Urinary (u) CXCL9 protein expression can noninvasively diagnose ACR and serial measurements during CNI withdrawal indicate that elevations in uCXCL9 detect subclinical rejection. How elevated uCXCL9 values respond to therapy as a symbol of reversing ACR is unknown. Clinical utility of such an assay further requires rapid detection that could be applied at point-of-care. We used biolayer interferometry (BLI) in which refraction of incident white light is proportional to binding of antigens to an antibody-linked probe in real time. Using a sandwich ELISA adaptation of BLI we detected uCXCL9 over a range of 36.9 pg/mL to > 5 ng/mL within 40 minutes. The assay is specific for CXCL9 without cross reactivity to CXCL10. When compared to results of standard ELISAs (n=180), we observed a correlation coefficient of 0.78; using 200 pg/mL as the threshold for defining a positive result (derived from published data), BLI and ELISA showed 100% concordance. We then serially and prospectively collected urine from 25 subjects beginning at a time of biopsy for acute transplant dysfunction over the ensuing 8 weeks, measured CXCL9 by BLI and correlated the results with pathological diagnoses, treatment and subsequent clinical course. Positive uCXCL9 (>200 pg/ml) was detected in all subjects with ACR (n=7, mean 5356 pg/ml), 5 with borderline ACR (mean 145 pg/mL), 1 subject with BKV (204 pg/mL), but none of the subjects with normal pathology (n=6, mean 64.3 pg/mL), IFTA (n=3, mean 76.7 pg/mL), or AMR (n=3, mean 110 pg/mL). Initial anti-rejection therapy normalized serum creatinine and resolved elevated uCXCL9 to <200 pg/mL within 7 days (n=4). Persistent elevation of uCXCL9 beyond day 7 signaled ongoing ACR and/or AMR unresponsive to initial therapy even in the context of resolving serum creatinine elevations (n=8). Rapid detection of uCXCL9 detection by BLI can potentially be used as a point of care approach for diagnosing ACR and guiding subsequent therapeutic decision making in kidney transplant recipients.

CITATION INFORMATION: Gandolfini I, Harris C, Purroy C, Nair V, Reid-Adam J, Bestard O, Heeger P. Rapid Detection of Urinary CXCL9 as a Diagnostic and Prognostic Tool for Managing Acute Cellular Rejection (ACR) in Kidney Transplantation. Am J Transplant. 2016;16 (suppl 3).

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