The natural killer gene complex genetic locus Chok encodes Ly-49D, a target recognition receptor that activates natural killing

Control of Viral Infection by Natural Killer Cell Inhibitory Receptors

Major histocompatibility complex class I (MHC-I)-restricted immune responses are largely attributed to cytotoxic T lymphocytes (CTLs). However, natural killer (NK) cells, as predicted by the missing-self hypothesis, have opposing requirements for MHC-I, suggesting that they may also demonstrate MHC-I-restricted effects. In mice, the Ly49 inhibitory receptors prevent NK cell killing of missing-self targets in effector responses, and they have a proposed second function in licensing or educating NK cells via self-MHC-I in vivo. Here we show MHC-I-restricted control of murine cytomegalovirus (MCMV) infection in vivo that is NK cell dependent. Using mice lacking specific Ly49 receptors, we show that control of MCMV requires inhibitory Ly49 receptors and an inhibitory signaling motif and the capacity for MCMV to downregulate MHC-I. Taken together, these data provide definitive evidence that the inhibitory receptors are required for missing-self rejection and are relevant to MHC-I-restricted NK cell control of a viral infection in vivo.

Parikh et al. show that major histocompatibility complex class I (MHC-I)-restricted control of viral infection is due to natural killer (NK) cells rather than cytotoxic T cells. Inhibitory NK cell receptors are essential for protection, requiring NK cell licensing (education) by self-MHC-I and missing-self recognition due to virus-induced MHC-I downregulation.

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.

A herpesvirus encoded Qa-1 mimic inhibits natural killer cell cytotoxicity through CD94/NKG2A receptor engagement

A recurrent theme in viral immune evasion is the sabotage of MHC-I antigen presentation, which brings virus the concomitant issue of ‘missing-self’ recognition by NK cells that use inhibitory receptors to detect surface MHC-I proteins. Here, we report that rodent herpesvirus Peru (RHVP) encodes a Qa-1 like protein (pQa-1) via RNA splicing to counteract NK activation. While pQa-1 surface expression is stabilized by the same canonical peptides presented by murine Qa-1, pQa-1 is GPI-anchored and resistant to the activity of RHVP pK3, a ubiquitin ligase that targets MHC-I for degradation. pQa-1 tetramer staining indicates that it recognizes CD94/NKG2A receptors. Consistently, pQa-1 selectively inhibits NKG2A⁺ NK cells and expression of pQa-1 can protect tumor cells from NK control in vivo. Collectively, these findings reveal an innovative NK evasion strategy wherein RHVP encodes a modified Qa-1 mimic refractory to MHC-I sabotage and capable of specifically engaging inhibitory receptors to circumvent NK activation.

Immunoreceptor tyrosine-based inhibitory motif-dependent functions of an MHC class I-specific NK cell receptor

Natural killer (NK) cells express MHC class I (MHC-I)-specific receptors, such as Ly49A, that inhibit killing of cells expressing self-MHC-I. Self-MHC-I also "licenses" NK cells to become responsive to activating stimuli and regulates the surface level of NK-cell inhibitory receptors. However, the mechanisms of action resulting from these interactions of the Ly49s with their MHC-I ligands, particularly in vivo, have been controversial. Definitive studies could be derived from mice with targeted mutations in inhibitory Ly49s, but there are inherent challenges in specifically altering a single gene within a multigene family. Herein, we generated a knock-in mouse with a targeted mutation in the immunoreceptor tyrosine-based inhibitory motif (ITIM) of Ly49A that abolished the inhibitory function of Ly49A in cytotoxicity assays. This mutant Ly49A caused a licensing defect in NK cells, but the surface expression of Ly49A was unaltered. Moreover, NK cells that expressed this mutant Ly49A exhibited an altered inhibitory receptor repertoire. These results demonstrate that Ly49A ITIM signaling is critical for NK-cell effector inhibition, licensing, and receptor repertoire development.

Genetic dissection of NK cell responses

The association of Natural Killer (NK) cell deficiencies with disease susceptibility has established a central role for NK cells in host defence. In this context, genetic approaches have been pivotal in elucidating and characterizing the molecular mechanisms underlying NK cell function. To this end, homozygosity mapping and linkage analysis in humans have identified mutations that impact NK cell function and cause life-threatening diseases. However, several critical restrictions accompany genetic studies in humans. Studying NK cell pathophysiology in a mouse model has therefore proven a useful tool. The relevance of the mouse model is underscored by the similarities that exist between cell-structure-sensing receptors and the downstream signaling that leads to NK cell activation. In this review, we provide an overview of how human and mouse quantitative trait locis (QTLs) have facilitated the identification of genes that modulate NK cell development, recognition, and killing of target cells.

Identification of the nephropathy-susceptibility locus HIVAN4

HIVAN1, HIVAN2, and HIVAN3 are nephropathy-susceptibility loci previously identified in the HIV-1 transgenic mouse, a model of collapsing glomerulopathy. The HIVAN1 and HIVAN2 loci modulate expression of Nphs2, which encodes podocin and several other podocyte-expressed genes. To identify additional loci predisposing to nephropathy, we performed a genome-wide scan in 165 backcross mice generated between the nephropathy-sensitive HIV-1-transgenic FVB/NJ (TgFVB) strain and the resistant Balb/cJ (BALB) strain. We identified a major susceptibility locus (HIVAN4) on chromosome 6 G3-F3, with BALB alleles conferring a twofold reduction in severity (peak LOD score = 4.0). Similar to HIVAN1 and HIVAN2, HIVAN4 modulated expression of Nphs2, indicating a common pathway underlying these loci. We independently confirmed the HIVAN4 locus in a sister TgFVB colony that experienced a dramatic loss of nephropathy subsequent to a breeding bottleneck. In this low-penetrance line, 3% of the genome was admixed with BALB alleles, suggesting a remote contamination event. The admixture localized to discrete segments on chromosome 2 and at the HIVAN4 locus. HIVAN4 candidate genes include killer lectin-like receptor genes as well as A2m and Ptpro, whose gene products are enriched in the glomerulus and interact with HIV-1 proteins. In summary, these data identify HIVAN4 as a major quantitative trait locus for nephropathy and a transregulator of Nphs2. Furthermore, similar selective breeding strategies may help identify further susceptibility loci.

