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NKG2D ligand MICA is retained in the cis-Golgi apparatus by human cytomegalovirus protein UL142.

ABSTRACT: Human cytomegalovirus (HCMV) evades T-cell recognition by down-regulating expression of major histocompatibility complex (MHC) class I and II molecules on the surfaces of infected cells. Contrary to the "missing-self" hypothesis, HCMV-infected cells are refractory to lysis by natural killer (NK) cells. Inhibition of NK cell function is mediated by a number of HCMV immune evasion molecules, which operate by delivering inhibitory signals to NK cells and preventing engagement of activating ligands. One such molecule is UL142, which is an MHC class I-related glycoprotein encoded by clinical isolates and low-passage-number strains of HCMV. UL142 is known to down-modulate surface expression of MHC class I-related chain A (MICA), which is a ligand of the activating NK receptor NKG2D. However, the mechanism by which UL142 interferes with MICA is unknown. Here, we show that UL142 localizes predominantly to the endoplasmic reticulum (ER) and cis-Golgi apparatus. The transmembrane domain of UL142 mediates its ER localization, while we propose that the UL142 luminal domain is involved in its cis-Golgi localization. We also confirm that UL142 down-modulates surface expression of full-length MICA alleles while having no effect on the truncated allele MICA*008. However, we demonstrate for the first time that UL142 retains full-length MICA alleles in the cis-Golgi apparatus. In addition, we propose that UL142 interacts with nascent MICA en route to the cell surface but not mature MICA at the cell surface. Our data also demonstrate that the UL142 luminal and transmembrane domains are involved in recognition and intracellular sequestration of full-length MICA alleles.

SUBMITTER: Ashiru O

PROVIDER: S-EPMC2786768 | biostudies-literature | 2009 Dec

REPOSITORIES: biostudies-literature

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NKG2D ligand MICA is retained in the cis-Golgi apparatus by human cytomegalovirus protein UL142.

Ashiru Omodele O Bennett Neil J NJ Boyle Louise H LH Thomas Mair M Trowsdale John J Wills Mark R MR

Journal of virology 20090930 23

Human cytomegalovirus (HCMV) evades T-cell recognition by down-regulating expression of major histocompatibility complex (MHC) class I and II molecules on the surfaces of infected cells. Contrary to the "missing-self" hypothesis, HCMV-infected cells are refractory to lysis by natural killer (NK) cells. Inhibition of NK cell function is mediated by a number of HCMV immune evasion molecules, which operate by delivering inhibitory signals to NK cells and preventing engagement of activating ligands. ...[more]

PMID: 19793804

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Project description:Natural killer (NK) cells are lymphocytes of the innate immune system capable of killing hazardous cells, including virally infected cells. NK cell-mediated killing is triggered by activating receptors. Prominent among these is the activating receptor NKG2D, which binds several stress-induced ligands, among them major histocompatibility complex (MHC) class I-related chain A (MICA). Most of the human population is persistently infected with human cytomegalovirus (HCMV), a virus which employs multiple immune evasion mechanisms, many of which target NK cell responses. HCMV infection is mostly asymptomatic, but in congenitally infected neonates and in immunosuppressed patients it can lead to serious complications and mortality. Here we discovered that an HCMV protein named UL148A whose role was hitherto unknown is required for evasion of NK cells. We demonstrate that UL148A-deficient HCMV strains are impaired in their ability to downregulate MICA expression. We further show that when expressed by itself, UL148A is not sufficient for MICA targeting, but rather acts in concert with an unknown viral factor. Using inhibitors of different cellular degradation pathways, we show that UL148A targets MICA for lysosomal degradation. Finally, we show that UL148A-mediated MICA downregulation hampers NK cell-mediated killing of HCMV-infected cells. Discovering the full repertoire of HCMV immune evasion mechanisms will lead to a better understanding of the ability of HCMV to persist in the host and may also promote the development of new vaccines and drugs against HCMV.IMPORTANCE Human cytomegalovirus (HCMV) is a ubiquitous pathogen which is usually asymptomatic but that can cause serious complications and mortality in congenital infections and in immunosuppressed patients. One of the difficulties in developing novel vaccines and treatments for HCMV is its remarkable ability to evade our immune system. In particular, HCMV directs significant efforts to thwarting cells of the innate immune system known as natural killer (NK) cells. These cells are crucial for successful control of HCMV infection, and yet our understanding of the mechanisms which HCMV utilizes to elude NK cells is partial at best. In the present study, we discovered that a protein encoded by HCMV which had no known function is important for preventing NK cells from killing HCMV-infected cells. This knowledge can be used in the future for designing more-efficient HCMV vaccines and for formulating novel therapies targeting this virus.

