• Proteinuria and elevated markers of complement activation at TMA diagnosis are associated with poor outcome.• Clinical interventions should be considered in HSCT patients with these high-risk features at the time TMA is diagnosed.Transplant-associated thrombotic microangiopathy (TMA) leads to generalized endothelial dysfunction that can progress to multiorgan injury, and severe cases are associated with poor outcomes after hematopoietic stem cell transplantation (HSCT). Identifying patients at highest risk for severe disease is challenging. We prospectively evaluated 100 consecutive HSCT recipients to determine the incidence of moderate and severe TMA and factors associated with poor overall outcomes. Thirty-nine subjects (39%) met previously published criteria for TMA. Subjects with TMA had a significantly higher nonrelapse mortality (43.6% vs 7.8%, P < .0001) at 1 year post-HSCT compared with those without TMA. Elevated lactate dehydrogenase, proteinuria on routine urinalysis, and hypertension were the earliest markers of TMA. Proteinuria (>30 mg/dL) and evidence of terminal complement activation (elevated sC5b-9) in the blood at the time of TMA diagnosis were associated with very poor survival (<20% at 1 year), whereas all TMA subjects without proteinuria and a normal sC5b-9 serum concentration survived (P < .01). Based on these prospective observations, we conclude that severe TMA occurred in 18% of HSCT recipients in our cohort and propose an algorithm to identify the highest-risk patients who might benefit from prompt clinical interventions. (Blood. 2014;124(4):645-653)

Hematopoietic stem cell transplantation (HSCT)-associated thrombotic microangiopathy (TA-TMA) is now a well-recognized and potentially severe complication of HSCT that carries a high risk of death. In those who survive, TA-TMA may be associated with long-term morbidity and chronic organ injury. Recently, there have been new insights into the incidence, pathophysiology, and management of TA-TMA. Specifically, TA-TMA can manifest as a multi-system disease occurring after various triggers of small vessel endothelial injury, leading to subsequent tissue damage in different organs. While the kidney is most commonly affected, TA-TMA involving organs such as the lung, bowel, heart, and brain is now known to have specific clinical presentations. We now review the most up-to-date research on TA-TMA, focusing on the pathogenesis of endothelial injury, the diagnosis of TA-TMA affecting the kidney and other organs, and new clinical approaches to the management of this complication after HSCT.

Transplantation-associated thrombotic microangiopathy (TA-TMA) is a challenging diagnosis after hematopoietic stem cell transplantation. Although endothelial injury represents the final common pathway of disease, the exact pathophysiology of TA-TMA remains unclear. Potential causes include infections, chemotherapy, radiation, and calcineurin inhibitors. Recent literature addresses the roles of cytokines, graft-versus-host disease, the coagulation cascade, and complement in the pathogenesis of TA-TMA. Current diagnostic criteria are unsatisfactory, because patients who have received a transplant can have multiple other reasons for the laboratory abnormalities currently used to diagnose TA-TMA. Moreover, our lack of understanding of the exact mechanism of disease limits the development and evaluation of potential treatments. Short- and long-term renal complications contribute to TA-TMA's overall poor prognosis. In light of these challenges, future research must validate novel markers of disease to aid in early diagnosis, guide current and future treatments, prevent long-term morbidity, and improve outcomes. We focus on TA-TMA as a distinct complication of hematopoietic stem cell transplantation, emphasizing the central role of the kidney in this disease.

Noninvasive biomarkers are needed to assess immune risk and ultimately guide therapeutic decision-making following kidney transplantation. A requisite step toward these goals is validation of markers that diagnose and/or predict relevant transplant endpoints. The Clinical Trials in Organ Transplantation-01 protocol is a multicenter observational study of biomarkers in 280 adult and pediatric first kidney transplant recipients. We compared and validated urinary mRNAs and proteins as biomarkers to diagnose biopsy-proven acute rejection (AR) and stratify patients into groups based on risk for developing AR or progressive renal dysfunction. Among markers tested for diagnosing AR, urinary CXCL9 mRNA (odds ratio [OR] 2.77, positive predictive value [PPV] 61.5%, negative predictive value [NPV] 83%) and CXCL9 protein (OR 3.40, PPV 67.6%, NPV 92%) were the most robust. Low urinary CXCL9 protein in 6-month posttransplant urines obtained from stable allograft recipients classified individuals least likely to develop future AR or a decrement in estimated glomerular filtration rate between 6 and 24 months (92.5–99.3% NPV). Our results support using urinary CXCL9 for clinical decision-making following kidney transplantation. In the context of acute dysfunction, low values can rule out infectious/immunological causes of injury. Absent urinary CXCL9 at 6 months posttransplant defines a subgroup at low risk for incipient immune injury.