Effect of Ly49 haplotype variance on NK cell function and education

The class I MHC-specific receptors expressed by murine NK cells exhibit remarkable variation. Specific activating killer Ig-related receptor/Ly49 have major effects on autoimmune and infectious disease induction and outcome in humans and mice. However, these studies are greatly affected by individual background genetics. Furthermore, the educational impact of variable inhibitory KIR/Ly49 gene numbers on NK cell development and the subsequent ability to survey for MHC class I (MHC-I) expression remain unknown. To address these questions, Ly49 congenic mice were generated that maintain a 129-derived Ly49 gene cluster on a C57BL/6 genetic background (B6.Ly49(129) mice), and the in vitro and in vivo NK cell function of these mice was compared with their inbred parental 129S1 and C57BL/6 counterparts. Notably, target cell recognition directed by activating Ly49 receptors was profoundly affected by allelic variation in B6.Ly49(129) congenic cells versus C57BL/6 NK cells. Furthermore, when assessing NK cell function based on education and subsequent recognition of the C57BL/6 MHC-I haplotype by inhibitory Ly49 receptors, B6.Ly49(129) congenic mice exhibited robust NK cell activity, demonstrating efficient NK cell education by the 129S1 Ly49 cluster during development. The responsiveness of NK cells expressing 129S1 Ly49 was shown to be mediated by subsets expressing one or more self-MHC receptors, including Ly49I, Ly49O, Ly49V, and NKG2A. These findings demonstrate that the genetically segregating and diverse MHC-I and Ly49 loci in mice exhibit independent and epistatic effects on NK cell education that can be uncoupled during the intercrossing of inbred strains.

Altered NK cell development and enhanced NK cell-mediated resistance to mouse cytomegalovirus in NKG2D-deficient mice

NKG2D is a potent activating receptor on natural killer (NK) cells and acts as a molecular sensor for stressed cells expressing NKG2D ligands such as infected or tumor-transformed cells. Although NKG2D is expressed on NK cell precursors, its role in NK cell development is not known. We have generated NKG2D-deficient mice by targeting the Klrk1 locus. Here we provide evidence for an important regulatory role of NKG2D in the development of NK cells. The absence of NKG2D caused faster division of NK cells, perturbation in size of some NK cell subpopulations, and their augmented sensitivity to apoptosis. As expected, Klrk1(-/-) NK cells are less responsive to tumor targets expressing NKG2D ligands. Klrk1(-/-) mice, however, showed an enhanced NK cell-mediated resistance to mouse cytomegalovirus infection as a consequence of NK cell dysregulation. Altogether, these findings provide evidence for regulatory function of NKG2D in NK cell physiology.

Ly49h-deficient C57BL/6 mice: a new mouse cytomegalovirus-susceptible model remains resistant to unrelated pathogens controlled by the NK gene complex

Cmv1 was the first mouse cytomegalovirus (MCMV) resistance locus identified in C57BL/6 mice. It encodes Ly49H, a NK cell-activating receptor that specifically recognizes the m157 viral protein at the surface of MCMV-infected cells. To dissect the effect of the Ly49h gene in host-pathogen interactions, we generated C57BL/6 mice lacking the Ly49h region. We found that 36 h after MCMV infection, the lack of Ly49h resulted in high viral replication in the spleen and dramatically enhanced proinflammatory cytokine production in the serum and spleen. At later points in time, we observed that MCMV induced a drastic loss in CD8(+) T cells in B6.Ly49h(-/-) mice, probably reflecting severe histological changes in the spleen. Overall, our results indicate that Ly49H(+) NK cells contain a systemic production of cytokines that may contribute to the MCMV-induced pathology and play a central role in maintaining normal spleen cell microarchitecture. Finally, we tested the ability of B6.Ly49h(-/-) mice to control replication of Leishmania major and ectromelia virus. Resistance to these pathogens has been previously mapped within the NK gene complex. We found that the lack of Ly49H(+) NK cells is not associated with an altered resistance to L. major. In contrast, absence of Ly49H(+) NK cells seems to afford additional protection against ectromelia infection in C57BL/6 mice, suggesting that Ly49H may recognize ectromelia-infected cells with detrimental effects. Taken together, these results confirm the pivotal role of the Ly49H receptor during MCMV infection and open the way for further investigations in host-pathogen interactions.

NK cell receptors in rodents and cattle

Natural killer (NK) cells discriminate between normal syngeneic cells and infected, neoplastic or MHC-disparate allogeneic cells. The reactivity of NK cells appears to be regulated by a balance between activating receptors that recognize non-self or altered self, and inhibitory receptors recognizing normal, self-encoded MHC class I molecules. Subfamilies of NK receptors undergo rapid evolution, and appear to co-evolve with the MHC. We here review present views on the evolution and function of NK cell receptors, with an emphasis on knowledge gained in cattle and rodents.

