Activating Ly-49 NK receptors: central role in cytokine and chemokine production

In vivo tumor surveillance by NK cells requires TYK2 but not TYK2 kinase activity

Tyrosine kinase 2 (TYK2) is a Janus kinase (JAK) that is crucially involved in inflammation, carcinogenesis and defense against infection. The cytotoxic activity of natural killer (NK) cells in TYK2-deficient (Tyk2^−/−) mice is severely reduced, although the underlying mechanisms are largely unknown. Using Tyk2^−/− mice and mice expressing a kinase-inactive version of TYK2 (Tyk2^K923E), we show that NK cell function is partly independent of the enzymatic activity of TYK2. Tyk2^−/− and Tyk2^K923E NK cells develop normally in the bone marrow, but the maturation of splenic Tyk2^−/− NK cells (and to a lesser extent of Tyk2^K923E NK cells) is impaired. In contrast, the production of interferon γ (IFNγ) in response to interleukin 12 (IL-12) or to stimulation through NK cell-activating receptors strictly depends on the presence of enzymatically active TYK2. The cytotoxic activity of Tyk2^K923E NK cells against a range of target cells in vitro is higher than that of Tyk2^−/− NK cells. Consistently, Tyk2^K923E mice control the growth of NK cell-targeted tumors significantly better than TYK2-deficient mice, showing the physiological relevance of the finding. Inhibitors of TYK2's kinase activity are being developed for the treatment of inflammatory diseases and cancers, but their effects on tumor immune surveillance have not been investigated. Our finding that TYK2 has kinase-independent functions in vivo suggests that such inhibitors will leave NK cell mediated tumor surveillance largely intact and that they will be suitable for use in cancer therapy.

Genomic expression profiling of NK cells in health and disease

NK cells are important components of innate and adaptive immunity. Functionally, they play key roles in host defense against tumors and infectious pathogens. Within the past few years, genomic-scale experiments have provided us with a plethora of gene expression data that reveal an extensive molecular and biological map underlying gene expression programs. In order to better explore and take advantage of existing datasets, we review here the genomic expression profiles of NK cells and their subpopulations in resting or stimulated states, in diseases, and in different organs; moreover, we contrast these expression data to those of other lymphocytes. We have also compiled a comprehensive list of genomic profiling studies of both human and murine NK cells in this review.

MicroRNA-155 tunes both the threshold and extent of NK cell activation via targeting of multiple signaling pathways

NK cells are innate lymphocytes important for host defense against viral infections and malignancy. However, the molecular programs orchestrating NK cell activation are incompletely understood. MicroRNA-155 (miR-155) is markedly upregulated following cytokine activation of human and mouse NK cells. Surprisingly, mature human and mouse NK cells transduced to overexpress miR-155, NK cells from mice with NK cell-specific miR-155 overexpression, and miR-155(-/-) NK cells all secreted more IFN-γ compared with controls. Investigating further, we found that activated NK cells with miR-155 overexpression had increased per-cell IFN-γ with normal IFN-γ(+) percentages, whereas greater percentages of miR-155(-/-) NK cells were IFN-γ(+). In vivo murine CMV-induced IFN-γ expression by NK cells in these miR-155 models recapitulated the in vitro phenotypes. We performed unbiased RNA-induced silencing complex sequencing on wild-type and miR-155(-/-) NK cells and found that mRNAs targeted by miR-155 were enriched in NK cell activation signaling pathways. Using specific inhibitors, we confirmed these pathways were mechanistically involved in regulating IFN-γ production by miR-155(-/-) NK cells. These data indicate that miR-155 regulation of NK cell activation is complex and that miR-155 functions as a dynamic tuner for NK cell activation via both setting the activation threshold as well as controlling the extent of activation in mature NK cells. In summary, miR-155(-/-) NK cells are more easily activated, through increased expression of proteins in the PI3K, NF-κB, and calcineurin pathways, and miR-155(-/-) and 155-overexpressing NK cells exhibit increased IFN-γ production through distinct cellular mechanisms.

Interleukin-15-dependent NKp46+ innate lymphoid cells control intestinal inflammation by recruiting inflammatory monocytes

With the goal in mind to define how interleukin-15 (IL-15) contributes to acute intestinal inflammation, we have used a mouse model of ileitis induced by oral infection with Toxoplasma gondii. We observed that a crosstalk between IL-15 and interleukin-18 (IL-18) promoted intestinal recruitment of inflammatory monocytes, where these cells participated in parasite control but also in tissue damage. A stromal source of IL-15 controlled the development of lamina propria NKp46(+)NK1.1(+) cells, whereas IL-18 produced during T. gondii infection stimulated their production of the chemokine CCL3. In turn, CCL3 attracted inflammatory monocytes via their chemokine receptor CCR1, which was indispensable for their recruitment into the inflamed gut. Collectively, these results identify the IL-15-dependent subset of intestinal NKp46(+) cells as an important source of CCL3, which can amplify intestinal inflammation via the recruitment of CCR1(+) inflammatory monocytes. Preliminary evidence suggests that this pathway might operate in Crohn's disease.

Activating and inhibitory receptors of natural killer cells

Natural killer (NK) cells are potent immune effector cells that can respond to infection and cancer, as well as allowing maternal adaptation to pregnancy. In response to malignant transformation or pathogenic invasion, NK cells can secrete cytokine and may be directly cytolytic, as well as exerting effects indirectly through other cells of the immune system. To recognize and respond to inflamed or infected tissues, NK cells express a variety of activating and inhibitory receptors including NKG2D, Ly49 or KIR, CD94-NKG2 heterodimers and natural cytotoxicity receptors, as well as co-stimulatory receptors. These receptors recognize cellular stress ligands as well as major histocompatibility complex class I and related molecules, which can lead to NK cell responses. Importantly, NK cells must remain tolerant of healthy tissue, and some of these receptors can also prevent activation of NK cells. In this review, we describe the expression of prominent NK cell receptors, as well as expression of their ligands and their role in immune responses. In addition, we describe the main signaling pathways used by NK cell receptors. Although we now appreciate that NK cell biology is more complicated than first thought, there are still facets of their biology that remain unclear. These will be highlighted and discussed in this review.

