Structural Immunology Section, Laboratory of Immunogenetics, National Institute of Allergy and Infectious Diseases, Rockville, MD 20852; and.
Experimental Immunology Branch, National Cancer Institute, Bethesda, MD 20892.
J Immunol. 2020 Jun 15;204(12):3351-3359. doi: 10.4049/jimmunol.1901084. Epub 2020 Apr 22.
During normal T cell development in the thymus, αβ TCRs signal immature thymocytes to differentiate into mature T cells by binding to peptide-MHC ligands together with CD4/CD8 coreceptors. Conversely, in MHC and CD4/CD8 coreceptor-deficient mice, the thymus generates mature T cells expressing MHC-independent TCRs that recognize native conformational epitopes rather than linear antigenic-peptides presented by MHC. To date, no structural information of MHC-independent TCRs is available, and their structural recognition of non-MHC ligand remains unknown. To our knowledge in this study, we determined the first structures of two murine MHC-independent TCRs (A11 and B12A) that bind with high nanomolar affinities to mouse adhesion receptor CD155. Solution binding demonstrated the Vαβ-domain is responsible for MHC-independent B12A recognition of its ligand. Analysis of A11 and B12A sequences against various MHC-restricted and -independent TCR sequence repertoires showed that individual V-genes of A11 and B12A did not exhibit preference against MHC-restriction. Likewise, CDR3 alone did not discriminate against MHC binding, suggesting VDJ recombination together with Vα/Vβ pairing determine their MHC-independent specificity for CD155. The structures of A11 and B12A TCR are nearly identical to those of MHC-restricted TCR, including the conformations of CDR1 and 2. Mutational analysis, together with negative-staining electron microscopy images, showed that the CDR regions of A11 and B12A recognized epitopes on D1 domain of CD155, a region also involved in CD155 binding to poliovirus and Tactile in human. Taken together, MHC-independent TCRs adopt canonical TCR structures to recognize native Ags, highlighting the importance of thymic selection in determining TCR ligand specificity.
在胸腺中正常 T 细胞发育过程中，αβTCR 通过与 MHC 配体结合以及与 CD4/CD8 核心受体结合来信号不成熟的胸腺细胞分化为成熟 T 细胞。相反，在 MHC 和 CD4/CD8 核心受体缺陷小鼠中，胸腺生成表达 MHC 非依赖性 TCR 的成熟 T 细胞，这些 TCR 识别非 MHC 配体的天然构象表位，而不是 MHC 呈递的线性抗原肽。迄今为止，尚无 MHC 非依赖性 TCR 的结构信息，其对非 MHC 配体的结构识别仍不清楚。据我们所知，本研究首次确定了两种与高纳摩尔亲和力结合的小鼠 MHC 非依赖性 TCR（A11 和 B12A）的结构，它们与小鼠黏附受体 CD155 结合。溶液结合实验表明，Vαβ 结构域负责 B12A 对其配体的 MHC 非依赖性识别。对 A11 和 B12A 序列与各种 MHC 受限和非 MHC 受限 TCR 序列库的分析表明，A11 和 B12A 的个体 V 基因没有表现出对 MHC 限制的偏好。同样，CDR3 本身不能区分 MHC 结合，这表明 VDJ 重组与 Vα/Vβ 配对一起决定了它们对 CD155 的 MHC 非依赖性特异性。A11 和 B12A TCR 的结构与 MHC 受限 TCR 几乎相同，包括 CDR1 和 2 的构象。突变分析以及负染电子显微镜图像表明，A11 和 B12A 的 CDR 区域识别 CD155 D1 结构域上的表位，该区域也参与 CD155 与脊髓灰质炎病毒和人 Tactile 的结合。总之，MHC 非依赖性 TCR 采用经典 TCR 结构来识别天然抗原，突出了胸腺选择在决定 TCR 配体特异性中的重要性。
