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

Expression of Class II Human Leukocyte Antigens on Human Endothelial Cells Shows High Interindividual and Intersubclass Heterogeneity

SIGNIFICANCE STATEMENT

Donor-specific antibodies against class II HLA are a major cause of chronic kidney graft rejection. Nonetheless, some patients presenting with these antibodies remain in stable histological and clinical condition. This study describes the use of endothelial colony-forming cell lines to test the hypothesis of the heterogeneous expression of HLA molecules on endothelial cells in humans. Flow cytometry and immunofluorescence staining revealed substantial interindividual and interlocus variability, with HLA-DQ the most variable. Our data suggest that the expression of HLA class II is predicted by locus. The measurement of endothelial expression of HLA class II in the graft could present a novel paradigm in the evaluation of the alloimmune risk in transplantation and certain diseases.

BACKGROUND

HLA antigens are important targets of alloantibodies and allospecific T cells involved in graft rejection. Compared with research into understanding alloantibody development, little is known about the variability in expression of their ligands on endothelial cells. We hypothesized individual variability in the expression of HLA molecules.

METHODS

We generated endothelial colony forming cell lines from human peripheral blood mononuclear cells ( n =39). Flow cytometry and immunofluorescence staining were used to analyze the cells, and we assessed the relationship between HLA-DQ expression and genotype. Two cohorts of kidney transplant recipients were analyzed to correlate HLA-DQ mismatches with the extent of intragraft microvascular injury.

RESULTS

Large variability was observed in the expression of HLA class II antigens, not only between individuals but also between subclasses. In particular, HLA-DQ antigens had a low and heterogeneous expression, ranging from 0% to 85% positive cells. On a within-patient basis, this expression was consistent between endothelial cell colonies and antigen-presenting cells. HLA-DQ5 and -DQ6 were associated with higher levels of expression, whereas HLA-DQ7, -DQ8, and -DQ9 with lower. HLA-DQ5 mismatches among kidney transplant recipients were associated with significant increase in graft microvascular.

CONCLUSION

These data challenge the current paradigm that HLA antigens, in particular HLA class II, are a single genetic and post-translational entity. Understanding and assessing the variability in the expression of HLA antigens could have clinical monitoring and treatment applications in transplantation, autoimmune diseases, and oncology.

Alphaherpesvirus gB Homologs Are Targeted to Extracellular Vesicles, but They Differentially Affect MHC Class II Molecules

Herpesvirus envelope glycoprotein B (gB) is one of the best-documented extracellular vesicle (EVs)-incorporated viral proteins. Regarding the sequence and structure conservation between gB homologs, we asked whether bovine herpesvirus-1 (BoHV-1) and pseudorabies virus (PRV)-encoded gB share the property of herpes simplex-1 (HSV-1) gB to be trafficked to EVs and affect major histocompatibility complex (MHC) class II. Our data highlight some conserved and differential features of the three gBs. We demonstrate that mature, fully processed BoHV-1 and PRV gBs localize to EVs isolated from constructed stable cell lines and EVs-enriched fractions from virus-infected cells. gB also shares the ability to co-localize with CD63 and MHC II in late endosomes. However, we report here a differential effect of the HSV-1, BoHV-1, and PRV glycoprotein on the surface MHC II levels, and MHC II loading to EVs in stable cell lines, which may result from their adverse ability to bind HLA-DR, with PRV gB being the most divergent. BoHV-1 and HSV-1 gB could retard HLA-DR exports to the plasma membrane. Our results confirm that the differential effect of gB on MHC II may require various mechanisms, either dependent on its complex formation or on inducing general alterations to the vesicular transport. EVs from virus-infected cells also contained other viral glycoproteins, like gD or gE, and they were enriched in MHC II. As shown for BoHV-1 gB- or BoHV-1-infected cell-derived vesicles, those EVs could bind anti-virus antibodies in ELISA, which supports the immunoregulatory potential of alphaherpesvirus gB.

