IFN-gamma production and degranulation are differentially regulated in response to stimulation in murine natural killer cells

Age-dependent natural killer cell and interferon γ deficits contribute to severe pertussis in infant mice

Many respiratory infections are selectively injurious to infants, yet the etiology of age-associated susceptibility is unknown. One such bacterial pathogen is Bordetella pertussis. In adult mice, innate interferon γ (IFN-γ) is produced by natural killer (NK) cells and restricts infection to the respiratory tract. In contrast, infant pertussis resembles disease in NK cell- and IFN-γ-deficient adult mice that experience disseminated lethal infection. We hypothesized that infants exhibit age-associated deficits in NK cell frequency, maturation, and responsiveness to B. pertussis, associated with low IFN-γ levels. To delineate mechanisms behind age-dependent susceptibility, we compared infant and adult mouse models of infection. Infection in infant mice resulted in impaired upregulation of IFN-γ and substantial bacterial dissemination. B. pertussis-infected infant mice displayed fewer pulmonary NK cells than adult mice. Furthermore, the NK cells in the infant mouse lungs had an immature phenotype, and the infant lung showed no upregulation of the IFN-γ-inducing cytokine IL-12p70. Adoptive transfer of adult NK cells into infants, or treatment with exogenous IFN-γ, significantly reduced bacterial dissemination. These data indicate that the lack of NK cell-produced IFN-γ significantly contributes to infant fulminant pertussis and could be the basis for other pathogen-induced, age-dependent respiratory diseases.

BACH2 restricts NK cell maturation and function, limiting immunity to cancer metastasis

Natural killer (NK) cells are critical to immune surveillance against infections and cancer. Their role in immune surveillance requires that NK cells are present within tissues in a quiescent state. Mechanisms by which NK cells remain quiescent in tissues are incompletely elucidated. The transcriptional repressor BACH2 plays a critical role within the adaptive immune system, but its function within innate lymphocytes has been unclear. Here, we show that BACH2 acts as an intrinsic negative regulator of NK cell maturation and function. BACH2 is expressed within developing and mature NK cells and promotes the maintenance of immature NK cells by restricting their maturation in the presence of weak stimulatory signals. Loss of BACH2 within NK cells results in accumulation of activated NK cells with unrestrained cytotoxic function within tissues, which mediate augmented immune surveillance to pulmonary cancer metastasis. These findings establish a critical function of BACH2 as a global negative regulator of innate cytotoxic function and tumor immune surveillance by NK cells.

A High-Salt Diet Disturbs the Development and Function of Natural Killer Cells in Mice

A high-salt diet (HSD) is common worldwide and can lead to cardiovascular disease, chronic inflammation, and autoimmune diseases. Moreover, increasing evidence shows that HSD is closely related to a variety of immune diseases. Natural killer (NK) cells are important innate immune cells that directly kill their targets via degranulation and secretion of interferon gamma (IFN-γ). NK cells play a vital role in resisting viruses and preventing the malignant transformation of cells; however, whether HSD affects the development and function of NK cells has not yet been elucidated. Therefore, the purpose of the present study was to understand the effects of HSD on the development and function of NK cells, in addition to investigating the underlying molecular mechanism. Our results show that the number of NK cells in the spleen and lungs of HSD-fed mice was significantly reduced, which may be due to the inhibition of NK cell proliferation. Further, the development of NK cells in mice was evaluated, and it was found that HSD reduced the effective NK cell subset (CD27⁺CD11b⁻). Moreover, it was also found that the ability of NK cells to secrete CD107a and IFN-γ in HSD-fed mice was decreased following stimulation with RMA-S and YAC-1 tumor cells. Finally, the underlying molecular mechanism was evaluated, and it was found that HSD increased the production of reactive oxygen species (ROS) by NK cells, while the expression of CD122 was decreased, suggesting that HSD downregulates CD122 expression in NK cells via ROS signaling, thereby reducing the responsiveness to IL-15 and ultimately inhibiting NK cell function. The present research discovered a novel mechanism by which HSD inhibits the function of NK cells, providing an alternative avenue for the treatment of immune diseases caused by HSD.

