Antigen processing in the endocytic compartment

Requirement for Phosphoinositide 3-Kinase p110δ Signaling in B Cell Antigen Receptor-Mediated Antigen Presentation

The BCR serves to both signal cellular activation and enhance uptake and presentation of Ags by B cells; however, the intracellular signaling mechanisms linking the BCR to Ag presentation functions have been controversial. PI3Ks are critical signaling enzymes controlling many cellular processes, with the p110delta isoform playing a critical role in BCR signaling. In this study, we used pharmacological and genetic approaches to evaluate the role of p110delta signaling in Ag presentation by primary B lymphocytes. It was found that activation of allogeneic T cells is significantly reduced when B cells are pretreated with global PI3K inhibitors, but was intact when p110delta signaling was specifically inactivated. In contrast, inactivation of p110delta significantly impaired the ability of B cells to activate T cells in a BCR-mediated Ag uptake and presentation model. Prestimulation of p110delta-inactivated B cells with anti-CD40 or LPS could not rescue their BCR-mediated Ag presentation ability to normal levels. p110delta signaling was required for efficient presentation of either anti-Ig or protein Ag via a lysozyme-specific BCR. p110delta-inactivated B cells were able to internalize Ag normally, and no defects in association of Ag with lysosome-associated membrane protein 1(+) late endosomes were observed; however, these cells were less effective in forming polarized conjugates with Ag-specific T cells. Our data demonstrate a role for p110delta signaling in B cell Ag presentation function, implicating 3-phosphoinositides and their targets in the latter stages of this process.

Antigen presentation by B lymphocytes: how receptor signaling directs membrane trafficking

Antigen capture and presentation onto MHC class II molecules by B lymphocytes is mediated by their surface antigen receptor - the B-cell receptor (BCR). The BCR must therefore coordinate the transport of MHC class II- and antigen-containing vesicles for them to converge and ensure efficient processing. Recently, progress has been made in understanding which and how these vesicular transport events are molecularly linked to BCR signaling. In particular, recent studies have emphasized the key roles of membrane microdomains and the actin cytoskeleton in regulation of membrane trafficking upon BCR engagement.

Increased proteolysis of diphtheria toxin by human monocytes after heat shock: a subsidiary role for heat-shock protein 70 in antigen processing

The expression of heat-shock proteins (hsp) increases after exposure to various stresses including elevated temperatures, oxidative injury, infection and inflammation. As molecular chaperones, hsp have been shown to participate in antigen processing and presentation, in part through increasing the stability and expression of major histocompatibility complex molecules. Heat shock selectively increases human T-cell responses to processed antigen, but does not affect T-cell proliferation induced by non-processed antigens. Here, we have analysed the mechanisms by which stress such as heat shock, and the ensuing hsp over-expression affect the processing of diphtheria toxin (DT) in peripheral blood monocytes. We found that heat shock increased DT proteolysis in endosomes and lysosomes while the activities of the cathepsins B and D, classically involved in DT proteolysis, were decreased. These effects correlated with the heat-shock-mediated increase in hsp 70 expression observed in endosomes and lysosomes. Actinomycin D or blocking anti-hsp 70 antibodies abolished the heat-shock-mediated increase in DT proteolysis. These data indicate that the increased expression of hsp 70 constitutes a subsidiary mechanism that facilitates antigen proteolysis in stressed cells. Confirming these data, presentation by formaldehyde-fixed cells of DT proteolysates that were obtained with endosomes and lysosomes from heat-shocked peripheral blood monocytes showed higher stimulation of T cells than those generated with endosomes and lysosomes from control peripheral blood monocytes.

A mutation in Epstein-Barr virus LMP2A reveals a role for phospholipase D in B-Cell antigen receptor trafficking

Epstein-Barr virus (EBV) latent infection of B cells blocks the interrelated signaling and antigen-trafficking functions of the BCR through the activity of its latent membrane protein 2A (LMP2A). At present, the molecular mechanisms by which LMP2A exerts its control of BCR functions are only poorly understood. Earlier studies showed that in B cells expressing LMP2A containing a tyrosine mutation at position 112 in its cytoplasmic domain (Y112-LMP2A), the BCR could initiate signaling but could not properly traffic antigen for processing. Here, we show that BCR signaling in Y112-LMP2A-expressing cells is attenuated with a reduction in both the degree and duration of phosphorylation of key components of the BCR signaling cascade including Syk, BLNK, PI3K, and Btk. Notably, Y112-LMP2A expression completely blocked the BCR-induced activation of phospholipase D (PLD), a lipase implicated in the intracellular trafficking of a variety of surface receptors. We show that blocking PLD activity, by expressing Y112-LMP2A, treating cells with the PLD inhibitor 1-butanol or reducing PLD expression by siRNA, blocked BCR trafficking to class II-containing compartments. Moreover, Y112-LMP2A expression blocked the recruitment of phosphorylated forms of the downstream BCR signaling components, Erk and JNK, through both PLD-dependent and PLD-independent mechanisms. Thus, the investigation of the mechanism by which Y112-LMP2A blocks BCR function revealed an essential role for PLD in BCR trafficking for antigen processing.

Molecular biology and immunology for clinicians, 14: Antigen presenting cells--Class II

Pivotal to immunity and auto-immunity is the ability of the human immune response to make antigen-specific responses, both cellular and humoral. T- and B-cells contain within themselves the ability to recognize and react to specific antigens, but they must be made aware of the presence of their target in the surrounding environment to respond. Turns out this part of the education of T-cells (not B-cells, which are activated by specific antigens in a different manner) is provided by a large number of cells, all coming under the umbrella term: antigen-presenting cells. Understanding how these cells take up molecules from the environment or acquire protein molecules from the intracellular milieu, manipulate them, and then offer the modified material to engage potentially responding cells in an immunological educational conversation is crucial to understanding normal immune function and, of course, auto-immunity and other forms of immune dysregulation. In the broadest of terms, there are two sources of proteins: endogenous (produced within the cell) and exogenous (produced outside of the cell), and there are two not entirely mutually exclusive pathways involved in antigen processing and presentation. To decrease confusion between these two separate pathways antigens, I will proceed with a description of the latter in this paper and cover the former in the next paper in this series. So, now on to antigen processing and presentation of proteins.