Assembly, Intracellular Transport, and Release of MHC Class II Peptide Receptors

MHC class II molecules play a pivotal role for the induction and maintenance of immune responses against pathogens, but are also implicated in pathological conditions like autoimmune diseases or rejection of transplanted organs. Human antigen-presenting cells express three human leukocyte antigen (HLA) class II isotypes (DR, DP, and DQ), which are, with the exception of DRα, composed of highly polymorphic α and β subunits. The combination of α- and β-chains results in a multitude of MHC-II αβ-heterodimers of the same isotype, but also isotype-mixed MHC class II molecules have been identified. Invariant chain chaperones the assembly of MHC-II molecules within the endoplasmatic reticulum and also facilitates the intracellular transport to MHC class II loading compartments (MIICs). MHC-II molecules are loaded with antigenic peptides and shuttled to the cell surface for inspection by CD4 T-cells. Alternatively, class-II molecules enriched on intraluminal vesicles can be released via exosomes into the extracellular space. Since some of the αβ-combinations may yield mismatched nonfunctional heterodimers, it is not entirely clear which type of HLA class II peptide receptors are transported to MIICs and found on the cell surface of antigen-presenting cells. We present techniques to inspect assembly, intracellular transport, cell surface expression, and exosomal release of MHC class II heterodimers.

Salmonella polarises peptide-MHC-II presentation towards an unconventional Type B CD4+ T-cell response

Distinct peptide-MHC-II complexes, recognised by Type A and B CD4(+) T-cell subsets, are generated when antigen is loaded in different intracellular compartments. Conventional Type A T cells recognize their peptide epitope regardless of the route of processing, whereas unconventional Type B T cells only recognise exogenously supplied peptide. Type B T cells are implicated in autoimmune conditions and may break tolerance by escaping negative selection. Here we show that Salmonella differentially influences presentation of antigen to Type A and B T cells. Infection of bone marrow-derived dendritic cells (BMDCs) with Salmonella enterica serovar Typhimurium (S. Typhimurium) reduced presentation of antigen to Type A T cells but enhanced presentation of exogenous peptide to Type B T cells. Exposure to S. Typhimurium was sufficient to enhance Type B T-cell activation. Salmonella Typhimurium infection reduced surface expression of MHC-II, by an invariant chain-independent trafficking mechanism, resulting in accumulation of MHC-II in multi-vesicular bodies. Reduced MHC-II surface expression in S. Typhimurium-infected BMDCs correlated with reduced antigen presentation to Type A T cells. Salmonella infection is implicated in reactive arthritis. Therefore, polarisation of antigen presentation towards a Type B response by Salmonella may be a predisposing factor in autoimmune conditions such as reactive arthritis.

Assembly of matched alpha/beta subunits to HLA class II peptide receptors

Human antigen presenting cells express three human leukocyte antigen (HLA) class II isotypes (DR, DP, and DQ), which are composed of polymorphic α and β subunits. The combination of polymorphic α- and β-chains results in cis (encoded on the same chromosome) or trans (encoded on different chromosomes) combinations. Since some of the α-β combinations may yield mismatched non-functional α-β heterodimers, it is not entirely clear which type of HLA class II peptide receptors are found on the cell surface of antigen presenting cells. We have developed a combination of biochemical techniques for inspection of the assembly and intracellular transport of isotype matched and mismatched class II heterodimers.

Dynamic modification of microtubule-dependent transport by effector proteins of intracellular Salmonella enterica

Intracellular Salmonella enterica translocate effector proteins that modify microtubule-dependent transport processes of the host cell and modulate the biogenesis of the Salmonella-containing vacuole (SCV). One functional consequence is the induction of tubular aggregates of endosomal membranes, termed Salmonella-induced filaments or SIFs, and further tubular membrane compartments have recently been described. SIFs are unique, highly dynamic compartments that form by modification of vesicular transport on microtubules. The molecular mechanism of the interference of intracellular Salmonella with host cell vesicular transport is still elusive, but recent studies demonstrate the complexity of pathogenic activities and the intricacy of manipulating host cell functions.

