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

Interleukin-2 from Adaptive T Cells Enhances Natural Killer Cell Activity against Human Cytomegalovirus-Infected Macrophages

Control of human cytomegalovirus (HCMV) requires a continuous immune surveillance, thus HCMV is the most important viral pathogen in severely immunocompromised individuals. Both innate and adaptive immunity contribute to the control of HCMV. Here, we report that peripheral blood natural killer cells (PBNKs) from HCMV-seropositive donors showed an enhanced activity toward HCMV-infected autologous macrophages. However, this enhanced response was abolished when purified NK cells were applied as effectors. We demonstrate that this enhanced PBNK activity was dependent on the interleukin-2 (IL-2) secretion of CD4(+) T cells when reexposed to the virus. Purified T cells enhanced the activity of purified NK cells in response to HCMV-infected macrophages. This effect could be suppressed by IL-2 blocking. Our findings not only extend the knowledge on the immune surveillance in HCMV-namely, that NK cell-mediated innate immunity can be enhanced by a preexisting T cell antiviral immunity-but also indicate a potential clinical implication for patients at risk for severe HCMV manifestations due to immunosuppressive drugs, which mainly suppress IL-2 production and T cell responsiveness.

IMPORTANCE

Human cytomegalovirus (HCMV) is never cleared by the host after primary infection but instead establishes a lifelong latent infection with possible reactivations when the host's immunity becomes suppressed. Both innate immunity and adaptive immunity are important for the control of viral infections. Natural killer (NK) cells are main innate effectors providing a rapid response to virus-infected cells. Virus-specific T cells are the main adaptive effectors that are critical for the control of the latent infection and limitation of reinfection. In this study, we found that IL-2 secreted by adaptive CD4(+) T cells after reexposure to HCMV enhances the activity of NK cells in response to HCMV-infected target cells. This is the first direct evidence that the adaptive T cells can help NK cells to act against HCMV infection.

Impaired NK Cell Responses to Pertussis and H1N1 Influenza Vaccine Antigens in Human Cytomegalovirus-Infected Individuals

NK cells contribute to postvaccination immune responses after activation by IL-2 from Ag-specific memory T cells or by cross-linking of the low-affinity IgG receptor, CD16, by Ag–Ab immune complexes. Sensitivity of NK cells to these signals from the adaptive immune system is heterogeneous and influenced by their stage of differentiation. CD56^dimCD57⁺ NK cells are less responsive to IL-2 and produce less IFN-γ in response to T cell–mediated activation than do CD56^bright or CD56^dimCD57⁻ NK cells. Conversely, NK cell cytotoxicity, as measured by degranulation, is maintained across the CD56^dim subsets. Human CMV (HCMV), a highly prevalent herpes virus causing lifelong, usually latent, infections, drives the expansion of the CD56^dimCD57⁺NKG2C⁺ NK cell population, skewing the NK cell repertoire in favor of cytotoxic responses at the expense of cytokine-driven responses. We hypothesized, therefore, that HCMV seropositivity would be associated with altered NK cell responses to vaccine Ags. In a cross-sectional study of 152 U.K. adults, with HCMV seroprevalence rate of 36%, we find that HCMV seropositivity is associated with lower NK cell IFN-γ production and degranulation after in vitro restimulation with pertussis or H1N1 influenza vaccine Ags. Higher expression of CD57/NKG2C and lower expression of IL-18Rα on NK cells from HCMV seropositive subjects do not fully explain these impaired responses, which are likely the result of multiple receptor–ligand interactions. This study demonstrates for the first time, to our knowledge, that HCMV serostatus influences NK cell contributions to adaptive immunity and raises important questions regarding the impact of HCMV infection on vaccine efficacy.

NK Cells and Cancer Immunoediting

Natural killer (NK) cells are innate lymphoid cells (ILC) known for their ability to recognize and rapidly eliminate infected or transformed cells. Consequently, NK cells are fundamental for host protection against virus infections and malignancies. Even though the critical role of NK cells in cancer immunosurveillance was suspected years ago, the underlying mechanisms took time to be unraveled. Today, it is clear that anti-tumor functions of NK cells are tightly regulated and expand far beyond the simple killing of malignant cells. In spite of tremendous steps made in understanding the NK cell biology, further work is warranted to fully exploit the anticancer potential of these cells. Indeed, tumor-mediated immune suppression hampers NK cell activity, thus complicating their stimulation for therapeutic purposes. Herein, we review the current knowledge of NK cell functions in anti-tumor immunity . We discuss NK cell activity in the cancer immunoediting process with particular emphasis on the elimination and escape phases.

SHP-1-mediated inhibitory signals promote responsiveness and anti-tumour functions of natural killer cells

Natural killer (NK) cells are cytotoxic innate lymphoid cells that are involved in immune defense. NK cell reactivity is controlled in part by MHC class I recognition by inhibitory receptors, but the underlying molecular mechanisms remain undefined. Using a mouse model of conditional deletion in NK cells, we show here that the protein tyrosine phosphatase SHP-1 is essential for the inhibitory function of NK cell MHC class I receptors. In the absence of SHP-1, NK cells are hyporesponsive to tumour cells in vitro and their early Ca(2+) signals are compromised. Mice without SHP-1 in NK cells are unable to reject MHC class I-deficient transplants and to control tumours in vivo. Thus, the inhibitory activity of SHP-1 is needed for setting the threshold of NK cell reactivity.

Striking the Right Balance Determines TB or Not TB

Mycobacterium tuberculosis continues to be one of the most successful pathogens on earth. Upon inhalation of M. tuberculosis by a healthy individual, the host immune system will attempt to eliminate these pathogens using a combination of immune defense strategies. These include the recruitment of macrophages and other phagocytes to the site of infection, production of cytokines that enhance the microbicidal capacity of the macrophages, as well as the activation of distinct subsets of leukocytes that work in concert to fight the infection. However, being as successful as it is, M. tuberculosis has evolved numerous strategies to subvert host immunity at virtual every level. As a consequence, one third of the world inhabitants carry M. tuberculosis, and tuberculosis continuous to cause disease in more than 8 million people with deadly consequences in well over 1 million patients each year. In this review, we discuss several of the strategies that M. tuberculosis employs to circumvent host immunity, as well as describe some of the mechanisms that the host uses to counter such subversive strategies. As for many other infectious diseases, the ultimate outcome is usually defined by the relative strength of the virulence strategies employed by the tubercle bacillus versus the arsenal of immune defense mechanisms of the infected host.

Conditional ablation of NKp46+ cells using a novel Ncr1(greenCre) mouse strain: NK cells are essential for protection against pulmonary B16 metastases

To study gene functions specifically in NKp46+ cells we developed novel Cre mice allowing for conditional gene targeting in cells expressing Ncr1 (encoding NKp46). We generated transgenic Ncr1(greenCre) mice carrying an EGFPcre fusion under the control of a proximal Ncr1 promoter that faithfully directed EGFPcre expression to NKp46+ cells from lymphoid and nonlymphoid tissues. This approach allowed for direct detection of Cre-expressing NKp46+ cells via their GFP signature by flow cytometry and histology. Cre was functional as evidenced by the NKp46+ cell-specific expression of RFP in Ncr1(greenCre) Rosa-dtRFP reporter mice. We generated Ncr1(greenCre) Il2rg(fl/fl) mice that lack NKp46+ cells in an otherwise intact hematopoietic environment. Il2rg encodes the common gamma chain (γc ), which is an essential receptor subunit for cytokines (IL-2, -4, -7, -9, -15, and -21) that stimulate lymphocyte development and function. In Ncr1(greenCre) Il2rg(fl/fl) mice, NK cells are severely reduced and the few remaining NKp46+ cells escaping γc deletion failed to express GFP. Using this new NK-cell-deficient model, we demonstrate that the homeostasis of NKp46+ cells from all tissues (including the recently described intraepithelial ILC1 subset) requires Il2rg. Finally, Ncr1(greenCre) Il2rg(fl/fl) mice are unable to reject B16 lung metastases demonstrating the essential role of NKp46+ cells in antimelanoma immune responses.

