Background Graft-derived cell-free DNA (GcfDNA), which is released into the
blood stream by necrotic and apoptotic cells, is a promising noninvasive organ
integrity biomarker. In liver transplantation (LTx), neither conventional
liver function tests (LTFs) nor immunosuppressive drug monitoring are very
effective for rejection monitoring. We therefore hypothesized that the
quantitative measurement of donor-derived cell-free DNA (cfDNA) would have
independent value for the assessment of graft integrity, including damage from
acute rejection. Methods and findings Traditional LFTs were performed and
plasma GcfDNA was monitored in 115 adults post-LTx at three German transplant
centers as part of a prospective, observational, multicenter cohort trial.
GcfDNA percentage (graft cfDNA/total cfDNA) was measured using droplet digital
PCR (ddPCR), based on a limited number of predefined single nucleotide
polymorphisms, enabling same-day turn-around. The same method was used to
quantify blood microchimerism. GcfDNA was increased >50% on day 1 post-LTx,
presumably from ischemia/reperfusion damage, but rapidly declined in patients
without graft injury within 7 to 10 d to a median <10%, where it remained for
the 1-y observation period. Of 115 patients, 107 provided samples that met
preestablished criteria. In 31 samples taken from 17 patients during biopsy-
proven acute rejection episodes, the percentage of GcfDNA was elevated
substantially (median 29.6%, 95% CI 23.6%–41.0%) compared with that in 282
samples from 88 patients during stable periods (median 3.3%, 95% CI 2.9%–3.7%;
p < 0.001). Only slightly higher values (median 5.9%, 95% CI 4.4%–10.3%) were
found in 68 samples from 17 hepatitis C virus (HCV)–positive, rejection-free
patients. LFTs had low overall correlations (r = 0.28–0.62) with GcfDNA and
showed greater overlap between patient subgroups, especially between acute
rejection and HCV+ patients. Multivariable logistic regression modeling
demonstrated that GcfDNA provided additional LFT-independent information on
graft integrity. Diagnostic sensitivity and specificity were 90.3% (95% CI
74.2%–98.0%) and 92.9% (95% CI 89.3%–95.6%), respectively, for GcfDNA at a
threshold value of 10%. The area under the receiver operator characteristic
curve was higher for GcfDNA (97.1%, 95% CI 93.4%–100%) than for same-day
conventional LFTs (AST: 95.7%; ALT: 95.2%; γ-GT: 94.5%; bilirubin: 82.6%). An
evaluation of microchimerism revealed that the maximum donor DNA in
circulating white blood cells was only 0.068%. GcfDNA percentage can be
influenced by major changes in host cfDNA (e.g., due to leukopenia or
leukocytosis). One limitation of our study is that exact time-matched GcfDNA
and LFT samples were not available for all patient visits. Conclusions In this
study, determination of GcfDNA in plasma by ddPCR allowed for earlier and more
sensitive discrimination of acute rejection in LTx patients as compared with
conventional LFTs. Potential blood microchimerism was quantitatively low and
had no significant influence on GcfDNA value. Further research, which should
ideally include protocol biopsies, will be needed to establish the practical
value of GcfDNA measurements in the management of LTx patients