Acquisition of Ly49 receptor expression by developing natural killer cells

Ly49G, but not Ly49C/I, is dispensable for diverse antigen-specific memory NK cell responses in H-2d and H-2b mice

Immunological memory is a hallmark of the adaptive immune system. However, considerable evidence indicates that the natural killer (NK) cell subset of innate lymphoid cells also mediates specific memory responses to diverse antigens, including peptides. Even though the existence of NK cell memory is established, the mechanism behind NK cell adaptive responses is yet to be elucidated. Previously, we observed that the Ly49 family of class-I MHC receptors in mice are critical for the formation of adaptive NK cell memory responses. To define the nature of Ly49 involvement in NK cell memory responses, we investigated the contribution of individual Ly49 receptors and their defined class-I MHC ligands. We determined that the Ly49 requirement for the generation of NK memory responses is not uniform. Specifically, Ly49C and/or Ly49I proteins are indispensable for the adaptive NK cell responses as assessed by contact hypersensitivity recall responses to haptens and peptides, in H-2b and H-2d MHC backgrounds. In contrast, the highly expressed inhibitory receptor, Ly49G, did not appear to play any role in NK cell memory responses as determined using antibody-mediated subset depletion and gene-deficient mouse models, even in strains containing known ligands for Ly49G. These findings point to a unique role for Ly49C/I in adaptive NK cell antigen recognition and provide further insight into the mechanism behind adaptive NK cell responses.

Expression of a single inhibitory member of the Ly49 receptor family is sufficient to license NK cells for effector functions

Natural killer (NK) cells recognize target cells through germline-encoded activation and inhibitory receptors enabling effective immunity against viruses and cancer. The Ly49 receptor family in the mouse and killer immunoglobin-like receptor family in humans play a central role in NK cell immunity through recognition of major histocompatibility complex class I (MHC-I) and related molecules. Functionally, these receptor families are involved in the licensing and rejection of MHC-I-deficient cells through missing-self. The Ly49 family is highly polymorphic, making it challenging to detail the contributions of individual Ly49 receptors to NK cell function. Herein, we showed mice lacking expression of all Ly49s were unable to reject missing-self target cells in vivo, were defective in NK cell licensing, and displayed lower KLRG1 on the surface of NK cells. Expression of Ly49A alone on an H-2Dd background restored missing-self target cell rejection, NK cell licensing, and NK cell KLRG1 expression. Thus, a single inhibitory Ly49 receptor is sufficient to license NK cells and mediate missing-self in vivo.

Expression of a single inhibitory member of the Ly49 receptor family is sufficient to license NK cells for effector functions

Natural killer (NK) cells recognize target cells through germline-encoded activation and inhibitory receptors enabling effective immunity against viruses and cancer. The Ly49 receptor family in the mouse and killer immunoglobin-like receptor family in humans play a central role in NK cell immunity through recognition of MHC class I and related molecules. Functionally, these receptor families are involved in licensing and rejection of MHC-I-deficient cells through missing-self. The Ly49 family is highly polymorphic, making it challenging to detail the contributions of individual Ly49 receptors to NK cell function. Herein, we showed mice lacking expression of all Ly49s were unable to reject missing-self target cells in vivo, were defective in NK cell licensing, and displayed lower KLRG1 on the surface of NK cells. Expression of Ly49A alone on a H-2Dd background restored missing-self target cell rejection, NK cell licensing, and NK cell KLRG1 expression. Thus, a single inhibitory Ly49 receptor is sufficient to license NK cells and mediate missing-self in vivo.

Polymorphic residues in HLA-B that mediate HIV control distinctly modulate peptide interactions with both TCR and KIR molecules

Immunogenetic studies have shown that specific HLA-B residues (67, 70, 97, and 156) mediate the impact of HLA class I on HIV infection, but the molecular basis is not well understood. Here we evaluate the function of these residues within the protective HLA-B∗5701 allele. While mutation of Met67, Ser70, and Leu156 disrupt CD8+ T cell recognition, substitution of Val97 had no significant impact. Thermal denaturation of HLA-B∗5701-peptide complexes revealed that Met67 and Leu156 maintain HLA-peptide stability, while Ser70 and Leu156 facilitate T cell receptor (TCR) interactions. Analyses of existing structures and structural models suggested that Val97 mediates HLA-peptide binding to inhibitory KIR3DL1 molecules, which was confirmed by experimental assays. These data thereby demonstrate that the genetic basis by which host immunity impacts HIV outcomes occurs by modulating HLA-B-peptide stability and conformation for interaction with TCR and killer immunoglobulin receptor (KIR) molecules. Moreover, they indicate a key role for epitope specificity and HLA-KIR interactions to HIV control.

Cis-regulatory evolution of the recently expanded Ly49 gene family

Comparative genomics has revealed the rapid expansion of multiple gene families involved in immunity. Members within each gene family often evolved distinct roles in immunity. However, less is known about the evolution of their epigenome and cis-regulation. Here we systematically profile the epigenome of the recently expanded murine Ly49 gene family that mainly encode either inhibitory or activating surface receptors on natural killer cells. We identify a set of cis-regulatory elements (CREs) for activating Ly49 genes. In addition, we show that in mice, inhibitory and activating Ly49 genes are regulated by two separate sets of proximal CREs, likely resulting from lineage-specific losses of CRE activity. Furthermore, we find that some Ly49 genes are cross-regulated by the CREs of other Ly49 genes, suggesting that the Ly49 family has begun to evolve a concerted cis-regulatory mechanism. Collectively, we demonstrate the different modes of cis-regulatory evolution for a rapidly expanding gene family.

MHC Class I–Dependent Shaping of the NK Cell Ly49 Receptor Repertoire Takes Place Early during Maturation in the Bone Marrow

MHC class I (MHC I) expression in the host influences NK cells in a process termed education. The result of this education is reflected in the responsiveness of NK cells at the level of individual cells as well as in the repertoire of inhibitory MHC I–specific receptors at the NK cell system level. The presence of MHC I molecules in the host environment gives rise to a skewed receptor repertoire in spleen NK cells where subsets expressing few (one or two) inhibitory receptors are expanded whereas subsets with many (three or more) receptors are contracted. It is not known whether this MHC I–dependent skewing is imposed during development or after maturation of NK cells. In this study, we tested the hypothesis that the NK cell receptor repertoire is shaped already early during NK cell development in the bone marrow. We used mice with a repertoire imposed by a single MHC I allele, as well as a C57BL/6 mutant strain with exaggerated repertoire skewing, to investigate Ly49 receptor repertoires at different stages of NK cell differentiation. Our results show that NK cell inhibitory receptor repertoire skewing can indeed be observed in the bone marrow, even during the earliest developmental steps where Ly49 receptors are expressed. This may partly be accounted for by selective proliferation of certain NK cell subsets, but other mechanisms must also be involved. We propose a model for how repertoire skewing is established during a developmental phase in the bone marrow, based on sequential receptor expression as well as selective proliferation.

