*Purpose: Immune checkpoint inhibitors (ICI) are an emerging treatment for numerous advanced diseases, but their safety and efficacy in immunosuppressed transplant patients is not known. This study undertakes a scoping review of research on ICI use in solid organ transplant (SOT) recipients to determine: 1) What is the effect of ICI on SOT recipients?; 2) What factors influence the effect of ICI on SOT recipients?

*Methods: Data Sources: Searches of PubMed, Scopus and MEDLINE were performed with language restrictions on September 1, 2020. Study Selection: Any studies that reported the use of ICI in patients with a history of SOT were included irrespective of study design. Data Extraction: Three reviewers independently screened citations and performed data abstractions. A variety of variables were extracted from each publication. Data Synthesis: Of the total 549 screened articles, 50 articles met inclusion criteria with a sum of 101 cases of ICI use in patients with a history of SOT.

*Results: Graft rejection occurred in 42% (n=42) of the cases. Kidneys were the most commonly rejected organ (n=28) with PD-1 inhibitors being the often-implicated etiology (n=32). Nearly 100% of the cases of graft rejection transpired within 2 months of ICI initiation. Patients on steroid monotherapy had higher rejection rates (72%, n=13) versus those on a steroid plus one or more immunosuppressive agents. In cases where graft preservation was pursued, salvage occurred 33% of the time (n=9). Regardless of graft outcomes, patient outcomes were overall poor due to advanced disease. In cases where patient outcome was reported, 71% (n=39) died directly from disease progression.

*Conclusions: ICI offer a promising therapeutic alternative to traditional chemoradiation for patients with advanced malignancies. However, their use in patients with a history of SOT poses a significant risk to the transplanted organ and cancer outcomes are worse in patients with SOT. Future studies are needed to delineate the risk and benefit more clearly.

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