Strategies of natural killer cell recognition and signaling

Autophagy-mediated NKG2D internalization impairs NK cell function and exacerbates radiation pneumonitis

Introduction

Radiation pneumonitis is a critical complication that constrains the use of radiation therapy for thoracic malignancies, leading to substantial morbidity via respiratory distress and lung function impairment. The role of Natural killer (NK) cells in inflammatory diseases is well-documented; however, their involvement in radiation pneumonitis is not fully understood.

Methods

To explore the involvement of NK cells in radiation pneumonitis, we analyzed tissue samples for NK cell presence and function. The study utilized immunofluorescence staining, western blotting, and immunoprecipitation to investigate CXCL10 and ROS levels, autophagy activity, and NKG2D receptor dynamics in NK cells derived from patients and animal models subjected to radiation.

Result

In this study, we observed an augmented infiltration of NK cells in tissues affected by radiation pneumonitis, although their function was markedly diminished. In animal models, enhancing NK cell activity appeared to decelerate the disease progression. Concomitant with the disease course, there was a notable upsurge in CXCL10 and ROS levels. CXCL10 was found to facilitate NK cell migration through CXCR3 receptor activation. Furthermore, evidence of excessive autophagy in patient NK cells was linked to ROS accumulation, as indicated by immunofluorescence and Western blot analyses. The association between the NKG2D receptor and its adaptor proteins (AP2 subunits AP2A1 and AP2M1), LC3, and lysosomes was intensified after radiation exposure, as demonstrated by immunoprecipitation. This interaction led to NKG2D receptor endocytosis and subsequent lysosomal degradation.

Conclusion

Our findings delineate a mechanism by which radiation-induced lung injury may suppress NK cell function through an autophagy-dependent pathway. The dysregulation observed suggests potential therapeutic targets; hence, modulating autophagy and enhancing NK cell activity could represent novel strategies for mitigating radiation pneumonitis.

Association between multiple coagulation-related factors and lymph node metastasis in patients with gastric cancer: A retrospective cohort study

Background

Patients with tumors generally present with accompanying activation of the coagulation system, which may be related to tumor stage. To our knowledge, few studies have examined the activation of the coagulation system in reference to lymph node metastasis within gastric cancer. This study aimed to investigate the correlation between multiple coagulation-related factors and lymph node metastasis in patients with gastric cancer after excluding the influence of tumor T stage.

Materials and methods

We retrospectively evaluated the relationship between lymph node metastasis and coagulation-related factors in 516 patients with T4a stage gastric cancer. We further analyzed influencing factors for lymph node metastasis and verified the predictive value of maximum amplitude (MA, a parameter of thromboelastography which is widely used to assess the strength of platelet-fibrinogen interaction in forming clots) in reference to lymph node metastasis.

Results

Platelet counts (P=0.011), fibrinogen levels (P=0.002) and MA values (P=0.006) were statistically significantly higher in patients with T4a stage gastric cancer presenting with lymph node metastasis than in those without lymph node metastasis. Moreover, tumor N stage was statistically significantly and positively correlated with platelet count (P<0.001), fibrinogen level (P=0.003), MA value (P<0.001), and D-dimer level (P=0.010). The MA value was an independent factor for lymph node metastasis (β=0.098, 95% CI: 1.020-1.193, P=0.014) and tumor N stage (β=0.059, 95% CI: 0.015-0.104, P=0.009), and could be used to predict the presence of lymph node metastasis in patients with gastric cancer (sensitivity 0.477, specificity 0.783, P=0.006). The independent influencing factors for MA value mainly included platelet levels, fibrinogen levels, D-dimer and hemoglobin levels; we found no statistically significant correlations with tumor diameter, tumor area, and other evaluated factors.

Conclusion

We conclude that MA value is an independent influencing factor for lymph node metastasis and tumor N stage in patients with T4a stage gastric cancer. The MA value has important value in predicting the presence or absence of lymph node metastasis in patients with gastric cancer.

Clinical trial registration

http://www.chictr.org.cn, identifier ChiCTR2200064936.

NKG2D Natural Killer Cell Receptor-A Short Description and Potential Clinical Applications

Natural Killer (NK) cells are natural cytotoxic, effector cells of the innate immune system. They can recognize transformed or infected cells. NK cells are armed with a set of activating and inhibitory receptors which are able to bind to their ligands on target cells. The right balance between expression and activation of those receptors is fundamental for the proper functionality of NK cells. One of the best known activating receptors is NKG2D, a member of the CD94/NKG2 family. Due to a specific NKG2D binding with its eight different ligands, which are overexpressed in transformed, infected and stressed cells, NK cells are able to recognize and attack their targets. The NKG2D receptor has an enormous significance in various, autoimmune diseases, viral and bacterial infections as well as for transplantation outcomes and complications. This review focuses on the NKG2D receptor, the mechanism of its action, clinical relevance of its gene polymorphisms and a potential application in various clinical settings.

