A subset of natural killer cells achieves self-tolerance without expressing inhibitory receptors specific for self-MHC molecules

Host genetic determinants of HIV pathogenesis: an immunologic perspective

PURPOSE OF REVIEW

The purpose of this review is to highlight recent advances in our understanding of host genetic determinants of HIV pathogenesis and to provide a theoretical framework for interpreting these studies in the context of our evolving understanding of HIV immunopathogenesis.

RECENT FINDINGS

The first genome-wide association analysis of host determinants of HIV pathogenesis and other recent studies evaluating the interaction between killer cell immunoglobulin-like receptors and human leukocyte antigen alleles have implicated both adaptive and innate immune responses in the control of HIV replication. Furthermore, genetic variation associated with the expression of CCR5 and its ligand have been strongly associated with both decreased susceptibility to HIV infection and delayed clinical progression, independent of their effects on viral replication, suggesting a potential role for CCR5 inhibitors as immune-based therapies in HIV disease.

SUMMARY

Host factors associated with the control of HIV replication may help identify important targets for vaccine design, while those associated with delayed clinical progression provide targets for future immune-based therapies against HIV infection.

NKG2D-deficient mice are defective in tumor surveillance in models of spontaneous malignancy

Ligands for the NKG2D stimulatory receptor are frequently upregulated on tumor lines, rendering them sensitive to natural killer (NK) cells, but the role of NKG2D in tumor surveillance has not been addressed in spontaneous cancer models. Here, we provided the first characterization of NKG2D-deficient mice, including evidence that NKG2D was not necessary for NK cell development but was critical for immunosurveillance of epithelial and lymphoid malignancies in two transgenic models of de novo tumorigenesis. In both models, we detected NKG2D ligands on the tumor cell surface ex vivo, providing needed evidence for ligand expression by primary tumors. In a prostate cancer model, aggressive tumors arising in NKG2D-deficient mice expressed higher amounts of NKG2D ligands than did similar tumors in wild-type mice, suggesting an NKG2D-dependent immunoediting of tumors in this model. These findings provide important genetic evidence for surveillance of primary tumors by an NK receptor.

Line of attack: NK cell specificity and integration of signals

Natural killer (NK) cells possess potent cytolytic activity and secrete immune modulating cytokines. The large repertoire of NK cell receptors provides versatility for the identification of infected and transformed cells and for their elimination by NK cells. NK cell responses also stimulate and regulate the adaptive arm of the immune system. We review current knowledge about the molecular specificity of NK cell receptors and about the regulation of NK cell effector functions upon encounter with target cells. Mechanisms of recognition, interplay among receptors, signal integration, and the dynamic fine-tuning of NK cell responses are discussed. New insights into the molecular checkpoints for NK cell effector function are highlighted, and underlying reasons for the complexity in NK cell recognition and signaling are proposed.

Distinct KIR/HLA compound genotypes affect the kinetics of human antiviral natural killer cell responses

Genetic studies suggest a role for killer cell immunoglobulin-like receptor/HLA (KIR/HLA) compound genotypes in the outcome of viral infections, but functional data to explain these epidemiological observations have not been reported. Using an in vitro model of infection with influenza A virus (IAV), we attribute functional differences in human NK cell activity to distinct KIR/HLA genotypes. Multicolor flow cytometry revealed that the HLA-C-inhibited NK cell subset in HLA-C1 homozygous subjects was larger and responded more rapidly in IFN-gamma secretion and CD107a degranulation assays than its counterpart in HLA-C2 homozygous subjects. The differential IFN-gamma response was also observed at the level of bulk NK cells and was independent of KIR3DL1/HLA-Bw4 interactions. Moreover, the differential response was not caused by differences in NK cell maturation status and phenotype, nor by differences in the type I IFN response of IAV-infected accessory cells between HLA-C1 and HLA-C2 homozygous subjects. These results provide functional evidence for differential NK cell responsiveness depending on KIR/HLA genotype and may provide useful insights into differential innate immune responsiveness to viral infections such as IAV.

Antiviral NK cell responses in HIV infection: I. NK cell receptor genes as determinants of HIV resistance and progression to AIDS

NK cells play an important role in controlling viral infections. They can kill virus-infected cells directly as well as indirectly via antibody-dependent, cell-mediated cytotoxicity. They need no prior sensitization and expansion for this killing. NK cells are also considered as important regulators of antiviral immune responses. They do so by secreting a multitude of soluble mediators and by directly interacting with other immune cells, e.g., dendritic cells. NK cells do not possess a single well-defined receptor to recognize antigens on target cells. Instead, they express an array of inhibitory and activating receptors and coreceptors, which bind to their cognate ligands expressed on the surface of target cells. These ligands include classical and nonclassical MHC class I antigens, MHC-like proteins, and a variety of other self- and virus-derived molecules. They may be expressed constitutively and/or de novo on the surface of virus-infected cells. NK cell receptors (NKRs) of the killer-cell Ig-like receptor (KIR) family, like their MHC class I ligands, are highly polymorphic. Several recent studies suggest that epistatic interactions between certain KIR and MHC class I genes may determine innate resistance of the host to viral infections, including HIV. In the first part of this review article, we provide an overview of the current state of knowledge of NK cell immunobiology and describe how NKR genes, alone and in combination with HLA genes, may determine genetic resistance/susceptibilty to HIV infection and the development of AIDS in humans.

Enhanced NK-cell development and function in BCAP-deficient mice

In B lymphocytes, the B-cell adaptor for phosphatidylinositol 3-kinase (BCAP) facilitates signaling from the antigen receptor. Mice lacking BCAP have a predominantly immature pool of B cells with impaired immune function and increased susceptibility to apoptosis. Unexpectedly, we have found that natural killer (NK) cells from BCAP-deficient mice are more mature, more long-lived, more resistant to apoptosis, and exhibit enhanced functional activity compared with NK cells from wild-type mice. Surprisingly, these effects are evident despite a severe impairment of the immunoreceptor tyrosine-based activation motif-mediated Akt signaling pathway. The seemingly paradoxical phenotype reveals inherent differences in the signals controlling the final maturation of B cells and NK cells, which depend on positive and negative signals, respectively. Both enhanced interferon-gamma responses and augmented maturation of NK cells in BCAP-deficient mice are independent of available MHC class I ligands. Our data support a model in which blunting of BCAP-mediated activation signaling in developing NK cells promotes functionality, terminal maturation, and long-term survival.

