In vivo and in vitro modulation of HLA-DM and HLA-DO is induced by B lymphocyte activation

HLA-DM and HLA-DO interplay for the peptide editing of HLA class II in healthy tissues and leukemia

HLA class II antigen presentation is modulated by the activity of the peptide editor HLA-DM and its antagonist HLA-DO, with their interplay controlling the peptide repertoires presented by normal and malignant cells. The role of these molecules in allogeneic hematopoietic cell transplantation (alloHCT) is poorly investigated. Balanced expression of HLA-DM and HLA-DO can influence the presentation of leukemia-associated antigens and peptides targeted by alloreactive T cells, therefore affecting both anti-leukemia immunity and the potential onset of Graft versus Host Disease. We leveraged on a large collection of bulk and single cell RNA sequencing data, available at different repositories, to comprehensively review the level and distribution of HLA-DM and HLA-DO in different cell types and tissues of the human body. The resulting expression atlas will help future investigations aiming to dissect the dual role of HLA class II peptide editing in alloHCT, and their potential impact on its clinical outcome.

How Does B Cell Antigen Presentation Affect Memory CD4 T Cell Differentiation and Longevity?

Dendritic cells are the antigen presenting cells that process antigens effectively and prime the immune system, a characteristic that have gained them the spotlights in recent years. B cell antigen presentation, although less prominent, deserves equal attention. B cells select antigen experienced CD4 T cells to become memory and initiate an orchestrated genetic program that maintains memory CD4 T cells for life of the individual. Over years of research, we have demonstrated that low levels of antigens captured by B cells during the resolution of an infection render antigen experienced CD4 T cells into a quiescent/resting state. Our studies suggest that in the absence of antigen, the resting state associated with low-energy utilization and proliferation can help memory CD4 T cells to survive nearly throughout the lifetime of mice. In this review we would discuss the primary findings from our lab as well as others that highlight our understanding of B cell antigen presentation and the contributions of the MHC Class II accessory molecules to this outcome. We propose that the quiescence induced by the low levels of antigen presentation might be a mechanism necessary to regulate long-term survival of CD4 memory T cells and to prevent cross-reactivity to autoantigens, hence autoimmunity.

The Role of Autoimmunity-Related Gene CLEC16A in the B Cell Receptor-Mediated HLA Class II Pathway

C-type lectin CLEC16A is located next to CIITA, the master transcription factor of HLA class II (HLA-II), at a susceptibility locus for several autoimmune diseases, including multiple sclerosis (MS). We previously found that CLEC16A promotes the biogenesis of HLA-II peptide-loading compartments (MIICs) in myeloid cells. Given the emerging role of B cells as APCs in these diseases, in this study, we addressed whether and how CLEC16A is involved in the BCR-dependent HLA-II pathway. CLEC16A was coexpressed with surface class II-associated invariant chain peptides (CLIP) in human EBV-positive and not EBV-negative B cell lines. Stable knockdown of CLEC16A in EBV-positive Raji B cells resulted in an upregulation of surface HLA-DR and CD74 (invariant chain), whereas CLIP was slightly but significantly reduced. In addition, IgM-mediated Salmonella uptake was decreased, and MIICs were less clustered in CLEC16A-silenced Raji cells, implying that CLEC16A controls both HLA-DR/CD74 and BCR/Ag processing in MIICs. In primary B cells, CLEC16A was only induced under CLIP-stimulating conditions in vitro and was predominantly expressed in CLIP^high naive populations. Finally, CLIP-loaded HLA-DR molecules were abnormally enriched, and coregulation with CLEC16A was abolished in blood B cells of patients who rapidly develop MS. These findings demonstrate that CLEC16A participates in the BCR-dependent HLA-II pathway in human B cells and that this regulation is impaired during MS disease onset. The abundance of CLIP already on naive B cells of MS patients may point to a chronically induced stage and a new mechanism underlying B cell-mediated autoimmune diseases such as MS.

The love and hate relationship of HLA-DM/DO in the selection of immunodominant epitopes

Successful activation of CD4 T cells is centered around the ability of antigen presenting cells to successfully process, select Class II immunodominant epitopes from exogenous antigens and to present it to cognate T cells. To achieve this, newly synthesized MHC-II molecules are transferred to a specialized compartment which contain both exogenous antigens and the Class II processing machinery. Here in a process known as 'editing,' the Class II accessory molecule DM (HLA-DM human; murine H2-M) facilitates the loading and selection of exogenous peptides to MHC class II molecules thereby assuring proper selection of immunodominant epitopes. A second Class II accessory molecule, DO (HLA-DO human; murine H2-O), mainly present in B cells and thymic epithelium also contributes to the selection of immunodominant epitopes. Yet, despite a wealth of mechanistic insights into how DM functions, understanding the contributions of DO to epitope selection has proven to be highly challenging. In this review, we have attempted to discuss published in vitro and in vivo data during the past three years with insights into the biology of DO.

