Whatever turns you on: accessory-cell-dependent activation of NK cells by pathogens

Killer-cell immunoglobulin-like receptors and malaria caused by Plasmodium falciparum in The Gambia

The relevance of innate immune responses to Plasmodium falciparum infection, in particular the central role of natural killer (NK) cell-derived interferon gamma (IFN-γ), is becoming increasingly recognised. Recently, it has been shown that IFN-γ production in response to P. falciparum antigens is in part regulated by killer-cell immunoglobulin-like receptor (KIR) genes, and a study from malaria-exposed Melanesians suggested an association between KIR genotypes and susceptibility to infection. This prompted us to determine and compare the frequencies of 15 KIR genes in Gambian children presenting with either severe malaria (n = 133) or uncomplicated malaria (n = 188) and in cord-blood population control samples (n = 314) collected from the same area. While no significant differences were observed between severe and uncomplicated cases, proportions of individuals with KIR2DS2+C1 and KIR2DL2+C1 were significantly higher among malaria cases overall than in population control samples. In an exploratory analysis, activating KIR genes KIR2DS2, KIR3DS1 and KIR2DS5 were slightly higher in children in disease subgroups associated with the highest mortality. In addition, our data suggest that homozygosity for KIR genotype A might be associated with different malaria outcomes including protection from infection and higher blood parasitaemia levels in those that do get infected. These findings are consistent with a probable role of KIR genes in determining susceptibility to malaria, and further studies are warranted in different populations.

Severe necroinflammatory reaction caused by natural killer cell-mediated Fas/Fas ligand interaction and dendritic cells in human hepatocyte chimeric mouse

The necroinflammatory reaction plays a central role in hepatitis B virus (HBV) elimination. Cluster of differentiation (CD)8-positive cytotoxic T lymphocytes (CTLs) are thought to be a main player in the elimination of infected cells, and a recent report suggests that natural killer (NK) cells also play an important role. Here, we demonstrate the elimination of HBV-infected hepatocytes by NK cells and dendritic cells (DCs) using urokinase-type plasminogen activator/severe combined immunodeficiency mice, in which the livers were highly repopulated with human hepatocytes. After establishing HBV infection, we injected human peripheral blood mononuclear cells (PBMCs) into the mice and analyzed liver pathology and infiltrating human immune cells with flow cytometry. Severe hepatocyte degeneration was observed only in HBV-infected mice transplanted with human PBMCs. We provide the first direct evidence that massive liver cell death can be caused by Fas/Fas ligand (FasL) interaction provided by NK cells activated by DCs. Treatment of mice with anti-Fas antibody completely prevented severe hepatocyte degeneration. Furthermore, severe hepatocyte death can be prevented by depletion of DCs, whereas depletion of CD8-positive CTLs did not disturb the development of massive liver cell apoptosis.

CONCLUSION

Our findings provide the first direct evidence that DC-activated NK cells induce massive HBV-infected hepatocyte degeneration through the Fas/FasL system and may indicate new therapeutic implications for acute severe/fulminant hepatitis B.

Infection-induced regulation of natural killer cells by macrophages and collagen at the lymph node subcapsular sinus

Infection leads to heightened activation of natural killer (NK) cells, a process that likely involves direct cell-to-cell contact, but how this occurs in vivo is poorly understood. We have used two-photon laser-scanning microscopy in conjunction with Toxoplasma gondii mouse infection models to address this question. We found that after infection, NK cells accumulated in the subcapsular region of the lymph node, where they formed low-motility contacts with collagen fibers and CD169(+) macrophages. We provide evidence that interactions with collagen regulate NK cell migration, whereas CD169(+) macrophages increase the activation state of NK cells. Interestingly, a subset of CD169(+) macrophages that coexpress the inflammatory monocyte marker Ly6C had the most potent ability to activate NK cells. Our data reveal pathways through which NK cell migration and function are regulated after infection and identify an important accessory cell population for activation of NK cell responses in lymph nodes.

NK cell tolerance to TLR agonists mediated by regulatory T cells after polymicrobial sepsis

As sensors of infection, innate immune cells are able to recognize pathogen-associated molecular patterns by receptors such as TLRs. NK cells present in many tissues contribute to inflammatory processes, particularly through the production of IFN-γ. They may display a protective role during infection but also a detrimental role during sterile or infectious systemic inflammatory response syndrome. Nevertheless, the exact status of NK cells during bacterial sepsis and their capacity directly to respond to TLR agonists remain unclear. The expression of TLRs in NK cells has been widely studied by analyzing the mRNA of these receptors. The aim of this study was to gain insight into TLR2/TLR4/TLR9 expression on/in murine NK cells at the protein level and determine if their agonists were able to induce cytokine production. We show, by flow cytometry, a strong intracellular expression of TLR2 and a low of TLR4 in freshly isolated murine spleen NK cells, similar to that of TLR9. In vitro, purified NK cells respond to TLR2, TLR4, and TLR9 agonists, in synergy with activating cytokines (IL-2, IL-15, and/or IL-18), and produce proinflammatory cytokines (IFN-γ and GM-CSF). Finally, we explored the possible tolerance of NK cells to TLR agonists after a polymicrobial sepsis (experimental peritonitis). For the first time, to our knowledge, NK cells are shown to become tolerant in terms of proinflammatory cytokines production after sepsis. We show that this tolerance is associated with a reduction of the CD27(+)CD11b(-) subset in the spleen related to the presence of regulatory T cells and mainly mediated by TGF-β.

Activation of cord blood myeloid dendritic cells by Trypanosoma cruzi and parasite-specific antibodies, proliferation of CD8+ T cells, and production of IFN-γ

We previously reported that Trypanosoma cruzi, the agent of Chagas disease, induces in congenitally infected fetuses a strong, adult-like parasite-specific CD8(+) T cell response producing IFN-γ (Hermann et al. in Blood 100:2153-2158, 2002). This suggests that the parasite is able to overcome the immaturity of neonatal antigen presenting cells, an issue which has not been previously addressed. We therefore investigated in vitro the ability of T. cruzi to activate cord blood DCs and compared its effect to that on adult cells. We show that T. cruzi induces phenotypic maturation of cord blood CD11c(+) myeloid DCs (mDCs), by enhancing surface expression of CD40, CD80, and CD83, and that parasite-specific IgG purified from cord blood of neonates born to T. cruzi-infected mothers amplify such expression. CD83, considered as the best marker of mature DCs, reaches higher level on cord blood than on adult mDCs. Allo-stimulation experiments showed that T. cruzi-activated cord blood mononuclear cells enriched in DCs (eDCs) stimulate proliferation of cord blood and adult CD3(+) T cells to a similar extent. Of note, T. cruzi-activated eDCs from cord blood trigger more potent proliferation of CD8(+) than CD8(-) (mainly CD4(+)) adult T cells, a feature not observed with adult eDCs. T cell proliferation is associated with IFN-γ release and down-regulation of IL-13 production. These data show that T. cruzi potently activates human cord blood mDCs and endows eDCs to trigger CD8(+) T cell proliferation and favor type 1 immune response. Interestingly, maternal antibodies can strengthen the development of mature DCs that might contribute to overcome the immunological immaturity associated with early life.

