temic expansion of ST2 + Tregs (29,30). IL-33 expressed by fibrogenic/adipogenic progenitors in skeletal muscle has also been shown to regulate skeletal muscle Treg homeostasis and support muscle regeneration (31). Related studies have suggested a direct, cardioprotective role for rIL-33 against hypertrophy resulting from cardiac overload (32) and fibrosis after myocardial infarction (33). However, delivery of rIL-33 also aggravates autoimmune eosinophilic pericarditis during coxsackievirus B3 infection (34), suggesting that IL-33 can contribute to cardiac inflammation. IL-33 expression has been reported in cardiac fibroblasts (32) and the vasculature ( 35), yet how the expression of this alarmin is modulated in cardiac allografts or impacts outcomes was unknown.Using IL-33-deficient heart grafts in a mouse chronic rejection model we have established that IL-33 stands out among identified alarmins and limits differentiation of proinflammatory macrophages to prevent chronic rejection. Specifically, transplants lacking IL-33 displayed dramatically accelerated chronic rejectionassociated vasculopathy and subsequent fibrosis orchestrated by graft-infiltrating recipient proinflammatory macrophages. IL-33expressing heart grafts in recipients with ST2-deficient macrophages also displayed increased graft infiltration by proinflammatory macrophages and accelerated graft loss. Mechanistic studies demonstrated that IL-33 promoted a reparative macrophage phenotype through a metabolic reprograming involving augmented oxidative phosphorylation (OXPHOS) and fatty acid (FA) uptake. We also revealed that IL-33 prevents proinflammatory stimuli-induced disruption of the tricarboxylic acid (TCA) cycle that shifts macrophage metabolism to anaerobic glycolysis and generates proinflammatory metabolites (36,37). Restoration of IL-33 to IL-33-deficient heart transplants using vesicles in ECM-derived hydrogel immediately after transplantation profoundly reduced the frequency of proinflammatory myeloid cells in the graft and prevented graft loss to chronic rejection. Thus, the local delivery of IL-33 in ECM-based materials after transplantation may be a practical and promising biologic for chronic rejection prophylaxis.