Regulation of MHC class II antigen presentation by sorting of recycling HLA-DM/DO and class II within the multivesicular body

The Small GTPase Rab7 Regulates Antigen Processing in B Cells in a Possible Interplay with Autophagy Machinery

In B cells, antigen processing and peptide-antigen (pAg) presentation is essential to ignite high-affinity antibody responses with the help of cognate T cells. B cells efficiently internalize and direct specific antigens for processing and loading onto MHCII. This critical step, which enables pAg presentation, occurs in MHCII compartments (MIICs) which possess the enzymatic machinery for pAg loading on MHCII. The intracellular transport systems that guide antigen and maintain this unique compartment remain enigmatic. Here, we probed the possible functional role of two known endosomal proteins, the Rab family small GTPases Rab7 and Rab9, that are both reported to colocalize with internalized antigen. As compared to Rab9, we found Rab7 to exhibit a higher overlap with antigen and MIIC components. Rab7 also showed a higher association with antigen degradation. The inhibition of Rab7 drastically decreased pAg presentation. Additionally, we detected the strong colocalization of perinuclearly clustered and presumably MIIC-associated antigen with autophagy protein LC3. When we pharmacologically inhibited autophagy, pAg presentation was inhibited. Together, our data promote Rab7 as an important regulator of antigen processing and, considering the previously reported functions of Rab7 in autophagy, this also raises the possibility of the involvement of autophagy-related machinery in this process.

Revisiting nonclassical HLA II functions in antigen presentation: Peptide editing and its modulation

The nonclassical major histocompatibility complex of class II molecules (ncMHCII) HLA-DM (DM) and HLA-DO (DO) feature essential functions for the selection of the peptides that are displayed by classical MHCII proteins (MHCII) for CD4⁺ T_h cell surveillance. Thus, although the binding groove of classical MHCII dictates the main features of the peptides displayed, ncMHCII function defines the preferential loading of peptides from specific cellular compartments and the extent to which they are presented. DM acts as a chaperone for classical MHCII molecules facilitating peptide exchange and thereby favoring the binding of peptide-MHCII complexes of high kinetic stability mostly in late endosomal compartments. DO on the other hand binds to DM blocking its peptide-editing function in B cells and thymic epithelial cells, limiting DM activity in these cellular subsets. DM and DO distinct expression patterns therefore define specific antigen presentation profiles that select unique peptide pools for each set of antigen presenting cell. We have come a long way understanding the mechanistic underpinnings of such distinct editing profiles and start to grasp the implications for ncMHCII biological function. DM acts as filter for the selection of immunodominant, pathogen-derived epitopes while DO blocks DM activity under certain physiological conditions to promote tolerance to self. Interestingly, recent findings have shown that the unexplored and neglected ncMHCII genetic diversity modulates retroviral infection in mouse, and affects human ncMHCII function. This review aims at highlighting the importance of ncMHCII function for CD4⁺ T_h cell responses while integrating and evaluating what could be the impact of distinct editing profiles because of natural genetic variations.

Alphaherpesvirus gB Homologs Are Targeted to Extracellular Vesicles, but They Differentially Affect MHC Class II Molecules

Herpesvirus envelope glycoprotein B (gB) is one of the best-documented extracellular vesicle (EVs)-incorporated viral proteins. Regarding the sequence and structure conservation between gB homologs, we asked whether bovine herpesvirus-1 (BoHV-1) and pseudorabies virus (PRV)-encoded gB share the property of herpes simplex-1 (HSV-1) gB to be trafficked to EVs and affect major histocompatibility complex (MHC) class II. Our data highlight some conserved and differential features of the three gBs. We demonstrate that mature, fully processed BoHV-1 and PRV gBs localize to EVs isolated from constructed stable cell lines and EVs-enriched fractions from virus-infected cells. gB also shares the ability to co-localize with CD63 and MHC II in late endosomes. However, we report here a differential effect of the HSV-1, BoHV-1, and PRV glycoprotein on the surface MHC II levels, and MHC II loading to EVs in stable cell lines, which may result from their adverse ability to bind HLA-DR, with PRV gB being the most divergent. BoHV-1 and HSV-1 gB could retard HLA-DR exports to the plasma membrane. Our results confirm that the differential effect of gB on MHC II may require various mechanisms, either dependent on its complex formation or on inducing general alterations to the vesicular transport. EVs from virus-infected cells also contained other viral glycoproteins, like gD or gE, and they were enriched in MHC II. As shown for BoHV-1 gB- or BoHV-1-infected cell-derived vesicles, those EVs could bind anti-virus antibodies in ELISA, which supports the immunoregulatory potential of alphaherpesvirus gB.

B cells rapidly target antigen and surface-derived MHCII into peripheral degradative compartments

In order to mount high-affinity antibody responses, B cells internalise specific antigens and process them into peptides loaded onto MHCII for presentation to T helper cells (T_H cells). While the biochemical principles of antigen processing and MHCII loading have been well dissected, how the endosomal vesicle system is wired to enable these specific functions remains much less studied. Here, we performed a systematic microscopy-based analysis of antigen trafficking in B cells to reveal its route to the MHCII peptide-loading compartment (MIIC). Surprisingly, we detected fast targeting of internalised antigen into peripheral acidic compartments that possessed the hallmarks of the MIIC and also showed degradative capacity. In these vesicles, internalised antigen converged rapidly with membrane-derived MHCII and partially overlapped with cathepsin-S and H2-M, both required for peptide loading. These early compartments appeared heterogenous and atypical as they contained a mixture of both early and late endosomal markers, indicating a specialized endosomal route. Together, our data suggest that, in addition to in the previously reported perinuclear late endosomal MIICs, antigen processing and peptide loading could have already started in these specialized early peripheral acidic vesicles (eMIIC) to support fast peptide-MHCII presentation.

