HLA-DM is localized to conventional and unconventional MHC class II-containing endocytic compartments

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

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

Positive and negative selection shape the human naïve B cell repertoire

Although negative selection of developing B cells in the periphery is well described, yet poorly understood, evidence of naive B cell positive selection remains elusive. Using 2 humanized mouse models, we demonstrate that there was strong skewing of the expressed immunoglobulin repertoire upon transit into the peripheral naive B cell pool. This positive selection of expanded naive B cells in humanized mice resembled that observed in healthy human donors and was independent of autologous thymic tissue. In contrast, negative selection of autoreactive B cells required thymus-derived Tregs and MHC class II–restricted self-antigen presentation by B cells. Indeed, both defective MHC class II expression on B cells of patients with rare bare lymphocyte syndrome and prevention of self-antigen presentation via HLA-DM inhibition in humanized mice resulted in the production of autoreactive naive B cells. These latter observations suggest that Tregs repressed autoreactive naive B cells continuously produced by the bone marrow. Thus, a model emerged, in which both positive and negative selection shaped the human naive B cell repertoire and that each process was mediated by fundamentally different molecular and cellular mechanisms.

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

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

Distinct editing functions of natural HLA-DM allotypes impact antigen presentation and CD4+ T cell activation

Classical human leukocyte antigen (HLA) molecules of the major histocompatibility class II (MHCII) complex present peptides for the development, surveillance and activation of CD4+ T cells. The nonclassical MHCII-like protein HLA-DM (DM) catalyzes the exchange and loading of peptides onto MHCII molecules, thereby shaping MHCII immunopeptidomes. Natural variations of DM in both chains of the protein (DMA and DMB) have been hypothesized to impact peptide presentation, but no evidence for altered function has been reported. Here we define the presence of DM allotypes in human populations covered by the 1000 Genomes Project and probe their activity. The functional properties of several allotypes are investigated and show strong enhancement of peptide-induced T cell activation for a particular combination of DMA and DMB. Biochemical evidence suggests a broader pH activity profile for the new variant relative to that of the most commonly expressed DM allotype. Immunopeptidome analysis indicates that the compartmental activity of the new DM heterodimer extends beyond the late endosome and suggests that the natural variation of DM has profound effects on adaptive immunity when antigens bypass the canonical processing pathway.

Human leukocyte Antigen-DM polymorphisms in autoimmune diseases

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

HLA-DMB in Amerindians: Specific linkage of DMB*01:03:01/DRB1 alleles

BACKGROUND

HLA-DMB proteins are important for intracellular microbial metabolism in order other major histocompatibility complex (MHC) molecules present peptides to lymphocytes. In addition, HLA-DMB alleles have been found linked to diseases in some ethnic groups and HLA-DMB molecules may be important to explain HLA disease association.

OBJECTIVE

To detect HLA-DMB alleles profile in Amerindians for the first time and compare them to other populations. This will establish the bases to study HLA-DMB linkage to disease in Amerindians.

METHOD

A group of 168 voluntary Amerindians have been typed for HLA-DMB alleles. They have been characterized both, by genetic and genealogical bases. Cloning and automated HLA-DMB DNA (exons 2, 3 and 4) sequencing have been performed for allele assignation.

RESULTS

HLA-DMB*01:01:01 and HLA-DMB*01:03:01 show the highest frequencies. These have been compared to other World wide populations. HLA-DMB*01:03:01 is tightly associated to certain specific HLA-DRB1 alleles in Amerindians.

CONCLUSION

The specific Amerindian HLA-DMB allele frequencies and their linkage disequilibrium with other MHC alleles may be crucial to determine HLA-DMB World wide variation, evolution and specific linkage to disease in Amerindians and other populations.

Variations in MHC Class II Antigen Processing and Presentation in Health and Disease

MHC class II (MHC-II) molecules are critical in the control of many immune responses. They are also involved in most autoimmune diseases and other pathologies. Here, we describe the biology of MHC-II and MHC-II variations that affect immune responses. We discuss the classic cell biology of MHC-II and various perturbations. Proteolysis is a major process in the biology of MHC-II, and we describe the various components forming and controlling this endosomal proteolytic machinery. This process ultimately determines the MHC-II-presented peptidome, including cryptic peptides, modified peptides, and other peptides that are relevant in autoimmune responses. MHC-II also variable in expression, glycosylation, and turnover. We illustrate that MHC-II is variable not only in amino acids (polymorphic) but also in its biology, with consequences for both health and disease.

MHC class II presentation of gp100 epitopes in melanoma cells requires the function of conventional endosomes and is influenced by melanosomes

Many human solid tumors express MHC class II (MHC-II) molecules, and proteins normally localized to melanosomes give rise to MHC-II-restricted epitopes in melanoma. However, the pathways by which this response occurs have not been defined. We analyzed the processing of one such epitope, gp100(44-59), derived from gp100/Pmel17. In melanomas that have down-regulated components of the melanosomal pathway, but constitutively express HLA-DR*0401, the majority of gp100 is sorted to LAMP-1(high)/MHC-II(+) late endosomes. Using mutant gp100 molecules with altered intracellular trafficking, we demonstrate that endosomal localization is necessary for gp100(44-59) presentation. By depletion of the AP-2 adaptor protein using small interfering RNA, we demonstrate that gp100 protein internalized from the plasma membrane to such endosomes is a major source for gp100(44-59) epitope production. The gp100 trapped in early endosomes gives rise to epitopes that are indistinguishable from those produced in late endosomes but their production is less sensitive to inhibition of lysosomal proteases. In melanomas containing melanosomes, gp100 is underrepresented in late endosomes, and accumulates in stage II melanosomes devoid of MHC-II molecules. The gp100(44-59) presentation is dramatically reduced, and processing occurs entirely in early endosomes or stage I melanosomes. This occurrence suggests that melanosomes are inefficient Ag-processing compartments. Thus, melanoma de-differentiation may be accompanied by increased presentation of MHC-II restricted epitopes from gp100 and other melanosome-localized proteins, leading to enhanced immune recognition.

