Protection from natural killer cell-mediated lysis by HLA-G expression on target cells

HLA class I antigen downregulation in human cancers: T-cell immunotherapy revives an old story

Human leukocyte antigen (HLA) class I molecule downregulation occurs frequently in many cancers, and this abnormality might adversely affect the clinical course of cancer and the outcome of T-cell-based immunotherapy. Mutations in the HLA class I genes themselves, abnormalities in their regulation and/or defects in HLA class I-dependent antigen processing can underlie HLA class I downregulation. These mutations modulate the susceptibility of tumor cells to in vitro lysis by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Immune selection of CTL- and NK-cell-resistant tumor cells might explain the rapid progression and poor prognosis of cancers that exhibit HLA class I downregulation. These findings provide compelling evidence that HLA class I downregulation represents a significant challenge for the successful application of T-cell-based immunotherapy of cancer.

Tissue distribution of the mRNA for a rhesus monkey major histocompatibility class Ib molecule, Mamu-AG

We identified recently a novel major histocompatibility complex (MHC) class I locus in the rhesus monkey, Mamu-AG, which is expressed in the placenta and encodes molecules that share unique characteristics of human HL4-G. We established locus-specific reverse transcription-polymerase chain reaction (RT-PCR) and ribonuclease protection assays to determine whether Mamu-AG is expressed in other rhesus monkey tissues. With an RT-PCR assay, Mamu-AG mRNA was detected in placenta, amniotic membranes, kidney, spleen, eye, brain, lung, spinal cord, liver and occasionally heart, but was undetectable in lymph nodes, salivary glands, peripheral blood lymphocytes (PBL), large and small intestine, skeletal muscle or skin. Examination of endocrine organs demonstrated the presence of Mamu-AG transcripts in pituitary, testes, ovary and adrenal glands but not in pancreas or thyroid. Quantitative analysis using a ribonuclease protection assay demonstrated that the highest level of Mamu-AG mRNA expression was consistently in the placenta and amniotic membranes, while expression was moderate in a few tissues (testis, adrenal) and low to undetectable in all other tissues. These results suggest that the Mamu-AG mRNA, like the mRNA for the human MHC class Ib gene HLA-G, is expressed at high levels in the placenta, but also has restricted low-level expression in other tissues.

Conservation and diversification of MHC class I and its related molecules in vertebrates

The elucidation of the complete peptide-binding domains of the highly polymorphic shark MHC class I genes offered us an opportunity to examine the characteristics of their predicted protein products in the light of the latest advance in the structural studies of the MHC class I molecules. The results suggest that the fundamental characteristics in the T-cell recognition of the MHC class I molecule/peptide complex are expected to have been established at the early stage of the vertebrate evolution. The elucidation of the typical classical class I molecules from fishes and also of some MHC class I-related molecules may help us-to explore the common denominator of the ancient class I molecules.

The expression of human leukocyte antigen-G on trophoblasts abolishes the growth-suppressing effect of interleukin-2 towards them

PROBLEM

We have shown the attenuated human leukocyte antigen (HLA)-G expression on trophoblasts and an aberrant expression of interleukin (IL)-2, a cytotoxic cytokine, in decidual tissue in preeclampsia, where deteriorated trophoblastic invasion into decidual layers may constitute a crucial pathogenesis. We hypothesized that the absence of HLA-G might make trophoblasts susceptible to compromise by IL-2.

METHOD OF STUDY

We analyzed the growth of HLA-G-negative and positive cell lines, all of which possessed IL-2 receptors, in the culture with or without IL-2 supplementation.

RESULTS

The proliferation of HLA-G positive trophoblastic cell lines (BeWo and JEG-3) was not influenced by the addition of IL-2, whereas a HLA-G-negative trophoblastic cell line (JAR) exhibited significantly decreased proliferation when cultured with IL-2. Interestingly, the transfection of JAR cells with HLA-G completely eliminates the growth-inhibitory effect of IL-2.

CONCLUSION

The expression of HLA-G may commit trophoblasts to evade cell damage by IL-2, which may be relevant to maternal tolerance of the fetus during pregnancy and its derangement as exemplified by preeclampsia.

Functions of nonclassical MHC and non-MHC-encoded class I molecules

Fascinating recent discoveries have focused attention on the nonclassical class I molecules. They can exert their function at most levels of the immune response, being part of both innate and adaptive immune systems. They not only have specialized antigen-presentation functions but also play important immunoregulatory roles: HLA-E regulates natural killer cells by interacting with CD94/NKG2 receptors; the MIC (MHC class I chain related) glycoproteins appear crucial to the activation of gammadelta T cells in the gastrointestinal epithelium; HLA-G may play a role in controlling the immune response to the fetus; and CD1 molecules are important in defense against bacterial infections, as well as in the development and regulation of a subset of NKT cells expressing a highly restricted TCR repertoire; however not all nonclassical class I molecules have an immunological function, as demonstrated by HFE which is implicated in iron metabolism.

