The truncated cytoplasmic tail of HLA-G serves a quality-control function in post-ER compartments

Efficient leukocyte Ig-like receptor signaling and crystal structure of disulfide-linked HLA-G dimer

HLA-G is a nonclassical major histocompatibility complex class I (MHCI) molecule, which is expressed in trophoblasts and confers immunological tolerance in the maternal-fetal interface by binding to leukocyte Ig-like receptors (LILRs, also called as LIR/ILT/CD85) and CD8. HLA-G is expressed in disulfide-linked dimer form both in solution and at the cell surface. Interestingly, MHCI dimer formations have been involved in pathogenesis and T cell activation. The structure and receptor binding characteristics of MHCI dimers have never been evaluated. Here we performed binding studies showing that the HLA-G dimer exhibited higher overall affinity to LILRB1/2 than the monomer by significant avidity effects. Furthermore, the cell reporter assay demonstrated that the dimer formation remarkably enhanced the LILRB1-mediated signaling at the cellular level. We further determined the crystal structure of the wild-type dimer of HLA-G with the intermolecular Cys(42)-Cys(42) disulfide bond. This dimer structure showed the oblique configuration to expose two LILR/CD8-binding sites upward from the membrane easily accessible for receptors, providing plausible 1:2 (HLA-G dimer:receptors) complex models. These results indicated that the HLA-G dimer conferred increased avidity in a proper structural orientation to induce efficient LILR signaling, resulting in the dominant immunosuppressive effects. Moreover, structural and functional implications for other MHCI dimers observed in activated T cells and the pathogenic allele, HLA-B27, are discussed.

HLA-G and immune tolerance in pregnancy

Multiple mechanisms underlie the surprising willingness of mothers to tolerate genetically different fetal tissues during pregnancy. Chief among these is the choice of HLA-G, a gene with few alleles, rather than the highly polymorphic HLA-A and -B genes, for expression by the placental cells that interface directly with maternal blood and tissues. Novel aspects of this major histocompatibility complex class Ib gene include alternative splicing to permit production of membrane and soluble isoforms, deletions that dampen responses to interferons, and a shortened cytoplasmic tail that affects expression at the cell surface. Placental cells migrating into the maternal uterus synthesize both membrane and soluble isoforms, which interact with inhibitory receptors on leukocytes such as ILT2 and ILT4. Cytotoxic T lymphocytes either die or reduce production of one of their major coreceptor/activator cell surface molecules, CD8; natural killer cells are immobilized and mononuclear phagocytes are programmed into suppressive modes characterized by high production of anti-inflammatory cytokines. The idea that placental HLA-G proteins facilitate semiallogeneic pregnancy by inhibiting maternal immune responses to foreign (paternal) antigens via these actions on immune cells is now well established, and the postulate that the recombinant counterparts of these proteins may be used as powerful tools for preventing immune rejection of transplanted organs is gaining in popularity.

Residues Met76 and Gln79 in HLA-G alpha1 domain involve in KIR2DL4 recognition

Human leukocyte antigen-G (HLA-G) has long been speculated as a beneficial factor for a successful pregnancy for its restricted expression on fetal-maternal extravillous cytotrophoblasts and its capability of modulating uterine natural killer cell (uNK) function such as cytotoxicity and cytokine production through NK cell receptors. HLA class I alpha1 domain is an important killer cell Ig-like receptor (KIR) recognition site and the Met76 and Gln79 are unique to HLA-G in this region. NK cell receptor KIR2DL4 is a specific receptor for HLA-G, yet the recognition site on HLA-G remains unknown. In this study, retroviral transduction was applied to express the wild type HLA-G (HLA-wtG), mutant HLA-G (HLA-mG) on the chronic myelogenous leukemia cell line K562 cells and KIR2DL4 molecule on NK-92 cells, respectively. KIR2DL4-IgG Fc fusion protein was generated to determine the binding specificity between KIR2DL4 and HLA-G. Our results showed that residue Met76, Gln79 mutated to Ala76,79 in the alpha1 domain of HLA-G protein could affect the binding affinity between KIR2DL4 and HLA-G, meanwhile, the KIR2DL4 transfected NK-92 cells (NK-92-2DL4) showed a considerably different cytolysis ability against the HLA-wtG and HLA-mG transfected K562 targets. Taken together, our data indicated that residue Met76 and Gln79 in HLA-G alpha1 domain plays a critical role in the recognition of KIR2DL4, which could be an explanation for the isoforms of HLA-G, all containing the a1 domain, with the potential to regulate NK functions.

