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

Natural killer cell-mediated killing of freshly isolated neuroblastoma cells: critical role of DNAX accessory molecule-1-poliovirus receptor interaction

In the present study, we assessed the susceptibility of freshly isolated neuroblastoma cells to killing mediated by normal human natural killer (NK) cells and analyzed the receptor-ligand interactions that regulate this event. We show that killing of freshly isolated neuroblasts, similar to neuroblastoma cell lines, involves NKp46 and NKp30 (natural cytotoxicity receptors). However, freshly isolated neuroblasts were generally more resistant to NK-mediated lysis than conventional neuroblastoma cell lines. Moreover, a significant heterogeneity in susceptibility to lysis existed among neuroblastomas derived from different patients. Remarkably, susceptibility to lysis directly correlated with the surface expression, on neuroblasts, of poliovirus receptor [PVR (CD155)], a ligand for the DNAX accessory molecule-1 [DNAM-1 (CD226)] triggering receptor expressed by NK cells. Indeed, PVR-expressing neuroblastomas were efficiently killed by NK cells. Moreover, monoclonal antibody-mediated masking of either DNAM-1 (on NK cells) or PVR (on neuroblasts) resulted in strong inhibition of tumor cell lysis. Thus, assessment of the PVR surface levels may represent a novel useful criterion to predict the susceptibility/resistance of neuroblastomas to NK-mediated killing.

Natural killer cells and innate immunity to protozoan pathogens

Natural killer (NK) cells are lymphoid cells that mediate significant cytotoxic activity and produce high levels of pro-inflammatory cytokines in response to infection. During viral infection, NK cell cytotoxicity and cytokine production is induced principally by monocyte-macrophage- and dendritic cell-derived cytokines but virally encoded ligands for NK cells are also beginning to be described. NK derived interferon-gamma (IFN-gamma) production is also essential for control of several protozoal infections including toxoplasmosis, trypanosomiasis, leishmaniasis and malaria. The activation of NK cells by protozoan pathogens is also believed to be cytokine-mediated although some recent studies suggest that direct recognition of parasites by NK cells also occurs. Both indirect signalling via accessory cell-derived cytokines and direct signalling, presumably through NK receptors, are needed in order for human malaria parasites (Plasmodium falciparum) to optimally stimulate NK activity.

Effects of Chlamydia trachomatis infection on the expression of natural killer (NK) cell ligands and susceptibility to NK cell lysis

Natural killer (NK) cells are an important component of the immediate immune response to infections, including infection by intracellular bacteria. We have investigated recognition of Chlamydia trachomatis (CT) by NK cells and show that these cells are activated to produce interferon (IFN)-gamma when peripheral blood mononuclear cells (PBMC) are stimulated with CT organisms. Furthermore, infection of epithelial cell lines with CT renders them susceptible to lysis by human NK cells. Susceptibility was observed 18-24 h following infection and required protein synthesis by the infecting chlamydiae, but not by the host cell; heat or UV inactivated chlamydiae did not induce susceptibility to NK cell lysis. CT infection was also shown to decrease the expression of classical and non-classical major histocompatibility complex (MHC) molecules on infected cells, thus allowing recognition by NK cells when combined with an activating signal. A candidate activating signal is MICA/B, which was shown to be expressed constitutively on epithelial cells.

Analysis of the receptor-ligand interactions in the natural killer-mediated lysis of freshly isolated myeloid or lymphoblastic leukemias: evidence for the involvement of the Poliovirus receptor (CD155) and Nectin-2 (CD112)

On the basis of recent clinical and experimental data, natural killer (NK) cells appear to play a crucial role in eradication of acute myeloid leukemias. In the present study, by exploiting our current knowledge on NK receptors and their ligands on target cells, we investigated the interactions between NK and leukemic cells. We show that the size of the NK cell subset expressing the killer immunoglobulin-like receptor (KIR) not engaged by the HLA-class I alleles of the patient parallels the degree of NK cytotoxicity against leukemic cells. A sharp down-regulation of HLA-class I molecules has been detected in various leukemias and it was more frequent in myeloid than in lymphoblastic leukemias. Analysis of the ligands for triggering NK receptors revealed the consistent expression of Poliovirus receptor (PVR) and Nectin-2 in myeloid leukemias. In contrast, major histocompatibility complex class I-related chain molecules A/B (MICA/B) and UL1b-binding protein (ULBPs) were either absent or weakly expressed. Accordingly, NK-mediated lysis of these leukemias was dependent on DNAM-1 but not NKG2D. The major role of NKp46 and NKp30 was also confirmed. The expression of PVR and/or Nectin-2 was less frequent in lymphoblastic leukemias. In most leukemias, both CD48 and NTBA were down-regulated. The correlation found between marker expression and susceptibility to lysis may reveal useful information for NK-based immunotherapy.

The corticosteroid-induced inhibitory effect on NK cell function reflects down-regulation and/or dysfunction of triggering receptors involved in natural cytotoxicity

Corticosteroids are known to inhibit NK cell functions. However no information is available on whether such inhibition may affect the expression and/or the function of receptors involved in NK cell activation. In an attempt to analyze this point, we studied peripheral blood NK cells isolated from pediatric patients undergoing allogeneic BM transplantation. NK cells were analyzed before, during and after methylprednisolone administration to treat acute graft-versus-host disease. In NK cells freshly isolated from peripheral blood during methylprednisolone treatment, the surface expression of activating receptors, particularly NKp46 and NKp30, was consistently reduced. Such impaired expression could also be detected after 5 days of culture in IL-2. Such cultured NK cells also failed to express the IL-2-inducible NKp44 receptor. Accordingly, cytotoxicity against different tumor target cell lines was sharply reduced. The effect on NK cells isolated from healthy individuals and cultured in the presence of methylprednisolone was also analyzed. A similar inhibitory effect occurred in the expression of activating NK receptors. In addition, a sharp impairment of NK cytotoxicity against different tumor target cell lines or immature DC was detected.

