Programmed cell death protein 1 (PD-1) is an inhibitory receptor on T lymphocytes that is critical for modulating adaptive immunity. As such, it has been successfully exploited for cancer immunotherapy. Programmed death ligand 1 (PD-L1) and PD-L2 are ligands for PD-1; the former is ubiquitously expressed in inflamed tissues, whereas the latter is restricted to antigen-presenting cells. PD-L2 binds to PD-1 with 3-fold stronger affinity compared with PD-L1. To date, this affinity discrepancy has been attributed to a tryptophan (W110PD-L2) that is unique to PD-L2 and has been assumed to fit snuggly into a pocket on the PD-1 surface. Contrary to this model, using surface plasmon resonance to monitor real-time binding of recombinantly-expressed and -purified proteins, we found that W110PD-L2 acts as an “elbow” that helps shorten PD-L2 engagement with PD-1 and therefore lower affinity. Furthermore, we identified a “latch” between the C and D β-strands of the binding face as the source of the PD-L2 affinity advantage. We show that the 3-fold affinity advantage of PD-L2 is the consequence of these two opposing features, the W110PD-L2 “elbow” and a C–D region “latch.” Interestingly, using phylogenetic analysis, we found that these features evolved simultaneously upon the emergence of placental mammals, suggesting that PD-L2–affinity tuning was part of the alterations to the adaptive immune system required for placental gestation.

中文翻译：

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在胎盘哺乳动物中出现了区分PD-L2与PD-L1的结构特征

程序性细胞死亡蛋白1（PD-1）是T淋巴细胞上的一种抑制性受体，对调节适应性免疫至关重要。因此，已成功地将其用于癌症免疫治疗。程序性死亡配体1（PD-L1）和PD-L2是PD-1的配体；前者在发炎的组织中普遍表达，而后者则限于抗原呈递细胞。与PD-L1相比，PD-L2与PD-1的结合亲和力为3倍。迄今为止，这种亲和力差异已归因于色氨酸（W110PD-L2），该色氨酸是PD-L2所独有的，并且被认为可以紧紧地适合PD-1表面上的口袋。与该模型相反，使用表面等离振子共振监测重组表达和纯化蛋白的实时结合，我们发现W110PD-L2充当“肘”，有助于缩短PD-L2与PD-1的结合，从而降低亲和力。此外，我们确定了结合面的C和Dβ链之间的“闩锁”是PD-L2亲和力优势的来源。我们表明，PD-L2的3倍亲和力优势是这两个相对特征W110PD-L2“肘”和C-D区“闩锁”的结果。有趣的是，使用系统发育分析，我们发现这些特征在胎盘哺乳动物出现时同时演化，这表明PD-L2亲和力调节是胎盘妊娠所需适应性免疫系统改变的一部分。我们表明，PD-L2的3倍亲和力优势是这两个相对特征W110PD-L2“肘”和C-D区“闩锁”的结果。有趣的是，使用系统发育分析，我们发现这些特征在胎盘哺乳动物出现时同时演化，这表明PD-L2亲和力调节是胎盘妊娠所需适应性免疫系统改变的一部分。我们表明，PD-L2的3倍亲和力优势是这两个相对特征W110PD-L2“肘”和C-D区“闩锁”的结果。有趣的是，使用系统发育分析，我们发现这些特征在胎盘哺乳动物出现时同时演化，这表明PD-L2亲和力调节是胎盘妊娠所需适应性免疫系统改变的一部分。