In the absence of the invariant chain, HLA-DR molecules display a distinct array of peptides which is influenced by the presence or absence of HLA-DM

Molecular Determinants Regulating the Plasticity of the MHC Class II Immunopeptidome

In the last few years, advancement in the analysis of the MHC class II (MHC-II) ligandome in several mouse and human haplotypes has increased our understanding of the molecular components that regulate the range and selection of the MHC-II presented peptides, from MHC class II molecule polymorphisms to the recognition of different conformers, functional differences in endosomal processing along the endocytic tract, and the interplay between the MHC class II chaperones DM and DO. The sum of all these variables contributes, qualitatively and quantitatively, to the composition of the MHC II ligandome, altogether ensuring that the immunopeptidome landscape is highly sensitive to any changes in the composition of the intra- and extracellular proteome for a comprehensive survey of the microenvironment for MHC II presentation to CD4 T cells.

Exposing the Specific Roles of the Invariant Chain Isoforms in Shaping the MHC Class II Peptidome

The peptide repertoire (peptidome) associated with MHC class II molecules (MHCIIs) is influenced by the polymorphic nature of the peptide binding groove but also by cell-intrinsic factors. The invariant chain (Ii) chaperones MHCIIs, affecting their folding and trafficking. Recent discoveries relating to Ii functions have provided insights as to how it edits the MHCII peptidome. In humans, the Ii gene encodes four different isoforms for which structure-function analyses have highlighted common properties but also some non-redundant roles. Another layer of complexity arises from the fact that Ii heterotrimerizes, a characteristic that has the potential to affect the maturation of associated MHCIIs in many different ways, depending on the isoform combinations. Here, we emphasize the peptide editing properties of Ii and discuss the impact of the various isoforms on the MHCII peptidome.

Interaction of HLA-DR and CD74 at the cell surface of antigen-presenting cells by single particle image analysis

Major histocompatibility complex (MHC) class II-associated antigen presentation involves an array of interacting molecules. CD74, the cell surface isoform of the MHC class II-associated invariant chain, is one such molecule; its role remains poorly defined. To address this, we have employed a high-resolution single-particle imaging method for quantifying the colocalization of CD74 with human leukocyte antigen (HLA)-DR molecules on human fibroblast cells known for their capacity to function as antigen-presenting cells. We have also examined whether the colocalization induces internalization of HLA-DR using HA(307-319), a "universal" peptide that binds specifically to the peptide-binding groove of all HLA-DR molecules, irrespective of their alleles. We have determined that 25 ± 1.3% of CD74 and 17 ± 0.3% of HLA-DR are colocalized, and the association of CD74 with HLA-DR and the internalization of HLA-DR are both inhibited by HA(307-319). A similar inhibition of HLA-DR internalization was observed in freshly isolated monocyte-derived dendritic cells. A key role of CD74 is to translocate HLA-DR molecules to early endosomes for reloading with peptides prior to recycling to the cell surface. We conclude that CD74 regulates the balance of peptide-occupied and peptide-free forms of MHC class II at the cell surface.

On the perils of poor editing: regulation of peptide loading by HLA-DQ and H2-A molecules associated with celiac disease and type 1 diabetes

This review discusses mechanisms that link allelic variants of major histocompatibility complex (MHC) class II molecules (MHCII) to immune pathology. We focus on HLA (human leukocyte antigen)-DQ (DQ) alleles associated with celiac disease (CD) and type 1 diabetes (T1D) and the role of the murine DQ-like allele, H2-Ag7 (I-Ag7 or Ag7), in murine T1D. MHCII molecules bind peptides, and alleles vary in their peptide-binding specificity. Disease-associated alleles permit binding of disease-inducing peptides, such as gluten-derived, Glu-/Pro-rich gliadin peptides in CD and peptides from islet autoantigens, including insulin, in T1D. In addition, the CD-associated DQ2.5 and DQ8 alleles are unusual in their interactions with factors that regulate their peptide loading, invariant chain (Ii) and HLA-DM (DM). The same alleles, as well as other T1D DQ risk alleles (and Ag7), share nonpolar residues in place of Asp at β57 and prefer peptides that place acidic side chains in a pocket in the MHCII groove (P9). Antigen-presenting cells from T1D-susceptible mice and humans retain CLIP because of poor DM editing, although underlying mechanisms differ between species. We propose that these effects on peptide presentation make key contributions to CD and T1D pathogenesis.

Beyond the classical: influenza virus and the elucidation of alternative MHC class II-restricted antigen processing pathways

CD4+ T cells (T(CD4+)) are activated by peptides, generally 13-17 amino acids in length, presented at the cell surface in combination with highly polymorphic MHC class II molecules. According to the classical model, these peptides are generated by endosomal digestion of internalized antigen and loaded onto MHC class II molecules in the late endosome. Historically, this "exogenous" pathway has been defined through the extensive use of purified proteins. However, the relatively recent use of clinically relevant antigens, those of influenza virus in our case, has revealed several additional pathways of peptide production, including some that are truly "endogenous", entailing synthesis of the protein within the infected cell. Indeed, some peptides appear to be created only via endogenous processing. The cell biology that underlies these alternative pathways remains poorly understood as do their relative contributions to defence against infectious agents and cancer, and the triggering of autoimmune diseases.

