*Purpose: The kSORT (Kidney Solid Organ Response Test) assay uses transcriptomic profiling of peripheral blood to predict the presence of acute rejection (AR) in renal transplant recipients on calcineurin-inhibitor (CNI)-based immunosuppression. This study seeks to determine if the kSORT assay can reliably predict the presence of AR under belatacept-based immunosuppression.

*Methods: 702 blood samples were collected from 212 renal transplants participating in a randomized trial comparing CNI-based (Group C, r-ATG/TAC/MMF) and belatacept-based [(Groups A (ALE/BELA/MMF) and B (r-ATG, BELA/MMF)] immunosuppression. Blood samples were collected at months 3, 6, and 12 and at the time of for-cause biopsy for kSORT analysis. Each of the 17 previously identified genes for the kSORT predictive assay were analyzed for correlation with rejection based upon treatment regimen. Results were reported as high risk (HR), indeterminate (IR) and low risk (LR) for rejection.

*Results: Sensitivity and specificity of the kSORT assay for predicting AR under belatacept was much lower than when used to predict AR under CNI based immunosuppression (Tables 1, 3). The kSORT analysis was able to predict AR at the time of biopsy proven acute rejection (BPAR) in 33.3% and 58.3% of patients in Groups A and B, respectively, with low prediction rates in samples collected between 0-3 months and >3months prior to BPAR (Tables 2, 4). No statistically significant correlation between any of the 17 gene kSORT validation set and acute rejection in the belatacept groups were observed (Figure 1).

*Conclusions: The kSORT assay performance in predicting rejection was low in patients receiving belatacept immunosuppression. Additional studies to identify alternative gene sets may enable better acute rejection predictability under belatacept-based immunosuppression.

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