The murine NK receptor 2B4 (CD244) exhibits inhibitory function independent of signaling lymphocytic activation molecule-associated protein expression

2B4 (CD244) is a receptor belonging to the CD2-signaling lymphocytic activation molecule family and is found on all murine NK cells and a subset of NKT and CD8+ T cells. Murine 2B4 is expressed as two isoforms (2B4 short and 2B4 long) that arise by alternative splicing. They differ only in their cytoplasmic domains and exhibit opposing function when expressed in the RNK-16 cell line. The ligand for 2B4, CD48, is expressed on all hemopoietic cells. Previous studies have shown that treatment of NK cells with a 2B4 mAb results in increased cytotoxicity and IFN-gamma production. In this report, we used CD48+/- variants of the P815 tumor cell line and 2B4 knockout mice to show that engagement of 2B4 by its counterreceptor, CD48, expressed on target cells leads to an inhibition in NK cytotoxicity. The addition of 2B4 or CD48 mAb relieves this inhibition resulting in enhanced target cell lysis. This 2B4-mediated inhibition acts independently of signaling lymphocytic activation molecule-associated protein expression. Imaging studies show that 2B4 preferentially accumulates at the interface between NK and target cells during nonlytic events also indicative of an inhibitory receptor. This predominant inhibitory function of murine 2B4 correlates with increased 2B4 long isoform level expression over 2B4 short.

Membrane-associated heparan sulfate proteoglycans are involved in the recognition of cellular targets by NKp30 and NKp46

Lysis of virus-infected and tumor cells by NK cells is mediated via natural cytotoxicity receptors (NCRs). We have recently shown that the NKp44 and NKp46 NCRs, but not the NKp30, recognize viral hemagglutinins. In this study we explored the nature of the cellular ligands recognized by the NKp30 and NKp46 NCRs. We demonstrate that target cell surface heparan sulfate proteoglycans (HSPGs) are recognized by NKp30 and NKp46 and that 6-O-sulfation and N-acetylation state of the glucose building unit affect this recognition and lysis by NK cells. Tumor cells expressing cell surface heparanase, CHO cells lacking membranal heparan sulfate and glypican-1-suppressed pancreatic cancer cells manifest reduced recognition by NKp30 and NKp46 and are lysed to a lesser extent by NK cells. Our results are the first clue for the identity of the ligands for NKp30 and NKp46. Whether the ligands are particular HSPGs, unusual heparan sulfate epitopes, or a complex of HSPGs and either other protein or lipid moieties remains to be further explored.

A novel epitope of N-CAM defines precursors of human adherent NK cells

Activated, adherent natural killer (A-NK) cells represent a distinct subpopulation of interleukin (IL)-2-stimulated NK cells, which are selectively endowed with the increased expression of integrins and ability to adhere to solid surfaces, migrate into, infiltrate, and destroy cancerous tissues. The present study defines the phenotype and functions of precursors of A-NK (pre-A-NK) cells in humans. Peripheral blood pre-A-NK cells, in contrast to the rest of NK cells, express a novel epitope of CD56 neuronal cell adhesion molecule, termed ANK-1, and increased cell-surface levels of integrins. Pre-A-NK cells also express low levels of CD56 and CD161, and some express CD162 receptor, do not express CD25 or activation markers, and are effective mediators of NK cytotoxicity. Thus, pre-A-NK cells are generally similar to CD56(dim) NK cells. However, pre-A-NK cells differ from the main NK cell subpopulation by having a lower expression level of CD16 and a lower ability to mediate redirected antibody-dependent, cell-mediated cytotoxicity. More importantly, pre-A-NK cells are preferentially endowed with the ability to rapidly respond to IL-2 by integrin-mediated adherence to endothelial cells, extracellular matrix, and plastic. This early, specific response of pre-A-NK cells to IL-2 is followed by their activation, vigorous proliferation, and differentiation into phenotypically and functionally similar A-NK cells. Pre-A-NK cells represent only approximately 26% of peripheral blood NK cells but encompass the majority of NK cells in normal and cancerous, solid tissues. We conclude that pre-A-NK cells represent a distinct subset of resting, mature NK cells with the characteristics indicative of their ability to migrate and reside in solid tissues.

Investigation of KIR diversity in immunosenecence and longevity within the Irish population

Natural killer (NK) cells play a pivotal role in the innate immune response. During the ageing process, variations occur in NK cell number and function. The cytolytic activity of NK cells is controlled by an array of activating and inhibitory cell surface receptors, including the killer cell immunoglobulin-like receptors (KIRs). In the present study, genetic diversity of the KIR loci was analysed with respect to successful ageing in the Irish population. A PCR-SSOP KIR gene identification system was employed to determine the frequency of the named KIR genes/pseudogenes and KIR genotypes within a healthy aged cohort and young control group. Although, two KIR genes (2DS3, 2DL5) displayed an initial increased frequency in the aged group, the significance of this association was lost when repeated in a second cohort. In view of the lack of studies to date, investigating the role of the KIR gene system in healthy ageing, further analysis of KIR diversity is required to fully elucidate it's role in respect to age-related disease and longevity.

Natural killer cells and cancer

Natural killer (NK) cells are lymphocytes that were first identified for their ability to kill tumor cells without deliberate immunization or activation. Subsequently, they were also found to be able to kill cells that are infected with certain viruses and to attack preferentially cells that lack expression of major histocompatibility complex (MHC) class I antigens. The recent discovery of novel NK receptors and their ligands has uncovered the molecular mechanisms that regulate NK activation and function. Several activating NK cell receptors and costimulatory molecules have been identified that permit these cells to recognize tumors and virus-infected cells. These are modulated by inhibitory receptors that sense the levels of MHC class I on prospective target cells to prevent unwanted destruction of healthy tissues. In vitro and in vivo, their cytotoxic ability can be enhanced by cytokines, such as interleukin (IL)-2, IL-12, IL-15 and interferon alpha/beta (IFN-alpha/beta). In animal studies, they have been shown to play a critical role in the control of tumor growth and metastasis and to provide innate immunity against infection with certain viruses. Following activation, NK cells release cytokines and chemokines that induce inflammatory responses; modulate monocyte, dendritic cells, and granulocyte growth and differentiation; and influence subsequent adaptive immune responses. The underlining mechanism of discriminating tumor cells and normal cells by NK cells has provided new insights into tumor immunosurveillance and has suggested new strategies for the treatment of human cancer.

