Finally, a comprehensive review of critical areas of onconephrology clinical practice is presented as practical value to the clinical practitioner and seeds of investigation to be sown among the community of atypical hemolytic uremic syndrome researchers.

Pathway-driven rare germline variants associated with transplant-associated thrombotic microangiopathy (TA-TMA)

The significance of rare germline mutations in transplant-associated thrombotic microangiopathy (TA-TMA) is not well studied. We performed a genetic association study in 100 adult TA-TMA patients vs. 98 post-transplant controls after matching by race, sex, and year. We focused on 5 pathways in complement, von Willebrand factor (VWF) function and related proteins, VWF clearance, ADAMTS13 function and related proteins, and endothelial activation (3641variants in 52 genes). In the primary analysis focused on 189 functional rare variants, no differential variant enrichment was observed in any of the pathways; specifically, 29 % TA-TMA and 33 % controls had at least 1 rare complement mutation. In the secondary analysis focused on 37 rare variants predicted to be pathogenic or likely pathogenic by ClinVar, Complement Database, or REVEL in-silico prediction tool, rare variants in the VWF clearance pathway were found to be significantly associated with TA-TMA (p = 0.008). On the gene level, LRP1 was the only one with significantly increased variants in TA-TMA in both analyses (p = 0.025 and 0.015). In conclusion, we did not find a significant association between rare variants in the complement pathway and TA-TMA; however, we discovered a new signal in the VWF clearance pathway driven by the gene LRP1 among likely pathogenic variants.

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy and the role of advanced practice providers and pharmacists

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy (HSCT-TMA) is a severe and potentially life-threatening complication. HSCT-TMA is often underdiagnosed due to multifactorial pathophysiology and a historic lack of standard diagnostic criteria. Identification of the multi-hit hypothesis and the key role of the complement system, particularly the lectin pathway of complement, has led to development of treatments targeting the underlying pathogenesis of HSCT-TMA. Additional research is ongoing to investigate the efficacy and safety of these targeted therapies in patients with HSCT-TMA. Advanced practice providers (APPs; nurse practitioners and physician assistants) and pharmacists are critical members of the multidisciplinary HSCT team and ensure management of patients throughout the continuum of care. Additionally, pharmacists and APPs can improve patient care through medication management of complex regimens; transplant education for patients, staff, and trainees; evidence-based protocol and clinical guideline development; assessment and reporting of transplant-related outcomes; and quality improvement initiatives to improve outcomes. Understanding the presentation, prognosis, pathophysiology, and treatment options for HSCT-TMA can improve each of these efforts. Collaborative practice model for monitoring and care of HSCT-TMA. Advanced practice providers and pharmacists contribute to many aspects of patient care in transplant centers, including medication management for complex regimens; transplant education for patients, staff, and trainees; evidence-based protocol and clinical guideline development; assessment and reporting of transplant-related outcomes; and quality improvement initiatives. HSCT-TMA is a severe and potentially life-threatening complication that is often underdiagnosed. The collaboration of a multidisciplinary team of advanced practice providers, pharmacists, and physicians can optimize recognition, diagnosis, management, and monitoring of patients with HSCT-TMA, thereby improving outcomes for these patients.

Molecular Advances in Sinusoidal Obstruction Syndrome/Veno-Occlusive Disease

Sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD) detected in the liver has been considered a severe complication of hematopoietic stem cell transplantation (HSCT). SOS/VOD is characterized by hepatomegaly, right upper quadrant pain, jaundice, and ascites. The severe forms of the disease may result in multi-organ dysfunction (MOD) with a high mortality rate (>80%). The development of SOS/VOD can be rapid and unpredictable. Therefore, early identification and severity assessment is crucial in facilitating prompt diagnosis and timely treatment. Effective treatment and potential prophylaxis with defibrotide highlight the need for characterizing a sub-group of patients at high risk for SOS/VOD. Moreover, antibodies that are conjugated with calicheamicin, gemtuzumab, and inotuzumab ozogamicin, have led to renewed interest in this syndrome. Evaluation and management of serious adverse events associated with gemtuzumab and inotuzumab ozogamicin are recommended. We review hepatic-, transplant- and patient-related risk factors, criteria for diagnosis and grading classification, and SOS/VOD potential biomarkers. Furthermore, we examine pathogenesis, clinical presentation, diagnostic criteria, risk factors, prophylaxis, and treatment of SOS/VOD occurring post HSCT. Moreover, we aim to provide an up-to-date summary of molecular advances in the diagnosis and management of SOS/VOD. We performed a comprehensive review of the literature and examined the recently available data, mostly using the PubMed and Medline search engines for original articles published over the last decade. In the era of precision medicine, our review provides up-to-date knowledge of genetic or sera markers for SOS/VOD with the goal of identifying a subset of high-risk patients.