Modulation of HLA-G expression in human neural cells after neurotropic viral infections

HLA-G is a nonclassical human major histocompatibility complex class I molecule. It may promote tolerance, leading to acceptance of the semiallogeneic fetus and tumor immune escape. We show here that two viruses-herpes simplex virus type 1 (HSV-1), a neuronotropic virus inducing acute infection and neuron latency; and rabies virus (RABV), a neuronotropic virus triggering acute neuron infection-upregulate the neuronal expression of several HLA-G isoforms, including HLA-G1 and HLA-G5, the two main biologically active isoforms. RABV induces mostly HLA-G1, and HSV-1 induces mostly HLA-G3 and HLA-G5. HLA-G expression is upregulated in infected cells and neighboring uninfected cells. Soluble mediators, such as beta interferon (IFN-beta) and IFN-gamma, upregulate HLA-G expression in uninfected cells. The membrane-bound HLA-G1 isoform was detected on the surface of cultured RABV-infected neurons but not on the surface of HSV-1-infected cells. Thus, neuronotropic viruses that escape the host immune response totally (RABV) or partially (HSV-1) regulate HLA-G expression on human neuronal cells differentially. HLA-G may therefore be involved in the escape of certain viruses from the immune response in the nervous system.

The optimization of peptide cargo bound to MHC class I molecules by the peptide-loading complex

Major histocompatibility complex (MHC) class I complexes present peptides from both self and foreign intracellular proteins on the surface of most nucleated cells. The assembled heterotrimeric complexes consist of a polymorphic glycosylated heavy chain, non-polymorphic beta(2) microglobulin, and a peptide of typically nine amino acids in length. Assembly of the class I complexes occurs in the endoplasmic reticulum and is assisted by a number of chaperone molecules. A multimolecular unit termed the peptide-loading complex (PLC) is integral to this process. The PLC contains a peptide transporter (transporter associated with antigen processing), a thiooxido-reductase (ERp57), a glycoprotein chaperone (calreticulin), and tapasin, a class I-specific chaperone. We suggest that class I assembly involves a process of optimization where the peptide cargo of the complex is edited by the PLC. Furthermore, this selective peptide loading is biased toward peptides that have a longer off-rate from the assembled complex. We suggest that tapasin is the key chaperone that directs this action of the PLC with secondary contributions from calreticulin and possibly ERp57. We provide a framework model for how this may operate at the molecular level and draw parallels with the proposed mechanism of action of human leukocyte antigen-DM for MHC class II complex optimization.

Viral interference with MHC class I antigen presentation pathway: the battle continues

CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in the defense against viral infections. In general, CD8+ CTLs recognize antigenic peptides in the context of the major histocompatibility complex (MHC) class I molecule. The MHC class I molecules are expressed on almost all the nucleated cells in the body. The trimolecular complex consisting of the class I heavy chain, beta2-microglobulin and the peptide are generated by the MHC class I antigen presentation pathway. This pathway is designed to sample the intracellular milieu and present the information to the CTLs trafficking the area. This rigorous sampling of intracellular environment enables the CTLs to quickly identify and eliminate the cells that synthesize non-self proteins as a result of a viral infection. Many viruses, including several viruses of veterinary importance, have evolved astounding strategies to interfere with the MHC class I antigen presentation pathway, as a means of evading the CTL response of the host. This review focuses on the diverse mechanisms of viral evasion of the MHC class I antigen presentation pathway with particular emphasis on viruses of veterinary importance.

