Cloning and characterization of 5E6(Ly-49C), a receptor molecule expressed on a subset of murine natural killer cells

Uterine natural killer cells and recurrent spontaneous abortion

Recurrent spontaneous abortion (RSA), termed as two or more consecutive pregnancy loss is a great problem for some women of childbearing age. A large number of evidence confirm that there may be an immune background of RSA. As a member of the innate immune system, uterine natural killer (uNK) cells account for about 70% of total lymphocytes during pregnancy and play a critical role in the establishment and maintenance of pregnancy. This review mainly introduces the phenotype, origin, receptor, and function of uNK cells to illuminate its relationship with RSA.

The Accidental Pathologist: A Curiosity-Driven Journey from Plant Evolution to Innate Immunity

I have had the singular opportunity to perform research and to participate in medical education. Not unexpectedly, people have asked me which of the two was more important to me. My answer has always been and remains that I am equally passionate about research and teaching. My research has been curiosity driven and not purposeful; hence, I was willing to take risks. That my research led to the discovery of natural killer cells and the unraveling of the molecular basis of a human disease was an unexpected reward. By contrast, my interest in medical education was purposeful, with the goal of improving healthcare by teaching pathology as the scientific foundation of medicine. It started with participation in Robbins pathology texts but progressed toward development of technology-based tools for medical education. This was driven by the belief that technology, by providing equal access to knowledge across the world, can be a powerful democratizing force.

Cancer and innate immune system interactions: translational potentials for cancer immunotherapy

Passive immunotherapy, including adoptive T-cell therapy and antibody therapy, has shown encouraging results in cancer treatment lately. However, active immunotherapy of solid cancers remains an elusive goal. It is now known that the human innate immune system recognizes pathogen-associated molecular patterns conserved among microbes or damage-associated molecular patterns released from tissue injuries to initiate adaptive immune responses during infection and tissue inflammation, respectively. In contrast, how the innate immune system recognizes endogenously arising cancer remains poorly understood at the molecular level, which poses a significant roadblock to the development of active cancer immunotherapy. We hereby review the current knowledge of how solid cancers directly and indirectly interact with cells of the human innate immune system, with a focus on the potential effect of such interactions to the resultant adaptive immune responses against cancer. We believe that understanding cancer and innate immune system interactions may allow us to better manipulate the adaptive immune system at the molecular level to develop effective active immunotherapy against cancer. Current and future perspectives in clinical development that exploits these molecular interactions are discussed.

Mistaken notions about natural killer cells

As natural killer (NK) cells were first described more than 30 years ago--a lifetime in recent immunological history--this is a good time to reflect on their transition from outcasts of mainstream immunology to prominent players in innate immunity. Whereas much of our existing knowledge about NK cells is taken as longstanding fact, it may be surprising to immunologists of a younger vintage, particularly those working on NK cells today, that some of this knowledge was, at least initially, unexpected. In this Perspective, I take an unconventional approach to discussing our progress in understanding NK cells.

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.

Mouse TCRalphabeta+CD8alphaalpha intraepithelial lymphocytes express genes that down-regulate their antigen reactivity and suppress immune responses

Mouse small intestine intraepithelial lymphocytes (IEL) that express alphabetaTCR and CD8alphaalpha homodimers are an enigmatic T cell subset, as their specificity and in vivo function remain to be defined. To gain insight into the nature of these cells, we performed global gene expression profiling using microarray analysis combined with real-time quantitative PCR and flow cytometry. Using these methods, TCRalphabeta(+)CD8alphaalpha IEL were compared with their TCRalphabeta(+)CD8beta(+) and TCRgammadelta(+) counterparts. Interestingly, TCRalphabeta(+)CD8alphaalpha IEL were found to preferentially express genes that would be expected to down-modulate their reactivity. They have a unique expression pattern of members of the Ly49 family of NK receptors and tend to express inhibitory receptors, along with some activating receptors. The signaling machinery of both TCRalphabeta(+)CD8alphaalpha and TCRgammadelta(+) IEL is constructed differently than other IEL and peripheral T cells, as evidenced by their low-level expression of the linker for activation of T cells and high expression of the non-T cell activation linker, which suppresses T cell activation. The TCRalphabeta(+)CD8alphaalpha IEL subset also has increased expression of genes that could be involved in immune regulation, including TGF-beta(3) and lymphocyte activation gene-3. Collectively, these data underscore the fact that, while TCRalphabeta(+)CD8alphaalpha IEL resemble TCRgammadelta(+) IEL, they are a unique population of cells with regulated Ag reactivity that could have regulatory function.

A rapid and efficient method for the generation and screening of monoclonal antibodies specific for cell surface antigens

The generation of monoclonal antibodies (mAb) of desired specificity to cell surface antigens can serve as a valuable tool to study protein expression and function. However, traditional approaches to mAb generation usually involve large-scale protein purification and intensive screening, and may not result in mAb specificities to the native protein of interest. We describe a simple, inexpensive, high-throughput method for the generation and screening of hybridomas secreting mAb specific for cell surface receptors. Intact reporter cells expressing a CD3zeta-fusion receptor of the protein of interest are plated in 96-well arrays of captured, plate-bound hybridoma supernatants. A mAb to the protein of interest generates a signal leading to reporter-cell expression of beta-galactosidase, and enzyme activity can be screened in a single day using a non-radioactive substrate. Importantly, a single cell line can be used for immunization, screening, semi-quantitative affinity comparisons, and subsequent screening for physiological ligand expression, if the protein of interest is a receptor. We describe an application of this approach to generate mAb specific for a protein of previously unknown expression and undocumented function.

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.

Molecular and genetic basis for strain-dependent NK1.1 alloreactivity of mouse NK cells

NK1.1 alloantigen expression can be used to define NK cells in certain mouse strains, such as B6 (NKR-P1C) and SJL (NKR-P1B). However, BALB/c NK cells do not react with the anti-NK1.1 mAb, PK136. To investigate the NK1.1(-) phenotype of BALB/c NK cells, we have undertaken NK1.1 epitope mapping and genomic analysis of the BALB/c Nkrp1 region. Bacterial artificial chromosome library analysis reveals that, unlike the Ly49 region, the Nkrp1-Ocil/Clr region displays limited genetic divergence between B6 and BALB/c mice. In fact, significant divergence is confined to the Nkrp1b and Nkrp1c genes. Strikingly, the B6 Nkrp1d gene appears to represent a divergent allele of the Nkrp1b gene in BALB/c mice and other strains. Importantly, BALB/c NK cells express abundant and functional Nkrp1 transcripts, and the BALB/c NKR-P1B receptor functionally binds Ocil/Clr-b ligand. However, the BALB/c NKR-P1B/C sequences differ from those of the known NK1.1 alloantigens, and epitope mapping demonstrates that directed mutation of a single amino acid in the NKR-P1B(BALB) protein confers NK1.1 reactivity. Thus, PK136 mAb recognizes, in part, a distal C-terminal epitope present in NKR-P1B(Sw/SJL) and NKR-P1C(B6), but absent in NKR-P1A/D/F(B6) and NKR-P1B/C(BALB). Allelic divergence of the Nkrp1b/c gene products and limited divergence of the BALB/c Nkrp1-Ocil/Clr region explain a longstanding confusion regarding the strain-specific NK1.1 alloantigen reactivity of mouse NK cells.

