Dissecting complement pathway activation after allogeneic hematopoietic cell transplant: Results of a monocentric longitudinal monitoring program.

Abstract

Abstract Systemic complement activation is a well-established pathophysiological mechanism involved in endothelial complications, particularly post-transplant microangiopathy (TMA), following allogeneic hematopoietic cell transplantation (allo-HCT). We previously demonstrated its impact on post-transplant outcomes in a prospective study involving patients who received a myeloablative conditioning (MAC) regimen. Here, we aimed to investigate the role of complement activation in the clinical course of an independent, contemporary cohort of patients prospectively enrolled in a longitudinal complement monitoring program at our institution between 2022 and 2024. The cohort included patients who received either MAC or reduced-intensity conditioning (RIC) regimens, covering all disease indications. Complement monitoring was performed weekly from baseline through week 12 post-transplant, assessing CH50 activity and C3/C4 levels. Activation profiles were defined by a drop in one or more of these markers below the lower limit of normal. A total of 125 patients were included in this study, with a male-to-female ratio of 2.3 and a median age of 59.4 years (interquartile range [IQR] 45.6–65.5). The cohort comprised 67 patients (54%) with acute myeloid leukemia, 13 (11%) acute lymphoblastic leukemia, 11 (8%) myelofibrosis, 6 (5%) other myeloproliferative neoplasms, 18 (14%) lymphoproliferative neoplasms or lymphoma, 9 (7%) myelodysplastic syndromes and 1 (1%) bone marrow failure. Donor types included matched related donors (n=20, 16%), matched unrelated donors (n=65, 53%), mismatched unrelated donors (n=24, 19%), and haploidentical donors (n=16, 12%). Most patients received grafts from mobilized peripheral blood (n=117, 94%), while bone marrow (n=6, 5%) and umbilical cord blood (n=2, 1%) were used less frequently. RIC regimens were administered in the majority of cases (n=107, 86%). We first evaluated the complement profile at baseline and observed that a substantial number of patients (n=20, 16%) exhibited isolated C3 activation, with median C3 levels of 30.5 mg/dL (IQR 26.7–35.2), suggesting a possible selective activation of the alternative pathway prior to the initiation of the conditioning regimen. One patient showed a global reduction across all complement markers, while another displayed isolated C4 consumption and other 4 reduction in CH50 activity, without any sign of infection or inflammation. After allo-HCT, 52 patients (41%) developed a complement activation profile at some point during follow-up between week 1 and 12. Among them, 16 patients had already exhibited an activated profile at baseline. Using time-dependent Cox regression models, we found that complement activation was associated with significantly worse overall survival (HR 3.84, 95% CI 1.55–9.49, p=0.0035) and higher non-relapse mortality (NRM, HR 5.44, 95% CI 1.49–20.16, p=0.0112), primarily driven by an increased risk of grade II–IV acute graft-versus-host disease (GvHD) (HR 2.25, 95% CI 1.28–3.95, p=0.0046). No significant associations were observed with relapse, chronic GvHD, or infections in this cohort. The limited number of endothelial events (veno-occlusive disease [VOD] n=7, post-transplant TMA n=6, capillary leak syndrome n=1) precluded firm conclusions, although most affected patients exhibited complement activation. In the TMA group, 4 of 6 patients showed persistent, isolated C3 consumption suggestive of sustained alternative pathway activation. In the VOD group, 3 patients had isolated C3 decline and one showed global complement consumption consistent with systemic classical or lectin pathway activation. Notably, the presence of a baseline (pre-transplant) complement activation profile was not associated with differences in survival or the incidence of acute GvHD. We assessed through time dependent models the immune reconstitution patterns. While no clear impact was seen for CD4 and CD8 T cell reconstitution, patients with complement activation tended to have a delayed immunoglobulin (p= 0.0801) and NK cell recovery (p= 0.0985). These findings underscore the importance of monitoring complement activity after allo-HSCT, since dynamic complement activation was associated with inferior survival and higher NRM, primarily driven by an increased risk of acute GvHD. Incorporating complement monitoring into routine post-transplant follow-up may help identify patients at risk and may open new avenues for the prevention of alloreactive complications.