Anti-HLA-antibody characteristics aid to risk-stratify patients and improve long-term renal graft outcomes. Complement activation by donor-specific antibody (DSA) is an important characteristic that may determine renal allograft outcome. There is heterogeneity in graft outcomes within the moderate to high immunological risk cases (cross-match-positive). We explored the role of C3d-positive DSAs in sub-stratification of cross-match-positive cases and relate to the graft outcomes. We investigated 139 cross-match-positive living-donor renal transplant recipients from four transplant centres in the United Kingdom. C3d assay was performed on serum samples obtained at pretreatment (predesensitization) and Day 14 post-transplant. C3d-positive DSAs were found in 52 (37%) patients at pretreatment and in 37 (27%) patients at Day 14 post-transplant. Median follow-up of patients was 48 months (IQR 20.47-77.57). In the multivariable analysis, pretreatment C3d-positive DSA was independently associated with reduced overall graft survival, the hazard ratio of 3.29 (95% CI 1.37-7.86). The relative risk of death-censored five-year graft failure was 2.83 (95% CI 1.56-5.13). Patients with both pretreatment and Day 14 C3d-positive DSAs had the worst five-year graft survival at 45.5% compared with 87.2% in both pretreatment and Day 14 C3d-negative DSA patients with the relative risk of death-censored five-year graft failure was 4.26 (95% CI 1.79, 10.09). In this multicentre study, we have demonstrated for the first time the utility of C3d analysis as a distinctive biomarker to sub-stratify the risk of poor graft outcome in cross-match-positive living-donor renal transplantation.

• Background: Kidney transplantation offers a better quality of life and a clear survival advantage compared to long-term dialysis. However, rates of transplantation are low for highly sensitized patients (HSP) who have a broad specificity of human leucocyte antigen (HLA) specific antibodies. This is because the presence of pre-formed HLA Donor Specific Antibodies can result in hyperacute rejection and is an immunological veto to transplantation. The proportion of eligible donors therefore decreases with increasing sensitization, resulting in increased waiting time prior to transplantation. This study aims to assess the efficacy of a combination of B cell depletion and proteasome inhibition, together with antibody removal and steroids, to reduce HLA antibodies in HSP, predicted to improve rates of transplantation, and to determine the feasibility of conducting a future definitive trial. • Methods: ITOPS is a multi-center, randomized parallel, non-blinded, controlled phase III feasibility trial. The participants will be allocated in a 1:1 ratio to either the trial intervention (rituximab followed by 1-2 cycles of plasmapheresis, bortezomib and dexamethasone) or control group (no intervention). • Discussion: The primary outcome is the proportion of patients achieving an absolute reduction in sensitization (calculated reaction frequency, cRF) of at least 10% at 12 weeks following their last intervention, compared to the control group. Secondary outcomes include recruitment and transplantation rates, durability of change and absolute percentage change in cRF, and acceptability of the intervention. We will proceed to development of a larger trial if at least 50% of the participants in the intervention arm achieve an absolute reduction in cRF of a minimum of 10% at 12 weeks following their last intervention. If the proportion is <50% in the intervention arm but 50% is included within the 95% confidence interval, then consideration to proceed to a larger randomized trial will be given alongside other aspects of feasibility.• Trial Registration: International Standard Randomized Controlled Trial Number ISRCTN66441193 https://doi.org/10.1186/ISRCTN66441193