Background: Cancer is a leading cause of death among KTR, but these patients (pts) have been excluded from trials of immune checkpoint inhibitors due to immunosuppression and risk of allograft loss. We report findings from the first prospective clinical trial of NIVO + TACRO + PRED +/- IPI in KTR with selected advanced cutaneous cancers. Methods: The primary composite endpoint was lack of tumor progression per RECIST v1.1 without allograft loss at 16 weeks (W) on NIVO. Adult KTR with advanced melanoma, basal, cutaneous squamous, or Merkel cell carcinoma (MEL, BCC, CSCC, MCC), for whom non-immune therapies were insufficient were eligible. Immunosuppression was standardized to low-dose TACRO (goal trough 2-5 ng/mL) + PRED 5mg daily; pts then received NIVO 480mg IV q4W. Pts with progressive disease (PD) could receive NIVO 3mg/kg + IPI 1mg/kg IV q3W x 4 followed by NIVO 480mg IV q4W. Donor-derived cell-free DNA (dd-cfDNA) levels were measured q2W as a potential predictor of allograft rejection. Results: From 11/2019 - 4/2021, of 12 pts enrolled, 8 pts with CSCC, MCC or MEL were evaluable for response (Table). All pts experienced PD on NIVO; treatment-related allograft loss (TRAL) occurred in 1 pt. 6 pts then received IPI + NIVO. Responses: 2 (33%) with marked tumor regression at 6W and eventual complete response (CR; 1 with TRAL), and 4 (67%) with PD (1 with TRAL). 7/8 pre-NIVO tumor biopsies contained a paucity of infiltrating immune cells. Only 2/5 on-NIVO biopsies demonstrated moderate immune infiltrates; both of these pts later developed a CR to IPI + NIVO. Rejecting allografts contained dense immune responses (plasma cells, CD4+ & CD8+ lymphocytes, PD-1+ lymphocytes, macrophages, PD-L1+ glomerular endothelium, and focal PD-1 & PD-L1 positivity in renal tubules). In 2/3 pts with TRAL, elevations in dd-cfDNA levels occurred 10 and 15 days earlier than increases in weekly serum creatinine levels. TRAL #3 occurred after discontinuation of study therapy (including TACRO) and dd-cfDNA monitoring. Conclusions: In KTR receiving low-dose TACRO + PRED, NIVO augments tumor immune cell infiltration in some pts but is insufficient to mediate tumor regression. Adding IPI can enhance anti-tumor immunity and mediate tumor regression. TACRO + PRED was insufficient to prevent allograft rejection after PD-1 +/- CTLA-4 blockade in 2/8 pts. In pts with TRAL, increased dd-cfDNA levels preceded increased serum creatinine. Based on these findings, we are modifying the trial therapy regimen to augment anti-tumor immunity and preserve allograft function. Clinical trial information: NCT03816332.