Continuous engagement of a self-specific activation receptor induces NK cell tolerance

Natural killer (NK) cell tolerance mechanisms are incompletely understood. One possibility is that they possess self-specific activation receptors that result in hyporesponsiveness unless modulated by self–major histocompatability complex (MHC)–specific inhibitory receptors. As putative self-specific activation receptors have not been well characterized, we studied a transgenic C57BL/6 mouse that ubiquitously expresses m157 (m157-Tg), which is the murine cytomegalovirus (MCMV)–encoded ligand for the Ly49H NK cell activation receptor. The transgenic mice were more susceptible to MCMV infection and were unable to reject m157-Tg bone marrow, suggesting defects in Ly49H⁺ NK cells. There was a reversible hyporesponsiveness of Ly49H⁺ NK cells that extended to Ly49H-independent stimuli. Continuous Ly49H–m157 interaction was necessary for the functional defects. Interestingly, functional defects occurred when mature wild-type NK cells were adoptively transferred to m157-Tg mice, suggesting that mature NK cells may acquire hyporesponsiveness. Importantly, NK cell tolerance caused by Ly49H–m157 interaction was similar in NK cells regardless of expression of Ly49C, an inhibitory receptor specific for a self-MHC allele in C57BL/6 mice. Thus, engagement of self-specific activation receptors in vivo induces an NK cell tolerance effect that is not affected by self-MHC–specific inhibitory receptors.

Evolution of the Ly49 and Nkrp1 recognition systems

The Ly49 and Nkrp1 loci encode structurally and functionally related cell surface proteins that positively or negatively regulate natural killer (NK) cell-mediated cytotoxicity and cytokine production. Yet despite their clear relatedness and genetic linkage within the NK gene complex (NKC), these two multi-gene families have adopted dissimilar evolutionary strategies. The Ly49 genes are extremely polymorphic and evolutionarily dynamic, with distinct gene numbers, remarkable allelic diversity, and varying MHC-I-ligand specificities and affinities among different murine haplotypes. In contrast, the Nkrp1 genes have opted for overall conservation of genomic organization, sequences, and ligand specificities, with only limited and focused allelic polymorphism. Possible selection pressures driving such varied evolution of the two gene families may include disequilibrium from ligand co-inheritance, pathogen immunoevasin strategies, flexibility in host counter-evolution mechanisms, and the prevalence and dynamics of inherent repetitive elements.

NK cells and their receptors

Despite early reports that natural killer (NK) cells are non-specific or have non-major histocompatibility complex (MHC)- restricted killing, it is now clear that NK cells express a panoply of receptors with defined specificity for ligands expressed on their cellular targets. The roles of these receptors in terms of physiological NK cell effector functions, such as cytotoxicity and cytokine production, are beginning to be unravelled. Inasmuch as NK cells accumulate in the uterus, an appreciation of NK cell receptor specificities and their physiological functions should provide valuable clues to the role of NK cells in the uterus and during pregnancy.

Cmv1 and natural killer cell responses to murine cytomegalovirus infection

The dissection of genetic resistance to murine cytomegalovirus infection in inbred laboratory mouse strains led to the identification of a natural killer cell activation receptor that recognizes a virus-encoded protein. Herein, we summarize the genetic approach and findings that have provided novel insights into innate immune control of virus infections.

Rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B cells and requires help from NK cells

Xenoantibody production directed at a wide variety of T lymphocyte–dependent and T lymphocyte–independent xenoantigens remains the major immunologic obstacle for successful xenotransplantation. The B lymphocyte subpopulations and their helper factors, involved in T-cell–independent xenoantibody production are only partially understood, and their identification will contribute to the clinical applicability of xenotransplantation. Here we show, using models involving T-cell–deficient athymic recipient mice, that rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B lymphocytes and requires help from natural killer (NK) cells. This collaboration neither required NK-cell–mediated IFN-γ production, nor NK-cell–mediated cytolytic killing of xenogeneic target cells. The T-cell–independent IgM xenoantibody response could be partially suppressed by CD40L blockade.

NKR-P1 biology: from prototype to missing self

Natural killer (NK) cells represent lymphocytes of the innate immune system capable of recognizing and destroying a broad array of target cells, including tumors, virus-infected cells, antibodycoated cells, foreign transplants, and "stressed" cells. NK cells eliminate their targets through two main effector mechanisms, cytokine secretion and cell-mediated cytotoxicity, which in turn depend on detection of target cells through a complex integration of stimulatory and inhibitory receptor-ligand interactions. The NKR-P1 molecules were the first family of NK cell receptors identified, yet they have remained enigmatic in their contribution to self-nonself discrimination until recently. Here, we outline a brief history of the NKR-P1 receptor family, then examine recent data providing insight into their genetic regulation, signaling function, cognate ligands, and gene organization and diversity.

Estrogenic regulation of host immunity against an estrogen receptor-negative human breast cancer

PURPOSE

The risk of developing breast cancer is positively correlated with exposure to increased levels of estrogen and/or an increased duration of estrogen exposure. Many different mechanisms have been proposed to explain the association of estrogens with breast cancer risk; however, the well-documented immune modulatory properties of estrogen have received little attention. In part, this is due to a lack of suitable models for studying this relationship.

EXPERIMENTAL DESIGN

We have developed an animal model using estrogen receptor (ER)-negative human breast cancer cell line, MDA-MB-468, xenografted into severe combined immunodeficient (SCID) mice. We also generated the ER-alpha knockout (ER-alphaKO) mice on the SCID background and then tested the ability of 17beta-estradiol to stimulate growth of xenografted ER-negative human breast cancer tumors in wild-type and ER-alphaKO SCID mice. We quantified vascularization of tumors, macrophage recruitment to the tumor site by immunocytochemistry, and inflammatory cytokine production.

RESULTS

We show that estrogen treatment of C57BL/6/SCID mice promotes the growth of xenografted ER-negative tumors in wild-type mice and this estrogen-induced tumor growth is abrogated in ER-alphaKO mice. Tumor neovascularization of estrogen-treated mice was unchanged versus control; however, estrogen treatment of the C57BL/6/SCID host suppressed macrophage recruitment to and inflammatory cytokine production at the tumor site.

CONCLUSIONS

These data are consistent with estrogen modulation of the inflammatory response as a contributing factor in estrogen-stimulated growth of an ER-negative tumor. This effect on the host innate immune response was mediated by ER-alpha.

