RESUMO
Superantigens (SAgs) are microbial toxins defined by their ability to activate T lymphocytes in a T cell receptor (TCR) ß-chain variable domain (Vß)-specific manner. Although existing structural information indicates that diverse bacterial SAgs all uniformly engage the Vß second complementarity determining region (CDR2ß) loop, the molecular rules that dictate SAg-mediated T cell activation and Vß specificity are not fully understood. Herein we report the crystal structure of human Vß2.1 (hVß2.1) in complex with the toxic shock syndrome toxin-1 (TSST-1) SAg, and mutagenesis of hVß2.1 indicates that the non-canonical length of CDR2ß is a critical determinant for recognition by TSST-1 as well as the distantly related SAg streptococcal pyrogenic exotoxin C. Frame work (FR) region 3 is uniquely critical for TSST-1 function explaining the fine Vß-specificity exhibited by this SAg. Furthermore, domain swapping experiments with SAgs, which use distinct domains to engage both CDR2ß and FR3/4ß revealed that the CDR2ß contacts dictate T lymphocyte Vß-specificity. These findings demonstrate that the TCR CDR2ß loop is the critical determinant for functional recognition and Vß-specificity by diverse bacterial SAgs.
Assuntos
Toxinas Bacterianas/química , Regiões Determinantes de Complementaridade/química , Enterotoxinas/química , Ativação Linfocitária , Receptores de Antígenos de Linfócitos T alfa-beta/química , Superantígenos/química , Linfócitos T/química , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Proteínas de Bactérias/imunologia , Toxinas Bacterianas/genética , Toxinas Bacterianas/imunologia , Linhagem Celular , Regiões Determinantes de Complementaridade/genética , Regiões Determinantes de Complementaridade/imunologia , Cristalografia por Raios X , Enterotoxinas/genética , Enterotoxinas/imunologia , Exotoxinas/química , Exotoxinas/genética , Exotoxinas/imunologia , Humanos , Mutagênese , Estrutura Quaternária de Proteína , Estrutura Terciária de Proteína , Receptores de Antígenos de Linfócitos T alfa-beta/genética , Receptores de Antígenos de Linfócitos T alfa-beta/imunologia , Superantígenos/genética , Superantígenos/imunologia , Linfócitos T/imunologiaRESUMO
Superantigens (SAGs) interact with host immune receptors to induce a massive release of inflammatory cytokines that can lead to toxic shock syndrome and death. Bacterial SAGs can be classified into five distinct evolutionary groups. Group V SAGs are characterized by the alpha3-beta8 loop, a unique approximately 15 amino acid residue extension that is required for optimal T cell activation. Here, we report the X-ray crystal structures of the group V SAG staphylococcal enterotoxin K (SEK) alone and in complex with the TCR hVbeta5.1 domain. SEK adopts a unique TCR binding orientation relative to other SAG-TCR complexes, which results in the alpha3-beta8 loop contacting the apical loop of framework region 4, thereby extending the known TCR recognition site of SAGs. These interactions are absolutely required for TCR binding and T cell activation by SEK, and dictate the TCR Vbeta domain specificity of SEK and other group V SAGs.