NK/ILC1 cells mediate neuroinflammation and brain pathology following congenital CMV infection

Congenital human cytomegalovirus (cHCMV) infection of the brain is associated with a wide range of neurocognitive sequelae. Using infection of newborn mice with mouse cytomegalovirus (MCMV) as a reliable model that recapitulates many aspects of cHCMV infection, including disseminated infection, CNS infection, altered neurodevelopment, and sensorineural hearing loss, we have previously shown that mitigation of inflammation prevented alterations in cerebellar development, suggesting that host inflammatory factors are key drivers of neurodevelopmental defects. Here, we show that MCMV infection causes a dramatic increase in the expression of the microglia-derived chemokines CXCL9/CXCL10, which recruit NK and ILC1 cells into the brain in a CXCR3-dependent manner. Surprisingly, brain-infiltrating innate immune cells not only were unable to control virus infection in the brain but also orchestrated pathological inflammatory responses, which lead to delays in cerebellar morphogenesis. Our results identify NK and ILC1 cells as the major mediators of immunopathology in response to virus infection in the developing CNS, which can be prevented by anti-IFN-γ antibodies.

Evidence for Prescribed NK Cell Ly-49 Developmental Pathways in Mice

Previous studies of NK cell inhibitory Ly-49 genes showed their expression is stochastic. However, relatively few studies have examined the mechanisms governing acquisition of inhibitory receptors in conjunction with activating Ly-49 receptors and NK cell development. We hypothesized that the surface expression of activating Ly-49 receptors is nonrandom and is influenced by inhibitory Ly-49 receptors. We analyzed NK cell "clusters" defined by combinatorial expression of activating (Ly-49H and Ly-49D) and inhibitory (Ly-49I and Ly-49G2) receptors in C57BL/6 mice. Using the product rule to evaluate the interdependencies of the Ly-49 receptors, we found evidence for a tightly regulated expression at the immature NK cell stage, with the highest interdependencies between clusters that express at least one activating receptor. Further analysis demonstrated that certain NK clusters predominated at the immature (CD27⁺CD11b^-), transitional (CD27⁺CD11b⁺), and mature (CD27^-CD11b^-) NK cell stages. Using parallel in vitro culture and in vivo transplantation of sorted NK clusters, we discovered nonrandom expression of Ly-49 receptors, suggesting that prescribed pathways of NK cluster differentiation exist. Our data infer that surface expression of Ly-49I is an important step in NK cell maturation. Ki-67 expression and cell counts confirmed that immature NK cells proliferate more than mature NK cells. We found that MHC class I is particularly important for regulation of Ly-49D and Ly-49G2, even though no known MHC class I ligand for these receptors is present in B6 mice. Our data indicate that surface expression of both activating and inhibitory Ly-49 receptors on NK cell clusters occurs in a nonrandom process correlated to their maturation stage.

Functional Characterization of Ly49+CD8 T-Cells in Both Normal Condition and During Anti-Viral Response

The role of Ly49+CD8 T-cells in the immune system is not clear. Previously, several papers suggested Ly49+CD8 T-cells as immunosuppressors, while multiple studies also suggested their role as potent participants of the immune response. The mechanism of Ly49 expression on CD8 T-cells is also not clear. We investigated phenotype, functions, and regulation of Ly49 expression on murine CD8 T-cells in both normal state and during LCMV infection. CD8 T-cells express different Ly49 receptors compared with NK-cells. In intact mice, Ly49+CD8 T-cells have a phenotype similar to resting central memory CD8 T-cells and do not show impaired proliferation and cytokine production. Conventional CD8 T-cells upregulate Ly49 receptors during TCR-induced stimulation, and IL-2, as well as IL-15, affect it. At the same time, Ly49+CD8 T-cells change the Ly49 expression profile dramatically upon re-stimulation downregulating inhibitory and upregulating activating Ly49 receptors. We observed the expression of Ly49 receptors on the virus-specific CD8 T-cells during LCMV infection, especially marked in the early stages, and participation of Ly49+CD8 T-cells in the anti-viral response. Thus, CD8 T-cells acquire Ly49 receptors during the T-cell activation and show dynamic regulation of Ly49 receptors during stimulation.

Synergized regulation of NK cell education by NKG2A and specific Ly49 family members

Mice lacking MHC class-I (MHC-I) display severe defects in natural killer (NK) cell functional maturation, a process designated as “education”. Whether self-MHC-I specific Ly49 family receptors and NKG2A, which are closely linked within the NK gene complex (NKC) locus, are essential for NK cell education is still unclear. Here we show, using CRISPR/Cas9-mediated gene deletion, that mice lacking all members of the Ly49 family exhibit a moderate defect in NK cell activity, while mice lacking only two inhibitory Ly49 members, Ly49C and Ly49I, have comparable phenotypes. Furthermore, the deficiency of NKG2A, which recognizes non-classical MHC-Ib molecules, mildly impairs NK cell function. Notably, the combined deletion of NKG2A and the Ly49 family severely compromises the ability of NK cells to mediate “missing-self” and “induced-self” recognition. Therefore, our data provide genetic evidence supporting that NKG2A and the inhibitory members of Ly49 family receptors synergize to regulate NK cell education.

MHC-I-induced signalling of various natural killer (NK) inhibitory receptors is critical for regulation NK cell education, but clear genetic evidence is still lacking. Here the authors generate multiple lines of mice differentially deficient in Ly49 family and/or NKG2A NK receptors, and find that self-MHCI specific Ly49 members and NKG2A synergize to regulate NK education.

Prenatal tobacco smoke exposure predisposes offspring mice to exacerbated allergic airway inflammation associated with altered innate effector function

Background

Epidemiological studies suggest that prenatal and early life environmental exposures have adverse effects on pulmonary function and are important contributors in the development of childhood asthma and allergic disease. The mechanism by which environmental tobacco smoke (ETS) exposure in utero promotes the development of allergic asthma remains unclear. In this study, we investigated the immunological consequences of prenatal exposure to ETS in order to understand events responsible for the development or exacerbation of allergic asthma.

Methods

Pregnant C57BL/6 mice were exposed to either ETS or filtered air throughout gestation and the effect on pulmonary inflammation in the offspring were examined and compared. Specifically, the effects on eosinophilic inflammation, airway hyperreactivity, goblet cell hyperplasia, properties of pulmonary natural killer (NK) cells and type 2 cytokines elicited in response to inhaled house dust mite (HDM) allergen were investigated in the progeny.

Results

Exposure to ETS prenatally significantly exacerbated HDM-induced airway eosinophilic inflammation, hyperreactivity, mucus secretion, cysteinyl leukotriene biosynthesis and type 2 cytokine production in the offspring. Consistently, lung mononuclear cells from ETS-exposed offspring secreted higher levels of IL-13 when stimulated in vitro with anti-αβ TCR antibody or HDM allergen. Moreover, offspring from ETS-exposed dams exhibited a higher frequency of CD11b⁺ dendritic cells and CD3⁺CD4⁺ T lymphocytes in the lungs following allergen inhalation compared to air-exposed mice. Unexpectedly, the exacerbated allergic inflammation in the ETS-exposed offspring was associated with a reduction in CD3⁻CD19⁻NK1.1⁺CD94⁺ NK cell numbers and their IFN-γ production, highlighting a role for altered innate immunity in the enhanced allergic response.

