Association of DAP12 with activating CD94/NKG2C NK cell receptors

Chimeric antigen receptor (CAR)-modified natural killer cell-based immunotherapy and immunological synapse formation in cancer and HIV

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells contribute to the body’s immune defenses. Current chimeric antigen receptor (CAR)-modified T cell immunotherapy shows strong promise for treating various cancers and infectious diseases. Although CAR-modified NK cell immunotherapy is rapidly gaining attention, its clinical applications are mainly focused on preclinical investigations using the NK92 cell line. Despite recent advances in CAR-modified T cell immunotherapy, cost and severe toxicity have hindered its widespread use. To alleviate these disadvantages of CAR-modified T cell immunotherapy, additional cytotoxic cell-mediated immunotherapies are urgently needed. The unique biology of NK cells allows them to serve as a safe, effective, alternative immunotherapeutic strategy to CAR-modified T cells in the clinic. While the fundamental mechanisms underlying the cytotoxicity and side effects of CAR-modified T and NK cell immunotherapies remain poorly understood, the formation of the immunological synapse (IS) between CAR-modified T or NK cells and their susceptible target cells is known to be essential. The role of the IS in CAR T and NK cell immunotherapies will allow scientists to harness the power of CAR-modified T and NK cells to treat cancer and infectious diseases. In this review, we highlight the potential applications of CAR-modified NK cells to treat cancer and human immunodeficiency virus (HIV), and discuss the challenges and possible future directions of CAR-modified NK cell immunotherapy, as well as the importance of understanding the molecular mechanisms of CAR-modified T cell- or NK cell-mediated cytotoxicity and side effects, with a focus on the CAR-modified NK cell IS.

Uterine Natural Killer Cells: Functional Distinctions and Influence on Pregnancy in Humans and Mice

Our understanding of development and function of natural killer (NK) cells has progressed significantly in recent years. However, exactly how uterine NK (uNK) cells develop and function is still unclear. To help investigators that are beginning to study tissue NK cells, we summarize in this review our current knowledge of the development and function of uNK cells, and what is yet to be elucidated. We compare and contrast the biology of human and mouse uNK cells in the broader context of the biology of innate lymphoid cells and with reference to peripheral NK cells. We also review how uNK cells may regulate trophoblast invasion and uterine spiral arterial remodeling in human and murine pregnancy.

A Mature NK Profile at the Time of HIV Primary Infection Is Associated with an Early Response to cART

Natural killer (NK) cells are major effectors of the innate immune response. Despite an overall defect in their function associated with chronic human immunodeficiency virus (HIV) infection, their role in primary HIV infection is poorly understood. We investigated the modifications of the NK cell compartment in patients from the ANRS-147-Optiprim trial, a study designed to examine the benefits of intensive combination antiretroviral therapy (cART) in patients with acute or early primary HIV infection. Multiparametric flow cytometry combined with bioinformatics analyses identified the NK phenotypes in blood samples from 30 primary HIV-infected patients collected at inclusion and after 3 months of cART. NK phenotypes were revealed by co-expression of CD56/CD16/NKG2A/NKG2C and CD57, five markers known to delineate stages of NK maturation. Three groups of patients were formed according to their distributions of the 12 NK cell phenotypes identified. Their virological and immunological characteristics were compared along with the early outcome of cART. At inclusion, HIV-infected individuals could be grouped into those with predominantly immature/early differentiated NK cells and those with predominantly mature NK cells. Several virological and immunological markers were improved in patients with mature NK profiles, including lower HIV viral loads, lower immune activation markers on NK and dendritic cell (DC), lower levels of plasma IL-6 and IP-10, and a trend to normal DC counts. Whereas all patients showed a decrease of viremia higher than 3 log₁₀ copies/ml after 3 months of treatment, patients with a mature NK profile at inclusion reached this threshold more rapidly than patients with an immature NK profile (70 vs. 38%). In conclusion, a better early response to cART is observed in patients whose NK profile is skewed to maturation at inclusion. Whether the mature NK cells contributed directly or indirectly to HIV control through a better immune environment under cART is unknown. The NK maturation status of primary infected patients should be considered as a relevant marker of an immune process contributing to the early outcome of cART that could help in the management of HIV-infected patients.

About Training and Memory: NK-Cell Adaptation to Viral Infections

Viral infections continuously challenge and shape our immune system. Due to their fine antigen recognition ability, adaptive lymphocytes protect against pathogen reencounter by generating specific immunological memory. Innate cells such as macrophages also adapt to pathogen challenge and mount resistance to reinfection, a phenomenon termed trained immunity. As part of the innate immunity, natural killer (NK) cells can display rapid effector functions and play a crucial role in the control of viral infections, especially by the β-herpesvirus cytomegalovirus (CMV). CMV activates the NK-cell pool by inducing proinflammatory signals, which prime NK cells, paralleling macrophage training. In addition, CMV dramatically shapes the NK-cell repertoire due to its ability to trigger specific NK cell-activating receptors, and enables the expansion and persistence of a specific NK-cell subset displaying adaptive and memory features. In this chapter, we will discuss how different signals during CMV infection contribute to NK-cell training and acquisition of classical memory properties and how these events can impact on reinfection and cross-resistance.

Vps35-dependent recycling of Trem2 regulates microglial function

Triggering receptor expressed on myeloid cells 2 (Trem2), an immune-modulatory receptor, is preferentially expressed in microglia of central nervous system. Trem2 might be involved in the development of Alzheimer's disease (AD) through regulating the inflammatory responses and phagocytosis of microglia. However, the intracellular trafficking of Trem2 remains unclear. In this study, we showed that Trem2 in the plasma membrane underwent endocytosis and recycling. Trem2 is internalized in a clathrin-dependent manner and then recycled back to the plasma membrane through vacuolar protein sorting 35 (Vps35), the key component of cargo recognition core of retromer complex, but not Rab11. When Vps35 is knocked down, Trem2 accumulated in the lysosomes but was not degraded. More importantly, Vps35 deficiency leads to excessive lipopolysaccharide (LPS)-induced inducible nitric oxide synthase (iNOS) expression and IL-6 production, which can be abolished by Trem2 overexpression. Furthermore, R47H Trem2, an AD-associated mutant, failed to interact with Vps35 and became unstable compared with wild-type Trem2. Our study suggests that Vps35/retromer is responsible for recycling of Trem2 in the regulation of microglial function such as proinflammatory responses, whereas R47H mutation impairs Trem2 trafficking, which might contribute to AD.

