Fc receptors on natural killer cells

Decoding NY-ESO-1 TCR T cells: transcriptomic insights reveal dual mechanisms of tumor targeting in a melanoma murine xenograft model

The development of T cell receptor-engineered T cells (TCR-T) targeting intracellular antigens is a promising strategy for treating solid tumors; however, the mechanisms underlying their effectiveness remain poorly understood. In this study, we employed advanced techniques to investigate the functional state of T cells engineered with retroviral vectors to express a TCR specific for the NY-ESO-1 157-165 peptide in the HLA-A*02:01 context. Flow cytometry revealed a predominance of naïve T cells. Gene expression profiling using NanoString technology revealed upregulation of genes encoding chemokine receptors CCR2 and CCR5, indicating enhanced migration towards tumor sites. In the SK-Mel-37 xenograft model, these transduced T cells achieved complete tumor eradication. Furthermore, single-cell RNA sequencing (scRNA-seq) conducted 14 days post-TCR T cell infusion provided a comprehensive analysis of the in vivo adaptation of these cells, identifying a distinct subset of CD8+ effector T cells with an NK cell-like gene expression profile. Our findings indicate that NY-ESO-1 TCR-transduced T cells have the potential to mediate dual antitumor effects through both antigen-independent NK-like and antigen-specific CTL-like responses. This study underscores the potential of NY-ESO-1 TCR-T cells as potent tumor-eradicating agents, highlighting the importance of harnessing their versatile functional capabilities to refine and enhance therapeutic strategies.

Combinatorial immunotherapy of N-803 (IL-15 superagonist) and dinutuximab with ex vivo expanded natural killer cells significantly enhances in vitro cytotoxicity against GD2+ pediatric solid tumors and in vivo survival of xenografted immunodeficient NSG mice

Background

Children with recurrent and/or metastatic osteosarcoma (OS), neuroblastoma (NB) and glioblastoma multiforme (GBM) have a dismal event-free survival (<25%). The majority of these solid tumors highly express GD2. Dinutuximab, an anti-GD2 monoclonal antibody, significantly improved event-free survival in children with GD2⁺ NB post autologous stem cell transplantation and enhanced natural killer (NK) cell-mediated antibody-dependent cell cytotoxicity. Thus, approaches to increase NK cell number and activity, improve persistence and trafficking, and enhance tumor targeting may further improve the clinical benefit of dinutuximab. N-803 is a superagonist of an interleukin-15 (IL-15) variant bound to an IL-15 receptor alpha Su-Fc fusion with enhanced biological activity.

Methods

The anti-tumor combinatorial effects of N-803, dinutuximab and ex vivo expanded peripheral blood NK cells (exPBNK) were performed in vitro using cytoxicity assays against GD2⁺ OS, NB and GBM cells. Perforin and interferon (IFN)-γ levels were measured by ELISA assays. Multiple cytokines/chemokines/growth factors released were measured by multiplex assays. Human OS, GBM or NB xenografted NOD/SCID/IL2rγnull (NSG) mice were used to investigate the anti-tumor combinatorial effects in vivo.

Results

N-803 increased the viability and proliferation of exPBNK. The increased viability and proliferation are associated with increased phosphorylation of Stat3, Stat5, AKT, p38MAPK and the expression of NK activating receptors. The combination of dinutuximab and N-803 significantly enhanced in vitro cytotoxicity of exPBNK with enhanced perforin and IFN-γ release against OS, GBM and NB. The combination of exPBNK+N-803+dinutuximab significantly reduced the secretion of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), platelet-derived growth factor-BB (PDGF-BB), and stem cell growth factor beta (SCGF-β) from OS or GBM tumor cells. Furthermore, OS or GBM significantly inhibited the secretion of regulated on activation, normal T cell expressed and presumably secreted (RANTES) and stromal cell-derived factor-1 alpha (SDF-1α) from exPBNK cells (p<0.001) but significantly enhanced monokine induced by gamma interferon (MIG) secretion from exPBNK cells (p<0.001). N-803 combined with dinutuximab and exPBNK cells significantly extended the survival of OS, GBM or NB xenografted NSG mice.

Conclusions

Our results provide the rationale for the development of a clinical trial of N-803 in combination with dinutuximab and ex vivo exPBNK cells in patients with recurrent or metastatic GD2⁺ solid tumors.

Novel cytokine-antibody fusion protein, N-820, to enhance the functions of ex vivo expanded natural killer cells against Burkitt lymphoma

BACKGROUND

The prognosis of patients with relapsed or progressive B cell (CD20⁺) non-Hodgkin's lymphoma (B-NHL), including Burkitt lymphoma (BL), is dismal due to chemoradiotherapy resistance. Novel therapeutic strategies are urgently needed. N-820 is a fusion protein of N-803 (formerly known as ALT-803) to four single-chains of rituximab. This agent has tri-specific binding activity to CD20 and enhanced antibody-dependent cell-mediated cytotoxicity.

METHODS

We investigated the anti-tumor combinatorial effects of N-820 with ex vivo expanded peripheral blood natural killer (exPBNK) cells against rituximab-sensitive and rituximab-resistant CD20⁺ BL in vitro using cytoxicity assays and in vivo using human BL xenografted NOD/SCID/IL2rγnull (NSG) mice. We also investigated the cytokines/chemokines/growth factors released using ELISA and multiplex assay. Gene expression changes were examined using real-time PCR arrays.

RESULTS

N-820 significantly enhanced the expression of NK activating receptors (p<0.001) and the proliferation of exPBNK cells with enhanced Ki67 expression and Stat5 phosphorylation (p<0.001). N-820 significantly enhanced the secretion of cytokines, chemokines, and growth factors including GM-CSF, RANTES, MIP-1B (p<0.001) from exPBNK cells as compared with the combination of rituximab+N-803. Importantly, N-820 significantly enhanced in vitro cytotoxicity (p<0.001) of exPBNK with enhanced granzyme B and IFN-γ release (p<0.001) against BL. Gene expression profiles in exPBNK stimulated by N-820+Raji-2R showed enhanced transcription of CXCL9, CXCL1, CSF2, CSF3, GZMB, and IFNG. Moreover, N-820 combined with exPBNK significantly inhibited rituximab-resistant BL growth (p<0.05) and extended the survival (p<0.05) of BL xenografted NSG mice.

