Coculture of mouse IL-3-dependent mast cells with 3T3 fibroblasts stimulates synthesis of globopentaosylceramide (Forssman glycolipid) by fibroblasts and surface expression on both populations

Mouse IL-3-dependent bone marrow culture-derived mast cells (BMMC) and mouse 3T3 fibroblasts, cultured separately or together, were examined for their cell surface expression and biosynthesis of globopentaosylceramide, a marker of the mouse serosal mast cell. As assessed by flow cytometric analysis, BMMC cultured for up to 7 wk in 50% WEHI 3-conditioned medium containing IL-3 did not bind the B1.1 anti-globopentaosylceramide mAb (six experiments). A total of 10 ± 4% (mean ± SD, three experiments) of 3T3 fibroblasts that had reached confluence in medium without IL-3 bound B1.1 antibody and, after an additional ~28 days of culture in that medium or in 50% WEHI 3-conditioned medium, 12 ± 3% (mean ± SD, five experiments) and 16 ± 7% (mean ± SD, three experiments) of the cells, respectively, bound the antibody. After coculture of BMMC and confluent 3T3 fibroblasts for 28 days in 50% WEHI 3-conditioned medium, followed by dispersal and purification of the cells, 92 ± 18% of the mast cells and 92 ± 16% (mean ± SD, seven experiments) of the fibroblasts were B1.1⁺. Whereas the increase in the expression of the epitope bound by B1.1 antibody on fibroblasts was noted by day 14 of coculture, expression of the epitope on mast cells did not occur until day 21 (three experiments). Biosynthesis of globopentaosylceramide was assessed by intrinsic radiolabeling of each cell population and identification of the extracted neutral glycosphingolipids by TLC and autoradiography. Synthesis of globopentaosylceramide was not detected in extracts of 9 x 10⁶ BMMC, 1 x 10⁶ confluent 3T3 fibroblasts cultured alone for 28 days, or 9 x 10⁶ mast cells purified from 28-day cocultures but was readily detected in extracts of 3 x 10⁶ fibroblasts purified from the same cocultures. These findings indicate that BMMC stimulate an increase in the synthesis and expression of globopentaosylceramide on 3T3 fibroblasts and suggest that the subsequent appearance of this neutral glycosphingolipid on the surface of the mast cells due to its secretion by fibroblasts and adsorption to the mast cell surface. Thus, the interactions between mast cells and fibroblasts during coculture alter the biochemical and Ag phenotypes of both populations.