Cytoskeletal stabilization of inhibitory interactions in immunologic synapses of mature human dendritic cells with natural killer cells

A Proximity-Dependent Biosensor System for Visualizing Cell-Cell Interactions Induced by Therapeutic Antibodies

Despite the promise of therapeutic antibodies in engaging the immune system to eliminate malignant cells, many aspects of the complex interplay between immune cells and cancer cells induced by antibody therapy remain incompletely understood. This study aimed to develop a biosensor system that can evaluate direct cell-cell physical contact and interactions between immune effector and target cells induced by therapeutic antibodies in physiologically relevant environments. The system uses two structural complementary luciferase units (SmBit and LgBit) expressed on the respective membranes of effector and target cells. Upon cell-cell contact, the two subunits form active NanoLuc, generating a luminescent signal, allowing for real-time monitoring of cell-cell interactions and quantitatively assessing the pharmacological effects of therapeutic antibodies. We optimized the system to ensure selectivity by adjusting the spacer lengths between two luciferase units to minimize interference from nonspecific intercellular contact. The system was applied to quantitatively monitor cell-cell interactions between NK and target cells induced by rituximab and between T and target cells induced by blinatumomab in a 3D cell culture system. The biosensor system has the potential to characterize antibody pharmacology through a deeper understanding of antibody-mediated cell-cell interactions.

Construction and Mechanism of IL-15-Based Coactivated Polymeric Micelles for NK Cell Immunotherapy

Natural killer (NK) cells are an important contributor to cancer immunotherapy, but their antitumor efficacy remains suboptimal. While cytokine-based priming shows promise in enhancing NK-cell activity, its clinical translation faces many challenges, including coactivation of multiple cytokines, poor pharmacokinetics, and limited mechanistic understanding. Here, this work develops a polymeric micelle-based IL-15/IL-2 codelivery system (IL-15/2-PEG-PTMC) for NK-cell activation. In vivo studies demonstrate that half-life of IL-15 and IL-2 and the recruitment of NK cell within tumor tissue are significantly increased after PEG-PTMC loading. Coupled with the coactivation effect of IL-15 and IL-2 conferred by this system, it noticeably delays the growth of tumors compared to conventional NK-cell activation approach, that is free IL-15 and IL-2. It is also surprisingly found that cholesterol metabolism is highly involved in the NK cell activation by IL-15/2-PEG-PTMC. Following stimulation with IL-15/2-PEG-PTMC or IL-15, NK cells undergo a series of cholesterol metabolism reprogramming, which elevates the cholesterol levels on NK cell membrane. This in turn promotes the formation of lipid rafts and activates immune synapses, effectively contributing to the enhancement of NK cell's antitumor activity. It is believed that it will open a new avenue for improving the efficacy of NK cell immunotherapy by regulating cholesterol metabolism.

Actin cytoskeleton remodeling at the cancer cell side of the immunological synapse: good, bad, or both?

Cytotoxic lymphocytes (CLs), specifically cytotoxic T lymphocytes and natural killer cells, are indispensable guardians of the immune system and orchestrate the recognition and elimination of cancer cells. Upon encountering a cancer cell, CLs establish a specialized cellular junction, known as the immunological synapse that stands as a pivotal determinant for effective cell killing. Extensive research has focused on the presynaptic side of the immunological synapse and elucidated the multiple functions of the CL actin cytoskeleton in synapse formation, organization, regulatory signaling, and lytic activity. In contrast, the postsynaptic (cancer cell) counterpart has remained relatively unexplored. Nevertheless, both indirect and direct evidence has begun to illuminate the significant and profound consequences of cytoskeletal changes within cancer cells on the outcome of the lytic immunological synapse. Here, we explore the understudied role of the cancer cell actin cytoskeleton in modulating the immune response within the immunological synapse. We shed light on the intricate interplay between actin dynamics and the evasion mechanisms employed by cancer cells, thus providing potential routes for future research and envisioning therapeutic interventions targeting the postsynaptic side of the immunological synapse in the realm of cancer immunotherapy. This review article highlights the importance of actin dynamics within the immunological synapse between cytotoxic lymphocytes and cancer cells focusing on the less-explored postsynaptic side of the synapse. It presents emerging evidence that actin dynamics in cancer cells can critically influence the outcome of cytotoxic lymphocyte interactions with cancer cells.

Cofilin-1 participates in the hyperfunction of myeloid dendritic cells in patients with severe aplastic anaemia

Cofilin‐1 interacts with actin to regulate cell movement. The importance of cofilin‐1 in immunity has been established, and its involvement in a number of autoimmune diseases has been confirmed. However, its role in severe aplastic anaemia (SAA) remains elusive. Thus, the aim of the current study was to investigate the role of cofilin‐1 in patients with SAA. Flow cytometry, Western blotting and real‐time quantitative reverse transcription‐polymerase chain reaction were performed to detect the mRNA and protein expression of cofilin‐1 in myeloid dendritic cells (mDCs) from patients with SAA. The expression of cofilin‐1 was then suppressed via siRNA, and its effects on mDCs and downstream effector T‐cell function were evaluated. Cofilin‐1 expression was higher in mDCs from patients with SAA and correlated with routine blood and immune indexes. Moreover, cofilin‐1 knockdown in mDCs from patients with SAA reduced their phagocytic capacity, migration capacity, and CD86 expression through F‐actin remodelling, downregulating the stimulatory capacity of mDCs on CD4⁺ and CD8⁺ T lymphocytes. Collectively, these findings indicate that cofilin‐1 participates in the hyperfunction of mDCs in patients with SAA and that the downregulation of cofilin‐1 in mDCs from patients with SAA could be a novel treatment approach for SAA.

