Enhancement of class II-restricted T cell responses by costimulatory NK receptors for class I MHC proteins

HLA and killer cell immunoglobulin-like receptor (KIRs) genotyping in patients with acute ischemic stroke

INTRODUCTION

In humans, a major component of natural killer (NK) and T cell target recognition depends on the surveillance of human leukocyte antigen (HLA) class I molecules by killer immunoglobulin-like receptors (KIRs).

AIMS

To implement the knowledge about the immunological genetic background of acute ischemic stroke susceptibility in relation to the frequency of the KIR genes and HLA alleles.

METHODS

Subjects with acute ischemic stroke and subjects without stroke were genotyped for the presence of KIR genes and of the three major KIR ligand groups, HLA-C1, HLA-C2, and HLA-Bw4, both HLA-B and HLA-A loci.

RESULTS

Between November 2013 and February 2016, consecutive patients with acute ischemic stroke were recruited. As healthy controls, we enrolled subjects without acute ischemic stroke. Subjects with acute ischemic stroke in comparison with controls showed a higher frequency of 2DL3, 2DL5B, 2DS2, and 2DS4 KIR genes and a lower frequency of HLA-B-Bw4I alleles. Subjects without acute ischemic stroke showed a higher frequency of interaction between KIR 2DS2 and HLAC2. We also observed a higher frequency of 2DL3 and 2 DL4 KIR genes in subjects with atherosclerotic (LAAS) subtype. Multiple logistic regression analysis showed a protective effect towards stroke of HLA-B-Bw4I and interaction between KIR 2DL2 and HLAC1 and 2DS2-HLAC2 and a detrimental effect of 2DL2-HLA-C1_A interactions.

CONCLUSION

Our findings of a higher frequency of activating KIR genes seem to be consistent with findings previously reported patients with coronary syndrome. This higher frequency of "proinflammatory" genes in subjects with ischemic stroke could also explain the immunoinflammatory activation of the acute phase of stroke.

Dan Davis: Up close and personal with immune cells

Davis uses microscopy and imaging approaches to study immune cell interactions.

The Role of Natural Killer T Cells in Cancer-A Phenotypical and Functional Approach

Natural killer T (NKT) cells are a subset of CD1d-restricted T cells at the interface between the innate and adaptive immune system. NKT cells can be subdivided into functional subsets that respond rapidly to a wide variety of glycolipids and stress-related proteins using T- or natural killer (NK) cell-like effector mechanisms. Because of their major modulating effects on immune responses via secretion of cytokines, NKT cells are also considered important players in tumor immunosurveillance. During early tumor development, T helper (T_H)1-like NKT cell subsets have the potential to rapidly stimulate tumor-specific T cells and effector NK cells that can eliminate tumor cells. In case of tumor progression, NKT cells may become overstimulated and anergic leading to deletion of a part of the NKT cell population in patients via activation-induced cell death. In addition, the remaining NKT cells become hyporesponsive, or switch to immunosuppressive T_H2-/T regulatory-like NKT cell subsets, thereby facilitating tumor progression and immune escape. In this review, we discuss this important role of NKT cells in tumor development and we conclude that there should be three important focuses of future research in cancer patients in relation with NKT cells: (1) expansion of the NKT cell population, (2) prevention and breaking of NKT cell anergy, and (3) skewing of NKT cells toward T_H1-like subsets with antitumor activity.

Haploidentical hematopoietic transplantation from KIR ligand-mismatched donors with activating KIRs reduces nonrelapse mortality

Because activating killer cell immunoglobulinlike receptors (KIRs) are heterogeneously expressed in the population, we investigated the role of donor activating KIRs in haploidentical hematopoietic transplants for acute leukemia. Transplants were grouped according to presence vs absence of KIR-ligand mismatches in the graft-vs-host direction (ie, of donor-vs-recipient natural killer [NK]-cell alloreactivity). In the absence of donor-vs-recipient NK-cell alloreactivity, donor activating KIRs had no effects on outcomes. In the 69 transplant pairs with donor-vs-recipient NK-cell alloreactivity, transplantation from donors with KIR2DS1 and/or KIR3DS1 was associated with reduced risk of nonrelapse mortality, largely infection related (KIR2DS1 present vs absent: hazard ratio [HR], 0.25; P = .01; KIR3DS1 present vs absent: HR, 0.18; P = .006), and better event-free survival (KIR2DS1 present vs absent: HR, 0.31; P = .011; KIR3DS1 present vs absent: HR, 0.30; P = .008). Transplantation from donors with KIR2DS1 and/or KIR3DS1 was also associated with a 50% reduction in infection rate (P = .003). In vitro analyses showed that KIR2DS1 binding to its HLA-C2 ligand upregulated inflammatory cytokine production by alloreactive NK cells in response to infectious challenges. Because ∼40% of donors able to exert donor-vs-recipient NK-cell alloreactivity carry KIR2DS1 and/or KIR3DS1, searching for them may become a feasible, additional criterion in donor selection.

Multiplex and genome-wide analyses reveal distinctive properties of KIR+ and CD56+ T cells in human blood

Killer cell Ig-like receptors (KIRs) on NK cells have been linked to a wide spectrum of health conditions such as chronic infections, autoimmune diseases, pregnancy complications, cancers, and transplant failures. A small subset of effector memory T cells also expresses KIRs. In this study, we use modern analytic tools including genome-wide and multiplex molecular, phenotypic, and functional assays to characterize the KIR(+) T cells in human blood. We find that KIR(+) T cells primarily reside in the CD56(+) T population that is distinctively DNAM-1(high) with a genome-wide quiescent transcriptome, short telomere, and limited TCR excision circles. During CMV reactivation in bone marrow transplant recipients, KIR(+)CD56(+) T cells rapidly expanded in real-time but not KIR(+)CD56(-) T cells or KIR(+) NK cells. In CMV(+) asymptomatic donors, as much as 50% of CD56(+) T cells are KIR(+), and most are distinguishably KIR2DL2/3(+)NKG2C(+)CD57(+). Functionally, the KIR(+)CD56(+) T cell subset lyses cancer cells and CMVpp65-pulsed target cells in a dual KIR-dependent and TCR-dependent manner. Analysis of metabolic transcriptome confirms the immunological memory status of KIR(+)CD56(+) T cells in contrast to KIR(-)CD56(+) T cells that are more active in energy metabolism and effector differentiation. KIR(-)CD56(+) T cells have >25-fold higher level of expression of RORC than the KIR(+) counterpart and are a previously unknown producer of IL-13 rather than IL-17 in multiplex cytokine arrays. Our data provide fundamental insights into KIR(+) T cells biologically and clinically.

