AbstractSevere bacterial or viral infections can induce a state of immune hyperactivation that can culminate in a potentially lethal cytokine storm. The classic example is toxic shock syndrome, a life-threatening complication of Staphylococcus aureus or Streptococcus pyogenes infection, which is driven by potent toxins known as superantigens (SAgs). SAgs are thought to promote immune evasion via the promiscuous activation of T cells, which subsequently become hyporesponsive, and act by cross-linking major histocompatibility complex class II molecules on antigen-presenting cells to particular β-chain variable (TRBV) regions of αβ T cell receptors (TCRs). Although some of these interactions have been defined previously, our knowledge of SAg-responsive TRBV regions is incomplete. In this study, we found that CD4⁺ and CD8⁺ T cells expressing TRBV12-3/12-4⁺ TCRs were highly responsive to streptococcal pyrogenic exotoxin C (SpeC) and toxic shock syndrome toxin-1 (TSST-1). In particular, SpeC and TSST-1 specifically induced effector cytokine production and the upregulation of multiple coinhibitory receptors among TRBV12-3/12-4⁺ CD4⁺ and CD8⁺ memory T cells, and importantly, these biological responses were dependent on human leukocyte antigen (HLA)-DR. Collectively, these data provided evidence of functionally determinative and therapeutically relevant interactions between SpeC and TSST-1 and CD4⁺ and CD8⁺ memory T cells expressing TRBV12-3/12-4⁺ TCRs, mediated via HLA-DR.