Multiple receptors for HLA-G on human natural killer cells

HLA-E-restricted HIV-1-specific CD8+ T cell responses in natural infection

CD8+ T cell responses restricted by MHC-E, a nonclassical MHC molecule, have been associated with protection in an SIV/rhesus macaque model. The biological relevance of HLA-E-restricted CD8+ T cell responses in HIV infection, however, remains unknown. In this study, CD8+ T cells responding to HIV-1 Gag peptides presented by HLA-E were analyzed. Using in vitro assays, we observed HLA-E-restricted T cell responses to what we believe to be a newly identified subdominant Gag-KL9 as well as a well-described immunodominant Gag-KF11 epitope in T cell lines derived from chronically HIV-infected patients and also primed from healthy donors. Blocking of the HLA-E/KF11 binding by the B7 signal peptide resulted in decreased CD8+ T cell responses. KF11 presented via HLA-E in HIV-infected cells was recognized by antigen-specific CD8+ T cells. Importantly, bulk CD8+ T cells obtained from HIV-infected individuals recognized infected cells via HLA-E presentation. Ex vivo analyses at the epitope level showed a higher responder frequency of HLA-E-restricted responses to KF11 compared with KL9. Taken together, our findings of HLA-E-restricted HIV-specific immune responses offer intriguing and possibly paradigm-shifting insights into factors that contribute to the immunodominance of CD8+ T cell responses in HIV infection.

miRNAs in NK Cell-Based Immune Responses and Cancer Immunotherapy

The incidence of certain forms of tumors has increased progressively in recent years and is expected to continue growing as life expectancy continues to increase. Tumor-infiltrating NK cells may contribute to develop an anti-tumor response. Optimized combinations of different cancer therapies, including NK cell-based approaches for targeting tumor cells, have the potential to open new avenues in cancer immunotherapy. Functional inhibitory receptors on NK cells are needed to prevent their attack on healthy cells. Nevertheless, disruption of inhibitory receptors function on NK cells increases the cytotoxic capacity of NK cells against cancer cells. MicroRNAs (miRNAs) are small non-coding RNA molecules that target mRNA and thus regulate the expression of genes involved in the development, maturation, and effector functions of NK cells. Therapeutic strategies that target the regulatory effects of miRNAs have the potential to improve the efficiency of cancer immunotherapy. Interestingly, emerging evidence points out that some miRNAs can, directly and indirectly, control the surface expression of immune checkpoints on NK cells or that of their ligands on tumor cells. This suggests a possible use of miRNAs in the context of anti-tumor therapy. This review provides the current overview of the connections between miRNAs and regulation of NK cell functions and discusses the potential of these miRNAs as innovative biomarkers/targets for cancer immunotherapy.

The immunology of the macaque placenta: A detailed analysis and critical comparison with the human placenta

The cynomolgus monkey is increasingly considered in toxicological research as the most appropriate model for humans due to the species' close physiological contiguity, including reproductive physiology. Here, literature on the cynomolgus monkey placenta is reviewed in regards to its similarity to the human placenta and particularly for its immunological role, which is not entirely mirrored in humans. Pertinent original data are included in this article. The cynomolgus monkey placenta is evaluated based on three aspects: first, morphological development; second, the spatial and temporal appearance of maternal and fetal immune cells and certain immune cell products of the innate and adaptive immune systems; and third, the expression of relevant immune tolerance-related molecules including the homologs of anti-human leucocyte antigen, indoleamine 2,3-dioxygenase, FAS/FAS-L, annexin II, and progesterone. Parameters relevant to the immunological role of the placenta are evaluated from the immunologically immature stage of gestational day (GD) 50 until more mature stages close to birth. Selected comparisons are drawn with human and other laboratory animal placentas. In conclusion, the cynomolgus monkey placenta has a high degree of morphological and physiological similarity to the human placenta. However, there are differences in the topographical distribution of cell types and immune tolerance-related molecules. Three basic features are recognized: (1) the immunological capacity of the placenta changes throughout the lifetime of the organ; (2) these immunological changes include multiple parameters such as morphological adaptations, cell type involvement, and changes in immune-relevant molecule expression; and (3) the immune systems of two genetically disparate individuals (mother and child) are functionally intertwined at the maternal-fetal interface.

Glioblastoma Stem-Like Cells Are More Susceptible Than Differentiated Cells to Natural Killer Cell Lysis Mediated Through Killer Immunoglobulin-Like Receptors-Human Leukocyte Antigen Ligand Mismatch and Activation Receptor-Ligand Interactions

Glioblastoma (GBM) is the most aggressive brain malignancy in adults, where survival is approximately 14.6 months. Novel therapies are urgently needed and immunotherapy has hailed a new dawn for treatment of solid tumors. Natural killer (NK) cells may be amenable therapeutic effectors against heterogeneous GBM, since they also do not require co-stimulation and antigen specificity. However, it is unclear how culture media routinely used in pre-clinical studies affect GBM cell responses to NK-mediated cytotoxicity. We hypothesized that the culture medium would affect GBM cell phenotype, proliferation, and responses to NK cytotoxicity. We investigated in paired analyses n = 6 patient-derived primary GBM cells propagated in stem cell or serum-containing medium for morphology, proliferation, as well as susceptibility to NK cytolysis and related this to expression of surface and intracellular lineage markers, as well as ligands for NK cell activating and inhibitory receptors. We genotyped the GBM cells for human leukocyte antigen (HLA) as well as the killer immunoglobulin-like receptors (KIR) of the n = 6 allogeneic NK cells used as effector cells. Culture in serum-containing medium induced a switch in GBM cell morphology from suspension neuropsheres to adherent epithelial-mesenchymal-like phenotypes, which was partially reversible. The differentiated cells diminished expression of nestin, CD133 (prominin-1), and A2B5 putative glioma stem-cell markers, attenuated growth, diminished expression of ligands for activating NK cell receptors, while upregulating class I HLA ligands for NK cell inhibitory receptors. When maintained in serum-containing medium, fewer GBM cells expressed intercellular cell adhesion molecule-1 (ICAM-1) and were less susceptible to lysis by NK cells expressing αLβ2 integrin receptor (LFA-1), mediated through combination of inhibitory KIR-HLA ligand mismatch and diminished activation receptor-ligand interactions compared to cells maintained in stem cell media. We conclude that development of preclinical immunotherapy strategies against GBM should not use cells propagated in serum-containing media to avoid misinterpretation of potential therapeutic responses.