On regulation of phagosome maturation and antigen presentation

Phagocytosis has essential functions in immunity. Here we highlight the presence of a subcellular level of self-non-self discrimination in dendritic cells that operates at the level of individual phagosomes. We discuss how engagement of Toll-like receptor signaling controls distinct programs of phagosome maturation. An inducible mode of phagosome maturation triggered by these receptors ensures the selection of microbial antigens for presentation by major histocompatibility class II molecules during the simultaneous phagocytosis of self and non-self.

Molecular cloning and characterization of a novel V-ATPase associated protein, DVA9.2, from human dendritic cells

The vacuolar proton-ATPase (V-ATPase) is a ubiquitous ATP-driven H(+) transporter that functions in numerous cell processes. Accumulating evidence shows important roles of V-ATPase in tumor metastasis and antigen presentation of dendritic cells (DC). A novel V-ATPase associated protein, designated as DVA9.2 (dendritic cell-derived V-ATPase associated protein of 9.2 kDa), has been identified from a human DC cDNA library by large-scale random sequencing. Full length cDNA of DVA9.2 encodes an 81-residue protein that shares 70-80% homology with human V-ATPase subunit M9.2. Distant relationship is also found with Vma21p, a yeast protein required for V-ATPase assembly. DVA9.2 contains a conserved domain, ATP synthase subunit H (pafm05493), and two membrane-spanning helices. DVA9.2 mRNA is detectable in several human tumor cell lines as well as some human normal cells and tissues. Moreover, the inducible expression of DVA9.2 mRNA in DC during maturation is observed. DVA9.2 displays integration with membrane and main localization in lysosome, endoplasmic reticulum and Golgi-associated organelles, only less at the plasma membrane. In addition, DVA9.2 is co-localized with V(0)-sector subunit a. Silencing of DVA9.2 by small interfering RNA (siRNA) does not affect the V-ATPase activity in cell membrane fractions or attenuate the migration and invasion in breast cancer MDA-MB-231 cells. These results indicate that DVA9.2, as a novel V-ATPase-associated protein, is not essential for the activity of V-ATPase complex and may be involved in functions of DC.

Small organic compounds enhance antigen loading of class II major histocompatibility complex proteins by targeting the polymorphic P1 pocket

Major histocompatibility complex (MHC) molecules are a key element of the cellular immune response. Encoded by the MHC they are a family of highly polymorphic peptide receptors presenting peptide antigens for the surveillance by T cells. We have shown that certain organic compounds can amplify immune responses by catalyzing the peptide loading of human class II MHC molecules HLA-DR. Here we show now that they achieve this by interacting with a defined binding site of the HLA-DR peptide receptor. Screening of a compound library revealed a set of adamantane derivatives that strongly accelerated the peptide loading rate. The effect was evident only for an allelic subset and strictly correlated with the presence of glycine at the dimorphic position beta86 of the HLA-DR molecule. The residue forms the floor of the conserved pocket P1, located in the peptide binding site of MHC molecule. Apparently, transient occupation of this pocket by the organic compound stabilizes the peptide-receptive conformation permitting rapid antigen loading. This interaction appeared restricted to the larger Gly(beta86) pocket and allowed striking enhancements of T cell responses for antigens presented by these "adamantyl-susceptible" MHC molecules. As catalysts of antigen loading, compounds targeting P1 may be useful molecular tools to amplify the immune response. The observation, however, that the ligand repertoire can be affected through polymorphic sites form the outside may also imply that environmental factors could induce allergic or autoimmune reactions in an allele-selective manner.

Antigen three-dimensional structure guides the processing and presentation of helper T-cell epitopes

Antigen three-dimensional structure potentially controls presentation of CD4(+) T-cell epitopes by limiting the access of proteolytic enzymes and MHC class II antigen-presenting proteins. The protease-sensitive mobile loops of Hsp10s from mycobacteria, Escherichia coli, and bacteriophage T4 (T4Hsp10) are associated with adjacent immunodominant helper T-cell epitopes, and a mobile-loop deletion in T4Hsp10 eliminated the protease sensitivity and the associated epitope immunodominance. In the present work, protease-sensitivity and epitope presentation was analyzed in a group of T4Hsp10 variants. Two mobile-loop sequence variants of T4Hsp10 were constructed by replacing different segments of the mobile loop with an irrelevant sequence from hen egg lysozyme. The variant proteins retained native-like structure, and the mobile loops retained protease sensitivity. Mobile-loop deletion and reconstruction affected the presentation of two epitopes according to whether the epitope was protease-independent or protease-dependent. The protease-independent epitope lies within the mobile loop, and the protease-dependent epitope lies in a well-ordered segment on the carboxy-terminal flank of the mobile loop. The results are consistent with a model for processing of the protease-dependent epitope in which an endoproteolytic nick in the mobile-loop unlocks T4Hsp10 three-dimensional structure, and then the epitope becomes available for binding to the MHC protein.

A vitellogenic-like carboxypeptidase expressed by human macrophages is localized in endoplasmic reticulum and membrane ruffles

Carboxypeptidase, vitellogenic-like (CPVL) is a serine carboxypeptidase of unknown function that was first characterized in human macrophages. Initial studies suggested that CPVL is largely restricted to the monocytic lineage, although it may also be expressed by cells outside the immune system. Here, we use a new monoclonal antibody to characterize the properties and localization of CPVL in human macrophages to elucidate a possible function for the protease. CPVL is up-regulated during the maturation of monocytes (MO) to macrophages, although the protein can be seen in both. In primary macrophages, CPVL is glycosylated with high mannose residues and colocalizes with markers for endoplasmic reticulum, while in MO it is more disperse and less clearly associated with endoplasmic reticulum. CPVL is highly expressed in lamellipodia and membrane ruffles, which also concentrate markers of the secretory pathway (MIP-1alpha and tumour necrosis factor-alpha) and major histocompatibility complex (MHC) class I and II molecules. CPVL can be seen on early latex bead and Candida albicans phagosomes, but it is not retained in the maturing phagosome, unlike MHC class I/II. CPVL has a mixed cytosolic and membrane-associated localization but is not detectable on the outer plasma membrane. We propose that CPVL may be involved in antigen processing, the secretory pathway and/or in actin remodelling and lamellipodium formation.

The impact of glycosylation on HLA-DR1-restricted T cell recognition of type II collagen in a mouse model

OBJECTIVE

Type II collagen (CII) is a candidate autoantigen implicated in the pathogenesis of rheumatoid arthritis (RA). Posttranslational glycosylation of CII could alter intracellular antigen processing, leading to the development of autoimmune T cell responses. To address this possibility, we studied the intracellular processing of CII for presentation of the arthritogenic glycosylated epitope CII(259-273) to CD4 T cells in macrophages from HLA-DR1-transgenic mice.

