Inclusion Criteria:  Age ≥ 18 years old Patients treated with ICI immunotherapy (monotherapy or combination), including anti-PD1, anti-PDL1, anti-CTLA4; and including any type of cancer (even those in which ICI is not currently approved by regulatory) Definite, probable or possible ICI-induced myocarditis according to the diagnostic criteria of the most recent expert consensus recommendations (e.g27, to be updated with any new recommendations to be published)  Severe or corticosteroid-resistant ICI-myocarditis:  Severe ICI-myocarditis is defined either 1/ by the appearance of an alteration of the LVEF<50% or a wall motion kinetics abnormality, or 2/ by the appearance of ventricular tachycardias or high-grade conductive disorders (atrioventricular block grade 2 or 3) or 3/ by the association with myasthenia gravis-like-syndrome (diplopia, ptosis, diaphragmatic dysfunction, dysarthria, dysphonia, dysphagia). Corticosteroid-resistant ICI-myocarditis is defined by the absence of decrease in troponin levels or the appearance/persistence of severity criteria despite receiving prednisone dose ≥0.5 mg/kg/day for ≥2 days. Signature of informed consent before any trial procedure from the patient or legal representative or the close relative Patients covered by social security regimen (excepting AME) Withhold of ICI  Exclusion Criteria:  Untreated and/or uncontrolled bacterial, fungal, or viral infection Pregnancy, breast-feeding or planning to become pregnant during the study period For women of childbearing age, lack of effective contraception throughout the duration of participation in the study Being treated with abatacept or belatacept within 3 months prior to inclusion Known hypersensitivity to abatacept or belatacept Being treated with anti-thymoglobulin, or alemtuzumab within 6 weeks of the first scheduled dose of abatacept Patient participating to another interventional study (RIPH 1 only) People under legal protection measure (tutorship, curatorship or safeguard measures)