Isoforms of human leukocyte antigen-G and their inhibitory receptors in human kidney allograft acceptance

Novel therapeutic strategies such as the modulation of dendritic cell and T-cell function have exhibited great potential in clinical transplantation. Human leukocyte antigen (HLA)-G is a molecule that plays a significant role in establishing complex mechanisms to protect semiallogeneic fetuses from rejection by the maternal immune system. The unique characteristics of both cell-surface and soluble isoforms of HLA-G, the formation of disulfide-bonded dimers with the potential to augment inhibitory receptor signaling, and the function of HLA-G as a preferential ligand for the immunoglobulin-like transcript receptors make HLA-G very important in fundamental approaches for the modulation of immune responses to improve allogeneic graft survival in clinical transplantation. Experimental data from several groups as well as our data from experiments involving HLA-G-mediated human tolerogenic dendritic cells in vitro and receptor transgenic mice in vivo indicate that different isoforms of HLA-G have various immunomodulatory effects through the inhibitory receptors. This knowledge is crucial in understanding mechanisms of prolongation of allograft survival. The analyses of HLA-G isoforms and inhibitory receptors in patients with kidney allograft and the relationship among different isoforms of HLA-G, inhibitory receptors, their mediated immunoregulation, and graft acceptance or failure will be discussed here.

Immune-refractory cancers and their little helpers--an extended role for immunetolerogenic MHC molecules HLA-G and HLA-E?

There is strong evidence to support a role for non-classical MHC class I (class Ib) molecules, most notably HLA-E and HLA-G in tumour immune escape. In this article, we summarize the current knowledge on their expression, regulation and functional relevance in various malignancies, particularly brain tumours. Special emphasis is devoted to the phenomenon that these tolerogenic molecules are expressed by non-transformed cells that are found in close neighborhood to tumour cells representing either parenchymal cells or immune cells attracted to the tumour microenvironment. Here they may act as "natural" or "inducible" suppressors of anti-tumoural immune responses. We thus speculate about the role of HLA-G expressing T cells, a novel population of natural regulatory cells that was identified recently. It is suggested that various cell types within a tumour cooperate in order to inhibit anti-tumour immunity-and that immunetolerogenic HLA-G may play a major role in this context.

HLA-G expression is induced in Epstein-Barr virus-transformed B-cell lines by culture conditions

Human leukocyte antigen-G (HLA-G) belongs to the nonclassical HLA class I family of genes presently designated as class Ib genes. It was found to be expressed mainly in placental tissue and in the thymus. Expression of HLA-G is induced by lymphokines such as interleukin-10 and has been associated with the escape of tumor cells from immune surveillance or with inhibition of graft rejection. In this report, Epstein-Barr virus-transformed B-cell lines established from peripheral blood lymphocytes from healthy volunteers were studied. Our results show that EBV-transformed B-cell lines, but not freshly separated peripheral blood lymphocytes, can be induced to express HLA-G either by subjecting the cultures to nutrient deficiency to hypoxia or to both, however, not all cell lines responded equally to stress conditions. The association of HLA-G with certain cancer transformations may suggest that the resistance to HLA-G expression could be related to susceptibility to the development of malignancy.

Human decidual NK cells form immature activating synapses and are not cytotoxic

In early pregnancy invading fetal trophoblasts encounter abundant maternal decidual natural killer cells (dNK). dNK express perforin, granzymes A and B and the activating receptors NKp30, NKp44, NKp46, NKG2D, and 2B4 as well as LFA-1. Even though they are granular and express the essential molecules required for lysis, fresh dNK displayed very reduced lytic activity on classical MHC I negative targets K562 and 721.221, approximately 15% of that of peripheral NK cells. dNK formed conjugates and activating immune synapses with 721.221 and K562 cells in which CD2, LFA-1 and actin were polarized toward the contact site. However, in contrast to peripheral NK cells, they failed to polarize their microtubule organizing centers and perforin-containing granules to the synapse, accounting for their lack of cytotoxicity.

