自然杀伤细胞参与肝脏恶性肿瘤免疫逃逸研究进展

自然杀伤细胞是机体固有免疫系统重要组成部分,在肝脏等免疫器官中含量丰富,而且免疫表型、功能等表现出器官特异性。在正常情况下,靶细胞表面的配体与自然杀伤细胞表面的活化性受体直接结合并释放细胞毒性物质,诱导活化靶细胞凋亡程序,从而发挥抗感染、抗肿瘤作用。然而肿瘤细胞仍能够通过多种途径逃逸机体的免疫监视功能,研究认为肿瘤细胞抗原异常表达、肿瘤微环境中细胞因子及其他免疫细胞相互作用等因素所引起的自然杀伤细胞活性降低对于诱导肿瘤免疫逃逸起重要作用。本文综述了自然杀伤细胞在肝脏恶性肿瘤发生过程中参与免疫逃逸的机制及研究进展,以期为临床抗肿瘤免疫治疗的研究提供参考。     

“Harpoon” Model for Cell–Cell Adhesion and Recognition of Target Cells by the Natural Killer Cells

The mechanism of recognition by natural killer (NK) cells is still unknown. A dynamic model is formulated describing recognition or NK-sensitive target cells (TCs) by NK cells of NK-like cells. This model does not assume the presence of the specific NK-receptor(s) on the membrane of NK cells and corresponding specific ligands on the NK-sensitive TCs. We suggest: (1) the expression of various kinds of “non-NK receptors” and corresponding ligands (counter-receptors) on the plasma membrane of the same NK cell and, possibly, of TCs (e.g. LFA-1 and ICAM-1-ICAM3, CD2 and LFA-3; receptors for TNF and corresponding ligand etc.); (2) the presence of multiple disorders in the organization of “extracellular matrix-surface membrane-submembrane cytoskeleton” assembly of the NK-sensitive TCs; (3) non-specific primary linking of NK cell with TCs, which induces a transfer of vesicles or membrane fragments from the NK surface to the target cell surface (and perhaps vice versa). These processes may also permit the transfer of many types of receptor and counter-receptor molecules from the surface of one conjugated cell to another by vesicles or membrane fragments. After transferral through the intercellular cleft, the free receptors and counter-receptors will be localized on both cell surfaces at the contact region between conjugated cells. By this model the NK cell can “harpoon” the TC and enhance the binding forces between cells up to the critical level and then switch on killing mechanisms for the TC. By means of this “harpoon” model of cell recognition, it seems possible to explain the nature of the wide polymorphism of TCs which are sensitive to the effect of NK and NK-like cells. A mathematical model of the NK cell cytotoxic reaction is described. The model describes many nonlinear peculiarities of the cytotoxic process and predicts some new phenomena. We suggest new approaches of manipulation of cell membranes which can transform NK-resistant target cells in NK sensitive cells and vice versa.     

Recognition of Lyso-Phospholipids by Human Natural Killer T Lymphocytes

Natural killer T (NKT) cells are a subset of T lymphocytes with potent immunoregulatory properties. Recognition of self-antigens presented by CD1d molecules is an important route of NKT cell activation; however, the molecular identity of specific autoantigens that stimulate human NKT cells remains unclear. Here, we have analyzed human NKT cell recognition of CD1d cellular ligands. The most clearly antigenic species was lyso-phosphatidylcholine (LPC). Diacylated phosphatidylcholine and lyso-phosphoglycerols differing in the chemistry of the head group stimulated only weak responses from human NKT cells. However, lyso-sphingomyelin, which shares the phosphocholine head group of LPC, also activated NKT cells. Antigen-presenting cells pulsed with LPC were capable of stimulating increased cytokine responses by NKT cell clones and by freshly isolated peripheral blood lymphocytes. These results demonstrate that human NKT cells recognize cholinated lyso-phospholipids as antigens presented by CD1d. Since these lyso-phospholipids serve as lipid messengers in normal physiological processes and are present at elevated levels during inflammatory responses, these findings point to a novel link between NKT cells and cellular signaling pathways that are associated with human disease pathophysiology.     