Ly49D-mediated ITAM signaling in immature thymocytes impairs development by bypassing the pre-TCR checkpoint

Activating and inhibitory NK receptors regulate the development and effector functions of NK cells via their ITAM and ITIM motifs, which recruit protein tyrosine kinases and phosphatases, respectively. In the T cell lineage, inhibitory Ly49 receptors are expressed by a subset of activated T cells and by CD1d-restricted NKT cells, but virtually no expression of activating Ly49 receptors is observed. Using mice transgenic for the activating receptor Ly49D and its associated ITAM signaling DAP12 chain, we show in this article that Ly49D-mediated ITAM signaling in immature thymocytes impairs development due to a block in maturation from the double negative (DN) to double positive (DP) stages. A large proportion of Ly49D/DAP12 transgenic thymocytes were able to bypass the pre-TCR checkpoint at the DN3 stage, leading to the appearance of unusual populations of DN4 and DP cells that lacked expression of intracellular (ic) TCRβ-chain. High levels of CD5 were expressed on ic TCRβ(-) DN and DP thymocytes from Ly49D/DAP12 transgenic mice, further suggesting that Ly49D-mediated ITAM signaling mimics physiological ITAM signaling via the pre-TCR. We also observed unusual ic TCRβ(-) single positive thymocytes with an immature CD24(high) phenotype that were not found in the periphery. Importantly, thymocyte development was completely rescued by expression of an Ly49A transgene in Ly49D/DAP12 transgenic mice, indicating that Ly49A-mediated ITIM signaling can fully counteract ITAM signaling via Ly49D/DAP12. Collectively, our data indicate that inappropriate ITAM signaling by activating NK receptors on immature thymocytes can subvert T cell development by bypassing the pre-TCR checkpoint.

The tortoise and the hare: slowly evolving T-cell responses take hastily evolving KIR

The killer cell immunoglobulin-like receptor (KIR) locus comprises a variable and rapidly evolving set of genes encoding multiple inhibitory and activating receptors. The activating receptors recently evolved from the inhibitory receptors and both bind HLA class I and probably also class I-like structures induced by viral infection. Although generally considered natural killer (NK) cell receptors, KIR are also expressed by a large fraction of effector memory T cells, which slowly accumulate during human life. These effector memory cells are functionally similar to NK cells, as they are immediate effector cells that are cytotoxic and produce IFN-γ. However, different rules apply to NK and T cells with respect to KIR expression and function. For example, KIR tend to modulate signals driven by the T-cell receptor (TCR) rather than to act independently, and use different signal transduction pathways to modulate only a subset of effector functions. The most important difference may lie in the rules governing tolerance: while NK cells with activating KIR binding self-HLA are hyporesponsive, the same is unlikely to apply to T cells. We argue that the expression of activating KIR on virus-specific T cells carrying TCR that weakly cross-react with autoantigens can unleash the autoreactive potential of these cells. This may be the case in rheumatoid arthritis, where cytomegalovirus-specific KIR2DS2(+) T cells might cause vasculitis. Thus, the rapid evolution of activating KIR may have allowed for efficient NK-cell control of viruses, but may also have increased the risk that slowly evolving T-cell responses to persistent pathogens derail into autoimmunity.

Lack of killer immunoglobulin-like receptor 2DS2 (KIR2DS2) and KIR2DL2 is associated with poor responses to therapy of recurrent hepatitis C virus in liver transplant recipients

Killer immunoglobulin-like receptors (KIRs) expressed on natural killer and natural killer T cells are involved in activation of these cells and can influence antiviral immunity in the liver. This study investigated the association between KIR genetic diversity and sustained virologic response (SVR) to Peginterferon and Ribavirin (Peg/RBV) therapy in liver transplant (LT) recipients with hepatitis C virus (HCV) recurrence. We tested KIR genotypes in 44 HCV-infected LT recipients treated with Peg/RBV for 48 weeks. Patients were categorized as having KIR genotypes A/A or B/x and analyzed for association with SVR. Fifteen of 44 (34%) patients had SVR. Only 2 of 18 (11%) who lacked KIR2DS2/KIR2DL2 achieved SVR compared to 13 of 26 (50%) who carried these two genes (odds ratio: 8.0, 95% confidence interval: 1.5-42.0, P = 0.008). The association between lack of KIR2DS2/KIR2DL2 and SVR remained significant after exclusion of 10 patients with non-genotype 1 HCV. No correlation was found with other activating or inhibitory KIR genes. Absence of KIR2DS2 and/or KIR2DL2 is associated with failure of Peg/RBV therapy in patients with recurrent HCV after LT. These findings support the role of natural killer and natural killer T cells in HCV clearance after LT and might be generalizable to treatment of HCV infection outside the setting of LT.

Ly49D engagement on T lymphocytes induces TCR-independent activation and CD8 effector functions that control tumor growth

Recent data showing expression of activating NK receptors (NKR) by conventional T lymphocytes raise the question of their role in the triggering of TCR-independent responses that could be damaging for the host. Transgenic mice expressing the activating receptor Ly49D/DAP12 offer the opportunity to better understand the relevance of ITAM signaling in the biology of T cells. In vitro experiments showed that Ly49D engagement on T lymphocytes by a cognate MHC class I ligand expressed by Chinese hamster ovary (CHO) cells or by specific Ab triggered cellular activation of both CD4 and CD8 populations with modulation of activation markers and cytokine production. The forced expression of the ITAM signaling chain DAP12 is mandatory for Ly49D-transgenic T cell activation. In addition, Ly49D stimulation induced T lymphocyte proliferation, which was much stronger for CD8 T cells. Phenotypic analysis of anti-Ly49D-stimulated CD8 T cells and their ability to produce high levels of IFN-gamma and to kill target cells indicate that Ly49D ligation generates effector cytotoxic CD8 T cells. Ly49D engagement by itself also triggered cytotoxic activity of activated CD8 T cells. Adoptive transfer experiments confirmed that Ly49D-transgenic CD8 T cells are able to control growth of CHO tumor cells or RMA cells transfected with Hm1-C4, the Ly49D ligand normally expressed by CHO. In conclusion, Ly49D engagement on T cells leads to T cell activation and to a full range of TCR-independent effector functions of CD8 T cells.

Human cytomegalovirus infection induces a rapid and sustained change in the expression of NK cell receptors on CD8+ T cells

The CD8(+) T cell compartment of human CMV-seropositive individuals characteristically contains a high proportion of cells that express NK cell receptors (NKRs) which may contribute to the surveillance of virus-infected cells. To test whether this enhanced expression is a direct and immediate result of CMV infection, we used DNA microarrays to analyze putative changes in the RNA expression level of 39 NKRs in CMV-specific CD8(+) T cells of renal transplant recipients experiencing primary CMV infection. Already in the acute phase of infection 29 NKRs were induced, of which 19 remained high 1 year after cessation of viral replication. Activating and inhibitory NKRs were induced to a similar extent. Detailed longitudinal flow cytometric analyses confirmed NKR changes at the protein level. Strikingly, a strong induction of CD94 on CD3(+) T cells was observed with surface expression of activating CD94(dim) NKG2C dimers appearing before inhibitory CD94(bright) NKG2A ones. After the acute phase of infection, the balance between inhibitory and activating receptors did not change. Thus, CMV infection induces a rapid and lasting change in the expression of NKRs on human CD8(+) T cells.

