PDCD1 and neoplasm: In addition, low response rates upon PD-1 blockade therapy are attributed to the paucity of pre-existing tumor infiltrating lymphocytes (TILs), of which CD3 + T cells (total T lymphocytes) and CD8 + T cells (T effector cells) act as major mediators of the adaptive immune response (Galon et al. 2006; Waldman et al. 2020).