H2-O inhibits presentation of bacterial superantigens, but not endogenous self antigens

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.

Expression Patterns of H2-O in Mouse B Cells and Dendritic Cells Correlate with Cell Function

In the endosomes of APCs, the MHC class II-like molecule H2-M catalyzes the exchange of class II-associated invariant chain peptides (CLIP) for antigenic peptides. H2-O is another class II-like molecule that modulates the peptide exchange activity of H2-M. Although the expression pattern of H2-O in mice has not been fully evaluated, H2-O is expressed by thymic epithelial cells, B cells, and dendritic cells (DCs). In this study, we investigated H2-O, H2-M, and I-A(b)-CLIP expression patterns in B cell subsets during B cell development and activation. H2-O was first detected in the transitional 1 B cell subset and high levels were maintained in marginal zone and follicular B cells. H2-O levels were down-regulated specifically in germinal center B cells. Unexpectedly, we found that mouse B cells may have a pool of H2-O that is not associated with H2-M. Additionally, we further evaluate H2-O and H2-M interactions in mouse DCs, as well as H2-O expression in bone marrow-derived DCs. We also evaluated H2-O, H2-M, I-A(b), and I-A(b)-CLIP expression in splenic DC subsets, in which H2-O expression levels varied among the splenic DC subsets. Although it has previously been shown that H2-O modifies the peptide repertoire, H2-O expression did not alter DC presentation of a number of endogenous and exogenous Ags. Our further characterization of H2-O expression in DCs, as well as the identification of a potential free pool of H2-O in mouse splenic B cells, suggest that H2-O may have a yet to be elucidated role in immune responses.

Interleukin-13-regulated M2 macrophages in combination with myeloid suppressor cells block immune surveillance against metastasis

CD1-deficient mice reject established, disseminated 4T1 metastatic mammary cancer and survive indefinitely if their primary mammary tumors are surgically removed. This highly effective immune surveillance is due to three interacting mechanisms: (a) the generation of inducible nitric oxide synthase (iNOS)-producing M1 macrophages that are tumoricidal for 4T1 tumor cells; (b) a rapid decrease in myeloid-derived Gr1(+)CD11b(+) suppressor cells that are elevated and down-regulate the CD3zeta chain when primary tumor is present and that suppress T cells by producing arginase; and (c) production of activated lymphocytes. Macrophages from wild-type BALB/c mice are polarized by interleukin-13 (IL-13) towards a tumor-promoting M2 phenotype, thereby inhibiting the generation of tumoricidal M1 macrophages. In contrast, CD1(-/-) mice, which are deficient for IL-13 because they lack IL-13-producting NKT cells, generate M1 macrophages that are cytotoxic for 4T1 via the production of nitric oxide. Although tumoricidal macrophages are a necessary component of immune surveillance in CD1(-/-) mice, they alone are not sufficient for tumor resistance because IL-4Ralpha(-/-) mice have M1 macrophages and retain high levels of myeloid suppressor cells after surgery; in addition, they are susceptible to 4T1 metastatic disease. These results show that effective immune surveillance against established metastatic disease is negatively regulated by IL-13 and requires the induction of tumoricidal M1 macrophages and lymphocytes combined with a reduction in tumor-induced myeloid suppressor cells.

The relationship between immunodominance, DM editing, and the kinetic stability of MHC class II:peptide complexes

Immunodominance refers to the restricted antigen specificity of T cells detected in the immune response after immunization with complex antigens. Despite the presence of many potential peptide epitopes within these immunogens, the elicited T-cell response apparently focuses on a very limited number of peptides. Over the last two decades, a number of distinct explanations have been put forth to explain this very restricted specificity of T cells, many of which suggest that endosomal antigen processing restricts the array of peptides available to recruit CD4 T cells. In this review, we present evidence from our laboratory that suggest that immunodominance in CD4 T-cell responses is primarily due to an intrinsic property of the peptide:class II complexes. The intrinsic kinetic stability of peptide:class II complexes controls DM editing within the antigen-presenting cells and thus the initial epitope density on priming dendritic cells. Additionally, we hypothesize that peptides that possess high kinetic stability interactions with class II molecules display persistence at the cell surface over time and will more efficiently promote T-cell signaling and differentiation than competing, lower-stability peptides contained within the antigen. We discuss this model in the context of the existing data in the field of immunodominance.