Killer Ig-like receptor (KIR) genotype and HLA ligand combinations in ulcerative colitis susceptibility

Killer immunoglobulin-like receptors (KIRs) are expressed on natural killer cells and some T-cell subsets and produce either activation or inhibitory signals upon binding with the appropriate human leucocyte antigen (HLA) ligand on target cells. Recent genetic association studies have implicated KIR genotype in the development of several inflammatory conditions. Ulcerative colitis (UC) is an inflammatory disorder of the colonic mucosa that results from an inappropriate activation of the immune system driven by host bacterial flora. We developed a polymerase chain reaction-sequence specific primer (SSP)-based assay to genotype 194 UC patients and 216 control individuals for 14 KIR genes, the HLA-Cw ligand epitopes of the KIR2D receptors and a polymorphism of the lectin-like-activating receptor NKG2D. Initial analysis found the phenotype frequency of KIR2DL2 and -2DS2 to be significantly increased in the UC cohort (P=0.030 and 0.038, respectively). Logistic regression analysis revealed a protective effect conferred by KIR2DL3 in the presence of its ligand HLA-Cw group 1 (P=0.019). These results suggest that KIR genotype and HLA ligand interaction may contribute to the genetic susceptibility of UC.

Nature of allelic sequence polymorphism at the KIR3DL3 locus

KIR3DL3 is a framework gene of the Leukocyte Receptor Complex, present in all individuals and haplotypes analysed to date. We describe 17 novel KIR3DL3 alleles, including seven single nucleotide polymorphic (SNP) positions within the coding region. Sequence variation within introns included a VNTR within intron 1. As KIR3DL3 mRNA is known to be expressed in decidual NK cells, we investigated the impact of KIR3DL3 allelic variation on pre-eclampsia. No statistical difference in allele frequency or polymorphism was observed between pre-eclampsia patient and control cohorts. Linkage disequilibrium (LD) analysis of exonic SNPs suggested that recombination may be a mechanism of generating sequence diversity within KIR3DL3. A potential recombination hotspot was located within intron 5. A strong LD was detected between polymorphism in exon 6 of KIR3DL3 and the KIR gene -2DL3 or -2DS2 loci, which define the centromeric end of two main haplotypes (A and B) of the KIR cluster. Comparison of primate KIR sequences indicated that the Ig domains of KIR3DL3 are highly conserved between chimpanzee, gorilla and humans. Investigation of KIR3DL3 dN/dS ratios indicated a greater level of synonymous mutations consistent with purifying selection, although positive selection was detected acting on two sites within the stem region.

The production, purification and crystallization of a soluble form of the nonclassical MHC HLA-G: the essential role of cobalt

HLA-G is a nonclassical class I major histocompatibility complex (MHC) molecule that is primarily expressed at the foetal-maternal interface. Although the role of HLA-G has not been fully elucidated, current evidence suggests it protects the foetus from the maternal immune response. In this report, HLA-G (44 kDa) is characterized by expression in Escherichia coli. The inclusion bodies were refolded in complex with a peptide derived from histone H2A (RIIPRHLQL), purified and subsequently crystallized. Correct refolding was determined using two conformation-dependent antibodies. Cobalt ions were shown to be an essential ingredient for obtaining diffraction-quality crystals. The crystals, which diffracted to 1.9 A resolution, belonged to space group P3(2)2(1), with unit-cell parameters a = b = 77.15, c = 151.72 A.

NK cell recognition

The integrated processing of signals transduced by activating and inhibitory cell surface receptors regulates NK cell effector functions. Here, I review the structure, function, and ligand specificity of the receptors responsible for NK cell recognition.

Patients with paroxysmal nocturnal hemoglobinuria have a high frequency of peripheral-blood T cells expressing activating isoforms of inhibiting superfamily receptors

Patients with paroxysmal nocturnal hemoglobinuria (PNH) have a large clonal population of blood cells deriving from hematopoietic stem cells (HSCs) deficient in glycosylphosphatidylinositol (GPI)-anchored surface molecules. A current model postulates that PNH arises through negative selection against normal HSCs exerted by autoreactive T cells, whereas PNH HSCs escape damage. We have investigated the inhibitory receptor superfamily (IRS) system in 13 patients with PNH. We found a slight increase in the proportion of T cells expressing IRS. In contrast to what applies to healthy donors, the engagement of IRS molecules on T cells from patients with PNH elicited a powerful cytolytic activity in a redirected killing assay, indicating that these IRSs belong to the activating type. This was confirmed by clonal analysis: 50% of IRS+ T-cell clones in patients with PNH were of the activating type, while only 5% were of the activating type in healthy donors. Moreover, the ligation of IRS induces (1) production of tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) and (2) brisk cytolytic activity against cells bearing appropriate IRS counter-ligands. In addition, these IRS+ T cells show natural killer (NK)-like cytolytic activity to which GPI- cells were less sensitive than GPI+ cells. Thus, T cells with NK-like features, expressing the activating isoforms of IRS, may include effector cells involved in the pathogenesis of PNH.

Distinct Transcriptional Control Mechanisms of Killer Immunoglobulin-like Receptors in Natural Killer (NK) and in T Cells

Killer immunoglobulin-like receptors (KIR) are expressed by natural killer (NK) cells and by subsets of CD4+ and CD8+ T cells, which are therefore thought to be subject to similar regulatory mechanisms. Here, we show that the transcriptional machinery to express KIR is limited to NK and T cells; however, the KIR transcriptional control differs between these two types of lymphocytes. T cells selectively express transcriptional activators binding to positions -52 to -61 of the KIR promoter, whereas an AML site around position-98 is relevant for transcription in NK cells. Although KIR expression is restricted to subsets of memory T cells, our studies demonstrate that transcriptional activators for KIRs are not acquired during T cell differentiation but are already present in naïve T cells, suggesting a basic role of KIRs in T cell biology. We suggest that the regulated expression of KIRs in T cells profoundly influences peripheral tolerance and antigen-specific immune responses.

Crystal structure of HLA-G: a nonclassical MHC class I molecule expressed at the fetal-maternal interface

HLA-G is a nonclassical major histocompatibility complex class I (MHC-I) molecule that is primarily expressed at the fetal-maternal interface, where it is thought to play a role in protecting the fetus from the maternal immune response. HLA-G binds a limited repertoire of peptides and interacts with the inhibitory leukocyte Ig-like receptors LIR-1 and LIR-2 and possibly with certain natural killer cell receptors. To gain further insights into HLA-G function, we determined the 1.9-A structure of a monomeric HLA-G complexed to a natural endogenous peptide ligand from histone H2A (RIIPRHLQL). An extensive network of contacts between the peptide and the antigen-binding cleft reveal a constrained mode of binding reminiscent of the nonclassical HLA-E molecule, thereby providing a structural basis for the limited peptide repertoire of HLA-G. The alpha3 domain of HLA-G, a candidate binding site for the LIR-1 and -2 inhibitory receptors, is structurally distinct from the alpha3 domains of classical MHC-I molecules, providing a rationale for the observed affinity differences for these ligands. The structural data suggest a head-to-tail mode of dimerization, mediated by an intermolecular disulfide bond, that is consistent with the observation of HLA-G dimers on the cell surface.

