Th17 and other IL-17–expressing T cells have recently emerged as crucial regulators of inflammatory responses. However, although IL-17 has been relatively well studied, the function of IL-17F is poorly understood. In this study, we describe the expression, signaling pathway, and in vivo function of IL-17F. Our study for the first time indicates that although IL-17F has very similar regulation and function as IL-17 in vitro, analysis of animals deficient in either of the two genes has revealed their distinct functions in inflammatory responses.
IL-17F is expressed in Th17 cells and other types of IL-17–expressing T cells in vivo. Previous papers describe similar regulation of IL-17 and IL-17F expression by cytokines IL-23, TGF-β, IL-6, and IL-21, as well as transcription factors RORγt, RORα, and STAT3 (8
). Moreover, we found that IL-17
gene promoters share the same pattern of chromatin remodeling in differentiated Th17 cells (15
). Several conserved noncoding sequences in this locus, including CNS2 (29
), likely mediate the coordinated expression of IL-17 and IL-17F. However, our analysis also revealed different ratios of IL-17 and IL-17F expression in different T cell populations in vitro and in vivo. This suggests differential cytokine expression in differentiated Th17 cells. What accounts for this differential regulation is unknown at this point. The biological or pathological significance of this regulation is also unclear. In Th2 cells, we previously found that the inducible co-stimulator–c-Maf pathway only controls IL-4 but not IL-5 or IL-10 expression in the effector stage, and inducible co-stimulator deficiency selectively abrogates IL-4–dependent IgE production but not IL-5–mediated airway eosinophilia (41
). Moreover, expression of IL-22 by Th17 cells appears to be more dependent on IL-23 than IL-17 and IL-17F (43
). More studies are necessary in the future to comprehend the differential regulation of Th17 cytokines. Interestingly, our recent work revealed that RORα mutant T cells had a selective defect in IL-17 but not IL-17F or IL-22 production (29
Compared with IL-17, IL-17F has weaker activity in inducing proinflammatory molecules in vitro. However, this activity is meaningful as in vivo, transgenic overexpression of IL-17F led to pathological airway phenotypes. Why, then, do we need two similar cytokines? The situation is even more complex considering the heterodimeric IL-17A/F molecule with intermediate activity in vitro as compared with the IL-17 and IL-17F homodimers (16
). One possible explanation is that these cytokines may differentially use different cytokine receptors that are differentially expressed or alternatively spliced. IL-17, IL-17F, and IL-17A/F depend on IL-17RA for signal transduction (16
). How IL-17RA mediates IL-17F signaling is unclear at this point. Previous literature showed that IL-17F did not bind to IL-17RA in vitro (18
). We also found normal binding of IL-17F–Ig to IL-17RA−/−
macrophages (unpublished data). Thus, IL-17RA may not directly mediate binding of IL-17F to the cell surface but rather may regulate its signaling with other receptor components. IL-17 signaling also requires IL-17RC, which forms a complex with IL-17RA (10
). In our preliminary analysis, we found that siRNA reduction of IL-17RC expression in MEFs also decreased IL-17F–induced gene expression (unpublished data), suggesting the involvement of IL-17RC in IL-17F signaling. These results are consistent with a recent paper demonstrating the binding of IL-17F to IL-17RC (20
). Further studies are necessary to elucidate the receptors used by these cytokines. Although IL-17 and IL-17F may potentially bind to different receptors on the target cells, they appear to use the same signaling components. In the current study, we for the first time show the requirements of Act1 and TRAF6 in IL-17F induction of downstream inflammatory genes.
IL-17F appears to have a weaker proinflammatory function. One may predict that IL-17F may be less important than IL-17 in vivo. This may be true in the EAE model in which IL-17F is not required for the initiation of EAE with only a minor role in maintaining inflammation in the CNS. Our results using IL-17F KO mice are consistent with our unpublished data that anti–IL-17F did not ameliorate EAE (unpublished data). In contrast, IL-17 KO mice, similar to mice treated with anti–IL-17 (3
), exhibited greatly delayed onset and progression of EAE. Thus, IL-17 is thus a dominating pathogenic factor in EAE. However, our current study using mice deficient in either the IL-17 or IL-17F gene has for the first time provided a surprising insight into the differential function of these two cytokines in immune responses. During T cell priming in vivo in response to KLH or MOG peptide immunization, IL-17– but not IL-17F–deficient mice exhibited increased IFN-γ expression in the spleen. Because IL-17 has not been found to regulate Th cell differentiation (39
), this regulation may be indirect, for example, by acting on myeloid cells. On the other hand, IFN-γ expression was found reduced in draining lymph nodes in these animals, suggesting a possibility that IL-17 regulates the localization of Th1 cells. In IL-17F–deficient animals, IL-17 expression was reduced in the spleen although not significantly in the lymph nodes. However, in the late phases of EAE, such a defect was not found, suggesting that IL-17F regulation of IL-17 expression could be overcome by chronic immune responses. Furthermore, IgG2a production was consistently up-regulated only in IL-17F–deficient animals. Because IFN-γ production was not affected significantly in the same setting, it is possible that IL-17F may regulate B cell responses to IFN-γ, which is known to be important for IgG2a switching. B cells express the receptors for IL-17F (unpublished data). Whether they respond to IL-17F and what effect will be caused need to be investigated further. Nonetheless, our data suggest differential but clear involvement of IL-17 and IL-17F in the early phases of immune responses.
Evidence such as its presence in asthmatic CD4 T cell clones (22
) or its mutations in asthma patients (45
) suggests that IL-17F might be relevant to asthma. Our current study using IL-17 and IL-17F KO animals indicates that IL-17 is not required for neutrophil recruitment in innate responses to an allergen; instead, IL-17F appears more prominent in recruiting neutrophils. However, when we examined the effect of IL-17 or IL-17F deficiency on an asthma model, IL-17 KO mice, as predicted from the literature, exhibited reduced Th2 cytokine expression, whereas IL-17F KO mice had enhanced clinic symptoms such as elevated type 2 cytokines and eosinophil functions compared with WT mice. Thus, IL-17 and IL-17F may have opposite functions in chronic allergic airway diseases. The contrasting effects of IL-17 and IL-17F have also been observed in acute experimental colitis induced by DSS. IL-17F KO mice were protected, whereas IL-17 deficiency increased the colon damage. At this stage, it is difficult to comprehend the functional differences of IL-17 and IL-17F in these models. Perhaps the receptors for them are differentially expressed on target cells or these cytokines transduce differential signals that may, in turn, activate feedback antiinflammatory mechanisms. Further studies will be needed, and our data represent a first step toward a better understanding of IL-17 and IL-17F functions in vivo.
In summary, in addition to IL-17, IL-17F is expressed in Th17 cells and IL-17–expressing γδ T cells. These two cytokines may use the same signaling components, such as IL-17RA, Act1, and TRAF6, to induce similar downstream inflammatory genes. In vivo, transgenic overexpression of IL-17 or IL-I7F in the lung led to similar pathological phenotypes. However, our analysis using mice deficient in either gene has revealed their distinct functions in different types of inflammatory responses. Our results have revealed complex mechanisms underlying tissue inflammation. A further understanding of these mechanisms may shed light on cytokine targeting in the treatment of inflammatory diseases such as allergic asthma and multiple sclerosis.