CONCLUSIONS

Our results provide the rationale for the development of a clinical trial of N-820 alone or in combination with endogenous or ex vivo expanded NK cells in patients with CD20⁺ B-NHL failing prior rituximab containing chemoimmunotherapy regimens.

Single-cell RNA sequencing reveals compromised immune microenvironment in precursor stages of multiple myeloma

Precursor states of Multiple Myeloma (MM) and its native tumor microenvironment need in-depth molecular characterization to better stratify and treat patients at risk. Using single-cell RNA sequencing of bone marrow cells from precursor stages, MGUS and smoldering myeloma (SMM), to full-blown MM alongside healthy donors, we demonstrate early immune changes during patient progression. We find NK cell abundance is frequently increased in early stages, and associated with altered chemokine receptor expression. As early as SMM, we show loss of GrK⁺ memory cytotoxic T-cells, and show their critical role in MM immunosurveillance in mouse models. Finally, we report MHC class II dysregulation in CD14⁺ monocytes, which results in T cell suppression in vitro. These results provide a comprehensive map of immune changes at play over the evolution of pre-malignant MM, which will help develop strategies for immune-based patient stratification.

Effect of Plasma-Derived Exosomes of Refractory/Relapsed or Responsive Patients with Diffuse Large B-Cell Lymphoma on Natural Killer Cells Functions

Objective

The purpose of this study was to investigate effect of plasma-derived exosomes of refractory/relapsed or responsive diffuse large B-cell lymphoma (DLBCL) patients on natural killer (NK) cell functions.

Materials and Methods

In this cross-sectional and experimental study, NK cells were purified from responsive patients (n=10) or refractory/relapsed patients (n=12) and healthy donors (n=12). NK cells were treated with plasma-derived exosomes of responsive or refractory/relapsed patients. We examined the expression levels of hsa-miR-155-5p, hsa- let-7g-5p, INPP5D (SHIP-1) and SOCS-1 in NK cells quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Percentages of NK cells expressing CD69, NKG2D and CD16, NK cell cytotoxicity and NK cell proliferation (using flow-cytometry) as well as interferon-gamma (IFN-γ) level in the supernatant of NK cells using ELISA were also investigated.

Results

We observed an increased level of hsa-miR-155-5p and a decreased level of SOCS-1 in NK cells treated with exosomes compared to untreated NK cell in healthy donors and DLBCL patients. An increase in hsa-miR-155-5p level was associated with an increased level of IFN-γ in healthy donors. The decreased levels of hsa-let-7g-5p were observed in NK cells treated with exosomes in comparison with untreated NK cells in DLBCL patients (P<0.05). There was no significant difference in the percentage of CD69⁺NK cells and NKG2D⁺ NK cells in the absence or presence of exosomes of DLBCL patients in each group. Furthermore, we observed significant reduction of NK cell proliferation in DLBCL patients and healthy donors in the presence of exosomes of refractory/relapsed patients (P<0.05). A significant decrease was observed in cytotoxicity of NK cell in patients with DLBCL treated with exosomes of responsive patients.

Conclusion

Our findings demonstrated adverse effect of plasma-derived exosomes of DLBCL patients on some functions of NK cell. It was also determined that low NK cell count might be associated with impaired response to R-CHOP and an increased recurrence risk of cancer.

Epigenetic and Posttranscriptional Regulation of CD16 Expression during Human NK Cell Development

The surface receptor FcγRIIIA (CD16a) is encoded by the FCGR3A gene and is acquired by human NK cells during maturation. NK cells bind the Fc portion of IgG via CD16a and execute Ab-dependent cell-mediated cytotoxicity, which is critical for the effectiveness of several antitumor mAb therapies. The role of epigenetic regulatory mechanisms controlling transcriptional and posttranscriptional CD16 expression in NK cells is unknown. In this study, we compared specific patterns of DNA methylation and expression of FCGR3A with FCGR3B, which differ in cell type-specific expression despite displaying nearly identical genomic sequences. We identified a sequence within the FCGR3A promoter that selectively exhibits reduced methylation in CD16a+ NK cells versus CD16a- NK cells and neutrophils. This region contained the transcriptional start site of the most highly expressed CD16a isoform in NK cells. Luciferase assays revealed remarkable cell-type specificity and methylation-dependent activity of FCGR3A- versus FCGR3B-derived sequences. Genomic differences between FCGR3A and FCGR3B are enriched at CpG dinucleotides, and mutation of variant CpGs reversed cell-type specificity. We further identified miR-218 as a posttranscriptional negative regulator of CD16a in NK cells. Forced overexpression of miR-218 in NK cells knocked down CD16a mRNA and protein expression. Moreover, miR-218 was highly expressed in CD16a- NK cells compared with CD16a+ NK cells. Taken together, we propose a system of FCGR3A regulation in human NK cells in which CpG dinucleotide sequences and concurrent DNA methylation confer developmental and cell type-specific transcriptional regulation, whereas miR-218 provides an additional layer of posttranscriptional regulation during the maturation process.

Gene expression profiling of the human natural killer cell response to Fc receptor activation: unique enhancement in the presence of interleukin-12

Background

Traditionally, the CD56^dimCD16⁺ subset of Natural Killer (NK) cells has been thought to mediate cellular cytotoxicity with modest cytokine secretion capacity. However, studies have suggested that this subset may exert a more diverse array of immunological functions. There exists a lack of well-developed functional models to describe the behavior of activated NK cells, and the interactions between signaling pathways that facilitate effector functions are not well understood. In the present study, a combination of genome-wide microarray analyses and systems-level bioinformatics approaches were utilized to elucidate the transcriptional landscape of NK cells activated via interactions with antibody-coated targets in the presence of interleukin-12 (IL-12).

Methods

We conducted differential gene expression analysis of CD56^dimCD16⁺ NK cells following FcR stimulation in the presence or absence of IL-12. Next, we functionally characterized gene sets according to patterns of gene expression and validated representative genes using RT-PCR. IPA was utilized for biological pathway analysis, and an enriched network of interacting genes was generated using GeneMANIA. Furthermore, PAJEK and the HITS algorithm were employed to identify important genes in the network according to betweeness centrality, hub, and authority node metrics.

Results

Analyses revealed that CD56^dimCD16⁺ NK cells co-stimulated via the Fc receptor (FcR) and IL-12R led to the expression of a unique set of genes, including genes encoding cytotoxicity receptors, apoptotic proteins, intracellular signaling molecules, and cytokines that may mediate enhanced cytotoxicity and interactions with other immune cells within inflammatory tissues. Network analyses identified a novel set of connected key players, BATF, IRF4, TBX21, and IFNG, within an integrated network composed of differentially expressed genes in NK cells stimulated by various conditions (immobilized IgG, IL-12, or the combination of IgG and IL-12).