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.

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.

The MHC class II cofactor HLA-DM interacts with Ig in B cells

B cells internalize extracellular Ag into endosomes using the Ig component of the BCR. In endosomes, Ag-derived peptides are loaded onto MHC class II proteins. How these pathways intersect remains unclear. We find that HLA-DM (DM), a catalyst for MHC class II peptide loading, coprecipitates with Ig in lysates from human tonsillar B cells and B cell lines. The molecules in the Ig/DM complexes have mature glycans, and the complexes colocalize with endosomal markers in intact cells. A larger fraction of Ig precipitates with DM after BCR crosslinking, implying that complexes can form when DM meets endocytosed Ig. In vitro, in the endosomal pH range, soluble DM directly binds the Ig Fab domain and increases levels of free Ag released from immune complexes. Taken together, these results argue that DM and Ig intersect in the endocytic pathway of B cells with potential functional consequences.

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.

TLR agonists downregulate H2-O in CD8alpha- dendritic cells

Peptide loading of MHC class II (MHCII) molecules is catalyzed by the nonclassical MHCII-related molecule H2-M. H2-O, another MHCII-like molecule, associates with H2-M and modulates H2-M function. The MHCII presentation pathway is tightly regulated in dendritic cells (DCs), yet how the key modulators of MHCII presentation, H2-M and H2-O, are affected in different DC subsets in response to maturation is unknown. In this study, we show that H2-O is markedly downregulated in vivo in mouse CD8α(-) DCs in response to a broad array of TLR agonists. In contrast, CD8α(+) DCs only modestly downregulated H2-O in response to TLR agonists. H2-M levels were slightly downmodulated in both CD8α(-) and CD8α(+) DCs. As a consequence, H2-M/H2-O ratios significantly increased for CD8α(-) but not for CD8α(+) DCs. The TLR-mediated downregulation was DC specific, as B cells did not show significant H2-O and H2-M downregulation. TLR4 signaling was required to mediate DC H2-O downregulation in response to LPS. Finally, our studies showed that the mechanism of H2-O downregulation was likely due to direct protein degradation of H2-O as well as downregulation of H2-O mRNA levels. The differential H2-O and H2-M modulation after DC maturation supports the proposed roles of CD8α(-) DCs in initiating CD4-restricted immune responses by optimal MHCII presentation and of CD8α(+) DCs in promoting immune tolerance via presentation of low levels of MHCII-peptide.

Burkitt lymphoma: pathogenesis and immune evasion

B-cell lymphomas arise at distinct stages of cellular development and maturation, potentially influencing antigen (Ag) presentation and T-cell recognition. Burkitt lymphoma (BL) is a highly malignant B-cell tumor associated with Epstein-Barr Virus (EBV) infection. Although BL can be effectively treated in adults and children, leading to high survival rates, its ability to mask itself from the immune system makes BL an intriguing disease to study. In this paper, we will provide an overview of BL and its association with EBV and the c-myc oncogene. The contributions of EBV and c-myc to B-cell transformation, proliferation, or attenuation of cellular network and immune recognition or evasion will be summarized. We will also discuss the various pathways by which BL escapes immune detection by inhibiting both HLA class I- and II-mediated Ag presentation to T cells. Finally, we will provide an overview of recent developments suggesting the existence of BL-associated inhibitory molecules that may block HLA class II-mediated Ag presentation to CD4+ T cells, facilitating immune escape of BL.

Targeted regulation of self-peptide presentation prevents type I diabetes in mice without disrupting general immunocompetence

Peptide loading of MHC class II (MHCII) molecules is directly catalyzed by the MHCII-like molecule HLA-DM (DM). Another MHCII-like molecule, HLA-DO (DO), associates with DM, thereby modulating DM function. The biological role of DO-mediated regulation of DM activity in vivo remains unknown; however, it has been postulated that DO expression dampens presentation of self antigens, thereby preventing inappropriate T cell activation that ultimately leads to autoimmunity. To test the idea that DO modulation of the MHCII self-peptide repertoire mediates self tolerance, we generated NOD mice that constitutively overexpressed DO in DCs (referred to herein as NOD.DO mice). NOD mice are a mouse model for type 1 diabetes, an autoimmune disease mediated by the destruction of insulin-secreting pancreatic beta cells. Our studies showed that diabetes development was completely blocked in NOD.DO mice. Similar to NOD mice, NOD.DO animals selected a diabetogenic T cell repertoire, and the numbers and function of Tregs were normal. Indeed, immune system function in NOD.DO mice was equivalent to that in NOD mice. NOD.DO DCs, however, presented an altered MHCII-bound self-peptide repertoire, thereby preventing the activation of diabetogenic T cells and subsequent diabetes development. These studies show that DO expression can shape the overall MHCII self-peptide repertoire to promote T cell tolerance.

