Linking T cell receptor sequence to T cell function

T细胞是适应性免疫系统的关键效应细胞，能识别宿主细胞主要组织相容性复合体(MHC)呈递的多肽形式的异源抗原。每个T细胞都有一个独特的抗原识别受体，即T细胞受体(TCR)，它决定了单个T细胞与多肽-MHC结合的高度特异性和亲和力。TCR是由一种称为末端脱氧核苷酸转移酶(TDT)的酶通过重新排列生殖系编码的基因片段和添加非模板(非生殖系编码的)核苷酸而产生的。TCR和多肽-MHC之间的相互作用强度是所有T细胞命运决定的关键参数，从发育期间的胸腺选择到感染期间的分化。然而，由于可以制造的TCR的巨大多样性(&gt；1015不同的受体)，从其TCR序列预测T细胞的功能特征是一个悬而未决的挑战。此外，尽管TDT在TCR基因片段重排过程中添加的T细胞内巨大的TCR多样性被认为是有效的宿主保护所必需的，但这一点尚未得到实验证明。因此，TCR序列多样性、pMHC结合强度和T细胞在宿主保护中的功能之间的联系仍然不清楚。 到目前为止，在没有预先知道每个TCR的多肽-MHC配体范围的情况下，很难辨别TCR序列的全局模式或T细胞群体的结合亲和力。为了克服这一局限性，我们提出了一种新的方法，以无偏见的方式询问TCR序列对T细胞功能的影响。我们以前曾描述过，T细胞膜蛋白CD5的表达与自身配体以及MHC背景下的外源抗原获得的TCR信号的强度成正比。因此，通过将CD4T细胞分为CD5hi和CD5lo两个群体，我们现在可以研究与多肽-MHC具有不同结合强度的TCR序列的性质，而不需要事先了解它们识别的多肽。我们假设CD5hi细胞包含主要由种系编码的TCRs，这些TCRs在进化上是保守的，可以结合我们作为一个物种经常遇到的外来肽-MHC。相反，我们预测CD5lo细胞富含TDT依赖的TCRs，以提供应对新的和不同的病原体所需的额外多样性。我们将确定由TDT介导的连接核苷酸加成产生的低亲和力TCR是否在免疫系统中发挥与高亲和力TCR根本不同的作用。总体而言，我的研究计划将提供对有效T细胞谱系的数量和质量特征的基本见解，并阐明导致使用TDT来增加TCR多样性的进化压力。

T cells are critical effector cells of the adaptive immune system which recognize foreign antigen in the form of peptides presented by host cell major histocompatibility complexes (MHC). Each T cell bears a unique antigen recognition receptor, the T cell receptor (TCR), which determines the exquisite specificity and the affinity with which an individual T cell binds peptide-MHC. The TCR is generated through rearrangement of germline encoded gene segments and the addition of non-templated (non-germline encoded) nucleotides by an enzyme called terminal deoxynucleotidyl transferase (TdT). The interaction strength between the TCR and peptide-MHC is a critical parameter in all T cell fate decisions, from selection in the thymus during development to differentiation during infection. Yet, due to the enormous diversity of TCRs that can be made (>1015 distinct receptors), predicting functional characteristics of a T cell from its TCR sequence is an unsolved challenge. Moreover, while the enormous TCR diversity within a T cell repertoire that is added by TdT during TCR gene segment rearrangement is thought to be essential in effective host protection, this has not been demonstrated experimentally. Thus, the link between TCR sequence diversity, pMHC binding strength, and T cell function in host protection remains unclear. To date, it has been difficult to discern global patterns in TCR sequence or binding affinity for a T cell population without prior knowledge of the range of peptide-MHC ligands for each TCR. To overcome this limitation, we propose a novel approach that interrogates the impact of the TCR sequence on T cell function in an unbiased manner. We previously described that T cell expression of the membrane protein CD5 is directly proportional to the strength of the TCR signal obtained from self-ligands as well as foreign antigens presented in the context of MHC. Thus, by sorting CD4 T cells into CD5hi and CD5lo populations, we can now investigate properties of TCR sequences with distinct binding strengths for peptide-MHC without requiring prior knowledge of the peptides that they recognize. We hypothesize that CD5hi cells contain predominantly germline-encoded TCRs that are evolutionarily conserved to bind foreign peptide-MHC that we frequently encounter as a species. In contrast, we predict that CD5lo cells are enriched for TdT-dependent TCRs to provide the additional diversity that is required to respond to new and divergent pathogens. We will determine whether lower affinity TCRs that are generated by TdT-mediated junctional nucleotide additions play a fundamentally different role in the immune system than do high affinity TCRs. Overall, my research program will provide fundamental insight into the quantitative and qualitative features of an effective T cell repertoire and shed light on the evolutionary pressures that led to the use of TdT to increase TCR diversity.