Understanding the focused CD4 T cell response to antigen and pathogenic organisms

Immunodominance is a term that reflects the final, very limited peptide specificity of T cells that are elicited during an immune response. Recent experiments in our laboratory compel us to propose a new paradigm for the control of immunodominance in CD4 T cell responses, stating that immunodominance is peptide-intrinsic and is dictated by the off-rate of peptides from MHC class II molecules. Our studies have revealed that persistence of peptide:class II complexes both predicts and controls CD4 T cell immunodominance and that this parameter can be rationally manipulated to either promote or eliminate immune responses. Mechanistically, we have determined that DM editing in APC is a key event that is influenced by the kinetic stability of class II:peptide complexes and that differential persistence of complexes also impacts the expansion phase of the immune response. These studies have important implications for rational vaccine design and for understanding the immunological mechanisms that limit the specificity of CD4 T cell responses.

An MHC class II restriction bias in CD4 T cell responses toward I-A is altered to I-E in DM-deficient mice

The MHC-encoded cofactor DM catalyzes endosomal loading of peptides onto MHC class II molecules. Despite evidence from in vitro experiments that DM acts to selectively edit the repertoire of class II:peptide complexes, the consequence of DM expression in vivo, or a predictive pattern of DM activity in the specificity of CD4 T cell responses has remained unresolved. Therefore, to characterize DM function in vivo we used wild-type (WT) or DM-deficient (DM(-/-)) mice of the H-2(d) MHC haplotype and tested the hypothesis that DM promotes narrowing of the repertoire of class II:peptide complexes displayed by APC, leading to a correspondingly selective CD4 T cell response. Surprisingly, our results indicated that DM(-/-) mice do not exhibit a broadened CD4 T cell response relative to WT mice, but rather shift their immunodominance pattern to new peptides, a pattern associated with a change in class II isotype-restriction. Specifically, we found that CD4 T cell responses in WT mice were primarily restricted to the I-A class II molecule, whereas DM(-/-) mice recognize peptides in the context of I-E. The observed shift in isotype-restriction appeared to be due in part to a modification in the peripheral CD4 T cell repertoire available for peptide recognition.

Loss of Invariant Chain Protects Nonobese Diabetic Mice against Type 1 Diabetes

The invariant (Ii) chain acts as an essential chaperone to promote MHC class II surface expression, Ag presentation, and selection of CD4(+) T cells. We have examined its role in the development of type 1 diabetes in NOD mice and show that Ii chain-deficient NOD mice fail to develop type 1 diabetes. Surprisingly, Ii chain functional loss fails to disrupt in vitro presentation of islet Ags, in the context of NOD I-A(g7) molecules. Moreover, pathogenic effector cells could be shown to be present in Ii chain-deficient NOD mice because they were able to transfer diabetes to NOD.scid recipients. The ability of these cells to transfer diabetes was markedly enhanced by depletion of CD25 cells coupled with in vivo anti-CD25 treatment of recipient mice. The numbers of CD4(+)CD25(+)Foxp3(+) T cells in thymus and periphery of Ii chain-deficient NOD mice were similar to those found in normal NOD mice, in contrast to conventional CD4(+) T cells whose numbers were reduced. This suggests that regulatory T cells are unaffected in their selection and survival by the absence of Ii chain and that an alteration in the balance of effector to regulatory T cells contributes to diabetes prevention.

The impact of DM on MHC class II-restricted antigen presentation can be altered by manipulation of MHC-peptide kinetic stability

DM edits the peptide repertoire presented by major histocompatibility complex class II molecules by professional antigen-presenting cells (APCs), favoring presentation of some peptides over others. Despite considerable research by many laboratories, there is still significant uncertainty regarding the biochemical attributes of class II–peptide complexes that govern their susceptibility to DM editing. Here, using APCs that either do or do not express DM and a set of unrelated antigens, we found that the intrinsic kinetic stability of class II–peptide complexes is tightly correlated with the effects of DM editing within APCs. Furthermore, through the use of kinetic stability variants of three independent peptides, we demonstrate that increasing or decreasing the kinetic stability of class II–peptide complexes causes a corresponding alteration in DM editing. Finally, we show that the spontaneous kinetic stability of class II complexes correlates directly with the efficiency of presentation by DM⁺ APCs and the immunodominance of that class II–peptide complex during an immune response. Collectively, these results suggest that the pattern of DM editing in APCs can be intentionally changed by modifying class II–peptide interactions, leading to the desired hierarchy of presentation on APCs, thereby promoting recruitment of CD4 T cells specific for the preferred peptides during an immune response.

Presence and regulation of messenger ribonucleic acids encoding components of the class II major histocompatibility complex-associated antigen processing pathway in the bovine corpus luteum

Luteal cells express class II major histocompatibility complex (MHC) molecules and can stimulate T lymphocyte proliferationin vitro. However, it is unknown whether luteal cells express the intracellular components necessary to process the peptides presented by class II MHC molecules. The objective of the present study was to examine the expression and regulation of three major class II-associated antigen processing components – class II MHC-associated invariant chain (Ii), DMα and DMβ – in luteal tissue. Corpora lutea were collected early in the estrous cycle, during midcycle and late in the estrous cycle, and at various times following administration of a luteolytic dose of prostaglandin F2α(PGF2α) to the cow. Northern analysis revealed the presence of mRNA encoding each of the class II MHC-associated antigen processing proteins in luteal tissue. Ii mRNA concentrations did not change during the estrous cycle, whereas DMα and DMβ mRNA concentrations were highest in midcycle luteal tissue compared with either early or late luteal tissue. Tumor necrosis factor-α (TNF-α) reduced DMα mRNA concentrations in cultured luteal cells in the presence of LH or PGF2α. DMα and DMβ mRNA were also present in highly enriched cultures of luteal endothelial (CLENDO) cells, and DMα mRNA concentrations were greater in CLENDO cultures compared with mixed luteal cell cultures. Expression of invariant chain, DMα and DMβ genes indicates that cells within the corpus luteum express the minimal requirements to act as functional antigen-presenting cells, and the observation that CLENDO cells are a source of DMα and DMβ mRNA indicates that non-immune cells within the corpus luteum may function as antigen-presenting cells.