Impaired antigen presentation in neoplasia: basic mechanisms and implications for acute myeloid leukemia

During onset, treatment and progression of acute myeloid leukemia (AML), inadequate immune responses against certain myeloid leukemic blasts might be associated with the occurrence of minimal residual disease and subsequent relapse. Several studies on this subject have demonstrated that, in general, solid tumor cells are able to avoid CD8+ cytotoxic T-cell recognition by downregulating HLA class I-restricted presentation of tumor-associated antigens. In tumor cells that can express HLA class II molecules, such as myeloid leukemic blasts, abnormalities in the processing pathways of endogenous antigens could also result in impaired HLA class II-restricted tumor-associated antigen presentation to CD4+ T helper cells. More insight into impaired tumor-associated antigen presentation by myeloid leukemic blasts could explain their escape from immune recognition and might be crucial for selecting appropriate strategies to improve whole-cell or dendritic cell-based tumor vaccine efficacy in the treatment of AML patients.

Detection of aberrant transcription of major histocompatibility complex class II antigen presentation genes in chronic lymphocytic leukaemia identifies HLA-DOA mRNA as a prognostic factor for survival

In human B cells, effective major histocompatibility complex (MHC) class II-antigen presentation depends not only on MHC class II, but also on the invariant chain (CD74 or Ii), HLA-DM (DM) and HLA-DO (DO), the chaperones regulating the antigen loading process of MHC class II molecules. We analysed immediate ex vivo expression of HLA-DR (DR), CD74, DM and DO in B cell chronic lymphocytic leukaemia (B-CLL). Real-time reverse transcription polymerase chain reaction demonstrated a highly significant upregulation of DRA, CD74, DMB, DOA and DOB mRNA in purified malignant cells compared to B cells from healthy donors. The increased mRNA levels were not translated into enhanced protein levels but could reflect aberrant transcriptional regulation. Indeed, upregulation of DRA, DMB, DOA and DOB mRNA correlated with enhanced expression of class II transactivator (CIITA). In-depth analysis of the various CIITA transcripts demonstrated a significant increased activity of the interferon-gamma-inducible promoter CIITA-PIV in B-CLL. Comparison of the aberrant mRNA levels with clinical outcome identified DOA mRNA as a prognostic indicator for survival. Multivariate analysis revealed that the prognostic value of DOA mRNA was independent of the mutational status of the IGHV genes. Thus, aberrant transcription of DOA forms a novel and additional prognostic indicator for survival in B-CLL.

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.

Genetic variants in major histocompatibility complex-linked genes associate with pediatric liver transplant rejection

BACKGROUND & AIMS

Limited access to large samples precludes genome-wide association studies of rare but complex traits. To localize candidate genes with family-based genome-wide association, a novel exploratory analysis was first tested on 1774 major histocompatibility complex single nucleotide polymorphisms (SNPs) in 240 DNA samples from 80 children with primary liver transplantation and their biologic parents.

METHODS

Initially, 57 SNPs with large differences (P < .05) in minor allele frequencies were selected when parents of children with early rejection (rejectors) were compared with parents of nonrejectors.

RESULTS

In hypothesis testing of selected SNPs, the gamete competition statistic identified the minor allele G of the SNP rs9296068, near HLA-DOA, as being significantly different (P = .018) between outcome groups in parent-to-child transmission. Subsequent simple association testing confirmed over- and undertransmission of rs9296068 based on the most significant differences between outcome groups, of 1774 SNPs tested (P = .002), and allele (G) frequencies that were greater among rejectors (51.4% vs 36.8%, respectively, P = .015) and lower among nonrejectors (26.8% vs 36.8%, respectively, P = .074) compared with 400 normal control Caucasian children. In early functional validation, rejectors demonstrated significant repression of the first HLA-DOA exon closest to rs9296068. Also, intragraft B lymphocytes, whose antigen-presenting function is selectively inhibited by HLA-DOA were 3-fold more numerous during rejection among rejectors with the risk allele, than those without.

CONCLUSIONS

The minor allele of the SNP rs9296068 is significantly associated with liver transplantation rejection and with enhanced B-lymphocyte participation in rejection, likely because of a dysfunctional HLA-DOA gene product.