High-Fat Diet and Feeding Regime Impairs Number, Phenotype, and Cytotoxicity of Natural Killer Cells in C57BL/6 Mice

Overweight and obesity are major public health challenges worldwide. Obesity is associated with a higher risk for the development of several cancer types, but specific mechanisms underlying the link of obesity and cancer are still unclear. Natural killer (NK) cells are circulating lymphoid cells promoting the elimination of virus-infected and tumor cells. Previous investigations demonstrated conflicting results concerning the influence of obesity on functional NK cell parameters in small animal models. The aim of the present study was to clarify potential obesity-associated alterations of murine NK cells in vivo, implementing different feeding regimes. Therefore, C57BL/6 mice were fed a normal-fat diet (NFD) or high-fat diet (HFD) under restrictive and ad libitum feeding regimes. Results showed diet and feeding-regime dependent differences in body weight, visceral fat mass and plasma cytokine concentrations. Flow cytometry analyses demonstrated significant changes in total cell counts as well as frequencies of immune cell populations in peripheral blood comparing mice fed NFD or HFD in an ad libitum or restrictive manner. Mice fed the HFD showed significantly decreased frequencies of total NK cells and the mature CD11b⁺CD27⁺ NK cell subset compared to mice fed the NFD. Feeding HFD resulted in significant changes in the expression of the maturation markers KLRG1 and CD127 in NK cells. Furthermore, real-time PCR analyses of NK-cell related functional parameters in adipose tissue revealed significant diet and feeding-regime dependent differences. Most notable, real-time cytotoxicity assays demonstrated an impaired cytolytic activity of splenic NK cells toward murine colon cancer cells in HFD-fed mice compared to NFD-fed mice. In conclusion, our data demonstrate that feeding a high-fat diet influences the frequency, phenotype and function of NK cells in C57BL/6 mice. Interestingly, restricted feeding of HFD compared to ad libitum feeding resulted in a partial prevention of the obesity-associated alterations on immune cells and especially on NK cells, nicely fitting with the current concept of an advantage for interval fasting for improved health.

Characterization of Surface Receptor Expression and Cytotoxicity of Human NK Cells and NK Cell Subsets in Overweight and Obese Humans

Obesity is associated with an increased risk for several cancer types and an altered phenotype and functionality of natural killer (NK) cells. This study aimed to investigate the association of overweight and obesity with NK cell functions and receptor expression profiles in humans. Therefore, peripheral blood mononuclear cells were isolated from normal weight, overweight, and obese healthy blood donors. In depth analysis of immune cell populations and 23 different surface markers, including NK cell receptors, NK-cell-related markers as well as functional intracellular markers on total NK cells and NK subgroups were performed by multicolor flow cytometry. The data revealed a decreased expression of the activating NK cell receptors KIR2DS4 and NKp46 as well as an increased expression of the inhibitory NK cell receptors NKG2A and Siglec-7 in overweight and obese compared to normal weight individuals. Additionally, the expression of the adhesion molecule CD62L and the maturation and differentiation marker CD27 was downregulated in NK cells of overweight and obese subjects. Furthermore, the cytotoxicity of NK cells against colorectal cancer cells was decreased in overweight and obese subjects. Investigations on underlying killing mechanisms demonstrated a reduced TRAIL expression on NK cells of obese subjects suggesting an impaired death receptor pathway in obesity. The present study gives new insights into an impaired functionality and phenotype of NK cells and NK cell subsets in overweight and obesity. These phenotypic alterations and dysfunction of NK cells might be an explanation for the increased cancer risk in obesity.

KIR Variation in Iranians Combines High Haplotype and Allotype Diversity With an Abundance of Functional Inhibitory Receptors

Natural killer (NK) cells are innate lymphocytes that eliminate infected and transformed cells. They discriminate healthy from diseased tissue through killer cell Ig-like receptor (KIR) recognition of HLA class I ligands. Directly impacting NK cell function, KIR polymorphism associates with infection control and multiple autoimmune and pregnancy syndromes. Here we analyze KIR diversity of 241 individuals from five groups of Iranians. These five populations represent Baloch, Kurd, and Lur, together comprising 15% of the ethnically diverse Iranian population. We identified 159 KIR alleles, including 11 not previously characterized. We also identified 170 centromeric and 94 telomeric haplotypes, and 15 different KIR haplotypes carrying either a deletion or duplication encompassing one or more complete KIR genes. As expected, comparing our data with those representing major worldwide populations revealed the greatest similarity between Iranians and Europeans. Despite this similarity we observed higher frequencies of KIR3DL1^*001 in Iran than any other population, and the highest frequency of HLA-B^*51, a Bw4-containing allotype that acts as a strong educator of KIR3DL1^*001⁺ NK cells. Compared to Europeans, the Iranians we studied also have a reduced frequency of 3DL1^*004, which encodes an allotype that is not expressed at the NK cell surface. Concurrent with the resulting high frequency of strong viable interactions between inhibitory KIR and polymorphic HLA class I, the majority of KIR-A haplotypes characterized do not express a functional activating receptor. By contrast, the most frequent KIR-B haplotype in Iran expresses only one functional inhibitory KIR and the maximum number of activating KIR. This first complete, high-resolution, characterization of the KIR locus of Iranians will form a valuable reference for future clinical and population studies.

