Cytokines regulate proteolysis in major histocompatibility complex class II-dependent antigen presentation by dendritic cells

Cathepsin G, and Not the Asparagine-Specific Endoprotease, Controls the Processing of Myelin Basic Protein in Lysosomes from Human B Lymphocytes

The asparagine-specific endoprotease (AEP) controls lysosomal processing of the potential autoantigen myelin basic protein (MBP) by human B lymphoblastoid cells, a feature implicated in the immunopathogenesis of multiple sclerosis. In this study, we demonstrate that freshly isolated human B lymphocytes lack significant AEP activity and that cleavage by AEP is dispensable for proteolytic processing of MBP in this type of cell. Instead, cathepsin (Cat) G, a serine protease that is not endogenously synthesized by B lymphocytes, is internalized from the plasma membrane and present in lysosomes from human B cells where it represents a major functional constituent of the proteolytic machinery. CatG initialized and dominated the destruction of intact MBP by B cell-derived lysosomal extracts, degrading the immunodominant MBP epitope and eliminating both its binding to MHC class II and a MBP-specific T cell response. Degradation of intact MBP by CatG was not restricted to a lysosomal environment, but was also performed by soluble CatG. Thus, the abundant protease CatG might participate in eliminating the immunodominant determinant of MBP. Internalization of exogenous CatG represents a novel mechanism of professional APC to acquire functionally dominant proteolytic activity that complements the panel of endogenous lysosomal enzymes.

Membrane specializations and endosome maturation in dendritic cells and B cells

Interest in the cell biology of antigen presentation is centered on dendritic cells (DCs) as initiators of the immune response. The ability to examine primary antigen-presenting cells, as opposed to cell lines, has opened a new window for study of antigen processing and peptide acquisition by Class II major histocompatibility complex (MHC) products, especially where intracellular trafficking of peptide-Class-II complexes is concerned. Here, we review the dynamics of Class II MHC-positive intracellular structures in dendritic cells as well as B cells. We focus on the generation of multivesicular bodies, where Class II MHC products acquire antigenic peptide, on the endosomal transport of peptide-loaded Class II MHC to the cell surface and on the importance of Class II MHC localization in membrane microdomains.

Disulfide bonds in merozoite surface protein 1 of the malaria parasite impede efficient antigen processing and affect thein vivoantibody response

The 19 kDa C-terminal fragment of the malaria parasite merozoite surface protein 1 (MSP1(19)) is a leading malaria vaccine candidate. In rodents, high antibody levels to this protein confer protective immunity, and can be generated by immunization with the antigen in adjuvants. In natural human infections, however, MSP1(19)-specific antibody responses can be short-lived and comparatively low, despite repeated exposure to infection. The tightly folded structure of MSP1(19) is stabilized by five or six disulfide bonds. These bonds impede antigen processing and, thereby, may affect the generation of CD4+ T cells providing help for B cells. Asparagine endopeptidase could digest unfolded, but not native MSP1(19) in vitro. Immunization with unfolded MSP1(19) resulted in a faster antibody response, and a combination of unfolded and native MSP1(19) increased antibody responses to the native form. Immunization with either form of the antigen activated similar numbers of CD4+ T cells, but, unlike the antibody response, CD4+ T cells immunized with one form of MSP119 were able to respond in vitro to the other form of the protein. Although the reduced form of MSP1(19) does not induce protective antibodies, our data suggest that inclusion of unfolded protein may improve the efficacy of MSP1(19) as a vaccine.

Human B lymphoblastoid cells contain distinct patterns of cathepsin activity in endocytic compartments and regulate MHC class II transport in a cathepsin S-independent manner

