Natural cytotoxicity receptors that trigger human NK-cell-mediated cytolysis

Early expression of triggering receptors and regulatory role of 2B4 in human natural killer cell precursors undergoing in vitro differentiation

In this study we analyzed the progression of cell surface receptor expression during the in vitro-induced human natural killer (NK) cell maturation from CD34(+) Lin(-) cell precursors. NKp46 and NKp30, two major triggering receptors that play a central role in natural cytotoxicity, were expressed before the HLA class I-specific inhibitory receptors. Moreover, their appearance at the cell surface correlated with the acquisition of cytolytic activity by developing NK cells. Although the early expression of triggering receptors may provide activating signals required for inducing further cell differentiation, it may also affect the self-tolerance of developing NK cells. Our data show that a fail-safe mechanism preventing killing of normal autologous cells may be provided by the 2B4 surface molecule, which, at early stages of NK cell differentiation, functions as an inhibitory rather than as an activating receptor.

Syk regulation of phosphoinositide 3-kinase-dependent NK cell function

Emerging evidence suggests that NK-activatory receptors use KARAP/DAP12, CD3zeta, and FcepsilonRIgamma adaptors that contain immunoreceptor tyrosine-based activatory motifs to mediate NK direct lysis of tumor cells via Syk tyrosine kinase. NK cells may also use DAP10 to drive natural cytotoxicity through phosphoinositide 3-kinase (PI3K). In contrast to our recently identified PI3K pathway controlling NK cytotoxicity, the signaling mechanism by which Syk associates with downstream effectors to drive NK lytic function has not been clearly defined. In NK92 cells, which express DAP12 but little DAP10/NKG2D, we now show that Syk acts upstream of PI3K, subsequently leading to the specific signaling of the PI3K-->Rac1-->PAK1-->mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase-->ERK cascade that we earlier described. Tumor cell ligation stimulated DAP12 tyrosine phosphorylation and its association with Syk in NK92 cells; Syk tyrosine phosphorylation and activation were also observed. Inhibition of Syk function by kinase-deficient Syk or piceatannol blocked target cell-induced PI3K, Rac1, PAK1, mitogen-activated protein/ERK kinase, and ERK activation, perforin movement, as well as NK cytotoxicity, indicating that Syk is upstream of all these signaling events. Confirming that Syk does not act downstream of PI3K, constitutively active PI3K reactivated all the downstream effectors as well as NK cytotoxicity suppressed in Syk-impaired NK cells. Our results are the first report documenting the instrumental role of Syk in control of PI3K-dependent natural cytotoxicity.

Human natural killer cell line modified with a chimeric immunoglobulin T-cell receptor gene leads to tumor growth inhibition in vivo

The gene transfer of tumor-specific chimeric immunoglobulin T-cell receptors (cIgTCRs) combining antibody-like specificity with the effector cell function could be an attractive tool in immunotherapy. In this study, we directed the human natural killer (NK) cell line YT to tumor cells by gene transfer of a cIgTCR with specificity against the human carcinoembryonic antigen (CEA). The cIgTCR was constructed of a CEA-specific humanized single-chain Fv antibody fragment fused to the IgG1 Fc domain and the CD3 zeta chain. YT cells were transfected with the cIgTCR gene by electroporation and cIgTCR-expressing cells were enriched by immunoaffinity purification. cIgTCR-expressing YT cells specifically lysed CEA(+) colon carcinoma cell lines, which were resistant to the parental YT cell line. The lysis was not inhibited in the presence of soluble CEA. Receptor gene-modified YT cells retained their CEA-specific cytolytic activity after gamma-irradiation in vitro and inhibited the tumor growth in vivo after adoptive transfer into NOD/SCID mice. This gene-modified NK cell line available in unlimited source might be useful in clinical immunotherapy of CEA(+) cancer.

Rolling adhesion of human NK cells to porcine endothelial cells mainly relies on CD49d-CD106 interactions

BACKGROUND

Acute vascular rejection in pig-to-primate xenotransplantation involves recognition and damage of porcine (po) endothelial cells (EC) by human (hu) leukocytes, probably including natural killer (NK) cells. To study such interactions we analyzed rolling and static adhesion of hu NK cells to po EC.

METHODS

The effects of blocking hu and po adhesion molecules on the adhesion hu NK cells to po EC monolayers was analyzed under shear stress (10 min, 37 degrees C, 0.7 dynes/cm2) or under static conditions (10 min, 37 degrees C). All used cell populations were phenotypically characterized by flow cytometry.

RESULTS

Blocking of CD106 on po EC or its ligand CD49d on hu NK cells decreased rolling adhesion of both fresh and activated hu NK cells by more than 75%. Masking of CD62L on fresh but not activated hu NK resulted in a 44% decrease in rolling adhesion, in line with the diminished cell surface expression of CD62L upon activation. Antibodies to CD31, CD54, CD62E, and CD62P on EC or CD11a, CD18, and CD162 on NK cells had only minor effects on rolling adhesion. The adhesion of the FcgammaRIII- hu NK cell line NK92 to po EC was inhibited by 95% after masking po CD106 whereas antibodies to po CD31, CD54, CD62E, or CD62P had no effect, thereby excluding effects of Fc-receptor-dependent binding of hu NK cells to po EC. Static adhesion of activated NK cells was reduced by approximately 60% by blocking either CD49d or CD106, by 47% by blocking CD11a, and by 82% upon simultaneous blocking of CD11a and CD49d.

CONCLUSIONS

Interactions between hu CD49d and po CD106 are crucial for both rolling and firm adhesion of hu NK cells to po EC and thus represent attractive targets for specific therapeutic interventions to prevent NK cell-mediated responses against po xenografts.

Natural killer cells: a mystery no more

Recent years have witnessed remarkable progress in our understanding of the molecular mechanism regulating natural killer (NK) cell function. NK cells can sense whether cells have lost the surface expression of major histocompatibility complex (MHC)-class I molecules. The discovery of MHC-class I-specific inhibitory receptors clarified the basis of this discrimination and elucidated the nature of the 'off' signal. However, the receptors responsible for the 'on' signal in the process of natural cytotoxicity remained mysterious until recently. Here, we describe the identification and characterization of such receptors and discuss the emerging implications of these findings in different diseases.

