CD4 and neoplasm: CTLA-4 (Cytotoxic T lymphocyte-associated antigen 4), a co-inhibitory receptor, is expressed on several immune cells, such as activated T lymphocytes (CD4+ T cells & CD8+ T cells), regulatory T cells, tumor-infiltrating NK cells, and splenic Kit+ CD11b− NK cells in tumor-bearing mice, and is induced on mouse NK cells upon IL-2 stimulation (298–300).