Immunocytochemical Colocalization of Fibroblast Growth Factor-1 with Neurotrophin-3 in Mouse Alveolar Macrophages

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Alveolar macrophages are known to express a variety of growth factors and neurotrophins. Fibroblast growth factor-1 (FGF-1) is abundantly present in the lung and has mitogenic and neurotrophic activities similarly to neurotrophins. In order to determine whether FGF-1 associates with neurotrophins in alveolar macrophages, we investigated the immunocytochemical colocalization of FGF-1 with neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), in mouse alveolar macrophages. The results showed that 34% of macrophages were immunoreactive for FGF-1, 10% for NGF, 9% for BDNF, and 17% for NT-3. Of FGF-1-immunoreactive (IR) macrophages, 16% were immunoreactive for NT-3, but only small percentages were immunoreactive for NGF (0.8%) and for BDNF (0.3%). FGF-1 and neurotrophins were all localized in the intracellular vesicles. In the vesicles, FGF-1 and NT-3 were frequently colocalized. All macrophages expressed lysosome-associated protein-2 (LAMP-2), a late endosomal and lysosomal marker, and early endosomes antigen 1 (EEA1), an early endosomal marker. FGF-1 and NT-3 were predominantly colocalized with LAMP-2 rather than with EEA1, whereas NGF and BDNF were colocalized with EEA1 rather than with LAMP-2. These results indicate that FGF-1 and NT-3 are substantially expressed in mouse alveolar macrophages and colocalized in vesicles, predominantly in late endosomes and lysosomes.