Lineage-specific transition of histone signatures in the killer cell Ig-like receptor locus from hematopoietic progenitor to NK cells

The clonal distribution and stable expression of killer cell Ig-like receptor (KIR) genes is epigenetically regulated. To assess the epigenetic changes that occur during hemopoietic development we examined DNA methylation and chromatin structure of the KIR locus in early hemopoietic progenitor cells and major lymphocyte lineages. In hemopoietic progenitor cells, KIR genes exhibited the major hallmarks of epigenetic repression, which are dense DNA methylation, inaccessibility of chromatin to Micrococcus nuclease digest, and a repressive histone signature, characterized by strong H3K9 dimethylation and reduced H4K8 acetylation. In contrast, KIR genes of NK cells showed active histone signatures characterized by absence of H3K9 dimethylation and presence of H4K8 acetylation. Histone modifications correlated well with the competence of different lymphocyte lineages to express KIR; whereas H4K8 acetylation was high in NK and CD8+ T cells, it was almost absent in CD4+ T cells and B cells and, in the latter case, replaced by H3K9 dimethylation. In KIR-competent lineages, active histone signatures were also observed in silent KIR genes and in this case found in combination with dense DNA methylation of the promoter and nearby regions. The study suggests a two-step model of epigenetic regulation in which lineage-specific acquisition of euchromatic histone marks is a prerequisite for subsequent gene-specific DNA demethylation and expression of KIR genes.

HLA alleles determine differences in human natural killer cell responsiveness and potency

Epidemiological studies have associated certain human disease outcomes with particular killer cell Ig-like receptor (KIR) and HLA genotypes. However, the functional explanation for these associations is poorly understood, because the KIRs were initially described as natural killer (NK) cell inhibitory receptors with specificity for HLA molecules on their cellular targets. Yet resolution of infections is often associated with genotypic pairing of inhibitory KIRs with their cognate HLA ligands. Recent studies in mice indicate a second role for MHC-specific inhibitory receptors, i.e., self-MHC recognition confers functional competence on the NK cell to be triggered through their activation receptors, a process termed licensing. As a result, licensed NK cells with self-MHC-specific receptors are more readily activated as compared with unlicensed NK cells without self-MHC-specific receptors. Such results predict that human NK cells may undergo a similar process. Here, we examined the human NK cell subset expressing KIR3DL1, the only known KIR specific for HLA-Bw4 alleles. The KIR3DL1(+) subset in normal donors with two HLA-B-Bw4 genes displayed increased responsiveness to tumor stimulation compared with the KIR3DL1(+) subset from individuals with only one or no Bw4 genes. By contrast, NK cells lacking KIR3DL1 showed no differences. Therefore, these data indicate that particular KIR and HLA alleles are associated with more responsive NK cells, strongly suggesting that human NK cells are also subjected to NK cell licensing, and providing a potential functional explanation for the influence of KIR and HLA genes in disease as well as interindividual differences in NK cell potency.

Self-tolerance of human natural killer cells lacking self-HLA-specific inhibitory receptors

Natural killer (NK) cells identify cells with altered human leucocyte antigen (HLA) expression as targets through lacking engagement of self-HLA-specific inhibitory receptors (e.g. killer cell immunoglobulin-like receptor, KIR). Thus, they eliminate cells with 'missing self' because of viral or malignant transformation. We performed analysis of HLA, KIR genotypes and KIR receptor expression patterns at single cell level in NK cells in 17 donors. The function of NK cell subsets is determined by degranulation assays using target cells expressing self, cognate, control or no HLA class I. Donors could be grouped into three groups: their NK cells possess potential for alloreactivity, autoreactivity based on the presence of NK cells expressing particular KIR only (mono-KIR) in the absence of its ligand or lack alloreactivity. All donors possess NK cells lacking all detectable inhibitory receptors. Both potential autoreactive subpopulations did not respond to HLA class I-positive target cells. They retain partial reactivity against HLA class I-negative tumour target cells. Mono-KIR NK cells without the corresponding ligands in the individuals and NK cells lacking all inhibitory receptors behave self-tolerant. Our results suggest alternative mechanisms than HLA-specific inhibitory receptors to control NK cell activity. But HLA seems to be involved in shaping effector function of the NK cell repertoire.

Humanized immune system (HIS) mice as a tool to study human NK cell development

The study of human hematopoiesis is conditioned by access to nondiseased human tissue samples that harbor the cellular substrates for this developmental process. Technical and ethical concerns limit the availability to tissues derived from the fetal and newborn periods, while adult samples are generally restricted to peripheral blood. Access to a small animal model that faithfully recapitulates the process of human hematopoiesis would provide an important tool. Natural killer (NK) cells comprise between 10% and 15% of human peripheral blood lymphocytes and appear conserved in several species. NK cells are implicated in the recognition of pathogen-infected cells and in the clearance of certain tumor cells. In this chapter, we discuss NK cell developmental pathways and the use of humanized murine models for the study of human hematopoiesis and, in particular, human NK cell development.

Killer cell immunoglobulin-like receptors on NK cells: the how, where and why

Natural killer (NK) cells have killer cell immunoglobulin-like receptors (KIR) that recognize and interact with HLA class I antigen. The KIRs are a multigene family and its members are often highly polymorphic. Evidence is emerging from disease-association studies that KIR receptors can play beneficial roles in viral infections, such as HIV, HCV, but may also predispose to certain autoimmune diseases. Knowledge regarding expression and function of KIR on human NK cells is lagging behind the rapid expansion of sequencing and genetic data already generated. This review focuses on recent discoveries that have been made, which help bridge this gap. We now appreciate the importance of phenotypic diversity of KIR receptor expression in NK cells and are starting to unravel some of the mysteries surrounding control of their complex expression patterns. In particular, the role that HLA ligand contributes to KIR receptor expression will be discussed. It is also becoming increasingly clear that genetic factors, such as promoters and epi-genetic mechanisms such as methylation, are hugely important in controlling NK cell receptor expression and function. The relevance of phenotypic diversity of NK cell receptors will be discussed in light of these recent findings.

IFN-gamma production and degranulation are differentially regulated in response to stimulation in murine natural killer cells

Activation of natural killer (NK) cells is induced via receptors like NKG2D, NKR-P1C and NKp46. This activation is balanced by interactions with inhibitory receptors. NK cell activation can lead to cytotoxicity mediated via polarized exocytosis of secretory lysosomes (degranulation) and interferon (IFN)-gamma production. We studied cell surface mobilization of a molecule present in secretory lysosomes, CD107a (LAMP-1), to monitor the relationship between degranulation of NK cells and their production of IFN-gamma at the single cell level. A comparison of responses in naive mouse NK cells and NK cells pre-activated with the type I interferon-inducer tilorone demonstrated a dramatic influence of pre-activation, allowing potent degranulation and IFN-gamma responses to NKG2D mediated stimulation that were not observed with naive NK cells. Degranulation and IFN-gamma production were performed by overlapping NK cell populations with generally higher frequencies of degranulating than IFN-gamma producing NK cells. An NK cell subset analysis based on expression of Mac-1 and CD27 revealed that immature NK cells (Mac-1(lo) CD27(hi)) are preferentially degranulating, Mac-1(hi) CD27(hi) cells perform both effector functions efficiently, while the most mature (Mac-1(hi) CD27(lo)) NK cells display reduced degranulation but with maintained IFN-gamma production.