Obesity-Associated Alterations of Natural Killer Cells and Immunosurveillance of Cancer

Obesity is accompanied by a systemic chronic low-grade inflammation as well as dysfunctions of several innate and adaptive immune cells. Recent findings emphasize an impaired functionality and phenotype of natural killer (NK) cells under obese conditions. This review provides a detailed overview on research related to overweight and obesity with a particular focus on NK cells. We discuss obesity-associated alterations in subsets, distribution, phenotype, cytotoxicity, cytokine secretion, and signaling cascades of NK cells investigated in vitro as well as in animal and human studies. In addition, we provide recent insights into the effects of physical activity and obesity-associated nutritional factors as well as the reduction of body weight and fat mass on NK cell functions of obese individuals. Finally, we highlight the impact of impaired NK cell physiology on obesity-associated diseases, focusing on the elevated susceptibility for viral infections and increased risk for cancer development and impaired treatment response.

Impaired natural killer cell subset phenotypes in human obesity

Obesity is associated with alterations in functionality of immune cells, like macrophages and natural killer (NK) cells, leading to an increased risk for severe infections and several cancer types. This study aimed to examine immune cell populations and functional NK cell parameters focusing on NK cell subset phenotypes in normal-weight and obese humans. Therefore, peripheral blood mononuclear cells (PBMCs) were isolated from normal-weight and obese individuals and analyzed by flow cytometry. Results show no significant changes in the frequency of monocytes, B lymphocytes, or NKT cells but a significantly increased frequency of T lymphocytes in obesity. The frequency of total NK cells was unaltered, whereas the number of low cytotoxic CD56bright NK cell subset was increased, and the number of high cytotoxic CD56dim NK cell subset was decreased in obese subjects. In addition, the frequency of CD56bright NK cells expressing the activating NK cell receptor NKG2D as well as intracellular interferon (IFN)-γ was elevated in the obese study group. In contrast, the frequency of NKG2D- and IFN-γ-positive CD56dim NK cells was lower in obesity compared to normal-weight individuals. Moreover, the expression of the activation marker CD69 was decreased in NK cells, which can be attributed to a reduction of CD69-positive CD56dim NK cells in obese subjects. In conclusion, data reveal an impaired NK cell phenotype and NK cell subset alterations in obese individuals. This NK cell dysfunction might be one link to the higher cancer risk and the elevated susceptibility for viral infections in obesity.

Leptin affects filopodia and cofilin in NK-92 cells in a dose- and time-dependent manner

Hyperleptinemia, associated with obesity, is related with immune dysfunction and carcinogenesis. Natural Killer (NK) cells, a major component of the innate immune system are mediators of anti-tumor immunity and the most actively migrating cells among leukocytes. Actin rearrangement, promoted by cofilin plays a central role in cellular migration. Leptin affects the phosphorylation-dependent activity of cofilin and thus actin remodeling. We used human NK-92 cells to explore the in vitro effects of leptin on co-localization of cofilin and F-actin and on morphological changes in NK cells. NK-92 cells were incubated with different leptin concentrations (10 and 100 ng/mL) for 30 min and 24 h and immunocytochemically stained. Results demonstrate a dose- and time-dependent influence of leptin on cellular morphology. Utilizing confocal microscopy, we observed that the co-localization of cofilin-1 and F-actin was slightly influenced by leptin. In summary, the present study demonstrates an impact of a physiological leptin stimulation on the filopodia length, and a time-dependent effect on the co-localization of cofilin and F-actin in NK-92 cells.

Diet-Induced Obesity Is Associated with an Impaired NK Cell Function and an Increased Colon Cancer Incidence

Obesity is associated with an increased colon cancer incidence, but underlying mechanisms remained unclear. Previous studies showed altered Natural killer (NK) cell functions in obese individuals. Therefore, we studied the impact of an impaired NK cell functionality on the increased colon cancer risk in obesity. In vitro investigations demonstrated a decreased IFN-γ secretion and cytotoxicity of human NK cells against colon tumor cells after NK cell preincubation with the adipokine leptin. In addition, leptin incubation decreased the expression of activating NK cell receptors. In animal studies, colon cancer growth was induced by injection of azoxymethane (AOM) in normal weight and diet-induced obese rats. Body weight and visceral fat mass were increased in obese animals compared to normal weight rats. AOM-treated obese rats showed an increased quantity, size, and weight of colon tumors compared to the normal weight tumor group. Immunohistochemical analyses demonstrated a decreased number of NK cells in spleen and liver in obesity. Additionally, the expression levels of activating NK cell receptors were lower in spleen and liver of obese rats. The results show for the first time that the decreased number and impaired NK cell function may be one cause for the higher colon cancer risk in obesity.

