NK cell-activating receptors require PKC-theta for sustained signaling, transcriptional activation, and IFN-gamma secretion

Euphohelioscopin A enhances NK cell antitumor immunity through GSDME-triggered pyroptosis

Immune evasion by cancer cells poses a significant challenge for natural killer cell-based immunotherapy. Pyroptosis, a newly discovered form of programmed cell death, has shown great potential for enhancing the antitumor immunity of natural killer cells. Consequently, targeting pyroptosis has become an attractive strategy for boosting natural killer cell activity against cancer. In this study, various assays were conducted, including natural killer cell cytotoxicity assays, flow cytometry, xenograft tumor models, real-time polymerase chain reaction, and enzyme-linked immunosorbent assay, to assess natural killer cell-mediated cell killing, as well as gene and protein expressions. The results indicated that euphohelioscopin A, a potential pyroptosis activator, enhances natural killer cell-mediated lysis of tumor cells, resulting in inhibiting tumor growth that could be reversed by natural killer cell depletion. Furthermore, we found that euphohelioscopin A significantly enhanced IFNγ production in natural killer cells and synergistically upregulated GSDME with IFNγ in cancer cells. Euphohelioscopin A also increased the cleavage of GSDME, promoting granzyme B-induced pyroptosis, which could be reversed by GSDME knockdown and IFNγ blockade. Overall, the findings suggested that euphohelioscopin A enhanced natural killer cell-mediated killing of cancer cells by triggering pyroptosis, making euphohelioscopin A a promising pyroptosis activator with great potential for use in natural killer cell-based cancer immunotherapy.

Defining the Effects of PKC Modulator HIV Latency-Reversing Agents on Natural Killer Cells

Background

Latency reversing agents (LRAs) such as protein kinase C (PKC) modulators can reduce rebound-competent HIV reservoirs in small animal models. Furthermore, administration of natural killer (NK) cells following LRA treatment improves this reservoir reduction. It is currently unknown why the combination of a PKC modulator and NK cells is so potent and whether exposure to PKC modulators may augment NK cell function in some way.

Methods

Primary human NK cells were treated with PKC modulators (bryostatin-1, prostratin, or the designed, synthetic bryostatin-1 analog SUW133), and evaluated by examining expression of activation markers by flow cytometry, analyzing transcriptomic profiles by RNA sequencing, measuring cytotoxicity by co-culturing with K562 cells, assessing cytokine production by Luminex assay, and examining the ability of cytokines and secreted factors to independently reverse HIV latency by co-culturing with Jurkat-Latency (J-Lat) cells.

Results

PKC modulators increased expression of proteins involved in NK cell activation. Transcriptomic profiles from PKC-treated NK cells displayed signatures of cellular activation and enrichment of genes associated with the NFκB pathway. NK cell cytotoxicity was unaffected by prostratin but significantly decreased by bryostatin-1 and SUW133. Cytokines from PKC-stimulated NK cells did not induce latency reversal in J-Lat cell lines.

Conclusions

Although PKC modulators have some significant effects on NK cells, their contribution in "kick and kill" strategies is likely due to upregulating HIV expression in CD4+ T cells, not directly enhancing the effector functions of NK cells. This suggests that PKC modulators are primarily augmenting the "kick" rather than the "kill" arm of this HIV cure approach.

T-BET and EOMES sustain mature human NK cell identity and antitumor function

Since the T-box transcription factors (TFs) T-BET and EOMES are necessary for initiation of NK cell development, their ongoing requirement for mature NK cell homeostasis, function, and molecular programming remains unclear. To address this, T-BET and EOMES were deleted in unexpanded primary human NK cells using CRISPR/Cas9. Deleting these TFs compromised in vivo antitumor response of human NK cells. Mechanistically, T-BET and EOMES were required for normal NK cell proliferation and persistence in vivo. NK cells lacking T-BET and EOMES also exhibited defective responses to cytokine stimulation. Single-cell RNA-Seq revealed a specific T-box transcriptional program in human NK cells, which was rapidly lost following T-BET and EOMES deletion. Further, T-BET- and EOMES-deleted CD56bright NK cells acquired an innate lymphoid cell precursor-like (ILCP-like) profile with increased expression of the ILC-3-associated TFs RORC and AHR, revealing a role for T-box TFs in maintaining mature NK cell phenotypes and an unexpected role of suppressing alternative ILC lineages. Our study reveals the critical importance of sustained EOMES and T-BET expression to orchestrate mature NK cell function and identity.

Human NK cells responses are enhanced by CD56 engagement

Natural Killer (NK) cells are important innate lymphocytes for effective immune responses against intracellular pathogens and tumors. CD56 is a well-known marker for human NK cells, but there is very limited information about a functional role of this surface receptor. Here we show that engagement of CD56 can induce NK cell activation resulting in degranulation, IFN-γ secretion and morphological changes, making CD56 a potential co-activating receptor in NK cells. Interestingly, this effect was only observed in cytokine pre-activated and not in freshly isolated human NK cells, demonstrating that NK cell reactivity upon CD56 engagement was dependent on cytokine stimulation. Inhibition of Syk, PI3K, Erk and src-family-kinases impaired CD56-mediated NK cell stimulation. Finally, we used CRISPR/Cas9 to delete CD56 from primary human NK cells. While this abolished the stimulatory effect of CD56 on pre-activated NK cells, the cytotoxic activity of NK cells against several tumor target cells was not affected by the absence of CD56. This demonstrates that the stimulating effect of CD56 on pre-activated NK cells does not have a major impact on their cytotoxic activity, but it may contribute to the function of CD56 as a fungal recognition receptor and in the NK cell developmental synapse. This article is protected by copyright. All rights reserved.

Inhibition of SHP-1 activity by PKC-θ regulates NK cell activation threshold and cytotoxicity

Natural killer (NK) cells play a crucial role in immunity, killing virally infected and cancerous cells. The balance of signals initiated upon engagement of activating and inhibitory NK receptors with cognate ligands determines killing or tolerance. Nevertheless, the molecular mechanisms regulating rapid NK cell discrimination between healthy and malignant cells in a heterogeneous tissue environment are incompletely understood. The SHP-1 tyrosine phosphatase is the central negative NK cell regulator that dephosphorylates key activating signaling proteins. Though the mechanism by which SHP-1 mediates NK cell inhibition has been partially elucidated, the pathways by which SHP-1 is itself regulated remain unclear. Here, we show that phosphorylation of SHP-1 in NK cells on the S591 residue by PKC-θ promotes the inhibited SHP-1 ‘folded’ state. Silencing PKC-θ maintains SHP-1 in the active conformation, reduces NK cell activation and cytotoxicity, and promotes tumor progression in vivo. This study reveals a molecular pathway that sustains the NK cell activation threshold through suppression of SHP-1 activity.