Upregulation of natural killer cells functions underlies the efficacy of intratumorally injected dendritic cells engineered to produce interleukin-12

OBJECTIVE

Injection of dendritic cells (DC) engineered with recombinant adenoviral vectors to produce interleukin-12 (IL-12) inside experimental murine tumors frequently achieves complete regressions. In such a system the function of CD8(+) T cells has been shown to be an absolute requirement, in contrast to observations made upon depletion of CD4(+) T cells, which minimally affected the outcome. The aim of this work was to study the possible involvement of natural killer (NK) cells in this setting.

MATERIALS, METHODS, AND RESULTS

Depletions with anti-AsialoGM1 antiserum showed only a small decrease in the proportion of complete regressions obtained that correlated with induction of NK activities in lymphatic tissues into which DC migrate, whereas combined depletions of CD4(+) and NK cells completely eliminated the antitumor effects. Likewise in vivo neutralization of interferon-gamma (IFN-gamma) also eliminated those therapeutic effects. Trying to define the cellular role played by NK cells in vivo, it was observed that injection of cultured DC inside the spleen of T- and B-cell-deficient (Rag1(-/-)) mice induced upregulation of NK activity only if DC had been adenovirally engineered to produce IL-12. In addition, identically transfected fibroblasts also activated NK cells, indicating that IL-12 transfection was the unique requirement. Equivalent human DC only activated in vitro the cytolytic and cytokine-secreting functions of autologous NK cells if transfected to express human IL-12.

CONCLUSIONS

Overall, these results point out an important role played by NK cell activation in the potent immunotherapeutic effects elicited by intratumoral injection of IL-12--secreting DC and that NK activation under these conditions is mainly, if not only, dependent on IL-12.

Selective expansion of human natural killer cells from peripheral blood mononuclear cells by the cell line, HFWT

An anchorage-dependent Wilms tumor cell line HFWT was found to stimulate selective and remarkable expansion of human natural killer (NK) cells from human peripheral blood mononuclear cells (PBMC). After PBMC of healthy donors were cultured on irradiated HFWT cells for 10 - 21 days, the lymphocytes expanded 58- to 401-fold. This NK cell expansion required direct contact of PBMC with live, but not fixed, HFWT cells. The PBMC from an end-stage brain tumor patient also expanded 156-fold, whereas those cultured with irradiated NK-sensitive K562 grew only 30.5-fold. CD16+ CD56+ NK cells accounted for more than 70% of the population expanded on HFWT cells. No essential difference in expression of NK receptors was observed in the expanded NK cells on HFWT and K562 and without feeder cells. The expanded NK cells killed not only fresh HFWT cells but, unexpectedly, also MHC class I-expressing autologous brain tumor cells at an effector/target ratio of 4 for 24 h. These results will contribute to the development of a large-scale preparation method for human NK cells, which will aid studies of NK cell biology and possible treatment of brain tumors.

Human dendritic cells activate resting natural killer (NK) cells and are recognized via the NKp30 receptor by activated NK cells

During the innate response to many inflammatory and infectious stimuli, dendritic cells (DCs) undergo a differentiation process termed maturation. Mature DCs activate antigen-specific naive T cells. Here we show that both immature and mature DCs activate resting human natural killer (NK) cells. Within 1 wk the NK cells increase two-- to fourfold in numbers, start secreting interferon (IFN)-gamma, and acquire cytolytic activity against the classical NK target LCL721.221. The DC-activated NK cells then kill immature DCs efficiently, even though the latter express substantial levels of major histocompatibility complex (MHC) class I. Similar results are seen with interleukin (IL)-2--activated NK cell lines and clones, i.e., these NK cells kill and secrete IFN-gamma in response to immature DCs. Mature DCs are protected from activated NK lysis, but lysis takes place if the NK inhibitory signal is blocked by a human histocompatibility leukocyte antigen (HLA)-A,B,C--specific antibody. The NK activating signal mainly involves the NKp30 natural cytotoxicity receptor, and not the NKp46 or NKp44 receptor. However, both immature and mature DCs seem to use a NKp30 independent mechanism to act as potent stimulators for resting NK cells. We suggest that DCs are able to control directly the expansion of NK cells and that the lysis of immature DCs can regulate the afferent limb of innate and adaptive immunity.

Human cytomegalovirus (HCMV)-infected endothelial cells and macrophages are less susceptible to natural killer lysis independent of the downregulation of classical HLA class I molecules or expression of the HCMV class I homologue, UL18

A number of reports have suggested that human cytomegalovirus (HCMV)-infected fibroblasts are resistant to natural killer (NK) lysis, and that the HCMV-encoded human leucocyte antigen (HLA) class I homologue UL18 may be responsible for this effect. While fibroblasts are easy to infect in vitro, their role in HCMV pathogenesis in vivo is unclear. Here, we have established systems to address NK recognition of infected endothelial cells and macrophages, two important HCMV cellular reservoirs in vivo. The HCMV-infected endothelial cells exhibited increased resistance to NK killing, and, in most experiments, infected macrophages demonstrated a decreased susceptibility to NK lysis. Infection with the mutant HCMV strain RV670, lacking the genes US1-9 and US11 that are responsible for downregulation of HLA class I molecules, also led to decreased NK susceptibility. Furthermore, reduced NK susceptibility was independent of the expression of the HLA class I homologue UL18, since cells infected with the UL18Delta HCMV strain were also less susceptible to NK killing. These results suggest that HCMV-induced resistance to NK cytotoxicity in endothelial cells and macrophages is independent of known pathways that interfere with the expression of cellular HLA class I A, B and C surface antigens and the HCMV encoded class I homologue UL18.

Tumor cell recognition by natural killer cells

Natural killer (NK) cells contribute to the immune defense against cancer and viruses. Tumor cells and infected cells that downregulate the HLA class I antigen expression are targets for NK cell responses because NK cell activation is controlled by a repertoire of inhibitory receptors with different HLA class I specificities. The clonal distribution of these inhibitory receptors permits NK cell recognition of target cells that have lost expression of a single HLA-B or HLA-C allotype. Several activation receptors on NK cells have been identified that contribute to tumor cell recognition. One such receptor, NKG2D, is expressed by all NK cells and binds to inducible ligands on tumor cells.