A bird's eye view of NK cell receptor interactions with their MHC class I ligands

The surveillance of target cells by natural killer (NK) cells utilizes an ensemble of inhibitory and activating receptors, many of which interact with major histocompatibility complex (MHC) class I molecules. NK cell recognition of MHC class I proteins is important developmentally for the acquisition of full NK cell effector capacity and during target cell recognition, where the engagement of inhibitory receptors and MHC class I molecules attenuates NK cell activation. Human NK cells have evolved two broad strategies for recognition of human leukocyte antigen (HLA) class I molecules: (i) direct recognition of polymorphic classical HLA class I proteins by diverse receptor families such as the killer cell immunoglobulin-like receptors (KIRs), and (ii) indirect recognition of conserved sets of HLA class I-derived peptides displayed on the non-classical HLA-E for recognition by CD94-NKG2 receptors. In this review, we assess the structural basis for the interaction between these NK receptors and their HLA class I ligands and, using the suite of published KIR and CD94-NKG2 ternary complexes, highlight the features that allow NK cells to orchestrate the recognition of a range of different HLA class I proteins.

Cytomegalovirus-Infected Primary Endothelial Cells Trigger NKG2C+ Natural Killer Cells

Among innate cells, natural killer (NK) cells play a crucial role in the defense against cytomegalovirus (CMV). In some individuals, CMV infection induces the expansion of NKG2C+ NK cells that persist after control of the infection. We have previously shown that KIR2DL+ NK cells, in contrast to NKG2C+ NK cells, contribute to controlling CMV infection using a CMV-infected monocyte-derived dendritic cell (MDDC) model. However, the nature of CMV-infected cells contributing to the expansion of the NKG2C+ NK cell subset remains unclear. To gain more insight into this question, we investigated the contribution of NKG2C+ NK cell activation by CMV-infected primary human aortic endothelial cells (EC) isolated from kidney transplant donors, which constitutively express the human leukocyte antigen (HLA)-E molecule. Here, we show that, although classic HLA class I expression was drastically downregulated, nonclassic HLA-E expression was maintained in CMV-infected EC. By comparing HLA expression patterns in CMV-infected EC, fibroblasts and MDDC, we demonstrate a cell-dependent modulation of HLA-E expression by CMV infection. NKG2C+ NK cell degranulation was significantly triggered by CMV-infected EC regardless of the nature of the HLA-E allele product. EC, predominantly present in vessels, may constitute a privileged site for CMV infection that drives a 'memory' NKG2C+ NK cell subset.

Critical Role of CD2 Co-stimulation in Adaptive Natural Killer Cell Responses Revealed in NKG2C-Deficient Humans

Infection by human cytomegalovirus (HCMV) leads to NKG2C-driven expansion of adaptive natural killer (NK) cells, contributing to host defense. However, approximately 4% of all humans carry a homozygous deletion of the gene that encodes NKG2C (NKG2C^−/−). Assessment of NK cell repertoires in 60 NKG2C^−/− donors revealed a broad range of NK cell populations displaying characteristic footprints of adaptive NK cells, including a terminally differentiated phenotype, functional reprogramming, and epigenetic remodeling of the interferon (IFN)-γ promoter. We found that both NKG2C⁻ and NKG2C⁺ adaptive NK cells expressed high levels of CD2, which synergistically enhanced ERK and S6RP phosphorylation following CD16 ligation. Notably, CD2 co-stimulation was critical for the ability of adaptive NK cells to respond to antibody-coated target cells. These results reveal an unexpected redundancy in the human NK cell response to HCMV and suggest that CD2 provides “signal 2” in antibody-driven adaptive NK cell responses.

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NKG2C^−/− donors have normal T cell immunity to cytomegalovirus

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NKG2C^−/− donors have normal frequencies of adaptive NK cells

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CD2 is critical for antibody-triggered responses by adaptive NK cells

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CD2 synergizes with NKG2C in classical adaptive NK cells

Microenvironmental stresses induce HLA-E/Qa-1 surface expression and thereby reduce CD8(+) T-cell recognition of stressed cells

Hypoxia and glucose deprivation are often observed in the microenvironment surrounding solid tumors in vivo. However, how they interfere with MHC class I antigen processing and CD8(+) T-cell responses remains unclear. In this study, we analyzed the production of antigenic peptides presented by classical MHC class I in mice, and showed that it is quantitatively decreased in the cells exposed to either hypoxia or glucose deprivation. In addition, we unexpectedly found increased surface expression of HLA-E in human and Qa-1 in mouse tumor cells exposed to combined oxygen and glucose deprivation. The induced Qa-1 on the stressed tumor model interacted with an inhibitory NKG2/CD94 receptor on activated CD8(+) T cells and attenuated their specific response to the antigen. Our results thus suggest that microenvironmental stresses modulate not only classical but also nonclassical MHC class I presentation, and confer the stressed cells the capability to escape from the CD8(+) T-cell recognition.

The role of NK cells in HIV-1 protection: autologous, allogeneic or both?

Natural killer (NK) cells specialize in killing virally infected- or tumor cells and are part of the innate immune system. The activational state of NK cells is determined by the balance of incoming activating and inhibitory signals mediated by receptor-ligand binding with the target cell. These receptor-ligand bonds mainly consist of the killer immunoglobulin-like receptors (KIR), which are expressed at the cell surface of NK cells, and their ligands: the highly variable human leukocyte antigen -class I molecules (HLA). Absence of an inhibitory receptor-ligand bond lowers the NK cell activation threshold, whereas an activating receptor-ligand bond stimulates the cell, potentially overcoming this threshold and triggering NK cell activation. NK cells influence the course of infection as well as the acquisition of HIV-1. Several lines of evidence relate the activating NK cell receptor KIR3DS1, in the presence or absence of its putative ligand HLA-Bw4, with slower disease progression as well as resistance to HIV-1 infection. Overall, resistance to HIV-1 infection predominantly correlates with activating KIR/HLA profiles, consisting of e.g. activating KIRs, group B haplotypes, or inhibitory KIRs in absence of their ligands. Such a conclusion is less evident for studies of HIV-1 disease progression, with studies reporting beneficial as well as detrimental effects of activating KIR/HLA genotypes. It is likely that KIR/HLA association studies are complicated by the complexity of the KIR and HLA loci and their mutual interactions, as well as by additional factors like route of HIV exposure, immune activation, presence of co-infections, and the effect of anti-HIV-1 antibodies. One newly discovered NK cell activation pathway associated with resistance to HIV-1 infection involves the presence of an iKIR/HLA mismatch between partners. The absence of such an iKIR/HLA bond renders donor-derived allogeneic HIV-1 infected cells vulnerable to NK cell responses during HIV-1 transmission. Therefore, theoretically, HIV-1 would be eliminated before it has the chance to infect the autologous cells in the recipient. While this “alloreactive” NK cell mechanism is especially relevant to HIV transmission in monogamous couples, it would be interesting to investigate how it could influence resistance to HIV in other settings. The objective of this review is to summarize the knowledge about these autologous and alloreactive NK cell responses with regard to HIV-1 outcome.