Epigenetic regulation of natural killer cell memory

Immunological memory is the underlying mechanism by which the immune system remembers previous encounters with pathogens to produce an enhanced secondary response upon re-encounter. It stands as the hallmark feature of the adaptive immune system and the cornerstone of vaccine development. Classic recall responses are executed by conventional T and B cells, which undergo somatic recombination and modify their receptor repertoire to ensure recognition of a vast number of antigens. However, recent evidence has challenged the dogma that memory responses are restricted to the adaptive immune system, which has prompted a reevaluation of what delineates "immune memory." Natural killer (NK) cells of the innate immune system have been at the forefront of these pushed boundaries, and have proved to be more "adaptable" than previously thought. Like T cells, we now appreciate that their "natural" abilities actually require a myriad of signals for optimal responses. In this review, we discuss the many signals required for effector and memory NK cell responses and the epigenetic mechanisms that ultimately endow their enhanced features.

Tumor-derived NKG2D ligand sMIC reprograms NK cells to an inflammatory phenotype through CBM signalosome activation

Natural Killer (NK) cell dysfunction is associated with poorer clinical outcome in cancer patients. What regulates NK cell dysfunction in tumor microenvironment is not well understood. Here, we demonstrate that the human tumor-derived NKG2D ligand soluble MIC (sMIC) reprograms NK cell to secrete pro-tumorigenic cytokines with diminished cytotoxicity and polyfunctional potential. Antibody clearing sMIC restores NK cell to a normal cytotoxic effector functional state. We discovered that sMIC selectively activates the CBM-signalosome inflammatory pathways in NK cells. Conversely, tumor cell membrane-bound MIC (mMIC) stimulates NK cell cytotoxicity through activating PLC2γ2/SLP-76/Vav1 pathway. Ultimately, antibody targeting sMIC effectuated the in vivo anti-tumor effect of adoptively transferred NK cells. Our findings uncover an unrecognized mechanism that could instruct NK cell to a dysfunctional state in response to cues in the tumor microenvironment. Our findings provide a rationale for co-targeting sMIC to enhance the efficacy of the ongoing NK cell-based cancer immunotherapy.

Immune checkpoint markers and anti-CD20-mediated NK cell activation

Anti-CD20 mAb is an effective therapy for most B-cell malignancies. Checkpoint blockade has been used to enhance T-cell-mediated antitumor response. Little is known about the biologic significance of immune checkpoints expressed by NK cells in anti-CD20-based therapy. To investigate the role of checkpoints in anti-CD20-mediated NK cell biology, Raji B-cell lymphoma cells, and PBMCs from normal donors were cocultured with rituximab (RTX), obinutuzumab (OBZ), or trastuzumab as a control mAb for between 20 h and 9 d. RTX and OBZ induced a dose-dependent NK cell up-regulation of T-cell immunoreceptor with Ig and ITIM domain (TIGIT) and T-cell immunoglobulin mucin-3 (TIM3), but not PD1, CTLA4, or LAG3. Resting CD56^dim NK had higher TIGIT and TIM3 expression than resting CD56^bright NK although TIGIT and TIM3 were up-regulated on both subsets. NK cells with the CD16 158VV single nucleotide polymorphism had greater TIM3 up-regulation than did NK from VF or FF donors. TIGIT⁺ and TIM3⁺ NK cells degranulated, produced cytokines, and expressed activation markers to a greater degree than did TIGIT^- or TIM3^- NK cells. Blockade of TIGIT, TIM3, or both had little impact on RTX-induced NK cell proliferation, degranulation, cytokine production, or activation. Taken together, TIGIT and TIM3 can serve as markers for anti-CD20-mediated NK cell activation, but may not serve well as targets for enhancing the anti-tumor activity of such therapy.

Coxiella burnetii-Infected NK Cells Release Infectious Bacteria by Degranulation

Natural killer (NK) cells are critically involved in the early immune response against various intracellular pathogens, including Coxiella burnetii and Chlamydia psittaci Chlamydia-infected NK cells functionally mature, induce cellular immunity, and protect themselves by killing the bacteria in secreted granules. Here, we report that infected NK cells do not allow intracellular multiday growth of Coxiella, as is usually observed in other host cell types. C. burnetii-infected NK cells display maturation and gamma interferon (IFN-γ) secretion, as well as the release of Coxiella-containing lytic granules. Thus, NK cells possess a potent program to restrain and expel different types of invading bacteria via degranulation. Strikingly, though, in contrast to Chlamydia, expulsed Coxiella organisms largely retain their infectivity and, hence, escape the cell-autonomous self-defense mechanism in NK cells.

Metalloimmunology: The metal ion-controlled immunity

Metals are essential components in all forms of life required for the function of nearly half of all enzymes and are critically involved in virtually all fundamental biological processes. Especially, the transition metals iron (Fe), zinc (Zn), manganese (Mn), nickel (Ni), copper (Cu) and cobalt (Co) are crucial micronutrients known to play vital roles in metabolism as well due to their unique redox properties. Metals carry out three major functions within metalloproteins: to provide structural support, to serve as enzymatic cofactors, and to mediate electron transportation. Metal ions are also involved in the immune system from metal allergies to nutritional immunity. Within the past decade, much attention has been drawn to the roles of metal ions in the immune system, since increasing evidence has mounted to suggest that metals are critically implicated in regulating both the innate immune sensing of and the host defense against invading pathogens. The importance of ions in immunity is also evidenced by the identification of various immunodeficiencies in patients with mutations in ion channels and transporters. In addition, cancer immunotherapy has recently been conclusively demonstrated to be effective and important for future tumor treatment, although only a small percentage of cancer patients respond to immunotherapy because of inadequate immune activation. Importantly, metal ion-activated immunotherapy is becoming an effective and potential way in tumor therapy for better clinical application. Nevertheless, we are still in a primary stage of discovering the diverse immunological functions of ions and mechanistically understanding the roles of these ions in immune regulation. This review summarizes recent advances in the understanding of metal-controlled immunity. Particular emphasis is put on the mechanisms of innate immune stimulation and T cell activation by the essential metal ions like calcium (Ca²⁺), zinc (Zn²⁺), manganese (Mn²⁺), iron (Fe²⁺/Fe³⁺), and potassium (K⁺), followed by a few unessential metals, in order to draw a general diagram of metalloimmunology.

Mirc11 Disrupts Inflammatory but Not Cytotoxic Responses of NK Cells

Natural killer (NK) cells generate proinflammatory cytokines that are required to contain infections and tumor growth. However, the posttranscriptional mechanisms that regulate NK cell functions are not fully understood. Here, we define the role of the microRNA cluster known as Mirc11 (which includes miRNA-23a, miRNA-24a, and miRNA-27a) in NK cell-mediated proinflammatory responses. Absence of Mirc11 did not alter the development or the antitumor cytotoxicity of NK cells. However, loss of Mirc11 reduced generation of proinflammatory factors in vitro and interferon-γ-dependent clearance of Listeria monocytogenes or B16F10 melanoma in vivo by NK cells. These functional changes resulted from Mirc11 silencing ubiquitin modifiers A20, Cbl-b, and Itch, allowing TRAF6-dependent activation of NF-κB and AP-1. Lack of Mirc11 caused increased translation of A20, Cbl-b, and Itch proteins, resulting in deubiquitylation of scaffolding K63 and addition of degradative K48 moieties on TRAF6. Collectively, our results describe a function of Mirc11 that regulates generation of proinflammatory cytokines from effector lymphocytes.