Heparanase is a regulator of natural killer cell activation and cytotoxicity

Heparanase is the only mammalian enzyme capable of cleaving heparan sulfate, a glycosaminoglycan of the extracellular matrix and cell surfaces. Most immune cells express heparanase that contributes to a range of functions including cell migration and cytokine expression. Heparanase also promotes natural killer (NK) cell migration; however, its role in other NK cell functions remains to be defined. In this study, heparanase-deficient (Hpse^-/- ) mice were used to assess the role of heparanase in NK cell cytotoxicity, activation, and cytokine production. Upon challenge with the immunostimulant polyinosinic:polycytidylic acid (poly(I:C)), NK cells isolated from Hpse^-/- mice displayed impaired cytotoxicity against EO771.LMB cells and reduced levels of activation markers CD69 and NKG2D. However, in vitro cytokine stimulation of wild-type and Hpse^-/- NK cells resulted in similar CD69 and NKG2D expression, suggesting the impaired NK cell activation in Hpse^-/- mice results from elements within the in vivo niche. NK cells are activated in vivo by dendritic cells (DCs) in response to poly(I:C). Poly(I:C)-stimulated Hpse^-/- bone marrow DCs (BMDCs) expressed less IL-12, and when cultured with Hpse^-/- NK cells, less MCP-1 mRNA and protein was detected. Although cell-cell contact is important for DC-mediated NK cell activation, co-cultures of Hpse^-/- BMDCs and NK cells showed similar levels of contact to wild-type cells, suggesting heparanase contributes to NK cell activation independently of cell-cell contact with DCs. These observations define a role for heparanase in NK cell cytotoxicity and activation and have important implications for how heparanase inhibitors currently in clinical trials for metastatic cancer may impact NK cell immunosurveillance.

NK cells eliminate Epstein-Barr virus bound to B cells through a specific antibody-mediated uptake

Epstein Barr virus (EBV) causes a highly prevalent and lifelong infection contributing to the development of some malignancies. In addition to the key role played by T cells in controlling this pathogen, NK cells mediate cytotoxicity and IFNγ production in response to EBV-infected B cells in lytic cycle, both directly and through antibody (Ab)-dependent activation. We recently described that EBV-specific Ab-dependent NK cell interaction with viral particles (VP) bound to B cells triggered degranulation and TNFα secretion but not B cell lysis nor IFNγ production. In this report we show that NK cell activation under these conditions reduced B cell transformation by EBV. NK cells eliminated VP from the surface of B cells through a specific and active process which required tyrosine kinase activation, actin polymerization and Ca2+, being independent of proteolysis and perforin. VP were displayed at the NK cell surface before being internalized and partially shuttled to early endosomes and lysosomes. VP transfer was encompassed by a trogocytosis process including the EBV receptor CD21, together with CD19 and CD20. Our study reveals a novel facet of the antibody-dependent NK cell mediated response to this viral infection.

Epstein-Barr virus (EBV) is a member of the herpesvirus family which causes a frequent and lifelong infection. The immune system is unable to fully eliminate the virus, which remains dormant in infected B lymphocytes. EBV reactivation leads to the production of new infective particles, spreading to other cells and favoring its transmission. EBV infection goes generally unnoticed in healthy individuals, though it may occasionally cause a disease termed Infectious Mononucleosis, as well as severe disorders in patients with a defective immune response. Remarkably, EBV has oncogenic potential contributing to the development of some tumors, and has been associated to autoimmune diseases. T lymphocytes and Natural Killer (NK) cells play an essential role in the defense against EBV, killing infected cells when the virus reactivates. Antiviral NK cell functions may be also triggered by antibodies (Ab) recognizing infected cells. In this report we provide the first evidence supporting that NK cells in combination with anti-EBV Ab are able to eliminate the virus attached to the surface of B cells, reducing their infection without killing them.

The Role of the Cytoskeleton in Regulating the Natural Killer Cell Immune Response in Health and Disease: From Signaling Dynamics to Function

Natural killer (NK) cells are innate lymphoid cells, which play key roles in elimination of virally infected and malignant cells. The balance between activating and inhibitory signals derived from NK surface receptors govern the NK cell immune response. The cytoskeleton facilitates most NK cell effector functions, such as motility, infiltration, conjugation with target cells, immunological synapse assembly, and cytotoxicity. Though many studies have characterized signaling pathways that promote actin reorganization in immune cells, it is not completely clear how particular cytoskeletal architectures at the immunological synapse promote effector functions, and how cytoskeletal dynamics impact downstream signaling pathways and activation. Moreover, pioneering studies employing advanced imaging techniques have only begun to uncover the architectural complexity dictating the NK cell activation threshold; it is becoming clear that a distinct organization of the cytoskeleton and signaling receptors at the NK immunological synapse plays a decisive role in activation and tolerance. Here, we review the roles of the actin cytoskeleton in NK cells. We focus on how actin dynamics impact cytolytic granule secretion, NK cell motility, and NK cell infiltration through tissues into inflammatory sites. We will also describe the additional cytoskeletal components, non-muscle Myosin II and microtubules that play pivotal roles in NK cell activity. Furthermore, special emphasis will be placed on the role of the cytoskeleton in assembly of immunological synapses, and how mutations or downregulation of cytoskeletal accessory proteins impact NK cell function in health and disease.

