Acute rejection after kidney transplantation should ideally be diagnosed prior to immunologic injury in a non-invasive fashion in order to improve long-term graft function. Donor-derived cell-free DNA (ddcfDNA) is a promising method to do so as it is elevated prior to acute rejection and has good predictive performance especially for antibody-mediated and high severity T-cell mediated rejection. Its ability to predict low severity T-cell mediated rejection and future graft function remains equivocal. Regulatory T cells (Tregs) are essential in transplant tolerance by suppressing effector immune responses. Circulating post-transplant highly suppressive HLA-DR+ Tregs were reduced in recipients who developed acute rejection. Preliminary results in a cohort including predominantly low severity T-cell mediated rejection also showed that pre-transplant circulating highly suppressive TNFR2+ Tregs were reduced in and could predict acute rejection. Integrating dd-cfDNA with HLA-DR+TNFR2+ Treg could improve the predictive performance for acute rejection especially of low severity and potentially predict graft function. Plasma dd-cfDNA and HLA-DR+TNFR2+ Tregs will be measured in 150 kidney transplant recipients at scheduled intervals during the first 6 months post-transplant. Predictive accuracy of a model integrating ddcfDNA and HLA-DR+TNFR2+ Treg for acute rejection will be tested using ROC curve analysis and multivariate logistic regression. Predictive accuracy for 1-year graft function will be tested using multivariate linear regression. High predictive performance for acute rejection and graft function using a model integrating dd-cfDNA and HLA-DR+TNFR2+ Treg would help identify kidney transplant recipients at immunologic risk early on and allow personalization of immunosuppression accordingly.