Linkage disequilibrium between human leukocyte antigen (HLA) class II and HLA-G--possible implications for human reproduction and autoimmune disease

A line of investigation indicates that one or several genes in the human major histocompatibility complex (MHC) influences reproductive success. Studies have revealed associations between human leukocyte antigen (HLA) class II genes and risk of recurrent spontaneous abortion (RSA) and pre-eclampsia. However, these genes are not expressed at the feto-maternal interface. Furthermore, associations between polymorphisms in the nonclassical HLA class Ib gene, HLA-G, and reproductive outcome have been demonstrated. HLA-G is expressed by extravillous trophoblast during pregnancy, making it a more obvious candidate gene for a possible influence on pregnancy outcome. HLA-G has immunomodulatory functions. We have studied linkage disequilibrium between HLA class II genes, primarily HLA-DRB1 alleles, and HLA-G alleles in women with RSA and their partners (n = 103) and in control women and their partners (n = 92). We found a significant linkage disequilibrium between HLA-DR3 and HLA-G*010102 in both the RSA and control populations. For all four studied HLA loci, the alleles in the haplotype HLA-DRB1*03.DQA1*05.DQB1*02.G*010102 was in clear linkage disequilibrium. This HLA haplotype has repeatedly been associated with different autoimmune diseases but also with RSA. The G*010102 allele includes a 14-bp sequence polymorphism in the 3' untranslated region of the gene, which has been associated with differences in HLA-G mRNA alternative splicing and stability. This 14-bp polymorphism has also been associated with RSA, pre-eclampsia, and outcome of in vitro fertilization. Implications of HLA polymorphism--and other polymorphic genes in the MHC for pregnancy outcome--and for autoimmune diseases during pregnancy are discussed.

The involvement of NK cells in ankylosing spondylitis

A role for NK cells in the regulation of autoimmunity has been demonstrated. Since there is a strong association between Ankylosing Spondylitis (AS) and HLA-B27, which is specifically recognized by the NK-inhibitory receptor KIR3DL1, this study evaluated the potential involvement of NK cells in AS. We studied 19 AS patients and 22 healthy volunteer donors and assessed the percentage, activity and receptor expression of peripheral blood NK cells. We also evaluated candidate-inflammatory mediators in sera. We found that AS patients have significantly higher percentages of NK cells. However, we found no differences between the ability of NK cells derived from AS and healthy controls to recognize target cells expressing HLA-B27. Remarkably, we observed that the NK-inhibitory receptor CEACAM1 (carcino-embryonic antigen-cell adhesion molecule) is highly expressed among AS-derived NK cells. Furthermore, engagement of CEACAM1 inhibited NK activity in these patients. Finally, we demonstrated that CEACAM1 expression is induced by IL-8 and SDF-1 (stromal cell derived factor), both of which are present in high levels in the sera of AS patients. These results may indicate that NK cells and CEACAM1 play a role in AS pathogenesis and implicate chemokines in the mechanism of CEACAM1 expression.

An update in recurrent spontaneous abortion

Recurrent spontaneous abortion (RSA) is defined as three or more consecutive pregnancy losses prior to the 20th week of gestation. The etiology of recurrent spontaneous abortion is often unclear and may be multifactorial, with much controversy regarding diagnosis and treatment. Reasonably accepted etiologic causes include, genetics, anatomical, endocrine, placental anomalies, hormonal problems, infection, smoking and alcohol consumption, exposure to environmental factors, psychological trauma and stressful life event, certain coagulation and immunoregulatory protein defects. Detection of an abnormality in any of these areas may result into specific therapeutic measures, with varying degrees of success. However, the majority of cases of RSA remains unexplained and is found to be associated with certain autoimmune (APA, ANA, ACA, ATA, AECA) and alloimmune (APCA, Ab2, MLR-Bf) antibodies that may play major role in the immunologic failure of pregnancy and may lead to abortion. Alteration in the expression of HLA-G molecules, T-helper-1 (Th-1) pattern of cytokines and natural killer (NK) cells activity may also induce abortion. Various forms of treatment like antithrombotic therapies such as aspirin and heparin, intravenous immunoglobulin (IVIg) therapy, immunotherapy with paternal lymphocytes and vitamin D3 therapy are effective mode of treatment for unexplained cause of fetal loss in women with RSA.

Mechanisms of resistance to natural killer cell-mediated cytotoxicity in acute lymphoblastic leukemia

OBJECTIVE

Natural killer (NK) cell-mediated cytotoxicity contributes to the innate immune response against numerous malignancies, including leukemias. Acute lymphoblastic leukemias (ALL) often display a high degree of resistance, the mechanisms of which have not been elucidated.

METHODS

We used the well-characterized NK cell line NK-92 as a model to investigate whether mechanisms commonly implicated in tumor escape from NK cell killing are relevant for ALL.

RESULTS

We demonstrate selective resistance of B-precursor ALL to NK-92 cytotoxicity even in the absence of inhibitory killer cell immunoglobulin-like receptors (KIR), except for KIR2DL4. We also show that human leukocyte antigen-G, a ligand of KIR2DL4, expressed on a subset of ALL, does not mediate resistance of NK-cell mediated lysis. Similarly, intracellular adhesion molecule/lymphocyte function-associated antigen-1 interaction did not contribute significantly to resistance. In contrast the NK-sensitive T-ALL (MOLT-4) expressed moderate amounts of MHC class I chain-related gene AB (MICA/B) a ligand for the NK cell activating receptor NKG2D, while expression of MICA/B was absent in resistant B-ALL cell lines.

CONCLUSIONS

The NK cell-resistance of B-lineage ALLs does not appear to involve inhibitory mechanisms, but suggests deficient NK cell activation. Thus, immunostrategies designed to enhance ALL sensitivity toward NK cell-mediated cytotoxicity should focus on mechanisms of NK cell activation.

