Miller Peter J, Pazy Yael, Conti Brian, Riddle David, Appella Ettore, Collins Edward J
Department of Biochemistry and Biophysics, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
J Mol Biol. 2007 Oct 19;373(2):315-27. doi: 10.1016/j.jmb.2007.07.028. Epub 2007 Jul 26.
The keystone of the adaptive immune response is T cell receptor (TCR) recognition of peptide presented by major histocompatibility complex (pMHC) molecules. The crystal structure of AHIII TCR bound to MHC, HLA-A2, showed a large interface with an atypical binding orientation. MHC mutations in the interface of the proteins were tested for changes in TCR recognition. From the range of responses observed, three representative HLA-A2 mutants, T163A, W167A, and K66A, were selected for further study. Binding constants and co-crystal structures of the AHIII TCR and the three mutants were determined. K66 in HLA-A2 makes contacts with both peptide and TCR, and has been identified as a critical residue for recognition by numerous TCR. The K66A mutation resulted in the lowest AHIII T cell response and the lowest binding affinity, which suggests that the T cell response may correlate with affinity. Importantly, the K66A mutation does not affect the conformation of the peptide. The change in affinity appears to be due to a loss in hydrogen bonds in the interface as a result of a conformational change in the TCR complementarity-determining region 3 (CDR3) loop. Isothermal titration calorimetry confirmed the loss of hydrogen bonding by a large loss in enthalpy. Our findings are inconsistent with the notion that the CDR1 and CDR2 loops of the TCR are responsible for MHC restriction, while the CDR3 loops interact solely with the peptide. Instead, we present here an MHC mutation that does not change the conformation of the peptide, yet results in an altered conformation of a CDR3.
适应性免疫反应的关键在于T细胞受体（TCR）对主要组织相容性复合体（pMHC）分子呈递的肽段的识别。与MHC、HLA - A2结合的AHIII TCR的晶体结构显示出一个具有非典型结合方向的大界面。对蛋白质界面处的MHC突变进行了TCR识别变化的测试。从观察到的一系列反应中，选择了三个具有代表性的HLA - A2突变体T163A、W167A和K66A进行进一步研究。测定了AHIII TCR与这三个突变体的结合常数和共晶体结构。HLA - A2中的K66与肽段和TCR都有接触，并且已被确定为众多TCR识别的关键残基。K66A突变导致AHIII T细胞反应最低以及结合亲和力最低，这表明T细胞反应可能与亲和力相关。重要的是，K66A突变不影响肽段的构象。亲和力的变化似乎是由于TCR互补决定区3（CDR3）环的构象变化导致界面处氢键的丧失。等温滴定量热法通过焓的大幅损失证实了氢键的丧失。我们的发现与TCR的CDR1和CDR2环负责MHC限制而CDR3环仅与肽段相互作用的观点不一致。相反，我们在此展示了一种MHC突变，该突变不会改变肽段的构象，但会导致CDR3的构象改变。