Synergy between B cell receptor/antigen uptake and MHCII peptide editing relies on HLA-DO tuning

B cell receptors and surface-displayed peptide/MHCII complexes constitute two key components of the B-cell machinery to sense signals and communicate with other cell types during antigen-triggered activation. However, critical pathways synergizing antigen-BCR interaction and antigenic peptide-MHCII presentation remain elusive. Here, we report the discovery of factors involved in establishing such synergy. We applied a single-cell measure coupled with super-resolution microscopy to investigate the integrated function of two lysosomal regulators for peptide loading, HLA-DM and HLA-DO. In model cell lines and human tonsillar B cells, we found that tunable DM/DO stoichiometry governs DM_free activity for exchange of placeholder CLIP peptides with high affinity MHCII ligands. Compared to their naïve counterparts, memory B cells with less DM_free concentrate a higher proportion of CLIP/MHCII in lysosomal compartments. Upon activation mediated by high affinity BCR, DO tuning is synchronized with antigen internalization and rapidly potentiates DM_free activity to optimize antigen presentation for T-cell recruitment.

Human leukocyte Antigen-DM polymorphisms in autoimmune diseases

Classical MHC class II (MHCII) proteins present peptides for CD4⁺ T-cell surveillance and are by far the most prominent risk factor for a number of autoimmune disorders. To date, many studies have shown that this link between particular MHCII alleles and disease depends on the MHCII's particular ability to bind and present certain peptides in specific physiological contexts. However, less attention has been paid to the non-classical MHCII molecule human leucocyte antigen-DM, which catalyses peptide exchange on classical MHCII proteins acting as a peptide editor. DM function impacts the presentation of both antigenic peptides in the periphery and key self-peptides during T-cell development in the thymus. In this way, DM activity directly influences the response to pathogens, as well as mechanisms of self-tolerance acquisition. While decreased DM editing of particular MHCII proteins has been proposed to be related to autoimmune disorders, no experimental evidence for different DM catalytic properties had been reported until recently. Biochemical and structural investigations, together with new animal models of loss of DM activity, have provided an attractive foundation for identifying different catalytic efficiencies for DM allotypes. Here, we revisit the current knowledge of DM function and discuss how DM function may impart autoimmunity at the organism level.

Introduction: MHC/KIR and governance of specificity

The MHC controls specificity, to ensure that appropriate immune responses are mounted to invading pathogens whilst maintaining tolerance to the host. It encodes molecules that act as sentinels, providing a snapshot of the health of the interior and exterior of the cell for immune surveillance. To maintain the ability to respond appropriately to any disease requires a delicate balance of expression and function, and many subtleties of the system have been described at the gene, individual and population level. The main players are the highly polymorphic classical MHC class I and class II molecules, as well as some non-classical loci of both types. Transporter associated with antigen processing (TAP) peptide transporters, proteasome components and Tapasin, encoded within the MHC, are also involved in selection of peptide for presentation. The plethora of mechanisms microorganisms use to subvert immune recognition, through blocking these antigen processing and presentation pathways, attests to the importance of HLA in resistance to infection. There is continued interest in MHC genetics in its own right, as well as in relation to KIR, to transplantation, infection, autoimmunity and reproduction. Also of topical interest, cancer immunotherapy through checkpoint inhibition depends on highly specific recognition of cancer peptide antigen and continued expression of HLA molecules. Here, we briefly introduce some background to the MHC/KIR axis in man. This special issue of immunogenetics expands on these topics, in humans and other model species.

The Other Function: Class II-Restricted Antigen Presentation by B Cells

Mature B lymphocytes (B cells) recognize antigens using their B cell receptor (BCR) and are activated to become antibody-producing cells. In addition, and integral to the development of a high-affinity antibodies, B cells utilize the specialized major histocompatibility complex class II (MHCII) antigen presentation pathway to process BCR-bound and internalized protein antigens and present selected peptides in complex with MHCII to CD4+ T cells. This interaction influences the fate of both types of lymphocytes and shapes immune outcomes. Specific, effective, and optimally timed antigen presentation by B cells requires well-controlled intracellular machinery, often regulated by the combined effects of several molecular events. Here, we delineate and summarize these events in four steps along the antigen presentation pathway: (1) antigen capture and uptake by B cells; (2) intersection of internalized antigen/BCRs complexes with MHCII in peptide-loading compartments; (3) generation and regulation of MHCII/peptide complexes; and (4) exocytic transport for presentation of MHCII/peptide complexes at the surface of B cells. Finally, we discuss modulation of the MHCII presentation pathway across B cell development and maturation to effector cells, with an emphasis on the shaping of the MHCII/peptide repertoire by two key antigen presentation regulators in B cells: HLA-DM and HLA-DO.

pH-susceptibility of HLA-DO tunes DO/DM ratios to regulate HLA-DM catalytic activity

The peptide-exchange catalyst, HLA-DM, and its inhibitor, HLA-DO control endosomal generation of peptide/class II major histocompatibility protein (MHC-II) complexes; these complexes traffic to the cell surface for inspection by CD4+ T cells. Some evidence suggests that pH influences DO regulation of DM function, but pH also affects the stability of polymorphic MHC-II proteins, spontaneous peptide loading, DM/MHC-II interactions and DM catalytic activity, imposing challenges on approaches to determine pH effects on DM-DO function and their mechanistic basis. Using optimized biochemical methods, we dissected pH-dependence of spontaneous and DM-DO-mediated class II peptide exchange and identified an MHC-II allele-independent relationship between pH, DO/DM ratio and efficient peptide exchange. We demonstrate that active, free DM is generated from DM-DO complexes at late endosomal/lysosomal pH due to irreversible, acid-promoted DO destruction rather than DO/DM molecular dissociation. Any soluble DM that remains in complex with DO stays inert. pH-exposure of DM-DO in cell lysates corroborates such a pH-regulated mechanism, suggesting acid-activated generation of functional DM in DO-expressing cells.