NK cell-intrinsic FcεRIγ limits CD8+ T-cell expansion and thereby turns an acute into a chronic viral infection

During viral infection, tight regulation of CD8⁺ T-cell functions determines the outcome of the disease. Recently, others and we determined that the natural killer (NK) cells kill hyperproliferative CD8⁺ T cells in the context of viral infection, but molecules that are involved in shaping the regulatory capability of NK cells remain virtually unknown. Here we used mice lacking the Fc-receptor common gamma chain (FcRγ, FcεRIγ, Fcer1g^–/– mice) to determine the role of Fc-receptor and NK-receptor signaling in the process of CD8⁺ T-cell regulation. We found that the lack of FcRγ on NK cells limits their ability to restrain virus-specific CD8⁺ T cells and that the lack of FcRγ in Fcer1g^–/– mice leads to enhanced CD8⁺ T-cell responses and rapid control of the chronic docile strain of the lymphocytic choriomeningitis virus (LCMV). Mechanistically, FcRγ stabilized the expression of NKp46 but not that of other killer cell–activating receptors on NK cells. Although FcRγ did not influence the development or activation of NK cell during LCMV infection, it specifically limited their ability to modulate CD8⁺ T-cell functions. In conclusion, we determined that FcRγ plays an important role in regulating CD8⁺ T-cell functions during chronic LCMV infection.

FcRγ is a signaling molecule for Fc receptors and NK cell killer activating receptor (KAR) complex. FcRγ is highly expressed by NK cells and involved in NK cell activity. NK cells are widely defined to regulate the expansion of T cells. Here using chronic LCMV model, we described the role of FcRγ in NK cell mediated shaping of CD8⁺ T cell response and viral control. We observed that FcRγ does not affect the early activity of NK cells which is mainly innate immune cytokines driven, but rather the specific activation due to NKp46 inadequacy. We detected that FcRγ stabilizes NKp46 protein by preventing it from proteasomal degradation. Due to lack of NKp46 expression in absence of FcRγ, we observed strong CD8⁺ T cell response and faster viral clearance during chronic LCMV infection. These data demonstrate that FcRγ is crucial for specific activity of NK cells for regulation of CD8⁺ T cell response during viral infection.

NK Cell-Mediated Processing Of Chlamydia psittaci Drives Potent Anti-Bacterial Th1 Immunity

Natural killer (NK) cells are innate immune cells critically involved in the early immune response against various pathogens including chlamydia. Here, we demonstrate that chlamydia-infected NK cells prevent the intracellular establishment and growth of the bacteria. Upon infection, they display functional maturation characterized by enhanced IFN-γ secretion, CD146 induction, PKCϴ activation, and granule secretion. Eventually, chlamydia are released in a non-infectious, highly immunogenic form driving a potent Th1 immune response. Further, anti-chlamydial antibodies generated during immunization neutralize the infection of epithelial cells. The release of chlamydia from NK cells requires PKCϴ function and active degranulation, while granule-associated granzyme B drives the loss of chlamydial infectivity. Cellular infection and bacterial release can be undergone repeatedly and do not affect NK cell function. Strikingly, NK cells passing through such an infection cycle significantly improve their cytotoxicity. Thus, NK cells not only protect themselves against productive chlamydial infections but also actively trigger potent anti-bacterial responses.

Transcription factor ID2 prevents E proteins from enforcing a naïve T lymphocyte gene program during NK cell development

All innate lymphoid cells (ILCs) require the small helix-loop-helix transcription factor ID2, but the functions of ID2 are not well understood in these cells. We show that mature natural killer (NK) cells, the prototypic ILCs, developed in mice lacking ID2 but remained as precursor CD27⁺CD11b^- cells that failed to differentiate into CD27^-CD11b⁺ cytotoxic effectors. We show that ID2 limited chromatin accessibility at E protein binding sites near naïve T lymphocyte-associated genes including multiple chemokine receptors, cytokine receptors, and signaling molecules and altered the NK cell response to inflammatory cytokines. In the absence of ID2, CD27⁺CD11b^- NK cells expressed ID3, a helix-loop-helix protein associated with naïve T cells, and they transitioned from a CD8 memory precursor-like to a naïve-like chromatin accessibility state. We demonstrate that ID3 was required for the development of ID2-deficient NK cells, indicating that completely unfettered E protein function is incompatible with NK cell development. These data solidify the roles of ID2 and ID3 as mediators of effector and naïve gene programs, respectively, and revealed a critical role for ID2 in promoting a chromatin state and transcriptional program in CD27⁺CD11b^- NK cells that supports cytotoxic effector differentiation and cytokine responses.

CD56bright cells respond to stimulation until very advanced age revealing increased expression of cellular protective proteins SIRT1, HSP70 and SOD2

BACKGROUND

NK cells are cytotoxic lymphocytes of innate immunity composed of: cytotoxic CD56dim and immunoregulatory CD56bright cells. The study aimed to analyze the expression of cellular protective proteins: sirtuin 1 (SIRT1), heat shock protein 70 (HSP70) and manganese superoxide dismutase (SOD2) in CD56dim and CD56bright NK cells of the young, seniors aged under 85 ('the old') and seniors aged over 85 ('the oldest'). We studied both non-stimulated NK cells and cells stimulated by IL-2, LPS or PMA with ionomycin. The expression level of proinflammatory cytokines TNF and IFN-γ was also assessed in NK cell subsets and some relationships between the studied parameters were analyzed.

RESULTS

CD56bright cells showed sensitivity to most of the applied stimulatory agents until very advanced age in regards to the expression of SIRT1 and intracellular HSP70. On the contrary, CD56dim cells, sensitive to stimulation by most of the stimulatory agents in the young and the old, in the oldest lost this sensitivity and presented rather high, constant expression of SIRT1 and HSP70, resistant to further stimulation. With reference to SOD2 expression, CD56dim cells were insensitive to stimulation in the young, but their sensitivity increased with ageing. CD56bright cells were sensitive to most of the applied agents in the young and the old but in the oldest they responded to all of the stimulatory agents used in the study. Similarly, both NK cell subsets were sensitive to stimulation until very advanced age in regards to the expression of TNF and IFN-γ.

CONCLUSIONS

CD56bright cells maintained sensitivity to stimulation until very advanced age presenting also an increased expression of SIRT1 and HSP70. CD56dim cells showed a constantly increased expression of these cellular protective proteins in the oldest, insensitive for further stimulation. The oldest, however, did not reveal an increased level of SOD2 expression, but it was significantly elevated in both NK cell subsets after stimulation.The pattern of expression of the studied cellular protective proteins in ageing process revealed the adaptation of NK cells to stress response in the oldest seniors which might accompany the immunosenescence and contribute to the long lifespan of this group of the elderly.

Epigenetic control of innate and adaptive immune memory

Clonal expansion and immunological memory are hallmark features of the mammalian adaptive immune response and essential for prolonged host control of pathogens. Recent work demonstrates that natural killer (NK) cells of the innate immune system also exhibit these adaptive traits during infection. Here we demonstrate that differentiating and 'memory' NK cells possess distinct chromatin accessibility states and that their epigenetic profiles reveal a 'poised' regulatory program at the memory stage. Furthermore, we elucidate how individual STAT transcription factors differentially control epigenetic and transcriptional states early during infection. Finally, concurrent chromatin profiling of the canonical CD8+ T cell response against the same infection demonstrated parallel and distinct epigenetic signatures defining NK cells and CD8+ T cells. Overall, our study reveals the dynamic nature of epigenetic modifications during the generation of innate and adaptive lymphocyte memory.