Regulation of CD1 antigen-presenting complex stability

For major histocompatibility complex class I and II molecules, the binding of specific peptide antigens is essential for assembly and trafficking and is at the center of their quality control mechanism. However, the role of lipid antigen binding in stabilization and quality control of CD1 heavy chain (HC)·β₂-microglobulin (β₂m) complexes is unclear. Furthermore, the distinct trafficking and loading routes of CD1 proteins take them from mildly acidic pH in early endososmal compartments (pH 6.0) to markedly acidic pH in lysosomes (pH 5.0) and back to neutral pH of the cell surface (pH 7.4). Here, we present evidence that the stability of each CD1 HC·β₂m complex is determined by the distinct pH optima identical to that of the intracellular compartments in which each CD1 isoform resides. Although stable at acidic endosomal pH, complexes are only stable at cell surface pH 7.4 when bound to specific lipid antigens. The proposed model outlines a quality control program that allows lipid exchange at low endosomal pH without dissociation of the CD1 HC·β₂m complex and then stabilizes the antigen-loaded complex at neutral pH at the cell surface.

Large Propeptides of Fungal β-N-Acetylhexosaminidases Are Novel Enzyme Regulators That Must Be Intracellularly Processed to Control Activity, Dimerization, and Secretion into the Extracellular Environment

Filamentous fungi produce and secrete beta-N-acetylhexosaminidases, Hex, as important components of the binary chitinolytic systems involved in the formation of septa and hyphenation. Enzyme reconstitution experiments published previously indicate that Hex can occur in the form of two molecular species containing either one or two molecules of the propeptide noncovalently associated with the enzyme dimer. Here, we describe a novel mechanism for the regulation of the activity of Hex based on the association of their catalytic subunits with the large N-terminal propeptides in vivo. We show that the enzyme precursor is processed early in the biosynthesis, shortly after the addition of N-glycans through the action of a dibasic peptidase, cleaving both before and after the dibasic sequence. The processing site for this unique dibasic peptidase, different from that of kexins, is conserved among the beta-N-acetylhexosaminidases from filamentous fungi, and inhibition of the dibasic peptidase abrogates enzyme folding and activation. Binding of the released propeptide to the catalytic subunit of Hex is essential for its activation. An examination of the kinetics of Hex activation and dimerization in vitro allowed us to understand the unusually high efficiency of the assembly of this enzyme. We also report that the fungus is able to actively regulate the concentration of the processed propeptide in endoplasmic reticulum and thus the specific activity of the produced Hex. This novel regulatory mechanism enables the control of the catalytic activity and architecture of the secreted enzyme according to the needs of the producing cell at various stages of its growth cycle.

Human cytomegalovirus alters localization of MHC class II and dendrite morphology in mature Langerhans cells

Hemopoietic stem cell-derived mature Langerhans-type dendritic cells (LC) are susceptible to productive infection by human CMV (HCMV). To investigate the impact of infection on this cell type, we examined HLA-DR biosynthesis and trafficking in mature LC cultures exposed to HCMV. We found decreased surface HLA-DR levels in viral Ag-positive as well as in Ag-negative mature LC. Inhibition of HLA-DR was independent of expression of unique short US2-US11 region gene products by HCMV. Indeed, exposure to UV-inactivated virus, but not to conditioned medium from infected cells, was sufficient to reduce HLA-DR on mature LC, implicating particle binding/penetration in this effect. Reduced surface levels reflected an altered distribution of HLA-DR because total cellular HLA-DR was not diminished. Accumulation of HLA-DR was not explained by altered cathepsin S activity. Mature, peptide-loaded HLA-DR molecules were retained within cells, as assessed by the proportion of SDS-stable HLA-DR dimers. A block in egress was implicated, as endocytosis of surface HLA-DR was not increased. Immunofluorescence microscopy corroborated the intracellular retention of HLA-DR and revealed markedly fewer HLA-DR-positive dendritic projections in infected mature LC. Unexpectedly, light microscopic analyses showed a dramatic loss of the dendrites themselves and immunofluorescence revealed that cytoskeletal elements crucial for the formation and maintenance of dendrites are disrupted in viral Ag-positive cells. Consistent with these dendrite effects, HCMV-infected mature LC exhibit markedly reduced chemotaxis in response to lymphoid chemokines. Thus, HCMV impedes MHC class II molecule trafficking, dendritic projections, and migration of mature LC. These changes likely contribute to the reduced activation of CD4+ T cells by HCMV-infected mature LC.