Dysfunction of dendritic cells in aged C57BL/6 mice leads to failure of natural killer cell activation and of tumor eradication

The reciprocal activation of dendritic cells (DCs) and natural killer cells (NKs) plays a key role in both innate and adaptive immunity. The effect of aging on this cross-talk, a critical step in virus disease control and tumor immunology, has not been reported. Splenic DCs and NKs were purified from both young and old C57BL/6 mice and cocultured in the presence of polyinosinic:polycytidylic acid (poly I:C). The resulting activation of NKs was measured as expression of CD69 and secretion of IFN-γ. However, DCs from old mice could not activate NKs from either young or old mice in vitro or in vivo. In contrast, DCs from young mice efficiently activated NKs from both young and old mice. DCs from old mice were deficient in poly I:C-stimulated secretion of IL-15, IL-18, and IFN-α. Gene expression analysis revealed many other differences between DCs of old and young mice. Young mice strongly eradicated MHC class I-negative NK-sensitive RMA-S lymphoma mutant tumor cells, but old mice did not, in concert with the previous report that mousepox kills aged, but not young, C57BL/6 mice. Furthermore, a similar dysfunction of DC and its key role in NK activation was found in 27 out of 55 healthy human donors.

Immune responses in neonates

Neonates have little immunological memory and a developing immune system, which increases their vulnerability to infectious agents. Recent advances in the understanding of neonatal immunity indicate that both innate and adaptive responses are dependent on precursor frequency of lymphocytes, antigenic dose and mode of exposure. Studies in neonatal mouse models and human umbilical cord blood cells demonstrate the capability of neonatal immune cells to produce immune responses similar to adults in some aspects but not others. This review focuses mainly on the developmental and functional mechanisms of the human neonatal immune system. In particular, the mechanism of innate and adaptive immunity and the role of neutrophils, antigen presenting cells, differences in subclasses of T lymphocytes (Th1, Th2, Tregs) and B cells are discussed. In addition, we have included the recent developments in the neonatal mouse immune system. Understanding neonatal immunity is essential to development of therapeutic vaccines to combat newly emerging infectious agents.

Partial Activation of natural killer and γδ T cells by classical swine fever viruses is associated with type I interferon elicited from plasmacytoid dendritic cells

Vaccination with live attenuated classical swine fever virus (CSFV) vaccines can rapidly confer protection in the absence of neutralizing antibodies. With an aim of providing information on the cellular mechanisms that may mediate this protection, we explored the interaction of porcine natural killer (NK) cells and γδ T cells with CSFV. Both NK and γδ T cells were refractory to infection with attenuated or virulent CSFV, and no stimulatory effects, as assessed by the expression of major histocompatibility complex (MHC) class II (MHC-II), perforin, and gamma interferon (IFN-γ), were observed when the cells were cultured in the presence of CSFV. Coculture with CSFV and myeloid dendritic cells (mDCs) or plasmacytoid dendritic cells (pDCs) showed that pDCs led to a partial activation of both NK and γδ T cells, with upregulation of MHC-II being observed. An analysis of cytokine expression by infected DC subsets suggested that this effect was due to IFN-α secreted by infected pDCs. These results were supported by ex vivo analyses of NK and γδ T cells in the tonsils and retropharyngeal lymph nodes from pigs that had been vaccinated with live attenuated CSFV and/or virulent CSFV. At 5 days postchallenge, there was evidence of significant upregulation of MHC-II but not perforin on NK and γδ T cells, which was observed only following a challenge of the unvaccinated pigs and correlated with increased CSFV replication and IFN-α expression in both the tonsils and serum. Together, these data suggest that it is unlikely that NK or γδ T cells contribute to the cellular effector mechanisms induced by live attenuated CSFV.

Syk signaling in dendritic cells orchestrates innate resistance to systemic fungal infection

Host protection from fungal infection is thought to ensue in part from the activity of Syk-coupled C-type lectin receptors and MyD88-coupled toll-like receptors in myeloid cells, including neutrophils, macrophages and dendritic cells (DCs). Given the multitude of cell types and receptors involved, elimination of a single pathway for fungal recognition in a cell type such as DCs, primarily known for their ability to prime T cell responses, would be expected to have little effect on innate resistance to fungal infection. Here we report that this is surprisingly not the case and that selective loss of Syk but not MyD88 in DCs abrogates innate resistance to acute systemic Candida albicans infection in mice. We show that Syk expression by DCs is necessary for IL-23p19 production in response to C. albicans, which is essential to transiently induce GM-CSF secretion by NK cells that are recruited to the site of fungal replication. NK cell-derived-GM-CSF in turn sustains the anti-microbial activity of neutrophils, the main fungicidal effectors. Thus, the activity of a single kinase in a single myeloid cell type orchestrates a complex series of molecular and cellular events that underlies innate resistance to fungal sepsis.

Differential activation of CD57-defined natural killer cell subsets during recall responses to vaccine antigens

Natural killer (NK) cells contribute to the effector phase of vaccine-induced adaptive immune responses, secreting cytokines and releasing cytotoxic granules. The proportion of responding NK cells varies between individuals and by vaccine, suggesting that functionally discrete subsets of NK cells with different activation requirements may be involved. Here, we have used responses to individual components of the DTP vaccine [tetanus toxoid (TT), diphtheria toxoid (DT), whole cell inactivated pertussis] to characterize the NK cell subsets involved in interleukin-2-dependent recall responses. Culture with TT, DT or pertussis induced NK cell CD25 expression and interferon-γ production in previously vaccinated individuals. Responses were the most robust against whole cell pertussis, with responses to TT being particularly low. Functional analysis of discrete NK cell subsets revealed that transition from CD56bright to CD56dim correlated with increased responsiveness to CD16 cross-linking, whereas increasing CD57 expression correlated with a loss of responsiveness to cytokines. A higher frequency of CD56dim CD57− NK cells expressed CD25 and interferon-γ following stimulation with vaccine antigen compared with CD56dim CD57+ NK cells and made the largest overall contribution to this response. CD56dim CD57int NK cells represent an intermediate functional phenotype in response to vaccine-induced and receptor-mediated stimuli. These findings have implications for the ability of NK cells to contribute to the effector response after vaccination and for vaccine-induced immunity in older individuals.