The HLA-G genotype is associated with IL-10 levels in activated PBMCs

Human leukocyte antigen (HLA)-G is an MHC class Ib molecule that is expressed at the feto-maternal interface during pregnancy. However, recent results have also shown that it may have important functions as an immuno-modulatory factor in adult life. Differences in the pattern of alternative splicing and in the stability of HLA-G mRNA transcripts have been associated with HLA-G polymorphisms, especially a 14 bp deletion/insertion polymorphism in the 3' untranslated region of the HLA-G gene. We have investigated the secretion of HLA-G5/soluble HLA-G1 and interleukin-10 (IL-10) in lipopolysaccharide (LPS)-activated peripheral blood mononuclear lymphocytes (PBMCs) in relation to the HLA-G 14 bp genotype. No HLA-G5/sHLA-G1 could be detected in the non-activated control PBMC culture media, and there were no significant differences among the three HLA-G 14 bp genotypes regarding IL-10 concentrations. In LPS-activated PBMC cultures, no significant differences among the three HLA-G 14 bp genotypes regarding HLA-G5/sHLA-G1 concentrations were observed. However, this was in contrast to the IL-10 levels (P=0.0004, Kruskal-Wallis test). The +14/+14 bp PBMC samples expressed higher levels of IL-10 when compared to the -14/+14 bp genotype and the -14/-14 bp genotype. Interestingly, the IL-10 G/G polymorphism at position -1082 was more frequent in the +14/+14 bp genotype (P=0.024, chi2 test). These results support an autocrine loop between HLA-G5/sHLA-G1 and IL-10 expression in activated PBMCs, which may result in higher IL-10 levels in +14/+14 bp HLA-G genotypes.

HLA-G is associated with pemphigus vulgaris in Jewish patients

Pemphigus is a group of life-threatening autoimmune blistering diseases of the skin and mucous membranes. The etiology and pathogenesis of this destructive autoimmune process remains unknown, but significant association with human leukocyte antigen (HLA) factors have been described in pemphigus vulgaris (PV) patient cohorts worldwide. We have recently analyzed DNA samples obtained from pemphigus patients and matched controls with a set of microsatellite markers, and found that markers mapped to HLA class I region are significantly associated with the disease. In order to narrow the region that is associated with the disease single nucleotide polymorphism (SNP) technology was used. In this study, a set of 26 SNP markers, which span a chromosomal region of about 600,000 bp, were used to screen DNA samples of the patients and their matched controls. Of the 26 SNPs, four markers were found informative, all mapped to HLA-G. Typing patients and controls for HLA-G polymorphism revealed significant differences in the exon 8 deletion/insertion variant. The latter is probably associated with the efficiency of transcription of this gene. Taken together, the results suggest that HLA-G is associated with PV in Jewish patients.

Express and protect yourself: the potential role of HLA-G on muscle cells and in inflammatory myopathies

Muscle is the site or the target of immunologic injury in several diseases. Whereas under physiologic conditions muscle fibers are negative for major histocompatibility complex (MHC) class I antigens, these are upregulated under pathologic conditions, thus rendering muscle a possible target for the recognition by cytotoxic CD8 T cells. Cultured muscle cells are capable of presenting antigens to CD4 and CD8 T cells, further indicating that muscle fibers in vivo are critically involved in the initiating or perpetuating steps of inflammatory responses. The finding that muscle fibers in autoimmune inflammatory myopathies in vivo and cultured muscle cells in vitro express the nonclassical major histocompatibility complex molecule HLA-G raises several hypothesis concerning its possible pathophysiologic role. We review present knowledge on the functional consequences of muscle-related HLA-G and provide concepts of its relevance under pathologic conditions. We further speculate on the potential therapeutic implications of HLA-G that relate to special approaches such as myoblast transplantation or strategies against inflammatory aggression in general.

Killer inhibitory receptor CD158a overexpression among natural killer cells in women with endometriosis is undiminished by laparoscopic surgery and gonadotropin releasing hormone agonist treatment

PROBLEM

Natural killer (NK) dysfunction is considered to contribute to the pathogenesis of endometriosis. In this study, we investigated the host immune response to endometriosis in terms of killer inhibitory receptor (KIR) expression by NK cells.

METHOD OF STUDY

We compared cells from Japanese women laparoscopically diagnosed with endometriosis and treated with laparoscopic surgery (n = 98), 1 month after laparoscopic surgery (n = 36), and 12 weeks after gonadotropin releasing hormone agonist (GnRHa) treatment (n = 18) to cells from 104 women with other laparoscopic diagnoses. KIR expression by NK cells was assessed in peripheral blood and peritoneal fluid samples by flow cytometry.

RESULTS

In women with endometriosis, the percentage of CD158a-expressing cells among CD16-expressing NK (CD158a(+)NK) cells in both peritoneal fluid and peripheral blood was significantly higher than in control subjects. No significant differences in proportion of CD158a(+)NK cells were identified between peripheral blood NK cells sampled before and 1 month after laparoscopic surgery, or 12 weeks after initiating GnRHa treatment.

CONCLUSIONS

Increased percentage of CD158a(+)NK cells in peripheral blood from women with endometriosis was undiminished by laparoscopic surgery and GnRHa treatment; the persistence of CD158a(+)NK cell excess is probably related to NK cell suppression in endometriosis. This overexpression may represent a risk factor for development of endometriosis and its recurrence after treatments.

Truncated HLA-G isoforms are retained in the endoplasmic reticulum and insufficiently provide HLA-E ligands

The preferential expression of the non-polymorphic human leukocyte antigen G (HLA-G) on invading extravillous cytotrophoblast cells that are, with the exception of HLA-C and -E, HLA class I negative led to the hypothesis that HLA-G plays a major role in controlling the effector functions of the large granular leukocytes (LGL), a specialized natural killer (NK) cell population present in large numbers in the decidua. Transcription of the HLA-G gene is characterized by extensive alternative splicing producing at least seven potentially membrane bound or secreted isoforms. Except for HLA-G1 and its soluble variant (HLA-G1s), there is still dispute as to whether any of the other isoforms displays a major immunological function. Here we describe that the membrane-bound isoforms HLA-G2, -G3, and G4 as well as the soluble variant of HLA-G2 (HLA-G2s) do not egress the endoplasmic reticulum as determined by Endo H sensitivity assays. Moreover these isoforms seem not to have a major immunological function with respect to NK cell inhibition by providing a ligand for HLA-E, which would allow the interaction of this molecule with the inhibitory CD94/NKG2A NK cell receptor.