KIR2DL4 is an IL-2-regulated NK cell receptor that exhibits limited expression in humans but triggers strong IFN-gamma production

Killer cell Ig-like receptor (KIR)2DL4 (2DL4, CD158d) was previously described as the only KIR expressed by every human NK cell. It is also structurally atypical among KIRs because it possesses a basic transmembrane residue, which is characteristic of many activating receptors, but also contains a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). We expressed epitope-tagged 2DL4 in an NK-like cell line to study receptor function. Three distinct 2DL4 cDNA clones were analyzed: one encoding the "conventional" 2DL4 with the cytoplasmic ITIM (2DL4.1) and two encoding different cytoplasmic truncated forms lacking the ITIM (2DL4.2 and 2DL4(*)). Surprisingly, one truncated receptor (2DL4.2), which is the product of a prevalent human 2DL4 allele, was not expressed on the cell surface, indicating that some individuals may lack functional 2DL4 protein expression. Conversely, both 2DL4.1 and 2DL4(*) were expressed on the cell surface and up-regulated by IL-2. Analysis of primary NK cells with anti-2DL4 mAb confirmed the lack of surface expression in a donor with the 2DL4.2 genotype. Donors with the 2DL4.1 genotype occasionally expressed receptor only on CD56(high) NK cells, although their expression was up-regulated by IL-2. Interestingly, Ab engagement of epitope-tagged 2DL4 triggered rapid and robust IFN-gamma production, but weak redirected cytotoxicity in an NK-like cell line, which was the opposite pattern to that observed upon engagement of another NK cell activating receptor, NKp44. Importantly, both 2DL4.1 and 2DL4(*) exhibited similar activation potential, indicating that the ITIM does not influence 2DL4.1 activating function. The unique activation properties of 2DL4 suggest linkage to a distinct signaling pathway.

Analysis of the natural killer mediated immune response in metastatic renal cell carcinoma patients

Metastatic renal cell carcinomas (MRCC) are considered as immunogeneic tumors on the basis of the clinical responses observed in patients treated by IL-2. However, renal cell carcinoma patients are also characterized by alterations of the immune response that may compromise the immunotherapeutic approaches. In our study, we have studied the phenotype and the functional capacities of peripheral NK cells in a panel of neprectomized metastatic renal cell carcinoma patients. NK cells were harvested by negative immunoselection from fresh peripheral blood samples. In most of MRCC patients analysed (23/28), the expression of NCR (NKp46 and NKp30) was similar to that of donors. Lytic capacities by activated immunoselected NK cells from MRCC patients assessed against K562 and 3 renal tumor cell lines were in the range of that observed in NK cells from normal donors. HLA-I- renal tumor cells UOK23 were killed with a good efficiency, whereas HLA-I renal tumor cells were more resistant. Although LFA-1/ICAM-1 interaction potentiates RCC cell lysis, HLA-I/NKR interaction clearly decreased RCC cell susceptibility to NK cells. In addition, proliferation of NK cells from MRCC patients in response to cytokines was altered.

Unravelling natural killer cell function: triggering and inhibitory human NK receptors

Natural killer (NK) cells represent a highly specialized lymphoid population characterized by a potent cytolytic activity against tumor or virally infected cells. Their function is finely regulated by a series of inhibitory or activating receptors. The inhibitory receptors, specific for major histocompatibility complex (MHC) class I molecules, allow NK cells to discriminate between normal cells and cells that have lost the expression of MHC class I (e.g., tumor cells). The major receptors responsible for NK cell triggering are NKp46, NKp30, NKp44 and NKG2D. The NK-mediated lysis of tumor cells involves several such receptors, while killing of dendritic cells involves only NKp30. The target-cell ligands recognized by some receptors have been identified, but those to which major receptors bind are not yet known. Nevertheless, functional data suggest that they are primarily expressed on cells upon activation, proliferation or tumor transformation. Thus, the ability of NK cells to lyse target cells requires both the lack of surface MHC class I molecules and the expression of appropriate ligands that trigger NK receptors.

Classification systems for acute and chronic leukaemias

Modern classification systems for acute and chronic leukaemias are based on cytomorphology, cytochemistry, immunophenotyping, immunogenetics and molecular cytogenetics. Morphology forms the initial diagnosis of leukaemia, but generally is not sufficient to identify biologically and clinically relevant subsets within the main categories of leukaemia. Immunophenotyping precisely defines the lineage and stage of differentiation of malignantly transformed haematopoietic cells. This is usually sufficient for precise classification of mature lymphoid malignancies, although immunogenetic and (molecular) cytogenetic studies might be helpful to confirm the diagnosis of disseminated non-Hodgkin's lymphomas. However, certain categories of disease that are clearly defined by cytomorphology and immunophenotyping, particularly acute leukaemias, are still heterogeneous, mainly owing to different underlying leukaemogenic events. Immunophenotyping can reveal subgroups highly suggestive of certain chromosome aberrations but reliable identification of such aberrations requires cytogenetic or molecular studies. Such combined diagnostic information forms the basis of current WHO classification of tumours of haematopoietic and lymphoid tissues. This will be complemented in the near future with novel criteria revealed by microarray gene expression profiling. This chapter summarizes and comments on the currently used immunophenotypic classification systems of acute and chronic leukaemias and on the added value of molecular diagnostics.

Crystal structure of the human natural killer (NK) cell activating receptor NKp46 reveals structural relationship to other leukocyte receptor complex immunoreceptors

Natural cytotoxicity receptors (NCR) mediate lysis of a variety of tumor and virus-infected cells by natural killer (NK) cells. Upon engagement, NCR trigger the cytolytic activity and cytokine release of NK cells through association with ITAM-containing signaling molecules. To further understand the function of these receptors in activation of natural cytotoxicity, we determined the crystal structure of the extracellular ligand binding domain of human NKp46, one of three known NCR, at 2.2-A resolution. The overall fold and disposition of the two C2-set immunoglobulin domains are similar to the D1D2 domains of inhibitory killer cell Ig-like receptor (KIR) and Ig-like transcript (ILT) receptors. As the cellular ligands of NKp46 have not yet been defined, the known ligand binding sites of KIR and ILT were compared with the corresponding structural regions of NKp46 in an effort to identify potential areas suitable for molecular recognition. A potential binding site for influenza hemagglutinin is located near the interdomain hinge, a region that mediates ligand binding in KIR. The structural similarity of NKp46 to inhibitory KIR receptors may have implications for how NK cells balance activating and inhibitory signals.

