The eosinophils appear to be particularly troublesome cells of inflammation. Eosinophils evolved to defend the body against parasites, much like IgE. Nevertheless, they are often present in great numbers in the blood of people with allergies . When they arrive at the site of the allergic reaction, they release chemicals that cause damage to the tissues and continue to promote the inflammation. Repeated episodes of this "late phase" reaction contribute to chronic allergic symptoms and make the tissues even more sensitive to subsequent exposure.
Using in situ hybridization and the reverse transcriptase polymerase chain reaction (RT-PCR) we show that messenger RNA for IL-4, IL-5 and tumor necrosis factor-α (TNF-α) is induced by cross-linkage of high-affinity FcΕ receptors (FcΕRI) on human skin mast cells, but that only TNF-α mRNA is selectively induced by substance P. Skin mast cells were purified using the Percoll density technique. T cells were removed by serial negative selection using a CD2 monoclonal antibody (mAb) to achieve a final mast cell purity > 95%. Purified mast cells were precultured with recombinant human stem cell factor (rhSCF; 10 ng/ml) and myeloma IgE (3 µg/ml) for 16 h before challenge with sheep polyclonal antihuman IgE antibody (anti-IgE; 1 or 10 µg/ml) in the presence of rhSCF (50 ng/ml). Using in situ hybridization, we demonstrated that IgE-dependent stimulation induces the expression of IL-4, IL-5 and TNF-α mRNA in skin mast cells. We have investigated the expression of IL-4, IL-5 and TNF-α mRNA by substance P, with the result that substance P, –30 µ M , selectively induced TNF-α mRNA. However, substance P did not induce IL-4 mRNA and did not enhance IL-5 mRNA. Furthermore, we confirmed the release of TNF-α by substance P from skin mast cells using an ELISA technique. These findings demonstrate the capacity of human skin mast cells to transcribe IL-4, IL-5 and TNF-α by immunological activation and to transcribe and release TNF-α by substance P.