The role of an exposed loop in the alpha2 domain in the mouse MHC class I H-2Dd molecule for recognition by the monoclonal antibody 34-5-8S and the NK-cell receptor Ly49A

Natural killer (NK) cells express major histocompatibility complex (MHC) class I-specific inhibitory receptors. The region mediating the protective effect of the MHC class I molecule H-2Dd (Dd), recognized by the inhibitory receptor Ly49A, has been mapped to the alpha1/alpha2 domains. Here we have focused on an exposed loop in the N-terminal part of the alpha2 domain, which constitutes a major structural motif that differs between Dd (strong binding to Ly49A) and Db (weak binding to Ly49A at best). We mutated the residues 103, 104 and 107 in Dd to the corresponding amino acids in Db. The Dd mutant molecule retained the ability to be stabilized by a Dd-binding peptide. However, the mutation totally abolished the recognition by the conformational dependent monoclonal antibody (MoAb) 34-5-8S, known to inhibit the interaction between Dd and Ly49A. In addition, there was a marked impairment of the binding to Ly49A as evaluated by the ability of tetramers of the Dd mutant molecule to bind to Ly49A-transfected reporter cells and spleen cells. These results demonstrate that the introduced changes at positions 103, 104 and 107 directly or indirectly affect the epitopes for the MoAb 34-5-8S and the Ly49A receptor.

Nigerooligosaccharides augments natural killer activity of hepatic mononuclear cells in mice

Nigerooligosaccharides (NOS), a mixture of nigerose and nigerosylmaltooligosaccharides, is immunopotentiating oligosaccharides found in foodstuffs. Augmentation of natural killer (NK) activity by NOS was studied in vitro and in vivo in mice. In vitro treatment of hepatic mononuclear cells (MNC) from normal mice with 1 micro/ml NOS 17 h or just prior to the cytotoxicity assay enhanced their cytotoxicity against YAC-1 cells. NK activity of hepatic MNC was also enhanced in mice injected intraperitoneally with 0.4 mg NOS or in mice given 1% NOS solution orally as a drinking fluid. In association with the augmentation of NK activity, increase in the liver weight induced by intravenous inoculation of EL-4 tumor cells was significantly suppressed by intraperitoneal pretreatment with 0.4 mg NOS. Moreover, drinking 1% NOS significantly improved the survival curve of mice intravenously inoculated with EL-4 cells. Our results suggest that the immunopotentiating activity of NOS is exerted partly through the augmentation of NK activity.

Inhibition of CD28-mediated natural cytotoxicity by KIR2DL2 does not require p56(lck) in the NK cell line YT-Indy

CD28 functions as a cytotoxicity activation receptor in the NK cell line YT-Indy. To analyze the requirement of p56(lck) kinase in the function of killer inhibitory receptors, we transfected the p56(lck) negative YT-Indy cell line with the cl43 gene encoding for KIR2DL2. Pervanadate treatment revealed KIR2DL2 phosphorylation in YT-Indy-cl43, as well as SHP1/SHP2 recruitment. YT-Indy-cl43 cells were inhibited in their ability to lyse target cells expressing HLA-Cw3, a ligand for KIR2DL2. This inhibition was blocked by anti-KIR2DL2 or anti-HLA class I mAb. CD28 crosslinking on YT-Indy-cl43 enhanced tyrosine phosphorylation of PLC-gamma1. The simultaneous ligation of KIR2DL2 with mAb resulted in a decrease in CD28-induced tyrosine phosphorylation of PLC-gamma1 confirming that dephosphorylation of this protein is involved in the KIR2DL2-induced inhibition of CD28-mediated cytotoxicity. As YT-Indy-cl43 did not express detectable levels of p56(lck), these results indicate that this kinase is not required for transmitting the negative signals generated by KIR2DL2 ligation.

Heterogeneity in the effect of different ixodid tick species on human natural killer cell activity

Tick saliva plays a vital role in blood-feeding, including manipulation of the host response to tick infestation. Furthermore, a diverse number of tick-borne pathogens are transmitted to vertebrate hosts via tick saliva, some of which exploit the immunomodulatory activities of their vector's saliva. We report that salivary gland extracts (SGE) derived from Dermacentor reticulatus adult ticks induce a decrease in the natural killer (NK) activity of effector cells obtained from healthy human blood donors. The decrease was observed with SGE from both female and male D. reticulatus fed for either 3 or 5 days on mice, but no significant effect was observed with SGE from unfed ticks or ticks that had fed for 1 day. These results indicate that the tick anti-NK factor(s) is only active after blood-feeding has commenced. Microscopic examination revealed that the first step of NK activity, namely effector/target cell conjugate formation, was affected by SGE. The observed reduction in conjugate formation occurred when effector (but not target) cells were treated with SGE for 30 min, and the effect persisted after 12 h of treatment. Similar but less potent anti-NK activity was detected for SGE from Amblyomma variegatum and Haemaphysalis inermis. By contrast, SGE derived from Ixodes ricinus and Rhipicephalus appendiculatus female ticks did not decrease NK activity. The apparent absence of such activity in these two important vectors of tick-borne viruses suggests that control of NK cells does not play an important role in promoting virus transmission, at least for these particular species.

Overexpression of Hsp25 in K1735 murine melanoma cells enhances susceptibility to natural killer cytotoxicity

In the present study we used a murine melanoma model to investigate the effect of the 25-kDa heat shock protein (Hsp25) on natural killer (NK) cytotoxicity. The melanoma lines K1735-C123 (low metastatic potential) and K1735-M2 (high metastatic potential) were transfected with hsp25 and a control plasmid. Highly purified interleukin (IL)-2-stimulated DX-5+ NK cells showed enhanced lysis of Hsp25-overexpressing K1735-C123 targets in comparison with controls. In contrast, there was no difference in susceptibility to lysis by purified IL-2-stimulated DX-5+ NK cells between Hsp25-overexpressing and control-transfected K1735-M2 targets. Fluorescence-activated cell sorter analysis revealed that Hsp25 is displayed on the cell surface independently of Hsp25 overexpression and metastatic phenotype. Thus, surface localization of Hsp25 does not correlate with the target cell susceptibility to killing. To sum up, a cytoplasmic overexpression of Hsp25 is associated with an increased susceptibility to lysis by DX-5+ NK cells in the low-metastatic murine melanoma model investigated.