Suppression of Natural Killer cell NKG2D and CD226 anti-tumour cascades by platelet cloaked cancer cells: Implications for the metastatic cascade

Tumour cell immune evasion is a principal hallmark of successful metastasis. Tumour cells in the vasculature adopt a platelet cloak that efficiently suppresses the innate immune system by directly inhibiting Natural Killer (NK) cells, which normally function to neutralise spreading cancers. Here we describe two novel mechanisms of tumour cell evasion of NK cell anti-tumour functions. The first, an ‘immune decoy’ mechanism in which platelets induce the release of soluble NKG2D ligands from the tumour cell to mask detection and actively suppress NK cell degranulation and inflammatory cytokine (IFNγ) production, concomitantly. This represents a double-hit to immune clearance of malignant cells during metastasis. The second mechanism, a platelet-derived TGFβ-mediated suppression of the CD226/CD96-CD112/CD155 axis, is a novel pathway with poorly understood anti-cancer functions. We have demonstrated that platelets robustly suppress surface expression of CD226 and CD96 on the NK cell surface and their associated ligands on the tumour cell to further enhance NK cell suppression. These highly evolved mechanisms promote successful tumour immune evasion during metastasis and provide a unique opportunity for studying the complexity of cellular interactions in the metastatic cascade and thus novel targets for cancer immunotherapy.

The Influence of Hepatitis C Viral Loads on Natural Killer Cell Function

Background

Hepatitis C virus (HCV) infection has a high rate of chronicity, attributable to its capacity to alter host immunity, including natural killer (NK) cell function. In this study, the interaction between NK cell activity and HCV viral load was investigated.

Methods

Peripheral blood NK cells were examined for cytotoxicity and interferon (IFN)-γ expression in HCV infected low (LVL, < 800,000 IU/mL, n = 10) and high (HVL, > 800,000 IU/mL, n = 13) viral load patient cohorts.

Results

Spontaneous NK cell cytotoxicity was more robust in the LVL cohort resulting in a negative correlation with viral loads (spontaneous, r = -0.437, P = 0.037; IFN-α activated, r = -0.372, P = 0.081). Although the percent of IFN-γ+ NK cells did not associate with viral load, within the LVL cohort there was a marked increase in IFN-γ+ NK cells upon IFN-α activation relative to medium alone (P < 0.01). To examine the inability of NK cells derived from HVL patients to be further activated, the expression of the exhaustion marker programmed cell death protein (PD)-1 was evaluated. PD-1 expression upon NK cell activation correlated with viral load (r = 0.649, P = 0.009). In addition, HCV proteins upregulated PD-1 expression in vitro (P < 0.05), suggesting that HCV can directly promote NK cell exhaustion. Cells from HVL patients were also more likely to produce IFN-γ in response to HCV core protein. The finding that NK cell PD-1 and IFN-γ expression are linked (r = 0.542, P < 0.05) suggests that increased IFN-γ levels may induce PD-1 as a negative feedback mechanism.

Conclusions

High HCV loads appear to promote NK exhaustion in chronic HCV infection.

Functional and Phenotypic Changes of Natural Killer Cells in Whole Blood during Mycobacterium tuberculosis Infection and Disease

Tuberculosis (TB) is still a global health concern, especially in resource-poor countries such as The Gambia. Defining protective immunity to TB is challenging: its pathogenesis is complex and involves several cellular components of the immune system. Recent works in vaccine development suggest important roles of the innate immunity in natural protection to TB, including natural killer (NK) cells. NK cells mediate cellular cytotoxicity and cytokine signaling in response to Mycobacterium tuberculosis (Mtb). NK cells can display specific memory-type markers to previous antigen exposure; thus, bridging innate and adaptive immunity. However, major knowledge gaps exist on the contribution of NK cells in protection against Mtb infection or TB. We performed a cross-sectional assessment of NK cells phenotype and function in four distinct groups of individuals: TB cases pre-treatment (n = 20) and post-treatment (n = 19), and household contacts with positive (n = 9) or negative (n = 18) tuberculin skin test (TST). While NK cells frequencies were similar between all groups, significant decreases in interferon-γ expression and degranulation were observed in NK cells from TB cases pre-treatment compared to post-treatment. Conversely, CD57 expression, a marker of advanced NK cells differentiation, was significantly lower in cases post-treatment compared to pre-treatment. Finally, NKG2C, an activation and imprinted-NK memory marker, was significantly increased in TST+ (latently infected) compared to TB cases pre-treatment and TST- (uninfected) individuals. The results of this study provide valuable insights into the role of NK cells in Mtb infection and TB disease, demonstrating potential markers for distinguishing between infection states and monitoring of TB treatment response.