Developmental pathways that generate natural-killer-cell diversity in mice and humans

Natural killer (NK) cells are large granular lymphocytes capable of producing inflammatory cytokines and spontaneously killing malignant, infected or 'stressed' cells. These NK-cell functions are controlled by cell-surface receptors that titrate stimulatory and inhibitory signals. However, we remain puzzled about where and when NK cells develop and differentiate, and this has fuelled the debate over the diversification of the peripheral NK-cell pool: are NK cells functionally homogeneous or are there subsets with specialized effector functions? In this Review, we consider the developmental relationships and biological significance of the diverse NK-cell subsets in mice and humans, and discuss how new humanized mouse models may help to characterize them further.

The combined actions of NK and T lymphocytes are necessary to reject an EGFP+ mesenchymal tumor through mechanisms dependent on NKG2D and IFN gamma

Better understanding of the mechanisms that mediate spontaneous immune rejections ought to be important in the quest for improvements in immunotherapy of cancer. A set of intraperitoneal tumors of mesenchymal origin that had been chemically induced in ubiquitously expressing EGFP transgenic mice provided a model in which both T and NK cells were absolutely required for tumor rejection. Tumor cells were traceable because of being fluorescent and readily grafted in RAG1(-/-) immunodeficient mice, whereas they were rejected in a majority of syngeneic C57BL/6 and EGFP-transgenic mice. Tumor-cell clones with the highest EGFP expression tended to be rejected, but a direct involvement of EGFP as the antigen recognized for the immune rejections was ruled out. Rejections were absolutely dependent on NK cells as well as on CD4(+) and CD8(+) T lymphocytes according to selective depletion studies. Furthermore, CD8(+) and CD4(+) T lymphocytes as well as NK cells were detected in the inflammatory infiltrate that mediates tumor rejection along with some DC. The effects of IFN gamma, produced at the tumor site by T and NK lymphocytes, were only required at the malignant cell level and were necessary for tumor eradication. NK recognition of tumor cells was mediated by the NKG2D-activating receptor and blocking its function in vivo partially interfered with rejection. Therefore, complete rejection of these mesenchymal tumors requires a concerted set of activities including direct tumor-cell destruction and IFN gamma production that are mediated by both NK and T cells.

Bcl10 plays a divergent role in NK cell-mediated cytotoxicity and cytokine generation

Activating receptors such as NKG2D and Ly49D mediate a multitude of effector functions including cytotoxicity and cytokine generation in NK cells. However, specific signaling events that are responsible for the divergence of distinct effector functions have yet to be determined. In this study, we show that lack of caspase recruitment domain-containing protein Bcl10 significantly affected receptor-mediated cytokine and chemokine generation, but not cytotoxicity against tumor cells representing "missing-self" or "induced-self." Lack of Bcl10 completely abrogated the generation of GM-CSF and chemokines and it significantly reduced the generation of IFN-gamma (>75%) in NK cells. Commitment, development, and terminal maturation of NK cells were largely unaffected in the absence of Bcl10. Although IL-2-activated NK cells could mediate cytotoxicity to the full extent, the ability of the freshly isolated NK cells to mediate cytotoxicity was somewhat reduced. Therefore, we conclude that the Carma1-Bcl10-Malt1 signaling axis is critical for cytokine and chemokine generation, although it is dispensable for cytotoxic granule release depending on the activation state of NK cells. These results indicate that Bcl10 represents an exclusive "molecular switch" that links the upstream receptor-mediated signaling to cytokine and chemokine generations.

NK cell alloreactivity and allogeneic hematopoietic stem cell transplantation

As only 60% of leukaemia patients find a matched donor, the Perugia Bone Marrow Transplant Centre developed transplantation from HLA haplotype-mismatched family donors to provide a cure for more patients [F. Aversa, A. Tabilio, A. Terenzi, et al., Successful engraftment of T-cell-depleted haploidentical "three-loci" incompatible transplants in leukemia patients by addition of recombinant human granulocyte colony-stimulating factor-mobilized peripheral blood progenitor cells to bone marrow inoculum, Blood 84 (1994) 3948-3955] [F. Aversa, A. Tabilio, A. Velardi, et al., Treatment of high-risk acute leukemia with T-cell-depleted stem cells from related donors with one fully mismatched HLA haplotype, N. Engl. J. Med. 339 (1998) 1186-1193] [F. Aversa, A. Terenzi, A. Tabilio, et al., Full haplotype-mismatched hematopoietic stem-cell transplantation: a phase II study in patients with acute leukemia at high risk of relapse, J. Clin. Oncol. 23 (2005) 3447-3454]. HLA-mismatches trigger donor vs. recipient NK cell alloreactivity which improves engraftment, protects from GvHD and reduces relapse in AML patients [L. Ruggeri, M. Capanni, E. Urbani, et al., Effectiveness of donor natural killer cell alloreactivity in mismatched hematopoietic transplants, Science 295 (2002) 2097-2100], [L. Ruggeri, A. Mancusi, M. Capanni, E. Urbani, A. Carotti, T. Aloisi, M. Stern, D. Pende, K. Perruccio, E. Burchielli, F. Topini, E. Bianchi, F. Aversa, M.F. Martelli, A. Velardi, Donor natural killer cell allorecognition of missing self in haploidentical hematopoietic transplantation for acute myeloid leukemia: challenging its predictive value, Blood, in press]. We are using murine transplant models to determine whether NK cell alloreactivity can be exploited to reduce transplant-related mortality (TRM) which remains a major issue. Data from these on-going studies show pre-transplant infusion of alloreactive NK cells: (1) ablates AML cells, (2) kills recipient T cells, permitting a reduced toxicity conditioning regimen, and (3) ablates the recipient dendritic cells (DCs) which trigger GvHD, thus protecting from GvHD while permitting a higher T cell content in the graft. We are designing a clinical haploidentical transplant trial using alloreactive NK cells in the conditioning regimen, with the aim of reducing TRM and improving outcomes and overall survival.

NKG2D signaling is coupled to the interleukin 15 receptor signaling pathway

The effector functions of natural killer cells are regulated by activating receptors, which recognize stress-inducible ligands expressed on target cells and signal through association with signaling adaptors. Here we developed a mouse model in which a fusion of the signaling adaptor DAP10 and ubiquitin efficiently downregulated expression of the activating receptor NKG2D on the surfaces of natural killer cells. With this system, we identified coupling of the signaling pathways triggered by NKG2D and DAP10 to those initiated by the interleukin 15 receptor. We suggest that this coupling of activating receptors to other receptor systems could function more generally to regulate cell type-specific signaling events in distinct physiological contexts.