CD56bright cells differ in their KIR repertoire and cytotoxic features from CD56dim NK cells

In this study we present new differential characteristics of NK cells expressing CD56 surface antigen in low (CD56dim) or high (CD56bright) density. In contrast to CD56bright NK cells CD56dim cells express killer cell immunoglobulin (Ig)-like receptors (KIR) such as CD158a, CD158b, and NKB1. However, c-type lectin-like receptors (KLR) CD94/NKG2 and CD161 are present on both subsets. The ability to form conjugates with susceptible targets is approximately twice as strongly pronounced in CD56dim vs. CD56bright NK cells. Last but not least, granules of CD56dim cells contain about tenfold more perforin and granzyme A enabling potentially more effective cytolysis compared to CD56bright NK cells. On the other hand, CD56bright NK cells are superior in producing the proinflammatory cytokines IFN-gamma (28.5% vs. 20.8%, p<0.05) and TNF-alpha (28% vs. 15.8%, p<0.001). The different NK cell populations retained their specific phenotype in vitro during culture in the presence of IL-2 contradicting that they simply display different stages of maturity. Taken together our data support the view that CD56bright cells are specialized NK cells that regulate immunological response mechanisms rather by cytokine supply than by their cytotoxic potential. The poor cytolytic capacity of CD56bright NK cells can be explained by weak ability in forming conjugates with target cells and low contents of perforin and granzyme A in their granules.

Phylogeny of natural cytotoxicity: cytotoxic activity of coelomocytes of the purple sea urchin, Arbacia punctulata

Coelomocyte-mediated nonspecific cell cytotoxic activity against human and murine target cells by the purple sea urchin Arbacia punctulata was investigated in vitro. Cytotoxic activity toward target cells was shown to be mediated by different coelomocyte populations isolated by discontinuous density gradient centrifugation. The population of phagocytic amebocytes showed the strongest cytotoxic activity and the highest binding to human NK markers by cytometry analysis. Our immunophenotypic studies showed that A. punctulata phagocytic amebocytes are CD14(+), CD56(+), CD158b(+), CD3(-), CD4(-), CD8(-), and CD16(-). The cytotoxic activity was independent of experimental incubation temperatures, required viable effector cells, and required cell-cell contact between the effector and target cells. Sodium azide significantly decreased coelomocyte cytotoxicity, indicating that cytotoxicity is metabolically dependent, and EDTA reduction of cytotoxic activity is consistent with the involvement of divalent cations in the cytotoxic process. These data describe a population of sea urchin coelomocytes (the phagocytic amebocyte) that are CD14(+), CD56(+), and CD158b(+), with cytotoxic activities.

HLA-G inhibits rolling adhesion of activated human NK cells on porcine endothelial cells

Human NK cells adhere to and lyse porcine endothelial cells (pEC) and therefore may contribute to the cell-mediated rejection of vascularized pig-to-human xenografts. Since MHC class I molecules inhibit the cytotoxic activity of NK cells, the expression of HLA genes in pEC has been proposed as a potential solution to overcome NK cell-mediated xenogeneic cytotoxicity. HLA-G, a minimally polymorphic HLA class I molecule that can inhibit a wide range of NK cells, is an especially attractive candidate for this purpose. In this study we tested whether the expression of HLA-G on pEC inhibits the molecular mechanisms that lead to adhesion of human NK cells to pEC and subsequent xenogeneic NK cytotoxicity. To this end two immortalized pEC lines (2A2 and PED) were stably transfected with HLA-G1. Rolling adhesion of activated human NK cells to pEC monolayers and xenogeneic cytotoxicity against pEC mediated by polyclonal human NK lines as well as NK clones were inhibited by the expression of HLA-G. The adhesion was partially reversed by masking HLA-G on pEC with anti-HLA mAbs or by masking the HLA-G-specific inhibitory receptor ILT-2 on NK cells with the mAb HP-F1. The inhibition of NK cytotoxicity by HLA-G was only partially mediated by ILT-2, indicating a role for other unknown NK receptors. In conclusion, transgenic expression of HLA-G may be useful to prevent human NK cell responses to porcine xenografts, but is probably not sufficient on its own. Moreover, the blocking of rolling adhesion by HLA-G provides evidence for a novel biological function of HLA molecules.

Viral infections induce abundant numbers of senescent CD8 T cells

Viral infections are often accompanied by extensive proliferation of reactive CD8 T cells. After a defined number of divisions, normal somatic cells enter a nonreplicative stage termed senescence. In the present study we have identified the inhibitory killer cell lectin-like receptor G1 (KLRG1) as a unique marker for replicative senescence of murine CD8 T cells. KLRG1 expression was induced in a substantial portion (30-60%) of CD8 T cells in C57BL/6 mice infected with lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus, or vaccinia virus. Similarly, KLRG1 was found on a large fraction of LCMV gp33 peptide-specific TCR-transgenic (tg) effector and memory cells activated in vivo using an adoptive transfer model. Transfer experiments with CFSE-labeled TCR-tg cells into LCMV-infected hosts further indicated that induction of KLRG1 expression required an extensive number of cell divisions. Most importantly, KLRG1(+) TCR-tg effector/memory cells could efficiently lyse target cells and secrete cytokines, but were severely impaired in their ability to proliferate after Ag stimulation. Thus, this study demonstrates that senescent CD8 T cells are induced in abundant numbers during viral infections in vivo.

The biology of natural killer cells in cancer, infection, and pregnancy

OBJECTIVE

NK cells are important cells of the immune system. They are ultimately derived from pluripotent hematopoietic stem cells. NK cell cytotoxicity and other functions are tightly regulated by numerous activating and inhibitory receptors including newly discovered receptors that selectively recognize major histocompatibility complex class I alleles. Based on their defining function of spontaneous cytotoxicity without prior immunization, NK cells have been thought to play a critical role in immune surveillance and cancer therapy. However, new insights into NK cell biology have suggested major roles for NK cells in infection control and uterine function. The purpose of this review is to provide an update on NK cell function, ontogeny, and biology in order to better understand the role of NK cells in health and disease.

DATA SOURCES

In the Medline database, the major subject heading "Natural Killer Cells" was introduced in 1983, identifying 16,848 citations as of December 31, 2000. Since 1986, there have been approximately 1000 citations per year under this subject heading. In this database, 68% of manuscripts are limited to human NK cells; 40% of citations cross with the major sub-heading of cytotoxicity, 40% with cytokines, 36% with neoplasm, 5% with antibody-dependent cellular cytotoxicity, 2.8% with pregnancy, and 1.3% with infection. Of references from the year 2000-2001, 46 were selected to combine with contributions from earlier literature.

