Over-shedding of donor-derived cell-free DNA at immune-related regions into plasma of lung transplant recipient.

Dear editor, The discovery of the positive correlation between the fraction of donor-derived cell-free DNA (cfDNA) in the recipient’s plasma (herein denoted as donor DNA fraction) and the risk of organ transplant rejection has empowered the development of non-invasive methods for the prediction and prevention of organ transplant failure.1,2 However, as previous studies mainly focused on a global estimation of donor DNA fraction and usually have been conducted months post-transplant, some important questions remained unanswered. For example, whether the released donor DNA is an even distribution of the graft genome; if otherwise there exists certain levels of over-representation, what biological insights are underlined; and how early a signal indicative of poor prognosis and potential needs for clinical interventions may occur. To address the above questions, we examined in depth the cfDNA of 15 plasma samples (denoted as Dx samples) from three lung transplant recipients at multiple time points (Day 1/4/7/10/13) during the first 2 weeks post-transplant, plus their genomic DNA obtained pretransplant (D0 samples), using deep (≈50X)whole genome sequencing (Figure 1A). We estimated the global donor DNA fraction for each transplant recipient at each time point based on genome-wide SNP genotyping1,2 (Supporting InformationMethods; Figure 1B). Consistent with previous findings, donor DNA fraction peaked immediately after transplantation (day 1) and fell quickly (by day 4). Interestingly, after the sharp decrease at day 4, one of the recipient, patient 3, showed an acute relapse during days 10–13, while patients 1 and 2 showed flattening/slowdecreasing trends. This early dynamics was in line with patient outcome (Table 1): patient 3 was detected positive of anti-HLA-II antibodies at 26 days post-transplant and developed pleural effusion, a sign of acute lung injury at 5 weeks post-transplant; while patients 1 and 2 showed no signs of allograft dysfunction 18 months post-transplant.