Influence of xenogeneic beta2-microglobulin on functional recognition of H-2Kb by the NK cell inhibitory receptor Ly49C

NK cells maintain self-tolerance through expression of inhibitory receptors that bind MHC class I (MHC-I) molecules. MHC-I can exist on the cell surface in several different forms, including "peptide-receptive" or PR-MHC-I that can bind exogenous peptide. PR-MHC-I molecules are short lived and, for H-2K(b), comprise approximately 10% of total MHC-I. In the present study, we confirm that signaling through the mouse NK inhibitory receptor Ly49C requires the presence of PR-K(b) and that this signaling is prevented when PR-K(b) is ablated by pulsing with a peptide that can bind to it with high affinity. Although crystallographic data indicate that Ly49C can engage H-2K(b) loaded with high-affinity peptide, our data suggest that this interaction does not generate an inhibitory signal. We also show that no signaling occurs when the PR-K(b) complex has mouse beta(2)-microglobulin (beta(2)m) replaced with human beta(2)m, although replacement with bovine beta(2)m has no effect. Furthermore, we show that beta(2)m exchange occurs preferentially in the PR-K(b) component of total H-2K(b). These conclusions were reached in studies modulating the sensitivity to lysis of both NK-resistant syngeneic lymphoblasts and NK-sensitive RMA-S tumor cells. We also show, using an in vivo model of lymphocyte recirculation, that engrafted lymphocytes are unable to survive NK attack when otherwise syngeneic lymphocytes express human beta(2)m. These findings suggest a qualitative extension of the "missing self" hypothesis to include NK inhibitory receptors that are restricted to the recognition of unstable forms of MHC-I, thus enabling NK cells to respond more quickly to events that decrease MHC-I synthesis.

Complete elucidation of a minimal class I MHC natural killer cell receptor haplotype

The BALB/c inbred mouse is widely used in models of infectious disease, transplantation, and cancer. The differences in the immune responses of BALB/c compared to C57BL/6 mice are especially valuable for the identification of immune regulation genes. One striking immune variance between these mice is in the function of natural killer (NK) cells, and there is strong evidence implicating differential expression of Ly49 genes. In this study, the complete BALB/c Ly49 gene cluster has been sequenced and found to contain six functional genes and two pseudogenes. Compared to C57BL/6 mice, there is a 200 kb region absent in the BALB/c cluster including a complete lack of Ly49h-related genes, which explains the increased susceptibility of BALB/c to cytomegalovirus infection. In addition, there is no BALB/c Ly49d allele, explaining the inability of BALB/c NK cells to kill certain tumor cells. The Ly49 region has now been sequenced in three different inbred mouse strains, and comparisons indicate that the evolution of each haplotype is not straightforward and has involved large-scale deletions/insertions, gene recombination, and unequal crossing over between divergent haplotypes. This study confirms that relatively small murine class I MHC receptor haplotypes exist, analogous to observations made of human killer cell Ig-like receptor gene haplotypes.

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.

Liver NK cells expressing TRAIL are toxic against self hepatocytes in mice

Although it is known that activation of natural killer (NK) cells causes liver injury, the mechanisms underlying NK cell-induced killing of self-hepatocytes are not clear. We demonstrated that liver NK cells have cytotoxicity against normal syngeneic hepatocytes in mice. Polyinosinic-polycytidylic acid (poly I:C) treatment enhanced hepatocyte toxicity of liver NK cells but not that of spleen NK cells. Unlike NK cells in other tissues, approximately 30%-40% of liver NK cells constitutively express tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). An in vitro NK cell cytotoxic assay revealed that hepatocyte toxicity of liver NK cells from both naïve and poly I:C-treated mice was inhibited partially by an anti-TRAIL monoclonal antibody (mAb) alone and completely by the combination with anti-Fas ligand (FasL) mAb and a perforin inhibitor, concanamycin A, indicating contribution of TRAIL to NK cell-mediated hepatocyte toxicity. The majority of TRAIL(+) NK cells lacked expression of Ly-49 inhibitory receptors recognizing self-major histocompatibility complex class I, indicating a propensity to targeting self-hepatocytes. Poly I:C treatment significantly upregulated the expression of Ly-49 receptors on TRAIL(-) NK cells. This might be a compensatory mechanism to protect self-class I-expressing cells from activated NK cell-mediated killing. However, such compensatory alteration was not seen at all in the TRAIL(+) NK cell fraction. Thus, liver TRAIL(+) NK cells have less capacity for self-recognition, and this might be involved in NK cell-dependent self-hepatocyte toxicity. In conclusion, our findings are consistent with a model in which TRAIL-expressing NK cells play a critical role in self-hepatocyte killing through poor recognition of MHC.

Independent Control ofLy49gAlleles: Implications for NK Cell Repertoire Selection and Tumor Cell Killing

A novel murine NK cell-reactive mAb, AT8, was generated. AT8 recognizes Ly49G from 129/J, BALB/c, and related mouse strains, but does not bind to Ly49G(B6). Costaining with AT8 and a Ly49G(B6)-restricted Ab (Cwy-3) provides the first direct evidence that Ly49G protein is expressed from both alleles on a significant proportion of NK cells from four different types of F(1) hybrid mice. The observed level of biallelic Ly49G expression reproducibly followed the product rule in both freshly isolated and cultured NK cells. Surprisingly, the percentage of NK cells expressing both Ly49G alleles could be dramatically increased in vitro and in vivo through IL-2R- and IFN receptor-dependent signaling pathways, respectively. Unexpectedly, Ly49G(B6+) NK cells in an H-2(d), but not H-2(b), background were more likely to lyse D(d+) and Chinese hamster ovary tumor cells than Ly49G(BALB/129+) NK cells. Furthermore, Ly49G(B6+) NK cells also proliferated to a higher degree in response to poly(I:C) than NK cells expressing a non-Ly49G(B6) allele in an H-2(d), but not H-2(b), background. These results suggest that Ly49G(B6) has a lower affinity for H-2D(d) than Ly49G(BALB/129), and the genetic background calibrates the responsiveness of NK cells bearing self-specific Ly49. Other H-2D(d) receptors on the different Ly49G(+) NK cell subsets were unequally coexpressed, possibly explaining the disparate responses of Ly49G(B6+) NK cells in different hybrid mice. These data indicate that the stochastic mono- and biallelic expression of divergent Ly49G alleles increases the range of MHC affinities and the functional potential in the total NK cell population of heterozygous mice.

Ly49Q, a member of the Ly49 family that is selectively expressed on myeloid lineage cells and involved in regulation of cytoskeletal architecture

Here, we identified and characterized a Ly49 family member, designated as Ly49Q. The Ly49q gene encodes a 273-aa protein with an immunoreceptor tyrosine-based inhibitory motif (ITIM) at the N terminus of its cytoplasmic domain. We show that the ITIM of Ly49Q can recruit SHP-2 and SHP-1 in a tyrosine phosphorylation-dependent manner. In contrast to other known members of the Ly49 family, Ly49Q was found not to be expressed on NK1.1(+) cells, but instead was detectable on virtually all Gr-1(+) cells, such as myeloid precursors in bone marrow. Monocytes/macrophages also expressed low levels of Ly49Q, and the expression was enhanced by the treatment of cells with IFN-gamma. Treatment of activated macrophages with anti-Ly49Q mAb induced rapid formation of polarized actin structures, showing filopodia-like structure on one side and lamellipodial-like structure on the other side. A panel of proteins became tyrosine-phosphorylated in myeloid cells when treated with the mAb. Induction of the phosphorylation depends on the ITIM of Ly49Q. Thus, Ly49Q has unique features different from other known Ly49 family members and appears to be involved in regulation of cytoskeletal architecture of macrophages through ITIM-mediated signaling.

