Distinct intracellular compartments involved in invariant chain degradation and antigenic peptide loading of major histocompatibility complex (MHC) class II molecules

Igβ ubiquitination activates PI3K signals required for endosomal sorting

Veselits et al. show that Igβ ubiquitination activates PI3K and the accumulation of PIP₃ on BCR-associated endosomal membranes, which is necessary and sufficient for sorting into classical antigen-processing compartments. Surprisingly, proper BCR sorting is critical for endosomal TLR activation yet dispensable for T-dependent humoral immunity.

A wealth of in vitro data has demonstrated a central role for receptor ubiquitination in endocytic sorting. However, how receptor ubiquitination functions in vivo is poorly understood. Herein, we report that ablation of B cell antigen receptor ubiquitination in vivo uncouples the receptor from CD19 phosphorylation and phosphatidylinositol 3-kinase (PI3K) signals. These signals are necessary and sufficient for accumulating phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) on B cell receptor–containing early endosomes and proper sorting into the MHC class II antigen-presenting compartment (MIIC). Surprisingly, MIIC targeting is dispensable for T cell–dependent immunity. Rather, it is critical for activating endosomal toll-like receptors and antiviral humoral immunity. These findings demonstrate a novel mechanism of receptor endosomal signaling required for specific peripheral immune responses.

Activation of the cAMP/protein kinase A signalling pathway by coronin 1 is regulated by cyclin-dependent kinase 5 activity

Coronins constitute a family of conserved proteins expressed in all eukaryotes that have been implicated in the regulation of a wide variety of cellular activities. Recent work showed an essential role for coronin 1 in the modulation of the cAMP/PKA pathway in neurons through the interaction of coronin 1 with the G protein subtype Gαs in a stimulus-dependent manner, but the molecular mechanism regulating coronin 1-Gαs interaction remains unclear. We here show that phosphorylation of coronin 1 on Thr(418/424) by cyclin-dependent kinase (CDK) 5 activity was responsible for coronin 1-Gαs association and the modulation of cAMP production. Together these results show an essential role for CDK5 activity in promoting the coronin 1-dependent cAMP/PKA pathway.

A Distinctive Cytoplasmic Tail Contributes to Low Surface Expression and Intracellular Retention of the Patr-AL MHC Class I Molecule

Chimpanzees have orthologs of the six fixed, functional human MHC class I genes. But, in addition, the chimpanzee has a seventh functional gene, Patr-AL, which is not polymorphic but contributes substantially to population diversity by its presence on only 50% of MHC haplotypes. The ancestral AL gene emerged long before the separation of human and chimpanzee ancestors and then subsequently and specifically lost function during human evolution, but was maintained in chimpanzees. Patr-AL is an alloantigen that participates in negative and positive selection of the T cell repertoire. The three-dimensional structure and the peptide-binding repertoire of Patr-AL and HLA-A*02 are surprisingly similar. In contrast, the expression of these two molecules is very different, as shown using specific mAbs and polyclonal Abs made against Patr-AL. Peripheral blood cells and B cell lines express low levels of Patr-AL at the cell surface. Higher levels are seen for 221-cell transfectants expressing Patr-AL, but in these cells a large majority of Patr-AL molecules are retained in the early compartments of the secretory pathway: mainly the endoplasmic reticulum, but also cis-Golgi. Replacing the cytoplasmic tail of Patr-AL with that of HLA-A*02 increased the cell-surface expression of Patr-AL substantially. Four substitutions distinguish the Patr-AL and HLA-A*02 cytoplasmic tails. Systematic mutagenesis showed that each substitution contributes changes in cell-surface expression. The combination of residues present in Patr-AL appears unique, but each individual residue is present in other primate MHC class I molecules, notably MHC-E, the most ancient of the functional human MHC class I molecules.

Recruitment of Cbl-b to B cell antigen receptor couples antigen recognition to Toll-like receptor 9 activation in late endosomes

Casitas B-lineage lymphoma-b (Cbl-b) is a ubiquitin ligase (E3) that modulates signaling by tagging molecules for degradation. It is a complex protein with multiple domains and binding partners that are not involved in ubiquitinating substrates. Herein, we demonstrate that Cbl-b, but not c-Cbl, is recruited to the clustered B cell antigen receptor (BCR) and that Cbl-b is required for entry of endocytosed BCRs into late endosomes. The E3 activity of Cbl-b is not necessary for BCR endocytic trafficking. Rather, the ubiquitin associated (UBA) domain is required. Furthermore, the Cbl-b UBA domain is sufficient to confer the receptor trafficking functions of Cbl-b on c-Cbl. Cbl-b is also required for entry of the Toll-like receptor 9 (TLR9) into late endosomes and for the in vitro activation of TLR9 by BCR-captured ligands. These data indicate that Cbl-b acts as a scaffolding molecule to coordinate the delivery of the BCR and TLR9 into subcellular compartments required for productively delivering BCR-captured ligands to TLR9.

