PDCD1 and neoplasm: Moreover, the unfavorable TME, which is characterized by hypoxia and elevated concentrations of immunosuppressive soluble factors, as well as various cells (e.g., myeloid cells and regulatory T cells) and proteins (e.g., programmed cell death protein 1 (PD‐1)) [23], hinders T cell activation [24] and prevents the infiltration of CAR‐T cells into the tumor site.