Human NK Cells Differ More in Their KIR2DL1-Dependent Thresholds for HLA-Cw6-Mediated Inhibition than in Their Maximal Killing Capacity

In this study we have addressed the question of how activation and inhibition of human NK cells is regulated by the expression level of MHC class I protein on target cells. Using target cell transfectants sorted to stably express different levels of the MHC class I protein HLA-Cw6, we show that induction of degranulation and that of IFN-γ secretion are not correlated. In contrast, the inhibition of these two processes by MHC class-I occurs at the same level of class I MHC protein. Primary human NK cell clones were found to differ in the amount of target MHC class I protein required for their inhibition, rather than in their maximum killing capacity. Importantly, we show that KIR2DL1 expression determines the thresholds (in terms of MHC I protein levels) required for NK cell inhibition, while the expression of other receptors such as LIR1 is less important. Furthermore, using mathematical models to explore the dynamics of target cell killing, we found that the observed delay in target cell killing is exhibited by a model in which NK cells require some activation or priming, such that each cell can lyse a target cell only after being activated by a first encounter with the same or a different target cell, but not by models which lack this feature.本研究针对人类自然杀伤（NK）细胞的激活与抑制如何受靶细胞表面主要组织相容性复合体I类（MHC class I）蛋白表达水平调控这一科学问题展开探究。我们通过分选获得稳定表达不同水平MHC I类蛋白人类白细胞抗原Cw6（HLA-Cw6）的靶细胞转染株，实验结果表明：NK细胞脱颗粒的诱导与干扰素-γ（IFN-γ）分泌的诱导二者并无相关性。与之相反，MHC I类蛋白对这两个过程的抑制作用，均在相同的MHC I类蛋白表达水平下发生。研究发现，原代人NK细胞克隆的差异并非体现在最大杀伤能力上，而是体现在实现抑制所需的靶细胞MHC I类蛋白剂量上。尤为重要的是，我们证实杀伤细胞免疫球蛋白样受体2DL1（KIR2DL1）的表达决定了NK细胞产生抑制所需的MHC I类蛋白水平阈值，而LIR1等其他受体的表达对该过程的影响则相对较小。此外，我们借助数学模型探究靶细胞杀伤的动力学过程，结果发现：观测到的靶细胞杀伤延迟现象，仅能在符合以下特征的模型中重现——NK细胞需要经历一定的激活或致敏过程，即每个NK细胞仅在首次接触相同或不同靶细胞并被激活后，才能裂解靶细胞；而缺乏该特征的模型无法复现这一现象。