METHODS

HLA-DR1-transgenic mice were generated on a class II major histocompatibility complex-deficient background, and T cell hybridomas specific for the glycosylated and nonglycosylated epitope CII(259-273) were developed. Subcellular fractionation of macrophages was used to localize CII degradation to particular compartments and to identify the catalytic subtype of proteinases involved.

RESULTS

We showed that the glycosylated CII(259-273) epitope required more extensive processing than did the nonglycosylated form of the same epitope. Dense fractions containing lysosomes were primarily engaged in the processing of CII for antigen presentation, since these compartments contained 1) enzyme activity that generated antigenic CII fragments bearing the arthritogenic glycosylated epitope, 2) the antigenic CII fragments themselves, 3) CII peptide-receptive HLA-DR1 molecules, and 4) peptide/HLA-DR1 complexes that could directly activate T cell hybridomas. Degradation of CII by dense fractions occurred optimally at pH 4.5 and was abrogated by inhibitors of serine and cysteine proteinases.

CONCLUSION

Processing of the arthritogenic glycosylated CII(259-273) epitope, which is implicated in the induction of autoimmune arthritis, is more stringently regulated than is processing of the nonglycosylated form of the same epitope. Mechanisms of intracellular processing of the glycosylated epitope may constitute novel therapeutic targets for the treatment of RA.

Major histocompatibility complex class I restricted T-cell autoreactivity in human peripheral blood mononuclear cells

During selection in the thymus or any subsequent response, T-cells recognize peptides bound to major histocompatibility complex (MHC) molecules. Peptides produced by lysosomes or by proteasome/immunoproteasome stimulate CD4+ or CD8+ T-cell, respectively. Inflammation alters components of both antigen-processing pathways resulting in the production of different peptides. The role of such changes in self/non-self discrimination was examined in autologous mixed peripheral blood mononuclear cell cultures. Stimulator cells were incubated in the presence or absence of INF-gamma, with or without lysosome inhibitors (ammonium chloride/chloroquine), cathepsin inhibitor (E-64), or proteasome/immunoproteasome inhibitor (epoxomicin). Responder cells were added and zeta-chain phosphorylated forms were used as read out. INF-gamma did not affect zeta-chain phosphorylated forms, which means that the expected INF-gamma induced alterations in antigen processing machinery do not influence self/non-self discrimination. Surprisingly, the completely phosphorylated 23-kDa zeta-chain was always present except in the case of epoxomicin, indicating the presence of MHC class I restricted autoreactive CD8+ T-cells but not of MHC class II restricted autoreactive CD4+ T-cells, possibly due to more efficient negative selection in the thymus of the latter. Autoimmunity is prevented due to absence of help by CD4+ T-cells. This conclusion was confirmed by the lack of differences in IL-2 levels in cell culture supernatants, as well as, by the absence of differences in cell proliferation under the various conditions described above.

Cell biology of T cell activation and differentiation

T cells are major components of the adaptive immune system. They can differentiate into two different populations of effector cells-type one and type two-and may also become tolerant. T cells respond to immune challenges by interacting with antigen-presenting cells of the innate immune system. These latter cells can identify the nature of any immune challenge and initiate adaptive immune responses. Dendritic cells are the most important antigen-presenting cells in the body. The T cell recognizes both peptides associated with MHC molecules on the antigen-presenting cells and also other molecules in a complex structure known as an immunological synapse. The nature of the antigen, the cytokine environment, and other molecules on the dendritic cell surface instruct the T cells as to the response required. A better understanding of the biology of T cell responses offers the prospect of more effective therapeutic interventions.

Identification of peptides from human pathogens able to cross-activate an HIV-1-gag-specific CD4+ T cell clone

Antigen recognition by T cells is degenerate both at the MHC and the TCR level. In this study, we analyzed the cross-reactivity of a human HIV-1 gag p24-specific CD4(+) T cell clone obtained from an HIV-1-seronegative donor using a positional scanning synthetic combinatorial peptide library (PS-SCL)-based biometrical analysis. A number of decapeptides able to activate the HIV-1 gag-specific clone were identified and shown to correspond to sequences found in other human pathogens. Two of these peptides activated the T cell clone with the same stimulatory potency as the original HIV-1 gag p24 peptide. These findings show that an HIV-1-specific human T helper clone can react efficiently with peptides from other pathogens and suggest that cellular immune responses identified as being specific for one human pathogen (HIV-1) could arise from exposure to other pathogens.

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.

Asparaginyl endopeptidase: case history of a class II MHC compartment protease

Although the endpoint of the class II antigen-processing pathway is well characterized, the processing events that lead to the production of class II major histocompatibility complex (MHC)/peptide complexes are not. It is generally assumed that protease action on native antigen substrates leads to unfolding and capture of either long or short peptides. Whether specific protease activities are needed for presentation of particular T-cell epitopes is largely unknown. Here, we review our recent studies that aim to identify the processing enzymes that initiate processing of different antigens. We suggest a general strategy that can potentially identify preferred relationships between substrates and processing enzymes in vitro and suggest ways in which these relationships can be tested in vivo. We draw heavily on the example of asparaginyl endopeptidase, which is involved in both productive and destructive processing of different antigen substrates. Overall, while there is undoubtedly redundancy in class II MHC antigen processing, the contributions of individual enzymes can be clearly dissected.

Dendritic cells, DRiPs, and DALIS in the control of antigen processing

The study of the cell biology of antigen processing and presentation has greatly contributed to our understanding of the immune response. The work of many immunologically inclined cell biologists has also permitted new insights into cellular mechanisms shared by many cell types. Here are described recent and particularly exciting findings on the regulation of major histocompatibility complex class I-restricted presentation in dendritic cells and their contribution to the deciphering of the cellular response to pathogen detection.

Compartmentalization of class II antigen presentation: contribution of cytoplasmic and endosomal processing

During antigen processing, peptides are generated and displayed in the context of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) to modulate immune responses to foreign antigens and guide self-tolerance. Exogenous and cytoplasmic antigens are processed by distinct routes within APCs to yield class II ligands. Exogenous antigens are internalized, processed, and bound to class II molecules within endosomal and lysosomal compartments of APCs. Studies reviewed here demonstrate the importance of reduction in regulating exogenous antigen presentation. The differential expression of a gamma-interferon-inducible lysosomal thiol reductase in professional APCs and melanomas is discussed in the context of tumor immune evasion. Cytoplasmic autoantigens, by contrast, are degraded by the proteasome and other enzymes in the cytosol, with the resulting peptides translocating to endosomal and lysosomal compartments for intersection with class II molecules. Processing and editing of these antigenic peptides within endosomes and lysosomes may be critical in regulating their display via class II proteins. Multiple pathways may regulate the transit of cytosolic peptides to class II molecules. The role of lysosome-associated membrane protein-2a and heat-shock cognate protein 70 in promoting cytoplasmic peptide presentation by MHC class II molecules is discussed.