We recently observed that dysregulation of the complement system may be involved in the pathogenesis of hematopoietic stem cell transplantation–associated thrombotic microangiopathy (HSCT-TMA). These findings suggest that the complement inhibitor eculizumab could be a therapeutic option for this severe HSCT complication with high mortality. However, the efficacy of eculizumab in children with HSCT-TMA and its dosing requirements are not known. We treated 6 children with severe HSCT-TMA using eculizumab and adjusted the dose to achieve a therapeutic level >99 μg/mL. HSCT-TMA resolved over time in 4 of 6 children after achieving therapeutic eculizumab levels and complete complement blockade, as measured by low total hemolytic complement activity (CH50). To achieve therapeutic drug levels and a clinical response, children with HSCT-TMA required higher doses or more frequent eculizumab infusions than currently recommended for children with atypical hemolytic uremic syndrome. Two critically ill patients failed to reach therapeutic eculizumab levels, even after dose escalation, and subsequently died. Our data indicate that eculizumab may be a therapeutic option for HSCT-TMA, but HSCT patients appear to require higher medication dosing than recommended for other conditions. We also observed that a CH50 level ≤ 4 complement activity enzyme units correlated with therapeutic eculizumab levels and clinical response, and therefore CH50 may be useful to guide eculizumab dosing in HSCT patients as drug level monitoring is not readily available.

Key Points Genetic variations in the alternative pathway of complement may be associated with thrombotic microangiopathy in children receiving HSCT. These findings may guide the development of novel treatment interventions for this poorly understood transplant complication.

Thrombotic microangiopathy (TMA) after hematopoietic stem cell transplant (HSCT) associated with terminal complement activation, as measured by elevated plasma terminal complement (sC5b-9) concentrations, has a very high mortality. The complement inhibitor eculizumab may be a therapeutic option for HSCT-associated TMA. We examined the pharmacokinetics and pharmacodynamics (PK/PD) of eculizumab in children and young adult HSCT recipients with TMA and activated complement to determine drug dosing requirements for future efficacy trials. We analyzed prospectively collected laboratory samples and clinical data from 18 HSCT recipients with high-risk TMA presenting with complement activation who were treated with eculizumab. We measured eculizumab serum concentrations, total hemolytic complement activity (CH50), and plasma sC5b-9 concentrations. Population PK/PD analyses correlated eculizumab concentrations with complement blockade and clinical response and determined inter-individual differences in PK parameters. We also compared transplant survival in patients treated with eculizumab (n=18) to patients with the same high-risk TMA features who did not receive any targeted therapy during a separate prospective observational study (n=11). In the PK analysis, we found significant inter-patient variability in eculizumab clearance, ranging from 16 to 237 mL/hr/70kg in the induction phase. The degree of complement activation measured by sC5b-9 concentrations at the start of therapy, in addition to actual body weight, were significant determinants of eculizumab clearance and disease response. Sixty one percent of treated patients had complete resolution of TMA and were able to safely discontinue eculizumab without disease recurrence. Overall survival was significantly higher in treated subjects compared to untreated patients (56% versus 9%, p=0.003). Complement blocking therapy is associated with improved survival in HSCT patients with high-risk TMA who historically have dismal outcomes, but eculizumab pharmacokinetics in HSCT recipients differ significantly from reports in other diseases like atypical hemolytic uremic syndrome and paroxysmal nocturnal hemoglobinurina. Our eculizumab dosing algorithm, including pre-treatment plasma sC5b-9 concentrations, patient’s actual body weight, and the first eculizumab dose (mg), accurately determined eculizumab concentration-time profiles for HSCT recipients with high-risk TMA. This algorithm may guide eculizumab treatment and ensure that future efficacy studies use the most clinically appropriate and cost-efficient dosing schedules.

In a cross-sectional study, we assessed effects of calcineurin inhibitor (CNI) or rapamycin on T-regulatory (Treg) cells from children with stable liver (n=53) or kidney (n=9) allografts several years post-transplant. We analyzed Treg number, phenotype, suppressive function, and methylation at the Treg-specific demethylation region (TSDR) using Tregs and peripheral blood mononuclear cells. 48 patients received CNI (39 as monotherapy) and 12 patients received rapamycin (9 as monotherapy). Treg numbers diminished over time on either regimen, but reached significance only with CNI (r=−0.424, p=0.017). CNI levels inversely correlated with Treg number (r=−0.371, p=0.026), and positively correlated with CD127+ expression by Tregs (r=0.437, p=0.023). Patients with CNI levels >3.6 ng/ml had weaker Treg function than those with levels <3.6 ng/ml, whereas rapamycin therapy positively correlated with Treg numbers (r=0.628, p=0.029) and their expression of CTLA4 (r=0.726, p=0.041). Overall, CTLA4 expression, TSDR demethylation and an absence of CD127 were important for Treg suppressive function. We conclude that rapamycin has beneficial effects on Treg biology, whereas long-term and high dose CNI use may impair Treg number, function and phenotype, potentially acting as a barrier to attaining host hyporesponsiveness to an allograft.