Intracellular cysteine residues in the tail of MHC class I proteins are crucial for extracellular recognition by leukocyte Ig-like receptor 1

Open MHC Class I Conformers: A Look through the Looking Glass

Studies carried out during the last few decades have consistently shown that cell surface MHC class I (MHC-I) molecules are endowed with functions unrelated with antigen presentation. These include cis–trans-interactions with inhibitory and activating KIR and LILR, and cis-interactions with receptors for hormones, growth factors, cytokines, and neurotransmitters. The mounting body of evidence indicates that these non-immunological MHC-I functions impact clinical and biomedical settings, including autoimmune responses, tumor escape, transplantation, and neuronal development. Notably, most of these functions appear to rely on the presence in hematopoietic and non-hematopoietic cells of heavy chains not associated with β2m and the peptide at the plasma membrane; these are known as open MHC-I conformers. Nowadays, open conformers are viewed as functional cis-trans structures capable of establishing physical associations with themselves, with other surface receptors, and being shed into the extracellular milieu. We review past and recent developments, strengthening the view that open conformers are multifunctional structures capable of fine-tuning cell signaling, growth, differentiation, and cell communication.

Protein Palmitoylation in Leukocyte Signaling and Function

Palmitoylation is a post-translational modification (PTM) based on thioester-linkage between palmitic acid and the cysteine residue of a protein. This covalent attachment of palmitate is reversibly and dynamically regulated by two opposing sets of enzymes: palmitoyl acyltransferases containing a zinc finger aspartate-histidine-histidine-cysteine motif (PAT-DHHCs) and thioesterases. The reversible nature of palmitoylation enables fine-tuned regulation of protein conformation, stability, and ability to interact with other proteins. More importantly, the proper function of many surface receptors and signaling proteins requires palmitoylation-meditated partitioning into lipid rafts. A growing number of leukocyte proteins have been reported to undergo palmitoylation, including cytokine/chemokine receptors, adhesion molecules, pattern recognition receptors, scavenger receptors, T cell co-receptors, transmembrane adaptor proteins, and signaling effectors including the Src family of protein kinases. This review provides the latest findings of palmitoylated proteins in leukocytes and focuses on the functional impact of palmitoylation in leukocyte function related to adhesion, transmigration, chemotaxis, phagocytosis, pathogen recognition, signaling activation, cytotoxicity, and cytokine production.

Measuring Synthesis and Degradation of MHC Class I Molecules

Major histocompatibility complex (MHC) class I molecules function to present pathogen derived peptides to cytotoxic T cells and act as ligands for Natural Killer cells, thus alerting the immune system to the presence of invading pathogens. However, some MHC class I molecules, most notably HLA-B27, can be strongly associated with autoimmune diseases. In addition, the MHC class I pathway is a target for numerous viral evasion strategies Understanding not only the antigen presenting functions, but also the biosynthesis and the degradation pathways of MHC class I molecules has therefore become important in determining their role in pathogen and autoimmune related diseases. Here, we describe how using epitope tagged MHC class I molecules can aid in the analysis of MHC class I molecule biosynthesis and degradation as well as complementary studies using conventional conformationally specific antibodies. Coupled together with pharmacological manipulation which can target both biosynthetic and degradative pathways, this offers a powerful tool in analyzing MHC class I molecules.

HLA-B and HLA-C Differ in Their Nanoscale Organization at Cell Surfaces

The particular HLA class I variants an individual carries influences their resistance and susceptibility to a multitude of diseases. Expression level and variation in the peptide binding region correlates with, for example, a person's progression to AIDS after HIV infection. One factor which has not yet been addressed is whether or not different HLA class I proteins organize differently in the cell membrane on a nanoscale. Here, we examined the organization of three HLA-B allotypes (B^*2705, B^*5301, and B^*5701) and two HLA-C allotypes (C^*0602 and C^*0702) in the membrane of 721.221 cells which otherwise lack expression of HLA-B or HLA-C. All these allotypes are ligands for the T cell receptor and leukocyte immunoglobulin-like receptors, but additionally, the HLA-B allotypes are ligands for the killer-cell immunoglobulin-like receptor family member KIR3DL1, HLA-C^*0602 is a ligand for KIR2DL1, and HLA-C^*0702 is a ligand for KIR2DL2/3. Using super-resolution microscopy, we found that both HLA-B and HLA-C formed more clusters and a greater proportion of HLA contributed to clusters, when expressed at lower levels. Thus, HLA class I organization is a covariate in genetic association studies of HLA class I expression level with disease progression. Surprisingly, we also found that HLA-C was more clustered than HLA-B when expression level was controlled. HLA-C consistently formed larger and more numerous clusters than HLA-B and a greater proportion of HLA-C contributed to clusters than for HLA-B. We also found that the organization of HLA class I proteins varied with cell type. T cells exhibited a particularly clustered organization of HLA class I while B cells expressed a more uniform distribution. In summary, HLA class I variants are organized differently in the cell surface membrane which may impact their functions.

