Hematopoietic stem cell transplantation (HSCT) is a recognized treatment for both malignant and non-malignant diseases. While this treatment has increased cure rates and reduced disease mortality, its complications remain life-threatening and of concern. Transplant-associated thrombotic microangiopathy (TA-TMA) is one of the most devastating complications of hematopoietic stem cell transplantation. A recent study reported a 3-year cumulative incidence rate of 3% for TA-TMA, and TA-TMA was associated with high mortality (HR = 3.1, 95% CI: 2.8–16.3%) (1). Treatment intensity, use of calcitonin inhibitors (CNIs), graft-versus-host disease (GvHD), and viral infection are risk factors for TA-TMA (2, 3). Patients with TA-TMA are characterized by microangiopathic hemolytic anemia, unexplained thrombocytopenia, elevated lactate dehydrogenase (LDH), and endothelial injury-related organ failure, such as hypertension, chronic kidney disease (CKD), pulmonary hypertension, gastrointestinal or central nervous system disease (4). TA-TMA is mainly defined with two standard diagnostic criteria. One is the International Working Group (IWG) (thrombocytopenia in the blood; new-onset, prolonged or progressive thrombocytopenia; sudden and persistent elevation of LDH; decreased hemoglobin or increased transfusion requirements, decreased serum hemoglobin) (5), and the other is overall-TMA (O-TMA) as demonstrated by Cho et al. (Elevated LDH; new-onset thrombocytopenia with a platelet count <50 × 10⁹/L or thrombocytopenia >50%; new-onset anemia with hemoglobin below the lower limit of normal or requiring transfusion support; the presence of typed cells in peripheral blood or histological evidence of microangiopathy in tissue specimens; no coagulation dysfunction, negative Coombs test) (6). The mechanism of how systemic microvascular endothelial injury leads to TA-TMA remains unclear. Due to its similar histomorphology to atypical hemolytic uremic syndrome (aHUS), most studies suggest that it is dysregulation of the complement system that causes TA-TMA to occur (7–9). Subsequently, C5 is cleaved to C5a and C5b, forming a cell membrane attack complex (MAC, C5b-9) on the surface of the endothelial cells, resulting in further endothelial cell damage (8). Significantly elevated plasma C3b, sC5b-9 levels were observed in TA-TMA patients (10).

There is no consensus on care strategies for TA-TMA. Conventional treatments, including supportive care measures, withdrawal of CNIs, therapeutic plasma exchange (TPE) and pharmacological treatments such as rituximab, defibrotide, and daclizumab have been used in the treatment of TA-TMA (8). Recently, the use of Eculizumab for the treatment of TA-TMA has raised concerns. With more and more case reports reaching remission (11–13), Eculizumab has shown its benefits in the treatment of patients with TA-TMA. Eculizumab is a terminal complement inhibitor that works by inhibiting the cleavage of C5 to C5a and C5b. C5b-9 is then blocked from forming on the surface of endothelial cells (14). Since the FDA approved Eculizumab for the treatment of atypical hemolytic uremic syndrome (aHUS) (15), TA-TMA patients treated with Eculizumab were treated according to the standard regimen of aHUS (16). Patients received induction therapy with Eculizumab 900mg once a week for four weeks. When hematological signs of TA-TMA resolved, maintenance therapy was continued with 1,200 mg given every two weeks (17, 18). Some researchers have paid attention to evaluate the therapeutic benefits of Eculizumab in patients with TA-TMA. A study by Joslyn et al. showed a hematological response rate of 70% in patients with TA-TMA after Eculizumab treatment (19), similar to the 67% reported by Michelle et al. (20), but much worse compared to the 93% reported by Stephan et al. (21). In addition, the survival rate of 60% (19) studied by Joslyn et al. was similar to that of Michelle et al.’s 67% (20) and more favorable than that of Stephan et al. (33%) (21). The efficacy results vary from study to study.

To determine which factors may contribute to diversity in response rates and survival, we systematically reviewed relevant studies of Eculizumab in patients with TA-TMA and performed a meta-analysis to better understand the efficacy and safety of Eculizumab.

In all six observational studies, including 116 patients, were included in a systematic review and meta-analysis to investigate the efficacy and safety of Eculizumab in patients with TA-TMA (19–21, 24–26). After Eculizumab treatment, almost 71% of patients responded to the therapy. However, the number of patients who reached full response was significantly lower (32%). Patients with TA-TMA treated with Eculizumab had a 52% survival rate at a median follow-up of 13.5 months after HSCT. Previous treatment strategies for TA-TMA after HSCT have focused on conventional therapies, including withdrawal of CNIs, plasmapheresis, defibrillation, rituximab, and combinations of several therapies (8). A retrospective study showed that 24% of TA-TMA patients (n = 33) underwent plasmapheresis and achieved a clinical response with an SR of 45% at 100 days after diagnosis (27). Corti et al. reported a total of 12 TA-TMA patients undergoing defibrillation in two centers with an ORR and SR of 67 and 50%, respectively (28). Another study by Au et al. showed an ORR of 80% (n = 4/5) for rituximab treatment and a study SR of 60% at a median follow-up of 305 (250–440) d (29). Due to the small sample size reported above, it is difficult to perform a systematic and comprehensive comparison between conventional therapy and Eculizumab. Nonetheless, the summary ORR and SR of Eculizumab from our meta-analysis appeared higher than traditional treatment.