Degranulation Response in Cytotoxic Rat Lymphocytes Measured with a Novel CD107a Antibody

Measuring degranulation through CD107a expression has become an advantageous tool for testing the functional capacity of cytotoxic cells. Such functional studies have been hampered in the rat by the lack of a suitable anti-rat CD107a antibody. In this study, we report a novel hybridoma generated by immunizing Armenian inbred hamsters with transfected Chinese hamster ovary cells expressing CD107a. The SIM1 clone exhibited specific reactivity with CD107a and measured degranulation from natural killer (NK) cells stimulated with target cells or mAb crosslinking of their activating receptors. Degranulation in IL-2-activated NK cells could also be measured, when using low effector to target ratios. SIM1 also stained activated CD8, but not CD4 T cells. This report characterizes the degranulation response in cytotoxic rat cells with a new antibody against rat CD107a.

Bone marrow produces sufficient alloreactive natural killer (NK) cells in vivo to cure mice from subcutaneously and intravascularly injected 4T1 breast cancer

Purpose

Administration of 5 million alloreactive natural killer (NK) cells after low-dose chemo-irradiation cured mice of 4T1 breast cancer, supposedly dose dependent. We now explored the efficacy of bone marrow as alternative in vivo source of NK cells for anti-breast cancer treatment, as methods for in vitro clinical scale NK cell expansion are still in developmental phases.

Methods

Progression-free survival (PFS) after treatment with different doses of spleen-derived alloreactive NK cells to 4T1-bearing Balb/c mice was measured to determine a dose–response relation. The potential of bone marrow as source of alloreactive NK cells was explored using MHC-mismatched mice as recipients of 4T1. Chemo-irradiation consisted of 2× 2 Gy total body irradiation and 200 mg/kg cyclophosphamide. Antibody-mediated in vivo NK cell depletion was applied to demonstrate the NK cell’s role.

Results

Administration of 2.5 instead of 5 million alloreactive NK cells significantly reduced PFS, evidencing dose responsiveness. Compared to MHC-matched receivers of subcutaneous 4T1, fewer MHC-mismatched mice developed tumors, which was due to NK cell alloreactivity because in vivo NK cell depletion facilitated tumor growth. Application of low-dose chemo-irradiation increased plasma levels of NK cell-activating cytokines, NK cell activity and enhanced NK cell-dependent elimination of subcutaneous tumors. Intravenously injected 4T1 was eliminated by alloreactive NK cells in MHC-mismatched recipients without the need for chemo-irradiation.

Conclusions

Bone marrow is a suitable source of sufficient alloreactive NK cells for the cure of 4T1 breast cancer. These results prompt clinical exploration of bone marrow transplantation from NK-alloreactive MHC-mismatched donors in patients with metastasized breast cancer.

Vaccinia virus Western Reserve induces rapid surface expression of a host molecule detected by the antibody 4C7 that is distinct from CLEC2D

In this study, the effect of active infection with vaccinia virus Western Reserve (VACV WR) on expression of C-type lectin domain family 2 (CLEC2D), a ligand of the human NK cell inhibitory receptor NKR-P1, was examined. As predicted, VACV infection led to a loss of CLEC2D mRNA in 221 cells, a B cell lymphoma line. Surprisingly, VACV infection of 221 cells caused a dramatic increase in cell surface staining for one CLEC2D-specific antibody, 4C7. There were no changes in other antibodies specific for CLEC2D and no indication that NK cells with NKR-P1A were inhibited, suggesting 4C7 detects a non-CLEC2D molecule following infection. The rapid increase in 4C7 signal requires virus attachment and is disrupted by UV treatment, but does not depend on new transcription or translation of either cellular or viral proteins. 4C7 does react with intracellular compartments, suggesting the molecule that is detected at the surface following infection is derived from an intracellular store. The phenomenon extends beyond lymphoid cells: it was observed in the non-human primate cell line Cos-7, but not with myxoma, a poxvirus distinct from VACV. To our knowledge, this is the first report of VACV or any poxvirus leading to rapid externalization of a host molecule. Among the VACV strains tested, the phenomenon was restricted to VACV WR and IHD-W, suggesting it has a virulence-, as opposed to a replication-related, function.

