Induction of NKG2D ligands on human dendritic cells by TLR ligand stimulation and RNA virus infection

An integrated view of the regulation of NKG2D ligands

NKG2D is one of the best characterized activating receptors and is expressed on natural killer cells and on various T-cell subsets. This receptor recognizes several different ligands that are induced by cellular stresses. In this review, we described the mechanisms controlling the expression of NKG2D ligands, with the emphasis on post-transcriptional and post-translational regulation.

The case of oncolytic viruses versus the immune system: waiting on the judgment of Solomon

The three-way interaction between oncolytic viruses, the tumor microenvironment, and the immune system is critical to the outcome of antitumor therapy. Classically, the immune system is thought to limit the efficacy of therapy, leading to viral clearance. However, preclinical and clinical data suggest that in some cases virotherapy may in fact act as cancer immunotherapy. In this review we discuss the ability of oncolytic viruses to alter the immunogenic milieu of the tumor microenvironment, and the role of innate and adaptive immunity in both restricting and augmenting therapy. Strategies to improve virotherapy by immunomodulation, including suppression or enhancement of the innate and adaptive responses, are discussed.

Enhancement of antitumor natural killer cell activation by orally administered Spirulina extract in mice

Oral administration of hot-water extract of Spirulina, cyanobacterium Spirulina platensis, leads to augmentation of NK cytotoxicity in humans. Here, we applied to syngeneic tumor-implant mice (C57BL/6 versus B16 melanoma) Spirulina to elucidate the mechanism of raising antitumor NK activation. A B16D8 subcell line barely expressed MHC class I but about 50% expressed Rae-1, a ligand for NK activation receptor NKG2D. The Rae-1-positive population of implant B16 melanoma was effectively eliminated in the tumor mass progressed in mice. This antitumor activity was induced in parallel with IFN-gamma and abolished in mice by treatment with asialoGM-1 but not CD8beta Ab, suggesting the effector is NK cell. NK cell activation occurred in the spleen of wild-type mice medicated with Spirulina. This Spirulina-mediated enhanced NK activation was abrogated in MyD88 -/- mice but not in TICAM-1 -/- mice. The NK activating properties of Spirulina depending on MyD88 were confirmed with in vitro bone marrow-derived dendritic cells expressing TLR2/4. In D16D8 tumor challenge studies, the antitumor effect of Spirulina was abolished in MyD88 -/- mice. Hence, orally administered Spirulina enhances tumoricidal NK activation through the MyD88 pathway. Spirulina exerted a synergistic antitumor activity with BCG-cell wall skeleton, which is known to activate the MyD88 pathway via TLR2/4 with no NK enhancing activity. Spirulina and BCG-cell wall skeleton synergistically augmented IFN-gamma production and antitumor potential in the B16D8 versus C57BL/6 system. We infer from these results that NK activation by Spirulina has some advantage in combinational use with BCG-cell wall skeleton for developing adjuvant-based antitumor immunotherapy.

Regulation of TLR7/9 responses in plasmacytoid dendritic cells by BST2 and ILT7 receptor interaction

Plasmacytoid dendritic cells (pDCs) produce copious type I interferon (IFN) upon sensing nucleic acids through Toll-like receptor (TLR) 7 and TLR9. Uncontrolled pDC activation and IFN production are implicated in lymphopenia and autoimmune diseases; therefore, a mechanism controlling pDC IFN production is essential. Human pDCs specifically express an orphan receptor, immunoglobulin-like transcript 7 (ILT7). Here, we discovered an ILT7 ligand expressed by human cell lines and identified it as bone marrow stromal cell antigen 2 (BST2; CD317). BST2 directly binds to purified ILT7 protein, initiates signaling via the ILT7–FcϵRIγ complex, and strongly inhibits production of IFN and proinflammatory cytokines by pDCs. Readily induced by IFN and other proinflammatory cytokines, BST2 may modulate the human pDC’s IFN responses through ILT7 in a negative feedback fashion.

