Bacterial superantigen toxins induce a lethal cytokine storm by enhancing B7-2/CD28 costimulatory receptor engagement, a critical immune checkpoint

Abstract

Formation of the costimulatory axis between the B7-2 and CD28 coreceptors is critical for T-cell activation.Superantigens, Gram-positive bacterial virulence factors, cause toxic shock and sepsis by hyperinducing inflammatory cytokines. We report a novel role for costimulatory receptors CD28 and B7-2 as obligatory receptors for superantigens, rendering them therapeutic targets. We show that by engaging not only CD28 but also its coligand B7-2 directly, superantigens potently enhance the interaction between B7-2 and CD28, inducing thereby T-cell hyperactivation. Using a conserved twelve amino-acid domain, superantigens engage both B7-2 and CD28 at their homodimer interfaces, sites far removed from where these receptors interact, implying that inflammatory signaling can be controlled through the receptor homodimer interfaces. Short B7-2 and CD28 dimer interface mimetic peptides bind diverse superantigens, prevent superantigen binding to cell-surface B7-2 or CD28, attenuate inflammatory cytokine overexpression, and protect mice from lethal superantigen challenge. Thus, superantigens induce a cytokine storm by mediating not only the interaction between MHC-II molecule and T-cell receptor but critically, by promoting B7-2/CD28 coreceptor engagement, forcing the principal costimulatory axis to signal excessively. Our findings highlight the B7/CD28 interaction as a bottleneck in signaling for expression of inflammatory cytokines. B7-2 and CD28 homodimer interface mimetic peptides prevent superantigen lethality by blocking the superantigen-host costimulatory receptor interaction.

Binding of Staphylococcal Enterotoxin B (SEB) to B7 Receptors Triggers TCR- and CD28-Mediated Inflammatory Signals in the Absence of MHC Class II Molecules

“…On the contrary, in CH7 cells, SEB was not able to promote neither CD3 recruitment (Figures 7G, H) nor actin polymerization (Figures 7G, I). Altogether, in line with the fact that SEB strongly promotes CD28/B7 engagement (15), these data indicate that binding of CD28 to B7, creating the costimulatory axis, is required for SEBinduced conjugate formation between T:APC and that TCR coengagement by SEB results in more stable contacts that will favour optimal immunological synapse formation and hyperactivation of the inflammatory responses (Figure 8).…”

“…The excessive activation of cellular signalling pathways leading to inflammatory cytokine production by SEB relies on its ability to bind directly not only to specific TCR Vb chains and MHC class II molecules (7,9,23) but also to CD28 and its ligands B7.1/ CD80 or B7.2/CD86 (13)(14)(15)(16). SEB is able to interact directly with CD28 and B7 molecules also in the absence of MHC class II or other coligands (13)(14)(15)(16)24), but the functional relevance of this interaction remains still unknown. Here, we examined the contribution of the CD28/B7 costimulatory axis on SEBinflammatory cytokine production independently of MHC class II.…”

“…The SEB/TCR/MHC class II ternary complex formed outside the TCR and MHC class II peptide-binding domains is also able to circumvent the restriction between the TCR and the peptide bound to MHC class II (11,12). Recent findings demonstrated that to elicit inflammatory cytokine production, SEB must also bind directly to the costimulatory receptor CD28 and its natural coligands B7.1/CD80 and B7.2/ CD86 (13)(14)(15)(16). CD28 is an important costimulatory receptor constitutively expressed as a homodimer on T cell surface that binds B7.1/CD80 and B7.2/CD86 on APCs and cooperates with the TCR to induce optimal T cell activation (17,18).…”

“…Notably, the region of SEB involved in binding of CD28 and B7 molecules is distal from TCR and MHC class II binding sites (23). Indeed, SEB is able to interact directly with CD28 and B7 molecules also in the absence of MHC class II or other coligands (13)(14)(15)(16)24). However, the functional relevance of this interaction without MHC class II still remains unknown.…”

“Small” Intestinal Immunopathology Plays a “Big” Role in Lethal Cytokine Release Syndrome, and Its Modulation by Interferon-γ, IL-17A, and a Janus Kinase Inhibitor

“…SAgs bind directly to MHC class II molecules outside of the peptide-binding groove without undergoing any processing. Subsequently, they interact directly with selected T cell receptor (TCR) variable region (Vβ or Vα) families, and CD28 on T cells, crosslink the TCRs and CD28, thereby causing a robust polyclonal activation of 40-60% of all CD4 + and CD8 + αβ TCR + adaptive T cells (12). T cells activated by SAgs rapidly produce abundant amounts of pro-inflammatory cytokines resulting in CRS.…”