Protein A-binding immunosuppressive mouse serum factors.

In this study protein A of Staphylococcus aureus has been used to isolate an immunosuppressive component present in mouse serum. The suppressive effect of mouse serum on lymphocyte activation was partially abrogated by prior adsorption on protein A, and also by ammonium sulfate precipitation or specific immune precipitation with anti-IgG but not with anti-IgM. Protein A-binding material was isolated after chromatography on protein A-Sepharose and studied in spleen cell cultures. Protein A eluates from normal or NZB/NZW mice were found to suppress concanavalin A (Con A)-activated normal mouse spleen cells, and suppression was more potent with NZB/NZW serum isolates. Suppressive activity was dependent upon the dose of eluate added to cell cultures. The suppressive effect of NZB/NZW protein A-binding material was apparent in both Con A- and lipopolysaccharide (LPS)-stimulated normal mouse spleen cells, and required early addition to the cell cultures or preincubation with target lymphocytes. The suppressive activity was not detectably cytotoxic during a suppressive preincubation period. The possible relevance of these observations to experimental strategies in tumor immunotherapy is discussed.