Natural Killer Cells: Development, Maturation, and Clinical Utilization

Natural killer (NK) cells are the predominant innate lymphocyte subsets that mediate anti-tumor and anti-viral responses, and therefore possess promising clinical utilization. NK cells do not express polymorphic clonotypic receptors and utilize inhibitory receptors (killer immunoglobulin-like receptor and Ly49) to develop, mature, and recognize “self” from “non-self.” The essential roles of common gamma cytokines such as interleukin (IL)-2, IL-7, and IL-15 in the commitment and development of NK cells are well established. However, the critical functions of pro-inflammatory cytokines IL-12, IL-18, IL-27, and IL-35 in the transcriptional-priming of NK cells are only starting to emerge. Recent studies have highlighted multiple shared characteristics between NK cells the adaptive immune lymphocytes. NK cells utilize unique signaling pathways that offer exclusive ways to genetically manipulate to improve their effector functions. Here, we summarize the recent advances made in the understanding of how NK cells develop, mature, and their potential translational use in the clinic.

In Vitro Exposure to Prostratin but Not Bryostatin-1 Improves Natural Killer Cell Functions Including Killing of CD4+ T Cells Harboring Reactivated Human Immunodeficiency Virus

In the attempt of purging the HIV-1 reservoir through the “shock-and-kill” strategy, it is important to select latency-reversing agents (LRAs) devoid of deleterious effects on the antiviral function of immune effector cells. Here, we investigated two LRAs with PKC agonist activity, prostratin (PRO) and bryostatin-1 (BRY), for their impact on the function of natural killer (NK) cells, the major effectors of innate immunity whose potential in HIV-1 eradication has emerged in recent clinical trials. Using NK cells of healthy donors, we found that exposure to either PRO or BRY potently activated NK cells, resulting in upmodulation of NKG2D and NKp44 activating receptors and matrix metalloprotease-mediated shedding of CD16 receptor. Despite PRO and BRY affected NK cell phenotype in the same manner, their impact on NK cell function was diverse and showed considerable donor-to-donor variation. Altogether, in most tested donors, the natural cytotoxicity and antibody-dependent cellular cytotoxicity (ADCC) of NK cells were either improved or maintained by PRO, while both activities were impaired by BRY. Moreover, we analyzed the effect of these drugs on the capacity of treated NK cells to kill autologous latently infected CD4⁺ T cells reactivated via the same treatment. First, we found that PRO but not BRY increased upmodulation of the ULBP2 ligand for NKG2D on reactivated p24⁺ cells. Importantly, we showed that clearance of reactivated p24⁺ cells by NK cells was enhanced when both targets and effectors were exposed to PRO but not to BRY. Overall, PRO had a superior potential compared with BRY as to the impact on key NK cell functions and on NK-cell-mediated clearance of the HIV-1 reservoir. Our results emphasize the importance of considering the effects on NK cells of candidate “shock-and-kill” interventions. With respect to combinative approaches, the impact on NK cells of each LRA should be re-evaluated upon combination with a second LRA, which may have analogous or opposite effects, or with immunotherapy targeting NK cells. In addition, avoiding co-administration of LRAs that negatively impact ADCC activity by NK cells might be essential for successful application of antibodies or vaccination to “shock-and-kill” strategies.

Lymphocytes in Placental Tissues: Immune Regulation and Translational Possibilities for Immunotherapy

Immune modulation at the fetomaternal interface is crucial to ensure that the fetal allograft is not rejected. In the present review, the focus is to describe basic functions of lymphocyte populations and how they may contribute to fetomaternal immune regulation, as well as determining what proportions and effector functions of these cells are reported to be present in placental tissues in humans. Also explored is the possibility that unique cell populations at the fetomaternal interface may be targets for adoptive cell therapy. Increasing the understanding of immune modulation during pregnancy can give valuable insight into other established fields such as allogeneic hematopoietic stem cell transplantation and solid organ transplantation. In these settings, lymphocytes are key components that contribute to inflammation and rejection of either patient or donor tissues following transplantation. In contrast, an allogeneic fetus eludes rejection by the maternal immune system.

In silico modeling identifies CD45 as a regulator of IL-2 synergy in the NKG2D-mediated activation of immature human NK cells

Natural killer (NK) cells perform immunosurveillance of virally infected and transformed cells, and their activation depends on the balance between signaling by inhibitory and activating receptors. Cytokine receptor signaling can synergize with activating receptor signaling to induce NK cell activation. We investigated the interplay between the signaling pathways stimulated by the cytokine interleukin-2 (IL-2) and the activating receptor NKG2D in immature (CD56^bright) and mature (CD56^dim) subsets of human primary NK cells using mass cytometry experiments and in silico modeling. Our analysis revealed that IL-2 changed the abundances of several key proteins, including NKG2D and the phosphatase CD45. Furthermore, we found differences in correlations between protein abundances, which were associated with the maturation state of the NK cells. The mass cytometry measurements also revealed that the signaling kinetics of key protein abundances induced by NKG2D stimulation depended on the maturation state and the pretreatment condition of the NK cells. Our in silico model, which described the multidimensional data with coupled first-order reactions, predicted that the increase in CD45 abundance was a major enhancer of NKG2D-mediated activation in IL-2-treated CD56^bright NK cells but not in IL-2-treated CD56^dim NK cells. This dependence on CD45 was verified by measurement of CD107a mobilization to the NK cell surface (a marker of activation). Our mathematical framework can be used to glean mechanisms underlying synergistic signaling pathways in other activated immune cells.

Protein kinase Cθ controls type 2 innate lymphoid cell and TH2 responses to house dust mite allergen

BACKGROUND

Protein kinase C (PKC) θ, a serine/threonine kinase, is involved in T_H2 cell activation and proliferation. Type 2 innate lymphoid cells (ILC2s) resemble T_H2 cells and produce the T_H2 cytokines IL-5 and IL-13 but lack antigen-specific receptors. The mechanism by which PKC-θ drives innate immune cells to instruct T_H2 responses in patients with allergic lung inflammation remains unknown.

OBJECTIVES

We hypothesized that PKC-θ contributes to ILC2 activation and might be necessary for ILC2s to instruct the T_H2 response.

METHODS

PRKCQ gene expression was assessed in innate lymphoid cell subsets purified from human PBMCs and mouse lung ILC2s. ILC2 activation and eosinophil recruitment, T_H2-related cytokine and chemokine production, lung histopathology, interferon regulatory factor 4 (IRF4) mRNA expression, and nuclear factor of activated T cells (NFAT1) protein expression were determined. Adoptive transfer of ILC2s from wild-type mice was performed in wild-type and PKC-θ-deficient (PKC-θ^-/-) mice.

RESULTS

Here we report that PKC-θ is expressed in both human and mouse ILC2s. Mice lacking PKC-θ had reduced ILC2 numbers, T_H2 cell numbers and activation, airway hyperresponsiveness, and expression of the transcription factors IRF4 and NFAT1. Importantly, adoptive transfer of ILC2s restored eosinophil influx and IL-4, IL-5 and IL-13 production in lung tissue, as well as T_H2 cell activation. The pharmacologic PKC-θ inhibitor (Compound 20) administered during allergen challenge reduced ILC2 numbers and activation, as well as airway inflammation and IRF4 and NFAT1 expression.

