Intracellular staining for analysis of the expression and phosphorylation of signal transducers and activators of transcription (STATs) in NK cells

PP2A negatively regulates NK cell T-bet expression and anti-tumor effector function

The transcription factor T-bet is essential for the anti-tumor effector function of natural killer (NK) cells, but the mechanism regulating its expression in NK cells remains unclear. In this study, we aimed to identify an NK cell-intrinsic regulator that controls T-bet expression. Using T-bet-luciferase reporter assay screening, we identified a protein phosphatase inhibitor as a potential activator of T-bet expression. A series of protein phosphatase 2A (PP2A)-specific inhibitors (PP2Ai) or PP2A siRNA induced the expression of T-bet. In PP2Ai-treated mice, the expression of T-bet and its downstream effector molecules, granzyme B and IFN-γ, was also upregulated in NK cells. Mechanistically, PP2Ai increased the phosphorylation of mTOR and ribosomal protein S6 in NK cells, and mTOR inhibitor canceled the effects of PP2Ai in NK cells. Importantly, NK cells isolated from PP2Ai-treated mice showed higher cytotoxicity and IFN-γ production; therefore, they increased the anti-tumor effector function of NK cells. Accordingly, PP2Ai treatment inhibited lung metastasis of B16 melanoma by NK cell- and mTOR-dependent mechanisms. These results suggest that PP2A negatively regulates NK cell T-bet expression and effector function by an mTOR-dependent mechanism.

Aberrant signaling of immune cells in Sjögren’s syndrome patient subgroups upon interferon stimulation

Background

Primary Sjögren’s syndrome (pSS) is a systemic autoimmune disease, characterized by mononuclear cell infiltrates in the salivary and lacrimal glands, leading to glandular atrophy and dryness. Patient heterogeneity and lack of knowledge regarding its pathogenesis makes pSS a difficult disease to manage.

Methods

An exploratory analysis using mass cytometry was conducted of MAPK/ERK and JAK/STAT signaling pathways in peripheral blood mononuclear cells (PBMC) from 16 female medication free pSS patients (8 anti-Sjögren’s syndrome-related antigen A negative/SSA- and 8 SSA+) and 8 female age-matched healthy donors after stimulation with interferons (IFNs).

Results

We found significant differences in the frequencies of memory B cells, CD8+ T central and effector memory cells and terminally differentiated CD4+ T cells among the healthy donors and patient subgroups. In addition, we observed an upregulation of HLA-DR and CD38 in many cell subsets in the patients. Upon IFNα2b stimulation, slightly increased signaling through pSTAT1 Y701 was observed in most cell types in pSS patients compared to controls, while phosphorylation of STAT3 Y705 and STAT5 Y694 were slightly reduced. IFNγ stimulation resulted in significantly increased pSTAT1 Y701 induction in conventional dendritic cells (cDCs) and classical and non-classical monocytes in the patients. Most of the observed differences were more prominent in the SSA+ subgroup, indicating greater disease severity in them.

Conclusions

Augmented activation status of certain cell types along with potentiated pSTAT1 Y701 signaling and reduced pSTAT3 Y705 and pSTAT5 Y694 induction may predispose pSS patients, especially the SSA+ subgroup, to upregulated expression of IFN-induced genes and production of autoantibodies. These patients may benefit from therapies targeting these pathways.

Assessing Phosphorylation of STAT Transcription Factors in Mouse Innate Lymphoid Cells

Innate lymphoid cells (ILCs) ensure protection against pathogens by quickly reacting to the alterations of the cytokine milieu taking place upon infection. More than 50 cytokines and growth factors activate the Janus kinases (JAKs), leading to phosphorylation of members of the signal transducer and activator of transcription (STAT) family. Activation of STATs induces specific transcriptional programs which are associated with distinct cellular outcomes. Thus, an efficient measurement of rapid STAT phosphorylation enables not only to dissect the spectrum of cytokine sensitivity among ILC subsets but also to pinpoint specific transcriptional programs and cellular functions initiated after activation. Using this method, we have previously dissected the downstream events of Interleukin (IL)-23 and IL-12 signaling in ILCs, shedding light on the differential usage of STATs among ILC subsets. Here, we provide an optimized and detailed protocol describing how to analyze phosphorylation of STAT transcription factors in murine NK and ILC subsets isolated from different tissues.