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.

MHC class II molecules in tumour immunology: prognostic marker and target for immune modulation

MHC class II molecules presenting MHC class II restricted antigens play an important role in the activation of CD4+ T cells, which are the central orchestrating cells of an immune response. This review focuses on the particular role of MHC class II molecules in tumour immunology. The MHC class II antigen presentation pathway and the expression of MHC class II molecules on tumour cells related to clinical outcome is discussed. Improving the MHC class II tumour antigen presentation pathway, for instance by downregulation of the invariant chain or modulation of HLA-DO expression, offers many opportunities for developing new modalities of immunotherapy.

Human Dendritic Cell Expression of HLA-DO Is Subset Specific and Regulated by Maturation

Expression of HLA-DO (DO) in cells that express HLA-DM (DM) results in an altered repertoire of MHC class II/peptide complexes, indicating that DO modulates DM function. Human and murine B cells and thymic epithelial cells express DO, while monocytes/macrophages do not. Monocyte-derived dendritic cells (DC) also have been found to be DO-negative, leading to the assumption that DC do not express DO. In this study, we report that, in fact, certain types of human primary DC express DO. These include Langerhans cells (LC) and some subtypes of circulating blood DC. Specifically, the majority of BDCA-3(+) DC, a small subset of uncertain function, are DO(+), while smaller proportions of CD11c(+), BDCA-1(+) (myeloid) DC, at most a minority of CD123(+)/BDCA-2(+) (plasmacytoid) DC, and no detectable CD16(+) (myeloid) DC, express DO. Immunohistochemistry of human tonsil sections demonstrates that tonsillar interdigitating DC are also DO(+). In a subset of immature LC with higher DO expression, an increased fraction of surface DR molecules carry CLIP peptides, indicating that DO functions as a DM inhibitor in these cells. LC expression of DO is down-regulated by maturation stimuli. DM levels also decrease under these conditions, but the DM:DO ratio generally increases. In the myeloid cell types tested, DO expression correlates with levels of DObeta, but not DOalpha, implying that modulation of DObeta regulates DO dimer abundance in these cells. The range of APC types shown to express DO suggests a broader role for DO in immune function than previously appreciated.

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.

DM and DO shape the repertoire of peptide–MHC-class-II complexes

The presentation of antigenic peptides by MHC class II molecules is essential for activation of CD4+ T cells. The formation of most peptide-MHC-class-II complexes is influenced by the actions of two specialized accessory proteins--DM and DO--located in the endosomal/lysosomal system where peptide loading occurs. DM removes class-II-associated invariant-chain peptide (CLIP) from newly synthesized class II molecules, but by now it is clearly established that this is only a special case of the general peptide-editing function of DM. Recent data have begun to explain the molecular basis for the editing activity. The other accessory protein, DO, modulates DM activity in vitro, but the physiological importance of DO is unclear. New evidence from several laboratories has provided clues that may soon change this.

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.

Class II-Associated Invariant Chain Peptide Expression on Myeloid Leukemic Blasts Predicts Poor Clinical Outcome

Effective antitumor responses need the activation of CD4+ T cells. MHC class II antigen presentation requires the release of class II-associated invariant chain peptide (CLIP) from the antigen-binding site. In antigen-presenting cells, human leukocyte antigen DM (HLA-DM; abbreviated DM in this article) catalyzes CLIP dissociation. In B cells, HLA-DO (DO) down-modulates DM function. Cell surface CLIP:HLA-DR (DR) ratio correlates to DO:DM ratio and the efficacy of antigen presentation. We examined 111 blood and bone marrow samples of patients with newly diagnosed acute myeloid leukemia (AML) for the expression of CLIP, DR, DM, and DO by flow cytometry. Patients with DR+/CLIP- blasts had a significant longer disease-free survival than patients with DR+/CLIP+ blasts. DO, until now believed to be restricted to lymphoid cells, could be demonstrated at protein level as well as by reverse transcription-PCR. DO:DM ratio correlated to CLIP:DR ratio, suggesting that, unlike in other antigen-presenting cells of the nonlymphoid cell type, both DO and DM mediate regulation of CLIP expression in AML blasts. We hypothesize that DR+/CLIP- AML blasts are able to present leukemia-specific antigens to CD4+ T helper cells initiating an effective and long-lasting antitumor response resulting in a prolonged disease-free survival.