Invariant chain as a vehicle to load antigenic peptides on human MHC class I for cytotoxic T-cell activation

Protective T-cell responses depend on efficient presentation of antigen (Ag) in the context of major histocompatibility complex class I (MHCI) and class II (MHCII) molecules. Invariant chain (Ii) serves as a chaperone for MHCII molecules and mediates trafficking to the endosomal pathway. The genetic exchange of the class II-associated Ii peptide (CLIP) with antigenic peptides has proven efficient for loading of MHCII and activation of specific CD4(+) T cells. Here, we investigated if Ii could similarly activate human CD8(+) T cells when used as a vehicle for cytotoxic T-cell (CTL) epitopes. The results show that wild type Ii, and Ii in which CLIP was replaced by known CTL epitopes from the cancer targets MART-1 or CD20, coprecipitated with HLA-A*02:01 and mediated colocalization in the endosomal pathway. Furthermore, HLA-A*02:01-positive cells expressing CLIP-replaced Ii efficiently activated Ag-specific CD8(+) T cells in a TAP- and proteasome-independent manner. Finally, dendritic cells transfected with mRNA encoding IiMART-1 or IiCD20 primed naïve CD8(+) T cells. The results show that Ii carrying antigenic peptides in the CLIP region can promote efficient presentation of the epitopes to CTLs independently of the classical MHCI peptide loading machinery, facilitating novel vaccination strategies against cancer.

HLA-DO increases bacterial superantigen binding to human MHC molecules by inhibiting dissociation of class II-associated invariant chain peptides

HLA-DO (H2-O in mice) is an intracellular non-classical MHC class II molecule (MHCII). It forms a stable complex with HLA-DM (H2-M in mice) and shapes the MHC class II-associated peptide repertoire. Here, we tested the impact of HLA-DO and H2-O on the binding of superantigens (SAgs), which has been shown previously to be sensitive to the structural nature of the class II-bound peptides. We found that the binding of staphylococcal enterotoxin (SE) A and B, as well as toxic shock syndrome toxin 1 (TSST-1), was similar on the HLA-DO(+) human B cell lines 721.45 and its HLA-DO(-) counterpart. However, overexpressing HLA-DO in MHC class II(+) HeLa cells (HeLa-CIITA-DO) improved binding of SEA and TSST-1. Accordingly, knocking down HLA-DO expression using specific siRNAs decreased SEA and TSST-1 binding. We tested directly the impact of the class II-associated invariant chain peptide (CLIP), which dissociation from MHC class II molecules is inhibited by overexpressed HLA-DO. Loading of synthetic CLIP on HLA-DR(+) cells increased SEA and TSST-1 binding. Accordingly, knocking down HLA-DM had a similar effect. In mice, H2-O deficiency had no impact on SAgs binding to isolated splenocytes. Altogether, our results demonstrate that the sensitivity of SAgs to the MHCII-associated peptide has physiological basis and that the effect of HLA-DO on SEA and TSST-1 is mediated through the inhibition of CLIP release.

Ubiquitination of HLA-DO by MARCH family E3 ligases

HLA-DO (DO) is a nonclassical MHC class II (MHCII) molecule that negatively regulates the ability of HLA-DM to catalyse the removal of invariant chain-derived CLIP peptides from classical MHCII molecules. Here, we show that DO is posttranslationally modified by ubiquitination. The location of the modified lysine residue is shared with all classical MHCII beta chains, suggesting a conserved function. Three membrane-associated RING-CH (MARCH1, 8 and 9) family E3 ligases that polyubiquitinate MHCII induce similar profiles of polyubiquitination on DOβ. All three MARCH proteins also influenced trafficking of DO indirectly by a mechanism that required the DOβ encoded di-leucine and tyrosine-based endocytosis motifs. This may be the result of MARCH-induced ubiquitination of components of the endocytic machinery. MARCH9 was by far the most efficient at inducing intracellular redistribution of DO but did not target molecules for lysosomal degradation. The specificity of MARCH9 for HLA-DQ and HLA-DO suggests a need for common regulation of these two MHC-encoded molecules.

Sibling rivalry: competition between MHC class II family members inhibits immunity

Peptide loading of major histocompatibility complex (MHC) class II molecules in the endosomes and lysosomes of antigen-presenting cells is catalyzed by human leukocyte antigen-DM (HLA-DM) and modulated by HLA-DO. In a structural study in this issue, Guce et al. show that HLA-DO is an MHC class II mimic and functions as a competitive and essentially irreversible inhibitor of HLA-DM activity, thereby inhibiting MHC class II antigen presentation.

Pathways of antigen processing

T cell recognition of antigen-presenting cells depends on their expression of a spectrum of peptides bound to major histocompatibility complex class I (MHC-I) and class II (MHC-II) molecules. Conversion of antigens from pathogens or transformed cells into MHC-I- and MHC-II-bound peptides is critical for mounting protective T cell responses, and similar processing of self proteins is necessary to establish and maintain tolerance. Cells use a variety of mechanisms to acquire protein antigens, from translation in the cytosol to variations on the theme of endocytosis, and to degrade them once acquired. In this review, we highlight the aspects of MHC-I and MHC-II biosynthesis and assembly that have evolved to intersect these pathways and sample the peptides that are produced.

HLA-DO acts as a substrate mimic to inhibit HLA-DM by a competitive mechanism

MHCII proteins bind peptide antigens in endosomal compartments of antigen-presenting cells. The non-classical MHCII protein HLA-DM chaperones peptide-free MHCII against inactivation and catalyzes peptide exchange on loaded MHCII. Another non-classical MHCII protein, HLA-DO, binds HLA-DM and influences the repertoire of peptides presented by MHCII proteins. However, the mechanism by which HLA-DO functions is unclear. Here we use x-ray crystallography, enzyme kinetics and mutagenesis approaches to investigate human HLA-DO structure and function. In complex with HLA-DM, HLA-DO adopts a classical MHCII structure, with alterations near the alpha subunit 3₁₀ helix. HLA-DO binds to HLA-DM at the same sites implicated in MHCII interaction, and kinetic analysis demonstrates that HLA-DO acts as a competitive inhibitor. These results show that HLA-DO inhibits HLA-DM function by acting as a substrate mimic and place constraints on possible functional roles for HLA-DO in antigen presentation.