Restoration of TLR3-activated myeloid dendritic cell activity leads to improved natural killer cell function in chronic hepatitis B virus infection

There is increasing evidence that the function of NK cells in patients with chronic hepatitis B (CHB) infection is impaired. The underlying mechanism for the impaired NK cell function is still unknown. Since myeloid dendritic cells (mDC) are potent inducers of NK cells, we investigated the functional interaction of mDC and NK cells in CHB and the influence of antiviral therapy. Blood BDCA1(+) mDC and NK cells were isolated from 16 healthy controls or 39 CHB patients at baseline and during 6 months of antiviral therapy. After activation of mDC with poly(I · C) and gamma interferon (IFN-γ), mDC were cocultured with NK cells. Phenotype and function were analyzed in detail by flow cytometry and enzyme-linked immunosorbent assay. Our findings demonstrate that on poly(I · C)/IFN-γ-stimulated mDC from CHB patients, the expression of costimulatory molecules was enhanced, while cytokine production was reduced. In cocultures of poly(I · C)/IFN-γ-stimulated mDC and NK cells obtained from CHB patients, reduced mDC-induced NK cell activation (i.e., CD69 expression) and IFN-γ production compared to those in healthy individuals was observed. Antiviral therapy normalized mDC activity, since decreased expression of CD80 and CD86 on DC and of HLA-E on NK cells was observed, while poly(I · C)/IFN-γ-induced cytokine production by mDC was enhanced. In parallel, successful antiviral therapy resulted in improved mDC-induced NK cell activation and IFN-γ production. These data demonstrate that CHB patients display a diminished functional interaction between poly(I · C)/IFN-γ activated mDC and NK cells due to impaired mDC function, which can be partially restored by antiviral therapy. Enhancing this reciprocal interaction could reinforce the innate and thus the adaptive T cell response, and this may be an important step in achieving effective antiviral immunity.

Natural killer (NK) cells in antibacterial innate immunity: angels or devils?

Natural killer (NK) cells were first described as immune leukocytes that could kill tumor cells and soon after were reported to kill virus-infected cells. In the mid-1980s, 10 years after their discovery, NK cells were also demonstrated to contribute to the fight against bacterial infection, particularly because of crosstalk with other leukocytes. A wide variety of immune cells are now recognized to interact with NK cells through the production of cytokines such as interleukin (IL)-2, IL-12, IL-15 and IL-18, which boost NK cell activities. The recent demonstration that NK cells express pattern recognition receptors, namely Toll-like and nucleotide oligomerization domain (NOD)-like receptors, led to the understanding that these cells are not only under the control of accessory cells, but can be directly involved in the antibacterial response thanks to their capacity to recognize pathogen-associated molecular patterns. Interferon (IFN)-γ is the predominant cytokine produced by activated NK cells. IFN-γ is a key contributor to antibacterial immune defense. However, in synergy with other inflammatory cytokines, IFN-γ can also lead to deleterious effects similar to those observed during sepsis. Accordingly, as the main source of IFN-γ in the early phase of infection, NK cells display both beneficial and deleterious effects, depending on the circumstances.

Neutrophil depletion impairs natural killer cell maturation, function, and homeostasis

Neutropenia in mice and humans results in the generation of NK cells with an immature and hyporesponsive phenotype.

Natural killer (NK) cells are bone marrow (BM)–derived granular lymphocytes involved in immune defense against microbial infections and tumors. In an N-ethyl N-nitrosourea (ENU) mutagenesis strategy, we identified a mouse mutant with impaired NK cell reactivity both in vitro and in vivo. Dissection of this phenotype showed that mature neutrophils were required both in the BM and in the periphery for proper NK cell development. In mice lacking neutrophils, NK cells displayed hyperproliferation and poor survival and were blocked at an immature stage associated with hyporesponsiveness. The role of neutrophils as key regulators of NK cell functions was confirmed in patients with severe congenital neutropenia and autoimmune neutropenia. In addition to their direct antimicrobial activity, mature neutrophils are thus endowed with immunoregulatory functions that are conserved across species. These findings reveal novel types of cooperation between cells of the innate immune system and prompt examination of NK cell functional deficiency in patients suffering from neutropenia-associated diseases.

Wild mice provide insights into natural killer cell maturation and memory

The immune system has evolved, and continues to evolve, in response to the selection pressure that infections exert on animals in their natural environments, yet much of our understanding about how the immune system functions comes from studies of model species maintained in the almost complete absence of such environmental selection. The scientific discipline of immunology has among its aims the improvement of human and animal health by the application of immunological knowledge. As research on humans and domesticated animals is highly constrained-ethically, logistically and financially-experimental animal models have become an invaluable tool for dissecting the functioning of the immune system. The house mouse (Mus musculus) is by far the most widely used animal model in immunological research but laboratory-reared mice provide a very narrow view of the immune system-that of a well-fed and comfortably housed animal with minimal exposure to microbial pathogens. Indeed, so much of our immunological knowledge comes from studies of a very few highly inbred mouse strains that-to all intents and purposes-our immunological knowledge is based on enormously detailed studies of very small numbers of individual mice. The limitations of studies in inbred strains of laboratory mice are well-recognized (Pedersen & Babayan 2011), but serious attempts to address these limitations have been few and far between. However, the emerging field of 'ecological immunology' where free-living populations are studied in their natural habitat is beginning to redress this imbalance (Viney et al. 2005; Martin et al. 2006; Owen et al. 2010; Abolins et al. 2011). As demonstrated in the work by Boysen et al. (2011) in this issue of Molecular Ecology, studies in wild animal populations-especially free-living M. musculus-represent a valuable bridge between studies in humans and livestock and studies of captive animals.

Mechanisms of NK cell-macrophage Bacillus anthracis crosstalk: a balance between stimulation by spores and differential disruption by toxins

NK cells are important immune effectors for preventing microbial invasion and dissemination, through natural cytotoxicity and cytokine secretion. Bacillus anthracis spores can efficiently drive IFN-γ production by NK cells. The present study provides insights into the mechanisms of cytokine and cellular signaling that underlie the process of NK-cell activation by B. anthracis and the bacterial strategies to subvert and evade this response. Infection with non-toxigenic encapsulated B. anthracis induced recruitment of NK cells and macrophages into the mouse draining lymph node. Production of edema (ET) or lethal (LT) toxin during infection impaired this cellular recruitment. NK cell depletion led to accelerated systemic bacterial dissemination. IFN-γ production by NK cells in response to B. anthracis spores was: i) contact-dependent through RAE-1-NKG2D interaction with macrophages; ii) IL-12, IL-18, and IL-15-dependent, where IL-12 played a key role and regulated both NK cell and macrophage activation; and iii) required IL-18 for only an initial short time window. B. anthracis toxins subverted both NK cell essential functions. ET and LT disrupted IFN-γ production through different mechanisms. LT acted both on macrophages and NK cells, whereas ET mainly affected macrophages and did not alter NK cell capacity of IFN-γ secretion. In contrast, ET and LT inhibited the natural cytotoxicity function of NK cells, both in vitro and in vivo. The subverting action of ET thus led to dissociation in NK cell function and blocked natural cytotoxicity without affecting IFN-γ secretion. The high efficiency of this process stresses the impact that this toxin may exert in anthrax pathogenesis, and highlights a potential usefulness for controlling excessive cytotoxic responses in immunopathological diseases. Our findings therefore exemplify the delicate balance between bacterial stimulation and evasion strategies. This highlights the potential implication of the crosstalk between host innate defences and B. anthracis in initial anthrax control mechanisms.