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.

Increased bone marrow allograft rejection by depletion of NK cells expressing inhibitory Ly49 NK receptors for donor class I antigens

Natural killer (NK) cells are the major effectors of acute rejection of incompatible bone marrow cell (BMC) grafts in lethally irradiated mice. The immunogenetics of BMC rejection are largely controlled by the coexpression (or not) of inhibitory and stimulatory Ly49 receptors whose ligands are class I major histocompatibility complex (MHC) molecules. The majority of the BMC rejection studies involved low numbers of BMCs that were resisted by host NK cells. In the present study, larger numbers of BMCs were given in which rejection was not detected and the role of different Ly49 NK subsets not presumably involved in the rejection of a particular BMC haplotype was examined. Surprisingly, the data show that the removal of NK cell subsets expressing Ly49 inhibitory receptors for donor class I antigens, which would be predicted to have no effect on the BMC rejection capability, resulted in the marked rejection of BMCs where no resistance was normally seen. These results extend the "missing self" hypothesis to suggest that NK Ly49 inhibitory receptors can both inhibit activation and killing by those cells, but also can in some way influence the function of NK cells that do not express that inhibitory receptor in a cell-cell interaction. This suggests that caution must be exercised before removal of host NK cell subset is applied clinically because enhanced BMC rejection may result. Altering the balance of Ly49 NK subsets may also affect other in vivo activities of these cells.

The role of natural killer cells in the induction of autoimmune gastritis

A number of experimental models of organ-specific autoimmunity involve a period of peripheral T cell lymphopenia prior to disease onset. In particular, experimental autoimmune gastritis, induced in susceptible mouse strains by neonatal thymectomy, is a CD4+ T cell mediated autoimmune disease. We have previously demonstrated that this disease displays the hallmarks of a Th1-mediated DTH inflammatory response with an essential role for IFN-gamma very early in the pathogenesis of disease. Given the interplay between the innate and adaptive immune responses, a potential source of early IFN-gamma production in these lymphopenic mice is the innate immune response. Here we have assessed the contribution of innate immunity to the induction of experimental autoimmune gastritis, in particular, the role of natural killer (NK) cells in production of IFN-gamma. Analysis of NK cells and macrophages revealed no difference in either the number or activation status between euthymic and neonatally thymectomised mice. Furthermore, in vivo depletion of NK cells immediately after neonatal thymectomy of (BALB/cCrSlcxC57BL/6) F1 mice demonstrated no reduction in disease incidence compared to control groups of neonatally thymectomised mice. Therefore, we conclude that NK cells are not the primary source of IFN-gamma required for the pathogenesis of autoimmune gastritis following neonatal thymectomy but rather the small cohort of T cells in the periphery of lymphopenic mice are likely to be responsible for the IFN-gamma production.

Binding of the natural killer cell inhibitory receptor Ly49A to its major histocompatibility complex class I ligand. Crucial contacts include both H-2Dd AND beta 2-microglobulin

Ly49A, an inhibitory C-type lectin-like mouse natural killer cell receptor, functions through interaction with the major histocompatibility complex class I molecule, H-2D(d). The x-ray crystal structure of the Ly49A.H-2D(d) complex revealed that homodimeric Ly49A interacts at two distinct sites of H-2D(d): Site 1, spanning one side of the alpha1 and alpha2 helices, and Site 2, involving the alpha1, alpha2, alpha3, and beta(2)m domains. Mutants of Ly49A, H-2D(d), and beta(2)-microglobulin at intermolecular contacts and the Ly49A dimer interface were examined for binding affinity and kinetics. Although mutations at Site 1 had little affect, several at Site 2 and at the dimer interface hampered the Ly49A.H-2D(d) interaction, with no effect on gross structure or T cell receptor interaction. The region surrounding the most critical residues (in H-2D(d), Asp(122); in Ly49A, Asp(229), Ser(236), Thr(238), Arg(239), and Asp(241); and in beta(2)-microglobulin, Gln(29) and Lys(58)) of the Ly49A.H-2D(d) interface at Site 2 includes a network of water molecules, suggesting a molecular basis for allelic specificity in natural killer cell recognition.

Critical role of NK but not NKT cells in acute rejection of parental bone marrow cells in F1 hybrid mice

F1 hybrid mice vigorously reject transplanted parental bone marrow (BM) cells, which is a phenomenon called "hybrid resistance (HR)". Since NK1.1(+) cells play crucial role in HR, both NK1.1(+)CD3(+) NKT cells and NK1.1(+)CD3(-) NK cells have been possible candidates of effector cells. To elucidate the major effector cells in HR, we employed Rag-2(-/-) mice devoid of T, B, and NKT cells and cytokine receptor common gamma subunit and Rag-2 double-deficient (gamma(c)(-/-(y))-Rag-2(-/-)) mice lacking all lymphoid cells including NK cells. Rag-2(-/-) F1 hybrid mice rejected parental BM cells to the extent similar to wild-type (WT) F1 hybrids. In contrast, male gamma(c)(-/y)-Rag-2(-/-) F1 hybrid mice were unable to reject parental BM cells. After reconstitution with NK but not NKT cells, male gamma(c)(-/y)-Rag-2(-/-) F1 hybrid mice restored the ability to reject parental BM cells. Collectively, it is concluded that NKT cells play little role, if any, and NK cells are the only cells involved in HR.

MHC Class I-Ly49 Interactions Shape the Ly49 Repertoire on Murine NK Cells

This study aims to determine how the interaction of Ly49 receptors with MHC class I molecules shapes the development of the Ly49 repertoire. We have examined the percentage of NK cells that expressed Ly49A, Ly49G2, and Ly49D in single and double Ly49A/C-transgenic mice on four different MHC backgrounds, H-2(b), H-2(d), H-2(b/d), and beta(2)-microglobulin(-/-). The results show that the total numbers of NK cells were not different among the strains. The prior expression of a Ly49 receptor capable of binding to self MHC class I altered the percentage of NK cells expressing endogenous Ly49A, Ly49G2, and Ly49D even in mice in which no MHC ligand was present for the latter receptors. The NK cells in the Ly49-transgenic mice expressed the same level of endogenous Ly49 receptors as wild-type mice of a similar MHC background. In contrast, the number of NK T cells was reduced in mice in which the Ly49 transgene could bind to a MHC class I molecule. The onset of Ly49 receptor expression on NK cells during ontogeny was not altered in the presence of transgenic Ly49 receptors. These data support a sequential model and argue against a selection model for Ly49 repertoire development on NK cells.