Conclusion

Our results reveal that prenatal exposure to ETS predisposes offspring to an exacerbated allergic airway inflammation that is associated with a reduction in pulmonary NK cell function, suggesting that NK cells play a key role in controlling asthma severity.

Expression of CD226 is associated to but not required for NK cell education

DNAX accessory molecule-1 (DNAM-1, also known as CD226) is an activating receptor expressed on subsets of natural killer (NK) and T cells, interacts with its ligands CD155 or CD112, and has co-varied expression with inhibitory receptors. Since inhibitory receptors control NK-cell activation and are necessary for MHC-I-dependent education, we investigated whether DNAM-1 expression is also involved in NK-cell education. Here we show an MHC-I-dependent correlation between DNAM-1 expression and NK-cell education, and an association between DNAM-1 and NKG2A that occurs even in MHC class I deficient mice. DNAM-1 is expressed early during NK-cell development, precedes the expression of MHC-I-specific inhibitory receptors, and is modulated in an education-dependent fashion. Cd226^−/− mice have missing self-responses and NK cells with a normal receptor repertoire. We propose a model in which NK-cell education prevents or delays downregulation of DNAM-1. This molecule endows educated NK cells with enhanced effector functions but is dispensable for education.

CD226 is an activating receptor expressed in a co-varied manner with inhibitory receptors on natural killer (NK) cells, but whether CD226 is involved in NK cell education is unclear. Here the authors show that CD226 expression is plastic depending on the MHC environment and endows educated NK cells enhanced effector functions.

Activating Receptor Signals Drive Receptor Diversity in Developing Natural Killer Cells

It has recently been appreciated that NK cells exhibit many features reminiscent of adaptive immune cells. Considerable heterogeneity exists with respect to the ligand specificity of individual NK cells and as such, a subset of NK cells can respond, expand, and differentiate into memory-like cells in a ligand-specific manner. MHC I-binding inhibitory receptors, including those belonging to the Ly49 and KIR families, are expressed in a variegated manner, which creates ligand-specific diversity within the NK cell pool. However, how NK cells determine which inhibitory receptors to express on their cell surface during a narrow window of development is largely unknown. In this manuscript, we demonstrate that signals from activating receptors are critical for induction of Ly49 and KIR receptors during NK cell development; activating receptor-derived signals increased the probability of the Ly49 bidirectional Pro1 promoter to transcribe in the forward versus the reverse direction, leading to stable expression of Ly49 receptors in mature NK cells. Our data support a model where the balance of activating and inhibitory receptor signaling in NK cells selects for the induction of appropriate inhibitory receptors during development, which NK cells use to create a diverse pool of ligand-specific NK cells.

Selection, tuning, and adaptation in mouse NK cell education

Natural killer (NK) cells recognize transformed cells with an array of germline-encoded inhibitory and activating receptors. Inhibitory Ly49 receptors bind major histocompatibility complex class I (MHC-I) molecules, providing a mechanism by which NK cells maintain self-tolerance yet eliminate cells expressing reduced levels of MHC-I. Additionally, MHC-I molecules are required for NK cell education, a process in which NK cells acquire responsiveness. In this review, we discuss three facets of MHC class I-dependent education of mouse NK cells: skewing of the inhibitory receptor repertoire, induction of functional responsiveness, and tuning in response to changes in MHC-I expression. We discuss prevailing models for education such as licensing and disarming and propose a model for positive selection of 'useful' NK cell subsets. Furthermore, we argue that both repertoire skewing and functional NK cell education may be altered in mature NK cells subject to changes in MHC-I input and suggest that this process may provide increased dynamics to the NK cell system.

Prenatal Allospecific NK Cell Tolerance Hinges on Instructive Allorecognition through the Activating Receptor during Development

Little is known about how the prenatal interaction between NK cells and alloantigens shapes the developing NK cell repertoire toward tolerance or immunity. Specifically, the effect on NK cell education arising from developmental corecognition of alloantigens by activating and inhibitory receptors with shared specificity is uncharacterized. Using a murine prenatal transplantation model, we examined the manner in which this seemingly conflicting input affects NK cell licensing and repertoire formation in mixed hematopoietic chimeras. We found that prenatal NK cell tolerance arose from the elimination of phenotypically hostile NK cells that express an allospecific activating receptor without coexpressing any allospecific inhibitory receptors. Importantly, the checkpoint for the system appeared to occur centrally within the bone marrow during the final stage of NK cell maturation and hinged on the instructive recognition of allogeneic ligand by the activating receptor rather than through the inhibitory receptor as classically proposed. Residual nondeleted hostile NK cells expressing only the activating receptor exhibited an immature, anergic phenotype, but retained the capacity to upregulate inhibitory receptor expression in peripheral sites. However, the potential for this adaptive change to occur was lost in developmentally mature chimeras. Collectively, these findings illuminate the intrinsic process in which developmental allorecognition through the activating receptor regulates the emergence of durable NK cell tolerance and establishes a new paradigm to fundamentally guide future investigations of prenatal NK cell-allospecific education.

NK cell tolerance as the final endorsement of prenatal tolerance after in utero hematopoietic cellular transplantation

The primary benefits of in utero hematopoietic cellular transplantation (IUHCT) arise from transplanting curative cells prior to the immunologic maturation of the fetus. However, this approach has been routinely successful only in the treatment of congenital immunodeficiency diseases that include an inherent NK cell deficiency despite the existence of normal maternal immunity in either setting. These observations raise the possibility that fetal NK cells function as an early barrier to allogeneic IUHCT. Herein, we summarize the findings of previous studies of prenatal NK cell allospecific tolerance in mice and in humans. Cumulatively, this new information reveals the complexity of the fetal immune response in the setting of rejection or tolerance and illustrates the role for fetal NK cells in the final endorsement of allospecific prenatal tolerance.

Pro-inflammatory role of natural killer cells in the development of allergic airway disease

BACKGROUND

Natural Killer (NK) cells have been implicated in the development of allergic airway inflammation. However, the in vivo role of NK cells has not been firmly established due to the lack of animal models with selective deficiencies in NK cells.

OBJECTIVE

To determine the specific contribution of NK cells in a murine model of allergic airway disease (AAD).

METHODS

The role of NK cells in AAD was studied using NK-deficient (NKD) mice, perforin(-/-) mice, and mice depleted of Ly49A/D/G(+) NK cell subsets in an ovalbumin-induced model of allergic airway disease (OVA-AAD).