Renal cell carcinoma-infiltrating natural killer cells express differential repertoires of activating and inhibitory receptors and are inhibited by specific HLA class I allotypes

Among tumor-infiltrating lymphocytes (TILs) directly isolated from renal cell carcinomas (RCCs), we found substantial numbers of natural killer (NK) cells in most tumor tissues. They could be identified reliably in situ with an antibody directed against the activating receptor (AR) NKp46 that is exclusively expressed by all NK cells. NK-enriched TILs (NK-TILs) showed cytotoxicity against major histocompatibility complex (MHC) class I-negative cell lines. The ability to detect lysis of target cells was dependent on the percentage of NK cells within the TILs, and cytotoxicity was only observed after overnight activation with low-dose interleukin-2 (IL-2). Infiltrating NK cells were found to express various inhibitory receptors (IRs); among these the CD94/NKG2A receptor complex was overrepresented compared to the autologous peripheral blood mononuclear cell (PBMC) population. Other IRs were underrepresented, indicating that NK subpopulations vary in their tumor-infiltrating capacity. IRs expressed by NK-TILs are functional since receptor engagement with MHC class I ligands presented by human leukocyte antigen (HLA)-transfected target cell lines was able to inhibit NK-mediated cytotoxicity. NK-TILs were also able to lyse autologous or allogeneic tumor cell lines in vitro. This activity correlated with low HLA class I surface expression since lysis could be inhibited by interferon (IFN)-gamma-expressing RCC transductants that displayed a higher surface density of HLA class I molecules. Therefore, NK cells infiltrating tumor tissues have an inherent ability to recognize transformed cells, but they require cytokine activation and are sensitive to inhibition by IR ligands.

Human natural killer cell receptors: insights into their molecular function and structure

NK cells express receptors characterized by opposite functions that finely regulate their activities. Among inhibitory receptors, some are specific for different groups of MHC class I alleles, while others are still orphan receptors. On the contrary, various activating receptors are involved in the triggering of NK-mediated natural cytotoxicity. In general, their engagement induces human NK cells to kill target cells that are either HLA class I-negative or -deficient. Thus, the process of NK cell triggering mediated by Natural Cytotoxicity Receptors can be mainly considered as a non MHC-restricted mechanism. Here, a brief description of the molecular nature of these receptors, as well as, of their 3D-structures and of the implications for ligand recognition, is given.

Structure of the human NK cell triggering receptor NKp46 ectodomain

NKp46, a natural killer (NK) cell-specific receptor, has been recently identified as one of the triggering receptors involved in NK cell activation mediated by non-HLA class I ligands. The structure of the NKp46 extracellular receptor region, here reported, consists of two Ig-like domains assembled similarly to leukocyte immunoglobulin-like receptors (LIRs) and killer inhibitory receptors (KIRs). The extensive NKp46 residue substitutions at sites structurally related to those mediating interaction with HLA antigens in LIRs and KIRs indicate that NKp46 recognition processes in vivo should involve non-HLA ligands. NKP46 is shown to stem from an ancestral KIR/LIR family. However, the absence of close paralogues, such as those found for LIR and KIR, indicates that NKp46 is the unique member of a distinct Ig-like subfamily and suggests a specific role, which appears to be maintained across primates and rodents.

Channel catfish NK-like cells are armed with IgM via a putative FcmicroR

Two-color flow cytometry demonstrated that 4-8% of channel catfish PBL are positive for both F and G IgL chain isotypes, suggesting that they passively acquire serum IgM via a putative FcmicroR. These cells show spontaneous killing toward allogeneic targets, and in vitro stimulation of PBL with allogeneic cells results in an increase of double IgL chain positive cells with a concomitant increase in nonspecific cytotoxicity. Long-term cultures of alloantigen-stimulated PBL contain both sIgM(+) and sIgM(-) cytotoxic cells that transcribe message for the catfish homolog of the FcepsilonR gamma chain, but not for Igmicro and TCR-alpha,-beta, or -gamma chains. Immunoprecipitation of lysates from sIgM(+) NK-like cells with anti-IgM co-immunoprecipitated a putative FcmicroR of approximately 64 kDa. Finally, removal of IgM from sIgM(+) NK-like cells and replacement with anti-hapten antibody enabled antibody-armed effectors to kill haptenated targets that were refractory to killing by effectors armed with normal IgM. This is the first report suggesting that teleost NK-like cells express a putative FcmicroR which participates in antibody-dependent cell-mediated cytotoxicity.

Fraction of the peripheral blood concentration of CD56+/CD16−/CD3− cells in total natural killer cells as an indication of fertility and infertility

OBJECTIVE

To determine whether the peripheral blood concentration of CD56(+)/CD16(-)/CD3(-) cells, the main natural killer (NK) subpopulation that colonizes the endometrium in the middle and late secretory phase, can be related to fertility or infertility status.

DESIGN

A case control study.

SETTING

Immunopathology department of an infertility laboratory.

PATIENTS

A total 99 women were selected (group I: consecutive spontaneous aborters, n = 25; group II: sporadic spontaneous aborters, n = 30; group III: infertile, n = 33; group IV: controls, n = 11).

INTERVENTION

Immunophenotyping of women grouped according to their fertility status.

MAIN OUTCOME MEASURES

Peripheral blood lymphocytes were examined by two- and three-color flow cytometry.

RESULTS

A statistically significant association between endometrial-type peripheral blood (PB) NK cell concentrations and fertility status (groups I and IV vs. groups II and III) was documented. The %(/TOTAL PB)(CD56+CD16-CD3-) NK cells was significantly higher [1] in fertile (groups I and IV) than in sporadic aborters/infertile (groups II and III) women, [2] in group I when compared with groups II and III, and [3] in group IV when compared with groups II and III.

CONCLUSIONS

This study indicates that diminished numbers of the %(/TOTAL PB)(CD56+CD16-CD3-) NK cells are related to sporadic aborters and infertile women. Thus, the fraction could be used as an indicator of subsequent successful implantation and maintenance of gestation.