Natural killer cell immune responses

Natural killer (NK) cells play a vital role in innate immune responses to infection; they express activation receptors that recognize virus-infected cells. Highly related to receptors recognizing tumor cells, the activation receptors trigger cytotoxicity and cytokine production. NK cells also express inhibitory receptors for major histocompatibility complex (MHC) class I molecules that block the action of the activation receptors. Although many ligands for NK cell receptors have MHC class I folds, recent studies also indicate ligands resembling the NK cell receptors themselves. A combination of immunologic, genetic, biophysical, and in vivo approaches is being employed to understand fully how these receptors contribute to NK cell activities in innate immunity to pathogens and tumors.

Regulation of Ly49D/DAP12 Signal Transduction by Src-Family Kinases and CD45

Activating, DAP12-coupled members of the Ly-49 family of NK cell receptors help control viral infections in mice. However, the kinases and/or phosphatases mediating tyrosine phosphorylation of Ly-49D-associated DAP12 have not been elucidated. In this study, we show for the first time that Src family tyrosine kinases are physically and functionally associated with Ly-49D/DAP12 signaling in murine NK cells. Specifically, we demonstrate the following: 1) inhibition of Src family kinases suppresses DAP12 phosphorylation and downstream DAP12 signals; 2) both Fyn and Lck are capable of phosphorylating DAP12; and 3) both kinases coimmunoprecipitate with the Ly-49D/DAP12 complex in NK cells. Although we detect enhanced phosphorylation of Fyn upon Ly-49D cross-linking in NK cells, Ly-49D-mediated events in both Fyn-/- and Fyn/Lck-/- mice appear normal, reinforcing the theme of redundancy in the ability of Src family kinases to initiate activation events. In contrast to disruption of specific Src family enzymes, Ly-49D/DAP12-mediated calcium mobilization and cytokine production by CD45 null NK cells are defective. Although others have ascribed the effects of CD45 mutation solely on the suppression of Src family activity, we demonstrate in this study that DAP12 is hyperphosphorylated in CD45 null NK cells, resulting in uncoordinated tyrosine-mediated signaling upon Ly-49D ligation. Therefore, although our data are consistent with a Src kinase activity proximally within DAP12 signaling, DAP12 also appears to be a substrate of CD45, suggesting a more complex role for this phosphatase than has been reported previously.

CD95L/FasL and TRAIL in tumour surveillance and cancer therapy

The membrane-bound death ligands CD95L/FasL and TRAIL, which activate the corresponding death receptors CD95/Fas, TRAILR1 and TRAILR2, induce apoptosis in many tumour cells, but can also elicit an inflammatory response. This chapter focuses on the relevance of CD95L/FasL and TRAIL for the tumour surveillance function of natural killer cells and cytotoxic T-cells and discuss current concepts of utilizing these ligands in tumour therapy.

NK Cell Receptors as Tools in Cancer Immunotherapy

Natural killer (NK) cells were identified 30 years ago based on their ability to "spontaneously" kill tumor cells. The basis for NK cell recognition and activation is due to a variety of receptors that bind to specific ligands on tumor cells and normal cells. Some of these receptors have the ability to inhibit NK cell function, and other receptors activate NK cell function. Therapeutic strategies for cancer therapy are being developed based on preventing NK cell inhibition or using NK cell receptors to activate NK cells or T cells. There are intriguing clinical data from studies of bone marrow transplantation that support the idea that preventing NK cell inhibition by human leukocyte antigen (HLA) class I molecules can be a means to promote graft-versus-leukemia (GvL) effects and limit graft-versus-host disease (GvHD) in acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) patients. Experimental findings also support the blockade of NK cell inhibitory receptors as a way to protect against leukemia relapse. It may be possible to use our knowledge of NK cell activating receptors and their ligands to immunize patients with modified tumor cells to promote beneficial NK cell responses and development of host antitumor cytotoxic T lymphocytes (CTLs). Finally, new data support the idea of using modified NK cell receptors as a means to target patients' T cells against their own tumor cells and induce long-term immunity against them. Tumors are essentially tissues that have overcome normal regulation mechanisms, and therefore the ability to distinguish normal cells from abnormal cells is a key part of selectively attacking tumor cells. NK cells have various receptor systems designed to recognize infected and abnormal cells. Understanding NK cell receptors and their recognition mechanisms provides new tools for the development of immunotherapies against cancer.

The natural killer complex regulates severe malarial pathogenesis and influences acquired immune responses to Plasmodium berghei ANKA

The natural killer complex (NKC) is a genetic region of highly linked genes encoding several receptors involved in the control of NK cell function. The NKC is highly polymorphic, and allelic variability of various NKC loci has been demonstrated in inbred mice. Making use of BALB.B6-Cmv1r congenic mice, in which the NKC from disease-susceptible C57BL/6 mice has been introduced into the disease-resistant BALB/c background, we show here that during murine malaria infection, the NKC regulates a range of pathophysiological syndromes such as cerebral malaria, pulmonary edema, and severe anemia, which contribute to morbidity and mortality in human malaria. Parasitemia levels were not affected by the NKC genotype, indicating that control of malarial fatalities by the NKC cells does not operate through effects on parasite growth rate. Parasite-specific antibody responses and the proinflammatory gene transcription profile, as well as the TH1/TH2 balance, also appeared to be influenced by NKC genotype, providing evidence that this region, known to control innate immune responses via NK and/or NK T-cell activation, can also significantly regulate acquired immunity to infection. To date, NKC-encoded innate system receptors have been shown mainly to regulate viral infections. Our data provide evidence for critical NKC involvement in the broad immunological responses to a protozoan parasite.