Mapping the HLA-DO/HLA-DM complex by FRET and mutagenesis

HLA-DO (DO) is a nonclassic class II heterodimer that inhibits the action of the class II peptide exchange catalyst, HLA-DM (DM), and influences DM localization within late endosomes and exosomes. In addition, DM acts as a chaperone for DO and is required for its egress from the endoplasmic reticulum (ER). These reciprocal functions are based on direct DO/DM binding, but the topology of DO/DM complexes is not known, in part, because of technical limitations stemming from DO instability. We generated two variants of recombinant soluble DO with increased stability [zippered DOαP11A (szDOv) and chimeric sDO-Fc] and confirmed their conformational integrity and ability to inhibit DM. Notably, we found that our constructs, as well as wild-type sDO, are inhibitory in the full pH range where DM is active (4.7 to ∼6.0). To probe the nature of DO/DM complexes, we used intermolecular fluorescence resonance energy transfer (FRET) and mutagenesis and identified a lateral surface spanning the α1 and α2 domains of szDO as the apparent binding site for sDM. We also analyzed several sDM mutants for binding to szDOv and susceptibility to DO inhibition. Results of these assays identified a region of DM important for interaction with DO. Collectively, our data define a putative binding surface and an overall orientation of the szDOv/sDM complex and have implications for the mechanism of DO inhibition of DM.

The "Bridge" in the Epstein-Barr virus alkaline exonuclease protein BGLF5 contributes to shutoff activity during productive infection

Replication of the human herpesvirus Epstein-Barr virus drastically impairs cellular protein synthesis. This shutoff phenotype results from mRNA degradation upon expression of the early lytic-phase protein BGLF5. Interestingly, BGLF5 is the viral DNase, or alkaline exonuclease, homologues of which are present throughout the herpesvirus family. During productive infection, this DNase is essential for processing and packaging of the viral genome. In contrast to this widely conserved DNase activity, shutoff is only mediated by the alkaline exonucleases of the subfamily of gammaherpesviruses. Here, we show that BGLF5 can degrade mRNAs of both cellular and viral origin, irrespective of polyadenylation. Furthermore, shutoff by BGLF5 induces nuclear relocalization of the cytosolic poly(A) binding protein. Guided by the recently resolved BGLF5 structure, mutants were generated and analyzed for functional consequences on DNase and shutoff activities. On the one hand, a point mutation destroying DNase activity also blocks RNase function, implying that both activities share a catalytic site. On the other hand, other mutations are more selective, having a more pronounced effect on either DNA degradation or shutoff. The latter results are indicative of an oligonucleotide-binding site that is partially shared by DNA and RNA. For this, the flexible "bridge" that crosses the active-site canyon of BGLF5 appears to contribute to the interaction with RNA substrates. These findings extend our understanding of the molecular basis for the shutoff function of BGLF5 that is conserved in gammaherpesviruses but not in alpha- and betaherpesviruses.

Cutting edge: HLA-DO impairs the incorporation of HLA-DM into exosomes

In multivesicular bodies, HLA-DM (DM) assists the loading of antigenic peptides on classical MHC class II molecules such as HLA-DR. In cells expressing HLA-DO (DO), DM is redistributed from the internal vesicles to the limiting membrane of these organelles. This suggests that DO might reduce DM incorporation into exosomes, which are shed upon fusion of multivesicular bodies with the plasma membrane. To test this hypothesis, we used the 721.45 B lymphoblastoid cell line and different HeLa cell transfectants. We demonstrate that the poor recovery of DM in exosomes as compared with HLA-DR is not the mere reflection of differences in protein expression. Indeed, we found that DO contributes to the inefficient transfer of DM to exosomes. This negative regulation requires an intact di-leucine endosomal sorting motif in the cytoplasmic tail of HLA-DOβ. These results demonstrate that canonical sorting signals and protein-protein interactions modulate the selection of MHC protein cargos.

Forced expression of HLA-DM at the surface of dendritic cells increases loading of synthetic peptides on MHC class II molecules and modulates T cell responses

Adoptive transfer of autologous dendritic cells (DCs) loaded with tumor-associated CD4 and CD8 T cell epitopes represents a promising avenue for the immunotherapy of cancer. In an effort to increase the loading of therapeutic synthetic peptides on MHC II molecules, we used a mutant of HLA-DM (DMY) devoid of its lysosomal sorting motif and that accumulates at the cell surface. Transfection of DMY into HLA-DR(+) cells resulted in increased loading of the exogenously supplied HA(307-318) peptide, as well as increased stimulation of HA-specific T cells. Also, on transduction in mouse and human DCs, DMY increased loading of HEL(48-61) and of the tumor Ag-derived gp100(174-190) peptides, respectively. Interestingly, expression of DMY at the surface of APCs favored Th1 differentiation over Th2. Finally, we found that DMY(-) and DMY(+) mouse APCs differentially stimulated T cell hybridomas sensitive to the fine conformation of peptide-MHC II complexes. Taken together, our results suggest that the overexpression of HLA-DMY at the plasma membrane of DCs may improve quantitatively, but also qualitatively, the presentation of CD4 T cell epitopes in cellular vaccine therapies for cancer.