Activation of Natural Killer Cells by Newcastle Disease Virus Hemagglutinin-Neuraminidase

The avian paramyxovirus Newcastle disease virus (NDV) selectively replicates in tumor cells and is known to stimulate T-cell-, macrophage-, and NK cell-mediated responses. The mechanisms of NK cell activation by NDV are poorly understood so far. We studied the expression of ligand structures for activating NK cell receptors on NDV-infected tumor cells. Upon infection with the nonlytic NDV strain Ulster and the lytic strain MTH-68/H, human carcinoma and melanoma cells showed enhanced expression of ligands for the natural cytotoxicity receptors NKp44 and NKp46, but not NKp30. Ligands for the activating receptor NKG2D were partially downregulated. Soluble NKp44-Fc and NKp46-Fc, but not NKp30-Fc, chimeric proteins bound specifically to NDV-infected tumor cells and to NDV particle-coated plates. Hemagglutinin-neuraminidase (HN) of the virus serves as a ligand structure for NKp44 and NKp46, as indicated by the blockade of binding to NDV-infected cells and viral particles in the presence of anti-HN antibodies and by binding to cells transfected with HN cDNA. Consistent with the recognition of sialic acid moieties by the viral lectin HN, the binding of NKp44-Fc and NKp46-Fc was lost after desialylation. NKp44- and NKp46-CD3ζ lacZ-inducible reporter cells were activated by NDV-infected cells. NDV-infected tumor cells stimulated NK cells to produce increased amounts of the effector lymphokines gamma interferon and tumor necrosis factor alpha. Primary NK cells and the NK line NK-92 lysed NDV-infected tumor cells with enhanced efficiency, an effect that was eliminated by the treatment of target cells with the neuraminidase inhibitor Neu5Ac2en. These results suggest that direct activation of NK cells contributes to the antitumor effects of NDV.Virulent strains of Newcastle disease virus (NDV) infect domestic poultry and other birds, causing a rapidly spreading viral disease that affects the alimentary and respiratory tracts as well as the central nervous system (55). In humans, however, NDV is well tolerated (17, 18). Other than mild fever for a day, only a few adverse effects have been reported. NDV, also known as avian paramyxovirus 1, is an enveloped virus containing a negative-sense, single-stranded RNA genome which codes for six proteins in the order (from 3′ to 5′) of nucleoprotein, phosphoprotein, matrix protein, fusion (F) protein, hemagglutinin-neuraminidase (HN), and large polymerase protein (19). There are many different strains of NDV, classified as either lytic or nonlytic for different types of cells. Lytic and nonlytic NDV strains both replicate much more efficiently in human cancer cells than they do in most normal human cells (43). Viruses of both strain types have been investigated as potential anticancer agents (30, 49, 52). The NDV strains that have been evaluated most widely for the treatment of cancer are 73-T, MTH-68, and Ulster (1, 7, 11, 17, 18, 53, 54, 56, 71).Initial binding of NDV to a host cell takes place through the interaction of HN molecules in the virus coat with sialic acid-containing molecules on the cell surface (31). NDV neuraminidase has strict specificity for the hydrolysis of the NeuAc-α2,3-Gal linkage, with no hydrolysis of the NeuAc-α2,6-Gal linkage (41).NDV infection of tumor cells not only improves T-cell responses (53, 58, 68), but has also been reported to vigorously stimulate innate immune responses. In the course of NDV infection, large amounts of alpha interferon (IFN-α) are released (68) and in turn activate dendritic cells and NK cells and polarize, in concert with interleukin-12 (IL-12), toward a Th1 T-cell response (33, 44, 47). In addition, NDV induces antitumor cytotoxicity in murine macrophages which produce increased amounts of tumor necrosis factor alpha (TNF-α) and nitric oxide (51, 60) and in human monocytes through the induction of TRAIL (64). Little is known about the NDV-mediated activation of NK cells. The coincubation of peripheral blood mononuclear cells with NDV was shown previously to stimulate NK-mediated cytotoxicity (70). Enhanced cytotoxicity correlates with the induction of IFN-α (70). It is not known, however, whether NDV-infected cells can directly activate NK cells and, if so, which molecular interactions are involved.The cytolytic activity of NK cells against virus-infected or tumor cells is regulated by the engagement of activating or inhibitory NK cell surface receptors, the actions of cytokines, and cross talk with other immune cells (32, 39). Most inhibitory receptors recognize particular major histocompatibility complex (MHC) class I alleles and thereby ensure the tolerance of NK cells against self antigens (38). Activating receptors on human NK cells include CD16; NKG2D; the natural cytotoxicity receptors (NCR) NKp30, NKp44, and NKp46; as well as NKp80; DNAM-1; and various stimulatory coreceptors (32).NCR are important activating receptors for the antitumor and antiviral activities of NK cells (5, 32, 37). Heparan sulfate has been discussed previously as a cellular ligand for NKp46, NKp44, and NKp30 (9, 26, 27), and nuclear factor BAT3, which can be released from tumor cells under stress conditions, has been described as a cellular ligand for NKp30 (42). Ligands for NKp30 and NKp44 can be detected on the surfaces and in the intracellular compartments of several kinds of tumor cells (10). Moreover, a number of pathogen-derived NCR ligands have been reported. The hemagglutinin protein of influenza virus and the HN of Sendai virus can bind to NKp46 and NKp44 and activate NK cells (3, 24, 34). The pp65 protein of human cytomegalovirus has been shown to bind NKp30 and inhibit its function (4). Human immunodeficiency virus, vaccinia virus, and herpes simplex virus have also been shown to upregulate the expression of cellular NCR ligands in infected cells (13, 14, 62). The Plasmodium falciparum erythrocyte membrane protein 1 is involved in the NCR-mediated NK cell attack against infected erythrocytes (36). Furthermore, NKp46 recognizes cells infected with mycobacteria (22, 61), and NKp44 was recently reported to directly bind to the surfaces of mycobacteria and other bacteria (21).In this study, we investigated the expression of ligand structures for NCR and NKG2D on NDV-infected cells. We demonstrate that NDV HN proteins which are strongly expressed on NDV-infected tumor cells function as activating ligand structures for NKp44 and NKp46 but that cellular ligands for NKG2D are partially downregulated during NDV infection.     