NK cells are important immune effectors that perform a surveillance task and react to transformed, stressed, and virally infected cells. They represent a first-line defence against cancer and pathogen invasion. Different pathogens trigger distinct NK-cell activation pathways. The Bacillus anthracis spore is the highly resistant form that enters the host and provokes anthrax. This microbe kills through a combination of acute bacterial infection and devastating toxemia. In the present study, we characterise the crosstalk between NK cells and spores, as well as the strategies used by B. anthracis to evade initial control mechanisms and impact anthrax pathogenesis. Our findings exemplify the spores' property to efficiently drive a high production of IFN-γ by NK cells, as well as the complex pathways used for activation which require both cytokine and cellular signaling. B. anthracis subverts this response through its toxins by paralysing essential NK cell functions. Furthermore, edema toxin from B. anthracis blocks natural cytotoxicity without affecting IFN-γ secretion. The CyaA toxin of Bordetella pertussis possesses the same enzymatic activity and has a similar effect. The high efficiency of these toxins in blocking cytotoxicity in vivo implies possible exploitation of their subverting activity to modulate excessive cytotoxic responses in immunopathological diseases.

Activation of natural killer cells during microbial infections

Natural killer (NK) cells are large granular lymphocytes that express a diverse array of germline encoded inhibitory and activating receptors for MHC Class I and Class I-like molecules, classical co-stimulatory ligands, and cytokines. The ability of NK cells to be very rapidly activated by inflammatory cytokines, to secrete effector cytokines, and to kill infected or stressed host cells, suggests that they may be among the very early responders during infection. Recent studies have also identified a small number of pathogen-derived ligands that can bind to NK cell surface receptors and directly induce their activation. Here we review recent studies that have begun to elucidate the various pathways by which viral, bacterial, and parasite pathogens activate NK cells. We also consider two emerging themes of NK cell–pathogen interactions, namely their contribution to adaptive immune responses and their potential to take on regulatory and immunomodulatory functions.

Negative regulation of Schistosoma japonicum egg-induced liver fibrosis by natural killer cells

The role of natural killer (NK) cells in infection-induced liver fibrosis remains obscure. In this study, we elucidated the effect of NK cells on Schistosoma japonicum (S. japonicum) egg-induced liver fibrosis. Liver fibrosis was induced by infecting C57BL/6 mice with 18–20 cercariae of S. japonicum. Anti-ASGM1 antibody was used to deplete NK cells. Toll-like receptor 3 ligand, polyinosinic-polycytidylic acid (poly I∶C) was used to enhance the activation of NK cells. Results showed that NK cells were accumulated and activated after S. japonicum infection, as evidenced by the elevation of CD69 expression and IFN-γ production. Depletion of NK cells markedly enhanced S. japonicum egg-induced liver fibrosis. Administration of poly I∶C further activated NK cells to produce IFN-γ and attenuated S. japonicum egg-induced liver fibrosis. The observed protective effect of poly I∶C on liver fibrosis was diminished through depletion of NK cells. Disruption of IFN-γ gene enhanced liver fibrosis and partially abolished the suppression of liver fibrosis by poly I∶C. Moreover, expression of retinoic acid early inducible 1 (RAE 1), the NKG2D ligand, was detectable at high levels on activated hepatic stellate cells derived from S. japonicum-infected mice, which made them more susceptible to hepatic NK cell killing. In conclusion, our findings suggest that the activated NK cells in the liver after S. japonicum infection negatively regulate egg-induced liver fibrosis via producing IFN-γ, and killing activated stellate cells.

Schistosomiasis continues to be a major public health problem in the developing world. Parasite egg-induced liver fibrosis is the principal cause of morbidity and mortality in human infected with schistosoma. Thus, elucidating the mechanisms that restrict tissue fibrosis may lead to more effective strategies for immunological intervention in this and a variety of chronic diseases. NK cells have been demonstrated to play an important role in suppressing carbon tetrachloride (CCl₄)-induced liver fibrosis. However, little is known about the role of NK cells in an infection-based model of fibrosis. In the current study, we determined, for the first time, the role of NK cells in S. japonicum egg-induced liver fibrosis. Our findings suggest that the activated NK cells in the liver after S. japonicum infection negatively regulate egg-induced liver fibrosis via producing IFN-γ, and killing activated stellate cells. These results further our understanding of the innate immune cells that regulate the development of S. japonicum-induced fibrosis and aid in the development of potential strategies to enhance immunity against this and other chronic inflammatory diseases of the liver where fibrosis is a common feature.

The role of humoral innate immunity in hepatitis C virus infection

Infection with Hepatitis C Virus (HCV) causes chronic disease in approximately 80% of cases, resulting in chronic inflammation and cirrhosis. Current treatments are not completely effective, and a vaccine has yet to be developed. Spontaneous resolution of infection is associated with effective host adaptive immunity to HCV, including production of both HCV-specific T cells and neutralizing antibodies. However, the supporting role of soluble innate factors in protection against HCV is less well understood. The innate immune system provides an immediate line of defense against infections, triggering inflammation and playing a critical role in activating adaptive immunity. Innate immunity comprises both cellular and humoral components, the humoral arm consisting of pattern recognition molecules such as complement C1q, collectins and ficolins. These molecules activate the complement cascade, neutralize pathogens, and recruit antigen presenting cells. Here we review the current understanding of anti-viral components of the humoral innate immune system that play a similar role to antibodies, describing their role in immunity to HCV and their potential contribution to HCV pathogenesis.

A LysM and SH3-domain containing region of the Listeria monocytogenes p60 protein stimulates accessory cells to promote activation of host NK cells

Listeria monocytogenes (Lm) infection induces rapid and robust activation of host natural killer (NK) cells. Here we define a region of the abundantly secreted Lm endopeptidase, p60, that potently but indirectly stimulates NK cell activation in vitro and in vivo. Lm expression of p60 resulted in increased IFNγ production by naïve NK cells co-cultured with treated dendritic cells (DCs). Moreover, recombinant p60 protein stimulated activation of naive NK cells when co-cultured with TLR or cytokine primed DCs in the absence of Lm. Intact p60 protein weakly digested bacterial peptidoglycan (PGN), but neither muropeptide recognition by RIP2 nor the catalytic activity of p60 was required for NK cell activation. Rather, the immune stimulating activity mapped to an N-terminal region of p60, termed L1S. Treatment of DCs with a recombinant L1S polypeptide stimulated them to activate naïve NK cells in a cell culture model. Further, L1S treatment activated NK cells in vivo and increased host resistance to infection with Francisella tularensis live vaccine strain (LVS). These studies demonstrate an immune stimulating function for a bacterial LysM domain-containing polypeptide and suggest that recombinant versions of L1S or other p60 derivatives can be used to promote NK cell activation in therapeutic contexts.

Mechanisms of NK cell activation: CD4(+) T cells enter the scene

Natural killer (NK) cells are innate lymphocytes involved in immunosurveillance through their cytotoxic activity and their capacity to secrete inflammatory cytokines. NK cell activation is necessary to initiate effector functions and results from a complex series of molecular and cellular events. We review here the signals that trigger NK cells and discuss recent findings showing that, besides antigen-presenting cells, T cells can play a central role in the initiation of NK cell activation in lymph nodes.

Regulating the adaptive immune response to blood-stage malaria: role of dendritic cells and CD4⁺Foxp3⁺ regulatory T cells

Although a clearer understanding of the underlying mechanisms involved in protection and immunopathology during blood-stage malaria has emerged, the mechanisms involved in regulating the adaptive immune response especially those required to maintain a balance between beneficial and deleterious responses remain unclear. Recent evidence suggests the importance of CD11c⁺ dendritic cells (DC) and CD4⁺Foxp3⁺ regulatory T cells in regulating immune responses during infection and autoimmune disease, but information concerning the contribution of these cells to regulating immunity to malaria is limited. Here, we review recent findings from our laboratory and others in experimental models of malaria in mice and in Plasmodium-infected humans on the roles of DC and natural regulatory T cells in regulating adaptive immunity to blood-stage malaria.

