Immune-checkpoint inhibitors (ICI) have revolutionized cancer therapy but are associated with infrequent but lethal myocarditis, for which management remains uncertain. Abatacept, a CTLA-4 fusion protein targeting CD86 on antigen presenting cells and leading to global T-cell anergy, has been described as a potential treatment in individual reports. Yet, abatacept treatment dosage, schedule and optimal combination with other immunosuppressive therapies are unclear. We describe a 25-year-old man who developed pembrolizumab (anti-PD1)-induced myocarditis 14 days after first injection for thymoma treatment, which deteriorated into cardiogenic shock, with sustained ventricular arrhythmia, requiring urgent extracorporeal life support implantation, despite prompt initiation of corticosteroids and mycophenolate-mofetil. Using a strategy of serial measurement ensuring with a target of >80% CD86 receptor occupancy on circulating monocytes, abatacept dose was adjusted and combined with ruxolitinib and methylprednisolone. This strategy resulted in high-dose of abatacept: 60 mg/kg in three doses (20 mg/kg each) within the first 10 days, followed by two doses. Clinical improvement occurred within 7 days, with resolution of systolic cardiac dysfunction, and ventricular arrhythmias resulting in successful discharge from hospital. We reversed a case of nearly lethal ICI-myocarditis, using specific patient-dose adjusted abatacept, which may serve as basis for personalized treatment of patients with severe ICI-adverse events. Trial registration number: NCT04294771 .