Chaperoning antigen presentation by MHC class II molecules and their role in oncogenesis

Tumor vaccine development aimed at stimulating the cellular immune response focuses mainly on MHC class I molecules. This is not surprising since most tumors do not express MHC class II or CD1 molecules. Nevertheless, the most successful targets for cancer immunotherapy, leukemia and melanoma, often do express MHC class II molecules, which leaves no obvious reason to ignore MHC class II molecules as a mediator in anticancer immune therapy. We review the current state of knowledge on the process of MHC class II-restricted antigen presentation and subsequently discuss the consequences of MHC class II expression on tumor surveillance and the induction of an efficient MHC class II mediated antitumor response in vivo and after vaccination.

Genetic approaches for the induction of a CD4+ T cell response in cancer immunotherapy

Recently, it has become more and more obvious that not only CD8+ cytotoxic T lymphocytes, but also CD4+ T helper cells are required for the induction of an optimal, long-lasting anti-tumor immune response. CD4+ T helper cells, and in particular IFN-gamma-secreting type 1 T helper cells, have been shown to fulfill a critical function in the mounting of a cancer-specific response. Consequently, targeting antigens into MHC class II molecules would greatly enhance the efficacy of an anti-cancer vaccine. The dissection of the MHC class II presentation pathway has paved the way for rational approaches to achieve this goal: novel systems have been developed to genetically manipulate the MHC class II presentation pathway. First, different genetic approaches have been used for the delivery of known epitopes into the MHC class II processing pathway or directly onto the peptide-binding groove of the MHC molecules. Second, several strategies exist for the targeting of whole tumor antigens, containing both MHC class I and class II restricted epitopes, to the MHC class II processing pathway. We review these data and describe how this knowledge is currently applied in vaccine development.

Inhibitory p41 isoform of invariant chain and its potential target enzymes cathepsins L and H in distinct populations of macrophages in human lymph nodes

Activation of the CD4(+) T-cell mediated immune response relies on the proteolytic capacity of enzymes involved in modulating major histocompatibility complex (MHC) II-associated antigen presentation in antigen-presenting cells (APC). The MHC II-associated chaperone molecule p41 isoform of invariant chain (inhibitory p41 Ii) has been suggested to regulate stability and activity of cathepsin L in these APC. In the present study the human lymph node distribution of non-inhibitory p31 Ii and inhibitory p41 Ii have been compared by differential labelling, using two specific monoclonal antibodies. The distribution of p41 Ii, but not p31 Ii, matched the distribution of cathepsins L and H in subcapsular and cortical sinuses and germinal centres. Co-localization of p41 Ii with cathepsin H was confirmed in strongly CD68(+) sinus-lining macrophages, acting as APC. Furthermore, p41 Ii was determined together with cathepsins L and H in tingible body macrophages, highly phagocytic, but not antigen-presenting cells inside germinal centres. With respect to the physiological function that these two populations of macrophages have in human lymph nodes, our results support a regulatory function of p41 Ii towards cathepsins L and H in human macrophages, associated with the processes of phagocytosis rather than antigen presentation.

Selective isolation and identification of HLA-DR-associated naturally processed and presented epitope peptides