Expression of inhibitory KIR is confined to CD8+ effector T cells and limits their proliferative capacity

A subset of effector/memory CD8(+) T cells expresses natural killer cell receptors (NKR). Expression of inhibitory NKR at that stage of T cell differentiation is poorly understood. Interestingly, recent studies in mice indicated that transgenic expression of an inhibitory NKR induced the accumulation of memory T cells by inhibiting activation-induced cell death (AICD). To further understand the role of inhibitory NKR on T cells, we characterized the subset of human peripheral T cells expressing the inhibitory NKR, CD158b, and studied the modulation of antigen-driven T cell expansion by an endogenous inhibitory NKR. We found that CD158b expression was confined to a population of CD8(+)TCRalphabeta(+) effector T cells as defined by a CD45RA(+)CCR7(-) phenotype and high constitutive expression of granzyme B1. Few cells expressed the activating form CD158j in the absence of CD158b. Functionally, engagement of CD158b by MHC ligands diminished early TCR signaling, as well as AICD. However, the reduced AICD did not rescue cells for proliferation, since T cell expansion in the presence of CD158b triggering was impaired. Expression of inhibitory NKR on effector CD8(+) T cells may explain in part the poor replicative capacity of T cells at that stage of differentiation.

Phenotypic and Functional Characterization of CD4 T Cells Expressing Killer Ig-Like Receptors

Killer Ig-like receptors (KIR) are commonly found on human NK cells, gammadelta T cells, and CD8 T cells. Although KIR(+) CD4 T cells are found in certain patients, their prevalence in healthy donors is controversial. We now provide definitive proof that such cells are present in most individuals, and report on their frequency, surface phenotype, cytokine profile, and Ag specificity. The number of KIR(+) CD4 T cells detected in peripheral blood increased with age. In contrast with regular KIR(-) CD4 T cells, the majority of KIR(+) CD4 T cells lacked surface expression of CD27, CD28, CCR4, and CCR7, but did express CD57 and 2B4. In addition, KIR were detected on approximately one-tenth of CD28(-) and CD57(+) memory CD4 T cells. In line with the absence of the Th2 marker CCR4, the KIR(+) CD4 cells produced mainly IFN-gamma and little IL-4, IL-10, or IL-17 upon TCR triggering. Furthermore, the KIR(+) population contained cells that responded to recall Ags in an HLA class II-restricted fashion. Together, our data indicate that KIR-expressing CD4 T cells are predominantly HLA class II-restricted effector memory Th1 cells, and that a significant, previously unrecognized fraction of effector memory Th1 cells expresses KIR.

KIR in type 1 diabetes: disparate distribution of activating and inhibitory natural killer cell receptors in patients versus HLA-matched control subjects

Killer cell immunoglobulin-like receptors (KIRs) modulate natural killer cell and T-cell function by interacting with HLA class 1 ligands on target cells. Both KIR and HLA are highly polymorphic. We studied the influence of KIR and HLA class 1 genes on the susceptibility to develop type 1 diabetes. The results showed increased numbers of activating KIR genes in patients compared with control subjects (P = 0.049). The combination of the activating KIR2DS2 gene, together with its putative HLA ligand, was present more frequently in patients than in diabetes high-risk HLA-matched control subjects (P = 0.030). Moreover, our results imply that an increase in activating KIR2DS2-HLA ligand pairs combined with a lack of inhibitory KIR-HLA ligand pairs is associated with an additional risk to develop type 1 diabetes in individuals with diabetes high-risk HLA alleles (P = 0.035). We propose that the genetic imbalance between KIR and their HLA class 1 ligands may enhance the activation of T-cells with a low affinity for pancreatic self-antigens, thereby contributing to the pathogenesis of type 1 diabetes.

Selective activation of the c-Jun NH2-terminal protein kinase signaling pathway by stimulatory KIR in the absence of KARAP/DAP12 in CD4+ T cells

Activation of CD4(+) T cells is governed by interplay between stimulatory and inhibitory receptors; predominance of stimulatory signals favors autoimmune reactions. In patients with rheumatoid arthritis, expression of the critical costimulatory molecule, CD28, is frequently lost. Instead, CD4(+)CD28(null) T cells express killer immunoglobulin-like receptors (KIRs) with a preferential expression of the stimulatory receptor, CD158j. The frequency of CD4(+)CD28(null) T cells in rheumatoid arthritis (RA) correlates with the risk for more severe disease. Moreover, the KIR2DS2 gene, which encodes for CD158j, is a genetic risk factor for rheumatoid vasculitis. CD158j signals through the adaptor molecule, KARAP/DAP12, to positively regulate cytotoxic activity in NK cells. However, the majority of CD4(+)CD28(null) T cell clones lacked the expression of KARAP/DAP12. Despite the absence of KARAP/DAP12, CD158j was functional and augmented interferon-gamma production after T cell receptor stimulation. Cross-linking of CD158j resulted in selective phosphorylation of c-Jun NH(2)-terminal protein kinase (JNK) and its upstream kinase, MKK4 that led to the expression of ATF-2 and c-Jun, all in the absence of extracellular signal-regulated kinase (ERK)1/2 phosphorylation. Mutation of the lysine residue within the transmembrane domain of CD158j abolished JNK activation, suggesting that an alternate adaptor molecule was being used. CD4(+)CD28(null) T cells expressed DAP10 and inhibition of phosphatidylinositol 3-kinase, which acts downstream of DAP10, inhibited JNK activation; however, no interaction of DAP10 with CD158j could be detected. Our data suggest that CD158j in T cells functions as a costimulatory molecule through the JNK pathway independent of KARAP/DAP12 and DAP10. Costimulation by CD158j may contribute to the autoreactivity of CD4(+)CD28(null) T cells in RA.

Prolongation of allogeneic skin graft survival by injection of anti-Ly49A monoclonal antibody YE1/48

Ly49A receptors expressed on NK, NKT, and T cells play inhibitory roles in regulating the immune responses in vivo and in vitro. Whether or not injection of anti-Ly49A monoclonal antibody (mAb) YE1/48 can block allograft rejection has not been evaluated. Balb/c mouse recipients received intraperitoneal injections of YE1/48 mAb (0.5 mg) or control mAb or phosphate-buffered saline on days -1 and 10. On day 0, fully MHC-mismatched allogeneic C57BL/6 (B6) skin grafts were implanted. The skin graft survival and anti-donor humoral responses were observed. Whereas allogeneic B6 skin grafts survived 14 days in isotopy control antibody-treated or nontreated Balb/c mice, injection of YE1/48 mAb significantly prolonged B6 skin graft survival to 19 days (P < 0.0005). Injection of YE1/48 mAb into presensitized Balb/c recipients did not significantly delay B6 skin graft rejection. On the other hand, after depleting recipient NK, NKT, and some cytotoxic T cells by injection of anti-asialo GM1, YE1/48 failed to prolong B6 skin graft survival in Balb/c recipients. The present studies indicate that injection of YE1/48 mAb significantly delays allogeneic skin graft rejection in nonsensitized recipients but not in sensitized recipients. The presence of NK, NKT cells, and some cytotoxic T cells may be essential for YE1/48-mediated immunosuppression in vivo.