Immunopathogenesis of psoriasis

Psoriasis is a chronic skin disease that affects about 1.5% of the Caucasian population and is characterized by typical macroscopic and microscopic skin alterations. Psoriatic lesions are sharply demarcated, red and slightly raised lesions with silver-whitish scales. The microscopic alterations of psoriatic plaques include an infiltration of immune cells in the dermis and epidermis, a dilatation and an increase in the number of blood vessels in the upper dermis, and a massively thickened epidermis with atypical keratinocyte differentiation. It is considered a fact that the immune system plays an important role in the pathogenesis of psoriasis. Since the early 1990s, it has been assumed that T1 cells play the dominant role in the initiation and maintenance of psoriasis. However, the profound success of anti-tumor necrosis factor-alpha therapy, when compared with T-cell depletion therapies, should provoke us to critically re-evaluate the current hypothesis for psoriasis pathogenesis. Recently made discoveries regarding other T-cell populations such as Th17 and regulatory T cells, dendritic cells, macrophages, the keratinocyte signal transduction and novel cytokines including interleukin (IL)-22, IL-23 and IL-20, let us postulate that the pathogenesis of psoriasis consists of distinct subsequent stages, in each of them different cell types playing a dominant role. Our model helps to explain the varied effectiveness of the currently tested immune modulating therapies and may enable the prediction of the success of future therapies.

Private Lives: Reflections and Challenges in Understanding the Cell Biology of the Immune System

The immune system comprises a variety of cell types whose activities must be carefully regulated to act as a coherent unit for the purpose of host defense. To perform their autonomous functions, immune cells must rely on the same basic organizational features that apply to all cells, although immune cells often exhibit remarkable degrees of specialization and adaptation. The study of these specializations has lagged behind advances in understanding the immune response and cell biology individually. As a result, there are great opportunities, but also great challenges, for new conceptual discoveries by taking a more cell-biological approach to probing the function of the immune system.

CTLA-4 Differentially Regulates the Immunological Synapse in CD4 T Cell Subsets

Primary murine Th1 and Th2 cells differ in the organization of the immunological synapse, with Th1 cells, but not Th2 cells, clustering signaling molecules at the T cell/B cell synapse site. We sought to determine whether differential costimulatory signals could account for the differences observed. We found that Th2 cells express higher levels of CTLA-4 than Th1 cells, and demonstrated that Th2 cells lacking CTLA-4 are now able to cluster the TCR with the same frequency as Th1 cells. Furthermore, reconstitution of CTLA-4 into CTLA-4-deficient Th2 cells, or into Th1 cells, inhibits the clustering of the TCR. We have also shown that Th2 cells, but not Th1 cells, show variations in the organization of the immunological synapse depending on levels of expression of CD80/CD86 on the APC. These studies demonstrate a unique role for CTLA-4 as a critical regulator of Th2 cells and the immunological synapse.

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.

Monocyte differentiation up-regulates the expression of the lysosomal sialidase, Neu1, and triggers its targeting to the plasma membrane via major histocompatibility complex class II-positive compartments

Human sialidase (neuraminidase) Neu1 catalyzes lysosomal catabolism of sialylated glycoconjugates. Here we show that during the differentiation of monocytes and the monocytic cell line, THP-1, into macrophages, the majority of Neu1 relocalizes from the lysosomes to the cell surface. In contrast to other cellular sialidases Neu2, Neu3, and Neu4, whose expression either remains unchanged or is down-regulated, Neu1 mRNA, protein and activity are specifically increased during the phorbol 12-myristate 13-acetate-induced differentiation, consistent with a significant induction of the transcriptional activity of the Neu1 gene promoter. The lysosomal carboxypeptidase, cathepsin A, which forms a complex with and activates Neu1 in the lysosome, is sorted to the plasma membrane of the differentiating cells similarly to Neu1. Both proteins are first targeted to the lysosome and then are sorted to the LAMP-2-negative, major histo-compatibility complex II-positive vesicles, which later merge with the plasma membrane. Similar trafficking was observed for the internalized fluorescent dextran or horseradish peroxidase initially stored in the lysosomal/endosomal compartment. The suppression of Neu1 expression in the THP-1-derived macrophages by small interfering RNA or with anti-Neu1 antibodies significantly reduced the ability of the cells to engulf bacteria or to produce cytokines. Altogether our data suggest that the upregulation of the Neu1 expression is important for the primary function of macrophages and establish the link between Neu1 and the cellular immune response.

MHC class II compartment subtypes: structure and function

Reports from the past couple of years point to an emerging association of the biogenesis, composition and ultrastructural morphology of MHC class II compartments (MIICs) with their functions in antigen processing and loading. Growth factors and cytokines involved in dendritic cell maturation have been shown to regulate MIIC biogenesis, and the MHC-class-II-associated invariant chain chaperone has been reported to regulate endosomal morphology and vacuolation. Differences among ultrastructurally distinct MIICs have begun to be appreciated with regard to variation in antigen loading capacity and to polarization of MHC class II conformational variants among different compartments. Finally, the MIIC ultrastructure organizes the mechanism of MHC class II surface trafficking. Together, these findings begin to shed light on the connection between MIIC protein content, MIIC morphology and MHC class II-related antigen processing.