Direct sequence comparison of two divergent class I MHC natural killer cell receptor haplotypes

The murine Ly49 gene family encoding natural killer cell receptors for class I MHC is an example of a rapidly evolving cluster of immune response genes. Determining the genomic sequence of the 129S6/SvEvTac (129S6) Ly49 cluster and comparing it to the known sequence of the C57BL/6 (B6) region provided insight into the mechanisms of Ly49 gene evolution. 129S6 contains 20 Ly49, many of which are pseudogenes and 40% of the genes have no counterpart in the B6 genome. The difference in gene content between these two strains is primarily the result of distinct patterns of gene duplication. Phylogenetic analyses of individual exons showed that Ly49 genes form distinct sub-families and an ancestral haplotype can be surmised. Dotplot analysis supports limited allelism in the two haplotypes; however, large regions of variation punctuate these islands of co-linearity. These variable regions contain a high concentration of repetitive elements that are predicted to contribute to the dynamic evolution of this cluster. The extreme variation in Ly49 haplotype content between mouse strains provides a genetic explanation for the documented differences in natural killer cell phenotype, and also indicates that differences in natural killer cell function observed between B6 and 129-derived gene-targeted mice should be interpreted with caution.

Mapping of the BALB/c Ly49 cluster defines a minimal natural killer cell receptor gene repertoire

The BALB/c inbred mouse strain is one of the most commonly used for immunological studies and is an animal model for natural killer (NK) cell function during pathogen infection and tumorigenesis. To understand better NK cell function in this strain, the complete BALB/c Ly49 haplotype was deduced. The BALB/c haplotype spans approximately 300 kb with a gene order and content of Ly49q, e, x, i, g, l, c, and a. Functional BALB/c alleles of Ly49q and e were isolated and found to be conserved. The BALB/c cluster represents a minimal haplotype as it contains many fewer functional genes than the 129 or B6 mouse strains. The small number of BALB/c Ly49 genes is due mainly to an absent group of genes (relative to B6 and 129) between Ly49x and i, although other smaller deletions are present. These gene deletions provide a genetic basis for the lack of certain Ly49-associated NK cell functions in this mouse strain. Finally, the mapping of a third Ly49 haplotype reveals that the basic murine Ly49 repertoire is composed of three framework gene pairs (Ly49q and e, Ly49i and g, and Ly49c and a) that are interspersed with variable numbers of strain-specific Ly49.

Escape of mutant double-stranded DNA virus from innate immune control

As innate immune system components, natural killer (NK) cells respond rapidly to infections and effectively control replication of pathogens, including murine cytomegalovirus (MCMV), a double-stranded DNA beta-herpesvirus. In the absence of NK cell control, MCMV infection results in early mortality due to uncontrolled viral replication. However, here we show that even in the face of initial NK cell control, there is late recrudescence of disease and mortality in immunodeficient mice due to the outgrowth of MCMV mutants that escape recognition by innate NK cells. These data suggest that viral infections in certain clinical settings also may be due to viral escape from innate immunity.

T-bet regulates the terminal maturation and homeostasis of NK and Valpha14i NKT cells

Natural killer (NK) and CD1d-restricted Valpha14i natural killer T (NKT) cells play a critical early role in host defense. Here we show that mice with a targeted deletion of T-bet, a T-box transcription factor required for Th1 cell differentiation, have a profound, stem cell-intrinsic defect in their ability to generate mature NK and Valpha14i NKT cells. Both cell types fail to complete normal terminal maturation and are present in decreased numbers in peripheral lymphoid organs of T-bet(-/-) mice. T-bet expression is regulated during NK cell differentiation by NK-activating receptors and cytokines known to control NK development and effector function. Our results identify T-bet as a key factor in the terminal maturation and peripheral homeostasis of NK and Valpha14i NKT cells.

The Rat RT1-A1cMHC Molecule Is a Xenogeneic Ligand Recognized by the Mouse Activating Ly-49W and Inhibitory Ly-49G Receptors

Mouse Ly-49 receptors are known to recognize xenogeneic ligands from hamster and rat. However, until now, there has been no description of a specific rat xenogeneic ligand for any mouse Ly-49 receptor. In this report, we identify RT1-A1c, a rat classical class I MHC molecule, as a ligand for the Ly-49G(BALB/c) inhibitory receptor and the closely related activating receptor, Ly-49W. Xenogeneic class I recognition of targets from PVG but not DA strain rats was mapped to the classical region of the RT1c haplotype by using Con A blasts from RT1c/RT1av1 intra-MHC recombinant rats as targets for RNK-16 cells expressing either Ly-49W or Ly-49G(BALB/c) receptors. Individual expression of class I molecules from PVG and DA rat strains in YB2/0 target cells demonstrate the xenogeneic recognition to be allele specific, because other class I molecules of the RT1c haplotype, RT1-A2c and RT1-U2c, and a classical class I molecule encoded by the RT1av1 haplotype, RT1-Aa, are not recognized by Ly-49W and -G(BALB/c). Furthermore, specificity for RT1-Ac can be transferred from Ly-49W to Ly-49P, which is normally unable to recognize RT1-Ac, by substitution of three residues shared by Ly-49W and -G(BALB/c) but not Ly-49P. These residues are located in the Ly-49 beta4-beta5 loop, which can determine class I allele specificity in mouse Ly-49 receptor interactions with mouse class I ligands, suggesting that mouse Ly-49 recognition of rat class I molecules follows similar principles of interaction. These findings have implications for xenotransplantation studies and for discerning Ly-49 recognition motifs present in MHC molecules.

Genetically linked C-type lectin-related ligands for the NKRP1 family of natural killer cell receptors

The natural killer (NK) gene complex (NKC) encodes orphan lectin-like NK cell receptors that may explain uncharacterized NK cell specificities. Unlike other NKC-encoded receptors that recognize molecules with major histocompatibility complex (MHC) class I folds, here we show that mouse Nkrp1d and Nkrp1f bind specific C-type lectin-related (Clr) molecules. Nkrp1d mediated inhibition when recognizing Clrb, a molecule expressed in dendritic cells and macrophages. Nkrp1 (official gene name, Klrb1) and Clr are intertwined in a genetically conserved NKC region showing recombination suppression, reminiscent of plant self-incompatibility loci. Thus, these findings broaden the 'missing-self' hypothesis from solely involving MHC class I to including related NK cell receptors for lectin-like ligands, and reflect genetic strategies for biological self-recognition processes in other species.