Structure of natural killer receptor 2B4 bound to CD48 reveals basis for heterophilic recognition in signaling lymphocyte activation molecule family

Natural killer (NK) cells eliminate virally infected and tumor cells. Among the receptors regulating NK cell function is 2B4 (CD244), a member of the signaling lymphocyte-activation molecule (SLAM) family that binds CD48. 2B4 is the only heterophilic receptor of the SLAM family, whose other members, e.g., NK-T-B-antigen (NTB-A), are self-ligands. We determined the structure of the complex between the N-terminal domains of mouse 2B4 and CD48, as well as the structures of unbound 2B4 and CD48. The complex displayed an association mode related to, yet distinct from, that of the NTB-A dimer. Binding was accompanied by the rigidification of flexible 2B4 regions containing most of the polymorphic residues across different species and receptor isoforms. We propose a model for 2B4-CD48 interactions that permits the intermixing of SLAM receptors with major histocompatibility complex-specific receptors in the NK cell immune synapse. This analysis revealed the basis for heterophilic recognition within the SLAM family.

HLA and KIR polymorphisms affect NK-cell anti-tumor activity

Killer cell immunoglobulin-like receptors (KIRs) and their cognate human leukocyte antigen (HLA) ligands are key to the maintenance of natural killer (NK) cell tolerance. The gene complexes encoding both KIRs and HLA ligands are extremely polymorphic. Because the extent of NK cell inhibition varies with the allelic forms expressed, NK cell tolerance can be broken more easily in some individuals than in others. This explains why particular combinations of KIR and HLA genes are associated with an increased risk of autoimmune diseases or with more efficient antiviral responses. Breaking of NK cell tolerance might be prerequisite to kill leukemic blasts. At present, there are ample indications that NK cells can eradicate acute myeloid leukemia blasts in patients with a favorable combination of HLA and KIR genes. Selecting these individuals for clinical trials should give insight into the feasibility of anti-tumor therapy mediated through NK cells.

KIR2DS1-positive NK cells mediate alloresponse against the C2 HLA-KIR ligand group in vitro

The inhibitory 2DL1 and activating 2DS1 killer Ig-like receptors (KIR) both have shared ligand specificity for codon sequences in the C2 group HLA-Cw Ags. In this study, we have investigated NK cell activation by allogeneic target cells expressing different combinations of the HLA-KIR ligand groups C1, C2, and Bw4. We demonstrate that fresh NK cells as well as IL-2-propagated NK cells from 2DS1-positive donors that are homozygous for the C1 ligand group are activated in vitro by B lymphoblastoid cell lines expressing the C2 group. This response is, in part, due to the absence of C1 group recognition mediated by the inhibitory receptor 2DL2/3. This "missing self" alloresponse to C2, however, is rarely observed in NK cells from donors lacking 2DS1. Even in presence of 2DS1, the NK alloresponse is dramatically reduced in donors that have C2 group as "self." Analysis of selected NK clones that express 2DS1 mRNA and lack mRNA for 2DL1 demonstrates that activation by the C2 ligand and mAb cross-linking of 2DS1 in these clones induces IFN-gamma. Furthermore, this C2 group-induced activation is inhibited by Abs to both HLA class I and the receptor. Collectively, these studies demonstrate that NK cells from 2DS1-positive donors are activated by target cells that express the C2 group as an alloantigen. This leads to increased IFN-gamma-positive fresh NK cells and induces NK allocytotoxicity in IL2-propagated polyclonal NK cells and NK clones. This study also provides support for the concept that incompatibility for the HLA-KIR ligand groups C1, C2, and Bw4 dominates NK alloactivation in vitro.

Defective cytotoxic lymphocyte degranulation in syntaxin-11 deficient familial hemophagocytic lymphohistiocytosis 4 (FHL4) patients

Familial hemophagocytic lymphohistiocytosis (FHL) is typically an early onset, fatal disease characterized by a sepsislike illness with cytopenia, hepatosplenomegaly, and deficient lymphocyte cytotoxicity. Disease-causing mutations have been identified in genes encoding perforin (PRF1/FHL2), Munc13-4 (UNC13D/FHL3), and syntaxin-11 (STX11/FHL4). In contrast to mutations leading to loss of perforin and Munc13-4 function, it is unclear how syntaxin-11 loss-of-function mutations contribute to disease. We show here that freshly isolated, resting natural killer (NK) cells and CD8(+) T cells express syntaxin-11. In infants, NK cells are the predominant perforin-containing cell type. NK cells from FHL4 patients fail to degranulate when encountering susceptible target cells. Unexpectedly, IL-2 stimulation partially restores degranulation and cytotoxicity by NK cells, which could explain the less severe disease progression observed in FHL4 patients, compared with FHL2 and FHL3 patients. Since the effector T-cell compartment is still immature in infants, our data suggest that the observed defect in NK-cell degranulation may contribute to the pathophysiology of FHL, that evaluation of NK-cell degranulation in suspected FHL patients may facilitate diagnosis, and that these new insights may offer novel therapeutic possibilities.

Inhibitory KIR-HLA receptor-ligand mismatch in autologous haematopoietic stem cell transplantation for solid tumour and lymphoma

Genes that encode killer Ig-like receptors (KIRs) and their HLA class I ligands segregate independently; thus, some individuals may express an inhibitory KIR gene but not its cognate ligand. We hypothesised that these patients with KIR-HLA receptor-ligand mismatch have a low risk of relapse after an autologous haematopoietic stem cell transplantation (HCT). Sixteen consecutive patients with lymphoma or solid tumour were enrolled onto a prospective study. They received high-dose busulphan and melphalan followed by autologous CD133(+) HCT. We found that 8 of the 16 patients experienced disease progression after autologous HCT, including 5 of the 6 patients (83%) with no inhibitory KIR-HLA mismatch and 3 of the 6 patients (50%) with 1 mismatched pair; none of the 4 (0%) patients with 2 mismatched pairs experienced disease progression. Survival analyses showed that inhibitory KIR-HLA mismatch was the only significant prognostic factor (P=0.01). The potential applicability of the receptor-ligand mismatch model to autologous HCTs and to patients with lymphoma or solid tumour is clinically significant because of the prevalence of the HCT procedure.