Coevolution of TCR-MHC interactions: conserved MHC tertiary structure is not sufficient for interactions with the TCR

The specificity for self-MHC that is necessary for T cell function is a consequence of intrathymic selection during which T cell antigen receptors (TCRs) expressed by immature thymocytes are tested for their affinity for self-peptide:self-MHC. The germ-line-encoded segments of the TCR, however, are believed to have an innate specificity for structural features of MHC molecules. We directly tested this hypothesis by generating a transgenic mouse system in which the protein HLA-DM is expressed at the surface of thymic cortical epithelial cells in the absence of classical MHC molecules. The specialized intracellular function of HLA-DM has removed this MHC class II-like protein from the evolutionary forces that have been hypothesized to shape TCR-MHC interactions. Our study shows that a structural mimic of MHC class II is not sufficient to appropriately interact with the TCRs expressed by developing thymocytes. This result emphasizes the unique complementarity of TCR-MHC interactions that are maintained by the evolutionary pressures dictated by positive selection.

CELL BIOLOGY OF ANTIGEN PROCESSING IN VITRO AND IN VIVO

The conversion of exogenous and endogenous proteins into immunogenic peptides recognized by T lymphocytes involves a series of proteolytic and other enzymatic events culminating in the formation of peptides bound to MHC class I or class II molecules. Although the biochemistry of these events has been studied in detail, only in the past few years has similar information begun to emerge describing the cellular context in which these events take place. This review thus concentrates on the properties of antigen-presenting cells, especially those aspects of their overall organization, regulation, and intracellular transport that both facilitate and modulate the processing of protein antigens. Emphasis is placed on dendritic cells and the specializations that help account for their marked efficiency at antigen processing and presentation both in vitro and, importantly, in vivo. How dendritic cells handle antigens is likely to be as important a determinant of immunogenicity and tolerance as is the nature of the antigens themselves.

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

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

MHC class II molecules traffic into lipid rafts during intracellular transport

There have been many studies demonstrating that a portion of MHC class II molecules reside in detergent-insoluble membrane domains (commonly referred to as lipid rafts). We have proposed that the function of raft association is to concentrate specific MHC class II-peptide complexes in plasma membrane microdomains that can facilitate efficient T cell activation. We now show that MHC class II becomes lipid raft associated before binding antigenic peptides. Using pulse-chase radiolabeling techniques, we find that newly synthesized MHC class II and MHC class II-invariant chain complexes initially reside in a detergent-soluble membrane fraction and acquire detergent insolubility as they traffic to lysosomal Ag processing compartments. Monensin, an inhibitor of protein transport through the Golgi apparatus, blocks association of newly synthesized MHC class II with lipid rafts. Treatment of cells with leupeptin, which inhibits invariant chain degradation, leads to the accumulation of MHC class II in lipid rafts within the lysosome-like Ag-processing compartments. Raft fractionation of lysosomal membranes confirmed the presence of MHC class II in detergent-insoluble microdomains in Ag-processing compartments. These findings indicate that newly synthesized MHC class II complexes are directed to detergent-insoluble lipid raft microdomains before peptide loading, a process that may facilitate the loading of similar peptides on MHC class II complexes in these microdomains.

Regulated Recruitment of MHC Class II and Costimulatory Molecules to Lipid Rafts in Dendritic Cells

T cell activation has long been associated with the partitioning of Ag receptors and associated molecules to lipid microdomains. We now show that dendritic cells (DCs) also accomplish the selective recruitment to lipid rafts of molecules critical for Ag presentation. Using mouse bone marrow-derived DCs, we demonstrate that MHC class II molecules become substantially localized to rafts upon DC maturation. Even more striking is the fact that CD86 is recruited to rafts upon T cell-DC interaction. Recruitment is Ag dependent and requires CD28 on T cells. Despite the regulated recruitment of MHC class II and CD86 to rafts, unlike the counter-receptors in T cells, DCs do not polarize these molecules to sites of DC-T cell contact. This difference may reflect the necessity for DCs to interact with multiple T cells simultaneously and emphasizes that the biochemical and morphological correlates of lipid rafts are not necessarily equivalent.

Cystatin F is secreted, but artificial modification of its C-terminus can induce its endocytic targeting

Cystatins constitute a superfamily of cysteine protease inhibitors. A member of the type II secreted cystatin family, cystatin F, has been identified through different gene array experiments to be specifically expressed in hematopoietic cells as well as to be associated with several malignant tumors, suggesting a role in immunity or cancer progression. Cystatin F specificity as a protease inhibitor is still elusive, and understanding the cellular traffic of this molecule is therefore a major step in its characterization. Although the mannosylation-6 phosphate of cystatin F has been suggested, no conclusive evidences of its endosomal targeting have been reported. Here we show using U937 cells that cystatin F is secreted as a disulfide bridge-linked dimer and is not associated with endosomes intracellularly. Interestingly, although cystatin F targeting to endosomes or lysosomes is not observed in U937, modification of its C-terminal end by the addition of several amino acids promotes its accumulation in the lysosomes of transfected HeLa cells. This observation suggests that cystatin F can be targeted to the endocytic pathway under specific conditions and its C-terminal domain might contribute to this event.

