*Purpose: Kidney transplantations are one of the most common transplantations in the world; 80.696 kidneys were transplanted in 2017. Despite tremendous improvements in quality and tolerability of immunosuppressive medications, numerous kidney grafts are rejected by the recipient’s body. To detect and treat acute rejection as early as possible follow-up examinations after kidney transplantation are mandatory. An acute T cell mediated rejection can only be confirmed by biopsy, which resembles the diagnostic gold standard. However, these sensitive examinations are not harmless and, like any operation, involve risks like bleeding and infection. Therefore, an easy-to-use lateral flow assay (LFA) for the patient was developed in the here presented project.

*Methods: Samples from kidney transplant recipients were analyzed by multiplex testing to quantify the concentration of sCD25 (also known as IL-2Rα). sCD25 is the soluble form of the α-subunit of the IL-2 receptor, which is produced by certain T cells after allogen contact. Based on the identified range of relevant sCD25 concentration in patient urine and plasma samples a LFA was established. Initially, spiked buffer was used and the lateral flow assay was established. Following, samples from patients with T cell mediated rejection as well as samples of unsuspicious patients as negative controls were applied onto the optimized LFA.

*Results: An association between acute T cell mediated rejections and sCD25 in urine could be determined (urine: N=7 rejectors, 3.7 ± 2.1 pM versus controls N=20 controls, 2.6 ± 1.0 pM, p<0.001; Mann Whitney U test). In the lateral flow assay using spiked buffer a limit of detection of 25 pM was realized. When samples from patients with T cell mediated rejection were applied onto the optimized LFA all rejections were successfully and clearly identified. Samples of unsuspicious patients were used as negative controls and did not show a rejection.

*Conclusions: In this study, the range of relevant sCD25 concentration in urine of patients with a T cell mediated rejection could be determined. Furthermore, a lateral flow assay which can detect this range was successfully developed.

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