Cell surface expression of HLA-E: interaction with human beta2-microglobulin and allelic differences

The formation of a trimeric complex composed of MHC class I heavy chain, beta2-microglobulin (beta2m) and peptide ligand is a prerequisite for its efficient transport to the cell surface. We have previously demonstrated impaired intracellular transport of the human class Ib molecule HLA-E in mouse myeloma X63 cells cotransfected with the genes for HLA-E and human beta2m (hbeta2m), which is most likely attributable to inefficient intracellular peptide loading of the HLA-E molecule. Here we demonstrate that cell surface expression of HLA-E in mouse cells strictly depends on the coexpression of hbeta2m and that soluble empty complexes of HLA-E and hbeta2m display a low degree of thermostability. Both observations imply that low affinity interaction of HLA-E with beta2m accounts to a considerable extent for the observed low degree of peptide uptake in the endoplasmic reticulum. Moreover, we show that the only allelic variation present in the caucasoid population located at amino acid position 107 (Gly or Arg) greatly affects intracellular transport and cell surface expression upon transfection of the respective alleles into mouse cells. No obvious difference was found with regard to the sequence of the peptide ligand.

HLA-G expression protects porcine endothelial cells against natural killer cell-mediated xenogeneic cytotoxicity

BACKGROUND

Natural killer (NK) cells are major component of the cellular response in xenotransplantation. NK cell activation and NK cell-mediated cytotoxicity can be a direct barrier to the potential use of xenogeneic organs in human transplantation.

METHODS

To determine if HLA-G would protect porcine xenogeneic cells from human NK cell lysis, human full-length HLA-G genomic DNA was transfected into porcine aortic endothelial cell (PAECs) by the lipofection method. Surface expression of HLA-G in transfected PAECs was confirmed by immunofluorescense staining with anti-HLA class I framework antibody, PA2.6. Fresh human peripheral blood lymphocytes were used as NK effector cells with HLA-G-transfected PAECs as targets in a 51Cr release assay. The inhibition of human polyclonal NK cells by HLA-G expression on PAECs was confirmed by antibody blocking using purified F(ab')2 portion of anti-human HLA class I antibody PA2.6.

RESULTS

Expression of HLA-G on PAECs conferred a significant protection against NK-mediated lysis (range: 52-100% inhibition) when peripheral blood lymphocytes from seven healthy donors, bearing either homozygous HLA-Cw3 or -Cw4 used as NK effector cells. Such protection was inhibited by purified F(ab')2 portion of anti-HLA class I antibody, indicating that the protection of PAECs was directly mediated by HLA-G expression.

CONCLUSIONS

Expression of HLA-G on PAECs protected xenogeneic PAECs against human polyclonal NK cell-mediated lysis. These results indicate that the expression of HLA-G alone in the absence of other nonclassical HLA class I molecules is sufficient to inhibit human NK cell lysis. These findings suggest methods utilizing HLA-G to overcome NK cell-mediated cytotoxicity against porcine endothelial cells, considered the first cell type effected during xenograft cellular rejection.

IL-10 is an autocrine inhibitor of human placental cytotrophoblast MMP-9 production and invasion

During human placentation, fetal cytotrophoblast stem cells differentiate and then invade the uterine wall and its associated spiral arteries. This process anchors the placenta to the uterus and supplies maternal blood to the fetus. Cytotrophoblast invasion in vitro requires the expression of matrix metalloproteinase-9 (MMP-9). Recently, we showed that cytotrophoblasts produce interleukin-10 (IL-10), a potent immunomodulatory cytokine that could have paracrine effects on the maternal immune system. IL-10 synthesis is dramatically downregulated after the first 12 h of culture, while MMP-9 secretion is rapidly upregulated and the cells acquire an invasive phenotype. These observations prompted us to investigate whether IL-10 is an autocrine regulator of cytotrophoblast MMP-9 production. We found that the cells expressed IL-10 receptor mRNA, suggesting that autocrine effects are possible. Adding recombinant IL-10 to cytotrophoblast cultures significantly decreased the cells' MMP-9 expression at both protein and mRNA levels, but did not affect mRNA levels of the tissue inhibitor of metalloproteinase-3. Thus, IL-10 may alter the proteinase/inhibitor balance. IL-10 treatment further caused a net decrease in MMP activity, thereby reducing cytotrophoblast invasiveness. An antibody that neutralized endogenous IL-10 function had the opposite effect in all experiments. Together, these data suggest that IL-10 is an autocrine inhibitor of cytotrophoblast MMP-9 activity and invasiveness.

The ILT2(LIR1) and CD94/NKG2A NK cell receptors respectively recognize HLA-G1 and HLA-E molecules co-expressed on target cells

Previous studies on NK recognition of HLA-G1 employed as targets 721.221 transfectants (.221-G1) that unknowingly co-expressed the HLA-E molecule, subsequently found to be a major ligand for the CD94/NKG2 receptors. In the present study we re-evaluated the relative role played by CD94/NKG2 and ILT2(LIR1) molecules in recognition of HLA-G1 by NK clones. We employed as targets .221-G1 cells and a surface HLA-E-negative transfectant, .221-G1(Eneg), generated by site-directed mutagenesis of the HLA-G1 leader sequence. The antagonistic effects of receptor- (ie. CD94/NKG2A, ILT2) and ligand-specific mAb (i.e. HLA-G, HLA-E) were assessed. In addition, binding of an ILT2-Ig fusion protein to the .221-AEH, expressing only HLA-E, and the .221-G1(Eneg) transfectants was analyzed. Our data demonstrate that NK recognition of cells expressing HLA-G1 involves at least two non-overlapping receptor-ligand systems: the CD94/NKG2 interaction with HLA-E, and the engagement of the ILT2(LIR1) receptor by HLA-G1 molecules.

