Allergic inflammation results from a complex interaction between environment, immune system and target organ that has not been well defined. It is still unclear whether a primary defect lies in a target organ that provides insufficient barrier function or in an inappropriate Th2 immune response that develops to innocuous antigen. While polymorphisms in genes required for skin barrier function are associated with AD (16
), it is unclear if these result in altered gene expression, or perhaps altered sensitivity to cytokines produced during an allergic response. In this report, we used a model of atopic dermatitis arising from T cell-specific expression of an active Stat6 to test whether predisposition to a Th2 response would result in specific changes in target organ gene expression. We demonstrated increased EDC gene expression and barrier function in the absence of endogenous IL-4, contrasting with decreased EDC gene expression and allergic skin inflammation when the immune system is predisposed towards a Th2 response. Importantly, IL-4-deficiency protects from the development of allergic skin inflammation and hastens the recovery of barrier function following skin irritation.
The ability of IL-4 to regulate basal expression of EDC genes could result from several mechanisms. Effects could be direct resulting from homeostatic levels of IL-4 present in the circulation or the tissue that act directly on the keratinocytes (5
). It has not been demonstrated whether IL-4-induced changes in keratinocyte gene expression are a result of STAT6 binding to EDC gene promoters, or interference with other transcription factors that positively regulate mRNA levels. Effects could also be indirect, and in the absence of IL-4 the production of Th1 or Th17 cytokines that affect expression of EDC genes could be altered (5
). However, we did not observe increases in mRNA levels of Ifng
in skin from Il4-/-
mice (data not shown). That expression of Ivl
was increased most by the absence of IL-4, while Flg
was decreased the most in the presence of a high Th2 environment (Stat6VT transgenic mice), suggests that EDC gene regulation is likely more complex than simply the presence or absence of IL-4.
The model of AD in Stat6VT transgenic mice is unique in that disease results from a perturbation of T cell function, not of ectopic gene expression within the skin, and is dependent on IL-4 for development. This is important as allergic skin inflammation in other mouse models of AD may appear as Th1-mediated inflammation in mice that are genetically-deficient in the ability to develop Th2 responses (3
). Thus, the dependence on IL-4 for disease development in this model is significant. The requirement for a specific effector cytokine suggests that while genetic predisposition to changes in barrier function including mutations in FLG
is an important risk factor in AD development (17
), a strong skewing of immune function can have similar effects. The increased Th2 cytokine production in Stat6VT transgenic mice decreases expression of multiple genes in the EDC complex resulting in decreased barrier function, which at least partially contributes to the development of allergic skin inflammation. Our results also provide a mechanism for the AD-like disease that develops in IL-4 transgenic models (9
While IL-4 is clearly required for the development of allergic skin inflammation in Stat6VT transgenic mice, the relative roles of IL-4 and IL-13 are not fully defined. We demonstrate that acute treatment of skin from Stat6VT transgenic mice with either anti-IL-4 or anti-IL-13 increased the level of Ivl
mRNA. Other reports have shown that treatment of keratinocytes with IL-4 and IL-13 can decrease EDC gene expression (4
). But while transgenic mice expressing IL-4, or IL-4 and IL-13, develop allergic skin inflammation (7
), the ability of IL-13 alone to promote allergic skin inflammation has not been determined. The decreases in IL-4 production caused by targeting the Il13
locus prevent a precise determination of the role of IL-13 in this model (15
). Importantly, the ability of IL-4-deficiency to prevent allergic skin inflammation in Stat6VT transgenic mice, and that IL-13-secreting cells develop in Il4-/-
Stat6VT transgenic mice (14
), suggests that while both cytokines can induce regulation of EDC genes, they are not functionally redundant in the development of allergic skin inflammation.
The development of AD in patients relies on multiple factors. As mentioned above, mutations in FLG
are a significant risk factor for AD (17
), and barrier function, as assessed by increased TEWL, is decreased (21
). Similarly, a family history of allergic disease, which likely reflects an increased propensity for development of allergic responses, is a significant risk factor (25
). Importantly our studies show that the effect of IL-4 on keratinocyte gene expression is not restricted to active lesions (4
), but that it also contributes to homeostatic expression of genes involved in skin barrier function. Thus, increases in systemic or local IL-4, through the ability to regulate barrier function, may be a predisposing factor for the development of AD. Modulating the levels of IL-4 in patients might be an effective method for not only treating lesions, but also limiting future exacerbations.