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Engagement of the T cell receptor/CD3 complex activates the serine/threonine kinase, Raf‐1, but the physiologic consequences of its activation have not been determined. The effects of Raf‐1 on interleukin 2 (IL2) production in T cells were examined using activated and inhibitory forms of Raf‐1. A truncated active form of Raf‐1 was expressed constitutively from the metallothionein promoter in a malignant T cell line, Jurkat. Treatment of the cells with zinc and cadmium greatly increased active Raf‐1 expression. This increase in Raf‐1 expression allowed antibodies to CD3 and to CD28 to stimulate IL2 production in the absence of phorbol myristate acetate (PMA) and enhanced IL2 production stimulated by these antibodies in the presence of PMA. The action of active Raf‐1 was to increase IL2 gene transcription as it enhanced transcription of a reporter gene linked to IL2 promoter. Finally, the dominant negative form of Raf‐1 inhibited transcription directed by the IL2 promoter that was induced by the mitogen phytohemagglutinin (PHA) and PMA. We conclude that Raf‐1 activity is necessary for IL2 gene transcription and secretion. These data indicate a role for Raf‐1 in the immune response.

Deletion of the amino-terminal domain of Raf-1, which contains the Ras-binding region, results in the constitutive activation of the liberated Raf-1 catalytic domain in fibroblast cell lines. We demonstrate that the MEK kinase activity of the isolated Raf-1 catalytic domain, Raf-BXB, is not constitutively active, but is regulated in Jurkat T cells. Raf-BXB is activated by engaging the antigen receptor-CD3 complex, or treating cells with phorbol myristate acetate or okadaic acid. Increasing intracellular cAMP inhibits Raf-1 activation stimulated by phorbol myristate acetate, but not the activation of Raf-BXB. Serine 621, but not serine 499, is essential for Raf-BXB MEK kinase activity. Because Raf-BXB does not bind Ras, the data establishes a Ras-independent signal in directly regulating the activity of the Raf-1 catalytic domain.

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