Biochemical and Functional Insights into the Integrated Regulation of Innate Immune Cell Responses by Teleost Leukocyte Immune-Type Receptors

Across vertebrates, innate immunity consists of a complex assortment of highly specialized cells capable of unleashing potent effector responses designed to destroy or mitigate foreign pathogens. The execution of various innate cellular behaviors such as phagocytosis, degranulation, or cell-mediated cytotoxicity are functionally indistinguishable when being performed by immune cells isolated from humans or teleost fishes; vertebrates that diverged from one another more than 450 million years ago. This suggests that vital components of the vertebrate innate defense machinery are conserved and investigating such processes in a range of model systems provides an important opportunity to identify fundamental features of vertebrate immunity. One characteristic that is highly conserved across vertebrate systems is that cellular immune responses are dependent on specialized immunoregulatory receptors that sense environmental stimuli and initiate intracellular cascades that can elicit appropriate effector responses. A wide variety of immunoregulatory receptor families have been extensively studied in mammals, and many have been identified as cell- and function-specific regulators of a range of innate responses. Although much less is known in fish, the growing database of genomic information has recently allowed for the identification of several immunoregulatory receptor gene families in teleosts. Many of these putative immunoregulatory receptors have yet to be assigned any specific role(s), and much of what is known has been based solely on structural and/or phylogenetic relationships with mammalian receptor families. As an attempt to address some of these shortcomings, this review will focus on our growing understanding of the functional roles played by specific members of the channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs), which appear to be important regulators of several innate cellular responses via classical as well as unique biochemical signaling networks.

Immune Adaptation to Environmental Influence: The Case of NK Cells and HCMV

The immune system of an individual human is determined by heritable traits and a continuous process of adaptation to a broad variety of extrinsic, non-heritable factors such as viruses, bacteria, dietary components and more. Cytomegalovirus (CMV) successfully infects the majority of the human population and establishes latency, thereby exerting a life-long influence on the immune system of its host. CMV has been shown to influence the majority of immune parameters in healthy individuals. Here we focus on adaptive changes induced by CMV in subsets of Natural Killer (NK) cells, changes that question our very definition of adaptive and innate immunity by suggesting that adaptations of immune cells to environmental influences occur across the entire human immune system and not restricted to the classical adaptive branch of the immune system.

Natural killer cell memory in infection, inflammation and cancer

Immunological memory can be defined as a quantitatively and qualitatively enhanced immune response upon rechallenge. For natural killer (NK) cells, two main types of memory exist. First, similarly to T cells and B cells, NK cells can exert immunological memory after encounters with stimuli such as haptens or viruses, resulting in the generation of antigen-specific memory NK cells. Second, NK cells can remember inflammatory cytokine milieus that imprint long-lasting non-antigen-specific NK cell effector function. The basic concepts derived from studying NK cell memory provide new insights about innate immunity and could lead to novel strategies to improve treatments for infectious diseases and cancer.

Harnessing adaptive natural killer cells in cancer immunotherapy

Natural killer (NK) cells are innate lymphocytes with a refined ability to recognize transformed cells through a broad array of activating receptors in combination with stochastically expressed inhibitory receptors that recognize MHC-class I. Recent advances in NK cell biology have revealed a high degree of functional plasticity that can be attributed to dynamic cell-to-cell interactions in concert with transcriptional and epigenetic reprogramming. Here, we discuss how new insights into the adaptive behavior of NK cells pave the way for next generation cell therapy based on guided differentiation and selective expansion of particularly cytotoxic NK cell subsets.

Functional anti-CD94/NKG2A and anti-CD94/NKG2C autoantibodies in patients with systemic lupus erythematosus

OBJECTIVE

Recently we serendipitously identified a patient with systemic lupus erythematosus (SLE) who was positive for autoantibodies to CD94/natural killer receptor group 2A (NKG2A). The present study was undertaken to investigate the occurrence and function of autoantibodies targeting lectin-like NK cell receptors in SLE.

METHODS

Sera from 203 SLE patients and 90 healthy individuals were analyzed, by flow cytometry, for Ig binding to Ba/F3 cells transfected with CD94/NKG2A, CD94/NKG2C, or NKG2D. Autoantibodies identified were characterized with regard to interference with HLA-E binding, effect on NK cell activation in response to HLA-E-transfected K562 cells, and capacity to facilitate antibody-dependent cell-mediated cytotoxicity (ADCC). Levels of autoantibodies were determined in longitudinally sampled sera, and correlations with disease activity (SLE Disease Activity Index 2000) and severity (Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index) were investigated.

RESULTS

Anti-CD94/NKG2A autoantibodies were identified in 7 SLE patients. The autoantibodies from 6 patients inhibited binding of HLA-E to CD94/NKG2A, whereas those from the seventh patient augmented this binding. Autoantibodies from 2 patients also reacted with the activating receptor CD94/NKG2C, with inhibition of the binding of HLA-E to CD94/NKG2C observed in 1 case and enhancement of this binding in the other. None of the sera contained anti-NKG2D autoantibodies. The levels of anti-CD94/NKG2A and anti-CD94/NKG2C autoantibodies correlated with disease activity and with a more severe SLE phenotype. Mechanistically, anti-CD94/NKG2A and anti-CD94/NKG2C autoantibodies both interfered with HLA-E-mediated regulation of NK cell activation and facilitated the elimination of target cells expressing CD94/NKG2A or CD94/NKG2C through ADCC.

CONCLUSION

Anti-CD94/NKG2A and anti-CD94/NKG2C autoantibodies occur in a subset of patients with clinically active SLE. Given their capacity to deplete certain NK cell subsets and interfere with particular NK cell function, such autoantibodies may promote the pathogenesis of SLE.

Transmembrane Complexes of DAP12 Crystallized in Lipid Membranes Provide Insights into Control of Oligomerization in Immunoreceptor Assembly

The membrane-spanning α helices of single-pass receptors play crucial roles in stabilizing oligomeric structures and transducing biochemical signals across the membrane. Probing intermolecular transmembrane interactions in single-pass receptors presents unique challenges, reflected in a gross underrepresentation of their membrane-embedded domains in structural databases. Here, we present two high-resolution structures of transmembrane assemblies from a eukaryotic single-pass protein crystallized in a lipidic membrane environment. Trimeric and tetrameric structures of the immunoreceptor signaling module DAP12, determined to 1.77-Å and 2.14-Å resolution, respectively, are organized by the same polar surfaces that govern intramembrane assembly with client receptors. We demonstrate that, in addition to the well-studied dimeric form, these trimeric and tetrameric structures are made in cells, and their formation is competitive with receptor association in the ER. The polar transmembrane sequences therefore act as primary determinants of oligomerization specificity through interplay between charge shielding and sequestration of polar surfaces within helix interfaces.

Hair regrowth in alopecia areata patients following Stem Cell Educator therapy

BACKGROUND

Alopecia areata (AA) is one of the most common autoimmune diseases and targets the hair follicles, with high impact on the quality of life and self-esteem of patients due to hair loss. Clinical management and outcomes are challenged by current limited immunosuppressive and immunomodulating regimens.

METHODS

We have developed a Stem Cell Educator therapy in which a patient's blood is circulated through a closed-loop system that separates mononuclear cells from the whole blood, allows the cells to briefly interact with adherent human cord blood-derived multipotent stem cells (CB-SC), and returns the "educated" autologous cells to the patient's circulation. In an open-label, phase 1/phase 2 study, patients (N = 9) with severe AA received one treatment with the Stem Cell Educator therapy. The median age was 20 years (median alopecic duration, 5 years).

