Drug toxicity is one of the most common causes of kidney injury in hospital. There is neither an effective therapy nor an effective diagnostic test. Extracellular vesicles (ECVs) are released into human urine from all regions of the nephron1, so downstream kidney tubular cells are exposed to ECVs originating proximally. ECV signalling could be contributing to pathological cell-to-cell signalling, raising the question of their role in renal injury and their potential as therapeutic targets. Collecting duct (CCD) immortalised cells were injured with lithium chloride (LiCl; vehicle, 52mM and 300mM) for 24 hours. ECVs were harvested from the supernatant using ultracentrifugation. These ECVs were co-cultured with healthy ‘recipient’ CCD cells for 24 hours, before these cells were, in turn, injured with LiCl (vehicle, 52mM and 100mM). ECV concentration was normalised using protein concentration. Cell injury was quantified by caspase 3/7 activity. Inhibition of endocytosis was achieved by treatment of the cells with Dynasore 150μM, 1 hour prior to ECV addition. Nanoparticle tracking analysis was used to quantify the ECV population, as per our group’s previously documented method2. Normalising the ECV pellet to protein concentration reliably normalises the concentration of ECVs with a 20-100nm diameter (Vehicle ECV: 95.8±22.5; 52mM LiCl ECV: 101.1±12.2; 300mM LiCl ECV: 116.7±3.0 AUC of number of particles x106/ml, n=3, NS). ECVs isolated from cells treated with LiCL (52mM) protected recipient cells against subsequent LiCL (100mM) induced apoptosis (-30.47±7.6 vs -2.96±9.6% for vehicle, n=6, p<0.05). This effect was not seen at a lesser concentration of subsequent injury (52mM LiCl; -21.3±8.96 vs-5.228±18.58% for vehicle, n=6, NS). Interestingly, ECVs from 300mM cells increase caspase activity in healthy CCD cells (fig. a). Inhibition of dynamin-dependant endocytosis, prior to inoculation with ECVs, nullifies this response (figure b). Our work has raised the exciting possibility that ECVs derived from severely injured cells increase apoptosis in healthy cells but may, at an earlier stage of nephrotoxic injury, confer protection to downstream cells. This is in line with previous observations implicating ECV signalling in apoptosis, a signal which may be mediated by mRNA and miRNA exchange3-5, raising the interesting physiological and pathophysiological role of ECV signalling in the nephron.