Possible role of natural killer cells in negative selection of mutant lymphocytes that fail to express the human leukocyte antigen-A2 allele

Increased frequencies of cells carrying mutations at several loci have been found in the blood cells of atomic-bomb (A-bomb) survivors upon testing four or five decades after the bombing. Interestingly, though, we have been unable to demonstrate any radiation-associated increases in the frequencies of mutant blood cells in which human leukocyte antigen (HLA)-A expression has been disrupted; this is true both of preliminary tests on the T cells of a small subset of A-bomb survivors and of the much more extensive study reported here in which we screened a much larger group of survivors for HLA-A2 loss mutations in B cells and granulocytes as well as in T cells. In attempting to explain our inability to detect any increases in HLA-A2-negative cell numbers in HLA-A2 heterozygous individuals exposed to A-bomb irradiation, we decided to test the hypothesis that HLA-A mutant lymphocytes might well have been induced by radiation exposure in much the same way as every other type of mutant we encountered, but may subsequently have been eliminated by the strong negative selection associated with their almost inevitable exposure to autologous natural killer (NK) cells in the bloodstream of each of the individuals concerned. We now report that mutant B lymphocyte cell lines that have lost the ability to express the HLA-A2 antigen do indeed appear to be much more readily eliminated than their parental heterozygous counterparts during co-culture in vitro with autologous NK cells. We make this claim first because we have observed that adding autologous NK cells to in vitro cultures of HLA-A2 heterozygous B or T cell lines appeared to cause a dose-dependent decrease in the numbers of HLA-A2-negative mutants that could be detected over a period of 3 days, and second because when we used peripheral blood HLA-A2 heterozygous lymphocyte cultures from which most of the autologous NK cells had been removed we found that we were able to detect newly-arising HLA-A2 mutant T cells in substantial numbers. Taken together, these results strongly support the hypothesis that autologous NK cells are responsible for eliminating mutant lymphocytes that have lost the ability to express self-HLA class I molecules in vivo, and may well therefore explain why we have been unable to detect increased frequencies of HLA-A2 mutants in samples from any of the 164 A-bomb survivors whose HLA-A2 heterozygote status made their lymphocytes suitable for our tests.

Class I MHC-binding characteristics of the 129/J Ly49 repertoire

The Ly49 family of NK cell receptors and its MHC-binding characteristics have only been well characterized in C57BL/6 (B6) mice. Previous studies have shown that 129/J mice express unique Ly49 genes that are not found in the B6 strain. Screening of a 129/J cDNA library led to the discovery of 10 distinct full-length Ly49-related coding sequences (Ly49e, g, i, o, p, r, s, t, u, and v). Although 129/J mice share identical class I MHC (K(b) and D(b)) transcripts with B6 mice, only one Ly49 is identical in the two strains (Ly49E). In addition to the previously characterized Ly49P, two new activating Ly49 proteins were discovered, Ly49R and U. The MHC specificity of the total 129/J Ly49 repertoire was evaluated with soluble class I MHC tetramers and found to be distinct compared with the B6 Ly49 repertoire. Ly49V bound to many types of class I MHC, suggesting that Ly49V(+) NK cells may monitor host cells for a global down-regulation in MHC levels. An activating receptor, Ly49R, was shown to bind soluble class I molecules to a moderate degree, a result not previously observed for other activating Ly49 proteins. Furthermore, tetramer-binding results were confirmed functionally with cytotoxicity assays using sorted 129/J NK cells. This study shows that the Ly49 repertoire and its MHC-binding characteristics can be very different among inbred mouse strains. Ly49 divergence should be considered when using 129-derived embryonic stem cells for the production of gene-targeted mice, especially when an immune or NK-derived phenotype is under scrutiny.

A "chimeric" C57l-derived Ly49 inhibitory receptor resembling the Ly49D activation receptor

Ly49D is a natural killer (NK) cell activation receptor that is responsible for differential mouse inbred strain-determined lysis of Chinese hamster ovary (CHO) cells. Whereas C57BL/6 NK cells kill CHO, BALB/c-derived NK cells cannot kill because they lack expression of Ly49D. Furthermore, the expression of Ly49D, as detected by monoclonal antibody 4E4, correlates well with CHO lysis by NK cells from different inbred strains. However, one discordant mouse strain was identified; C57L NK cells express the mAb 4E4 epitope but fail to lyse CHO cells. Herein we describe a Ly49 molecule isolated from C57L mice that is recognized by mAb 4E4 (anti-Ly49D). Interestingly, this molecule shares extensive similarity to Ly49D(B6) in its extracellular domain, but its cytoplasmic and transmembrane domains are identical to the inhibitory receptor Ly49A(B6), including a cytoplasmic ITIM. This molecule bears substantial overall homology to the previously cloned Ly49O molecule from 129 mice the serologic reactivity and function of which were undefined. Cytotoxicity experiments revealed that 4E4(+) LAK cells from C57L mice failed to lyse CHO cells and inhibited NK cell function in redirected inhibition assays. MHC class I tetramer staining revealed that the Ly49O(C57L)-bound H-2D(d) and lysis by 4E4(+) C57L LAK cells is inhibited by target H-2D(d). The structural basis for ligand binding was also examined in the context of the recent crystallization of a Ly49A-H-2D(d) complex. Therefore, this apparently "chimeric" Ly49 molecule serologically resembles an NK cell activation receptor but functions as an inhibitory receptor.

Expression of Ly49E and CD94/NKG2 on fetal and adult NK cells

Murine NK cells express inhibitory receptors belonging to the Ly49 and CD94/NKG2 family. Ly49E and CD94 are the only NK cell receptor transcripts detectable in fetal NK cells. Still unproved is the surface expression of Ly49E on NK cells. Here we generated two novel mAbs, a mAb recognizing Ly49E with cross-reactivity to Ly49C, and a mAb against NKG2A/C/E. Ly49E was immunoprecipitated as a disulfide-linked homodimer with 46-kDa subunits. Removal of N-linked carbohydrates revealed a 31-kDa protein backbone. NKG2A was immunoprecipitated as a 38-kDa protein. Although the frequency of fetal NK cells expressing Ly49E was higher than 25%, it decreased drastically from 2 wk after birth. Phenotypic analysis showed that approximately 90% of fetal NK cells and approximately 50% of adult NK cells express high levels of CD94/NKG2. The remaining 50% of adult NK cells expressed low surface levels of CD94/NKG2. Expression of Ly49E and CD94/NKG2 was not restricted to NK cells, but was also observed on NK T and memory T cells. Functional analysis showed that sorted Ly49E(+) and CD94/NKG2(+) fetal NK cells could discriminate between MHC class I-positive and MHC class I-negative tumor cells. We also demonstrated that Ly49E becomes phosphorylated following pervanadate stimulation of fetal NK cells. The expression levels of Ly49E and CD94/NKG2 were similar in wild-type compared with beta(2)-microglobulin(-/-) mice. In conclusion, generation of mAbs against Ly49E and NKG2 extended the phenotypic and functional characterization of NK cells.

NK-mediated elimination of mutant lymphocytes that have lost expression of MHC class I molecules

Mutant cells generated in vivo can be eliminated when mutated gene products are presented as altered MHC/peptide complexes and recognized by T cells. Diminished expression of MHC/peptide complexes enables mutant cells to escape recognition by T cells. In the present study, we tested the hypothesis that mutant lymphocytes lacking expression of MHC class I molecules are eliminated by autologous NK cells. In H-2b/k F1 mice, the frequency of H-2Kb-negative T cells was higher than that of H-2Kk-negative T cells. The frequency of H-2K-deficient T cells increased transiently after total body irradiation. During recovery from irradiation, H-2Kk-negative T cells disappeared more rapidly than H-2Kb-negative T cells. The disappearance of H-2K-deficient T cells was inhibited by administration of Ab against asialo-GM1. H-2Kk-negative T cells showed higher sensitivity to autologous NK cells in vitro than H-2Kb/k heterozygous or H-2Kb-negative T cells. Adding syngeneic NK cells to in vitro cultures prevented emergence of mutant cells lacking H-2Kk expression but had little effect on the emergence of mutant cells lacking H-2Kb expression. Results in the H-2b/k F1 strain correspond with the sensitivity of parental H-2-homozygous cells in models of marrow graft rejection. In H-2b/d F1 mice, there was no significant difference between the frequencies of H-2Kb-negative and H-2Kd-negative T cells, although the frequencies of mutant cells were different after radiation exposure among the strains examined. H-2b/d F1 mice also showed rapid disappearance of the mutant T cells after irradiation, and administration of Ab against asialo-GM1 inhibited the disappearance of H-2K-deficient T cells in H-2b/d F1 mice. Our results provide direct evidence that autologous NK cells eliminate mutant cell populations that have lost expression of self-MHC class I molecules.