Tumor Vasculature Targeted TNFα Therapy: Reversion of Microenvironment Anergy and Enhancement of the Anti-tumor Efficiency

Tumor cells and tumor-associated stromal cells such as immune, endothelial and mesenchimal cells create a Tumor Microenvironment (TME) which allows tumor cell promotion, growth and dissemination while dampening the anti-tumor immune response. Efficient anti-tumor interventions have to keep into consideration the complexity of the TME and take advantage of immunotherapy and chemotherapy combined approaches. Thus, the aim of tumor therapy is to directly hit tumor cells and reverse endothelial and immune cell anergy. Selective targeting of tumor vasculature using TNFα-associated peptides or antibody fragments in association with chemotherapeutic agents, has been shown to exert a potent stimulatory effect on endothelial cells as well as on innate and adaptive immune responses. These drug combinations reducing the dose of single agents employed have led to minimize the associated side effects. In this review, we will analyze different TNFα-mediated tumor vesseltargeted therapies in both humans and tumor mouse models, with emphasis on the role played by the cross-talk between natural killer and dendritic cells and on the ability of TNFα to trigger tumor vessel activation and normalization. The improvement of the TNFα-based therapy with anti-angiogenic immunomodulatory drugs that may convert the TME from immunosuppressive to immunostimulant, will be discussed as well.

Trans-endocytosis of intact IL-15Rα-IL-15 complex from presenting cells into NK cells favors signaling for proliferation

Interleukin 15 (IL-15) is an essential cytokine for the survival and proliferation of natural killer (NK) cells. IL-15 activates signaling by the β and common γ (γ_c) chain heterodimer of the IL-2 receptor through trans-presentation by cells expressing IL-15 bound to the α chain of the IL-15 receptor (IL-15Rα). We show here that membrane-associated IL-15Rα-IL-15 complexes are transferred from presenting cells to NK cells through trans-endocytosis and contribute to the phosphorylation of ribosomal protein S6 and NK cell proliferation. NK cell interaction with soluble or surface-bound IL-15Rα-IL-15 complex resulted in Stat5 phosphorylation and NK cell survival at a concentration or density of the complex much lower than required to stimulate S6 phosphorylation. Despite this efficient response, Stat5 phosphorylation was reduced after inhibition of metalloprotease-induced IL-15Rα-IL-15 shedding from trans-presenting cells, whereas S6 phosphorylation was unaffected. Conversely, inhibition of trans-endocytosis by silencing of the small GTPase TC21 or expression of a dominant-negative TC21 reduced S6 phosphorylation but not Stat5 phosphorylation. Thus, trans-endocytosis of membrane-associated IL-15Rα-IL-15 provides a mode of regulating NK cells that is not afforded to IL-2 and is distinct from activation by soluble IL-15. These results may explain the strict IL-15 dependence of NK cells and illustrate how the cellular compartment in which receptor-ligand interaction occurs can influence functional outcome.

Actin Cytoskeleton Straddling the Immunological Synapse between Cytotoxic Lymphocytes and Cancer Cells

The immune system is a fundamental part of the tumor microenvironment. In particular, cytotoxic lymphocytes, such as cytolytic T cells and natural killer cells, control tumor growth and disease progression by interacting and eliminating tumor cells. The actin cytoskeleton of cytotoxic lymphocytes engaged in an immunological synapse has received considerable research attention. It has been recognized as a central mediator of the formation and maturation of the immunological synapse, and its signaling and cytolytic activities. In comparison, fewer studies have explored the organization and function of actin filaments on the target cancer cell side of the immunological synapse. However, there is growing evidence that the actin cytoskeleton of cancer cells also undergoes extensive remodeling upon cytotoxic lymphocyte attack, and that such remodeling can alter physical and functional interactions at the immunological synapse. In this article, we review the current knowledge of actin organization and functions at both sides of the immunological synapse between cytotoxic lymphocytes and cancer cells, with particular focus on synapse formation, signaling and cytolytic activity, and immune evasion.