1α,25(OH)2 Vitamin D3 Modulates Avian T Lymphocyte Functions without Inducing CTL Unresponsiveness

1,25-Dihydroxyvitamin D3 (Vitamin D) is a naturally synthesized fat soluble vitamin shown to have immunomodulatory, anti-inflammatory and cancer prevention properties in human and murine models. Here, we studied the effects of Vitamin D on the functional abilities of avian T lymphocytes using chicken Interferon (IFN)-γ ELISPOT assay, BrdU proliferation assay, Annexin V apoptosis assay and PhosFlow for detecting phosphorylated signalling molecules. The results demonstrate that Vitamin D significantly inhibited the abilities of T lymphocytes to produce IFN-γ and proliferate in vitro (P≤0.05), but retained their ability to undergo degranulation, which is a maker for cytotoxicity of these cells. Similarly, Vitamin D did not inhibit Extracellular signal-Regulated Kinase (ERK) 1/2 phosphorylation, a key mediator in T cell signalling, in the stimulated T lymphocytes population, while reduced ERK1/2 phosphorylation levels in the unstimulated cells. Our data provide evidence that Vitamin D has immuno-modulatory properties on chicken T lymphocytes without inducing unresponsiveness and by limiting immuno-pathology can promote protective immunity against infectious diseases of poultry.

CD160 is essential for NK-mediated IFN-γ production

Tu et al. generated a novel CD160-deficient mouse and showed impaired NK cell–mediated tumor elimination and IFN-γ production. CD160⁺ NK cells are functionally distinct in secretion of IFN-γ from their CD160⁻ NK cell counterparts.

NK-derived cytokines play important roles for natural killer (NK) function, but how the cytokines are regulated is poorly understood. CD160 is expressed on activated NK or T cells in humans but its function is unknown. We generated CD160-deficient mice to probe its function. Although CD160^−/− mice showed no abnormalities in lymphocyte development, the control of NK-sensitive tumors was severely compromised in CD160^−/− mice. Surprisingly, the cytotoxicity of NK cells was not impaired, but interferon-γ (IFN-γ) secretion by NK cells was markedly reduced in CD160^−/− mice. Functionally targeting CD160 signaling with a soluble CD160-Ig also impaired tumor control and IFN-γ production, suggesting an active role of CD160 signaling. Using reciprocal bone marrow transfer and cell culture, we have identified the intrinsic role of CD160 on NK cells, as well as its receptor on non-NK cells, for regulating cytokine production. To demonstrate sufficiency of the CD160⁺ NK cell subset in controlling NK-dependent tumor growth, intratumoral transfer of the CD160⁺ NK fraction led to tumor regression in CD160^−/− tumor-bearing mice, indicating demonstrable therapeutic potential for controlling early tumors. Therefore, CD160 is not only an important biomarker but also functionally controls cytokine production by NK cells.

Recombinant TLR5 agonist CBLB502 promotes NK cell-mediated anti-CMV immunity in mice

Prior work using allogeneic bone marrow transplantation (allo-BMT) models showed that peritransplant administration of flagellin, a toll-like receptor 5 (TLR5) agonist protected murine allo-BMT recipients from CMV infection while limiting graft-vs-host disease (GvHD). However, the mechanism by which flagellin-TLR5 interaction promotes anti-CMV immunity was not defined. Here, we investigated the anti-CMV immunity of NK cells in C57BL/6 (B6) mice treated with a highly purified cGMP grade recombinant flagellin variant CBLB502 (rflagellin) followed by murine CMV (mCMV) infection. A single dose of rflagellin administered to mice between 48 to 72 hours prior to MCMV infection resulted in optimal protection from mCMV lethality. Anti-mCMV immunity in rflagellin-treated mice correlated with a significantly reduced liver viral load and increased numbers of Ly49H+ and Ly49D+ activated cytotoxic NK cells. Additionally, the increased anti-mCMV immunity of NK cells was directly correlated with increased numbers of IFN-γ, granzyme B- and CD107a producing NK cells following mCMV infection. rFlagellin-induced anti-mCMV immunity was TLR5-dependent as rflagellin-treated TLR5 KO mice had ∼10-fold increased liver viral load compared with rflagellin-treated WT B6 mice. However, the increased anti-mCMV immunity of NK cells in rflagellin-treated mice is regulated indirectly as mouse NK cells do not express TLR5. Collectively, these data suggest that rflagellin treatment indirectly leads to activation of NK cells, which may be an important adjunct benefit of administering rflagellin in allo-BMT recipients.