Beyond NK cells: the expanding universe of innate lymphoid cells

For a long time, natural killer (NK) cells were thought to be the only innate immune lymphoid population capable of responding to invading pathogens under the influence of changing environmental cues. In the last few years, an increasing amount of evidence has shown that a number of different innate lymphoid cell (ILC) populations found at mucosal sites rapidly respond to locally produced cytokines in order to establish or maintain homeostasis. These ILC populations closely mirror the phenotype of adaptive T helper subsets in their repertoire of secreted soluble factors. Early in the immune response, ILCs are responsible for setting the stage to mount an adaptive T cell response that is appropriate for the incoming insult. Here, we review the diversity of ILC subsets and discuss similarities and differences between ILCs and NK cells in function and key transcriptional factors required for their development.

Increased numbers of NK cells, NKT-like cells, and NK inhibitory receptors in peripheral blood of patients with chronic obstructive pulmonary disease

T cells and B cells participate in the pathogenesis of COPD. Currently, NK cells and NKT cells have gained increasing attention. In the present study, 19 COPD patients and 12 healthy nonsmokers (HNS) were recruited, and their pulmonary function was assessed. The frequencies of CD3⁺ T, CD4⁺ T, CD8⁺ T, B, NK, and NKT-like cells were determined using flow cytometry. The frequencies of spontaneous and inducible IFN-γ⁺ or CD107a⁺ NK and NKT-like cells as well as activating or inhibitory receptors were also detected. The potential association of lymphocyte subsets with disease severity was further analyzed. Significantly decreased numbers of CD3⁺ and CD4⁺ T cells, and the CD4⁺/CD8⁺ ratio, but increased numbers of CD3⁻CD56⁺ NK and CD3⁺CD56⁺ NKT-like cells were observed in COPD patients compared to HNS. The frequencies of inducible IFN-γ-secreting NK and NKT-like cells were less in COPD patients. The frequencies of CD158a and CD158b on NK cells and CD158b on NKT-like cells were greater. The frequency of CD158b⁺ NK cells was negatively correlated with FEV₁% prediction and FEV₁/FVC. Our data indicate that COPD patients have immune dysfunction, and higher frequencies of inhibitory NK cells and NKT-like cells may participate in the pathogenesis of COPD.

Coagulation and metastasis: what does the experimental literature tell us?

Inhibition of coagulation greatly limits cancer metastasis in many experimental models. Cancer cells trigger coagulation, through expression of tissue factor or P-selectin ligands that have correlated with worse prognosis in human clinical studies. Cancer cells also affect coagulation through expression of thrombin and release of microparticles that augment coagulation. In the cancer-bearing host, coagulation facilitates tumour progression through release of platelet granule contents, inhibition of Natural Killer cells and recruitment of macrophages. We are revisiting this literature in the light of recent studies in which treatment of clinical cohorts with anticoagulant drugs led to diminished metastasis.

The SCHOOL of nature: IV. Learning from viruses

During the co-evolution of viruses and their hosts, the latter have equipped themselves with an elaborate immune system to defend themselves from the invading viruses. In order to establish a successful infection, replicate and persist in the host, viruses have evolved numerous strategies to counter and evade host antiviral immune responses as well as exploit them for productive viral replication. These strategies include those that modulate signaling mediated by cell surface receptors. Despite tremendous advancement in recent years, the exact molecular mechanisms underlying these critical points in viral pathogenesis remain unknown. In this work, based on a novel platform of receptor signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) platform, I suggest specific mechanisms used by different viruses such as human immunodeficiency virus (HIV), cytomegalovirus (CMV), severe acute respiratory syndrome coronavirus, human herpesvirus 6 and others, to modulate receptor signaling. I also use the example of HIV and CMV to illustrate how two unrelated enveloped viruses use a similar SCHOOL mechanism to modulate the host immune response mediated by two functionally different receptors: T cell antigen receptor and natural killer cell receptor, NKp30. This suggests that it is very likely that similar general mechanisms can be or are used by other viral and possibly non-viral pathogens. Learning from viruses how to target cell surface receptors not only helps us understand viral strategies to escape from the host immune surveillance, but also provides novel avenues in rational drug design and the development of new therapies for immune disorders.

Memory-like responses of natural killer cells

Natural killer (NK) cells are lymphocytes with the capacity to produce cytokines and kill target cells upon activation. NK cells have long been categorized as members of the innate immune system and as such have been thought to follow the 'rules' of innate immunity, including the principle that they have no immunologic memory, a property thought to be strictly limited to adaptive immunity. However, recent studies have suggested that NK cells have the capacity to alter their behavior based on prior activation. This property is analogous to adaptive immune memory; however, some NK cell memory-like functions are not strictly antigen dependent and can be demonstrated following cytokine stimulation. Here, we discuss the recent evidence that NK cells can exhibit properties of immunologic memory, focusing on the ability of cytokines to non-specifically induce memory-like NK cells with enhanced responses to restimulation.

