We conducted a systematic review and meta-analysis to evaluate the incidence and survival of kidney allografts in patients with de novo TMA. This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines and The Cochrane Collaboration form [30, 31].

We systematically searched PubMed, Cochrane Library, and EMBASE (until April 2022). A manual search of the reference lists of relevant studies was performed to complement our search results. Search terms included kidney transplantation, de novo, and thrombotic microangiopathy, including all subheadings of the Medical Subject Headings and text searches for articles that were not indexed. No language restrictions were used to reduce funnel plot asymmetry. Automatic e-mail updates were built to periodically acquire new research results from the databases. Full details of the search strategy are presented in Supplementary Table S1. The reference lists of the relevant reports were manually searched to identify any missing relevant research articles or strategies.

All randomized controlled trials, observational studies, case reports, and case series were included in this systematic review and meta-analysis if they reported the following: 1) kidney allograft recipient; 2) de novo TMA; 3) incidence; or 4) graft survival. The exclusion criteria were as follows: 1) studies without retrievable endpoints; 2) studies with recurrent TMA; and 3) studies with posters or editorial comments only. The titles, abstracts, and contents were screened by three authors (C-YHs, S-HW, and C-YHu) to determine whether the studies met the inclusion criteria. The full texts of potentially relevant studies were retrieved and assessed in more detail.

Three reviewers (C-YHu, S-HW, and C-YHs) independently assessed the studies for eligibility and extracted the data using a standardized data extraction form. Disagreements were resolved through a discussion with a fourth author (H-YC). The following parameters were extracted from each study: general characteristics (first author, year of publication, study terms, study design, and country), patient characteristics (number of patients in each treatment arm, patient age, sex, kidney donor types, genetic variants for complement dysregulation, cause of end-stage renal disease [ESRD], anti-rejection regimen, kidney pathological features, treatment of TMA, and follow-up duration), TMA incidence, and kidney allograft survival.

All cohort studies that met the inclusion criteria were subjected to quality appraisal using the Newcastle–Ottawa Scale, which contains 8 items within 3 domains and a total maximum score of 9 for cohort studies. Scores of 7–9 indicate high quality, 4–6 indicate high risk, and 0–3 indicate a very high risk of bias. All the case reports that met the inclusion criteria were subjected to quality appraisal using the CARE checklist and were recorded as “YES,” “PARTLY,” or “NO,” according to information reported by the included studies. The responses were assigned scores of 1, 0.5, and 0, respectively. The overall score was the sum of the 21 sub-items and was defined as “high” (more than 15), “medium” (10.5–14.5), and “low” (less than 10) [32]. These quality assessments were judged independently by two reviewers (S-HW and C-YHs), and any conflict was discussed with the third reviewer (C-YHu).

The study outcomes were the de novo TMA incidence and graft survival rates. De novo TMA incidence was divided into systemic and renal-limited TMA. Some studies that were not classified as systemic or renal-limited TMA were classified as unknown type of TMA. Therefore, we reported the following four different TMA incidences: 1) systemic TMA, 2) renal-limited TMA, 3) total TMA, and 4) unknown type of TMA.

The graft outcomes included graft loss and graft survival. Some studies showed graft survival of 1, 2, 3, 4, 5, 8, and 10 years.

De novo TMA incidence was reported as a percentage and event per person-year. The pooled estimated incidence of de novo TMA was reported with a 95% confidence interval (CI). The graft survival rate was reported as a percentage. The pooled estimated graft survival was also reported with a 95% CI.

The effect of baseline characteristics on the incidence of de novo TMA was analyzed. These factors included C4d, acute AMR, acute cell-mediated rejection, and the use of tacrolimus or cyclosporine. A random effects model was used for the meta-analysis.

We used the MedCalc statistical software version Medal 20.110 (Acacialaan 22 8400 Ostend Belgium) to conduct meta-analyses and SPSS version 23.0 (IBM Corp., Armonk, New York) for descriptive analyses. Statistical heterogeneity of studies was assessed using I² (inconsistency) from the fixed-effects model. All results were analyzed using a random-effects model if I² was greater than 50% to minimize the potential heterogeneity effect and between-study variance. For descriptive analyses, continuous data were reported as mean ± standard deviation. p < 0.05 was considered statistically significant.

Overall, 229 potentially relevant articles were identified in the literature search. Based on the review of the titles and abstracts, 126 studies were excluded. Further, 103 full-text articles were assessed for their eligibility; 31 records were excluded for the reasons of posters, insufficient data, or an editorial protocol. Finally, 75 studies met the inclusion criteria. Supplementary Figure S1 summarizes the flowchart of the search. Of the 75 included studies, 46 were case reports and case series, 21 were single-arm studies, and 8 were cohort studies. Eculizumab was approved for aHUS treatment by the Food and Drug Administration of the United States in 2011. Most of the studies published in 2012 collected data before 2012. Therefore, we categorized studies as published before 2013 and after 2013 (Supplementary Figure S2).

Supplementary Table S4 summarizes the characteristics of single-arm and cohort studies. The percentage of males in the included studies ranged from 17% to 77.8%. The recruitment years of the studies ranged from 1980 to 2019, and 13 studies were published before 2013. The mean age was not reported in 9 studies, while the mean age reported in the other 20 studies was >23 years. The proportion of sex was not reported in 15 studies, whereas the male sex percentage was ranged from 0% to 77.7% in 14 studies. The study population was divided into kidney allograft recipients and renal biopsy recipients. The causes of ESRD included presumed chronic glomerulonephritis, presumed chronic interstitial nephritis, IgA nephropathy, focal segmental glomerulosclerosis, diabetic nephropathy, nephrosclerosis, lupus nephropathy, polycystic kidney disease, and hypertensive nephrosclerosis; however, they were not reported in 22 studies. Regarding management, CNI adjustment was reported in seven studies, two studies reported the efficacy of plasma exchange (PE), and three studies reported eculizumab therapy. Moreover, the proportion of AMR was mentioned in five studies, whereas the proportion of ABO-incompatible cases was mentioned in one study. Finally, one study reported pregnancy outcomes.