Antibacterial role for natural killer cells in host defense to Bacillus anthracis

Natural killer (NK) cells have innate antibacterial activity that could be targeted for clinical interventions for infectious disease caused by naturally occurring or weaponized bacterial pathogens. To determine a potential role for NK cells in immunity to Bacillus anthracis, we utilized primary human and murine NK cells, in vitro assays, and in vivo NK cell depletion in a murine model of inhalational anthrax. Our results demonstrate potent antibacterial activity by human NK cells against B. anthracis bacilli within infected autologous monocytes. Surprisingly, NK cells also mediate moderate antibacterial effects on extracellular vegetative bacilli but do not have activity against extracellular or intracellular spores. The immunosuppressive anthrax lethal toxin impairs NK gamma interferon (IFN-γ) expression, but neither lethal nor edema toxin significantly alters the viability or cytotoxic effector function of NK cells. Compared to human NK cells, murine NK cells have a similar, though less potent, activity against intracellular and extracellular B. anthracis. The in vivo depletion of murine NK cells does not alter animal survival following intranasal infection with B. anthracis spores in our studies but significantly increases the bacterial load in the blood of infected animals. Our studies demonstrate that NK cells participate in the innate immune response against B. anthracis and suggest that immune modulation to augment NK cell function in early stages of anthrax should be further explored in animal models as a clinical intervention strategy.

Natural killer cell function is altered during the primary response of aged mice to influenza infection

Influenza is a public health concern, especially for the elderly. While influenza vaccination is efficacious in the young, it offers only limited protection in the elderly. Thus, it becomes imperative to understand age-related changes in the primary response to influenza infection. This study identified potential age-related defects in natural killer (NK) cell function during influenza infection. We showed that NK cells from aged mice were reduced and had impaired function and altered phenotype in lungs during influenza infection. Aged NK cells demonstrated decreased IFN-γ production, but not degranulation, after influenza infection. However, after ex vivo activation with YAC-1 cells, aged NK cells demonstrated both reduced IFN-γ production and degranulation. IFN-γ was also reduced in aged NK cells after activation with anti-NKp46 and soluble cytokines. IFN-β, and IL-12p40 mRNA expression was not significantly different from that observed in adult mice. Analysis of NK cell subsets indicated that aged mice had more immature and less terminally mature NK cells. These data suggest that aging affects the numbers, function and phenotype of NK cells. Thus, these defects in NK cell function could impair the ability of aged mice to induce a strong antiviral immune response during the early stages of the infection.

Plasmid vector-linked maturation of natural killer (NK) cells is coupled to antigen-dependent NK cell activation during DNA-based immunization in mice

Plasmid DNA vaccines serve in a wide array of applications ranging from prophylactic vaccines to potential therapeutic tools against infectious diseases and cancer. In this study, we analyzed the mechanisms underlying the activation of natural killer (NK) cells and their potential role in adaptive immunity during DNA-based immunization against hepatitis B virus surface antigen in mice. We observed that the mature Mac-1(+) CD27(-) NK cell subset increased in the liver of mice early after DNA injection, whereas the number of the less mature Mac-1(+) CD27(+) NK cells in the liver and spleen was significantly reduced. This effect was attributed to bacterial sequences present in the plasmid backbone rather than to the encoded antigen and was not observed in immunized MyD88-deficient mice. The activation of NK cells by plasmid-DNA injection was associated with an increase in their effector functions that depended on the expressed antigen. Maturation of NK cells was abrogated in the absence of T cells, suggesting that cross talk exists between NK cells and antigen-specific T cells. Taken together, our data unravel the mechanics of plasmid vector-induced maturation of NK cells and plasmid-encoded antigen-dependent activation of NK cells required for a crucial role of NK cells in DNA vaccine-induced immunogenicity.