Natural Killer Cells Control Tumor Growth by Sensing a Growth Factor

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Killing of Avian and Swine Influenza Virus by Natural Killer Cells

Today, global attention is focused on two influenza virus strains: the current pandemic strain, swine origin influenza virus (H1N1-2009), and the highly pathogenic avian influenza virus, H5N1. At present, the infection caused by the H1N1-2009 is moderate, with mortality rates of less <1%. In contrast, infection with the H5N1 virus resulted in high mortality rates, and ca. 60% of the infected patients succumb to the infection. Thus, one of the world greatest concerns is that the H5N1 virus will evolve to allow an efficient human infection and human-to-human transmission. Natural killer (NK) cells are one of the innate immune components playing an important role in fighting against influenza viruses. One of the major NK activating receptors involved in NK cell cytotoxicity is NKp46. We previously demonstrated that NKp46 recognizes the hemagglutinin proteins of B and A influenza virus strains. Whether NKp46 could also interact with H1N1-2009 virus or with the avian influenza virus is still unknown. We analyzed the immunological properties of both the avian and the H1N1-2009 influenza viruses. We show that NKp46 recognizes the hemagglutinins of H1N1-2009 and H5 and that this recognition leads to virus killing both in vitro and in vivo. However, importantly, while the swine H1-NKp46 interactions lead to the direct killing of the infected cells, the H5-NKp46 interactions were unable to elicit direct killing, probably because the NKp46 binding sites for these two viruses are different.Natural killer (NK) cells, which comprise 5 to 15% of peripheral blood lymphocytes, are a key frontline defense against a number of pathogens, including intracellular bacteria, parasites, and most importantly with respect to the present study, viruses (6, 40). The antiviral mechanisms by which NK cells operate include both cytotoxic activity and cytokine/chemokine secretion (21). The NK killing activity is executed by numerous receptors, including NKG2D, NKp80, CD16, and the natural cytotoxic receptors (NCRs): NKp30, NKp44, and NKp46 (7, 10, 25).Although the cellular ligands for NKG2D were identified (31, 38), the identity of several of the cellular ligands for the human NCRs is still unknown, except for BAT3 and B7-H6, which are ligands for NKp30 (8, 30). In contrast, viral ligands were identified for the NCRs, and we demonstrated that pp65 of HCMV interacts with NKp30 (3) and that various influenza virus hemagglutinins (HAs) are ligands for the NKp44 and NKp46 receptors (5, 22). Supporting these observations, it was recently shown that the HA-neuraminidase of Newcastle disease virus could also interact with NKp46 and NKp44 but not with NKp30 (17). Furthermore, we have shown in vivo that in the absence of NCR1 (the mouse homologue of NKp46), A/PR8 influenza virus infection is lethal (14).Human influenza virus (H1 and H3 subtype) infections pose a major threat to the entire population, as exemplified by the three major influenza pandemics that occurred during the 20th century. The Asian (A/H2N2) in 1957 to 1958 and the Hong Kong (A/H3N2) pandemics in 1968 to 1969 resulted in the deaths of 1 to 2 million people and the 1918 “Spanish flu” (A/H1N1) pandemic killed around 50 million people (18). At present, the worldwide concern regarding influenza pandemics concentrates mainly on two viruses: the A/H1N1 swine origin influenza virus (H1N1-2009), which currently causes only a moderate pandemic (the mortality rates are ca. 1%) but is more pathogenic than a