Expression and function of NKp46 W32R: the human homologous protein of mouse NKp46 W32R (Noé)

Natural killer (NK) cells eradicate infected cells and tumors following the triggering of activating receptors, like the Natural Cytotoxicity Receptors (NCRs), which include NKp30, NKp44 and NKp46. NKp46 is the only NCR expressed in mice (mNKp46), and except for some Innate Lymphoid Cell (ILC) populations (ILC1/3 subsets), its expression is restricted to NK cells. Previously, a mouse named Noé was generated in which a random point mutation (W32R) impaired the cell surface expression of mNKp46. Interestingly, the Noé mice NK cells expressed twice as much of the transcription factor Helios, and displayed general non-NKp46 specific hyperactivity. We recently showed that the mNKp46 W32R (Noé) protein was expressed on the surface of various cells; albeit slowly and unstably, that it is aberrantly glycosylated and accumulates in the ER. Interestingly, the Tryptophan (Trp) residue in position 32 is conserved between humans and mice. Therefore, we studied here the human orthologue protein of mNKp46 W32R, the human NKp46 W32R. We demonstrated that NKp46 W32R is aberrantly glycosylated, accumulates in the ER, and is unstable on the cell surface. Furthermore, we showed that overexpression of NKp46 W32R or Helios resulted in augmented NK cell activation, which may be applied to boost NK activity for therapeutic applications.

Cryptic protein-protein interaction motifs in the cytoplasmic domain of MHCI proteins

BACKGROUND

Major histocompatibility complex class I (MHCI) proteins present antigenic peptides for immune surveillance and play critical roles in nervous system development and plasticity. Most MHCI are transmembrane proteins. The extracellular domain of MHCI interacts with immunoreceptors, peptides, and co-receptors to mediate immune signaling. While the cytoplasmic domain also plays important roles in endocytic trafficking, cross-presentation of extracellularly derived antigens, and CTL priming, the molecular mediators of cytoplasmic signaling by MHCI remain largely unknown.

RESULTS

Here we show that the cytoplasmic domain of MHCI contains putative protein-protein interaction domains known as PDZ (PSD95/disc large/zonula occludens-1) ligands. PDZ ligands are motifs that bind to PDZ domains to organize and mediate signaling at cell-cell contacts. PDZ ligands are short, degenerate motifs, and are therefore difficult to identify via sequence homology alone, but several lines of evidence suggest that putative PDZ ligand motifs in MHCI are under positive selective pressure. Putative PDZ ligands are found in all of the 99 MHCI proteins examined from diverse species, and are enriched in the cytoplasmic domain, where PDZ interactions occur. Both the position of the PDZ ligand and the class of ligand motif are conserved across species, as well as among genes within a species. Non-synonymous substitutions, when they occur, frequently preserve the motif. Of the many specific possible PDZ ligand motifs, a handful are strikingly and selectively overrepresented in MHCI's cytoplasmic domain, but not elsewhere in the same proteins. Putative PDZ ligands in MHCI encompass conserved serine and tyrosine residues that are targets of phosphorylation, a post-translational modification that can regulate PDZ interactions. Finally, proof-of-principle in vitro interaction assays demonstrate that the cytoplasmic domains of particular MHCI proteins can bind directly and specifically to PDZ1 and PDZ4&5 of MAGI-1, and identify a conserved PDZ ligand motif in the classical MHCI H2-K that is required for this interaction.

