PDCD1 and neoplasm: The majority of the three TIL subsets express low to intermediate levels of CD44, whereas the CD44int cells are the lowest in TIM-3+PD-1+T cells.30 Owing to these results, we speculate that exhausted T cells in TME favor the differentiation of effector/memory T cells instead of central memory T cells, through which the long-term maintenance of anti-tumor immunity is impaired (Figure 1).