Specific binding of nuclear factors to the HLA-G gene promoter correlates with a lack of HLA-G transcripts in first trimester human fetal liver

The nonclassical MHC class I HLA-G antigen is expressed in cytotrophoblasts during pregnancy and may play a role in inhibiting lysis by maternal natural killer cells. HLA-G gene transcription was analyzed in human fetal liver of 6-8 wk of gestation, a development stage where classical HLA class I expression is very reduced. We demonstrated that HLA-G transcription is undetectable in these cells and we investigated the molecular mechanisms that control the lack of HLA-G gene transcription. We compared protein interactions of nuclear extracts from first trimester fetal livers, YT2C2-PR (HLA-G negative) and JEG-3 (HLA-G positive) cell lines to a 244-bp EcoR I/Hind III DNA region located 1.2 kb from the HLA-G gene, previously shown to direct HLA-G expression in transgenic mouse placenta. A strong specific C7-factor was specifically detected in first trimester fetal liver that could account for the inhibition of HLA-G transcription. Interaction of C7-factor and cell-specific factors previously detected in YT2C2 cell line (C5, C6) with two distinct regulatory regions identify this 244-bp EcoR I/Hind III fragment as a putative target for inhibition of HLA-G transcription.

Expanded genomic organization of conserved mammalian MHC class I-related genes, human MR1 and its murine ortholog

MR1 is a major histocompatibility complex (MHC) class I-related gene located outside the human MHC. Among several divergent class I molecules, the predicted MR1 molecule is closest, in the alpha1 and alpha2 domains, to the class I group to which the vertebrate classical class I molecules belong. We report here the genomic organizations of the human MR1 and mouse Mr1 genes. Both genes exhibit genomic structures largely similar to those of the MHC class I genes. However, they are highly expanded in their scale in contrast to the classical MHC class I genes. Inclusion of transposable elements into introns seems to partly contribute to these genomic structures. Several other MHC class I-related genes also show relatively large genomic structures. The present study extended heterogeneity in the genomic organization among the class I gene family by revealing a highly expanded structure of the human MR1 gene and its murine ortholog.

HLA-G polymorphisms: ethnic differences and implications for potential molecule function

PROBLEM

Human leukocyte antigen (HLA)-G is uniquely expressed on extravillous cytotrophoblasts of the placenta and is postulated to be a mediator of maternal immune tolerance. Although it was originally considered to be nonpolymorphic, variations of the HLA-G DNA sequence have been reported, and a limited number of HLA-G alleles been defined.

METHOD OF STUDY

The HLA-G wild-type sequence was compared with HLA-A2 with regard to the conservation of functionally essential parts of classical HLA-I molecules. HLA-G polymorphisms were analyzed under the aspect of ethnic differences, site, and consequences for postulated molecule functions.

RESULTS

HLA-G exhibits a high degree of conservation relative to HLA-A2 in functionally relevant sites of HLA-class I molecules. However, polymorphic sites in HLA-G and classical HLA loci are not congruent.

CONCLUSION

The type and localization of HLA-G polymorphisms suggest that different parts of HLA-G molecule underlie different selective constraints.

Expression of allogeneic MHC class I antigens by transgenic mouse trophoblast does not interfere with the normal course of pregnancy

Mammalian embryos express paternal histocompatibility antigens which make them potential targets for maternal immune responses. Yet, the histoincompatible fetus survives and develops normally. Down regulation of classical MHC antigen expression by trophoblast cells which are in direct contact with maternal circulation has been repeatedly shown. The trophoblast cells are unable to function properly in antigen presentation and do not induce allogeneic rejection reactions. In the present study we have created transgenic mice that express an allogeneic class I transgene whose transcription is controlled by the transferrin receptor promoter. The expression patterns of the transgene product mice from a single transgenic line were studied in each of the typical placental subpopulations. The allogeneic class I antigen was expressed in the allantoic plate region of the trophoblast, and this expression was not restricted to the endothelial region but extended also to the spongiotrophoblast, as well as the major blood vessels and in the endodermal sinuses. In contrast to the normal class I expression, prominent levels of allogeneic H-2 antigens were detected in the labyrinthine trophoblast. The fetal resorption rate in females mated with these transgenic males was not higher then the normal rate, and the embryos survived and developed normally. These data imply that the unusual expression of allogeneic class I antigens in certain trophoblast subpopulations does not affect fetal development.

Unique biochemical properties of human leukocyte antigen-E allow for a highly specific function in immune recognition

PROBLEM

Does a correlation exist between the expression of human leukocyte antigen (HLA) class Ia and HLA-E and what is its biological significance?

METHOD OF STUDY

HLA-E transcripts were detected by reverse transcriptase-polymerase chain reaction. Metabolically labeled HLA-E heavy chains were immunoprecipited and analyzed by one-dimensional isoelectric focusing. Mouse RMA-S cells defective with regard to transporter associated with antigen processing (TAP) function were transfected with HLA-E and human beta 2-microglobulin to investigate TAP dependence of the cell-surface expression of HLA-E.

RESULTS

HLA-E is transcribed regardless of the down-regulation of polymorphic HLA class Ia expression. HLA-E is transported to the cell surface in the absence of TAP-controlled peptide loading. In human cells, the amount of HLA-E protein is very low regardless of the presence of correct peptide ligands.