Cooperation of adenosine and prostaglandin E2 (PGE2) in amplification of cAMP-PKA signaling and immunosuppression

INTRODUCTION

We hypothesize that adenosine and PGE2 could have a complementary immunosuppressive effect that is mediated via common cAMP-PKA signaling.

MATERIALS AND METHODS

To test this hypothesis, the effect of adenosine and PGE2 on the cytotoxic activity and cytokine production of lymphokine activated killer (LAK) cells was investigated.

RESULTS

PGE2 and adenosine inhibited LAK cells cytotoxic activity and production of INF-gamma, GM-CSF and TNF-alpha. In combination they showed substantially higher inhibition than each modality used alone. Using agonists and antagonists specific for PGE2 and adenosine receptors we found that cooperation of PGE2 and adenosine in their inhibitory effects are mediated via EP2 and A2A receptors, respectively. LAK cells have 35-fold higher expression of EP2 than A2A. Combined PGE2 and adenosine treatment resulted in augmentation of cAMP production, PKA activity, CREB phosphorylation and inhibition of Akt phosphorylation. Wortmannin and LY294002 enhanced the suppressive effects of adenosine and PGE2. In contrast, Rp-8-Br-cAMPS, an inhibitor of PKA type I blocked their immunosuppressive effects, suggesting that the inhibitory effects of PGE2 and adenosine are mediated via common pathway with activation of cAMP-PKA and inhibition of Akt.

CONCLUSION

In comparison to other immunosuppressive molecules (TGF-beta and IL-10), adenosine and PGE2 are unique in their ability to inhibit the executive function of highly cytotoxic cells. High intratumor levels of adenosine and PGE2 could protect tumor from immune-mediated destruction by inactivation of the tumor infiltrating functionally active immune cells.

In vivo regulation of experimental autoimmune encephalomyelitis by NK cells: alteration of primary adaptive responses

Innate immune responses provide the host with its first line of defense against infections. Signals generated by subsets of lymphocytes, including NK cells, NKT cells, and APC during this early host response determine the nature of downstream adaptive immune responses. In the present study, we have examined the role of innate NK cells in an autoimmune model through the use of primary immunization with the myelin oligodendrocyte glycoprotein peptide to induce experimental autoimmune encephalomyelitis (EAE). Our studies have shown that in vivo depletion of NK cells can affect the adaptive immune responses, because NK cells were found to regulate the degree of clinical paralysis and to alter immune adaptive responses to the myelin oligodendrocyte glycoprotein peptide. The requirement for NK cells was reflected by changes in the T cell responses and diminished clinical disease seen in mice treated with anti-NK1.1, anti-asialo GM1, and selected Ly49 subtype-depleted mice. In addition to alteration in T cell responses, the maturational status of dendritic cells in lymph nodes was altered both quantitatively and qualitatively. Finally, examination of TCR Vbeta usage of the brain lymphocytes from EAE mice indicated a spectra-type change in receptor expression in NK- depleted mice as compared with non-NK-depleted EAE mice. These findings further establish a recently postulated link between NK cells and the generation of autoreactive T cells.

Modulation of lymphocyte function with inhibitory CD2: loss of NK and NKT cells

Analysis of the NK cell developmental pathway suggests that CD2 expression may be important in regulating NK maturation. To test this hypothesis, we developed mice containing only an inhibitory CD2 molecule by linking the extracellular domain of CD2 to an intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM) motif. Mice containing the CD2 Tg(ITIM) transgene, introduced into a CD2 KO background, have no morphologically detectable lymph nodes, although development of the thymus appears normal. In addition, these mice had major loss of both NK and NKT subsets in peripheral organs, while T and B cell frequencies were intact. Expression of CD2 was low on T cells and lacking on B cells and functional defects were observed in these populations. NKT cells expressing CD4 were absent, while the CD8+ and double negative NKT cells were retained. Small subsets of NK cells were detected but expression of CD2 on these cells was very low or absent, and their maturation was impaired. Based on the phenotype described here, we believe that these mice represent a unique model to study lymphoid organ and lymphocyte development.

Type 1 cytokine/chemokine production by mouse NK cells following activation of their TLR/MyD88-mediated pathways

It is well established that IL-18R- and toll-like receptor (TLR)-mediated signalings share a common signal pathway mediated by signal adaptor, MyD88, and that IL-18 synergizes with IL-12 for IFN-gamma production by NK cells. Here, we investigated whether TLR agonists can replace IL-18 for production of IFN-gamma by NK cells. Freshly isolated NK cells possessed functional LPS receptor composed of TLR4/MD2 complex and of CD14, and also expressed other various tlrs. Hepatic CD3(-)DX5(+) NK cells produced IFN-gamma in response to TLR2 or TLR7 agonists only when co-stimulated with IL-12, indicating that TLR agonists synergize with IL-12 for IFN-gamma. The tlr2(-/-) or tlr7(-/-) NK cells could not produce IFN-gamma in response to IL-12 plus TLR2 or TLR7 ligands, respectively, indicating requirement of the corresponding TLRs. Furthermore, upon stimulation with these combinations, wild-type NK cells produced type 1 chemokines, such as CCL3, CCL4 and CCL5 as well. NK cells from bacterium (e.g. Propionibacterium acnes)-inoculated rag2(-/-) mice, when compared with those from naive mice, exhibited significantly enhanced capacity to produce these CC chemokines and IFN-gamma, suggesting that microbial infection enhances responsiveness of NK cells to TLR agonists. These results indicate that upon microbial infection, macrophages produce IL-12 that renders NK cells highly responsive to TLR agonists to produce IFN-gamma and chemokines, which might in turn recruit and fully activate macrophages, leading to the development of inflammatory foci presumably necessary for efficient microbial eradication. Thus, NK cells, like T cells, induce orchestrated immune responses in collaboration with macrophages to show potent host defense effects during early infectious phase.