Function and regulation of MHC class II molecules in T-lymphocytes: of mice and men

The main function of major histocompatibility complex (MHC) class II molecules is to present processed antigens, which are derived primarily from exogenous sources, to CD4(+) T-lymphocytes. MHC class II molecules thereby are critical for the initiation of the antigen-specific immune response. Besides antigen presentation, growing evidence is showing that ligation of MHC class II molecules also activates intracellular signaling pathways, frequently leading to apoptosis. Constitutive expression of MHC class II molecules is confined to professional antigen-presenting cells (APC) of the immune system, and in nonprofessional APCs MHC class II molecules can be induced by a variety of immune regulators. Interestingly, activated T cells from many species, with the exception of mice, synthesize and express MHC class II molecules at their cell surface. In this review, we discuss our current knowledge on the transcriptional regulation of MHC class II expression in activated human and mouse T cells, and the contribution of DNA methylation of the T-cell employed class II transactivator promoter III to the MHC class II deficiency of mouse T cells. We also discuss the proposed functions of the activated T cell synthesized and expressed MHC class II molecules, including antigen presentation, T-T cell interactions, and MHC class II-mediated intracellular signaling.

Analysis of H2-O influence on antigen presentation by B cells

HLA-DM (DM; in mouse H2-DM) promotes the exchange of MHC class II-associated peptides, resulting in the accumulation of stable MHC class II-peptide complexes. In naive (but not germinal center) B cells, a large part of DM is tightly associated with HLA-DO (DO; in mouse H2-O), but the functional consequence of this association for Ag presentation is debated. Here, we have extended previous studies by examining the presentation of multiple epitopes after Ag internalization by fluid phase endocytosis or receptor-mediated uptake by membrane Ig (mIg) receptors. We find that the effects of H2-O are more complex than previously appreciated; thus, while only minor influences on Ag presentation could be detected after fluid phase uptake, many epitopes were substantially affected after mIg-mediated uptake. Unexpectedly, the presentation of different epitopes was found to be enhanced, diminished, or unaffected in the absence of H2-O, depending on the specificity of the mIg used for Ag internalization. Interestingly, epitopes from the same Ag did not necessarily show the same H2-O dependency. This finding suggests that H2-O may control the repertoire of peptides presented by B cells depending on the mIg-Ag interaction. The absence of DO/H2-O from germinal center B cells suggests that this control may be released during B cell maturation.

Editing of an immunodominant epitope of glutamate decarboxylase by HLA-DM

HLA-DM stabilizes peptide-receptive class II alphabeta dimers and facilitates the capture of high affinity peptides, thus influencing the peptide repertoire presented by class II molecules. Variations in DM levels may therefore have a profound effect on the antigenic focus of T cell-mediated immune responses. Specifically, DM expression may influence susceptibility and resistance to autoimmune diseases. In this study the role of DM in HLA-DR4-restricted presentation of an insulin-dependent diabetes mellitus autoantigen, glutamate decarboxylase (GAD), was tested. Presentation of immunodominant GAD epitope 273-285 was regulated by endogenous DM levels in human B lymphoblasts. T cell responses to exogenous GAD as well as an endogenous cytoplasmic form of this Ag were significantly diminished with increasing cellular expression of DM. Epitope editing by DM was observed only using Ag and not small synthetic peptides, suggesting that this process occurred within endosomes. Results with cytoplasmic GAD also indicated that peptides from this compartment intersect class II proteins in endocytic vesicles where DM editing was facilitated. Changes in DM levels within APC may therefore influence the presentation of autoantigens and the development of autoimmune disorders such as type I diabetes.

Class II transactivator-induced expression of HLA-DO(beta) in HeLa cells

HLA-DO is an intracellular nonclassical MHC class II molecule expressed in the endocytic pathway of B lymphocytes. It shapes the repertoire of peptides bound to classical class II molecules such as HLA-DR by regulating the activity of HLA-DM. Using a peptide corresponding to the cytoplasmic tail of HLA-DO(beta), we have developed a mouse monoclonal antibody, HKC5. Immunofluorescence microscopy revealed that HKC5 recognizes HLA-DO molecules present in the endoplasmic reticulum as well as those in vesicular compartments of the endocytic pathway. In addition, the antibody detects the isolated beta chain on Western blots. Using mutants of the DO(beta) cytoplasmic tail fused to a reporter molecule and expressed in epithelial cells, we showed by flow cytometry that the antibody epitope includes one or both of the leucine residues forming the lysosomal sorting signal. Finally, we have used HKC5 to evaluate the presence of the HLA-DO(beta) chain in HeLa cells expressing the class II transactivator protein CIITA. Our flow cytometry and confocal microscopy analyses showed a marked expression of DO(beta) suggesting that HLA-DO could accumulate under the influence of CIITA in non-B cells.