Human NK Cells Lyse Th2-Polarizing Dendritic Cells via NKp30 and DNAM-1

Cross-talk between NK cells and dendritic cells (DCs) is important in Th1 immune responses, including antitumor immunity and responses to infections. DCs also play a crucial role in polarizing Th2 immunity, but the impact of NK cell-DC interactions in this context remains unknown. In this study, we stimulated human monocyte-derived DCs in vitro with different pathogen-associated molecules: LPS or polyinosinic-polycytidylic acid, which polarize a Th1 response, or soluble egg Ag from the helminth worm Schistosoma mansoni, a potent Th2-inducing Ag. Th2-polarizing DCs were functionally distinguishable from Th1-polarizing DCs, and both showed distinct morphology and dynamics from immature DCs. We then assessed the outcome of autologous NK cells interacting with these differently stimulated DCs. Confocal microscopy showed polarization of the NK cell microtubule organizing center and accumulation of LFA-1 at contacts between NK cells and immature or Th2-polarizing DCs but not Th1-polarizing DCs, indicative of the assembly of an activating immune synapse. Autologous NK cells lysed immature DCs but not DCs treated with LPS or polyinosinic-polycytidylic acid as reported previously. In this study, we demonstrated that NK cells also degranulated in the presence of Th2-polarizing DCs. Moreover, time-lapse live-cell microscopy showed that DCs that had internalized fluorescently labeled soluble egg Ag were efficiently lysed. Ab blockade of NK cell-activating receptors NKp30 or DNAM-1 abrogated NK cell lysis of Th2-polarizing DCs. Thus, these data indicate a previously unrecognized role of NK cell cytotoxicity and NK cell-activating receptors NKp30 and DNAM-1 in restricting the pool of DCs involved in Th2 immune responses.

A correlative and quantitative imaging approach enabling characterization of primary cell-cell communication: Case of human CD4+ T cell-macrophage immunological synapses

Cell-to-cell communication engages signaling and spatiotemporal reorganization events driven by highly context-dependent and dynamic intercellular interactions, which are difficult to capture within heterogeneous primary cell cultures. Here, we present a straightforward correlative imaging approach utilizing commonly available instrumentation to sample large numbers of cell-cell interaction events, allowing qualitative and quantitative characterization of rare functioning cell-conjugates based on calcium signals. We applied this approach to examine a previously uncharacterized immunological synapse, investigating autologous human blood CD4+ T cells and monocyte-derived macrophages (MDMs) forming functional conjugates in vitro. Populations of signaling conjugates were visualized, tracked and analyzed by combining live imaging, calcium recording and multivariate statistical analysis. Correlative immunofluorescence was added to quantify endogenous molecular recruitments at the cell-cell junction. By analyzing a large number of rare conjugates, we were able to define calcium signatures associated with different states of CD4+ T cell-MDM interactions. Quantitative image analysis of immunostained conjugates detected the propensity of endogenous T cell surface markers and intracellular organelles to polarize towards cell-cell junctions with high and sustained calcium signaling profiles, hence defining immunological synapses. Overall, we developed a broadly applicable approach enabling detailed single cell- and population-based investigations of rare cell-cell communication events with primary cells.

SWAP70 is a universal GEF-like adaptor for tethering actin to phagosomes

We recently identified a key role for SWAP70 as the tethering factor stabilizing F-actin filaments on the surface of phagosomes in human dendritic cells by interacting both with Rho-family GTPases and the lipid phosphatidylinositol (3,4)-bisphosphate. In this study, we aimed to investigate whether this role of SWAP70 was general among immune phagocytes. Our data reveal that SWAP70 is recruited to early phagosomes of macrophages and dendritic cells from both human and mouse. The putative inhibitor of SWAP70 sanguinarine blocked phagocytosis and F-actin polymerization, supporting a key role for SWAP70 in phagocytosis as demonstrated previously with knock-down. Moreover, SWAP70 was recently shown to sequester the F-actin severing protein cofilin and we investigated this relationship in phagocytosis. Our data show an increased activation of cellular cofilin upon siRNA knockdown of SWAP70. Finally, we explored whether SWAP70 would be recruited to the immune synapse between dendritic cells and T cells required for antigen presentation, as the formation of such synapses depends on F-actin. However, we observed that SWAP70 was depleted at immune synapses and specifically was recruited to phagosomes. Our data support an essential and specific role for SWAP70 in tethering and stabilizing F-actin to the phagosomal surface in a wide range of phagocytes.

Quantitative Imaging Approaches to Study the CAR Immunological Synapse

The lytic immunological synapse (IS) is a discrete structural entity formed after the ligation of specific activating receptors that leads to the destruction of a cancerous cell. The formation of an effector cell IS in cytotoxic T lymphocytes or natural killer cells is a hierarchical and stepwise rearrangement of structural and signaling components and targeted release of the contents of lytic granules. While recent advances in the generation and testing of cytotoxic lymphocytes expressing chimeric antigen receptors (CARs) has demonstrated their efficacy in the targeted lysis of tumor targets, the contribution and dynamics of IS components have not yet been extensively investigated in the context of engineered CAR cells. Understanding the biology of the CAR IS will be a powerful approach to efficiently guide the engineering of new CARs and help identify mechanistic problems in existing CARs. Here, we review the formation of the lytic IS and describe quantitative imaging-based measurements using multiple microscopy techniques at a single cell level that can be used in conjunction with established population-based assays to provide insight into the important cytotoxic function of CAR cells. The inclusion of this approach in the pipeline of CAR product design could be a novel and valuable innovation for the field.