EBV protein BNLF2a exploits host tail-anchored protein integration machinery to inhibit TAP

EBV, the prototypic human γ(1)-herpesvirus, persists for life in infected individuals, despite the presence of vigorous antiviral immunity. CTLs play an important role in the protection against viral infections, which they detect through recognition of virus-encoded peptides presented in the context of HLA class I molecules at the cell surface. The viral peptides are generated in the cytosol and are transported into the endoplasmic reticulum (ER) by TAP. The EBV-encoded lytic-phase protein BNLF2a acts as a powerful inhibitor of TAP. Consequently, loading of antigenic peptides onto HLA class I molecules is hampered, and recognition of BNLF2a-expressing cells by cytotoxic T cells is avoided. In this study, we characterize BNLF2a as a tail-anchored (TA) protein and elucidate its mode of action. Its hydrophilic N-terminal domain is located in the cytosol, whereas its hydrophobic C-terminal domain is inserted into membranes posttranslationally. TAP has no role in membrane insertion of BNLF2a. Instead, Asna1 (also named TRC40), a cellular protein involved in posttranslational membrane insertion of TA proteins, is responsible for integration of BNLF2a into the ER membrane. Asna1 is thereby required for efficient BNLF2a-mediated HLA class I downregulation. To optimally accomplish immune evasion, BNLF2a is composed of two specialized domains: its C-terminal tail anchor ensures membrane integration and ER retention, whereas its cytosolic N terminus accomplishes inhibition of TAP function. These results illustrate how EBV exploits a cellular pathway for TA protein biogenesis to achieve immune evasion, and they highlight the exquisite adaptation of this virus to its host.

Major histocompatibility complex polymorphism: dynamics and consequences of parasite-mediated local adaptation in fishes

Parasitism is a common form of life and represents a strong selective pressure for host organisms. In response to this evolutionary pressure, vertebrates have developed genetically coded defences such as the major histocompatibility complex (MHC). Mechanisms of parasite-mediated selection not only maintain outstanding polymorphism in these genes but have also been proposed to further promote host population divergence and ultimately speciation because it can drive evolution of local adaptation in which MHC genes play a crucial role. This review first highlights the dynamics and complexity of parasite-mediated selection in natural systems, which not only depends on dominating parasite strategies and on the taxonomic diversity of the parasite community but also includes the differences in parasite communities between habitats and niches, creating divergent selection on locally adapted populations. Then the different ways in which MHC genes potentially allow vertebrates to respond to these dynamics and to adapt locally are outlined. Finally, it is proposed that varying selection strength in time and space may lead to variation in the strength of precopulatory reproductive isolation which has evolved to maintain local adaptation.

Dendritic cell-based immunotherapy in myeloid leukaemia: translating fundamental mechanisms into clinical applications

Immunotherapy for leukaemia patients, aiming at the generation of anti-leukaemic T cell responses, could provide a new therapeutic approach to eliminate minimal residual disease (MRD) cells in acute myeloid leukaemia (AML). Leukaemic blasts harbour several ways to escape the immune system including deficient MHC class II expression, low levels of co-stimulatory molecules and suppressive cytokines. Therapeutic vaccination with dendritic cells (DC) is now recognized as an important investigational therapy. Due to their unique antigen presenting capacity, immunosuppressive features of the leukaemic blasts can be circumvented. DC can be successfully cultured from leukaemic blasts in 60-70% of patients and show functional potential in vivo. Alternatively, monocyte derived DC obtained at time of complete remission loaded with leukaemia-specific antigens can be used as vaccine. Several sources of leukaemia-associated antigen and different methods of loading antigen onto DC have been used in an attempt to optimize antitumour responses including apoptotic cells, necrotic cell lysates and tumour-associated pep-tides. Currently, the AML-derived cell line MUTZ-3, an immortalized equivalent of CD34(+) DC precursor cells, is under investigation for vaccination purposes. For effective DC vaccination the intrinsic tolerant state of the patient must be overcome. Therefore, the development of efficient and safe adjuvants in antigen specific immunotherapeutic programs should be encouraged.

The HLA-DRalpha chain is modified by polyubiquitination

Ubiquitination plays a major role in regulating cell surface and intracellular localization of major histocompatibility complex class II molecules. Two E3 ligases, MARCH I and MARCH VIII, have been shown to polyubiquitinate lysine residue 225 in the cytoplasmic tail of I-Abeta and HLA-DRbeta. We show that lysine residue 219 in the cytoplasmic tail of DRalpha is also subject to polyubiquitination. Each chain of the HLA-DR heterodimer is independently recognized and ubiquitinated, but DRbeta is more extensively modified. In the cytoplasmic tail of DRbeta lysine, residue 225 is the only residue that is absolutely required for ubiquitination; all other residues can be deleted or substituted without loss of function. In contrast, although lysine 219 is absolutely required for modification of DRalpha, other features of the DRalpha tail act to limit the extent of ubiquitination.

Characterization of intracellular HLA-DR, DM and DO profile in K562 and HL-60 leukemic cells

Surface class-II antigen expression fires-up the antigen presentation process and development of immune response. The absence of surface HLA-DR is used in various systems to avoid immune recognition. Most leukemic cells use such mechanism to escape immune surveillance. Here, K562 and HL-60 leukemic cells were examined as to intracellular HLA-DR, -DM and -DO expression, if any. Immunofluorescence scored by UV-microscopy, flow cytometry or confocal microscope analysis detected intracellular pools of HLA-DR, -DO and to a lesser degree HLA-DM, whereas sub-cellular fractionation localized these molecules within endosomes. RT-PCR experiments revealed the presence of HLA-DRalphabeta, HLA-DMalphabeta and HLA-DObeta but not HLA-DOalpha transcripts. Despite the absence of the HLA-DOalpha chain, stable transfectants of K562 with a full length HLA-DObeta-EGFP construct showed that DObeta chain could be translocated to endosomes and form stable complexes with HLA-DR. Such complexes could be responsible for arresting HLA-DR molecules within endosomes, maintaining their surface class-II negative state.