CONCLUSIONS

These results identify cryptic protein interaction motifs in the cytoplasmic domain of MHCI. In so doing, they suggest that the cytoplasmic domain of MHCI could participate in previously unsuspected PDZ mediated protein-protein interactions at neuronal as well as immunological synapses.

Interaction of the LILRB1 inhibitory receptor with HLA class Ia dimers

Leukocyte immunoglobulin-like receptor subfamily B member 1 (LILRB1) has been reported to interact with a wide spectrum of HLA class I (HLA-I) molecules, albeit with different affinities determined by allelic polymorphisms and conformational features. HLA-G dimerization and the presence of intracellular Cys residues in HLA-B7 have been shown to be critical for their recognition by LILRB1. We hypothesized that dimerization of classical HLA class Ia molecules, previously detected in exosomes, might enhance their interaction with LILRB1. A soluble LILRB1-Fc fusion protein and a sensitive cellular reporter system expressing a LILRB1-ζ chimera were employed to assess receptor interaction with different HLA class Ia molecules transfected in the human lymphoblastoid 721.221 cell line. Under these conditions, intracellular Cys residues and HLA-I dimerization appeared associated with increased LILRB1 recognition. On the other hand, a marginal interaction of LILRB1 with primary monocytic cells, irrespective of their high HLA-I expression, was enhanced by type I interferon (IFN). This effect appeared disproportionate to the cytokine-induced increase of surface HLA-I expression and was accompanied by detection of HLA class Ia dimers. Altogether, the results support that a regulated assembly of these noncanonical HLA-I conformers during the immune response may enhance the avidity of their interaction with LILRB1.

LILRB2 interaction with HLA class I correlates with control of HIV-1 infection

Natural progression of HIV-1 infection depends on genetic variation in the human major histocompatibility complex (MHC) class I locus, and the CD8+ T cell response is thought to be a primary mechanism of this effect. However, polymorphism within the MHC may also alter innate immune activity against human immunodeficiency virus type 1 (HIV-1) by changing interactions of human leukocyte antigen (HLA) class I molecules with leukocyte immunoglobulin-like receptors (LILR), a group of immunoregulatory receptors mainly expressed on myelomonocytic cells including dendritic cells (DCs). We used previously characterized HLA allotype-specific binding capacities of LILRB1 and LILRB2 as well as data from a large cohort of HIV-1-infected individuals (N = 5126) to test whether LILR-HLA class I interactions influence viral load in HIV-1 infection. Our analyses in persons of European descent, the largest ethnic group examined, show that the effect of HLA-B alleles on HIV-1 control correlates with the binding strength between corresponding HLA-B allotypes and LILRB2 (p = 10(-2)). Moreover, overall binding strength of LILRB2 to classical HLA class I allotypes, defined by the HLA-A/B/C genotypes in each patient, positively associates with viral replication in the absence of therapy in patients of both European (p = 10(-11)-10(-9)) and African (p = 10(-5)-10(-3)) descent. This effect appears to be driven by variations in LILRB2 binding affinities to HLA-B and is independent of individual class I allelic effects that are not related to the LILRB2 function. Correspondingly, in vitro experiments suggest that strong LILRB2-HLA binding negatively affects antigen-presenting properties of DCs. Thus, we propose an impact of LILRB2 on HIV-1 disease outcomes through altered regulation of DCs by LILRB2-HLA engagement.

The biochemistry and immunology of non-canonical forms of HLA-B27

HLA-B27 (B27) is strongly associated with the spondyloarthritides. B27 is expressed at the cell surface of antigen presenting cells (APC) both as canonical β2m-associated and non-canonical β2m-free heavy chain (FHC) forms which include B27 dimers (termed B272). B27 FHC forms arise in an endosomal compartment from recycling β2m-associated B27. Formation of cell surface FHC dimers is critically dependent on an unpaired reactive cysteine 67 in the α1 helix of the class I heavy chain. HLA-B27 also form redox-inducible β2m-associated dimers on exosomes and apoptosing cells. By contrast with cell surface expressed cysteine 67-dependent heavy chain dimers these dimers are dependent on a cytoplasmic cysteine 325 for their formation. HLA-B27 binds to immunoregulatory receptors including members of the Killer cell Immunoglobulin-like (KIR) and Leukocyte Immunoglobulin-like receptor family. B27 FHC bind to different but overlapping sets of these immunoreceptors compared to classical β2m-associated HLA-B27. B27 FHC bind more strongly to KIR3DL2 and LILRB2 immune receptor than other β2m-associated HLA-class I ligands. Genetic studies have implicated genes which control production of the important proinflammatory cytokine IL-17 in the pathogenesis of spondyloarthritis. Cell surface HLA-B27 FHC binding to these immune receptors or acting through other mechanisms could impact on the pathogenesis of spondyloarthritis by promoting immune cell production of IL-17. Here we review the literature on these non-canonical forms of HLA-B27 and the immune receptors they bind to and discuss the possible relevance of these interactions to the pathogenesis of spondyloarthropathy.

