Toxic Shock Syndrome is a disease mediated by the Staphylococcal Toxic shock syndrome toxin 1 (TSST1). This toxin binds simultaneously to a T-cell receptor (outside the antigen-binding cleft) and an MHC II molecule.
This cross-linkage produces an activating signal that induces T-cell activation and proliferation independent of antigen-specificity. The result of this polyclonal activation of T cells leads to overproduction of cytokines and toxicity.
Which of the following terms BEST describes molecules such as TSST1, that activate T cells in this fashion?