Abstract
Requirements for stimulation of cytotoxic T cells (CTL) and for their lytic recognition have been compared in T cell lines repeatedly stimulated with trinitrobenzene sulfonateโtreated syngeneic murine spleen cells. Differences were observed between the requirements for cells to stimulate or to be lysed by the CTL, which included: (a) the expression of major histocompatibility complex (MHC = Hโ2) encoded allelic products, and (b) the hapten density. Propagation of the CTL within the line required IโA intraโHโ2 homology between haptenโtreated stimulating cells and the line cells, whereas the lytic interaction required Hโ2K region homology between haptenโtreated target cells and CTL. The hapten density requirement was analyzed for a responder (Hโ2^k^) and a nonโresponder (Hโ2^b^) strain to low hapten density modified syngeneic cells. This property was found to be a characteristic of the lytic phase rather than of the stimulation of CTL. CTL clones could be derived by growing the line cells under conditions of limiting dilution in the presence of T cell growth factors. Such CTL clones were unable to be stimulated by their target antigens and were dependent on T cell growth factors for their propagation. These results are discussed in terms of the dependence of the development and growth of CTL on T helper cells.