Structural insights into activation of antiviral NK cell responses

Natural killer (NK) cells are key components of innate immune responses, providing surveillance against cells undergoing tumorigenesis or infection, by viruses or internal pathogens. NK cells can directly eliminate compromised cells and regulate downstream responses of the innate and acquired immune systems through the release of immune modulators (cytokines, interferons). The importance of the role NK cells play in immune defense was demonstrated originally in herpes viral infections, usually mild or localized, which become severe and life threatening in NK-deficient patients . NK cell effector functions are governed by balancing opposing signals from a diverse array of activating and inhibitory receptors. Many NK receptors occur in paired activating and inhibitory isoforms and recognize major histocompatibility complex (MHC) class I proteins with varying degrees of peptide specificity. Structural studies have made considerable inroads into understanding the molecular mechanisms employed to broadly recognize multiple MHC ligands or specific pathogen-associated antigens and the strategies employed by viruses to thwart these defenses. Although many details of NK development, signaling, and integration remain mysterious, it is clear that NK receptors are key components of a system exquisitely tuned to sense any dysregulation in MHC class I expression, or the expression of certain viral antigens, resulting in the elimination of affected cells.

Emerging roles for protein S-palmitoylation in immunity from chemical proteomics

The activation of innate and adaptive immune signaling pathways and effector functions often occur at cellular membranes and are regulated by complex mechanisms. Here we review the growing number of proteins which are known to be regulated by S-palmitoylation in immune cells emerging from recent advances in chemical proteomics. These chemical proteomic studies have highlighted the roles of this dynamic lipid modification in regulating the specificity and strength of immune responses in different lymphocyte populations.

Measuring synthesis and degradation of MHC class I molecules

Major histocompatibility complex (MHC) class I molecules function to present pathogen-derived peptides to cytotoxic T cells or act as ligands for Natural Killer cells, thus alerting the immune system to the presence of invading pathogens. Furthermore MHC class I molecules can be strongly associated with autoimmune diseases. Therefore understanding not only the biosynthesis and the degradation pathways of MHC class I molecules has become important in determining their role in pathogen and autoimmune-related diseases. Here we describe how using epitope-tagged MHC class I molecules can aid in the analysis of MHC class I molecule biosynthesis and degradation and also complement studies using conventional conformationally specific antibodies. Coupled together with pharmacological manipulation which can target both biosynthetic and degradative pathways, this offers a powerful tool in analyzing MHC class I molecules.

The multi-faceted nature of HLA class I dimer molecules

The canonical role of major histocompatibility complex class I (MHCI) molecules in antigen presentation involves the recognition of a short peptide of intracellular origin, bound to the upper surface of the class I molecule, by CD8(+) T lymphocytes. Assembly and loading of the MHCI is a highly regulated, chaperone-mediated process and only when the fully folded MHCI molecule is correctly loaded with peptide is it released from the endoplasmic reticulum for trafficking to the cell surface. Current models of the interactions of MHCI molecules with their cognate receptors visualize them functioning as monomeric entities. However, in recent years, new data have revealed MHCI molecules with the ability to form disulphide-linked dimeric structures, with several distinct dimer entities being elucidated. We describe here three types of MHCI dimers; HLA-B27 dimers formed predominantly through the possession of an unpaired cysteine within the peptide-binding groove; HLA-G dimers, which form through a cysteine on its external surface; and a novel population we term redox-induced dimers, which can form between cysteine residues in the cytoplasmic tail domains. The characteristics of these dimeric MHCI molecules and their role in both normal immune responses and in disease pathogenesis are reviewed in this article.