Prolonged contact with dendritic cells turns lymph node-resident NK cells into anti-tumor effectors

Natural killer (NK) cells are critical players against tumors. The outcome of anti‐tumor vaccination protocols depends on the efficiency of NK‐cell activation, and efforts are constantly made to manipulate them for immunotherapeutic approaches. Thus, a better understanding of NK‐cell activation dynamics is needed. NK‐cell interactions with accessory cells and trafficking between secondary lymphoid organs and tumoral tissues remain poorly characterized. Here, we show that upon triggering innate immunity with lipopolysaccharide (LPS), NK cells are transiently activated, leave the lymph node, and infiltrate the tumor, delaying its growth. Interestingly, NK cells are not actively recruited at the draining lymph node early after LPS administration, but continue their regular homeostatic turnover. Therefore, NK cells resident in the lymph node at the time of LPS administration become activated and exert anti‐tumor functions. NK‐cell activation correlates with the establishment of prolonged interactions with dendritic cells (DCs) in lymph nodes, as observed by two‐photon microscopy. Close DC and NK‐cell contacts are essential for the localized delivery of DC‐derived IL‐18 to NK cells, a strict requirement in NK‐cell activation.

IL15 promotes growth and invasion of endometrial stromal cells and inhibits killing activity of NK cells in endometriosis

Endometriosis (EMS) is associated with an abnormal immune response to endometrial cells, which can facilitate the implantation and proliferation of ectopic endometrial tissues. It has been reported that human endometrial stromal cells (ESCs) express interleukin (IL)15. The aim of our study was to elucidate whether or not IL15 regulates the cross talk between ESCs and natural killer (NK) cells in the endometriotic milieu and, if so, how this regulation occurs. The ESC behaviors in vitro were verified by Cell Counting Kit-8 (CCK-8), Annexin/PI, and Matrigel invasion assays, respectively. To imitate the local immune microenvironment, the co-culture system between ESCs and NK cells was constructed. The effect of IL15 on NK cells in the co-culture unit was investigated by flow cytometry (FCM). In this study, we found that ectopic endometrium from patients with EMS highly expressed IL15. Rapamycin, an autophagy inducer, decreased the level of IL15 receptors (i.e. IL15Rα and IL2Rβ). IL15 inhibits apoptosis and promotes the invasiveness, viability, and proliferation of ESCs. Meanwhile, a co-culture with ESCs led to a decrease in CD16 on NK cells. In the co-culture system, IL15 treatment downregulated the levels of Granzyme B and IFN-γ in CD16+NK cells, NKG2D in CD56dimCD16-NK cells, and NKP44 in CD56brightCD16-NK cells. On the one hand, these results indicated that IL15 derived from ESCs directly stimulates the growth and invasion of ESCs. On the other hand, IL15 may help the immune escape of ESCs by suppressing the cytotoxic activity of NK cells in the ectopic milieu, thereby facilitating the progression of EMS.

Human NK cell development requires CD56-mediated motility and formation of the developmental synapse

While distinct stages of natural killer (NK) cell development have been defined, the molecular interactions that shape human NK cell maturation are poorly understood. Here we define intercellular interactions between developing NK cells and stromal cells which, through contact-dependent mechanisms, promote the generation of mature, functional human NK cells from CD34(+) precursors. We show that developing NK cells undergo unique, developmental stage-specific sustained and transient interactions with developmentally supportive stromal cells, and that the relative motility of NK cells increases as they move through development in vitro and ex vivo. These interactions include the formation of a synapse between developing NK cells and stromal cells, which we term the developmental synapse. Finally, we identify a role for CD56 in developmental synapse structure, NK cell motility and NK cell development. Thus, we define the developmental synapse leading to human NK cell functional maturation.

Adoptive transfer of natural killer cells promotes the anti-tumor efficacy of T cells

The density of NK cells in tumors correlates positively with prognosis in many types of cancers. The average number of infiltrating NK cells is, however, quite modest (approximately 30 NK cells/sq.mm), even in tumors deemed to have a "high" density of infiltrating NK cells. It is unclear how such low numbers of tumor-infiltrating NK cells can influence outcome. Here, we used ovalbumin-expressing tumor cell lines and TCR transgenic, OVA-specific cytotoxic T lymphocytes (OT-I-CTLs) to determine whether the simultaneous attack by anti-tumor CTLs and IL-2-activated NK (A-NK) cells synergistically increases the overall tumor cell kill and whether upregulation of tumor MHC class-I by NK cell-derived interferon-gamma (IFNγ) improves tumor-recognition and kill by anti-tumor CTLs. At equal E:T ratios, A-NK cells killed OVA-expressing tumor cells better than OT-I-CTLs. The cytotoxicity against OVA-expressing tumor cells increased by combining OT-I-CTLs and A-NK cells, but the increase was additive rather than synergistic. A-NK cells adenovirally-transduced to produce IL-12 (A-NK^IL-12) produced high amounts of IFNγ. The addition of a low number of A-NK^IL-12 cells to OT-I-CTLs resulted in a synergistic, albeit modest, increase in overall cytotoxicity. Pre-treatment of tumor cells with NK cell-conditioned medium increased tumor MHC expression and sensitivity to CTL-mediated killing. Pre-treatment of CTLs with NK cell-conditioned medium had no effect on CTL cytotoxicity. In vivo, MHC class-I expression by OVA-expressing B16 melanoma lung metastases increased significantly within 24-48h after adoptive transfer of A-NK^IL-12 cells. OT-I-CTLs and A-NK^IL-12 cells localized selectively and equally well into OVA-expressing B16 lung metastases and treatment of mice bearing 7-days-old OVA-B16 lung metastases with both A-NK^IL-12 cells and OT-I-CTLs lead to a significant prolongation of survival. Thus, an important function of tumor-infiltrating NK cells may be to increase tumor cell expression of MHC class-I through secretion of IFNγ, to prepare them for recognition by tumor-specific CTLs.