CD4(+) NKG2D(+) T cells induce NKG2D down-regulation in natural killer cells in CD86-RAE-1ε transgenic mice

The binding of NKG2D to its ligands strengthens the cross-talk between natural killer (NK) cells and dendritic cells, particularly at early stages, before the initiation of the adaptive immune response. We found that retinoic acid early transcript-1ε (RAE-1ε), one of the ligands of NKG2D, was persistently expressed on antigen-presenting cells in a transgenic mouse model (pCD86-RAE-1ε). By contrast, NKG2D expression on NK cells, NKG2D-dependent cytotoxicity and tumour rejection, and dextran sodium sulphate-induced colitis were all down-regulated in this mouse model. The down-regulation of NKG2D on NK cells was reversed by stimulation with poly (I:C). The ectopic expression of RAE-1ε on dendritic cells maintained NKG2D expression levels and stimulated the activity of NK cells ex vivo, but the higher frequency of CD4(+) NKG2D(+) T cells in transgenic mice led to the down-regulation of NKG2D on NK cells in vivo. Hence, high levels of RAE-1ε expression on antigen-presenting cells would be expected to induce the down-regulation of NK cell activation by a regulatory T-cell subset.

The bacterial pathogen Listeria monocytogenes and the interferon family: type I, type II and type III interferons

Interferons (IFNs) are secreted proteins of the cytokine family that regulate innate and adaptive immune responses to infection. Although the importance of IFNs in the antiviral response has long been appreciated, their role in bacterial infections is more complex and is currently a major focus of investigation. This review summarizes our current knowledge of the role of these cytokines in host defense against the bacterial pathogen Listeria monocytogenes and highlights recent discoveries on the molecular mechanisms evolved by this intracellular bacterium to subvert IFN responses.

Effects of Peptide on NK cell-mediated MHC I recognition

The inhibitory receptors for MHC class I have a central role in controlling natural killer (NK) cell activity. Soon after their discovery, it was found that these receptors have a degree of peptide selectivity. Such peptide selectivity has been demonstrated for all inhibitory killer cell immunoglobulin-like receptor (KIR) tested to date, certain activating KIR, and also members of the C-type lectin-like family of receptors. This selectivity is much broader than the peptide specificity of T cell receptors, with NK cell receptors recognizing peptide motifs, rather than individual peptides. Inhibitory receptors on NK cells can survey the peptide:MHC complexes expressed on the surface of target cells, therefore subsequent transduction of an inhibitory signal depends on the overall peptide content of these MHC class I complexes. Functionally, KIR-expressing NK cells have been shown to be unexpectedly sensitive to changes in the peptide content of MHC class I, as peptide:MHC class I complexes that weakly engage KIR can antagonize the inhibitory signals generated by engagement of stronger KIR-binding peptide:MHC class I complexes. This property provides KIR-expressing NK cells with the potential to recognize changes in the peptide:MHC class I repertoire, which may occur during viral infections and tumorigenesis. By contrast, in the presence of HLA class I leader peptides, virus-derived peptides can induce a synergistic inhibition of CD94:NKG2A-expressing NK cells through recruitment of CD94 in the absence of NKG2A. On the other hand, CD94:NKG2A-positive NK cells can be exquisitely sensitive to changes in the levels of MHC class I. Peptide antagonism and sensitivity to changes in MHC class I levels are properties that distinguish KIR and CD94:NKG2A. The subtle difference in the properties of NK cells expressing these receptors provides a rationale for having complementary inhibitory receptor systems for MHC class I.

The quantitative and functional changes of NK cells in mice infected with Angiostrongylus cantonensis

Angiostrongylus cantonensis is a neurotropic parasite which can cause injury to central nervous system and eosinophilic meningitis to human. Natural killer (NK) cells are specialized innate lymphocytes important in early defense against pathogens as in a variety of intracellular bacterial, viral, and protozoan infections. However, the number and function of NK cells in extracellular parasitic infection of A. cantonensis are unclear. In this study, on A. cantonensis infected mice which may mimic the human's infection, we found that the percentage of splenic NK cells and the absolute number of peripheral blood NK cells were decreased at 21-day post infection compared with that of controls. When administrating with albendazole treatment at early stage of the infection, the changes of NK cells could be avoided. Further analysis confirmed that the reduction of NK cells was due to their apoptosis manifested as increased expressions of annexin V and activated caspase-3 after 16-day post infection. Moreover, both activated and inhibitory receptors such as CD16, CD69, NKG2D, and Ly49a on NK cells were down-regulated after 16-day post infection. Interestingly, NK cells isolated from mice of 21-day post infection showed enhanced IFN-γ production when stimulated with IL-12 for 24 h and cytotoxicity to YAC-1 cells, as well as elevated CD107a expression. It is evident that NK cell population and its function were changed in A. cantonensis infected mice, suggesting their involvement in pathogenesis of the infection.

NK cell intrinsic regulation of MIP-1α by granzyme M

Granzymes are generally recognized for their capacity to induce various pathways of perforin-dependent target cell death. Within this serine protease family, Granzyme M (GrzM) is unique owing to its preferential expression in innate effectors such as natural killer (NK) cells. During Listeria monocytogenes infection, we observed markedly reduced secretion of macrophage inflammatory protein-1 alpha (MIP-1α) in livers of GrzM-deficient mice, which resulted in significantly impaired NK cell recruitment. Direct stimulation with IL-12 and IL-15 demonstrated that GrzM was required for maximal secretion of active MIP-1α. This effect was not due to reduced protein induction but resulted from heightened intracellular accumulation of MIP-1α, with reduced release. These results demonstrate that GrzM is a critical mediator of innate immunity that can regulate chemotactic networks and has an important role in the initiation of immune responses and pathogen control.

Intrahepatic natural killer cell activation, but not function, is associated with HBsAg levels in patients with HBeAg-negative chronic hepatitis B

BACKGROUND & AIMS

Natural killer (NK) cells play an important role in the immune response to viruses. As the hepatitis B virus (HBV) replicates in hepatocytes, examination of the liver of chronic hepatitis B (CHB) patients is crucial to better understand the role of NK cells in HBV. HBeAg-negative CHB differs in many aspects from HBeAg-positive CHB, and until now little is known about the intrahepatic NK cell response in HBeAg-negative patients. Intrahepatic immune control might be different in HBeAg-negative as compared with HBeAg-positive patients.

METHODS

Liver NK cells were investigated in 21 HBeAg-positive and 35 HBeAg-negative CHB patients. Biopsy specimens were processed for routine histopathology and staging according to Ishak scores. Intrahepatic and blood NK cell frequencies, activation status and function of NK cells were analysed by flow cytometry.

RESULTS

In HBeAg-negative CHB patients, compared to blood, liver NK cells displayed a more activated phenotype and stimulation further increased the activation status, but production of IFN-γ was markedly less. There was no difference with HBeAg-positive CHB. Only in HBeAg-negative CHB, but not in HBeAg-positive CHB, NK cell activation was inversely correlated with HBsAg levels.

CONCLUSIONS

The present study indicates that liver NK cells of CHB have a higher activation status compared to blood. However, they are not capable to increase cytokine production above levels reached by activated blood NK cells. In HBeAg-negative CHB, the levels of HBsAg may contribute to the incapacity of activated liver NK cells to increase cytokine production.

