Overexpression of IL-4 alters the homeostasis in the skin

IL-4 has been implicated to play an important role in the pathogenesis of many inflammatory diseases including skin diseases such as atopic dermatitis. Because it is not clear which pathologic features of atopic dermatitis are dependent on IL-4, we assessed the consequences of IL-4 overexpression in the skin, using transgenic mice overexpressing IL-4 ubiquitously. Although transgenic mice display no clinical signs of skin inflammation, IL-4 induced a wide spectrum of pathologies including an increased number of mast cells and Langerhans cells in dermis and epidermis, respectively, focal deposition of collagen and a considerably reduced adipocyte layer in the dermis as well as an increased mitotic activity of keratinocytes, reflected in acanthosis and hyperkeratosis. The increase in Langerhans cell number may be explained in part by the substantially reduced Langerhans cell emigration from the epidermis in transgenic mice. The molecular mechanism behind this phenomenon remains to be clarified. Under in vitro culture conditions, Langerhans cells from transgenic mice undergo a maturation process similar to that of Langerhans cells from control mice, and their immunostimulatory capacity is also comparable. In contrast, transgenic Langerhans cells are superior to control Langerhans cells in their antigen-processing capacity. We conclude that the overexpression of IL-4 in the skin is, by itself, not sufficient for the induction of a full-blown atopic dermatitis phenotype, but several changes seen in the skin of transgenic mice mirror the cardinal pathologic manifestations of this disease.