Macrophage engulfment of mucosal mast cells in rats treated with dexamethasone

The effects of corticosteroid treatment on mucosal mast cells in rat jejunal mucosa were examined. Rats previously infected with Nippostrongylus brasiliensis received a single IP injection of 1 mg dexamethasone. Three hours later, one third of mucosal mast cells demonstrated minor granular changes (fusion or peripheral clear zones) by electron microscopy. At 7 hours, by light microscopy, the majority of mucosal mast cells appeared abnormal with clustering of granules. By electron microscopy, 151 of 233 (65%) mucosal mast cells had been engulfed by enlarged macrophages and were in various stages of degeneration inside large phagosomes. By 24 hours, the number of mucosal mast cells had decreased to less than 10% of the initial number with parallel decreases in tissue rat mast cell protease II and histamine levels. Serum levels of rat mast cell protease II did not increase, and intestinal morphology was invariably normal with no evidence of inflammatory changes up to and including 24 hours. Observations were similar in uninfected animals. In contrast, in rats undergoing antigen-induced anaphylaxis, a significant elevation of serum rat mast cell protease II level was evident at 3 and 7 hours, and macrophage engulfment of mucosal mast cells was never seen, although tissue edema, enterocyte loss, and hemorrhage were observed. It is concluded that dexamethasone treatment results in macrophage engulfment and destruction of mucosal mast cells that occurs without granular mediator release and local inflammatory effects.