In organ-specific autoimmunity, the balance of cytokines is a key determinant of resistance or susceptibility. In EAE, disease susceptibility is thought to correlate with the expression of proinflammatory cytokines such as IL-17, IFNγ, TNF, IL-6, and IL-1β. On the other hand, Th2 cytokines such as IL-4 and IL-13 have been shown to be important for preventing or ameliorating disease (26
). In the current study, we show that IL-25, a Th2-promoting cytokine, can protect mice from EAE, and loss of IL-25 results in enhanced proinflammatory cytokine production and accelerated CNS pathology. This accelerated disease is also associated with an increased number of IL-17–, IFNγ- and TNF-producing T cells that invade the CNS. Even though IFNγ-producing cells can induce EAE and their presence within the CNS likely contributed to acute inflammation, neutralization of IFNγ in il25−/−
mice did not lead to disease amelioration. In contrast, anti–IL-17 mAb treatment significantly inhibited EAE in il25−/−
mice, suggesting that the IL-17 immune pathway plays an important role in the disease process. We also demonstrate that IL-25 treatment protects mice from EAE, and this protection clearly depends on the suppression of IL-17–producing T cells. Thus, the presence of IL-25 supports a cytokine environment that limits chronic inflammatory responses.
IL-25 is constitutively expressed in the CNS (37
). Interestingly, EAE-resistant BALB/c mice constitutively expressed higher CNS levels of IL-25 mRNA compared with moderately susceptible C57BL/6 (unpublished data). In addition, we found that resident microglia is a primary cellular source of IL-25 in a normal CNS. Importantly, there is a ninefold induction of IL-25 expression in microglia during CNS inflammation. This IL-25 expression pattern supports the hypothesis that microglia have protective roles during chronic CNS inflammation. The observation of tissue-specific expression of IL-25 is consistent with our recent study in a helminth infection model, where we showed that locally expressed IL-25 is crucial for the ability of mice to clear the gut parasite Trichuris muris
). Moreover, IL-25 is also constitutively expressed in the lungs (1
). Again, highly specialized tissue-resident cells, in this case, alveolar macrophages, were shown to be the source of IL-25 (4
). Thus, IL-25 is expressed in organ systems where regulation of inflammation is of critical importance. In healthy digestive and respiratory tracts, an anti-inflammatory environment has to be maintained because of the constant exposure to commensal microbes. In immune-privileged sites, such as the eyes or the CNS, inflammation must be constrained because of the detrimental consequences of tissue swelling responses in these organs.
During EAE priming, IL-25 induces IL-4, IL-5, and IL-13 in the periphery. Although IL-4 and IL-13 have been reported to be protective in EAE, there is little evidence that IL-5 plays a regulatory role in EAE (38
). In contrast, the role of IL-4 in suppressing organ-specific autoimmunity is well documented (29
). IL-4 is required for the differentiation of Th cells toward the Th2 pathway (42
). In EAE, deviation of the immune system toward a Th2 response correlates with disease resistance (44
), and delivery of IL-4 in vivo can modulate the immune response to ameliorate disease (29
). Surprisingly, IL-25 treatment still protected il4−/−
mice from EAE, suggesting that the levels of IL-4 induced by IL-25 treatment are not sufficient for EAE suppression. In contrast to IL-4, we found that IL-13 is necessary for IL-25–mediated EAE suppression. Both IL-4Rα– (required for IL-13 signaling) and IL-13–deficient mice were not protected by IL-25 treatment, suggesting that IL-25 is acting via IL-13 to suppress disease. On its own, IL-13 protects rats from EAE, through its ability to deactivate inflammatory macrophages, as well as suppressing the production of nitric oxide, IL-1β, and TNF (27
Here, we demonstrate a novel role for IL-13 in inhibiting Th17 differentiation and function. However, IL-13 can only partially inhibit IL-17 production. In contrast, IL-4 can completely block IL-17 response, consistent with two recent reports (21
). These results suggest that IL-13 and IL-4 may use different mechanisms to inhibit Th17 cells. Indeed, although T cells are shown to express the shared IL-4/IL-13 receptor chain IL-4Rα, the specific IL-13 receptor is not expressed on T cells (35
). Therefore, IL-13 might act by inhibiting DC function (27
). Indeed, we observed that IL-13 blocked Th17-promoting factors such as IL-1β, IL-6, and IL-23.
It is intriguing that in our Th17 cultures (DC-derived IL-6, IL-1β, TGF-β plus exogenous IL-23, anti–IL-4, and anti-IFNγ), IL-25 still induced IL-13 production. We have observed that IL-25 led to IL-13 production in CD4+
T cell populations in vitro (unpublished data). However, it is still unclear which T cell populations are responsive to IL-25 in vivo. It is also possible that IL-25 acts on NKT cells, which play an important role in the early IL-4 production during an immune response (47
). NKT cells secrete IL-4, IL-13, and IFNγ upon activation and have an important immunoregulatory role (49
). Interestingly, activation of NKT cells by the synthetic ligand α-galactosylceramide can induce IL-4 and IL-13, which protects mice from autoimmune inflammation (51
). Another possible target of IL-25 may be antigen-presenting cells such as DCs. The IL-25 receptor, IL-17RB, has been found to be up-regulated in DCs under Th2-inducing conditions (54
). Consistent with pervious reports (55
), we observed that IL-25 could induce IL-13 mRNA expression in cultured DCs (unpublished data). Further studies are needed to understand how these IL-13–expressing cells regulate Th17 development.
We have shown that although IL-25 and IL-17 are members of the same cytokine family, they play opposing roles in the regulation of organ-specific autoimmunity. The type 2 responses promoted by IL-25 drive a novel regulatory mechanism for controlling Th17 responses. Unexpectedly, this regulation relies on IL-13 rather than IL-4, suggesting that IL-13 may be secreted at higher levels in the target organs during autoimmune inflammation. This protective mechanism is important because the lack of IL-25 causes devastating inflammatory responses during EAE. Thus, IL-25 expression by CNS-resident microglia may provide an important mechanism that limits brain and spinal cord inflammation. This unique property suggests that IL-25 may be a potential therapeutic agent for a range of organ-specific inflammatory disorders.