With complements: C3 inhibition in the clinic

C3 is a key complement protein, located at the nexus of all complement activation pathways. Extracellular, tissue, cell-derived, and intracellular C3 plays critical roles in the immune response that is dysregulated in many diseases, making it an attractive therapeutic target. However, challenges such as very high concentration in blood, increased acute expression, and the elevated risk of infections have historically posed significant challenges in the development of C3-targeted therapeutics. This is further complicated because C3 activation fragments and their receptors trigger a complex network of downstream effects; therefore, a clear understanding of these is needed to provide context for a better understanding of the mechanism of action (MoA) of C3 inhibitors, such as pegcetacoplan. Because of C3's differential upstream position to C5 in the complement cascade, there are mechanistic differences between pegcetacoplan and eculizumab that determine their efficacy in patients with paroxysmal nocturnal hemoglobinuria. In this review, we compare the MoA of pegcetacoplan and eculizumab in paroxysmal nocturnal hemoglobinuria and discuss the complement-mediated disease that might be amenable to C3 inhibition. We further discuss the current state and outlook for C3-targeted therapeutics and provide our perspective on which diseases might be the next success stories in the C3 therapeutics journey.

Comparison of multiple treatments in the management of transplant-related thrombotic microangiopathy: a network meta-analysis

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy (TA-TMA) is a fatal post-transplant complication. It has a high mortality rate and worse prognosis, but treatment strategies remain controversial. We screened 6 out of 3453 studies on the treatment of TA-TMA. These investigations compared 5 treatment strategies with a network meta-analysis approach. The final outcome was the proportion of patients who responded to these therapies. There were significant differences in response rates for each treatment. Achieving analysis through direct and indirect evidence in the rank probabilities shows that rTM (recombinant human soluble thrombomodulin) is most likely to be rank 1 (64.98%), Eculizumab intervention rank 2 (48.66%), ISM (immunosuppression manipulation) rank 3 (32.24%), TPE (therapeutic plasma exchange) intervention rank 4 (69.56%), and supportive care intervention rank 5 (70.20%). Eculizumab and ISM have significantly higher efficacy than supportive care (odds ratio (OR): 18.04, 18.21 respectively); and TPE having lower efficacy than all other TA-TMA therapies exception to supportive care. In our study, rTM and Eculizumab may be the best choice when treating TA-TMA.

Pitfalls in complement analysis: A systematic literature review of assessing complement activation

Background

The complement system is an essential component of our innate defense and plays a vital role in the pathogenesis of many diseases. Assessment of complement activation is critical in monitoring both disease progression and response to therapy. Complement analysis requires accurate and standardized sampling and assay procedures, which has proven to be challenging.

Objective

We performed a systematic analysis of the current methods used to assess complement components and reviewed whether the identified studies performed their complement measurements according to the recommended practice regarding pre-analytical sample handling and assay technique. Results are supplemented with own data regarding the assessment of key complement biomarkers to illustrate the importance of accurate sampling and measuring of complement components.

Methods

A literature search using the Pubmed/MEDLINE database was performed focusing on studies measuring the key complement components C3, C5 and/or their split products and/or the soluble variant of the terminal C5b-9 complement complex (sTCC) in human blood samples that were published between February 2017 and February 2022. The identified studies were reviewed whether they had used the correct sample type and techniques for their analyses.

Results

A total of 92 out of 376 studies were selected for full-text analysis. Forty-five studies (49%) were identified as using the correct sample type and techniques for their complement analyses, while 25 studies (27%) did not use the correct sample type or technique. For 22 studies (24%), it was not specified which sample type was used.

Conclusion

A substantial part of the reviewed studies did not use the appropriate sample type for assessing complement activation or did not mention which sample type was used. This deviation from the standardized procedure can lead to misinterpretation of complement biomarker levels and hampers proper comparison of complement measurements between studies. Therefore, this study underlines the necessity of general guidelines for accurate and standardized complement analysis

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.

Mannan-Binding Lectin Promotes Murine Graft-versus-Host Disease by Amplifying Lipopolysaccharide-Initiated Inflammation

Conditioning regimens used for hematopoietic stem cell transplantation (HCT) can escalate the severity of acute T cell-mediated graft-versus-host disease (GVHD) by disrupting gastrointestinal integrity and initiating lipopolysaccharide (LPS)-dependent innate immune cell activation. Activation of the complement cascade has been associated with murine GVHD, and previous work has shown that alternative pathway complement activation can amplify T cell immunity. Whether and how mannan-binding lectin (MBL), a component of the complement system that binds mannose as well as oligosaccharide components of LPS and lipoteichoic acid, affects GVHD is unknown. In this study, we tested the hypothesis that MBL modulates murine GVHD and examined the mechanisms by which it does so. We adoptively transferred C3.SW bone marrow (BM) cells ± T cells into irradiated wild type (WT) or MBL-deficient C57Bl/6 (B6) recipients with or without inhibiting MBL-initiated complement activation using C1-esterase inhibitor (C1-INH). We analyzed the clinical severity of disease expression and analyzed intestinal gene and cell infiltration. In vitro studies assessed MBL expression on antigen-presenting cells (APCs) and compared LPS-induced responses of WT and MBL-deficient APCs. MBL-deficient recipients of donor BM ± T cells exhibited significantly less weight loss over the first 2 weeks post-transplantation weeks compared with B6 controls (P < .05), with similar donor engraftment in the 2 groups. In recipients of C3.SW BM + T cells, the clinical expression of GVHD was less severe (P < .05) and overall survival was better (P < .05) in MBL-deficient mice compared with WT mice. On day-7 post-transplantation, analyses showed that the MBL-deficient recipients exhibited less intestinal IL1b, IL17, and IL12 p40 gene expression (P < .05 for each) and fewer infiltrating intestinal CD11c+, CD11b+, and F4/80+ cells and TCRβ+, CD4+, CD4+IL17+, and CD8+ T cells (P < .05 for each). Ovalbumin or allogeneic cell immunizations induced equivalent T cell responses in MBL-deficient and WT mice, demonstrating that MBL-deficiency does not directly impact T cell immunity in the absence of irradiation conditioning. Administration of C1-INH did not alter the clinical expression of GVHD in preconditioned WT B6 recipients, suggesting that MBL amplifies clinical expression of GVHD via a complement-independent mechanism. WT, but not MBL-deficient, APCs express MBL on their surfaces. LPS-stimulated APCs from MBL-deficient mice produced less proinflammatory cytokines (P < .05) and induced weaker alloreactive T cell responses (P < .05) compared with WT APCs. Together, our data show that MBL modulates murine GVHD, likely by amplifying complement-independent, LPS-initiated gastrointestinal inflammation. The results suggest that devising strategies to block LPS/MBL ligation on APCs has the potential to reduce the clinical expression of GVHD.