Synergistic effects of in vivo depletion of Ly-49A and Ly-49G2 natural killer cell subsets in the rejection of H2b bone marrow cell allografts

Subsets of murine natural killer (NK) cells exist that express the Ly-49 family of molecules that recognize different major histocompatibility complex (MHC) determinants. Bone marrow transplantation studies were performed to examine the in vivo functions of 2 of these subsets. Subsets of Ly-49A and Ly-49G2 NK share specificity for the same MHC class 1 ligand, Dd, binding of which results in an inhibitory signal to the NK cell but allows them to lyse H2b targets in vitro. We therefore examined the ability of these subsets to reject H2b bone marrow cell allografts in lethally irradiated mice. Surprisingly, depletion of Ly-49A+ NK cells in BALB/c or B10.D2 mice (both H2d) had no effect on the rejection of H2b BMC. However, Ly-49A depletion did partially abrogate the ability of B10.BR (H2k) mice to reject H2ballografts. Although depletion of either Ly-49A+ or Ly-49G2+ NK cells alone had no effect on the ability of B10.D2 mice to reject H2b BMC, depletion of both subsets dramatically and synergistically abrogated rejection. Studies with various B10 congenic mice and their F1 hybrids indicate that this synergy between Ly49A and Ly4G2 depletion occurs in every instance. Thus, Ly-49A+ NK cells appear to play a role in the rejection H2b bone marrow allografts, but, in most strains of mice studied, Ly-49G2+ NK cells must also be eliminated. The putative roles of these NK cell subsets in clinical transplantation remains to be elucidated.

Synergistic effects of in vivo depletion of Ly-49A and Ly-49G2 natural killer cell subsets in the rejection of H2b bone marrow cell allografts

Subsets of murine natural killer (NK) cells exist that express the Ly-49 family of molecules that recognize different major histocompatibility complex (MHC) determinants. Bone marrow transplantation studies were performed to examine the in vivo functions of 2 of these subsets. Subsets of Ly-49A and Ly-49G2 NK share specificity for the same MHC class 1 ligand, Dd, binding of which results in an inhibitory signal to the NK cell but allows them to lyse H2b targets in vitro. We therefore examined the ability of these subsets to reject H2b bone marrow cell allografts in lethally irradiated mice. Surprisingly, depletion of Ly-49A+ NK cells in BALB/c or B10.D2 mice (both H2d) had no effect on the rejection of H2b BMC. However, Ly-49A depletion did partially abrogate the ability of B10.BR (H2k) mice to reject H2ballografts. Although depletion of either Ly-49A+ or Ly-49G2+ NK cells alone had no effect on the ability of B10.D2 mice to reject H2b BMC, depletion of both subsets dramatically and synergistically abrogated rejection. Studies with various B10 congenic mice and their F1 hybrids indicate that this synergy between Ly49A and Ly4G2 depletion occurs in every instance. Thus, Ly-49A+ NK cells appear to play a role in the rejection H2b bone marrow allografts, but, in most strains of mice studied, Ly-49G2+ NK cells must also be eliminated. The putative roles of these NK cell subsets in clinical transplantation remains to be elucidated.

Ly49I NK cell receptor transgene inhibition of rejection of H2b mouse bone marrow transplants

The Ly49 family of genes encode NK cell receptors that bind class I MHC Ags and transmit negative signals if the cytoplasmic domains have immunoregulatory tyrosine-based inhibitory motifs (ITIMs). 5E6 mAbs recognize Ly49C and Ly49I receptors and depletion of 5E6+ NK cells prevents rejection of allogeneic or parental-strain H2d bone marrow cell (BMC) grafts. To determine the function of the Ly49I gene in the rejection of BMC grafts, we transfected fertilized eggs of FVB mice with a vector containing DNA for B6 strain Ly49I (Ly49IB6). Ly49IB6 is ITIM+ and is recognized by 5E6 as well as Ly49I-specific 8H7 mAbs. Normal FVB H2q mice reject H2b but not H2d BMC allografts, and the rejection of H2b BMC was inhibited partially by anti-NK1.1 and completely by anti-asialo GM1, but not by anti-CD8, Abs. In FVB mice, NK1.1 is expressed on only 60% NK cells. FVB. Ly49IB6 hosts failed to reject H2d or H2b BMC, but did reject class I-deficient TAP-1-/- BMC, indicating that NK cells were functional. Nondepleting doses of anti-Ly49I Abs reversed the acceptance of H2b BMC by FVB.Ly49IB6 mice. FVB.Ly49IB6+/- mice were crossed and back-crossed with 129 mice-H2b, 5E6-, poor responders to H2d BMC grafts. While transgene-negative H2b/q F1 or first-generation back-crossed mice rejected H2b marrow grafts (hybrid resistance), transgene-positive mice did not. Thus B6 strain Ly49I receptors transmit inhibitory signals from H2b MHC class I molecules. Moreover, Ly49IB6 has no positive influence on the rejection of H2d allografts.

Interaction of Ly-49D+ NK cells with H-2Dd target cells leads to Dap-12 phosphorylation and IFN-gamma secretion

Murine Ly-49D augments NK cell function upon recognition of target cells expressing H-2Dd. Ly-49D activation is mediated by the immunoreceptor tyrosine-based activation motif-containing signaling moiety Dap-12. In this report we demonstrate that Ly-49D receptor ligation can lead to the rapid and potent secretion of IFN-gamma. Cytokine secretion can be induced from Ly-49D+ NK cells after receptor ligation with Ab or after interaction with target cells expressing their H-2Dd ligand. Consistent with the dominant inhibitory function of Ly-49G, NK cells coexpressing Ly-49D and Ly-49G show a profound reduction in IFN-gamma secretion after interaction with targets expressing their common ligand, H-2Dd. Importantly, we are able to demonstrate for the first time that effector/target cell interactions using Ly-49D+ NK cells and H-2Dd targets result in the rapid phosphorylation of Dap-12. However, Dap-12 is not phosphorylated when Ly-49D+ NK cells coexpress the inhibitory receptor, Ly-49G. These studies are novel in describing Ly-49 activation vs inhibition, where two Ly-49 receptors recognize the same class I ligand, with the dominant inhibitory receptor down-regulating phosphorylation of Dap-12, cytokine secretion, and cytotoxicity in NK cells.