Hide-and-seek in the brain: a role for HLA-G mediating immune privilege for glioma cells

This review summarizes the current knowledge on the expression and functional role of HLA-G in normal CNS cells and brain tumor cells in vitro and in vivo. The CNS has classically been viewed as an immune-privileged organ. Here we discuss some of the particularities of anti-tumoral responses within this compartment. Special emphasis is dedicated to the possible role of the non-classical MHC molecule HLA-G as an alternative mechanism of immune escape. We review the mechanisms how glioma cell-derived HLA-G may paralyze the immune system and which cellular subsets of the immune system are affected. Possible therapeutic implications derived from these observations include the targeting of HLA-G expression within the framework of inducing glioma-specific immunity.

CXCL12 expression by invasive trophoblasts induces the specific migration of CD16- human natural killer cells

In the maternal decidua, natural killer (NK) cells, characterized by lack of CD16, are found in direct contact with the fetal extravillous trophoblasts (EVTs). It is yet unknown which factors contribute to the specific homing of this unique NK subset to the decidua. In this study we analyze the chemokine receptor repertoire on various NK populations derived from the peripheral blood and decidua. We show that CXCR4 and CXCR3 receptors are preferentially expressed on CD16- NK subsets derived either from the peripheral blood or the decidua and that these receptors are involved in migration of all NK subsets to their ligands. We further demonstrate in vivo that invading EVTs that eventually perform endovascular invasion express CXCL12, the ligand for CXCR4, but not ligands for CXCR3. Indeed, specific accumulation of the CD16- NK cells at the expense of CD16+ cells was observed only when in vitro migration was performed with ligands for CXCR4. Finally, incubation of the peripheral blood CD16- NK cells with cytokines present in the decidua, especially interleukin 15 (IL-15), resulted in the expression of chemokine receptor repertoire similar to that observed on decidual NK cells, suggesting an additional important regulatory effect of local decidual cytokines.

Soluble nonclassical HLA generated by the metalloproteinase pathway

Soluble human leukocyte antigens (HLA-A, -B, and -C) proteins can be generated by a membrane-bound metalloproteinase (MPase). The MPase-mediated pathway produces soluble nonconformed HLA proteins susceptible to further degradation, and also HLA proteins with high affinity peptides stable at physiologic temperatures. Accessibility of classical HLA to the MPase cleavage inversely correlates with stability of heavy chain (HC) interactions with beta2-microglobulin (beta(2)m). Whether a MPase is involved in release of soluble nonclassical HLA or CD1 proteins is unknown. We have investigated this question with transfectants expressing full-length HLA proteins. Native surface HLA-E and -G complexes, similar to HLA-A2, were unstable at low pH and dissociated giving rise to beta(2)m-free HC. Furthermore, HLA-E and -G proteins, similar to HLA-A2, were readily released from cell surface into supernatants as soluble 37-kilodalton beta(2)m-free HC. However, the stability of surface CD1d complexes was not affected by pH changes and no soluble CD1d was detected. Because beta(2)m-free CD1d HC were expressed on cells, the lack of cleaved soluble products cannot be explained by high stability of native complexes. Instead, absence of a CD1d-specific MPase in these cells or its impaired interactions with substrate HC may be responsible.

Complexes of HLA-G protein on the cell surface are important for leukocyte Ig-like receptor-1 function

The nonclassical class I MHC molecule HLA-G is selectively expressed on extravillous cytotrophoblast cells at the maternal-fetal interface during pregnancy. HLA-G can inhibit the killing mediated by NK cells via interaction with the inhibitory NK cell receptor, leukocyte Ig-like receptor-1 (LIR-1). Comparison of the sequence of the HLA-G molecule to other class I MHC proteins revealed two unique cysteine residues located in positions 42 and 147. Mutating these cysteine residues resulted in a dramatic decrease in LIR-1 Ig binding. Accordingly, the mutated HLA-G transfectants were less effective in the inhibition of NK killing and RBL/LIR-1 induced serotonin release. Immunoprecipitation experiments demonstrated the involvement of the cysteine residues in the formation of HLA-G protein oligomers on the cell surface. The cysteine residue located at position 42 is shown to be critical for the expression of such complexes. These oligomers, unique among the class I MHC proteins, probably bind to LIR-1 with increased avidity, resulting in an enhanced inhibitory function of LIR-1 and an impaired killing function of NK cells.