Hostile communication of measles virus with host innate immunity and dendritic cells

Following measles virus (MV) infection, host innate immune responses promptly operate to purge the virus. Detection of alerting measles viral components or replication intermediates by pattern-recognizing host machinery of Toll-like receptors and RNA helicases triggers signaling to synthesize array of anti-viral and immunoregulatory molecules, including type I interferon (IFN). Diverse subtypes of dendritic cells (DCs) play pivotal roles in both host innate immunity on the primary MV-infected site and initiating adaptive immune responses on secondary lymphoid tissues. Responding to the predictable host immune responses, MV appears to have devised multiple strategies to evade, suppress, or even utilize host innate immunity and DC responses. This review focuses on versatile actions of MV-induced type I IFNs causing beneficial or deleterious influence on host immunity and the interplay between MV and heterogeneous DCs at distinct locations.

Beyond Stressed Self: Evidence for NKG2D Ligand Expression on Healthy Cells

The activity of cytotoxic lymphocytes is regulated by the opposing function of stimulatory and inhibitory cell surface receptors. According to the now classical model of Natural Killer (NK) cell activity, the ligands for inhibitory receptors are constitutively expressed on healthy cells but can be lost on infection and on malignant cells. Loss of inhibitory checks will then allow activating signals to predominate, forming the basis of 'missing self recognition'. Natural Killer Group 2D (NKG2D) is an important member of the cohort of activating receptors expressed on Natural Killer (NK) cells and subsets of T cells. Ligands for the NKG2D receptor comprise a diverse array of self-proteins structurally related to MHC class I molecules. Expression of NKG2D ligands can be induced in cells during infection with pathogens, tumourigenesis, and by stimuli such as DNA damage, oxidative stress, and heat shock. Consequently NKG2D has been widely described as participating in 'stressed self' or 'damaged self' recognition. However, a body of evidence has recently emerged to suggest that this intuitive model of NKG2D function may be an oversimplification. NKG2D ligand expression has now widely been reported on cells that could not be described as stressed or damaged. For example activated T cells can express NKG2D ligands, and constitutive expression of NKG2D ligands has been reported on normal myelomonocytic cells, dendritic cells, and epithelial cells of the gut mucosa. In this article we will review the literature suggesting that NKG2D may function to recognise non-stressed cells and discuss the role NKG2D ligands could be playing in apparently healthy cells.

HCV and innate immunity

Hepatitis C virus (HCV) is a single-strand, positive sense RNA virus belonging to the flaviviridae family. HCV develops persistent infection in >70% of infected patients, and eventually causes chronic hepatitis, cirrhosis, and hepatocellular carcinoma in some patients. Once chronic infection is established in patients with HCV, spontaneous viral clearance fails, although how HCV remains persistently infecting the liver is largely unknown. Insufficient immune response, involving antiviral innate immune response including dendritic cells (DCs), has been focused. A number of controversial studies have been reported as to HCV genome replication and HCV-mediated immune responses in human DCs. A tantalizing point of these earlier studies is the lack of the system for viral propagation in HCV. Recently, an in vitro system was exploited to propagate HCV particles using the JFH1 strain. In this review, we review the previous reports about the subversion of innate immunity by HCV and show the innate response of monocyte-derived dendritic cells (MoDCs) against the JFH1 strain. We could not observe HCV direct interaction with MoDC maturation. MoDCs maturated by phagocytosing HCV-infected apoptotic cells containing virus-derived dsRNA, which interacted with TLR3 in the phagosomes. All of these data suggests the importance of TLR3 signal for the induction of anti-HCV innate immunity.

Measles virus-induced immunosuppression

Immunosuppression is the major cause of infant death associated with acute measles and therefore of substantial clinical importance. Major hallmarks of this generalized modulation of immune functions are (1) lymphopenia, (2) a prolonged cytokine imbalance consistent with suppression of cellular immunity to secondary infections, and (3) silencing of peripheral blood lymphocytes, which cannot expand in response to ex vivo stimulation. Lymphopenia results from depletion, which can occur basically at any stage of lymphocyte development, and evidently, expression of the major MV receptor CD150 plays an important role in targeting these cells. Virus transfer to T cells is thought to be mediated by dendritic cells (DCs), which are considered central to the induction of T cell silencing and functional skewing. As a consequence of MV interaction, viability and functional differentiation of DCs and thereby their expression pattern of co-stimulatory molecules and soluble mediators are modulated. Moreover, MV proteins expressed by these cells actively silence T cells by interfering with signaling pathways essential for T cell activation.