Helicobacter pylori-driven modulation of NK cell expansion, intracellular cytokine expression and cytotoxic activity

During Helicobacter pylori (Hp) infections, innate immune cells may be positively or negatively modulated by Hp compounds or by Hp-induced cytokines. We have shown previously that the natural cytotoxic activity of PBMC was lower in Hp-infected [Hp(+)] than Hp-uninfected individuals [Hp(-)]. Here, we asked whether the Hp-modulated cytotoxic amplitude is associated with changes in the number of NK cells, their activation or intracellular cytokine expression. Flow cytometry immunophenotyping of PBMC was performed with regard to the surface receptors CD3, CD56 and CD25, and intracellular cytokine expression of IL-2, IFN-γ and IL-10 after in vitro stimulation with Hp glycine acid extract (GE), Hp LPS or standard Escherichia coli LPS. Hp GE-driven enhancement of lymphocyte cytotoxic activity was associated with the expansion of CD3(-)CD56(+)CD25(+) NK cells and the up-regulation of IFN-γ and/or IL-2 synthesis, up to the higher level in Hp(-) than in Hp(+), while Hp LPS-mediated decrease in lymphocyte cytotoxicity was accompanied by the lack of CD3(-)CD56(+)CD25(+) NK propagation, the inhibition of pro-inflammatory cytokine expression and intense expansion of IL-10-producing NK cells. Thus, the cytotoxic and cytokine activities of NK cells were dependent on the type of antigenic challenge and the Hp status, that is, NK cells could be modulated positively by Hp GE Ags and negatively by Hp LPS.

Utilizing cytokines to function-enable human NK cells for the immunotherapy of cancer

Natural killer (NK) cells are innate lymphoid cells important for host defense against pathogens and mediate antitumor immunity. Cytokine receptors transduce important signals that regulate proliferation, survival, activation status, and trigger effector functions. Here, we review the roles of major cytokines that regulate human NK cell development, survival, and function, including IL-2, IL-12, IL-15, IL-18, and IL-21, and their translation to the clinic as immunotherapy agents. We highlight a recent development in NK cell biology, the identification of innate NK cell memory, and focus on cytokine-induced memory-like (CIML) NK cells that result from a brief, combined activation with IL-12, IL-15, and IL-18. This activation results in long lived NK cells that exhibit enhanced functionality when they encounter a secondary stimulation and provides a new approach to enable NK cells for enhanced responsiveness to infection and cancer. An improved understanding of the cellular and molecular aspects of cytokine-cytokine receptor signals has led to a resurgence of interest in the clinical use of cytokines that sustain and/or activate NK cell antitumor potential. In the future, such strategies will be combined with negative regulatory signal blockade and enhanced recognition to comprehensively enhance NK cells for immunotherapy.

TLR-mediated activation of NK cells and their role in bacterial/viral immune responses in mammals

Natural killer (NK) cells are important in innate immunity, first described as guardians for the detection and clearance of transformed or virus-infected cells. Later, this cell type was revealed to be also able to recognize and respond to bacteria-infected cells. NK cells possess receptors allowing them to sense and respond to viral and bacterial patterns, including Toll-like receptors (TLRs). Initially described in other innate immune cells, particularly monocytes/macrophages, TLRs have more recently been characterized in NK cells. Controversies remain regarding the TLR expression in NK cells and their responsiveness to agonists, specifically the requirement for the presence of accessory cells, such as dendritic cells, or of accessory cytokines (IL-2, IL-12, IL-15 and IL-18) to respond to TLR agonists. Upon TLR activation, NK cells are an important source of IFN-γ and granulocyte macrophage colony-stimulating factor, cytokines necessary to fight infection but that can also contribute to deleterious inflammation if produced in excessive amounts. Here, we review the current knowledge concerning the expression of TLRs in and on NK cells and the responsiveness to their agonists and review the literature on the role of NK cells in the sensing of bacterial or viral patterns and in combatting infection.

Modeling leukocyte-leukocyte non-contact interactions in a lymph node

The interaction among leukocytes is at the basis of the innate and adaptive immune-response and it is largely ascribed to direct cell-cell contacts. However, the exchange of a number of chemical stimuli (chemokines) allows also non-contact interaction during the immunological response. We want here to evaluate the extent of the effect of the non-contact interactions on the observed leukocyte-leukocyte kinematics and their interaction duration. To this aim we adopt a simplified mean field description inspired by the Keller-Segel chemotaxis model, of which we report an analytical solution suited for slowly varying sources of chemokines. Since our focus is on the non-contact interactions, leukocyte-leukocyte contact interactions are simulated only by means of a space dependent friction coefficient of the cells. The analytical solution of the Keller-Segel model is then taken as the basis of numerical simulations of interactions between leukocytes and their duration. The mean field interaction force that we derive has a time-space separable form and depends on the chemotaxis sensitivity parameter as well as on the chemokines diffusion coefficient and their degradation rate. All these parameters affect the distribution of the interaction durations. We draw a successful qualitative comparison between simulated data and sets of experimental data for DC-NK cells interaction duration and other kinematic parameters. Remarkably, the predicted percentage of the leukocyte-leukocyte interactions falls in the experimental range and depends (≅25% increase) upon the chemotactic parameter indicating a non-negligible direct effect of the non-contact interaction on the leukocyte interactions.

Coagulation, protease-activated receptors, and viral myocarditis

The coagulation protease cascade plays an essential role in hemostasis. In addition, a clot contributes to host defense by limiting the spread of pathogens. Coagulation proteases induce intracellular signaling by cleavage of cell surface receptors called protease-activated receptors (PARs). These receptors allow cells to sense changes in the extracellular environment, such as infection. Viruses activate the coagulation cascade by inducing tissue factor expression and by disrupting the endothelium. Virus infection of the heart can cause myocarditis, cardiac remodeling, and heart failure. A recent study using a mouse model have shown that tissue factor, thrombin, and PAR-1 signaling all positively regulate the innate immune during viral myocarditis. In contrast, PAR-2 signaling was found to inhibit interferon-β expression and the innate immune response. These observations suggest that anticoagulants may impair the innate immune response to viral infection and that inhibition of PAR-2 may be a new strategy to reduce viral myocarditis.

Myeloid-derived suppressor cells confer tumor-suppressive functions on natural killer cells via polyinosinic:polycytidylic acid treatment in mouse tumor models

Polyinosinic:polycytidylic acid (poly I:C), a synthetic double-stranded RNA, acts on myeloid cells and induces potent antitumor immune responses including natural killer (NK) cell activation. Myeloid-derived suppressor cells (MDSCs) systemically exist in tumor-bearing hosts and have strong immunosuppressive activity against antitumor effector cells, thereby dampening the efficacy of cancer immunotherapy. Here we tested what happened in MDSCs in poly I:C-treated mice. NK-sensitive syngenic tumor (B16)-bearing C57BL/6 mice were employed for this study. Intraperitoneal poly I:C treatment induced MDSC activation, driving CD69 expression and interferon (IFN)-γ production in NK cells. IFN-γ directly inhibited proliferation of B16 cells. This NK cell priming led to growth retardation of B16 tumors, although no direct tumoricidal activity was induced in NK cells. Mechanistic analysis using KO mice and function-blocking monclonal antibody revealed that MDSCs produced IFN-α via the mitochondrial antiviral signaling protein (MAVS) pathway after in vivo administration of poly I:C, and activated NK cells through the IFNAR pathway. MDSC-mediated NK cell priming was reconstituted by IFN-α in a coculture system. Either the MAVS or IFNAR signaling pathway was required for activation of MDSCs that led to growth retardation of B16 tumor in vivo. The results infer that MDSC is a target of poly I:C to prime NK cells, which exert antitumor activity to NK-sensitive tumor cells.