CONCLUSIONS

Therefore our findings identify PKC-θ as a critical factor for ILC2 activation that contributes to T_H2 cell differentiation, which is associated with IRF4 and NFAT1 expression in allergic lung inflammation.

Signaling in Effector Lymphocytes: Insights toward Safer Immunotherapy

Receptors on T and NK cells systematically propagate highly complex signaling cascades that direct immune effector functions, leading to protective immunity. While extensive studies have delineated hundreds of signaling events that take place upon receptor engagement, the precise molecular mechanism that differentially regulates the induction or repression of a unique effector function is yet to be fully defined. Such knowledge can potentiate the tailoring of signal transductions and transform cancer immunotherapies. Targeted manipulations of signaling cascades can augment one effector function such as antitumor cytotoxicity while contain the overt generation of pro-inflammatory cytokines that contribute to treatment-related toxicity such as “cytokine storm” and “cytokine-release syndrome” or lead to autoimmune diseases. Here, we summarize how individual signaling molecules or nodes may be optimally targeted to permit selective ablation of toxic immune side effects.

Targeting PKCθ in skeletal muscle and muscle diseases: good or bad?

Protein kinase Cθ (PKCθ) is a member of the novel calcium-independent PKC family, with a relatively selective tissue distribution. Most studies have focused on its unique role in T-lymphocyte activation and suggest that inhibition of PKCθ could represent a novel therapeutic approach in the treatment of chronic inflammation, autoimmunity and allograft rejection. However, considering that PKCθ is also expressed in other cell types, including skeletal muscle cells, it is important to understand its function in different tissues before proposing it as a molecular target for the treatment of immune-mediated diseases. A number of studies have highlighted the role of PKCθ in mediating several intracellular pathways, regulating muscle cell development, homoeostasis and remodelling, although a comprehensive picture is still lacking. Moreover, we recently showed that lack of PKCθ in a mouse model of Duchenne muscular dystrophy (DMD) ameliorates the progression of the disease. In the present article, we review new developments in our understanding of the involvement of PKCθ in intracellular mechanisms regulating skeletal muscle development, growth and maintenance under physiological conditions and recent advances showing a hitherto unrecognized role of PKCθ in promoting muscular dystrophy.

The Novel PKCθ from Benchtop to Clinic

The protein kinases C (PKCs) are a family of serine/threonine kinases involved in regulating multiple essential cellular processes such as survival, proliferation, and differentiation. Of particular interest is the novel, calcium-independent PKCθ which plays a central role in immune responses. PKCθ shares structural similarities with other PKC family members, mainly consisting of an N-terminal regulatory domain and a C-terminal catalytic domain tethered by a hinge region. This isozyme, however, is unique in that it translocates to the immunological synapse between a T cell and an antigen-presenting cell (APC) upon T cell receptor-peptide MHC recognition. Thereafter, PKCθ interacts physically and functionally with downstream effectors to mediate T cell activation and differentiation, subsequently leading to inflammation. PKCθ-specific perturbations have been identified in several diseases, most notably autoimmune disorders, and hence the modulation of its activity presents an attractive therapeutic intervention. To that end, many inhibitors of PKCs and PKCθ have been developed and tested in preclinical and clinical studies. And although selectivity remains a challenge, results are promising for the future development of effective PKCθ inhibitors that would greatly advance the treatment of several T-cell mediated diseases.

The immune suppressive function of transforming growth factor-β (TGF-β) in human diseases

Transforming growth factor-β (TGF-β) functions as an immune suppressor by influencing immune cells' development, differentiation, tolerance induction and homeostasis. In human diseases, TGF-β has been revealed as an essential regulator of both innate and adaptive functions in autoimmune diseases. Furthermore, it plays a significant role in cancer by inhibiting immunosurveillance in the tumor-bearing host. A variety of TGF-β neutralizing anti-cancer therapies have been investigated based on the role of TGF-β in immunosuppression. New studies are focusing on combining TGF-β blockade with tumor vaccinations and immunogene therapies.

Analysis of protein kinase C theta inhibitors for the control of HIV-1 replication in human CD4+ T cells reveals an effect on retrotranscription in addition to viral transcription

HIV-1 infection cannot be cured due to reservoirs formed early after infection. Decreasing the massive CD4+ T cell activation that occurs at the beginning of the disease would delay reservoir seeding, providing a better prognosis for patients. CD4+ T cell activation is mediated by protein kinase C (PKC) theta (θ), which is involved in T-cell proliferation, as well as NF-κB, NF-AT, and AP-1 activation. We found that PKCθ activity increased viral replication, but also that HIV-1 induced higher activation of PKCθ in infected CD4+ T cells, creating a feedback loop. Therefore, specific inhibition of PKCθ activity could contribute to control HIV-1 replication. We tested the efficacy of seven PKCθ specific inhibitors to control HIV-1 replication in CD4+ T cells and selected two of the more potent and safer: CGX1079 and CGX0471. They reduced PKCθ phosphorylation at T538 and its translocation to the plasma membrane, which correlated with decreased HIV-1 retrotranscription through partial inhibition of SAMHD1 antiviral activity, rendering lower proviral integration. CGX1079 and CGX0471 also interfered with viral transcription, which would reduce the production of new virions, as well as the subsequent spread and infection of new targets that would increase the reservoir size. CGX1079 and CGX0471 did not completely abrogate T-cell functions such as proliferation and CD8-mediated release of IFN-γ in PBMCs from HIV-infected patients, thereby avoiding general immunosuppresion. Consequently, using PKCθ inhibitors as adjuvant of antiretroviral therapy in recently infected patients would decrease the pool of activated CD4+ T cells, thwarting proviral integration and reducing the reservoir size.

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.

PDK1 orchestrates early NK cell development through induction of E4BP4 expression and maintenance of IL-15 responsiveness

Yang et al. provide new mechanistic insight into the role of kinase PDK1 in early NK cell development. Genetic deletion of PDK1 in mice led to a severe loss of NK cells and impaired antitumor activity in vivo. Their results indicate that PDK1 acts as a regulator of IL-15 signaling to induce expression of the circadian protein E4BP4, also modulating CD122 upregulation.

E4BP4, a circadian protein, is indispensable for NK cell development. It remains largely unknown which signal is required to induce E4BP4 expression and what effects it has during NK cell differentiation. Here, we reveal that PDK1, a kinase upstream of mTOR, connects IL-15 signaling to E4BP4. Early deletion of PDK1 caused a severe loss of NK cells and compromised antitumor activity in vivo. PDK1-deficient NK cells displayed much weaker IL-15–induced mTOR activation and E4BP4 induction, as well as remarkable reduction in CD122, a receptor subunit specifying NK cell responsiveness to IL-15. The phenotypes were partially reversible by ectopic expression of E4BP4 or bypassed activation of mTOR. We also determined that PDK1-mediated metabolic signaling was dispensable for NK cell terminal maturation and survival. Thus, we identify a role for PDK1 signaling as a key mediator in regulating E4BP4 expression during early NK cell development. Our findings underscore the importance of IL-15 self-responsiveness through a positive feedback loop that involves PDK1–mTOR–E4BP4–CD122 signaling.