Conclusions

These results are the first to address the global mechanisms by which NK cells mediate their biological functions when encountering antibody-coated targets within inflammatory sites. Moreover, this study has identified a set of high-priority targets for subsequent investigation into strategies to combat cancer by enhancing the anti-tumor activity of CD56^dimCD16⁺ NK cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-015-0142-9) contains supplementary material, which is available to authorized users.

Lenalidomide augments actin remodeling and lowers NK-cell activation thresholds

As multiple myeloma (MM) progresses, natural killer (NK)-cell responses decline against malignant plasma cells. The immunomodulatory drug lenalidomide is widely used for treatment of MM but its influence on NK-cell biology is unclear. Here, we report that lenalidomide lowers the threshold for NK-cell activation, causing a 66% decrease in the 50% effective concentration (EC50) for activation through CD16, and a 38% decrease in EC50 for NK group 2 member D (NKG2D)-mediated activation, allowing NK cells to respond to lower doses of ligand. In addition, lenalidomide augments NK-cell responses, causing a twofold increase in the proportion of primary NK cells producing interferon-γ (IFN-γ), and a 20-fold increase in the amount of IFN-γ produced per cell. Importantly, lenalidomide did not trigger IFN-γ production in unstimulated NK cells. Thus, lenalidomide enhances the NK-cell arm of the immune response, without activating NK cells inappropriately. Of particular clinical importance, lenalidomide also allowed NK cells to be activated by lower doses of rituximab, an anti-CD20 monoclonal antibody (mAb) widely used to treat B-cell malignancies. This supports combined use of lenalidomide and rituximab in a clinical setting. Finally, superresolution microscopy revealed that lenalidomide increased the periodicity of cortical actin at immune synapses, resulting in an increase in the area of the actin mesh predicted to be penetrable to vesicles containing IFN-γ. NK cells from MM patients also responded to lenalidomide in this way. This indicates that nanometer-scale rearrangements in cortical actin, a recently discovered step in immune synapse assembly, are a potential new target for therapeutic compounds.

Tumor-associated and immunochemotherapy-dependent long-term alterations of the peripheral blood NK cell compartment in DLBCL patients

Natural Killer (NK) cells are a key component of tumor immunosurveillance and thus play an important role in rituximab-dependent killing of lymphoma cells via an antibody-dependent cellular cytotoxicity (ADCC) mechanism. We evaluated the phenotypic and functional assets of peripheral blood NK cell subsets in 32 newly-diagnosed diffuse large B-cell lymphoma (DLBCL) patients and in 27 healthy controls. We further monitored long-term modifications of patient NK cells for up to 12 months after rituximab-based immunochemotherapy. At diagnosis, patients showed a higher percentage of CD56^dim and CD16⁺ NK cells, and a higher frequency of GrzB⁺ cells in CD56^dim, CD56^bright, and CD16⁺ NK cell subsets than healthy controls. Conversely, DLBCL NK cell killing and interferon γ (IFNγ) production capability were comparable to those derived from healthy subjects. Notably, NK cells from refractory/relapsed patients exhibited a lower "natural" cytotoxicity. A marked and prolonged therapy-induced reduction of both "natural" and CD16-dependent NK cytotoxic activities was accompanied by the down-modulation of CD16 and NKG2D activating receptors, particularly in the CD56^dim subset. However, reduced NK cell killing was not associated with defective lytic granule content or IFNγ production capability. This study firstly describes tumor-associated and therapy-induced alterations of the systemic NK cell compartment in DLBCL patients. As these alterations may negatively impact rituximab-based therapy efficacy, our work may provide useful information for improving immunochemotherapeutic strategies.

The quantitative and functional changes of NK cells in mice infected with Angiostrongylus cantonensis

Angiostrongylus cantonensis is a neurotropic parasite which can cause injury to central nervous system and eosinophilic meningitis to human. Natural killer (NK) cells are specialized innate lymphocytes important in early defense against pathogens as in a variety of intracellular bacterial, viral, and protozoan infections. However, the number and function of NK cells in extracellular parasitic infection of A. cantonensis are unclear. In this study, on A. cantonensis infected mice which may mimic the human's infection, we found that the percentage of splenic NK cells and the absolute number of peripheral blood NK cells were decreased at 21-day post infection compared with that of controls. When administrating with albendazole treatment at early stage of the infection, the changes of NK cells could be avoided. Further analysis confirmed that the reduction of NK cells was due to their apoptosis manifested as increased expressions of annexin V and activated caspase-3 after 16-day post infection. Moreover, both activated and inhibitory receptors such as CD16, CD69, NKG2D, and Ly49a on NK cells were down-regulated after 16-day post infection. Interestingly, NK cells isolated from mice of 21-day post infection showed enhanced IFN-γ production when stimulated with IL-12 for 24 h and cytotoxicity to YAC-1 cells, as well as elevated CD107a expression. It is evident that NK cell population and its function were changed in A. cantonensis infected mice, suggesting their involvement in pathogenesis of the infection.

Fc gamma receptors in respiratory syncytial virus infections: implications for innate immunity

RSV infections are a major burden in infants less than 3 months of age. Newborns and infants express a distinct immune system that is largely dependent on innate immunity and passive immunity from maternal antibodies. Antibodies can regulate immune responses against viruses through interaction with Fc gamma receptors leading to enhancement or neutralization of viral infections. The mechanisms underlying the immunomodulatory effect of Fc gamma receptors on viral infections have yet to be elucidated in infants. Herein, we will discuss current knowledge of the effects of antibodies and Fc gamma receptors on infant innate immunity to RSV. A better understanding of the pathogenesis of RSV infections in young infants may provide insight into novel therapeutic strategies such as vaccination.