Roles of the protein tyrosine phosphatase PTPN22 in immunity and autoimmunity

PTPN22 is a protein tyrosine phosphatase expressed by the majority of cells belonging to the innate and adaptive immune systems. Polymorphisms in PTPN22 are associated with several autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis and type 1 diabetes. This review discusses the role of PTPN22 in T and B cells, and its function in innate immune cells, such as monocytes, dendritic cells and NK cells. We focus particularly on the complexity that underlies the function of PTPN22 in the biological processes of the immune system; such complexity has led various research groups to produce rather conflicting data.

IFN-γ production by human natural killer cells in response to HCV-infected hepatoma cells is dependent on accessory cells

BACKGROUND & AIMS

Interferon-γ (IFN-γ), a cytokine produced by activated natural killer cells (NK) and T lymphocytes, is an important regulator of innate and adaptive immunity during hepatitis C virus (HCV) infection. However, the cellular sources and mechanisms of IFN-γ induction in HCV-infection are not fully understood.

METHODS

We cultured normal human peripheral blood mononuclear cells (PBMCs) with different populations of immune cells and JFH-1 HCV-infected HuH7.5 (JFH-1/HuH7.5) cells.

RESULTS

We found that PBMCs produced large amounts of IFN-γ after co-culture with JFH-1/HuH7.5 cells. Using intracellular cytokine staining, we confirmed that NK cells and NKT cells (to a lesser extent) were the major IFN-γ producers within PBMCs. Purified NK/NKT cells did not produce IFN-γ in response to JFH-1/HuH7.5 cells and depletion of accessory (HLA-DR(+)) cells prevented IFN-γ induction in PBMCs. Through selective cell depletion of dendritic cells or monocytes from PBMCs, we determined that plasmacytoid dendritic cells (pDCs) were indispensable for NK-IFN-γ induction and the presence of monocytes was needed for maximal NK-IFN-γ induction. We further revealed that NK-IFN-γ induction depended on pDC-derived IFN-α while other IFN-γ inducing cytokines, IL-12, and IL-18, played minimal roles. Close contact between JFH-1/HuH7.5 cells and NK cells was required for IFN-γ production and monocyte-derived IL-15 significantly augmented IFN-γ induction.

CONCLUSIONS

We discovered a novel mechanism where NK cells interact with pDCs and monocytes, efficiently producing IFN-γ in response to HCV-infected cells. This indicates that co-operation between NK cells and accessory cells is critical for IFN-γ production and regulation of immunity during HCV infection.

Bench-to-bedside review: Natural killer cells in sepsis - guilty or not guilty?

Bacterial sepsis and septic shock are complex inflammatory disorders associated with a systemic inflammatory response syndrome. In the most severe cases of infection, an overzealous release of pro-inflammatory cytokines and inflammatory mediators by activated leukocytes, epithelial cells and endothelial cells, known as a 'cytokine storm', leads to deleterious effects such as organ dysfunction and even death. By the end of the 20th century, natural killer (NK) cells were for the first time identified as important players during sepsis. The role of this cell type was, however, double-edged, either 'angel' or 'devil' depending upon the bacterial infection model under study. Bacterial sensors (such as Toll-like receptors) have recently been shown to be expressed at the protein level in these cells. In addition, NK cells are important sources of interferon-γ and granulocyte-macrophage colony-stimulating factor, which are pro-inflammatory cytokines necessary to fight infection but can contribute to deleterious inflammation as well. Interestingly, an adaptative response occurs aimed to silence them, similar to the well-known phenomenon of endotoxin reprogramming.

TLR3 impairment in human newborns

Newborns are highly susceptible to viral infections. We hypothesized that this susceptibility could be due to a dysregulated expression of innate virus-sensing receptors, i.e., TLR3, TLR7, TLR8, and TLR9 and the cytosolic receptors retinoic acid-inducible gene I, melanoma differentiation-associated protein 5, protein kinase R, and IFN-γ-inducible protein 16. Cord blood mononuclear cells (CBMCs) expressed mRNA for all these receptors except for TLR3. In peripheral blood mononuclear cells (PBMCs), TLR3 mRNA was preferentially expressed in cytotoxic cells, particularly CD56(dim) NK cells. Cord NK cells in contrast showed low TLR3 mRNA expression and lacked TLR3 protein expression. Cord NK cells did not produce IFN-γ in response to polyinosinic-polycytidylic acid [poly(I:C)], whereas strong IFN-γ production was observed in poly(I:C)-stimulated adult NK cells. Cord NK cells had poor cytotoxic function that was only marginally enhanced by exposure to the TLR3 ligand poly(I:C). Opposite to NK cells from adults, their cytotoxicity was not improved by herpes simplex virus (HSV) exposure and they were unable to kill HSV-infected cells. There were no differences in the TLR3 mRNA levels among men, women, and pregnant women, implying that TLR3 is not under sex hormone control. However, decidual NK cells expressed low levels of TLR3 mRNA, which was attributed to their CD56(bright) phenotype. Our data show that cord blood NK cells have deficient TLR3 expression associated with an inability to respond to poly(I:C) and HSV activation and to kill HSV-infected cells. This might explain why newborns are particularly sensitive to neonatal HSV infections.

Endoplasmic reticulum aminopeptidase-1 functions regulate key aspects of the innate immune response

Endoplasmic reticulum aminopeptidase-1 (ERAP1) is a multifunctional, ubiquitously expressed enzyme whose peptide-trimming role during antigen processing for presentation by MHC I molecules is well established, however, a role for ERAP1 in modulating global innate immune responses has not been described to date. Here we demonstrate that, relative to wild type mice, mice lacking ERAP1 exhibit exaggerated innate immune responses early during pathogen recognition, as characterized by increased activation of splenic and hepatic NK and NKT cells and enhanced production of pro-inflammatory cytokines such as IL12 and MCP1. Our data also revealed that ERAP1 is playing a critical role in NK cell development and function. We observed higher frequencies of terminally matured NK cells, as well as higher frequencies of licensed NK cells (expressing the Ly49C and Ly49I receptors) in ERAP1-KO mice, results that positively correlated with an enhanced NK activation and IFNγ production by ERAP1-KO mice challenged with pro-inflammatory stimuli. Furthermore, during pathogen recognition, ERAP1 regulates IL12 production by CD11c(+) DCs specifically, with increases in IL12 production positively correlated with an increased phagocytic activity of splenic DCs and macrophages. Collectively, our results demonstrate a previously unrecognized, more central role for the ERAP1 protein in modulating several aspects of both the development of the innate immune system, and its responses during the initial stages of pathogen recognition. Such a role may explain why ERAP1 has been implicated by GWAS in the pathogenesis of autoimmune diseases that may be precipitated by aberrant responses to pathogen encounters.

Role of CD14 in host protection against infections and in metabolism regulation

CD14 is a glycosylphosphatidylinositol (GPI)-anchored receptor known to serve as a co-receptor for several Toll-like Receptors (TLRs) both at the cell surface and in the endosomal compartment. CD14 can be expressed by cells of both hematopoietic and non-hematopoietic origin as a cell membrane or secreted protein. Although CD14 was discovered more than 20 years ago, its activities remain largely to be defined. Most of the information available concerns CD14's role as a co-receptor working with TLR4 and facilitating cellular responses to low doses of lipopolysaccharide (LPS). Recent studies have highlighted and molecularly defined many other functions of this pattern recognition receptor (PRR). These functions include the mechanisms through which CD14 allows the activation of the TLR4-TRAM-TRIF pathway upon LPS stimulation; the capacity of CD14 to transduce a TLR4-independent signaling pathway leading to the activation of NFAT transcription factor family members with important consequences in myeloid cells; the CD14 influence on cell metabolism in conditions predisposing to obesity. In this review, we summarize recent progresses toward the molecular definition of the multiple roles exerted by CD14 in innate immune cells in response to LPS and the consequences of CD14 activation in physiologic and pathologic conditions.