Activation of CD4 helper T-cells is mediated by the presentation of antigenic peptides in context of self-MHC class II molecules. So far, the rules after which antigen-presenting cells (APC) select a particular epitope within a given protein antigen have been not fully elucidated. Nevertheless, immunoaffinity purification of APC-derived MHC class II molecules and the subsequent elutions of their with associated naturally processed and presented peptide epitopes (NPPE) have helped tremendously in understanding the nature of this rather complex process. In the present study, a novel approach for identifying such NPPEs is introduced, which is based on the culture of APCs in a completely protein-free medium during the antigen presenting process. These APCs do still express a high level of MHC class II as determined by HLA-DR cell surface staining, but the repertoire of the associated NPPEs is drastically reduced when compared to peptides eluted from cells maintained under normal culture condition. Actually, reverse phase-high pressure liquid chromatography (RP-HPLC) revealed that the entire NPPE repertoire consisted of less than ten major peaks, which is more than a 100-fold reduction of background peptide peaks as seen in cells from serum-containing culture conditions. Feeding APCs with exogenous antigens further confirmed the advantage of this novel system. While exogenous antigen-derived peptide peaks in an NPPE-eluate from RP-HPLC are hardly to detect by conventional procedures, the very low background of serum- and protein-free cultured APCs immensely facilitated this process, providing an improved tool for the identification and characterization of NPPEs.

MHC class II invariant chain homologues in rainbow trout (Oncorhynchus mykiss)

The MHC class II invariant chain (Ii or CD74) in higher vertebrates is necessary for normal MHC class II loading in endosomal compartments. Detection of an Ii chain in fish would greatly support the idea that MHC class II function in fish and higher vertebrates is similar. Before this study only Ii homologues had been reported in fish that are unlikely to perform true Ii function. In the present study two Ii-like genes, Onmy-Iclp-1 and Onmy-Iclp-2, were detected in rainbow trout. Conservation of elements, particularly in Onmy-Iclp-1, suggests that the encoded proteins may be involved in MHC class II transport and peptide loading as is the Ii protein. The expression pattern of both rainbow trout genes was similar to that of the MHC class II beta chain, with strong expression in the lymphoid tissues, gills and intestine. Analysis of separated peripheral blood leucocyte fractions indicated that expression of Onmy-Iclp-1, Onmy-Iclp-2 and the MHC class II beta chain were all highest in B lymphocytes. This agrees with the expectation that the functions of the products of the new genes are closely associated with MHC class II. It is interesting why in rainbow trout there are two proteins that may function similar to Ii in higher vertebrates.

Proteinases and their inhibitors in the immune system

The most important roles of proteinases in the immune system are found in apoptosis and major histocompatibility complex (MHC) class II-mediated antigen presentation. A variety of cysteine proteinases, serine proteinases, and aspartic proteinases as well as their inhibitors are involved in the regulation of apoptosis in neutrophils, monocytes, and dendritic cells, in selection of specific B and T lymphocytes, and in killing of target cells by cytotoxic T cells and natural killer cells. In antigen presentation, endocytosed antigens are digested into antigenic peptides by both aspartic and cysteine proteinases. In parallel, MHC class II molecules are processed by aspartic and cysteine proteinases to degrade the invariant chain that occupies the peptide-binding site. Proteinase activity in these processes is highly regulated, particularly by posttranslational activation and the balance between active proteinases and specific endogenous inhibitors such as cystatins, thyropins, and serpins. This article discusses the regulation of proteolytic processes in apoptosis and antigen presentation in immune cells and the consequences of therapeutic interference in the balance of proteinases and their inhibitors.

The complexity of HLA class II (DRB1, DQB1, DM) associations with disseminated Mycobacterium avium complex infection among HIV-1-seropositive whites

Earlier associations of polymorphism in classic HLA class II (DRB1 and DQB1) genes have been extended to include the accessory genes DMA and DMB as determinants of disseminated Mycobacterium avium complex (DMAC) infection among HIV-1-seropositive whites. From the Multicenter AIDS Cohort study, 176 DMAC cases were matched with 176 controls in a nested case-control study. PCR-based HLA genotyping techniques were used to resolve variants of DRB1 and DQB1 to their four-digit or five-digit alleles, and single-strand conformation polymorphism was used to resolve sequences in exon 3 at each DM locus. The DMA*0102 allele occurred less frequently among DMAC cases than among controls (OR = 0.46, p =.02). Combinations of DRB1 alleles with or without specific DMA and DMB variants showed significant differences in distributions between the cases and controls, but both of the previously associated class II alleles (DRB1*1501 and DRB1*0701) showed stronger positive associations with DMAC in the absence than in the presence of DMA*0102. Apparent joint effects of DRB1 and DM allelic combinations on occurrence and timing of DMAC suggest that class II disease relationships may be better predicted by biologically plausible interactive combinations than by polymorphisms in individual genes.