RESULTS

Clinical data demonstrated that patients with severe AA achieved improved hair regrowth and quality of life after receiving Stem Cell Educator therapy. Flow cytometry revealed the up-regulation of Th2 cytokines and restoration of balancing Th1/Th2/Th3 cytokine production in the peripheral blood of AA subjects. Immunohistochemistry indicated the formation of a "ring of transforming growth factor beta 1 (TGF-β1)" around the hair follicles, leading to the restoration of immune privilege of hair follicles and the protection of newly generated hair follicles against autoimmune destruction. Mechanistic studies revealed that co-culture with CB-SC may up-regulate the expression of coinhibitory molecules B and T lymphocyte attenuator (BTLA) and programmed death-1 receptor (PD-1) on CD8β(+)NKG2D(+) effector T cells and suppress their proliferation via herpesvirus entry mediator (HVEM) ligands and programmed death-1 ligand (PD-L1) on CB-SCs.

CONCLUSIONS

Current clinical data demonstrated the safety and efficacy of the Stem Cell Educator therapy for the treatment of AA. This innovative approach produced lasting improvement in hair regrowth in subjects with moderate or severe AA.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT01673789, 21 August 2012.

Balancing natural killer cell activation through paired receptors

Natural killer (NK) cells are innate lymphocytes that are crucial for the control of infections and malignancies. NK cells express a variety of inhibitory and activating receptors that facilitate fine discrimination between damaged and healthy cells. Among them, a family of molecules that bind nectin and nectin-like proteins has recently emerged and has been shown to function as an important regulator of NK cell functions. These molecules include CD226, T cell immunoreceptor with immunoglobulin and ITIM domains (TIGIT), CD96, and cytotoxic and regulatory T cell molecule (CRTAM). In this Review, we focus on the recent advances in our understanding of how these receptors regulate NK cell biology and of their roles in pathologies such as cancer, infection and autoimmunity.

DAP12-based activating chimeric antigen receptor for NK cell tumor immunotherapy

NK cells are emerging as new effectors for immunotherapy of cancer. In particular, the genetic engraftment of chimeric Ag receptors (CARs) in NK cells is a promising strategy to redirect NK cells to otherwise NK cell-resistant tumor cells. On the basis of DNAX-activation protein 12 (DAP12), a signaling adaptor molecule involved in signal transduction of activating NK cell receptors, we generated a new type of CAR targeting the prostate stem cell Ag (PSCA). We demonstrate in this article that this CAR, designated anti-PSCA-DAP12, consisting of DAP12 fused to the anti-PSCA single-chain Ab fragment scFv(AM1) confers improved cytotoxicity to the NK cell line YTS against PSCA-positive tumor cells when compared with a CAR containing the CD3ζ signaling chain. Further analyses revealed phosphorylation of the DAP12-associated ZAP-70 kinase and IFN-γ release of CAR-engineered cells after contact with PSCA-positive target cells. YTS cells modified with DAP12 alone or with a CAR bearing a phosphorylation-defective ITAM were not activated. Notably, infused YTS cells armed with anti-PSCA-DAP12 caused delayed tumor xenograft growth and resulted in complete tumor eradication in a significant fraction of treated mice. The feasibility of the DAP12-based CAR was further tested in human primary NK cells and confers specific cytotoxicity against KIR/HLA-matched PSCA-positive tumor cells, which was further enhanced by KIR-HLA mismatches. We conclude that NK cells engineered with DAP12-based CARs are a promising tool for adoptive tumor immunotherapy.

NKG2C copy number variations in five distinct populations in mainland China and susceptibility to nasopharyngeal carcinoma (NPC)

In this study, copy number variation (CNV) of NKG2C gene was investigated in 1129 normal, unrelated individuals representing two southern Chinese Han populations (Hunan Han and Guangdong Han), two northern Chinese populations (Inner Mongolia Han and Inner Mongolia Mongol) and one southeastern Chinese Han population (Fujian Han) using polymerase chain reaction-sequence-specific priming (PCR-SSP) method. CNV of NKG2C gene did not vary significantly among the five Chinese populations, with NKG2C gene deletion showing a frequency ranging from 0.2031 to 0.2688. Compared with worldwide ethnic groups, very significant difference was observed between the five Chinese populations and the Mexican mestizos (all Pcorrected=0.0025), and between the Fujian Han population and the German population (Pcorrected=0.005). We further examined CNV of NKG2C and HLA-E allelic distribution in 653 patients afflicted with nasopharyngeal carcinoma (NPC) in Hunan province. Neither CNV of NKG2C nor HLA-E was associated with NPC. There was a trend of reduced NPC risk in individuals who were homozygous for both HLA-E(∗)01:03 and NKG2C deletion (0.46% vs. 2.51%, P=0.0076, Pcorrected=0.0684, OR (95% CI)=0.1794 (0.0473-0.6809)). Taken together, our results suggest that NKG2C deletion and HLA-E signalling pathway does not play a major role in determining genetic susceptibility to NPC.

DNAX-activating protein of 12 kDa impairs host defense in pneumococcal pneumonia

OBJECTIVES

Streptococcus pneumoniae is the most common causative organism in community-acquired pneumonia responsible for millions of deaths every year. DNAX-activating protein of 12 kDa is an adaptor molecule for different myeloid expressed receptors involved in innate immunity.

DESIGN

Animal study.

SETTING

University research laboratory.

SUBJECTS

DNAX-activating protein of 12 kDa-deficient (dap12) and wild-type mice.

INTERVENTIONS

Mice were intranasally infected with S. pneumoniae. In addition, ex vivo responsiveness of alveolar macrophages was examined.

MEASUREMENTS AND MAIN RESULTS

dap12 alveolar macrophages released more tumor necrosis factor-α upon stimulation with S. pneumoniae and displayed increased phagocytosis of this pathogen compared with wild-type cells. After infection with S. pneumoniae via the airways, dap12 mice demonstrated reduced bacterial outgrowth in the lungs together with delayed dissemination to distant body sites relative to wild-type mice. This favorable response in dap12 mice was accompanied by reduced lung inflammation and an improved survival.

CONCLUSIONS

These data suggest that DNAX-activating protein of 12 kDa impairs host defense during pneumococcal pneumonia at the primary site of infection at least in part by inhibiting phagocytosis by alveolar macrophages.

Mammalian non-classical major histocompatibility complex I and its receptors: Important contexts of gene, evolution, and immunity

The evolutionary conserved, less-polymorphic, nonclassical major histocompatibility complex (MHC) class I molecules: Qa-1 and its human homologue human leukocyte antigen-E (HLA-E) along with HLA-F, G and H cross-talk with the T-cell receptors and also interact with natural killer T-cells and other lymphocytes. Moreover, these nonclassical MHC molecules are known to interact with CD94/NKG2 heterodimeric receptors to induce immune responses and immune regulations. This dual role of Qa-1/HLA-E in terms of innate and adaptive immunity makes them more interesting. This review highlights the new updates of the mammalian nonclassical MHC-I molecules in terms of their gene organization, evolutionary perspective and their role in immunity.