CD74: an emerging opportunity as a therapeutic target in cancer and autoimmune disease

INTRODUCTION

CD74, also known as the invariant chain, participates in several key processes of the immune system, including antigen presentation, B-cell differentiation and inflammatory signaling. Despite being described more than 3 decades ago, new functions and novel interactions for this evolutionarily conserved molecule are still being unraveled. As a participant in several immunological processes and an indicator of disease in some conditions, it has potential as a therapeutic target.

AREAS COVERED

The relationship between the structure of CD74 variants and their physiological functions is detailed in this review. The function of CD74 in several cell lineages is examined with a focus on the interactions with cathepsins and, in an inflammatory milieu, the pro-inflammatory cytokine macrophage migratory inhibitory factor. The role of CD74 signaling in inflammatory and carcinogenic processes is outlined as is the use of CD74 as a therapeutic target (in cancer) and tool (as a vaccine).

EXPERT OPINION

CD74 has several roles within the cell and throughout the immune system. Most prominent amongst these are the complex relationships with MIF and cathepsins. Modulation of CD74 function shows promise for the effective amelioration of disease.

Receptors, subcellular compartments and the regulation of peripheral B cell responses: the illuminating state of anergy

Signals through the B cell antigen receptor (BCR) are necessary but not sufficient for cellular activation. Co-stimulatory signals must be provided through other immune recognition receptor systems, such as MHC class II/CD40 and the toll-like receptor (TLR) 9 that can only productively acquire their ligands in the processive environment of specialized late endosomes (MHC class II containing compartment or MIIC). It has long been appreciated that the BCR, by effectively capturing complex antigens and delivering them to late endosomes, is the link between activation events on the cell surface and those dependent on late endosomes. However, it has become increasingly apparent that the BCR also directs the translocation of MHC class II and TLR9 into the MIIC and that the endocytic flow of these receptors coincides with that of the BCR. This likely ensures close apposition of receptor complexes within the MIIC and the efficient transfer of ligands from the BCR to MHC class II and TLR9. This complex orchestration of receptor endocytic movement is dependent upon the quality of signals elicited through the BCR. Failure to activate specific signaling pathways, such as occurs in anergic B cells, prevents the entry of the BCR and TLR9 into the MIIC and abrogates TLR9 activation. Like anergy, this block in endocytic trafficking is rapidly reversible. These findings indicate that cellular responsiveness can be determined by mechanisms that control the subcellular location of important immune recognition receptors.

Endocytic sequestration of the B cell antigen receptor and toll-like receptor 9 in anergic cells

In autoimmune prone murine strains, sequential engagement of the B cell antigen receptor (BCR) on the cell surface and toll-like receptors (TLRs) in late endosomes is necessary and sufficient for secretion of autoantibodies. However, ubiquitous nucleoprotein self-antigens fail to elicit productive TLR activation, and break self-tolerance in anergic DNA-reactive B cells. The mechanisms limiting TLR activation in these cells are largely unknown. Here, we demonstrate that in anergic 3H9/Vkappa8 and Ars/A1 B cells the normal endocytic transit of both the ligated BCR and TLR9 into late endosomes is abrogated. The BCR and TLR9 arrest together just outside late endosomes, indicating that they enter this compartment along a single, regulated endocytic route. Access to late endosomes could be restored by reversing anergy through several methods, including conferring genetic susceptibility to autoimmunity, complementing proximal BCR signaling or by preventing BCR binding to self-antigen. Downstream of the BCR, JNK, which is activated in naive but not anergic B cells, regulated entry into late endosomes. Restoration of BCR and TLR9 endocytic trafficking rescued TLR9 activation by BCR-captured ligands. These results indicate that B cell anergy is reinforced by the exclusion of both TLRs and their BCR captured ligands from subcellular environments necessary for TLR activation.

RNA interference in J774 macrophages reveals a role for coronin 1 in mycobacterial trafficking but not in actin-dependent processes

Macrophages are crucial for innate immunity, apoptosis, and tissue remodeling, processes that rely on the capacity of macrophages to internalize and process cargo through phagocytosis. Coronin 1, a member of the WD repeat protein family of coronins specifically expressed in leukocytes, was originally identified as a molecule that is recruited to mycobacterial phagosomes and prevents the delivery of mycobacteria to lysosomes, allowing these to survive within phagosomes. However, a role for coronin 1 in mycobacterial pathogenesis has been disputed in favor for its role in mediating phagocytosis and cell motility. In this study, a role for coronin 1 in actin-mediated cellular processes was addressed using RNA interference in the murine macrophage cell line J774. It is shown that the absence of coronin 1 in J774 macrophages expressing small interfering RNA constructs specific for coronin 1 does not affect phagocytosis, macropinocytosis, cell locomotion, or regulation of NADPH oxidase activity. However, in coronin 1-negative J774 cells, internalized mycobacteria were rapidly transferred to lysosomes and killed. Therefore, these results show that in J774 cells coronin 1 has a specific role in modulating phagosome-lysosome transport upon mycobacterial infection and that it is dispensable for most F-actin-mediated cytoskeletal rearrangements.