Induction of HLA-G expression in a melanoma cell line OCM-1A following the treatment with 5-aza-2′-deoxycytidine

The non-classical HLA class I antigen HLA-G is an immune modulator which inhibits the functions of T cells, NK cells, and the Dendritic cells (DC). As a result, HLA-G expression in malignant cells may provide them with a mechanism to escape the immune surveillance. In melanoma, HLA-G antigen expression has been found in 30% of surgically removed lesions but in less than 1% of established cell lines. One possible mechanism underlying the differential HLA-G expression in vivo and in vitro is that the HLA-G gene is epigenetically repressed in melanoma cells in vitro. To test this hypothesis, we treated the HLA-G negative melanoma cell line OCM-1A with the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-AC) and analyzed whether HLA-G expression can be restored. Our data strongly suggest that HLA-G is silenced as a result of CpG hypermethylation within a 5' regulatory region encompassing 220 bp upstream of the start codon. After treatment, HLA-G mRNA expression was dramatically increased. Western blot and flow cytometry showed that HLA-G protein was induced. Interestingly, HLA-G cell surface expression on the 5-AC treated OCM-1A cells is much less than that on the HLA-G positive JEG-3 cells while a similar amount of total HLA-G was observed. Possible mechanisms for the difference were analyzed in the study such as cell cold-treatment, peptide loading and antigen processing machinery components (APM) as well as beta2 microglobulin (beta2-m) expression. Data revealed that the APM component calreticulin might be involved in the lower HLA-G surface expression on OCM-1A cells. Taken together, our results indicated that DNA methylation is an important epigenetic mechanism by which HLA-G antigen expression is modulated in melanoma cells in vitro. Furthermore, to the first time, we hypothesized that the deficiency of calreticulin might be involved in the low HLA-G surface expression on the 5-AC treated OCM-1A cells.

Tolerization of dendritic cells by HLA-G

The expression of HLA-G at the fetal-maternal interface during pregnancy and in transplanted tissue makes this a key molecule in the acceptance of a semiallogeneic fetus and allogeneic transplant. Dendritic cells (DC) play a critical role in the control of innate and adaptive immune responses. DC are present in maternal decidua, but must be kept under tight control. Here we describe the mechanism of tolerization of DC by HLA-G through inhibitory receptor interactions. The HLA-G-ILT (immunoglobulin-like transcript) interaction leads to development of tolerogenic DC with the induction of anergic and immunosuppressive T cells. Using human monocyte-derived DC and ILT4-transgenic mice, we show that (i) HLA-G induces the development of tolerogenic DC with arrest maturation/activation of myeloid DC, (ii) HLA-G-modified DC induce differentiation of anergic and immunosuppressive CD4(+) and CD8(+) effector T cells, and (iii) the gene expression profile provides evidence that HLA-G induces tolerogenic DC by disruption of the MHC class II presentation pathway. Ligation of ILT4 receptor on DC from transgenic mice diminished peptide presentation by MHC class II molecules and significantly prolonged allograft survival. These findings provide support that HLA-G is an important tolerogenic molecule on DC for the acceptance of a semiallogeneic fetus and transplanted tissue/organ.

Tails of wonder: endocytic-sorting motifs key for exogenous antigen presentation

Antigen-presenting molecules, including MHC I, II and CD1, have central roles in the induction of T cell-mediated immunity against pathogens and tumors and also in the maintenance of tolerance towards self-antigens. The presentation of exogenously derived peptide and lipid antigens to specific T cells by professional antigen-presenting cells (pAPCs) is an essential part of both processes. Exogenous antigen loading takes place mostly within specialized endocytic and phagocytic compartments of pAPCs and targeting of antigen-presenting molecules to these intracellular compartments is mediated by highly conserved cytoplasmic sorting motifs. Recent data have revealed that the cytoplasmic tails of antigen-presenting molecules, by controlling the access of these molecules to exogenously derived antigens, have a crucially important and largely underappreciated role in the generation of tolerance and T-cell mediated immunity.