B cell activation regulates exosomal HLA production

Exosomes are nanovesicles produced constitutively and inducibly by several types of cells. They are generated as intraluminal vesicles of multivesicular bodies and express MHC and several endosomal/lysosomal proteins. In spite of their potential role in cellular immunity, the regulatory mechanisms of exosome production are largely unknown. In this study, we have established a novel ELISA system to quantify exosomal HLA using a combination of anti-HLA class I and anti-HLA-DR mAb. We found that exosomal HLA production of B cells was enhanced by contact with CD4(+) T cells. Neutralizing anti-CD154 (CD40L) mAb inhibited this effect, and a soluble CD40L significantly increased production of exosomal HLA in B cells. In addition, B cell stimulation via BCR and TLR9 enhanced their production while IL-4 stimulation alone failed to do so. Strikingly, an inhibitor of the classical NF-kappaB pathway drastically inhibited exosomal HLA production in stimulated B cells, indicating that the classical NF-kappaB pathway is critical for exosomal HLA production in B cells. Together, these findings suggest a pivotal role of B cell activation in exosomal HLA production in vivo.

The DMalpha and DMbeta chain cooperate in the oxidation and folding of HLA-DM

HLA-DM (DM) is a heterodimeric MHC molecule that catalyzes the peptide loading of classical MHC class II molecules in the endosomal/lysosomal compartments of APCs. Although the function of DM is well-established, little is known about how DMalpha and beta-chains fold, oxidize, and form a complex in the endoplasmic reticulum (ER). In this study, we show that glycosylation promotes, but is not essential for, DMalphabeta ER exit. However, glycosylation of DMalpha N15 is required for oxidation of the alpha-chain. The DMalpha and beta-chains direct each others fate: single DMalpha chains cannot fully oxidize without DMbeta, while DMbeta forms disulfide-linked homodimers without DMalpha. Correct oxidation and subsequent ER egress depend on the unique DMbeta C25 and C35 residues. This suggests that the C25-C35 disulfide bond in the peptide-binding domain overcomes the need for stabilizing peptides required by other MHC molecules.

In vivo control of endosomal architecture by class II-associated invariant chain and cathepsin S

The invariant chain (Ii) is a chaperone that regulates assembly and transport of class II MHC molecules. In the absence of the lysosomal protease cathepsin S (CatS), degradation of Ii is impaired and an Ii remnant that extends from the N terminus to about residue 110 accumulates in class II MHC-positive endosomal compartments, which are enlarged in size and lack multivesicular morphology. In primary B cells examined in vitro and in lymph nodes examined by immuno-electron microscopy, CatS controls architecture of class II-positive endosomal compartments. In a compound mutant mouse that lacks both CatS and Ii, the normal size of endosomes in class II-positive cells is restored, although internal endosomal membranes are absent. Proper degradation of Ii is thus essential for normal endosomal morphology in antigen-presenting cells in vivo.

Right place, right time, right peptide: DO keeps DM focused

Peptide loading of major histocompatibility class II molecules is catalyzed in late endosomal and lysosomal compartments of cells by the catalytic action of human leukocyte antigen (HLA)-DM (H-2M in mice). In B cells, dendritic cells and thymic epithelial cells, the peptide loading of class II molecules is modified by the expression of the non-classical class II molecule, HLA-DO (H-2O in mice). Collectively, studies to date support that DO/H-2O expression inhibits the presentation of antigens acquired by cells via fluid phase endocytosis. However, in B cells, the expression of H-2O promotes the presentation of antigens internalized by the B-cell receptor. In this review, we summarize the literature pertaining to DO assembly, transport, and function, with an emphasis on the function of DO/H-2O.

Chaperoning antigen presentation by MHC class II molecules and their role in oncogenesis

Tumor vaccine development aimed at stimulating the cellular immune response focuses mainly on MHC class I molecules. This is not surprising since most tumors do not express MHC class II or CD1 molecules. Nevertheless, the most successful targets for cancer immunotherapy, leukemia and melanoma, often do express MHC class II molecules, which leaves no obvious reason to ignore MHC class II molecules as a mediator in anticancer immune therapy. We review the current state of knowledge on the process of MHC class II-restricted antigen presentation and subsequently discuss the consequences of MHC class II expression on tumor surveillance and the induction of an efficient MHC class II mediated antitumor response in vivo and after vaccination.

A point mutation in the groove of HLA-DO allows egress from the endoplasmic reticulum independent of HLA-DM

B lymphocytes express the nonclassical class II molecule HLA-DO, which modulates the peptide loading activity of HLA-DM in the endocytic pathway. Binding to HLA-DM is required for HLA-DO to egress from the endoplasmic reticulum (ER). To gain insights into the mode of action of DO and on the role of DM in ER release, we sought to identify DM-binding residues on DO. Our results show that DOalpha encompasses the binding site for HLA-DM. More specifically, mutation of residue DOalpha41 on an exposed lateral loop of the alpha1 domain affects the binding to DM, ER egress, and activity of DO. Using a series of chimeric DR/DO molecules, we confirmed the role of the alpha chain and established that a second DM-binding region is located C-terminal to the DOalpha80 residue, most probably in the alpha2 domain. Interestingly, after mutation of a buried proline (alpha11) on the floor of the putative peptide-binding groove, HLA-DO remained functional but became independent of HLA-DM for ER egress and intracellular trafficking. Collectively, these results suggest that the binding of HLA-DM to DOalpha allows the complex to egress from the ER by stabilizing intramolecular contacts between the N-terminal antiparallel beta-strands of the DOalphabeta heterodimer.