RESULTS

Induction of OVA-AAD in C57BL/6 wild-type (WT) mice resulted in the expansion of airway NK cells and the development of pronounced airway eosinophilia. In the absence of NK cells or specific subsets of NK cells, either in NKD mice, or after the depletion of Ly49A/D/G(+) NK cells, the development of OVA-AAD was significantly impaired as seen by decreased airway inflammation and eosinophilia, decreased secretion of the Th2 cytokines IL-4, IL-5 and IL-13 and diminished OVA-specific antibody production. Furthermore, while OVA-exposure induced a dramatic expansion of dendritic cells (DCs) in WT mice, their induction was significantly attenuated in NKD mice. Development of OVA-AAD in perforin(-/-) mice suggested that the proinflammatory role of NK cells is not dependent on perforin-mediated cytotoxicity. Lastly, induction of allergic disease by OVA-specific CD4 T cells from WT but not NK-depleted or NKD mice in RAG(-/-) recipients, demonstrates that NK cells are essential for T cell priming.

CONCLUSIONS AND CLINICAL RELEVANCE

Our data demonstrate that conventional NK cells play an important and distinct role in the development of AAD. The presence of activated NK cells has been noted in patients with asthma. Understanding the mechanisms by which NK cells regulate allergic disease is therefore an important component of treatment approaches.

Adipocyte IL-15 regulates local and systemic NK cell development

NK cell development and homeostasis require IL-15 produced by both hematopoietic and parenchymal cells. Certain hematopoietic IL-15 sources, such as macrophages and dendritic cells, are known, whereas the source of parenchymal IL-15 remains elusive. Using two types of adipocyte-specific Il15(-/-) mice, we identified adipocytes as a parenchymal IL-15 source that supported NK cell development nonredundantly. Both adipocyte-specific Il15(-/-) mice showed reduced IL-15 production specifically in the adipose tissue but impaired NK cell development in the spleen and liver in addition to the adipose tissue. We also found that the adipose tissue harbored NK progenitors as other niches (e.g. spleen) for NK cell development, and that NK cells derived from transplanted adipose tissue populated the recipient's spleen and liver. These findings suggest that adipocyte IL-15 contributes to systemic NK cell development by supporting NK cell development in the adipose tissue, which serves as a source of NK cells for other organs.

The unique neonatal NK cells: a critical component required for neonatal autoimmune disease induction by maternal autoantibody

Human maternal autoantibodies can trigger autoimmune diseases such as congenital heart block (CHB) in the progeny of women with lupus or Sjogren’s disease. The pathogenic effect of early autoantibody (autoAb) exposure has been investigated in a murine neonatal autoimmune ovarian disease (nAOD) model triggered by a unique ZP3 antibody. Although immune complexes (IC) are formed in adult and neonatal ovaries, ZP3 antibody triggers severe nAOD only in <7-day-old neonatal mice. Propensity to nAOD is due to the uniquely hyper-responsive neonatal natural killer (NK) cells that lack the inhibitory Ly49C/I receptors. In nAOD, the neonatal NK cells directly mediate ovarian inflammation and oocyte depletion while simultaneously promoting de novo pathogenic ovarian-specific T cell responses. Resistance to nAOD in older mice results from the emergence of the Ly49C/I⁺ NK cells that regulate effector NK cells and from CD25⁺ regulatory T cell control. In preliminary studies, FcγRIII⁺ NK cells as well as the ovarian resident FcγRIII⁺ macrophages and/or dendritic cells were found to be as indispensable players. Activated by ovarian IC, they migrate to lymphoid organs where NK cell priming occurs. Remarkably, the findings in nAOD are very similar to those reported for neonatal responses to a retrovirus and its cognate antibody that lead to long-lasting immunity. Studies on nAOD therefore provide insights into maternal autoAb-mediated neonatal autoimmunity, including CHB, while simultaneously uncovering new properties of the neonatal innate and adaptive responses, lethality of premature infant infection, and novel neonatal antiviral vaccine design.

Ly49 receptors: innate and adaptive immune paradigms

The Ly49 receptors are type II C-type lectin-like membrane glycoproteins encoded by a family of highly polymorphic and polygenic genes within the mouse natural killer (NK) gene complex. This gene family is designated Klra, and includes genes that encode both inhibitory and activating Ly49 receptors in mice. Ly49 receptors recognize class I major histocompatibility complex-I (MHC-I) and MHC-I-like proteins on normal as well as altered cells. Their functional homologs in humans are the killer cell immunoglobulin-like receptors, which recognize HLA class I molecules as ligands. Classically, Ly49 receptors are described as being expressed on both the developing and mature NK cells. The inhibitory Ly49 receptors are involved in NK cell education, a process in which NK cells acquire function and tolerance toward cells that express “self-MHC-I.” On the other hand, the activating Ly49 receptors recognize altered cells expressing activating ligands. New evidence shows a broader Ly49 expression pattern on both innate and adaptive immune cells. Ly49 receptors have been described on multiple NK cell subsets, such as uterine NK and memory NK cells, as well as NKT cells, dendritic cells, plasmacytoid dendritic cells, macrophages, neutrophils, and cells of the adaptive immune system, such as activated T cells and regulatory CD8⁺ T cells. In this review, we discuss the expression pattern and proposed functions of Ly49 receptors on various immune cells and their contribution to immunity.

Multiple levels of chemokine receptor regulation in the control of mouse natural killer cell development

Chemokines play a fundamental role in lymphocyte development, mainly attributable to the control of the correct localization in the proper microenvironments of cells undergoing maturation. Natural killer (NK) cell development occurs in the bone marrow (BM) where their localization is regulated by the balance of chemokine function in cell retention into tissues and mobilization into circulation. In addition, NK cells from several extra-medullary tissues are phenotypically and functionally different from their circulating counterpart suggesting that maturation can be completed in organs other than BM. Indeed, a role of chemokines in NK cell localization into tissues during homeostatic conditions is also documented. In this review, we summarize the current notion related to the relevance of several chemokine/chemokine receptor axes in NK cell development with a focus on the regulation of their expression and function.

Tissue-resident natural killer (NK) cells are cell lineages distinct from thymic and conventional splenic NK cells

Natural killer (NK) cells belong to the innate immune system; they can control virus infections and developing tumors by cytotoxicity and producing inflammatory cytokines. Most studies of mouse NK cells, however, have focused on conventional NK (cNK) cells in the spleen. Recently, we described two populations of liver NK cells, tissue-resident NK (trNK) cells and those resembling splenic cNK cells. However, their lineage relationship was unclear; trNK cells could be developing cNK cells, related to thymic NK cells, or a lineage distinct from both cNK and thymic NK cells. Herein we used detailed transcriptomic, flow cytometric, and functional analysis and transcription factor-deficient mice to determine that liver trNK cells form a distinct lineage from cNK and thymic NK cells. Taken together with analysis of trNK cells in other tissues, there are at least four distinct lineages of NK cells: cNK, thymic, liver (and skin) trNK, and uterine trNK cells.