β-Adrenergic receptor mediated increases in activation and function of natural killer cells following repeated social disruption

Natural killer (NK) cells are specialized innate lymphocytes important in the early defense against tumor and virus bearing cells. Many factors influence the immune system's effectiveness against pathogens, including stress. Social disruption (SDR) "primes" macrophages/monocytes and dendritic cells thereby enhancing their anti-microbial function. What remains unclear is whether similar responses are evident in NK cells. Current studies investigated the cellular distribution and activation/inhibitory phenotypes of NK cells in the spleen, lung, and blood of C57BL/6 male mice following SDR. Furthermore, cytolytic activity and anti-viral cytokine production of splenic NK cells were determined. Lastly, β-adrenergic receptor (β-AR) signaling was investigated to determine possible mechanisms behind the SDR-induced NK cell alterations. Results indicated NK cells from SDR mice have increased expression of CD16 and CD69 and reduced NKG2a and Ly49a expression on splenic CD3-/DX5+ NK cells indicative of an activated phenotype, both immediately and 14h post-SDR. Administration of propranolol (10mg/kg; non-selective β-adrenergic receptor antagonist) was shown to block these "priming" effects at the 14h time-point. In the lung, SDR had similar effects on activation and inhibitory receptors 14h post-SDR, however no alterations were evident in the blood besides increased NK cells directly after SDR. Additionally, splenic NK cells from SDR mice had increased CD107a surface expression, cytolytic activity, and IFN-γ production was increased upon costimulation with IgG and IL-2 ex vivo. Collectively, these data suggest that social stress "primes" NK cells in the spleen and lung to be more proficient in their cytolytic and anti-viral/tumor effecter functions through β-adrenergic receptor dependent signaling.

Super-resolution imaging of remodeled synaptic actin reveals different synergies between NK cell receptors and integrins

Natural killer (NK) cells secrete lytic granules to directly kill virus-infected or transformed cells and secrete cytokines to communicate with other cells. Three-dimensional super-resolved images of F-actin, lytic granules, and IFN-γ in primary human NK cells stimulated through different activating receptors reveal that both IFN-γ and lytic granules accumulated in domains where the periodicity of the cortical actin mesh at the synapse opened up to be penetrable. Ligation of some activating receptors alone (eg, CD16 or NKG2D) was sufficient to increase the periodicity of the actin mesh, but surprisingly, ligation of others (eg, NKp46 or CD2) was not sufficient to induce cortical actin remodeling unless LFA-1 was coligated. Importantly, influenza virus particles that can be recognized by NK cells similarly did not open the actin mesh but could if LFA-1 was coligated. This leads us to propose that immune cells using germline-encoded receptors to directly recognize foreign proteins can use integrin recognition to differentiate between free pathogens and pathogen-infected cells that will both be present in blood. This distinction would not be required for NK cell receptors, such as NKG2D, which recognize host cell-encoded proteins that can only be found on diseased cells and not pathogens.

miR-155 regulates IFN-γ production in natural killer cells

MicroRNAs (miRs) are small, noncoding RNA molecules with important regulatory functions whose role in regulating natural killer (NK) cell biology is not well defined. Here, we show that miR-155 is synergistically induced in primary human NK cells after costimulation with IL-12 and IL-18, or with IL-12 and CD16 clustering. Over-expression of miR-155 enhanced induction of IFN-γ by IL-12 and IL-18 or CD16 stimulation, whereas knockdown of miR-155 or its disruption suppressed IFN-γ induction in monokine and/or CD16-stimulated NK cells. These effects on the regulation of NK cell IFN-γ expression were found to be mediated at least in part via miR-155's direct effects on the inositol phosphatase SHIP1. Consistent with this, we observed that modulation of miR-155 overrides IL-12 and IL-18-mediated regulation of SHIP1 expression in NK cells. Collectively, our data indicate that miR-155 expression is regulated by stimuli that strongly induce IFN-γ in NK cells such as IL-12, IL-18, and CD16 activation, and that miR-155 functions as a positive regulator of IFN-γ production in human NK cells, at least in part via down-regulating SHIP1. These findings may have clinical relevance for targeting miR-155 in neoplastic disease.

Tim-3 is an inducible human natural killer cell receptor that enhances interferon gamma production in response to galectin-9

NK-cell function is regulated by the integration of signals received from activating and inhibitory receptors. Here we show that a novel immune receptor, T-cell Ig and mucin-containing domain-3 (Tim-3), is expressed on resting human NK cells and is up-regulated on activation. The NK92 NK-cell line engineered to overexpress Tim-3 showed a marked increase in IFN-γ production in the presence of soluble rhGal-9 or Raji tumor cells engineered to express Gal-9. The Tim-3(+) population of low-dose IL-12/IL-18-activated primary NK cells significantly increased IFN-γ production in response to soluble rhGal-9, Gal-9 presented by cell lines, and primary acute myelogenous leukemia (AML) targets that endogenously express Gal-9. This effect is highly specific as Tim-3 Ab blockade significantly decreased IFN-γ production, and Tim-3 cross-linking induced ERK activation and degradation of IκBα. Exposure to Gal-9-expressing target cells had little effect on CD107a degranulation. Reconstituted NK cells obtained from patients after hematopoietic cell transplantation had diminished expression of Tim-3 compared with paired donors. This observation correlates with the known IFN-γ defect seen early posttransplantation. In conclusion, we show that Tim-3 functions as a human NK-cell coreceptor to enhance IFN-γ production, which has important implications for control of infectious disease and cancer.

The PP2A inhibitor SET regulates granzyme B expression in human natural killer cells

The ability of natural killer (NK) cells to kill malignant or infected cells depends on the integration of signals from different families of cell surface receptors, including cytokine receptors. How such signals then regulate NK-cell cytotoxicity is incompletely understood. Here we analyzed an endogenous inhibitor of protein phosphatase 2A (PP2A) activity called SET, and its role in regulating human NK-cell cytotoxicity and its mechanism of action in human NK cells. RNAi-mediated suppression of SET down-modulates NK-cell cytotoxicity, whereas ectopic overexpression of SET enhances cytotoxicity. SET knockdown inhibits both mRNA and protein granzyme B expression, as well as perforin expression, whereas SET overexpression enhances granzyme B expression. Treatment of NK cells with the PP2A activator 1,9-dideoxy-forskolin also inhibits both granzyme B expression and cytotoxicity. In addition, pretreatment with the PP2A inhibitor okadaic acid rescues declining granzyme B mRNA levels in SET knockdown cells. Down-modulation of SET expression or activation of PP2A also decreases human NK-cell antibody-dependent cellular cytotoxicity. Finally, the induction of granzyme B gene expression by interleukin-2 and interleukin-15 is inhibited by SET knockdown. These data provide evidence that granzyme B gene expression and therefore human NK-cell cytotoxicity can be regulated by the PP2A-SET interplay.