NK cells: immune cross-talk and therapeutic implications

Increased evidence of cross-talk between NK cells and other immune cells has enhanced the possibilities of exploiting the interplay between the activation and inhibition of NK cells for immunotherapeutic purposes. The battery of receptors possessed by NK cells help them to efficiently detect aberrant and infected cells and embark on the signaling pathways necessary to eliminate them. Endogenous expansion of NK cells and their effector mechanisms are under exploration for enhancing adoptive immunotherapy prospects in combination with immunostimulatory and cell-death-sensitizing treatments against cancer, viral infections and other pathophysiological autoimmune conditions. Various modes of NK cell manipulation are being undertaken to overcome issues such as relapse and graft rejections associated with adoptive immunotherapy. While tracing the remarkable properties of NK cells and the major developments in this field, we highlight the role of immune cooperativity in the betterment of current immunotherapeutic approaches.

Unconventional repertoire profile is imprinted during acute chikungunya infection for natural killer cells polarization toward cytotoxicity

Chikungunya virus (CHIKV) is a worldwide emerging pathogen. In humans it causes a syndrome characterized by high fever, polyarthritis, and in some cases lethal encephalitis. Growing evidence indicates that the innate immune response plays a role in controlling CHIKV infection. We show here that CHIKV induces major but transient modifications in NK-cell phenotype and function soon after the onset of acute infection. We report a transient clonal expansion of NK cells that coexpress CD94/NKG2C and inhibitory receptors for HLA-C1 alleles and are correlated with the viral load. Functional tests reveal cytolytic capacity driven by NK cells in the absence of exogenous signals and severely impaired IFN-γ production. Collectively these data provide insight into the role of this unique subset of NK cells in controlling CHIKV infection by subset-specific expansion in response to acute infection, followed by a contraction phase after viral clearance.

Chikungunya virus (CHIKV) infection, which is responsible for devastating human illness, is rapidly becoming a global concern. The spread of this disease throughout tropical areas, where it now affects nearly 40 countries, underlines the need to improve our understanding of this infection. In 2008, CHIKV was listed as a US National Institute of Allergy and Infectious Disease (NIAID) category C priority pathogen. Natural killer (NK) cells are cytotoxic effector cells that play a vital role in the innate immune system by limiting acute infection, as previously described for several other diseases. This report describes the first phenotypic and functional analysis of NK cells soon after infection by this virus. The key element of this study was the detailed analysis of the expansion of NK cells. Coexpression of NKG2C activating receptors and HLA-C1 ligands is associated with viral load, impaired IFN-γ production, and significant cytolytic functions. We found that NK cells were able to sense CHIKV from the beginning of infection and contributed to the clearance of the infected cells through the expansion of a unique NK-cell subset.

The interplay between Leishmania promastigotes and human Natural Killer cells in vitro leads to direct lysis of Leishmania by NK cells and modulation of NK cell activity by Leishmania promastigotes

NK cells represent one of the first lines of defence in the immune reaction after invasion of Leishmania parasites. Depletion of mouse natural killer (NK) cells dramatically enhances susceptibility of normally resistant mice. In this study we evaluated the fate of NK cells and parasites after contact formation. The hydrophilic fluorescent dye CMFDA (chloro-methylfluorescin diacetate) that allows analysis of cytotoxicity in flow cytometry and microscopy was used. Furthermore, these findings were confirmed with scanning and transmission electron microscopy. Direct contact points were found between Leishmania promastigotes and naïve human NK cells. These contacts were associated with transfer of cytosol by membrane bridges and cytotoxicity of NK cells against Leishmania. However, in contrast to other target cells which allow repeated exocytosis of lytic granules, contact with Leishmania causes immediate destruction of NK cells in a non-apoptotic way. Our results give a reasonable explanation for ex vivo observations of reduced NK cell numbers and impaired NK response in patients with acute cutaneous leishmaniasis. Animal models have clearly shown that NK cells play a key role in the induction and direction of the immune response. Thus inhibition of NK cells at the onset of infection would be advantageous for the survival of the parasite.

Cutting Edge: A dual role for type I IFNs during polyinosinic-polycytidylic acid-induced NK cell activation

NK cells are cytotoxic lymphocytes that are most efficient at fulfilling their functions after a phase of priming provided by cytokines and/or accessory cells. Although type I IFNs are known to be important in this process, it remains unclear whether they act directly on NK cells or indirectly on accessory cells. We used adoptive transfer experiments and mixed bone marrow chimeras to dissect the requirement for type I IFN signaling in response to the dsRNA analog polyinosinic-polycytidylic acid. We demonstrate that optimal NK cell priming requires type I IFNs to signal on both NK cells and accessory cells. In the absence of IL-15, the residual NK cell activation was strictly dependent on cell-intrinsic IFNAR signaling in NK cells. Our results suggest that type I IFNs produced following viral infection simultaneously target accessory cells for IL-15 transpresentation and NK cells themselves and that these two pathways cooperate for NK cell priming.

Leishmania-infected macrophages are targets of NK cell-derived cytokines but not of NK cell cytotoxicity

Natural killer (NK) cells are important components of a protective immune response against intracellular pathogens such as Leishmania parasites, which reside within myeloid cells. Previous in vivo studies in murine cutaneous or visceral leishmaniasis showed that NK cells are activated by conventional dendritic cells in a Toll-like receptor 9-, interleukin-12 (IL-12)-, and IL-18-dependent manner during the early phase of infection and help to restrict the tissue parasite burden by unknown mechanisms. Here, we tested whether NK cells contribute to the control of Leishmania infections by lysing or by activating infected host cells. Coculture experiments revealed that activated NK cells from poly(I:C)-treated mice readily killed tumor target cells, whereas Leishmania infantum- or L. major-infected macrophages or dendritic cells remained viable. Infection with Leishmania did not significantly alter the expression of NK cell-activating molecules (retinoic acid early transcript alpha [Rae-1α], mouse UL16-binding protein-like transcript 1 [MULT-1], CD48) or inhibitory molecules (major histocompatibility complex [MHC] class I, nonclassical MHC class 1b molecule Qa-1) on the surface of myeloid cells, which offers an explanation for their protection from NK cell cytotoxicity. Consistent with these in vitro data, in vivo cytotoxicity assays revealed poor cytolytic activity of NK cells against adoptively transferred infected wild-type macrophages, whereas MHC class I-deficient macrophages were efficiently eliminated. NK cells activated by IL-12 and IL-18 stimulated macrophages to kill intracellular Leishmania in a cell contact-independent but gamma interferon-, tumor necrosis factor-, and inducible nitric oxide synthase-dependent manner. We conclude that Leishmania parasites, unlike viruses, do not render infected myeloid cells susceptible to the cytotoxicity of NK cells. Instead, soluble products of NK cells trigger the leishmanicidal activity of macrophages.

Major histocompatibility complex class-I-interacting natural killer cell receptors of nonhuman primates

Human natural killer (NK) cell receptors are known to be highly polymorphic, to show complex genetics and to be associated with susceptibility to a variety of immunological diseases. Nonhuman primates are used as important models of these diseases, yet the knowledge of nonhuman primate NK cell receptors and ligands is not as advanced as in humans. Recently published data indicated that diversity and polymorphism of NK cell receptors are similar between nonhuman primates and humans. Comparative genomics revealed instructive insights into the evolution and function of primate NK cell receptor genes and contributed to the understanding of how present-day NK cell receptors and their ligands have evolved. Here, I review the current knowledge of nonhuman primate NK cell receptors that interact with major histocompatibility complex class I proteins.