CD4 Raft Association and Signaling Regulate Molecular Clustering at the Immunological Synapse Site

T cell activation is associated with the partitioning of TCRs and other signaling proteins, forming an immunological synapse. This study demonstrates a novel function for the CD4 coreceptor in regulating molecular clustering at the immunological synapse site. We show using transgenic mouse and retroviral reconstitution studies that CD4 is required for TCR/protein kinase C (PKC) theta clustering. Specifically, we demonstrate that CD4 palmitoylation sequences are required for TCR/PKCtheta raft association and subsequent clustering, indicating a particular role for raft-associated CD4 molecules in regulating immune synapse organization. Although raft association of CD4 is necessary, it is not sufficient to mediate clustering, as cytoplasmic tail deletion mutants are able to localize to rafts, but are unable to mediate TCR/PKCtheta clustering, indicating an additional requirement for CD4 signaling. These studies suggest that CD4 coreceptor function is regulated not only through its known signaling function, but also by posttranslational lipid modifications which regulate localization of CD4 in lipid rafts.

Tab2 is a novel conserved RNA binding protein required for translation of the chloroplast psaB mRNA

The chloroplast psaB mRNA encodes one of the reaction centre polypeptides of photosystem I. Protein pulse-labelling profiles indicate that the mutant strain of Chlamydomonas reinhardtii, F14, affected at the nuclear locus TAB2, is deficient in the translation of psaB mRNA and thus deficient in photosystem I activity. Genetic studies reveal that the target site for Tab2 is situated within the psaB 5'UTR. We have used genomic complementation to isolate the nuclear Tab2 gene. The deduced amino acid sequence of Tab2 (358 residues) displays 31-46% sequence identity with several orthologues found only in eukaryotic and prokaryotic organisms performing oxygenic photosynthesis. Directed mutagenesis indicates the importance of a highly conserved C-terminal tripeptide in Tab2 for normal psaB translation. The Tab2 protein is localized in the chloroplast stroma where it is associated with a high molecular mass protein complex containing the psaB mRNA. Gel mobility shift assays reveal a direct and specific interaction between Tab2 and the psaB 5'UTR. We propose that Tab2 plays a key role in the initial steps of PsaB translation and photosystem I assembly.

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.

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.

Differentiation of immunostimulatory stem-cell- and monocyte-derived dendritic cells involves maturation of intracellular compartments responsible for antigen presentation and secretion

Dendritic cells (DCs) are important for the induction of primary T-cell responses and may serve as "biologic adjuvants" in therapeutic protocols. However, given the "plasticity" of this antigen-presenting cell, it remains unclear which DC type (source, subtype, and stage of differentiation) should be applied clinically. To provide additional insight in this selection process, we have, for the first time, analyzed the in vitro differentiation of CD34(+) precursor-derived and monocyte-derived DCs for ultrastructure, phenotype, and function. The ultrastructural intracytoplasmic differentiation of DCs correlated with increasing T-cell stimulatory activity of these cells. "Early-stage"-DCs proliferate, exhibit high levels of soluble antigen uptake, and moderate T-cell stimulatory capacity, and are characterized by centrally located nuclei and numerous enlarged mitochondria. "Intermediate-stage"-DCs are enlarged cells with enhanced T-cell stimulatory activity and pronounced cytoplasmic protein synthesis machinery. "Late-stage" (LS)-DCs exhibit a mature secretory cell phenotype and low proliferative index. They express high levels of the HLA-DR, CD40L, B7-1, and B7-2 molecules and CD83, a specific marker of mature DCs, and appear maximally stimulatory to T cells. Ultrastructurally, LS-DCs feature an accentric nucleus, an enlarged cytoplasm, containing numerous secretory storage vesicles, along with a fully developed Golgi complex. LS-DCs exhibited numerous multivesicular and multilaminar structures containing major histocompatibility complex class II molecules, consistent with the MIIC (peptide-loading) compartment. In extended studies, cultured CD14(+) monocyte-derived DCs displayed a similar, but accelerated, temporal differentiation staging pattern.

Dendritic cell maturation triggers retrograde MHC class II transport from lysosomes to the plasma membrane

Central to the initiation of immune responses is recognition of peptide antigen by T lymphocytes. The cell biology of dendritic cells makes them ideally suited for the essential process of antigen presentation. Their life cycle includes several stages characterized by distinct functions and mechanisms of regulation. Immature dendritic cells synthesize large amounts of major histocompatibility complex class II molecules (MHC II), but the alpha beta-dimers are targeted to late endosomes and lysosomes (often referred to as MHC class II compartments) where they reside unproductively with internalized antigens. After exposure to microbial products or inflammatory mediators, endocytosis is downregulated, the expression of co-stimulatory molecules is enhanced, and newly formed immunogenic MHC II-peptide complexes are transported to the cell surface. That these MHC II molecules reach the surface is surprising, as the lysosomes comprise the terminal degradative compartment of the endocytic pathway from which exogenous components generally cannot be recovered intact. Here we have visualized this pathway in live dendritic cells by video microscopy, using cells expressing MHC II tagged with green fluorescent protein (GFP). We show that on stimulation, dendritic cells generate tubules from lysosomal compartments that go on to fuse directly with the plasma membrane.

CD1d endosomal trafficking is independently regulated by an intrinsic CD1d-encoded tyrosine motif and by the invariant chain

Endosomal trafficking is an essential component of the CD1 pathway of lipid antigen presentation to T cells. We demonstrate that CD1d access to endosomal compartments is under dual regulation by an intrinsic tyrosine-based motif, which governs intense recycling between the plasma membrane and the endosome, and by the invariant chain, with which CD1d associates in the endoplasmic reticulum. Both pathways independently enhance antigen presentation to V(alpha)14(+) NKT cells, the main subset of CD1d-restricted T cells. These results reveal the complexity of CD1d trafficking and suggest that the invariant chain was a component of ancestral antigen presentation pathways prior to the evolution of MHC and CD1.