Upregulation of HIF-1α contributes to complement activation in transplantation-associated thrombotic microangiopathy

Transplantation-associated thrombotic microangiopathy (TA-TMA) is a severe complication of haematopoietic stem cell transplantation (HSCT). Complement activation is involved in the development of TA-TMA. However, the underlying mechanism is unclear. Therefore, 21 samples of TA-TMA and 1:1 matched controls were measured for hypoxia-inducible factor-1α (HIF-1α) and complement protein. The mechanism was investigated both in vitro and in vivo. In this study, we found that levels of HIF-1α were significantly higher in TA-TMA patients than that in non-TA-TMA controls. Upregulation of HIF-1α induced an increase in membrane-bound complement C3 and dysfunction of human umbilical vein endothelial cells (HUVECs) in vitro. Increasing HIF-1α in vivo led to C3 and C5b-9 deposition in the glomerular endothelial capillary complex, thrombocytopenia, anaemia, and increased serum lactate dehydrogenase (LDH) levels in wild-type (WT) but not in C3^-/- mice subjected to HSCT. High platelet aggregation in peripheral blood and CD41-positive microthrombi in the kidney were also found in dimethyloxallyl glycine (DMOG)-treated mice, recapitulating the TA-TMA phenotype seen in patients. Comprehensive analysis, including DNA array, luciferase reporter assay, chromatin immunoprecipitation (ChIP)-seq, and quantitative polymerase chain reaction (PCR), revealed that HIF-1α interacted with the promoter of complement factor H (CFH) to inhibit its transcription. Decreased CFH led to complement activation in endothelial cells.

Complement Mediated Endothelial Damage in Thrombotic Microangiopathies

Thrombotic microangiopathies (TMA) constitute a group of different disorders that have a common underlying mechanism: the endothelial damage. These disorders may exhibit different mechanisms of endothelial injury depending on the pathological trigger. However, over the last decades, the potential role of the complement system (CS) has gained prominence in their pathogenesis. This is partly due to the great efficacy of complement-inhibitors in atypical hemolytic syndrome (aHUS), a TMA form where the primary defect is an alternative complement pathway dysregulation over endothelial cells (genetic and/or adquired). Complement involvement has also been demonstrated in other forms of TMA, such as thrombotic thrombocytopenic purpura (TTP) and in Shiga toxin-producing Escherichia coli hemolytic uremic syndrome (STEC-HUS), as well as in secondary TMAs, in which complement activation occurs in the context of other diseases. However, at present, there is scarce evidence about the efficacy of complement-targeted therapies in these entities. The relationship between complement dysregulation and endothelial damage as the main causes of TMA will be reviewed here. Moreover, the different clinical trials evaluating the use of complement-inhibitors for the treatment of patients suffering from different TMA-associated disorders are summarized, as a clear example of the entry into a new era of personalized medicine in its management.

Acute graft-versus-host disease increase risk and accuracy in prediction model of transplantation-associated thrombotic microangiopathy in patients with myelodysplastic syndrome

Allogeneic hematopoietic stem cell transplantation is the only curative therapy for patients with myelodysplastic syndrome. Transplantation-associated thrombotic microangiopathy (TA-TMA) remains a cause of death after transplantation. This study assessed the risk factors of TA-TMA and established a prediction model for this complication. We launched a real-world study from 303 MDS patients after allo-HSCT from Dec 1, 2007, to Jun 1, 2018. Logistic regression was used to analyze risk factors and to establish a nomogram. The accuracy of the model was assessed by C-index and calibration curve. TA-TMA class was associated with an over twofold increase in the risk of death (HR 2.66, 95% CI 1.39-5.09, p = 0.003). Stage III or IV acute graft-versus-host disease (aGVHD) (OR: 6.17, 95% CI: 2.19-17.18, p < 0.001) and occurrence time of aGVHD were the risk factors for TA-TMA. Next, we put these two variants and the other three variants into the prediction model via multivariate Lasso regression. In order to quantify the contribution of each factor, a nomogram was generated and displayed (C index of 0.783). TA-TMA predicts worsened outcomes of overall survival. A cross-validated multivariate score including aGVHD occurrence showed excellent concordance and efficacy of predicting TA-TMA in HSCT patients.

