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

Abstract

The inflammatory activity of staphylococcal enterotoxin B (SEB) relies on its capacity to trigger polyclonal T-cell activation by binding both T-cell receptor (TCR) and costimulatory receptor CD28 on T cells and MHC class II and B7 molecules on antigen presenting cells (APC). Previous studies highlighted that SEB may bind TCR and CD28 molecules independently of MHC class II, yet the relative contribution of these interactions to the pro-inflammatory function of SEB remained unclear. Here, we show that binding to MHC class II is dispensable for the inflammatory activity of SEB, whereas binding to TCR, CD28 and B7 molecules is pivotal, in both human primary T cells and Jurkat T cell lines. In particular, our finding is that binding of SEB to B7 molecules suffices to trigger both TCR- and CD28-mediated inflammatory signalling. We also provide evidence that, by strengthening the interaction between CD28 and B7, SEB favours the recruitment of the TCR into the immunological synapse, thus inducing lethal inflammatory signalling.

SARS-CoV-2 Spike Does Not Possess Intrinsic Superantigen-like Inflammatory Activity

“…Recent computational simulations generated on the basis of the cryoelectron microscopy of the SARS-CoV-2 spike [ 60 ] and the X-ray structure of human TCR TRAV27/TRBV19 in a ternary complex with HLA-DR1 and the staphyloccoccal SAg, SEH [ 61 ], suggested a unique putative binding site for the TCR Vβ chain near the S1/S2 cleavage site of the SARS-CoV-2 spike [ 19 ]. Further examination of the TCR Vβ-binding region on the SARS-CoV-2 spike (T 678 NSPRRARSVASQ 690 ) also suggested strong structural similarities to the superantigenic fragment of staphylococcal SEB that has been recently identified to bind to CD28 costimulatory molecules, thus triggering an inflammatory cytokine storm [ 34 , 37 , 38 , 62 ]. However, the functional relevance of this putative SAg activity of the SARS-CoV-2 spike is presently unknown [ 63 ].…”

“…After verifying the purity of the SARS-CoV-2 spike and SEB by SDS-PAGE ( Supplementary Figure S1 ), we firstly analysed the SAg activity of the SARS-CoV-2 spike on a CD28 + CH7C17 Jurkat T cell line expressing TCRVβ3.1 [ 47 , 48 ] that specifically interacts with SEB [ 67 ] and potentially with the putative CD28-binding site of the SARS-CoV-2 spike [ 19 ]. Although we have recently demonstrated that MHC-II molecules are dispensable for SEB inflammatory activity [ 38 ], the models and simulations of the SAg activity of the SARS-CoV-2 Spike have been conducted on a trimolecular complex involving HLA-DR1 [ 19 , 61 ]. Accordingly, we used murine 5-3.1/B7 cells co-expressing human HLA-DRB1*0101 and B7.1/CD80 molecules as APCs, which efficiently bind to SAgs and activate Jurkat and primary T cells without processing and presenting the derived peptides in association to MHC-II [ 38 , 47 , 68 ].…”

“…Although we have recently demonstrated that MHC-II molecules are dispensable for SEB inflammatory activity [ 38 ], the models and simulations of the SAg activity of the SARS-CoV-2 Spike have been conducted on a trimolecular complex involving HLA-DR1 [ 19 , 61 ]. Accordingly, we used murine 5-3.1/B7 cells co-expressing human HLA-DRB1*0101 and B7.1/CD80 molecules as APCs, which efficiently bind to SAgs and activate Jurkat and primary T cells without processing and presenting the derived peptides in association to MHC-II [ 38 , 47 , 68 ]. The stimulation of CD28 + TCRVβ3.1 + Jurkat cells with SEB alone induced significant TNF- α production (mean = 114 pg mL −1 ) that further increased by at least six-fold (mean = 697 pg mL −1 ) in the presence of MHC-II/B7-expressing 5-3.1/B7 cells ( Figure 1 a).…”

“…SEB is a staphylococcal SAg that, by binding to the outer leaflet of major histocompatibility class II (MHC-II) molecules on antigen-presenting cells (APCs) and specific TCRVβ chain elements, induces the polyclonal activation of T cells [ 28 , 29 , 30 , 31 , 32 , 33 ] with high levels of inflammatory cytokines, including TNF-α, IL-2 and IFN-γ, which contribute to respiratory failure, multiorgan system breakdown, and death [ 32 , 33 ]. To induce optimal inflammatory cytokine production, in addition to TCR, SEB also binds to CD28 [ 34 , 35 , 36 , 37 , 38 ], an important costimulatory molecule [ 39 , 40 ] that, in human T cells, triggers TCR-independent inflammatory signals [ 41 , 42 , 43 , 44 , 45 ].…”

SARS-CoV-2-Specific Immune Response and the Pathogenesis of COVID-19

“…The area containing this motif has a strong affinity for the V-domains of both the αand β-chains of the TCR, and it shares structural similarities with Staphylococcal enterotoxin B (SEB), one of the most potent SAg [292,293]. SEB binds not only MHC-II and TCR proteins, but also contact interaction receptors CD28 (on T cells) and B7 (on APC), which play a key role in the activation of these cells, including the generation of proinflammatory cytokines [294].…”