CONCLUSIONS

HLA-E regulates immune functions in cells that have down-regulated the expression of polymorphic HLA-class Ia molecules, either by preventing harmful natural killer cells from attacking targets that have physiologically decreased HLA-class Ia expression or by activating effector cells against virus-infected and tumor cells with impaired HLA-class Ia expression.

HLA-E-bound peptides influence recognition by inhibitory and triggering CD94/NKG2 receptors: preferential response to an HLA-G-derived nonamer

The HLA-E class Ib molecule constitutes a major ligand for the lectin-like CD94/NKG2 natural killer (NK) cell receptors. Specific HLA class I leader sequence-derived nonapeptides bind to endogenous HLA-E molecules in the HLA-defective cell line 721.221, inducing HLA-E surface expression, and promote CD94/NKG2A-mediated recognition. We compared the ability of NK clones which expressed either inhibitory or activating CD94/NKG2 receptors to recognize HLA-E molecules on the surface of 721.221 cells loaded with a panel of synthetic nonamers derived from the leader sequences of most HLA class I molecules. Our results support the notion that the primary structure of the HLA-E-bound peptides influences CD94/ NKG2-mediated recognition, beyond their ability to stabilize surface HLA-E. Further, CD94/ NKG2A+ NK clones appeared more sensitive to the interaction with most HLA-E-peptide complexes than did effector cells expressing the activating CD94/NKG2C receptor. However, a significant exception to this pattern was HLA-E loaded with the HLA-G-derived nonamer. This complex triggered cytotoxicity very efficiently over a wide range of peptide concentrations, suggesting that the HLA-E/G-nonamer complex interacts with the CD94/NKG2 triggering receptor with a significantly higher affinity. These results raise the possibility that CD94/NKG2-mediated recognition of HLA-E expressed on extravillous cytotrophoblasts plays an important role in maternal-fetal cellular interactions.

Is the expression of classical HLA class I antigens on trophoblast of importance for human pregnancy?

PROBLEM

Human leukocyte antigen (HLA)-C and possibly also HLA-B seem to be expressed on the extravillous trophoblast. These antigens carry epitopes that function as ligands for natural killer (NK)-cell-inhibitory receptors. Antitrophoblast cytotoxicity mediated by decidual NK cells might be involved in miscarriage. We thus found it relevant to elucidate whether parental HLA-C and -Bw polymorphism play a role in recurrent miscarriage (RM).

METHOD OF STUDY

HLA-C and -Bw investigations by DNA-based techniques were undertaken in 35 couples with unexplained RM and in 30 couples with normal fecundity. The number of HLA-C- and -Bw-related supertypic specificities that can bind NK-cell-inhibitory receptors was evaluated in selected couples.

RESULTS

The proportions of couples with RM and control couples carrying four HLA-C alleles with the same NK-cell-inhibitory supertypic specificities were equal. In 46% of studied couples with RM, all four HLA-B alleles carried the HLA-Bw6 supertypic specificity, which was significantly higher than the corresponding frequency (17%) in the control couples (P < 0.02).

CONCLUSIONS

The expression of polymorphic HLA-C on trophoblasts does not seem to play a role in RM. Assuming that HLA-B is expressed on trophoblasts, we may suggest that the revealed predominance of HLA-Bw6 expression (which excludes the presence of HLA-Bw4-protective antigens) may predispose a particular couple to the RM phenomenon.

Trophoblast class I major histocompatibility complex (MHC) products are resistant to rapid degradation imposed by the human cytomegalovirus (HCMV) gene products US2 and US11

US11 and US2 encode gene products expressed early in the replicative cycle of human cytomegalovirus (HCMV), which cause dislocation of human and murine major histocompatibility complex (MHC) class I molecules from the lumen of the endoplasmic reticulum to the cytosol, where the class I heavy chains are rapidly degraded. Human histocompatibility leukocyte antigens (HLA)-C and HLA-G are uniquely resistant to the effects of both US11 and US2 in a human trophoblast cell line as well as in porcine endothelial cells stably transfected with human class I genes. Dislocation and degradation of MHC class I heavy chains do not appear to involve cell type-specific factors, as US11 and US2 are fully active in this xenogeneic model. Importantly, trophoblasts HLA-G and HLA-C possess unique characteristics that allow their escape from HCMV-associated MHC class I degradation. Trophoblast class I molecules could serve not only to block recognition by natural killer cells, but also to guide virus-specific HLA-C- and possibly HLA-G-restricted cytotoxic T-lymphocytes to their targets.

Soluble HLA-G in human placentas: synthesis in trophoblasts and interferon-gamma-activated macrophages but not placental fibroblasts

The HLA class Ib antigen, HLA-G, is highly expressed in early gestation placentas where it is believed to modulate maternal-fetal immunological interactions. In this study, soluble isoforms (sHLA-G) encoded by intron 4-retaining transcripts were identified in first trimester placentas by immunohistochemistry using a mAb specific for the C-terminus of sHLA-G. Immunoreactive sHLA-G protein was localized to trophoblast cells and to villous mesenchymal cells with the morphological features of macrophages. Reverse transcriptase polymerase chain reaction analysis which used primers specific for intron 4 and the 3' untranslated region of the HLA-G gene showed that transcripts encoding sHLA-G were present in the trophoblast-derived Jeg-3 cells as well as interferon-gamma-activated myelomonocytic U937 cells but were absent and uninducible in placental fibroblasts. These results indicate that placental sHLA-G is synthesized in trophoblast cells and activated placental macrophages and support the postulate that placenta-derived sHLA-G modulates maternal and fetal immune cell functions during pregnancy.