Natural killer cell responses to infections in early life

Natural killer (NK) cells are an important component of innate immune responses to infectious diseases. They mediate protection by being able to rapidly lyse infected cells and produce cytokines (primarily interferon-γ) that shape innate and adaptive immune responses. This review summarizes current knowledge on the phenotype and functional abilities of NK cells from healthy newborns/infants and on NK cell responses against viral, bacterial and protozoan infections in early life. Interestingly, NK cell blood counts are higher in newborns than in adults but they do not display striking differences in phenotype, except for an increased frequency of expression of the inhibitory CD94/NKG2A receptor. They display some inherent functional defects, mainly a lower cytolytic capacity that may contribute to the immaturity of the neonatal immune system. Changes in circulating levels of NK cells observed during pediatric infections and the ability of NK cells from newborns and children to produce interferon-γ at the encounter with pathogens indicate that NK cells participate in the immune response to infectious diseases in early life. Unfortunately, information is currently insufficient to assess whether these NK cell responses really contribute to control infections, either vertically transmitted or acquired in infancy.

Construction and immunogenicity of a recombinant pseudotype baculovirus expressing the glycoprotein of rabies virus in mice

A pseudotype baculovirus with the glycoprotein of vesicular stomatitis virus (VSV-G) on the envelope was used as a vector for the construction of recombinant baculovirus expressing the G protein of rabies virus (RABV) under the cytomegalovirus (CMV) promoter. The generated recombinant baculovirus (BV-G) efficiently expressed the RABV G proteins in mammalian cells. Intramuscular vaccination with BV-G (10(9) PFU/mouse) induced the production of RABV G-specific neutralizing antibodies and strong T cell responses in mice. Our data clearly indicate that pseudotype baculovirus-mediated gene delivery can be utilized as an alternative strategy to develop a new generation of vaccine against RABV infection.

Human natural killer cells exhibit direct activity against Aspergillus fumigatus hyphae, but not against resting conidia

Because natural killer (NK) cells kill tumor cells and combat infections, there is growing interest in adoptively transferring NK cells to hematopoietic stem cell recipients. Unfortunately, in humans, the activity of NK cells against Aspergillus species, the major cause of invasive fungal infection in stem cell recipients, are poorly characterized. Our results show that unstimulated and interleukin-2 prestimulated human NK cells kill Aspergillus fumigatus hyphae but do not affect resting conidia. Killing is also induced by the supernatant of prestimulated NK cells and human perforin. The high levels of interferon-γ and granulocyte macrophage colony-stimulating factor produced by prestimulated NK cells are significantly reduced by Aspergillus, indicating an immunosuppressive effect of the fungus. Whereas Aspergillus hyphae activate NK cells, resting, and germinating, conidia and conidia of ΔrodA mutants lacking the hydrophobic surface layer do not. Our results suggest that adoptively transferred human NK cells may be a potential antifungal tool in the transplantation context.

Human neutrophils interact with both 6-sulfo LacNAc+ DC and NK cells to amplify NK-derived IFN{gamma}: role of CD18, ICAM-1, and ICAM-3

The role of neutrophils as key players in the regulation of innate and adaptive immune responses is increasingly being recognized. We report that human neutrophils establish a network with both natural killer (NK) cells and 6-sulfo LacNAc(+) dendritic cells (slanDCs), which ultimately serves to up-regulate NK-derived interferonγ (IFNγ). This network involves direct reciprocal interactions and positive amplification loops mediated by cell-derived cytokines. Accordingly, we show that after lipopolysaccharide + interleukin-2 (IL-2) or IL-15/IL-18 stimulation, neutrophils directly interact with and potentiate the activity of both slanDCs and NK cells. On the one hand, neutrophils augment the release of IL-12p70 by slanDCs via a CD18/ intercellular adhesion molecule-1 (ICAM-1) interaction that stimulates activated NK cells to produce IFNγ. IFNγ further potentiates the interaction between neutrophils and slanDCs and the release of slanDC-derived IL-12p70, thus creating a positive feedback loop. On the other hand, neutrophils directly co-stimulate NK cells via CD18/ICAM-3, leading to the production of IFNγ. Colocalization of neutrophils, NK cells, and slanDCs, as well as of IL-12p70 and IFNγ, in inflamed tissues of Crohn disease and psoriasis provides strong evidence for a novel cellular and cytokine cooperation within the innate immune system in which neutrophils act as amplifiers of NK cell/slanDC-mediated responses.

NKG2D is required for NK cell activation and function in response to E1-deleted adenovirus

Despite high transduction efficiency in vivo, the application of recombinant E1-deleted adenoviral vectors for in vivo gene therapy has been limited by the attendant innate and adaptive immune responses to adenoviral vectors. NK cells have been shown to play an important role in innate immune elimination of adenoviral vectors in vivo. However, the mechanisms underlying NK cell activation and function in response to adenoviral vectors remain largely undefined. In this study, we showed that NK cell activation upon adenoviral infection was dependent on accessory cells such as dendritic cells and macrophages and that cell contact-dependent signals from the accessory cells are necessary for NK cell activation. We further demonstrated that ligands of the NK activating receptor NKG2D were upregulated in accessory cells upon adenoviral infection and that blockade of NKG2D inhibited NK cell activation upon adenoviral infection, leading to a delay in adenoviral clearance in vivo. In addition, NKG2D was required for NK cell-mediated cytolysis on adenovirus-infected targets. Taken together, these results suggest that efficient NK cell activation and function in response to adenoviral infection is critically dependent on the NKG2D pathway, which understanding may assist in the design of effective strategies to improve the outcome of adenovirus-mediated gene therapy.

Identification of a polyI:C-inducible membrane protein that participates in dendritic cell-mediated natural killer cell activation

The novel polyI:C-inducible membrane protein INAM triggers dendritic cell–mediated natural killer cell activation.

In myeloid dendritic cells (mDCs), TLR3 is expressed in the endosomal membrane and interacts with the adaptor toll/interleukin 1 receptor homology domain–containing adaptor molecule 1 (TICAM-1; TRIF). TICAM-1 signals culminate in interferon (IFN) regulatory factor (IRF) 3 activation. Co-culture of mDC pretreated with the TLR3 ligand polyI:C and natural killer (NK) cells resulted in NK cell activation. This activation was triggered by cell-to-cell contact but not cytokines. Using expression profiling and gain/loss-of-function analyses of mDC genes, we tried to identify a TICAM-1–inducing membrane protein that participates in mDC-mediated NK activation. Of the nine candidates screened, one contained a tetraspanin-like sequence and satisfied the screening criteria. The protein, referred to as IRF-3–dependent NK-activating molecule (INAM), functioned in both the mDC and NK cell to facilitate NK activation. In the mDC, TICAM-1, IFN promoter stimulator 1, and IRF-3, but not IRF-7, were required for mDC-mediated NK activation. INAM was minimally expressed on NK cells, was up-regulated in response to polyI:C, and contributed to mDC–NK reciprocal activation via its cytoplasmic tail, which was crucial for the activation signal in NK cells. Adoptive transfer of INAM-expressing mDCs into mice implanted with NK-sensitive tumors caused NK-mediated tumor regression. We identify a new pathway for mDC–NK contact-mediated NK activation that is governed by a TLR signal-derived membrane molecule.