Distinct patterns of membrane microdomain partitioning in Th1 and th2 cells

Here we show that activated Th1 and Th2 cells have distinct patterns of membrane compartmentalization into lipid rafts. TCR complex members are recruited efficiently to rafts and aggregate with rafts at the site of MHC/peptide contact in Th1 cells but not Th2 cells. TCR/raft association in Th1 cells is deficient in the absence of CD4, suggesting that CD4 aids recruitment of the TCR to rafts. We show differential utilization of rafts in Th1 and Th2 cells by cholesterol depletion studies, which alters calcium signaling in Th1 but not Th2 cells. Furthermore, Th2 cells have a decreased ability to respond to low-affinity peptide stimulation. These studies indicate that components of membrane microdomains are differentially regulated in functionally distinct CD4 T cells.

CD8(+) T lymphocytes induce polarized exocytosis of secretory lysosomes by dendritic cells with release of interleukin-1beta and cathepsin D

We recently reported that human dendritic cells release the leaderless secretory protein interleukin-1beta (IL-1beta) following specific interaction with alloreactive T lymphocytes. To clarify the molecular mechanism underlying this secretion, this study investigated the intracellular trafficking of IL-1beta in dendritic cells and the signal(s) regulating its release. Results show that a fraction of the intracellular IL-1beta precursor colocalizes with the hydrolase cathepsin D in endolysosomes of dendritic cells; secretion of both proteins is elicited by stimuli that induce intracellular calcium increases. Alloreactive CD8(+) T lymphocytes generate a Ca(++) influx in dendritic cells followed by enrichment in endolysosomes containing IL-1beta and cathepsin D beneath the membrane in contact with T cells. These events result in polarized exocytosis of secretory lysosomes, mediated by microtubules, with release of IL-1beta and cathepsin D toward the interacting CD8(+) T cell.

Presentation of antigens by MHC class II molecules: getting the most out of them

The function of MHC class II molecules is to bind peptides derived from antigens that access the endocytic route of antigen presenting cells and display them on the plasma membrane for recognition by CD4(+) T cells. Formation of the MHC II-peptide complexes entails the confluence of the antigens and the MHC II molecules in the same compartments of the endocytic route. There, both the antigens and the MHC II molecules undergo a series of orchestrated changes that involve proteases, other hydrolases and chaperones, culminating in the generation of a wide repertoire of MHC II-peptide combinations. All the events that lead to formation of MHC II-peptide complexes show a considerable degree of flexibility; this lack of strict rules is advantageous in that it provides T cells with the maximum amount of information, ensuring that pathogens do not go undetected.

Prolonged antigen persistence within nonterminal late endocytic compartments of antigen-specific B lymphocytes

Although Ag-specific B lymphocytes can process Ag and express peptide-class II complexes as little as 1 h after Ag exposure, it requires 3-5 days for the immune system to develop a population of Ag-specific effector CD4 T lymphocytes to interact with these complexes. Presently, it is unclear how B cells maintain the expression of cell surface antigenic peptide-class II complexes until effector CD4 T lymphocytes become available. Therefore, we investigated B cell receptor (BCR)-mediated Ag processing and presentation by normal B lymphocytes to determine whether these cells have a mechanism to prolong the cell surface expression of peptide-class II complexes derived from the processing of cognate AG: Interestingly, after transit of early endocytic compartments, internalized Ag-BCR complexes are delivered to nonterminal late endosomes where they persist for a prolonged period of time. In contrast, Ags internalized via fluid phase endocytosis are rapidly delivered to terminal lysosomes and degraded. Moreover, persisting Ag-BCR complexes within nonterminal late endosomes exhibit a higher degree of colocalization with the class II chaperone HLA-DM/H2-M than with the HLA-DM/H2-M regulator HLA-DO/H2-O. Finally, B cells harboring persistent Ag-BCR complexes exhibit prolonged cell surface expression of antigenic peptide-class II complexes. These results demonstrate that B lymphocytes possess a mechanism for prolonging the intracellular persistence of Ag-BCR complexes within nonterminal late endosomes and suggest that this intracellular Ag persistence allows for the prolonged cell surface expression of peptide-class II complexes derived from the processing of specific AG:

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

MHC class II molecules bind antigenic peptides in the late endosomal/lysosomal MHC class II compartments (MIIC) before cell surface presentation. The class II modulatory molecules HLA-DM and HLA-DO mainly localize to the MIICs. Here we show that DM/DO complexes continuously recycle between the plasma membrane and the lysosomal MIICs. Like DMbeta and the class II-associated invariant chain, the DObeta cytoplasmic tail contains potential lysosomal targeting signals. The DObeta signals, however, are not essential for internalization of the DM/DO complex from the plasma membrane or targeting to the MIICs. Instead, the DObeta tail determines the distribution of both DM/DO and class II within the multivesicular MIIC by preferentially localizing them to the limiting membrane and, in lesser amounts, to the internal membranes. This distribution augments the efficiency of class II antigenic peptide loading by affecting the efficacy of lateral interaction between DM/DO and class II molecules. Sorting of DM/DO and class II molecules to specific localizations within the MIIC represents a novel way of regulating MHC class II Ag presentation.

Cutting edge: editing of recycling class II:peptide complexes by HLA-DM

HLA-DM catalyzes the exchange and selection of ligands for MHC class II molecules within mature endosomal/lysosomal compartments. Here, evidence is provided that DM edits peptides in early endosomes, thus influencing presentation via recycling class II molecules. Maximal class II-restricted presentation of an albumin-derived peptide, dependent on endocytosis and recycling class II molecules, was observed in cells lacking HLA-DM. DM editing of this epitope was observed in early endocytic compartments as shown using inhibitors of early to late endosomal transport. Editing was tempered by coexpression of HLA-DO, suggesting that DM:DO ratio may be important in guiding epitope editing in early endosomal compartments. Thus, HLA-DM appears to interact with, and edit epitopes displayed by, recycling class II molecules.

