CD4 and neoplasm: In addition, Treg cells (C5–CD4) were suggested to interact intensively with CD8+ Tex cells (C1–CD8 and C4–CD8) through HLA-E-KLRK1 and its subtypes, which were classified as immune inhibitors because they promote senescent cell accumulation [36] and suppress tumor-specific cytotoxic T lymphocytes [37], and HLA-E-KLRK1 was identified as a novel checkpoint in the tumor microenvironment (TME) [38].