CONCLUSIONS

NK cells should no longer be thought of as direct cytotoxic killers alone as they clearly serve a critical role in cytokine production which may be important to control cancer, infection, and fetal implantation. Understanding mechanisms of NK cell functions may lead to novel therapeutic strategies for the treatment of human disease.

T-cell immunity in acute coronary syndromes

Acute coronary syndromes (ACS) are complications of atherosclerotic vascular disease that are triggered by the sudden rupture of an atheroma. Atherosclerotic plaque stability is determined by multiple factors, of which immune and inflammatory pathways are critical. Unstable plaque is characterized by an infiltrate of T cells and macrophages, thereby resembling a delayed hypersensitivity reaction. On activation, T cells secrete cytokines that regulate the activity of macrophages, or the T cells may differentiate into effector cells with tissue-damaging potential. Constitutive stimulation of T cells and macrophages in ACS is not limited to the vascular lesion but also involves peripheral immune cells, suggesting fundamental abnormalities in homeostatic mechanisms that control the assembly, turnover, and diversity of the immune system as a whole. This review gives particular attention to the emergence of a specialized T-cell subset, natural killer T cells, in patients with ACS. Natural killer T cells have proinflammatory properties and the capability of directly contributing to vascular injury.

Natural killer cells, viruses and cancer

Natural killer cells are innate immune cells that control certain microbial infections and tumours. The function of natural killer cells is regulated by a balance between signals transmitted by activating receptors, which recognize ligands on tumours and virus-infected cells, and inhibitory receptors specific for major histocompatibility complex class I molecules. Here, we review the emerging evidence that natural killer cells have an important role in vivo in immune defence.

The relationship between major histocompatibility receptors and innate immunity in teleost fish

Studies of the innate immune system have recently shown that, in addition to its role in producing the primary response that slows down pathogens, it may also play an important role in initiating and directing the type of response that the adaptive immune system makes. These discoveries have shown a complex web of control containing new roles for the innate immune system in organizing responses of T-cell to antigens being presented by major histocompatibility receptors, as well as new roles for those receptors in innate immune responses. Both of these activities are managed through feedback networks involving elements of both the innate and adaptive immune system. This paper will discuss these newly discovered interactions and how they are influencing current theories regarding the initiation of adaptive immune responses. In particular, it will highlight the recent progress that is being made towards understanding these relationships in the immune systems of teleost fish.

CD56bright cells differ in their KIR repertoire and cytotoxic features from CD56dim NK cells

In this study we present new differential characteristics of NK cells expressing CD56 surface antigen in low (CD56dim) or high (CD56bright) density. In contrast to CD56bright NK cells CD56dim cells express killer cell immunoglobulin (Ig)-like receptors (KIR) such as CD158a, CD158b, and NKB1. However, c-type lectin-like receptors (KLR) CD94/NKG2 and CD161 are present on both subsets. The ability to form conjugates with susceptible targets is approximately twice as strongly pronounced in CD56dim vs. CD56bright NK cells. Last but not least, granules of CD56dim cells contain about tenfold more perforin and granzyme A enabling potentially more effective cytolysis compared to CD56bright NK cells. On the other hand, CD56bright NK cells are superior in producing the proinflammatory cytokines IFN-gamma (28.5% vs. 20.8%, p<0.05) and TNF-alpha (28% vs. 15.8%, p<0.001). The different NK cell populations retained their specific phenotype in vitro during culture in the presence of IL-2 contradicting that they simply display different stages of maturity. Taken together our data support the view that CD56bright cells are specialized NK cells that regulate immunological response mechanisms rather by cytokine supply than by their cytotoxic potential. The poor cytolytic capacity of CD56bright NK cells can be explained by weak ability in forming conjugates with target cells and low contents of perforin and granzyme A in their granules.

Human monocyte-derived and CD83(+) blood dendritic cells enhance NK cell-mediated cytotoxicity

Dendritic cells (DC) are known to be the most potent APC and to stimulate antigen-specific T cell responses. Recently it was reported that murine DC were also capable of modulating the innate immunity by stimulating NK cells through cell-to-cell contact. In the present study, we examined whether human DC could affect NK activity. Both monocyte-derived and CD83(+) blood DC were tested. The addition of DC to cultures of CD56(+) cells resulted in the significant dose-dependent enhancement of the killing activity against various NK-sensitive targets. The resultant activity was comparable to that induced by optimal concentrations of various cytokines, including IL-2, IL-12, IL-15 and IFN-gamma. Interestingly, DC enhanced the cytotoxicity of CD3(-)CD56(+) NK cells, but not that of CD3(+)CD56(+) T cells. Experiments using transwells clearly demonstrated that the enhancement of NK activity by DC was mediated by soluble factors produced by DC. The culture supernatants of DC also stimulated NK activity. The treatment of both DC and their supernatants with anti-human IL-12 or IL-18 antibodies did not block the enhancement of NK cell-mediated cytolysis by DC, indicating that other factor(s) produced by DC were responsible for the enhancement of NK activity. These results suggest that human myeloid DC can modulate innate immunity by enhancing NK activity.

Genetic and functional relationships between MHC and NK receptor genes

HLA class I and NK receptors are encoded within dense clusters of immune loci. The MHC, at 6p21.3, and the complex containing the KIR loci, at 19q13.4, both feature variation in the number of genes, as well as sequence polymorphism. In addition to T cell receptors, several variable class I-related molecules interact with polymorphic NK receptors. Some of the lectin-related NK receptor genes, at 12p13.1, also have ligands belonging to the extended class I family. The expanding clusters of class I-related sequences and their receptors, some of which evolved recently, reveal further complexity in immune recognition of disease.

Cutting edge: induction of IFN-gamma production but not cytotoxicity by the killer cell Ig-like receptor KIR2DL4 (CD158d) in resting NK cells

Activated NK cells lyse tumor cells and virus-infected cells and produce IFN-gamma upon contact with sensitive target cells. The regulation of these effector responses in resting NK cells is not well understood. We now describe a receptor, KIR2DL4, that has the unique property of inducing IFN-gamma production, but not cytotoxicity, by resting NK cells in the absence of cytokines. In contrast, the NK cell-activation receptors CD16 and 2B4 induced cytotoxicity but not IFN-gamma production. The induction by KIR2DL4 of IFN-gamma production by resting NK cells was blocked by an inhibitor of the p38 mitogen-activated protein kinase signaling pathway, in contrast to the IL-2-induced IFN-gamma secretion that was sensitive to inhibition of the extracellular signal-regulated kinase mitogen-activated protein kinase pathway. These results reveal a functional dichotomy (cytokine production vs cytotoxicity) in the response of resting NK cells, as dictated by the signals of individual receptors.

