Regulation of the growth rate of mouse fibroblasts by IL-3-activated mouse bone marrow-derived mast cells

When mouse bone marrow-derived mast cells (BMMC) are cocultured with a confluent layer of mouse 3T3 fibroblasts in the presence of WEHI-3-conditioned medium, the mast cells undergo a phenotypic change toward that of a connective tissue mast cell, and the fibroblasts increase their synthesis of globopentaosylceramide. We now demonstrate that fibroblasts lose their contact inhibition and multiply such that by the 2nd and the 4th wk of coculture there are, respectively, approximately four-fold and six-fold more fibroblasts than in the cultures that are not exposed to BMMC. This in vitro increase in the number of fibroblasts is dependent on the number of mast cells (over the range of 6 x 10(4) to 1 x 10(6) BMMC/culture) initially seeded with the fibroblasts and on the concentration of WEHI-3-conditioned medium present during the coculture. That the fibroblasts also multiply in BMMC/fibroblast cocultures exposed to synthetic IL-3 or to purified IL-3 indicates that IL-3 is a component in WEHI-3-conditioned medium that induces mast cells to produce the fibroblast growth factor. The number of fibroblasts does not increase if fibroblasts are exposed to lysates of BMMC, or to BMMC-derived conditioned medium, or if the two cell types are separated from one another during the coculture with a 3-microns filter or a 0.4-microns filter. Thus, IL-3-activated BMMC must be in proximity to fibroblasts to induce them to multiply. Because of their increased numbers per culture dish, total fibroblasts that were cocultured with mast cells synthesized approximately two-fold more 35S-labeled proteoglycans, incorporated approximately 3-fold more 3H] proline into collagenase-sensitive proteins, and had substantially more alpha 2(I) collagen mRNA than fibroblasts that were maintained in the absence of mast cells. These is vitro studies reveal a sequence by which IL-3-activated mast cells may play a role in the induction of fibrosis.