Memory-like responses of natural killer cells

Natural killer (NK) cells are lymphocytes with the capacity to produce cytokines and kill target cells upon activation. NK cells have long been categorized as members of the innate immune system and as such have been thought to follow the 'rules' of innate immunity, including the principle that they have no immunologic memory, a property thought to be strictly limited to adaptive immunity. However, recent studies have suggested that NK cells have the capacity to alter their behavior based on prior activation. This property is analogous to adaptive immune memory; however, some NK cell memory-like functions are not strictly antigen dependent and can be demonstrated following cytokine stimulation. Here, we discuss the recent evidence that NK cells can exhibit properties of immunologic memory, focusing on the ability of cytokines to non-specifically induce memory-like NK cells with enhanced responses to restimulation.

Memory-like responses of natural killer cells

Natural killer (NK) cells are lymphocytes with the capacity to produce cytokines and kill target cells upon activation. NK cells have long been categorized as members of the innate immune system and as such have been thought to follow the 'rules' of innate immunity, including the principle that they have no immunologic memory, a property thought to be strictly limited to adaptive immunity. However, recent studies have suggested that NK cells have the capacity to alter their behavior based on prior activation. This property is analogous to adaptive immune memory; however, some NK cell memory-like functions are not strictly antigen dependent and can be demonstrated following cytokine stimulation. Here, we discuss the recent evidence that NK cells can exhibit properties of immunologic memory, focusing on the ability of cytokines to non-specifically induce memory-like NK cells with enhanced responses to restimulation.

The SCHOOL of nature: II. Protein order, disorder and oligomericity in transmembrane signaling

Recent reports have revealed that many proteins that do not adopt globular structures under native conditions, thus termed intrinsically disordered proteins (IDPs), are involved in cell signaling. Intriguingly, physiologically relevant oligomerization of IDPs has been recently observed and shown to exhibit unique biophysical characteristics, including the lack of significant changes in chemical shift and peak intensity upon binding. In this work, I summarize several distinct features of protein disorder that are especially important as related to receptor-mediated transmembrane signal transduction. I also hypothesize that interactions of IDPs with their protein or lipid partners represent a general biphasic process with the "no disorder-to-order" fast interaction which, depending on the interacting partner, may or may not be accompanied by the slow formation of a secondary structure. Further, I suggest signaling-related functional connections between protein order, disorder, and oligomericity and hypothesize that receptor oligomerization induced or tuned upon ligand binding outside the cell is translated across the membrane into protein oligomerization inside the cell, thus providing a general platform, the Signaling Chain HOmoOLigomerization (SCHOOL) platform, for receptor-mediated signaling. This structures our current multidisciplinary knowledge and views of the mechanisms governing the coupling of recognition to signal transduction and cell response. Importantly, this approach not only reveals previously unrecognized striking similarities in the basic mechanistic principles of function of numerous functionally diverse and unrelated surface membrane receptors, but also suggests the similarity between therapeutic targets, thus opening new horizons for both fundamental and clinically relevant studies.

The SCHOOL of nature: I. Transmembrane signaling

Receptor-mediated transmembrane signaling plays an important role in health and disease. Recent significant advances in our understanding of the molecular mechanisms linking ligand binding to receptor activation revealed previously unrecognized striking similarities in the basic structural principles of function of numerous cell surface receptors. In this work, I demonstrate that the Signaling Chain Homooligomerization (SCHOOL)-based mechanism represents a general biological mechanism of transmembrane signal transduction mediated by a variety of functionally unrelated single- and multichain activating receptors. within the SCHOOL platform, ligand binding-induced receptor clustering is translated across the membrane into protein oligomerization in cytoplasmic milieu. This platform resolves a long-standing puzzle in transmembrane signal transduction and reveals the major driving forces coupling recognition and activation functions at the level of protein-protein interactions-biochemical processes that can be influenced and controlled. The basic principles of transmembrane signaling learned from the SCHOOL model can be used in different fields of immunology, virology, molecular and cell biology and others to describe, explain and predict various phenomena and processes mediated by a variety of functionally diverse and unrelated receptors. Beyond providing novel perspectives for fundamental research, the platform opens new avenues for drug discovery and development.

Innate immune responses in primary HIV-1 infection

PURPOSE OF REVIEW

Events occurring in acute HIV-1 infection are now recognized to be critical determinants of the subsequent disease course. Innate responses constitute the first line of defence against pathogens, and also play a key role in triggering the early adaptive response; as such, the innate responses activated in acute HIV-1 infection and their contribution to control of viral replication or disease pathogenesis are the focus of much current research. We review recent advances in this area.