HLA-G Isoforms Produced by Placental Cytotrophoblasts and Found in Amniotic Fluid Are Due to Unusual Glycosylation

The human placenta expresses HLA-G, a nonclassical (class Ib) MHC molecule that could play a central role in maternal tolerance of the semiallogeneic fetus. In this work, we report the production of a new mAb, 4H84, that specifically reacts with HLA-G in two formats: immunocytochemistry and immunoblotting. Immunolocalization experiments with 4H84 confirmed our previous finding that cytotrophoblasts within the uterine wall are the only cells in tissue sections of placenta that express the HLA-G protein. Additional experiments showed that both amniocytes and cytotrophoblasts in the amnion-chorion express this protein. Since multiple HLA-G transcripts have been described, we used immunoblotting to study the HLA-G isoforms produced by cytotrophoblasts in vitro and by the amnion-chorion in vivo. Cytotrophoblasts, their conditioned medium, and amniotic fluid samples contained heterodisperse immunoreactive bands (Mr 35,000–50,000). N-deglycosylation by peptide-N-glycosidase F digestion resolved these isoforms into two distinct bands. Cell samples contained primarily an Mr 37,000–42,000 protein, most likely encoded by the full-length mRNA. Conditioned medium and amniotic fluid contained a slightly smaller protein, most likely the secreted form lacking the transmembrane and cytoplasmic regions. Removal of polylactosamine chains by endo-β d-galactosidase digestion significantly reduced the electrophoretic mobility of the immunoreactive bands, suggesting that HLA-G, unlike class Ib molecules studied to date, carries N-acetyllactosamine units. These data show that Mr heterogeneity of HLA-G is due to its novel glycosylation, rather than to the translation of alternatively spliced mRNAs. We postulate that the unusual carbohydrate structures this molecule carries could interact with maternal immune cells and/or stabilize the molecule.

Structure and function of the human MHC class Ib molecules HLA-E, HLA-F and HLA-G

The major histocompatibility (MHC) class Ib molecules HLA-E, HLA-F and HLA-G are relatively non-polymorphic compared to class Ia molecules. Both HLA-E and HLA-G bind peptides and are involved in natural killer (NK)-cell recognition, but the role of HLA-F is unclear. HLA-E binds specifically to the conserved leader sequence peptides from the class Ia MHC molecules and interacts on the cell surface with the CD94/NKG2 class of NK-cell receptors. The framework structure of HLA-E is similar to that of the MHC class Ia molecules, but the peptide-binding groove is highly adapted for the specific binding of the leader sequence peptides. This is different from class Ia molecules, which have highly promiscuous peptide-binding grooves. The HLA-E groove makes full use of all the available pockets and imposes specificity along the entire length of the peptide. HLA-G binds nonamer peptides with leucine or isoleucine at position 2, proline at position 3 and leucine at position 9. Expression of HLA-G inhibits NK cells expressing the CD94/NKG2 class of receptors, though an interaction with these receptors has not been directly demonstrated.

NK cell receptors

NK cells are regulated by opposing signals from receptors that activate and inhibit effector function. While positive stimulation may be initiated by an array of costimulatory receptors, specificity is provided by inhibitory signals transduced by receptors for MHC class I. Three distinct receptor families, Ly49, CD94/NKG2, and KIR, are involved in NK cell recognition of polymorphic MHC class I molecules. A common pathway of inhibitory signaling is provided by ITIM sequences in the cytoplasmic domains of these otherwise structurally diverse receptors. Upon ligand binding and activation, the inhibitory NK cell receptors become tyrosine phosphorylated and recruit tyrosine phosphatases, SHP-1 and possibly SHP-2, resulting in inhibition of NK cell-mediated cytotoxicity and cytokine expression. Recent studies suggest these inhibitory NK cell receptors are members of a larger superfamily containing ITIM sequences, the inhibitory receptor superfamily (IRS).

HLA-E Surface Expression Depends on Binding of TAP-Dependent Peptides Derived from Certain HLA Class I Signal Sequences

Previous studies showed that HLA-E was expressed in lymphoblastoid cell line (LCL) 721.221 cells, but surface expression was lacking. To determine the signals controlling surface expression, we constructed a series of hybrid genes using complementary portions derived from the HLA-E and HLA-A2 genes. In this manner, a hybrid of HLA-E was identified, designated AEH, which differed from HLA-E by having the HLA-A2 signal sequence substituting for the HLA-E leader peptide. Transfection of LCL 721.221 cells with AEH induced HLA-E surface expression. Analysis of peptides bound to HLA-E revealed that a nonamer peptide derived from the A2 signal sequence was the predominant peptide bound. LCL 721.221 cells transfected with certain class I genes, including HLA-G, were also sufficient to promote peptide binding and HLA-E surface expression without increasing the level of HLA-E heavy chain synthesis. Peptides bound to HLA-E consisted of nine amino acids, with methionine at position 2 and leucine in the carboxyl-terminal position, and were nearly identical to the leader sequence-derived peptide previously shown to be a predominant peptide bound to the murine Qa-1 Ag. Signal peptides derived from certain HLA-B proteins with threonine in position 2 only marginally up-regulated HLA-E surface expression in .221 cells. An examination of HLA-E peptide binding in the TAP negative cell line .134 indicated that peptide binding to HLA-E was dependent on a functional TAP heterodimer regardless of whether peptide was available in cis, as in the AEH construct, or in trans, as in the class I transfectants of .221 cells.

