We aimed to investigate costimulatory receptor expression, particularly 4-1BB (CD137 or tumor necrosis factor receptor superfamily member 9), on tumor-infiltrating CD8<sup>+</sup> T cells (CD8<sup>+</sup> tumor-infiltrating lymphocytes [TILs]) and its association with distinct T-cell activation features among exhausted CD8<sup>+</sup> TILs in hepatocellular carcinoma (HCC).
Furthermore, CD137L expression found on tumor cell lines was functional because the ligation of CD137L on lung squamous carcinoma cells L78 with CD137 on T cells induced IFN-gamma production by T cells, and ligation of CD137L on hepatocarcinoma cells HepG2.2.15 with CD137 triggered tumor cells to produce IL-8.