Helminth parasites--masters of regulation

Immune regulation by parasites is a global concept that includes suppression, diversion, and conversion of the host immune response to the benefit of the pathogen. While many microparasites escape immune attack by antigenic variation or sequestration in specialized niches, helminths appear to thrive in exposed extracellular locations, such as the lymphatics, bloodstream, or gastrointestinal tract. We review here the multiple layers of immunoregulation that have now been discovered in helminth infection and discuss both the cellular and the molecular interactions involved. Key events among the host cell population are dominance of the T-helper 2 cell (Th2) phenotype and the selective loss of effector activity, against a background of regulatory T cells, alternatively activated macrophages, and Th2-inducing dendritic cells. Increasingly, there is evidence of important effects on other innate cell types, particularly mast cells and eosinophils. The sum effect of these changes to host reactivity is to create an anti-inflammatory environment, which is most favorable to parasite survival. We hypothesize therefore that parasites have evolved specific molecular strategies to induce this conducive landscape, and we review the foremost candidate immunomodulators released by helminths, including cytokine homologs, protease inhibitors, and an intriguing set of novel products implicated in immune suppression.

Dendritic cells, pro-inflammatory responses, and antigen presentation in a rodent malaria infection

An infection of mice with Plasmodium chabaudi is characterized by a rapid and marked inflammatory response with a rapid but regulated production of interleukin-12 (IL-12), tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma). Recent studies have shown that dendritic cells (DCs) are activated in vivo in the spleen, are able to process and present malaria antigens during infection, and may provide a source of cytokines that contribute to polarization of the CD4 T-cell response. P. chabaudi-infected erythrocytes are phagocytosed by DCs, and peptides of malaria proteins are presented on major histocompatibility complex (MHC) class II. The complex disulfide-bonded structure of some malaria proteins can impede their processing in DCs, which may affect the magnitude of the CD4 T-cell response and influence T-helper 1 (Th1) or Th2 polarization. DCs exhibit a wide range of responses to parasite-infected erythrocytes depending on their source, their maturational state, and the Plasmodium species or strain. P. chabaudi-infected erythrocytes stimulate an increase in the expression of costimulatory molecules and MHC class II on mouse bone marrow-derived DCs, and they are able to induce the production of pro-inflammatory cytokines such as IL-12, TNF-alpha, and IL-6, thus enhancing the Th1 response of naïve T cells. IFN-gamma and TNF-alpha play a role in both protective immunity and the pathology of the infection, and the inflammatory disease may be regulated by IL-10 and transforming growth factor-beta. It will therefore be important to elucidate the host and parasite molecules that are involved in activation or suppression of the DCs and to understand the interplay between these opposing forces on the host response in vivo during a malaria infection.

Multiple Intracellular Routes in the Cross-Presentation of a Soluble Protein by Murine Dendritic Cells

Soluble heat shock fusion proteins (Hsfp) stimulate mice to produce CD8+ CTL, indicating that these proteins are cross-presented by dendritic cells (DC) to naive CD8 T cells. We report that cross-presentation of these proteins depends upon their binding to DC receptors, likely belonging to the scavenger receptor superfamily. Hsfp entered DC by receptor-mediated endocytosis that was either inhibitable by cytochalasin D or not inhibitable, depending upon aggregation state and time. Most endocytosed Hsfp was transported to lysosomes, but not the small cross-presented fraction that exited early from the endocytic pathway and required access to proteasomes and TAP. Naive CD8 T cell (2C and OT-I) responses to DC incubated with Hsfp at 1 microM were matched by incubating DC with cognate octapeptides at 1-10 pM, indicating that display of very few class I MHC-peptide complexes per DC can be sufficient for cross-presentation. With an Hsfp (heat shock protein-OVA) having peptide sequences for both CD4+ (OT-II) and CD8+ (OT-I) cells, the CD4 cells responded far more vigorously than the CD8 cells and many more class II MHC-peptide than class I MHC-peptide complexes were displayed.

Bm-CPI-2, a cystatin from Brugia malayi nematode parasites, differs from Caenorhabditis elegans cystatins in a specific site mediating inhibition of the antigen-processing enzyme AEP

The filarial parasite Brugia malayi survives for many years in the human lymphatic system. One immune evasion mechanism employed by Brugia is thought to be the release of cysteine protease inhibitors (cystatins), and we have previously shown that the recombinant cystatin Bm-CPI-2 interferes with protease-dependent antigen processing in the MHC class II antigen presentation pathway. Analogy with vertebrate cystatins suggested that Bm-CPI-2 is bi-functional, with one face of the protein blocking papain-like proteases, and the other able to inhibit legumains such as asparaginyl endopeptidase (AEP). Site-directed mutagenesis was carried out on Bm-CPI-2 at Asn-77, the residue on which AEP inhibition is dependent in vertebrate homologues. Two mutations at this site (to Asp and Lys) showed 10-fold diminished and ablated activity respectively, in assays of AEP inhibition, while blocking of papain-like proteases was reduced by only a small degree. Comparison of the B. malayi cystatins with two homologues encoded by the free-living model organism, Caenorhabditis elegans, suggested that while the papain site may be intact, the AEP site would not be functional. This supposition was tested with recombinant C. elegans proteins, Ce-CPI-1 (K08B4.6) and Ce-CPI-2 (R01B10.1), both of which block cathepsins and neither of which possess the ability to block AEP. Thus, Brugia CPI-2 may have convergently evolved to inhibit an enzyme important only in the mammalian environment.

