Polarized HLA Class I Expression on Renal Tubules Hinders the Detection of Donor-Specific Urinary Extracellular Vesicles

Liang Wu,1,2 Martijn H van Heugten,3 Thierry PP van den Bosch,4 Hans Duimel,5 Carmen López-Iglesias,5 Dennis A Hesselink,2 Carla C Baan,2 Karin Boer2 1Department of Nephrology, the First Affiliated Hospital of Shaoyang University, Shaoyang, Hunan, People’s Republic of China; 2Erasmus MC Transplant Institute, University Medical Center Rotterdam, Department of Internal Medicine, Division of Nephrology and Transplantation, Rotterdam, the Netherlands; 3University Medical Center Rotterdam, Department of Internal Medicine, Division of Nephrology and Transplantation, Rotterdam, the Netherlands; 4Department of Pathology, University Medical Center Rotterdam, Rotterdam, the Netherlands; 5The Microscopy CORE Laboratory at the Faculty of Health, Medicine and Life Sciences, Maastricht University, Maastricht, the NetherlandsCorrespondence: Liang Wu, Erasmus MC Transplant Institute, Department of Internal Medicine, University Medical Center Rotterdam Erasmus MC, Room No. Na-514, Doctor Molewaterplein 40, 3015 GD, Rotterdam, the Netherlands, Tel +31 0622145029, Email l.wu.1@erasmusmc.nlPurpose: Kidney transplantation is the optimal treatment for patients with end-stage kidney disease. Donor-specific urinary extracellular vesicles (uEVs) hold potential as biomarkers for assessing allograft status. We aimed to develop a method for identifying donor-specific uEVs based on human leukocyte antigen (HLA) mismatching with the kidney transplant recipients (KTRs).Patients and Methods: Urine and plasma were obtained from HLA-A2+ donors and HLA-A2- KTRs pre-transplant. CD9 (tetraspanin, EV marker) and HLA-A2 double-positive (CD9+ HLA-A2+) EVs were quantified using isolation-free imaging flow cytometry (IFCM). Healthy individuals’ urine was used to investigate CD9+ HLA-class-I+ uEV quantification using IFCM, time-resolved fluoroimmunoassay (TR-FIA), and immunogold staining cryo-electron microscopy (cryo-EM). Culture-derived CD9+ HLA-class-I+ EVs were spiked into the urine to investigate urine matrix effects on uEV HLA detection. Deceased donor kidneys and peritumoral kidney tissue were used for HLA class I detection with histochemistry.Results: The concentrations of CD9+ HLA-A2+ EVs in both donor and recipient urine approached the negative (detergent-treated) control levels for IFCM and were significantly lower than those observed in donor plasma. In parallel, universal HLA class I+ uEVs were similarly undetectable in the urine and uEV isolates compared with plasma, as verified by IFCM, TR-FIA, and cryogenic electron microscopy. Culture supernatant containing HLA class I+ vesicles from B, T, and human proximal tubule cells were spiked into the urine, and these EVs remained stable at 37°C for 8 hours. Immunohistochemistry revealed that HLA class I was predominantly expressed on the basolateral side of renal tubules, with limited expression on their urine/apical side.Conclusion: The detection of donor-specific uEVs is hindered by the limited release of HLA class I+ EVs from the kidney into the urine, primarily due to the polarized HLA class I expression on renal tubules. Identifying donor-specific uEVs requires further advancements in recognizing transplant-specific uEVs and urine-associated markers.Keywords: kidney transplantation, donor-specific biomarker, HLA, extracellular vesicles, human urine, renal tubule