Survival of mycobacteria in macrophages is mediated by coronin 1-dependent activation of calcineurin

Pathogenic mycobacteria survive within macrophages by avoiding lysosomal delivery, instead residing in mycobacterial phagosomes. Upon infection, the leukocyte-specific protein coronin 1 is actively recruited to mycobacterial phagosomes, where it blocks lysosomal delivery by an unknown mechanism. Analysis of macrophages from coronin 1-deficient mice showed that coronin 1 is dispensable for F-actin-dependent processes such as phagocytosis, motility, and membrane ruffling. However, upon mycobacterial infection, coronin 1 was required for activation of the Ca(2+)-dependent phosphatase calcineurin, thereby blocking lysosomal delivery of mycobacteria. In the absence of coronin 1, calcineurin activity did not occur, resulting in lysosomal delivery and killing of mycobacteria. Furthermore, blocking calcineurin activation with cyclosporin A or FK506 led to lysosomal delivery and intracellular mycobacterial killing. These results demonstrate a role for coronin 1 in activating Ca(2+) dependent signaling processes in macrophages and reveal a function for calcineurin in the regulation of phagosome-lysosome fusion upon mycobacterial infection.

Ubiquitinylation of Igβ Dictates the Endocytic Fate of the B Cell Antigen Receptor

In both infection and autoimmunity, the development of high-affinity Abs and memory requires B cells to efficiently capture and process Ags for presentation to cognate T cells. Although a great deal is known about how Ags are processed, the molecular mechanisms by which the BCR captures Ag for processing are still obscure. In this study, we demonstrate that the Ig beta component of the BCR is diubiquitinylated and that this is dependent on the E3 ligase Itch. Itch-/- B lymphocytes manifest both a defect in ligand-induced BCR internalization and endocytic trafficking to late endosomal Ag-processing compartments. In contrast, analysis of ubiquitinylation-defective receptors demonstrated that the attachment of ubiquitins to Ig beta is required for endosomal sorting and for the presentation of Ag to T cells, yet, ubiquitinylation is dispensable for receptor internalization. Membrane-bound Ig mu was not detectably ubiquitinylated nor were the conserved lysines in the mu cytosolic tail required for trafficking to late endosomes. These results demonstrate that ubiquitinylation of a singular substrate, Ig beta, is required for a specific receptor trafficking event. However, they also reveal that E3 ligases play a broader role in multiple processes that determine the fate of Ag-engaged BCR complexes.

Inhibition of phagosome maturation by mycobacteria does not interfere with presentation of mycobacterial antigens by MHC molecules

Pathogenic mycobacteria escape host innate immune responses by surviving within phagosomes of host macrophages and blocking their delivery to lysosomes. Avoiding lysosomal delivery may also be involved in the capacity of living mycobacteria to modulate MHC class I- or II-dependent T cell responses, which may contribute to their pathogenicity in vivo. In this study, we show that the presentation of mycobacterial Ags is independent of the site of intracellular residence inside professional APCs. Infection of mouse macrophages or dendritic cells in vitro with mycobacterial mutants that are unable to escape lysosomal transfer resulted in an identical efficiency of Ag presentation compared with wild-type mycobacteria. Moreover, in vivo, such mutants induced CD4(+) Th1 or CD8(+) CTL responses in mice against various mycobacterial Ags that were comparable to those induced by their wild-type counterparts. These results suggest that the limiting factor for the generation of an adaptive immune response against mycobacteria is not the degree of lysosomal delivery. These findings are important in the rational design of improved vaccines to combat mycobacterial diseases.

A translocated bacterial protein protects vascular endothelial cells from apoptosis

The modulation of host cell apoptosis by bacterial pathogens is of critical importance for the outcome of the infection process. The capacity of Bartonella henselae and B. quintana to cause vascular tumor formation in immunocompromised patients is linked to the inhibition of vascular endothelial cell (EC) apoptosis. Here, we show that translocation of BepA, a type IV secretion (T4S) substrate, is necessary and sufficient to inhibit EC apoptosis. Ectopic expression in ECs allowed mapping of the anti-apoptotic activity of BepA to the Bep intracellular delivery domain, which, as part of the signal for T4S, is conserved in other T4S substrates. The anti-apoptotic activity appeared to be limited to BepA orthologs of B. henselae and B. quintana and correlated with (i) protein localization to the host cell plasma membrane, (ii) elevated levels of intracellular cyclic adenosine monophosphate (cAMP), and (iii) increased expression of cAMP-responsive genes. The pharmacological elevation of cAMP levels protected ECs from apoptosis, indicating that BepA mediates anti-apoptosis by heightening cAMP levels by a plasma membrane-associated mechanism. Finally, we demonstrate that BepA mediates protection of ECs against apoptosis triggered by cytotoxic T lymphocytes, suggesting a physiological context in which the anti-apoptotic activity of BepA contributes to tumor formation in the chronically infected vascular endothelium.

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.