The role of regulatory T cells in the control of natural killer cells: relevance during tumor progression

Tumor immunosurveillance relies on cognate immune effectors [lymphocytes and interferon-gamma (IFN-gamma)] and innate immunity [natural killer (NK) cells, natural killer group 2, member D (NKG2D) ligands, perforin/granzyme, and tumor necrosis factor-related apoptosis-inducing ligand]. In parallel, tumor cells promote the expansion of CD4(+)CD25(+) regulatory T cells (Tregs) that counteract T-cell-based anti-tumor immunity. Moreover, accumulating evidence points to a critical role for Tregs in dampening NK cell immune responses. This review summarizes the findings showing that Tregs suppress NK cell effector functions in vitro and in vivo, i.e. homeostatic proliferation, cytotoxicity, and interleukin-12-mediated IFN-gamma production. The molecular mechanism involve selective expression of membrane-bound transforming growth factor-beta on Tregs, which downregulate NKG2D expression on NK cells in vitro and in vivo. The regulatory events dictating NK cell suppression by Tregs have been studied and are discussed. The pathological relevance of the Treg-NK cell interaction has been brought up in tumor models and in patients with cancer. Consequently, inhibition of Tregs through pharmacological interventions should be considered during NK-cell-based immunotherapy of cancer.

Natural killer cell-dendritic cell crosstalk in the initiation of immune responses

Dendritic cells (DCs) and natural killer (NK) cells play a critical role in early defences against cancer and infections. They specialise in complementary functions, including IL-12 or IFN-alpha/beta secretion and antigen presentation for the former, and IFN-gamma secretion and killing of infected or tumour cells for the latter. Both DCs and NK cells are also sensors of the immune system that have developed different, but partially overlapping, systems to identify pathology associated danger signals. Evidence of NK-DC interaction has accumulated recently. This interaction may lead to NK cell activation, DC activation, or apoptosis depending on the activation status of both cell types. Thus, the outcome of NK-DC crosstalk is likely to influence both innate and adaptive immune responses. This review addresses the molecular mechanisms under-lying the different NK-DC interactions, and their in vivo significance in anti-tumour or antimicrobial immunity. Finally, we discuss the potential clinical implications of this new field.

Adenosine-Mediated Inhibition of the Cytotoxic Activity and Cytokine Production by Activated Natural Killer Cells

Adenosine is an important signaling molecule that regulates multiple physiologic processes and exerts major anti-inflammatory actions. Tumors have high concentrations of adenosine, which could inhibit the function of tumor-infiltrating lymphoid cells. We investigated the ability of adenosine and its stable analogue 2-chloroadenosine (CADO) to inhibit cytokine production and cytotoxic activity of lymphokine-activated killer (LAK) cells and determined whether both these effects are initiated via a common pathway. CADO strongly inhibited cytotoxic activity of LAK cells and attenuated the production of IFN-gamma, granulocyte macrophage colony-stimulating factor, tumor necrosis factor alpha, and macrophage inflammatory protein-1alpha by LAK cells stimulated by cross-linking of the Ly49D receptor. These inhibitory effects were associated with the ability of CADO to stimulate cyclic AMP (cAMP) production and activate protein kinase A (PKA). Using cAMP analogues with different affinities for the A and B sites of the regulatory subunits of PKA types I and II, we found that activation of PKA I, but not PKA II, mimicked the inhibitory effects of CADO on LAK cell cytotoxic activity and cytokine production. Inhibitors of the PKA catalytic subunits (H89 and PKI(14-22) peptide) failed to abrogate the inhibitory effects of CADO whereas Rp-8-Br-cAMPS, an antagonist of the RI subunit, blocked the inhibitory effects of CADO. We conclude that the inhibitory effects of adenosine are probably mediated via cAMP-dependent activation of the RI subunits of PKA I but are independent of the catalytic activity of PKA. Tumor-produced adenosine could be a potent tumor microenvironmental factor inhibiting the functional activity of tumor-infiltrating immune cells.

Regulation of Ly49D/DAP12 Signal Transduction by Src-Family Kinases and CD45

Activating, DAP12-coupled members of the Ly-49 family of NK cell receptors help control viral infections in mice. However, the kinases and/or phosphatases mediating tyrosine phosphorylation of Ly-49D-associated DAP12 have not been elucidated. In this study, we show for the first time that Src family tyrosine kinases are physically and functionally associated with Ly-49D/DAP12 signaling in murine NK cells. Specifically, we demonstrate the following: 1) inhibition of Src family kinases suppresses DAP12 phosphorylation and downstream DAP12 signals; 2) both Fyn and Lck are capable of phosphorylating DAP12; and 3) both kinases coimmunoprecipitate with the Ly-49D/DAP12 complex in NK cells. Although we detect enhanced phosphorylation of Fyn upon Ly-49D cross-linking in NK cells, Ly-49D-mediated events in both Fyn-/- and Fyn/Lck-/- mice appear normal, reinforcing the theme of redundancy in the ability of Src family kinases to initiate activation events. In contrast to disruption of specific Src family enzymes, Ly-49D/DAP12-mediated calcium mobilization and cytokine production by CD45 null NK cells are defective. Although others have ascribed the effects of CD45 mutation solely on the suppression of Src family activity, we demonstrate in this study that DAP12 is hyperphosphorylated in CD45 null NK cells, resulting in uncoordinated tyrosine-mediated signaling upon Ly-49D ligation. Therefore, although our data are consistent with a Src kinase activity proximally within DAP12 signaling, DAP12 also appears to be a substrate of CD45, suggesting a more complex role for this phosphatase than has been reported previously.

