Cell‐free factor substitute for “signal 2" in generating cytotoxic reactions

Abstract Cytotoxic T lymphocytes in the responding cell population recognize allogeneic target antigens on ultraviolet light (uv)‐treated stimulating cells in a mixed leukocyte culture. However, under these conditions no cytotoxic response is generated unless the responding cells are also stimulated with x‐irradiated or mitomycin C‐treated cells differing by the H‐2 I region‐determined LD antigens. We refer to the signal given to the cytotoxic T lymphocyte by its target antigen as signal 1, and to the help which the cytotoxic T lymphocyte receives presumably from the LD‐responsive T helper cell as signal 2. We have developed a method in which different cell populations are separated by an agar layer to demonstrate that a soluble “factor(s)” can both potentiate a weak cytotoxic reaction as well as allow the development of cytotoxicity against uv‐treated stimulating cells. These results support the two‐signal model in which a “factor”, presumably produced by helper T cells responding to LD antigens, can give signal 2 to the cytotoxic T lymphocyte that has recognized target determinants.