CD28 and neoplasm: CD28 costimulation also plays a crucial role during the activation of antigen-experienced T cells such as tumor-specific effector T cells (Agarwal et al., 2023; Duraiswamy et al., 2021; Kamphorst et al., 2017; Magen et al., 2023), and the rescue of exhausted CD8+ T cells by PD-1–targeted therapies (Kamphorst et al., 2017).