Achieving stability through editing and chaperoning: regulation of MHC class II peptide binding and expression

In antigen-presenting cells (APCs), loading of major histocompatibility complex class II (MHC II) molecules with peptides is regulated by invariant chain (Ii), which blocks MHC II antigen-binding sites in pre-endosomal compartments. Several molecules then act upon MHC II molecules in endosomes to facilitate peptide loading: Ii-degrading proteases, the peptide exchange factor, human leukocyte antigen-DM (HLA-DM), and its modulator, HLA-DO (DO). Here, we review our findings arguing that DM stabilizes a globally altered conformation of the antigen-binding groove by binding to a lateral surface of the MHC II molecule. Our data imply changes in the interactions between specificity pockets and peptide side chains, complementing data from others that suggest DM affects hydrogen bonds. Selective weakening of peptide/MHC interactions allows DM to alter the peptide repertoire. We also review our studies in cells that highlight the ability of several factors to modulate surface expression of MHC II molecules via post-Golgi mechanisms; these factors include MHC class II-associated Ii peptides (CLIP), DM, and microbial products that modulate MHC II traffic from endosomes to the plasma membrane. In this context, we discuss possible mechanisms by which the association of some MHC II alleles with autoimmune diseases may be linked to their low CLIP affinity.

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.

Dissecting MHC class II export, B cell maturation, and DM stability defects in invariant chain mutant mice

Invariant (Ii) chain loss causes defective class II export, B cell maturation, and reduced DM stability. In this study, we compare Ii chain and class II mutant mouse phenotypes to dissect these disturbances. The present results demonstrate that ER retention of alphabeta complexes, and not beta-chain aggregates, disrupts B cell development. In contrast, we fail to detect class II aggregates in Ii chain mutant thymi. Ii chain loss in NOD mice leads to defective class II export and formation of alphabeta aggregates, but in this background, downstream signals are misregulated and mature B cells develop normally. Finally, Ii chain mutant strains all display reduced levels of DM, but mice expressing either p31 or p41 alone, and class II single chain mutants, are indistinguishable from wild type. We conclude that Ii chain contributions as a DM chaperone are independent of its role during class II export. This Ii chain/DM partnership favors class II peptide loading via conventional pathway(s).

Dissection of the HLA-DR4 peptide repertoire in endocrine epithelial cells: strong influence of invariant chain and HLA-DM expression on the nature of ligands

Class II MHC (MHC II) expression is restricted to professional APCs and thymic epithelium but it also occurs in the epithelial cells of autoimmune organs which are the unique targets of the CD4 autoreactive T cells in endocrine autoimmune diseases. This specificity is presumably conditioned by an epithelium-specific peptide repertoire associated to MHC II at the cell surface. MHC II expression and function is dependent on the action of two main chaperones, invariant chain (Ii) and DM, whose expression is coregulated with MHC II. However, there is limited information about the in vivo expression levels of these molecules and uncoordinated expression has been demonstrated in class II-positive epithelial cells that may influence the MHC-associated peptide repertoires and the outcome of the autoimmune response. We have examined the pool of peptides associated to DR4 molecules expressed by a neuroendocrine epithelial cell and the consequences of Ii and DM coexpression. The RINm5F rat insulinoma cell line was transfected with HLA-DRB1*0401, Ii, and DM molecules in four different combinations: RIN-DR4, -DR4Ii, -DR4DM, and -DR4IiDM. The analysis of the peptide repertoire and the identification of the DR4 naturally processed ligands in each transfected cell were achieved by mass spectrometry. The results demonstrate that 1) the expression of Ii and DM affected the DR4 peptide repertoires by producing important variations in their content and in the origin of peptides; 2) these restrictions affected the stability and sequence of the peptides of each repertoire; and 3) Ii and DM had both independent and coordinate effects on these repertoires.

Effects of prostaglandin F2alpha and progesterone on the ability of bovine luteal cells to stimulate T lymphocyte proliferation

Bovine luteal cells express class I and II major histocompatibility complex molecules and stimulate T lymphocyte proliferation in vitro. Proliferation of T lymphocytes is greater in cocultures of luteal cells and T lymphocytes collected following administration of a luteolytic dose of prostaglandin (PG) F2alpha to the cow. Whether this results from changes in luteal cells that increase their ability to stimulate T lymphocyte proliferation or from changes in T lymphocytes that enhance their ability to respond to luteal cells is unclear. To determine which is the case, luteal cell-T lymphocyte cocultures were performed using luteal cells and T lymphocytes isolated from the same animals before and 8 h after administration of PGF2alpha. In the presence of T lymphocytes collected before PGF2alpha administration, luteal cells isolated after PGF2alpha were more potent stimulators of T lymphocyte proliferation than were luteal cells collected before PGF2alpha (P<0.05). The effect of progesterone on luteal cell-stimulated T lymphocyte proliferation was also evaluated. Proliferation of T lymphocytes was greater (P<0.05) in cultures containing the cytochrome P450 side-chain cleavage enzyme-inhibitor aminoglutethimide. Exogenous progesterone caused a dose-dependent inhibition of luteal cell-stimulated T lymphocyte proliferation (P<0.05). Progesterone-receptor mRNA was undetectable in peripheral blood mononuclear cells collected before and after PGF2alpha administration, indicating that the effect of progesterone was not mediated via progesterone receptors in lymphocytes. These results imply that specific changes in luteal cells in response to PGF2alpha enhance the ability of these cells to stimulate T lymphocyte proliferation. These results also demonstrate that progesterone can suppress luteal cell-stimulated T lymphocyte proliferation.