Human NK cell development in NOD/SCID mice receiving grafts of cord blood CD34+ cells

Definition of the cytokine environment, which regulates the maturation of human natural killer (NK) cells, has been largely based on in vitro assays because of the lack of suitable animal models. Here we describe conditions leading to the development of human NK cells in NOD/SCID mice receiving grafts of hematopoietic CD34+ precursor cells from cord blood. After 1-week-long in vivo treatment with various combinations of interleukin (IL)-15, flt3 ligand, stem cell factor, IL-2, IL-12, and megakaryocyte growth and differentiation factor, CD56+CD3- cells were detected in bone marrow (BM), spleen, and peripheral blood (PB), comprising 5% to 15% of human CD45+ cells. Human NK cells of NOD/SCID mouse origin closely resembled NK cells from human PB with respect to phenotypic characteristics, interferon (IFN)-gamma production, and cytotoxicity against HLA class 1-deficient K562 targets in vitro and antitumor activity against K562 erythroleukemia in vivo. In the absence of growth factor treatment, CD56+ cells were present only at background levels, but CD34+CD7+ and CD34-CD7+ lymphoid precursors with NK cell differentiation potential were detected in BM and spleen of chimeric NOD/SCID mice for up to 5 months after transplantation. Our results demonstrate that limitations in human NK cell development in the murine microenvironment can be overcome by treatment with NK cell growth-promoting human cytokines, resulting in the maturation of IFN-gamma-producing cytotoxic NK cells. These studies establish conditions to explore human NK cell development and function in vivo in the NOD/SCID mouse model.

Protein kinase C is involved in 2B4 (CD244)-mediated cytotoxicity and AP-1 activation in natural killer cells

2B4 (CD244) is a member of the CD2 subset of the immunoglobulin superfamily and functions as a triggering molecule on natural killer (NK) cells. Previously, we have found that 2B4-mediated activation of NK cells involves complex interactions involving LAT, Ras, Raf, ERK and p38 and that cytolytic function and cytokine production may be regulated by distinct pathways. Here we assessed the role of protein kinase C (PKC) in 2B4-mediated cytotoxicity of YT cells, a human NK cell line. Our data indicate that PKC-delta is activated upon stimulation with monoclonal antibody against 2B4. Treatment with the PKC inhibitor, bisindolylmaleimide I (Gö6850), of YT cells or YT cells depleted of Ca2+-dependent isoforms of PKC prior to 2B4 stimulation, resulted in inhibition of natural cytotoxicity and redirected antibody-dependent cellular cytotoxicity. However, inhibition of PKC failed to block 2B4 stimulation of interferon-gamma secretion as opposed to pretreatment with LY294002, a phosphoinositide 3-kinase inhibitor. We also examined the effect of phorbol 12-myristate 13-acetate (PMA) induction on 2B4 gene transcription. PMA induction resulted in a more than two-fold increase of 2B4 transcription. However, when we introduced a three-base substitution mutation to disrupt the activator protein-1 binding site at (-106 to -100) in the 2B4 promoter, we found complete loss of transcriptional activity, including the two-fold increase due to PMA induction of PKC. The present study indicated that PKC may play an important role in 2B4 signalling and activator protein-1 activation.

Stepwise cytoskeletal polarization as a series of checkpoints in innate but not adaptive cytolytic killing

Cytolytic killing is a major effector mechanism in the elimination of virally infected and tumor cells. The innate cytolytic effectors, natural killer (NK) cells, and the adaptive effectors, cytotoxic T cells (CTL), despite differential immune recognition, both use the same lytic mechanism, cytolytic granule release. Using live cell video fluorescence microscopy in various primary cell models of NK cell and CTL killing, we show here that on tight target cell contact, a majority of the NK cells established cytoskeletal polarity required for effective lytic function slowly or incompletely. In contrast, CTLs established cytoskeletal polarity rapidly. In addition, NK cell killing was uniquely sensitive to minor interference with cytoskeletal dynamics. We propose that the stepwise NK cell cytoskeletal polarization constitutes a series of checkpoints in NK cell killing. In addition, the use of more deliberate progression to effector function to compensate for inferior immune recognition specificity provides a mechanistic explanation for how the same effector function can be used in the different functional contexts of the innate and adaptive immune response.

The three-dimensional structure of the human NK cell receptor NKp44, a triggering partner in natural cytotoxicity

Natural killer (NK) cells direct cytotoxicity against tumor or virally infected cells. NK cell activation depends on a fine balance between inhibitory and activating receptors. NKp44 is a cytotoxicity activating receptor composed of one Ig-like extracellular domain, a transmembrane segment, and a cytoplasmic domain. The 2.2 A crystal structure shows that the NKp44 Ig domain forms a saddle-shaped dimer, where a charged surface groove protrudes from the core structure in each subunit. NKp44 Ig domain disulfide bridge topology defines a new Ig structural subfamily. The data presented are a first step toward understanding the molecular basis for ligand recognition by natural cytotoxicity receptors, whose key role in the immune system is established, but whose cellular ligands are still elusive.

Human natural killer cell function and their interactions with dendritic cells

Natural killer (NK) cells have long been considered as "primitive" and "non-specific" effector cells. However, the past 10 years have witnessed dramatic progress in our understanding of how NK cells function and their role in innate defenses. Thanks to specialized inhibitory receptors specific for MHC-class I molecules, they can sense the decrease or loss of these molecules, a typical condition of potentially dangerous cells such as tumor or virally-infected cells. NK cell triggering and lysis of these cells is mediated by several activating receptors and co-receptors that have recently been identified and cloned. While normal cells are usually resistant to the NK-mediated attack, a remarkable exception is represented by dendritic cells (DC). In their immature form (iDC), they are susceptible to NK-mediated lysis because of the expression of low levels of surface MHC-class I molecules. Since the process of DC maturation (mDC) is characterized by the surface expression of high levels of MHC-class I molecules, mDC become resistant to NK cells. Exposure to live bacteria induces rapid DC maturation and, thus, resistance to NK cells. The cross-talk between DC and NK cells is more complex and involves also a DC-dependent NK cell activation and proliferation. Thus, two important players of the innate immunity may be involved in a coordinated regulation of critical events occurring at the interface between innate and adaptive immunity.

Innate and adaptive mucosal immunity in protection against HIV infection

Control of the HIV pandemic requires an effective vaccine. The difficulties in developing a preventive vaccine are generally believed to be due to the rapid rate of mutation of HIV that escapes cytotoxic lymphocytes (CTL) and the problems in induction of neutralising antibodies to wild strains of HIV. These difficulties should re-orientate vaccine strategy into four somewhat neglected areas of immunisation. Innate immunity, with its rapid protective response to infection that is independent of memory and relies on an optimal mucosal adjuvant. Targeting the genital and rectal mucosa, with the associated lymph nodes, as an immune response has to be elicited directly on encountering HIV during sexual intercourse. Stimulating a broadly based adaptive immune response that enhances the memory CD4(+) and CD8(+) T cells and B cells, induces maturation of dendritic cells and results in Th1 polarised immunity. Taking advantage of "experiments of nature", by utilising host antigens, as manifested by protection against HIV infection in homozygous Delta32 CCR5 individuals and in allo-immunity.