A subpopulation of human peripheral blood NK cells that lacks inhibitory receptors for self-MHC is developmentally immature

How receptor acquisition correlates with the functional maturation of natural killer (NK) cells is poorly understood. We used quantitative real-time polymerase chain reaction (PCR) assays to compare NKG2 and killer immunoglobulin-like receptor (KIR) gene expression in NK cells from allogeneic transplant recipients and their donors. Marked differences were observed in the NK subsets of recipients who had 8-fold more CD56(bright) cells, diminished KIR expression (except 2DL4), and increased NKG2A. In normal blood not all CD56(dim) cells express KIR, and a novel subpopulation of cells committed to the NK-cell lineage was defined. These cells, which comprise 19.4% +/- 2.8% of the CD56(dim) NK population in healthy donors, express the activating NKG2D and NKG2E receptors but no KIR or NKG2A. Although the CD56(dim) NKG2A(-) KIR(-) NK cells lack "at least one" inhibitory receptor for autologous MHC class I, they are not fully responsive, but rather functionally immature cells with poor cytotoxicity and IFN-gamma production. Upon culture with IL-15 and a stromal cell line, CD56(dim) and CD56(bright) KIR(-) NK cells proliferate, express KIR, and develop cytotoxicity and cytokine-producing potential. These findings have implications for the function of NK cells reconstituting after transplantation and support a model for in vivo development in which CD56(bright) cells precede CD56(dim) cells.

Innate partnership of HLA-B and KIR3DL1 subtypes against HIV-1

Allotypes of the natural killer (NK) cell receptor KIR3DL1 vary in both NK cell expression patterns and inhibitory capacity upon binding to their ligands, HLA-B Bw4 molecules, present on target cells. Using a sample size of over 1,500 human immunodeficiency virus (HIV)+ individuals, we show that various distinct allelic combinations of the KIR3DL1 and HLA-B loci significantly and strongly influence both AIDS progression and plasma HIV RNA abundance in a consistent manner. These genetic data correlate very well with previously defined functional differences that distinguish KIR3DL1 allotypes. The various epistatic effects observed here for common, distinct KIR3DL1 and HLA-B Bw4 combinations are unprecedented with regard to any pair of genetic loci in human disease, and indicate that NK cells may have a critical role in the natural history of HIV infection.

Relevance of C1 and C2 epitopes for hemopoietic stem cell transplantation: role for sequential acquisition of HLA-C-specific inhibitory killer Ig-like receptor

Killer Ig-like receptors (KIR) and HLA class I ligands were studied in unrelated hemopoietic stem cell transplantation for chronic myeloid leukemia (n = 108). Significantly improved overall survival was observed in patients, which were homozygous for HLA-C-encoded group 1 (C1) ligands compared with those with group 2 (C2) ligands. Favorable outcome in the former patient group was an early effect that was highly significant in patients transplanted with G-CSF-mobilized peripheral blood and patients with advanced disease stages. In contrast, presence of C1 ligands in the donor was associated with significantly reduced patient survival. The differential roles of the two HLA-C ligands are explained in the context of a biased NK cell reconstitution, which is generally dominated by the presence of C1- but absence of C2-specific NK cells. The clinical observations are corroborated by in vitro experiments showing that NK cells derived from hemopoietic progenitor cells generally acquire the C1-specific inhibitory KIR2DL2/3 at earlier time points and with higher frequency than the C2-specific KIR2DL1. These findings define a novel determinant for understanding the role of NK cells in clinical hemopoietic stem cell transplantation.

CD56brightCD16- killer Ig-like receptor- NK cells display longer telomeres and acquire features of CD56dim NK cells upon activation

Human NK cells can be divided into CD56(dim)CD16(+) killer Ig-like receptors (KIR)(+/-) and CD56(bright)CD16(-) KIR(-) subsets that have been characterized extensively regarding their different functions, phenotype, and tissue localization. Nonetheless, the developmental relationship between these two NK cell subsets remains controversial. We report that, upon cytokine activation, peripheral blood (PB)-CD56(bright) NK cells mainly gain the signature of CD56(dim) NK cells. Remarkably, KIR can be induced not only on CD56(bright), but also on CD56(dim) KIR(-) NK cells, and their expression correlates with lower proliferative response. In addition, we demonstrate for the first time that PB-CD56(dim) display shorter telomeres than PB- and lymph node (LN)-derived CD56(bright) NK cells. Along this line, although human NK cells collected from nonreactive LN display almost no KIR and CD16 expression, NK cells derived from highly reactive LN, efferent lymph, and PB express significant amounts of KIR and CD16, implying that CD56(bright) NK cells could acquire these molecules in the LN during inflammation and then circulate through the efferent lymph into PB as KIR(+)CD16(+) NK cells. Altogether, our results suggest that CD56(bright)CD16(-) KIR(-) and CD56(dim)CD16(+)KIR(+/-) NK cells correspond to sequential steps of differentiation and support the hypothesis that secondary lymphoid organs can be sites of NK cell final maturation and self-tolerance acquisition during immune reaction.

Prospects for the use of NK cells in immunotherapy of human cancer

Current insights into the molecular specificities that regulate natural killer (NK)-cell function suggest that it might be possible to design NK-cell-based immunotherapeutic strategies against human cancer. Here, we describe evidence for NK-cell targeting of human tumours and address crucial questions that, in our opinion, require consideration for the development of successful NK-cell-based therapies. Appropriately used, we predict that NK cells will have a role, both directly and in combination with other treatment modalities, in future treatment of cancer.

Self-tolerance, dendritic cell (DC)-mediated activation and tissue distribution of natural killer (NK) cells

Natural killer (NK) cells are lymphocytes of the innate immune system that exert a potent function against infected and tumor cells. Although NK cells were originally defined by their capacity to lyse target cells and produce interferon (IFN)-gamma without prior activation, more recent studies found that NK cells display also a potent regulatory function. Following engagement of surface receptors by other cells or signalling by soluble molecules, NK cells release cytokines able to influence the outcome of an immune response. Since their discovery in the 1970s, the biology of NK cells has been deeply investigated; nevertheless some aspects of their maturation process, activation mechanisms, and tissue distribution remain still obscure. These review will focus on three major issues regarding NK cell regulation. In particular we aim to discuss: (i) how NK cells become tolerant to self-tissues during their maturation; (ii) how NK cells become activated, with a particular attention to dendritic cell (DC)-mediated mechanisms of NK cell priming; (iii) where NK cells play their functions and how NK cell tissue distribution can favour their capacity to skew T cell responses.

Stromal-cell regulation of natural killer cell differentiation

Natural killer (NK) cells are bone-marrow-derived lymphocytes that play a crucial role in host defense against some viral and bacterial infections, as well as against tumors. Their phenotypic and functional maturation requires intimate interactions between the bone marrow stroma and committed precursors. In parallel to the identification of several phenotypic and functional stages of NK cell development, recent studies have shed new light on the role of stromal cells in driving functional maturation of NK cells. In this review, we provide an overview of the role of bone marrow microenvironment in NK cell differentiation.

