Differential activation of T cell cytokine production by the extracellular signal‐regulated kinase (ERK) signaling pathway

Stimulation of T cells via the T cell receptor (TCR) activates a number of signaling pathways that are potentially involved in the elicitation of physiological responses, such as the production of cytokines. The extracellular signal‐regulated kinases (ERK) are a group of molecules activated in response to TCR ligation, whose role in T cell cytokine production is controversial. In this study, we have asked whether ERK activation is coupled to the production of a number of T cell‐derived cytokines, and whether particular cytokines are differentially affected by ERK activation. To address these questions, we have utilized a constitutively active version of the immediate upstream activator of both ERK1 and ERK2, mitogen‐activated/extracellular signal‐regulated kinase 1 (MEK1), to activate ERK signaling selectively in the absence of other TCR‐activated signaling pathways. The effect of constitutive MEK/ERK activation on T cell cytokine production was measured by transiently co‐transfecting newly activated mouse T cells with DNA encoding constitutively active MEK1 (CA‐MEK1) and the human interleukin‐2 (IL‐2) receptor α chain (hCD25), purifying hCD25+ transfectants by flow‐cytometric cell sorting, and measuring the production of IL‐3, IL‐4, interferon (IFN)‐γ and granulocyte/macrophage‐colony‐stimulating factor (GM‐CSF) either in the presence or absence of ionomycin stimulation. Newly activated T cells were used in these experiments as they more closely resemble T cells activated in vivo than do transformed T cells or long‐term established T cell clones. CA‐MEK1 expression led to constitutive ERK activation, which acted synergystically with ionomycin treatment to stimulate cytokine production. Furthermore, these experiments revealed a hierarchy of cytokine responsiveness to MEK/ERK activation, such that the production of IL‐3 was most affected, followed by GM‐CSF, IFN‐γ, and IL‐4.