Spatial separation of HLA-DM/HLA-DR interactions within MIIC and phagosome-induced immune escape

Major Histocompatibility Complex (MHC) class II molecules, including Human Leukocyte Antigen (HLA)-DR, present peptide fragments from proteins degraded in the endocytic pathway. HLA-DR is targeted to late-endocytic structures named MHC class II-containing Compartments (MIIC), where it interacts with HLA-DM. This chaperone stabilizes HLA-DR during peptide exchange and is critical for successful peptide loading. To follow this process in living cells, we have generated cells containing HLA-DR3/Cyan Fluorescent Protein (CFP), HLA-DM/Yellow Fluorescent Protein (YFP), and invariant chain. HLA-DR/DM interactions were observed by Fluorescence Resonance Energy Transfer (FRET). These interactions were pH insensitive, yet occurred only in internal structures and not at the limiting membrane of MIIC. In a cellular model of infection, phagosomes formed a limiting membrane surrounding internalized Salmonella. HLA-DR and HLA-DM did not interact in Salmonella-induced vacuoles, and HLA-DR was not loaded with antigens. The absence of HLA-DR/DM interactions at the limiting membrane prevents local loading of MHC class II molecules in phagosomes. This may allow these bacteria to successfully evade the immune system.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

The expression of HLA-DO (H2-O) in B lymphocytes

HLA-DO (H2-O in mice) is a nonpolymorphic transmembrane alphabeta heterodimer encoded in the class II region of the major histocompatibility complex (MHC). It is expressed selectively in B lymphocytes and thymic medullary epithelial cells. DO forms a stable complex with the peptide-loading catalyst HLA-DM in the endoplasmic reticulum (ER); in the absence of DM, DO is unstable. During intracellular transport and distribution in the endosomal compartments, the ratio of DO to DM changes. In primary B cells, only approx 50% of DM molecules are associated with DO. DO appears to regulate the peptide-loading function of DM in the MHC class II antigen-presentation pathway. Although certain discrepancies are present, results from most studies indicate that DO (as well as H2-O) inhibits DM (H2-M) function; this inhibition is pH-dependent. As a consequence, DO restrains presentation of exogenous antigens delivered through nonreceptor-mediated mechanisms; in addition, DO alters the peptide repertoire that is associated with cell-surface class II molecules. The biological function of DO remains obscure, partially because of the lack of striking phenotypes in the H2-O knockout mice. Results from recent studies indicate that DO expression in B cells is dynamic, and highly regulated during B-cell development and B-cell activation, suggesting that the physiological role of DO is to tailor the antigen presentation function of the B-lineage cells to meet their primary function at each stage of B-cell development and maturation. Further investigations are needed in this direction.

Subcellular localization of Toll-like receptor 3 in human dendritic cells

Toll-like receptor (TLR)3 recognizes dsRNA and transduces signals to activate NF-kappaB and IFN-beta promoter. Type I IFNs (IFN-alpha/beta) function as key cytokines in anti-viral host defense. Human fibroblasts express TLR3 on the cell surface, and anti-TLR3 mAb inhibits dsRNA-induced IFN-beta secretion by fibroblasts, suggesting that TLR3 acts on the cell surface to sense viral infection. In this study, we examined the expression and localization of human TLR3 in various DC subsets using anti-TLR3 mAb. In monocyte-derived immature dendritic cells (iDCs), TLR3 predominantly resided inside the cells but not on the cell surface. iDCs produced IL-12p70 and IFN-alpha and -beta in response to poly(I:C). Similar response was observed in iDCs treated with rotavirus-derived dsRNA. These responses could not be blocked by pretreatment of the cells with anti-TLR3 mAb. In CD11c(+) blood DCs, cytoplasmic retention of TLR3 was also observed as in monocyte-derived iDCs, again endorsing a different TLR3 distribution profile from fibroblasts. In precursor DC2, however, TLR3 could not be detected inside or outside the cells. Of note, there was a putative centrosomal protein that shared an epitope with TLR3 in myeloid DCs and precursor DC2, but not peripheral blood monocytes. Immunoelectron microscopic analysis revealed that TLR3, when stably expressed in the murine B cell line Ba/F3, was specifically accumulated in multivesicular bodies, a subcellular compartment situated in endocytic trafficking pathways. Thus, regulation and localization of TLR3 are different in each cell type, which may reflect participation of cell type-specific multiple pathways in antiviral IFN induction via TLR3.

Clustering of MHC-peptide complexes prior to their engagement in the immunological synapse: lipid raft and tetraspan microdomains

Protein reorganization at the interface of a T cell and an antigen-presenting cell (APC) plays an important role in T cell activation. Imaging techniques reveal that reorganization of particular receptor-ligand pairs gives rise to an intercellular junction, termed the immunological synapse. In this synapse antigenic peptides associated with major histocompatibility complex (MHC) molecules form multimolecular arrays on the APC side, engaging an equivalent number of clustered T cell receptors (TCRs) on the T cell. The accumulation of MHC molecules carrying cognate peptide in the APC-T cell interface was thought to depend on the specificity and presence of TCRs. Recent evidence, however, suggests that the APC is equipped to preorganize MHC-peptide complexes in the absence of T cells. To this end, MHC molecules become incorporated into two types of membrane microdomains: (i) cholesterol- and glycosphingolipid-enriched domains, denoted lipid rafts, that preconcentrate MHC class II molecules; and (ii) microdomains made up of tetraspan proteins, such as CD9, CD63, CD81 or CD82, that mediate enrichment of MHC class II molecules loaded with a select set of peptides. It follows that the integrity, composition and dynamics of these microdomains are candidate determinants favoring activation or silencing of T cells.

HLA-DR4 molecules in neuroendocrine epithelial cells associate to a heterogeneous repertoire of cytoplasmic and surface self peptides

Expression of MHC class II genes by epithelial cells is induced in inflammatory conditions such as autoimmunity and organ transplantation. Class II ligands generated by the epithelial cell processing mechanisms are unknown, although some unique epitopes have been described in epithelial cells that B cells could not generate. Epithelial cells are the targets of autoreactive T cell responses in autoimmune diseases and of transplant rejection processes, which may involve recognition of cell type-specific epitopes. In the present report, we have compared the DR4-associated repertoire and the intracellular distribution of class II, invariant chain (Ii), and DM molecules between a human DR4-, Ii-, and DM-transfected rat neuroendocrine epithelial cell line and a homozygous DR4 (DRB1*0401) lymphoblastoid B cell line, by mass spectrometry sequencing techniques, and immunoelectron microscopy. The epithelial cells chosen for transfection, RINm5F, are rat insular cells widely used for human studies of autoimmune diabetes. The results revealed a remarkably heterogeneous pool of self protein-derived peptides from the cell surface and various intracellular compartments, including the cytosol and secretory vesicles in epithelial cells, compared with a very restricted homogeneous repertoire in lymphoblastoid B cell lines, where few epitopes from surface molecules were predominant. The generation of distinct DR4-associated peptide repertoires in these two cell types could be due to the effect of several factors including differences in subcellular location of Ii and DM molecules, differential DO expression, and cell type-specific mechanisms of class II ligand generation. This is specially relevant to processes involving epithelial T cell interactions such as organ-specific autoimmunity and transplant rejection.

