ICOS and thymoma: Arbour et al. investigated the expression of PD-L1 in the membrane of tumor cells, CD3+ and CD8+ tumor-infiltrating lymphocytes (TILs), co-stimulatory such as CD137, glucocorticoid-induced TNFR-related protein (GITR) and inducible T cell costimulator (ICOS), and co-inhibitory immune checkpoint molecules (PD-1, CTLA-4, and T cell immunoglobulin and mucin-domain-containing protein, TIM-3) in tissue samples from 23 patients with TET (12 thymomas and 11 thymic carcinomas) [11].