Sustained Type I Interferon Reinforces NK Cell-Mediated Cancer Immunosurveillance during Chronic Virus Infection

The importance of natural killer (NK) cells in the early immune response to viral or bacterial infection is well known. However, the phenotype, function, and physiologic role of NK cells during the late stage of persistent viral infection have not been extensively studied. Here, we characterized NK cells in mice persistently infected with lymphocytic choriomeningitis virus clone 13 and showed that in contrast to NK cells from acutely infected or uninfected mice, NK cells from chronically infected mice expressed a terminally differentiated phenotype, stronger cytotoxicity, and reduced inhibitory receptor expression. In an in vivo tumor model, chronically infected mice exhibited significantly delayed tumor progression in an NK cell-dependent manner. NK cells from chronically infected mice also expressed high STAT1, and blocking the type I interferon (IFN) receptor revealed that type I IFN signaling directly regulated NK cell cytotoxicity. Our findings indicate that sustained type I IFN signaling during chronic viral infection potentiates the cytolytic function of NK cells and contributes to NK cell-dependent host immune surveillance.

Aberrant cell signalling in PBMCs upon IFN-α stimulation in primary Sjögren's syndrome patients associates with type I interferon signature

Primary Sjögren's syndrome (pSS) is a complex systemic autoimmune disease with heterogeneous disease manifestations. Genetic predisposition, hormonal and environmental factors are all thought to contribute to disease etiology and pathogenesis. A better understanding of the disease pathogenesis is required in order to establish new targeted therapies. We analysed MAPK/ERK and JAK/STAT signalling networks in peripheral blood mononuclear cells (PBMCs) upon stimulation with interferon alpha 2b (IFN‐α2b) by flow cytometry to define potentially dysfunctional intracellular signalling pathways involved in disease pathogenesis. Cells derived from pSS patients displayed small but significant increases in basal phosphorylation levels of numerous signalling proteins compared to cells from healthy donors. The phosphorylation profiles following stimulation with IFNα2b differed significantly between pSS patients and healthy donors, especially regarding STAT1 Y701. PCA further grouped patients according to clinical characteristics. Type I IFN induced gene expression was found to negatively correlate with the IFN‐α2b induced phosphorylation of STAT3 S727 in T cells and positively with pSTAT1 Y701 in B cells. Increases in pSTAT1 Y701 were associated with the presence of autoantibodies. Our results indicate involvement of both STAT3 S727 and STAT1 Y701 pathways in pSS patients. Therapies targeting these pathways might therefore be beneficial for certain subgroups of patients.

Immune Cell Profiling of IFN-λ Response Shows pDCs Express Highest Level of IFN-λR1 and Are Directly Responsive via the JAK-STAT Pathway

The interferon lambda (IFN-λ) cytokines have well-known antiviral properties, yet their contribution to immune regulation is not well understood. Epithelial cells represent the major target cell of IFN-λ; peripheral blood mononuclear cells are generally considered nonresponsive, with the exception of plasmacytoid dendritic cells (pDCs). In this study we aimed to define the potential for discrete subpopulations of cells to directly respond to IFN-λ. Analysis of peripheral blood leukocytes reveals that, while pDCs uniformly express the highest levels of IFN-λ receptor, a small proportion of B cells and monocytes also express the receptor. Nevertheless, B cells and monocytes respond poorly to IFN-λ stimulation in vitro, with minimal STAT phosphorylation and interferon-stimulated gene (ISG) induction observed. We confirm that pDCs respond to IFN-λ in vitro, upregulating their expression of pSTAT1, pSTAT3, and pSTAT5. However, we found that pDCs do not upregulate pSTAT6 in response to IFN-λ treatment. Our results highlight unique aspects of the response to IFN-λ and confirm that while the IFN-λ receptor is expressed by a small proportion of several different circulating immune cell lineages, under normal conditions only pDCs respond to IFN-λ stimulation with robust STAT phosphorylation and ISG induction. The difference in STAT6 responsiveness of pDCs to type I and type III interferons may help explain the divergence in their biological activities.

The Critical Role of IL-15-PI3K-mTOR Pathway in Natural Killer Cell Effector Functions

Natural killer (NK) cells were so named for their uniqueness in killing certain tumor and virus-infected cells without prior sensitization. Their functions are modulated in vivo by several soluble immune mediators; interleukin-15 (IL-15) being the most potent among them in enabling NK cell homeostasis, maturation, and activation. During microbial infections, NK cells stimulated with IL-15 display enhanced cytokine responses. This priming effect has previously been shown with respect to increased IFN-γ production in NK cells upon IL-12 and IL-15/IL-2 co-stimulation. In this study, we explored if this effect of IL-15 priming can be extended to various other cytokines and observed enhanced NK cell responses to stimulation with IL-4, IL-21, IFN-α, and IL-2 in addition to IL-12. Notably, we also observed elevated IFN-γ production in primed NK cells upon stimulation through the Ly49H activation receptor. Currently, the fundamental processes required for priming and whether these signaling pathways work collaboratively or independently for NK cell functions are poorly understood. To identify the key signaling events for NK cell priming, we examined IL-15 effects on NK cells in which the pathways emanating from IL-15 receptor activation were blocked with specific inhibitors. Our results demonstrate that the PI3K–AKT–mTOR pathway is critical for cytokine responses in IL-15 primed NK cells. Furthermore, this pathway is also implicated in a broad range of IL-15-induced NK cell effector functions such as proliferation and cytotoxicity. Likewise, NK cells from mice treated with rapamycin to block the mTOR pathway displayed defects in proliferation, and IFN-γ and granzyme B productions resulting in elevated viral burdens upon murine cytomegalovirus infection. Taken together, our data demonstrate the requirement of PI3K–mTOR pathway for enhanced NK cell functions by IL-15, thereby coupling the metabolic sensor mTOR to NK cell anti-viral responses.

