Characteristics of an antigen-specific helper factor that enhances the in vitro generation of cytotoxic cells directed against a syngeneic tumor.

An antigen-specific helper factor (Hf) that enhances the in vitro generation of tumor-specific cytotoxic T cells (CL) has been isolated from the spleens of DBA/2 mice bearing the syngeneic mastocytoma, P815. This factor enhances the CL response to P815 but not to L1210 (a syngeneic T cell leukemia in DBA/2 mice), B10 minor antigens, or DBA/2 alloantigens. The factor binds specifically to P815 membrane immunoadsorbent columns and can be eluted with 2 M NaCl. P815 Hf did not resemble the lymphokine Il 2 because it did not enhance the proliferative or CL response of thymocytes nor would it support the growth of an IL 2-requiring line of T cells. This factor acted optimally if added late in the culture period in contrast to IL 2, which was more effective if added early. In limiting dilution experiments P815 Hf enhanced the level of CL activity generated but not the number of precursors triggered. IL 2 markedly amplified both of these parameters.