Biomarker Dynamics in Pediatric Transplantation-Associated Thrombotic Microangiopathy (TA-TMA): A Prospective Cohort and Nested Case-Control Study

Introduction: Transplant-associated thrombotic microangiopathy (TA-TMA) is a potentially fatal complication following allogeneic hematopoietic cell transplantation (allo-HCT). While complement activation and endothelial injury are thought to be pathogenic for TA-TMA, specific prognostic and diagnostic biomarkers remain elusive. This study aims to identify biomarkers that can distinguish patients at high risk for severe TA-TMA.

Methods: We performed a prospective cohort study at Texas Children's Hospital 2021-2024. Patients who underwent allo-HCT were enrolled and blood samples were collected sequentially. TA-TMA screening was performed prospectively every week using an electronic health record trigger tool to identify patients with concurrent schistocytosis, anemia, thrombocytopenia, and high LDH. Patients with positive screens were independently reviewed by two hematologists to determine TA-TMA diagnosis as described previously.

Patients with at least 100 days of follow-up were included in the analytic cohort. Cumulative incidence was assessed while treating early death as a competing risk. Overall survival (OS) was analyzed using the Kaplan-Meier estimator. For biomarkers, a subset of patients with sequential blood samples were chosen and a nested case/control study with 1:2 frequency matching on the underlying disease was done. Potential biomarkers tested included soluble complement 5b-9 (sC5b9), complement activation product Ba (Ba), and angiopoietin-2 (ANG2). All assay values were presented in median [1st quartile-3rd quartile] ng/mL and compared with rank sum tests.

Results: The final analytic cohort had 88 patients. The median age was 8.9 years (range 0.1-22.1), 36% were female, and 59% were Hispanic. Indications for HCT was non-malignant in 53% and first-ever allo-HCT occurred in 92%. Conditioning regimen was 43% TBI-based and 46% Bu-based. Donor type included 25% matched related, 18% matched unrelated, 21% haploidentical, and 14% cord.

The cumulative incidence of TA-TMA was 11.5% and 12.9% at 100 days and 180 days, respectively (n=14/88). The median time to diagnosis was 79 days [53-121]. At pre-HCT, in the TA-TMA group, there was a higher proportion of female patients (57% vs. 32%), those of Hispanic ethnicity (71% vs. 57%), and with non-malignant diseases (71% vs. 50%) compared to the control group. Of 14 TA-TMA patients, 8 received eculizumab and 6 did not. The two groups had similar unadjusted OS at 6 months after TA-TMA diagnosis (60% in treated vs. 67% in untreated, p=0.76).

For biomarker analysis, 12 TA-TMA were matched with 24 non-TMA controls. All assays were measured in ng/mL. At pre-HCT, only sC5b9 level was significantly higher in cases than controls (293 [228-398] vs. 232 [181-285], p=0.04). At early post-transplant (closest sample to day 15 after stem cell infusion and before TA-TMA), sC5b9 levels remained significantly higher in cases than controls (380 [290-500] vs. 236 [169-287], p=0.01). Ba levels became elevated (906 [627-1425] vs. 547 [446-826], p=0.02), but ANG2 was not significantly different (3597 [2910-7904] vs. 2721 [1939-5147], p=0.17). At the onset of TA-TMA (before TMA-directed therapy) or median time to TMA in controls (day 79), all 3 biomarkers were higher in cases than controls: sC5b9 (359 [267-473] vs. 201 [168-268], p=0.01), Ba (1556 [717-2087] vs. 699 [566-939], p=0.02), and ANG2 (7281 [4142-12788] vs. 3462 [2905-4459], p=0.01).

Conclusions: In this prospective cohort study in a large tertiary pediatric transplant center, the cumulative incidence for clinically significant TA-TMA was 13% in patients undergoing allo-HCT at 6 months. There was no unadjusted difference in survival in eculizumab-treated and untreated group. Among those who developed TA-TMA, the terminal complement activation biomarker (sC5b9) was elevated at all time points, while the upstream alternative complement activation (Ba) occurred early post-transplant around engraftment, and the endothelial injury biomarker (ANG2) later increased at the onset of TA-TMA diagnosis. The temporal patterns of these biomarkers suggest an interplay between complement activation and endothelial injury/inflammation and provide more mechanistic insight to the prognosis and diagnosis of the disease.