HLA-G polymorphisms and allele frequencies in Caucasians

HLA-G, a nonclassical class I MHC molecule, is uniquely expressed on extravillous cytotrophoblasts of the maternal-fetal interface and is suggested to be essential for establishment of maternal-fetal immune tolerance. Although the level of polymorphism in HLA-G has originally been considered low, number, nature and site of polymorphisms seem to vary between different ethnic populations. We investigated HLA-G polymorphisms in a population of German and Croatian origin by SSCP-analysis and direct sequencing as well as RFLP analysis for presence of the 1597delC mutation. HLA-A alleles associated with the different HLA-G alleles were determined by SSP PCR-typing. In Caucasians, HLA-G exhibits a low degree of polymorphism on the amino-acid level and only slightly higher variability on the nucleotide level. In 264 independent chromosomes, 4 HLA-G alleles on the level of amino acid polymorphisms and an additional 6 variations of nucleotide sequences could be identified. The null-allele G*0105N was present at an allele frequency of 2.3%, which is higher than initially suggested for Caucasians but lower than in Hispanics and African-Americans. Furthermore, some HLA-G alleles exhibit strong linkage disequilibrium with HLA-A.

HLA-E is a major ligand for the natural killer inhibitory receptor CD94/NKG2A

We previously showed that the availability of a nonamer peptide derived from certain HLA class I signal sequences is a necessary requirement for the stabilization of endogenous HLA-E expression on the surface of 721.221 cells. This led us to examine the ability of HLA-E to protect HLA class I transfectants from natural killer (NK) cell-mediated lysis. It was possible to implicate the CD94/NKG2A complex as an inhibitory receptor recognizing this class Ib molecule by using as target a .221 transfectant selectively expressing surface HLA-E. HLA-E had no apparent inhibitory effect mediated through the identified Ig superfamily (Ig-SF) human killer cell inhibitory receptors or ILT2/LIR1. Further studies of CD94/NKG2+ NK cell-mediated recognition of .221 cells transfected with different HLA class I allotypes (i.e., -Cw4, -Cw3, -B7) confirmed that the inhibitory interaction was mediated by CD94/NKG2A recognizing the surface HLA-E molecule, because only antibodies directed against either HLA-E, CD94, or CD94/NKG2A specifically restored lysis. Surface stabilization of HLA-E in cold-treated .221 cells loaded with appropriate peptides was sufficient to confer protection, resulting from recognition of the HLA class Ib molecule by the CD94/NKG2A inhibitory receptor. Consistent with the prediction that the ligand for CD94/NKG2A is expressed ubiquitously, our examination of HLA-E antigen distribution indicated that it is detectable on the surface of a wide variety of cell types.

HLA-G expression in melanoma: a way for tumor cells to escape from immunosurveillance

Considering the well established role of nonclassical HLA-G class I molecules in inhibiting natural killer (NK) cell function, the consequence of abnormal HLA-G expression in malignant cells should be the escape of tumors from immunosurveillance. To examine this hypothesis, we analyzed HLA-G expression and NK sensitivity in human malignant melanoma cells. Our analysis of three melanoma cell lines and ex vivo biopsy demonstrated that (i) IGR and M74 human melanoma cell lines exhibit a high level of HLA-G transcription with differential HLA-G isoform transcription and protein expression patterns, (ii) a higher level of HLA-G transcription ex vivo is detected in a skin melanoma metastasis biopsy compared with a healthy skin fragment from the same individual, and (iii) HLA-G protein isoforms other than membrane-bound HLA-G1 protect IGR from NK lysis. It thus appears of critical importance to consider the specific role of HLA-G expression in tumors in the design of future cancer immunotherapies.

Recognition of human histocompatibility leukocyte antigen (HLA)-E complexed with HLA class I signal sequence-derived peptides by CD94/NKG2 confers protection from natural killer cell-mediated lysis

Human histocompatibility leukocyte antigen (HLA)-E is a nonclassical HLA class I molecule, the gene for which is transcribed in most tissues. It has recently been reported that this molecule binds peptides derived from the signal sequence of HLA class I proteins; however, no function for HLA-E has yet been described. We show that natural killer (NK) cells can recognize target cells expressing HLA-E molecules on the cell surface and this interaction results in inhibition of the lytic process. Furthermore, HLA-E recognition is mediated primarily through the CD94/NKG2-A heterodimer, as CD94-specific, but not killer cell inhibitory receptor (KIR)-specific mAbs block HLA-E-mediated protection of target cells. Cell surface HLA-E could be increased by incubation with synthetic peptides corresponding to residues 3-11 from the signal sequences of a number of HLA class I molecules; however, only peptides which contained a Met at position 2 were capable of conferring resistance to NK-mediated lysis, whereas those having Thr at position 2 had no effect. Interestingly, HLA class I molecules previously correlated with CD94/NKG2 recognition all have Met at residue 4 of the signal sequence (position 2 of the HLA-E binding peptide), whereas those which have been reported not to interact with CD94/NKG2 have Thr at this position. Thus, these data show a function for HLA-E and suggest an alternative explanation for the apparent broad reactivity of CD94/NKG2 with HLA class I molecules; that CD94/NKG2 interacts with HLA-E complexed with signal sequence peptides derived from "protective" HLA class I alleles rather than directly interacting with classical HLA class I proteins.

