Acute rejection is the main contributor to early allograft failure. Current immunosuppressive regimens have shortcomings, such as toxicity and minimal inhibitory effects on B cells. Hence, developing novel, effective, and selective anti-acute rejection drugs is crucial. Therefore, this study aimed to investigate the inhibitory effect of ritlecitinib (PF-06651600), a new janus kinase 3 inhibitor, on T and B cells, as well its preventive effects on acute allograft rejection. A murine cardiac transplantation model coupled with cell culture- and immunohistochemistry-based techniques was used. In vitro assays demonstrated that ritlecitinib inhibits naïve CD4