Adsorption of follicular dendritic cell-secreted protein (FDC-SP) onto mineral deposits. Application of a new stable gene expression system.

Follicular dendritic cell-secreted protein (FDC-SP) is a small secretory protein having structural similarities to statherin, a protein in saliva thought to play a role in calcium retention in saliva. In contrast, FDC-SP is thought to play a role in the immune system associated with germinal centers. We report here the very specific expression of FDC-SP in junctional epithelium at the gingival crevice. This region is very important for the host defense against pathogens and for periodontal protection. To be able to better understand the function of FDC-SP, we developed a novel gene expression system that exploited gene trapping and site-specific gene integration to introduce the protein into a mammalian cell culture system. Using this system we were able to express FDC-SP as a fusion protein with green fluorescent protein in an osteogenic progenitor cell line with long term stability, which we then used to find that the fusion protein specifically adsorbs onto mineral deposits and the surface of hydroxyapatite particles exogenously added to the culture. This adsorption was highly dependent on the structural integrity of FDC-SP. These results suggest that FDC-SP may play an important role, adsorbing onto the surface of cementum and alveolar bone adjacent to periodontal ligament and onto tooth surface at the gingival crevice.