Thymus dependence of compound 48/80-induced mucosal mast cell proliferation

The mucosal and connective-tissue mast cells (MMC and CTMC, respectively) of the rat are histochemically and functionally distinct. MMC, unlike CTMC, are insensitive to the degranulating action of the mast cell secretagogue Compound 48/80 and instead increase in number after treatment with this drug. T cell-derived growth factors are necessary for the growth of MMC-like cells from hemopoietic tissues in vitro, as well as for nematode-induced MMC proliferation in vivo. We therefore examined the possible role of the thymus in the pharmacologically induced MMC expansion. Mast cell numbers and histamine content after treatment with Compound 48/80 were determined in the skin, tongue and gut of athymic male LEW/MOL-rnu/rnu rats and their normal littermates at the age of 6-7 weeks. The influence of the strain of rat and the mode of administration of the drug was assessed by studying age- and sex-matched Sprague-Dawley rats as well. Injections of Compound 48/80 for 5 days resulted in a statistically significant increase in the number of intestinal MMC and the histamine content of all thymus-bearing rats. The increase was more pronounced in the Sprague-Dawley rats, however, and unrelated to the mode of administration of the drug, indicating that the MMC-stimulating action of Compound 48/80 is at least partly controlled by genetic factors. Athymic rats showed no statistically significant MMC expansion. This suggests that the MMC response after Compound 48/80 treatment may be at least partly dependent on the thymus. The mast cell numbers and histamine content in the tissues of athymic control rats were higher than in their thymus-bearing littermates.(ABSTRACT TRUNCATED AT 250 WORDS)