NTB-A [correction of GNTB-A], a novel SH2D1A-associated surface molecule contributing to the inability of natural killer cells to kill Epstein-Barr virus-infected B cells in X-linked lymphoproliferative disease

In humans, natural killer (NK) cell function is regulated by a series of receptors and coreceptors with either triggering or inhibitory activity. Here we describe a novel 60-kD glycoprotein, termed NTB-A, that is expressed by all human NK, T, and B lymphocytes. Monoclonal antibody (mAb)-mediated cross-linking of NTB-A results in the induction of NK-mediated cytotoxicity. Similar to 2B4 (CD244) functioning as a coreceptor in the NK cell activation, NTB-A also triggers cytolytic activity only in NK cells expressing high surface densities of natural cytotoxicity receptors. This suggests that also NTB-A may function as a coreceptor in the process of NK cell activation. Molecular cloning of the cDNA coding for NTB-A molecule revealed a novel member of the immunoglobulin superfamily belonging to the CD2 subfamily. NTB-A is characterized, in its extracellular portion, by a distal V-type and a proximal C2-type domain and by a cytoplasmic portion containing three tyrosine-based motifs. NTB-A undergoes tyrosine phosphorylation and associates with the Src homology 2 domain-containing protein (SH2D1A) as well as with SH2 domain-containing phosphatases (SHPs). Importantly, analysis of NK cells derived from patients with X-linked lymphoproliferative disease (XLP) showed that the lack of SH2D1A protein profoundly affects the function not only of 2B4 but also of NTB-A. Thus, in XLP-NK cells, NTB-A mediates inhibitory rather than activating signals. These inhibitory signals are induced by the interaction of NTB-A with still undefined ligands expressed on Epstein-Barr virus (EBV)-infected target cells. Moreover, mAb-mediated masking of NTB-A can partially revert this inhibitory effect while a maximal recovery of target cell lysis can be obtained when both 2B4 and NTB-A are simultaneously masked. Thus, the altered function of NTB-A appears to play an important role in the inability of XLP-NK cells to kill EBV-infected target cells.

Activating receptors and coreceptors involved in human natural killer cell-mediated cytolysis

Natural killer cells can discriminate between normal cells and cells that do not express adequate amounts of major histocompatibility complex (MHC) class I molecules. The discovery, both in mouse and in human, of MHC-specific inhibitory receptors clarified the molecular basis of this important NK cell function. However, the triggering receptors responsible for positive NK cell stimulation remained elusive until recently. Some of these receptors have now been identified in humans, thus shedding some light on the molecular mechanisms involved in NK cell activation during the process of natural cytotoxicity. Three novel, NK-specific, triggering surface molecules (NKp46, NKp30, and NKp44) have been identified. They represent the first members of a novel emerging group of receptors collectively termed natural cytotoxicity receptors (NCR). Monoclonal antibodies (mAbs) to NCR block to differing extents the NK-mediated lysis of various tumors. Moreover, lysis of certain tumors can be virtually abrogated by the simultaneous masking of the three NCRs. There is a coordinated surface expression of the three NCRs, their surface density varying in different individuals and also in the NK cells isolated from a given individual. A direct correlation exists between the surface density of NCR and the ability of NK cells to kill various tumors. NKp46 is the only NCR involved in human NK-mediated killing of murine target cells. Accordingly, a homologue of NKp46 has been detected in mouse. Molecular cloning of NCR revealed novel members of the Ig superfamily displaying a low degree of similarity to each other and to known human molecules. NCRs are coupled to different signal transducing adaptor proteins, including CD3 zeta, Fc epsilon RI gamma, and KARAP/DAP12. Another triggering NK receptor is NKG2D. It appears to play either a complementary or a synergistic role with NCRs. Thus, the triggering of NK cells in the process of tumor cell lysis may often depend on the concerted action of NCR and NKG2D. In some instances, however, it may uniquely depend upon the activity of NCR or NKG2D only. Strict NKG2D-dependency can be appreciated using clones that, in spite of their NCR(dull) phenotype, efficiently lyse certain epithelial tumors or leukemic cell lines. Other triggering surface molecules including 2B4 and the novel NKp80 appear to function as coreceptors rather than as true receptors. Indeed, they can induce natural cytotoxicity only when co-engaged with a triggering receptor. While an altered expression or function of NCR or NKG2D is being explored as a possible cause of immunological disorders, 2B4 dysfunction has already been associated with a severe form of immunodeficiency. Indeed, in patients with the X-linked lymphoproliferative disease, the inability to control Epstein-Barr virus infections may be consequent to a major dysfunction of 2B4 that exerts inhibitory instead of activating functions.

The biology of natural killer cells and implications for therapy of human disease

Natural killer (NK) cells are unique lymphocytes capable of lysing target cells without prior immunization. NK cells activated with cytokines, like interleukin-2 (IL-2), have been used since the 1980s as adoptive immunotherapy against metastatic solid tumors, but their effectiveness has been limited. The mechanisms by which NK cells recognize their targets are complex, including newly identified receptors that recognize class I MHC molecules. Understanding these mechanisms may support the use of NK cells as clinical therapy against infectious diseases and cancer. We have been interested in the use of NK cells clinically for their potential to eradicate minimal residual disease and prevent relapses after autologous stem cell transplantation. Several strategies are discussed to increase the specificity and efficacy of NK cell therapy. One method is to increase the targeting of NK cells by the use of monoclonal antibodies. Another approach uses allogeneic NK cells to overcome the inhibitory receptor mechanisms that may block target cell lysis by recognition of class I molecules. These and other novel strategies may prove to be attractive and effective immunotherapeutic tools to manipulate NK cells to fight human disease.