The dendritic cell side of the immunological synapse

Immune responses are initiated by the interactions between antigen-presenting cells (APCs), such as dendritic cells (DCs), with responder cells, such as T cells, via a tight cellular contact interface called the immunological synapse. The immunological synapse is a highly organized subcellular structure that provides a platform for the presentation of antigen in major histocompatibility class I and II complexes (MHC class I and II) on the surface of the APC to receptors on the surface of the responder cells. In T cells, these contacts lead to highly polarized membrane trafficking that results in the local release of lytic granules and in the delivery and recycling of T cell receptors at the immunological synapse. Localized trafficking also occurs at the APC side of the immunological synapse, especially in DCs where antigen loaded in MHC class I and II is presented and cytokines are released specifically at the synapse. Whereas the molecular mechanisms underlying polarized membrane trafficking at the T cell side of the immunological synapse are increasingly well understood, these are still very unclear at the APC side. In this review, we discuss the organization of the APC side of the immunological synapse. We focus on the directional trafficking and release of membrane vesicles carrying MHC molecules and cytokines at the immunological synapses of DCs. We hypothesize that the specific delivery of MHC and the release of cytokines at the immunological synapse mechanistically resemble that of lytic granule release from T cells.

NK Cell Proliferation Induced by IL-15 Transpresentation Is Negatively Regulated by Inhibitory Receptors

IL-15 bound to the IL-15Rα-chain (IL-15Rα) is presented in trans to cells bearing the IL-2Rβ-chain and common γ-chain. As IL-15 transpresentation occurs in the context of cell-to-cell contacts, it has the potential for regulation by and of other receptor-ligand interactions. In this study, human NK cells were tested for the sensitivity of IL-15 transpresentation to inhibitory receptors. Human cells expressing HLA class I ligands for inhibitory receptors KIR2DL1, KIR2DL2/3, or CD94-NKG2A were transfected with IL-15Rα. Proliferation of primary NK cells in response to transpresented IL-15 was reduced by engagement of either KIR2DL1 or KIR2DL2/3 by cognate HLA-C ligands. Inhibitory KIR-HLA-C interactions did not reduce the proliferation induced by soluble IL-15. Therefore, transpresentation of IL-15 is subject to downregulation by MHC class I-specific inhibitory receptors. Similarly, proliferation of the NKG2A(+) cell line NKL induced by IL-15 transpresentation was inhibited by HLA-E. Coengagement of inhibitory receptors, either KIR2DL1 or CD94-NKG2A, did not inhibit phosphorylation of Stat5 but inhibited selectively phosphorylation of Akt and S6 ribosomal protein. IL-15Rα was not excluded from, but was evenly distributed across, inhibitory synapses. These findings demonstrate a novel mechanism to attenuate IL-15-dependent NK cell proliferation and suggest that inhibitory NK cell receptors contribute to NK cell homeostasis.

TLR7/8 agonists promote NK-DC cross-talk to enhance NK cell anti-tumor effects in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common cancer worldwide and the third leading cause of cancer death. Immunotherapy is considered a promising treatment with the aim to boost or arouse HCC-specific immune responses. TLR7 and TLR8 agonists are effective immunomodulators and have been applied topically for the treatment of certain skin tumors and viral infections. Here, we explored the role of TLR7 and TLR8 agonists on the activation of dendritic cells (DCs) and natural killer (NK) cells. We demonstrated that these agonists could directly activate NK cells, promoting the maturation of immature DCs. Meanwhile, DCs also assisted in the function of NK cells, resulting in enhanced anti-tumor immune responses to HCC. Importantly, the combination therapy with NK cells stimulated with DCs and TLR7/8 agonist Gardiquimod (GDQ) significantly suppresses the growth of human HepG2 liver carcinoma xenografts. This study provides a new immunotherapeutic approach for human HCC based on DC-NK cross-talk and also suggests that TLR7 and/or TLR8 agonists, particularly GDQ, may serve as potent innate and adaptive immune response immunomodulators in tumor therapy.

Animal models of Epstein Barr virus infection

Epstein Barr virus (EBV) was the first human tumor virus to be described. Despite its discovery now more than fifty years ago, immune control of this virus is still not very well understood and no vaccine is available. This knowledge gap is due in part to the lack of a preclinical small animal model which can faithfully recapitulate EBV infection and immune control, and would allow testing of EBV specific vaccine candidates. With the advent of mice with reconstituted human immune system compartments (HIS mice) during the past decade this is changing. We will discuss which aspects of EBV infection and its immune control can already be modeled in HIS mice, and which shortcomings still need to be overcome in order to recapitulate the immunobiology of oncogenic EBV infection.