Kidney Disease Following Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell transplantation (SCT) provides a curative option for the treatment of several malignancies. Its growing use is associated with an increased burden of kidney disease. Acute kidney injury is usually seen within the first 100 days of transplantation and has an incidence ranging between 12 and 73%, with the highest rate in myeloablative allogeneic SCT. A large subset of patients after SCT develop chronic kidney disease. They can be broadly classified into thrombotic microangiopathy, nephrotic syndrome, and calcineurin toxicity. Dialysis requirement after SCT is associated with mortality exceeding 80%. Given the higher morbidity and mortality related to development kidney disease, nephrologists need to be aware of the various causes and best treatment options.

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.

A systematic review of diagnostic, prognostic, and risk blood and urine biomarkers of transplant-associated thrombotic microangiopathy

Transplant-associated thrombotic microangiopathy (TA-TMA) is an increasingly recognized complication of allogeneic and autologous hematopoietic cellular therapy (HCT), associated with significant morbidity and mortality. Although the central drivers of the disease are thought to be endothelial damage and complement activation, no specific diagnostic biomarkers have been identified. TA-TMA is typically diagnosed using criteria comprised of non-specific clinical and laboratory features. Some patients will have a self-remitting course, but more than half develop multi-organ dysfunction or die, making prognostic biomarkers critical. Prevention of TA-TMA, an approach central to other HCT complications such as graft-versus-host disease, is largely untested in part due to a lack of identified early high-risk biomarkers. We conducted a systematic review to summarize the diagnostic, early risk, and prognostic biomarkers of TA-TMA. We screened the titles and abstracts of 1524 citations. After screening out duplications, we read the abstracts of 979 papers and fully reviewed 132 full-text publications. Thirty-one publications fulfilled the inclusion criteria of more than five patients with TA-TMA and a reported measure of association with diagnosis, prognosis, or risk of later development of the disease. Fourteen studies (45%) were with adults, 12 (39%) were with children <18 years old, three included both children and adults, and two did not report age. There were 53 biomarker or biomarker signature entries, and a total of 27 unique biomarkers. Only four biomarkers reported sensitivity and specificity. The single biomarker with the most robust data was sC5b-9, which conferred diagnostic, prognostic, and risk implications. Studies of combinations of biomarkers were rare. No meta-analyses were performed because of significant heterogeneity between studies. The limitations of studies included small sample size, study designs with a high risk of bias (i.e., case-control), the timing of sample collection, and the selection of controls. Furthermore, only two (6%) studies included a training and validation cohort. Cut-off points are needed to stratify groups, as most biomarkers do not have normal values, or normal values cannot be assumed in the HCT setting. In the future, multi-institutional, collaborative efforts are needed to perform rigorously designed, prospective studies with serially enrolled patients, with samples collected at the time of TA-TMA diagnosis, careful selection of controls, and validation of selected biomarkers and cut-off points in a separate cohort.

N-Acetyl-L-Cysteine Potentially Inhibits Complement Activation in Transplantation-Associated Thrombotic Microangiopathy

Transplant-associated thrombotic microangiopathy (TA-TMA) has a high mortality rate and lacks effective treatments. We searched the GEO database and analyzed RNA-seq data and whole-genome sequencing data from patients' blood samples. We identified N-acetyl-L-cysteine (NAC) as a possible therapeutic target for TA-TMA. In vitro experiments showed that NAC reduced complement activation and VWF multimerization in HUVECs. We also treated a 36-year-old female TA-TMA patient with NAC. Hemoglobin, platelet counts, lactate dehydrogenase levels, and sC5b-9 levels and schistocytes were normalized after using NAC. It shows that NAC may be an effective drug to improve TA-TMA symptoms by inhibiting complement activation.

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.

Role of the lectin pathway of complement in hematopoietic stem cell transplantation-associated endothelial injury and thrombotic microangiopathy

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy (HSCT-TMA) is a life-threatening syndrome that occurs in adult and pediatric patients after hematopoietic stem cell transplantation. Nonspecific symptoms, heterogeneity within study populations, and variability among current diagnostic criteria contribute to misdiagnosis and underdiagnosis of this syndrome. Hematopoietic stem cell transplantation and associated risk factors precipitate endothelial injury, leading to HSCT-TMA and other endothelial injury syndromes such as hepatic veno-occlusive disease/sinusoidal obstruction syndrome, idiopathic pneumonia syndrome, diffuse alveolar hemorrhage, capillary leak syndrome, and graft-versus-host disease. Endothelial injury can trigger activation of the complement system, promoting inflammation and the development of endothelial injury syndromes, ultimately leading to organ damage and failure. In particular, the lectin pathway of complement is activated by damage-associated molecular patterns (DAMPs) on the surface of injured endothelial cells. Pattern-recognition molecules such as mannose-binding lectin (MBL), collectins, and ficolins—collectively termed lectins—bind to DAMPs on injured host cells, forming activation complexes with MBL-associated serine proteases 1, 2, and 3 (MASP-1, MASP-2, and MASP-3). Activation of the lectin pathway may also trigger the coagulation cascade via MASP-2 cleavage of prothrombin to thrombin. Together, activation of complement and the coagulation cascade lead to a procoagulant state that may result in development of HSCT-TMA. Several complement inhibitors targeting various complement pathways are in clinical trials for the treatment of HSCT-TMA. In this article, we review the role of the complement system in HSCT-TMA pathogenesis, with a focus on the lectin pathway.

