INTRAEPITHELIAL INFILTRATION BY MAST-CELLS WITH BOTH CONNECTIVE TISSUE-TYPE AND MUCOSAL-TYPE CHARACTERISTICS IN GUT, TRACHEA, AND KIDNEYS OF IL-9 TRANSGENIC MICE

IL-9 transgenic mice were analyzed for the presence of mast cells in d ifferent tissues, In these mice, increased mast cell infiltration was found in the gastric and intestinal epithelium as well as in the upper airways and kidney epithelium, but not in other organs, such as skin, IL-9 transgenic mast cells do not show signs of massive degranulation such as that found in IL-4 transgenic mice and are not involved in sp ontaneous pathologic changes, Gastric mast cells showed a phenotype re lated to connective-type mast cells, since they were stained by safran in, and strong expression of mouse mast cell protease-4 and -5 was fou nd in this organ, However, they also expressed proteases related to th e mucosal cell type, such as mouse mast cell protease-l and -2, In vit ro, although IL-9 by itself did not induce mast cell development from bone marrow progenitors, it strongly synergized with stem cell factor for the growth and differentiation of mast cells expressing the same p rotease pattern as that observed in IL-9 transgenic mice, Since consti tutive stem cell factor expression was observed in vivo, and anti-c-Ki t Abs inhibited IL-9 transgenic mastocytosis in the gut, this synergis tic combination of factors is likely to be responsible for the mastocy tosis observed in IL-9 transgenic mice, Taken together, these data dem onstrate that IL-9 induces the in vivo amplification of a nonclassical mast cell subset,vith a mucosal localization but expressing proteases characteristic of both connective tissue-type and mucosal mast cells.