Distinct Characteristics of Murine STAT4 Activation in Response to IL-12 and IFN-α

Abstract

The role of type I IFN in Th1 development, STAT4 activation, and IFN-γ production in murine T cells has remained unresolved despite extensive examination. Initial studies indicated that IFN-α induced Th1 development and IFN-γ production in human, but not murine, T cells, suggesting species-specific differences in signaling. Later studies suggested that IFN-α also induced Th1 development in mice, similar to IL-12. More recent studies have questioned whether IFN-α actually induces Th1 development even in the human system. In the present study, we compared the capacity of IL-12 and IFN-α to induce Th1 differentiation, STAT4 phosphorylation, and IFN-γ production in murine T cells. First, we show that IFN-α, in contrast to IL-12, cannot induce Th1 development. However, in differentiated Th1 cells, IFN-α can induce transient, but not sustained, STAT4 phosphorylation and, in synergy with IL-18, can induce transient, but not sustained, IFN-γ production in Th1 cells, in contrast to the sustained actions of IL-12. Furthermore, loss of STAT1 increases IFN-α-induced STAT4 phosphorylation, but does not generate levels of STAT4 activation or IFN-γ production achieved by IL-12 or convert transient STAT4 activation into a sustained response. Our findings agree with recent observations in human T cells that IFN-α-induced STAT4 activation is transient and unable to induce Th1 development, and indicate that IFN-α may act similarly in human and murine T cells.

The JAK-STAT Signaling Pathway: Input and Output Integration

“…However, the data can just as easily be resolved when we consider that STAT4 activation from the IFN-␣␤R is increased in the absence of STAT1. Recent data confirm this interpretation but also show that STAT4 activation by the IFN-␣␤R, although increased, cannot sustain IFN-␥ production from T cells when compared with IL-12 (38). This is probably because of the stronger differential activity of SOCS1 on the IFN-␣␤R versus the IL-12R (discussed below).…”

“…This pathway is a central regulatory event in the development of the Th1 type T cell responses. IFN-␣␤, via the IFN-␣␤R, also activates STAT4 (in addition to STAT1 and STAT2 that forms a complex with IRF-9 to mediate anti-viral gene expression) but cannot activate strong IFN-␥ production and therefore cannot drive Th1 development (38). However, in the absence of STAT1, IFN-␣␤ causes a large increase in IFN-␥ production, especially in vivo during viral infection (39,40).…”

IFN-α Is Not Sufficient to Drive Th1 Development Due to Lack of Stable T-bet Expression

“…One explanation was that IL-12 stimulation was able to maintain STAT4 phosphorylation over a longer period of time compared with IFN-␣␤. We have recently confirmed this observation in murine CD4 ϩ T cells (26); however, a molecular explanation for this difference in signaling between IFN-␣ and IL-12 has not been examined. At the core of this issue is whether IL-12 and IFN-␣␤ share redundant roles in Th commitment and whether there truly is a species-specific pathway that operates in human but not in murine T cells.…”

Activating Transcription Factor 3 Is a Positive Regulator of Human IFNG Gene Expression

“…The percentage of IFN-g-positive cells, especially cells that stained highly positive for IFN-g, was higher in the cell population expressing ectopic ATF3 than in the mock-transfected cells. The two distinct IFN-g-positive cell populations are typically detected by intracellular staining and flow cytometry (21,42,43) and can be considered as IFN-g + (high) and IFN-g + (low) cell populations. To confirm the role of ATF3 in IFN-g expression by an alternative approach, specific siRNA oligonucleotides targeting ATF3 mRNA were designed.…”