regular seasonal influenza virus (19, 26, 27), and the avian influenza virus carrying the unique H5 HA (20). The avian influenza virus is quite deadly and, although it remains a zoonotic infection, ca. 60% of infected humans died due to the infection (28).The unique properties of the H5 protein of the avian influenza virus are one of the main reasons for the virulence of the virus. The H5 of the avian influenza virus binds to cell surface glycoproteins or glycolipids containing terminal sialyl-galactosyl residues linked by 2-3-linkage [Neu5Ac(α2-3)Gal] that are found in the human conjunctiva and ciliated portion of the respiratory columnar epithelium (33). In contrast, human viruses (including all three strains that caused the pandemics described above and the H1N1-2009) bind to receptors that mostly contain terminal 2-6-linked sialyl-galactosyl moieties [Neu5Ac(α2-6)Gal]. Such glycosylations are predominant on epithelial cells in the nasal mucosa, paranasal sinuses, pharynx, trachea, and bronchi (33, 37). It has been suggested that the lack of human-to-human transmission of avian influenza viruses is due to their α2,3-SA receptor binding preference, and the concern is that genetic changes in H5 might alter its preference from α2,3-SA to α2,6-SA, allowing human-to-human transmission.In our previous studies (4, 22) we showed that the interaction between NKp46 and influenza virus HAs depends on the sialylation of the NKp46 receptor. We further demonstrated that the sialic acid residues, which are linked via α2,6 to the threonine 225 residue of NKp46, are crucial for the NKp46 interactions with the various influenza virus HAs (4).We show that, both in vitro and in vivo, the killing of H1N1-2009-infected cells is correlated with the degree of NKp46 binding. Surprisingly, we observed that although NKp46 efficiently recognized the avian H5 HA, such interactions were unable to elicit the direct killing of the infected cells. By using mutagenesis analysis experiments and killing assays we demonstrate that NKp46 interacts with H1 and H5 at distinct sites, since we show that the sugar carrying residue at position 225 is crucial for the NKp46-H1N1-2009 interactions, whereas the interaction of H5 with NKp46 depends on both residues 216 and 225.     

Suppression of Natural Killer Cells by Sorafenib Contributes to Prometastatic Effects in Hepatocellular Carcinoma

Sorafenib, a multi-tyrosine kinase inhibitor, is a standard treatment for advanced hepatocellular carcinoma (HCC). The present study was undertaken to determine whether the growth and metastasis of HCC were influenced in mice receiving sorafenib prior to implantation with tumors, and to investigate the in-vivo and in-vitro effect of sorafenib on natural killer (NK) cells. In sorafenib-pretreated BALB/c nu/nu mice and C57BL/6 mice, tumor growth was accelerated, mouse survival was decreased, and lung metastasis was increased. However, the depletion of NK1.1⁺ cells in C57BL/6 mice eliminated sorafenib-mediated pro-metastatic effects. Sorafenib significantly reduced the number of NK cells and inhibited reactivity of NK cells against tumor cells, in both tumor-bearing and tumor-free C57BL/6 mice. Sorafenib down-regulated the stimulatory receptor CD69 in NK cells of tumor-bearing mice, but not in tumor-free mice, and inhibited proliferation of NK92-MI cells, which is associated with the blocking of the PI3K/AKT pathway, and inhibited cytotoxicity of NK cells in response to tumor targets, which was due to impaired ERK phosphorylation. These results suggest immunotherapeutic approaches activating NK cells may enhance the therapeutic efficacy of sorafenib in HCC patients.     

