Background Developing a noninvasive clinical test to accurately diagnose kidney allograft rejection is critical to improve allograft outcomes. Urinary exosomes, tiny vesicles released into the urine that carry parent cells' proteins and nucleic acids, reflect the biologic function of the parent cells within the kidney, including immune cells. Their stability in urine makes thema potentially powerful tool for liquid biopsy and a noninvasive diagnostic biomarker for kidney-transplant rejection. MethodsUsing 192 of 220 urine samples withmatched biopsy samples from175 patients who underwent a clinically indicated kidney-transplant biopsy, we isolated urinary exosomal mRNAs and developed rejection signatures on the basis of differential gene expression. We used crossvalidation to assess the performance of the signatures on multiple data subsets. Results An exosomal mRNA signature discriminated between biopsy samples from patients with all-cause rejection and those with no rejection, yielding an area under the curve (AUC) of 0.93 (95% CI, 0.87 to 0.98), which is significantly better than the current standard of care (increase in eGFRAUCof 0.57;95%CI, 0.49 to0.65). The exosome-based signature's negativepredictive value was 93.3% and its positive predictive valuewas 86.2%. Using the same approach, we identified an additional gene signature that discriminated patients with T cell-mediated rejection from those with antibody-mediated rejection (with anAUCof 0.87; 95%CI, 0.76 to 0.97). This signature's negative predictive value was 90.6% and its positive predictive value was 77.8%. Conclusions Our findings show that mRNA signatures derived from urinary exosomes represent a powerful and noninvasive tool to screen for kidney allograft rejection. This finding has the potential to assist clinicians in therapeutic decision making.
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