CD16 (FcRgammaIII) as a potential marker of osteoclast precursors in psoriatic arthritis

Introduction

Psoriatic arthritis (PsA) is a chronic inflammatory arthritis characterized by bone erosion mediated by osteoclasts (OC). Our previous studies showed an elevated frequency of OC precursors (OCP) in PsA patients. Here, we examined if OC arise from CD16-positive monocytes in PsA.

Methods

Peripheral blood mononuclear cells (PBMC) or monocytes were isolated from human peripheral blood and sorted based on CD16 expression. Sorted cells were cultured alone or with bone wafers in the presence of receptor activator of nuclear factor kappa-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). Enumeration and bone erosion activity of OC were examined after culture. The effects of tumor necrosis factor-alpha (TNFα), OC-promoting (M-CSF plus RANKL), and dendritic cell (DC)-promoting (GM-CSF plus interleukin (IL)-4) cytokines on CD16 surface expression were examined by flow cytometry.

Results

PsA and psoriasis (Ps) subjects had a higher percentage of circulating inflammatory CD14+CD16+ cells than healthy controls (HC). Exposure of cells to OC-promoting, but not DC-promoting media, was associated with CD16 up-regulation. PBMC of Ps and PsA had a higher frequency of cells expressing intermediate levels of CD16. OC were mainly derived from CD16+ cells in PsA. Increased CD16 expression was associated with a higher bone erosion activity in PsA.

Conclusions

An increased frequency of circulating CD14+CD16+ cells was noted in PsA compared to controls, and intermediate levels of CD16 may suggest a transitional state of OCP during osteoclastogenesis. Intriguingly, TNFα blocked CD16 expression on a subset of CD14+ monocytes. Collectively, our data suggest that CD16 has the potential to serve as an OCP marker in inflammatory arthritis.

A phase I trial of paclitaxel and trastuzumab in combination with interleukin-12 in patients with HER2/neu-expressing malignancies

Our preclinical work showed a dramatic synergy between interleukin-12 (IL-12) and trastuzumab for stimulation of natural killer cell cytokine secretion. We aimed to determine the safety profile of IL-12 when given in combination with trastuzumab and paclitaxel to patients with metastatic HER2-overexpressing cancers. Paclitaxel was given i.v. at 175 mg/m(2) every 3 weeks. Trastuzumab was given on day 1 each week (4 mg/kg initially and 2 mg/kg thereafter) in combination with injections of IL-12 on days 2 and 5 starting in cycle 2. This trial accrued 21 patients with metastatic HER2-positive tumors (breast, 7; colon, 6; esophagus, 4; stomach, 2; pancreas, 1; thyroid, 1). The IL-12 component was dose-escalated in cohorts of three patients. The dose-limiting toxicity was grade 3 fatigue at the 300 ng/kg dose level in two patients. The recommended phase II dose was 200 ng/kg administered s.c. There was one complete response in a patient with breast cancer, partial responses in 4 patients (breast, 2; esophageal, 2), and stabilization of disease lasting 3 months or greater (SD) in 6 other patients. All but one response occurred in patients with HER2 3+ disease. Two SD patients completed 1 year of therapy. Ten patients had progressive disease. There was increased activation of extracellular signal-regulated kinase in peripheral blood mononuclear cells and increased levels of IFN-gamma and several chemokines in patients with clinical benefit (complete response, partial response, or SD), but not in patients with progressive disease. IL-12 in combination with trastuzumab and paclitaxel therefore exhibits an acceptable toxicity profile and has activity in patients with HER2-overexpressing cancers.

Minimal requirement for induction of natural cytotoxicity and intersection of activation signals by inhibitory receptors

Natural killer (NK) cells provide innate control of infected and neoplastic cells. Multiple receptors have been implicated in natural cytotoxicity, but their individual contribution remains unclear. Here, we studied the activation of primary, resting human NK cells by Drosophila cells expressing ligands for receptors NKG2D, DNAM-1, 2B4, CD2, and LFA-1. Each receptor was capable of inducing inside-out signals for LFA-1, promoting adhesion, but none induced degranulation. Rather, release of cytolytic granules required synergistic activation through coengagement of receptors, shown here for NKG2D and 2B4. Although engagement of NKG2D and 2B4 was not sufficient for strong target cell lysis, collective engagement of LFA-1, NKG2D, and 2B4 defined a minimal requirement for natural cytotoxicity. Remarkably, inside-out signaling induced by each one of these receptors, including LFA-1, was inhibited by receptor CD94/NKG2A binding to HLA-E. Strong inside-out signals induced by the combination of NKG2D and 2B4 or by CD16 could overcome CD94/NKG2A inhibition. In contrast, degranulation induced by these receptors was still subject to inhibition by CD94/NKG2A. These results reveal multiple layers in the activation pathway for natural cytotoxicity and that steps as distinct as inside-out signaling to LFA-1 and signals for granule release are sensitive to inhibition by CD94/NKG2A.

TGF-beta utilizes SMAD3 to inhibit CD16-mediated IFN-gamma production and antibody-dependent cellular cytotoxicity in human NK cells

TGF-beta can be a potent suppressor of lymphocyte effector cell functions and can mediate these effects via distinct molecular pathways. The role of TGF-beta in regulating CD16-mediated NK cell IFN-gamma production and antibody-dependent cellular cytotoxicity (ADCC) is unclear, as are the signaling pathways that may be utilized. Treatment of primary human NK cells with TGF-beta inhibited IFN-gamma production induced by CD16 activation with or without IL-12 or IL-2, and it did so without affecting the phosphorylation/activation of MAP kinases ERK and p38, as well as STAT4. TGF-beta treatment induced SMAD3 phosphorylation, and ectopic overexpression of SMAD3 resulted in a significant decrease in IFN-gamma gene expression following CD16 activation with or without IL-12 or IL-2. Likewise, NK cells obtained from smad3(-/-) mice produced more IFN-gamma in response to CD16 activation plus IL-12 when compared with NK cells obtained from wild-type mice. Coactivation of human NK cells via CD16 and IL-12 induced expression of T-BET, the positive regulator of IFN-gamma, and T-BET was suppressed by TGF-beta and by SMAD3 overexpression. An extended treatment of primary NK cells with TGF-beta was required to inhibit ADCC, and it did so by inhibiting granzyme A and granzyme B expression. This effect was accentuated in cells overexpressing SMAD3. Collectively, our results indicate that TGF-beta inhibits CD16-mediated human NK cell IFN-gamma production and ADCC, and these effects are mediated via SMAD3.