HLA-G allelic variants are associated with differences in the HLA-G mRNA isoform profile and HLA-G mRNA levels

During pregnancy, the human extra-villous trophoblast in the contact zone between maternal and fetal tissue in the placenta does not express the classical MHC class I and II molecules. Instead, HLA-G and -C, and possibly HLA-E, are expressed. HLA-G may modulate the immunological relationship between mother and fetus in several ways. Finally, the expression of membrane-bound HLA-G and soluble HLA-G has been proposed to influence the outcome of pregnancy, and an aberrant HLA-G expression in pre-eclamptic placentas and spontaneous abortions has been reported. Here, an association between certain HLA-G polymorphisms and the mRNA levels of the different alternatively spliced HLA-G isoforms in first trimester trophoblast cell populations is reported. Several alternatively spliced HLA-G mRNA isoforms, including a 14-bp polymorphism in the 3'UTR end (exon 8) of the HLA-G gene, are expressed at a significantly lower level than the corresponding HLA-G mRNA isoforms with the 14-bp sequence deleted. Furthermore, characteristic HLA-G mRNA isoform expression patterns were associated with specific HLA-G genotypes and alleles. In the HLA-G*01012 and - G*01013 alleles that include the 14-bp sequence, an additional alternative splicing was observed, with the first 92-bp of exon 8 spliced out. This was most pronounced in HLA-G genotypes with G*01013. These findings may have functional implications for the recent reports of aberrant HLA-G expression and reproductive success.

The non-classical MHC molecule HLA-G protects human muscle cells from immune-mediated lysis: implications for myoblast transplantation and gene therapy

HLA-G is a non-classical MHC class I molecule with highly limited tissue distribution which has been attributed chiefly immune-regulatory functions. We previously have reported that HLA-G is expressed in inflamed muscle in vivo and by cultured myoblasts in vitro. Here, we used the in vitro models of human myoblasts or TE671 muscle rhabdomyosarcoma cells to characterize the functional role of HLA-G for muscle immune cell interactions. Gene transfer of the two major isoforms of HLA-G (transmembranous HLA-G1 and soluble HLA-G5) into TE671 rendered these cells resistant to alloreactive lysis by direct inhibition of natural killer (NK) cells, and CD4 and CD8 T cells. Further, HLA-G reduced alloproliferation, interfered with effective priming of antigen-specific cytotoxic T cells and reduced antigen-specific alloreactive lysis. HLA-G pre-induced on cultured myoblasts inhibited lysis by alloreactive peripheral blood mononuclear cells. This protection was reversed by a neutralizing HLA-G antibody. Interestingly, a few HLA-G-positive cells within a population of HLA-G-negative muscle target cells conveyed significant inhibitory effects on alloreactive lysis. Our results reveal further insights into the immunobiology of muscle and suggest that ectopic expression of HLA-G may promote the survival of transplanted myoblasts in the future treatment of hereditary muscle diseases. Further, HLA-G could represent a novel self-derived anti-inflammatory principle applicable in strategies against inflammatory aggression.

Disulfide bond-mediated dimerization of HLA-G on the cell surface

HLA-G is a nonclassical class I MHC molecule with an unknown function and with unusual characteristics that distinguish it from other class I MHC molecules. Here, we demonstrate that HLA-G forms disulfide-linked dimers that are present on the cell surface. Immunoprecipitation of HLA-G from surface biotinylated transfectants using the anti-beta2-microglobulin mAb BBM.1 revealed the presence of an approximately equal 78-kDa form of HLA-G heavy chain that was reduced by using DTT to a 39-kDa form. Mutation of Cys-42 to a serine completely abrogated dimerization of HLA-G, suggesting that the disulfide linkage formed exclusively through this residue. A possible interaction between the HLA-G monomer or dimer and the KIR2DL4 receptor was also investigated, but no interaction between these molecules could be detected through several approaches. The cell-surface expression of dimerized HLA-G molecules may have implications for HLA-Greceptor interactions and for the search for specific receptors that bind HLA-G.

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.

A functional role of HLA-G expression in human gliomas: an alternative strategy of immune escape

HLA-G is a nonclassical MHC molecule with highly limited tissue distribution that has been attributed chiefly immune regulatory functions. Glioblastoma is paradigmatic for the capability of human cancers to paralyze the immune system. To delineate the potential role of HLA-G in glioblastoma immunobiology, expression patterns and functional relevance of this MHC class Ib molecule were investigated in glioma cells and brain tissues. HLA-G mRNA expression was detected in six of 12 glioma cell lines in the absence of IFN-gamma and in 10 of 12 cell lines in the presence of IFN-gamma. HLA-G protein was detected in four of 12 cell lines in the absence of IFN-gamma and in eight of 12 cell lines in the presence of IFN-gamma. Immunohistochemical analysis of human brain tumors revealed expression of HLA-G in four of five tissue samples. Functional studies on the role of HLA-G in glioma cells were conducted with alloreactive PBMCs, NK cells, and T cell subpopulations. Expression of membrane-bound HLA-G1 and soluble HLA-G5 inhibited alloreactive and Ag-specific immune responses. Gene transfer of HLA-G1 or HLA-G5 into HLA-G-negative glioma cells (U87MG) rendered cells highly resistant to direct alloreactive lysis, inhibited the alloproliferative response, and prevented efficient priming of cytotoxic T cells. The inhibitory effects of HLA-G were directed against CD8 and CD4 T cells, but appeared to be NK cell independent. Interestingly, few HLA-G-positive cells within a population of HLA-G-negative tumor cells exerted significant immune inhibitory effects. We conclude that the aberrant expression of HLA-G may contribute to immune escape in human glioblastoma.

Increased killer inhibitory receptor KIR2DL1 expression among natural killer cells in women with pelvic endometriosis

OBJECTIVE

To investigate the host immunologic response to endometriosis in terms of killer inhibitory receptor (KIR) expression by natural killer (NK) cells.

DESIGN

Case-control study of immunologic markers.

SETTING

University hospital.

PATIENT(S)

We compared cells from Japanese women with laparoscopically diagnosed endometriosis to cells from 40 women with other laparoscopic diagnoses.