Antigen Stability Controls Antigen Presentation

We investigated whether protein stability controls antigen presentation using a four disulfide-containing snake toxin and three derivatives carrying one or two mutations (L1A, L1A/H4Y, and H4Y). These mutations were anticipated to increase (H4Y) or decrease (L1A) the antigen non-covalent stabilizing interactions, H4Y being naturally and frequently observed in neurotoxins. The chemically synthesized derivatives shared similar three-dimensional structure, biological activity, and T epitope pattern. However, they displayed differential thermal unfolding capacities, ranging from 65 to 98 degrees C. Using these differentially stable derivatives, we demonstrated that antigen stability controls antigen proteolysis, antigen processing in antigen-presenting cells, T cell stimulation, and kinetics of expression of T cell determinants. Therefore, non-covalent interactions that control the unfolding capacity of an antigen are key parameters in the efficacy of antigen presentation. By affecting the stabilizing interaction network of proteins, some natural mutations may modulate the subsequent T-cell stimulation and might help microorganisms to escape the immune response.

The potentials of MS-based subproteomic approaches in medical science: the case of lysosomes and breast cancer

Because of the great number of women who are diagnosed with breast cancer each year, and though this disease presents the lowest mortality rate among cancers, breast cancer remains a major public health problem. As for any cancer, the tumorigenic and metastatic processes are still hardly understood, and the biochemical markers that allow either a precise monitoring of the disease or the classification of the numerous forms of breast cancer remain too scarce. Therefore, great hopes are put on the development of high-throughput genomic and proteomic technologies. Such comprehensive techniques should help in understanding the processes and in defining steps of the disease by depicting specific genes or protein profiles. Because techniques dedicated to the current proteomic challenges are continuously improving, the probability of the discovery of new potential protein biomarkers is rapidly increasing. In addition, the identification of such markers should be eased by lowering the sample complexity; e.g., by sample fractionation, either according to specific physico-chemical properties of the proteins, or by focusing on definite subcellular compartments. In particular, proteins of the lysosomal compartment have been shown to be prone to alterations in their localization, expression, or post-translational modifications (PTMs) during the cancer process. Some of them, such as the aspartic protease cathepsin D (CatD), have even been proven as participating actively in the disease progression. The present review aims at giving an overview of the implication of the lysosome in breast cancer, and at showing how subproteomics and the constantly refining MS-based proteomic techniques may help in making breast cancer research progress, and thus, hopefully, in improving disease treatment.

Enhancement to the RANKPEP resource for the prediction of peptide binding to MHC molecules using profiles

We introduced previously an on-line resource, RANKPEP that uses position specific scoring matrices (PSSMs) or profiles for the prediction of peptide-MHC class I (MHCI) binding as a basis for CD8 T-cell epitope identification. Here, using PSSMs that are structurally consistent with the binding mode of MHC class II (MHCII) ligands, we have extended RANKPEP to prediction of peptide-MHCII binding and anticipation of CD4 T-cell epitopes. Currently, 88 and 50 different MHCI and MHCII molecules, respectively, can be targeted for peptide binding predictions in RANKPEP. Because appropriate processing of antigenic peptides must occur prior to major histocompatibility complex (MHC) binding, cleavage site prediction methods are important adjuncts for T-cell epitope discovery. Given that the C-terminus of most MHCI-restricted epitopes results from proteasomal cleavage, we have modeled the cleavage site from known MHCI-restricted epitopes using statistical language models. The RANKPEP server now determines whether the C-terminus of any predicted MHCI ligand may result from such proteasomal cleavage. Also implemented is a variability masking function. This feature focuses prediction on conserved rather than highly variable protein segments encoded by infectious genomes, thereby offering identification of invariant T-cell epitopes to thwart mutation as an immune evasion mechanism.

Peptides from common viral and bacterial pathogens can efficiently activate diabetogenic T-cells

Cross-reactivity between an autoantigen and unknown microbial epitopes has been proposed as a molecular mechanism involved in the development of insulin-dependent diabetes (type 1 diabetes). Type 1 diabetes is an autoimmune disease that occurs in humans and the nonobese diabetic (NOD) mouse. BDC2.5 is an islet-specific CD4+ T-cell clone derived from the NOD mouse whose natural target antigen is unknown. A biometrical analysis of screening data from BDC2.5 T-cells and a positional scanning synthetic combinatorial library (PS-SCL) was used to analyze and rank all peptides in public viral and bacterial protein databases and identify potential molecular mimic sequences with predicted reactivity. Selected sequences were synthesized and tested for stimulatory activity with BDC2.5 T-cells. Active peptides were identified, and some of them were also able to stimulate spontaneously activated T-cells derived from young, pre-diabetic NOD mice, indicating that the reactivity of the BDC2.5 T-cell is directed at numerous mouse peptides. Our results provide evidence for their possible role as T-cell ligands involved in the activation of diabetogenic T-cells.

The exogenous pathway for antigen presentation on major histocompatibility complex class II and CD1 molecules

The endosomes and lysosomes of antigen-presenting cells host the processing and assembly reactions that result in the display of peptides on major histocompatibility complex (MHC) class II molecules and lipid-linked products on CD1 molecules. This environment is potentially hostile for T cell epitope and MHC class II survival, and the influence of regulators of protease activity and specialized chaperones that assist MHC class II assembly is crucial. At present, evidence indicates that individual proteases make both constructive and destructive contributions to antigen processing for MHC class II presentation to CD4 T cells. Some features of CD1 antigen capture within the endocytic pathway are also discussed.

Mycobacterial phosphatidylinositol mannoside is a natural antigen for CD1d-restricted T cells

A group of T cells recognizes glycolipids presented by molecules of the CD1 family. The CD1d-restricted natural killer T cells (NKT cells) are primarily considered to be self-reactive. By employing CD1d-binding and T cell assays, the following structural parameters for presentation by CD1d were defined for a number of mycobacterial and mammalian lipids: two acyl chains facilitated binding, and a polar head group was essential for T cell recognition. Of the mycobacterial lipids tested, only a phosphatidylinositol mannoside (PIM) fulfilled the requirements for CD1d binding and NKT cell stimulation. This PIM activated human and murine NKT cells via CD1d, thereby triggering antigen-specific IFN-gamma production and cell-mediated cytotoxicity, and PIM-loaded CD1d tetramers identified a subpopulation of murine and human NKT cells. This phospholipid, therefore, represents a mycobacterial antigen recognized by T cells in the context of CD1d.