NKG2D triggers cytotoxicity in mouse NK cells lacking DAP12 or Syk family kinases

In activated mouse natural killer (NK) cells, the NKG2D receptor associates with two intracellular adaptors, DAP10 and DAP12, which trigger phosphatidyl inositol 3 kinase (PI3K) and Syk family protein tyrosine kinases, respectively. Here we show that cytotoxicity, but not cytokine production, is triggered by NKG2D in activated NK cells lacking either DAP12 or the Syk family members Syk and ZAP70. Inhibition of PI3K blocks this cytotoxicity, suggesting that the DAP10-PI3K pathway is sufficient to initiate NKG2D-mediated killing of target cells. Our results highlight signaling divergence in the effector functions of NKG2D and indicate that alternative associations between a receptor and its adaptors may provide a single receptor with a dual 'on-switch', giving mouse NK cells more choices through which to trigger cytotoxicity.

The NK cell receptor repertoire: formation, adaptation and exploitation

The identification of NK cell receptors specific for MHC class I molecules has greatly improved our knowledge of NK cell reactivity and specificity. Inhibitory receptors prevent NK cell activation directed against cells expressing self-MHC class I molecules. Consequently, diseased cells that do not express self-MHC class I molecules become susceptible to NK cell-mediated attack. Because of the specificity and distribution of inhibitory NK cell receptors, cells that express non-self (allogeneic) MHC class I molecules are also susceptible to NK cell reactions. This feature has been exploited in a clinical setting to treat leukemia patients.

Immune functions encoded by the natural killer gene complex

There has been marked progress in our understanding of the role of natural killer (NK) cells in immune responses, mainly due to the identification of NK-cell receptors and their ligands. The genes encoding many NK-cell receptors are located in the NK-gene complex (NKC). Here, we review the properties of NKC-encoded receptors, and provide a genomic and conceptual framework for an insight into NK-cell function and biology.

Expression of IFN-gamma upon triggering of activating Ly49D NK receptors in vitro and in vivo: costimulation with IL-12 or IL-18 overrides inhibitory receptors

NK cells can express both activating and inhibitory Ly49 receptors on their cell surface. When cells expressing both receptors are presented with a ligand, inhibition dominates the functional outcome. In this report we demonstrate that costimulation of the activating Ly49D murine NK cell receptor with IL-12 or IL-18 is capable of over-riding the inhibitory Ly49G2 receptor blockade for cytokine production both in vitro and in vivo. This synergy is mediated by and dependent upon Ly49D-expressing NK cells and results in significant systemic expression of IFN-gamma. This would place NK cells and their activating Ly-49 receptors as important initiators of microbial, antiviral, and antitumor immunity and provide a mechanism for the release of activating Ly49 receptors from inhibitory receptor blockade.

Virus-driven evolution of natural killer cell receptors

Natural killer (NK) cells can mount an immediate response against viral infection, secreting cytokines and killing virus-infected cells. However, viruses have devised strategies to avoid immune detection. Here, we discuss NK cell recognition of viruses and propose that viruses may provide the evolutionary pressure causing the diversification of the NK cell receptors.

Natural killer cell activation receptors in innate immunity to infection

Natural killer (NK) cells are best known for their capacity to kill tumors but they are also critical in early innate responses to infection, especially herpesviruses. Recent studies indicate that NK cell receptors involved in tumor target specificity are also involved in responses to viral infections.

NKG2D recruits two distinct adapters to trigger NK cell activation and costimulation

NKG2D is a receptor on natural killer (NK) cells and cytotoxic T lymphocytes that binds major histocompatibility complex (MHC) class I-like ligands expressed primarily on virally infected and neoplastic cells. In vitro studies indicate that NKG2D provides costimulation through an associated adapter, DAP10, which recruits phosphatidylinositol-3 kinase. Here we show that in DAP10-deficient mice, CD8+ T cells lack NKG2D expression and are incapable of mounting tumor-specific responses. However, DAP10-deficient NK cells express a functional NKG2D receptor due to the association of NKG2D with another adapter molecule, DAP12 (also known as KARAP), which recruits protein tyrosine kinases. Thus, NKG2D is a versatile receptor that, depending on the availability of adapter partners, mediates costimulation in T cells and/or activation in NK cells.

Innate immunity to cytomegalovirus: the Cmv1 locus and its role in natural killer cell function

The identification and characterization of genetic loci that contribute to patterns of susceptibility/resistance to infection provide important insights into the mechanisms of innate and adaptive immunity. Genetic heterogeneity across the population makes the characterization of such traits in humans technically difficult; however, inbred animal models represent an ideal tool for such analyses. This review illustrates the power of mouse genetics as utilized for the identification and characterization of the locus conferring early resistance to murine cytomegalovirus infection, Cmv1. This locus encodes an activating C-type lectin receptor of the Ly49 family that promotes natural killer (NK) cell cytolysis of infected cells. Although NK cells are usually able to detect and destroy virally infected cells via recognition of the downregulation of MHC class I molecules, the Cmv1 locus provides the first example of an NK receptor that is able to mediate clearance of viral infection via direct recognition of a virally encoded protein.