Dysregulation of signaling pathways in CD45-deficient NK cells leads to differentially regulated cytotoxicity and cytokine production

CD45, a protein tyrosine phosphatase that regulates Src family kinases, is important for regulating T cell and B cell receptor signaling; however, little is known about how CD45 regulates immunoreceptor tyrosine-based activation motif (ITAM)-dependent natural killer (NK) cell receptor signaling and the resulting effector functions. NK cells from CD45-deficient mice are relatively competent for ITAM receptor-induced cell-mediated cytotoxicity, yet completely deficient for cytokine secretion after stimulation with ligands to or antibodies against NK1.1, CD16, Ly49H, Ly49D, and NKG2D. This deficiency in cytokine/chemokine production occurs at the level of mRNA expression. After receptor engagement, extracellular signal-regulated kinase and c-Jun N-terminal kinase activation was markedly perturbed, whereas p38 activation was not substantially affected. The pattern and amounts of basal tyrosine phosphorylation were altered in freshly isolated NK cells and were surprisingly and markedly increased in IL-2-expanded NK cells from CD45-/- mice. These findings indicate that CD45-dependent regulation of ITAM-dependent signaling pathways is essential for NK cell-mediated cytokine production but not cytolytic activity.

Reduction of virus burden-induced splenectomy in patients with liver cirrhosis related to hepatitis C virus infection

AIM: To examine the hepatitis C virus (HCV) levels and immunological markers in cirrhotic patients after splenectomy.

METHODS: HCV RNA titers as well as cellular and humoral immune markers were determined in 20 cirrhotic patients after splenectomy and in 32 cirrhotic controls with an intact spleen.

RESULTS: Serum HCV RNA titers were lower in the splenectomized patients than in the controls (186  ±  225 × 10³ copies/mL vs 541  ±  417 × 10³ copies/mL, P < 0.01). HCV RNA was judged to have been spontaneously eradicated in 4 splenectomized patients, but in none of the controls. Natural killer cell activity was higher in the splenectomized patients than in the controls (41.2  ±  19.3% vs 24.7  ±  15.3%, P < 0.01), and natural killer cell activity was negatively correlated to HCV RNA titers in the splenectomized patients except in those with serotype 2-related infection. The CD4/CD8 ratio was significantly lower in the splenectomized patients than in the controls.

CONCLUSION: The findings suggest that splenectomy may diminish virus burden in cirrhotic patients with HCV infection at least in part, through augmentation of natural killer cell activity.

Cytokines as critical co-stimulatory molecules in modulating the immune response of natural killer cells

Cytokines are involved in directing the activation of natural killer (NK) cells. NK cells are involved in the recognition of cells that have been altered; thus they do not recognize specific insults to the host, but when activated, are capable of destroying infected cells directly, as well as promoting the recruitment and response of the other components of the immune system by the release of cytokines and chemokines. It is these properties that have made NK cells a critical part of innate immunity and adaptive immunity, and they play a principal role linking innate and adaptive immunity by the recruitment of an adaptive immune response to an innate immune reaction.

LNFPIII/LeX-stimulated macrophages activate natural killer cells via CD40-CD40L interaction

Lacto-N-fucopentaose III (LNFPIII) is a human milk sugar containing the biologically active Lewis X (LeX) trisaccharide. LNFPIII/LeX is also expressed by immunosuppressive helminth parasites, by bacteria, and on a number of tumor/cancer cells. In this report, we first demonstrate that LNFPIII activates macrophages in vitro as indicated by upregulation of Gr-1 expression on F4/80(+) cells. Further, we investigated the effect of LNFPIII-activated macrophages on NK cell activity. We found that LNFPIII-stimulated F4/80(+) cells were able to activate NK cells, inducing upregulation of CD69 expression and gamma interferon (IFN-gamma) production. The experiments show that NK cell activation is macrophage dependent, since NK cells alone did not secrete IFN-gamma in response to LNFPIII. Furthermore, we found that activation of NK cells by glycan-stimulated macrophages required cell-cell contact. As part of the cell-cell contact mechanism, we determined that CD40-CD40L interaction was critical for IFN-gamma secretion by NK cells, as the addition of anti-CD40L antibodies to the coculture blocked IFN-gamma production. We also demonstrated that LNFPIII-stimulated macrophages secrete prostaglandin E(2), interleukin-10 (IL-10), and tumor necrosis factor alpha (TNF-alpha) but a very low level of IL-12. Interestingly, addition of anti-TNF-alpha, anti-IL-10, or anti-IL-12 monoclonal antibodies did not significantly alter NK cell activity. Our data show that these soluble mediators are not critical for LNFPIII-stimulated macrophage activation of NK cells and provide further evidence for the importance of cell-cell contact and CD40-CD40L interactions between macrophages and NK cells.

A requirement for CD45 distinguishes Ly49D-mediated cytokine and chemokine production from killing in primary natural killer cells

Engagement of receptors on the surface of natural killer (NK) cells initiates a biochemical cascade ultimately triggering cytokine production and cytotoxicity, although the interrelationship between these two outcomes is currently unclear. In this study we investigate the role of the cell surface phosphatase CD45 in NK cell development and intracellular signaling from activating receptors. Stimulation via the major histocompatibility complex I-binding receptor, Ly49D on CD45(-/-) primary NK cells resulted in the activation of phosphoinositide-3-kinase and normal cytotoxicity but failed to elicit a range of cytokines and chemokines. This blockage is associated with impaired phosphorylation of Syk, Vav1, JNK, and p38, which mimics data obtained using inhibitors of the src-family kinases (SFK). These data, supported by analogous findings after CD16 and NKG2D stimulation of CD45(-/-) primary NK cells, place CD45 upstream of SFK in NK cells after stimulation via immunoreceptor tyrosine-based activation motif-containing receptors. Thus we identify CD45 as a pivotal enzyme in eliciting a precise subset of NK cell responses.