Natural killer and dendritic cells collaborate in the immune response induced by the vaccine against uterine cervical cancer

Virus-like particles (VLPs) of human papillomavirus (HPV) are used as a vaccine against HPV-induced cancer, and recently we have shown that these VLPs are able to activate natural killer (NK) cells. Since NK cells collaborate with dendritic cells (DCs) to induce an immune response against viral infections and tumors, we studied the impact of this crosstalk in the context of HPV vaccination. NK cells in the presence of HPV-VLPs enhanced DC-maturation as shown by an upregulation of CD86 and HLA-DR and an increased production of IL-12p70, but not of the immunosuppressive cytokine IL-10. This activation was bidirectional. Indeed, in the presence of HPV-VLPs, DCs further activated NK cells by inducing the upregulation of cell surface activation markers (CD69 and HLA-DR). The function of NK cells was also improved as shown by an increase in IFN-γ secretion and cytotoxic activity against an HPV(+) cell line. This crosstalk between NK cells and DCs needed CD40 interaction and IL-12p70 secretion, whereas NKG2D was not implicated. Our results provide insight into how VLPs interact with innate immune cells and how NK cells and DCs play a role in the immune response induced by this vaccine agent.

NK/DC crosstalk in immunosurveillance: a broken relationship caused by WASP-deficiency

Wiskott-Aldrich syndrome (WAS) is a primary immunodeficiency, which is characterized by abnormal immune system functions caused by the lack of expression of WAS protein (WASp). A higher tumor susceptibility is observed in WAS patients; whether this is a direct consequence of impaired immunosurveillance due to WAS deficiency in immune cells is, however, an open question. In this issue of the European Journal of Immunology, Catucci et al. [Eur. J. Immunol. 2014. 44: 1039-1045] shed light on the link between Was deficiency and immunosurveillance in a tumor-prone mouse model and report a role for the impaired crosstalk between natural killer (NK) cells and dendritic cells (DCs) in mediating this process. The potential mechanisms involved in WASp regulation of NK/DC-mediated immunosurveillance are the focus of this Commentary.

Dynamics of natural killer cell receptor revealed by quantitative analysis of photoswitchable protein

Natural Killer (NK) cell activation is dynamically regulated by numerous activating and inhibitory surface receptors that accumulate at the immune synapse. Quantitative analysis of receptor dynamics has been limited by methodologies that rely on indirect measurements such as fluorescence recovery after photobleaching. Here, we report an apparently novel approach to study how proteins traffic to and from the immune synapse using NK cell receptors tagged with the photoswitchable fluorescent protein tdEosFP, which can be irreversibly photoswitched from a green to red fluorescent state by ultraviolet light. Thus, after a localized switching event, the movement of the photoswitched molecules can be temporally and spatially resolved by monitoring fluorescence in two regions of interest. By comparing images with mathematical models, we evaluated the diffusion coefficient of the receptor KIR2DL1 (0.23 ± 0.06 μm(2) s(-1)) and assessed how synapse formation affects receptor dynamics. Our data conclude that the inhibitory NK cell receptor KIR2DL1 is continually trafficked into the synapse, and remains surprisingly stable there. Unexpectedly, however, in NK cells forming synapses with multiple target cells simultaneously, KIR2DL1 at one synapse can relocate to another synapse. Thus, our results reveal a previously undetected intersynaptic exchange of protein.

IL-15.IL-15Rα complex shedding following trans-presentation is essential for the survival of IL-15 responding NK and T cells

Interleukin (IL)-15 and its specific receptor chain, IL-15Rα, support the development of various effector cells, including NK and CD8 T cells via a mechanism called trans-presentation. Whereas the dynamic of trans-presentation has been shown to involve the recycling of IL-15Rα by presenting cells, the way responding cells integrate, or take advantage of this process has not been evaluated yet. To address this question, we set up a trans-presentation model using a membrane-bound IL-15.IL-15Rα fusion protein, and found that IL-15 is detectable within responding cells following IL-15 trans-presentation. The role of the proteolytic cleavage of IL-15Rα in this process was investigated by generating an uncleavable form of IL-15Rα. We showed that IL-15 entry into responding cells necessitates the cleavage of IL-15.IL-15Rα complex from the surface of IL-15 presenting cells, and observed that IL-15Rα cleavage is associated with a decrease of the duration of Stat5 signaling. Once separated from presenting cells, responding cells are able to recycle IL-15.IL-15Rα complexes via intracellular compartments, for residual proliferation in a time-limited manner. These studies define an unprecedented cytokine pathway in which the IL-15.IL-15Rα complex cleaved from presenting cells allows responding cells to internalize, store and use IL-15.IL-15Rα complex for their own proliferation and survival.