INTERVENTION(S)

Peripheral venous blood sampling and laparoscopic peritoneal fluid collection.

MAIN OUTCOME MEASURE(S)

Flow cytometry was used to assess expression of KIR by NK cells in the cell samples.

RESULT(S)

The percentage of cells that expressed KIR2DL1 among NK (KIR2DL1(+)NK) cells in peritoneal fluid and peripheral blood was significantly higher in women with endometriosis than in controls. The proportion of KIR2DL1(+)NK cells in peripheral blood NK cells before and 1 month after laparoscopic surgery did not differ significantly.

CONCLUSION(S)

The proportion of KIR2DL1(+)NK cells was increased in peritoneal fluid and peripheral blood in women with endometriosis; this difference is probably related to NK cell suppression in endometriosis. This increase in KIR2DL1 expression by NK cells may represent a risk factor in the pathogenesis of endometriosis.

Maturation of antigen-presenting cells is compromised in HLA-G transgenic mice

The human MHC class Ib antigen HLA-G is thought to regulate maternal immune responses during pregnancy. Here we show that expression of HLA-G in transgenic mice diminished cellular immunity by inhibiting maturation of myelomonocytic cells into functional antigen-presenting cells (APC). Skin allografts applied to HLA-G transgenic mice survived longer and resultant T cell responses were less potent compared to control mice. T cells from HLA-G mice responded normally to allogeneic APC and immunohistological analyses of spleen revealed no marked abnormalities. However, spontaneous outgrowths of myeloid cells were observed when bone marrow or splenocytes from HLA-G mice were cultured in vitro, but functionally competent APC did not develop spontaneously in bone marrow cultures supplemented with granulocyte macrophage colony stimulating factor (GM-CSF). Addition of lipopolysaccharide (LPS) to GM-CSF-derived bone marrow cultures rescued APC maturation. Studies using HLA-G tetrameric reagents revealed that HLA-G-specific binding activity was associated with CD11c(+) myelomonocytic cells, while binding to lymphoid and NK cell subsets was undetectable. These data show that spontaneous maturation of functionally competent dendritic cells (DC) is compromised in HLA-G mice. We hypothesize that HLA-G inhibits maturation of DC via receptor-mediated interactions with myelomonocytic precursors, which render immature DC precursors unable to receive signals from activated T cells.

Expression of the CD80 and CD86 molecules enhances cytotoxicity by human natural killer cells

In contrast to the inhibitory pathway of NK cell regulation, much less is known about stimulatory or activation signals in NK cells. Both CD80 and CD86 function as costimulatory molecules in T-cell cytotoxicity. Several previous reports, most of them in the murine system, have indirectly or directly indicated the possible role of B7 molecules (CD80 and CD86) triggering NK cell-mediated cytotoxicity in vitro. Nevertheless, only little is known about the role of these molecules on human target cells. Therefore, anti-CD80 and anti-CD86 mAbs were used in blocking experiments and both were shown to inhibit lysis by human NK cells. The degree of inhibition observed was variable. 64% of these NK clones were strongly inhibited by both anti-CD80 and anti-CD86 (Type 1). A small number (19%) were only moderately inhibited by both of these antibodies (Type 2), and 17% of these NK clones were inhibited strongly by anti-CD86 but weakly or not at all by anti-CD80 (Type 3). To further examine the importance of these proteins, B7.1 (CD80) and B7.2 (CD86) genes were transfected into the mouse mastocytoma P815 cell line that could not be killed by the human NK cells. These transfectant cell lines were then tested in cytotoxicity assays using a number of human NK lines. Expression of the CD80 and CD86 molecules resulted in enhanced lysis of P815 by most of the NK lines tested. Thus, both CD80 and CD86 molecules are involved in triggering of human NK cells.

Melanomas and melanoma cell lines do not express HLA-G, and the expression cannot be induced by gammaIFN treatment

HLA-G is an effective ligand of natural killer (NK) inhibitory receptors, HLA-G transcripts have been detected in several human tumors, and cytokines like gamma interferon (IFN) enable HLA-G molecules to be expressed. These findings are particularly upsetting in case of melanomas: IFN treatment is frequently included in melanoma therapeutic protocols, and downregulation of classical class I molecules occurs in nearly half of these tumors. Therefore, a melanoma cell downregulating classical class I and de novo expressing HLA-G, either constitutively or upon IFN treatment, is probably a stealthy target for the immune system, having inhibited both the cytotoxic T lymphocyte (CTL) and the NK activity. To elucidate this point we have investigated the expression of HLA-G molecules in 45 melanoma cell lines before and after gammaIFN treatment. Analysis was performed by immunofluorescence and flow cytometry, using the anti-HLA-G MoAbs 87G and G233, by Western blot, using the anti-HLA-G MEM/G1 MoAb and PAG1 antiserum, and by RT-PCR analysis. In addition, 8 melanoma tissues from patients free from therapy and 6 nevi were studied by immunohistochemistry using the 87G MoAb. No evidence was gathered of HLA-G expression, neither constitutive nor, in cell lines, after gammaIFN treatment. We therefore conclude that HLA-G expression is an uncommon event in melanomas, and that a therapy including IFNs cannot harm the patient by inducing the de novo expression of HLA-G molecules at least in its G1 isoform.