Intracellular transport of MHC class II and associated invariant chain in antigen presenting cells from AP-3-deficient mocha mice

MHC class II-restricted antigen presentation requires trafficking of newly synthesized class II-invariant chain complexes from the trans-Golgi network to endosomal, peptide-loading compartments. This transport is mediated by dileucine-like motifs within the cytosolic tail of the invariant chain. Although these signals have been well characterized, the cytosolic proteins that interact with these dileucine signals and mediate Golgi sorting and endosomal transport have not been identified. Recently, an adaptor complex, AP-3, has been identified that interacts with dileucine motifs and mediates endosomal/lysosomal transport in yeast, Drosophila, and mammals. In this report, we have assessed class II-invariant chain trafficking in a strain of mice (mocha) which lacks expression of AP-3. Our studies demonstrate that the lack of AP-3 does not affect the kinetics of invariant chain degradation, the route of class II-invariant chain transport, or the rate and extent of class II-peptide binding as assessed by the generation of SDS-stable dimers. The possible role of other known or unknown adaptor complexes in class II-invariant chain transport is discussed.

The transmembrane segment of invariant chain mediates binding to MHC class II molecules in a CLIP-independent manner

Invariant chain (Ii) association with MHC class II molecules is strongly dependent upon interaction of CLIP (Ii exon 3, residues 81 - 104) with the peptide binding groove of the class II dimer. This dominant interaction does not adequately explain, however, the efficient association of Ii with class II molecules of diverse allelic and isotypic origin, which have markedly different affinities for synthetic peptides corresponding to CLIP. In agreement with other recent observations, we demonstrate here that class II molecules with occupied binding sites unable to engage CLIP maintain association with Ii in mild detergent. The association is direct and not mediated through unoccupied class II chains bound to properly assembled and loaded class II dimers, nor is it mediated through chaperones. The site of this CLIP-independent binding has been mapped using truncation mutants and an Ii-human transferrin receptor chimeric protein to the transmembrane segment of Ii. The existence of multiple low-affinity sites of interaction between MHC class II and Ii helps explain how effective occupancy of all newly synthesized class II molecules can occur despite substantial variations in the strength of CLIP-dependent association that arise from class II binding domain polymorphism. These data establishing a site of Ii-MHC class II association N-terminal to CLIP also provide new insight into the possible functional relationship between the sequential endocytic proteolysis of Ii from its C terminus and a series of contact sites with MHC class II molecules spread from the transmembrane region through to the tip of the lumenal segment of Ii.

Exosome: from internal vesicle of the multivesicular body to intercellular signaling device

Exosomes are small membrane vesicles that are secreted by a multitude of cell types as a consequence of fusion of multivesicular late endosomes/lysosomes with the plasma membrane. Depending on their origin, exosomes can play roles in different physiological processes. Maturing reticulocytes externalize obsolete membrane proteins such as the transferrin receptor by means of exosomes, whereas activated platelets release exosomes whose function is not yet known. Exosomes are also secreted by cytotoxic T cells, and these might ensure specific and efficient targeting of cytolytic substances to target cells. Antigen presenting cells, such as B lymphocytes and dendritic cells, secrete MHC class-I- and class-II-carrying exosomes that stimulate T cell proliferation in vitro. In addition, dendritic-cell-derived exosomes, when used as a cell-free vaccine, can eradicate established murine tumors. Although the precise physiological target(s) and functions of exosomes remain largely to be resolved, follicular dendritic cells (accessory cells in the germinal centers of secondary lymphoid organs) have recently been shown to bind B-lymphocyte-derived exosomes at their cell surface, which supports the notion that exosomes play an immunoregulatory role. Finally, since exosomes are derived from multivesicular bodies, their molecular composition might provide clues to the mechanism of protein and lipid sorting in endosomes.

Nonclassical MHC class II molecules

Major histocompatibility complex (MHC) class II molecules are cell surface proteins that present peptides to CD4(+) T cells. In addition to these wellcharacterized molecules, two other class II-like proteins are produced from the class II region of the MHC, HLA-DM (DM) and HLA-DO (DO) (called H2-M, or H2-DM and H2-O in the mouse). The function of DM is well established; it promotes peptide loading of class II molecules in the endosomal/lysosomal system by catalyzing the release of CLIP peptides (derived from the class II-associated invariant chain) in exchange for more stably binding peptides. While DM is present in all class II- expressing antigen presenting cells, DO is expressed mainly in B cells. In this cell type the majority of DM molecules are not present as free heterodimers but are instead associated with DO in tight heterotetrameric complexes. The association with DM is essential for the intracellular transport of DO, and the two molecules remain associated in the endosomal system. DO can clearly modify the peptide exchange activity of DM both in vitro and in vivo, but the physiological relevance of this interaction is still only partly understood.