Proteomic analysis of S-acylated proteins in human B cells reveals palmitoylation of the immune regulators CD20 and CD23

S-palmitoylation is a reversible post-translational modification important for controlling the membrane targeting and function of numerous membrane proteins with diverse roles in signalling, scaffolding, and trafficking. We sought to identify novel palmitoylated proteins in B lymphocytes using acyl-biotin exchange chemistry, coupled with differential analysis by liquid-chromatography tandem mass spectrometry. In total, we identified 57 novel palmitoylated protein candidates from human EBV-transformed lymphoid cells. Two of them, namely CD20 and CD23 (low affinity immunoglobulin epsilon Fc receptor), are immune regulators that are effective/potential therapeutic targets for haematological malignancies, autoimmune diseases and allergic disorders. Palmitoylation of CD20 and CD23 was confirmed by heterologous expression of alanine mutants coupled with bioorthogonal metabolic labeling. This study demonstrates a new subset of palmitoylated proteins in B cells, illustrating the ubiquitous role of protein palmitoylation in immune regulation.

MHC class I dimer formation by alteration of the cellular redox environment and induction of apoptosis

Many MHC class I molecules contain unpaired cysteine residues in their cytoplasmic tail domains, the function of which remains relatively uncharacterized. Recently, it has been shown that in the small secretory vesicles known as exosomes, fully folded MHC class I dimers can form through a disulphide bond between the cytoplasmic tail domain cysteines, induced by the low levels of glutathione in these extracellular vesicles. Here we address whether similar MHC class I dimers form in whole cells by alteration of the redox environment. Treatment of the HLA-B27-expressing Epstein-Barr virus-transformed B-cell line Jesthom, and the leukaemic T-cell line CEM transfected with HLA-B27 with the strong oxidant diamide, and the apoptosis-inducing and glutathione-depleting agents hydrogen peroxide and thimerosal, induced MHC class I dimers. Furthermore, induction of apoptosis by cross-linking FasR/CD95 on CEM cells with monoclonal antibody CH-11 also induced MHC class I dimers. As with exosomal MHC class I dimers, the formation of these structures on cells is controlled by the cysteine at position 325 in the cytoplasmic tail domain of HLA-B27. Therefore, the redox environment of cells intimately controls induction of MHC class I dimers, the formation of which may provide novel structures for recognition by the immune system.

The emerging role of leukocyte immunoglobulin-like receptors (LILRs) in HIV-1 infection

LILRs represent a group of immunomodulatory molecules that regulate the functional properties of professional APCs and influence immune activation in a variety of disease contexts. Many members of the LILR family recognize peptide/MHC class I complexes as their physiological ligands, and increasing evidence suggests that such interactions are prominently influenced by polymorphisms in HLA class I alleles or sequence variations in the presented antigenic peptides. Emerging data show that LILRs are involved in multiple, different aspects of HIV-1 disease pathogenesis and may critically influence spontaneous HIV-1 disease progression. Here, we review recent progress in understanding the role of LILR during HIV-1 infection by focusing on the dynamic interplay between LILR and HLA class I molecules in determining HIV-1 disease progression, the effects of HIV-1 mutational escape on LILR-mediated immune recognition, the contribution of LILR to HIV-1-associated immune dysfunction, and the unique expression patterns of LILR on circulating myeloid DCs from elite controllers, a small subset of HIV-1-infected patients with natural control of HIV-1 replication. Obtaining a more complete understanding of LILR-mediated immune regulation during HIV-1 infection may ultimately allow for improved strategies to treat or prevent HIV-1-associated disease manifestations.

Human NK cells differ more in their KIR2DL1-dependent thresholds for HLA-Cw6-mediated inhibition than in their maximal killing capacity

In this study we have addressed the question of how activation and inhibition of human NK cells is regulated by the expression level of MHC class I protein on target cells. Using target cell transfectants sorted to stably express different levels of the MHC class I protein HLA-Cw6, we show that induction of degranulation and that of IFN-γ secretion are not correlated. In contrast, the inhibition of these two processes by MHC class-I occurs at the same level of class I MHC protein. Primary human NK cell clones were found to differ in the amount of target MHC class I protein required for their inhibition, rather than in their maximum killing capacity. Importantly, we show that KIR2DL1 expression determines the thresholds (in terms of MHC I protein levels) required for NK cell inhibition, while the expression of other receptors such as LIR1 is less important. Furthermore, using mathematical models to explore the dynamics of target cell killing, we found that the observed delay in target cell killing is exhibited by a model in which NK cells require some activation or priming, such that each cell can lyse a target cell only after being activated by a first encounter with the same or a different target cell, but not by models which lack this feature.