The short cytoplasmic tail of HLA-G determines its resistance to HIV-1 Nef-mediated cell surface downregulation

During infection with the human immunodeficiency virus type 1 (HIV-1), selective downregulation of major histocompatibility complex (MHC) class I molecules by Nef protein allows infected cells to be protected from natural killer (NK) cell lysis and to escape the HIV-specific cytotoxic T-lymphocyte response. The nonclassical MHC class I molecule human leukocyte antigen (HLA)-G is mainly expressed in placental tissues and in thymic epithelial cells. Using chimeric molecules and flow cytometry, we show that in contrast with HLA-A2, the non classical MHC class I molecule HLA-G is resistant to Nef-induced cell surface downregulation solely because of the length of its intracytoplasmic domain. Moreover, confocal microscopy analysis indicates that Nef does not delocalize HLA-G molecules from the cell surface, whereas HLA-G molecules extended with the cytoplasmic tail of HLA-A2 accumulate intracellularly with Nef. Together, these data demonstrate that the short cytoplasmic tail of HLA-G confers resistance to Nef-induced downregulation and intracellular accumulation. This resistance may have functional consequences during the course of HIV infection.

Human immunodeficiency virus 1 downregulates cell surface expression of the non-classical major histocompatibility class I molecule HLA-G1

Human immunodeficiency virus 1 (HIV-1) downregulates cell surface expression of HLA-A and HLA-B but not HLA-C or HLA-E to ultimately escape immune defences. Here, it is shown that cell surface expression of the non-classical HLA-G1 is also downregulated by HIV-1, by using co-transfection experiments and infection with cell-free HIV-1 of HLA-G1-expressing U87 glioma cells or macrophages in primary culture. Moreover, co-transfection experiments using proviruses deleted in either nef or vpu or plasmids encoding HIV-1 Nef and Vpu mixed together with a HLA-G1-expressing construct demonstrated that HLA-G1 downregulation is Nef-independent and Vpu-dependent, contrasting with the Nef- and Vpu-dependent HLA-A2 downregulation. Together, these results show that the decrease of HLA-A2 and HLA-G1 caused by HIV-1 occurs through distinct mechanisms.

Soluble HLA-G generated by proteolytic shedding inhibits NK-mediated cell lysis

In contrast to the classical HLA class Ia molecules, the nonclassical HLA-G primary transcript is alternatively spliced to generate several mRNAs that encode four membrane-bound and three soluble isoforms. This study demonstrated that the soluble form of HLA-G can also be generated by metalloproteinase-dependent shedding at post-translational level. These soluble HLA-G1 molecules generated by the cleavage of membrane-bound HLA-G1 associate with beta2-microglobulin and contain bound peptides that are stable at physiological conditions. This report further showed that the soluble HLA-G1 is able to protect HLA class I-negative K562 cells from NK lysis, suggesting that soluble HLA-G could act as an immunoregulator in NK cell recognition and possibly in other immune responses.

Quality control of MHC class I maturation

Assembly of MHC class I molecules in the ER is regulated by the so-called loading complex (LC). This multiprotein complex is of definite importance for class I maturation, but its exact organization and order of assembly are not known. Evidence implies that the quality of peptides loaded onto class I molecules is controlled at multiple stages during MHC class I assembly. We recently found that tapasin, an important component of the LC, interacts with COPI-coated vesicles. Biochemical studies suggested that the tapa-sin-COPI interaction regulates the retrograde transport of immature MHC class I molecules from the Golgi network back to the ER. Also other findings now propose that in addition to the peptide-loading control, the quality control of MHC class I antigen presentation includes the restriction of export of suboptimally loaded MHC class I molecules to the cell surface. In this review, we use recent studies of tapasin to examine the efficiency of TAP, the LC constitution, ER quality control of class I assembly, and peptide optimization. The concepts of MHC class I recycling and ER retention are also discussed.

Quality Control and Protein Folding in the Secretory Pathway

The biosynthesis of secretory and membrane proteins in the endoplasmic reticulum (ER) yields mostly properly folded and assembled structures with full biological activity. Such fidelity is maintained by quality control (QC) mechanisms that avoid the production of nonnative structures. QC relies on chaperone systems in the ER that monitor and assist in the folding process. When folding promotion is not sufficient, proteins are retained in the ER and eventually retranslocated to the cytosol for degradation by the ubiquitin proteasome pathway. Retention of proteins that fail QC can sometimes occur beyond the ER, and degradation can take place in lysosomes. Several diseases are associated with proteins that do not pass QC, fail to be degraded efficiently, and accumulate as aggregates. In other cases, pathology arises from the downregulation of mutated but potentially functional proteins that are retained and degraded by the QC system.