Klebsiella pneumoniae-triggered DC recruit human NK cells in a CCR5-dependent manner leading to increased CCL19-responsiveness and activation of NK cells

Besides their role in destruction of altered self-cells, NK cells have been shown to potentiate T-cell responses by interacting with DC. To take advantage of NK-DC crosstalk in therapeutic DC-based vaccination for infectious diseases and cancer, it is essential to understand the biology of this crosstalk. We aimed to elucidate the in vitro mechanisms responsible for NK-cell recruitment and activation by DC during infection. To mimic bacterial infection, DC were exposed to a membrane fraction of Klebsiella pneumoniae, which triggers TLR2/4. DC matured with these bacterial fragments can actively recruit NK cells in a CCR5-dependent manner. An additional mechanism of DC-induced NK-cell recruitment is characterized by the induction of CCR7 expression on CD56(dim) CD16(+) NK cells after physical contact with membrane fraction of K. pneumoniae-matured DC, resulting in an enhanced migratory responsiveness to the lymph node-associated chemokine CCL19. Bacterial fragment-matured DC do not only mediate NK-cell migration but also meet the prerequisites needed for augmentation of NK-cell cytotoxicity and IFN-γ production, the latter of which contributes to Th1 polarization.

Neutrophil activation and survival are modulated by interaction with NK cells

It is increasingly evident that neutrophils are able to cross-talk with other leukocytes to shape ongoing inflammatory and immune responses. In this study, we analyzed whether human NK cells may influence the survival and activation of neutrophils under co-culture conditions. We report that NK cells exposed to either IL-15 or IL-18 alone strongly protect the survival of neutrophils via the release of IFNγ and granulocyte macrophage colony-stimulating factor (GM-CSF) plus IFNγ, respectively, and cause a slight up-regulation of neutrophil CD64 and CD11b expression. In comparison, NK cells exposed to both IL-15 and IL-18 show a lesser ability to increase the survival of neutrophils but can more potently up-regulate CD64 and CD11b expression, as well as induce the de novo surface expression of CD69, in neutrophils. Analysis of the events occurring in neutrophil/NK co-cultures exposed to IL-15 plus IL-18 revealed that (i) neutrophil survival is positively affected by NK-derived GM-CSF but negatively influenced by a CD18-dependent neutrophil/NK contact, (ii) NK-derived IFNγ is almost entirely responsible for the induction of CD64, (iii) both soluble factors (primarily GM-CSF) and direct cell-cell contact up-regulate CD11b and CD69 and (iv) NK-derived GM-CSF induces the expression of biologically active heparin-binding EGF-like growth factor (HB-EGF) in neutrophils. Finally, we demonstrate that NK cells can also express HB-EGF when stimulated with either IL-2 or IL-15, yet independently of endogenous GM-CSF. Altogether, our results define a novel interaction within the innate immune system whereby NK cells, by directly modulating neutrophil functions, might contribute to the pathogenesis of inflammatory diseases.

Interactions between lymphocytes and myeloid cells regulate pro- versus anti-tumor immunity

Tumor-associated myeloid cells have been implicated in regulating many of the “hallmarks of cancer” and thus fostering solid tumor development and metastasis. However, the same innate leukocytes also participate in anti-tumor immunity and restraint of malignant disease. While many factors regulate the propensity of myeloid cells to promote or repress cancerous growths, polarized adaptive immune responses by B and T lymphocytes have been identified as regulators of many aspects of myeloid cell biology by specifically regulating their functional capabilities. Here, we detail the diversity of heterogeneous B and T lymphocyte populations and their impacts on solid tumor development through their abilities to regulate myeloid cell function in solid tumors.

The defensive alliance between neutrophils and NK cells as a novel arm of innate immunity

The immune system is equipped with a plethora of mechanisms that protect the host from the harmful effects of environmental insults. However, the traditional "hierarchical" view of the immune response, in which innate, "nonspecific" cells are first recruited to the site of damage, before the highly "specific", adaptive immune response develops, has been questioned recently. First, the innate response is much more specific than recognized previously: indeed, each cell of the innate system is not only endowed with an ever-expanding array of germ-line-encoded receptors, which differentiate between distinct insults, but also is modulated continuously by other leukocytes that concomitantly interact with and respond to that particular insult. The other reason is that the cells of the innate system are instrumental for the adaptive system to accomplish its function, as they can also modulate the activity of lymphocytes reciprocally during the entire course of the immune response. This complex pattern of interactions is illustrated by recent advances on the functions of PMNs, clearly showing that unexpectedly, these cells also contribute to the regulation of the host immune response by crosstalk with innate and adaptive leukocytes, including NK cells. Herein, given the peculiar role of neutrophils and NK cells in inflammation, clearance of pathogens/viral-infected cells, and cancer immunosurveillance, we summarize the current knowledge about the mechanisms whereby neutrophils and NK cells interact and regulate the activities of one another, as well as discuss their potential implications involved in the pathogenesis of chronic, inflammatory pathologies, infections, and tumors.

NK cells as effectors of acquired immune responses: effector CD4+ T cell-dependent activation of NK cells following vaccination

We characterized vaccine-induced cellular responses to rabies virus in naive adult volunteers. Contrary to current paradigms, we observed potent and prolonged in vitro NK cell cytokine production and degranulation responses after restimulation of PBMCs with inactivated rabies virus in vaccinated, but not in unvaccinated, individuals. This "recall" NK cell response was absolutely dependent on Ag-specific IL-2 from CD45RO(+) CD4(+) T cells as well as IL-12 and IL-18 from accessory cells. Importantly, NK cells represented over 70% of all IFN-gamma-secreting and degranulating cells in the first 12-18 h after virus rechallenge indicating they may be required for rapid control of infection after vaccination. Activation of NK cells may be a critical function of IL-2-secreting effector memory T cells. Although IL-2-dependent postvaccination NK cell activation has been reported previously, this is the first time the magnitude of this effect and its contribution to the overall vaccine-induced response has been appreciated and the mechanisms of NK activation postvaccination have been elucidated. Our data will allow standard protocols for evaluating vaccine-induced immunity to be adapted to assess NK cell effector responses.

IL-18, but not IL-15, contributes to the IL-12-dependent induction of NK-cell effector functions by Leishmania infantum in vivo

Activation of NK cells is a hallmark of infections with intracellular pathogens. We previously showed that the protozoan parasite Leishmania infantum triggered a rapid NK-cell response in mice that required TLR9-positive myeloid DC and IL-12, but no IFN-α/β. Here, we investigated whether IL-15 or IL-18 mediate the activity of IL-12 or function as independent activators of NK cells. In contrast to earlier studies that described IL-15 as crucial for NK-cell priming in response to TLR ligands, the expression of IFN-γ, FasL, perforin and granzyme B by NK cells in L. infantum-infected mice was completely preserved in the absence of IL-15, whereas the proliferative capacity of NK cells was lower than in WT mice. IFN-γ secretion, cytotoxicity and FasL expression of NK cells from infected IL-18^−/− mice were significantly reduced compared with controls, but, unlike IL-12, IL-18 was not essential for NK-cell effector functions. Part of the NK-cell-stimulatory effect of IL-12 was dependent on IL-18. We conclude that IL-15 is not functioning as a universal NK-cell priming signal and that IL-18 contributes to the NK-cell response in visceral leishmaniasis. The cytokine requirements for NK-cell activation appear to differ contingent upon the infectious pathogen.

Primed antigen-specific CD4+ T cells are required for NK cell activation in vivo upon Leishmania major infection

The ability of NK cells to rapidly produce IFN-gamma is an important innate mechanism of resistance to many pathogens including Leishmania major. Molecular and cellular components involved in NK cell activation in vivo are still poorly defined, although a central role for dendritic cells has been described. In this study, we demonstrate that Ag-specific CD4(+) T cells are required to initiate NK cell activation early on in draining lymph nodes of L. major-infected mice. We show that early IFN-gamma secretion by NK cells is controlled by IL-2 and IL-12 and is dependent on CD40/CD40L interaction. These findings suggest that newly primed Ag-specific CD4(+) T cells could directly activate NK cells through the secretion of IL-2 but also indirectly through the regulation of IL-12 secretion by dendritic cells. Our results reveal an unappreciated role for Ag-specific CD4(+) T cells in the initiation of NK cell activation in vivo upon L. major infection and demonstrate bidirectional regulations between innate and adaptive immunity.