Endothelial cells potentiate interferon-γ production in a novel tripartite culture model of human cerebral malaria

We have established a novel in vitro co-culture system of human brain endothelial cells (HBEC), Plasmodium falciparum parasitised red blood cells (iRBC) and peripheral blood mononuclear cells (PBMC), in order to simulate the chief pathophysiological lesion in cerebral malaria (CM). This approach has revealed a previously unsuspected pro-inflammatory role of the endothelial cell through potentiating the production of interferon (IFN)-γ by PBMC and concurrent reduction of interleukin (IL)-10. The IFN-γ increased the expression of CXCL10 and intercellular adhesion molecule (ICAM)-1, both of which have been shown to be crucial in the pathogenesis of CM. There was a shift in the ratio of IL-10:IFN-γ protein from >1 to <1 in the presence of HBEC, associated with the pro-inflammatory process in this model. For this to occur, a direct contact between PBMC and HBEC, but not PBMC and iRBC, was necessary. These results support HBEC playing an active role in the pathogenesis of CM. Thus, if these findings reflect the pathogenesis of CM, inhibition of HBEC and PBMC interactions might reduce the occurrence, or improve the prognosis, of the condition.

Dynamics of NK cell interactions in vivo

Natural killer (NK) cells are innate lymphocytes endowed with the capacity to survey and eliminate infected and transformed cells. Like T cells and B cells, NK cells fulfill their task by responding to soluble factors and to signals exchanged during cell-cell contacts. In this respect, cellular interactions established by NK cells are critical at every stage of their life. They regulate their survival and tune their responsiveness in the periphery. They are also important during their activation and for the targeted delivery of their cytotoxic granules. With the help of two-photon imaging, the occurrence and dynamics of these interactions as they occur in vivo are being uncovered. Interestingly, several of these initial observations were not predicted by in vitro studies and revealed the transient nature of many NK cell interactions. We review here our recent understanding of NK cell interactions in vivo at steady state, during inflammation, and during tumor elimination, emphasizing the similarities and differences with T cells.

Monocytes play an IL-12-dependent crucial role in driving cord blood NK cells to produce IFN-g in response to Trypanosoma cruzi

We previously reported that foetuses congenitally infected with Trypanosoma cruzi, the agent of Chagas disease, mount an adult-like parasite-specific CD8⁺ T-cell response, producing IFN-g, and present an altered NK cell phenotype, possibly reflecting a post-activation state supported by the ability of the parasite to trigger IFN-g synthesis by NK cells in vitro. We here extended our knowledge on NK cell activation by the parasite. We compared the ability of T. cruzi to activate cord blood and adult NK cells from healthy individuals. Twenty-four hours co-culture of cord blood mononuclear cells with T. cruzi trypomastigotes and IL-15 induced high accumulation of IFN-g transcripts and IFN-g release. TNF-a, but not IL-10, was also produced. This was associated with up-regulation of CD69 and CD54, and down-regulation of CD62L on NK cells. The CD56^bright NK cell subset was the major IFN-g responding subset (up to 70% IFN-g-positive cells), while CD56^dim NK cells produced IFN-g to a lesser extent. The response points to a synergy between parasites and IL-15. The neonatal response, observed in all newborns, remained however slightly inferior to that of adults. Activation of IL-15-sensitized cord blood NK cells by the parasite required contacts with live/intact parasites. In addition, it depended on the engagement of TLR-2 and 4 and involved IL-12 and cross-talk with monocytes but not with myeloid dendritic cells, as shown by the use of neutralizing antibodies and cell depletion. This work highlights the ability of T. cruzi to trigger a robust IFN-g response by IL-15-sensitized human neonatal NK cells and the important role of monocytes in it, which might perhaps partially compensate for the neonatal defects of DCs. It suggests that monocyte- and IL-12- dependent IFN-g release by NK cells is a potentially important innate immune response pathway allowing T. cruzi to favour a type 1 immune response in neonates.

IFN-g release by NK cells is essential in early control of infections with intracellular pathogens by driving protective type 1 immune response. NK cell activation requires integration of signals delivered by cytokines, dendritic cells, monocytes/macrophages and/or pathogens. Little information is available about this topic in neonates, known to be deficient in mounting type 1 immune response. We show that Trypanosoma cruzi, the protozoa agent of Chagas disease, rapidly and strongly up-regulates the production of IFN-g by IL-15-primed cord blood NK cells to a level close to that produced by adult NK cells. This neonatal NK cell response was dependent on cross-talk with monocytes and engagement of TLR2 and TLR4 by the parasite. Importantly, IL-12 synthesis by monocytes, but not by dendritic cells, was central in driving NK cell IFN-g release. This study suggests that monocytes may compensate for the known defects of neonatal DCs to produce IL-12. This innate pathway may allow a pathogen to circumvent the defect to mount type 1 immune response in early life. This observation may be relevant in vivo in T. cruzi congenital infection, since such newborns have previously been shown to mount an adult like type 1 immune response.

Immunophenotype of peripheral blood natural killer cells and IL-10 serum levels in relation to Helicobacter pylori status

Recent findings suggest that NK (Natural Killer) cells may directly modulate the antimicrobial immune responses. In this study, we performed immunophenotypic analysis of peripheral blood NK cells with regard to CD56, CD16, Nkp46, and CD25 markers, as well as IL-10 levels quantification in the sera samples of asymptomatic, H. pylori (Hp)-infected or uninfected individuals, and combined these results with our previous findings on lymphocyte cytotoxic activity. Twenty healthy volunteers [10 Hp(-);10 Hp(+)] were included in the study. The percentages of classic lymphocytes (CD3(+) ) and NK cells (CD3(-) CD56(+) , CD3(-) Nkp46(+) , CD3(-) CD16(+) ) with or without CD25 receptor were evaluated by fluorochrome-conjugated monoclonal antibody staining and flow cytometry analysis. IL-10 quantification was performed by enzyme-linked immunosorbent assay-ELISA. Our study showed elevated levels of IL-10 and higher NK cell numbers of both CD3(-) CD56(+) CD25(+) and CD3(-) Nkp46(+) CD25(+) phenotypes, as well as CD3(+) CD25(+) classic lymphocytes in Hp(+) compared with Hp(-) individuals. No differences between Hp(-) and Hp(+) individuals were found either in total number of classic lymphocytes or NK cell subtypes. Our data suggest that in Hp(+) donors, there is a domination of lymphocytes and NK cells co-expressing CD25 marker, which might be influenced by the regulatory IL-10. This phenomenon may be a result of H. pylori adaptation to a changing environment in vivo leading to a chronic infection and lack of severe gastric pathologies.

