Under the influence of continuous stimulation of tumor antigens and various factors in TME (such as the immunosuppressive cytokines, immunosuppressive cells, hypoxia, lack of nutrition, acidic physical and chemical environment), the function of CD8+ T cell gradually decreases, manifested by reduced proliferation ability, susceptibility to apoptosis, and decreased levels of secreted effector cytokines (such as IL-2, IFN-γ, TNF-α), which is called “T cell exhaustion” [12]. Here, CD8A is linked to neoplasm.