Remarkably low affinity of CD4/peptide-major histocompatibility complex class II protein interactions
Remarkably low affinity of CD4/peptide-major histocompatibility complex class II protein interactions
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DOI：
10.1073/pnas.1513918113
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发表时间：
2016-05-17
影响因子：
11.1
通讯作者：
Klenerman, David
中科院分区：
文献类型：
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作者：
Jonsson, Peter;Southcombe, Jennifer H.;Klenerman, David
α β T细胞辅助受体CD 4在某些测定中增强免疫应答超过100万倍，但CD 4对其配体抗原呈递细胞上的肽-主要组织相容性II类（pMHC II）的亲和力如此之弱，以至于以前无法定量。在这里，我们报告说，可溶性形式的CD 4未能结合可检测到pMHC II在表面等离子体共振为基础的测定，建立一个新的上限为2.5 mM的溶液亲和力。然而，当提出多价磁珠，可溶性CD 4结合pMHC II表达B细胞，确认它是积极的，并允许映射的原生辅助受体结合位点pMHC II。尽管在溶液中无法检测到结合，但可以使用CD 4和粘附分子功能化的支持脂质双层在2D中测量CD 4/pMHC II相互作用的亲和力，产生类似于5，000分子/μ m的2D Kd（2）。该值比先前测量的相互作用白细胞表面蛋白的2D K-d值高两到三个数量级。然而，计算表明，CD 4/pMHC II结合将通过募集Lck使T细胞受体（TCR）复合物磷酸化的速率增加三倍，TCR对pMHC II的有效亲和力仅增加2-20%。因此，CD 4/pMHC II的亲和力似乎被设定在通过增强磷酸化而增加T细胞敏感性的值，而不损害配体识别。
The alpha beta T-cell coreceptor CD4 enhances immune responses more than 1 million-fold in some assays, and yet the affinity of CD4 for its ligand, peptide-major histocompatibility class II (pMHC II) on antigen-presenting cells, is so weak that it was previously un-quantifiable. Here, we report that a soluble form of CD4 failed to bind detectably to pMHC II in surface plasmon resonance-based assays, establishing a new upper limit for the solution affinity at 2.5 mM. However, when presented multivalently on magnetic beads, soluble CD4 bound pMHC II-expressing B cells, confirming that it is active and allowing mapping of the native coreceptor binding site on pMHC II. Whereas binding was undetectable in solution, the affinity of the CD4/pMHC II interaction could be measured in 2D using CD4-and adhesion molecule-functionalized, supported lipid bilayers, yielding a 2D K-d of similar to 5,000 molecules/mu m(2). This value is two to three orders of magnitude higher than previously measured 2D K-d values for interacting leukocyte surface proteins. Calculations indicated, however, that CD4/pMHC II binding would increase rates of T-cell receptor (TCR) complex phosphorylation by threefold via the recruitment of Lck, with only a small, 2-20% increase in the effective affinity of the TCR for pMHC II. The affinity of CD4/pMHC II therefore seems to be set at a value that increases T-cell sensitivity by enhancing phosphorylation, without compromising ligand discrimination.