PDCD1 and cancer: While CD8+ T cell function (such as cytolytic capacity; Rooney et al., 2015) are well established, CD4+ T cell help is important for CD8+ T cell responses in cancer (Borst et al., 2018) and both subsets are required in syngeneic tumor models in mice for anti-PD1 pathway responsiveness (Homet Moreno et al., 2016).