Lack of induced co-stimulation as a result of complement receptor 2 (CR2) ligation on mouse splenic B cells

B cells act as efficient antigen-presenting cells if they acquire antigen via membrane-bound Ig [termed the B cell receptor (BCR)]. Ligation of the BCR leads to antigen internalization, processing and presentation to CD4+ T cells in association with MHC class II molecules. Ligation of the BCR also leads to the generation of activation signals. One short-term consequence of this is the up-regulation of co-stimulatory molecule expression by the B cell, allowing full T cell activation. Other antigen receptors expressed by B cells can also mediate efficient antigen presentation to CD4+ T cells. Ligating one such receptor, complement receptor 2 (CR2), has also been described to induce co-stimulatory molecule expression. If correct, this may have serious consequences for ensuring the specificity of the resultant B cell response. We have therefore investigated the effects of ligating both the BCR and CR2 independently of each other, as well as with reagents to cross-link the two receptors, in order to clarify these findings. In contrast to the effects seen upon BCR ligation, we find no evidence for co-stimulatory molecule up-regulation following CR2 ligation. As antigen presentation in the absence of co-stimulation may lead to the induction of tolerogenic or regulatory signals being delivered to T cell populations, these findings imply that the role of CR2 in B cell-mediated antigen presentation is different from that of the BCR.

Selective expression of vacuolar H+-ATPase subunit d2 by particular subsets of dendritic cells among leukocytes

Dendritic cells (DC) are far more potent to activate T cells than other antigen presenting cells (e.g., macrophages) and distributed to many organs where DC develop to functionally and phenotypically distinctive subsets. To isolate DC-differentially expressed genes, we used a subtractive cDNA cloning (XS52 DC minus J774 macrophages), resulting in the identification of d2 isoform of vacuolar (V) H+-ATPase subunit d. Unlike the ubiquitously expressed isoform (d1), d2 mRNA manifested expression restricted to particular subsets of DC (e.g., skin- and bone marrow-derived DC) among leukocytes and encoded two transcripts (1.6 and 3.0 kb) that differed in the length of the 3'-untranslated region. The d2 protein displayed association with membranes and the localization in lysosomes and antigen-containing endosomes. Interestingly, XS52 DC expressed seven-fold higher V-ATPase proton-pump activity than J774 macrophages and distinguished from the macrophage by high levels of isoforms a1 and a2 expression among V-ATPase subunits. These results indicated that d2 is a new marker for DC and it may, co-operatively with subunit a isoforms, regulate V-ATPase activity.

Association of the leukocyte plasma membrane with the actin cytoskeleton through coiled coil-mediated trimeric coronin 1 molecules

Coronin 1 is a member of the coronin protein family specifically expressed in leukocytes and accumulates at sites of rearrangements of the F-actin cytoskeleton. Here, we describe that coronin 1 molecules are coiled coil-mediated homotrimeric complexes, which associate with the plasma membrane and with the cytoskeleton via two distinct domains. Association with the cytoskeleton was mediated by trimerization of a stretch of positively charged residues within a linker region between the N-terminal, WD repeat-containing domain and the C-terminal coiled coil. In contrast, neither the coiled coil nor the positively charged residues within the linker domain were required for plasma membrane binding, suggesting that the N-terminal, WD repeat-containing domain mediates membrane interaction. The capacity of coronin 1 to link the leukocyte cytoskeleton to the plasma membrane may serve to integrate outside-inside signaling with modulation of the cytoskeleton.

Protein kinase G from pathogenic mycobacteria promotes survival within macrophages

Pathogenic mycobacteria resist lysosomal delivery after uptake into macrophages, allowing them to survive intracellularly. We found that the eukaryotic-like serine/threonine protein kinase G from pathogenic mycobacteria was secreted within macrophage phagosomes, inhibiting phagosome-lysosome fusion and mediating intracellular survival of mycobacteria. Inactivation of protein kinase G by gene disruption or chemical inhibition resulted in lysosomal localization and mycobacterial cell death in infected macrophages. Besides identifying a target for the control of mycobacterial infections, these findings suggest that pathogenic mycobacteria have evolved eukaryotic-like signal transduction mechanisms capable of modulating host cell trafficking pathways.

Lowering the affinity between antigen and the B cell receptor can enhance antigen presentation

The B cell receptor (BCR) enables antigen-specific B cells to bind, internalize and target antigens for processing into small peptide fragments. These epitopes are then expressed on the plasma membrane in association with MHC class II molecules for recognition by CD4+ T cells. The affinity of the interaction between the BCR and antigen plays an important part in determining T cell epitope generation. In this report we provide evidence that the efficiency of antigen presentation by specific B cells does not need to be directly proportional to antigen/BCR affinity. We show that increased presentation can result from lowering the affinity of the antigen/BCR interaction. This finding suggests a novel mechanism by which B cells can recruit T cell help and obtain survival signals. Activation of these cells may have consequences for the generation of the B cell repertoire.