What happens to microglial TREM2 in Alzheimer's disease: Immunoregulatory turned into immunopathogenic?

Microglia play major roles in initiation, coordination and execution of innate immunity in the brain. In the adult brain, these include maintenance of homeostasis, neuron and tissue repair, and eliminating infectious agents, apoptotic cells, and misfolded proteins. Some of these activities are accompanied by inflammatory reactions; and others are performed with no inflammatory effects. Under normal conditions, triggering receptor expressed on myeloid cells 2 (TREM2) belongs to the second category. It pairs with the adaptor protein DNAX-activating protein of 12kDa (DAP12) to induce phagocytosis of apoptotic neurons without inflammatory responses, and to regulate Toll-like receptor-mediated inflammatory responses, and microglial activation. Although ligands for TREM2 are largely unknown, the mitochondrial heat shock protein 60, expressed on cell surface of apoptotic neurons, is a specific ligand that activates TREM2-mediated phagocytosis by microglia. TREM2 also phagocytoses amyloid beta peptide in cultured cells. Several TREM2 mutations have been identified recently that increase the risk of Alzheimer's disease, Frontotemporal dementia, Parkinson's disease, and amyotrophic lateral sclerosis. Some of these mutations cause impaired proteolysis of full-length TREM2 at the plasma membrane to different degrees. The defects in the intramembrane cleavage result in dysfunction of phagocytosis signaling. The association of TREM2 mutations with neurodegenerative disease also calls for the understanding of the biology and pathological role of non-mutated TREM2 on human brains and microglia. This review provides a summary of current literature in TREM2 and DAP12 from several aspects, and proposes a theory that loss of TREM2 functions might contribute to the immunopathogenic role of microglia in Alzheimer's disease.

Molecular dynamic simulation of the self-assembly of DAP12-NKG2C activating immunoreceptor complex

The DAP12-NKG2C activating immunoreceptor complex is one of the multisubunit transmembrane protein complexes in which ligand-binding receptor chains assemble with dimeric signal-transducing modules through non-covalent associations in their transmembrane (TM) domains. In this work, both coarse grained and atomistic molecular dynamic simulation methods were applied to investigate the self-assembly dynamics of the transmembrane domains of the DAP12-NKG2C activating immunoreceptor complex. Through simulating the dynamics of DAP12-NKG2C TM heterotrimer and point mutations, we demonstrated that a five-polar-residue motif including: 2 Asps and 2 Thrs in DAP12 dimer, as well as 1 Lys in NKG2C TM plays an important role in the assembly structure of the DAP12-NKG2C TM heterotrimer. Furthermore, we provided clear evidences to exclude the possibility that another NKG2C could stably associate with the DAP12-NKG2C heterotrimer. Based on the simulation results, we proposed a revised model for the self-assembly of DAP12-NKG2C activating immunoreceptor complex, along with a plausible explanation for the association of only one NKG2C with a DAP12 dimer.

Human NKG2E is expressed and forms an intracytoplasmic complex with CD94 and DAP12

The NKG2 family of NK receptors includes activating and inhibitory members. With the exception of the homodimer-forming NKG2D, NKG2 receptors recognize the nonclassical MHC class I molecule HLA-E, and they can be subdivided into two groups: those that associate with and signal through DAP12 to activate cells, and those that contain an ITIM motif to promote inhibition. The function of NKG2 family member NKG2E is unclear in humans, and its surface expression has never been conclusively established, largely because there is no Ab that binds specifically to NKG2E. Seeking to determine a role for this molecule, we chose to investigate its expression and ability to form complexes with intracellular signaling molecules. We found that NKG2E was capable of associating with CD94 and DAP12 but that the complex was retained intracellularly at the endoplasmic reticulum instead of being expressed on cell surfaces, and that this localization was dependent on a sequence of hydrophobic amino acids in the extracellular domain of NKG2E. Because this particular sequence has emerged and been conserved selectively among higher order primates evolutionarily, this observation raises the intriguing possibility that NKG2E may function as an intracellular protein.

NKG2C zygosity influences CD94/NKG2C receptor function and the NK-cell compartment redistribution in response to human cytomegalovirus

Human cytomegalovirus (HCMV) infection promotes a persistent expansion of a functionally competent NK-cell subset expressing the activating CD94/NKG2C receptor. Factors underlying the wide variability of this effect observed in HCMV-seropositive healthy individuals and exacerbated in immunocompromized patients are uncertain. A deletion of the NKG2C gene has been reported, and an apparent relation of NKG2C genotype with circulating NKG2C(+) NK-cell numbers was observed in HCMV(+) children. We have assessed the influence of NKG2C gene dose on the NK-cell repertoire in a cohort of young healthy adults (N = 130, median age 19 years). Our results revealed a relation of NKG2C copy number with surface receptor levels and with NKG2C(+) NK-cell numbers in HCMV(+) subjects, independently of HLA-E dimorphism. Functional studies showed quantitative differences in signaling (i.e. iCa(2+) influx), degranulation, and IL-15-dependent proliferation, in response to NKG2C engagement, between NK cells from NKG2C(+/+) and hemizygous subjects. These observations provide a mechanistic interpretation on the way the NKG2C genotype influences steady-state NKG2C(+) NK-cell numbers, further supporting an active involvement of the receptor in the HCMV-induced reconfiguration of the NK-cell compartment. The putative implications of NKG2C zygosity over viral control and other clinical variables deserve attention.

Low gene expression levels of activating receptors of natural killer cells (NKG2E and CD94) in patients with fulminant type 1 diabetes

Fulminant type 1 diabetes is an independent subtype of type 1 diabetes characterized by extremely rapid onset and absence of islet-related autoantibodies. However, detailed pathophysiology of this subtype is poorly understood. In this study, a comprehensive approach was applied to understand the pathogenesis of fulminant type 1 diabetes. We determined the genes that were differentially expressed in fulminant type 1 diabetes compared with type 1A diabetes and healthy control, using gene expression microarray in peripheral blood cells. Using volcano plot analysis, we found reduced expression of killer cell lectin-like receptor subfamily C, member 3 (KLRC3) which encodes NKG2E, a natural killer (NK) cell activating receptor, in fulminant type 1 diabetes, compared with healthy controls. This difference was confirmed by real-time RT-PCR among NK-enriched cells. The expression of KLRD1 (CD94), which forms heterodimer with NKG2E (KLRC3), was also reduced in NK-enriched cells in fulminant type 1 diabetes. Furthermore, flow cytometry showed significantly lower proportion of NK cells among peripheral blood mononuclear cells (PBMCs) in fulminant type 1 diabetes than in healthy controls. In patients with fulminant type 1 diabetes, the relative proportion of NK cells correlated significantly with the time period between onset of fever to the appearance of hyperglycemic-related symptoms. We conclude the presence of reduced NK activating receptor gene expression and low proportion of NK cells in fulminant type 1 diabetes.

