Usefulness of C5b9 deposition analysis for the diagnosis, treatment, and monitoring of patients with transplant-associated thrombotic microangiopathy

Abstract

Abstract Background: Transplant-associated thrombotic microangiopathy (TA-TMA) is a life-threatening complication of allogeneic hematopoietic cell transplantation (allo-HCT), with reported incidence rates ranging from 10% to 35%, and associated with substantial morbidity and mortality. Its pathogenesis is multifactorial, with endothelial injury and complement activation recognized as key pathogenic drivers. Although tissue biopsy is the diagnostic gold standard, it is often impractical due to procedural risks. Therefore, indirect diagnostic approaches using laboratory markers have been developed. Among these, soluble C5b9—a terminal product of complement activation— is considered a useful biomarker of complement activation, and is used, among others, for establishing an indirect diagnosis of TA-TMA. However, similar to what has been observed in atypical hemolytic uremic syndrome, we hypothesize that direct detection of membrane attack complex (C5b9) deposition on endothelial cells may provide a more specific indicator of pathogenic complement activation at the tissue level. This study explores the potential role of C5b9 deposition as a diagnostic and monitoring biomarker in TA-TMA. METHODS: Plasma samples from 7 adult allo-HCT recipients with clinically diagnosed TA-TMA from two Spanish centers were collected at different disease stages. C5b9 deposition on cultured endothelial cells was evaluated via immunofluorescence, quantified by the stained area over total cell area, and expressed as fold increase (x) over control plasma. Clinical characteristics and treatment responses were retrospectively analyzed. RESULTS: Among the 7 patients evaluated, TA-TMA was diagnosed between day +25- and +8-months post-transplant. Underlying diseases included ALL (n=5), myelofibrosis (n=1), and aplastic anemia (n=1). All patients received PTCy-based prophylaxis, with donor types including matched related and unrelated donors. Clinically, patients showed malignant hypertension (71%), acute kidney injury (57%), myelitis (29%), and/or neurological symptoms (14%). Concomitant complications included severe acute GVHD (n=3) and viral infections (HHV-6, EBV, BK virus, enterovirus). Diagnostic criteria were met in all cases using ≥4 of 7 standard parameters, including schistocytosis, thrombocytopenia, anemia (Hb <85 g/L), elevated LDH, and proteinuria. Plasma soluble C5b9 levels were elevated in all tested patients. No biopsies were performed. Six cases were classified into high-risk and one was standard-risk. At diagnosis, C5b9 deposition on endothelial cells was markedly increased in all patients (mean 6.9-fold over control plasma; range: 4.9–8.3), significantly higher than in controls. Targeted treatment of potential triggers (GVHD, infections) was administered in 3 (43%) cases, while 4 (57%) patients received anticomplement therapy. Initially, clinical benefit was observed in 6 of 7 patients (86%): 3 achieved complete responses, 3 had partial responses, 1 remained with stable disease. All patients were alive at the last follow-up, and 4 (66.6%) remained on therapeutic or maintenance anticomplement therapy. Follow-up assessments of C5b9 deposition were conducted in 6 patients, observing a decreasing trend in C5b9 deposition in 4 patients. Two of these patients were treated with anticomplement therapy and showed a rapid decline from x4.9 to x0.6 in 28 days and from x7.7 to x1.0 in 14 days, respectively. The other two patients, where TA-TMA was managed indirectly by treating the underlying trigger, C5b9 deposition also decreased, but showing more gradually trends (from x7.0 to x2.3 over 8 months, and from x8.3 to x6.3 over 3 months). In the remaining 2 cases, C5b9 deposition remained elevated despite treatment. Both patients experienced neurologic worsening during anticomplement maintenance with clinical signs of central nervous system involvement. Both required reinduction due to loss of response. Interestingly, this clinical decline was associated with persistently elevated endothelial C5b9 deposition, despite normal levels of soluble C5b9 in plasma. CONCLUSION:Overall, a reduction in endothelial C5b9 deposition correlated with active TA-TMA and with clinical improvement, suggesting its utility for diagnosis, disease monitoring and treatment guidance. It offers potential for guiding early, targeted treatment and for tracking therapeutic response over time, particularly in patients receiving complement inhibitors.