It has taken two to three decades for Th17 cells to be identified as a new lineage of effector CD4+
T cells following the original description of Th1 and Th2 cells. This delayed recognition reflects, in part, the relative paucity of Th17 cells after conventional immunization with antigen. In the absence of a cytokine milieu required to induce and expand Th17 development, stimulation of T cells from antigen-immunized mice often generates less than 1% Th17 cells, compared with much higher proportions of Th1 cells (Park et al., 2005
; Gocke et al., 2007
). Because generation of murine Th17 cells from naïve cells requires IL-6 and TGFβ (Veldhoen et al., 2006
; Bettelli et al., 2006
), and IL-23 for Th17 expansion (Park et al., 2005
; Harrington et al., 2005
), a relative insufficiency of these cytokines may dampen the Th17 response. However, a more efficient mechanism responsible for pre-emption of Th17 responses probably depends on the IFNAR-dependent inhibitory pathway described in this report.
The negative impact of IFNAR engagement on Th17 responses was mediated through inhibition of Opn-i expression, which in turn enhanced IL-27-dependent inhibition of Th17 development. We have not ruled out the possibility that IFNAR signaling in T cells may also inhibit expansion or survival of Th17 cells. Although extremely low amounts of IFN-I were sufficient to effectively initiate this IFNAR-dependent pathway in DC and attenuate Th17 development, increasing amounts of IFN-I, e.g. after certain viral infections, may be necessary to fully inhibit Th17 responses. According to this view, IFN-I constitutively produced at low amounts in the absence of viral infection may chronically dampen Th17 responses, whereas increased IFNAR signaling secondary to rapid increases in IFN-I amounts after viral infections may be necessary for increased IL-27-dependent inhibition of Th17 responses. In contrast, certain bacterial or fungal infections that disrupt this inhibitory pathway may provoke robust Th17 responses and increase the risk of autoimmune disease. It may be relevant that Th17 generation is also suppressed by expression of the IFNγ receptor on APC (Batten et al., 2006
), and the potential similarities between IFNα and IFNγ-dependent suppression of the Th17 response via interactions with DC deserve further investigation.
IL-27, a potent inhibitor of Th17 cell development, can also induce IL-10-producing T cells (Awasthi et al., 2007
; Fitzgerald et al., 2007
). Our DC-T cell co-cultures do not produce sufficiently high IL-27 concentrations to enhance IL-10-producing cells, suggesting that inhibition of Th17 responses that we observe does not reflect enhanced production of IL-10. In addition to being a potent inhibitor of Th17 cell development (Stumhofer et al., 2006
), Huber et al
. have shown that IL-27 also inhibits TGFβ-mediated expansion of Treg (Huber et al., 2008
). Low concentrations of IL-27 sufficient to suppress Th17 responses in cultures containing Opn-deficient DC did not substantially alter TGFβ-dependent generation of Foxp3+
Treg cells in this study, emphasizing the striking susceptibility of the Th17 response to low concentrations of IL-27 and also indicating that Treg induction is not required to inhibit the Th17 response in our experiments.
Previous studies have indicated that Opn-i expression in pDC promotes expression of IFNα and contributes to early protective responses against HSV-1 infection and tumor growth (Shinohara et al., 2006
). Here we show that Opn-i expression by conventional DC (cDC) (and microglia) contributes to Th17 commitment in vitro
and in vivo
in the setting of EAE. Since Opn expression is induced after TLR engagement of pDC but not cDC (Shinohara et al., 2006
), Th17 generation may be negatively regulated by pDC-dependent production of IFNα through engagement of IFNAR expressed by cDC. Additional experiments are necessary to evaluate the contribution of an inhibitory interaction between DC subsets: IFN-I-producing pDC (which do not themselves promote detectable Th17 responses) and IFNAR-producing cDC in regulating the Th17 response.
) gene expression in T cells is essential for efficient Th1 development (Ashkar et al., 2000
; O’Regan et al., 2000
; Chabas et al., 2001
; Shinohara et al., 2005
; Sato et al., 2005
; Renkl et al., 2005
), whereas expression in cDC promotes Th17 differentiation. The contribution of Opn to the development of these Th lineages reflects cell-type specific expression of the two Opn isoforms. Secretion of Opn (Opn-s) by T cells leads to an interaction with its receptors on macrophages that upregulates IL-12 production and enhances Th1 development. In addition, Opn-deficient T cells produced less IL-17 than Opn wt T cells in cultures containing Opn wt DC, suggesting that secreted Opn (Opn-s) may also augment the Th17 response under some circumstances, possibly through a direct interaction that inhibits apoptosis (Hur et al., 2007
). In addition to inhibition of IL-27, Opn-i may also promote Th17 responses through enhancement of DC-T cell interactions. Opn-i-dependent polarization of the actin cytoskeleton may enhance formation of immunological synapses necessary for efficient T cell activation and helper lineage commitment (Al Alwan et al., 2003
; Shapiro et al., 2003
; Benvenuti et al., 2004a
; Benvenuti et al., 2004b
; Maldonado et al., 2004
; West et al., 2004
We tested two different models of EAE: (1) naïve 2D2 CD4+
T cell transferred into Rag2−/−
hosts and (2) irradiation BM chimera reconstituted with 2D2 BM cells (in both models, transferred cells were Opn-deficient to rule out excess Opn-s). EAE development was robust in the former system, characterized by efficient T cell activation and infiltration into the CNS (data not shown), allowing us to examine full-blown EAE in Opn-deficient mice. The latter model, which allowed analysis of the role of Opn in APC, revealed that Opn-deficient APC promoted a dominant Th1-type of EAE. Enhanced Th1 responses in the CNS of Opn-deficient Rag2−/−
hosts were associated with increased IL-27 secretion by DC and/or microglia, that may have resulted in enhanced IFNγ responses (Pflanz et al., 2002
; Cao et al., 2008
). Analyses of Th17 and Th1 components of CNS cellular infiltrates at different stages of EAE induced by different autoantigens may define clinically distinct subsets of this murine model of MS, and provide insight into clinical subtypes of MS.
Definition of the IFNAR signaling pathway in DC leading to suppression of Th17 responses described in this report provides insight into the essential elements that govern the Th17 response. In addition, these findings are relevant to the use of IFNβ as the leading treatment for patients suffering from MS (Paty and Li, 1993
; Hafler et al., 2005
). The ability of IFNβ, an innate immune cytokine with anti-viral properties, to inhibit autoimmune inflammation and reduce the incidence of disease relapse has been poorly understood. Suppression of the development of pro-inflammatory Th17 cells provides a mechanistic explanation for the therapeutic effects of Type I interferon therapy in MS. These findings also suggest that analysis of the effects of IFNAR engagement by antibodies or mutant IFNs that differential engage the Opn IL-27 pathway described here may yield new and effective therapies for MS.