Purpose: In kidney transplant (KT) biopsies, ABMR is characterized by histological and molecular findings, donor-specific antibodies (DSA) and C4d staining (C4D). Recently, the presence of ABMR without detectable DSA has been reported in about 50% of ABMR cases at the time of indication biopsy, and donor-derived cell-free DNA (dd-cfDNA) was found to be superior to DSA in predicting ABMR. We investigated the differences in dd-cfDNA levels, presence of C4d staining and time to biopsy in patients with DSA-positive and DSA-negative ABMR.
Methods: KT recipients at a single site (Hospital do Rim, Sao Paulo, Brazil)  with indication biopsies were enrolled; matched blood samples were analyzed for dd-cfDNA fraction (the ProsperaTM test, Natera, Austin, TX).
Results: Of the 452 enrolled patients, median time from transplant to biopsy was 50.2 months (IQR 29.4-101.2) and 21.6 months (IQR 5.1-61.5) in DSA-positive and DSA-negative ABMR groups, respectively (n.s.). 60 patients (13%) had AR at the time of indication biopsy (10 ABMR, 36 TCMR, 14 mixed features). Among the 24 patients with ABMR ( ABMR only and episodes of mixed rejection), 12 (50%) were DSA-positive and 12 (50%) DSA-negative at the time of biopsy. Median dd-cfDNA fraction was 1.64% (IQR: 1.13%-6.58%) among DSA-positive ABMR cases compared to 3.30% (IQR 0.60%-5.47%) among DSA-negative ABMR (n.s.). Mean dd-cfDNA fraction was 3.79% among DSA-positive ABMR cases compared to a mean of 3.73% among DSA-negative ABMR (n.s.). Within the DSA-positive ABMR group, 17% (n=2) of cases were C4d-positive. None of the DSA-negative cases were C4d-positive. In all ABMR biopsies, dd-cfDNA was more frequently positive (≥1%) than DSA (71% vs. 50%).
Conclusions: Similar to previous reports, this analysis shows that 50% of ABMR cases do not have detectable DSA. In both DSA-positive and DSA-negative ABMR cases, dd-cfDNA levels were elevated compared to non-rejection cases and the difference in median dd-cfDNA between the two ABMR cohorts was not significant. These data support previous reports that at the time of indication biopsy, dd-cfDNA fraction may be a better indicator of ABMR activity than DSA status alone. Clinicians assessing graft dysfunction should consider dd-cfDNA testing in DSA-negative KT recipients as it may have implications in the management of these patients.