Coculture of interleukin 3-dependent mouse mast cells with fibroblasts results in a phenotypic change of the mast cells

The heparin-containing mast cells that reside in the connective tissue of the mouse, but not the chondroitin sulfate-containing mast cells in the gastrointestinal mucosa, stain with safranin when exposed to alcian blue/safranin. Mouse bone marrow-derived mast cells (BMMC), the probable in vitro counterparts of in vivo mucosal mast cells, were cultured for 14 days with mouse skin-derived 3T3 fibroblasts in RPMI 1640 medium containing 10% fetal calf serum and 50% WEHI-3 conditioned medium. Although the BMMC adhered to the fibroblast monolayer, they continued to divide, probably due to the presence of interleukin 3 in the conditioned medium. The mast cells remained viable throughout the period of coculture, since they failed to release lactate dehydrogenase and because they increased their histamine content ~ 15-fold. After 12-14 days of coculture, > 50% of the BMMC changed histochemically to become safranin⁺; 30-40% of the ³⁵S-labeled glycosaminoglycans on the proteoglycans synthesized by these cocultured mast cells were heparin, whereas heparin was not detected in the initial BMMC. In the absence of WEHI-3 conditioned medium, BMMC adhered to the fibroblast monolayer, and after 8 days of coculture, the number of mast cells did not change and their histamine content remained the same. However, these mast cells also became safranin⁺ and synthesized 40% heparin glycosaminoglycans. Thus, coculture of BMMC with fibroblasts induces a phenotypic change so that the resulting mast cells stain safranin⁺ and synthesize heparin proteoglycans, whereas the presence of WEHI-3 conditioned medium stimulates proliferation and an increase in histamine content.