DOI:http://dx.doi.org/10.7554/eLife.01659.001

Our immune system has white blood cells that migrate throughout the body in search of invading microbes or diseased and damaged cells. When these events are encountered, the white blood cells move into the affected tissue and launch an immune response to eliminate the threat.

Natural killer cells are white blood cells that kill cells that are infected with viruses or are cancerous. Most of what is known about conventional natural killer cells is derived from studying the spleen, which filters the blood and contains many immune cells. Natural killer cells also circulate around the body or are found within other tissues, and it was thought that both types of cells were either the same, or that one type could develop into the other. However, the thymus—an organ that is another source of white blood cells—contains a sub-population of natural killer cells that are distinct from the conventional splenic natural killer cells. Furthermore, recent work revealed the existence of two types of natural killer cells within the liver: some of these cells were similar to the conventional splenic natural killer cells that circulate throughout the body, while others appeared to be ‘tissue-resident’ natural killer cells that were poised to deliver an immune response.

Now Sojka et al. show that the tissue-resident natural killer cells found in the liver are a distinct lineage of cells. These cells mature independently from the conventional natural killer cells found in the spleen, and the natural killer cells found in the thymus. Moreover, the skin contains tissue-resident natural killer cells similar to those in the liver; whilst natural killer cells that had previously been discovered in the uterus were shown to contain a fourth distinct tissue-resident lineage.

The work of Sojka et al. will encourage a full re-evaluation of the roles played by natural killer cells to determine which populations of these cells are responsible for implementing immune responses. Furthermore, a more thorough understanding of how tissue-resident natural killer cells function to eliminate diseased or damaged cells, such as cancerous cells, could also contribute to future efforts to develop new anti-cancer treatments.

DOI:http://dx.doi.org/10.7554/eLife.01659.002

Location and cellular stages of natural killer cell development

The identification of distinct tissue-specific natural killer (NK) cell populations that apparently mature from local precursor populations has brought new insight into the diversity and developmental regulation of this important lymphoid subset. NK cells provide a necessary link between the early (innate) and late (adaptive) immune responses to infection. Gaining a better understanding of the processes that govern NK cell development should allow us to harness better NK cell functions in multiple clinical settings, as well as to gain further insight into how these cells undergo malignant transformation. In this review, we summarize recent advances in understanding sites and cellular stages of NK cell development in humans and mice.

Cutting edge: Ly49C/I⁻ neonatal NK cells predispose newborns to autoimmune ovarian disease induced by maternal autoantibody

NK cells are critical in immune responses against pathogens. However, their role in autoimmunity is still controversial. In this study, we demonstrate that neonatal NK cells render newborns more susceptible to neonatal autoimmunity induced by maternal autoantibodies (neonatal autoimmune ovarian disease); thus, neonatal but not adult NK cells are pathogenic after transfer into NK cell-deficient pups. The inhibitory receptors Ly49C/I are expressed in ∼5% of neonatal and ∼50% of adult NK cells. In this study, we show that the presence of Ly49C/I⁺ adult NK cells inhibits neonatal autoimmune ovarian disease induction. Thus, the ontogenetic regulation of Ly49C/I expression determines the propensity to autoantibody-induced autoimmunity. In summary, this study provides definitive evidence of a pathogenic role of NK cells in neonatal autoimmunity and also elucidates a novel mechanism by which neonatal NK cells render newborns more susceptible to autoantibody-induced autoimmunity.

CD62L is critical for maturation and accumulation of murine hepatic NK cells in response to viral infection

NK cells play critical roles in the first line of defense against viruses and other pathogens. However, the factors that control NK cell recruitment into local sites to exert effector functions during viral infection remain poorly understood. In this study, we found that murine NK cells in various organs could be divided into CD62L(-) and CD62L(+) subsets, the latter of which were less abundant in the liver and exhibited a relatively mature NK cell phenotype and a stronger cytotoxic function. Moreover, NK cells acquired CD62L expression after birth, and the frequency of CD62L(+) NK cells gradually increased during postnatal development. In models of polyinosinic-polycytidylic acid administration and adenovirus infection in vivo, CD62L(+) NK cell frequency and absolute numbers in the liver rapidly and markedly increased as a result of the augmented differentiation of CD62L(-) to CD62L(+) NK cells and recruitment of peripheral mature NK cells to the liver. However, blocking CD62L prior to administering viral stimuli in vivo abolished viral stimulation-induced NK cell accumulation and maturation in the liver. Collectively, these data suggest that CD62L marks a mature NK cell subset, as well as affects the magnitude of the local NK cell response to viral infection.

Cytotoxic lymphocyte microRNAs as prospective biomarkers for Chronic Fatigue Syndrome/Myalgic Encephalomyelitis

BACKGROUND

Immune dysfunction associated with a disease often has a molecular basis. A novel group of molecules known as microRNAs (miRNAs) have been associated with suppression of translational processes involved in cellular development and proliferation, protein secretion, apoptosis, immune function and inflammatory processes. MicroRNAs may be implicated in Chronic Fatigue Syndrome/Myalgic Encephalomyelitis (CFS/ME), where immune function is impaired. The objective of this study was to determine the association between miRNAs in cytotoxic cells and CFS/ME.

METHODS

Natural Killer (NK) and CD8(+)T cells were preferentially isolated from peripheral blood mononuclear cells from all participants (CFS/ME, n=28; mean age=41.8±9.6 years and controls, n=28; mean age=45.3±11.7 years), via negative cell enrichment. Following total RNA extraction and subsequent synthesis of cDNA, reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) was used to determine the expression levels of nineteen miRNAs.

RESULTS

There was a significant reduction in the expression levels of miR-21, in both the NK and CD8(+)T cells in the CFS/ME sufferers. Additionally, the expression of miR-17-5p, miR-10a, miR-103, miR-152, miR-146a, miR-106, miR-223 and miR-191 was significantly decreased in NK cells of CFS/ME patients in comparison to the non-fatigued controls.

LIMITATIONS

The results from these investigations are not yet transferable into the clinical setting, further validatory studies are now required.

CONCLUSIONS

Collectively these miRNAs have been associated with apoptosis, cell cycle, development and immune function. Changes in miRNAs in cytotoxic cells may reduce the functional capacity of these cells and disrupt effective cytotoxic activity along with other immune functions in CFS/ME patients.

TGF-β is responsible for NK cell immaturity during ontogeny and increased susceptibility to infection during mouse infancy

A major gap in our understanding of infant immunity is why natural killer (NK) cellresponses are deficient, making infants more prone to viral infection. Here we demonstrate that transforming growth factor-β (TGF-β) was responsible for NK cell immaturity during infancy. Higher numbers of fully mature NK cells were found in CD11c^dnR mice, whose NK cells lack TGF-βR signaling. Importantly, ontogenic maturation of NK cells progressed faster in the absence of TGF-β signaling, resulting in the formation of mature NK cell pool early in life. As a consequence, infant CD11c^dnR mice efficiently controlled viral infections. These data thus demonstrate an unprecedented role for TGF-β in ontogeny that can explain why NK cell responses are deficient early in life.