Natural killer cells and nitric oxide

Natural killer (NK) cells and nitric oxide (NO) are both important components of the natural or innate immune response. NK cells are large granular lymphocytes capable of destroying cells infected by virus or bacteria and susceptible tumor cells without prior sensitization and restriction by MHC antigens. They are abundant in blood, spleen, liver and lungs and are distinct from both T and B lymphocytes in their circulation patterns, profile of surface antigens, receptor repertoire and the way in which they discriminate between self and non-self. Uniquely, NK cells express receptors that can recognize and discriminate between normal and altered MHC class I determinants. NK cell cytotoxic activity is strongly induced by cytokines such as IL-2 and IL-12, and this activation is associated with synthesis of NO. Inhibitors of NO synthesis impair NK cell-mediated target cell killing, demonstrating a role for NO in NK cell function. Furthermore, NO itself can regulate NK cell activation. In this article, evidence that NO is a mediator of NK cell-mediated target cell killing, and that NO is a regulator of NK cell activation will be reviewed. Results of NO synthase gene deletion studies will be discussed, and rodent and human NK cells will be compared.

Abrogation of tumor induced Ly49 expression on mouse spleen cells by mitomycin C

Mouse spleen cells were activated with IL2 for 4 days in the presence or absence of paraformaldehyde fixed YAC (PFY) tumor cells (spleen cell: PFY ratio 100:1). Fresh spleen cells had poor expression of Ly49A but the expression was significantly upregulated by IL2 activation. Addition of PFYs resulted in a further boosting of Ly49A expression. Besides Ly49A, the Ly49C receptors were also upregulated by PFYs. Upregulated Ly49 expression was not restricted to NK cells (NK1.1 positive cells) but was also seen on T cells (TCRbeta positive cells). Time kinetics studies indicated that maximum upregulation of Ly49 in response to PFY cells occurred on day 3 and 4 and the expression declined thereafter. Ly49 expression in response to PFYs was completely blocked if spleen cells were pre-treated with Mitomycin C, an inhibitor of DNA synthesis. These results show that the addition of a small number of paraformaldehyde fixed YAC cells to spleen cell cultures undergoing IL2 activation, resulted in a significant upregulation of Ly49 receptors and this process was dependent upon cell proliferative activity.

Adhesive interactions between human NK cells and porcine endothelial cells

Human natural killer (NK) cells are able to adhere to xenogeneic porcine endothelial cells (EC) and evidence from in vitro studies as well as animal models suggests a potential role for NK cells in the cellular recognition and damage of porcine xenogeneic tissues. One possible explanation for the observed NK cell-mediated xenogeneic cytotoxicity against porcine EC is the molecular incompatibility between porcine major histocompatibility complex (MHC) class I molecules and MHC-specific inhibitory receptors on human NK cells. In this review we attempt to summarize the current knowledge concerning adhesive interactions between human NK cells and porcine EC under special considerations of the cross-species receptor-ligand interactions. Methodological differences in assessing adhesion between various studies are reviewed and comparisons to the syngeneic/allogeneic adhesion mechanisms are made. Finally, the therapeutic potential of blocking antibodies and transgenic HLA expression in preventing NK-cell adhesion and xenogeneic cytotoxicity is discussed.

Functional dichotomy in natural killer cell signaling: Vav1-dependent and -independent mechanisms

The product of the protooncogene Vav1 participates in multiple signaling pathways and is a critical regulator of antigen-receptor signaling in B and T lymphocytes, but its role during in vivo natural killer (NK) cell differentiation is not known. Here we have studied NK cell development in Vav1-/- mice and found that, in contrast to T and NK-T cells, the absolute numbers of phenotypically mature NK cells were not reduced. Vav1-/- mice produced normal amounts of interferon (IFN)-gamma in response to Listeria monocytogenes and controlled early infection but showed reduced tumor clearance in vivo. In vitro stimulation of surface receptors in Vav1-/- NK cells resulted in normal IFN-gamma production but reduced tumor cell lysis. Vav1 was found to control activation of extracellular signal-regulated kinases and exocytosis of cytotoxic granules. In contrast, conjugate formation appeared to be only mildly affected, and calcium mobilization was normal in Vav1-/- NK cells. These results highlight fundamental differences between proximal signaling events in T and NK cells and suggest a functional dichotomy for Vav1 in NK cells: a role in cytotoxicity but not for IFN-gamma production.

NK cell-mediated lysis of autologous antigen-presenting cells is triggered by the engagement of the phosphatidylinositol 3-kinase upon ligation of the natural cytotoxicity receptors NKp30 and NKp46

Interleukin-2 (IL-2)-activated polyclonal or clonal NK cells lysed autologous antigen presenting cells (APC) through the engagement of the natural cytotoxicity receptors (NCR) NKp30 and NKp46. NK cell-mediated cytolysis of APC correlated with the surface density of these NCR. Indeed, NK cell clones bearing low amounts of NKp30 and NKp46 did not lyse autologous APC, whereas NK cell clones with bright expression of these NCR efficiently killed autologous APC. Upon masking of NKp30 or NKp46 by specific monoclonal antibodies a strong reduction (by 50%) of APC lysis could be detected and the complete inhibition was achieved by the simultaneous masking of these NCR. Interestingly, NK cell-mediated APC lysis was impaired by the phosphatidylinositol 3-kinase (PI-3 K) inhibitors LY294002 or wortmannin. Similarly, these drugs strongly reduced NK cell activation triggered by NKp30 or NKp46 in a re-directed killing assay as well as the activation of Akt/PKB, substrate of PI-3 K, induced by the engagement of these receptors. Altogether, these findings strongly suggest that NCR are responsible for the killing of autologous APC through the activation of PI-3 K.

A fresh look at tumor immunosurveillance and immunotherapy

Despite major advances in our understanding of adaptive immunity and dendritic cells, consistent and durable responses to cancer vaccines remain elusive and active immunotherapy is still not an established treatment modality. The key to developing an effective anti-tumor response is understanding why, initially, the immune system is unable to detect transformed cells and is subsequently tolerant of tumor growth and metastasis. Ineffective antigen presentation limits the adaptive immune response; however, we are now learning that the host's innate immune system may first fail to recognize the tumor as posing a danger. Recent descriptions of stress-induced ligands on tumor cells recognized by innate effector cells, new subsets of T cells that regulate tumor tolerance and the development of spontaneous tumors in mice that lack immune effector molecules, beckon a reflection on our current perspectives on the interaction of transformed cells with the immune system and offer new hope of stimulating therapeutic immunity to cancer.

