Ly49C-Dependent Control of MCMV Infection by NK Cells Is Cis-Regulated by MHC Class I Molecules

Natural Killer (NK) cells are crucial in early resistance to murine cytomegalovirus (MCMV) infection. In B6 mice, the activating Ly49H receptor recognizes the viral m157 glycoprotein on infected cells. We previously identified a mutant strain (MCMVG1F) whose variant m157 also binds the inhibitory Ly49C receptor. Here we show that simultaneous binding of m157 to the two receptors hampers Ly49H-dependent NK cell activation as Ly49C-mediated inhibition destabilizes NK cell conjugation with their targets and prevents the cytoskeleton reorganization that precedes killing. In B6 mice, as most Ly49H+ NK cells do not co-express Ly49C, the overall NK cell response remains able to control MCMVm157G1F infection. However, in B6 Ly49C transgenic mice where all NK cells express the inhibitory receptor, MCMV infection results in altered NK cell activation associated with increased viral replication. Ly49C-mediated inhibition also regulates Ly49H-independent NK cell activation. Most interestingly, MHC class I regulates Ly49C function through cis-interactions that mask the receptor and restricts m157 binding. B6 Ly49C Tg, β2m ko mice, whose Ly49C receptors are unmasked due to MHC class I deficient expression, are highly susceptible to MCMVm157G1F and are unable to control a low-dose infection. Our study provides novel insights into the mechanisms that regulate NK cell activation during viral infection.自然杀伤（Natural Killer, NK）细胞在抵御鼠巨细胞病毒（murine cytomegalovirus, MCMV）感染的早期免疫防御中发挥关键作用。在B6小鼠体内，激活性Ly49H受体可识别感染细胞表面的病毒m157糖蛋白。我们此前已鉴定出一株突变毒株（MCMVG1F），其变异型m157还可结合抑制性Ly49C受体。本研究证实，m157同时结合这两种受体会抑制Ly49H依赖型NK细胞活化——这是因为Ly49C介导的抑制作用会瓦解NK细胞与靶细胞的结合，并阻断杀伤效应发生前的细胞骨架重排。 在B6小鼠中，由于多数Ly49H阳性NK细胞不会共表达Ly49C，整体NK细胞应答仍可有效控制MCMVm157G1F感染。但在所有NK细胞均表达该抑制性受体的B6 Ly49C转基因小鼠中，MCMV感染会导致NK细胞活化异常，并伴随病毒复制水平升高。此外，Ly49C介导的抑制作用还可调控非Ly49H依赖型NK细胞活化。 尤为值得关注的是，主要组织相容性复合体I类（MHC class I）通过顺式相互作用遮蔽Ly49C受体、限制其与m157结合，从而调控该受体的功能。 在B6 Ly49C转基因且β2微球蛋白敲除（β2m ko）的小鼠中，由于MHC I类分子表达缺失导致Ly49C受体未被遮蔽，该小鼠对MCMVm157G1F高度易感，且无法控制低剂量病毒感染。本研究为病毒感染过程中NK细胞活化的调控机制提供了全新的学术见解。