Genomic Modifiers of Natural Killer Cells,Immune Responsiveness and Lymphoid Tissue Remodeling Together Increase Host Resistance to Viral Infection

The MHC class I D^k molecule supplies vital host resistance during murine cytomegalovirus (MCMV) infection. Natural killer (NK) cells expressing the Ly49G2 inhibitory receptor, which specifically binds D^k, are required to control viral spread. The extent of D^k-dependent host resistance, however, differs significantly amongst related strains of mice, C57L and MA/My. As a result, we predicted that relatively small-effect modifier genetic loci might together shape immune cell features, NK cell reactivity, and the host immune response to MCMV. A robust D^k-dependent genetic effect, however, has so far hindered attempts to identify additional host resistance factors. Thus, we applied genomic mapping strategies and multicolor flow cytometric analysis of immune cells in naive and virus-infected hosts to identify genetic modifiers of the host immune response to MCMV. We discovered and validated many quantitative trait loci (QTL); these were mapped to at least 19 positions on 16 chromosomes. Intriguingly, one newly discovered non-MHC locus (Cmv5) controlled splenic NK cell accrual, secondary lymphoid organ structure, and lymphoid follicle development during MCMV infection. We infer that Cmv5 aids host resistance to MCMV infection by expanding NK cells needed to preserve and protect essential tissue structural elements, to enhance lymphoid remodeling and to increase viral clearance in spleen.     

Polymorphism in Human Cytomegalovirus UL40 Impacts on Recognition of Human Leukocyte Antigen-E (HLA-E) by Natural Killer Cells

Natural killer (NK) cell recognition of the nonclassical human leukocyte antigen (HLA) molecule HLA-E is dependent on the presentation of a nonamer peptide derived from the leader sequence of other HLA molecules to CD94-NKG2 receptors. However, human cytomegalovirus can manipulate this central innate interaction through the provision of a “mimic” of the HLA-encoded peptide derived from the immunomodulatory glycoprotein UL40. Here, we analyzed UL40 sequences isolated from 32 hematopoietic stem cell transplantation recipients experiencing cytomegalovirus reactivation. The UL40 protein showed a “polymorphic hot spot” within the region that encodes the HLA leader sequence mimic. Although all sequences that were identical to those encoded within HLA-I genes permitted the interaction between HLA-E and CD94-NKG2 receptors, other UL40 polymorphisms reduced the affinity of the interaction between HLA-E and CD94-NKG2 receptors. Furthermore, functional studies using NK cell clones expressing either the inhibitory receptor CD94-NKG2A or the activating receptor CD94-NKG2C identified UL40-encoded peptides that were capable of inhibiting target cell lysis via interaction with CD94-NKG2A, yet had little capacity to activate NK cells through CD94-NKG2C. The data suggest that UL40 polymorphisms may aid evasion of NK cell immunosurveillance by modulating the affinity of the interaction with CD94-NKG2 receptors.     

Stress-Induced In Vivo Recruitment of Human Cytotoxic Natural Killer Cells Favors Subsets with Distinct Receptor Profiles and Associates with Increased Epinephrine Levels

Background

Acute stress drives a ‘high-alert’ response in the immune system. Psychoactive drugs induce distinct stress hormone profiles, offering a sought-after opportunity to dissect the in vivo immunological effects of acute stress in humans.

Methods

3,4-methylenedioxymethamphetamine (MDMA), methylphenidate (MPH), or both, were administered to healthy volunteers in a randomized, double-blind, placebo-controlled crossover-study. Lymphocyte subset frequencies, natural killer (NK) cell immune-phenotypes, and changes in effector function were assessed, and linked to stress hormone levels and expression of CD62L, CX3CR1, CD18, and stress hormone receptors on NK cells.

Results

MDMA/MPH > MDMA > MPH robustly induced an epinephrine-dominant stress response. Immunologically, rapid redistribution of peripheral blood lymphocyte-subsets towards phenotypically mature NK cells occurred. NK cytotoxicity was unaltered, but they expressed slightly reduced levels of the activating receptor NKG2D. Preferential circulation of mature NK cells was associated with high epinephrine receptor expression among this subset, as well as expression of integrin ligands previously linked to epinephrine-induced endothelial detachment.

