PDCD1 and neoplasm: Conversely, chronic exposure of antigens to tumor-infiltrating lymphocytes (TILs) can cause the expression of PD-1 on T cells leading to blunted immune responses The higher frequency of PD-1 expression of intratumoral CD8+ T cells [Figs. 2(b) and 2(c)] suggests potential overstimulation of the TCR by sustained exposure to tumor antigens.