Differential Susceptibility of RAE-1 Isoforms to Mouse Cytomegalovirus

Abstract

The NKG2D receptor is one of the most potent activating natural killer cell receptors involved in antiviral responses. The mouse NKG2D ligands MULT-1, RAE-1, and H60 are regulated by murine cytomegalovirus (MCMV) proteins m145, m152, and m155, respectively. In addition, the m138 protein interferes with the expression of both MULT-1 and H60. We show here that one of five RAE-1 isoforms, RAE-1␦, is resistant to downregulation by MCMV and that this escape has functional importance in vivo. Although m152 retained newly synthesized RAE-1␦ and RAE-1␥ in the endoplasmic reticulum, no viral regulator was able to affect the mature RAE-1␦ form which remains expressed on the surfaces of infected cells. This differential susceptibility to downregulation by MCMV is not a consequence of faster maturation of RAE-1␦ compared to RAE-1␥ but rather an intrinsic property of the mature surface-resident protein. This difference can be attributed to the absence of a PLWY motif from RAE-1␦. Altogether, these findings provide evidence for a novel mechanism of host escape from viral immunoevasion of NKG2D-dependent control.Cytomegaloviruses (CMVs) are ubiquitous pathogens causing morbidity in immune suppressed and immunodeficient hosts (34). Since CMVs are strictly species-specific viruses, the infection of mice with murine CMV (MCMV) represents a widely used model for studying CMV infection and disease (22,40).Natural killer (NK) cells play a crucial role in the control of many viruses and are among the first cells to sense proinflammatory cytokines, as well as the perturbations in the expression of major histocompatibility complex (MHC) class I molecules and other surface molecules induced by viral infection (13). Both human CMV (HCMV) and MCMV have evolved strategies to compromise innate immunity-mediated by NK cells (20,49).Although proinflammatory cytokines released during the early stage of MCMV infection induce NK cell activation, this is usually not sufficient for virus control (11). Namely, most mouse strains fail to mount an effector phase of NK cell response against infected cells (42), in spite of the fact that MCMV infection causes the downmodulation of MHC I molecules (17), which should activate NK cells via a "missing-self" mechanism (28). The lack of NK cell activation by MCMV is even more puzzling considering that NK cells possess activating receptors that recognize cellular ligands induced by infection. Among these is the activating receptor NKG2D, a C-type lectinlike receptor encoded by a single gene in humans and rodents (39). Engagement of NKG2D transduces a strong activating signal to promote NK cell stimulation. NKG2D also serves as a costimulatory receptor on CD8 ϩ T cells (2). Several NKG2D ligands have been described in mice: MULT-1, H60a, H60b, H60c, and RAE-1␣, -1␤, -1␥, -1␦, and -1ε isoforms (4-6, 10, 14, 32, 35, 44). What prevents the activation of NK cells via the NKG2D receptor during MCMV infection? We and others have characterized four MCMV proteins involved in the downmodulation of NKG2D ligands (15,23...

Intracellular Sequestration of the NKG2D Ligand ULBP3 by Human Cytomegalovirus

“…HCMV is particularly adept at minimizing NK cell activation through the NKG2D pathway, with at least four viral genes (UL16, UL142, UL112-1, and an unidentified gene) (27, 28, 30, 31, 33, 34, 37, 38, 52) combining to prevent expression of MICA/B and ULBP1/ 2/6 at the cell surface. With so many mechanisms already identified, it seems possible that HCMV is particularly susceptible to NKG2D-mediated responses, a hypothesis supported by evidence from mouse models in which MCMV is the only virus known to downregulate all ligands of murine NKG2D (41)(42)(43)(44)(45)(46)(47).…”

“…MCMV also downregulates surface levels of mH60 with the m155 gene (43) in a proteasome-dependent manner (44). Interestingly, the virus also shows a certain amount of redundancy: specifically, the m138 gene product, originally identified as a viral FcR (45), is able to downregulate expression of mH60, MULT-1 (46), and some isoforms of the RAE genes (47).…”

NKG2D Receptor and Its Ligands in Host Defense

“…Mouse CMV has evolved its own viral proteins to counter NKG2D-dependent immunity. Mouse CMV m152 protein blocks surface expression of Raet1 proteins (52, 53); m155 blocks H60 (54), m145 blocks MULT1 (55), and m138 blocks expression of MULT1 and H60 (56). The Nef protein in HIV-1 inhibits expression of MICA, RAET1I (ULBP1), and RAET1H (ULBP2) (57), as well as inhibiting HLA-A and HLA-B.…”

γδ T Cells and the Lymphoid Stress-Surveillance Response

“…Perhaps different ligands exist to outflank viral immunoevasion of single ligands (Eagle and Trowsdale, 2007;Arapovic et al, 2009). Alternatively, by engaging NKG2D with different affinities (O'Callaghan et al, 2001), different ligands may stimulate NKG2D + cells to deploy different effectors appropriate to different forms of afferent stress.…”