Natural killer cells promote tissue injury and systemic inflammatory responses during fatal Ehrlichia-induced toxic shock-like syndrome

Human monocytotropic ehrlichiosis is caused by Ehrlichia chaffeensis, a Gram-negative bacterium lacking lipopolysaccharide. We have shown that fatal murine ehrlichiosis is associated with CD8(+)T cell-mediated tissue damage, tumor necrosis factor-alpha, and interleukin (IL)-10 overproduction, and CD4(+)Th1 hyporesponsiveness. In this study, we examined the relative contributions of natural killer (NK) and NKT cells in Ehrlichia-induced toxic shock. Lethal ehrlichial infection in wild-type mice induced a decline in NKT cell numbers, and late expansion and migration of activated NK cells to the liver, a main infection site that coincided with development of hepatic injury. The spatial and temporal changes in NK and NKT cells in lethally infected mice correlated with higher NK cell cytotoxic activity, higher expression of cytotoxic molecules such as granzyme B, higher production of interferon-gamma and tumor necrosis factor-alpha, increased hepatic infiltration with CD8alphaCD11c(+) dendritic cells and CD8(+)T cells, decreased splenic CD4(+)T cells, increased serum concentrations of IL-12p40, IL-18, RANTES, and monocyte chemotactic protein-1, and elevated production of IL-18 by liver mononuclear cells compared with nonlethally infected mice. Depletion of NK cells prevented development of severe liver injury, decreased serum levels of interferon-gamma, tumor necrosis factor-alpha, and IL-10, and enhanced bacterial elimination. These data indicate that NK cells promote immunopathology and defective anti-ehrlichial immunity, possibly via decreasing the protective immune response mediated by interferon-gamma producing CD4(+)Th1 and NKT cells.

Cytokine regulation of natural killer cell effector functions

Initially described as effectors of natural cytotoxicity and critical players for the control of viral infections and tumor growth, recent investigations unraveled more widespread functions for the natural killer (NK) cells. Through the establishment of a crosstalk with dendritic cells, NK cells promote T helper-1- and cytotoxic T lymphocyte-mediated immunity, whereas through the establishment of a crosstalk with macrophages, NK cells contribute to the activation of their microbicidal functions. Recent evidence has shown that NK cells also display memory, a characteristic thought to be privative of T and B cells, and that NK cells acquire their mature phenotype during a complex ontogeny program which tunes their activation threshold. Cytokines play critical roles in regulating all aspects of immune responses, including lymphoid development, homeostasis, differentiation, tolerance, and memory. Cytokines such as interleukin (IL)-2, IL-12, IL-15, IL-18, IL-21, and type I interferons constitute pivotal factors involved in the maturation, activation, and survival of NK cells. In addition, the discovery of novel cytokines is increasing the spectrum of soluble mediators that regulate NK cell immunobiology. In this review, we summarize and integrate novel concepts about the role of different cytokines in the regulation of NK cell function. We believe that a full understanding of how NK cells become activated and develop their effector functions in response to cytokines and other stimuli may lead to the development of novel immunotherapeutic strategies for the treatment of different types of cancer, viral infections, and chronic autoimmune diseases.

Polyphenols from red wine are potent modulators of innate and adaptive immune responsiveness

It is well known that the consumption of dietary polyphenols leads to beneficial effects for human health as in the case of prevention and/or attenuation of cardiovascular, inflammatory, neurodegenerative and neoplastic diseases. This review summarizes the role of polyphenols from red wine in the immune function. In particular, using healthy human peripheral blood mononuclear cells, we have demonstrated the in vitro ability of Negroamaro, an Italian red wine, to induce the release of nitric oxide and both pro-inflammatory and anti-inflammatory cytokines, thus leading to the maintenance of the immmune homeostasis in the host. All these effects were abrogated by deprivation of polyphenols from red wine samples. We have also provided evidence that Negromaro polyphenols are able to activate extracellular regulated kinase and p38 kinase and switch off the NF-kappaB pathway via an increased expression with time of the IkappaBalpha phosphorylated form. These mechanisms may represent key molecular events leading to inhibition of the pro-inflammatory cascade and atherogenesis. In conclusion, according to the current literature and our own data, moderate consumption of red wine seems to be protective for the host in the prevention of several diseases, even including aged-related diseases by virtue of its immunomodulating properties.

FimH can directly activate human and murine natural killer cells via TLR4

Although the importance of natural killer (NK) cells in innate immune responses against tumors or viral infections are well documented, their ability to directly recognize pathogens is less well defined. We have recently reported FimH, a bacterial fimbrial protein, as a novel Toll-like receptor (TLR)4 ligand that potently induces antiviral responses. Here, we investigated whether FimH either directly or indirectly can activate human and murine NK cells. We demonstrate that FimH potently activates both human and murine NK cells in vitro to induce cytokines [interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha] and cytotoxicity. Importantly, NK cells directly recognize FimH-expressing pathogens as FimH(+), but not FimH(-), bacteria were able to activate human NK cells. FimH activation of NK cells required TLR4 and MyD88 signaling, as NK cells from both TLR4(-/-) and MyD88(-/-) mice as well as human NK-92 cells, which lack TLR4, were all unresponsive to FimH. In addition, TLR4 neutralization significantly abrogated the response of human NK cells to FimH. Activation of purified NK cells by FimH was independent of lipopolysaccharide (LPS) or other bacterial contaminations. These data demonstrate for the first time that highly purified NK cells directly recognize and respond to FimH via TLR4-MyD88 pathways to aid innate protection against cancer or microbial infections.

Tumor immunoediting by NKp46

NK cells interact with a wide variety of hazardous cells including pathogen-infected and tumor cells. NKp46 is a specific NK killer receptor that recognizes various influenza hemagglutinins and unknown tumor ligands. It was recently shown that NKp46 plays a significant role in the in vivo eradication of tumor cells; however, the role played by NKp46 in vivo with regard to tumor development is still unclear. In this study, we used the 3-methylcholanthrene (MCA)-induced fibrosarcoma model in NKp46-deficient mice to test the NKp46 recognition of carcinogen-induced tumors. We show that although the rate of MCA-induced tumor formation was similar in the presence and in the absence of NKp46, the expression of its unknown ligands was NKp46 dependent. The unknown NKp46 ligands were nearly absent in tumors that originated in wild-type mice, whereas they were detected in tumors that originated in the NKp46-deficient mice. We demonstrate that the interactions between NKp46 and its MCA tumor-derived ligands lead to the secretion of IFN-gamma but not to the elimination of the MCA-derived tumor cells. In addition, we show that the in vivo growth of MCA-derived tumor cells expressing high levels of the NKp46 ligands is NKp46 and IFN-gamma dependent. Thus, we present in this study a novel NKp46-mediated mechanism of tumor editing.