Prevalence, clinical relevance and characterization of circulating cytotoxic CD4+CD28- T cells in ankylosing spondylitis

Circulating CD3+CD4+CD28- cells exhibit reduced apoptosis and were found to be more enriched in patients with ankylosing spondylitis than in age-matched healthy control individuals (7.40 +/- 6.6% versus 1.03 +/- 1.0%; P < 0.001). Levels of CD4+CD28- T cells correlate with disease status as measured using a modified metrology score, but they are independent of age and duration of ankylosing spondylitis. CD4+CD28- T cells produce IFN-gamma and perforin, and thus they must be considered proinflammatory and cytotoxic. These T cells share phenotypic and functional properties of natural killer cells, strongly expressing CD57 but lacking the lymphocyte marker CD7. MHC class I recognizing and activating natural killer cell receptors on the surface of CD4+CD28- T cells may be involved in a HLA-B27 mediated co-stimulation of these proinflammatory and cytotoxic cells.

Soluble HLA class I induces NK cell apoptosis upon the engagement of killer-activating HLA class I receptors through FasL-Fas interaction

The engagement of the activating isoforms of C-type lectin inhibitory receptor (CLIR) or killer Ig-like receptor (KIR) by their natural ligands, represented by soluble HLA-I (sHLA-I) molecules, induced programmed cell death of natural killer (NK) cells. Indeed, NK cell apoptosis elicited by either putative HLA-E and HLA-F (sHLA-I non-A, -B, -C, and -G) or sHLA-I-Cw4 or -Cw3 from untransfected or -Cw4 or -Cw3 alleles transfected HLA-A(-), B(-), C(-), G(-), E(+), F(+) 721.221 lymphoblastoid cell line, respectively, was blocked by covering the corresponding activating receptor with either anti-CLIR- or anti-KIR-specific monoclonal antibodies (mAbs). After sHLA-I-activating receptor interaction, NK cells produced and released Fas ligand (FasL), which in turn led to NK cell apoptosis by interacting with Fas at the NK cell surface. Blocking anti-Fas mAb, or anti-FasL mAb, inhibited sHLA-I-mediated apoptosis via activating receptor in NK cell clones. This apoptosis was inhibited by NK cell treatment with cyclosporin A, whereas this drug had no effect on activating receptor-mediated activation of cytolysis. Conversely, concanamycin A, an inhibitor of vacuolar type H(+)-adenosine triphosphatase (H(+)-ATPase) of granules, inhibited activating receptor-induced NK cell cytolysis, suggesting that activating receptor-mediated apoptosis and cytolysis can use different intracellular pathways. Furthermore, a large amount of interferon-gamma (IFN-gamma) was detectable in culture supernatant of activating receptor(+) NK cells incubated with the appropriate sHLA-I ligand. Again, cyclosporin A, but not concanamycin A, strongly reduced activating receptor-mediated IFN-gamma production. This suggests that activating receptor-induced apoptosis of NK cells could play a role in eliminating potentially harmful NK cell clones and, at the same time, it leads to production of IFN-gamma, an antiviral cytokine able to amplify immune responses.

The killer cell immunoglobulin-like receptor (KIR) genomic region: gene-order, haplotypes and allelic polymorphism

Recent genetic studies have established that the killer cell immunoglobulin-like receptor (KIR) genomic region displays extensive diversity through variation in gene content and allelic polymorphism within individual KIR genes. It is demonstrated by family segregation analysis, genomic sequencing, and gene order determination that genomic diversity by gene content alone gives rise to more than 20 different KIR haplotypes and at least 40-50 KIR genotypes. In the most reductionist format, KIR haplotypes can be accommodated within one of 10 different prototypes, each with multiple permutations. Our haplotype model considers the KIR haplotype as two separate halves: the centromeric half bordered upstream by KIR3DL3 and downstream by 2DL4, and the telomeric half bordered upstream by 2DL4 and downstream by 3DL2. There are rare KIR haplotypes that do not fit into this model. Recombination, gene duplication, and inversion can however, readily explain these haplotypes. Additional allelic polymorphism imposes extensive individual variability. Accordingly, this segment of the human genome displays a level of diversity similar to the one observed for the human major histocompatibility complex. Recent application of immunogenetic analysis of KIR genes in patient populations implicates these genes as important genetic disease susceptibility factors.

Pivotal role of CEACAM1 protein in the inhibition of activated decidual lymphocyte functions

Lymphocytes in direct contact with embryonic extravillous trophoblasts constitute more than 40% of decidual cells and appear to play major roles in implantation and early gestation. A unique subset of NK cells, making up 70-80% of decidual lymphocytes, express high levels of CD56 but lack CD16. We have recently demonstrated a novel class I MHC-independent inhibitory mechanism of NK cell cytotoxicity that is mediated by CEACAM1 homotypic interactions. This mechanism is used by some melanoma cells to avoid attack, mainly by CD16(-) NK cells. We now demonstrate that CEACAM1 is expressed on primary extravillous trophoblasts and is upregulated on the vast majority of IL-2-activated decidual lymphocytes, including NK, T, and NKT cells. Importantly, we present evidence that CEACAM1 interactions inhibit the lysis, proliferation, and cytokine secretion of activated decidual NK, T, and NKT cells, respectively. In vivo analysis of decidual lymphocytes isolated from cytomegalovirus-infected (CMV-infected) pregnant women revealed a dramatic increase in the expression of CEACAM1. Finally, we suggest that a novel ligand for this adhesion molecule is present on the surface of CMV-infected fibroblasts. These combined results demonstrate a major role for the CEACAM1 protein in controlling local decidual immune responses.

The lymphoproliferative defect in CTLA-4-deficient mice is ameliorated by an inhibitory NK cell receptor

T-cell responses are regulated by activating and inhibiting signals. CD28 and its homologue, cytotoxic T-lymphocyte antigen 4 (CTLA-4), are the primary regulatory molecules that enhance or inhibit T-cell activation, respectively. Recently it has been shown that inhibitory natural killer (NK) cell receptors (NKRs) are expressed on subsets of T cells. It has been proposed that these receptors may also play an important role in regulating T-cell responses. However, the extent to which the NKRs modulate peripheral T-cell homeostasis and activation in vivo remains unclear. In this report we show that NK cell inhibitory receptor Ly49A engagement on T cells dramatically limits T-cell activation and the resultant lymphoproliferative disorder that occurs in CTLA-4-deficient mice. Prevention of activation and expansion of the potentially autoreactive CTLA-4(-/-) T cells by the Ly49A-mediated inhibitory signal demonstrates that NKR expression can play an important regulatory role in T-cell homeostasis in vivo. These results demonstrate the importance of inhibitory signals in T-cell homeostasis and suggest the common biochemical basis of inhibitory signaling pathways in T lymphocytes.

