Staphylococcal enterotoxin B (SEB) binds specifically to major histocompatibility complex class II molecules on the surface of accessory cells and stimulates virtually all T cells bearing certain, but not all, T cell-receptor Vβ alleles. We have previously shown that this superantigen is a potent inducer of multiple regulatory T cell populations. In the present report we show that SEB induces a population of suppressor T cells which inhibits the generation of alloantigen-induced cytotoxic T cell activity. Using both negative- and positive-selection analysis, we found that this suppressor population is a CD4- CD8- CD5+ IL-2R+ T cell. This cell population inhibited both syngeneic and allogeneic cytotoxic T lymphocyte activity, but the cell population which inhibited allogeneic CTL activity was radiation sensitive. In addition, allogeneic SEB-primed cells appeared to develop cytolytic activity as a result of the additional stimulation in the mixed-lymphocyte reaction culture. The relationship of the SEB-primed CD4- CD8- CD5+ T cells to related regulatory T cell populations is discussed.
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