Allelic variation in the ectodomain of the inhibitory Ly-49G2 receptor alters its specificity for allogeneic and xenogeneic ligands

The Ly-49 multigene receptor family regulates mouse NK cell functions. A number of Ly-49 genes exhibit allelic variation, but the functional significance of allelic differences in extracellular domains of Ly-49 receptors regarding ligand specificity is largely unknown. Amino acid differences exist in the extracellular domains of the B6 and BALB/c allele products of the inhibitory Ly-49G receptor. We constructed chimeric Ly-49 receptors consisting of common cytoplasmic and transmembrane regions of the activating Ly-49W receptor fused with the ectodomains of the B6 and BALB/c alleles of Ly-49G. Expression of these chimeras in the RNK-16 rat NK cell line allowed us to study the specificity of inhibitory receptor ectodomains as they stimulated NK lytic activity. We found that the ectodomain of the BALB/c allele of Ly-49G recognizes both H-2D(d) and D(k) class I MHC alleles, whereas the ectodomain of the B6 allele of Ly-49G recognizes D(d), and not D(k). The specificity for D(k) as well as D(d) of the wild-type Ly-49G(BALB/c) allele product was confirmed with RNK-16 transfectants of this inhibitory receptor. Furthermore, the ectodomain of the Ly-49G(BALB/c) allele recognizes a distinct repertoire of xenogeneic ligands that only partially overlaps with that recognized by Ly-49G(B6). Our results indicate that allelic variation in Ly-49 extracellular domains can have functional significance by altering Ly-49 receptor specificity for mouse class I MHC and xenogeneic ligands.

Variable receptors controlling activation and inhibition of NK cells

NK cells are important effector lymphocytes of innate immunity; they kill infected cells and produce cytokines that stimulate other immune effects. Once considered relatively homogeneous, NK cells are now seen to be highly diverse. Within an individual, expression of different combinations of inhibitory and stimulatory receptors creates a diverse NK cell repertoire, which exhibits specificity in the immune response. Rapid evolution of NK cell receptor gene families distinguishes members of a species and causes substantial species-specific differences in NK cell receptor systems. All known ligands for these diverse receptors are MHC class I molecules, or molecules of host or pathogen origin that are homologous to MHC class I.

Aberrant DAP12 signaling in the 129 strain of mice: implications for the analysis of gene-targeted mice

NK cells are implicated in antiviral responses, bone marrow transplantation and tumor immunosurveillance. Their function is controlled, in part, through the Ly49 family of class I binding receptors. Inhibitory Ly49s suppress signaling, while activating Ly49s (i.e., Ly49D) activate NK cells via the DAP12 signaling chain. Activating Ly49 signaling has been studied primarily in C57BL/6 mice, however, 129 substrains are commonly used in gene-targeting experiments. In this study, we show that in contrast to C57BL/6 NK cells, cross-linking of DAP12-coupled receptors in 129/J mice induces phosphorylation of DAP12 but not calcium mobilization or cytokine production. Consistent with poor-activating Ly49 function, 129/J mice reject bone marrow less efficiently than C57BL/6 mice. Sequence analysis of receptors and DAP12 suggests no structural basis for inactivity, and both the 129/J and C57BL/6 receptors demonstrate normal function in a reconstituted receptor system. Most importantly, reconstitution of Ly49D in 129/J NK cells demonstrated that the signaling deficit is within the NK cells themselves. These unexpected findings bring into question any NK analysis of 129/J, 129Sv, or gene-targeted mice derived from these strains before complete backcrossing, and provide a possible explanation for the differences observed in the immune response of 129 mice in a variety of models.

Genetic control of innate immune responses against cytomegalovirus: MCMV meets its match

Cytomegalovirus (CMV) is a widespread pathogen that is responsible for severe disease in immunocompromised individuals and probably, associated with vascular disease in the general population. There is increasing evidence that cells of the innate immune system play a key role in controlling this important pathogen. This is particularly evident in the experimental murine CMV (MCMV) model of infection which has revealed an important role for natural killer (NK) cells in controlling early viral replication after infection with MCMV. In this model, different strains of inbred mice exhibit striking differences in their level of susceptibility to MCMV infection. Genetic studies, performed almost 10 years ago, revealed that this pattern of susceptibility/resistance can be attributed to a single genetic locus termed Cmv1 and recently several groups that have been working on the mapping and identification of Cmv1 have met with success. Interestingly, Cmv1 is allelic to a member of the Ly49 gene family, which encode activating or inhibitory transmembrane receptors present on the surface of NK cells. All Ly49 receptors characterized to date interact with MHC class I molecules on potential target cells, resulting in the accumulation of signals to the NK to either 'kill' or 'ignore' the cell based upon the repertoire of MHC class I molecules expressed. The identification of Cmv1 as Ly49H, a stimulatory member of the Ly49 family, adds an interesting twist to the Ly49 story. Although the ligand of Ly49H is not yet known, there is already compelling evidence that the ligand is upregulated on virally infected cells, resulting in specific activation of Ly49H-expressing NK cells. This review provides an historical perspective of the MCMV infection model from its inception to the discovery of the gene responsible for the phenotype and provides a basis for further experiments aimed at understanding the role of NK cells, in general, and Ly49H, in particular, in mediating resistance to cytomegalovirus.

Ly-49s3 is a promiscuous activating rat NK cell receptor for nonclassical MHC class I-encoded target ligands

Previous studies of the rapid rejection of MHC-disparate lymphocytes in rats, named allogeneic lymphocyte cytotoxicity, have indicated that rat NK cells express activating receptors for nonclassical MHC class I allodeterminants from the RT1-C/E/M region. Using an expression cloning system that identifies activating receptors associated with the transmembrane adapter molecule DAP12, we have cloned a novel rat Ly-49 receptor that we have termed Ly-49 stimulatory receptor 3 (Ly-49s3). A newly generated anti-Ly-49s3 Ab, mAb DAR13, identified subpopulations of resting and IL-2-activated NK cells, but not T or B lymphocytes. Depletion of Ly-49s3-expressing NK cells drastically reduced alloreactivity in vitro, indicating that this subpopulation is responsible for a major part of the observed NK alloreactivity. DAR13-mediated blockade of Ly-49s3 inhibited killing of MHC-congenic target cells from the av1, n, lv1, and c haplotypes, but not from the u or b haplotypes. A putative ligand was mapped to the nonclassical MHC class I region (RT1-C/E/M) using intra-MHC recombinant strains. Relative numbers of Ly-49s3(+) NK cells were reduced, and surface levels of Ly-49s3 were lower, in MHC congenic strains expressing the putative Ly-49s3 ligand(s). In conclusion, we have identified a novel Ly-49 receptor that triggers rat NK cell-mediated responses.