HLA-G1 protein expression is not essential for fetal survival

HLA-G is a nonclassical, class I HLA gene that is primarily expressed by fetal cells at the maternal-fetal interface and is thought to play a key role in the induction of tolerance in pregnancy. This paper reports the identification of a single base pair deletion at position 1597 (1597delC) in exon 3 (encoding the alpha2-domain) of HLA-G on 20 of 272 (7.4 per cent) African American chromosomes, three of 102 (2.9 per cent) Hispanic chromosomes, and none of 134 Caucasian chromosomes. This relatively common frameshift mutation results in amino acid substitutions in all of the residues in the second half of exon 3 including the conserved cysteine at codon 164. An adult individual was identified who was homozygous for this 'null' allele, and a first trimester placenta that was homozygous for 1597delC had no detectable HLA-G1 protein. These data indicate that expression of HLA-G1 protein is not essential for fetal survival.

NK Cell Natural Cytotoxicity and IFN-γ Production Are Not Always Coordinately Regulated: Engagement of DX9 KIR+ NK Cells by HLA-B7 Variants and Target Cells

DX9 mAb-binding killer cell-inhibitory receptors (KIR) recognize HLA-B molecules that express the Bw4 public serologic epitope. We assessed DX9+ NK cell fine specificity recognition of HLA-B7 variants and HLA-B27 alleles by 51Cr release natural cytotoxicity assays and by flow cytometry and enzyme-linked immunospot (ELISPOT) IFN-γ synthesis and release assays. 721.221 target cell expression of Bw4+ HLA-B27 alleles specifically inhibited DX9+ NK cell natural cytotoxicity and IFN-γ synthesis and release. A triple substitution of HLA-B7 at residues 80, 82, and 83 known to induce expression of the Bw4 serologic epitope also specifically inhibited DX9+ NK cell natural cytotoxicity and IFN-γ responses. Single HLA-B7 amino acid substitution variants were recognized in the same decreasing rank order by DX9+ NK cells and Bw4-reactive mAbs: G83R &gt; R82L &gt; N80T = HLA-B7. Natural cytotoxicity inhibition was reversed by the presence of blocking DX9 mAb. Natural cytotoxicity and IFN-γ production were coordinately regulated by a panel of HLA-B7 variants expressed on 721.221 cells, suggesting that these two effector functions are inhibited by the same KIR-mediated signaling mechanisms. In contrast, some NK cell clones killed 721.221 and K562 target cells equally well but released much more IFN-γ in response to K562 target cells. Differential regulation of natural cytotoxicity and IFN-γ release shows that NK cell effector functions respond to distinct signals.

Co-dominant expression of the HLA-G gene and various forms of alternatively spliced HLA-G mRNA in human first trimester trophoblast

Genes may be silenced at the transcriptional level by 'genomic imprinting' in such a way that only one of the parental alleles is expressed. Imprinting may be tissue-specific and in some cases it seems also to be time-dependent during development. The phenomenon has been studied in pre- and post-implantation developmental processes. Animal studies of genomic imprinting of major histocompatibility complex (MHC) antigens in the placenta have shown discordant results. To address this issue in the human placenta, we examined the expression of the non-classical human leukocyte antigen (HLA) class I gene, HLA-G. Genomic imprinting of the HLA-G locus could have implications for the interaction in the feto-maternal relationship. Restriction Fragment Length Polymorphism (RFLP), allele-specific amplification and Single Strand Conformation Polymorphism (SSCP) analysis followed by DNA sequencing were performed on Reverse Transcription (RT) Polymerase Chain Reaction (PCR) products of HLA-G mRNA to examine the expression of maternal and paternal alleles. Our results demonstrate that HLA-G is co-dominantly expressed in first trimester trophoblast cells. A "new" non-synonymous base substitution in exon 4 was detected. We also investigated the different alternatively spliced forms of HLA-G mRNA in first trimester trophoblast and found the full-length transcript to be the far most abundant.

Why certain antibodies cross-react with HLA-A and HLA-G: epitope mapping of two common MHC class I reagents

Antigen presentation at the maternal-fetal interface has been characterized by a reported lack of classical MHC class I products and the presence of a tissue-restricted, non-classical class I product with limited polymorphism, HLA-G. The lack of HLA-A, -B, and -C products at this interface would allow escape from T-cell mediated attack, while the presence of HLA-G may enable evasion of NK cell-mediated destruction. We provide evidence that in addition to HLA-G, the classical class I product HLA-C is also present in trophoblast. Specifically, cDNA from the trophoblast-derived JEG 3 cell line encodes the HLA-C-locus product, HLA-Cw*0401. This protein, obtained by in vitro transcription/translation, has biochemical characteristics identical to MHC class I products immunoprecipitated directly from the same cells. These findings are in agreement with RNA analysis and immunohistochemistry on both cell lines and primary trophoblast tissues. We report here the preferential reactivity in JEG 3 cells of two widely used monoclonal anti-MHC class I heavy chain antibodies, HC10 and HCA2, with HLA-C and HLA-G, respectively. We have mapped the epitopes recognized by these reagents to distinct areas of the alpha1 domain of the MHC class I heavy chain. HCA2 recognizes the motif xLxTLRGx spanning amino acids 77 84 present in both HLA-A and HLA-G. In contrast, HC10 may recognize a discontinuous epitope, with essential elements of the recognized motif surrounding residue 60 in the alpha1 domain of the class I heavy chain, as shown by truncation analysis. These results adequately explain the immunochemical cross-reactivity of HLA-A and HLA-G.