Lipid rafts, major histocompatibility complex molecules, and immune regulation

Glycolipid-enriched membrane microdomains ("rafts") are critical sites for signal transduction and other processes such as intracellular transport. While the participation of T-cell rafts in the formation of the immunological synapse is well established, the role of rafts on antigen-presenting cells (APCs) as well as the relationship between these domains and major histocompatibility complex (MHC) molecules is less clearly defined. We therefore investigated whether MHC class I or II molecules are found in rafts of the human macrophage-monocytic cell line U937. We detected the preferential localization of MHC class II, but not class I, molecules in rafts. Furthermore, raft disruption resulted in a decrease in constitutive protein tyrosine phosphorylation events in U937 cells. Our findings are reviewed in the context of results from other groups who also found important associations of MHC class II molecules with APC rafts. Additional, and at times contradictory, findings by others regarding the relationship between rafts and MHC molecules are also discussed. It is concluded that class II MHC molecules can localize in rafts of APCs and that this localization may be relevant for APC function and thus immune regulation.

Regulated expression of human histocompatibility leukocyte antigen (HLA)-DO during antigen-dependent and antigen-independent phases of B cell development

Human histocompatibility leukocyte antigen (HLA)-DO, a lysosomal resident major histocompatibility complex class II molecule expressed in B cells, has previously been shown to be a negative regulator of HLA-DM peptide loading function. We analyze the expression of DO in human peripheral blood, lymph node, tonsil, and bone marrow to determine if DO expression is modulated in the physiological setting. B cells, but not monocytes or monocyte-derived dendritic cells, are observed to express this protein. Preclearing experiments demonstrate that approximately 50% of HLA-DM is bound to DO in peripheral blood B cells. HLA-DM and HLA-DR expression is demonstrated early in B cell development, beginning at the pro-B stage in adult human bone marrow. In contrast, DO expression is initiated only after B cell development is complete. In all situations, there is a striking correlation between intracellular DO expression and cell surface class II-associated invariant chain peptide expression, which suggests that DO substantially inhibits DM function in primary human B cells. We report that the expression of DO is markedly downmodulated in human germinal center B cells. Modulation of DO expression may provide a mechanism to regulate peptide loading activity and antigen presentation by B cells during the development of humoral immune responses.

HLA-DM, HLA-DO and tapasin: functional similarities and differences

In both the MHC class II and class I pathways of antigen presentation, accessory molecules influence formation of MHC-peptide complexes. In the MHC class II pathway, DM functions in the loading and editing of peptides; recent work demonstrated that it is acting not only in late endosomal compartments but also in recycling compartments and on the surface of B cells and immature dendritic cells. DM activity is modulated by another accessory molecule, DO, but this modulation is mainly operative in B cells, where it may lead to preferential activation of B cells producing high-affinity antibodies. In the MHC class I pathway of antigen presentation, recent in vivo experiments with knockout mice confirmed the role of tapasin in antigen presentation and indicate that it acts as a peptide editor and as a chaperone for TAP and the MHC class I heavy chain. In the class I loading complex, calreticulin and the thiol-dependent oxidoreductase ER60/ERp57 appear to support the function of tapasin in an as-yet-unknown fashion. The picture emerges that DM and tapasin have analogous functions in shaping the peptide repertoire presented by the respective MHC class II and class I molecules.

Tetraspan microdomains distinct from lipid rafts enrich select peptide-MHC class II complexes

Complexes of peptide and major histocompatibility complex (MHC) class II are expressed on the surface of antigen-presenting cells but their molecular organization is unknown. Here we show that subsets of MHC class II molecules localize to membrane microdomains together with tetraspan proteins, the peptide editor HLA-DM and the costimulator CD86. Tetraspan microdomains differ from other membrane areas such as lipid rafts, as they enrich MHC class II molecules carrying a selected set of peptide antigens. Antigen-presenting cells deficient in tetraspan microdomains have a reduced capacity to activate CD4+ T cells. Thus, the organization of uniformly loaded peptide-MHC class II complexes in tetraspan domains may be a very early event that determines both the composition of the immunological synapse and the quality of the subsequent T helper cell response.

Transmembrane domain-mediated colocalization of HLA-DM and HLA-DR is required for optimal HLA-DM catalytic activity

HLA-DM catalyzes peptide loading and exchange reactions by MHC class II molecules. Soluble recombinant DM, lacking transmembrane and cytoplasmic domains, was observed to have 200- to 400-fold less activity compared with the full-length protein in assays measuring DM-catalyzed peptide dissociation from purified HLA-DR1 in detergent solutions. Additional studies with truncated soluble DR1 demonstrated that transmembrane domains in DR1 molecules are also required for optimal activity. The potential requirement for specific interaction between the transmembrane domains of DM and DR was ruled out in experiments with chimeric DR1 molecules containing transmembrane domains from either DM or the unrelated protein CD80. These results suggested that the major role of the transmembrane domains is to facilitate colocalization of DM and DR in detergent micelles. The latter conclusion was further supported by the observation that HLA-DM-catalyzed peptide binding to certain murine class II proteins is increased by reducing the volume of detergent micelles. The importance of membrane colocalization was directly demonstrated in experiments in which DM and DR were reconstituted separately or together into membrane bilayers in unilamellar liposomes. Our findings demonstrate the importance of membrane anchoring in DM activity and underscore the potential importance of membrane localization in regulating peptide exchange by class II molecules.