Signal transduction during natural killer cell activation

Understanding of transmembrane signaling during NK-cell activation has greatly expanded during the past few years. The discovery and characterization of novel triggering and inhibitory receptors have revealed the complexity of these processes. This unit focuses on receptor-initiated signaling pathways that modulate NK functions. Establishing the roles of different signaling pathways in NK cells is a crucial step in the design of therapeutic approaches for selective enhancement or suppression of NK-cell activation.

Characterization of murine cytomegalovirus m157 from infected cells and identification of critical residues mediating recognition by the NK cell receptor Ly49H

Activated NK cells mediate potent cytolytic and secretory effector functions and are vital components of the early antiviral immune response. NK cell activities are regulated by the assortment of inhibitory receptors that recognize MHC class I ligands expressed on healthy cells and activating receptors that recognize inducible host ligands or ligands that are not well characterized. The activating Ly49H receptor of mouse NK cells is unique in that it specifically recognizes a virally encoded ligand, the m157 glycoprotein of murine CMV (MCMV). The Ly49H-m157 interaction underlies a potent resistance mechanism (Cmv1) in C57BL/6 mice and serves as an excellent model in which to understand how NK cells are specifically activated in vivo, as similar receptor systems are operative for human NK cells. For transduced cells expressing m157 in isolation and for MCMV-infected cells, we show that m157 is expressed in multiple isoforms with marked differences in abundance between infected fibroblasts (high) and macrophages (low). At the cell surface, m157 is exclusively a glycosylphosphatidylinositol-associated protein in MCMV-infected cells. Through random and site-directed mutagenesis of m157, we identify unique residues that provide for efficient cell surface expression of m157 but fail to activate Ly49H-expressing reporter cells. These m157 mutations are predicted to alter the conformation of a putative m157 interface with Ly49H, one that relies on the position of a critical alpha0 helix of m157. These findings support an emerging model for a novel interaction between this important NK cell receptor and its viral ligand.

Activation, coactivation, and costimulation of resting human natural killer cells

Natural killer (NK) cells possess potent perforin- and interferon-gamma-dependent effector functions that are tightly regulated. Inhibitory receptors for major histocompatibility complex class I display variegated expression among NK cells, which confers specificity to individual NK cells. Specificity is also provided by engagement of an array of NK cell activation receptors. Target cells may express ligands for a multitude of activation receptors, many of which signal through different pathways. How inhibitory receptors intersect different signaling cascades is not fully understood. This review focuses on advances in understanding how activation receptors cooperate to induce cytotoxicity in resting NK cells. The role of activating receptors in determining specificity and providing redundancy of target cell recognition is discussed. Using Drosophila insect cells as targets, we have examined the contribution of individual receptors. Interestingly, the strength of activation is not determined simply by additive effects of parallel activation pathways. Combinations of signals from different receptors can have different outcomes: synergy, no enhancement over individual signals, or additive effects. Cytotoxicity requires combined signals for granule polarization and degranulation. The integrin leukocyte function-associated antigen-1 contributes a signal for polarization but not for degranulation. Conversely, CD16 alone or in synergistic combinations, such as NKG2D and 2B4, signals for phospholipase-C-gamma- and phosphatidylinositol-3-kinase-dependent degranulation.

Activation and self-tolerance of natural killer cells

Natural killer (NK) cells are regulated by numerous stimulatory and inhibitory receptors that recognize various classes of cell surface ligands, some of which are expressed by normal healthy cells. We review two key issues in NK cell biology. How do NK cells achieve tolerance to healthy self-cells, despite great potential variability in inhibitory and stimulatory receptor engagement? How is the disease status of unhealthy cells translated into changes in ligand expression and consequent sensitivity to NK cell lysis? Concerning the second question, we review evidence that ligands for one key NK receptor, NKG2D, are induced by the DNA damage response, which is activated in cells exposed to genotoxic stress. Because cancer cells and some infected cells are subject to genotoxic stress, these findings suggest a new concept for how diseased cells are discriminated by the immune system. Second, we review studies that have overturned the prevalent notion that NK cells achieve self-tolerance by expressing inhibitory receptors specific for self-major histocompatibility complex class I molecules. A subset of NK cells lacks such receptors. These NK cells are hyporesponsive when stimulatory receptors are engaged, suggesting that alterations in signaling pathways that dampen stimulatory receptor signals contribute to self-tolerance of NK cells.

Cytoskeleton rearrangement induced by tetraspanin engagement modulates the activation of T and NK cells

The hepatitis C virus (HCV) binds to human cells through the interaction of its envelope glycoprotein E2 with the tetraspanin CD81. We have previously reported that engagement of CD81 has opposite effects on T and NK cell function, as it enhances T cell receptor-mediated T cell activation and inhibits CD16- or IL-12-mediated NK cell activation. We further investigated this dichotomy and found that another tetraspanin, CD82, induces the same opposing effects on human primary T and NK cells. Activation by other unrelated stimuli such as NKG2D- and beta-1 integrin is also reduced by CD81 ligation on NK cells. CD81 engagement by monoclonal antibody or HCV-E2 enhances zeta and Erk phosphorylation in T cells and reduces them in NK cells, reflecting the opposite functional outcomes. CD81 engagement induces dramatic morphological changes and local F-actin accumulation in both NK and T cells, indicating rearrangement of the actin cytoskeleton. Pharmacological inhibition of actin polymerization reduces T cell activation, whereas it greatly enhances NK cell activation. Importantly, treatment with actin blockers abolishes the inhibitory effect of CD81 ligation on NK cells. We propose that tetraspanin engagement leads to comparable cytoskeleton reorganization in T and NK cells, which in turn results in opposite functional outcomes.

Cytolytic granule polarization and degranulation controlled by different receptors in resting NK cells

The relative contribution to cytotoxicity of each of the multiple NK cell activation receptors has been difficult to assess. Using Drosophila insect cells, which express ligands of human NK cell receptors, we show that target cell lysis by resting NK cells is controlled by different receptor signals for cytolytic granule polarization and degranulation. Intercellular adhesion molecule (ICAM)-1 on insect cells was sufficient to induce polarization of granules, but not degranulation, in resting NK cells. Conversely, engagement of the Fc receptor CD16 by rabbit IgG on insect cells induced degranulation without specific polarization. Lysis by resting NK cells occurred when polarization and degranulation were induced by the combined presence of ICAM-1 and IgG on insect cells. Engagement of receptor 2B4 by CD48 on insect cells induced weak polarization and no degranulation. However, coengagement of 2B4 and CD16 by their respective ligands resulted in granule polarization and cytotoxicity in the absence of leukocyte functional antigen-1-mediated adhesion to target cells. These data show that cytotoxicity by resting NK cells is controlled tightly by separate or cooperative signals from different receptors for granule polarization and degranulation.

