Ultrastructural immunolocalization of basic fibroblast growth factor-like molecules in cytoplasmic granules of mast cells in inflamed tissue

We recently showed that mast cells (MCs) serve as a source of basic fibroblast growth factor (bFGF), a potent angiogenic and mitogenic polypeptide, suggesting that bFGF may mediate MC-relaied neovascularization and fibroproliferation. However, bFGF lacks a classic peptide sequence for its secretion and the mechanism(s) for its release remains controversial. Since MCs release a wide spectrum of bioactive products via degranulation, we hypothesized that MC degranulation may be a mechanism of bFGF release. We used ultrastructural immunohistochemistry to test our hypothesis, following the rationale that if bFGF is released through degranulation, it should be localized within MC granules. In both rat dermal granuloma and human synovial tissue samples examined, positive staining for bFGF was predominantly localized to the cytoplasmic granules of MCs. Moderate bFGF immunoreactivity was also found in the nucleus, while the cytosol and other subcellular organelles exhibited negligible staining. Moderate positive staining was also present in the cytoplasm and nucleus of fibroblasts and endothelial cells. In contrast, no staining for bFGF was observed in lymphocytes or plasma cells. Two different anti-bFGF antibodies gave similar staining patterns, while non immune IgG gave no staining. Our data suggest that, despite the lack of a classic secretory peptide in its structure, bFGF is localized to the secretory granules in MCs and may be released through degranulation.