RECENT FINDINGS

Dendritic cell subsets can play pleiotropic roles in acute HIV-1 infection, with in-vitro studies illustrating that HIV-dendritic cell interactions may have outcomes as diverse as virion destruction, virus dissemination, T-cell triggering or subversion of dendritic cell functions. Natural killer cells can be activated in acute HIV-1 infection, and mounting evidence suggests that they contribute to determining the ensuing course of disease; however, much remains to be learned about how they mediate their effects.

SUMMARY

The importance of innate responses as determinants of the outcome of HIV infection is increasingly evident, but more work is needed to understand how innate immunity can be harnessed to combat this infection.

NK cells from tuberculous pleurisy express high ICAM-1 levels and exert stimulatory effect on local T cells

Tuberculous pleurisy, one of the most common manifestations of extrapulmonary tuberculosis, is characterized by a T-cell-mediated hypersensitivity reaction along with a Th1 immune profile. In this study, we investigated functional cross-talk among T and NK cells in human tuberculous pleurisy. We found that endogenously activated pleural fluid-derived NK cells express high ICAM-1 levels and induce T-cell activation ex vivo through ICAM-1. Besides, upon in vitro stimulation with monokines and PAMP, resting peripheral blood NK cells increased ICAM-1 expression leading to cellular activation and Th1 polarization of autologous T cells. Furthermore, these effects were abolished by anti-ICAM-1 Ab. Hence, NK cells may contribute to the adaptive immune response by a direct cell-contact-dependent mechanism in the context of Mycobacterium tuberculosis infection.

FCRL6 distinguishes mature cytotoxic lymphocytes and is upregulated in patients with B-cell chronic lymphocytic leukemia

Fc receptor-like 6 (FCRL6), the most recently characterized member of the FCRL family, is a cell surface glycoprotein with tyrosine-based regulatory potential. An extensive survey of human hematopoietic tissues disclosed that FCRL6 expression by NK- and T-cell subpopulations increases as a function of differentiation and is remarkably restricted to mature lymphocytes with cytotoxic capability. In particular, FCRL6 distinguishes perforin-expressing CD56dim NK cells, Vdelta1+ and Vdelta2+ gammadelta T cells, effector and effector memory CD8+ T cells, and rare cytotoxic CD4+ T cells in adult tissues. Analysis of this receptor in B-cell chronic lymphocytic leukemia (CLL) was also performed. FCRL6 was found to mark significantly expanded populations of cytotoxic CD8+ T, CD4+ T, and NK cells in patients with CLL. Despite sequence homology with the known Fc receptors for IgG and IgE, FCRL6 did not bind Ig. Although FCRL6 can be tyrosine-phosphorylated, its antibody-mediated ligation was unable to influence cellular activation. Collectively, these results demonstrate that FCRL6 is a distinct indicator of cytotoxic effector lymphocytes that is upregulated in diseases characterized by chronic immune stimulation.

A subgroup of Tourette's patients overexpress specific natural killer cell genes in blood: a preliminary report

Gilles de la Tourette Syndrome (TS) is a heritable, neurodevelopmental disorder characterized by motor and vocal tics. As no single gene or region has emerged from standard linkage approaches, TS may result from several as-yet-unidentified genetic factors, and may also occur due to infection-triggered, autoimmune processes. Etiological or pathogenic differences might result in clinically indistinguishable TS subgroups. We have previously used whole genome human oligonucleotide microarrays in an attempt to identify patterns of gene expression in blood linked with TS. In this proof-of-principle study, we applied Principal Components Analysis to a previously collected set of 16 familial TS and 16 control blood samples to identify subgroups. Fourteen genes, primarily Natural Killer Cell (NK) genes, discriminated between TS and all controls. Granzyme B and NKG7 were confirmed using RT-PCR. Five probesets (four genes) reside in chromosomal regions previously linked to familial TS or obsessive-compulsive disorder. Using the 14 genes, a Principal Components Analysis as well as a cluster analysis identified a TS subgroup (n = 10/16) that overexpressed the NK genes. 7/10 subjects within this subgroup were diagnosed with attention-deficit hyperactivity disorder (ADHD), suggesting that this expression profile might be associated with TS and co-morbid ADHD. Principal Components Analysis of gene expression in blood may be useful for identifying subgroups of other complex neurodevelopmental diseases, and the gene expression profile identified in this study may provide a biomarker for at least one subgroup of heritable TS.