Control of dendritic cell cross-presentation by the major histocompatibility complex class I cytoplasmic domain

Dendritic cells (DCs) can present extracellularly derived antigens in the context of major histocompatibility complex (MHC) class I molecules, a process called cross-presentation. Although recognized to be important for priming of T cell responses to many viral, bacterial and tumor antigens, the mechanistic details of this alternative antigen-presentation pathway are poorly understood. We demonstrate here the existence of an endolysosomal compartment in DCs where exogenously derived peptides can be acquired for presentation to T cells, and show that the MHC class I cytoplasmic domain contains a tyrosine-based targeting signal required for routing MHC class I molecules through these compartments. We also report that transgenic mice expressing H-2K(b) with a tyrosine mutation mount inferior H-2K(b)-restricted cytotoxic T lymphocyte responses against two immunodominant viral epitopes, providing evidence of a crucial function for cross-priming in antiviral immunity.

Secretion of sHLA-G molecules in malignancies

Our clinical studies revealed significantly increased soluble HLA-G (sHLA-G) plasma levels in patients suffering from malignant melanoma, glioma, breast and ovarian cancer. Specific ELISpot assays demonstrate that sHLA-G molecules expressing intron-4 sequences are preferentially secreted by peripheral blood monocytes. In vitro, the sHLA-G secretion of monocytes and tumor cells was strongly enhanced by TH1 cytokines like IFN-alpha, -beta, -gamma whereas TH2 cytokines (e.g. IL-4, -10) had minor effects. As sHLA-G can inhibit the functions of T and NK cells high concentration of these molecules should systemically or at the tumor side reduce the immune surveillance and thus favour the progression of cancer.

Biology and functions of human leukocyte antigen-G in health and sickness

In 1998, the first International Conference on human leukocyte antigen-G (HLA-G) was held in Paris. At that time, HLA-G was still a new HLA class I molecule, few aspects of its immunological functions were known, and its expression by tumors was just being described. In 1998, tools to properly study HLA-G were lacking, especially monoclonal antibodies, and three conclusions were drawn after the congress: (i) animal models were needed, (ii) the biology of HLA-G isoforms had to be confirmed, and (iii) HLA-G expression by tumors required clarification. Five years later, these three issues have been addressed. HLA-G is now gaining pace and is investigated for its immuno-inhibitory functions in the context of multiple pathologies. Eighty five oral presentations were given this year for more than 200 investigators working on HLA-G by speakers from over 20 countries. The success of the 3rd International Conference on HLA-G reflects the interest and tremendous work of the many research teams which, over the years, contributed to the publication of more than 500 peer-review articles. We summarize the key points that were presented and discussed during this meeting.

HLA-G, pre-eclampsia, immunity and vascular events

Pre-eclampsia, one of the main complications in pregnancy, is characterised by shallow cytotrophoblast invasion of decidua as well as by vascular endothelial cell dysfunction, leading to a poor perfusion of placenta. A striking feature of pre-eclamptic pregnancies is that expression of HLA-G protein is reduced in term placentas compared with normal pregnancy. How such HLA-G deficient expression may be related to the pre-eclamptic pathology is unknown. Here, we review the major structural characteristics of HLA-G and some of its functions that have been recently characterised. Soluble HLA-G1 isoform down-regulates both CD8(+) and CD4(+) T cell reactivity. HLA-G also modulates innate immunity by binding to several NK and/or decidual receptors, inducing particular cytokine secretion. HLA-G was shown to be less susceptible to human cytomegalovirus-derived US protein down-modulation. Finally, soluble HLA-G1 down-regulates endothelial cell proliferation and migration. In view of these different HLA-G properties, we will briefly discuss how defective HLA-G function may contribute to the low trophoblast invasion and vascular abnormalities observed in pre-eclamptic placentas.