Structural factors contributing to DM susceptibility of MHC class II/peptide complexes

Peptide loading of MHC class II (MHCII) molecules is assisted by HLA-DM, which releases invariant chain peptides from newly synthesized MHCII and edits the peptide repertoire. Determinants of susceptibility of peptide/MHCII complexes to DM remain controversial, however. Here we have measured peptide dissociation in the presence and the absence of DM for 36 different complexes of varying intrinsic stability. We found large variations in DM susceptibility for different complexes using either soluble or full-length HLA-DM. The DM effect was significantly less for unstable complexes than for stable ones, although this correlation was modest. Peptide sequence- and allele-dependent interactions along the entire length of the Ag binding groove influenced DM susceptibility. We also observed differences in DM susceptibility during peptide association. Thus, the peptide repertoire displayed to CD4(+) T cells is the result of a mechanistically complicated editing process and cannot be simply predicted from the intrinsic stability of the complexes in the absence of DM.

Binding interactions between peptides and proteins of the class II major histocompatibility complex

The activation of helper T cells by peptides bound to proteins of the class II Major Histocompatibility Complex (MHC II) is pivotal to the initiation of an immune response. The primary functional requirement imposed on MHC II proteins is the ability to efficiently bind thousands of different peptides. Structurally, this is reflected in a unique architecture of binding interactions. The peptide is bound in an extended conformation within a groove on the membrane distal surface of the protein that is lined with several pockets that can accommodate peptide side-chains. Conserved MHC II protein residues also form hydrogen bonds along the length of the peptide main-chain. Here we review recent advances in the study of peptide-MHC II protein reactions that have led to an enhanced understanding of binding energetics. These results demonstrate that peptide-MHC II protein complexes achieve high affinity binding from the array of hydrogen bonds that are energetically segregated from the pocket interactions, which can then add to an intrinsic hydrogen bond-mediated affinity. Thus, MHC II proteins are unlike antibodies, which utilize cooperativity among binding interactions to achieve high affinity and specificity. The significance of these observations is discussed within the context of possible mechanisms for the HLA-DM protein that regulates peptide presentation in vivo and the design of non-peptide molecules that can bind MHC II proteins and act as vaccines or immune modulators.

T-cell anergy and peripheral T-cell tolerance

The discovery that T-cell recognition of antigen can have distinct outcomes has advanced understanding of peripheral T-cell tolerance, and opened up new possibilities in immunotherapy. Anergy is one such outcome, and results from partial T-cell activation. This can arise either due to subtle alteration of the antigen, leading to a lower-affinity cognate interaction, or due to a lack of adequate co-stimulation. The signalling defects in anergic T cells are partially defined, and suggest that T-cell receptor (TCR) proximal, as well as downstream defects negatively regulate the anergic T cell's ability to be activated. Most importantly, the use of TCR-transgenic mice has provided compelling evidence that anergy is an in vivo phenomenon, and not merely an in vitro artefact. These findings raise the question as to whether anergic T cells have any biological function. Studies in rodents and in man suggest that anergic T cells acquire regulatory properties; the regulatory effects of anergic T cells require cell to cell contact, and appear to be mediated by inhibition of antigen-presenting cell immunogenicity. Close similarities exist between anergic T cells, and the recently defined CD4+ CD25+ population of spontaneously arising regulatory cells that serve to inhibit autoimmunity in mice. Taken together, these findings suggest that a spectrum of regulatory T cells exists. At one end of the spectrum are cells, such as anergic and CD4+ CD25+ T cells, which regulate via cell-to-cell contact. At the other end of the spectrum are cells which secrete antiinflammatory cytokines such as interleukin 10 and transforming growth factor-beta. The challenge is to devise strategies that reliably induce T-cell anergy in vivo, as a means of inhibiting immunity to allo- and autoantigens.

Conformational alterations during biosynthesis of HLA-DR3 molecules controlled by invariant chain and HLA-DM

HLA-DM is known to catalyze the exchange of class II-associated invariant chain (Ii) peptide (CLIP) for cognate peptide during biosynthesis. In DM-negative cells HLA-DR3 molecules have been shown to predominantly present CLIP and to lack the DR3-specific mAb epitope 16.23, which has led to the assumption that CLIP prevents binding of mAb 16.23. In the present study we show that CLIP does not prohibit 16.23 epitope expression, but that the formation of this epitope is directly influenced by interactions of the DR molecule with Ii and DM. Detergent solubilized DR3 from wild-type as well as DM(-) cells bound CLIP in a 16.23(+) mode. On cells, however, neither CLIP nor antigenic peptide bound to DR3 in a 16.23(+) conformation, unless HLA-DM was expressed. Thus, HLA-DM appears to alter the conformation of DR3 in a peptide-independent fashion. Since in DM-deficient cells that also lack Ii, DR3 molecules assembled in a 16.23(+) conformation, we conclude that during biosynthesis Ii and DM exert opposing conformational constraints, characterized by suppressing or releasing 16.23 epitope expression. These results imply that DR3/peptide complexes, including DR3/ CLIP, can exist in two conformations depending on previous interaction with DM, but independent of the nature of the peptide bound. We show that these naturally occurring class II conformers can be selectively recognized by T cells.