Role of interleukin-10 in the intracellular sequestration of human leukocyte antigen-DR in monocytes during septic shock

Monocytes from many critically ill patients show a low level of major histocompatibility complex type II (MHC II) expression. This phenomenon is believed to play a role in these patients' increased susceptibility to secondary infections. In the present study, we show that the level of monocyte human leukocyte antigen (HLA)-DR expression inversely correlates with the degree of severity of the sepsis syndrome. The defect of the monocyte HLA-DR expression resides in an intracellular sequestration of the MHC II molecules, a posttranslational effect. No significant decrease in the rate of transcription of HLA-DR, or its major transcriptional inducer, Class II transactivator, was noted. The levels of HLA-DR protein produced by monocytes from patients with septic shock were comparable to those from healthy volunteers. Plasma from patients with septic shock induced significant HLA-DR endocytosis resulting in decreased surface HLA-DR expression of normal donor monocytes. This effect was partially blocked by anti-interleukin (IL)-10 monoclonal antibody, but not by antagonists to transforming growth factor-beta1, prostaglandins, or beta-adrenergic agonists. Altogether, these data suggest that HLA-DR molecules are re-endocytosed and retained intracellularly in monocytes from patients with septic shock, and that this phenomenon is partially mediated by IL-10. IL-10 may represent a future target for immunomodulating patients with the sepsis syndrome or critically ill patients at risk of developing infections.

Human cytomegalovirus disrupts constitutive MHC class II expression

CD8(+) and CD4(+) T lymphocytes are important in controlling human CMV (HCMV) infection, but the virus has evolved protean mechanisms to inhibit MHC-based Ag presentation and escape T lymphocyte immunosurveillance. Herein, the interaction of HCMV with the MHC class II Ag presentation pathway was investigated in cells stably transfected with class II transactivator. Flow cytometry experiments demonstrate that HCMV infection decreases cell-surface MHC class II expression. HCMV down-regulates MHC class II surface expression without a significant effect on class II RNA or steady-state protein levels. SDS-stability and confocal microscopy experiments demonstrate normal levels of steady-state peptide-loaded class II molecules in infected cells and that class II molecules reach late endosomal and HLA-DM positive peptide-loading compartments. However, MHC class II positive vesicles are retained in an abnormal perinuclear distribution. Finally, experiments with a mutant HCMV strain demonstrate that this novel mechanism of decreased MHC class II expression is not mediated by one of the known HCMV immunomodulatory genes. These defects in MHC class II expression combined with previously identified CMV strategies for decreasing MHC class I expression enables infected cells to evade T lymphocyte immunosurveillance.

Specific immunotherapy of cancer in elderly patients

The concept of immunotherapy of cancer is more than a century old, but only recently have molecularly defined therapeutic approaches been developed. In this review, we focus on the most promising approach, active therapeutic vaccination. The identification of tumour antigens can now be accelerated by methods allowing the amplification of gene products selectively or preferentially transcribed in the tumour. However, determining the potential immunogenicity of such gene products remains a demanding task, since major histocompatibility complex (MHC) restriction of T cells implies that for any newly defined antigen, immunogenicity will have to be defined for any individual MHC haplotype. Tumour-derived peptides eluted from MHC molecules of tumour tissue are also a promising source of antigen. Tumour antigens are mostly of weak immunogenicity, because the vast majority are tumour-associated differentiation antigens already 'seen' by the patient's immune system. Effective therapeutic vaccination will thus require adjuvant support, possibly by new approaches to immunomodulation such as bispecific antibodies or antibody-cytokine fusion proteins. Tumour-specific antigens, which could be a more potent target for immunotherapy, mostly arise by point mutations and have the disadvantage of being not only tumour-specific, but also individual-specific. Therapeutic vaccination will probably focus on defined antigens offered as protein, peptide or nucleic acid. Irrespective of the form in which the antigen is applied, emphasis will be given to the activation of dendritic cells as professional antigen presenters. Dendritic cells may be loaded in vitro with antigen, or, alternatively, initiation of an immune response may be approached in vivo by vaccination with RNA or DNA, given as such or packed into attenuated bacteria. The importance of activation of T helper cells has only recently been taken into account in cancer vaccination. Activation of cytotoxic T cells is facilitated by the provision of T helper cell-derived cytokines. T helper cell-dependent recruitment of elements of non-adaptive defence, such as leucocytes, natural killer cells and monocytes, is of particular importance when the tumour has lost MHC class I expression. Barriers to successful therapeutic vaccination include: (i) the escape mechanisms developed by tumour cells in response to immune attack; (ii) tolerance or anergy of the evoked immune response; (iii) the theoretical possibility of provoking an autoimmune reaction by vaccination against tumour-associated antigens; and (iv) the advanced age of many patients, implying reduced responsiveness of the senescent immune system.