Inhibitory and regulatory immune synapses

Cell contact-dependent inhibition and regulation of immune responses play an essential role in balancing the need for rapid and efficient responses to a wide variety of pathological challenges, while at the same time maintaining self-tolerance. Much attention has been given to immune synapses that lead to the activation of, for example, cell-mediated cytotoxicity, and here we compare the supramolecular dynamics of synapses that lead to inhibition or regulatory functions. We focus on natural killer cells where such different synapses have been best studied. An emergent principle is that inhibition or regulatory responses are commonly achieved by selective recruitment of signalling proteins to the synapse and exclusion of membrane-proximal intracellular proteins needed for activation. We also discuss evidence that an inhibitory synapse triggers or maintains effector cells in a migratory configuration, which serves to break the synapse before the steps needed for effector cell activation can be completed. This model implies that the concept of kinetic-proofreading, previously used to describe activation of individual T-cell receptors, can also apply in determining the outcome of intercellular conjugation.

Chronic cigarette smoke exposure primes NK cell activation in a mouse model of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a debilitating, progressive lung disease punctuated by exacerbations of symptoms. COPD exacerbations are most often associated with viral infections, and exposure to cigarette smoke (CS) followed by viral infection has been shown experimentally to enhance lung inflammation, tissue destruction, and airway fibrosis. Despite this, however, the cellular mechanisms responsible for this effect are unknown. In this study, we examined NK cell function in a mouse model of COPD given the vital role of NK cells following viral infection. Ex vivo stimulation of lung leukocytes with poly(I:C), ssRNA40, or ODN1826 enhanced production of NK cell-derived IFN-gamma in CS-exposed mice. NK cells from CS-exposed mice exhibited a novel form of priming; highly purified NK cells from CS-exposed mice, relative to NK cells from filtered air-exposed mice, produced more IFN-gamma following stimulation with IL-12, IL-18, or both. Further, NK cell priming was lost following smoking cessation. NKG2D stimulation through overexpression of Raet1 on the lung epithelium primed NK cell responsiveness to poly(I:C), ssRNA40, or ODN1826 stimulation, but not cytokine stimulation. In addition, NK cells from CS-exposed mice expressed more cell surface CD107a upon stimulation, demonstrating that the NK cell degranulation response was also primed. Together, these results reveal a novel mechanism of activation of the innate immune system and highlight NK cells as important cellular targets in controlling COPD exacerbations.

The marginating-pulmonary immune compartment in rats: characteristics of continuous inflammation and activated NK cells

A significant role has been indicated for cellular immunity in controlling circulating cancer cells, but most autologous tumor cells seem resistant, in vitro, to natural killer cell (NKC) and cytotoxic T lymphocytes cytotoxicity. Addressing this apparent contradiction, we recently identified a unique leukocyte population, marginating-pulmonary (MP)-leukocytes, which exhibit potent natural killer (NK) cytotoxicity. Here, we characterize the MP-compartment in naive and immunostimulated rats, and assessed its cytotoxicity against "NK-resistant" tumors cells. Animals were treated with poly I-C (3x0.2 mg/kg) or saline, and circulating-leukocytes and MP-leukocytes were collected and analyzed in terms of cellular composition, cellular activation markers, and NK cytotoxicity of leukocytes and purified NKCs. Compared with circulating-leukocytes, MP-leukocytes showed greater proportion of granulocytes, monocytes, NKCs, and large NKCs; higher expression of activation and adhesion markers (CD25, CD11a, CD11b, and NKR-P1, IFN-gamma); and elevated NK cytotoxicity of leukocytes and purified NKCs against several syngeneic and xenogeneic NK-resistant target cells (from both F344 and BDX inbred rats). In immunostimulated animals (treated with poly I-C), but not in naive animals, purified NKCs from the MP-compartment showed markedly superior cytotoxicity, suggesting that poly I-C immunostimulation uniquely affect MP-NKCs, and that in naive animals other MP-leukocytes support NK cytotoxicity. Overall, the results suggest that the MP-compartment is characterized by a continuous activated inflammatory microenvironment uniquely affected by immunostimulation. If similarly potent MP-NKCs exist in patients, then circulating autologous tumor cells that are considered "NK-resistant" could actually be controlled by MP-NKCs. Innate immunity may assume greater role in controlling malignant spread, especially after immunostimulation.

Hepatitis C virions subvert natural killer cell activation to generate a cytokine environment permissive for infection

BACKGROUND & AIMS

Hepatitis C virus (HCV) is remarkably successful in establishing persistent infections due to its ability to evade host immune responses through a combination of mechanisms including modulation of interferon (IFN) signalling in infected cells, interference with effector cell function of the immune system and continual viral genetic variation. We have previously demonstrated that natural killer (NK) cells can be inhibited in vitro by recombinant HCV glycoprotein E2 via cross-linking of CD81, a cellular co-receptor for the virus.

METHODS

Taking advantage of the recently established tissue-culture system for HCV, we have studied the effects of CD81 engagement by the HCV envelope glycoprotein E2 when the protein is part of complete, infectious viral particles. Specifically, we asked whether exposure to HCV viral particles (HCVcc) affects activation of NK cells and whether altered NK cell activation, in turn, impacts on HCV infectivity.

RESULTS

We found that immobilized HCVcc, unlike soluble HCVcc, inhibited IFN-gamma production by interleukin (IL)-12 activated NK cells, and that this effect was mediated by engagement of cellular CD81 by HCV-virion displayed E2. In contrast, NK-production of IL-8 was increased in the presence of HCV. The cytokines produced by IL-12 activated NK cells strongly reduced the establishment of productive HCV infection. Importantly, NK-cell derived cytokines secreted in the presence of HCVcc showed a diminished antiviral effect that correlated with IFN-gamma reduction, while IL-8 concentrations had no impact on HCV infectivity.

CONCLUSIONS

Exposure to HCVcc modulates the pattern of cytokines produced by NK cells, leading to reduced antiviral activity.

Influenza infection leads to increased susceptibility to subsequent bacterial superinfection by impairing NK cell responses in the lung

Influenza viral infection is well-known to predispose to subsequent bacterial superinfection in the lung but the mechanisms have remained poorly defined. We have established a murine model of heterologous infections by an H1N1 influenza virus and Staphylococcus aureus. We found that indeed prior influenza infection markedly increased the susceptibility of mice to secondary S. aureus superinfection. Severe sickness and heightened bacterial infection in flu and S. aureus dual-infected animals were associated with severe immunopathology in the lung. We further found that flu-experienced lungs had an impaired NK cell response in the airway to subsequent S. aureus bacterial infection. Thus, adoptive transfer of naive NK cells to the airway of prior flu-infected mice restored flu-impaired antibacterial host defense. We identified that TNF-alpha production of NK cells played an important role in NK cell-mediated antibacterial host defense as NK cells in flu-experienced lungs had reduced TNF-alpha expression and adoptive transfer of TNF-alpha-deficient NK cells to the airway of flu-infected mice failed to restore flu-impaired antibacterial host defense. Defected NK cell function was found to be an upstream mechanism of depressed antibacterial activities by alveolar macrophages as contrast to naive wild-type NK cells, the NK cells from flu-infected or TNF-alpha-deficient mice failed to enhance S. aureus phagocytosis by alveolar macrophages. Together, our study identifies the weakened NK cell response in the lung to be a novel critical mechanism for flu-mediated susceptibility to bacterial superinfection.

Activation of human NK cells by malaria-infected red blood cells

This chapter describes a protocol to assess activation of human NK cells following in vitro stimulation with malaria-infected red blood cells. Activation is assessed by flow cytometry, staining for cell surface expression of CD69 and accumulation of intracellular IFN-gamma. Procedures are described for in vitro propagation and purification of Plasmodium falciparum parasites, separation of peripheral blood mononuclear cells from heparinised blood by density centrifugation, in vitro culture of PBMC and for staining and analysis of PBMC by flow cytometry. Some examples of typical FACS plots are shown.