Emerging therapeutic and preventive approaches to transplant-associated thrombotic microangiopathy

PURPOSE OF REVIEW

Transplant-associated thrombotic microangiopathy (TA-TMA) is a complication that can occur in both allogeneic and autologous haematopoietic cellular therapy (HCT) recipients and is associated with significant morbidity and mortality. Although TA-TMA is a complex disease, there is emerging evidence that complement activation and endothelial dysfunction play a key role in the pathophysiology of the disease. The use of eculizumab has improved survival in patients with high risk and severe disease, but mortality rates in treated patients still exceed 30%, highlighting the need for novel approaches.

RECENT FINDINGS

There are multiple ongoing and planned clinical trials investigating novel complement agents in TA-TMA and other TMAs. Drugs vary by targets of the complement system, mechanism, and form of administration. Clinical trial designs include single arm studies that span across multiple age groups including children, and double-blind, randomized, placebo-controlled studies. These studies will provide robust data to inform the care of patients with TA-TMA in the future. In addition to multiple promising therapeutic agents, preventing TA-TMA is an emerging strategy. Agents known to protect the endothelium from damage and augment endothelial function by promoting anti-inflammatory and antithrombotic effects may have a role in preventing TA-TMA or ameliorating the severity, though additional studies are needed.

SUMMARY

Novel therapeutic agents for TA-TMA inhibition of the complement system are under investigation and prophylactic strategies of endothelial protection are emerging. Further understanding of the pathophysiology of the disease may identify additional therapeutic targets. Multiinstitutional, collaborative clinical trials are needed to determine the safety and efficacy of these agents going forward.

Insights Into the Structure-Function Relationships of Dimeric C3d Fragments

Cleavage of C3 to C3a and C3b plays a central role in the generation of complement-mediated defences. Although the thioester-mediated surface deposition of C3b has been well-studied, fluid phase dimers of C3 fragments remain largely unexplored. Here we show C3 cleavage results in the spontaneous formation of C3b dimers and present the first X-ray crystal structure of a disulphide-linked human C3d dimer. Binding studies reveal these dimers are capable of crosslinking complement receptor 2 and preliminary cell-based analyses suggest they could modulate B cell activation to influence tolerogenic pathways. Altogether, insights into the physiologically-relevant functions of C3d(g) dimers gained from our findings will pave the way to enhancing our understanding surrounding the importance of complement in the fluid phase and could inform the design of novel therapies for immune system disorders in the future.

Early Elevation of Complement Factor Ba Is a Predictive Biomarker for Transplant-Associated Thrombotic Microangiopathy

Transplant-associated thrombotic microangiopathy (TA-TMA) is a fatal complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Previous reports suggest that TA-TMA is caused by complement activation by complement-related genetic variants; however, this needs to be verified, especially in adults. Here, we performed a nested case-control study of allo-HSCT-treated adults at a single center. Fifteen TA-TMA patients and 15 non-TA-TMA patients, matched according to the propensity score, were enrolled. Based on a previous report showing an association between complement-related genes and development of TA-TMA, we first sequenced these 17 genes. Both cohorts harbored several genetic variants with rare allele frequencies; however, there was no difference in the percentage of patients in the TA-TMA and non-TA-TMA groups with the rare variants, or in the average number of rare variants per patient. Second, we measured plasma concentrations of complement proteins. Notably, levels of Ba protein on Day 7 following allo-HSCT were abnormally and significantly higher in TA-TMA than in non-TA-TMA cases, suggesting that complement activation via the alternative pathway contributes to TA-TMA. All other parameters, including soluble C5b-9, on Day 7 were similar between the groups. The levels of C3, C4, CH50, and complement factors H and I in the TA-TMA group after Day 28 were significantly lower than those in the non-TA-TMA group. Complement-related genetic variants did not predict TA-TMA development. By contrast, abnormally high levels of Ba on Day 7 did predict development of TA-TMA and non-relapse mortality. Thus, Ba levels on Day 7 after allo-HSCT are a sensitive and prognostic biomarker of TA-TMA.