KIR: diverse, rapidly evolving receptors of innate and adaptive immunity

KIR genes have evolved in primates to generate a diverse family of receptors with unique structures that enable them to recognize MHC-class I molecules with locus and allele-specificity. Their combinatorial expression creates a repertoire of NK cells that surveys the expression of almost every MHC molecule independently, thus antagonizing the spread of pathogens and tumors that subvert innate and adaptive defense by selectively downregulating certain MHC class I molecules. The genes encoding KIR that recognize classical MHC molecules have diversified rapidly in human and primates; this contrasts with conservation of immunoglobulin- and lectin-like receptors for nonclassical MHC molecules. As a result of the variable KIR-gene content in the genome and the polymorphism of the HLA system, dissimilar numbers and qualities of KIR:HLA pairs function in different humans. This diversity likely contributes variability to the function of NK cells and T-lymphocytes by modulating innate and adaptive immune responses to specific challenges.

The natural killer cell -- friend or foe in autoimmune disease?

Autoimmune diseases are chronic conditions resulting from a loss of immunological tolerance to self-antigens. Recent observations have supported an ever-broader role for innate immune responses in directing and regulating adaptive immunity, including responses to self. This review summarizes recent findings supporting important functions of natural killer (NK) cells in regulating autoimmunity. A close survey of the current literature reveals multiple steps where NK cells can regulate inflammation and intervene in loss of self-tolerance. Importantly, the findings also caution against inferring a similar role for NK cells in all autoimmune phenomena or during separate stages of the same disease. Indeed, NK cells may have different influences during the priming and the effector phases of disease. Hence, an increased understanding of the involvement of NK cells in inflammation and infection should provide new insights into the pathogenesis of autoimmune disease.

NKG2D receptors induced by IL-15 costimulate CD28-negative effector CTL in the tissue microenvironment

Unlike primary T cells in lymph nodes, effector CD8(+) CTL in tissues do not express the costimulatory receptor CD28. We report that NKG2D, the receptor for stress-induced MICA and MICB molecules expressed in the intestine, serves as a potent costimulatory receptor for CTL freshly isolated from the human intestinal epithelium. Expression and function of NKG2D are selectively up-regulated by the cytokine IL-15, which is released by the inflamed intestinal epithelium. These findings identify a novel CTL costimulatory pathway regulated by IL-15 and suggest that tissues can fine-tune the activation of effector T cells based on the presence or absence of stress and inflammation. Uncontrolled secretion of IL-15 could lead to excessive induction of NKG2D and thus contribute to the development of autoimmune disease by facilitating the activation of autoreactive T cells.

Leukocyte receptor complex-encoded immunomodulatory receptors show differing specificity for alternative HLA-B27 structures

We studied recognition of the disease-associated HLA-B27 allele by immunomodulatory receptors encoded within the leukocyte receptor complex. HLA class I are ligands for members of the killer Ig receptor (KIR) and Ig-like transcript (ILT)/LIR/LILR families (the new LILR nomenclature is described at www. gene.ucl.ac.uk/nomenclature/genefamily/lilr.html). Members of these families bound HLA-B27 in both classical and beta(2) microglobulin-independent forms. Classical complexes bound ILT2, ILT4, and LIR6 transfectants but not ILT1, ILT3, or ILT5. A free H chain form of HLA-B27 bound ILT4 and LIR6. Both forms of HLA-B27 bound KIR3DL1 transfectants. HLA-B27 free H chain bound CD14(+) cells in PBL from healthy controls, consistent with ILT4 expression on monocytes. Alternative recognition of different forms of HLA-B27 by KIR or ILT could influence their immunomodulatory function and may imply a role in inflammatory disease.

Regulation of the natural killer cell receptor repertoire

Natural killer cells express inhibitory receptors specific for MHC class I proteins and stimulatory receptors with diverse specificities. The MHC-specific receptors discriminate among different MHC class I alleles and are expressed in a variegated, overlapping fashion, such that each NK cell expresses several inhibitory and stimulatory receptors. Evidence suggests that individual developing NK cells initiate expression of inhibitory receptor genes in a sequential, cumulative, and stochastic fashion. Superimposed on the receptor acquisition process are multiple education mechanisms, which act to coordinate the stimulatory and inhibitory specificities of developing NK cells. One process influences the complement of receptors expressed by individual NK cells. Other mechanisms may prevent NK cell autoaggression even when the developing NK cell fails to express self-MHC-specific inhibitory receptors. Together, these mechanisms ensure a self-tolerant and maximally discriminating NK cell population. Like NK cells, a fraction of memory phenotype CD8(+) T cells, as well as other T cell subsets, express inhibitory class I--specific receptors in a variegated, overlapping fashion. The characteristics of these cells suggest that inhibitory receptor expression may be a response to prior antigenic stimulation as well as to poorly defined additional signals. A unifying hypothesis is that both NK cells and certain T cell subsets initiate expression of inhibitory receptors in response to stimulation.

Expression and function of NK cell receptors in CD8+ T cells

A wide variety of inhibitory and stimulatory NK cell receptors are expressed by some CD8+ cytotoxic T lymphocytes in mice and humans. Recent data address the induction of these receptors on activated or memory CD8+ T cells and have led to hypotheses addressing their function in the CD8+ T cell response.

Recognition of HLA-Cw4 but not HLA-Cw6 by the NK cell receptor killer cell Ig-like receptor two-domain short tail number 4

NK cells are cytotoxic to virus-infected and tumor cells that have lost surface expression of class I MHC proteins. Target cell expression of class I MHC proteins inhibits NK cytotoxicity through binding to inhibitory NK receptors. In contrast, a similar family of activating NK receptors, characterized by the presence of a charged residue in their transmembrane portion and a truncated cytoplasmic tail, augment lysis by NK cells when ligated by an appropriate class I MHC protein. However, the class I MHC specificity of many of these activating NK receptors is still unknown. Here, we show enhanced lysis of HLA-Cw4 but not HLA-Cw6-expressing cells, by a subset of NK clones. This subset may express killer cell Ig-like receptor two-domain short tail number 4 (KIR2DS4), as suggested by staining with various mAb. It is still possible, however, that these clones may express receptors other than KIR2DS4 that might recognize HLA-Cw4. Binding of KIR2DS4-Ig fusion protein to cells expressing HLA-Cw4 but not to those expressing HLA-Cw6 was also observed. The binding of KIR2DS4-Ig to HLA-Cw4 is weaker than that of killer cell Ig-like receptor two-domain long tail number 1 (KIR2DL1)-Ig fusion protein; however, such weak recognition is capable of inhibiting lysis by an NK transfectant expressing a chimeric molecule of KIR2DS4 fused to the transmembrane and cytoplasmic portion of KIR2DL1. Residue alpha14 is shown to be important in the KIR2DS4 binding to HLA-Cw4. Implications of the role of the activating NK receptors in immunosurveillance are discussed.

