Transformation from SAA2-fibrils to AA-fibrils in amyloid fibrillogenesis: in vivo observations in murine spleen using anti-SAA and anti-AA antibodies.

Early amyloid fibrillogenesis from serum amyloid A protein (SAA) has been observed in the murine spleen after an injection of casein-Freund's complete adjuvant in the presence of amyloid enhancing factor, using anti-SAA C-terminal (anti-SAA) and anti-amyloid A (AA) antibodies. In Western immunoblotting of sera, both SAA1 and SAA2 reached a maximum after 24 h and began to decrease after 48 h. In spleen extracts, SAA2, but not SAA1 or AA, was found from 48 h, when amyloid was first deposited in the marginal zone. Electron microscopic immunohistochemistry of this stage showed reaction products from SAA in the marginal zone as fine granules along the cell membrane of mononuclear cells and focal intercellular aggregates, which contained fine fibrils originating from the cell membrane. Amyloid nodules, surrounded by mononuclear cells, developed from this stage. In the nodules, fibrils were positive for anti-SAA only in the vicinity of the cell membrane, while anti-AA stained fibrils throughout. Our hypothesis for fibrillogenesis is thus as follows: Serum SAA2 is specifically deposited on mononuclear cells in the marginal zone and polymerized extracellularly into fibrils, retaining its antigenicity (SAA2 amyloid fibrils); these fibrils are then processed to AA amyloid fibrils in situ by cleavage of the C-terminal portion of SAA2.