The immunological synapse

The adaptive immune response is initiated by the interaction of T cell antigen receptors with major histocompatibility complex molecule-peptide complexes in the nanometer scale gap between a T cell and an antigen-presenting cell, referred to as an immunological synapse. In this review we focus on the concept of immunological synapse formation as it relates to membrane structure, T cell polarity, signaling pathways, and the antigen-presenting cell. Membrane domains provide an organizational principle for compartmentalization within the immunological synapse. T cell polarization by chemokines increases T cell sensitivity to antigen. The current model is that signaling and formation of the immunological synapse are tightly interwoven in mature T cells. We also extend this model to natural killer cell activation, where the inhibitory NK synapse provides a striking example in which inhibition of signaling leaves the synapse in its nascent, inverted state. The APC may also play an active role in immunological synapse formation, particularly for activation of naïve T cells.

Immunocompetent astrocytes and microglia display major differences in the processing of the invariant chain and in the expression of active cathepsin L and cathepsin S

The role of astrocytes and microglia as antigen-presenting cells in the brain is still controversial. In this study we have analyzed and compared aspects of the molecular machinery that underlies MHC class II trafficking in immunocompetent astrocytes and microglia. We show that IFN-gamma-stimulated microglia possess active cathepsin L and cathepsin S, and efficiently degrade the invariant chain, unlike IFN-gamma-stimulated astrocytes that express cathepsin L but not cathepsin S. The lack of cathepsin S proves to be dramatic for the antigen-presentation capacity of astrocytes, which is nearly abolished when these cells are stimulated by a combination of IFN-gamma and TNF-alpha. TNF-alpha indeed decreases cathepsin L activity as we show here, leading to alterations in invariant chain processing, and hence in MHC class II trafficking in astrocytes. Cystatin C inhibits cathepsin L activity in astrocytes, but does not regulate cathepsin L and cathepsin S activity in microglia. We therefore identify cathepsin L and cathepsin S as key components in the regulation of the immune potential of astrocytes and microglia, and provide evidence for a cell-specific regulation exerted by IFN-gamma and TNF-alpha on the expression and activity of cathepsins.

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.

Expression of invariant chain and pro-cathepsin L in Alzheimer's brain

Inflammatory and immune systems are involved in the pathogenesis of Alzheimer's disease (AD), but those systems in the human brain have not been well identified. Cathepsin L might play a predominant role in the degradation of the invariant chain (Ii), which plays a critical role in antigen presentation to block the antigen-binding site of the major histocompatibility complex class II. We examined the expression of Ii and pro-cathepsin L (pCPL) in AD and normal brains by using immunohistochemistry. Ii expresses only in resting or mildly activated microglia, whereas pCPL strongly expresses in fully activated microglia but not in resting or mildly activated microglia in AD. Normal brain tissues have rarely been stained for Ii or pCPL. These results suggest that the activation of microglia leads to expression of a complex of Ii and human leukocyte antigen class II at first, and that further activation, which is followed by cluster formation and enlargement of microglia frequently seen in the AD brain, might cause pCPL expression to degrade Ii. Our study confirmed that microglia plays a central role in the immune system of the brain, and that an activation of microglia is involved in the pathogenesis of AD.