Synergy of IL-12 and IL-18 for IFN-γ Gene Expression: IL-12-Induced STAT4 Contributes to IFN-γ Promoter Activation by Up-Regulating the Binding Activity of IL-18-Induced Activator Protein 1

Abstract

IL-12 and IL-18 synergistically enhance IFN-γ mRNA transcription by activating STAT4 and AP-1, respectively. However, it is still unknown how STAT4/AP-1 elicit IFN-γ promoter activation. Using an IL-12/IL-18-responsive T cell clone, we investigated the mechanisms underlying synergistic enhancement of IFN-γ mRNA expression induced by these two cytokines. Synergy was observed in a reporter gene assay using an IFN-γ promoter fragment that binds AP-1, but not STAT4. An increase in c-Jun, a component of AP-1, in the nuclear compartment was elicited by stimulation with either IL-12 or IL-18, but accumulation of serine-phosphorylated c-Jun was induced only by IL-18 capable of activating c-Jun N-terminal kinase. The binding of AP-1 to the relevant promoter sequence depended on the presence of STAT4. STAT4 bound with c-Jun, and a phosphorylated c-Jun-STAT4 complex most efficiently interacted with the AP-1-relevant promoter sequence. Enhanced cobinding of STAT4 and c-Jun to the AP-1 sequence was also observed when activated lymph node T cells were exposed to IL-12 plus IL-18. These results show that STAT4 up-regulates AP-1-mediated IFN-γ promoter activation without directly binding to the promoter sequence, providing a mechanistic explanation for IL-12/IL-18-induced synergistic enhancement of IFN-γ gene expression.

Cytokines as critical co-stimulatory molecules in modulating the immune response of natural killer cells

“…By using pharmacological inhibitors of these pathways (SB203580 for p38 and U0126 for Erk), we completely blocked the synergistic expression of IFNg [11], thus demonstrating that both pathways are critical for the synergistic increase in gene expression. These results are most consistent with the model proposed by Nakahira and colleagues [13] where increased pSTAT4 (resulting from the IL-12 treatment) results in increased recruitment of AP-1 to the IFNg promoter [note that in our model, cross-linking LY49D results in increased AP-1 activity, (McVicar DW personal communication)]…”

CD40 Ligand Trimer Enhances the Response of CD8+ T Cells to Mycobacterium tuberculosis

“…CD40LT elicited production of the c-Jun, but not the c-Fos, component of AP-1. The combination of IL-12 and IL-18 markedly increases production and phosphorylation of c-Jun, enhancing binding to AP-1 sites of the IFN-␥ promoter (25). Because we found that the capacity of CD40LT to enhance IFN-␥ production by CD8 ϩ T cells was dependent on the presence of IL-12 and IL-18, we speculate that this effect is mediated in part through c-Jun.…”

“…Binding of CD40L on T cells to CD40 on APCs elicits production of IL-12 and IL-18 (22)(23)(24), and IL-12 and IL-18 enhance production of IFN-␥ by CD4 ϩ T cells (25,26). IL-15 also contributes to IFN-␥ production by CD8 ϩ cells in response to mycobacteria (27).…”

“…The transcription factor AP-1 is composed of dimers of members of the Jun and Fos families, and IL-12 and IL-18 markedly increase production and phosphorylation of c-Jun, enhancing binding to the IFN-␥ promoter (25). Because the experiments above suggested that CD40LT enhanced IFN-␥ production through production of IL-12 and IL-18, we studied the effect of CD40LT on expression of c-Jun in CD8 ϩ T cells from five healthy tuberculin reactors cultured with M. tuberculosis-infected monocytes.…”

cis-Element clustering correlates with dose-dependent pro- and antisignaling effects of IL18

“…IL12 cooperation with IL18 has been speculated to function via IL18 receptor upregulation and STAT4 activation. 17,20 Apart from interferon gamma expression, little is known about other genes regulated by IL18 alone or in combination with other cytokines.…”