Upregulation of Nrf2 Attenuates Oxidative Stress-Induced, Complement Activation-Associated Endothelial Injury and Apoptosis in Transplant-Associated Thrombotic Microangiopathy

Transplant-associated thrombotic microangiopathy (TA-TMA) is a serious and life-threatening complication after hematopoietic stem cell transplantation. Studies have reported that the main pathological manifestation of the disease is an endothelial injury associated with complement activation, but its molecular biological mechanisms remain unclear. Our previous studies have shown that oxidative stress may induce complement activation in TA-TMA. Nuclear factor erythroid 2-related factor 2 (Nrf2), a molecule that regulates oxidative stress, can inhibit endothelial stimulation by reactive oxygen species (ROS). We assessed Nrf2 expression in peripheral blood mononuclear cells (PBMCs) from patients with TA-TMA compared with healthy donors. Nrf2 expression, ROS accumulation, complement activation, and apoptosis were then assessed in human umbilical vein endothelial cells (HUVECs) incubated with TA-TMA plasma to identify whether complement-associated endothelial damage induced by oxidative stress occurs in TA-TMA. The protective effect of Nrf2 pathway activation on TA-TMA-induced endothelial injury was also investigated to explore a new avenue for TA-TMA prevention and treatment. In this study, peripheral blood was collected from six patients with TA-TMA, and healthy donors served as negative controls. We determined the expression of Nrf2 through in vitro and in vivo experiments and measured the level of apoptosis. We found increased expression of Nrf2 in PBMCs from patients with TA-TMA. HUVECs were then incubated with plasma from patients with TA-TMA or with plasma from healthy donors, and we found that complement 3 (C3) levels were increased in HUVECs treated with TA-TMA plasma. In contrast, total Nrf2 levels were decreased, and ROS production and apoptosis levels were increased. To determine whether complement activation and apoptosis were caused by oxidative stress, we added N-acetyl-L-cysteine to HUVECs incubated with TA-TMA plasma. As a result, ROS production, complement activation, and apoptosis levels were reduced. Finally, we upregulated Nrf2 in HUVECs by rescue experiments, and we found that activation of Nrf2 attenuated endothelial cell apoptosis and ROS production and reduced C3 and C5b-9 levels. These results suggest that oxidative stress-induced, complement activation-associated endothelial injury occurs in TA-TMA and that upregulation of Nrf2 protects endothelial cells from damage. Activation of the Nrf2 pathway may be a potential target for the treatment of complement activation-associated endothelial injury in TA-TMA.

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.

Thrombotic microangiopathy in oncology - a review

Thrombotic microangiopathy is a syndrome triggered by a wide spectrum of situations, some of which are specific to the Oncology setting. It is characterized by a Coombs-negative microangiopathic haemolytic anemia, thrombocytopenia and organ injury, with characteristic pathological features, resulting from platelet microvascular occlusion.

TMA is rare and its cancer-related subset even more so. TMA triggered by drugs is the most common within this group, including classic chemotherapy and the latest targeted therapies. The neoplastic disease itself and hematopoietic stem-cell transplantation could also be potential triggers.

Evidence-based medical guidance in the management of cancer-related TMA is scarce and the previous knowledge about primary TMA is valuable to understand the disease mechanisms and the potential treatments.

Given the wide spectrum of potential causes for TMA in cancer patients, the aim of this review is to gather the vast information available. For each entity, pathophysiology, clinical features, therapeutic approaches and prognosis will be covered.

Efficacy and Safety of Eculizumab in the Treatment of Transplant-Associated Thrombotic Microangiopathy: A Systematic Review and Meta-Analysis

Background

Transplant-associated thrombotic microangiopathy (TA-TMA) is a dangerous and life-threatening complication in patients undergoing hematopoietic stem cell transplantation (HSCT). Eculizumab has been used in the treatment of TA-TMA, and several studies have confirmed the benefit of Eculizumab in patients with TA-TMA. However, the results remain controversial. We conducted a systematic review and meta-analysis to evaluate the efficacy and safety of Eculizumab for TA-TMA.

Materials and Methods

We searched PubMed and Embase for studies on the efficacy and safety of Eculizumab in TA-TMA patients. Efficacy outcomes consisted of overall response rate (ORR), complete response rate (CRR), and survival rate at the last follow-up (SR). Safety outcomes were adverse events (AEs), including infection, sepsis, impaired liver function, infusion reactions, and death.

Results

A total of 116 patients from six studies were subjected to meta-analysis. The pooled estimates of ORR, CRR, and SR for TA-TMA patients were 71% (95% CI: 58-82%), 32% (95% CI: 11-56%), and 52% (95% CI: 40-65%), respectively. Only one patient presented with a severe rash, and infection was the most common AEs. The main causes of death were infection and GvHD.

Conclusion

Current evidence suggests that Eculizumab improves SR and ORR in patients with TA-TMA and that Eculizumab is well tolerated. However, the number of studies is limited, and the findings are based mainly on data from observational studies. Higher quality randomized controlled trials and more extensive prospective cohort studies are needed.

Prognostic Biomarkers for Thrombotic Microangiopathy after Acute Graft-versus-Host Disease: A Nested Case-Control Study