An essential function of tapasin in quality control of HLA-G molecules

Tapasin plays an important role in the quality control of major histocompatibility complex (MHC) class I assembly, but its precise function in this process remains controversial. Whether tapasin participates in the assembly of HLA-G has not been studied. HLA-G, an MHC class Ib molecule that binds a more restricted set of peptides than class Ia molecules, is a particularly interesting molecule, because during assembly, it recycles between the endoplasmic reticulum (ER) and the cis-Golgi until it is loaded with a high affinity peptide. We have taken advantage of this unusual trafficking property of HLA-G and its requirement for high affinity peptides to demonstrate that a critical function of tapasin is to transform class I molecules into a high affinity, peptide-receptive form. In the absence of tapasin, HLA-G molecules cannot bind high affinity peptides, and an abundant supply of peptides cannot overcome the tapasin requirement for high affinity peptide loading. The addition of tapasin renders HLA-G molecules capable of loading high affinity peptides and of transporting to the surface, suggesting that tapasin is a prerequisite for the binding of high-affinity ligands. Interestingly, the "tapasin-dependent" HLA-G molecules are not empty in the absence of tapasin but are in fact associated with suboptimal peptides and continue to recycle between the ER and the cis-Golgi. Together with the finding that empty HLA-G heterodimers are strictly retained in the ER and degraded, our data suggest that MHC class I molecules bind any available peptides to avoid ER-mediated degradation and that the peptides are in turn replaced by higher affinity peptides with the aid of tapasin.

Induction of HLA-G-restricted human cytomegalovirus pp65 (UL83)-specific cytotoxic T lymphocytes in HLA-G transgenic mice

The non-classical major histocompatibility complex class I molecule HLA-G is expressed mainly by extravillous trophoblasts at the materno-foetal interface. HLA-G has been found to bind endogenously processed nonameric peptides but its function as a restriction element for a cytotoxic T cell response to viruses with tropism for trophoblastic cells has never been demonstrated. In this study, candidate viral peptides derived from human cytomegalovirus (HCMV) pp65 (UL83), which stabilized the HLA-G molecule on HLA-G-transfected T2 cells, were identified. The specific anti-pp65 cytotoxic T lymphocyte (CTL) response restricted by HLA-G in triple transgenic mice (HLA-G, human beta2m, human CD8alpha) was then investigated by injection of dendritic cells loaded with synthetic pp65-derived peptides or by infection with canarypox virus expressing pp65. Results showed that CTLs from HLA-G mice have the capacity to kill target cells either infected with recombinant vaccinia viruses expressing pp65 or loaded with specific pp65-derived peptides using HLA-G as an antigen-presenting molecule. It was also demonstrated that these HLA-G-restricted pp65-specific T cells are able to kill the human astrocytoma cell line U373, which was transfected with HLA-G and infected with HCMV. Moreover, using HLA-G tetramers refolded with a synthetic pp65-derived peptide, peptide-specific CD8(+) cells restricted by HLA-G have been detected in vivo. These findings provide the first evidence that HLA-G can select anti-HCMV-restricted CTLs in vivo, although the potency of this cytolytic response is limited (20-25 %). The weak HLA-G-restricted anti-HCMV response is probably due to HLA-G-mediated inhibitory signals on the development of an antiviral CTL response.

HLA-G and MIC expression in tumors and their role in anti-tumor immunity

Non-classical MHC class Ib molecules have attracted growing interest in recent years, especially because they interact with non-T-cell inhibitory or triggering receptors expressed on natural killer (NK) and T cells, suggesting that they have a role in immune recognition. Abnormalities in MHC class Ib expression are frequently found in human tumors of various histologies and might be associated with poor clinical outcome despite the local accumulation of immune competent cells. Available data suggest that the balance between activating and suppressing signals significantly influences the efficacy of the immune response and consequently of tumor progression.

