MAST-CELLS AND THEIR MICROENVIRONMENT - THE INFLUENCE OF FIBRONECTIN AND FIBROBLASTS ON THE FUNCTIONAL REPERTOIRE OF RAT BASOPHILIC LEUKEMIA-CELLS

TO DETERMINE HOW THE MICROENVIRONMENT in which mast cells are located may influence their function, we explored the effects of fibronectin a nd fibroblasts on histamine secretion in vitro from a mast cell model, the rat basophilic leukemia (RBL-2H3) cell line. RBL-2H3 cells bound specifically to fibronectin-coated surfaces. Binding was maximal by 1 hour, was not detectable at 0-degrees-C or in the absence of Ca++, and was inhibited by preincubating the cells with a synthetic peptide con taining the RGD sequence. Adherence to fibronectin stimulated RBL-2H3 cell spreading with a concomitant reorganization of the cytoskeleton a nd a repositioning of the cytoplasmic granules to the cell periphery. Although adherence to fibronectin did not by itself induce histamine r elease, when stimulated by either immunologic or non-immunologic means , fibronectin-adherent cells released dramatically more histamine than cells plated in wells coated with BSA only. Thus, RBL-2H3 cells bind specifically to fibronectin, and in so doing are stimulated to undergo changes in morphology and enhanced responsiveness to secretory stimul i. RBL-2H3 cells grown in coculture with 3T3 fibroblasts, but not RBL- 2H3 cells grown alone, became responsive to the polymeric synthetic se cretagogue Compound 48/80 and the neuropeptide Substance P. Maximum se nsitivity to Compound 48/80 was attained by the second week in cocultu re. Histamine release was dose-dependent, non-cytotoxic and occurred e ven in the absence of extracellular Ca++. Contact between the 2 cell t ypes appeared to be a critical factor. RBL-2H3 cells, separated from 3 T3 cells by a 0.45 mum filter, failed to secrete histamine in response to Compound 48/80. Additionally, RBL-2H3 cells grown in Steel factor, a newly identified mast cell growth factor produced by fibroblasts, d id not secrete histamine, suggesting that unidentified factors produce d by fibroblasts may control mast cell function. The results of our st udies show that the local microenvironment can have profound effects o n mast cell structure and function.