Src Homology 2–Containing Inositol 5′-Phosphatase 1 Negatively Regulates IFN-γ Production by Natural Killer Cells Stimulated with Antibody-Coated Tumor Cells and Interleukin-12

We have previously shown that natural killer (NK) cells secrete a distinct profile of immunomodulatory cytokines in response to dual stimulation with antibody-coated tumor cells and interleukin-12 (IL-12). This NK cell cytokine response is dependent on synergistic signals mediated by the activating receptor for the Fc portion of IgG (FcgammaRIIIa) and the IL-12 receptor (IL-12R), both constitutively expressed on NK cells. The phosphatase Src homology 2-containing inositol 5'-phosphatase 1 (SHIP1) is known to exert inhibitory effects on Fc receptor (FcR) signaling via its enzymatic activity on phosphatidylinositol 3-kinase (PI3-K) products within many cells of the immune system, most notably mast cells, B cells, and monocytes. However, its activity in the context of FcR activation on NK cells has not been fully explored. The current study focused on the regulation of FcgammaRIIIa-induced NK cell cytokine production by SHIP1. Inhibitor studies showed that NK cell IFN-gamma production following FcR stimulation in the presence of IL-12 depended, in part, on the downstream products of PI3-K. Overexpression of wild-type (WT) SHIP1, but not a catalytic-deficient mutant, via retroviral transfection of primary human NK cells, resulted in a >70% reduction of NK cell IFN-gamma production in response to costimulation. In addition, NK cells from SHIP1-/- mice produced 10-fold greater amounts of IFN-gamma following culture with antibody-coated tumor cells plus IL-12 compared with NK cells from WT mice. Further, activation of the mitogen-activated protein kinase (MAPK) family member extracellular signal-regulated kinase (Erk; a downstream target of PI3-K) was significantly enhanced within SHIP1-/- NK cells compared with WT NK cells following costimulation. Pharmacologic inhibition of Erk activity, but not Jnk MAPK activity, led to significantly decreased IFN-gamma production from both SHIP1-/- and WT NK cells under these conditions. These results are the first to show a physiologic role for SHIP1 in the regulation of NK cell cytokine production and implicate PI3-K in the induction of MAPK signal transduction following costimulation of NK cells via the FcR and the IL-12R.

p38 MAPK activation controls the TLR3-mediated up-regulation of cytotoxicity and cytokine production in human NK cells

Natural killer (NK) cells are a component of the innate immunity against viral infections through their rapid cytotoxic activity and cytokine production. Although the synthetic double-stranded (ds) RNA polyinosinic-polycytidylic acid (poly I:C), a mimic of a common product of viral infections, is known to rapidly up-regulate their in vivo functions, NK cell ability to directly respond to dsRNA is still mostly unknown. Our results show that treatment with poly I:C significantly up-regulates both natural and CD16-mediated cytotoxicity of highly purified human NK cells. Poly I:C also induces the novel capability of producing CXCL10 chemokine in human NK cells and synergistically enhances interferon-gamma (IFN-gamma) production induced by either adaptive or innate cytokines. In accordance with the expression of Toll-like receptor-3 (TLR3) and of TRIF/TICAM-1 adaptor, poly I:C stimulation induces the activation of interferon regulatory factor-3 (IRF-3) transcription factor and of p38 mitogen-activated protein kinase (MAPK) in human NK cells. Finally, we demonstrate that p38 MAPK activity is required for the dsRNA-dependent enhancement of cytotoxicity and CXCL10 production. The occurrence of dsRNA-induced signaling and functional events closely correlates with the TLR3 mRNAprofile in different NK cell populations. Taken together, these data identify p38 as a central component of NK cell ability to directly respond to dsRNA pathogen-associated molecular pattern (PAMP).

A phase I study of interleukin 12 with trastuzumab in patients with human epidermal growth factor receptor-2-overexpressing malignancies: analysis of sustained interferon gamma production in a subset of patients

PURPOSE

On the basis of preclinical studies, we hypothesized that interleukin (IL)12 would potentiate the antitumor actions of an antihuman epidermal growth factor receptor-2 (HER2) monoclonal antibody (trastuzumab). We conducted a Phase I trial to determine the safety and optimal biological dose of IL-12 when given in combination with trastuzumab.

PATIENTS AND METHODS

Patients with metastatic HER2-positive malignancies received trastuzumab on day 1 of each weekly cycle. Beginning in week 3, patients also received intravenous injections of IL-12 on days 2 and 5. The IL-12 component was dose-escalated within cohorts of 3 patients (30, 100, 300, or 500 ng/kg). Correlative assays were conducted using serum samples and peripheral blood cells obtained during the course of therapy.

RESULTS

Fifteen patients were treated, including 12 with HER2 2+ or 3+ breast cancer. The regimen was well tolerated with IL-12-induced grade 1 nausea and grade 2 fatigue predominating. Evaluation of dose-limiting toxicity and biological end points suggested that the 300 ng/kg dose was both the maximally tolerated dose and the optimal biological dose of IL-12 for use in combination with trastuzumab. Two patients with HER2 3+ breast cancer within the 500 ng/kg dose level experienced grade 1 asymptomatic decreases in left ventricular ejection fraction of 12% and 19% after 3 and 10 months of therapy, respectively. There was one complete response in a patient with HER2 3+ breast cancer metastatic to the axillary, mediastinal, and supraclavicular nodes, and 2 patients with stabilization of bone disease lasting 10 months and >12 months, respectively. Correlative assays showed sustained production of interferon (IFN)gamma by natural killer cells only in those patients experiencing a clinical response or stabilization of disease. Elevated serum levels of macrophage inflammatory protein-1alpha, tumor necrosis factor-alpha, and the antiangiogenic factors IFN-gamma inducible protein-10 and monokine induced by gamma were also observed in these patients. Patient genotyping suggested that a specific IFN-gamma gene polymorphism might have been associated with increased IFN-gamma production. The ability of patient peripheral blood cells to conduct antibody-dependent cellular cytotoxicity against tumor targets in vitro did not correlate with clinical response or dose of IL-12.

