Bacterial superantigen toxins bind directly to the dimer interface of CD28, the principal co-stimulatory receptor, to induce a lethal cytokine storm, and peptides that prevent this binding can suppress superantigen lethality.

Full T-cell activation requires interaction between the costimulatory receptors B7-2 and CD28. By binding CD28, bacterial superantigens elicit harmful inflammatory cytokine overexpression through an unknown mechanism. We show that, by engaging not only CD28 but also its coligand B7-2 directly, superantigens potently enhance the avidity between B7-2 and CD28, inducing thereby T-cell hyperactivation. Using the same 12-aa β-strand-hinge-α-helix domain, superantigens engage both B7-2 and CD28 at their homodimer interfaces, areas remote from where these coreceptors interact, implying that inflammatory signaling can be controlled through the receptor homodimer interfaces. Short B7-2 dimer interface mimetic peptides bind diverse superantigens, prevent superantigen binding to cellsurface B7-2 or CD28, attenuate inflammatory cytokine overexpression, and protect mice from lethal superantigen challenge. Thus, superantigens induce a cytokine storm not only by mediating the interaction between MHC-II molecule and T-cell receptor but also, critically, by promoting B7-2/CD28 coreceptor engagement, forcing the principal costimulatory axis to signal excessively. Our results reveal a role for B7-2 as obligatory receptor for superantigens. B7-2 homodimer interface mimotopes prevent superantigen lethality by blocking the superantigen-host costimulatory receptor interaction.superantigen | cytokine storm | costimulatory receptor | B7-2 dimer interfaceA s principal costimulatory receptor, CD28 is a critical regulator of the immune response (1-3). Expressed constitutively on T cells, CD28 is a homodimer that interacts with its B7 coligands, transducing the signal essential for an immediate T-cell response (2-5). CD28 coligand B7-2 (CD86) is expressed constitutively on antigen-presenting cells whereas B7-1 (CD80) is induced only later (5, 6); thus, B7-2/CD28 interaction regulates early antigen signaling (7,8).The inflammatory cytokine response is indispensable for protective immunity, yet bacterial and viral infections often elicit an exaggerated response ("cytokine storm") harmful to the host. Thus, superantigens from Staphylococcus aureus and Streptococcus pyogenes induce toxic shock by activating an immune response, orders of magnitude beyond that elicited by regular antigens. Superantigens exploit the main axis of T-cell activation by binding directly as intact proteins to most major histocompatibility class II (MHC-II) and T-cell receptor (TCR) molecules outside their antigen-binding domains, linking them and bypassing restricted presentation of conventional antigens that typically activate <1% of T cells, thereby activating up to 20 to 30% of T cells (9-11). Moreover, T-cell activation by superantigens requires their direct binding to CD28 (12), the second signaling molecule mandatory for T-cell activation, which results in massive induction of inflammatory cytokines that mediate toxic shock, including IL-2, IFN-γ, and TNF.Induction of human inflammatory cytokine gene expression by divergent superantigens is inhibited by a short p...

Every adaptive immune response requires costimulation through the B7/CD28 axis, with CD28 on T-cells functioning as principal costimulatory receptor. Staphylococcal and streptococcal superantigen toxins hyperstimulate the T-cell-mediated immune response by orders of magnitude, inducing a lethal cytokine storm. We show that to elicit an inflammatory cytokine storm and lethality, superantigens must bind directly to CD28. Blocking access of the superantigen to its CD28 receptor with peptides mimicking the contact domains in either toxin or CD28 suffices to protect mice effectively from lethal shock. Our finding that CD28 is a direct receptor of superantigen toxins broadens the scope of microbial pathogen recognition mechanisms.

Staphylococcal and streptococcal superantigens are virulence factors that cause toxic shock by hyperinducing inflammatory cytokines. Effective T-cell activation requires interaction between the principal costimulatory receptor CD28 and its two coligands, B7-1 (CD80) and B7-2 (CD86). To elicit an inflammatory cytokine storm, bacterial superantigens must bind directly into the homodimer interfaces of CD28 and B7-2. Recent evidence revealed that by engaging CD28 and B7-2 directly at their dimer interface, staphylococcal enterotoxin B (SEB) potently enhances intercellular synapse formation mediated by B7-2 and CD28, resulting in T-cell hyperactivation. Here, we addressed the question, whether diverse bacterial superantigens share the property of triggering B7-2/CD28 receptor engagement and if so, whether they are capable of enhancing also the interaction between B7-1 and CD28, which occurs with an order-of-magnitude higher affinity. To this end, we compared the ability of distinct staphylococcal and streptococcal superantigens to enhance intercellular B7-2/CD28 engagement. Each of these diverse superantigens promoted B7-2/CD28 engagement to a comparable extent. Moreover, they were capable of triggering the intercellular B7-1/CD28 interaction, analyzed by flow cytometry of co-cultured cell populations transfected separately to express human CD28 or B7-1. Streptococcal mitogenic exotoxin Z (SMEZ), the most potent superantigen known, was as sensitive as SEB, SEA and toxic shock syndrome toxin-1 (TSST-1) to inhibition of inflammatory cytokine induction by CD28 and B7-2 dimer interface mimetic peptides. Thus, superantigens act not only by mediating unconventional interaction between MHC-II molecule and T-cell receptor but especially, by strongly promoting engagement of CD28 by its B7-2 and B7-1 coligands, a critical immune checkpoint, forcing the principal costimulatory axis to signal excessively. Our results show that the diverse superantigens use a common mechanism to subvert the inflammatory response, strongly enhancing B7-1/CD28 and B7-2/CD28 costimulatory receptor engagement.

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