9,10 Virtually all cells have the inherent capacity to secrete some level of IFN-α/β in response to certain viral infections. However, professional antigen-presenting cells, selleck particularly plasmacytoid dendritic cells (pDCs), are a key source of IFN-α/β. Plasmacytoid DCs are a specialized subset of
DCs whose maturation is guided by innate cytokines [interleukin-3 (IL-3), Flt2 ligand, granulocyte–macrophage colony-stimulating factor and IL-4] and signalling through pattern recognition receptors during infections.11,12 These signals promote the secretion of a variety of innate cytokines, notably IL-12, IL-18, and importantly, IFN-α/β.11,13,14 Although these cells are not as efficient at activating CD4+ T cells as monocyte-derived DCs because of their
lower expression of MHC-II, pDCs play a significant role in promoting T helper priming through cytokine secretion.15,16 In this review, we will survey recent advances in delineating the direct from the indirect effects of IFN-α/β in regulating the PLX4032 ic50 development of T-cell effector responses and its novel role in promoting T-cell memory. Since the discovery of CD4+ T-cell subsets, a major quest in T-cell biology has been to understand the signals that control the differentiation of these subpopulations. One of the first signals identified was found to control T helper type 1 (Th1) differentiation, with IL-12 being the key cytokine governing this pathway.17–19 Binding of IL-12 to its receptor (IL-12R) on CD4+ cells triggers the activation of the JAKs Jak2 and Tyk2,20 leading to the phosphorylation and activation of STAT4.21,22 Phosphorylated STAT4 plays a critical role during Th1 commitment by promoting expression of T-bet,23–26 and recent studies have defined unique roles for both STAT4 and T-bet Baf-A1 in regulating IFN-γ gene expression within committed Th1 cells.27 Finally, IFN-γ enhances both T-bet and IL-12Rβ2 expression, reinforcing IL-12-mediated Th1 commitment.28,29 Hence, in both mice and humans, IL-12 signalling through STAT4 and T-bet was established as a key pathway to IFN-γ production and the Th1 phenotype.
In parallel studies, the role of IFN-α/β in Th1 development was examined with seemingly conflicting results. In mouse, STAT4 activation was not detected in response to IFN-α/β compared with IL-12,22 yet studies with human cells reported just the opposite, suggesting a species difference in IFN-α/β-mediated STAT4 phosphorylation.30–32 However, as new and more specific reagents became available, low levels of phosphorylated STAT4 could be detected in mouse cells in response to IFN-α/β.33 The apparent species difference in STAT4 activation was found to involve STAT2.32 Like the IFNAR, STAT2 is also highly divergent across species, and the mouse sequence harbours a unique minisatellite sequence in the C-terminus that is not found in any other species.