Natural Killer Cell Evasion Is Essential for Infection by Rhesus Cytomegalovirus

The natural killer cell receptor NKG2D activates NK cells by engaging one of several ligands (NKG2DLs) belonging to either the MIC or ULBP families. Human cytomegalovirus (HCMV) UL16 and UL142 counteract this activation by retaining NKG2DLs and US18 and US20 act via lysomal degradation but the importance of NK cell evasion for infection is unknown. Since NKG2DLs are highly conserved in rhesus macaques, we characterized how NKG2DL interception by rhesus cytomegalovirus (RhCMV) impacts infection in vivo. Interestingly, RhCMV lacks homologs of UL16 and UL142 but instead employs Rh159, the homolog of UL148, to prevent NKG2DL surface expression. Rh159 resides in the endoplasmic reticulum and retains several NKG2DLs whereas UL148 does not interfere with NKG2DL expression. Deletion of Rh159 releases human and rhesus MIC proteins, but not ULBPs, from retention while increasing NK cell stimulation by infected cells. Importantly, RhCMV lacking Rh159 cannot infect CMV-naïve animals unless CD8+ cells, including NK cells, are depleted. However, infection can be rescued by replacing Rh159 with HCMV UL16 suggesting that Rh159 and UL16 perform similar functions in vivo. We therefore conclude that cytomegaloviral interference with NK cell activation is essential to establish but not to maintain chronic infection.

自然杀伤细胞受体NKG2D（natural killer cell receptor NKG2D）可通过结合属于MIC家族或ULBP家族的多种配体（NKG2DLs）激活NK细胞。人巨细胞病毒（Human cytomegalovirus, HCMV）的UL16与UL142可通过滞留NKG2DLs拮抗这一激活过程，US18与US20则通过溶酶体降解途径发挥作用，但NK细胞逃逸在病毒感染中的重要性仍未明确。鉴于NKG2DLs在恒河猴（rhesus macaques）中高度保守，本研究对恒河猴巨细胞病毒（rhesus cytomegalovirus, RhCMV）介导的NKG2DL扣留对体内感染的影响进行了系统表征。有趣的是，RhCMV并不携带UL16与UL142的同源物，而是利用UL148的同源物Rh159来阻断NKG2DL的细胞表面表达。Rh159定位于内质网（endoplasmic reticulum）并可滞留多种NKG2DLs，而UL148则不会干扰NKG2DL的表达。敲除Rh159可使被感染细胞释放滞留的人源与恒河猴源MIC蛋白，但无法释放ULBPs，同时可增强感染细胞对NK细胞的刺激能力。至关重要的是，缺失Rh159的RhCMV无法感染未接触过CMV的动物，除非同时清除包括NK细胞在内的CD8阳性（CD8+）细胞。但若将Rh159替换为HCMV UL16则可恢复其感染能力，这表明Rh159与UL16在体内发挥相似的功能。因此本研究得出结论：巨细胞病毒对NK细胞激活的干预对于建立初次感染至关重要，但并非维持慢性感染（chronic infection）所必需。