Ly-49CB6 NK Inhibitory Receptor Recognizes Peptide-Receptive H-2Kb 1

NK-mediated cytotoxicity involves two families of receptors: activating receptors that trigger lysis of the target cells being recognized and inhibitory receptors specific primarily for MHC I on the target cell surface that can override the activating signal. MHC I molecules on the cell surface can be classified into molecules made stable by the binding of peptide with high affinity or unstable molecules potentially capable of binding high affinity peptide (hence, peptide receptive) and being converted into stable molecules. It has been previously shown that the Ly-49A inhibitory receptor recognizes stable Dd molecules. We show in this study that the inhibitory receptor Ly-49CB6 recognizes peptide-receptive Kb molecules, but does not recognize Kb molecules once they have bound high affinity peptide.

Molecular cloning and characterization of the promoter region of murine natural killer cell receptor 2B4

Natural killer (NK) cells are bone marrow-derived lymphocytes that have the ability to kill certain tumor cells and virally infected cells. The activation of NK cells is mediated by a balance of negative and positive signals from cell-cell interactions and from responses to cytokines. However, the molecular basis of NK cell activation and recognition of target cells is poorly understood. We have previously identified, cloned and characterized a receptor, 2B4, expressed on murine NK cells. 2B4 is not only expressed on all NK cells, but also on a subset of T-cells which have NK-like killing properties. Structural analysis indicated that 2B4 belongs to the CD2 subset of immunoglobulin superfamily. We have also shown 2B4 to interact with CD48 with nine times more affinity than that of CD2-CD48 interaction. In order to understand the transcriptional regulation as well as the mechanisms controlling the restricted expression of the 2B4 gene, we obtained a genomic 2B4 clone including the sequence of the 5'-flanking region. To define the start site of transcription, we performed primer extension and 5'-RACE assays and found that the 2B4 gene may be initiated at multiple start sites and driven by a TATA-less promoter. Transient transfections of nested 5'-fragments of the 2B4 promoter to drive CAT expression revealed tissue specific expression in CTLL-2 cells, a mouse T-cell line. A promoter fragment of 348 bases upstream from the first base of the mouse 2B4 cDNA clone p2B4.8 produced maximal CAT activity in CTLL-2 cells. The presence of the region -653 to -540 on the other hand, drastically reduced transcription. Sequence analysis of this promoter region has identified potential recognition motifs for a number of lymphocyte-restricted in addition to ubiquitous transcription factors, which may play a role in the transcriptional regulation of the mouse 2B4 gene.

NK and CTL Recognition of a Single Chain H-2Dd Molecule: Distinct Sites of H-2Dd Interact with NK and TCR

We generated transgenic mice expressing a single-chain β2-microglobulin (β2m)-H-2Dd. The cell-surface β2m-H-2Dd molecule was expressed on a β2m-deficient background and reacted with appropriate mAbs. It was of the expected m.w. and directed the normal development of CD8+ T cells in the thymus of a broad TCR repertoire. It also presented both exogenously provided and endogenous peptide Ags to effector CD8+ T cells. In tests of NK cell education and function, it failed to reveal any interaction with NK cells, suggesting that the site of the interaction of NK receptors with H-2Dd was disrupted. Thus, the sites of TCR and NK receptor interaction with H-2Dd are distinct, an observation consistent with independent modes of TCR and NK receptor evolution and function.

Levels of Ly-49 Receptor Expression Are Determined by the Frequency of Interactions with MHC Ligands: Evidence Against Receptor Calibration to a “Useful” Level

Ly-49 receptor expression was studied in NK cells that developed in fully MHC-mismatched mixed bone marrow chimeras, in which host and donor MHC ligands were expressed solely on various proportions of hemopoietic cells or on both hemopoietic and nonhemopoietic cells. When hemopoietic cells were the only source of MHC ligand, a strong correlation between the level of down-regulation of Ly-49A, Ly-49C, and Ly-49G2 and the number of hemopoietic cells expressing their MHC ligands was observed on both donor and host NK cells. In some animals with low levels of donor hemopoietic chimerism, NK cells of donor origin expressed Ly-49 receptors at higher levels than was observed in normal mice of the same strain. This unexpected observation is inconsistent with the receptor calibration theory, which states that expression of Ly-49 inhibitory receptors is calibrated to an optimal level to maintain an NK cell repertoire that is sensitive to perturbations in normal class I ligand expression. Our data suggest a model in which Ly-49 receptors down-modulate in accordance with the frequency of their interactions with ligand-bearing cells, rather than a model in which these receptors calibrate to a specific “useful” level in response to ligands present in their environment.

Peptide dependency and selectivity of the NK cell inhibitory receptor Ly-49C

MHC class I molecules can prevent NK cell-mediated cytotoxicity by interacting with inhibitory receptors on the effector cells. Different conclusions have been reached regarding possible peptide selectivity of these receptors. To address whether peptide selectivity is an exclusive feature of human or immunoglobulin-superfamily receptors, we have studied a system based on the murine NK receptor Ly-49C in the lectin-superfamily. Loading of TAP-deficient RMA-S cells with the H-2Kb-restricted, ovalbumin-derived peptide OVA(257 - 264) (pOVA) induced their ability to bind Ly-49C-transfected reporter cells, and also protected them from killing by Ly-49C+ NK cells. Other peptides that bound and stabilized H-2Kb equally well differed in their NK protective capacity. Comparison of the MHC class I peptide complexes (crystal structures and molecular models) revealed a conformational motif encompassing the C-terminal parts of the alpha1 helix (73 - 77) and the bound peptide that was common for the protective complexes. Substitution analysis of pOVA suggested that position 7 in the peptide may be critical for optimal protection as well as for the conformational motif at position 73 - 77. In conclusion, protection mediated by the murine C-type lectin receptor Ly-49C is peptide dependent and selective.

Tolerance and Alloreactivity of the Ly49D Subset of Murine NK Cells

Class I-specific stimulatory and inhibitory receptors expressed by NK cell subsets contribute to the alloreactive potential of the self-tolerant murine NK cell repertoire. In this report, we have studied potential mechanisms of tolerance to the function of the positive signaling Ly49D receptor in mice that express one of its ligands, H2-Dd. Our results demonstrate that H2-Dd-expressing mice possess a large Ly49D+ subset of NK cells that is functionally capable of rejecting bone marrow cell (BMC) allografts in vivo and lysing allogeneic Con A lymphoblasts in vitro. Also, we show that the Ly49D receptor is responsible for the ability of H2b/d F1 hybrid mice to reject H2d/d parental BMC (hybrid resistance). Thus, deletion or anergy of Ly49D+ cells in H2-Dd+ hosts cannot explain self tolerance. Our functional studies revealed that coexpression of the Dd-specific Ly49A or Ly49G2 inhibitory receptors by Ly49D+ cells resulted in tolerance to Dd+ targets, while coexpression of Kb-specific inhibitory receptors Ly49C/I resulted in tolerance to Kb+ targets. Only in H2d/d cells did Ly49C/I dominantly inhibit Ly49D-Dd stimulation. This correlated with an increased mean fluorescence intensity of Ly49C expression, as well as an increased percentage of Ly49C+ cells in the Ly49D+A/G2− compartment. Therefore, we conclude that self tolerance of the Ly49D subset can be achieved through coexpression of a sufficient level of self-specific inhibitory receptors.