Innate immune control of HIV

Mounting evidence suggests a role for innate immunity in the early control of HIV infection, before the induction of adaptive immune responses. Among the early innate immune effector cells, dendritic cells (DCs) respond rapidly following infection aimed at arming the immune system, through the recognition of viral products via pattern recognition receptors. This early response results in the potent induction of a cascade of inflammatory cytokines, intimately involved in directly setting up an antiviral state, and indirectly activating other antiviral cells of the innate immune system. However, epidemiologic data strongly support a role for natural killer (NK) cells as critical innate mediators of antiviral control, through the recognition of virally infected cells through a network of receptors called the killer immunoglobulin-like receptors (KIRs). In this review, the early events in innate immune recognition of HIV, focused on defining the biology underlying KIR-mediated NK-cell control of HIV viral replication, are discussed.

Natural killer cell tolerance: control by self or self-control?

A major challenge for the immune system is to control pathogens and stressed cells, such as infected or tumors cells, while sparing healthy self-cells. To achieve this tolerance to self, immune cells must recognize and differentiate "self" versus "nonself" and "self" versus "altered self." In the absence of self-tolerance, cells of the adaptive immune system attack healthy cells and cause autoimmune diseases such as lupus, psoriasis, and type I diabetes. Mechanisms at work to ensure tolerance in the innate immune system are still poorly understood. Natural killer cells are innate immune lymphocytes, which have the capacity to kill cellular targets and produce cytokines without prior specific sensitization. Because of these intrinsic effector capacities, tolerance mechanisms must exist to prevent autoreactivity. Herein, we will review the present knowledge on NK cell tolerance.

Immunosenescence and Challenges of Vaccination against Influenza in the Aging Population

Influenza is an important contributor to morbidity and mortality worldwide. Accumulation of genetic mutations termed antigenic drift, allows influenza viruses to inflict yearly epidemics that may result in 250,000 to 500,000 deaths annually. Over 90% of influenza-related deaths occur in the older adult population. This is at least in part a result of increasing dysregulation of the immune system with age, termed immunosenescence. This dysregulation results in reduced capacity to cope with infections and decreased responsiveness to vaccination. The older adult population is in dire need of improved vaccines capable of eliciting protective responses in the face of a waning immune system. This review focuses on the status of immunity, responses to influenza vaccination, and strategies that are currently being explored to elicit enhanced immune responses in this high risk population.

Development of murine hepatic NK cells during ontogeny: comparison with spleen NK cells

The phenotype of developing liver NK cells (CD3(-)NK1.1(+)) was investigated during mouse ontogeny comparing with spleen NK cells. The highest percentage of hepatic CD27(-)CD11b(-) NK cells occurred at the fetal stage. After birth, the percentage of CD27(-)CD11b(-)NK cells in both the liver and spleen gradually decreased to their lowest level at 6 weeks. More CD27(+)CD11b(-)NK cells were detected in the liver than that in spleen from week 1 to 6. Expression of NKG2A on liver NK cells was decreased but still much higher than that of spleen NK cells after 1 week. The NKG2D expression on liver NK cells increased to its highest level and was significantly higher than on spleen NK cells till 4 weeks. During mouse ontogeny, weaker expression of NKp46 and CD2 and stronger expression of CD69, CD11c, 2B4, and CD73 were observed on liver NK cells. Furthermore, neonatal liver NK cells express higher IFN-γ and perforin than adult .These results suggest that the maturation process of NK cells is unique in the livers, and liver microenvironments might play critical roles to keep NK cells in an immature status.

Identification of the earliest natural killer cell-committed progenitor in murine bone marrow

Natural killer (NK) cells develop in the bone marrow and are known to gradually acquire the ability to eliminate infected and malignant cells, yet the cellular stages of NK lineage commitment and maturation are incompletely understood. Using 12-color flow cytometry, we identified a novel NK-committed progenitor (pre-NKP) that is a developmental intermediate between the upstream common lymphoid progenitor and the downstream NKP, previously assumed to represent the first stage of NK lineage commitment. Our analysis also refined the purity of NKPs (rNKP) by 6-fold such that 50% of both pre-NKP and rNKP cells gave rise to NKp46+ NK cells at the single-cell level. On transplantation into unconditioned Rag2-/-Il2rγc-/- recipients, both pre-NKPs and rNKPs generated mature NK cells expressing a repertoire of Ly49 family members that degranulated on stimulation ex vivo. Intrathymic injection of these progenitors, however, yielded no NK cells, suggesting a separate origin of thymic NK cells. Unlike the rNKP, the pre-NKP does not express IL-2Rβ (CD122), yet it is lineage committed toward the NK cell fate, adding support to the theory that IL-15 signaling is not required for NK commitment. Taken together, our data provide a high-resolution in vivo analysis of the earliest steps of NK cell commitment and maturation.

Unlicensed natural born killers

In this issue of Blood, Barao and colleagues describe repopulation of Ly49G2 single-positive NK cells after congenic hematopoietic stem cell transplantation (HSCT) that appear fully functional, having tumor cytolytic capacity despite recipients lacking MHC molecules thought necessary to license them.

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.

Regulation of the earliest immune response to in utero hematopoietic cellular transplantation

In Utero Hematopoietic Cellular Transplantation (IUHCT) is a promising intervention to treat a wide range of congenital disease. Through the presentation of donor cells to the immature immune system, mixed hematopoietic chimerism and donor-specific tolerance can be achieved. However, the failure of engraftment in prenatal recipients in which no immunodeficiency exists suggests the existence of a fetal immune barrier to transplantation. Although the possible barriers include effectors of the adaptive and innate immune system, our recent findings and ongoing investigations indicate that the barrier most likely resides in the developing NK cells. A chimerism level above a certain threshold during NK cell development is necessary to overcome rejection. Clinically, this transplantation barrier might also exist in early human fetal NK cells. Understanding the fetal immune barrier to allotransplantation is essential in advancing clinical application of IUHCT. Herein, we provide a short summary and new evidence for the earliest immune response to prenatal transplantation.

Innate and adaptive immunosenescence

OBJECTIVE

To review the effect of increasing age on the immune system and some of its clinical implications.

DATA SOURCES

MEDLINE and PubMed searches were performed cross-referencing the keywords immunosenescence, aging, and immunity. Articles were reviewed for additional citations.

STUDY SELECTION

Articles were reviewed and selected based on relevance to subject matter.

RESULTS

The study of immunosenescence is complex and not completely understood. Aging affects both the innate and adaptive arms of the immune response. With increased age, there may be a decrease in phagocytosis, alteration of cellular migration, changes in cell populations and numbers, and a decreased ability to produce specific antibodies. Clinically, these changes potentially increase morbidity and mortality in elderly individuals through an increased rate of infections, malignancy, and autoimmunity.