Interaction of human immunodeficiency virus type 2 Vpx and invariant chain

Vpx is a virion-associated protein of human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency viruses. The yeast two-hybrid system was used to identify invariant chain (Ii) as a cellular protein that interacts with HIV-2 Vpx. Vpx-Ii interaction was confirmed in cell-free reactions using bacterially expressed glutathione S-transferase fusion proteins and by coimmunoprecipitation in transfected and infected cells. In chronically infected cells expressing Vpx, Ii levels were markedly decreased, presumably due to enhanced degradation. These findings suggest that Vpx may disrupt major histocompatibility complex class II antigen presentation.

Human major histocompatibility molecules have the intrinsic ability to form homotypic associations

We have investigated the homotypic associations of major histocompatibilty, class II and class I molecules using immunoprecipitation from detergent solubilised cell extracts. A 120-kDa structure corresponding to an HLA-DR dimer of dimers was immunoprecipitated by the HLA-DR specific mAb L243 from both biotinylated cell-surface and metabolically labeled B cells and transfectant fibroblasts. The thermostability of this structure in SDS was examined. It was detected at 4 degrees C, 22 degrees C, and 37 degrees C, but not at 50 degrees C or 100 degrees C. Experiments performed with L243 Fab fragments and with purified HLA-DR molecules, indicated the presence of HLA-DR dimers of dimers and single heterodimers on B cells. HLA-DQ was also found to form SDS-stable dimers of dimers and single heterodimers on the cell surface of B cells, demonstrating that HLA class II isotypes, other than HLA-DR, also form homotypic associations. Similar experiments performed with HLA class I specific mAb, W632, revealed the existence of a 90 kDa and a 135-kDa structure corresponding to a MHC class I multimers. Under the same conditions, non-MHC molecules such as CD14 were found not to self-associate. These findings indicate that major histocompatibility molecules have the intrinsic ability to form homotypic associations at the cell surface of antigen presenting cells.

Genetic modulation of tumor antigen presentation

An effective cancer-cell vaccine is created by expressing major-histocompatibility-complex (MHC) class II molecules without the invariant chain protein (Ii) that normally blocks the antigenic-peptide-binding site of MHC class II molecules at their synthesis in the endoplasmic reticulum. Such tumor-cell constructs are created either by the transfer of genes for MHC class IIalpha and beta chains, or by the induction of MHC class II molecules and Ii protein with a transacting factor, followed by Ii suppression using antisense methods. Preclinical validation of this approach is reviewed with the goal of using this immunotherapy for metastatic human cancers.

MHC class II presentation of endogenous tumor antigen by cellular vaccines depends on the endocytic pathway but not H2-M

We have developed cell-based cancer vaccines that activate anti-tumor immunity by directly presenting endogenously synthesized tumor antigens to CD4+ T helper lymphocytes via MHC class II molecules. The vaccines are non-conventional antigen-presenting cells because they express MHC class II, do not express invariant chain or H-2M, and preferentially present endogenous antigen. To further improve therapeutic efficacy we have studied the intracellular trafficking pathway of MHC class II molecules in the vaccines using endoplasmic reticulumlocalized lysozyme as a model antigen. Experiments using endocytic and cytosolic pathway inhibitors (chloroquine, primaquine, and brefeldin A) and protease inhibitors (lactacystin, LLnL, E64, and leupeptin) indicate antigen presentation depends on the endocytic pathway, although antigen degradation is not mediated by endosomal or proteasomal proteases. Because H2-M facilitates presentation of exogenous antigen via the endocytic pathway, we investigated whether transfection of vaccine cells with H-2M could potentiate endogenous antigen presentation. In contrast to its role in conventional antigen presentation, H-2M had no effect on endogenous antigen presentation by vaccine cells or on vaccine efficacy. These results suggest that antigen/MHC class II complexes in the vaccines may follow a novel route for processing and presentation and may produce a repertoire of class II-restricted peptides different from those presented by professional APC. The therapeutic efficacy of the vaccines, therefore, may reside in their ability to present novel tumor peptides, consequently activating tumor-specific CD4+ T cells that would not otherwise be activated.

Accessory molecules for MHC class II peptide loading

Accessory molecules, such as HLA-DM and invariant chain, modulate the ligands bound to MHC class II molecules in antigen-presenting cells. Recent investigations, including gene targeting experiments, have shed light on the functions of these molecules, their mechanisms of action, interactions with class II molecules, and the relationships with associated molecules such as tetraspanins and HLA-DO.

Peptide exchange in MHC molecules

Major histocompatibility complex (MHC)-encoded glycoproteins bind peptide antigens through non-covalent interactions to generate complexes that are displayed on the surface of antigen-presenting cells (APC) for recognition by T cells. Peptide-binding site occupancy is necessary for stable assembly of newly synthesized MHC proteins and export from the endoplasmic reticulum (ER). The MHC class II antigen-processing pathway provides a mechanism for presentation of peptides generated in the endosomal pathway of APC. The chaperone protein, invariant chain, includes a surrogate peptide that stabilizes newly synthesized class II molecules during transport to endosomal compartments. The invariant chain-derived peptide must be replaced through a peptide exchange reaction that is promoted by acidic pH and the MHC-encoded co-factor HLA-DM. Peptide exchange reactions are not required for presentation of antigens by MHC class I molecules because they bind antigens during initial assembly in the ER. However, exchange reactions may play an important role in editing the repertoire of peptides presented by both class II and class I molecules, thus influencing the specificity of immunity and tolerance.