Targeting dendritic cells for priming cellular immune responses

The cardinal role of dendritic cells (DC) in priming adaptive immunity and in orchestrating immune responses against all classes of pathogens and also against tumors is well established. Their unique potential both to maintain self-tolerance and to initiate protective immune responses against foreign and/or dangerous structures is based on the functional diversity and flexibility of these cells. Tissue DC lining antigenic portals such as mucosal surfaces and the skin are specialized to take up a wide array of compounds including proteins, lipids, carbohydrates, glycoproteins, glycolipids and oligonucleotides, particles carrying such structures and apoptotic or necrotic cells. This process is facilitated by specialized receptors with high endocytic capacity, which provides potential targets for delivering designed molecules. The best route for targeting B- and/or T cell epitopes, however, is still the subject of intense investigation. Immature DC, which reside in various tissues, can be activated by pathogens, stress and inflammation or modified metabolic products, which induce mobilization of cells to draining lymph nodes where they act as highly potent professional antigen presenting cells. This is brought about by the ability to present their accumulated intracellular content for both CD4+ helper (Th) and CD8+ cytotoxic/cytolytic T lymphocytes (Tc/CTL). Engulfed proteins are processed intracellularly and their peptide fragments are transported to the cell surface in the context of major histocompatibility complex encoded class I and II molecules for presentation to Th cells and CTLs, respectively. The T cell priming capacity of DC, however, depends not only on antigen presentation but also on other features of DC. Human monocyte-derived DC provide an excellent tool to study the internalizing, antigen-presenting and T cell-activating functions of DC at their immature and activated differentiation states. These biological activities of DC, however, are highly dependent on their migratory potential from the peripheral non-lymphoid tissues to the lymph nodes, on the expression of adhesion molecules, which support the interaction of DC with T lymphocytes, and the cytokines secreted by DC, which polarize immune responses to Th1-mediated cellular or Th2-mediated antibody responses. These results altogether demonstrate that monocyte-derived DC are useful candidates for in vitro or in vivo targeting of antigens to induce efficient adaptive immune responses against pathogens and also against tumors.

A survival game of hide and seek: cytomegaloviruses and MHC class I antigen presentation pathways

Cytomegaloviruses (CMV) are members of the ubiquitous family of herpesviruses, which escape immunological clearance and persist throughout life in the infected host. Cytomegaloviruses have developed numerous strategies that permit them to co-exist with their host even as an anti-virus immune response endangers their long-term survival. A considerable number of these strategies are aimed at MHC class I presentation of viral proteins to CD8+ T cells (TCD8+ ). Although the gamut of CMV immune evasion will be briefly examined, the primary focus of this review is on the host ability to counteract the strategies developed by CMV to inhibit antigen processing and presentation. A primary mechanism used by the immune system is the recognition of very early virus proteins including recognition of the immunomodulatory proteins themselves. We further speculate that cross-presentation of antigen is an adaptive immune response to the inhibition of direct presentation. Other mechanisms, such as the evolution of pAPC subsets, may also allow the immune system to adapt to a variety of different infectious pathogens while preventing cytopathic infection of all pAPCs.

Functional proteomics of the active cysteine protease content in Drosophila S2 cells

The fruit fly genome is characterized by an evolutionary expansion of proteases and immunity-related genes. In order to characterize the proteases that are active in a phagocytic Drosophila model cell line (S2 cells), we have applied a functional proteomics approach that allows simultaneous detection and identification of multiple protease species. DCG-04, a biotinylated, mechanism-based probe that covalently targets mammalian cysteine proteases of the papain family was found to detect Drosophila polypeptides in an activity-dependent manner. Chemical tagging combined with tandem mass spectrometry permitted retrieval and identification of these polypeptides. Among them was thiol-ester motif-containing protein (TEP) 4 which is involved in insect innate immunity and shares structural and functional similarities with the mammalian complement system factor C3 and the pan-protease inhibitor alpha2-macroglobulin. We also found four cysteine proteases with homologies to lysosomal cathepsin (CTS) L, K, B, and F, which have been implicated in mammalian adaptive immunity. The Drosophila CTS equivalents were most active at a pH of 4.5. This suggests that Drosophila CTS are, similar to their mammalian counterparts, predominantly active in lysosomal compartments. In support of this concept, we found CTS activity in phagosomes of Drosophila S2 cells. These results underscore the utility of activity profiling to address the functional role of insect proteases in immunity.

Differential processing of CD4 T-cell epitopes from the protective antigen of Bacillus anthracis

We have mapped CD4+ T-cell epitopes located in three domains of the recombinant protective antigen of Bacillus anthracis. Mouse T-cell hybridomas specific for these epitopes were generated to study the mechanisms of proteolytic processing of recombinant protective antigen for antigen presentation by bone marrow-derived macrophages. Overall, epitopes differed considerably in their processing requirements. In particular, the kinetics of presentation, ranging from 15 (fast) to 120 min (slow), suggested sequential liberation of epitopes during proteolytic processing of the intact PA molecule. Pretreatment of macrophages with ammonium chloride or inhibitors of the major enzyme families showed that T-cell responses to an epitope presented with fast kinetics were unaffected by raising endosomal pH or inhibiting cysteine or aspartic proteinases, suggesting presentation independent of lysosomal processing. In contrast, responses to epitopes presented with slower kinetics were dependent on low pH and the activity of cysteine or aspartic proteinases indicating a requirement for lysosomal processing. In addition, responses to all epitopes, whether their presentation was dependent on low pH or not, were prevented by treatment of macrophages with broad spectrum serine proteinase inhibitors. Thus, our data are consistent with a model of sequential antigen processing within the endosomal system, beginning with a pre-processing step mediated by serine or metalloproteinases prior to further processing by lysosomal enzymes. Rapidly presented epitopes seemed to require only limited proteolysis at earlier stages of endocytosis, whereas the majority of epitopes required more extensive processing by neutral proteinases followed by lysosomal enzymes.

Phylogeny of Antigen-Processing Enzymes: Cathepsins of a Cephalochordate, an Agnathan and a Bony Fish

Cathepsins are enzymes that have been cleaving peptide bonds of lysosomal proteins probably since lysosomes appeared in early eucaryotes. When the adaptive system emerged in gnathostomes, cathepsins were recruited to produce peptides for loading onto the major histocompatibility complex class II molecules and for degrading the class II-associated invariant chain just before the loading. The circumstances under which this recruitment took place are unclear because the knowledge about vertebrate cathepsins is limited largely to mammals. To shed light on the recruitment, 10 amphioxus, one lamprey and one cichlid fish cathepsin cDNA clone were characterized and analysed phylogenetically. Disregarding cathepsin O, whose phylogenetic position is uncertain, the analysis confirms the existence of two old lines of descent, the B and the L lineages of cathepsins, which diverged from each other early in the evolution of eucaryotes. The B lineage encompasses cathepsins B, C and Z (X). The L lineage splits off sublineages encompassing cathepsins F and W before the plant-animal separation and cathepsin H early in the evolution of the metazoa. The remaining cathepsins belonging to the L lineage diverged from one another during the evolution of vertebrates: S, K and L before the emergence of bony fishes, and the group of rodent placentally expressed cathepsins [J (P), M, Q, R, 3, 6, 7 and 8] as well as the testis/ova-expressed cathepsins (testins) probably after the divergence of rodents from primates. The part possibly played by the adaptive immune system in some of these divergences is discussed.