Recognition of H-2Kb by Ly49Q suggests a role for class Ia MHC regulation of plasmacytoid dendritic cell function

Ly49Q is a member of the polymorphic Ly49 family of NK cell receptors that displays both a high degree of conservation and a unique expression pattern restricted to myeloid lineage cells, including plasmacytoid dendritic cells (pDC). The function and ligand specificity of Ly49Q are unknown. Here, we use reporter cell analysis to demonstrate that a high-affinity ligand for Ly49Q is present on H-2(b), but not H-2(d), H-2(k), H-2(q), or H-2(a)-derived tumor cells and normal cells ex vivo. The ligand is peptide-dependent and MHC Ia-like, as revealed by its functional absence on cells deficient in TAP-1, beta(2)m, or H-2K(b)D(b) expression. Furthermore, Ly49Q is specific for H-2K(b), as the receptor binds peptide-loaded H-2K(b) but not H-2D(b) complexes, and Ly49Q recognition can be blocked using anti-K(b) but not anti-D(b) mAb. Greater soluble H-2K(b) binding to ligand-deficient pDC also suggests cis interactions of Ly49Q and H-2K(b). These results demonstrate that Ly49Q efficiently binds H-2K(b) ligand, and suggest that pDC function, like that of NK cells, is regulated by classical MHC Ia molecules. MHC recognition capability by pDC has important implications for the role of this cell type during innate immune responses.

Quantifying the reduction in accessibility of the inhibitory NK cell receptor Ly49A caused by binding MHC class I proteins in cis

Murine natural killer (NK) cells are inhibited by target cell MHC class I molecules via Ly49 receptors. However, Ly49 receptors can be made inaccessible to target cell MHC class I by a cis interaction with its MHC class I ligand within the NK cell membrane. It has recently been demonstrated that MHC class I proteins transfer from the target cells to the NK cell. Here, we establish that the number of transferred MHC class I proteins is proportional to the number of Ly49A receptors at the NK cell surface. Ly49A+ NK cells from mice expressing the Ly49A ligand H-2D(d) showed a 90% reduction in Ly49A accessibility compared to Ly49A+ NK cells from H-2D(d)-negative mice. The reduction was caused both by lower expression of Ly49A and interactions in cis between Ly49A and H-2D(d) at the NK cell surface. Approximately 75% of the Ly49A receptors on H-2D(d)-expressing NK cells were occupied in cis with endogenous H-2D(d) and only 25% were free to interact with H-2D(d) molecules in trans. Thus, H-2D(d) ligands control Ly49A receptor accessibility through interactions both in cis and in trans.

Dendritic cells prime natural killer cells by trans-presenting interleukin 15

Natural killer (NK) cells are important effector cells in the control of infections. The cellular and molecular signals required for NK cell activation in vivo remain poorly defined. By using a mouse model for the inducible ablation of dendritic cells (DCs), we showed that the in vivo priming of NK cell responses to viral and bacterial pathogens required the presence of CD11c(high) DCs. After peripheral Toll-like receptor (TLR) stimulation, NK cells were recruited to local lymph nodes, and their interaction with DCs resulted in the emergence of effector NK cells in the periphery. NK cell priming was dependent on the recognition of type I IFN signals by DCs and the subsequent production and trans-presentation of IL-15 by DCs to resting NK cells. CD11c(high) DC-derived IL-15 was necessary and sufficient for the priming of NK cells. Our data define a unique in vivo role of DCs for the priming of NK cells, revealing a striking and previously unappreciated homology to T lymphocytes of the adaptive immune system.

Donor natural killer cell allorecognition of missing self in haploidentical hematopoietic transplantation for acute myeloid leukemia: challenging its predictive value

We analyzed 112 patients with high-risk acute myeloid leukemia (61 in complete remission [CR]; 51 in relapse), who received human leukocyte-antigen (HLA)-haploidentical transplants from natural killer (NK) alloreactive (n = 51) or non-NK alloreactive donors (n = 61). NK alloreactive donors possessed HLA class I, killer-cell immunoglobulin-like receptor (KIR) ligand(s) which were missing in the recipients, KIR gene(s) for missing self recognition on recipient targets, and alloreactive NK clones against recipient targets. Transplantation from NK-alloreactive donors was associated with a significantly lower relapse rate in patients transplanted in CR (3% versus 47%) (P > .003), better event-free survival in patients transplanted in relapse (34% versus 6%, P = .04) and in remission (67% versus 18%, P = .02), and reduced risk of relapse or death (relative risk versus non-NK-alloreactive donor, 0.48; 95% CI, 0.29-0.78; P > .001). In all patients we tested the "missing ligand" model which pools KIR ligand mismatched transplants and KIR ligand-matched transplants from donors possessing KIR(s) for which neither donor nor recipient have HLA ligand(s). Only transplantation from NK-alloreactive donors is associated with a survival advantage.

Differential and nonredundant roles of phospholipase Cgamma2 and phospholipase Cgamma1 in the terminal maturation of NK cells

NK cells play a central role in mediating innate immune responses. Activation of NK cells results in cytotoxicity, cytokine, and chemokine secretions. In this study, we show that in mice with targeted deletion of phospholipase Cgamma (PLCgamma)2, one of the key signal transducers, there are profound effects on the development and terminal maturation of NK cells. Lack of PLCgamma2 significantly impaired the ability of lineage-committed NK precursor cells to acquire subset-specific Ly49 receptors and thereby terminal maturation of NK cells. Overexpression of isozyme, PLCgamma1, in PLCgamma2-deficient NK cells resulted in the successful Ly49 acquisition and terminal maturation of the NK cells; however, it could only partially rescue NKG2D-mediated cytotoxicity with no cytokine production. Furthermore, PLCgamma2-deficient NK cells failed to mediate antitumor cytotoxicity and inflammatory cytokine production, displaying a generalized hyporesponsiveness. Our results strongly demonstrate that PLCgamma1 and PLCgamma2 play nonredundant and obligatory roles in NK cell ontogeny and in its effector functions.

Regulation of natural killer cell activity

Information regarding the role of natural killer (NK) cells in the response to viruses, intracellular bacteria and parasites continues to emerge. NK cells can directly lyse infected cells, secrete cytokines and interact with dendritic cells to drive the adaptive immune response. There are a large number of activating and inhibitory receptors that govern NK cell activity. Recent studies have revealed how signals are transmitted and integrated from the variety of receptors, how particular receptors influence NK development and functional status, and how NK cells access lymph nodes and sites of infection. The potential for NK cells to exhibit specific and memory-like responses has begun to blur the 'innate' definition of NK cells.

Regulation of class I major histocompatibility complex receptor expression in natural killer cells: one promoter is not enough!