Differential post-translational modification of CD63 molecules during maturation of human dendritic cells

The capacity of dendritic cells to initiate T cell responses is related to their ability to redistribute MHC class II molecules from the intracellular MHC class II compartments to the cell surface. This redistribution occurs during dendritic cell development as they are converted from an antigen capturing, immature dendritic cell into an MHC class II-peptide presenting mature dendritic cell. During this maturation, antigen uptake and processing are down-regulated and peptide-loaded class II complexes become expressed in a stable manner on the cell surface. Here we report that the tetraspanin CD63, that associates with intracellularly localized MHC class II molecules in immature dendritic cells, was modified post-translationally by poly N-acetyl lactosamine addition during maturation. This modification of CD63 was accompanied by a change in morphology of MHC class II compartments from typical multivesicular organelles to structures containing densely packed lipid moieties. Post-translational modification of CD63 may be involved in the functional and morphological changes of MHC class II compartments that occur during dendritic cell maturation.

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.

Destructive processing by asparagine endopeptidase limits presentation of a dominant T cell epitope in MBP

Little is known about the processing of putative human autoantigens and why tolerance is established to some T cell epitopes but not others. Here we show that a principal human HLA-DR2-restricted epitope--amino acids 85-99 of myelin basic protein, MBP(85-99)--contains a processing site for the cysteine protease asparagine endopeptidase (AEP). Presentation of this epitope by human antigen-presenting cells is inversely proportional to the amount of cellular AEP activity: inhibition of AEP in living cells greatly enhances presentation of the MBP(85-99) epitope, whereas overexpression of AEP diminishes presentation. These results indicate that central tolerance to this encephalitogenic MBP epitope may not be established because destructive processing limits its display in the thymus. Consistent with this hypothesis, AEP is expressed abundantly in thymic antigen-presenting cells.

Rheumatoid arthritis (RA)-associated HLA-DR alleles form less stable complexes with class II-associated invariant chain peptide than non-RA-associated HLA-DR alleles

Certain HLA-DR alleles confer strong susceptibility to the autoimmune disease rheumatoid arthritis (RA). We compared RA-associated alleles, HLA-DR*0401, HLA-DR*0404, and HLA-DR*0405, with closely related, non-RA-associated alleles, HLA-DR*0402 and HLA-DR*0403, to determine whether they differ in their interactions with the class II chaperone, invariant chain (Ii). Ii binds to class II molecules in the endoplasmic reticulum, inhibits binding of other ligands, and directs class II-Ii complexes to endosomes, where Ii is degraded to class II-associated Ii peptide (CLIP). To evaluate the interaction of Ii and CLIP with these DR4 alleles, we introduced HLA-DR*0401, *0402, and *0404 alleles into a human B cell line that lacked endogenous HLA-DR or HLA-DM molecules. In a similar experiment, we introduced HLA-DR*0403 and *0405 into an HLA-DM-expressing B cell line, 8.1.6, and its DM-negative derivative, 9.5.3. Surface abundance of DR4-CLIP peptide complexes and their susceptibility to SDS-induced denaturation suggested that the different DR4-CLIP complexes had different stabilities. Pulse-chase experiments showed CLIP dissociated more rapidly from RA-associated DR molecules in B cell lines. In vitro assays using soluble rDR4 molecules showed that DR-CLIP complexes of DR*0401 and DR*0404 were less stable than complexes of DR*0402. Using CLIP peptide variants, we mapped the reduced CLIP interaction of RA-associated alleles to the shared epitope region. The reduced interaction of RA-associated HLA-DR4 molecules with CLIP may contribute to the pathophysiology of autoimmunity in RA.

Trafficking of surface-linked and encapsulated liposomal antigens in macrophages: an immunocytochemical study

Liposomal antigens are potent adjuvants of humoral and cell-mediated immunity. Although this property requires as an essential condition a physical association between the antigen and the phospholipid vehicle, the nature of the association, i.e., encapsulation or surface linkage, markedly influences the outcome of the elicited response. Available evidence suggests that macrophages are involved in this fine tuning of the immune response in a manner that is not yet clearly established. It is postulated that this might be related to their capacity to interact differently with surface-linked and encapsulated formulations. Using conalbumin as a model antigen, we address the question by analyzing the movements of encapsulated and surface-linked antigen as well as those of MHC-II molecules in macrophages in a pulse-chase immunoelectron microscopic study carried out over a 24-hr period. The antigen was followed using a polyclonal serum specifically raised against fragmented conalbumin (fCA) that allows the detection of processed antigen and of some MHC-peptide complexes. The results indicate that, in macrophages, the two liposomal formulations affect macrophage morphology in distinct ways and circulate through the various subcellular compartments with different kinetics. On the basis of the overall results, we conclude that surface-linked antigen gains access less readily to the endogenous presentation pathway than encapsulated antigen but can favor a more sustained activation of the immune system through its production of exosome-like structures and its more thorough utilization of the MHC-II pathway.