Anti-NKG2A autoantibodies in a patient with systemic lupus erythematosus

OBJECTIVES

To characterize a novel anti-NKG2A autoantibody detected in a patient with SLE during a severe flare, and in a cross-sectional study investigate the occurrence of such autoantibodies in patients with SLE and primary SS (pSS).

METHODS

Serum or IgG from patients with SLE, pSS and healthy volunteers were assayed for blocking of anti-NKG2A or HLA-E binding to peripheral blood mononuclear cells or CD94/NKG2A- and CD94/NKG2C-transfected Ba/F3 cells. The anti-NKG2A autoantibodies were evaluated for effect on NK cell degranulation in response to HLA-E-transfected K562 cells. IFN-α was determined by an immunoassay and disease activity by the SLEDAI score.

RESULTS

Anti-NKG2A autoantibodies, which blocked binding of HLA-E tetramers to CD94/NKG2A-transfected cells and impaired NKG2A-mediated inhibition of NK cell activation, were observed in a patient with SLE. The presence of anti-NKG2A autoantibodies was associated with high SLE disease activity (SLEDAI score 14 and 16) and increased serum IFN-α. Of 94 SLE, 60 pSS and 30 healthy donor sera, only the index patient serum contained anti-NKG2A autoantibodies.

CONCLUSION

The presence of autoantibodies targeting NKG2A is a rare event, but when such autoantibodies occur they may promote excessive NK cell function. This can contribute to the pathogenesis by increasing the killing of cells and the release of autoantigens. Our findings highlight the possible importance of NK cells in the SLE disease process.

The association of polar residues in the DAP12 homodimer: TOXCAT and molecular dynamics simulation studies

Dimerization of the transmembrane (TM) adaptor protein DAP12 plays a key role in mediating activation signals through TM-TM association with cell-surface receptors. Herein, we apply the TOXCAT assay and molecular dynamics simulation to analyze dynamics and dimerization of the TM helix of DAP12 in the membrane bilayer. In the TOXCAT assay, we performed site-specific mutagenesis of potential dimerization motifs in the DAP12 TM domain. Instead of the common GxxxG dimerization motif, mutating either of the polar residues Asp-50 and Thr-54 significantly decreased the TOXCAT signal for the dimerization of DAP12 TM domain. Furthermore, through the conformational difference between wild-type and mutant DAP12 TM homodimers, a combined coarse-grained and atomistic molecular dynamics simulation has identified both Asp-50 and Thr-54 at the dimerization interface. The experimental and computational results of the DAP12 TM dimer are in excellent agreement with the previously reported NMR structure obtained in detergent micelles. Such a combination of dynamics simulation and cell-based experiments can be applied to produce insights at the molecular level into the TM-TM association of many other transmembrane proteins.

Dissection of agonistic and blocking effects of CD200 receptor antibodies

The CD200 receptor (CD200R) is present mainly on myeloid cells and gives inhibitory signals when engaged by its ligand CD200. The interaction is currently of therapeutic interest in cancer and inflammation. However functional effects are complicated by the fact that CD200R is itself polymorphic and also a member of a paired receptor family with four closely related gene products in mice called CD200RLa etc. We show that a second allele of CD200R (termed CD200R(2)) that differs in 7 amino acids also binds CD200 but did not react with the widely used CD200R antibody OX110. Biochemical and functional analysis showed that the CD200/CD200R interaction was blocked by the OX131, mAb that recognises both CD200R(1) and CD200R(2), but not by OX110 mAb. Both mAb can give agonistic inhibitory signals but functional analysis shows OX131 mAb also has the potential to block inhibition by preventing the ligand-receptor interaction and hence gives opposing effects. Although OX131 mAb cross-reacts with the activating receptor CD200RLe, it is specific for CD200R in C57BL/6 whilst OX110 mAb cross-reacts on CD200RLc. The results show the importance of the repertoire of paired receptors in strains or individuals and mAb used with implications for paired receptor analysis and therapeutics.

Theoretical elucidation of the origin for assembly of the DAP12 dimer with only one NKG2C in the lipid membrane

In this work, we have investigated in details the origin of the assembly of the DAP12 dimer with only one NKG2C in the activating immunoreceptor complex from thew two aspects of electronic properties and dynamic structures by performing density functional theory (DFT) calculations and molecular dynamics (MD) simulations. In the DFT calculations, we studied the aggregation ability of the NKG2C(TM) with the DAP12(TM) dimer and the DAP12(TM)-DAP12(TM)-NKG2C(TM) complex by analyzing the electrostatic potentials and frontier molecular orbitals (FMOs), and in the MD simulations we mainly investigated the dynamic structures of the DAP12(TM)-DAP12(TM)-NKG2C(TM) complex and its mutants, as well as the tetramer complex consisting of two DAP12(TM) and two NKG2C(TM) helixes without any restriction. Through the studies of the electrostatic potential, the FMOs, and the dynamic structures, we have provided reasonable explanations to some extent for the experimental observation that only one NKG2C can associate with the DAP12 homodimer. The present theoretical results are expected to give valuable information for further studying the assembly between receptors and signaling subunits.

Cytokine production by non-stimulated peripheral blood NK cells and lymphocytes in early-onset severe pre-eclampsia without HELLP

Preeclampsia involves an exacerbated maternal inflammatory response that suggests a possible role of innate immunity. NK cells can promote this kind of response through cytokine production and the expression of activating or inhibitory receptors. The aims of the present study were to explore cytokine production by peripheral blood mononuclear cells, as well as cytotoxic ability and receptor expression for HLA-E and HLA-G molecules in peripheral natural killer (NK) cells of women with early-onset severe preeclampsia without HELLP (hemolysis, elevated liver enzyme levels and a low platelet count) syndrome. The expression of the ILT2, KIRDL4, NKG2A, and NKG2C receptors and of cytotoxic activity was measured in non-stimulated NK cells, whereas the intracellular expression of IL-4, IL-10, IL-13, IL-12, IFNγ, TNF and VEGF, was assessed in non-stimulated peripheral blood mononuclear cells subsets using flow cytometry. Circulating soluble HLA-G was also determined by ELISA. The intracellular cytokines tested were significantly higher in NK cell subsets from severely preeclamptic women compared with the control group. On the other hand, the percentage of NK cells expressing NKG2A or NKG2C and the cytotoxic activity of NK cells were significantly higher in severely preeclamptic women. Furthermore, there was a significant correlation between urine protein concentration and soluble human leukocyte antigen G (soluble HLA-G) in serum. We conclude that patients with early-onset severe preeclampsia without HELLP syndrome have increased NK cell function related to cytokine production, cytotoxicity and expression of lectin-like receptors such as NKG2.