Mycobacterium tuberculosis-induced gamma interferon production by natural killer cells requires cross talk with antigen-presenting cells involving Toll-like receptors 2 and 4 and the mannose receptor in tuberculous pleurisy

Tuberculous pleurisy allows the study of human cells at the site of active Mycobacterium tuberculosis infection. In this study, we found that among pleural fluid (PF) lymphocytes, natural killer (NK) cells are a major source of early gamma interferon (IFN-gamma) upon M. tuberculosis stimulation, leading us to investigate the mechanisms and molecules involved in this process. We show that the whole bacterium is the best inducer of IFN-gamma, although a high-molecular-weight fraction of culture filtrate proteins from M. tuberculosis H37Rv and the whole-cell lysate also induce its expression. The mannose receptor seems to mediate the inhibitory effect of mannosylated lipoarabinomannan, and Toll-like receptor 2 and 4 agonists activate NK cells but do not induce IFN-gamma like M. tuberculosis does. Antigen-presenting cells (APC) and NK cells bind M. tuberculosis, and although interleukin-12 is required, it is not sufficient to induce IFN-gamma expression, indicating that NK cell-APC contact takes place. Indeed, major histocompatibility complex class I, adhesion, and costimulatory molecules as well as NK receptors regulate IFN-gamma induction. The signaling pathway is partially inhibited by dexamethasone and sensitive to Ca2+ flux and cyclosporine. Inhibition of p38 and extracellular-regulated kinase mitogen-activated protein kinase pathways reduces the number of IFN-gamma+ NK cells. Phosphorylated p38 (p-p38) is detected in ex vivo PF-NK cells, and M. tuberculosis triggers p-p38 in PF-NK cells at the same time that binding between NK and M. tuberculosis reaches its maximum value. Thus, interplay between M. tuberculosis and NK cells/APC triggering IFN-gamma would be expected to play a beneficial role in tuberculous pleurisy by helping to maintain a type 1 profile.

Mature natural killer cell and lymphoid tissue-inducing cell development requires Id2-mediated suppression of E protein activity

The Id2 transcriptional repressor is essential for development of natural killer (NK) cells, lymphoid tissue–inducing (LTi) cells, and secondary lymphoid tissues. Id2 was proposed to regulate NK and LTi lineage specification from multipotent progenitors through suppression of E proteins. We report that NK cell progenitors are not reduced in the bone marrow (BM) of Id2^−/− mice, demonstrating that Id2 is not essential for NK lineage specification. Rather, Id2 is required for development of mature (m) NK cells. We define the mechanism by which Id2 functions by showing that a reduction in E protein activity, through deletion of E2A, overcomes the need for Id2 in development of BM mNK cells, LTi cells, and secondary lymphoid tissues. However, mNK cells are not restored in the blood or spleen of Id2^−/−E2A^−/− mice, suggesting a role for Id2 in suppression of alternative E proteins after maturation. Interestingly, the few splenic mNK cells in Id2^−/− and Id2^−/−E2A^−/− mice have characteristics of thymus-derived NK cells, which develop in the absence of Id2, implying a differential requirement for Id2 in BM and thymic mNK development. Our findings redefine the essential functions of Id2 in lymphoid development and provide insight into the dynamic regulation of E and Id proteins during this process.

Identification, activation, and selective in vivo ablation of mouse NK cells via NKp46

Natural killer (NK) cells contribute to a variety of innate immune responses to viruses, tumors and allogeneic cells. However, our understanding of NK cell biology is severely limited by the lack of consensus phenotypic definition of these cells across species, by the lack of specific marker to visualize them in situ, and by the lack of a genetic model where NK cells may be selectively ablated. NKp46/CD335 is an Ig-like superfamily cell surface receptor involved in human NK cell activation. In addition to human, we show here that NKp46 is expressed by NK cells in all mouse strains analyzed, as well as in three common monkey species, prompting a unifying phenotypic definition of NK cells across species based on NKp46 cell surface expression. Mouse NKp46 triggers NK cell effector function and allows the detection of NK cells in situ. NKp46 expression parallels cell engagement into NK differentiation programs because it is detected on all NK cells from the immature CD122(+)NK1.1(+)DX5(-) stage and on a minute fraction of NK-like T cells, but not on CD1d-restricted NKT cells. Moreover, human NKp46 promoter drives NK cell selective expression both in vitro and in vivo. Using NKp46 promoter, we generated transgenic mice expressing EGFP and the diphtheria toxin (DT) receptor in NK cells. DT injection in these mice leads to a complete and selective NK cell ablation. This model paves a way for the in vivo characterization and preclinical assessment of NK cell biological function.

Distinctive NK-cell receptor repertoires sustain high-level constitutive NK-cell activation in HIV-exposed uninfected individuals

We have previously associated high natural killer (NK)–cell activity and protection against HIV-1 infection in Vietnamese exposed uninfected intravascular drug users (EUs). Considering that activating and inhibitory signals sensed by NK-cell receptors regulate NK-cell activation, we performed phenotypic and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) transcript analyses of the NK-cell receptor (NKR) repertoire in 25 EUs, 19 HIV+ intravenous drug users, and 26 uninfected blood donors. Although NK-cell activation was not linked to a unique NKR repertoire in EUs, various patterns consistent with NK-cell activation were detected in EUs: high KIR3DS1/KIR3DL1 ratio associated with down-regulated KIR3DL1 transcript levels, KIR2DL3+ low-affinity receptor expansion associated to group HLA-C1 ligand in 2DS2−/2DL2− EUs, enhanced NKG2C/NKG2A ratio, and increased CD69 expression. Remarkably, EUs exhibited high constitutive degranulation activity in the absence of exogenous stimulation, as shown by the CD107a assay. Furthermore, CD161 expression was increased within the CD107a+ NK-cell compartment. Our results suggest that in response to viral exposition, particular genetic or regulated features of the NKR repertoire of EUs contribute to their high constitutive NK-cell potential. This might allow NK cells to generate a more rapid and effective immune response to HIV-1, thereby contributing to prevention toward infection.