Efficient presentation of both cytosolic and endogenous transmembrane protein antigens on MHC class II is dependent on cytoplasmic proteolysis

Peptides from extracellular proteins presented on MHC class II are mostly generated and loaded in endolysosomal compartments, but the major pathways responsible for loading peptides from APC-endogenous sources on MHC class II are as yet unclear. In this study, we show that MHC class II molecules present peptides from proteins such as OVA or conalbumin introduced into the cytoplasm by hyperosmotic pinosome lysis, with efficiencies comparable to their presentation via extracellular fluid-phase endocytosis. This cytosolic presentation pathway is sensitive to proteasomal inhibitors, whereas the presentation of exogenous Ags taken up by endocytosis is not. Inhibitors of nonproteasomal cytosolic proteases can also inhibit MHC class II-restricted presentation of cytosolically delivered protein, without inhibiting MHC class I-restricted presentation from the same protein. Cytosolic processing of a soluble fusion protein containing the peptide epitope I-Ealpha(52-68) yields an epitope that is similar to the one generated during constitutive presentation of I-Ealpha as an endogenous transmembrane protein, but is subtly different from the one generated in the exogenous pathway. Constitutive MHC class II-mediated presentation of the endogenous transmembrane protein I-Ealpha is also specifically inhibited over time by inhibitors of cytosolic proteolysis. Thus, Ag processing in the cytoplasm appears to be essential for the efficient presentation of endogenous proteins, even transmembrane ones, on MHC class II, and the proteolytic pathways involved may differ from those used for MHC class I-mediated presentation.

Association of distinct tetraspanins with MHC class II molecules at different subcellular locations in human immature dendritic cells

Dendritic cells have the capacity to trigger T cell responses in lymphoid organs against antigens captured in the periphery. T cell stimulation depends on the ability of MHC class II molecules to present peptides at the cell surface that are acquired in MHC class II compartments. The high capacity of dendritic cells to stimulate T lymphocytes is related to their ability to regulate the distribution of MHC class II molecules intracellularly. To analyze the molecular components involved in the generation of MHC class II-peptide complexes in human immature dendritic cells, mAb were raised against purified MHC class II compartments. One of the antigens turned out to be CD63, a member of the tetraspanin superfamily. CD63 localized exclusively intracellularly where it associated with peptide-loaded class II molecules. In contrast, the tetraspanins CD9, CD53 and CD81 associated with class II molecules at the plasma membrane. Selective association of distinct tetraspanins may be involved in the regulation of MHC class II distribution in human dendritic cells.

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.

Proteases involved in MHC class II antigen presentation

Major histocompatibility complex class II antigen presentation requires the participation of lysosomal proteases in two convergent processes. First, the antigens endocytosed by the antigen-presenting cells must be broken down into antigenic peptides. Second, class II molecules are synthesized with their peptide-binding site blocked by invariant chain (Ii), and they acquire the capacity to bind antigens only after Ii has been degraded in the compartments where peptides reside. The study of genetically modified mice deficient in single lysosomal proteases has allowed us to determine their role in these processes. Cathepsins (Cat) B and D, previously considered major players in MHC class II antigen presentation, are dispensable for degradation of Ii and for generation of several antigenic determinants. By contrast, Cat S plays an essential role in removal of Ii in B cells and dendritic cells, whereas Cat L apparently does so in thymic epithelial cells. Accordingly, the absence of Cat S and L have major consequences for the onset of humoral immune responses and for T-cell selection, respectively. It is likely that other as yet uncharacterized lysosomal enzymes also play a role in Ii degradation and in generation of antigenic determinants. Experiments involving drugs that interfere with protein traffic suggest that more than one mechanism for Ii removal, probably involving different proteases, can co-exist in the same antigen-presenting cell. These findings may allow the development of protease inhibitors with possible therapeutic applications.

Direct transport of newly synthesized HLA-DR from the trans-Golgi network to major histocompatibility complex class II containing compartments (MIICS) demonstrated using a novel tyrosine-sulfated chimera

Binding of antigenic peptides to major histocompatibility complex (MHC) class II glycoproteins occurs in specialized endocytic compartments of antigen-presenting cells, which in man are termed MIICs. Newly synthesized MHC class II molecules are transported from the trans-Golgi network to MIICs, but previous studies of this important step in antigen processing have failed to conclusively determine whether most immature MHC class II complexes are transported directly to the processing compartments or are first transiently exposed at the cell surface. To attempt to resolve this question, I constructed a chimeric HLA-DRalpha chain containing two optimal tyrosine sulfation motifs. When expressed in a human B lymphoblastoid cell line lacking functional DRalpha chains, the chimera was correctly incorporated into complexes containing endogenous beta and invariant chains, transported to the trans-Golgi network, and efficiently sulfated. Pulse-chase experiments showed that the sulfated complexes were rapidly transported to processing compartments with kinetics consistent with direct transport from the trans-Golgi network. The rate of maturation was not significantly altered in cells expressing a temperature-sensitive mutant of dynamin under conditions where the endocytosis of transferrin was inhibited by 95%, confirming that endocytosis was not required for delivery to MIICs. Maturation of MHC class II-containing complexes was inhibited by aluminum fluoride and brefeldin A, indicating the involvement of heterotrimeric G-proteins and ADP-ribosylation factor in the transport event(s). The procedure described provides a unique mechanism to study critical events in antigen processing and presentation.