NK cell recognition of non-classical HLA class I molecules

NK cells recognize several HLA class Ib molecules employing both immunoglobulin-like (Ig-like) and C-type lectin receptors. The CD94/NKG2 and NKG2D lectin-like molecules, respectively, interact with HLA-E and MICA; CD94/NKG2A functions as an inhibitory receptor, while CD94/NKG2C and NKG2D trigger NK cell activity. HLA-E predominantly presents nonamers from the leader sequences of other class I molecules; a peptide derived from HLA-G1 constitutes the highest affinity ligand for both CD94/NKG2 receptors. Members of the Ig-like transcript (ILT) or leucocyte Ig-like receptor (LIR) family (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leucocyte lineages, interact with a broad spectrum of HLA class Ia molecules and HLA-G1. Among Ig-like KIRs, the KIR2DL4 (p49) receptor has been shown to specifically recognize HLA-G1; this molecule displays an unusual hybrid structure, sharing features with inhibitory and triggering KIRs.

Recognition of a Shared Human Prostate Cancer-Associated Antigen by Nonclassical MHC-Restricted CD8+ T Cells

To identify prostate cancer-associated Ags, tumor-reactive T lymphocytes were generated using iterative stimulations of PBMC from a prostate cancer patient with an autologous IFN-γ-treated carcinoma cell line in the presence of IL-2. A CD8+ T cell line and TCR αβ+ T cell clone were isolated that secreted IFN-γ and TNF-α in response to autologous prostate cancer cells but not to autologous fibroblasts or lymphoblastoid cells. However, these T cells recognized several normal and malignant prostate epithelial cell lines without evidence of shared classical HLA molecules. The T cell line and clone also recognized colon cancers, but not melanomas, sarcomas, or lymphomas, suggesting recognition of a shared epithelium-associated Ag presented by nonclassical MHC or MHC-like molecules. Although Ag recognition by T cells was inhibited by mAb against CD8 and the TCR complex (anti-TCR αβ, CD3, Vβ12), it was not inhibited by mAb directed against MHC class Ia or MHC class II molecules. Neither target expression of CD1 molecules nor HLA-G correlated with T cell recognition, but β2-microglobulin expression was essential. Ag expression was diminished by brefeldin A, lactacystin, and cycloheximide, but not by chloroquine, consistent with an endogenous/cytosolic Ag processed through the classical class I pathway. These results suggest that prostate cancer and colon cancer cells can process and present a shared peptidic Ag to TCR αβ+ T cells via a nonclassical MHC I-like molecule yet to be defined.

MHC class Ia molecules alone control NK-mediated bone marrow graft rejection

Mice with functionally deleted genes encoding MHC class I heavy (H-2K(b), H-2D(b)) and light (beta2-microglobulin) chains were used in bone marrow cell transfer experiments to study the role of class Ia and Ib molecules in NK cell function. Absence of H-2K(b) and absence of H-2D(b) on bone marrow cells resulted in complete and in almost complete NK-mediated rejection, respectively. Absence of either H-2 class Ib (at least when expressed in H-2 class Ia-deficient mice) or cell surface class Ia free heavy chains did not result in bone marrow rejection. Thus, in C57BL/6 adult mice, the inactivation of NK cells required for bone marrow cell engraftment relies entirely upon-H-2 class Ia molecules. These results imply the existence of an inhibitory receptor which recognizes either directly or indirectly H-2D(b) molecules and further suggest that in C57BL/6 mice the NK cells which do not express a H-2K(b) specific inhibitory receptor necessarily express an H-2D(b)-specific one.

HLA-G-mediated inhibition of antigen-specific cytotoxic T lymphocytes

In the present study, we demonstrate that the non-classical MHC class I molecule HLA-G impairs specific cytolytic T cell functions in addition to its well-established inhibition of NK lysis. The antigen-specific cytotoxic T lymphocyte (CTL) response analyzed was mediated by CD8(+) T cells specific for the influenza virus matrix epitope, M58-66, presented by HLA-A2. The transfection of HLA-G1 cDNA in target cells carrying the M58-66 epitope reduced their lysis by these virus-specific CTL. This HLA-G-mediated inhibition of antigen-specific CTL lysis was (i) peptide dose dependent, (ii) reversed by blocking HLA-G with a specific mAb and (iii) still observed despite the blockade of HLA-E/CD94/NKG2A interaction. By inhibiting both CTL and NK functions, HLA-G appears to have an extensive role in immune tolerance.

Evolution of MHC-G in primates: a different kind of molecule for each group of species

When MHC-G molecules in primates (New World and Old World monkeys, Anthropoids and humans) were compared phylogenetically, very different evolutionary patterns within each species were found; their molecules did not have a straight forward and linear development throughout the postulated evolutionary pathway of primates. The earlier New World monkeys (South America) had relatively more alleles and the polymorphism was placed in the T-cell receptor (TcR), NK receptors and antigen binding sites; MHC-G probably works as a classical class I presenting molecule in these monkeys. MHC-G intron 2 from New World monkeys does not show the typical 23 bp deletion found in all other more recent primate species. Thus, it is possible that MHC-G molecules in New World monkeys belong to a different lineage than the MHC from higher primates. Another early lineage, Eurasian Old World monkeys, shows stop codons at exon 3: MHC-G proteins lacking the alpha2 domain may functionally suffice or otherwise reading-through stop-codon translational mechanisms may exist, as shown for other genes. Orangutans show lower (but significant) polymorphism than New World monkeys at NK, TcR and antigen binding regions; gorilla and chimpanzee show very low polymorphism. Humans only show three different HLA-G proteins with changes not affecting NK, TcR or antigen binding sites. It is observed that the more exposed the mother to allogeneic fetuses (polygamy), the less polymorphic HLA-G is observed within a given species. The data are concordant with the postulated immune inhibitory function for MHC-G in Old World monkeys, anthropoids and humans both at placental and inflammatory level.

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.

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.