The formation of immunogenic major histocompatibility complex class II-peptide ligands in lysosomal compartments of dendritic cells is regulated by inflammatory stimuli

During their final differentiation or maturation, dendritic cells (DCs) redistribute their major histocompatibility complex (MHC) class II products from intracellular compartments to the plasma membrane. Using cells arrested in the immature state, we now find that DCs also regulate the initial intracellular formation of immunogenic MHC class II-peptide complexes. Immature DCs internalize the protein antigen, hen egg lysozyme (HEL), into late endosomes and lysosomes rich in MHC class II molecules. There, despite extensive colocalization of HEL protein and MHC class II products, MHC class II-peptide complexes do not form unless the DCs are exposed to inflammatory mediators such as tumor necrosis factor alpha, CD40 ligand, or lipoplolysaccharide. The control of T cell receptor (TCR) ligand formation was observed using the C4H3 monoclonal antibody to detect MHC class II-HEL peptide complexes by flow cytometry and confocal microscopy, and with HEL-specific 3A9 transgenic T cells to detect downregulation of the TCR upon MHC-peptide encounter. Even the binding of preprocessed HEL peptide to MHC class II is blocked in immature DCs, including the formation of C4H3 epitope in MHC class II compartments, suggesting an arrest to antigen presentation at the peptide-loading step, rather than an enhanced degradation of MHC class II-peptide complexes at the cell surface, as described in previous work. Therefore, the capacity of late endosomes and lysosomes to produce MHC class II-peptide complexes can be strictly controlled during DC differentiation, helping to coordinate antigen acquisition and inflammatory stimuli with formation of TCR ligands. The increased ability of maturing DCs to load MHC class II molecules with antigenic cargo contributes to the >100-fold enhancement of the subsequent primary immune response observed when immature and mature DCs are compared as immune adjuvants in culture and in mice.

Proteolysis in MHC class II antigen presentation: who's in charge?

Antigen-presenting cells (APC) degrade proteins intracellularly to generate peptides, which are then bound by products of the major histocompatibility complex (MHC) and exposed on the surface of the APC for recognition by T cells. The supply of antigenic peptides and their association with MHC molecules requires the concerted action of a cohort of accessory molecules that includes chaperones, transporters of peptides, and the proteases that degrade the antigens.

Intracellular transport and peptide loading of MHC class II molecules: regulation by chaperones and motors

MHC class II molecules are important in the onset and modulation of cellular immune responses. Studies on the intracellular transport of these molecules has provided insight into the way pathogens are processed and presented at the cell surface and may result in future immunological intervention strategies. Recent reviews have extensively described structural properties and early events in the biosynthesis of MHC class II (1-3). In this review, the focus will be on the function of the dedicated chaperone proteins Ii, DM and DO in the class II assembly, transport and peptide loading as well on proteins involved in transport steps late in the intracellular transport of MHC class II.

The role of H2-O and HLA-DO in major histocompatibility complex class II-restricted antigen processing and presentation

The function of major histocompatibility complex (MHC) class II molecules is to sample exogenous antigens for presentation to CD4+ T helper cells. After synthesis in the endoplasmic reticulum, class II molecules are directed into the endosomal system by association with the invariant chain (Ii), which is sequentially cleaved, generating class II dimers loaded with Ii-derived peptides (CLIP). These class II-peptide complexes are physiological substrates for H2-M/HLA-DM, a resident of the endosomal/lysosomal system which facilitates the removal of CLIP from newly synthesised class II alpha beta dimers. Exchange of CLIP for antigenic class II-binding peptides is also promoted by the action of H2-M/HLA-DM, resulting in stable peptide-class II complexes that are transported to the cell surface for presentation to CD4+ T cells. Recent evidence suggests that this H2-M/HLA-DM-mediated 'peptide editing' is influenced by another MHC class II-encoded molecule, H2-O/HLA-DO. This non-polymorphic alpha beta heterodimer is associated with H2-M/HLA-DM during intracellular transport and within the endosomal system of B cells. H2-O/HLA-DO alters the peptide exchange function of H2-M/HLA-DM in a pH-dependent manner, so that H2-M/HLA-DM activity is limited to more acidic conditions, corresponding to lysosomal compartments. Indeed, H2-O/HLA-DO may serve to limit the presentation of antigens after fluid phase uptake by B cells, while augmenting presentation of antigens internalised via membrane Ig receptors. Such a mechanism may maintain the fidelity of the B-cell-CD4+ T-cell interaction, counteracting self reactivity arising from less stringent lymphocyte activation. Here, data evaluating the role of H2-O/HLA-DO shall be reviewed and its putative function discussed.

Characterization of a peptide-loading compartment by monoclonal antibodies

Whether or not peptide-loading compartments are classical or specialized compartments of the endocytic pathway of antigen presenting cells is still a matter of debate. One way to solve this discrepancy would be to characterize specific markers for the peptide-loading compartment. We chose to generate monoclonal antibodies against the peptide-loading compartment that we previously characterized as lysozyme loading compartment (LLC) [Escola, J.M., Grivel, J.C., Chavrier, P., Gorvel, J.P., 1995. Different endocytic compartments are involved in the tight association of class II molecules with processed hen egg lysozyme and ribonuclease A in B cells. J. Cell Sci. 108, 2337; Escola, J.M., Deleuil, F., Stang, E., Boretto, J., Chavrier, P., Gorvel, J.P., 1996. Characterization of a lysozyme-major histocompatibility complex class II molecule-loading compartment as a specialized recycling endosome in murine B lymphocytes. J. Biol Chem. 271, 27360]. A preliminary screening by dot blot enabled us to identify several monoclonal antibodies recognizing the LLC and not early and late endosomes. One of these antibodies, the 20C4, was then characterized. It is directed against mature class II molecules of all murine haplotypes. By electron microscopy, 20C4 labeling was restricted to both the plasma membrane and the LLC. These reagents may be useful in the further characterization of the specialized function of these intracellular organelles.

