IL-9 was originally described as a TH
2-derived cytokine. Unlike classical TH
2 cytokines, IL-4 alone cannot induce IL-9 expression 6
. Indeed, TGF-β was found to be a crucial factor for inducing IL-9 and maximal production could be achieved by adding IL-4. Moreover, recent data have suggested that IL-9-producing T cells are distinct from a conventional TH
2 lineage 7,8
. However, the regulation of these cells remains unclear. Here, we found that IL-17RB is expressed by IL-9-producing T cells and regulates IL-9 production in vitro
and in vivo
IL-17RB is a single transmembrane protein that binds to IL-25 with high affinity. We previously showed that IL-17RB is expressed by TH
2 but not by TH
1 or TH
17 cells and IL-25 functions to promote the differentiation of TH
2 cells 12
. In this study, we describe that TH
9 cells also expressed IL-17RB and IL-25 plays additional role in promoting TH
9 differentiation. . Because IL-25 could not replace IL-4 in inducing TH
9 differentiation and IL-4 induced IL-17RB expression, it is probable that optimal IL-4 induced IL-17RB expression on T cells is necessary to promote IL-9 production. Indeed, naïve T cells from CD4-IL-17RB transgenic mice secreted substantial amounts of IL-9 in response to IL-25 in the absence of IL-4. In contrast, TGF-β appears not only to regulate IL-17RB expression but also acts with IL-25 signaling to induce IL-9 expression. Future studies will be needed to elucidate how downstream signals of the receptors for TGF-β, IL-4 and IL-25 act in inducing and/or enhancing IL-9 expression.
In addition to IL-17RB, IL-17RA is essential for IL-25 function. IL-25-mediated type-2 immune responses requires IL-17RA and IL-17RB 15
, however direct function of both receptors for IL-25 activities on T cells is unclear. Our data analysis of TH
cell differentiation in vitro
indicate that IL-17RA and IL-17RB is essential for IL-25-enhanced TH
2 and TH
9 differentiation in vitro. Thus, endogenous expression of IL-17RA in T cells and inducible expression of IL-17RB may contribute to the determination of TH
lineages. The physical interaction of IL-17RA and IL-17RB indicate that IL-25 functions need association of both receptors to form receptor complex. In addition, the cytoplasmic domain of IL-17RB was required for IL-25 function in enhancing the production of Th2 cytokines and IL-9, suggesting that not only IL-17RB mediates ligand recognition, it contributes to signal transduction in T cells. Recent studies indicate the essential of adaptor protein Act1, also known as CIKS (originally derived from connection to IκB kinase and stress-activated protein kinases) in IL-25-mediated allergic inflammation 18,19
. Requirement of Act1 in IL-25-enhanced Th2 and Th9 differentiation remains elusive. Dissecting the signal transduction pathways involved in IL-25 activities on T cells requires further investigation. Previous work has shown IL-17R-deficient mice show reduced airway inflammation in association with attenuated TH
2 responses 22
; these defects may therefore not be due solely to a defect in IL-17 signaling but also to a defect in IL-25-signaling.
TGF-β, IL-2, and IL-4 can regulate IL-9 expression6,23
. In CD4-IL-17RB transgenic T cells that secrete large amounts of IL-9 in response to IL-25, inhibition of IL-4 and IL-2 had little effect on IL-9 expression. However, neutralization of TGF-β abolished the production of IL-9. Furthermore, over-expression of IL-17RB in IL-4-deficient T cells resulted in normal induction of IL-9 expression, although it did not affect IL-5 and IL-13 expression. These data suggest that IL-25 uses different mechanisms to mediate TH
2 differentiation and IL-9 expression. IL-25 may promote TH
2 polarization through an IL-4-dependent pathway, whereas the regulation of IL-9 expression by IL-25 is independent of IL-4 but dependent of TGF-β signaling. Our in vitro
data not only indicate IL-25 as a novel IL-9 regulator, but also support the notion that IL-9 and TH
2 cytokines expression is regulated by common but also by distinct regulatory mechanisms.
Our in vivo
experiments confirm the importance of IL-25 in regulating IL-9 expression. Because IL-25 is an important regulator in allergic asthma 12,13,24,25
and IL-9 is a key player in airways inflammation and asthma 4,5,26
, IL-25 may contribute to IL-9-mediated lung inflammation. In mouse model of allergic lung diseases, the enhanced IL-9 expression in IL-17RB transgenic mice and reduced IL-9 expression in IL-25-deficient mice indicate the importance of IL-25 in the regulation of IL-9. Inhibition of IL-9 production in IL-17RB transgenic mice that led to reduced allergic inflammation in these mice clearly suggest that IL-25 is one regulator of IL-9 in vivo. Because of IL-25 can regulate the production of Th2 cytokines from non-T-non-B cells and the differentiation of Th2 and Th9 in allergic diseases, it may serve as an important therapeutic target for treatment of allergy and asthma.
In conclusion, we have provided in vitro and in vivo evidence that IL-25 acts as a regulator of IL-9-producing T cells through the expression of IL-17RB on these cells. Our results indicate an important function of IL-25 in coordinating both conventional TH2 cytokine and IL-9 expression. Further research on the regulation of TH9/TH2 subsets and their function in immune-mediated diseases are required to understand the complex effector mechanisms involved in immune responses.