Both NK cell-intrinsic and -extrinsic STAT1 signaling are required for NK cell response against vaccinia virus

NK cells play an important role in innate immune control of the infection with vaccinia virus (VV). However, it remains incompletely defined how the activation of NK cells in response to VV is regulated. In this study, we showed that STAT1 was critical for NK cell activation upon VV infection and the subsequent clearance of VV infection in vivo. We further demonstrated that STAT1 signaling in both NK and accessory cells such as dendritic cells was required for efficient NK cell activation upon VV infection. Mechanistically, STAT1 signaling in dendritic cells promoted the expression of NKG2D ligands, which is required for NK cell activation via the NKG2D pathway. Taken together, our data suggest that STAT1 mediates anti-VV effect by promoting NK cell activation through both NK-intrinsic and extrinsic mechanisms and may provide insights into the design of effective NK cell-based therapies for viral infections.

IL-18 provided in dying bacterial-infected macrophages induces IFN-γ production in functional T-cell hybridoma B6HO3 through cell conjugates

We have previously reported that the co-culture of functional T-cell hybridoma B6HO3 with dying J774 macrophage cells infected with Listeria monocytogenes (LM) results in the production of IFN-γ by B6HO3 cells. Here, we explore the mechanism underlying this phenomenon. We found that IFN-γ production was dependent on IL-18, but that the dying LM-infected macrophages produced no more than 100 pg/ml of IL-18, much less than the amount of IL-18 required for stimulating B6HO3 cells to produce IFN-γ. Furthermore, IL-18 binding protein added to the co-culture was unable to easily gain access to IL-18 for neutralisation. B6HO3 cells formed cell conjugates with J774 macrophages, and IFN-γ-producing B6HO3 cells were spatially and temporally associated with LM-infected macrophage cell death that exhibited neither pyroptosis nor pyronecrosis. These results suggest that the IL-18 produced by dying LM-infected macrophages is released to the interface of the cell conjugates, thereby inducing B6HO3 cells to produce IFN-γ. Based on the present and also previous findings, we propose that IL-18 released from macrophages because of cell death caused by bacteria may be the primary cytokine that triggers the innate IFN-γ production that is required for activating the bactericidal functions of macrophages at early stages of bacterial infection.

Yersinia enterocolitica inactivates NK cells

Natural Killer (NK) cells serve as an important source of proinflammatory cytokines early during infection. Hypothesizing that Yersinia enterocolitica might interact with and inactivate NK cells, we examined NK cell-Y. enterocolitica interactions in vitro and in vivo. Y. enterocolitica adheres to NK cells in an Invasin dependent manner and inhibits NK cell cytotoxicity and IFN-γ production induced by IL-12+IL-18 or IL-12 alone. YopP, an acetyltransferase known to inhibit MAPK and NFκB signaling, suppresses IL-12 and IL-12+IL-18 mediated IFN-γ production in NK cells by inhibiting phosphorylation of Tyk2 and STAT4 in addition to MAPK. YopP inhibits induction of all genes whose expression is induced by IL-12+IL-18 in NK cells. Y. enterocolitica-mediated adherence to and inactivation of NK cells also occurs after infection in vivo. Thus, we present the first report of a bacterial pathogen inactivating NK cells, and report interaction with Tyk2-STAT4 signaling as a novel function of YopP.

Induction of B7-H6, a ligand for the natural killer cell-activating receptor NKp30, in inflammatory conditions

B7-H6, a member of the B7 family of immunoreceptors, is as a cell-surface ligand for the NKp30-activating receptor expressed on natural killer cells. B7-H6 is not detected in normal human tissues at steady state but is expressed on tumor cells. However, whether B7-H6 can be expressed in other conditions remains unknown. We analyzed here the pathways that lead to the expression of B7-H6 in nontransformed cells. In vitro, B7-H6 was induced at the surface of CD14(+)CD16(+) proinflammatory monocytes and neutrophils upon stimulation by ligands of Toll-like receptors or proinflammatory cytokines such as interleukin-1β and tumor necrosis factor α. In these conditions, a soluble form of B7-H6 (sB7-H6) was also produced by activated monocytes and neutrophils. In vivo, B7-H6 was expressed on circulating proinflammatory CD14(+)CD16(+) monocytes in a group of patients in sepsis conditions, and was linked to an increased mortality. sB7-H6 was selectively detected in the sera of patients with gram-negative sepsis and was associated with membrane vesicles that co-sedimented with the exosomal fraction. These findings reveal that B7-H6 is not only implicated in tumor immunosurveillance but also participates in the inflammatory response in infectious conditions.

Probiotics in the management of lung diseases

The physiology and pathology of the respiratory and gastrointestinal tracts are closely related. This similarity between the two organs may underlie why dysfunction in one organ may induce illness in the other. For example, smoking is a major risk factor for COPD and IBD and increases the risk of developing Crohn's disease. Probiotics have been defined as "live microorganisms which, when administered in adequate amounts, confer health benefits on the host." In model systems probiotics regulate innate and inflammatory immune responses. Commonly used probiotics include lactic acid bacteria, particularly Lactobacillus, Bifidobacterium, and Saccharomyces, and these are often used as dietary supplements to provide a health benefit in gastrointestinal diseases including infections, inflammatory bowel disease, and colon cancer. In this respect, probiotics probably act as immunomodulatory agents and activators of host defence pathways which suggest that they could influence disease severity and incidence at sites distal to the gut. There is increasing evidence that orally delivered probiotics are able to regulate immune responses in the respiratory system. This review provides an overview of the possible role of probiotics and their mechanisms of action in the prevention and treatment of respiratory diseases.

Exacerbated type II interferon response drives hypervirulence and toxic shock by an emergent epidemic strain of Streptococcus suis

Streptococcus suis, a major porcine pathogen, can be transmitted to humans and cause severe symptoms. A large human outbreak associated with an unusual streptococcal toxic shock-like syndrome (STSLS) was described in China. Albeit an early burst of proinflammatory cytokines following Chinese S. suis infection was suggested to be responsible for STSLS case severity, the mechanisms involved are still poorly understood. Using a mouse model, the host response to S. suis infection with a North American intermediately pathogenic strain, a European highly pathogenic strain, and the Chinese epidemic strain was investigated by a whole-genome microarray approach. Proinflammatory genes were expressed at higher levels in mice infected with the Chinese strain than those infected with the European strain. The Chinese strain induced a fast and strong gamma interferon (IFN-γ) response by natural killer (NK) cells. In fact, IFN-γ-knockout mice infected with the Chinese strain showed significantly better survival than wild-type mice. Conversely, infection with the less virulent North American strain resulted in an IFN-β-subjugated, low inflammatory response that might be beneficial for the host to clear the infection. Overall, our data suggest that a highly virulent epidemic strain has evolved to massively activate IFN-γ production, mainly by NK cells, leading to a rapid and lethal STSLS.

Monocyte/macrophage-elicited natural killer cell dysfunction in hepatocellular carcinoma is mediated by CD48/2B4 interactions

Defects in natural killer (NK) cell functions are necessary for tumor immune escape, but their underlying regulatory mechanisms in human cancers remain largely unknown. Here we show, in detailed studies of NK cells in 294 untreated patients with hepatocellular carcinoma (HCC), that accumulation of functional NK cells in HCC tissues could predict improved survival of patients. However, in patients with advanced-stage HCC, NK cells were significantly decreased in number with impaired tumor necrosis factor alpha (TNF-α) and interferon-gamma (IFN-γ) production. High infiltration of peritumoral stroma monocytes/macrophages was positively correlated with impaired functional activities of NK cells in intratumoral areas. Further kinetic experiments revealed that soon after exposure to tumor-derived monocytes, NK cells underwent a rapid, transient activation, but then they became exhausted, and eventually died. The monocytes from HCC tissues, but not from nontumoral liver, strongly express CD48 proteins; and such monocyte-induced NK cell dysfunction was markedly attenuated by blocking CD48 receptor 2B4 on NK cells, but not by blockade of NKG2D and NKp30.

