Ficolin-1 is present in gelatinase granules and also in a previously unknown highly mobilizable subset of granules; once released, ficolin-1 binds to the neutrophil surface.

Ficolins are soluble molecules that bind carbohydrate present on the surface of microorganisms and function as recognition molecules in the lectin complement pathway. Three ficolins have been identified in humans: ficolin-1, ficolin-2, and ficolin-3. Ficolin-1 is synthesized in monocytes and type II alveolar epithelial cells. Ficolin-1 has been shown to be present in secretory granules of human neutrophils, but it is not known which subset of the neutrophils’ secretory granules harbors ficolin-1. To determine the exact subcellular localization of ficolin-1 in neutrophils, recombinant ficolin-1 was expressed in Chinese hamster ovary cells and used for generation of polyclonal antibodies. This allowed detection of ficolin-1 in subcellular fractions of human neutrophils by ELISA, by Western blotting, and by immunohistochemistry. Real-time PCR examination of normal human bone marrow showed FCN1 gene expression largely in myelocytes, metamyelocytes, and band cells with a profile quite similar to that of gelatinase. In accordance with this, biosynthesis studies of neutrophils precursor cells showed that ficolin-1 was primarily synthesized in myelocytes, metamyelocytes, and band cells. Immunohistochemistry and subcellular fractionation demonstrated that ficolin-1 is primarily localized in gelatinase granules but also in highly exocytosable gelatinase-poor granules, not described previously. Ficolin-1 is released from neutrophil granules by stimulation with fMLP or PMA, and the majority becomes associated with the surface membrane of the cells and can be detected by flow cytometry. Our studies show that neutrophils are a major source of ficolin-1, which can be readily exocytosed by stimulation.