Characterization of subepithelial interstitial cells in normal and pathological human prostate

AIMS

To characterize the subepithelial (SE) stromal cells in normal human prostate and to study phenotypical changes in these SE stromal cells in benign prostate hyperplasia (BPH) and prostate cancer (PCa).

METHODS AND RESULTS

Tissue was obtained from normal, hyperplastic and tumoral areas in prostatectomy specimens. Tissue samples were processed for morphology, immunohistochemistry, immunofluorescence and electron microscopy. An ultrastructural and immunohistochemical phenotype for the SE stromal cells was assessed, and changes occurring in BPH and PCa were looked for. Based on the ultrastructural findings the dominant SE stromal cell types in normal prostate were interstitial Cajal-like cells and spindle-shaped myoid cells with both myoid and fibroblastic features. An immunohistochemical correlate was found with selective expression of mesenchymal and myoid cell markers. In BPH and PCa, a changed stromal cell composition of the SE region was found. Furthermore, direct contacts between spindle-shaped myoid cells and tumour cells were observed in PCa.

CONCLUSIONS

The present study shows for the first time that the SE area in the human prostate houses different stromal cell types with distinct phenotypes. In BPH and PCa specific disease-related changes were observed in the organization and phenotypes of SE interstitial cells.

Characterization of NCR1+ cells residing in lymphoid tissues in the gut of lambs indicates that the majority are NK cells

Natural killer (NK) cells are important for immune protection of the gut mucosa. Previous studies have shown that under pathologic conditions NK cells, T cells and dendritic cells are found co-localised in secondary lymphoid organs where their interaction coordinates immune responses. However, in the gut-associated lymphoid tissues (GALTs), there are few detailed reports on the distribution of NK cells. Sheep harbour several types of organised lymphoid tissues in the gut that have different functions. The ileal Peyer's patch (IPP) functions as a primary lymphoid tissue for B cell generation, while the jejunal Peyer's patches (JPPs) and colon patches (CPs) are considered secondary lymphoid tissues. In the present study, we analysed tissues from healthy lambs by flow cytometry and in situ multicolour immunofluorescence, using recently described NCR1 antibodies to identify ovine NK cells. Most NCR1+ cells isolated from all tissues were negative for the pan T cell marker CD3, and thus comply with the general definition of NK cells. The majority of NCR1+ cells in blood as well as secondary lymphoid organs expressed CD16, but in the GALT around half of the NCR1+ cells were negative for CD16. A semi-quantitative morphometric study on tissue sections was used to compare the density of NK cells in four compartments of the IPPs, JPP and CPs. NCR1+ cells were found in all gut segments. Statistical analysis revealed significant differences between compartments of the primary lymphoid organ IPP and the secondary lymphoid organs of the JPPs and CP. NK cells co-localised and made close contact with T cells, dendritic cells and other NK cells, but did not show signs of proliferation. We conclude that NK cells are present in all investigated segments of the sheep gut, but that presence of other innate lymphoid cells expressing NCR1 cannot be excluded.

Dendritic cell derived cytokines in human natural killer cell differentiation and activation

Dendritic cells (DCs) and natural killer (NK) cells shape each other’s functions early during immune responses. DCs activate NK cells and NK cells can mature or kill DCs. In this review we will discuss which DC and NK cell subsets are mainly affected by this interaction, where these encounters might take place and which signals are exchanged. Finally, we will point out what the clinical benefit of understanding this interaction might be and how it changed our view on NK cells as innate lymphocytes.

Dynamics of NK cell interactions in vivo

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

IL12B expression is sustained by a heterogenous population of myeloid lineages during tuberculosis

IL12B is required for resistance to Mycobacterium tuberculosis (Mtb) infection, promoting the initiation and maintenance of Mtb-specific effector responses. While this makes the IL12-pathway an attractive target for experimental tuberculosis (TB) therapies, data regarding what lineages express IL12B after infection is established are limited. This is not obvious in the lung, an organ in which both hematopoietic and non-hematopoietic lineages produce IL12p40 upon pathogen encounter. Here, we use radiation bone marrow chimeras and Yet40 reporter mice to determine what lineages produce IL12p40 during experimental TB. We observed that hematopoietic IL12p40-production was sufficient to control Mtb, with no contribution by non-hematopoietic lineages. Furthermore, rather than being produced by a single subset, IL12p40 was produced by cells that were heterogenous in their size, granularity, autofluorescence and expression of CD11c, CD11b and CD8α. While depending on the timepoint and tissue examined, the surface phenotype of IL12p40-producers most closely resembled macrophages based on previous surveys of lung myeloid lineages. Importantly, depletion of CD11c(hi) cells during infection had no affect on lung IL12p40-concentrations. Collectively, our data demonstrate that IL12p40 production is sustained by a heterogenous population of myeloid lineages during experimental TB, and that redundant mechanisms of IL12p40-production exist when CD11c(hi) lineages are absent.

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.

Multiple distinct NK-cell synapses

In this issue of Blood, Barreira da Silva and colleagues further unravel the mysterious interaction between human NKcells and dendritic cells (DCs) through detailed study of their synaptic interface.1 Here, actin integrity in the DCs works to restrain cytolysis and enable civil cross-talk thus defining a regulatory NK-cell synapse.