Transplantation-associated thrombotic microangiopathy (TA-TMA) is a complication of allogeneic hematopoietic cell transplantation (HCT) that often occurs following the development of acute graft-versus-host disease (aGVHD). In this study, we aimed to identify early TMA biomarkers among patients with aGVHD. We performed a nested-case-control study from a prospective cohort of allogeneic HCT recipients, matching on the timing and severity of antecedent aGVHD. We identified 13 TMA cases and 25 non-TMA controls from 208 patients in the cohort. Using multivariable conditional logistic regression, the odds ratio for TMA compared with non-TMA was 2.65 (95% confidence interval [CI], 1.00 to 7.04) for every 100 ng/mL increase in terminal complement complex sC5b9 and 2.62 (95% CI, 1.56 to 4.38) for every 1000 pg/mL increase in angiopoietin-2 (ANG2) at the onset of aGVHD. ADAMTS13 and von Willebrand factor (VWF) antigens were not appreciably associated with TMA. Using a Cox regression model incorporating sC5b9 >300 ng/mL and ANG2 >3000 pg/mL at the onset of aGVHD, the adjusted hazard ratio for mortality was 5.33 (95% CI, 1.57 to 18.03) for the high-risk group (both elevated) and 4.40 (95% CI, 1.60 to 12.07) for the intermediate-risk group (one elevated) compared with the low-risk group (neither elevated). In conclusion, we found that elevated sC5b9 and ANG2 levels at the onset of aGVHD were associated with the development of TMA and possibly mortality after accounting for the timing and severity of aGVHD. The results suggest important roles of complement activation and endothelial dysfunction in the pathogenesis of TMA. Measurement of these biomarkers at the onset of aGVHD may inform prognostic enrichment for preventive trials and improve clinical care.

Tear proteomic profile in three distinct ocular surface diseases: keratoconus, pterygium, and dry eye related to graft-versus-host disease

BACKGROUND

Diseases of the anterior segment of the eye may present different mechanisms, intensity of symptoms, and impact on the patients' quality of life and vision. The tear film is in direct contact with the ocular surface and cornea and can be easily accessed for sample collection, figuring as a promising source of potential biomarkers for diagnosis and treatment control. This study aimed to evaluate tear proteomic profile in 3 distinct ocular diseases: keratoconus (corneal ectasia), severe dry eye related to graft-versus-host-disease (tear film dysfunction and ocular inflammatory condition) and pterygium (conjunctival fibrovascular degenerative disease).

METHODS

Tear samples were collected from patients of each condition and a control group. By using mass spectrometric analysis combined with statistics and bioinformatics tools, a detailed comparison of protein profile was performed.

RESULTS

After Student's t-test analyses comparing each condition to the control group, we found the following number of differentially expressed proteins: 7 in keratoconus group, 29 in pterygium group, and 79 in GVHD group. Following multivariate analyses, we also report potential candidates as biomarkers for each disease.

CONCLUSIONS

We demonstrated herein that mass spectrometry-based proteomics was able to indicate proteins that differentiate three distinct ocular conditions, which is a promising tool for the diagnosis of ocular diseases.

Validation of Early Increase in Complement Activation Marker sC5b-9 as a Predictive Biomarker for the Development of Thrombotic Microangiopathy After Stem Cell Transplantation

Hematopoietic stem cell transplantation (HSCT)-associated thrombotic microangiopathy (TA-TMA) is a multifactorial complication. Complement dysregulation may play an important role in the pathogenesis of TA-TMA. Our previous observations suggested that early increase of soluble C5b-9 (sC5b-9), before the development of other complications, can predict the development of later TA-TMA. The present study aims to validate our earlier findings in an independent cohort enrolling 67 pediatric patients who underwent allogeneic HSCT during the study period (October 2015–January 2019). Five different TA-TMA diagnostic criteria were applied, and all important clinical and laboratory parameters of TA-TMA activity were registered. Complement pathway activities, components and sC5b-9 levels were systematically measured before transplantation and on days 28, 56, and 100 after HSCT. A strong and remarkable association still have been found between early increase of sC5b-9 (10 of 10 patients with TA-TMA vs. 27 of 57 without TA-TMA; P = 0.002) and the development of TA-TMA during 100 days post-transplantation. An increase in sC5b-9 concentration had 100% sensitivity and 53% specificity for TA-TMA in the cohort. All TA-TMA cases have been observed during cyclosporine immunosuppression, no TA-TMA was diagnosed during tacrolimus or mycophenolat mofetil therapy. In the majority of patients TA-TMA was mild and self-limiting, without any signs of organ damage. No additional complement parameters were closely associated with the development of TA-TMA. Early raise of the sC5b-9 activation marker was predictive for later development of TA-TMA throughout the whole study period. In patients with a marked increase, early and frequent monitoring of TA-TMA activity markers should be attempted, to facilitate subsequent therapy decisions in time. However, patients with TA-TMA were only identified during or after cyclosporine immunosuppression. Further studies enrolling higher number of patients are necessary to determine the role of immunosuppression in the pathogenesis of TA-TMA.

A New Era in Endothelial Injury Syndromes: Toxicity of CAR-T Cells and the Role of Immunity

Immunotherapy with chimeric antigen receptor T (CAR-T cells) has been recently approved for patients with relapsed/refractory B-lymphoproliferative neoplasms. Along with great efficacy in patients with poor prognosis, CAR-T cells have been also linked with novel toxicities in a significant portion of patients. Cytokine release syndrome (CRS) and neurotoxicity present with unique clinical phenotypes that have not been previously observed. Nevertheless, they share similar characteristics with endothelial injury syndromes developing post hematopoietic cell transplantation (HCT). Evolution in complement therapeutics has attracted renewed interest in these life-threatening syndromes, primarily concerning transplant-associated thrombotic microangiopathy (TA-TMA). The immune system emerges as a key player not only mediating cytokine responses but potentially contributing to endothelial injury in CAR-T cell toxicity. The interplay between complement, endothelial dysfunction, hypercoagulability, and inflammation seems to be a common denominator in these syndromes. As the indications for CAR-T cells and patient populations expand, there in an unmet clinical need of better understanding of the pathophysiology of CAR-T cell toxicity. Therefore, this review aims to provide state-of-the-art knowledge on cellular therapies in clinical practice (indications and toxicities), endothelial injury syndromes and immunity, as well as potential therapeutic targets.