Supplementary Table S2 and Supplementary Figure S3 summarize the characteristics of the case reports and case series. A total of 46 case reports and case series of 62 kidney allograft de novo TMA recipients were identified. A total of 42 (68%) recipients were tacrolimus users, 15 (24%) were cyclosporine users, and 5 (8%) were sirolimus users. The gene mutation data were limited. Of the 42 tacrolimus users, 9 possessed a complement factor H mutation, 2 possessed a complement factor I mutation, 1 possessed a factor II mutation, and 1 had a factor V mutation. Further, 5 of the 62 patients had a history of kidney transplants, 20 were living donor recipients, and 38 were deceased donor recipients. The onset timings (mean ± SD) of TMA were 11.26 ± 37.38, 16.68 ± 32.99, and 1.71 ± 2.96 months among tacrolimus, cyclosporine, and sirolimus users, respectively.

Six patients had AMR, six had cell-mediated rejection, and six had C4d+ on kidney pathology. Five patients were ABO-incompatible. The management of TMA included tapering the CNI and sirolimus dose, and then shifting to other immunosuppressive agents, eculizumab therapy, PE or infusion therapy, or belatacept therapy. A total of 34 (55%) patients received PE or infusion, and 18 (29%) patients received eculizumab therapy. The follow-up periods, months (mean ± SD) were 19.10 ± 37.23, 14.81 ± 13.74, and 4.45 ± 4.68 among tacrolimus, cyclosporine, and sirolimus users, respectively. Finally, 8 of the 62 individuals showed graft loss, whereas 48 individuals showed improvement in serum creatinine levels.

The detailed de novo TMA incidence in the individual studies is summarized in Table 1. Among the studies included in our analysis, 20 reported on the incidence of de novo TMA. Of them, 18 studies focused on de novo TMA in kidney allograft recipients, whereas the remaining 2 studies [16, 36] reported on de novo TMA detected in kidney allograft biopsies. Two studies reported only on the incidence of TMA, without specifying the number of kidney allograft recipients or biopsies involved [34, 36]. Therefore, these studies were excluded from the meta-analysis.

Among kidney allograft recipients, the overall incidence of de novo TMA was 3.2% (95% CI: 1.93–4.77) (Figure 1A). The incidence of systemic and renal-limited de novo TMA was 1.38% (95% CI: 0.65–2.39) and 2.79% (95% CI: 1.27–4.91), respectively (Figures 1B, C). The unknown type of TMA incidence was 3.64% (95% CI: 1.50–6.67) (Figure 1D). All the outcomes showed significant heterogeneity (I² > 89%). Stratifying the analysis based on distinct follow-up periods provided information on the overall incidence of thrombotic microangiopathy (TMA). Within 5 years follow up time, the TMA incidence was 1.04% (95% CI: 0.16–2.68) with significant heterogeneity (I²: 92.17%) (Supplementary Figure S4). As the follow-up duration extended to the 5–10 years, the TMA incidence was 3.02% (95% CI: 2.23–3.92) with low heterogeneity (I²: 0.00%) (Supplementary Figure S5). If follow-up was more than 10 years, the TMA incidence was 4.15% (95% CI: 1.64–7.75) with significant heterogeneity (I²: 95.54%) (Supplementary Figure S6). In a cohort study conducted in 2003, the incidence of de novo TMA in kidney allograft recipients was 4.9 episodes per 1,000 person-years [3].

The incidence of de novo TMA among kidney allograft biopsies ranged from 0.26% to 4.8% across the studies [34, 36, 39]. The incidence of systemic de novo TMA was 0.26% [39].

The detailed individual de novo TMA graft survival rate is summarized in Table 2. Our analysis included a total of 18 studies that reported on kidney allograft survival, of which, 8 were eligible for inclusion in the meta-analysis [16, 17, 37, 38, 41, 45, 47, 50]. The overall graft loss rate of de novo TMA was 33.79% (95% CI: 26.14–41.88). No significant heterogeneity (I² = 18.04%) was observed (Figure 2). The meta-analysis of seven studies reporting 1-year graft survival outcomes revealed a rate of 55.39% (95% CI: 36.46–73.54). However, a substantial degree of heterogeneity (I² = 88.12%) was observed (Supplementary Figure S7).

33% of patients with CNI-related TMA (4 out of 12) developed ESRD, while all patients with rejection-associated TMA developed ESRD [37]. The overall 1-year graft survival rate was 47%, whereas the 5- and 10-year graft survival rates were 35%. Additionally, there was no significant difference in the graft survival rate between the renal-limited and systemic TMAs (p = 0.4) [37].

In one study, among 33 C4d-positive TMA patients, 23 (70%) underwent plasmapheresis, with a graft loss rate of 35% (8 out of 23). Conversely, the remaining 30% (10 patients) did not receive plasmapheresis, and among these, the graft loss rate was higher at 50% [16].

All observational studies scored from 6 to 9 on the Newcastle–Ottawa Scale criteria and were included in the quantitative analysis (Supplementary Table S3). Five cohort studies were considered to be of high quality (Newcastle–Ottawa score ≥ 7).