Natural killing of MHC class I(-) lymphoblasts by NK cells from long-term bone marrow culture requires effector cell expression of Ly49 receptors

NK cells from long-term bone marrow culture (LTBMC) were compared with IL-2-activated splenic NK cells [short-term spleen cell culture (STSC)] with regard to expression of inhibitory Ly49 receptors and cytotoxic function. In the LTBMC, the total number of NK cells expressing either one of the Ly49 molecules A, C/I and G2 was strongly reduced (10-15% of NK1.1(+) cells) compared to the STSC (80-90% of NK1.1(+) cells). With regard to cytotoxic function, we confirmed that LTBMC-derived NK cells efficiently killed the prototype NK target YAC-1. However, against other targets, killing was more variable. First, while STSC-derived NK cells clearly distinguished MHC class I(-) from MHC class I(+) tumor cell targets, LTBMC-derived NK cells did not; they either killed both targets equally well or not at all. Secondly, LTBMC-derived NK cells were largely incapable of killing lymphoblast targets deficient in MHC class I expression. To test whether this cytotoxic defect was due to the low number of Ly49(+) NK cells in the LTBMC, we separated Ly49(+) and Ly49(-) NK cells by cell sorting and tested them individually. This experiment showed that only Ly49(+) NK cells in the LTBMC were able to kill MHC class I(-) lymphoblasts (and to distinguish them from MHC class I(+)), despite good cytotoxicity against YAC-1 cells in both populations. These data suggest that certain modes of NK cell triggering are dependent on Ly49 receptor expression. From our results, we speculate that inhibitory receptors are expressed before triggering receptors for normal self cells during NK cell development, which may be an important mechanism to preserve self tolerance during the early stages of NK cell maturation.

Diversity of NK Cell Receptor Repertoire in Adult and Neonatal Mice

Murine NK cytotoxicity is regulated by two families of MHC class I-specific receptors, namely Ly49 and CD94/NKG2. We developed a single-cell RT-PCR method to analyze expression of all known Ly49 and NKG2A genes in individual NK cells and determined the receptor repertoires of NK cells from adult and neonatal (1-wk-old) C57BL/6 mice. In adult mouse NK cells, up to six different receptors were coexpressed in random combinations. Of 62 NK cells examined, 42 different patterns of receptor expression were observed. Most of them expressed at least one Ly49, whereas NKG2A was detected in 32% of the cells. Over 75% of them expressed Ly49C, I, or NKG2A, which are thought to recognize self-class I MHC (H-2b). Coexpression of multiple Ly49 receptors and NKG2A was stochastic. In contrast, very few neonatal NK cells expressed any Ly49, but almost 60% of them expressed NKG2A. These results demonstrate that adult NK cells are quite heterogeneous and have diverse receptor repertoires. They also suggest that the expression of NKG2A precedes Ly49 expression in NK cell ontogeny, and NKG2A is a major inhibitory receptor in neonatal NK cells.

Direct assessment of MHC class I binding by seven Ly49 inhibitory NK cell receptors

Mouse NK cells express at least seven inhibitory Ly49 receptors. Here we employ a semiquantitative cell-cell adhesion assay as well as class I/peptide tetramers to provide a comprehensive analysis of specificities of Ly49 receptors for class I MHC molecules in eight MHC haplotypes. Different Ly49 receptors exhibited diverse binding properties. The degree of class I binding was related to the extent of functional inhibition. The tetramer studies demonstrated that neither glycosylation nor coreceptors were necessary for class I binding to Ly49 receptors and uncovered peptide-specific recognition by a Ly49 receptor. The results provide a foundation for interpreting and integrating many existing functional studies as well as for designing tests of NK cell development and self-tolerance.

Recognition of Class I MHC by NK Receptor Ly-49C: Identification of Critical Residues

The Ly-49 family of inhibitory receptors plays a major role in regulating mouse NK cell cytotoxicity. Two of its members, Ly-49C and I, are recognized by the mAb 5E6, which also defines a subset of NK cells involved in the hybrid resistance phenomenon. Previous studies have shown that Ly-49C binds to a broad spectrum of class I MHC molecules, while Ly-49I apparently does not bind to any class I MHC molecules tested. In the present investigation we have defined the amino acid residues of Ly-49C that are critical for determining its ligand specificities. First, using quantitative COS cell adhesion assays, we demonstrated that Ly-49CB6 bound to Dd, Db, Kb, or Kk as well as to murine leukemic cell lines GM979 (H-2s) and IC-21 (H-2b). In contrast, COS cells expressing Ly-49IB6 did not significantly bind to any of the class I MHC tested. To determine which amino acid residues of Ly-49C are critical for their specific binding to class I MHC, a series of chimeric and mutant Ly-49C and I were generated and tested. Exchanging the critical residues between Ly-49C and I significantly affected their binding specificities. Finally, we identified the epitopes on Ly-49C recognized by mAbs 5E6 and 4LO3311 that functionally inhibit Ly-49C recognition of its ligands. These results further define the class I specificities of Ly-49C and provide insight into the structural basis for how class I MHC is recognized by the Ly-49 family of NK receptors.

Cloning of murine NKG2A, B and C: second family of C-type lectin receptors on murine NK cells

Multiple NK cell receptors for MHC class I have been identified. They include killer inhibitory receptors and CD94/NKG2 heterodimers in humans and the Ly49 family in mice. Here we report the cloning of murine NKG2A, B and C. The deduced amino acid sequence of mouse NKG2A contains only one consensus cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). NKG2A from B6 and BALB/c mice differ by six amino acid residues in the extracellular domain. Murine NKG2B, like its human conterpart, appears to be a splice variant of NKG2A and lacks a large portion of the stalk region. Murine NKG2C lacks an ITIM in its cytoplasmic domain, a feature shared by human and rat NKG2C. However, unlike the human counterpart, the transmembrane domain of mouse NKG2C does not contain a charged amino acid residue. Mouse NKG2A mRNA was detected in IL-2-activated NK cells and spleen cells but not in other tissues. The NKG2A gene was localized on the distal portion of chromosome 6 where the NK complex has been located. These results further extend the repertoire of C-type lectin receptors on murine NK cells.

Inhibition of NK cells by murine CMV-encoded class I MHC homologue m144

Murine cytomegalovirus (CMV)-encoded protein m144 is homologous to class I MHC heavy-chain and is thought to regulate NK-cell-mediated immune responses in vivo. To examine the effects of m144 on NK cytotoxicity in vitro, various cell lines were transfected with wild-type m144 or a chimeric construct in which the cytoplasmic domain of m144 was replaced with green fluorescence protein. Burkitt lymphoma line Raji expressed a significant level of m144 as determined by anti-m144 mAb binding or the green fluorescence of the fusion protein. The level of m144 expression was relatively low compared with that of transfected murine class I MHC Dd. However, m144 on Raji cells partially inhibited antibody-dependent cell-mediated cytotoxicity of IL-2-activated NK cells. NK cells from the CMV-susceptible BALB/c as well as those from the resistant C57BL/6 mice were inhibited by m144. Antibodies against the known murine NK inhibitory receptors Ly-49A, C, G, and I did not affect the inhibitory effect of m144. These results suggest that the murine CMV class I MHC homologue m144 partially inhibits NK cells by interacting with a novel inhibitory receptor.