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.

Eculizumab and Beyond: The Past, Present, and Future of Complement Therapeutics

Dysregulation of the complement system underlies the pathophysiology of many diseases. Renewed interest in complement occurred with the recognition that its therapeutic inhibition was possible. Terminal complement blockade with the anti-C5 monoclonal antibody eculizumab significantly changed management and clinical outcomes of patients with paroxysmal nocturnal hemoglobinuria, and served as a proof of concept for other complement-mediated diseases. Eculizumab is also approved for atypical hemolytic uremic syndrome and myasthenia gravis. Multiple new disease indications have been identified, and novel complement inhibitors are in various stages of development, with several currently in human trials. Beyond C5, these new drugs block proximal complement, pathway-specific targets, convertase activity, and anaphylatoxin function. Though monoclonal antibodies are still common, peptides, RNAi, and small molecule inhibitors provide the opportunity for different administration routes and schedules. Several challenges still exist or will soon present themselves, including mitigation of infection risk, effective monitoring strategies, and how to choose between therapeutics when more than one is available. In this review, we will describe the lessons learned from the "eculizumab era," present many of the novel therapeutics currently or soon to be in trials, and highlight some of the challenges that will require attention as the field progresses.

Complement-mediated thrombotic microangiopathy as a link between endothelial damage and steroid-refractory GVHD

Transplant-associated thrombotic microangiopathy (TA-TMA), a complication of hematopoietic cell transplant (HCT), is associated with significant morbidity and mortality. The pathophysiology and overlap of TA-TMA with other posttransplant complications such as graft-versus-host disease (GVHD) is poorly understood. We retrospectively identified cases of TA-TMA among patients with grade 3/4 gastrointestinal (GI) GVHD, reviewed intestinal biopsy specimens, and performed correlative testing of biomarkers associated with TA-TMA. TA-TMA was more common in patients with steroid-refractory GVHD compared with steroid-responsive GVHD (79.3% vs 42.1%; P = .001). Among patients surviving 100 days post-HCT, 1-year survival from day 100 was significantly better for patients who had not developed TA-TMA in the first 100 days (69.5% vs 36.7%; P < .001). Only 1 of 7 proposed TA-TMA histology criteria (mucosal hemorrhage) differed significantly based on GVHD steroid response. In multivariable modeling, steroid-refractory GVHD was a risk factor for development of TA-TMA (hazard ratio, 3.09; 95% confidence interval, 1.68-5.67; P < .001). There were no differences in complement activation at GVHD onset; however, 2 to 6 weeks later, patients with TA-TMA had higher levels of BBPlus and C5b-9, markers of alternative and terminal pathway activation (BBPlus: median, 600 vs 209.3 ng/mL; P = .0045) (C5b-9: median, 425.9 vs 258.4 ng/mL; P = .029). TA-TMA is associated with poor overall survival (OS) following HCT and may be detected early by histologic findings and may be differentiated from GVHD by measurement of alternative and terminal complement pathway activation. It is unknown whether treatment of TA-TMA will improve survival in steroid-refractory GVHD.

Clinical promise of next-generation complement therapeutics

The complement system plays a key role in pathogen immunosurveillance and tissue homeostasis. However, subversion of its tight regulatory control can fuel a vicious cycle of inflammatory damage that exacerbates pathology. The clinical merit of targeting the complement system has been established for rare clinical disorders such as paroxysmal nocturnal haemoglobinuria and atypical haemolytic uraemic syndrome. Evidence from preclinical studies and human genome-wide analyses, supported by new molecular and structural insights, has revealed new pathomechanisms and unmet clinical needs that have thrust a new generation of complement inhibitors into clinical development for a variety of indications. This review critically discusses recent clinical milestones in complement drug discovery, providing an updated translational perspective that may guide optimal target selection and disease-tailored complement intervention.

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.

Complement in Pathophysiology and Treatment of Transplant-Associated Thrombotic Microangiopathies

Transplant-associated thrombotic microangiopathy (TA-TMA) is a form of microangiopathy specifically occurring in the context of hematopoietic stem cell transplantation. Similarly, to other microangiopathies, TA-TMA is characterized by hemolytic anemia, thrombocytopenia, and organ failure due to endothelial injury. Although its clinical association with medications (eg, calcineurin inhibitors), immune reactions (eg, graft vs host disease) or infectious complications is well established, the pathophysiology remains largely unknown. Recent data have highlighted the role of complement in the pathophysiology of TA-TMA, which are frequently associated with a functional impairment (either inherited or acquired) of the endogenous regulation of the complement classic and alternative pathway. This manuscript will review the data supporting the involvement of complement in the pathophysiology of TA-TMA, as well as the clinical data supporting the use of anticomplement agents in this rare condition.