Human natural killer cell receptors and signal transduction

Natural killer (NK) cells express numerous receptors, which continually engage with ligands on cell surfaces. Until 1995, only a handful of these receptors were characterized and the molecular basis of NK cell activation was obscure. Recently, considerable advances have been made in characterizing the receptor repertoire on human NK cells. Both activating and inhibitory receptors can transduce positive or negative signals to regulate NK cell cytotoxicity and cytokine release responses. The inhibitory receptors normally predominate in this balance of signals. Certain tumor cells and virally infected cells that lack major histocompatibility complex (MHC) class I molecules, however, can rapidly trigger NK cell activation. The basis of this activation is the loss of negative signals that are normally transmitted by MHC class I-binding inhibitory receptors, and the corresponding domination of activating receptor signals. While ligand specificity for a number of the recently described receptors is still a mystery, their signal transduction properties have begun to be defined. The dynamic crosstalk between these receptors ultimately governs the NK cell activation state. Although the complexities of NK cell signalling are only marginally understood, several overall themes have been defined by characterizing the roles of distinct pathways during NK cell responses.

Major histocompatibility complex class I-recognizing receptors are disease risk genes in rheumatoid arthritis

Rheumatoid arthritis (RA) is a heterogeneous syndrome of which a subset of patients develops vascular inflammation. The genetic determinants that confer risk for rheumatoid vasculitis are not known, but patients with vascular complications are known to have an expansion of CD4(+)CD28(null) T cells, a cell population potentially involved in endothelial damage. CD4(+)CD28(null) T cell clones isolated from RA patients with vasculitis were found to express killer cell immunoglobulin-like receptors (KIRs) with the stimulatory KIR2DS2 often present in the absence of opposing inhibitory receptors with related specificities. To test the hypothesis that the KIR2DS2 gene is involved in the development of vasculitis, association studies were performed. The KIR2DS2 gene was significantly enriched among patients with rheumatoid vasculitis compared with normal individuals (odds ratio 5.56, P = 0.001) and patients with RA but no vasculitis (odds ratio 7.96, P = 0.001). Also, the distribution of human histocompatibility leukocyte antigen (HLA)-C, the putative ligand for KIRs, was significantly different in patients with rheumatoid vasculitis in comparison with the control populations. These data suggest that HLA class I-recognizing receptors and HLA class I genes are genetic risk determinants that modulate the pattern of RA expression. Specifically, KIR2DS2 in conjunction with the appropriate HLA-C ligand may have a role in vascular damage by regulating CD4(+)CD28(null) T cells.

Killer cell activating receptors function as costimulatory molecules on CD4+CD28null T cells clonally expanded in rheumatoid arthritis

Expansion of CD4+CD28null T cells is a characteristic finding in patients with rheumatoid arthritis. Despite lacking CD28 molecules, these unusual CD4 T cells undergo clonal proliferation and form large and long-lived clonal populations. They produce high levels of IFN-gamma, exhibit autoreactivity, and have cytolytic function. The mechanisms facilitating the expansion and longevity of CD4+CD28null T cell clones in vivo are unknown. Here, we report that CD4+CD28null, but not CD4+CD28+, T cells express MHC class I-recognizing receptors normally found on NK cells. CD4+CD28null T cells preferentially expressed killer cell activating receptors (KAR), often in the absence of killer cell inhibitory receptors. Cross-linking of KAR molecules enhanced the proliferative response to TCR-mediated stimulation, but not the cytolytic function of CD4+CD28null T cells, suggesting different signaling pathways in CD4 T cells and NK cells. Triggering of KAR signaling led to the phosphorylation of several cellular targets, although the pattern of phosphorylation differed from that induced by the TCR. Aberrant expression of KAR molecules in the absence of inhibitory receptors and in the appropriate HLA setting may lead to the clonal outgrowth of autoreactive CD4+CD28null T cells commonly seen in rheumatoid arthritis.

Nonstochastic coexpression of activation receptors on murine natural killer cells

Murine natural killer cells (NK) express lectin-like activation and inhibitory receptors, including the CD94/NKG2 family of receptors that bind Qa-1, and the Ly-49 family that recognizes major histocompatibility complex class I molecules. Here, we demonstrate that cross-linking of NK cells with a new specific anti-Ly-49H mAb induced NK cell cytotoxicity and cytokine production. Ly-49H is expressed on a subset of NK cells and can be coexpressed with Ly-49 inhibitory receptors. However, unlike Ly-49 inhibitory receptors, Ly-49H is not detectable on naive splenic CD3(+) T cells, indicating that Ly-49H may be an NK cell-specific activation receptor. In further contrast to the stochastically expressed Ly-49 inhibitory receptors, Ly-49H is preferentially expressed with the Ly-49D activation receptor, and expression of both Ly-49H and Ly-49D is augmented on NK cells that lack receptors for Qa-1 tetramers. On developing splenic NK1.1(+) cells, Ly-49D and Ly-49H are expressed later than the inhibitory receptors. These results directly demonstrate that Ly-49H activates primary NK cells, and suggest that expression of Ly-49 activation receptors by NK cells may be specifically regulated on NK cell subsets. The simultaneous expression of multiple activation receptors by individual NK cells contrasts with that of T cell antigen receptors and is relevant to the role of NK cells in innate immunity.

Increased expression of non-functional killer inhibitory receptor CD94 in CD8+ cells of myeloma patients

Different MHC class I-specific killer inhibitory receptors (KIRs) are expressed in vivo by a minor fraction of activated memory CD8+ cells. It has been postulated that KIRs may 'fine-tune' specific responses by altering their threshold of activation by the TCR-CD3 complex. We have previously shown that, in multiple myeloma (MM) patients, a large fraction of peripheral blood CD8+ cells display the phenotype of chronically activated memory T cells (CD38+, HLA-DR+, CD25-, CD45R0+, CD28-). We investigated the expression of KIRs on MM T cells and determined their possible influence on cytolytic responses elicited via the CD3-TCR complex. The expression of CD94, a molecule that is part of a heterodimeric KIR recognizing the non-classical MHC surface HLA-E molecule, was almost threefold higher in MM T cells than in age-matched normal control subjects (P < 0.0001). CD94 expression was preferentially confined to CD8+ cells but not restricted to activated (HLA-DR+) and/or memory (CD45R0+) T cells. Unlike normal T cells, in which CD94 is assembled with glycoproteins of the NKG2 family to form functional receptors with activating or inhibitory properties, most CD94+ MM T cells were devoid of both the NKG2-A and NKG2-C glycoproteins detected in the inhibitory or activating form respectively. CD94 blockade did not significantly affect either T-cell proliferation or cytotoxic T-lymphocyte generation induced by the myeloma-derived cell lines NCI and RPMI 8226. Similarly, the cytolytic activity induced by direct anti-CD3-mediated targeting of MM T cells to FCR+ P815 target cells was unaffected by the addition of anti-CD94 and/or anti-NKG2-A/C monoclonal antibodies (mAbs). These data indicate that the large majority of MM CD8+ cells do not express a functional CD94 receptor. Thus, their ability to 'fine-tune' an appropriate immune response against tumour cells can be impaired.