CONCLUSIONS

The process of aging is accompanied by diverse changes in immunity. Several therapeutic approaches are under investigation, including cytokine therapy, hormonal replacement, antioxidant supplementation, and caloric restriction, to attenuate or potentially reverse immunosenescence.

Two complementary rat NK cell subsets, Ly49s3+ and NKR-P1B+, differ in phenotypic characteristics and responsiveness to cytokines

Two major subsets of rat NK cells can be distinguished based on their expression of the Ly49s3 or the NKR-P1B lectin-like receptor. Ly49s3(+) NK cells, but not NKR-P1B(+) NK cells, express a wide range of Ly49 receptors. Here, we have examined differences between these two subsets in their expression of certain NK cell-associated molecules as well as their responses to cytokines. A microarray analysis suggested several differentially expressed genes, including preferential expression of NKG2A/C receptors by NKR-P1B(+) NK cells. This was confirmed by staining with tetramers of RT.BM1, the putative ligand of CD94/NKG2, indicating that Ly49 and CD94/NKG2 receptors separate into distinct NK cell compartments. Further, expression of CD25 by Ly49s3(+) NK cells was associated with more rapid proliferation in response to IL-2 as compared with NKR-P1B(+) NK cells. Thus, certain inflammatory situations may preferentially expand the Ly49s3(+) NK cells. Moreover, freshly isolated Ly49s3(+) and NKR-P1B(+) NK cells produce similar amounts of cytokines, and a minor Ly49s3(-)NKR-P1B(-) double-negative NK subset appears to be hyporesponsive based on its significantly lower IFN-gamma production. Collectively, our data demonstrate divergent profiles of NKR-P1B(+) and Ly49s3(+) NK cells, indicating distinct tasks in vivo.

Intrahepatic IL-10 maintains NKG2A+Ly49- liver NK cells in a functionally hyporesponsive state

The tolerogenic nature of the liver allows daily exposure to gut-derived foreign Ags without causing inflammation, but it may facilitate persistent infection in the liver. NK cells play a central role in innate immunity, as well as in shaping the adaptive immune response. We hypothesized that the naive mouse liver maintains intrahepatic NK cells in a functionally hyporesponsive state. Compared with splenic NK cells, liver NK cells displayed a dampened IFN-gamma response to IL-12/IL-18 stimulation. Importantly, 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. Adoptively transferred splenic NK cells that migrate to the liver displayed phenotypic and functional changes, suggesting that the liver environment modifies NK cell receptor expression and functional responsiveness. Notably, IL-10 is present at high levels within the liver, and in vivo blockade of IL-10R resulted in a decreased percentage of intrahepatic NKG2A(+)Ly49(-) NK cells. These data suggest that the liver environment regulates NK cell receptor expression and that IL-10 contributes to the regulation of liver NK cells, in part, by maintaining a greater percentage of the hyporesponsive NKG2A(+)Ly49(-) NK cells in the liver.

Use of stem cell radiation chimeras to analyze how domains of specific proteins impact on murine NK cell development in vivo

Although the use of mutant mice has been extremely useful in identifying those proteins and molecules specifically required for the development of NK cells, the establishment of a well-defined protocol to replicate in vitro the major steps corresponding to the process of NK cell differentiation and maturation has enabled us to dissect the molecular events governing certain aspects of NK cell development. This chapter describes a protocol that combines both the use of mutant mice and the in vitro bone marrow (BM) culture system for examining the role of proteins and their putative signaling domains in NK cell development. BM-derived Lin-c-kit(+) stem cells expressing the protein of interest are first cultured for 6 days in a cocktail of cytokines that promote lymphoid development. The semi-differentiated cells are then transplanted into mice to complete their development in vivo. While all hematopoietic lineages can develop from these transplanted cells, we focus primarily on assessing the effect of the protein on the production of NK cells, as well as the acquisition of Ly49 receptors. The most prevalent advantage of this method is the ability to potentially link signaling regulators to known aspects of NK cell development.

Natural killer cells: tolerance to self and innate immunity to viral infection and malignancy

Natural killer (NK) cells are lymphocytes whose ability to identify and kill virally infected and malignant cells while sparing normal cells was poorly understood until the late 1980’s and the introduction of the “missing self’ hypothesis. According to this hypothesis, downregulation of major histocompatibility complex (MHC) class I molecules during viral infection or malignant transformation triggers NK activation (1 ). Since this hypothesis was first proposed, much has been learned about NK cell surface receptors, their role in the molecular basis of missing-self recognition, and the mechanisms underlying NK cell tolerance. In this review, we will discuss these mechanisms, as well as their relevance to viral infection and tumor immunity and stem cell transplantation.

Inhibitory Ly49 receptors on mouse natural killer cells

The Ly49 receptors, which are expressed in a stochastic manner on subsets of murine natural killer (NK) cells, T cells, and other cells, are encoded by the Klra gene family and include receptors with either inhibitory or activating function. All of the inhibitory Ly49 receptors are characterized by an immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic domain, which upon phosphorylation recruits tyrosine or lipid phosphatases to dampen signals transmitted through other activating receptors. Most of the inhibitory Ly49 receptors recognize polymorphic epitopes on major histocompatibility complex (MHC) class I proteins as ligands. Here, we review the polymorphism, ligand specificity, and signaling capacity of the inhibitory Ly49 receptors and discuss how these molecules regulate NK cell development and function.

KIR acquisition probabilities are independent of self-HLA class I ligands and increase with cellular KIR expression

Inhibitory killer cell immunoglobulin-like receptors (KIRs) preserve tolerance to self and shape the functional response of human natural killer (NK) cells. Here, we have evaluated the influence of selection processes in the formation of inhibitory KIR repertoires in a cohort of 44 donors homozygous for the group A KIR haplotype. Coexpression of multiple KIRs was more frequent than expected by the product rule that describes random association of independent events. In line with this observation, the probability of KIR acquisition increased with the cellular expression of KIRs. Three types of KIR repertoires were distinguished that differed in frequencies of KIR- and NKG2A-positive cells but showed no dependency on the number of self-HLA class I ligands. Furthermore, the distribution of self- and nonself-KIRs at the cell surface reflected a random combination of receptors rather than a selection process conferred by cognate HLA class I molecules. Finally, NKG2A was found to buffer overall functional responses in KIR repertoires characterized by low-KIR expression frequencies. The results provide new insights into the formation of inhibitory KIR repertoires on human NK cells and support a model in which variegated KIR repertoires are generated through sequential and random acquisition of KIRs in the absence of selection.