Heterogeneity in the ability of cytotoxic murine NK cell clones to enhance Ig secretion in vitro

We recently described a panel of cytotoxic murine NK cell clones that also enhanced Ig secretion by B cells activated in an in vitro model of T cell-independent type 2 (TI-2) responses. We employed dextran-conjugated anti-IgD (alphadelta-dex) as a model antigen. Here we study the mechanism of Ig induction by these clones. Addition of the various NK clones to sort-purified B cells stimulated with alphadelta-dex and IL-2 resulted in a markedly heterogeneous increase in Ig secretion, which varied from 3-fold, as mediated by clone PKO 56, to 15-fold, as induced by clone PKO 101. The other NK cells showed intermediate levels of Ig induction. Furthermore, while addition of as few as 0.04% of PKO 101 cells stimulated significant increases and 1% induced near maximum Ig production, a 3% addition of PKO 56 cells was required for significant enhancement of Ig secretion. Supernatant material collected from the NK clones mediated Ig production at levels that mirrored the induction by the corresponding cells. Cytokine analysis showed that while all members of the NK panel produced IFN-gamma only two secreted granulocyte macrophage colony stimulating factor and that the levels of Ig induction mediated by the NK clones correlated only with their levels of IFN-gamma secretion. Culture of B and NK cells in the presence of anti-IFN-gamma demonstrated that IFN-gamma was the critical cytokine in NK-induced Ig production. These findings establish heterogeneity in the ability of NK cells to increase Ig secretion in vitro and show that NK-produced IFN-gamma is an important factor in determining this heterogeneity.

Direct and indirect T cell priming by dendritic cell vaccines

The mechanisms by which dendritic cell (DC) vaccines prime host T cells in vivo was analyzed. Mice were immunized with syngeneic bone marrow-derived DC and as surrogate antigen beta-galactosidase (beta-gal) was used. DC either pulsed with peptide, loaded with beta-gal antigen or gene-modified induced beta-gal-specific cytotoxic T lymphocytes (CTL) and moderate rejection of an in vivo challenge with beta-gal expressing tumors. In addition, beta-gal-specific CTL lysed the syngeneic DC that were used as vaccines. Using SCID mice reconstituted with F1 lymphocytes, direct priming by gene-modified DC vaccines was demonstrated by the presence of beta-gal-specific CTL of the haplotype exclusively expressed by DC while indirect priming by host antigen-presenting cells (APC) was shown by the detection of CTL of the haplotype exclusively present on host APC and absent on DC vaccines. Since DC immunization in syngeneic mice was associated with an increase in NK1.1+/Ly49C- cells and detectable lysis of DC in vitro by lymphokine-activated killer cells, DC vaccines appear to interact with host natural killer cells as well as with antigen-specific T cells. These effector cells in turn may lyse DC vaccines thereby leading to the release of antigens that can be taken up by host APC.

Genetic control of natural killing and in vivo tumor elimination by the Chok locus

The molecular mechanisms underlying target recognition during natural killing are not well understood. One approach to dissect the complexities of natural killer (NK) cell recognition is through exploitation of genetic differences among inbred mouse strains. In this study, we determined that interleukin 2-activated BALB/c-derived NK cells could not lyse Chinese hamster ovary (CHO) cells as efficiently as C57BL/6-derived NK cells, despite equivalent capacity to kill other targets. This strain-determined difference was also exhibited by freshly isolated NK cells, and was determined to be independent of host major histocompatibility haplotype. Furthermore, CHO killing did not correlate with expression of NK1.1 or 2B4 activation molecules. Genetic mapping studies revealed linkage between the locus influencing CHO killing, termed Chok, and loci encoded within the NK gene complex (NKC), suggesting that Chok encodes an NK cell receptor specific for CHO cells. In vivo assays recapitulated the in vitro data, and both studies determined that Chok regulates an NK perforin-dependent cytotoxic process. These results may have implications for the role of NK cells in xenograft rejection. Our genetic analysis suggests Chok is a single locus that affects NK cell-mediated cytotoxicity similar to other NKC loci that also regulate the complex activity of NK cells.

Natural killer cells and their role in graft rejection

Natural killer cells can weakly resist engraftment of allogeneic bone marrow transplants in mice. Functional studies suggest that natural killer cell tolerance can be induced by bone marrow transplantation. Human natural killer cell inhibitory receptor repertoires differ between individuals, depending on their MHC genotype. This supports the concept that the human natural killer cell repertoire, like that of mice, adapts to the MHC molecules presented in its environment. Natural killer cells play a greater role in rejecting xenogeneic than allogeneic bone marrow and have been implicated in the rejection of xenogeneic solid organ transplants. Natural killer cell inhibitory receptors may have a lower likelihood of cross-reacting with xenogeneic than with allogeneic MHC class I molecules; important glycosylation differences between species may affect the propensity of natural killer cells to kill xenogeneic targets.

An allele-specific, stochastic gene expression process controls the expression of multiple Ly49 family genes and generates a diverse, MHC-specific NK cell receptor repertoire

Mouse NK cells express MHC class I-specific inhibitory Ly49 receptors. Since these receptors display distinct ligand specificities and are clonally distributed, their expression generates a diverse NK cell receptor repertoire specific for MHC class I molecules. We have previously found that the Dd (or Dk)-specific Ly49A receptor is usually expressed from a single allele. However, a small fraction of short-term NK cell clones expressed both Ly49A alleles, suggesting that the two Ly49A alleles are independently and randomly expressed. Here we show that the genes for two additional Ly49 receptors (Ly49C and Ly49G2) are also expressed in a (predominantly) mono-allelic fashion. Since single NK cells can co-express multiple Ly49 receptors, we also investigated whether mono-allelic expression from within the tightly linked Ly49 gene cluster is coordinate or independent. Our clonal analysis suggests that the expression of alleles of distinct Ly49 genes is not coordinate. Thus Ly49 alleles are apparently independently and randomly chosen for stable expression, a process that directly restricts the number of Ly49 receptors expressed per single NK cell. We propose that the Ly49 receptor repertoire specific for MHC class I is generated by an allele-specific, stochastic gene expression process that acts on the entire Ly49 gene cluster.

The α2 Domain of H-2Dd Restricts the Allelic Specificity of the Murine NK Cell Inhibitory Receptor Ly-49A

Mouse NK lymphocytes express Ly-49 receptors, which inhibit cytotoxicity upon ligation by specific MHC I molecules on targets. Different members of the lectin-like mouse Ly-49 receptor family recognize distinct subsets of murine H-2 molecules, but the molecular basis for the allelic specificity of Ly-49 has not been defined. We analyzed inhibition of natural killing by chimeric MHC I molecules in which the α1, α2, or α3 domains of the Ly-49A-binding allele H-2Dd were exchanged for the corresponding domains of the nonbinding allele H-2Db. Using the Ly-49A-transfected rat NK cell line, RNK-mLy-49A.9, we demonstrated that the H-2Dd α2 domain alone accounts for allelic specificity in protection of rat YB2/0 targets in vitro. We also showed that the H-2Dd α2 domain is sufficient to account for the allele-specific in vivo protection of H-2b mouse RBL-5 tumors from NK cell-mediated rejection in D8 mice. Thus, in striking contrast to the α1 specificity of Ig-like killer inhibitory receptors for human HLA, the lectin-like mouse Ly-49A receptor is predominantly restricted by the H-2Dd α2 domain in vitro and in vivo.

Natural killer cell receptors

In killing of cellular targets, natural killer cells employ receptors that activate them and receptors specific for MHC class I that inhibit their activation. Progress in understanding the inhibitory receptors has been rapid, and indications are that they fall into two distinct structural types that appear to utilize the same inhibitory signaling cascade; meanwhile, components of the activation cascade are being elucidated, permitting us to integrate the pathways involved.