The class I major histocompatibility complex (MHC) receptors expressed by natural killer (NK) cells play an important role in regulating their function. The number and type of inhibitory receptors expressed by NK cells must be tightly controlled in order to avoid the generation of dominantly inhibited NK cells. The selective stochastic expression of the class I MHC receptors generates a variegated NK cell population capable of discriminating subtle changes in MHC expression on potential target cells. The molecular mechanisms controlling the cell-specific and probabilistic expression of these receptors are without doubt very complex. The traditional approach of considering a core promoter modulated by upstream enhancer elements is likely too simplistic a paradigm to adequately explain the regulation of these genes, as well as other gene clusters that are not expressed in an 'all or none' fashion. Our studies on the regulation of the mouse Ly49 and human killer immunoglobulin-like receptor (KIR) clusters of class I MHC receptor genes have revealed the presence of multiple transcripts in both sense and antisense orientations. In both systems, an antisense promoter overlaps a promoter that produces sense transcripts, creating a bidirectional element. In the Ly49 genes, the competing promoters behave as probabilistic switches, and it is likely that the human bidirectional promoters will have a similar property. The antisense transcripts generated in the Ly49 genes are far removed from the promoter responsible for Ly49 expression in mature NK cells, whereas the antisense KIR transcripts detected are within the adult promoter region. This finding suggests that the mechanism of promoter regulation in the KIR genes may be quite different from that of the Ly49 genes. This review summarizes the current state of knowledge regarding class I MHC receptor gene regulation. The models proposed for the control of the probabilistic expression of the Ly49 and KIR genes are discussed in the context of current knowledge regarding the complex control of other well-studied gene clusters such as the beta-globin and cytokine clusters.

Bone marrow versus thymic pathways of natural killer cell development

Understanding natural killer (NK) cell developmental pathways is crucial for harnessing the potential therapeutic benefits of this specialized lymphocyte subset. The bone marrow (BM) plays a major role in NK cell development, providing the appropriate environmental cues for NK cell commitment and subsequent NK cell differentiation. Nevertheless, the molecular signals provided in this context remain enigmatic. It is widely assumed that BM seeds the periphery with NK cells. However, the precise origins of NK cells found in lymphoid organs and tissues are not defined. Recently, we found that thymic NK cells bear molecular markers and functional attributes that distinguish them from most peripheral NK cells. We find that NK cells are actively exported from the thymus to the periphery, suggesting that thymus-derived NK cells may have unique roles both intrathymically and in secondary lymphoid organs. Here we compare the properties of thymic NK cells with properties of other NK cell subsets that have been identified in the mouse. We propose that heterogeneity in NK cell function can be achieved through distinct thymic and bone marrow pathways of NK cell development.

KIR ligands and prediction of relapse after unrelated donor hematopoietic cell transplantation for hematologic malignancy

Recurrent malignancy remains a significant complication after allogeneic hematopoietic cell transplantation (HCT). Efforts to decrease relapse have included donor lymphocyte infusion to stimulate donor anti-recipient T-cell allorecognition of major and minor histocompatibility differences. Recently, alloreactive effects of donor natural killer cell-mediated inhibitory killer immunoglobulin-like receptor (KIR) recognition of recipient HLA-C and -B ligands have been described. We examined KIR ligand effects on risk of relapse in 1770 patients undergoing myeloablative T-replete HCT from HLA-matched or -mismatched unrelated donors for the treatment of myeloid and lymphoid leukemias. KIR ligands defined by HLA-B and -C genotypes were used to determine donor-recipient ligand incompatibility or recipient lack of KIR ligand. Among HLA-mismatched transplantations, recipient homozygosity for HLA-B or -C KIR epitopes predicted lack of KIR ligand and was associated with a decreased hazard of relapse (hazard ratio, 0.61; 95% confidence interval, .043-0.85; P = .004). Absence of HLA-C group 2 or HLA-Bw4 KIR ligands was associated with lower hazards of relapse (hazard ratio, 0.47; 95% confidence interval, 0.28-0.79, P = .004; hazard ratio, 0.56; 95% confidence interval, 0.33-0.97; P = .04, respectively). The decrease in hazard of relapse in patients with acute myelogenous leukemia was similar to that in patients with chronic myelogenous leukemia and acute lymphoblastic leukemia (P = .95). Recipient homozygosity for HLA-B or -C epitopes that define KIR ligands is likely to be a predictive factor for leukemia relapse after myeloablative HCT from HLA-mismatched unrelated donors. This effect was not observed in HLA-identical unrelated transplants.

Licensing of natural killer cells by self-major histocompatibility complex class I

Natural killer (NK) cells have potent capacities to immediately kill cellular targets and produce cytokines that may potentially damage normal self-tissues unless they are kept in check. Such tolerance mechanisms are incompletely understood. Here we discuss recent studies suggesting that NK cells undergo a host major histocompatibility complex (MHC) class I-dependent functional maturation process, termed 'licensing'. Ironically, licensing directly involves inhibitory receptors that recognize target cell MHC class I molecules and block activation of NK cells in effector responses. This process results in two types of tolerant NK cells: functionally competent (licensed) NK cells, whose effector responses are inhibited by self-MHC class I molecules through the same receptors that conferred licensing, and functionally incompetent (unlicensed) NK cells.

Allogeneic hematopoietic transplantation and natural killer cell recognition of missing self

Although the optimal donor for allogeneic hematopoietic stem cell transplantation (HSCT) is a human leukocyte antigen-matched sibling, 75% of patients do not have a match, and alternatives are matched unrelated volunteers, unrelated umbilical cord blood units, and full-haplotype-mismatched family members. To cure leukemia, allogeneic HSCT relies on donor T cells in the allograft, which promote engraftment, eradicate malignant cells, and reconstitute immunity. Here, we focus on the open issues of rejection, graft-versus-host disease (GVHD), and infections and the benefits of natural killer (NK) cell alloreactivity and its underlying mechanisms. Donor-versus-recipient NK cell alloreactivity derives from a mismatch between inhibitory receptors for self-major histocompatibility complex (MHC) class I molecules on donor NK clones and the MHC class I ligands on recipient cells. These NK clones sense the missing expression of the self-MHC class I allele on the allogeneic targets and mediate alloreactions. HSCT from 'NK alloreactive' donors controls acute myeloid relapse without causing GVHD. We review the translation of NK cell recognition of missing self into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia.

