MMTV superantigens coerce an unconventional topology between the TCR and MHC class II.
Mouse mammary tumor virus superantigens (vSAGs) are notorious for defying structural characterization, and a consensus has yet to be reached regarding their ability to bridge the TCR to MHC class II (MHCII). In this study, we determined the topology of the T cell signaling complex by examining the respective relation of vSAG7 with the MHCII molecule, MHCII-associated peptide, and TCR. We used covalently linked peptide/MHCII complexes to demonstrate that vSAG presentation is tolerant to variation in the protruding side chains of the peptide, but can be sensitive to the nature of the protruding N-terminal extension. An original approach in which vSAG was covalently linked to either MHCII chain confirmed that vSAG binds outside the peptide binding groove. Also, whereas the C-terminal vSAG segment binds to the MHCII α-chain in a conformation-sensitive manner, the membrane-proximal N-terminal domain binds the β-chain. Because both moieties of the mature vSAG remain noncovalently associated after processing, our results suggest that vSAG crosslinks MHCII molecules. Comparing different T cell hybridomas, we identified key residues on the MHCII α-chain that are differentially recognized by the CDR3β when engaged by vSAG. Finally, we show that the highly conserved tyrosine residue found in the vSAg TGXY motif is required for T cell activation. Our results reveal a novel SAG/MHCII/TCR architecture in which vSAGs coerce a near-canonical docking between MHCII and TCR that allows eschewing of traditional CDR3 binding with the associated peptide in favor of MHCII α-chain binding. Our findings highlight the plasticity of the TCR CDRs.
Laboratoire d'Immunologie Moléculaire, Département de Microbiologie, Infectiologie et Immunologie, Université de Montréal, Montreal, Quebec HC3 3J7, Canada;
Jean-Simon
JS
Fortin
Laetitia
L
Genève
Catherine
C
Gauthier
Naglaa H
NH
Shoukry
Georges A
GA
Azar
Souheil
S
Younes
Bader
B
Yassine-Diab
Rafick-Pierre
RP
Sékaly
Daved H
DH
Fremont
Jacques
J
Thibodeau
D000956:Antigens, Viral / Q000276:immunology*; D001665:Binding Sites / Q000276:immunology; D057809:HEK293 Cells; D006367:HeLa Cells; D000949:Histocompatibility Antigens Class II / Q000276:immunology*; D006801:Humans; D019169:Jurkat Cells; D008213:Lymphocyte Activation / Q000276:immunology; D008324:Mammary Tumor Virus, Mouse / Q000276:immunology*; D011485:Protein Binding / Q000276:immunology; D017434:Protein Structure, Tertiary; D016693:Receptors, Antigen, T-Cell, alpha-beta / Q000276:immunology*; D015398:Signal Transduction / Q000276:immunology; D018089:Superantigens / Q000276:immunology*; D013601:T-Lymphocytes / Q000276:immunology
D000956:Antigens, Viral; D000949:Histocompatibility Antigens Class II; D016693:Receptors, Antigen, T-Cell, alpha-beta; D018089:Superantigens