BCR/ABL alters the function of NK cells and the acquisition of killer immunoglobulin-like receptors (KIRs)

Natural killer (NK) cells decrease in function during chronic myelogenous leukemia (CML) progression from chronic phase to blast crisis, and they can become BCR/ABL(+) late in the disease course. To study this altered function, NK92 cells were transduced with the BCR/ABL oncogene. In contrast to the parental cells, which died when deprived of interleukin 2 (IL-2), p210(+) NK92 cells proliferated and survived indefinitely in the absence of IL-2. BCR/ABL also decreased the natural cytotoxicity of NK92 cells against K562 targets, without affecting IL-2, interferon gamma (IFN-gamma), or tumor necrosis factor alpha (TNF-alpha) production. Although the ABL-specific tyrosine kinase inhibitor imatinib mesylate (STI-571) had no effect on parental NK92 cells, it markedly decreased the growth and survival of IL-2-independent p210(+) NK92 cells. In contrast to the parental cell line, serial analysis of p210(+) NK92 cells detected small populations that clonally expressed one or more killer immunoglobulin-like receptors (KIRs). Unlike the decreased natural cytotoxicity, the function of the activating CD158j receptor remained intact. Southern blotting and hybridization with an enhanced green fluorescence protein (eGFP) probe showed that KIR(-) and KIR(+) NK92 cells were all derived from the same clone, suggesting that KIR acquisition remains dynamic at the maturational stage represented by the NK92 cell line. When tested in primary CD56(+bright) NK cells, p210 induced partial IL-2-independent growth and increased KIR expression similar to findings in NK92 cells. This is the first study to show that BCR/ABL, well known for its effects on the myeloid lineage, can alter the function of lymphoid cells, which may be associated with the defect in innate immunity associated with CML progression.

Transforming growth factor beta 1 inhibits expression of NKp30 and NKG2D receptors: consequences for the NK-mediated killing of dendritic cells

The surface density of the triggering receptors responsible for the natural killer (NK)-mediated cytotoxicity is crucial for the ability of NK cells to kill susceptible target cells. In this study, we show that transforming growth factor beta1 (TGFbeta1) down-regulates the surface expression of NKp30 and in part of NKG2D but not that of other triggering receptors such as NKp46. The TGFbeta1-mediated inhibition of NKp30 surface expression reflects gene regulation at the transcriptional level. NKp30 has been shown to represent the major receptor involved in the NK-mediated killing of dendritic cells. Accordingly, the TGFbeta1-dependent down-regulation of NKp30 expression profoundly inhibited the NK-mediated killing of dendritic cells. On the contrary, killing of different NK-susceptible tumor cell lines was variably affected, reflecting the differential usage of NKp30 and/or NKG2D in the lysis of such tumors. Our present data suggest a possible mechanism by which TGFbeta1-producing dendritic cells may acquire resistance to the NK-mediated attack.

The size of the synaptic cleft and distinct distributions of filamentous actin, ezrin, CD43, and CD45 at activating and inhibitory human NK cell immune synapses

In this study, we report the organization of cytoskeletal and large transmembrane proteins at the inhibitory and activating NK cell immunological or immune synapse (IS). Filamentous actin accumulates at the activating, but not the inhibitory, NK cell IS. However, surprisingly, ezrin and the associated protein CD43 are excluded from the inhibitory, but not the activating, NK cell IS. This distribution of ezrin and CD43 at the inhibitory NK cell IS is similar to that previously seen at the activating T cell IS. CD45 is also excluded from the inhibitory, but not activating, NK cell IS. In addition, electron microscopy reveals wide and narrow domains across the synaptic cleft. Target cell HLA-C, located by immunogold labeling, clusters where the synaptic cleft spans the size of HLA-C bound to the inhibitory killer Ig-like receptor. These data are consistent with assembly of the NK cell IS involving a combination of cytoskeletal-driven mechanisms and thermodynamics favoring the organization of receptor/ligand pairs according to the size of their extracellular domains.

The promotion of tumor metastasis by surgery and stress: immunological basis and implications for psychoneuroimmunology

This mini-review emphasizes a psychoneuroimmunology (PNI) perspective of the hypothesis that stress and surgical excision of the primary tumor can promote tumor metastasis. It first establishes the empirical and theoretical basis for control of metastasis by cell-mediated immunity (CMI), as well as the interactive role of non-immunological risk factors. It then describes the various aspects of surgery that suppress CMI, and the neuroendocrine mechanisms mediating suppression by stress and surgery. Last, it briefly reviews the empirical evidence, from animal and human studies, for the promotion of metastasis by stress and surgery, with specific reference to the mediating role of CMI. It is concluded that: (a) Immunological mechanisms most likely play a role in limiting metastasis in patients with solid tumors. (b) Immunosuppression can be deleterious, especially when surgery is conducted early, before the tumor develops insurmountable mechanisms to escape immune destruction. (c) The most sensitive period for the establishment of metastases is the immediate aftermath of surgery. Interventions aiming at reducing stress and immunosuppression should thus strive to start beforehand. (d) 'Psychological and physiological insults activate similar neuroendocrine mechanisms of immunosuppression. Therefore, a multimodal therapeutic approach should be used to prevent tumor metastasis during the perioperative period. (e) Studies employing interventions aimed at reducing the surgical stress response should preferably assess immunological indices with an established clinical relevance, and follow up long-term recurrence provided sample size assure statistical power. (f) The progress toward earlier detection of cancer, and our growing understanding of immunosuppression, continuously improves the chances for successful PNI interventions.

