Background: Chronic graft loss after the first year remains substantial; Early detection of rejection after kidney transplantation is key to maintain long-term graft function. The current diagnosis of allograft rejection mainly   relies on clinical monitoring and a kidney biopsy. There is a need for a non-invasive diagnostic technique with good early predictive values to determine graft injury. Exosomes are important mediators of intercellular communication and contain molecular cargo from parental cells. Our hypothesis is that exosomes from the injured kidney can have a unique profile of altered proteins compared to a stable allograft/control.  The aim of the study was to isolate and characterize recipient's post-transplant urinary Exosome protein profile.
Methods: urine from pediatric and adult kidney transplant recipients at SCMCI and RMC and healthy controls was collected. Exosomes were extracted using an ultracentrifuge method and quantified using Nanocyte. Proteins were analyzed using LC/MS compared between patients with acute rejection and stable/chronic glomerulopathy.
Results: Twenty-six patients ages 3-24 years old, 11 females (42%) were included. The cohort included 5 (18.5%) healthy controls, 8 patients (30%) with chronic allograft glomerulopathy, 9 (33%) with stable allograft function and 5 (18.5%) patients with acute rejection. A total of 1,888 valid proteins were identified, of which 96 distinct proteins were significantly higher (75) or lower (21) in the acute rejection group compared to any of the other patient groups (FDR p-value < 0.1). Pathway enrichment analysis revealed a highly significant enrichment of complement and coagulation cascades (count=22, FDR p-value = 2.96E-25) and many other KEGG and GO pathways. Fifteen proteins had significantly higher expression in the acute rejection group compared to all of the other patient groups (FDR p-value < 0.1), 6 of which belonged to the complement and coagulation cascades (enrichment FDR p-value = 8.35E-08).
Conclusions: 
There was a differential expression of proteins comparing acute rejection and controls.
Exosomal proteome serves as a promising avenue of biomarker discovery for acute kidney allograft rejection.