Activation, coactivation, and costimulation of resting human natural killer cells

Natural killer (NK) cells possess potent perforin- and interferon-gamma-dependent effector functions that are tightly regulated. Inhibitory receptors for major histocompatibility complex class I display variegated expression among NK cells, which confers specificity to individual NK cells. Specificity is also provided by engagement of an array of NK cell activation receptors. Target cells may express ligands for a multitude of activation receptors, many of which signal through different pathways. How inhibitory receptors intersect different signaling cascades is not fully understood. This review focuses on advances in understanding how activation receptors cooperate to induce cytotoxicity in resting NK cells. The role of activating receptors in determining specificity and providing redundancy of target cell recognition is discussed. Using Drosophila insect cells as targets, we have examined the contribution of individual receptors. Interestingly, the strength of activation is not determined simply by additive effects of parallel activation pathways. Combinations of signals from different receptors can have different outcomes: synergy, no enhancement over individual signals, or additive effects. Cytotoxicity requires combined signals for granule polarization and degranulation. The integrin leukocyte function-associated antigen-1 contributes a signal for polarization but not for degranulation. Conversely, CD16 alone or in synergistic combinations, such as NKG2D and 2B4, signals for phospholipase-C-gamma- and phosphatidylinositol-3-kinase-dependent degranulation.

Activation and self-tolerance of natural killer cells

Natural killer (NK) cells are regulated by numerous stimulatory and inhibitory receptors that recognize various classes of cell surface ligands, some of which are expressed by normal healthy cells. We review two key issues in NK cell biology. How do NK cells achieve tolerance to healthy self-cells, despite great potential variability in inhibitory and stimulatory receptor engagement? How is the disease status of unhealthy cells translated into changes in ligand expression and consequent sensitivity to NK cell lysis? Concerning the second question, we review evidence that ligands for one key NK receptor, NKG2D, are induced by the DNA damage response, which is activated in cells exposed to genotoxic stress. Because cancer cells and some infected cells are subject to genotoxic stress, these findings suggest a new concept for how diseased cells are discriminated by the immune system. Second, we review studies that have overturned the prevalent notion that NK cells achieve self-tolerance by expressing inhibitory receptors specific for self-major histocompatibility complex class I molecules. A subset of NK cells lacks such receptors. These NK cells are hyporesponsive when stimulatory receptors are engaged, suggesting that alterations in signaling pathways that dampen stimulatory receptor signals contribute to self-tolerance of NK cells.

Receptor-ligand analyses define minimal killer cell Ig-like receptor (KIR) in humans

Interactions between inhibitory killer cell immunoglobulin-like receptors (iKIR) and human leukocyte antigen (HLA) class I molecules regulate natural killer (NK) cell responses to eliminate infected and transformed cells while maintaining tolerance to healthy cells. Unlinked polymorphic gene families encode KIR receptors and HLA class I ligands and their independent segregation results in a variable number and type of iKIR + HLA pairs inherited in individuals. The diversity in the co-inheritance of iKIR + HLA pairs and activating KIR (aKIR) genes in 759 unrelated individuals from four ethnic populations was analyzed. Every individual studied inherited a minimum of one iKIR + HLA pair; suggesting that major histocompatibility complex class I-dependent inhibitory KIR signaling is essential for human NK cell function. In contrast, 13.4% of the study group lacked all aKIR genes. Twenty percent of the study group carried only one of the four iKIR + HLA pairs. Interestingly, 3% of the study group carrying only KIR2DL3 + HLA-C1 as an iKIR + HLA pair lacked aKIR genes. These data suggest that a single iKIR can constitute the minimal KIR repertoire for human NK cells. Genotypes carrying an equal number of iKIR + HLA pairs and aKIR genes represented 20% of the study group. The remaining individuals had either a dominant inhibitory KIR genotype (iKIR + HLA > aKIR) or a dominant activating KIR genotype (iKIR + HLA < aKIR). Genotypes encoding these imbalanced inhibitory and activating interactions may contribute to susceptibility or resistance to human diseases.

Deficient major histocompatibility complex-linked innate murine cytomegalovirus immunity in MA/My.L-H2b mice and viral downregulation of H-2k class I proteins

NK cells are key effectors of innate immunity and host survival during cytomegalovirus (CMV) infection. Innate murine CMV (MCMV) resistance in MA/My mice requires Ly49H/m157-independent H-2k-linked NK cell control. Here we show that replacement of MA/My H-2k with C57L H-2b susceptibility genes led to a remarkable loss of innate virus immunity, though NK gamma interferon was induced in H-2b and H-2k strains shortly after infection. Thus, H-2b genes expressed in C57L or MA/My.L-H2b are sufficient in alerting NK cells to intrusion but fail to support NK restraint of viral infection. In addition, novel H-2 recombinant strains were produced and utilized in a further refinement of a critical genetic interval controlling innate H-2k-linked MCMV resistance. Importantly, this analysis excluded the gene interval from Kk class I through class II. The responsible gene(s) therefore resides in an interval spanning Dk class Ia and more-distal major histocompatibility complex (MHC) nonclassical class Ib genes. Recently, the NK activation receptor Ly49P and MHC class I Dk proteins were genetically implicated in MCMV resistance, in part because Ly49P-expressing reporter T cells could specifically bind Dk molecules on MCMV-infected mouse embryonic fibroblasts (MEFs). However, as we found that H-2k innate resistance differs in the C57L or MA/My backgrounds and because MCMV very efficiently downregulates H-2k class I proteins in L929 cells and primary MEFs shortly after infection, a Ly49P/Dk model should not fully explain H-2k-linked MCMV resistance.

The dynamics of natural killer cell tolerance

Natural killer (NK) cells are important mediators of resistance against tumor growth and metastasis. NK cell reactivity is regulated by a balance of signals from activating and inhibitory receptors. While reactivity against tumor cells is beneficial, it is essential that NK cells do not attack normal tissue. The distinction between tumor cells and normal cells is partly made at the level of activating receptors: transformation often results in induction of ligands for such receptors. In addition, NK cells discriminate self from non-self using MHC class I-binding inhibitory receptors. Host MHC class I molecules regulate development of NK cell reactivity and tolerance, a process that is not well understood. Recent data suggest that functional maturation may not be a binary phenomenon: quantitative aspects, with regards to avidity and frequency in interactions between developing NK cells and normal cells, may be important for the generation of NK cells that are 'tuned' to optimally sensing the absence of self-MHC class I. In this article, we discuss models for development of NK cell reactivity and tolerance. Our understanding of this process may have significant implications for the use of NK cells in cancer therapy.

Missing self recognition and self tolerance of natural killer (NK) cells

Natural killer cells express inhibitory receptors specific for polymorphic MHC molecules, which enables them to mediate "missing self recognition", the capacity to attack self cells that extinguish expression of MHC class I molecules. A key question is: how are NK cells rendered self-tolerant? It was proposed that all NK cells express at least one inhibitory receptor specific for self MHC, but we recently identified an NK cell subset that does not. Instead, these NK cells, like anergic B and T cells, are hyporesponsive to stimulation. These findings indicate that NK cell activity can be modulated independently of inhibitory receptors specific for MHC molecules, and that such modulation may contribute to self tolerance. This review summarizes current understanding of NK cell recognition and self tolerance.