Molecular analyses of the interactions between human NK receptors and their HLA ligands

NK cell cytotoxicity is regulated by the action of multiple families of receptors. The interactions of these receptors with their ligands control different activating/inhibiting signal pathways and it is the balance of these signals which determines the behavior of the NK cell. The major described inhibitory pathways begin either with the recognition of a target cell classical class I HLA molecule by a killer cell immunologlobulin-like receptor (KIR) or the binding of the non-classical class I molecule HLA-E to the CD94/NKG2-A heterodimer. Activating counterparts to these inhibitory NK receptors have also been described and this review focuses on the molecular details of the binding of the inhibitory and activating receptors to their HLA ligands.

Impairment of natural killer cell activity in Chlamydia trachomatis infected individuals

Natural killer (NK) cell activity is impaired in Chlamydia trachomatis-infected patients. The mechanisms behind the altered NK functions are not clear, but data concerning NK and antibody-dependent cellular cytotoxicity (ADCC) activity have been reported. To investigate whether this impairment is related to a defect at the target cell binding and/or the postbinding level, we evaluated highly purified NK cells obtained from 125 C. trachomatis-infected patients and compared them with 101 normal controls for their ability to kill K-562 and U-937 cell lines using a 51Cr release assay; release tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma); and kill anti-IgM preincubated P-815 cell line (ADCC activity). We found a decrease in the lytic capability of NK cells from C. trachomatis-infected patients against target cell lines; decreased ability to kill bound target cells; and low levels of released TNF-alpha and INF-gamma after incubation with U-937 cells. Taken together, these findings suggest that the impaired NK cell reaction during chlamydial infection is related to defects both at the target and postbinding levels. However, the precise mechanisms remain to be determined. The inability to restore normal NK activity after long-term culture in the presence of high levels of recombinant IL-2 support the hypothesis of an anergic process during chlamydial infection.

Interaction of the NK cell inhibitory receptor Ly49A with H-2Dd: identification of a site distinct from the TCR site

Natural killer cell function is controlled by interaction of NK receptors with MHC I molecules expressed on target cells. We describe the binding of bacterially expressed Ly49A, the prototype murine NK inhibitory receptor, to similarly engineered H-2Dd. Despite its homology to C-type lectins, Ly49A binds independently of carbohydrate and Ca2+ and shows specificity for MHC I but not bound peptide. The affinity of the Ly49A/H-2Dd interaction as determined by surface plasmon resonance is from 6 to 26 microM at 25 degrees C and is greater by ultracentrifugation at 4 degrees C. Biotinylated Ly49A stains H-2Dd-expressing cells. Competition experiments indicate that the Ly49A and T cell receptor (TCR) binding sites on MHC I are distinct, suggesting complex regulation of cells that bear both TCR and NK cell receptors.

Differing MHC class I requirements for induction and propagation of experimental systemic lupus erythematosus

Mice deficient in beta2-microglobulin expression are resistant to the induction of experimental systemic lupus erythematosus (SLE). The present studies were designed to identify the beta2-microglobulin-dependent cell surface molecule(s) that confers sensitivity to experimental SLE, and to determine its role in disease development. We report hat mice lacking the transporter associated with antigen presentation (TAP-/-) were also resistant to disease, whereas CD1-/- and CD8-/- mice were susceptible; susceptibility also did not correlate with neonatal Fc receptor or HEPH expression. These data indicate that disease susceptibility is determined by expression of MHC class I. Furthermore, by analyzing both adoptive transfer and radiation bone marrow chimeras, we demonstrate that MHC class I expression is necessary for propagation of disease, but not for induction of pathogenic cells.

Peptide presentation and NK inhibition by HLA-G

The expression of the nonclassical MHC class Ib molecule HLA-G is nearly exclusively restricted to the feto-maternal interface during pregnancy. There it probably serves the same physiological functions already known for classical MHC class I molecules; these include peptide presentation, natural killer cell (NK) inhibition and probably also T cell restriction. In this study a comparison between HLA-G and HLA-A2 as far as the amount and complexity of bound peptides is concerned revealed no significant differences. The peptide motif of HLA-G, as determined by analysis of naturally eluted peptides allows the construction of a peptide library that is efficient in binding to HLA-G and thereby confirms the rules of peptide binding to this nonclassical MHC class I molecule. In addition, we demonstrate that the inhibition of NK cells by HLA-G varies remarkably among the NK repertoires of different donors. The function of HLA-G as a survival factor in the development of the fetus during pregnancy is discussed in detail.

Regulation of immune responses through inhibitory receptors

Major histocompatibility complex class I-specific inhibitory receptors on natural killer cells prevent the lysis of healthy autologous cells. The outcome of this negative signal is not anergy or apoptosis of natural killer cells but a transient abortion of activation signals. The natural killer inhibitory receptors fulfill this function by recruiting the tyrosine phosphatase SHP-1 through a cytoplasmic immunoreceptor tyrosine-based inhibition motif. This immunoreceptor tyrosine-based inhibition motif has become the hallmark of a growing family of receptors with inhibitory potential, which are expressed in various cell types such as monocytes, macrophages, dendritic cells, leukocytes, and mast cells. Most of the natural killer inhibitory receptors and two members of a monocyte inhibitory-receptor family bind major histocompatibility complex class I molecules. Ligands for many of the other receptors have yet to be identified. The inhibitory-receptor superfamily appears to regulate many types of immune responses by blocking cellular activation signals.

Immunopathogenesis of multiple sclerosis: the role of T cells

Multiple sclerosis is considered to be an autoimmune disease that results from aberrant immune responses to central nervous system antigens. T cells are considered to be crucial in orchestrating an immunopathological cascade that culminates in damage to the myelin sheath, oligodendrocytes and axons.

The transmembrane sequence of human histocompatibility leukocyte antigen (HLA)-C as a determinant in inhibition of a subset of natural killer cells

Molecular interactions with the extracellular domains of class I major histocompatibility complex proteins are major determinants of immune recognition that have been extensively studied both physically and biochemically. However, no immunological function has yet been placed on the transmembrane or cytoplasmic amino acid sequences of these proteins despite strict conservation of unique features within each class I major histocompatibility complex locus. Here we report that lysis by a subset of natural killer (NK) cells inhibited by target cell expression of human histocompatibility leukocyte antigen (HLA)-Cw6 or -Cw7 was not inhibited by expression of chimeric proteins consisting of the extracellular domains of HLA-C and the COOH-terminal portion of HLA-G. Assays using transfectants expressing a variety of HLA-Cw6 mutants identified the transmembrane sequence and, in particular, cysteine at position 309 as necessary for inhibition of 68% (25/37) of NK cell lines and 23% (33/145) of NK clones tested. Moreover, these NK clones inhibited by target cell expression of HLA-Cw6 and dependent upon the transmembrane sequence were found not to express or to only dimly express NK inhibitory receptors (NKIR1) that are EB6/HP3E4-positive. Furthermore, assays using monoclonal antibody blocking suggest that an NK receptor other than NKIR1 or CD94 is responsible for recognition dependent upon the transmembrane sequence of HLA-Cw6.