CD28 and pulmonary arterial hypertension: Antigen presenting cells (APCs) such as dendritic cells andmacrophages present antigens to T cells, priming their activation.Optimal activation of T cells requires a co-stimulatory signalprovided by the interaction between CD28 on the surface of T cellsand CD80/CD86 on APCs.[76] Abatacept, a homologue of CD28,competitively binds to CD80/CD86 resulting in suppression of T cellactivation.[76] Pulmonary hypertension and acute respiratory failurehave been reported in patients receiving abatacept.[77,78]