AbstractThe requirements for activation and growth of T lymphocytes capable of mediating either cytolytic activity or help to B lymphocytes were studied in unprimed splenic T cell populations. The selectivity of expression of Lyt‐2 antigens, the reactivity to soluble concanavalin A (Con A), to partially purified interleukin 2 (IL2, T cell growth factors) and to lectin‐pulsed macrophages (MΦ) were used in this analysis. Lectin‐dependent cytotoxicity assays and a novel method that allows for the detection of all effector helper cells, regardless of their clonal specificities, were used for the functional identification of the responding T cells.The results show a marked contrast between cytolytic and helper T cells in their growth and activation requirements. Thus, while Lyt‐2+cytotoxic T lymphocyte precursors grow exponentially in IL2 after a short pulse with soluble Con A in the absence of accessory cells, Lyt‐2−helper cell precursors completely fail to proliferate under the same conditions and require the continuous presence of lectin‐pulsed MΦ for significant growth. Furthermore, addition of IL2 to MΦ‐stimulated cultures of Lyt‐2−cells has no effect. T cells which produce IL2 have the same growth characteristics as helper cells. In both cases, effector helper functions could be expanded more than 10‐fold on a per cell basis by a 5‐day‐culture period under those growth supporting conditions. The development of effector helper functions, however, was strongly inhibited by t
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