Scand. J. Immunol., Vol. 6, 1977- Identification in Human Placentae of Antisenic Activity Related to the Amyloid Serum Protein SAA p. M. JOHNSON, G. HUSBY, J. B. NATVIG, R. 1-. ANDERS & E. LINDER Institute of immunology and Rheumatolog), Rtk!ihospit;iiier Universitj' Hospital, Oslo, Norway, and Department of Serolojjj' and Baclcriolog)-, IVth Department of Medicine, University Central Hospical, Helsinki, FinUnd Johnson, P. M.. Husby, G., Natvig, J. B., Anders, R. F, & Linder, E. Identi- fication in Human Placentae of Antigenic Activity Related to the Amyloid Serum Protein SAA. Scand. j . Immuttol. (>, 319-325, 1977. Antigenic activity' related to the amyloid serum protein SAA was observed in indirect Immunofluorescence studies on human piacental tissue. Positive staining with anti-SAA antisera was localized to the cytoplasm of cells scattered witliin the mesenchymal stroma, thought to be fibroblasts, and to foetal stem vessel endothelium and some individual fibrillar structures in villous stroma and peri- vascular tissue. This immunofluorescent staining was specifically inhibited by protein SAA. In contrast, no immunoflunresci'nt staining!! wa.s achieved using anti- sera to the amyloid protein AA. Absorption and immunodiffusion studies have further suggested that anti-SAA antisera may recognize in human placentae only 3 very limited number of the antigenic determinants present in protein SAA but not in the smaller protein AA. The results support previous obser\'ation5 that protein SAA-like antigenic material can be found in normal human tissue. P. M. Johnson, Ph.D., Bone & Joint Research Institute, London Hospital Medical College, TufHer Street, London Rl 2AD, England A unique component of amyloid fibrils, protein AA, has been found in various clinical types of amyloidosis (11, 18). A serum protein (SAA) with similar antigenic properties has also been obscned In low levels in normal sera, and in increased levels in patients with amyloidosis or diseases predisposed to amy- loidosis, during pregnancy, and in old age (8, 20). The proteins A A and SAA have identical N-tcrminal amino acid scc^uences and subunit molecular weights of approximately 9,000 and M,000, respectively (1, 2, 21). It has not been established, however, whether SAA is a circulating precursor to AA, or whether both proteins are derived independent- ly from a commoti precursor. Protein SAA behaves as an acute-phase reactant, since high levels of this protein are found in acute in- flammator)' conditions (6), Furthermore, recent evidence from immunofluorescent staining of foetal tissues and cultured fibroblasts indicates that protein SAA-like material may be pro- duced by fibroblasts and be a normal con- stituent of some developing extracellular con- nective tissue fibres ( H ) . Similar immunofluo- rescence studies using anti-amyloid antibodies have onlj' been reported on tissues from pa- tients with amyloidosis (7). The human placenta is a foetal structure endowed with both maternal and paternal gene products. In the full-term piacental villous stroma there is an extensive connective tissue