Antigen presentation on MHC molecules as a diversity filter that enhances immune efficacy

We consider the way in which antigen is presented to T cells on MHC molecules and ask how MHC peptide presentation could be optimized so as to obtain an effective and safe immune response. By analysing this problem with a mathematical model of T-cell activation, we deduce the need for both MHC restriction and high presentation selectivity. We find that the optimal selectivity is such that about one pathogen-derived peptide is presented per MHC isoform, on the average. We also indicate upper and lower bounds to the number of MHC isoforms per individual based on detectability requirements. Thus we deduce that an important role of MHC presentation is to act as a filter that limits the diversity of antigen presentation.

Genomic organization of the mammalian MHC

The Human Genome Project transformed the quest of more than 50 years to understand the major histocompatibility complex (Mhc). The sequence of the Mhc from human and mouse, together with a large amount of sequence and mapping information from several other species, allows us to draw general conclusions about the organization and origin of this crucial part of the immune system. The Mhc is a mosaic of stretches formed by conserved and nonconserved genes. Surprisingly, of the approximately 3.6-Mb Mhc, the stretches that encode the class I and class II genes, which epitomize the Mhc, are the least conserved part, whereas the approximately 1.7-Mb stretches that encode at least 115 other genes are highly conserved. We summarize the available data to answer the questions (a) What is the Mhc? and (b) How can we define it in a general, not species-specific, way? Knowing what is essential and what is incidental helps us understand the fundamentals of the Mhc, and defining the species differences makes the model organisms more useful.

Understanding the cell biology of antigen presentation: the dendritic cell contribution

The study of the cell biology of antigen processing and presentation has greatly contributed to our understanding of the immune response. The work of many immunologically inclined cell biologists has also permitted us to gain new insights on cellular mechanisms shared by many cell types. Dendritic cells are master regulators of the immune system and consequently have received a lot of attention in recent years. With the aim of controlling antigen processing and presentation, the solutions used by dendritic cells to respond to environmental changes are numerous and surprising. In the presence of pathogens, dendritic cells regulate strongly their endocytic pathway by interfering with uptake, proteolysis, membrane dynamics and transport in and out of the lysosome to become the most potent antigen-presenting cells known.

Mhc-guided processing: binding of large antigen fragments

Ever since the emergence of models for the processing and presentation of antigenic determinants by MHC class II molecules, the main view has been that proteins are unfolded, enzymatically cleaved into peptide lengths of about 12-25 amino acids and then loaded onto MHC class II molecules. There is, however, an alternative model stating that partially intact unfolding antigens are first bound by MHC class II molecules and then trimmed to fragments of a smaller size while remaining bound to the MHC class II molecule. In this analysis, we make the case that a considerable portion of the elutable peptide cargo belongs to this latter class.

Mass and composition matrix-assisted laser desorption ionization time of flight analysis enabling inference of the sequence of most peptides where the protein of origin is known

Mass spectrometers combining matrix-assisted laser-desorption ionization (MALDI) and time-of-flight analysis are among the most widely used in peptide analysis. They excel at accurate mass determinations on complex samples but, compared with tandem instruments, have very limited capacity to determine amino acid sequence through daughter ion analysis. Here we have investigated the sequence information that can be inferred from the masses of peptides in the special circumstance in which the peptides are known to be sub-sequences of known parent sequences. We show how sequence can be inferred from the measured m/z of a peptide (mass analysis) and examine the parameters that influence the level of confidence that can be placed in "inferred sequences". We further describe how specific amino acid modifications can be used with MALDI-TOF analysis to obtain partial composition information and demonstrate that combined mass and composition (MAC) analysis enables the sequences of most peptide ions to be inferred with very high confidence.

Insights into the roles of cathepsins in antigen processing and presentation revealed by specific inhibitors

Eleven human cathepsins have been identified, however, the in vivo roles of individual cathepsins are still largely unknown. In this brief review we will summarize the functions of individual cathepsins in antigen processing and presentation, which are the initial steps of the immune response. Two general inhibitors of papain-like cysteine proteases, E-64 and pyridoxal phosphate, can completely suppress antigen presentation in vivo. To evaluate the contribution of individual cathepsins, specific inhibitors have been developed based on cathepsin tertiary structures: CA-074 for cathepsin B, CLIK-148 and -195 for cathepsin L, CLIK-60 for cathepsin S. Administration of CA-074, a cathepsin B inhibitor, suppresses the response to exogenous antigens, such as hepatitis B virus antigen, ovalbumin and Leishmania major antigen, and induces switching of the helper T cell responses from Th-2 to Th-1 of CD4+ T cells, thereby downregulating the production of IgE and IgG1. Administration of the cathepsin S inhibitor CLIK-60 impairs presentation of an autoantigen, alpha-fodrin, in Sjogren's syndrome and suppresses the Th-1 response and autoantibody production.

Human cathepsin S, but not cathepsin L, degrades efficiently MHC class II-associated invariant chain in nonprofessional APCs

MHC class II-restricted antigen presentation plays a central role in the immune response against exogenous antigens. The association of invariant (Ii) chain with MHC class II dimers is required for proper antigen presentation to CD4+ T cells by antigen-presenting cells. MHC class II complexes first traffic through the endocytic pathway to allow Ii chain degradation and antigenic peptide loading before their arrival at the cell surface. In recent years, a considerable effort has been directed toward the identification of proteases responsible for Ii chain degradation. Targeted gene deletion in mice has allowed a precise description of the cysteine proteases involved in the last step of Ii chain degradation. By using nonspecialized cellular models expressing MHC II molecules, we are now exploring the contribution of known cysteine proteases to human Ii chain processing. Surprisingly and contrary to the situation in mouse, cathepsin S was found to be the only human cysteine protease able to efficiently degrade the Ii-p10 fragment in epithelial cells. This selectivity has implications for thymic selection and indicates that differences between man and mice are probably more profound at this level than expected.

