Effect of Purified Protein SAA on Immune Response in vitro : Mechanisms of Suppression

Abstract

Studies were done to characterize the immunosuppressive effect of the serum α-globulin SAA. The in vitro antibody response of CBA/J murine spleen cells was studied in the Marbrook culture system. Addition of 10% SAA-containing serum from casein-treated CBA/J mice suppressed the primary antibody response to SRBC as measured by the hemolytic plaque assay by up to 90%. This suppression was reversed by addition of rabbit anti-amyloid protein AA. Addition of SAA 24 hr or longer after initiation of cultures failed to give suppression, showing that the suppression by SAA occurs in the early recognition phase of the in vitro antibody response. Extensive washing of spleen cells reversed the suppression, suggesting an effect at the level of the cell membrane. A lack of suppression of the response to DNP-Ficoll (a T-independent antigen) indicated that there was no direct effect of SAA on B lymphocytes.Purified SAA (100 µg/ml) isolated from the serum of casein-treated mice gave 95% suppression of SRBC response, and concentrations as low as 10 µg/ml gave significant suppression. This is much lower than the serum levels of SAA attained in humans and experimental animals after antigen challenge or immunization. It is suggested that a normal function of SAA may be to regulate immune response to certain foreign antigens or altered autologous antigens.