Activation mechanisms of natural killer cells during influenza virus infection

During early viral infection, activation of natural killer (NK) cells elicits the effector functions of target cell lysis and cytokine production. However, the cellular and molecular mechanisms leading to NK cell activation during viral infections are incompletely understood. In this study, using a model of acute viral infection, we investigated the mechanisms controlling cytotoxic activity and cytokine production in response to influenza (flu) virus. Analysis of cytokine receptor deficient mice demonstrated that type I interferons (IFNs), but not IL-12 or IL-18, were critical for the NK cell expression of both IFN-γ and granzyme B in response to flu infection. Further, adoptive transfer experiments revealed that NK cell activation was mediated by type I IFNs acting directly on NK cells. Analysis of signal transduction molecules showed that during flu infection, STAT1 activation in NK cells was completely dependent on direct type I IFN signaling, whereas STAT4 activation was only partially dependent. In addition, granzyme B induction in NK cells was mediated by signaling primarily through STAT1, but not STAT4, while IFN-γ production was mediated by signaling through STAT4, but not STAT1. Therefore, our findings demonstrate the importance of direct action of type I IFNs on NK cells to mount effective NK cell responses in the context of flu infection and delineate NK cell signaling pathways responsible for controlling cytotoxic activity and cytokine production.

Regulating type 1 IFN effects in CD8 T cells during viral infections: changing STAT4 and STAT1 expression for function

Type 1 IFNs can conditionally activate all of the signal transducers and activators of transcription molecules (STATs), including STAT4. The best-characterized signaling pathways use STAT1, however, and type 1 IFN inhibition of cell proliferation is STAT1 dependent. We report that type 1 IFNs can basally stimulate STAT1- and STAT4-dependent effects in CD8 T cells, but that CD8 T cells responding to infections of mice with lymphocytic choriomenigitis virus have elevated STAT4 and lower STAT1 expression with significant consequences for modifying the effects of type 1 IFN exposure. The phenotype was associated with preferential type 1 IFN activation of STAT4 compared with STAT1. Stimulation through the TCR induced elevated STAT4 expression, and STAT4 was required for peak expansion of antigen-specific CD8 T cells, low STAT1 levels, and resistance to type 1 IFN-mediated inhibition of proliferation. Thus, a mechanism is discovered for regulating the consequences of type 1 IFN exposure in CD8 T cells, with STAT4 acting as a key molecule in driving optimal antigen-specific responses and overcoming STAT1-dependent inhibition of proliferation.

Type 1 interferon induction of natural killer cell gamma interferon production for defense during lymphocytic choriomeningitis virus infection

Natural killer (NK) cells are equipped to innately produce the cytokine gamma interferon (IFN-γ) in part because they basally express high levels of the signal transducer and activator of transcription 4 (STAT4). Type 1 interferons (IFNs) have the potential to activate STAT4 and promote IFN-γ expression, but concurrent induction of elevated STAT1 negatively regulates access to the pathway. As a consequence, it has been difficult to detect type 1 IFN stimulation of NK cell IFN-γ during viral infections in the presence of STAT1 and to understand the evolutionary advantage for maintaining the pathway. The studies reported here evaluated NK cell responses following infections with lymphocytic choriomeningitis virus (LCMV) in the compartment handling the earliest events after infection, the peritoneal cavity. The production of type 1 IFNs, both IFN-α and IFN-β, was shown to be early and of short duration, peaking at 30 h after challenge. NK cell IFN-γ expression was detected with overlapping kinetics and required activating signals delivered through type 1 IFN receptors and STAT4. It took place under conditions of high STAT4 levels but preceded elevated STAT1 expression in NK cells. The IFN-γ response reduced viral burdens. Interestingly, increases in STAT1 were delayed in NK cells compared to other peritoneal exudate cell (PEC) populations. Taken together, the studies demonstrate a novel mechanism for stimulating IFN-γ production and elucidate a biological role for type 1 IFN access to STAT4 in NK cells.