Crystal structures of two rat MHC class Ia (RT1-A) molecules that are associated differentially with peptide transporter alleles TAP-A and TAP-B

Antigenic peptides are loaded onto class I MHC molecules in the endoplasmic reticulum (ER) by a complex consisting of the MHC class I heavy chain, beta(2)-microglobulin, calreticulin, tapasin, Erp57 (ER60) and the transporter associated with antigen processing (TAP). While most mammalian species transport these peptides into the ER via a single allele of TAP, rats have evolved different TAPs, TAP-A and TAP-B, that are present in different inbred strains. Each TAP delivers a different spectrum of peptides and is associated genetically with distinct subsets of MHC class Ia alleles, but the molecular basis for the conservation (or co-evolution) of the two transporter alleles is unknown. We have determined the crystal structures of a representative of each MHC subset, viz RT1-A(a) and RT1-A1(c), in association with high-affinity nonamer peptides. The structures reveal how the chemical properties of the two different rat MHC F-pockets match those of the corresponding C termini of the peptides, corroborating biochemical data on the rates of peptide-MHC complex assembly. An unusual sequence in RT1-A1(c) leads to a major deviation from the highly conserved beta(3)/alpha(1) loop (residues 40-59) conformation in mouse and human MHC class I structures. This loop change contributes to profound changes in the shape of the A-pocket in the peptide-binding groove and may explain the function of RT1-A1(c) as an inhibitory natural killer cell ligand.

Human NK cells and their receptors

The past decade has witnessed important progress in our understanding of how natural killer (NK) cells function. This is primarily consequent to the identification and functional characterization of MHC-specific inhibitory receptors that allow NK cells to discriminate between normal cells and potentially harmful cells that have lost or express insufficient amounts of MHC class I molecules. More recently, a number of activating receptors or coreceptors have been identified that are involved in the process of natural cytotoxicity but may also play a role in the direct recognition of pathogen-associated structures. Surprisingly, none of the triggering receptors identified in NK cells appears to be involved in the "NK-like activity" of a subset of CD8(+) cytolytic T lymphocytes. In this case, lysis of NK-susceptible tumor target cells is the result of the TCR alpha/beta-mediated recognition of HLA-E. The potent cytolytic activity of NK cells as well as their unique mode of functioning may be exploited in therapy. An important breakthrough is the recent report that "alloreactive" NK cells, generated in haploidentical bone marrow transplantation in patients with acute myeloid leukemias, may efficiently prevent leukemic relapses as well as graft rejection and graft-vs.-host disease. This may lead to a true revolution in bone marrow transplantation, based on the exploitation of appropriate HLA-Cl I mismatches that can put NK cells in action.

Hemophagocytic lymphohistiocytosis is associated with deficiencies of cellular cytolysis but normal expression of transcripts relevant to killer-cell-induced apoptosis

In 65 patients with hemophagocytic lymphohistiocytosis (HLH), we found an as yet undescribed heterogeneity of defects in cellular cytotoxicity when assay conditions were modified by the incubation time, the presence of mitogen, or interleukin-2 (IL-2). The standard 4-hour natural killer (NK) test against K562 targets was negative in all patients. In patients deficient in type 1 (n = 21), type 2 (n = 5), and type 4 (n = 8) HLH, negative NK function could be reconstituted by mitogen, by IL-2, or by prolongation of the incubation time (16 hours), respectively. Most patients (n = 31) displayed the type 3 defect, defined by a lack of any cellular cytotoxicity independent of assay variations. The characteristic hypercytokinemia also concerned counterregulatory cytokines, such as proinflammatory interferon-gamma (IFN-gamma), simultaneously elevated with suppressive IL-10 in 38% of types 1-, 2-, and 4-deficient patients and in 71% of type 3-deficient patients. Elevated IFN-gamma alone correlated with high liver enzymes, but sCD95-ligand and sCD25 did not-though these markers were expected to indicate the extent of histiocytic organ infiltration. Outcome analysis revealed more deaths in patients with type 3 deficiency (P =.017). Molecular defects were associated with homozygously mutated perforin only in 4 patients, but other type 3 patients expressed normal transcripts of effector molecules for target-cell apoptosis, including perforin and granzyme family members, as demonstrated by RNase protection analysis. Thus, target-cell recognition or differentiation defects are likely to explain this severe phenotype in HLH. Hyperactive phagocytes combined with NK defects may imply defects on the level of the antigen-presenting cell.

NK cells: a lesson from mismatched hematopoietic transplantation

The past ten years have witnessed dramatic progress in our understanding of how natural killer (NK) cells function, their role in innate defenses and their possible exploitation in therapy. This article traces the major advances in these formerly mysterious cells, from the 'missing self' hypothesis and the first discovery of HLA-class I-specific inhibitory receptors to a recent major breakthrough that highlighted important perspectives and major expectations regarding the cure of life-threatening leukemias. The key role of 'alloreactive' NK cells in eradicating acute myeloid leukemias and in preventing both graft rejection and graft-versus-host disease, might lead to a true revolution in bone marrow transplantation. Thus, it might now be possible to search for appropriate HLA class I mismatches to set NK cells in action.

Xenogeneic human NK cytotoxicity against porcine endothelial cells is perforin/granzyme B dependent and not inhibited by Bcl-2 overexpression

Because of organ shortages in clinical allotransplantation, the potential of pig-to-human xenotransplantation is currently being explored showing a possible critical role for natural killer (NK) cells in the immune response against xenografts. Therefore, we analyzed the cytotoxic pathways utilized by human natural killer cells (hNK) against porcine endothelial cells (pEC). Transmission electron microscopy of pEC cocultured with hNK cells showed both apoptotic and necrotic cell death, whereas soluble factors such as Fas ligand or TNFalpha did not induce apoptosis in pEC. NK lysis of pEC was abrogated by concanamycin A and ammonium chloride, reagents inhibiting the perforin/granzyme B (grB) pathway, but only partially blocked by caspase inhibition with z-VAD-fmk. Overexpression of bcl-2 protected pEC against apoptosis induced by staurosporine or actinomycin D, but failed to prevent hNK cell-mediated lysis. In conclusion, pEC are lysed in vitro by hNK cells via the perforin/grB pathway and are not protected from NK lysis by overexpression of bcl-2.

Lipid solubility- and concentration-dependent attenuation of in vitro natural killer cell cytotoxicity by local anesthetics

BACKGROUND

Natural killer (NK) cells constitute an essential component of the innate immune system in the defence against infected and malignant cells. In this study the in vitro effect on NK cell activity of three different local anesthetics with different lipid solubility was investigated.