Structural insights into activation of antiviral NK cell responses

Natural killer (NK) cells are key components of innate immune responses, providing surveillance against cells undergoing tumorigenesis or infection, by viruses or internal pathogens. NK cells can directly eliminate compromised cells and regulate downstream responses of the innate and acquired immune systems through the release of immune modulators (cytokines, interferons). The importance of the role NK cells play in immune defense was demonstrated originally in herpes viral infections, usually mild or localized, which become severe and life threatening in NK-deficient patients . NK cell effector functions are governed by balancing opposing signals from a diverse array of activating and inhibitory receptors. Many NK receptors occur in paired activating and inhibitory isoforms and recognize major histocompatibility complex (MHC) class I proteins with varying degrees of peptide specificity. Structural studies have made considerable inroads into understanding the molecular mechanisms employed to broadly recognize multiple MHC ligands or specific pathogen-associated antigens and the strategies employed by viruses to thwart these defenses. Although many details of NK development, signaling, and integration remain mysterious, it is clear that NK receptors are key components of a system exquisitely tuned to sense any dysregulation in MHC class I expression, or the expression of certain viral antigens, resulting in the elimination of affected cells.

Intraepithelial lymphocytes in celiac disease immunopathology

Celiac disease is a T cell-mediated immune disorder induced by dietary gluten that is characterized by the development of an inflammatory anti-gluten CD4 T cell response, anti-gluten antibodies, and autoantibodies against tissue transglutaminase 2 and the activation of intraepithelial lymphocytes (IELs) leading to the destruction of the intestinal epithelium. Intraepithelial lymphocytes represent a heterogeneous population of T cells composed mainly of cytotoxic CD8 T cells residing within the epithelial layer, whose main role is to maintain the integrity of the epithelium by eliminating infected cells and promoting epithelial repair. Dysregulated activation of IELs is a hallmark of CD and is critically involved in epithelial cell destruction and the subsequent development of villous atrophy. In this review, we compare and contrast the phenotype and function of human and mouse small intestinal IELs under physiological conditions. Furthermore, we discuss how conditions of epithelial distress associated with overexpression of IL-15 and non-classical MHC class I molecules induce cytotoxic IELs to become licensed killer cells that upregulate activating NKG2D and CD94/NKG2C natural killer receptors, acquiring lymphokine killer activity. Pathways leading to dysregulated IEL activation could eventually be targeted to prevent villous atrophy and treat patients who respond poorly to gluten-free diet.

Cloning and characterization of CD300d, a novel member of the human CD300 family of immune receptors

Herein we present the cloning and molecular characterization of CD300d, a member of the human CD300 family of immune receptors. CD300d cDNA was cloned from RNA obtained from human peripheral blood mononuclear cells, and RT-PCR revealed the gene to be expressed in cells of myeloid lineage. The cloned cDNA encoded for a type I protein with a single extracellular Ig V-type domain and a predicted molecular mass of 21.5 kDa. The short cytoplasmic tail is lacking in any known signaling motif, but there is a negatively charged residue (glutamic acid) within the transmembrane domain. CD300d forms complexes with the CD300 family members, with the exception of CD300c. Contrary to other activating members of the CD300 family of receptors, surface expression of CD300d in COS-7-transfected cells required the presence of an immunoreceptor tyrosine-based activating motif-bearing adaptor (FcεRγ). Accordingly, we found that CD300d was able to recruit FcεRγ. Unexpectedly, we could not detect CD300d on the surface of cells expressing FcεRγ, suggesting the existence of unknown mechanisms regulating the trafficking of this molecule. The presence of other CD300 molecules also did not modify the intracellular expression of CD300d. In fact, the presence of CD300d decreased the levels of surface expression of CD300f but not CD300c. Our data suggest that the function of CD300d would be related to the regulation of the expression of other CD300 molecules and the composition of CD300 complexes on the cell surface.

Natural killer group 2A (NKG2A) and natural killer group 2C (NKG2C) bind to sulfated glycans and α2,3-NeuAc-containing glycoproteins

Killer lectin-like receptors on natural killer (NK) cells mediate cytotoxicity through glycans on target cells. We prepared recombinant glutathione S-transferase-fused extracellular lectin-like domains (AA 94-231) of natural killer group 2A (NKG2A) (rGST-NKG2A) and NKG2C (rGST-NKG2C) and determined the binding of these receptors to plates coated with heparin-conjugated bovine serum albumin (heparin-BSA) and glycoproteins. rGST-NKG2A and rGST-NKG2C directly bound to heparin-BSA with K(d) values of 20 and 40 nM, respectively. Binding of rGST-NKG2A and rGST-NKG2C to heparin-BSA was suppressed in the presence of soluble heparin, heparan sulfate, fucoidan, λ-carrageenan, and dextran sulfate. 2-O-Sulfate residues in heparin were essential for the binding of rGST-NKG2A and rGST-NKG2C. Moreover, rGST-NKG2A and rGST-NKG2C bound to multimeric sialyl Lewis X expressing transferrin secreted by HepG2 cells with K(d) values of 80 and 114 nM, respectively. This is the first report showing that NKG2A and NKG2C bind to heparin and α2,3-NeuAc-containing glycoproteins.

Selenite induces posttranscriptional blockade of HLA-E expression and sensitizes tumor cells to CD94/NKG2A-positive NK cells

CD94/NKG2A is an inhibitory receptor that controls the activity of a large proportion of human NK cells following interactions with the nonclassical HLA class Ib molecule HLA-E expressed on target cells. In this study, we show that selenite (SeO(3)(2-)), an inorganic selenium compound, induces an almost complete loss of cell surface expression of HLA-E on tumor cells of various origins. Selenite abrogated the HLA-E expression at a posttranscriptional level, since selenite exposure led to a dose-dependent decrease in cellular HLA-E protein expression whereas the mRNA levels remained intact. The loss of HLA-E expression following selenite treatment was associated with decreased levels of intracellular free thiols in the tumor cells, suggesting that the reduced HLA-E protein synthesis was caused by oxidative stress. Indeed, HLA-E expression and the level of free thiols remained intact following treatment with selenomethionine, a selenium compound that does not generate oxidative stress. Loss of HLA-E expression, but not of total HLA class I expression, on tumor cells resulted in increased susceptibility to CD94/NK group 2A-positive NK cells. Our results suggest that selenite may be used to potentiate the anti-tumor cytotoxicity in settings of NK cell-based immunotherapies.

Clinical significance of the HLA-E and CD94/NKG2 interaction

HLA-E belongs to the non-classical HLA (class Ib family) broadly defined by a limited polymorphism and a restricted pattern of cellular expression. So far, only two functional alleles differing at only one amino acid position (non-synonymous mutation) in the α2 heavy chain domain, where an arginine in position 107 in HLA-E*0101 is replaced by a glycine in HLA-E*0103, have been reported. The interaction between non-classical HLA-E molecule and CD94/NKG2A receptor plays a crucial role in the immunological response involving natural killer (NK) cells and cytotoxic T lymphocytes. All proteins forming CD94/NKG2 receptors are encoded by genes situated in the same cluster on chromosome 12, allowing tight control over the order of their expression. The inhibitory members of the NKG2 receptor family are available on the cell surface before activating the members to prevent autoimmune incidents during immune cells' ontogenesis. In the present review, the potential role of this interaction in viral infection, pregnancy and transplantation of allogeneic hematopoietic stem cells (HSC) is presented and discussed. The review will also include the effect of HLA-E polymorphism on the outcome of HSC transplants in humans.