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.

Presentation of cytosolic antigens via MHC class II molecules

Major histocompatibility (MHC) class II molecules function to present antigenic peptides to CD4 T lymphocytes. The pathways by which these molecules present exogenous antigens have been extensively studied. However by contrast, far less is known about the processing and trafficking of cytosolic antigens, which can also serve as an alternative source of ligands for MHC class II molecules. Self-proteins, tumor antigens, as well as viral proteins found within the cytosol of cells, can be presented via MHC class II molecules, resulting in the activation of specific CD4 T cells. Studies have begun to reveal unique steps as well as some similarities in the pathways for cytosolic and exogenous antigen presentation. Recent developments in this area are summarized here.

Presentation of Endogenously Synthesized MHC Class II-Restricted Epitopes by MHC Class II Cancer Vaccines Is Independent of Transporter Associated with Ag Processing and the Proteasome

Cell-based vaccines consisting of invariant chain-negative tumor cells transfected with syngeneic MHC class II (MHC II) and costimulatory molecule genes are prophylactic and therapeutic agents for the treatment of murine primary and metastatic cancers. Vaccine efficacy is due to direct presentation of endogenously synthesized, MHC II-restricted tumor peptides to CD4+ T cells. Because the vaccine cells lack invariant chain, we have hypothesized that, unlike professional APC, the peptide-binding groove of newly synthesized MHC II molecules may be accessible to peptides, allowing newly synthesized MHC II molecules to bind peptides that have been generated in the proteasome and transported into the endoplasmic reticulum via the TAP complex. To test this hypothesis, we have compared the Ag presentation activity of multiple clones of TAP-negative and TAP-positive tumor cells transfected with I-Ak genes and the model Ag hen egg white lysozyme targeted to the endoplasmic reticulum or cytoplasm. Absence of TAP does not diminish Ag presentation of three hen egg white lysozyme epitopes. Likewise, cells treated with proteasomal and autophagy inhibitors are as effective APC as untreated cells. In contrast, drugs that block endosome function significantly inhibit Ag presentation. Coculture experiments demonstrate that the vaccine cells do not release endogenously synthesized molecules that are subsequently endocytosed and processed in endosomal compartments. Collectively, these data indicate that vaccine cell presentation of MHC II-restricted endogenously synthesized epitopes occurs via a mechanism independent of the proteasome and TAP complex, and uses a pathway that overlaps with the classical endosomal pathway for presentation of exogenously synthesized molecules.

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.

Ectopic expression of HLA-DO in mouse dendritic cells diminishes MHC class II antigen presentation

The MHC class II-like molecule HLA-DM (DM) (H-2M in mice) catalyzes the exchange of CLIP for antigenic peptides in the endosomes of APCs. HLA-DO (DO) (H-2O in mice) is another class II-like molecule that is expressed in B cells, but not in other APCs. Studies have shown that DO impairs or modifies the peptide exchange activity of DM. To further evaluate the role of DO in Ag processing and presentation, we generated transgenic mice that expressed the human HLA-DOA and HLA-DOB genes under the control of a dendritic cell (DC)-specific promoter. Our analyses of DCs from these mice showed that as DO levels increased, cell surface levels of A(b)-CLIP also increased while class II-peptide levels decreased. The presentation of some, but not all, exogenous Ags to T cells or T hybridomas was significantly inhibited by DO. Surprisingly, H-2M accumulated in DO-expressing DCs and B cells, suggesting that H-2O/DO prolongs the half-life of H-2M. Overall, our studies showed that DO expression impaired H-2M function, resulting in Ag-specific down-modulation of class II Ag processing and presentation.

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.