METHODS

Venous blood from seven healthy volunteers was incubated with three amide local anesthetics with three different concentrations of lipid solubility: lidocaine 0.50, 1.00 and 2.00 mg/ml, ropivacaine 0.375, 0.75 and 1.50 mg/ml, and bupivacaine 0.25, 0.50 and 1.00 mg/ml. After 1 h of incubation, mononuclear cells were isolated and cryopreserved until tested for NK cell cytotoxicity in a 4-h 51Cr-release assay against K-562 target cells. Natural killer cell cytotoxicity of mononuclear cells incubated with isotonic saline was used as the control.

RESULTS

A significant suppression in NK cell cytotoxicity was demonstrated for all three local anesthetic agents when the NK cell cytotoxicity was compared with the cytotoxicity estimated after incubation with the isotonic saline (P<0.004). Moreover a significant lipid solubility-dependent effect (P=0.0001) as well as an overall concentration-dependent effect (P<0.0001) on the NK cell cytotoxicity was found.

CONCLUSION

The results of the present in vitro study suggest a negative association between the estimated NK cell cytotoxicity and the lipid solubility as well as the concentrations of the three local anesthetic agents tested.

Natural killer cells and their receptors

Natural killer (NK) cells have been known for a long time to be a very important component of the innate immune system. However, it is only during the last 10 years that knowledge of their receptors has emerged. Described in the present review are those receptor families killer inhibitory receptor (KIR) (belonging to the immunoglobulin superfamily), and killer lectin like receptor (KLR) CD94/NKG2, that both use HLA as a ligand and have inhibiting and activating types of receptors, and natural cytotoxic receptors (NCR) which do not associate with HLA. Association of the receptor gives rise to either an inhibiting or activating signal leading to either failure or success in lysing a target cell. The KIR receptors are very polymorphic both in the number of genes expressed in an individual and the alleles present for a gene. They would appear to have had a rapid evolution compared to the CD94/NKG2 receptors. The roles that NK cells and their receptors have with various facets of transplantation, disease, pregnancy and control of virus infection in humans are described.

KIR: diverse, rapidly evolving receptors of innate and adaptive immunity

KIR genes have evolved in primates to generate a diverse family of receptors with unique structures that enable them to recognize MHC-class I molecules with locus and allele-specificity. Their combinatorial expression creates a repertoire of NK cells that surveys the expression of almost every MHC molecule independently, thus antagonizing the spread of pathogens and tumors that subvert innate and adaptive defense by selectively downregulating certain MHC class I molecules. The genes encoding KIR that recognize classical MHC molecules have diversified rapidly in human and primates; this contrasts with conservation of immunoglobulin- and lectin-like receptors for nonclassical MHC molecules. As a result of the variable KIR-gene content in the genome and the polymorphism of the HLA system, dissimilar numbers and qualities of KIR:HLA pairs function in different humans. This diversity likely contributes variability to the function of NK cells and T-lymphocytes by modulating innate and adaptive immune responses to specific challenges.

Defective expression and function of natural killer cell-triggering receptors in patients with acute myeloid leukemia

The cytolytic function of natural killer (NK) cells is induced by the engagement of a series of activating receptors and coreceptors some of which have recently been identified and collectively termed natural cytotoxicity receptors (NCRs). Here, we analyzed the cytolytic function of NK cells obtained from patients with acute myeloid leukemia (AML). In sharp contrast with healthy donors, in most (16 of 18) patients with AML the majority of NK cells displayed low NCR surface density (NCR(dull)). This phenotype correlated with a weak cytolytic activity against autologous leukemic cells that could not be reversed by the monoclonal antibody-mediated disruption of HLA class I/killer immunoglobulinlike receptor interaction. The remaining 2 patients were characterized by NK cells having an NCR(bright) phenotype. Surprisingly, although displaying NCR-mediated cytolytic activity, these NCR(bright) NK cells were unable to kill autologous leukemic blasts. Importantly, the leukemic blasts from these 2 patients were also resistant to lysis mediated by normal NCR(bright) allogeneic NK cells. Our study suggests that in most instances the inability of NK cells to kill autologous leukemic blasts is consequent to low NCR surface expression. In few cases, however, this failure appears to involve a mechanism of tumor escape based on down-regulation of ligands relevant for NCR-mediated target cell recognition.

Mistletoe viscotoxins increase natural killer cell-mediated cytotoxicity

Mistletoe extracts have immunomodulatory activity. We show that nontoxic concentrations of Viscum album extracts increase natural killer (NK) cell-mediated killing of tumor cells but spare nontarget cells from NK lysis. The compounds responsible for this bioactivity were isolated from mistletoe and characterized. They have low molecular mass and are thermostable and protease-resistant. After complete purification by HPLC, they were identified by tandem MS as viscotoxins A1, A2 and A3 (VTA1, VTA2 and VTA3, respectively). Whereas micromolar concentrations of these viscotoxins are cytotoxic to the targets, the bioactivity with respect to NK lysis is within the nanomolar range and differs between viscotoxin isoforms: VTA1 (85 nm), VTA2 (18 nm) and VTA3 (8 nm). Microphysiometry and assays of cell killing indicate that, within such nontoxic concentrations, viscotoxins do not activate NK cells, but act on cell conjugates to increase the resulting lysis.

NK cells and transplantation

The requirement for cytotoxic T lymphocytes during allograft rejection is controversial. We have demonstrated that CD8+ T cells are not essential for allograft rejection or for the induction of apoptosis in two experimental models of transplantation. To determine candidate cells types which may play a role in the events leading to graft rejection, the cellular composition of rejecting allografts was determined. We demonstrate that substantial numbers of NK cells, of recipient origin, infiltrate allografts as early as 12 h after transplantation. These NK cells produce cytokines and express cytotoxic mediators such as granzyme B and FasL. It is unknown which NK cell receptors are expressed and activated during transplantation. NK cells express multiple cell surface receptors, including MHC class I binding inhibitory receptors, which deliver a negative signal, and activation receptors, which stimulate cytokine secretion and cytotoxicity of NK cells. To begin to understand NK cell activation in the context of transplantation, we have recently cloned a novel rat immunoglobulin-like surface receptor from a rejecting liver allograft. Sequence analysis demonstrates that this putative activation receptor contains 71% identity to human NKp30 at the DNA level, suggesting that it is the rat homologue (rNKp30). Characterization of NK activation receptors may lead to better understanding of the interactions between the innate and adaptive immune responses in transplantation.