NKG2D ligands: key targets of the immune response

NKG2D is an activating receptor expressed by NK and T cells. NKG2D ligands show a restricted expression in normal tissues, but they are frequently overexpressed in cancer and infected cells. The binding of NKG2D to its ligands activates NK and T cells and promotes cytotoxic lysis of the cells expressing these molecules. The mechanisms involved in the expression of the ligands of NKG2D play a key role in the recognition of stressed cells by the immune system and represent a promising therapeutic target for improving the immune response against cancer or autoimmune disease. In this review, we analyse the recent advances in understanding the regulation of NKG2D ligand expression and their therapeutic implications.

Hepatitis C virus-infected hepatocytes extrinsically modulate dendritic cell maturation to activate T cells and natural killer cells

Dendritic cell maturation critically modulates antiviral immune responses, and facilitates viral clearance. Hepatitis C virus (HCV) is characterized by its high predisposition to persistent infection. Here, we examined the immune response of human monocyte-derived dendritic cells (MoDCs) to the JFH1 strain of HCV, which can efficiently replicate in cell culture. However, neither HCV RNA replication nor antigen production was detected in MoDCs inoculated with JFH1. None of the indicators of HCV interacting with MoDCs we evaluated were affected, including expression of maturation markers (CD80, 83, 86), cytokines (interleukin-6 and interferon-beta), the mixed lymphocyte reaction, and natural killer (NK) cell cytotoxicity. Strikingly, MoDCs matured by phagocytosing extrinsically-infected vesicles containing HCV-derived double-stranded RNA (dsRNA). When MoDCs were cocultured with HCV-infected apoptotic Huh7.5.1 hepatic cells, there was increased CD86 expression and interleukin-6 and interferon-beta production in MoDCs, which were characterized by the potential to activate NK cells and induce CD4+ T cells into the T helper 1 type. Lipid raft-dependent phagocytosis of HCV-infected apoptotic vesicles containing dsRNA was indispensable to MoDC maturation. Colocalization of dsRNA with Toll-like receptor 3 (TLR3) in phagosomes suggested the importance of TLR3 signaling in the MoDC response against HCV.

CONCLUSION

The JFH1 strain does not directly stimulate MoDCs to activate T cells and NK cells, but phagocytosing HCV-infected apoptotic cells and their interaction with the TLR3 pathway in MoDCs plays a critical role in MoDC maturation and reciprocal activation of T and NK cells.

NKG2D receptor signaling enhances cytolytic activity by virus-specific CD8+ T cells: evidence for a protective role in virus-induced encephalitis

Inoculation with the neurotropic JHM strain of mouse hepatitis virus (JHMV) into the central nervous system (CNS) of mice results in an acute encephalitis associated with an immune-mediated demyelinating disease. During acute disease, infiltrating CD8(+) T cells secrete gamma interferon (IFN-gamma) that controls replication in oligodendrocytes, while infected astrocytes and microglia are susceptible to perforin-mediated lysis. The present study was undertaken to reveal the functional contributions of the activating NKG2D receptor in host defense and disease following JHMV infection. NKG2D ligands RAE-1, MULT1, and H60 were expressed within the CNS following JHMV infection. The immunophenotyping of infiltrating cells revealed that NKG2D was expressed on approximately 90% of infiltrating CD8(+) T cells during acute and chronic disease. Blocking NKG2D following JHMV infection resulted in increased mortality that correlated with increased viral titers within the CNS. Anti-NKG2D treatment did not alter T-cell infiltration into the CNS or the generation of virus-specific CD8(+) T cells, and the expression of IFN-gamma was not affected. However, cytotoxic T-lymphocyte (CTL) activity was dependent on NKG2D expression, because anti-NKG2D treatment resulted in a dramatic reduction in lytic activity by virus-specific CD8(+) T cells. Blocking NKG2D during chronic disease did not affect either T-cell or macrophage infiltration or the severity of demyelination, indicating that NKG2D does not contribute to virus-induced demyelination. These findings demonstrate a functional role for NKG2D in host defense during acute viral encephalitis by selectively enhancing CTL activity by infiltrating virus-specific CD8(+) T cells.