Darley Rachel, Illing Patricia T, Duriez Patrick, Bailey Alistair, Purcell Anthony W, van Hateren Andy, Elliott Tim
Institute for Life Sciences and Centre for Cancer Immunology, Faculty of Medicine, University of Southampton, Southampton, United Kingdom.
Department of Biochemistry and Molecular Biology and Biomedicine Discovery Institute, Monash University, Melbourne, VIC, Australia.
Front Immunol. 2025 May 8;16:1563789. doi: 10.3389/fimmu.2025.1563789. eCollection 2025.
Major Histocompatibility Complex class I (MHC-I) molecules bind and present peptides to cytotoxic T cells, protecting against pathogens and cancer. MHC-I is highly polymorphic and each allotype is promiscuous, and capable of binding a unique and diverse repertoire of peptide ligands. The peptide editing chaperone tapasin optimizes this allotype specific repertoire of peptides, resulting in the selection of high affinity peptides. MHC-I allotypes differ in the extent they engage tapasin. This suggests that tapasin-dependent MHC-I allotypes should present a less diverse repertoire that is enriched in higher-affinity peptides, and which are present in higher abundance, than tapasin independent MHC-I allotypes, which should present a broader repertoire containing peptides with a lower average affinity. Experimental verification of this hypothesis has been confounded by the different peptide binding specificities of MHC-I allotypes. Here, we independently investigated the peptide focusing function of tapasin by introducing a point mutation into a tapasin independent MHC-I allotype that dramatically increased its tapasin dependence without substantially altering its peptide binding specificity. This allowed us to demonstrate ligand focusing by tapasin at both the repertoire level , and by using an system in which tapasin was artificially tethered to MHC-I, at the individual peptide level. We found that tapasin had a greater influence on tapasin dependent MHC-I molecules, and that tapasin modulated peptide selection according to peptide-MHC-I complex stability, disfavoring short-lived peptide-MHC-I complexes. Thus, tapasin dependent MHC-I molecules experience greater tapasin filtering, resulting in less diverse MHC-I immunopeptidomes that are enriched in high affinity peptide-MHC-I complexes.
主要组织相容性复合体I类（MHC-I）分子结合并向细胞毒性T细胞呈递肽段，从而抵御病原体和癌症。MHC-I具有高度多态性，每种同种异型都具有混杂性，能够结合独特且多样的肽配体库。肽编辑伴侣蛋白塔帕辛（tapasin）优化了这种同种异型特异性的肽库，从而筛选出高亲和力肽段。MHC-I同种异型在与塔帕辛结合的程度上存在差异。这表明，与不依赖塔帕辛的MHC-I同种异型相比，依赖塔帕辛的MHC-I同种异型所呈现的肽库多样性较低，其中高亲和力肽段更为富集且丰度更高，而不依赖塔帕辛的MHC-I同种异型应呈现出包含平均亲和力较低的肽段的更广泛肽库。由于MHC-I同种异型具有不同的肽结合特异性，这一假设的实验验证受到了干扰。在此，我们通过在一种不依赖塔帕辛的MHC-I同种异型中引入一个点突变，独立研究了塔帕辛的肽聚焦功能，该突变显著增加了其对塔帕辛的依赖性，而基本未改变其肽结合特异性。这使我们能够在肽库水平以及通过使用一种将塔帕辛人工连接到MHC-I的系统在单个肽段水平上证明塔帕辛的配体聚焦作用。我们发现塔帕辛对依赖塔帕辛的MHC-I分子影响更大，并且塔帕辛根据肽-MHC-I复合物的稳定性调节肽段选择，不利于寿命较短的肽-MHC-I复合物。因此，依赖塔帕辛的MHC-I分子经历了更大程度的塔帕辛筛选，从而导致MHC-I免疫肽组的多样性降低，其中高亲和力肽-MHC-I复合物更为富集。