Pathways regulating the complex and sometimes paradoxical effects of cytokines are poorly understood. Accumulating evidence indicates that the biological consequences of type 1 interferon (IFN) exposure are shaped by modifying the concentrations of particular STATs to change access to the different signaling molecules. The results of the experiments presented conclusively demonstrate that NK cell IFN-γ can be induced through type 1 IFN and STAT4 at the first site of infection during a period with high STAT4 but prior to induction of elevated STAT1 in the cells. The response mediates a role in viral defense. Thus, a very early pathway to and source of IFN-γ in evolving immune responses to infections are identified by this work. The information obtained helps resolve long-standing controversies and advances the understanding of mechanisms regulating key type 1 IFN functions, in different cells and compartments and at different times of infection, for accessing biologically important functions.

Differential alteration of CD56(bright) and CD56 (dim) natural killer cells in frequency, phenotype, and cytokine response in chronic hepatitis C virus infection

BACKGROUND

Natural killer (NK) cells play an important role in immune responses to virus infection. The cell population consists of CD56(bright) (bright-subset) and CD56(dim) (dim-subset) subsets that possess armed functions of cytokine production and cytolysis, respectively. How these subsets are involved in chronic hepatitis C virus infection (CHC) remains obscure.

METHODS

We investigated the frequency, phenotype, and cytokine response of these subsets in blood from CHC patients and healthy subjects (HS).

RESULTS

Dim-subset, but not bright-subset, showed lower frequency in the patients than in HS. Bright-subset from the patients more frequently expressed the NKG2A/CD94 inhibitory receptor than that from HS, while both subsets from the patients expressed lower levels of the NKG2D activating receptor. Both subsets from the patients displayed a significantly higher level of the signal transducer and activator of transcription (STAT) 1, compared with the HS. Upon stimulation with interferon-α, bright-subset activated less STAT4, required for interferon-γ production, and dim-subset activated more STAT1, required for cytolysis, in the patients than in HS.

CONCLUSIONS

These results indicate alterations of NK cell subsets in frequency, phenotype, and cytokine response in CHC, which might be associated with the immune pathogenesis of CHC.

Severe impairment of IFN-γ and IFN-α responses in cells of a patient with a novel STAT1 splicing mutation

Subjects affected by Signal Transducer and Activator of Transcription 1 (STAT1) deficiency have lethal bacterial and viral infections. Complete STAT1 deficiency is inherited as an autosomal recessive disease; partial STAT1 deficiency is inherited as an autosomal recessive or autosomal dominant trait. Here, we report a patient who developed disseminated mycobacteriosis early in life and had several viral infections, including herpetic skin infection and interstitial pneumonia by cytomegalovirus with severe respiratory distress. Molecular analysis of STAT1 showed a novel homozygous mutation affecting a splice site, leading to exon 3 skipping and to synthesis of a lower molecular weight STAT1 protein. This mutation leads to marked reduction of STAT1 phosphorylation; the electromobility shift assay showed a complete defect of DNA-binding activity, which accounts for the complete impairment of peripheral blood mononuclear cell functional response to both IFN-γ and IFN-α. Moreover, analysis of natural killer cells showed a defective STAT1 phosphorylation in response to IFN-α and impaired basal cytolytic activity, suggesting that the STAT1-dependent pathway might be important for natural killer cell function. These results suggested that exon 3 skipping of STAT1 leads to abnormal signaling in response to IFN-γ and IFN-α, which is associated with susceptibility to intracellular pathogens and viruses.

Tetherin restricts direct cell-to-cell infection of HIV-1

Background

Tetherin (BST-2/CD317/HM1.24) is an interferon (IFN)-inducible factor of the innate immune system, recently shown to exert antiviral activity against HIV-1 and other enveloped viruses by tethering nascent viral particles to the cell surface, thereby inhibiting viral release. In HIV-1 infection, the viral protein U (Vpu) counteracts this antiviral action by down-modulating tetherin from the cell surface. Viral dissemination between T-cells can occur via cell-free transmission or the more efficient direct cell-to-cell route through lipid raft-rich virological synapses, to which tetherin localizes.

Results

We established a flow cytometry-based co-culture assay to distinguish viral transfer from viral transmission and investigated the influence of tetherin on cell-to-cell spread of HIV-1. Sup-T1 cells inducible for tetherin expression were used to examine the impact of effector and target cell tetherin expression on virus transfer and transmission. Using this assay, we showed that tetherin inhibits direct cell-to-cell virus transfer and transmission. Viral Vpu promoted viral transmission from tetherin-expressing cells by down-modulating tetherin from the effector cell surface. Further, we showed that tetherin on the target cell promotes viral transfer and transmission. Viral infectivity in itself was not affected by tetherin.

Conclusion

In addition to inhibiting viral release, tetherin also inhibits direct cell-to-cell spread. Viral protein Vpu counteracts this restriction, outweighing its possible cost of fitness in cell-to-cell transmission. The differential role of tetherin in effector and target cells suggest a role for tetherin in cell-cell contacts and virological synapses.