Conclusion

The acute epinephrine-induced stress response was characterized by rapid accumulation of mature and functional NK cells in the peripheral circulation. This is in line with studies using other acute stressors and supports the role of the acute stress response in rapidly mobilizing the innate immune system to counteract incoming threats.     

Myeloid-Derived Suppressor Cells Regulate Natural Killer Cell Response to Adenovirus-Mediated Gene Transfer

The attendant innate and adaptive immune responses to viral vectors have posed a significant hurdle for clinical application of viral vector-mediated gene therapy. Previous studies have shown that natural killer (NK) cells play a critical role in innate immune elimination of adenoviral vectors in the liver. However, it is not clear how the NK cell response to adenoviral vectors is regulated. In this study, we identified a role for granulocytic myeloid-derived suppressor cells (G-MDSCs) in this process. We show that in vivo administration of adenoviral vectors results in rapid accumulation of G-MDSCs early during adenoviral infection. In vivo depletion of both MDSC populations, but not monocytic MDSCs (M-MDSCs) alone, resulted in accelerated clearance of adenoviral vectors in the liver. This was accompanied by enhanced NK cell proliferation and activation, suggesting a role for MDSCs, probably G-MDSCs, in suppressing NK cell activation and function in vivo. We further demonstrate in vitro that G-MDSCs, but not M-MDSCs, are responsible for the suppression of NK cell activation. In addition, we show that adenoviral infection activated G-MDSCs to produce higher levels of reactive oxygen species (ROS) and that G-MDSC-mediated suppression of NK cells is mediated by ROS, specifically, H₂O₂. This study demonstrates for the first time that the NK cell response to adenoviral vectors is negatively regulated by G-MDSCs and suggests that G-MDSC-based strategies could potentially improve the outcome of viral vector-mediated gene therapy.     

Dendritic Cells/Natural Killer Cross-Talk: A Novel Target for Human Immunodeficiency Virus Type-1 Protease Inhibitors

Background

HIV-1 Protease Inhibitors, namely PIs, originally designed to inhibit HIV-1 aspartic protease, can modulate the immune response by mechanisms largely unknown, and independent from their activity on viral replication. Here, we analyzed the ability of PIs to interfere with differentiation program of monocytes toward dendritic cell (DCs) lineage, a key process in the inflammatory response.

Methodology/Principal Findings

Monocytes from healthy donors were isolated and induced to differentiate in vitro in the presence or absence of saquinavir, ritonavir, nelfinavir, indinavir or amprenavir (sqv, rtv, nlfv, idv, apv, respectively). These drugs demonstrated a differential ability to sustain the generation of immature DCs (iDCs) with an altered phenotype, including low levels of CD1a, CD86, CD36 and CD209. DCs generated in the presence of rtv also failed to acquire the typical phenotype of mature DCs (mDCs), and secreted lower amounts of IL-12 and IL-15. Accordingly, these aberrant mDCs failed to support activation of autologous Natural Killer (NK) cells, and resulted highly susceptible to NK cell-mediated cytotoxicity.

Conclusions/Significance

Our findings uncover novel functional properties of PIs within the DC-NK cell cross-talk, unveiling the heterogeneous ability of members of this class drugs to drive the generation of atypical monocyte-derived DCs (MDDCs) showing an aberrant phenotype, a failure to respond appropriately to bacterial endotoxin, a weak ability to prime autologous NK cells, and a high susceptibility to NK cell killing. These unexpected properties might contribute to limit inflammation and viral spreading in HIV-1 infected patients under PIs treatment, and open novel therapeutical perspectives for this class drugs as immunomodulators in autoimmunity and cancer.     

Immune Receptors Involved in Streptococcus suis Recognition by Dendritic Cells

Streptococcus suis is an important swine pathogen and an emerging zoonotic agent of septicemia and meningitis. Knowledge on host immune responses towards S. suis, and strategies used by this pathogen for subversion of these responses is scarce. The objective of this study was to identify the immune receptors involved in S. suis recognition by dendritic cells (DCs). Production of cytokines and expression of co-stimulatory molecules by DCs were shown to strongly rely on MyD88-dependent signaling pathways, suggesting that DCs recognize S. suis and become activated mostly through Toll-like receptor (TLR) signaling. Supporting this fact, TLR2^−/− DCs were severely impaired in the release of several cytokines and the surface expression of CD86 and MHC-II. The release of IL-12p70 and CXC10, and the expression of CD40 were found to depend on signaling by both TLR2 and TLR9. The release of IL-23 and CXCL1 were partially dependent on NOD2. Finally, despite the fact that MyD88 signaling was crucial for DC activation and maturation, MyD88-dependent pathways were not implicated in S. suis internalization by DCs. This first study on receptors involved in DC activation by S. suis suggests a major involvement of MyD88 signaling pathways, mainly (but not exclusively) through TLR2. A multimodal recognition involving a combination of different receptors seems essential for DC effective response to S. suis.     