A secreted form of the major histocompatibility complex class II-associated invariant chain inhibiting T cell activation

Major histocompatibility complex (MHC) class II molecules function at the cell surface to present antigenic peptides to T helper cells. Intracellularly, MHC class II molecules are associated with the invariant chain (Ii). Ii can modulate MHC class II-dependent T cell activation through (i) assistance in the export of MHC class II molecules from the endoplasmic reticulum, (ii) providing a targeting signal for endosomal/lysosomal compartments, and (iii) preventing peptides from associating prematurely with MHC class II molecules. Here we describe the generation and subsequent secretion of a lumenal form of Ii, IiP25. IiP25 lacked the targeting sequences for transport to MHC class II compartments but contained part of the CLIP region that is known to compete with antigenic peptides for binding to MHC class II molecules. When added to an antigenic peptide presentation model system, IiP25 inhibited T cell activation by competing for the CLIP binding site at the plasma membrane. Secretion of a lumenal Ii fragment may represent an additional mechanism to modulate T cell activation by MHC class II molecules.

A rapid method to separate endosomes from lysosomal contents using differential centrifugation and hypotonic lysis of lysosomes

Here we describe a fast and efficient subcellular fractionation procedure that permits lysosomes to be separated from endosomes. Differential centrifugation is used to isolate a subcellular fraction containing both endosomes and lysosomes. Because lysosomes are sensitive to osmotic stress, hypotonic conditions destroy them, whereas endosomes, which are osmotically insensitive, stay intact. We demonstrate that hypotonic lysis of an endosome-lysosome-pool releases 85% of the lysosomes into the supernatant as measured by the activity of the lysosomal marker enzyme N-acetyl-beta-D-glucosaminidase (beta-AGA). The endosomal fraction is thoroughly characterised using a variety of subcellular markers. After pulsing cells with fluorescein isothiocyanate labelled transferrin (FITC-Tf), only about 12% of the marker is released under hypotonic conditions. A typical fractionation procedure takes about 1-2 h from initial cell homogenisation. The fractionation gives a pure lysosomal fraction (fraction L) containing high activities of lysosomal enzymes and an endosomal fraction (fraction E) reflecting different stages of endosomes.

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.

Differences in IgG subclass do not effect immune complex-enhanced T cell activation despite differential binding to antigen presenting cells

Ag presentation to CD4 T cells is a critical event in the generation of protective immunity. IgG, in the form of IgG-pathogen (Ag) complexes, is capable of mediating FcgammaR-dependent Ag presentation, and thereby enhanced T cell activation. Therefore, it is important to understand the ability of the individual human IgG subclasses to function in enhanced T cell activation. We hypothesized that increased delivery of Ag to monocyte FcgammaR by high affinity human IgG subclasses, IgG1 and IgG3, would lead to increased Ag presentation, as compared to low affinity IgG subclasses, IgG2 and IgG4. To create immune complexes, we linked biotinylated IgG subclasses to biotinylated Ag via an avidin bridge, and examined T cell responses to them. Although IgG2- and IgG4-Ag complexes bound to monocytes at significantly lower levels than those made with IgG1 and IgG3, we observed no significant difference in the ability of the four human IgG subclasses to mediate enhanced T cell activation. Studies suggest the explanation for this dichotomy lies within the first 24 h of Ag processing, and that processing efficiency may vary with IgG subclass. They also suggest the existence of a highly efficient, and selective processing pathway, which is dependent on IgG subclass, and can compensate for low level production and FcgammaR binding of IgG2- and IgG4-Ag complexes.

A coat protein on phagosomes involved in the intracellular survival of mycobacteria

Mycobacteria are intracellular pathogens that can survive within macrophage phagosomes, thereby evading host defense strategies by largely unknown mechanisms. We have identified a WD repeat host protein that was recruited to and actively retained on phagosomes by living, but not dead, mycobacteria. This protein, termed TACO, represents a component of the phagosome coat that is normally released prior to phagosome fusion with or maturation into lysosomes. In macrophages lacking TACO, mycobacteria were readily transported to lysosomes followed by their degradation. Expression of TACO in nonmacrophages prevented lysosomal delivery of mycobacteria and prolonged their intracellular survival. Active retention of TACO on phagosomes by living mycobacteria thus represents a mechanism preventing cargo delivery to lysosomes, allowing mycobacteria to survive within macrophages.

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.

Lectin-induced retardation of subcellular organelles during preparative density gradient electrophoresis: selective purification of plasma membranes

Plasma membranes (PM) are difficult to separate by conventional means from other cellular compartments. Using a density gradient electrophoresis (DGE) apparatus (7 cm, x 2.2 cm), mammalian subcellular organelles were separated from a total postnuclear supernatant. The sialic acid-binding lectin wheat germ agglutinin (WGA) permitted 1.5-fold electrophoretic retardation of plasma membranes lagging far behind endoplasmic reticulum, endosomes, Golgi and lysosomes (in order of increasing electrophoretic mobility). Mobilities of the latter organelles were not affected by wheat germ agglutinin. The retarded plasma membrane was monitored by surface iodination, the presence of Ca(++)- and Na+/K(+)-ATPases and by the presence of clathrin-coated pits using Western immunoblotting. In the presence of WGA two clathrin-containing compartments were detected; in the absence of WGA three clathrin populations were seen in the electropherogram: clathrin-coated vesicles, clathrin-coated pits (on the PM) and clathrin-coated structures on the trans-Golgi network (TGN). Both in the presence and absence of WGA, plasma membrane domains of different electrophoretic mobilities were detected.