Recognition of major histocompatibility complex class II antigens by two anti-HLA-DR monoclonal antibodies on canine marrow cells correlates with effects on in vitro and in vivo hematopoiesis

BACKGROUND

The role of major histocompatibility complex class II antigens in hematopoiesis is not well defined. We have shown that in vitro depletion of HLA-DR+ cells from canine marrow (e.g., by anti-HLA-DR monoclonal antibody [mAb] H81.9 and complement) prevents hematopoietic recovery. In vivo administration of the same mAb H81.9 after transplantation of unmanipulated autologous marrow results in graft failure. In vitro mAb H81.9 inhibited colony formation from short-term and long-term marrow cultures.

METHODS AND RESULTS

We investigated the effect of another mAb, Ca1.41, which also recognizes nonpolymorphic determinants on human (HLA-DR) and canine major histocompatibility complex class II antigens but is reactive with a narrower spectrum of cells in both canine peripheral blood and marrow than mAb H81.9 (and other anti-HLA-DR mAbs). In contrast to all other anti-HLA-DR mAbs tested, Ca1.41 did not interfere with colony formation in short-term or long-term marrow cultures and spared a population of small mononuclear cells with low forward light scatter that was eliminated via apoptosis by exposure to mAb H81.9. These target cells included lymphocytes and CD34+ hemopoietic precursors that expressed MHC class II molecules as determined by mAb H81.9 but not by mAb Ca1.41. In addition, transmembrane signaling and up-regulation of interleukin-1beta mRNA occurred with mAb H81.9 but not with Ca1.41. Transplantation of autologous marrow treated in vitro cytolytically with mAb Ca1.41 allowed for complete hematopoietic reconstitution. Further, in vivo administration of Ca1.41 posttransplant did not lead to autologous graft failure as had been observed with mAb H81.9.

CONCLUSIONS

These results support the notion that major histocompatibility complex class II is expressed on early hematopoietic precursor cells but recognition is dependent upon the mAb used. Preliminary studies show that mAb H81.9 triggered transmembrane signaling, resulting in up-regulation of interleukin-1beta and apoptosis, although mAb Ca1.41 did not. The fact that Ca1.41 binding was modified in the presence of exogenous invariant chain-derived peptide suggests that both binding and signaling are peptide dependent.

Discoordinate Expression of Invariant Chain and MHC Class II Genes in Class II Transactivator-Transfected Fibroblasts Defective for RFX5

MHC class II deficiency or bare lymphocyte syndrome is a severe combined immunodeficiency caused by defects in MHC-specific transcription factors. In the present study, we show that fibroblasts derived from a recently identified bare lymphocyte syndrome patient, SSI, were mutated for RFX5, one of the DNA-binding components of the RFX complex. Despite the lack of functional RFX5 and resulting MHC class II-deficient phenotype, transfection of exogenous class II transactivator (CIITA) in these fibroblasts can overcome this defect, resulting in the expression of HLA-DR, but not of DP, DQ, and invariant chain. The lack of invariant chain expression correlated with lack of CIITA-mediated transactivation of the invariant chain promoter in transient transfection assays in SSI fibroblast cells. Consequently, these CIITA transfectants lacked Ag-presenting functions.

BALB/c Invariant Chain Mutant Mice Display Relatively Efficient Maturation of CD4+ T Cells in the Periphery and Secondary Proliferative Responses Elicited upon Peptide Challenge

Allelic differences are known to influence many important aspects of class II biosynthesis, including subunit assembly, Ii chain associations, and DM-mediated peptide loading. Mutant mouse strains lacking Ii chain expression have been previously studied on mixed genetic backgrounds. The present experiments describe cellular and functional characteristics of congenic BALB/c Ii chain mutants. As expected, class II surface expression was markedly decreased, but in contrast to I-Ad-transfected cell lines, serological analysis of BALB/c Ii chain-deficient spleen cells gave no evidence for discordant expression of class II conformational epitopes. Thus, we conclude that properly folded class II molecules are exported via the Ii chain-independent pathway. Functional assays demonstrate consistently superior peptide-loading capabilities, suggesting that these I-Ad molecules are empty or occupied by an easily displaced peptide(s). Defective B cell development was observed for three mutant strains established on diverse genetic backgrounds. Ii chain function is also essential for optimal class II surface expression by mature splenic dendritic cells. Surprisingly, we observe in BALB/c Ii chain mutants, relatively efficient maturation of CD4+ T cells in the periphery and secondary proliferative responses elicited upon peptide challenge. The milder phenotype displayed by BALB/c Ii chain mutants in comparison with class II functional defects previously described for mouse strains lacking Ii chain is likely to have an effect on disease susceptibility.

HLA-DM can partially replace the invariant chain for HLA-DR transport and surface expression in transfected endocrine epithelial cells

The function of HLA class II molecules as peptide presenters to CD4+ T cells depends on the expression of associated molecules such as the invariant chain (Ii) and DM responsible for the correct transport of and high-stability peptide binding to the class II dimers. In organs affected by autoimmune diseases, endocrine epithelial cells express class II molecules, which presumably are involved in the presentation of self-peptides to autoreactive T cells. We have transfected the rat insulinoma cell line RINm5F with different combinations of HLA-DR, Ii and HLA-DM cDNAs and have studied how Ii and DM affect the transport and stability of class II molecules expressed by the different transfectants. Immunofluorescence and biochemical analysis showed that cells transfected with DR and DM in the absence of Ii expressed mostly stable molecules in their surface, and showed a lower accumulation of DR molecules in the endoplasmic reticulum (ER) than cells expressing only DR. This suggests that, in the absence of invariant chain, DM molecules can not only exchange peptides other than class II-associated invariant chain peptide (CLIP) but may also be involved in the transport of class II molecules out of the ER towards the endosomal route. In addition, these data confirm that expression of DR alone or DR+Ii do not allow the formation of sodium dodecyl sulphate (SDS)-stable complexes, that cells expressing DR+Ii have most DR molecules occupied by CLIP and that Ii and DM molecules allow regular routing and peptide loading of class II molecules.