Endocytic proteolysis represents a major functional component of the major histocompatibility complex class II antigen-presentation machinery. Although transport and assembly of class II molecules in the endocytic compartment are well characterized, we lack information about the pattern of endocytic protease activity along this pathway. Here, we used chemical tools that visualize endocytic proteases in an activity-dependent manner in combination with subcellular fractionation to dissect the subcellular distribution of the major cathepsins (Cat) CatS, CatB, CatH, CatD, CatC, and CatZ as well as the asparagine-specific endoprotease (AEP) in human B-lymphoblastoid cells (BLC). Endocytic proteases were distributed in two distinct patterns: CatB and CatZ were most prominent in early and late endosomes but absent from lysosomes, and CatH, CatS, CatD, CatC, and AEP distributed between late endosomes and lysosomes, suggesting that CatB and CatZ might be involved in the initial proteolytic attack on a given antigen. The entire spectrum of protease activity colocalized with human leukocyte antigen-DM and the C-terminal and N-terminal processing of invariant chain (Ii) in late endosomes. CatS was active in all endocytic compartments. Surprisingly and in contrast with results from dendritic cells, inhibition of CatS activity by leucine-homophenylalanine-vinylsulfone-phenol prevented N-terminal processing of Ii but did not alter the subcellular trafficking or surface delivery of class II complexes, as deferred from pulse-chase analysis in combination with subcellular fractionation and biotinylation of cell-surface protein. Thus, BLC contain distinct activity patterns of proteases in endocytic compartments and regulate the intracellular transport and surface-delivery of class II in a CatS-independent manner.

Dissection of the dislocation pathway for type I membrane proteins with a new small molecule inhibitor, eeyarestatin

The mammalian endoplasmic reticulum (ER)-to-cytosol degradation pathway for disposal of misfolded proteins is an attractive target for therapeutic intervention in diseases that are characterized by impaired protein degradation. The ability to do so is hampered by the small number of specific inhibitors available and by our limited understanding of the individual steps involved in this pathway. Cells that express a class I major histocompatibility complex (MHC) heavy chain-enhanced green fluorescent protein (EGFP) fusion protein and the human cytomegalovirus protein US11, which catalyzes dislocation of the class I MHC EGFP reporter, show only little fluorescence. Treatment with proteasome inhibitors increases their fluorescence by stabilizing EGFP-tagged MHC class I molecules. We used this change in signal intensity as a readout to screen a chemical library of 16,320 compounds and identified two structurally related compounds (eeyarestatin I and II) that interfered with the degradation of both EGFP-heavy chain and its endogenous unmodified class I MHC heavy chain counterpart. Eeyarestatin I also inhibited degradation of a second misfolded type I membrane protein, T-cell receptor alpha. Both compounds stabilize these dislocation substrates in the ER membrane, without preventing proteasomal turnover of cytosolic substrates. The new inhibitors must therefore interfere with a step that precedes proteasomal degradation. The use of eeyarestatin I thus allows the definition of a new intermediate in dislocation.

Identification of a potent and selective noncovalent cathepsin S inhibitor

Cathepsin S is considered crucial for normal presentation of major histocompatibility complex (MHC) class II-restricted antigens by antigen presenting cells to CD4+ T cells. It is a key enzyme for the degradation of the class II-associated invariant chain, a process that is required for effective antigen loading of class II molecules. Here, we report a selective, orally available, high-affinity cathepsin S inhibitor, 1-[3-[4-(6-Chloro-2,3-dihydro-3-methyl-2-oxo-1H-benzimidazol-1-yl)-1-piperidinyl]propyl]-4,5,6,7-tetrahydro-5-(methylsulfonyl)-3-[4-(trifluoromethyl)phenyl]-1H-pyrazolo[4,3-c]pyridine. (JNJ 10329670), that represents a novel class of immunosuppressive compounds. JNJ 10329670 is a highly potent (Ki of approximately 30 nM), nonpeptidic, noncovalent inhibitor of human cathepsin S, but it is much less active against the mouse, dog, monkey, and bovine enzymes. The compound is inactive against other proteases, including the closely related cathepsins L, F, and K. This selectivity makes JNJ 10329670 an excellent tool for exploring the role of cathepsin S in human systems. Treatment of human B cell lines and primary human dendritic cells with JNJ 10329670 resulted in the accumulation of the p10 fragment of the invariant chain (IC50 of approximately 1 microM). In contrast, inhibition of invariant chain proteolysis was much less effective in a human monocytic cell line, suggesting that other enzymes may degrade the invariant chain in this cell type. JNJ 10329670 was shown to block the proteolysis of the invariant chain in vivo by using immunocompromised mice injected with human peripheral blood mononuclear cells (PBMCs). Furthermore, this inhibitor blocks the presentation of tetanus toxoid and giant ragweed by human PBMCs. The properties of JNJ 10329670 make it a candidate for immunosuppressive therapy of allergies and autoimmune diseases.