A retrospective study by Prajwal et al. provided an evaluation of efficacy for TA-TMA patients treated with Eculizumab (25). Although they also demonstrated higher response rates and survival in TA-TMA patients treated with Eculizumab, the articles they included were mostly cases (11–13). Cases usually report successful treatment rather than unsuccessful treatment. The benefit of Eculizumab may be significantly overestimated, and it is difficult to explain the heterogeneity between cases. In addition, this retrospective analysis focused on patients with refractory discontinuation of calcineurin inhibitors and plasma exchange in patients with TA-TMA. Eculizumab was considered second-line therapy, whereas patients receiving Eculizumab as first-line therapy were not considered. What’s more, adverse events during Eculizumab treatment were not analyzed. Our current meta-analysis incorporates the largest number of observational studies to date. Among these included studies, we joined the largest cohort to date, which included the terminal complement blocker Eculizumab as first-line treatment in patients with TA-TMA, which provides a higher weighting in the meta-data. Subgroup analysis and meta-regression were performed to detect heterogeneous sources. A sensitivity analysis was performed to demonstrate the stability and reliability of the findings. More importantly, our study provides not only a pooled assessment of efficacy but also of AEs and cause of death. Therefore, the current meta-analysis is a more comprehensive and credible analysis of the effectiveness and safety of Eculizumab for TA-TMA.

Because efficacy outcomes vary widely across centers, our current study analyzed which factors may contribute to differences in response rates and survival rates. Since the p-values of meta-regression were all greater than 0.05, the subgroup analysis of ORR could not explain the source of the difference. Flore et al. (24) studied the lowest ORR compared to four other studies (19–21, 26). Due to the multicenter and retrospective nature of the study, there was heterogeneity in patient inclusion criteria, primary disease, stem cell transplantation characteristics, and Eculizumab regimens. It seems that this setup may be the source of the ORR discrepancy. More importantly, the overall response in the study results was good, but the complete response rate was meagre. This indicates that the kidney damage is advanced by the time of Eculizumab treatment. As a result, most patients do not get organ recovery. From this, it appears that the delay in Eculizumab treatment may prevent patients from achieving optimal response and maximum recovery of organ function. However, in the study by Joslyn et al., the median duration between diagnosis and Eculizumab therapy was shorter, at four days, compared to other studies. Only one patient achieved a complete response and organ function was restored (19). This suggests that earlier initiation of Eculizumab may have no significant effect on restoring organ function. How to improve the full response rate remains a big question. Our subgroup analysis of TA-TMA patient survival revealed that the time between HSCT and TA-TMA diagnosis is a potential source of SR heterogeneity, as the p value for meta-regression for days between transplantation and TA-TMA is 0.0266. TA-TMA diagnosed within the first 100 days after transplantation is more susceptible to publication bias than TA-TMA more than 100 days after transplantation. Therefore, early diagnosis of TA-TMA is the key to successful treatment of TA-TMA. However, the early diagnosis of TA-TMA faces challenges due to overlapping clinical features and the lack of standard diagnostic criteria as most studies support the idea that unregulated complement activation leads to the development of TA-TMA occurrence (7–9). The search for sensitivity and specificity of complement activation monitoring biomarkers should be of interest. Recently, Orsolya et al. showed that early elevation of sC5b-9 is a predictor of late development of TA-TMA (30). In this study, sC5b-9 levels increased from baseline levels to day 28 in patients with TA-TMA (n = 10), while the same trend was observed in only nine patients (p = 0.031) without TA-TMA (n = 23). In our meta-analysis, sC5b-9 levels were documented in three articles, and elevated sC5b-9 was observed in TA-TMA patients in two studies. However, the timing of the detection of sC5b-9 levels was not elaborated. Further studies are needed to determine whether terminal pathway activation is an independent predictor of TA-TMA after HSCT. Median age and primary disease may be another two factors contributing to significant differences between studies though their p-values of meta-regression are both 0.0827. Children seem to achieve higher SR than adults. In addition, SR of TA-TMA patients under treatment of Eculizumab reported in the studies which focused on hematological disease is lower than that of other studies. Sensitivity analysis of ORR and SR showed that the investigation by Stephan et al. (21) was a source of heterogeneity in SR of this meta-analysis. The SR (33%) in this single-center analysis was significantly lower compared to other studies. Patients in this study were diagnosed as hematological disease and the median age was 48 years, which was older than that of other five reports included. Additionally, the median days from hematopoietic stem cell transplantation to TA-TMA diagnosis were 264 days, the longest in all the studies.