CONCLUSIONS

The addition of IL-12 to trastuzumab therapy did not appear to enhance the efficacy of this antibody treatment. Sustained production of IFN-gamma and other cytokines were observed in three patients: One who exhibited a complete response and two others who had stabilization of disease lasting over 6 months. Given the small sample size and heterogeneity of the patient population, the effects of IL-12 on the innate immune response to trastuzumab therapy should be further explored in the context of a larger clinical trial.

Inhibition of natural killer cells through engagement of CD81 by the major hepatitis C virus envelope protein

The immune response against hepatitis C virus (HCV) is rarely effective at clearing the virus, resulting in approximately 170 million chronic HCV infections worldwide. Here we report that ligation of an HCV receptor (CD81) inhibits natural killer (NK) cells. Cross-linking of CD81 by the major envelope protein of HCV (HCV-E2) or anti-CD81 antibodies blocks NK cell activation, cytokine production, cytotoxic granule release, and proliferation. This inhibitory effect was observed using both activated and resting NK cells. Conversely, on NK-like T cell clones, including those expressing NK cell inhibitory receptors, CD81 ligation delivered a costimulatory signal. Engagement of CD81 on NK cells blocks tyrosine phosphorylation through a mechanism which is distinct from the negative signaling pathways associated with NK cell inhibitory receptors for major histocompatibility complex class I. These results implicate HCV-E2-mediated inhibition of NK cells as an efficient HCV evasion strategy targeting the early antiviral activities of NK cells and allowing the virus to establish itself as a chronic infection.

MHC class I-related neonatal Fc receptor for IgG is functionally expressed in monocytes, intestinal macrophages, and dendritic cells

The neonatal Fc receptor (FcRn) for IgG, an MHC class I-related molecule, functions to transport IgG across polarized epithelial cells and protect IgG from degradation. However, little is known about whether FcRn is functionally expressed in immune cells. We show here that FcRn mRNA was identifiable in human monocytes, macrophages, and dendritic cells. FcRn heavy chain was detectable as a 45-kDa protein in monocytic U937 and THP-1 cells and in purified human intestinal macrophages, peripheral blood monocytes, and dendritic cells by Western blot analysis. FcRn colocalized in vivo with macrosialin (CD68) and Ncl-Macro, two macrophage markers, in the lamina propria of human small intestine. The heavy chain of FcRn was associated with the beta(2)-microglobulin (beta(2)m) light chain in U937 and THP-1 cells. FcRn bound human IgG at pH 6.0, but not at pH 7.5. This binding could be inhibited by human IgG Fc, but not Fab. FcRn could be detected on the cell surface of activated, but not resting, THP-1 cells. Furthermore, FcRn was uniformly present intracellularly in all blood monocytes and intestinal macrophages. FcRn was detectable on the cell surface of a significant fraction of monocytes at lower levels and on a small subset of tissue macrophages that expressed high levels of FcRn on the cell surface. These data show that FcRn is functionally expressed and its cellular distribution is regulated in monocytes, macrophages, and dendritic cells, suggesting that it may confer novel IgG binding functions upon these cell types relative to typical Fc gamma Rs: Fc gamma RI, Fc gamma RII, and Fc gamma RIII.

CD94/NKG2-A Inhibitory Complex Blocks CD16-Triggered Syk and Extracellular Regulated Kinase Activation, Leading to Cytotoxic Function of Human NK Cells

The CD94/NKG2-A complex is the inhibitory receptor for the nonclassical MHC class I molecule HLA-E on human NK cells. Here we studied the molecular mechanisms underlying the inhibitory activity of CD94/NKG2-A on NK cell functions by analyzing its interference on CD16-initiated signaling pathways involved in the control of cytolytic activity. Both tyrosine phosphorylation and activation of Syk kinase together with tyrosine phosphorylation of CD16 receptor ζ subunit are markedly inhibited by the coengagement of CD94/NKG2-A complex. As a downstream consequence, CD94/NKG2-A cross-linking impairs the CD16-induced activation of extracellular regulated kinases (ERKs), a pathway involved in NK cytotoxic function. The block of ERK activation is exerted at an early, PTK-dependent stage in the events leading to p21ras activation, as the CD16-induced tyrosine phosphorylation of Shc adaptor protein and the formation of Shc/Grb-2 complex are abrogated by CD94/NKG2-A simultaneous engagement. Our observations indicate that CD94/NKG2-A inhibits the CD16-triggered activation of two signaling pathways involved in the cytotoxic activity of NK cells. They thus provide molecular evidence to explain the inhibitory function of CD94/NKG2-A receptor on NK effector functions.

Dependence of Both Spontaneous and Antibody-Dependent, Granule Exocytosis-Mediated NK Cell Cytotoxicity on Extracellular Signal-Regulated Kinases

Extracellular signal-regulated kinases (ERK, also known as mitogen-activated protein kinases) are serine-threonine kinases transducing signals elicited upon ligand binding to several tyrosine kinase-associated receptors. We have reported that ERK2 phosphorylation and activation follows engagement of the low affinity receptor for the Fc portion of IgG (CD16) on NK cells, and is necessary for CD16-induced TNF-α mRNA expression. Here, we analyzed the involvement of ERK in NK cell-mediated cytotoxicity and IFN-γ expression induced upon stimulation with targets cells, coated or not with Abs. Our data indicate that, as with immune complexes, ERK2 phosphorylation occurs in human primary NK cells upon interaction with target cells sensitive to granule exocytosis-mediated spontaneous cytotoxicity, and that this regulates both target cell- and immune complex-induced cytotoxicity and IFN-γ mRNA expression. A specific inhibitor of mitogen-activated protein kinase kinase reduced both spontaneous and Ab-dependent cytotoxicity in a dose-dependent manner involving, at least in part, inhibition of granule exocytosis without affecting effector/target cell interaction and rearrangement of the cytoskeleton proteins actin and tubulin. Involvement of ERK in the regulation of Ca2+-dependent cell-mediated cytotoxicity was confirmed, using a genetic approach, in primary NK cells infected with a recombinant vaccinia virus encoding an ERK inactive mutant. These data indicate that the biochemical pathways elicited in NK cells upon engagement of receptors responsible for either spontaneous or Ab-dependent recognition of target cells, although distinct, utilize ERK as one of their downstream molecules to regulate effector functions.