The oxidative folding and misfolding of human leukocyte antigen-b27

The major histocompatibility complex class I molecule human leukocyte antigen (HLA)-B27 is strongly associated with a group of inflammatory arthritic disorders known as the spondyloarthropathies. Many autoimmune diseases exhibit associations with major histocompatibility complex molecules encoded within the class II locus with defined immune responses either mediated by T or B-lymphocytes. Despite the association being known for over 30 years, no defined immune response and target autoantigens have been characterized for the spondyloarthropathies. Thus, the mechanism and role of HLA-B27 in disease pathogenesis remains undetermined. One hypothesis that has recently received much attention has focused around the enhanced propensity for HLA-B27 to misfold and the increased tendency of the heavy chain to dimerize. The misfolding of HLA-B27 has been associated with its redox status and this is postulated to be involved in disease development. Here we discuss the impact of the redox status on HLA-B27 biosynthesis and function.

Transcription variants of SLA-7, a swine non classical MHC class I gene

In pig, very little information is available on the non classical class I (Ib) genes of the Major Histocompatibility Complex (MHC) i.e. SLA-6, -7 and -8. Our aim was to focus on the transcription pattern of the SLA-7 gene. RT-PCR experiments were carried out with SLA-7 specific primers targeting either the full coding sequence (CDS) from exon 1 to the 3 prime untranslated region (3UTR) or a partial CDS from exon 4 to the 3UTR. We show that the SLA-7 gene expresses a full length transcript not yet identified that refines annotation of the gene with eight exons instead of seven as initially described from the existing RefSeq RNA. These two RNAs encode molecules that differ in cytoplasmic tail length. In this study, another SLA-7 transcript variant was characterized, which encodes a protein with a shorter alpha 3 domain, as a consequence of a splicing site within exon 4. Surprisingly, a cryptic non canonical GA-AG splicing site is used to generate this transcript variant. An additional SLA-7 variant was also identified in the 3UTR with a splicing site occurring 31 nucleotides downstream to the stop codon. In conclusion, the pig SLA-7 MHC class Ib gene presents a complex transcription pattern with two transcripts encoding various molecules and transcripts that do not alter the CDS and may be subject to post-transcriptional regulation.

Interacting regions of CD81 and two of its partners, EWI-2 and EWI-2wint, and their effect on hepatitis C virus infection

CD81 is a tetraspanin protein that is involved in several essential cellular functions, as well as in the hepatitis C virus (HCV) infection. CD81 interacts with a high stoichiometry with its partner proteins EWI-2, EWI-2wint, and EWI-F. These latter proteins modify the functions of CD81 and can thereby potentially inhibit infection or modulate cell migration. Here, we characterized the cleavage of EWI-2 leading to the production of EWI-2wint, which has been shown to inhibit HCV infection. We determined the regions of EWI-2/EWI-2wint and CD81 that are important for their interaction and their functionality. More precisely, we identified a glycine zipper motif in the transmembrane domain of EWI-2/EWI-2wint that is essential for the interaction with CD81. In addition, we found that palmitoylation on two juxtamembranous cysteines in the cytosolic tail of EWI-2/EWI-2wint is required for their interaction with CD81 as well as with CD9, another tetraspanin. Thus, we have shown that palmitoylation of a tetraspanin partner protein can influence the interaction with a tetraspanin. We therefore propose that palmitoylation not only of tetraspanins, but also of their partner proteins is important in regulating the composition of complexes in tetraspanin networks. Finally, we identified the regions in CD81 that are necessary for its functionality in HCV entry and we demonstrated that EWI-2wint needs to interact with CD81 to exert its inhibitory effect on HCV infection.