Natural Killer Cell Tolerance Persists Despite Significant Reduction of Self MHC Class I on Normal Target Cells in Mice

Background

A major group of murine inhibitory receptors on Natural Killer (NK) cells belong to the Ly49 receptor family and recognize MHC class I molecules. Infected or transformed target cells frequently downmodulate MHC class I molecules and can thus avoid CD8⁺ T cell attack, but may at the same time develop NK cell sensitivity, due to failure to express inhibitory ligands for Ly49 receptors. The extent of MHC class I downregulation needed on normal cells to trigger NK cell effector functions is not known.

Methodology/Principal Findings

In this study, we show that cells expressing MHC class I to levels well below half of the host level are tolerated in an in vivo assay in mice. Hemizygous expression (expression from only one allele) of MHC class I was sufficient to induce Ly49 receptor downmodulation on NK cells to a similar degree as homozygous expression, despite a strongly reduced cell surface level of MHC class I. Co-expression of weaker MHC class I ligands in the host did not have any further effect on the degree of Ly49 downmodulation. Furthermore, a single MHC class I allele could downmodulate up to three Ly49 receptors on individual NK cells. Only when NK cells simultaneously expressed several Ly49 receptors and hemizygous MHC class I levels, a putative threshold for Ly49 downmodulation was reached.

Conclusion

Collectively, our findings suggest that in interactions between NK cells and normal untransformed cells, MHC class I molecules are in most cases expressed in excess compared to what is functionally needed to ensure self tolerance and to induce maximal Ly49 downmodulation. We speculate that the reason for this is to maintain a safety margin for otherwise normal, autologous cells over a range of MHC class I expression levels, in order to ensure robustness in NK cell tolerance.     

人乳头瘤病毒的免疫逃逸机制

人乳头瘤病毒(human papillomavirus,HPV)是一类通过性传播的环状双链DNA病毒,与宫颈癌、阴道癌、头颈部癌症、阴茎癌和肛门癌等许多癌症的发生有密切关系.该病毒的持续感染是危险的致癌因素.HPV持续感染宿主在于它能够逃避宿主的免疫攻击.现就近年来关于HPV的免疫逃逸机制的最新研究进展做一简要综述.     

Prostate Tumor-Derived Exosomes Down-Regulate NKG2D Expression on Natural Killer Cells and CD8+ T Cells: Mechanism of Immune Evasion

Tumor-derived exosomes, which are nanometer-sized extracellular vesicles of endosomal origin, have emerged as promoters of tumor immune evasion but their role in prostate cancer (PC) progression is poorly understood. In this study, we investigated the ability of prostate tumor-derived exosomes to downregulate NKG2D expression on natural killer (NK) and CD8⁺ T cells. NKG2D is an activating cytotoxicity receptor whose aberrant loss in cancer plays an important role in immune suppression. Using flow cytometry, we found that exosomes produced by human PC cells express ligands for NKG2D on their surface. The NKG2D ligand-expressing prostate tumor-derived exosomes selectively induced downregulation of NKG2D on NK and CD8⁺ T cells in a dose-dependent manner, leading to impaired cytotoxic function in vitro. Consistent with these findings, patients with castration-resistant PC (CRPC) showed a significant decrease in surface NKG2D expression on circulating NK and CD8⁺ T cells compared to healthy individuals. Tumor-derived exosomes are likely involved in this NKG2D downregulation, since incubation of healthy lymphocytes with exosomes isolated from serum or plasma of CRPC patients triggered downregulation of NKG2D expression in effector lymphocytes. These data suggest prostate tumor-derived exosomes as down-regulators of the NKG2D-mediated cytotoxic response in PC patients, thus promoting immune suppression and tumor escape.