In Vivo Hydrodynamic Delivery of cDNA Encoding IL-2: Rapid, Sustained Redistribution, Activation of Mouse NK Cells, and Therapeutic Potential in the Absence of NKT Cells

In the present study, we have tested the ability of hydrodynamically delivered IL-2 cDNA to modulate the number and function of murine leukocyte subsets in different organs and in mice of different genetic backgrounds, and we have evaluated effects of this mode of gene delivery on established murine tumor metastases. Hydrodynamic administration of the IL-2 gene resulted in the rapid and transient production of up to 160 ng/ml IL-2 in the serum. The appearance of IL-2 was followed by transient production of IFN-gamma and a dramatic and sustained increase in NK cell numbers and NK-mediated cytolytic activity in liver and spleen leukocytes. In addition, significant increases in other lymphocyte subpopulations (e.g., NKT, T, and B cells) that are known to be responsive to IL-2 were observed following IL-2 cDNA plasmid delivery. Finally, hydrodynamic delivery of only 4 mug of the IL-2 plasmid to mice bearing established lung and liver metastases was as effective in inhibiting progression of metastases as was the administration of large amounts (100,000 IU/twice daily) of IL-2 protein. Studies performed in mice bearing metastatic renal cell tumors demonstrated that the IL-2 cDNA plasmid was an effective treatment against liver metastasis and moderately effective against lung metastasis. Collectively, these results demonstrate that hydrodynamic delivery of relatively small amounts of IL-2 cDNA provides a simple and inexpensive method to increase the numbers of NK and NKT cells, to induce the biological effects of IL-2 in vivo for use in combination with other biological agents, and for studies of its antitumor activity.

NK cell recognition

The integrated processing of signals transduced by activating and inhibitory cell surface receptors regulates NK cell effector functions. Here, I review the structure, function, and ligand specificity of the receptors responsible for NK cell recognition.

Natural-killer cells and dendritic cells: "l'union fait la force"

Several recent publications have focused on the newly described interactions between natural-killer (NK) cells and dendritic cells (DCs). Activated NK cells induce DC maturation either directly or in synergy with suboptimal levels of microbial signals. Immature DCs appear susceptible to autologous NK-cell-mediated cytolysis while mature DCs are protected. NK-cell-induced DC activation is dependent on both tumor necrosis factor-alpha (TNF-alpha)/interferon-gamma (IFN-gamma) secretion and a cell-cell contact involving NKp30. In vitro, interleukin-12 (IL-12)/IL-18, IL-15, and IFN-alpha/beta production by activated DCs enhance, in turn, NK-cell IFN-gamma production, proliferation, and cytotoxic potential, respectively. In vivo, NK-cell/DC interactions may occur in lymphoid organs as well as in nonlymphoid tissues, and their consequences are multiple. By inducing DC activation, NK-cell activation induced by tumor cells can indirectly promote antitumoral T-cell responses. Reciprocally, DCs activated through Toll-like receptors (TLRs) induce potent NK-cell activation in antiviral responses. Thus, DCs and NK cells are equipped with complementary sets of receptors that allow the recognition of various pathogenic agents, emphasizing the role of NK-cell/DC crosstalk in the coordination of innate and adaptive immune responses.

Mouse Ly49 NK receptors: balancing activation and inhibition

Previous studies from numerous laboratories have demonstrated that inhibitory class I binding NK receptors dominate functional interactions in vitro. Our previous studies have shown that in addition to lysis, a major consequence of triggering the murine activating NK receptor Ly49D is the expression of cytokines and chemokines. We have recently shown that the activating Ly49D murine NK cell receptor can potently synergize during co-stimulation with IL-12 and IL-18 for selective production of IFN-gamma. Activation both in vitro and in vivo and synergistic production of IFN-gamma by Ly49D expressing NK cells results from cytokine stimulation combined with co-receptor ligation. In addition, IL-12 is capable of overriding the inhibitory receptor blockade for cytokine production, both in vitro and in vivo. Our current studies will expand this finding of IL-12 synergy to other receptors in the NK repertoire and evaluate potential biochemical mechanisms involved in this synergy. These findings place NK cells and their activating Ly49 receptors as important initiators of microbial, antiviral and anti-tumor immunity and provide a mechanism for the release of activating Ly49 receptors from an inhibitory receptor blockade. Discussion of how activation of the innate immune system provides important initiators of adaptive immune responses by receptor cross-linking and cytokine co-receptor engagement will ensue.

Anatomy of a murder—signal transduction pathways leading to activation of natural killer cells

Natural killer (NK) cells control the early phases of viral infections, modulate antigen-specific immune responses, and participate in the rejection of tumours and bone marrow grafts. A fine balance between inhibitory and activating receptors tightly regulates NK cell activation. Biochemical studies in human cell lines and primary cells have revealed some of the activating NK cell signalling pathways, however animal models are instrumental to understand the physiological implications of these findings for immune responses in vivo. Gene targeting in mice and biochemical studies in cells are helping to dissect out the various signal transduction pathways that control NK cell activation. A clearer view of these pathways may eventually help designing more effective immune therapies based on the use of NK cells.