HLA-G Molecules: from Maternal–Fetal Tolerance to Tissue Acceptance

Over the past few years, HLA-G, the non-classical HLA class I molecule, has been the center of investigations that have led to the description of its specific structural and functional properties. Although located in the HLA class I region of chromosome six, the HLA-G gene may be distinguished from other HLA class I genes by its low polymorphism and alternative splicing that generates seven HLA-G proteins, whose tissue-distribution is restricted to normal fetal and adult tissues that display a tolerogeneic function toward both innate and acquired immune cells. We review these points, with special emphasis on the role of HLA-G in human pathologies, such as cancer, viral infection, and inflammatory diseases, as well as in organ transplantation.

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.

Secretory pathway quality control operating in Golgi, plasmalemmal, and endosomal systems

Exportable proteins that have significant defects in nascent polypeptide folding or subunit assembly are frequently retained in the endoplasmic reticulum and subject to endoplasmic reticulum-associated degradation by the ubiquitin-proteasome system. In addition to this, however, there is growing evidence for post-endoplasmic reticulum quality control mechanisms in which mutant or non-native exportable proteins may undergo anterograde transport to the Golgi complex and post-Golgi compartments before intracellular disposal. In some instances, these proteins may undergo retrograde transport back to the endoplasmic reticulum with re-targeting to the endoplasmic reticulum-associated degradation pathway; in other typical cases, they are targeted into the endosomal system for degradation by vacuolar/lysosomal proteases. Such quality control targeting is likely to involve recognition of features more commonly expressed in mutant proteins, but may also be expressed by wild-type proteins, especially in cells with perturbation of local environments that are essential for normal protein trafficking and stability in the secretory pathway and at the cell surface.

1st class ticket to class I: protein toxins as pathfinders for antigen presentation

A number of bacterial toxins have evolved diverse strategies for crossing membrane barriers in order to reach their substrates in the mammalian cytosol. Recent studies show that this property can be exploited for the delivery of fused antigens into the major histocompatibility complex class I-restricted presentation pathway, with the goal of eliciting a specific immune response. Here we discuss the peculiarities of the trafficking pathways of a variety of toxins, and how these may allow the toxins to be used as delivery vehicles for therapeutic and diagnostic purposes.

The MHC class I homolog of human cytomegalovirus is resistant to down-regulation mediated by the unique short region protein (US)2, US3, US6, and US11 gene products

Human CMV encodes four unique short region proteins (US), US2, US3, US6, and US11, each independently sufficient for causing the down-regulation of MHC class I molecules on the cell surface. This down-regulation allows infected cells to evade recognition by cytotoxic T cells but leaves them susceptible to NK cells, which lyse cells that lack class I molecules. Another human CMV-encoded protein, unique long region protein 18 (UL18), is an MHC class I homolog that might provide a mechanism for inhibiting the NK cell response. The sequence similarities between MHC class I molecules and UL18 along with the ability of UL18 to form trimeric complexes with beta(2)-microglobulin and peptides led to the hypothesis that if the US and UL18 gene products coexist temporally during infection, the US proteins might down-regulate UL18 molecules, similar to their action on MHC class I molecules. We show here that temporal expression of US and UL18 genes partially overlaps during infection. However, unlike MHC class I molecules, the MHC class I homolog, UL18, is fully resistant to the down-regulation associated with the US2, US3, US6, and US11 gene products. The specific effect of US proteins on MHC class I molecules, but not on UL18, represents another example of how viral proteins have evolved to evade immune surveillance, avoiding fratricide by specifically targeting host proteins.

The cell biology of MHC class I antigen presentation

MHC class I antigen presentation refers to the co-ordinated activities of many intracellular pathways that promote the cell surface appearance of MHC class I/beta2m heterodimers loaded with a spectrum of self or foreign peptides. These MHC class I peptide complexes form ligands for CD8 positive T cells and NK cells. MHC class I heterodimers are loaded within the endoplasmic reticulum (ER) with peptides derived from intracellular proteins. Alternatively, MHC class I molecules may be loaded with peptides derived from extracellular proteins in a process called MHC class I cross presentation. This pathway is less well defined but can overlap those pathways operating in classical MHC class I presentation and has recently been reviewed elsewhere (1). This review will address the current concepts regarding the intracellular assembly of MHC class I molecules with their peptide cargo within the ER and their subsequent progress to the cell surface.