Functional and biochemical characteristics of a murine interleukin 2 receptor‐inducing factor

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Abstract
High density (resting) murine Lyt‐2+ T cells exposed in vitro to the ligand concanavalin A (Con A) remain interleukin 2 (IL2) unresponsive, i.e. do not express functional IL2 receptors, unless reconstituted with accessory cells. This finding provides a bio‐assay to define functional and biochemical characteristics of an IL2 receptor‐inducing factor (RIF). RIF bioactivity as secreted from the macrophage cell line P388‐D1 is associated with a trypsin‐sensitive protein of 44 kDa which does not need to be glycosylated and which binds to and can be eluted from hydroxylapatite and phenyl‐Sepharose. While both RIF and IL1 are produced by accessory cells the lymphokines separate from each other according to functional and biochemical criteria.Either accessory cells, RIF or the protein kinase C activator phorbol myristate acetate can substitute for each other and are equally active for the induction of IL2 responsiveness in high‐density Lyt‐2+ T cells exposed to Con A. To explain these results we conclude that in the mitogen system used, induction of IL2 responsiveness (activation) represents a two‐step event in which first cross‐linking of cell surface structures by the ligand Con A excites the responder T cells, which subsequently respond to the accessory cell product RIF.

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