An Exosomal mRNA Urine Test for Detection and Risk Stratification of Human Kidney Transplant Rejection.

Introduction: We recently discovered 2 urinary exosomal mRNA signatures to identify and differentiate T-cell-mediated rejection (TCMR) from antibody-mediated rejection (ABMR) in kidney transplant recipients. Here, we developed Exosome Transplant Rejection Urine (ExoTRU), a urinetest based on a 4-gene signature from the previous discovery cohort, showed its clinical utility in a new cohort of kidney transplant recipients undergoing clinically indicated biopsies, and validated it through a separate laboratory in an independent-cohort of patients. Methods: A workflow suited for clinical laboratories was developed, allowing for smaller urine volumes and widely standardized qPCR instrumentation. A total of 226 urine samples from 214 patients were paired with clinically indicated biopsies. Urinary exosomal mRNAs levels were evaluated for previously defined targets. Results: Four mRNAs (IL32, B2M, CXCL11, and PGK1) performed well in distinguishing biopsies with rejection or significant inflammation from those without inflammation, achieving 94% sensitivity, 62% positive predictive value, and 52% specificity. Patients who tested positive by the signature but negative by biopsy were nearly twice as likely to experience adverse outcomes in the 5-year follow-up period, including subsequent rejection, thereby showing the limitations of kidney biopsies and the prognostic potential of molecular signatures. The evaluation of an independent validation cohort showed similar performance, achieving an area under the curve (AUC) of 0.838. Another 6-gene signature distinguished TCMR from ABMR, with an AUC of 0.756. Conclusion: Exosomal mRNA gene signatures identified patients with different stages and classes of rejection, including early stage and significant inflammation, enabling improved decision-making and patient management and reducing unnecessary biopsies by 45%. This represents a potential tool for risk stratification based on poor outcomes in patients with positive signatures.