The effect of 4α,5α-epoxy-10α,14-dihydro-inuviscolide, a novel immunosuppressant isolated from Carpesium abrotanoides, on the cytokine profile in vitro and in vivo

The plant Carpesium abrotanoides (CA) is used in Asian herbal medicines as an insecticide and to treat bruises. However, the effect of single compounds from CA blooms and the mechanism of its immunosuppressive effect remain poorly understood. The aim of this study was to investigate the mechanism of the immunosuppressive effect in the three kinds of immune cells, and the immunosuppressive effect of CA bloom extract (CAE) in acute inflammation models (LPS and ConA-induced inflammation). Interleukin-6, IL-4, IL-13, IFNγ, and IL-10-but not TNFα-were significantly reduced in a dose-dependent manner by 4α,5α-epoxy-10α,14-dihydro-inuviscolide (INV). Furthermore, INV inhibited NF-κB transcriptional activation and IL-10 promoter activity in the same manner as for Bay11. Meanwhile, treatment with dexamethasone reduced the levels of IFNγ, but not IL-10, and resulted in no change in NF-κB transcriptional activation or the IL-10 promoter. INV did not affect PMA-induced IκB kinase complex phosphorylation, IκB degradation, or MAPK and the nuclear translocation of p65, as with DEX. The in vivo, CAE has an immunosuppressive effect on the LPS-induced inflammation response model by inhibiting the plasma level of IFNγ and IL-6 levels. CAE treatment also tends to attenuate the plasma level of IFNγ, IL-4, and IL-6 in ConA-induced inflammation. These findings indicate that INV causes the reduction of the cytokine profile by blocking the NF-κB transcription factor activation and the molecular mechanism by which INV operates could provide new insights into the unique mechanisms responsible for NF-κB inhibition, in contrast to established immunosuppressants, as a therapeutic agent for immunopathological treatment.

Genkwadaphnin induces IFN-γ via PKD1/NF-κB/STAT1 dependent pathway in NK-92 cells

The flower buds of Daphne genkwa Sieb. et Zucc. have been used as a traditional Chinese medicine although their functional mechanisms have not been discovered yet. We have studied the potential effects of the plant extracts on natural killer (NK) cell activation, and isolated an active fraction. Genkwadaphnin (GD-1) displayed a potent efficacy to induce IFN-γ transcription in NK cells with concentration- and time-dependent manners. GD-1 treatment triggered the phosphorylation of PKD1, a member of PKC family, MEK and ERK, resulting in IKK activation to induce IκB degradation, and the nuclear localization of p65, an NF-κB subunit, which regulates IFN-γ transcription. GD-1 effect on IFN-γ production was blocked by the addition of Rottlerin, a PKC inhibitor, CID 755673, a PKD inhibitor, or Bay11-7082, an IKKα inhibitor. The nuclear localization of p65 was also inhibited by the kinase inhibitors. Secreted IFN-γ activates STAT1 phosphorylation as autocrine-loops to sustain its secretion. GD-1 induced the phosphorylation of STAT1 probably through the increase of IFN-γ. STAT1 inhibitor also abrogated the sustained IFN-γ secretion. These results suggest that GD-1 is involved in the activation of PKD1 and/or ERK pathway, which activate NK-κB triggering IFN-γ production. As positive feedback loops, secreted IFN-γ activates STAT1 and elongates its production in NK-92 cells.

Specific dysregulation of IFNγ production by natural killer cells confers susceptibility to viral infection

Natural Killer (NK) cells contribute to the control of viral infection by directly killing target cells and mediating cytokine release. In C57BL/6 mice, the Ly49H activating NK cell receptor plays a key role in early resistance to mouse cytomegalovirus (MCMV) infection through specific recognition of the MCMV-encoded MHC class I-like molecule m157 expressed on infected cells. Here we show that transgenic expression of Ly49H failed to provide protection against MCMV infection in the naturally susceptible A/J mouse strain. Characterization of Ly49H⁺ NK cells from Ly49h-A transgenic animals showed that they were able to mount a robust cytotoxic response and proliferate to high numbers during the course of infection. However, compared to NK cells from C57BL/6 mice, we observed an intrinsic defect in their ability to produce IFNγ when challenged by either m157-expressing target cells, exogenous cytokines or chemical stimulants. This effect was limited to NK cells as T cells from C57BL/6 and Ly49h-A mice produced comparable cytokine levels. Using a panel of recombinant congenic strains derived from A/J and C57BL/6 progenitors, we mapped the genetic basis of defective IFNγ production to a single 6.6 Mb genetic interval overlapping the Ifng gene on chromosome 10. Inspection of the genetic interval failed to reveal molecular differences between A/J and several mouse strains showing normal IFNγ production. The chromosome 10 locus is independent of MAPK signalling or decreased mRNA stability and linked to MCMV susceptibility. This study highlights the existence of a previously uncovered NK cell-specific cis-regulatory mechanism of Ifnγ transcript expression potentially relevant to NK cell function in health and disease.

Cytomegalovirus (CMV) is a ubiquitous herpesvirus that largely infects the human population leading to a significant cause of disease and death in the immunocompromised and elderly. The study of CMV in animal models has helped understand the pathogenic consequences of CMV infection and adds substantial understanding of the complex interplay of host and virus in living systems. Natural Killer (NK) cells have emerged as an important player during CMV infection trough their specific recognition of viral particles determinants and subsequent secretion of cytokines and cytolytic granules. In the present study, we have generated different mouse models to specifically investigate quantify viral recognition and cytokine expression by NK cells during CMV infection as a measure of NK cell function. We found that even after proper recognition of infected cells by NK cells, the adequate production of IFNγ is crucial to restrain viral infection. Moreover, we demonstrated that IFNγ production by NK cells is genetically determined and directly linked to the IFNγ locus. Hence, we provide the first evidence for of a unique mechanism of IFNγ production by NK cells which regulates susceptibility to viral infection.

IL-27 stimulates human NK-cell effector functions and primes NK cells for IL-18 responsiveness

IL-27, a member of the IL-12 family of cytokines, is produced by APCs, and displays pro- and anti-inflammatory effects. How IL-27 affects human NK cells still remains unknown. In this study, we observed that mature DCs secreted IL-27 and that blockade of IL-27R (CD130) reduced the amount of IFN-γ produced by NK cells during their coculture, showing the importance of IL-27 during DC-NK-cell crosstalk. Accordingly, human rIL-27 stimulated IFN-γ secretion by NK cells in a STAT1-dependent manner, induced upregulation of CD25 and CD69 on NK cells, and displayed a synergistic effect with IL-18. Preincubation experiments demonstrated that IL-27 primed NK cells for IL-18-induced IFN-γ secretion, which was associated with an IL-27-driven upregulation of T-bet expression. Also, IL-27 triggered NKp46-dependent NK-cell-mediated cytotoxicity against Raji, T-47D, and HCT116 cells, and IL-18 enhanced this cytotoxic response. Such NK-cell-mediated cytotoxicity involved upregulation of perforin, granule exocytosis, and TRAIL-mediated cytotoxicity but not Fas-FasL interaction. Moreover, IL-27 also potentiated Ab-dependent cell-mediated cytotoxicity against mAb-coated target cells. Taken together, IL-27 stimulates NK-cell effector functions, which might be relevant in different physiological and pathological situations.

