Several reports have shown the presence of T-helper lymphocytes with Th2 characteristics in the skin lesions of atopic dermatitis (AD). However. Th2 cells themselves require an exogenous pulse of IL-4 to initiate their differentiation and synthesis of IL-4. As mast cells have recently been shown to contain IL-4, this finding prompted us to investigate IL-4 in mast cells of AD lesions, to determine if they might provide the IL-4 pulse needed by the Th2 cells.

In this study, we measured IL-4 immunoreactivity in mast cells of non-lesional and lesional skin sections from 20 patients with AD. Ten patients with nummular eczema (NE) without any atopic features or background, and five healthy subjects, served as the control groups. Mast cells were first identified using an enzyme-histochemical staining method for tryptase. Subsequently, the sections were photographed, the tryptase stain was removed, and IL-4 was demonstrated with a polyclonal antibody. The sections were photographed again, and the percentage of IL-4-positive mast cells was calculated.

The percentage of mast cells exhibiting IL-4 immunoreactivity in the upper dermis in lesional vs, non-lesional skin was 66±18% vs. 37±18% in AD (P<0.0001, paired t-test), but only 46±19% vs. 31±22% in NE. In the skin of healthy controls, only 23±25% of the mast cells were positive for IL-4. In addition, a significant difference was found between lesional skin of AD vs. NE patients (P<0.008, unpaired t-test). Moreover, the staining intensity of IL-4 in mast cells was clearly increased in the lesional compared with the non-lesional AD skin. Mast cells appeared to be the main cell type containing IL-4 in 40% of the lesional AD skin specimens, whereas in the remaining 60% prominent IL-4-positive mononuclear cell infiltrates were also present. In the non-lesional skin, mast cells appeared to be the main cell type containing IL-4 in all specimens. These results indicate that mast cells are one major source of IL-4 in lesional and non-lesional AD skin, and they could contribute significantly to the development of AD.