Project description:NKG2D is a potent activating immunoreceptor expressed on nearly all cytotoxic lymphocytes promoting their cytotoxicity against self-cells expressing NKG2D ligands (NKG2DLs). NKG2DLs are MHC class I-like glycoproteins that usually are not expressed on "healthy" cells. Rather, their surface expression is induced by various forms of cellular stress, viral infection, or malignant transformation. Hence, cell surface NKG2DLs mark "dangerous" cells for elimination by cytotoxic lymphocytes and therefore can be considered as "kill-me" signals. In addition, NKG2DLs are up-regulated on activated leukocytes, which facilitates containment of immune responses. While the NKG2D receptor is conserved among mammals, NKG2DL genes have rapidly diversified during mammalian speciation, likely due to strong selective pressures exerted by species-specific pathogens. Consequently, NKG2DL genes are not conserved in man and mice, although their NKG2D-binding domains maintained structural homology. Human NKG2DLs comprise two members of the MIC (MICA/MICB) and six members of the ULBP family of glycoproteins (ULBP1-6) with MICA representing the best-studied human NKG2DLs by far. Many of these studies implicate a role of MICA in various malignant, infectious, or autoimmune diseases. However, conclusions from these studies often were limited in default of supporting in vivo experiments. Here, we report a MICA transgenic (MICAgen) mouse model that replicates central features of human MICA expression and function and, therefore, constitutes a novel tool to critically assess and extend conclusions from previous in vitro studies on MICA. Similarly to humans, MICA transcripts are broadly present in organs of MICAgen mice, while MICA glycoproteins are barely detectable. Upon activation, hematopoietic cells up-regulate and proteolytically shed surface MICA. Shed soluble MICA (sMICA) is also present in plasma of MICAgen mice but affects neither surface NKG2D expression of circulating NK cells nor their functional recognition of MICA-expressing tumor cells. Accordingly, MICAgen mice also show a delayed growth of MICA-expressing B16F10 tumors, not accompanied by an emergence of MICA-specific antibodies. Such immunotolerance for the xenoantigen MICA ideally suits MICAgen mice for anti-MICA-based immunotherapies. Altogether, MICAgen mice represent a valuable model to study regulation, function, disease relevance, and therapeutic targeting of MICA in vivo.

Project description:The replication of picornaviruses has been described to cause fragmentation of the Golgi apparatus that blocks the secretory pathway. The inhibition of major histocompatibility complex class I upregulation and cytokine, chemokine and interferon secretion may have important implications for host defense. Previous studies have shown that disruption of the secretory pathway can be replicated by expression of individual nonstructural proteins; however the situation with different serotypes of human rhinovirus (HRV) is unclear. The expression of 3A protein from HRV14 or HRV2 did not cause Golgi apparatus disruption or a block in secretion, whereas other studies showed that infection of cells with HRV1A did cause Golgi apparatus disruption which was replicated by the expression of 3A. HRV16 is the serotype most widely used in clinical HRV challenge studies; consequently, to address the issue of Golgi apparatus disruption for HRV16, we have systematically and quantitatively examined the effect of HRV16 on both Golgi apparatus fragmentation and protein secretion in HeLa cells. First, we expressed each individual nonstructural protein and examined their cellular localization and their disruption of endoplasmic reticulum and Golgi apparatus architecture. We quantified their effects on the secretory pathway by measuring secretion of the reporter protein Gaussia luciferase. Finally, we examined the same outcomes following infection of cells with live virus. We demonstrate that expression of HRV16 3A and 3AB and, to a lesser extent, 2B caused dispersal of the Golgi structure, and these three nonstructural proteins also inhibited protein secretion. The infection of cells with HRV16 also caused significant Golgi apparatus dispersal; however, this did not result in the inhibition of protein secretion. Importance: The ability of replicating picornaviruses to influence the function of the secretory pathway has important implications for host defense. However, there appear to be differences between different members of the family and inconsistent results when comparing infection with live virus to expression of individual nonstructural proteins. We demonstrate that individual nonstructural HRV16 proteins, when expressed in HeLa cells, can both fragment the Golgi apparatus and block secretion, whereas viral infection fragments the Golgi apparatus without blocking secretion. This has major implications for how we interpret mechanistic evidence derived from the expression of single viral proteins.

Project description:BackgroundTumour-derived exosomes can be released to serum and provide information on the features of the malignancy, however, in order to perform systematic studies in biological samples, faster diagnostic techniques are needed, especially for detection of low abundance proteins. Most human cancer cells are positive for at least one ligand for the activating immune receptor NKG2D and the presence in plasma of NKG2D-ligands can be associated with prognosis.MethodsUsing MICA as example of a tumour-derived antigen, endogenously expressed in metastatic melanoma and recruited to exosomes, we have developed two immunocapture-based assays for detection of different epitopes in nanovesicles. Although both techniques, enzyme-linked immunosorbent assay (ELISA) and Lateral flow immunoassays (LFIA) have the same theoretical basis, that is, using capture and detection antibodies for a colorimetric read-out, analysis of exosome-bound proteins poses methodological problems that do not occur when these techniques are used for detection of soluble molecules, due to the presence of multiple epitopes on the vesicle.ResultsHere we demonstrate that, in ELISA, the signal obtained was directly proportional to the amount of epitopes per exosome. In LFIA, the amount of detection antibody immobilized in Au-nanoparticles needs to be low for efficient detection, otherwise steric hindrance results in lower signal. We describe the conditions for detection of MICA in exosomes and prove, for the first time using both techniques, the co-existence in one vesicle of exosomal markers (the tetraspanins CD9, CD63 and CD81) and an endogenously expressed tumour-derived antigen. The study also reveals that scarce proteins can be used as targets for detection antibody in LFIA with a better result than very abundant proteins and that the conditions can be optimized for detection of the protein in plasma.ConclusionsThese results open the possibility of analyzing biological samples for the presence of tumour-derived exosomes using high throughput techniques.

Dataset Information

NKG2D ligand MICA is retained in the cis-Golgi apparatus by human cytomegalovirus protein UL142.

Publications

NKG2D ligand MICA is retained in the cis-Golgi apparatus by human cytomegalovirus protein UL142.

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