Generation of monoclonal antibodies to coelomocytes of the purple sea urchin Arbacia punctulata: characterization and phenotyping

Cellular cytotoxicity is a key component of animal innate immune responses that is one of the first lines of defense against invaders. There is increasing interest in the study of the cellular immune response, particularly non-specific cytotoxic cells and natural killer cells and their receptors. Studies of non-specific cytotoxic cell and natural killer cell recognition and killing (and the receptors involved) will reveal new and important insights into cellular mechanisms of host defense. Here we describe mAbs specific for coelomocyte sub-populations of the purple sea urchin, Arbacia punctulata, using highly purified coelomocyte populations as the antigen source. Monoclonal antibodies were selected using flow cytometric screening methods. Several of the mAbs were shown to bind to two sub-types of coelomocytes when assayed by fluorescence microscopy. Furthermore, these mAbs inhibited coelomocyte cytotoxicity against vertebrate target cells in a functional assay. The mAbs have been used in immunoprecipitation studies.

[NK cells and innate immunity to malaria]

Innate immune response against Plasmodium falciparum (Pf), a causative agent of human malaria, is the result of several thousand years of co-evolution between the parasite and his host. An early IFN-gamma production during infection is associated with a better evolution of the disease. Natural killer (NK) cells are among the first cells in peripheral blood to produce IFN-gamma in response to Pf-infected erythrocytes (Pf-E). NK cells are found in blood, in secondary lymphoid organs as well as in peripheral non-lymphoid tissues. They participate in host innate responses that occur upon viral and intracytoplasmic bacterial infections, but also during the course of tumor development and allogeneic transplantation. These lymphocytes are not only important players of innate effector responses, but also participate in the initiation and development of adaptive immune responses. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine IL-8, suggesting a role for NK cells in the recruitment and the activation of other cells during malaria infection. Several other cell subsets are involved in the innate immune response to Pf. Dendritic cells, macrophages, gamma delta T cells, NKT cells are able to sense the presence of the parasite. Along this line, the presence of IL-12 is necessary to NK cell IFN-gamma production and a functional cooperation takes place between macrophages and NK cells in the context of this parasitic infection. In particular, IL-18 produced by macrophages is a key factor for this NK response. However, the molecular basis of Pf-E recognition by NK cells as well as the functional role of NK cell responses during the course of the disease remain to be adressed.

An improved RT-PCR method for the detection of killer-cell immunoglobulin-like receptor (KIR) transcripts

Killer cell immunoglobulin-like receptors (KIRs) are expressed on human natural killer (NK) cells and a proportion of T cells. As the specificity of these NK and T cells is, at least in part, determined by the combination of KIRs they express, it is important to be able to determine the KIR expression pattern of NK and T cell clones to understand their function. However, for most KIR genes, specific reagents to detect expression are currently either unavailable or sensitive to allelic variations. In this study, a reverse transcriptase-polymerase chain reaction (RT-PCR) that uses new primer sets for the gene-specific detection of KIR transcripts is presented and validated. The key advantage of this RT-PCR method over previously published ones is that it was designed to detect transcripts of all confirmed allelic variants of the KIR genes, while remaining gene-specific.

What is natural in natural killer cells?

Denomination is a not a trivial issue in human activities in general, and in science in particular. Natural killer (NK) cells have been originally characterized as cytolytic effector lymphocytes present in mouse spleen and human peripheral blood. The "NK" name was originally coined after the NK cell's ability to kill targets cells without requirement for a prior exposure to these targets, in contrast to cytolytic T cells (Kiessling, R., Klein, E., Wigzell, H. 1975. "Natural" killer cells in the mouse. I. Cytotoxic cells with specificity for mouse Moloney leukemia cells. Specificity and distribution according to genotype. Eur J Immunol 5:112-117). In this review, an attempt is made to revisit the basis for this vastly admitted definition of NK cells, and to enumerate some of the unsolved questions related to NK cell biology. An emphasis will be given to recent data showing that circulating NK cells are not steady state killers unless they have gone through a process of functional maturation. This process that is referred as to "NK cell education" thereafter involves self MHC class I recognition via inhibitory receptors, but also some still unidentified factors. Therefore, the NK denomination does not reflect a genuine property of these lymphocytes that appear to more and more resemble T and B lymphocytes.