The safety profile seems to indicate that Eculizumab is well tolerated. A more substantial observational data set covering a 5-year registry of patients with aHUS reported that no new safety issues were identified in patients treated with adult or pediatric Eculizumab (31). In our meta-analysis, only one case of TRAEs, i.e. one patient with a severe rash, was reported, resulting in discontinuation of Eculizumab therapy. The most commonly reported AEs are infections. Eculizumab is a monoclonal antibody that inhibits C5 cleavage and prevents terminal complement activation (4). Patients treated with Eculizumab have an increased risk of infection, especially meningococcal infections, due to the lack of adequate functional complement (17). Whereas a study by Sonata et al. reported no cases of meningococcal infections in patients who had not received the meningococcal vaccine (32). And in our present study, no meningococcal infections were reported, which corresponds to the findings of Sonata et al. However, among patients treated with Eculizumab, the highest number of deaths due to infection was seen in the study by Stephan et al. Based on their report, an increase in mortality due to infection in the group treated with Eculizumab was found compared to the conventional treatment group (21). Therefore, precaution and treatment of infection are equally urgent during the treatment of TA-TMA. GvHD is a risk factor that leads not only to TA-TMA but also to death during treatment. It has been shown that GvHD almost always precedes the diagnosis of TA-TMA, and there may be a mechanical link between TA-TMA, GvHD, and endothelial injury (33). Another study reported that the occurrence of TA-TMA was associated with risk factors such as aGvHD grade ≥2, steroid-refractory aGvHD, and CMV reactivation/end-organ disease, but not with conditioning regimen (RIC or MAC), TBI use or TBI dose, primary condition, donor type, age, or gender. More importantly, patients diagnosed with TA-TMA combined with aGvHD had significantly lower overall survival compared to patients with TA-TMA alone or GvHD (median 5.6 vs. 7.6 vs. 55.4 months; p < 0.0001) (34). The relationship between TA-TMA and GvHD is unclear. Future studies should provide information on the relationship between GvHD and Evidence of the TA-TMA link. TA-TMA itself is another major cause of death because of endothelial injury-related organ failure. It is necessary to explore how endothelial cells are damaged. One study from our center has reported that heme oxygenase-1 (HO-1) was significantly decreased in patients with TA-TMA and suppressed oxidative stress could attenuate complement deposition in TMA plasma-challenged HUVECs (35). The nuclear factor erythroid 2-related factor 2 (Nrf2), a transcription factor, initiates transcription of the HO-1 gene to protect cells from oxidative stress (36). Further experimental study about Nrf2 and endothelial injury is undertaken in our center.

Another area that needs to be discussed is how much Eculizumab needs to be given to achieve a hematological response and the duration of Eculizumab treatment for TA-TMA. For adult patients, the dose of induction therapy was 900 mg per week for four weeks. If the patient responds to induction therapy, the treatment is maintained at 1,200 mg administered every two weeks. For pediatric patients, the initial dose is based on body weight, and subsequent dose adjustments are based on maintaining total hemolytic complement activity (CH50) levels. Patients weighing less than 40 kg started at 600 mg and others at 900 mg. Induction therapy is also administered weekly for four weeks, and CH50 should be maintained at complement activator enzyme (CAE) levels of 0 to 3 (18, 37). The treatment then transitions to maintenance therapy, which adequately suppresses CH50 to a scale of three CAE (4) and then to maintenance therapy. Regarding the question of when Eculizumab can be safely discontinued, the study by Prajwal et al. proposed that ECU can be suspended after determining clinical symptoms and laboratory manifestations (38).

There are still some potential limitations to our study. First, there is a complete lack of randomized controlled trials and a limited study population size, and investigators have conducted limited studies on the efficacy of Eculizumab for TA-TMA. Second, although there is a great deal of heterogeneity among the included studies, the limited number of included studies prevents us from analyzing the sources of heterogeneity. Third, AEs are generalized in the article, so we do not have access to security data for AEs. Despite these limitations, our review is the first comprehensive meta-analysis of all eligible studies that analyzed the efficacy and safety of Eculizumab in patients with TA-TMA.

This systematic review and meta-analysis suggest that Eculizumab improves SR and ORR in patients with TA-TMA. Furthermore, patients with TA-TMA diagnosed within the first 100 days after HSCT are more likely to achieve better outcomes with Eculizumab compared to patients with TA-TMA diagnosed more than 100 days after HSCT. In addition, Eculizumab is well-tolerated, but the prevention and treatment of infection still require attention. Further RCTs and extensive prospective cohort studies are needed to evaluate efficacy and safety, particularly for Eculizumab for TA-TMA.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

RZ, MZ, and JQ contributed to the study conception and design, and writing the manuscript. RZ, WM, and ZZ performed data collection and analysis. DW and YH commented on the research design, data analysis, writing the manuscript, and supervision of the study. All authors contributed to the article and approved the submitted version.

This work was supported by the National Natural Science Foundation of China (81873432 and 81670132), grants from the Jiangsu Province of China (BE2016665, SBE2016740635 and ZDRCA2016047), The Natural Science Foundation of the Jiangsu Higher Education Institution of China (18KJA320006), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.