T-bet regulates the terminal maturation and homeostasis of NK and Valpha14i NKT cells

Natural killer (NK) and CD1d-restricted Valpha14i natural killer T (NKT) cells play a critical early role in host defense. Here we show that mice with a targeted deletion of T-bet, a T-box transcription factor required for Th1 cell differentiation, have a profound, stem cell-intrinsic defect in their ability to generate mature NK and Valpha14i NKT cells. Both cell types fail to complete normal terminal maturation and are present in decreased numbers in peripheral lymphoid organs of T-bet(-/-) mice. T-bet expression is regulated during NK cell differentiation by NK-activating receptors and cytokines known to control NK development and effector function. Our results identify T-bet as a key factor in the terminal maturation and peripheral homeostasis of NK and Valpha14i NKT cells.

Aging of innate immunity: functional comparisons of NK/LAK cells obtained from bulk cultures of young and aged mouse spleen cells in high concentrations of interleukin-2

The technique of bulk cultivation of aged mouse spleen cells in high concentration of IL-2 was employed to obtain NK/LAK cells in sufficient number and enrichment for studies on the effects of aging on their functions. The yield and enrichment were equivalent to that of young mouse spleen cells. The aged and young mouse NK/LAK cells were equivalent also in their functional competence to proliferate, kill target cells and produce IFNgamma; i.e. they did not display age-associated defects typical of freshly-isolated NK/LAK cells. In two respects, however, the NK/LAK cells derived from aged mouse spleen were altered: (a) in the efficiency of nuclear translocation of transcription factors STAT 5A and 5B, and (b) in the deficiency in production of mRNA transcripts representing several chemokines. We recommend caution in the use of bulk cultivation in IL-2 to obtain NK/LAK cells for studies on aging. However, it does appear from this study that aging may severely affect chemokine production, at least in the case of NK/LAK cells.

Coordinate Expression of Cytokines and Chemokines by NK Cells during Murine Cytomegalovirus Infection

Cytokines and chemokines activate and direct effector cells during infection. We previously identified a functional group of five cytokines and chemokines, namely, IFN-gamma, activation-induced T cell-derived and chemokine-related cytokine/lymphotactin, macrophage-inflammatory protein 1alpha, macrophage-inflammatory protein 1beta, and RANTES, coexpressed in individual activated NK cells, CD8(+) T cells, and CD4(+) Th1 cells in vitro and during in vivo infections. However, the stimuli during infection were not known. In murine CMV (MCMV) infection, the DAP12/KARAP-associated Ly49H NK cell activation receptor is crucial for resistance through recognition of MCMV-encoded m157 but NK cells also undergo in vivo nonspecific responses to uncharacterized stimuli. In this study, we show that Ly49H ligation by m157 resulted in a coordinated release of all five cytokines/chemokines from Ly49H(+) NK cells. Whereas other cytokines also triggered the release of these cytokines/chemokines, stimulation was not confined to the Ly49H(+) population. At the single-cell level, the production of the five mediators showed strong positive correlation with each other. Interestingly, NK cells were a major source of these five cytokines/chemokines in vitro and in vivo, whereas infected macrophages produced only limited amounts of macrophage-inflammatory protein 1alpha, macrophage-inflammatory protein1beta, and RANTES. These findings suggest that both virus-specific and nonspecific NK cells play crucial roles in activating and directing other inflammatory cells during MCMV infection.

Dissociation of NKT Stimulation, Cytokine Induction, and NK Activation In Vivo by the Use of Distinct TCR-Binding Ceramides

NKT and NK cells are important immune regulatory cells. The only efficient means to selectively stimulate NKT cells in vivo is alpha-galactosylceramide (alphaGalCer). However, alphaGalCer effectively stimulates and then diminishes the number of detectable NKT cells. It also exhibits a potent, indirect ability to activate NK cells. We have now discovered another ceramide compound, beta-galactosylceramide (betaGalCer) (C12), that efficiently diminishes the number of detectable mouse NKT cells in vivo without inducing significant cytokine expression or activation of NK cells. Binding studies using CD1d tetramers loaded with betaGalCer (C12) demonstrated significant but lower intensity binding to NKT cells when compared with alphaGalCer, but both ceramides were equally efficient in reducing the number of NKT cells. However, betaGalCer (C12), in contrast to alphaGalCer, failed to increase NK cell size, number, and cytolytic activity. Also in contrast to alphaGalCer, betaGalCer (C12) is a poor inducer of IFN-gamma, TNF-alpha, GM-CSF, and IL-4 gene expression. These qualitative differences in NKT perturbation/NK activation have important implications for delineating the unique in vivo roles of NKT vs NK cells. Thus, alphaGalCer (which triggers NKT cells and activates NK cells) efficiently increases the resistance to allogeneic bone marrow transplantation while betaGalCer (C12) (which triggers NKT cells but does not activate NK cells) fails to enhance bone marrow graft rejection. Our results show betaGalCer (C12) can effectively discriminate between NKT- and NK-mediated responses in vivo. These results indicate the use of different TCR-binding ceramides can provide a unique approach for understanding the intricate immunoregulatory contributions of these two cell types.

Ly49D Receptor Expressed on Immature B Cells Regulates Their IFN-γ Secretion, Actin Polymerization, and Homing

Low levels of IFN-gamma secreted by immature B cells prevent their own migration and homing to the lymph nodes and premature encounter with Ag. In this study we followed the mechanism regulating IFN-gamma secretion by immature B cells. We show that the MHC class I receptor, Ly49D, is expressed on immature B cells and is down-regulated during maturation. Activation of this receptor leads to increase in IFN-gamma transcription and translation and results in the altered ability of B cells to polymerize actin in response to chemokine stimulation. Moreover, we show that H2-D blockage inhibits the ability of immature B cells to transcribe the IFN-gamma gene and results in rescue of cytoskeletal rearrangement. Thus, Ly49D that is expressed on immature B cells recognizes MHC class I on the peripheral tissues, inducing the secretion of low levels of IFN-gamma and thereby down-regulating immature B cell homing to the lymph nodes or to sites of inflammation.