Cutting edge: killer Ig-like receptors mediate "missing self" recognition in vivo

Although it is well established that human NK cells are able to detect the absence of autologous HLA class I in vitro by virtue of inhibitory killer Ig-like receptors (KIR), direct evidence that KIR can mediate "missing self" recognition in vivo is lacking. To test this, we generated mice transgenic for a human KIR B-haplotype and HLA-Cw3 on a C57BL/6 background. NK cells in these mice expressed multiple KIR in a stochastic manner, including the HLA-Cw3-specific inhibitory receptor KIR2DL2. KIR and HLA transgenic mice rejected wild-type C57BL/6 spleen cells upon i.v. injection. This rejection was dependent on the presence of the KIR transgene in the host and on the absence of HLA-Cw3 from the injected target cells. Hence, the KIR transgene mediated "missing self" recognition in vivo. We anticipate that this KIR and HLA transgenic mouse will help shed light on KIR and HLA effects in disease and transplantation.

Natural killer cell tolerance licensing and other mechanisms

Armed with potent cytotoxic and immunostimulatory effector functions, natural killer (NK) cells have the potential to cause significant damage to normal self cells unless controlled by self-tolerance mechanisms. NK cells identify and attack target cells based on integration of signals from activation and inhibitory receptors, whose ligands exhibit complex expression and/or binding patterns. Preservation of NK cell self-tolerance must therefore go beyond mere engagement of inhibitory receptors during effector functions. Herein, we review recent work that has uncovered a number of mechanisms to ensure self-tolerance of NK cells. For example, licensing of NK cells allows only NK cells that can engage self-MHC to become functionally competent, or licensed. The molecular mechanism of this phenomenon appears to require signaling by receptors that were originally identified in effector inhibition. However, the nature of the signaling event has not yet been defined, but new interpretations of several published experiments provide valuable clues. In addition, several other cell-intrinsic and -extrinsic mechanisms of NK cell tolerance are discussed, including activation receptor cooperation and synergy, cytokine stimulation, and the opposing roles of accessory and regulatory cells. Finally, NK cell tolerance is discussed as it relates to the clinic, such as KIR-HLA disease associations, tumor immunotherapy, and fetal tolerance.

Early chimerism threshold predicts sustained engraftment and NK-cell tolerance in prenatal allogeneic chimeras

The failure of engraftment in human cases of in utero hematopoietic cell transplantation (IUHCT) in which no immunodeficiency exists suggests the presence of an unrecognized fetal immune barrier. A similar barrier in murine IUHCT appears to be dependent on the chimerism level and is poorly explained by a lack of T-cell tolerance induction. Therefore, we studied the effect of the chimerism level on engraftment and host natural killer (NK)-cell education in a murine model of IUHCT. The dose of transplanted cells was found to exhibit a strong correlation with both the engraftment rate and chimerism level. More specifically, a threshold level of initial chimerism (> 1.8%) was identified that predicted durable engraftment for allogeneic IUHCT, whereas low initial chimerism (< 1.8%) predicted a loss of engraftment. NK cells taken from chimeras above the "chimerism threshold" displayed durable calibration of alloresponsive Ly49A receptors and tolerance to donor antigens. Depletion of recipient NK cells stabilized engraftment in low-level chimeras (< 1.8%). These studies illustrate the importance of the early chimerism threshold in predicting long-term engraftment and host NK-cell tolerance after in utero transplantation.

Regulation of NK cell responsiveness to achieve self-tolerance and maximal responses to diseased target cells

Inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules govern the capacity of natural killer (NK) cells to attack class I-deficient cells ('missing-self recognition'). These receptors are expressed stochastically, such that the panel of expressed receptors varies between NK cells. This review addresses how the activity of NK cells is coordinated in the face of this variation to achieve a repertoire that is self-tolerant and optimally reactive with diseased cells. Recent studies show that NK cells arise in normal animals or humans that lack any known inhibitory receptors specific for self-MHC class I. These NK cells exhibit self-tolerance and exhibit functional hyporesponsiveness to stimulation through various activating receptors. Evidence suggests that hyporesponsiveness is induced because these NK cells cannot engage inhibitory MHC class I molecules and are therefore persistently over-stimulated by normal cells in the environment. Finally, we discuss evidence that hyporesponsiveness is a quantitative trait that varies depending on the balance of signals encountered by developing NK cells. Thus, a tuning process determines the functional set-point of NK cells, providing a basis for discriminating self from missing-self, and at the same time endowing each NK cell with the highest inherent responsiveness compatible with self-tolerance.

A combined genotype of KIR3DL1 high expressing alleles and HLA-B*57 is associated with a reduced risk of HIV infection

OBJECTIVES

Coexpression of certain combinations of natural killer cell receptor KIR3DL1 and HLA-B alleles is associated with slower time to AIDS. The strongest protection in terms of disease outcome in KIR3DL1 homozygotes (3DL1 hmz) is coexpression of HLA-B*57 and a set of KIR3DL1 genotypes (3DL1*h/*y) lacking alleles expressed at low levels on natural killer cells. We questioned whether this allele combination could also influence resistance to infection.

DESIGN

The genetic distribution of 3DL1*h/*y and HLA-B*57 was compared in 41 HIV-exposed uninfected and 186 recently HIV-infected 3DL1 hmz.

METHODS

KIR3DL1 subtyping was performed by sequencing the exons 3, 4, 5, 7-9. The major histocompatibility complex class IB locus was typed by sequence specific oligonucleotide PCR and sequencing to resolve Bw4 and Bw6 alleles and the amino acid present at position 80.

RESULTS

Percentage carriers of HLA-B*57 in HIV-exposed uninfected and individuals in a primary infection cohort was 12.2 and 4.3%, respectively (P = 0.0631), whereas that of 3DL1*h/*y was similar in both populations (P = 0.221). The 3DL1*h/*y-HLA-B*57 combined genotype was more frequent in exposed uninfected individuals (12.2%) than individuals in primary infection (2.7%) (P = 0.019; odds ratio, 5.03; 95% confidence intervals, 1.38-18.3).

CONCLUSION

Coexpression of 3DL1*h/*y and B*57, which has been associated with a reduced risk of progressing to AIDS in HIV-infected individuals also lowers the risk of HIV infection in exposed uninfected individuals.

NK cell tolerance and the maternal-fetal interface

Natural killer (NK) cells play a fundamental role in the innate immune response through their ability to secrete cytokines and kill target cells without prior sensitization. These effector functions are central to NK cell anti-viral and anti-tumor abilities. Due to their cytotoxic nature, it is vital that NK cells have the capacity to recognize normal self-tissue and thus prevent their destruction. In addition to their role in host defense, NK cells accumulate at the maternal-fetal interface and are thought to play a critical role during pregnancy. The close proximity of uterine NK (uNK) cells to fetal trophoblast cells of the placenta would seemingly lead to catastrophic consequences, as the trophoblast cells are semi-allogeneic. A fundamental enigma of pregnancy is that the fetal cells constitute an allograft but, in normal pregnancies, they are in effect not perceived as foreign and are not rejected by the maternal immune system. Although the mechanisms involved in achieving NK cell tolerance are becoming increasingly well-defined, further clarification is required, given the clinical implications of this work in the areas of infection, transplantation, cancer and pregnancy. Herein, we discuss several mechanisms of NK cell tolerance and speculate as to how they may apply to uNK cells at the maternal-fetal interface.