Early endosomes are required for major histocompatiblity complex class II transport to peptide-loading compartments

Antigen presentation to CD4(+) T lymphocytes requires transport of newly synthesized major histocompatibility complex (MHC) class II molecules to the endocytic pathway, where peptide loading occurs. This step is mediated by a signal located in the cytoplasmic tail of the MHC class II-associated Ii chain, which directs the MHC class II-Ii complexes from the trans-Golgi network (TGN) to endosomes. The subcellular machinery responsible for the specific targeting of MHC class II molecules to the endocytic pathway, as well as the first compartments these molecules enter after exit from the TGN, remain unclear. We have designed an original experimental approach to selectively analyze this step of MHC class II transport. Newly synthesized MHC class II molecules were caused to accumulate in the Golgi apparatus and TGN by incubating the cells at 19 degrees C, and early endosomes were functionally inactivated by in vivo cross-linking of transferrin (Tf) receptor-containing endosomes using Tf-HRP complexes and the HRP-insoluble substrate diaminobenzidine. Inactivation of Tf-containing endosomes caused a marked delay in Ii chain degradation, peptide loading, and MHC class II transport to the cell surface. Thus, early endosomes appear to be required for delivery of MHC class II molecules to the endocytic pathway. Under cross-linking conditions, most alphabetaIi complexes accumulated in tubules and vesicles devoid of gamma-adaptin and/or mannose-6-phosphate receptor, suggesting an AP1-independent pathway for the delivery of newly synthesized MHC class II molecules from the TGN to endosomes.

Polarized Transport of MHC Class II Molecules in Madin-Darby Canine Kidney Cells Is Directed by a Leucine-Based Signal in the Cytoplasmic Tail of the β-Chain

MHC class II molecules are found on the basolateral plasma membrane domain of polarized epithelial cells, where they can present Ag to intraepithelial lymphocytes in the vascular space. We have analyzed the sorting information required for efficient intracellular localization and polarized distribution of MHC class II molecules in stably transfected Madin-Darby canine kidney cells. These cells were able to present influenza virus particles to HLA-DR1-restricted T cell clones. Wild-type MHC class II molecules were located on the basolateral plasma membrane domain, in basolateral early endosomes, and in late multivesicular endosomes, the latter also containing the MHC class II-associated invariant chain and an HLA-DM fusion protein. A phenylalanine-leucine residue within the cytoplasmic tail of the β-chain was required for basolateral distribution, efficient internalization, and localization of the MHC class II molecules to basolateral early endosomes. However, distribution to apically located, late multivesicular endosomes did not depend on signals in the class II cytoplasmic tails as both wild-type class II molecules and mutant molecules lacking the phenylalanine-leucine motif were found in these compartments. Our results demonstrate that sorting information in the tails of class II dimers is an absolute requirement for their basolateral surface distribution and intracellular localization.

Igα and Igβ Are Required for Efficient Trafficking to Late Endosomes and to Enhance Antigen Presentation

The B cell Ag receptor (BCR) is a multimeric complex, containing Igα and Igβ, capable of internalizing and delivering specific Ags to specialized late endosomes, where they are processed into peptides for loading onto MHC class II molecules. By this mechanism, the presentation of receptor-selected epitopes to T cells is enhanced by several orders of magnitude. Previously, it has been reported that, under some circumstances, either Igα or Igβ can facilitate the presentation of Ags. However, we now demonstrate that if these Ags are at low concentrations and temporally restricted, both Igα and Igβ are required. When compared with the BCR, chimeric complexes containing either chain alone were internalized but failed to access the MHC class II-enriched compartment (MIIC) or induce the aggregation and fusion of its constituent vesicles. Furthermore, Igα/Igβ complexes in which the immunoreceptor tyrosine-based activation motif tyrosines of Igα were mutated were also incapable of accessing the MIIC or of facilitating the presentation of Ag. These data indicate that both Igα and Igβ contribute signaling, and possibly other functions, to the BCR that are necessary and sufficient to reconstitute the trafficking and Ag-processing enhancing capacities of the intact receptor complex.

CIIV, MIIC and other compartments for MHC class II loading

Virtually every endocytic compartment has been claimed to be an MIIC, a site where class II molecules accumulate. Here, it is argued that the definition of MIIC is not accurate and often pointless. MIIC can better be used as a working title for a collection of late endocytic compartments that contain the goodies necessary for efficient peptide loading of class II molecules.

The secretory route of the leaderless protein interleukin 1beta involves exocytosis of endolysosome-related vesicles

Interleukin 1beta (IL-1beta), a secretory protein lacking a signal peptide, does not follow the classical endoplasmic reticulum-to-Golgi pathway of secretion. Here we provide the evidence for a "leaderless" secretory route that uses regulated exocytosis of preterminal endocytic vesicles to transport cytosolic IL-1beta out of the cell. Indeed, although most of the IL-1beta precursor (proIL-1beta) localizes in the cytosol of activated human monocytes, a fraction is contained within vesicles that cofractionate with late endosomes and early lysosomes on Percoll density gradients and display ultrastructural features and markers typical of these organelles. The observation of organelles positive for both IL-1beta and the endolysosomal hydrolase cathepsin D or for both IL-1beta and the lysosomal marker Lamp-1 further suggests that they belong to the preterminal endocytic compartment. In addition, similarly to lysosomal hydrolases, secretion of IL-1beta is induced by acidotropic drugs. Treatment of monocytes with the sulfonylurea glibenclamide inhibits both IL-1beta secretion and vesicular accumulation, suggesting that this drug prevents the translocation of proIL-1beta from the cytosol into the vesicles. A high concentration of extracellular ATP and hypotonic medium increase secretion of IL-1beta but deplete the vesicular proIL-1beta content, indicating that exocytosis of proIL-1beta-containing vesicles is regulated by ATP and osmotic conditions.