Update on pathways regulating the activation of uterine Natural Killer cells, their interactions with decidual spiral arteries and homing of their precursors to the uterus

Virgin adult C57Bl/6J mouse uterus contains a population of small, non-granulated Natural Killer (NK) cells with balanced expression of NK cell activating and inhibiting LY49 receptors. Coincident with blastocyst implantation and decidualization, uterine (u)NK cells become activated. The surface glycoslyation of uNK changes, the cells proliferate and they induce production of interferon (IFN)gamma, perforin, serine esterases and other molecules, including angiogenic factors. Mouse strains genetically ablated in uNK cells fail to undergo modification of spiral artery segments that branch from the uterine artery and feed into the placenta and these mice do not sustain a robust decidualization response. IFN-gamma is thought, from bone marrow transplantation and therapeutic studies, to be the key uNK-cell derived mediator regulating gene expression in vascular and decidual tissues. Here, we review recent studies showing that IL-15 is the critical cytokine controlling uNK cell differentiation and that uNK cells are activated by either IL-12 or IL-18 and by other factors when both IL-12 and IL-18 are genetically absent from implantation sites. We address possible roles of the IFN-gamma regulated gene alpha2-macroglobulin (alpha2-M) in regulation of the position of fetal trophoblast within the walls of the spiral arteries, and we discuss approaches that have been successful in evaluating mechanisms involved in homing of mouse uNK cell precursors to the uterus. These approaches maybe applicable to studies in women. Our studies show that complex immuno-physiological events contribute to spiral artery modification by mid-gestation in mice.

Expression of activation-induced, T cell-derived, and chemokine-related cytokine/lymphotactin and its functional role in rheumatoid arthritis

OBJECTIVE

To evaluate the possible role of activation-induced, T cell-derived, and chemokine-related cytokine (ATAC)/lymphotactin (Lptn) in the pathogenesis of rheumatoid arthritis (RA).

METHODS

ATAC/Lptn levels in serum and synovial fluid samples were measured by sandwich enzyme-linked immunosorbent assay. Expression of messenger RNA for ATAC/Lptn in synovial tissues was analyzed by reverse transcription-polymerase chain reaction (PCR) and by in situ hybridization, and was quantitated by real-time PCR. The phenotype of peripheral blood mononuclear cells (PBMCs) expressing ATAC/Lptn was analyzed by intracellular cytokine staining and flow cytometry.

RESULTS

Levels of ATAC/Lptn were similar in sera and synovial fluids from RA patients (n = 20) and osteoarthritis controls (n = 15). In phorbol myristate acetate/ionomycin-stimulated PBMCs, ATAC/Lptn expression was detected in CD8+ T cells and in a significantly increased proportion of CD4+,CD28- T cells from RA patients as compared with healthy controls. In synovial tissues, ATAC/Lptn was predominantly localized in CD3+ T cells in the sublining layer. Lymphocytes, synovial macrophages, and, unexpectedly, fibroblast-like synoviocytes (FLS) were identified as major target cells for ATAC/Lptn in RA synovium, as determined by analysis of the ATAC/Lptn receptor XCR1. In vitro, ATAC/Lptn stimulation of FLS resulted in a marked down-regulation of matrix metalloproteinase 2 production.

CONCLUSION

These data indicate that in RA synovium, ATAC/Lptn is mainly produced by T cells. Considering its function as a lymphocyte-specific chemoattractant, ATAC/Lptn might be a key modulator for T cell trafficking in the pathogenesis of RA. In addition, functional studies suggest that ATAC/Lptn may exert additional immunomodulatory effects in RA.

Natural killer cell receptor signaling

Natural killer (NK) cell immune responses are regulated by a balance of activating and inhibitory signals transmitted by cell surface receptors. Immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic domains of inhibitory NK receptors recruit tyrosine or lipid phosphatases, which modulate the activation signals transmitted by receptors linked to the Syk and ZAP70 tyrosine kinases and phosphatidylinositol-3 kinases. In addition, recent studies of gene-deficient animals, in particular Syk and ZAP70 double-deficient mice, suggest that NK cells possess a robust and potentially redundant receptor system to ensure their development and function.

Expression of IFN-gamma upon triggering of activating Ly49D NK receptors in vitro and in vivo: costimulation with IL-12 or IL-18 overrides inhibitory receptors

NK cells can express both activating and inhibitory Ly49 receptors on their cell surface. When cells expressing both receptors are presented with a ligand, inhibition dominates the functional outcome. In this report we demonstrate that costimulation of the activating Ly49D murine NK cell receptor with IL-12 or IL-18 is capable of over-riding the inhibitory Ly49G2 receptor blockade for cytokine production both in vitro and in vivo. This synergy is mediated by and dependent upon Ly49D-expressing NK cells and results in significant systemic expression of IFN-gamma. This would place NK cells and their activating Ly-49 receptors as important initiators of microbial, antiviral, and antitumor immunity and provide a mechanism for the release of activating Ly49 receptors from inhibitory receptor blockade.

Dynamics of Ly49 expressing cytotoxic lymphocyte subsets in response to virus infection

Viral infections induce first a loss and then an increase in natural killer (NK) and CD8(+) T cells. NK cells expressing Ly49G2 were selectively expanded by several viruses and poly I:C. CD8(+) T cells expressing Ly49G2 were selectively expanded by poly I:C and participated in the antigen-specific response to lymphocytic choriomeningitis virus.