A ligand for the murine NK activation receptor Ly-49D: activation of tolerized NK cells from beta 2-microglobulin-deficient mice

Mouse NK cells express inhibitory NK receptors that recognize target cell MHC class I molecules and activation receptors that are less well defined. The Ly-49D activation receptor on C57BL/6 NK cells recognizes Chinese hamster ovary cells and triggers natural killing. In this study, we demonstrate that a Chinese hamster classical MHC class I molecule is the ligand for Ly-49D in a reporter gene assay system as well as in NK cell killing assays. Ly-49D recognizes the Chinese hamster class I molecule better when it is expressed with Chinese hamster beta(2)-microglobulin (beta(2)m) than murine beta(2)m. However, it is still controversial that Ly-49D recognizes H-2D(d), as we were unable to demonstrate the specificity previously reported. Using this one ligand-one receptor recognition system, function of an NK activation receptor was, for the first time, investigated in NK cells that are tolerized in beta(2)m-deficient mice. Surprisingly, Ly-49D-killing activity against ligand-expressing targets was observed with beta(2)m-deficient mouse NK cells, albeit reduced, even though "tolerized" function of Ly-49D was expected. These results indicate that Ly-49D specifically recognizes the Chinese hamster MHC class I molecule associated with Chinese hamster beta(2)m, and indicate that the Ly-49D NK cell activation receptor is not tolerized in beta(2)m deficiency.

Recognition of a virus-encoded ligand by a natural killer cell activation receptor

Natural killer (NK) cells express inhibitory and activation receptors that recognize MHC class I-like molecules on target cells. These receptors may be involved in the critical role of NK cells in controlling initial phases of certain viral infections. Indeed, the Ly49H NK cell activation receptor confers in vivo genetic resistance to murine cytomegalovirus (MCMV) infections, but its ligand was previously unknown. Herein, we use heterologous reporter cells to demonstrate that Ly49H recognizes MCMV-infected cells and a ligand encoded by MCMV itself. Exploiting a bioinformatics approach to the MCMV genome, we find at least 11 ORFs for molecules with previously unrecognized features of predicted MHC-like folds and limited MHC sequence homology. We identify one of these, m157, as the ligand for Ly49H. m157 triggers Ly49H-mediated cytotoxicity, and cytokine and chemokine production by freshly isolated NK cells. We hypothesize that the other ORFs with predicted MHC-like folds may be involved in immune evasion or interactions with other NK cell receptors.

In vivo developmental stages in murine natural killer cell maturation

Natural killer (NK) cells develop in the bone marrow, but their in vivo stages of maturation, expansion and acquisition of receptors that guide target cell specificity are not well defined. We describe here such stages of development. We also show that developing NK cells actively proliferate at a phenotypically distinguishable immature stage after they have acquired expression of Ly49 and CD94-NKG2 receptors. These studies provide a developmental framework for NK cell maturation in vivo and suggest the possible involvement of the Ly49 and CD94-NKG2 receptors themselves in modulating expansion of NK cell populations with a given NK cell receptor repertoire.

Natural cytotoxicity uncoupled from the Syk and ZAP-70 intracellular kinases

The intracellular signals that trigger natural cytotoxicity have not been clearly determined. The Syk and ZAP-70 tyrosine kinases are essential for cellular activation initiated by B and T cell antigen receptors and may drive natural killer (NK) cell cytotoxicity via receptors bearing immunoreceptor tyrosine-based activation motifs (ITAMs). However, we found that, unlike B and T cells, NK cells developed in Syk-/-ZAP-70-/- mice and, despite their nonfunctional ITAMs, lysed various tumor targets in vitro and eliminated tumor cells in vivo, including those without NKG2D ligands. The simultaneous inhibition of phosphatidyl inositol 3 kinase and Src kinases abrogated the cytolytic activity of Syk-/-ZAP-70-/- NK cells and strongly reduced that of wild-type NK cells. This suggests that distinct and redundant signaling pathways act synergistically to trigger natural cytotoxicity.

Specific and nonspecific NK cell activation during virus infection

The natural killer (NK) cell activation receptor Ly49H is required for resistance to murine cytomegalovirus (MCMV). We show here that NK cell proliferation and production of interferon-gamma (IFN-gamma) was not dependent on Ly49H expression during early MCMV infection. During a later phase of infection, however, Ly49H+ NK cells selectively proliferated and this expansion was blocked by anti-Ly49H administration. With vaccinia virus infection, neither the early nor late phase of NK cell proliferation was selective for Ly49H+ NK cells. These findings indicated that Ly49H+ NK cells were specifically activated by MCMV and that MCMV infection was characterized by nonspecific and specific phases of NK cell activation in vivo.

Vital involvement of a natural killer cell activation receptor in resistance to viral infection

Natural killer (NK) cells are lymphocytes that can be distinguished from T and B cells through their involvement in innate immunity and their lack of rearranged antigen receptors. Although NK cells and their receptors were initially characterized in terms of tumor killing in vitro, we have determined that the NK cell activation receptor, Ly-49H, is critically involved in resistance to murine cytomegalovirus in vivo. Ly-49H requires an immunoreceptor tyrosine-based activation motif (ITAM)-containing transmembrane molecule for expression and signal transduction. Thus, NK cells use receptors functionally resembling ITAM-coupled T and B cell antigen receptors to provide vital innate host defense.