HLA-DMB expression by thyrocytes: indication of the antigen-processing and possible presenting capability of thyroid cells

Expression of HLA class II molecules on thyrocytes is a characteristic feature of autoimmune thyroid disease and may lead the thyroid cells to present autoantigens to CD4+ T lymphocytes. Since HLA-DM is a critical molecule in class II-restricted antigen processing and presentation, we assessed the expression of HLA-DMB, -invariant chain (Ii), class II transactivator (CIITA) and DRA in an untransformed, pure thyrocyte strain HTV-59A. Here we report that both HLA-DMB mRNA and the protein are expressed in thyrocytes and that CIITA expression is enhanced by interferon-gamma (IFN-gamma) treatment and occurs before DMB, Ii and DRA up-regulation, suggesting CIITA expression is a requirement for antigen processing in thyrocytes. These results indicate that thyrocytes are capable of antigen processing and possibly antigen presentation to T cells.

Understanding the mechanism of action of bacterial superantigens from a decade of research

In the face of the unique diversity and plasticity of the immune system pathogenic organisms have developed multiple mechanisms in adaptation to their hosts, including the expression of a particular class of molecules called superantigens. Bacterial superantigens are the most potent stimulators of T cells. The functional consequences of the expression of superantigens by bacteria can be extended not only to T lymphocytes, but also to B lymphocytes and to cells of the myeloid compartment, including antigen-presenting cells and phagocytes. The biological effects of bacterial superantigens as well as their molecular aspects have now been studied for a decade. Although there is still a long way to go to clearly understand the role these molecules play in the establishment of disease, recently acquired knowledge of their biochemistry now offers unique experimental opportunities in defining the molecular rules of T-cell activation. Here, we present some of the most recent functional and molecular aspects of the interaction of bacterial superantigens with MHC class II molecules and the T-cell receptor.

Identification of destabilizing residues in HLA class II-selected bacteriophage display libraries edited by HLA-DM

HLA-DM (DM) functions as a peptide editor by catalyzing the release of class II-associated invariant chain peptides (CLIP) and other unstable peptides, thus supporting the formation of stable class II-peptide complexes for presentation. To investigate the general features that determine the DM susceptibility of HLA-DR1/peptide complexes, we generated a large DM-sensitive peptide repertoire from an M13 bacteriophage display library using a novel double selection protocol: we selected bacteriophage capable of binding to DR1 molecules and, subsequently, we enriched DR1-bound bacteriophage susceptible to elution by purified DM molecules. Sequence and mutational analyses of the DR1/DM double-selected peptides revealed that the amino acids Gly and Pro play a destabilizing role in the dissociation kinetics of DR1 ligands. This observation was confirmed also in natural peptide sequences such as CLIP 89-101, HA 307-319 and bovine collagen II (CII) 261-273. Our results demonstrate that DM susceptibility does not only depend on the number and nature of anchor residues, or the peptide length. Instead, less obvious sequence characteristics play a major role in the DM editing process and ultimately in the composition of peptide repertoires presented to T cells.

HLA-DM and the MHC class II antigen presentation pathway

The MHC class II antigen processing pathway provides a mechanism to selectively present peptides generated in the endosomal compartments of antigen presenting cells to CD4+ T cells. Transport of newly synthesized class II molecules to the endosomal pathway requires the function of an accessory protein, invariant chain, which contains a region that interacts directly with the class II peptide binding site. Release of invariant chain and peptide loading by class II molecules are facilitated by a second accessory protein, HLA-DM. This MHC-encoded membrane protein catalyzes peptide exchange reactions, influencing the repertoire of peptides that are available for recognition by T cells.

The Ii41 isoform of invariant chain mediates both positive and negative selection events in T-cell receptor transgenic mice

The functional role of invariant chain in T-cell selection events and antigen presentation is well established. The invariant chain gene encodes differentially spliced isoforms, Ii31 and Ii41. The Ii41 isoform has been described to increase the efficiency of antigen presentation. We have analysed the effect of the Ii41 isoform on positive and negative selection of transgenic CD4 T cells with specificity for a natural self antigen (C5) which are crucially dependent on invariant chain for their development and functional antigen recognition. The data show that Ii41 fully substitutes for wild-type invariant chain in both positive and negative selection events during functional maturation of T cells with specificity for a natural, blood-borne self antigen.