Subcellular fractionation, electromigration analysis and mapping of organelles

Subcellular fractionation has provided the means required to analyze the composition and properties of purified cellular elements. In particular, subcellular fractionation has helped to define membrane boundaries and became necessary for the development of cell-free assays that reconstitute complicated cellular processes. Although cell fractionation techniques have improved over the last decades the purification of organelles to homogeneity is still a barely accessible goal in cell biology. In this article, we will first briefly review the basic principles of subcellular fractionation, and the establishment of different organelle fractions by density centrifugation, using tissue culture cells as a paradigm. Then we will discuss some of the intrinsic problems and will compare gradient purification of cellular extracts with electromigration analysis. Finally, we will describe alternative approaches, such as immunoisolation and flow cytometry to purify organelles from tissue culture cells.

Endogenous and exogenous forms of the same antigen are processed from different pools to bind MHC class II molecules in endocytic compartments

The current studies were carried out to examine the basis for the differences in the antigenic peptides generated from exogenous and endogenous forms of hen egg white lysozyme (HEL). The role of different intracellular compartments in the generation and binding of HEL peptides derived from two endogenous forms of HEL, either secreted (sHEL) or retained in the endoplasmic reticulum (ER, KDELHEL), presented by MHC class II molecules was examined and compared to exogenous HEL. Initially it was found that antigen-presenting cells bearing both intracellular forms of HEL generated and presented a number of IAk-restricted HEL epitopes to T cell hybridomas, although sHEL was processed more efficiently than KDEL-HEL. There were differences, however, for some determinants between endogenous and exogenous HEL. At equivalent antigen-presenting efficiencies, endogenous HEL-bearing cells displayed a lower surface density of IAk-bound HEL-52-61-related peptides than cells pulsed with exogenous HEL, as detected by a specific monoclonal antibody. Neither endogenous HEL degradation nor peptide binding to MHC class II molecules occurred in the ER. Processing of sHEL and KDELHEL appears to take place either in a post-trans-Golgi network acidic compartment or in the cytosol, whereas peptide binding to MHC class II molecules occurs in endocytic compartments. Furthermore, the peptides generated were derived from an endogenous source rather than from secreted and re-endocytosed HEL. Thus, processing of endogenous HEL is from a different pool than exogenous HEL and occurs in different compartments.

Cutting Edge: Signals from the B Lymphocyte Antigen Receptor Regulate MHC Class II Containing Late Endosomes

The B lymphocyte response to protein Ag is dependent upon the successful presentation to T cells of Ag-derived, MHC class II-restricted peptides. The B cell Ag receptor (BCR) facilitates this process by internalizing ligand and delivering it to specialized compartment(s) (MHC class II peptide-loading compartments (MIIC)) where it is processed into peptides and loaded onto MHC class II. In addition to efficiently targeting Ag, the BCR can provide tyrosine kinase-dependent signals that augment the presentation of Ag, possibly by enhancing the generation of immunogenic peptides. However, the mechanism by which this occurs is unclear. Herein, we report that the BCR signals a reorganization, fusion, and acidification of an MIIC-like compartment into an invariant chain- and MHC class II-rich complex of large vesicles. This complex becomes the primary target for endocytosed receptors. These data suggest that signals generated by the BCR regulate the site of Ag processing.

Subcellular fractionation by organelle electrophoresis: separation of phagosomes containing heat-killed yeast particles

Uptake of foreign material and its subsequent lysosomal degradation is an important function of macrophages. The mechanisms involved in the binding, uptake and delivery of such material to lysosomal organelles, are, however, poorly understood. Here we describe a method using organelle electrophoresis to study the uptake and trafficking of heat-killed yeast particles within murine macrophages. Such yeast particles, which were fluorescently labeled, could be readily detected in intact cells as well as in subcellular fractions. Organelle electrophoresis of a homogenate from macrophages that had internalized yeast particles resulted in the separation of yeast-containing organelles from most other subcellular membranes. In addition, this method was used to follow the kinetics of yeast particle transport within macrophages could be followed readily. Organelle electrophoresis may be a valuable tool for the analysis of phagocytosis.

Exploring the mechanisms of antigen processing by cell fractionation

It is becoming increasingly clear that most of the intracellular compartments that contain MHC class II products in antigen-presenting cells simply represent the conventional endosomes and lysosomes that are expressed in all cell types. Data from recent cell fractionation studies, however, predominantly those using electrophoresis techniques, show that a population of class-II-containing vesicles exists that may comprise a class of endosomes that are specialized for antigen processing. Strong support for this possibility comes from the observation that such specialized structures, designated class II vesicles (CIIV), are particularly abundant in mature dendritic cells.