Numerous studies have implicated cells of the lymphoid series as agents responsible for the destruction of solid tissue homografts in vivo (1)(2)(3)(4)(5). This view has been substantiated by recent demonstrations that lymphoid cells (6-15) and macrophages (16, 17) procured from specifically immunized animals have a cytocidal effect on appropriate homologous "target" cells in vitro. It was previously reported that by the 6th to 7th day after grafting with homologous skiu, lymphocytes from the regional lymph nodes of mice or rats had acquired the capacity to injure foreign target renal cells in culture, and that neither isoimmune serum nor complement was required for this destructive reactivity (15). The present communication is concerned with the application of a sensitive, reproducible in vitro assay system to elucidate further quantitative aspects of the destruction of homologous target cells by sensitized lymphocytic cells. Particular attention has been paid to the influence of (a) variation in the ratio of attacking lymphoid cells to target ceils (b) the duration of contact with lymphoid cells and (c) the possible influence of the presence or absence of isoimmune serum on the cytocidal activities of sensitized lymphocytes.
Materials and MethodsPrinciple of the Test System.--Rats were immunized to the foreign transplantation antigens of a homologous strain. Cell suspensions consisting of known numbers of lymphocytes prepared from the lymph nodes of normal, i.e. non-immune, or sensitized animals were then added to previously "seeded" monolayer cultures of knoWn numbers of target cells of donor strain origin. After a specified period of incubation, the number of target cells remaining in the