Palmitoylation of MIR2 is required for its function

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes two RING finger E3 ubiquitin ligases (MIR1 and MIR2) that mediate ubiquitination and degradation of cellular proteins important for the establishment of an efficient antiviral immune response. MIR1 and MIR2 share 30% sequence identity; however, their substrate preferences are varied. MIR1 has been shown to primarily downregulate major histocompatibility complex class I (MHC-I), whereas MIR2 can downregulate a wide range of cell surface proteins. Many of the MIR substrates are thought to be present in lipid raft microdomains, a subregion of the plasma membrane known to be important for a wide range of signal transduction events. Palmitoylation is a posttranslational modification that increases recruitment of transmembrane proteins to lipid rafts. In this study, we investigated the importance of palmitoylation for MIR function. We present evidence that MIR2-mediated downregulation of MHC-I and platelet endothelial cell adhesion molecule 1 (PECAM-1) but not other substrates is inhibited in the presence of the drug 2-bromohexadecanoic acid (2-Br), a chemical inhibitor of palmitoylation. Biochemical analysis indicates that MIR2 is directly palmitoylated on cysteine 146. Mutation of this cysteine to a phenylalanine prevents MIR2 palmitoylation and blocks the ability of MIR2 to downregulate MHC-I and PECAM-I but not B7.2 and intercellular adhesion molecule 1 (ICAM-I), consistent with the phenotype observed after 2-Br treatment. Unpalmitoylated MIR2 does not interact with MHC-I and is thus unable to ubiquitinate and downregulate MHC-I from the cell surface. Furthermore, we observed that MIR2 is palmitoylated in vivo during lytic infection. Palmitoylation may act to regulate MIR2 function and localization during viral infection by allowing MIR2 to properly interact with and downregulate multiple substrates known to play an important role in the host immune response.

HLA-DP, HLA-DQ, and HLA-DR have different requirements for invariant chain and HLA-DM

The MHC is central to the adaptive immune response. The human MHC class II is encoded by three different isotypes, HLA-DR, -DQ, and -DP, each being highly polymorphic. In contrast to HLA-DR, the intracellular assembly and trafficking of HLA-DP molecules have not been studied extensively. However, different HLA-DP variants can be either protective or risk factors for infectious diseases (e.g. hepatitis B), immune dysfunction (e.g. berylliosis), and autoimmunity (e.g. myasthenia gravis). Here, we establish a system to analyze the chaperone requirements for HLA-DP and to compare the assembly and trafficking of HLA-DP, -DQ, and -DR directly. Unlike HLA-DR1, HLA-DQ5 and HLA-DP4 can form SDS-stable dimers supported by invariant chain (Ii) in the absence of HLA-DM. Uniquely, HLA-DP also forms dimers in the presence of HLA-DM alone. In model antigen-presenting cells, SDS-stable HLA-DP complexes are resistant to treatments that prevent formation of SDS-stable HLA-DR complexes. The unexpected properties of HLA-DP molecules may help explain why they bind to a more restricted range of peptides than other human MHC class II proteins and frequently present viral peptides.

Erratum to: rhesus macaque MHC class I molecules show differential subcellular localizations

The MHC class I gene family of rhesus macaques is characterised by considerable gene duplications. While a HLA-C-orthologous gene is absent, the Mamu-A and in particular the Mamu-B genes have expanded, giving rise to plastic haplotypes with differential gene content. Although some of the rhesus macaque MHC class I genes are known to be associated with susceptibility/resistance to infectious diseases, the functional significance of duplicated Mamu-A and Mamu-B genes and the expression pattern of their encoded proteins are largely unknown. Here, we present data of the subcellular localization of AcGFP-tagged Mamu-A and Mamu-B molecules. We found strong cell surface and low intracellular expression for Mamu-A1, Mamu-A2 and Mamu-A3-encoded molecules as well as for Mamu-B*01704, Mamu-B*02101, Mamu-B*04801, Mamu-B*06002 and Mamu-B*13401. In contrast, weak cell surface and strong intracellular expression was seen for Mamu-A4*1403, Mamu-B*01202, Mamu-B*02804, Mamu-B*03002, Mamu-B*05704, Mamu-I*010201 and Mamu-I*0121. The different expression patterns were assigned to the antigen-binding alpha1 and alpha2 domains, suggesting failure of peptide binding is responsible for retaining 'intracellular' Mamu class I molecules in the endoplasmic reticulum. These findings indicate a diverse functional role of the duplicated rhesus macaque MHC class I genes.