Aberrant DAP12 signaling in the 129 strain of mice: implications for the analysis of gene-targeted mice

NK cells are implicated in antiviral responses, bone marrow transplantation and tumor immunosurveillance. Their function is controlled, in part, through the Ly49 family of class I binding receptors. Inhibitory Ly49s suppress signaling, while activating Ly49s (i.e., Ly49D) activate NK cells via the DAP12 signaling chain. Activating Ly49 signaling has been studied primarily in C57BL/6 mice, however, 129 substrains are commonly used in gene-targeting experiments. In this study, we show that in contrast to C57BL/6 NK cells, cross-linking of DAP12-coupled receptors in 129/J mice induces phosphorylation of DAP12 but not calcium mobilization or cytokine production. Consistent with poor-activating Ly49 function, 129/J mice reject bone marrow less efficiently than C57BL/6 mice. Sequence analysis of receptors and DAP12 suggests no structural basis for inactivity, and both the 129/J and C57BL/6 receptors demonstrate normal function in a reconstituted receptor system. Most importantly, reconstitution of Ly49D in 129/J NK cells demonstrated that the signaling deficit is within the NK cells themselves. These unexpected findings bring into question any NK analysis of 129/J, 129Sv, or gene-targeted mice derived from these strains before complete backcrossing, and provide a possible explanation for the differences observed in the immune response of 129 mice in a variety of models.

Genetic control of innate immune responses against cytomegalovirus: MCMV meets its match

Cytomegalovirus (CMV) is a widespread pathogen that is responsible for severe disease in immunocompromised individuals and probably, associated with vascular disease in the general population. There is increasing evidence that cells of the innate immune system play a key role in controlling this important pathogen. This is particularly evident in the experimental murine CMV (MCMV) model of infection which has revealed an important role for natural killer (NK) cells in controlling early viral replication after infection with MCMV. In this model, different strains of inbred mice exhibit striking differences in their level of susceptibility to MCMV infection. Genetic studies, performed almost 10 years ago, revealed that this pattern of susceptibility/resistance can be attributed to a single genetic locus termed Cmv1 and recently several groups that have been working on the mapping and identification of Cmv1 have met with success. Interestingly, Cmv1 is allelic to a member of the Ly49 gene family, which encode activating or inhibitory transmembrane receptors present on the surface of NK cells. All Ly49 receptors characterized to date interact with MHC class I molecules on potential target cells, resulting in the accumulation of signals to the NK to either 'kill' or 'ignore' the cell based upon the repertoire of MHC class I molecules expressed. The identification of Cmv1 as Ly49H, a stimulatory member of the Ly49 family, adds an interesting twist to the Ly49 story. Although the ligand of Ly49H is not yet known, there is already compelling evidence that the ligand is upregulated on virally infected cells, resulting in specific activation of Ly49H-expressing NK cells. This review provides an historical perspective of the MCMV infection model from its inception to the discovery of the gene responsible for the phenotype and provides a basis for further experiments aimed at understanding the role of NK cells, in general, and Ly49H, in particular, in mediating resistance to cytomegalovirus.

MIP-1alpha, MIP-1beta, RANTES, and ATAC/lymphotactin function together with IFN-gamma as type 1 cytokines

We analyzed for the first time the expression of chemokines in subpopulations of the murine immune system at the single-cell level. We demonstrate in vitro and in a model of murine listeriosis that macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, regulated on activation normal T cell expressed and secreted (RANTES), and activation-induced, T cell-derived, and chemokine-related cytokine (ATAC)/lymphotactin are cosecreted to a high degree with IFN-gamma by activated individual natural killer (NK), CD8(+) T, and CD4(+) T helper 1 (Th1) cells. Functionally, ATAC and the CC chemokines cooperate with IFN-gamma in the up-regulation of CD40, IL-12, and tumor necrosis factor-alpha, molecules playing a central role in the effector phase of macrophages. Our data indicate that (i) MIP-1alpha, MIP-1beta, RANTES, and ATAC are not only chemoattractants but also coactivators of macrophages, (ii) MIP-1alpha, MIP-1beta, RANTES, and ATAC constitute together with IFN-gamma a group of "type 1 cytokines," and (iii) these cytokines act together as a functional unit that is used by NK cells in the innate phase and then "handed over" to CD8(+) T cells in the antigen-specific phase of the immune defense, thus bridging the two components of a Th1 immune reaction.

Regulation of nuclear gamma interferon gene expression by interleukin 12 (IL-12) and IL-2 represents a novel form of posttranscriptional control

Posttranscriptional control of gamma interferon (IFN-gamma) gene expression has not been extensively studied and is poorly understood. Our work describes a posttranscriptional mechanism that modulates IFN-gamma mRNA expression in stimulated natural killer (NK) cells through nuclear retention of the IFN-gamma mRNA. This is evidenced by the elevated and sustained nuclear accumulation of both precursor and processed IFN-gamma mRNAs in NK cells stimulated with interleukin-12 (IL-12). The elevated nuclear mRNA accumulation persists long after transcriptional activity has subsided and the rate of cytoplasmic IFN-gamma mRNA accumulation has dropped. The IL-12-induced nuclear retention of the IFN-gamma mRNA prevails until a secondary cytokine stimulus is received. The secondary stimulus, which is initiated by IL-2, mediates transcription-independent movement of the nuclear IFN-gamma mRNA. Concurrent with the nucleocytoplasmic movement of the IFN-gamma mRNA, we have observed increases in the amount of processed nuclear IFN-gamma mRNA that are greater than that seen for the unprocessed IFN-gamma mRNA. The increase in processed IFN-gamma mRNA appears to be due to increased mRNA stability which then promotes increased nucleocytoplasmic shuttling of the mature IFN-gamma mRNA. These data support a model whereby mobilization of nuclear IFN-gamma mRNA stores allows NK cells to rapidly and robustly respond to secondary cytokine activators in a transcription-independent manner, thus shortening the time for overall cellular response to inflammatory signals.