CD4 and atrial fibrillation: Senescent CD8+ T cells (marked by CD28 loss) and their transition from persistently activated CD8+ T cells (Effros et al., 2005), along with CD28 loss in CD4+CD28− T cells and reduced PD-1 positivity in CD4+ lymphocytes (Kounis et al., 2020; Chang et al., 2022), are linked to AF pathogenesis.