Intracellular signaling by the killer immunoglobulin-like receptors and Ly49

Once thought to be promiscuous killers, it is now known that natural killer (NK) cells possess an elaborate array of receptors that regulate NK cytotoxic and secretory functions upon interaction with target cell MHC class I proteins. These receptors, known as killer cell immunoglobulin-like receptors (KIRs) in humans, and Ly49 receptors in the mouse, have become the focus of intense study in an effort to discern the underlying biology of these large receptor families. These receptor families include both inhibitory and activating receptors. Interrogation of a target expressing KIR ligands leads to coengagement of the inhibitory receptor with as-yet poorly defined activation receptors. Kinases activated during engagement mediate the phosphorylation of the KIR or Ly49 cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs). The phosphorylated ITIMs serve as efficient recruitment points for the cytosolic protein tyrosine phosphatases, SHP-1 and SHP-2, resulting in the dephosphorylation of substrates critical for cellular activation. In contrast, some KIRs and Ly49s lack the ITIM and possess a charged residue in their transmembrane domains that mediates interaction with the DAP12 signal transduction chain. DAP12 uses its cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM) to mediate cellular activation. Engagement of a DAP12 coupled KIR or Ly49 results in phosphorylation of DAP12, and other key substrates, including the Syk tryosine kinase, phospholipase C, and c-Cbl. DAP12 activation then leads to the Mapk cascade and ultimately to enhanced degranulation, and production of cytokines and chemokines. Although the context in which inhibitory and activating KIR and Ly49s function is not yet known, the dissection of the activating and inhibitory signal transduction pathways should shed light on their method of integration into the activation sequela of NK cells. Ultimately, this work will lead to concrete understanding of the immunobiology of these seemingly antagonistic receptor systems.

Crystal structure of the murine NK cell-activating receptor NKG2D at 1.95 A

NKG2D, a homodimeric lectin-like receptor, is a unique stimulatory molecule that is found on natural killer cells,T cells and activated macrophages. The natural ligands for murine NKG2D are distant major histocompatibility complex homologs, retinoic acid early transcript (Rae1) and H-60 minor histocompatibility antigen. The crystal structure of the extracellular region of murine NKG2D reveals close homology with other C-type lectin receptors such as CD94, Ly49A, rat MBP-A and CD69. However, the precise mode of dimeric assembly varies among these natural killer receptors, as well as their surface topography and electrostatic properties. The NKG2D structure provides the first structural insights into the role and ligand specificity of this stimulatory receptor in the innate and adaptive immune system.

Divergent and convergent evolution of NK-cell receptors

Natural killer (NK)-cell receptors specific for major histocompatibility complex (MHC) class I molecules have been identified in humans and mice. Some of the most important receptors are structurally unrelated in the two species: the murine Ly-49 receptors are C-type lectins, while human killer-cell inhibitory receptors (KIRs) belong to the immunoglobulin superfamily. Here, Roland Barten and colleagues describe the divergent and convergent evolution of NK-cell receptors.

Identification of NKp80, a novel triggering molecule expressed by human NK cells

The ability of NK cells to kill a wide range of tumor or virally infected target cells as well as normal allogeneic T cell blasts appears to depend upon the concerted action of multiple triggering NK receptors. In this study, using two specific monoclonal antibodies [(mAb) MA152 and LAP171], we identified a triggering NK receptor expressed at the cell surface as a dimer of approximately 80 kDa (NKp80). NKp80 is expressed by virtually all fresh or activated NK cells and by a minor subset of T cells characterized by the CD56 surface antigen. NKp80 surface expression was also detected in all CD3- and in 6 / 10 CD3+ large granular lymphocyte expansions derived from patients with lymphoproliferative disease of granular lymphocytes. In polyclonal NK cells, mAb-mediated cross-linking of NKp80 resulted in induction of cytolytic activity and Ca2+ mobilization. A marked heterogeneity existed in the magnitude of the cytolytic responses of different NK cell clones to anti-NKp80 mAb. This heterogeneity correlated with the surface density of NKp46 molecules expressed by different NK clones. The mAb-mediated masking of NKp80 led to a partial inhibition of the NK-mediated lysis of appropriate allogeneic phytohemagglutinin-induced T cell blasts, while it had no effect on the lysis of different tumor target cells, including T cell leukemia cells. These data suggest that NKp80 recognizes a ligand on normal T cells that may be down-regulated during tumor transformation. Molecular cloning of the cDNA coding for NKp80 revealed a type II transmembrane molecule of 231 amino acids identical to the putative protein encoded by a recently identified cDNA termed KLRF1.

V gamma 9V delta 2 T cells impair intracellular multiplication of Brucella suis in autologous monocytes through soluble factor release and contact-dependent cytotoxic effect

Human Vgamma9Vdelta2 T cells are considered to play an important role in brucellosis, as this population is dramatically increased in peripheral blood of patients during the acute phase of the infection. This T lymphocyte population has been largely demonstrated to be activated by small m.w. nonpeptidic molecules from natural or synthetic origin. We recently identified a nonpeptidic fraction of Brucella suis that specifically activates human Vgamma9Vdelta2 T cells. Using a two-separate-chambers system, we showed that Brucella fraction, as well as isopentenyl pyrophosphate-activated Vgamma9Vdelta2 T cells, impaired the multiplication of B. suis in differentiated THP-1 cells through TNF-alpha and IFN-gamma release. In the present study, using circulating Vgamma9Vdelta2 T cells and autologous monocytes infected with B. suis, we provide evidence that 1) intramonocytic multiplication of B. suis is impaired by supernatants of activated Vgamma9Vdelta2 T cells in part via TNF-alpha and IFN-gamma, this impairment occurring without host cell lysis; 2) unstimulated Vgamma9Vdelta2 T cells can impair intracellular bacterial multiplication after their activation by soluble factors released by infected monocytes; and 3) activated Vgamma9Vdelta2 T cells lyse Brucella-infected monocytes in a contact-dependent manner. Taken together, these results provide evidence that Vgamma9Vdelta2 T cells, in addition to being directly activated by soluble nonpeptidic molecules, can be stimulated to become highly cytotoxic in the specific presence of infected monocytes; moreover, they suggest how Vgamma9Vdelta2 T cells could be triggered and respond as antibacterial effector cells in the early stages of Brucella infection.