Current concepts in mucosal immunity. II. One size fits all: nonclassical MHC molecules fulfill multiple roles in epithelial cell function

The human major histocompatibility complex (MHC) on chromosome 6 encodes three classical class I genes: human leukocyte antigen-A (HLA-A), HLA-B, and HLA-C. These polymorphic genes encode a 43- to 45-kDa cell surface glycoprotein that, in association with the 12-kDa beta 2-microglobulin molecule, functions in the presentation of nine amino acid peptides to the T cell receptor of CD8-bearing T lymphocytes and killer inhibitory receptors on natural killer cells. In addition to these ubiquitously expressed polymorphic proteins, the human genome also encodes a number of nonclassical MHC class I-like, or class Ib, genes that in general encode nonpolymorphic molecules involved in a variety of specific immunologic functions. Many of these genes, including CD1, the neonatal Fc receptor for immunoglobulin G, HLA-G, the MHC class I chain-related gene A, and Hfe, are prominently displayed on epithelial cells, suggesting an important role in epithelial cell biology.

Multiple receptors for HLA-G on human natural killer cells

HLA-G is the putative natural killer (NK) cell inhibitory ligand expressed on the extravillous cytotrophoblast of the human placenta. Killing of the class I negative human B cell line 721.221 by NK cells is inhibited by the expression of HLA-G. This inhibition is dependent on a high level of HLA-G expression. In the present study, the nature of the receptors that mediate the inhibition has been studied with 140 NK cell lines from two donors and 246 NK clones from 5 donors by blocking the inhibition using monoclonal antibodies against the known NK inhibitory receptors: CD158a, CD158b, and CD94. Both CD94 and the two CD158 proteins can function as receptors, although the former clearly predominates. In many cases, a combination of antibodies to these receptors is required to achieve maximal reversal of inhibition. Moreover, in at least one-third of the NK cells that are inhibited by HLA-G, these antibodies alone or in combination do not reverse inhibition, strongly suggesting the existence of a third major unidentified receptor for HLA-G.

HLA-C and -Bw typing of couples with unexplained recurrent miscarriages

There is now evidence that the classical HLA class I molecule HLA-C is expressed on extravillous trophoblasts together with non-classical HLA-G molecules. Since clones of NK-cells are inhibited differently by supertypic epitopes associated with HLA-C and -B alleles we found it of interest to study HLA-C and -Bw polymorphism in 35 couples with recurrent miscarriage and 30 control couples with normal fecundity. All HLA assignments were undertaken by DNA techniques. The distribution of HLA-C alleles or the HLA-C associated supertypic epitopes recognized by NK1 or NK2 clones was not significantly different between patients and controls. The distribution of couples according to the number of NK1 and NK2 epitopes in the couple was similar in patients and controls. With respect to the HLA-Bw epitopes recognized by NKB1 clones, in 46% of the couples with recurrent miscarriage none of the spouses carried the HLA-Bw4 epitope compared with only 17% of the control couples (P < 0.02). It is concluded that the HLA-Bw4 epitope is carried more frequently by couples with normal fecundity than couples with recurrent miscarriage. The fetuses of couples with recurrent miscarriage are thus expected to lack expression of HLA-Bw4 epitopes on the trophoblast more often than fetuses of normal couples which might be of importance for the inhibition of NK-cell mediated antitrophoblast cytotoxicity.

Endothelial cells in chorionic fetal vessels of first trimester placenta express HLA-G

Using four different HLA-G-recognizing monoclonal antibodies (mAb), we investigated whether this nonclassical HLA class I molecule could be expressed in placental cell types other than extravillous cytotrophoblasts (evct) in which HLA-G has already been detected. Immunohistochemical analysis was performed on serial cryosections of first trimester placenta as well as on maternal decidual tissue. In addition to some proliferative evct, the recently described BFL.1 mAb also slightly stained some villous cytotrophoblast (vct) stem cells located near cell columns and cell islands, which until now have been considered as HLA-G negative. The same staining pattern was obtained with the 16G1 mAb raised against the soluble HLA-G isoform, whereas neither 87G nor HCA2 reacted with vct but did strongly label the invasive populations of evct, including interstitial and endovascular trophoblasts. Surprisingly, BFL.1 strongly and reproducibly stained endothelial cells in the fetal capillaries present in the mesenchymal core of the chorionic villi, whereas none of the other surrounding cellular components were stained. The same specific labeling was obtained, although with less intensity, with the three other HLA-G-recognizing mAb. In contrast, maternal endothelial cells present in spiral arteries of the decidua parietalis remained unstained. This unexpected cellular localization suggests that HLA-G may be present as a soluble form during the whole period of fetal vascularization and/or exert a nonimmunological function related to the endothelial cell type, in particular in the angiogenesis process which is highly active, until term, in chorionic villi.

HLA-G polymorphisms: neutral evolution or novel function?

HLA-G is a non-classical class Ib gene with many unusual features. Because of its unique expression pattern, which is primarily limited to fetal cells at the maternal fetal interface, this gene has gained the attention of many investigators. In this paper we review some of the novel features of HLA-G, with particular reference to polymorphic variants in the gene, and discuss the implications of these features for the potential function and evolutionary history of HLA-G.