IFNα signaling through PKC-θ is essential for antitumor NK cell function

We have previously shown that the development of a major histocompatibility complex class I (MHC-I)-deficient tumor was favored in protein kinase C-θ knockout (PKC-θ^-/-) mice compared to that occurring in wild-type mice. This phenomenon was associated with scarce recruitment of natural killer (NK) cells to the tumor site, as well as impaired NK cell activation and reduced cytotoxicity ex vivo. Poly-inosinic:cytidylic acid (poly I:C) treatment activated PKC-θ in NK cells depending on the presence of a soluble factor produced by a different splenocyte subset. In the present work, we sought to analyze whether interleukin-15 (IL-15) and/or interferon-α (IFNα) mediate PKC-θ-dependent antitumor NK cell function. We found that IL-15 improves NK cell viability, granzyme B expression, degranulation capacity and interferon-γ (IFNγ) secretion independently of PKC-θ. In contrast, we found that IFNα improves the degranulation capability of NK cells against target cancer cells in a PKC-θ-dependent fashion both ex vivo and in vivo. Furthermore, IFNα induces PKC-θ auto-phosphorylation in NK cells, in a signal transduction pathway involving both phosphatidylinositol-3-kinase (PI3K) and phospholipase-C (PLC) activation. PKC-θ dependence was further implicated in IFNα-induced transcriptional upregulation of chemokine (C-X-C motif) ligand 10 (CXCL10), a signal transducer and activator of transcription-1 (STAT-1)-dependent target of IFNα. The absence of PKC-θ did not affect IFNα-induced STAT-1 Tyr701 phosphorylation but affected the increase in STAT-1 phosphorylation on Ser727, attenuating CXCL10 secretion. This connection between IFNα and PKC-θ in NK cells may be exploited in NK cell-based tumor immunotherapy.

Costimulatory molecule DNAM-1 is essential for optimal differentiation of memory natural killer cells during mouse cytomegalovirus infection

Recent studies demonstrate that natural killer (NK) cells have adaptive immune features. Here, we investigated the role of the costimulatory molecule DNAM-1 in the differentiation of NK cells in a mouse model of cytomegalovirus (MCMV) infection. Antibody blockade of DNAM-1 suppressed the expansion of MCMV-specific Ly49H(+) cells during viral infection and inhibited the generation of memory NK cells. Similarly, DNAM-1-deficient (Cd226(-/-)) Ly49H(+) NK cells exhibited intrinsic defects in expansion and differentiation into memory cells. Src-family tyrosine kinase Fyn and serine-threonine protein kinase C isoform eta (PKCη) signaling through DNAM-1 played distinct roles in the generation of MCMV-specific effector and memory NK cells. Thus, cooperative signaling through DNAM-1 and Ly49H are required for NK cell-mediated host defense against MCMV infection.

Protein Kinase Inhibitors CK59 and CID755673 Alter Primary Human NK Cell Effector Functions

Natural killer (NK) cells are part of the innate immune response and play a crucial role in the defense against tumors and virus-infected cells. Their effector functions include the specific killing of target cells, as well as the modulation of other immune cells by cytokine release. Kinases constitute a relevant part in signaling, are prime targets in drug research and the protein kinase inhibitor Dasatinib is already used for immune-modulatory therapies. In this study, we tested the effects of the kinase inhibitors CK59 and CID755673. These inhibitors are directed against calmodulin kinase II (CaMKII; CK59) and PKD family kinases (CID755673) that were previously suggested as novel components of NK activation pathways. Here, we use a multi-parameter, FACS-based assay to validate the influence of CK59 and CID755673 on the effector functions of primary NK cells. Treatment with CK59 and CID755673 indeed resulted in a significant dose-dependent reduction of NK cell degranulation markers and cytokine release in freshly isolated Peripheral blood mononuclear cell populations from healthy blood donors. These results underline the importance of CaMKII for NK cell signaling and suggest protein kinase D2 as a novel signaling component in NK cell activation. Notably, kinase inhibition studies on pure NK cell populations indicate significant donor variations.

In and out of the bull's eye: protein kinase Cs in the immunological synapse

The immunological synapse (IS) formed between immune cells and antigen-presenting cells (APCs) provides a platform for signaling. Protein kinase C (PKC)θ localizes in the T cell IS within the central supramolecular activation cluster (cSMAC), where it associates with CD28 and mediates T cell receptor (TCR)/CD28 signals leading to effector T (Teff) cell activation. In regulatory T (Treg) cells, PKCθ is sequestered away from the IS, and inhibits suppressive function. Other PKCs localizing in the IS mediate additional functions in various immune cells. Further work is needed to identify mechanisms underlying PKC recruitment or exclusion at the IS, potential redundancy among IS-localized PKCs, and the relevance of PKC localization for IS dynamics and lymphocyte activation.

Controlling natural killer cell responses: integration of signals for activation and inhibition

Understanding how signals are integrated to control natural killer (NK) cell responsiveness in the absence of antigen-specific receptors has been a challenge, but recent work has revealed some underlying principles that govern NK cell responses. NK cells use an array of innate receptors to sense their environment and respond to alterations caused by infections, cellular stress, and transformation. No single activation receptor dominates; instead, synergistic signals from combinations of receptors are integrated to activate natural cytotoxicity and cytokine production. Inhibitory receptors for major histocompatibility complex class I (MHC-I) have a critical role in controlling NK cell responses and, paradoxically, in maintaining NK cells in a state of responsiveness to subsequent activation events, a process referred to as licensing. MHC-I-specific inhibitory receptors both block activation signals and trigger signals to phosphorylate and inactivate the small adaptor Crk. These different facets of inhibitory signaling are incorporated into a revocable license model for the reversible tuning of NK cell responsiveness.

The yin and yang of protein kinase C-theta (PKCθ): a novel drug target for selective immunosuppression

Protein kinase C-theta (PKCθ) is a protein kinase C (PKC) family member expressed predominantly in T lymphocytes, and extensive studies addressing its function have been conducted. PKCθ is the only T cell-expressed PKC that localizes selectively to the center of the immunological synapse (IS) following conventional T cell antigen stimulation, and this unique localization is essential for PKCθ-mediated downstream signaling. While playing a minor role in T cell development, early in vitro studies relying, among others, on the use of PKCθ-deficient (Prkcq(-/-)) T cells revealed that PKCθ is required for the activation and proliferation of mature T cells, reflecting its importance in activating the transcription factors nuclear factor kappa B, activator protein-1, and nuclear factor of activated T cells, as well as for the survival of activated T cells. Upon subsequent analysis of in vivo immune responses in Prkcq(-/-) mice, it became clear that PKCθ has a selective role in the immune system: it is required for experimental Th2- and Th17-mediated allergic and autoimmune diseases, respectively, and for alloimmune responses, but is dispensable for protective responses against pathogens and for graft-versus-leukemia responses. Surprisingly, PKCθ was recently found to be excluded from the IS of regulatory T cells and to negatively regulate their suppressive function. These attributes of PKCθ make it an attractive target for catalytic or allosteric inhibitors that are expected to selectively suppress harmful inflammatory and alloimmune responses without interfering with beneficial immunity to infections. Early progress in developing such drugs is being made, but additional studies on the role of PKCθ in the human immune system are urgently needed.

