A Positive Feedback Loop for Staphylococcal Enterotoxin-A-Stimulated IFN-Gamma Production Requires Macrophage Immune-Associated Antigen Upregulation

Abstract

The C57B1/6-derived T cell line, L12-R4, produced murine interferon-gamma (IFNγ) in response to mitogenic stimulation by phorbol myristate acetate (PMA) or concanavalin A (Con A), but not by staphylococcal enterotoxin A (SEA). Low levels of IFNγ were produced by SEA stimulation of L12-R4 cells cocultured with C57B1/6 bone marrow macrophages (BMM). Significantly increased yields of IFNγ resulted from 48-hour pretreatment of the BMM with recombinant IFNγ (100 U/ml) prior to coculture. Polyclonal anti-IFNγ and anti-IFNα/β were used to characterize the interferon as IFNγ. Paraformaldehyde (0.1%) treatment of IFNγ-pretreated BMM did not affect IFNγ production, suggesting that processing of SEA was not required. IFNγ treatment of BMM resulted in significantly increased expression of immune-associated (la) antigen as determined by flow cytometric analysis, suggesting that the accessory cell role of BMM involved la antigen. Polyclonal anti-Ia antibody selectively inhibited the production of IFNγ by SEA-stimulated whole spleen cell cultures, consistent with the necessity of la antigen for BMM help in SEA induction of IFNγ. These findings suggest that la antigen is necessary for BMM accessory function in SEA induction of IFNγ. More interestingly, the results implicate class II molecules in a positive feedback loop for IFNγ production by SEA.