Evolutionary analysis of two classical MHC class I loci of the medaka fish, Oryzias latipes: haplotype-specific genomic diversity, locus-specific polymorphisms, and interlocus homogenization

The major histocompatibility complex (MHC) region of the teleost medaka (Oryzias latipes) contains two classical class I loci, UAA and UBA, whereas most lower vertebrates possess or express a single locus. To elucidate the allelic diversification and evolutionary relationships of these loci, we compared the BAC-based complete genomic sequences of the MHC class I region of three medaka strains and the PCR-based cDNA sequences of two more strains and two wild individuals, representing nine haplotypes. These were derived from two geographically distinct medaka populations isolated for four to five million years. Comparison of the genomic sequences showed a marked diversity in the region encompassing UAA and UBA even between the strains derived from the same population, and also showed an ancient divergence of these loci. cDNA analysis indicated that the peptide-binding domains of both UAA and UBA are highly polymorphic and that most of the polymorphisms were established in a locus-specific manner before the divergence of the two populations. Interallelic recombination between exons 2 and 3 encoding these domains was observed. The second intron of the UAA genes contains a highly conserved region with a palindromic sequence, suggesting that this region contributed to the recombination events. In contrast, the alpha3 domain is extremely homogenized not only within each locus but also between UAA and UBA regardless of populations. Two lineages of the transmembrane and cytoplasmic regions are also shared by UAA and UBA, suggesting that these two loci evolved with intimate genetic interaction through gene conversion or unequal crossing over.

The association of MHC class I proteins with the 2B4 receptor inhibits self-killing of human NK cells

The killing activity of NK cells is carried out by several activating NK receptors, which includes NKp46, NKp44, NKp30, NKp80, NKG2D, and 2B4. The ligands of these receptors are either self-derived, pathogen-derived, stress-induced ligands or tumor ligands. Importantly, none of these killer ligands are expressed on NK cells and thus self-killing of NK cells is prevented. A notable exception with this regard, is the ligand of the 2B4 receptor. This unusual receptor can exert both activating and inhibiting signals; however, in human NK cells, it serves mainly as an activating receptor. The ligand of 2B4 is CD48 and in contrast to the ligands of all the other NK activating receptors, CD48 is also present on NK cells. Thus, NK cells might be at risk for self-killing that is mediated via the 2B4-CD48 interaction. In this study, we identify a novel mechanism that prevents this self-killing as we show that the association of the MHC class I proteins with the 2B4 receptor, both present on NK cells, results in the attenuation of the 2B4-mediated self-killing of NK cells.

Rhesus macaque MHC class I molecules show differential subcellular localizations

The MHC class I gene family of rhesus macaques is characterised by considerable gene duplications. While a HLA-C-orthologous gene is absent, the Mamu-A and in particular the Mamu-B genes have expanded, giving rise to plastic haplotypes with differential gene content. Although some of the rhesus macaque MHC class I genes are known to be associated with susceptibility/resistance to infectious diseases, the functional significance of duplicated Mamu-A and Mamu-B genes and the expression pattern of their encoded proteins are largely unknown. Here, we present data of the subcellular localization of AcGFP-tagged Mamu-A and Mamu-B molecules. We found strong cell surface and low intracellular expression for Mamu-A1, Mamu-A2 and Mamu-A3-encoded molecules as well as for Mamu-B*01704, Mamu-B*02101, Mamu-B*04801, Mamu-B*06002 and Mamu-B*13401. In contrast, weak cell surface and strong intracellular expression was seen for Mamu-A4*1403, Mamu-B*01202, Mamu-B*02804, Mamu-B*03002, Mamu-B*05704, Mamu-I*010201 and Mamu-I*0121. The different expression patterns were assigned to the antigen-binding α1 and α2 domains, suggesting failure of peptide binding is responsible for retaining ‘intracellular’ Mamu class I molecules in the endoplasmic reticulum. These findings indicate a diverse functional role of the duplicated rhesus macaque MHC class I genes.

Molecular mechanisms of MHC class I-antigen processing: redox considerations

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.