CD27-deficient mice show normal NK-cell differentiation but impaired function upon stimulation

Natural killer (NK) cells are part of the first line defense against tumors, parasites and virus-infected cells. Therefore, factors that control NK-cell numbers and their function are important. CD27 is constitutively expressed on NK cells and its expression correlates with sequential phases in NK-cell development, discriminating phenotypically and functionally different subsets within the NK-cell population. Although CD27 has been described to have an important regulatory role in effector and memory T and B lymphocytes, its role in NK-cell biology remains to be addressed. In this study, we used CD27(-/-) mice to investigate the role of CD27 in NK-cell development and function, both during the resting state and upon stimulation. The results show that NK-cell numbers are not impaired in CD27(-/-) mice. Moreover, CD27(-/-) NK cells reach full phenotypic maturity, evidenced by normal expression of CD49b, CD43 and CD11b. Expression of activating receptors is unaltered, whereas expression of several inhibitory receptors is increased. Cytotoxicity and interferon-γ production by NK cells from CD27(-/-) mice in the resting state are normal. However, upon in vivo anti-CD40- or poly-I:C-mediated activation, or in vitro interleukin-15 priming plus anti-NKp46 stimulation, the absence of CD27 results in decreased cytolytic activity and cytokine production by spleen and liver NK cells. In conclusion, this study demonstrates that CD27 is dispensable for the development of functional NK cells. However, upon stimulation of NK cells, CD27 displays an important role in their activation and functionality.

Probing natural killer cell education by Ly49 receptor expression analysis and computational modelling in single MHC class I mice

Murine natural killer (NK) cells express inhibitory Ly49 receptors for MHC class I molecules, which allows for “missing self” recognition of cells that downregulate MHC class I expression. During murine NK cell development, host MHC class I molecules impose an “educating impact” on the NK cell pool. As a result, mice with different MHC class I expression display different frequency distributions of Ly49 receptor combinations on NK cells. Two models have been put forward to explain this impact. The two-step selection model proposes a stochastic Ly49 receptor expression followed by selection for NK cells expressing appropriate receptor combinations. The sequential model, on the other hand, proposes that each NK cell sequentially expresses Ly49 receptors until an interaction of sufficient magnitude with self-class I MHC is reached for the NK cell to mature. With the aim to clarify which one of these models is most likely to reflect the actual biological process, we simulated the two educational schemes by mathematical modelling, and fitted the results to Ly49 expression patterns, which were analyzed in mice expressing single MHC class I molecules. Our results favour the two-step selection model over the sequential model. Furthermore, the MHC class I environment favoured maturation of NK cells expressing one or a few self receptors, suggesting a possible step of positive selection in NK cell education. Based on the predicted Ly49 binding preferences revealed by the model, we also propose, that Ly49 receptors are more promiscuous than previously thought in their interactions with MHC class I molecules, which was supported by functional studies of NK cell subsets expressing individual Ly49 receptors.

Myosin IIA associates with NK cell lytic granules to enable their interaction with F-actin and function at the immunological synapse

NK cell cytotoxicity requires the formation of an actin-rich immunological synapse (IS) with a target cell and the polarization of perforin-containing lytic granules toward the IS. Following the polarization of lytic granules, they traverse through the actin-rich IS to join the NK cell membrane in order for directed secretion of their contents to occur. We examined the role of myosin IIA as a candidate for facilitating this prefinal step in lytic NK cell IS function. Lytic granules in and derived from a human NK cell line, or ex vivo human NK cells, were constitutively associated with myosin IIA. When isolated using density gradients, myosin IIA-associated NK cell lytic granules directly bound to F-actin and the interaction was sensitive to the presence of ATP under conditions of flow. In NK cells from patients with a truncation mutation in myosin IIA, NK cell cytotoxicity, lytic granule penetration into F-actin at the IS, and interaction of isolated granules with F-actin were all decreased. Similarly, inhibition of myosin function also diminished the penetration of lytic granules into F-actin at the IS, as well as the final approach of lytic granules to and their dynamics at the IS. Thus, NK cell lytic granule-associated myosin IIA enables their interaction with actin and final transit through the actin-rich IS to the synaptic membrane, and can be defective in the context of naturally occurring human myosin IIA mutation.