Secondary structure composition and pH-dependent conformational changes of soluble recombinant HLA-DM

HLA-DM catalyzes the release of invariant chain fragments from newly synthesized major histocompatibility complex (MHC) class II molecules, stabilizes empty class II molecules, and edits class II-associated peptides by preferentially releasing those that are loosely bound. The ability of HLA-DM to carry out these functions in vitro is pH dependent, with an optimum at pH 4.5-5.5 and poor activity at pH 7. The structural basis for these properties of HLA-DM is unknown. Sequence homology suggests that HLA-DM resembles classical, peptide-binding MHC class II molecules. In this study, we examined whether HLA-DM has a secondary structure composition consistent with an MHC fold and whether HLA-DM changes conformation between pH 5 and pH 7. Far-UV circular dichroism (CD) spectra of recombinant soluble HLA-DM (sDM) indicate that HLA-DM belongs to the alpha/beta class of proteins and structurally resembles both MHC class I and class II molecules. The CD peak around 198 nm increases upon going from neutral to endosomal pH and drops sharply upon denaturation below pH 3.5, distinguishing at least three states of sDM: the denatured state and two highly similar folded states. Fluorescence emission spectra show a slight blue-shift and a approximately 20% drop in intensity at pH 5 compared with pH 7. Unfolding experiments using guanidinium chloride show that the stability of sDM is somewhat reduced but not lost at pH 5. These results indicate that sDM undergoes a pH-dependent conformational change between neutral and endosomal pH. The change seems to involve both hydrogen bonding patterns and the hydrophobic core of sDM and may contribute to the pH dependence of DM activity.

Dendritic cell maturation and antigen presentation in the absence of invariant chain

In immature dendritic cells (DCs), major histocompatibility complex class II molecules accumulate in peptide-loading compartments and, during DC maturation, are exported to the cell surface in response to inflammatory stimuli. Moreover, it has recently been proposed that DCs have specific mechanisms of antigen uptake and delivery into major histocompatibility complex class II-loading compartments. B cells bearing a genetically disrupted invariant chain gene (Ii -/-) show alterations in the transport and function of class II molecules. We herein report that DCs derived from Ii -/- H2(k) but not Ii -/- H2(b) mice undergo normal maturation in response to tumor necrosis factor alpha and show a high degree of class II surface expression. Class II molecules are accumulated in cathepsin D- and H2-M-positive compartments in immature Ii -/- DC and, during DC maturation, are exported to the cell membrane as compact dimers. Ii -/- DCs present putative Ii-dependent hen egg lysozyme-derived epitopes to T cells. These data support the existence of Ii-independent molecular requirements for class II transport and peptide loading in DCs.

MHC Class II Expression in Double Mutant Mice Lacking Invariant Chain and DM Functions

Invariant (Ii) chain and DM functions are required at distinct stages during class II maturation to promote occupancy by diverse peptide ligands. The class II molecules expressed by mutant mouse strains lacking Ii chain or DM activities display discrete structural and functional abnormalities. The present report describes the cellular and biochemical characteristics of Ii−DM− doubly deficient mice. As for Ii chain mutants, their mature AαbAβb dimers similarly exhibit reduced mobilities in SDS-PAGE, and in functional assays these molecules behave as if empty or occupied by an easily displaced peptide. Additionally, the present experiments demonstrate that the production of floppy AαbAβb dimers is TAP independent. In comparison with Ii chain mutants, Ii−DM− doubly deficient cell populations exhibit increased peptide binding activities and consistently greater presentation abilities in T cell stimulation assays. These functional differences appear to reflect higher class II surface expression associated with their increased representation of B lymphocytes. We also observe defective B cell maturation in mice lacking Ii chain or DM expression, and interestingly, B cell development appears more severely compromised in Ii−DM− double mutants. These mutant mice lacking both Ii chain and DM activities should prove useful for analyzing nonconventional class II Ag presentation under normal physiological conditions in the intact animal.

Cutting Edge: A Critical, Invariant Chain-Independent Role for H2-M in Antigen Presentation

Antigen presentation by MHC class II (class II) is facilitated by the accessory molecules, invariant chain (Ii) and H2-M. Ii associates with class II during biosynthesis and promotes transport of class II to Ag-loading compartments. One function of H2-M is the removal of Ii fragments from MHC class II. We have previously demonstrated that Ii-deficient mice, unlike class II-deficient mice, are resistant to L. major infection. In the present study, we found that H2-M-deficient (H2-M0) mice were susceptible to progressive infection with L. major. The dispensability of Ii for control of L. major allowed genetic analysis of whether H2-M functions by association with or independently of Ii. In contrast to Ii-deficient (Ii0) mice, Ii0H2-M0 mice were as susceptible to L. major as H2-M0 mice. Thus, H2-M has an essential, Ii-independent function during presentation of microbial pathogens.

Major histocompatibility complex class II-dependent unfolding, transport, and degradation of endogenous proteins

We have analyzed the ability of major histocompatibility (MHC) class II molecules to capture proteins in the biosynthetic pathway and whether this may be associated with MHC class II-dependent antigen processing. When coexpressed with HLA-DR 4 molecules in HeLa cells, influenza hemagglutinin was inhibited from folding and trimerization in the biosynthetic pathway, targeted to endosomal compartments, and rapidly degraded. Due to the interaction with MHC class II molecules, therefore, unfolded forms of hemagglutinin were bypassing the quality control mechanism of the secretory pathway. More important, however, the transport, endocytosis, and rapid degradation of unfolded hemagglutinin in the presence of MHC class II molecules suggest that proteins captured in the endoplasmic reticulum by class II molecules may become substrates for antigen processing and presentation to CD4-positive T cells. In insect cells we show that this phenomenon is not restricted to a few proteins such as hemagglutinin. A highly heterogeneous mixture of proteins from the endoplasmic reticulum including coexpressed hemagglutinin can form stable complexes with soluble HLA-DR alpha and beta chains that were transported into the supernatant. This mechanism may gain biological significance in abnormal situations associated with accumulation of unfolded or malfolded proteins in the endoplasmic reticulum, for example during viral infections.