Protein Kinase C-θ (PKC-θ) in Natural Killer Cell Function and Anti-Tumor Immunity

The protein kinase C-θ (PKCθ), which is essential for T cell function and survival, is also required for efficient anti-tumor immune surveillance. Natural killer (NK) cells, which express PKCθ, play a prominent role in this process, mainly by elimination of tumor cells with reduced or absent major histocompatibility complex class-I (MHC-I) expression. This justifies the increased interest of the use of activated NK cells in anti-tumor immunotherapy in the clinic. The in vivo development of MHC-I-deficient tumors is much favored in PKCθ^−/− mice compared with wild-type mice. Recent data offer some clues on the mechanism that could explain the important role of PKCθ in NK cell-mediated anti-tumor immune surveillance: some studies show that PKCθ is implicated in signal transduction and anti-tumoral activity of NK cells elicited by interleukin (IL)-12 or IL-15, while others show that it is implicated in NK cell functional activation mediated by certain killer-activating receptors. Alternatively, the possibility that PKCθ is involved in NK cell degranulation is discussed, since recent data indicate that it is implicated in microtubule-organizing center polarization to the immune synapse in CD4⁺ T cells. The implication of PKC isoforms in degranulation has been more extensively studied in cytotoxic T lymphocyte, and these studies will be also summarized.

NFAT control of innate immunity

The calcineurin/nuclear factor of activated T cells (NFAT) signaling pathway mediates multiple adaptive T-cell functions, but recent studies have shown that calcineurin/NFAT signaling also contributes to innate immunity and regulates the homeostasis of innate cells. Myeloid cells, including granulocytes and dendritic cells, can promote inflammation, regulate adaptive immunity, and are essential mediators of early responses to pathogens. Microbial ligation of pattern-recognition receptors, such as TLR4, CD14, and dectin 1, is now known to induce the activation of calcineurin/NFAT signaling in myeloid cells, a finding that has provided new insights into the molecular pathways that regulate host protection. Inhibitors of calcineurin/NFAT binding, such as cyclosporine A and FK506, are broadly used in organ transplantation and can act as potent immunosuppressive drugs in a variety of different disorders. There is increasing evidence that these agents influence innate responses as well as inhibiting adaptive T-cell functions. This review focuses on the role of calcineurin/NFAT signaling in myeloid cells, which may contribute to the various unexplained effects of immunosuppressive drugs already being used in the clinic.

Kinome analysis of receptor-induced phosphorylation in human natural killer cells

BACKGROUND

Natural killer (NK) cells contribute to the defense against infected and transformed cells through the engagement of multiple germline-encoded activation receptors. Stimulation of the Fc receptor CD16 alone is sufficient for NK cell activation, whereas other receptors, such as 2B4 (CD244) and DNAM-1 (CD226), act synergistically. After receptor engagement, protein kinases play a major role in signaling networks controlling NK cell effector functions. However, it has not been characterized systematically which of all kinases encoded by the human genome (kinome) are involved in NK cell activation.

RESULTS

A kinase-selective phosphoproteome approach enabled the determination of 188 kinases expressed in human NK cells. Crosslinking of CD16 as well as 2B4 and DNAM-1 revealed a total of 313 distinct kinase phosphorylation sites on 109 different kinases. Phosphorylation sites on 21 kinases were similarly regulated after engagement of either CD16 or co-engagement of 2B4 and DNAM-1. Among those, increased phosphorylation of FYN, KCC2G (CAMK2), FES, and AAK1, as well as the reduced phosphorylation of MARK2, were reproducibly observed both after engagement of CD16 and co-engagement of 2B4 and DNAM-1. Notably, only one phosphorylation on PAK4 was differentally regulated.

CONCLUSIONS

The present study has identified a significant portion of the NK cell kinome and defined novel phosphorylation sites in primary lymphocytes. Regulated phosphorylations observed in the early phase of NK cell activation imply these kinases are involved in NK cell signaling. Taken together, this study suggests a largely shared signaling pathway downstream of distinct activation receptors and constitutes a valuable resource for further elucidating the regulation of NK cell effector responses.

Transcriptional control of natural killer cell differentiation and function

Gene expression can be modulated depending on physiological and developmental requirements. A multitude of regulatory genes, which are organized in interdependent networks, guide development and eventually generate specific phenotypes. Transcription factors (TF) are a key element in the regulatory cascade controlling cell fate and effector functions. In this review, we discuss recent data on the diversity of TF that determine natural killer (NK) cell fate and NK cell function.

Molecular mechanisms of natural killer cell activation

With an array of activating and inhibitory receptors, natural killer (NK) cells can specifically eradicate infected and transformed cells. Target cell killing is achieved through directed release of lytic granules. Recognition of target cells also induces production of chemokines and cytokines that can coordinate immune responses. Upon contact with susceptible cells, a multiplicity of activating receptors can induce signals for adhesion. Engagement of the integrin leukocyte functional antigen-1 mediates firm adhesion, provides signals for granule polarization and orchestrates the structure of an immunological synapse that facilitates efficient target cell killing. Other activating receptors apart from leukocyte functional antigen-1 signal for lytic granule exocytosis, a process that requires overcoming a threshold for activation of phospholipase C-γ, which in turn induces STIM1- and ORAI1-dependent store-operated Ca²+ entry as well as exocytosis mediated by the SNARE-containing protein syntaxin-11 and regulators thereof. Cytokine and chemokine release follows a different secretory pathway which also requires phospholipase C-γ activation and store-operated Ca²+ entry. Recent studies of human NK cells have provided insights into a hierarchy of effector functions that result in graded responses by NK cell populations. Responses display cellular heterogeneity and are influenced by environmental cues. This review highlights recent knowledge gained on the molecular pathways for and regulation of NK cell activation.

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.

L-CBM signaling in lymphocyte development and function

The nuclear factor-κB (NF-κB) plays a central role in the activation and survival of lymphocytes. NF-κB, therefore, is pivotal for acquired immunity, but the dysregulation of NF-κB signaling leads to inflammatory diseases and lymphomagenesis. Accumulating evidence has demonstrated that the mucosa-associated lymphoid tissue (MALT) lymphoma-related molecules, B-cell lymphoma 10 (BCL10) and MALT-lymphoma-translocation gene1 (MALT1), are essential signaling components for NF-κB and mitogen-activated protein kinase (MAPK) activation, mediated by the immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors involved in both innate and adaptive immunity. CARMA1 (also referred to as CARD11 and Bimp3) is a crucial regulator for ITAM-mediated signaling as it forms a complex with BCL10-MALT1 in lymphoid lineage cells such as T, B, natural killer (NK), and natural killer T (NKT) cells, known as the lymphoid CARMA1-BCL10-MALT1 (L-CBM) complex. In this review, recent understanding of the molecular and biological functions and the signal regulation mechanisms of the L-CBM complex are described and its role in disease development and potential as a therapeutic target is further discussed.