CONCLUSION

These data reveal that human NK cells are regulated by a fine-tuned collaborative action between different types of immune cells, which may reflect a novel immune-escape mechanism by which tumors dynamically regulate their functions at distinct tumor microenvironments.

The top three areas of basic research on Aspergillus fumigatus in 2011

Over 450 peer-reviewed papers containing the keyword Aspergillus fumigatus were published in 2011. Although this method may be an impossible task, I have selected three clusters of papers describing exciting recent advances in research on A. fumigatus. The first is the novel approach of in vivo imagining of experimental aspergillosis by the use of (68) Ga-labeled siderophores, internalized by the fungus, and detected via positron emission tomography to image the site infection. This work may lead to improved diagnosis of aspergillosis. The second important finding is that NK lymphocytes, not thought to be involved in host resistance to aspergillosis, can kill aspergilli through direct contact, either through perforin or interferon-γ, or both. The third area pertains to a novel first-in-class antifungal drug, E1210 (Eisai), which inhibits GPI anchoring of fungal-associated cell wall proteins. Thus far, it shows promising in vitro activity against a broad range of fungi including Aspergilli, as well as those that are difficult to treat with currently available therapies. Overall, these three areas demonstrate the exciting promise, progress, and utility of basic research against A. fumigatus.

Consequences of the crosstalk between monocytes/macrophages and natural killer cells

The interaction between natural killer (NK) cells and different other immune cells like T cells and dendritic cells is well-described, but the crosstalk with monocytes or macrophages and the nature of ligands/receptors implicated are just emerging. The macrophage-NK interaction is a major first-line defense against pathogens (bacteria, viruses, fungi, and parasites). The recruitment and the activation of NK cells to perform cytotoxicity or produce cytokines at the sites of inflammation are important to fight infections. The two main mechanisms by which macrophages can prime NK cells are (1) activation through soluble mediators such as IL-12, IL-18, and (2) stimulation through direct cell-to-cell contact. We will discuss the progress in matters of modulation of NK cell functions by monocytes and macrophages, in the steady state and during diseases.

Activation of human NK cells by Plasmodium-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-γ. Procedures are described for in vitro propagation and purification of Plasmodium falciparum parasites, separation of peripheral blood mononuclear cells from heparinized 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.

Subcapsular sinus macrophages promote NK cell accumulation and activation in response to lymph-borne viral particles

Natural killer (NK) cells become activated during viral infection in response to cytokines or to engagement of NK cell activating receptors. However, the identity of cells sensing viral particles and mediating NK cell activation has not been defined. Here, we show that local administration of a modified vaccinia virus Ankara vaccine in mice results in the accumulation of NK cells in the subcapsular area of the draining lymph node and their activation, a process that is strictly dependent on type I IFN signaling. NK cells located in the subcapsular area exhibited reduced motility and were found associated with CD169+-positive subcapsular sinus (SCS) macrophages and collagen fibers. Moreover, depletion of SCS macrophages using clodronate liposomes abolished NK cell accumulation and activation. Our results identify SCS macrophages as primary mediators of NK cell activation in response to lymph-borne viral particles suggesting that they act as early sensors of local infection or delivery of viral-based vaccines.

IL-15 and macrophage secretory factors facilitate immune activation of neonatal natural killer cells by lipoteichoic acid

Neonates possess a relatively "naive", yet inducible immune system. Our hypothesis is that upon strategic antigen exposure, cytokine priming and sensitization by accessory cells, natural killer (NK) cells could be activated to become a functional phenotype. We investigated the in vitro stimulation of cord blood (CB) and adult NK cells upon challenge with lipoteichoic acid (LTA), interleukin (IL)-15 and LTA-primed autologous macrophage-conditioned medium, using CD107a and CD69 phenotypes as indicators of activation. We also examined response of CB macrophages to LTA, in terms of P44/42 extracellular signal-regulated kinases (ERK1/2) activation and cytokine secretion. LTA significantly induced secretion of inflammatory cytokines tumor necrotic factor (TNF)-α, IL-6, IL-12 and activated the upstream signal of ERK1/2 phosphorylation in neonatal macrophages. The magnitude of responses to stimulation differed between neonatal and adult NK cells. Co-stimulation with IL-15 was critical for expansion of the CD69 and CD107a NK subpopulations in both neonatal and adult cells, upon a LTA challenge. NK cell activation could be enhanced by LTA-primed autologous macrophages through secretory factors. Our results indicated that neonatal macrophages and NK cells can evoke immunologic responses to a Gram-positive bacterial antigen. The combinatory priming strategy is relevant for development of novel protocols, such as IL-15 treatment, to compensate for the immaturity of the innate immune system in newborns against bacterial infections.

Direct interaction studies between Aspergillus fumigatus and human immune cells; what have we learned about pathogenicity and host immunity?

Invasive aspergillosis is a significant threat to health and is a major cause of mortality in immunocompromised individuals. Understanding the interaction between the fungus and the immune system is important in determining how the immunocompetent host remains disease free. Several studies examining the direct interaction between Aspergillus fumigatus and purified innate immune cells have been conducted to measure the responses of both the host cells and the pathogen. It has been revealed that innate immune cells have different modes of action ranging from effective fungal killing by neutrophils to the less aggressive response of dendritic cells. Natural killer cells do not phagocytose the fungus unlike the other innate immune cells mentioned but appear to mediate their antifungal effect through the release of gamma interferon. Transcriptional analysis of A. fumigatus interacting with these cells has indicated that it can adapt to the harsh microenvironment of the phagosome and produces toxins, ribotoxin and gliotoxin, that can induce cell death in the majority of innate immune cells. These data point toward potential novel antifungal treatments including the use of innate immune cells as antifungal vaccines.

Naïve rat NK cells control the onset of T cell response

NK cell function in the rat is only defined in a rudimentary way due to missing tools for clear NK cell identification. The present study introduces the congenic LEW.BH-NKC rat strain which allows distinct detection of rat NK cells using commercial antibodies. LEW.BH-NKC rats were exposed in vivo to the porcine B cell line L23 by subcutaneous transfer of L23 cell suspension. We used Luciferase transgeneic L23 cells to follow the course of rejection by living imaging. L23 cells were rejected within five days after placement under the skin thus the rejection is mediated by innate immune responses in the first place. Indeed we found increased percentages of NK cells in the blood, spleen and in draining lymph nodes using flow cytometry methods. Surprisingly, we found as a consequence a decrease in proliferative T cell response in the draining lymph nodes. We identified NK cells as mediators of this regulation by in vitro performed mixed lymphocyte reactions. The remarkable feature was the naive state of NK cells exhibiting the regulative capacity. Furthermore, the regulation was not exclusively mediated by IL-10 as it has been reported before for influence of T cell response by activated NK cells but predominantly by TGF-β. Interestingly, after initiation of the adaptive immune response, NK cells failed to take influence on the proliferation of T cells. We conclude that naive NK cells build up a threshold of activation impulse that T cells have to overcome.