Creation versus destruction of T cell epitopes in the class II MHC pathway

Proteases perform two key roles in the class II MHC antigen processing pathway. They initiate removal of the invariant chain chaperone for class II MHC and they generate peptides from foreign and self proteins for eventual capture and display to T cells. How a balance is achieved between generation of suitable peptides versus their complete destruction in an aggressive proteolytic environment is not known. Nor is it known in most cases which proteases are actually involved in antigen processing. Our recent studies have identified asparagine endopeptidase (AEP or legumain) as an enzyme that contributes to both productive and destructive antigen processing in the class II MHC pathway. The emerging consensus seems to be that individual proteolytic enzymes make clear and non-redundant contributions to antigen processing.

Molecular mechanisms of B cell antigen receptor trafficking

B lymphocytes are among the most efficient cells of the immune system in capturing, processing, and presenting MHC class II restricted peptides to T cells. Antigen capture is essentially restricted by the specificity of the clonotypic antigen receptor expressed on each B lymphocyte. However, receptor recognition is only one factor determining whether an antigen is processed and presented. The context of antigen encounter is crucial. In particular, polyvalent arrays of repetitive epitopes, indicative of infection, accelerate the delivery of antigen to specialized processing compartments, and up-regulate the surface expression of MHC class II and co-stimulatory molecules such as B7. Recent studies have demonstrated that receptor-mediated signaling and receptor-facilitated peptide presentation to T cells are intimately related. For example, rapid sorting of endocytosed receptor complexes through early endosomes requires the activation of the tyrosine Syk. This proximal kinase initiates all BCR-dependent signaling pathways. Subsequent entry into the antigen-processing compartment requires the tyrosine phosphorylation of the BCR constituent Igalpha and direct recruitment of the linker protein BLNK. Signals from the BCR also regulate the biophysical and biochemical properties of the targeted antigen-processing compartments. These observations indicate that the activation and recruitment of signaling molecules by the BCR orchestrate a complex series of cellular responses that favor the presentation of even rare or low-affinity antigens if encountered in contexts indicative of infection. The requirement for BCR signaling provides possible mechanisms by which cognate B:T cell interactions can be controlled by the milieu in which antigen engagement occurs.

Asparagine endopeptidase can initiate the removal of the MHC class II invariant chain chaperone

The invariant chain (Ii) chaperone for MHC class II molecules is crucial for their effective function. Equally important is its removal. Cathepsins S or L are known to be required for the final stages of Ii removal in different APCs, but the enzymes which initiate Ii processing have not been identified. Here we show that this step can be performed in B lymphocytes by asparagine endopeptidase (AEP), which targets different asparagine residues in the lumenal domain of human and mouse invariant chain. Inhibition of AEP activity slows invariant chain processing and hinders the expression of an antigenic peptide engineered to replace the groove binding region of Ii (CLIP). However, the initiation of Ii removal can also be performed by other proteases, reflecting the importance of this step.

Promiscuous binding of proinsulin peptides to Type 1 diabetes-permissive and -protective HLA class II molecules

AIMS/HYPOTHESIS

Presentation of peptide epitopes derived from beta-cell autoantigens, such as insulin and its precursor molecules, by MHC class II molecules to autoreactive T-cells is believed to play a role in the development of Type 1 diabetes. However, little is known about the interaction between peptides of (prepro)insulin and MHC class II molecules permissive and protective for Type 1 diabetes. In this study therefore, peptides spanning the human preproinsulin sequence were assessed for their binding characteristics to Type 1 diabetes-protective and -permissive HLA molecules.

METHODS

HLA-DR2, -DQ6.2 (Type 1 diabetes-protective) and HLA-DR4, -DQ8 (Type 1 diabetes permissive) molecule binding affinity for overlapping synthetic 20mer peptides spanning human preproinsulin was measured in a direct competition binding assay against a biotinylated indicator peptide.

RESULTS

All HLA molecules tested showed similarity in their binding characteristics across the preproinsulin molecule, with regions of the insulin A-chain showing the highest affinity and C-peptide regions the lowest affinity for all HLA molecules tested. Furthermore, an insulin peptide implicated as a major CD4+ T-cell target in disease pathogenesis (B9-23) had high affinity binding to both protective and permissive HLA molecules but did not represent the highest affinity region of (prepro)insulin identified in either case.

CONCLUSION/INTERPRETATION

The results suggest that peptide binding affinity alone is unlikely to be the major determinant of disease susceptibility in relation to interactions between (prepro)insulin epitopes and HLA molecules. The identification of epitopes derived from beta-cell autoantigens that bind promiscuously to diabetes-permissive HLA molecules could be important in the design of peptide-based immunotherapeutic strategies for the prevention of Type 1 diabetes.

Activation of lysosomal function during dendritic cell maturation

In response to a variety of stimuli, dendritic cells (DCs) transform from immature cells specialized for antigen capture into mature cells specialized for T cell stimulation. During maturation, the DCs acquire an enhanced capacity to form and accumulate peptide-MHC (major histocompatibility complex) class II complexes. Here we show that a key mechanism responsible for this alteration was the generalized activation of lysosomal function. In immature DCs, internalized antigens were slowly degraded and inefficiently used for peptide loading. Maturation induced activation of the vacuolar proton pump that enhanced lysosomal acidification and antigen proteolysis, facilitating efficient formation of peptide-MHC class II complexes. Lysosomal function in DCs thus appears to be specialized for the developmentally regulated processing of internalized antigens.

Complex carbohydrates are not removed during processing of glycoproteins by dendritic cells: processing of tumor antigen MUC1 glycopeptides for presentation to major histocompatibility complex class II-restricted T cells

In contrast to protein antigens, processing of glycoproteins by dendritic cells (DCs) for presentation to T cells has not been well studied. We developed mouse T cell hybridomas to study processing and presentation of the tumor antigen MUC1 as a model glycoprotein. MUC1 is expressed on the surface as well as secreted by human adenocarcinomas. Circulating soluble MUC1 is available for uptake, processing, and presentation by DCs in vivo and better understanding of how that process functions in the case of glycosylated antigens may shed light on antitumor immune responses that could be initiated against this glycoprotein. We show that DCs endocytose MUC1 glycopeptides, transport them to acidic compartments, process them into smaller peptides, and present them on major histocompatability complex (MHC) class II molecules without removing the carbohydrates. Glycopeptides that are presented on DCs are recognized by T cells. This suggests that a much broader repertoire of T cells could be elicited against MUC1 and other glycoproteins than expected based only on their peptide sequences.