Expansion of a unique CD57⁺NKG2Chi natural killer cell subset during acute human cytomegalovirus infection

During human CMV infection, there is a preferential expansion of natural killer (NK) cells expressing the activating CD94-NKG2C receptor complex, implicating this receptor in the recognition of CMV-infected cells. We hypothesized that NK cells expanded in response to pathogens will be marked by expression of CD57, a carbohydrate antigen expressed on highly mature cells within the CD56(dim)CD16(+) NK cell compartment. Here we demonstrate the preferential expansion of a unique subset of NK cells coexpressing the activating CD94-NKG2C receptor and CD57 in CMV(+) donors. These CD57(+)NKG2C(hi) NK cells degranulated in response to stimulation through their NKG2C receptor. Furthermore, CD57(+)NKG2C(hi) NK cells preferentially lack expression of the inhibitory NKG2A receptor and the inhibitory KIR3DL1 receptor in individuals expressing its HLA-Bw4 ligand. Moreover, in solid-organ transplant recipients with active CMV infection, the percentage of CD57(+)NKG2C(hi) NK cells in the total NK cell population preferentially increased. During acute CMV infection, the NKG2C(+) NK cells proliferated, became NKG2C(hi), and finally acquired CD57. Thus, we propose that CD57 might provide a marker of "memory" NK cells that have been expanded in response to infection.

Paired opposing leukocyte receptors recognizing rapidly evolving ligands are subject to homogenization of their ligand binding domains

Some leukocyte receptors come in groups of two or more where the partners share ligand(s) but transmit opposite signals. Some of the ligands, such as MHC class I, are fast evolving, raising the problem of how paired opposing receptors manage to change in step with respect to ligand binding properties and at the same time conserve opposite signaling functions. An example is the KLRC (NKG2) family, where opposing variants have been conserved in both rodents and primates. Phylogenetic analyses of the KLRC receptors within and between the two orders show that the opposing partners have been subject to post-speciation gene homogenization restricted mainly to the parts of the genes that encode the ligand binding domains. Concerted evolution similarly restricted is demonstrated also for the KLRI, KLRB (NKR-P1), KLRA (Ly49), and PIR receptor families. We propose the term merohomogenization for this phenomenon and discuss its significance for the evolution of immune receptors.

NKp46 identifies an NKT cell subset susceptible to leukemic transformation in mouse and human

IL-15 may have a role in the development of T cell large granular lymphocyte (T-LGL) or NKT leukemias. However, the mechanisms of action and the identity of the cell subset that undergoes leukemic transformation remain elusive. Here we show that in both mice and humans, NKp46 expression marks a minute population of WT NKT cells with higher activity and potency to become leukemic. Virtually 100% of T-LGL leukemias in IL-15 transgenic mice expressed NKp46, as did a majority of human T-LGL leukemias. The minute NKp46+ NKT population, but not the NKp46⁻ NKT population, was selectively expanded by overexpression of endogenous IL-15. Importantly, IL-15 transgenic NKp46⁻ NKT cells did not become NKp46+ in vivo, suggesting that NKp46+ T-LGL leukemia cells were the malignant counterpart of the minute WT NKp46+ NKT population. Mechanistically, NKp46+ NKT cells possessed higher responsiveness to IL-15 in vitro and in vivo compared with that of their NKp46⁻ NKT counterparts. Furthermore, interruption of IL-15 signaling using a neutralizing antibody could prevent LGL leukemia in IL-15 transgenic mice. Collectively, our data demonstrate that NKp46 identifies a functionally distinct NKT subset in mice and humans that appears to be directly susceptible to leukemic transformation when IL-15 is overexpressed. Thus, IL-15 signaling and NKp46 may be useful targets in the treatment of patients with T-LGL or NKT leukemia.

Regulation of NK cell repertoire and function in the liver

NK cells represent a large proportion of the lymphocyte population in the liver and are involved in early innate immunity to pathogen infection. As a result of liver endothelial cell fenestrations, parenchymal cells are not separated by a basal membrane, and thereby pathogen-infected hepatocytes are extensively capable of interacting with innate immune cells including NK cells. In addition, hepatic NK cells interact with surrounding DC and alter their differentiation and function. Recent studies reveal that NK cells exhibit a regulatory function that modulates T cell responses through their interaction with DC and/or direct effect on T cells. Thus, NK cells play a central role, not only in innate immunity, but also in shaping the adaptive immune response. During pathogen infection, there is a remarkable increase of hepatic NK cells, possibly due to the expansion of resident liver NK cells and/or recruitement of NK cells from the blood. The liver microenvironment is believed to modulate hepatic NK cell function through the induction of activating/inhibitory receptor expression and inflammatory cytokine secretion. Particularly, the liver maintains intrahepatic NK cells in a functionally hyporesponsive state compared to splenic NK cells: liver NK cells displayed a dampened IFN-γ response to IL-12/IL-18 stimulation. Notably, the liver contains a significant population of functionally hyporesponsive NK cells that express high levels of the inhibitory receptor NKG2A and lack expression of MHC class I-binding Ly49 receptors. Importantly, adoptively transferred splenic NK cells that migrate to the liver displayed phenotypic and functional changes, supporting a view that the liver environment modifies NK cell receptor expression and functional responsiveness. In this article, we will review studies on the regulation of NK cell repertoire and function in the hepatic environment and the impact of liver NK cell immunoregulatory function on influencing adaptive immunity.

CD8⁺ T cells with an intraepithelial phenotype upregulate cytotoxic function upon influenza infection in human lung

The human lung T cell compartment contains many CD8⁺ T cells specific for respiratory viruses, suggesting that the lung is protected from recurring respiratory infections by a resident T cell pool. The entry site for respiratory viruses is the epithelium, in which a subset of lung CD8⁺ T cells expressing CD103 (αE integrin) resides. Here, we determined the specificity and function of CD103⁺CD8⁺ T cells in protecting human lung against viral infection. Mononuclear cells were isolated from human blood and lung resection samples. Variable numbers of CD103⁺CD8⁺ T cells were retrieved from the lung tissue. Interestingly, expression of CD103 was seen only in lung CD8⁺ T cells specific for influenza but not in those specific for EBV or CMV. CD103⁺ and influenza-reactive cells preferentially expressed NKG2A, an inhibitor of CD8⁺ T cell cytotoxic function. In contrast to CD103⁻CD8⁺ T cells, most CD103⁺CD8⁺ cells did not contain perforin or granzyme B. However, they could quickly upregulate these cytotoxic mediators when exposed to a type I IFN milieu or via contact with their specific antigen. This mechanism may provide a rapid and efficient response to influenza infection, without inducing cytotoxic damage to the delicate epithelial barrier.