Primed Before Procurement: Rethinking Complement, Brain Death, and the Window for Donor-targeted Therapy

Donation after brain death (DBD) remains the predominant source of organs for transplantation. Although outcomes for DBD allografts are generally acceptable, they remain inferior to those from living donors, even when HLA mismatches exist.1 This disparity underscores the unique pathophysiology of brain death, characterized by hemodynamic instability, hormonal dysregulation, and systemic inflammation. Complement activation has long been implicated as a key mediator of this injury, contributing to ischemia/reperfusion injury and delayed graft function (DGF). Yet the precise dynamics of complement activation during donor management have not been well defined. In this issue of Transplantation, Laura et al,2 present the most comprehensive analysis to date of systemic and intrarenal complement activity in DBD kidney donors. Using the UK Quality in Organ Donation biobank, they analyzed EDTA-plasma and biopsies collected at 3 standardized points: immediately after brain death declaration (DB2), at the end of donor management (DB3), and just before cross-clamping (DB4). This approach allowed the temporal evolution of complement activation to be mapped with unprecedented resolution and directly linked to transplant outcomes. The first key observation is that systemic complement activation is both rapid and sustained in DBD donors. Across all time points, plasma levels of C4d, Bb, C3c, and C5b-9 were significantly higher than in living donor controls, confirming that both classical/lectin and alternative pathways are triggered soon after brain death. Elevated activation fragments were already present at the earliest sampling point, with upstream markers particularly pronounced. Although C3c and C5b-9 remained stable during donor management, C4d and Bb declined over time but never approached the levels of living donors. These results confirm that complement is rapidly and robustly engaged after brain death, with activation sustained through to procurement. Importantly, variability at the earliest time point was not explained by donor characteristics such as sex, hypertension, cytomegalovirus status, or cause of death, suggesting intrinsic differences in inflammatory response. These observations are consistent with earlier reports from rodent and human studies,3–6 but the present study advances the field by defining precise temporal kinetics and showing that activation persists throughout donor management rather than being confined to procurement. The second important finding is that complement activation decreases over time with prolonged brain death support. Stratification by duration showed no cross-sectional differences at single time points, but paired analyses revealed significant within-donor declines in upstream fragments. C4d and Bb fell between DB2 and DB3 in donors with long brain death duration, with similar downward trends in C3c. Even short-duration donors exhibited a measurable decline in C4d. This pattern suggests that the most intense complement activity occurs soon after brain death induction, with subsequent attenuation under standardized donor management. Whether this decline reflects true downregulation or consumption of complement proteins remains uncertain, as intact proteins were not measured. Nonetheless, the findings challenge the assumption that every additional hour of brain death inevitably worsens graft injury. Instead, they raise the possibility that careful hemodynamic support and stabilization may allow the immune system to partially recalibrate, reframing time as a potential ally when donor care is optimized. Beyond allotransplantation, this observation has implications for xenotransplantation research, where decedent donor models with prolonged brain death support are increasingly used.7,8 These models reflect not only the early inflammatory surge but also the subsequent reduction in innate immune signaling, particularly complement activity. Such inherent downregulation may complicate the interpretation of xenograft outcomes, as responses could be influenced by both diminished donor-derived complement activity and xenogeneic incompatibility. The third key finding is that localized complement deposition in the kidney correlates with outcomes. Biopsy analysis revealed widespread staining for C4d, C3d, and C5b-9 in DBD kidneys compared with living donor controls, but deposition at the vascular pole was particularly enriched in grafts that later developed DGF. This microanatomic site, adjacent to the juxtaglomerular apparatus, is physiologically critical for renal hemodynamics and may represent a focal point where hemodynamic stress and innate immunity converge. The association between vascular-pole deposition and early dysfunction underscores the importance of tissue-level readouts, which may serve as actionable biomarkers for assessing organ function. Together, these findings carry several important implications. The demonstration that complement activation is early, dynamic, and partially attenuated over time reframes the period between brain death declaration and procurement as a therapeutic window. Targeted interventions during donor management or on ex vivo perfusion circuits may blunt complement initiation before reperfusion amplifies injury. Near-patient testing could make this strategy feasible. Rapid assays for complement activation products in EDTA-plasma, combined with point-of-care platforms and digital pathology, could enable real-time donor stratification and immediate tailoring of therapy. Several inhibitors are already clinically available and could be repurposed, including C1s blockade (sutimlimab), MASP-2 inhibition (narsoplimab), factor B/D inhibition (iptacopan and danicopan), and terminal C5 blockade (eculizumab and ravulizumab).9 The observation that upstream products peak early suggests that interventions targeting initiation and amplification cascades may be particularly effective in donors or during ex vivo perfusion. As with all biobank-based studies, limitations must be acknowledged. Biopsies were available for only half of the donor cohort, limiting power for compartment-specific analyses. The focus on short-term outcomes (DGF and 3-mo estimated glomerular filtration rate) is appropriate for a donor-centered study, but longer-term endpoints remain unexplored. Critically, intact complement proteins were not measured, leaving open whether reductions reflect true attenuation or consumption. Finally, although complement activity correlated with dysfunction, causality cannot be assumed, as complement may act as both a mediator and a biomarker of broader inflammatory stress. In summary, this study provides compelling evidence that complement activation in DBD donors is an early, dynamic, and anatomically focused process. The most intense activity occurs shortly after brain death induction, followed by partial attenuation under continued management, reframing time as a potential therapeutic ally rather than a liability. Localized deposition at the vascular pole correlates with early graft dysfunction, offering a tissue-based biomarker for organ assessment. By linking systemic dynamics, temporal attenuation, and histologic localization to outcomes, this work provides a strong rationale for precision donor management and for integrating complement monitoring and pathway-specific inhibition into transplantation practice.