Characterization of the MHC class I cross-presentation pathway for cell-associated antigens by human dendritic cells

Major histocompatibility complex (MHC) class I presentation of exogenous antigens is the mechanism enabling professional antigen-presenting cells (APCs) to induce CD8+ T-cell responses against viruses and tumors that do not have access to the classical MHC class I pathway. We have characterized the uptake, processing, and MHC class I cross-presentation by human dendritic cells (DCs) of cell-associated antigens derived from physiologically relevant sources, namely, vaccinia virus-infected apoptotic and necrotic cells. We show that cross-presentation is a rapid process, detectable within 2 to 4 hours after uptake of dead cells, and that proteolysis by cathepsin D in an acidic endosomal compartment is essential for cross-presentation. The presentation is abolished when the phagocytic or macropinocytic functions of the cells are inhibited and is dependent on transporter associated with antigen processing, sensitive to brefeldin A, and requires functional proteasomes. Altogether, these data suggest that antigens derived from apoptotic and necrotic cells require access to the cytosol to intersect with the conventional MHC class I pathway for presentation of cytosolic proteins.

Cathepsin S supports acid-independent infection by some reoviruses

In murine fibroblasts, efficient proteolysis of reovirus outer capsid protein sigma3 during cell entry by virions requires the acid-dependent lysosomal cysteine protease cathepsin L. The importance of cathepsin L for infection of other cell types is unknown. Here we report that the acid-independent lysosomal cysteine protease cathepsin S mediates outer capsid processing in macrophage-like P388D cells. P388D cells supported infection by virions of strain Lang, but not strain c43. Genetic studies revealed that this difference is determined by S4, the viral gene segment that encodes sigma3. c43-derived subvirion particles that lack sigma3 replicated normally in P388D cells, suggesting that the difference in infectivity of Lang and c43 virions is at the level of sigma3 processing. Infection of P388D cells with Lang virions was inhibited by the broad spectrum cysteine protease inhibitor trans-epoxysuccinyl-l-leucylamido-(4-guanidino)butane but not by NH(4)Cl, which raises the endocytic pH and thereby inhibits acid-dependent proteases such as cathepsins L and B. Outer capsid processing and infection of P388D cells with Lang virions were also inhibited by a cathepsin S-specific inhibitor. Furthermore, in the presence of NH(4)Cl, cell lines engineered to express cathepsin S supported infection by Lang, but not c43, virions. Our results thus indicate that differences in susceptibility to cathepsin S-mediated sigma3 processing are responsible for strain differences in reovirus infection of macrophage-like P388D cells and other cathepsin S-expressing cells. Additionally, our data suggest that the acid dependence of reovirus infections of most other cell types may reflect the low pH requirement for the activities of most other lysosomal proteases rather, than some other acid-dependent aspect of cell entry.

Definition of TCR epitopes for CTL-mediated attack of cutaneous T cell lymphoma

Therapeutic vaccination against cutaneous T cell lymphoma (CTCL) requires the characterization of cancer cell-specific CTL epitopes. Despite reported evidence for tumor-reactive cytotoxicity in CTCL patients, the nature of the recognized determinants remains elusive. The clonotypic TCR of CTCL cells is a promising candidate tumor-specific Ag. In this study, we report that the clonotypic and framework regions of the TCRs expressed in the malignant T cell clones of six CTCL patients contain multiple peptides with anchor residues fitting the patients' MHC class I molecules. We demonstrate that TCR peptide-specific T cells from the blood of healthy donors and patients can be induced to become cytotoxic effectors after repeated stimulation with 6 of 11 selected peptides with experimentally proven affinity for HLA-A*0201. Importantly, 4 of these 6 CTL lines reproducibly recognize and lyse autologous primary CTCL cells in MHC class I/CD8-dependent fashion. These tumoricidal CTL lines are directed against epitopes from V, hypervariable, and C regions of TCRalpha. We therefore conclude that recombined as well as V framework regions of the tumor cell TCRs contain predictable epitopes for CTL-mediated attack of CTCL cells. Our data further suggest that such peptides represent valuable tools for future anti-CTCL vaccination approaches.

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

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

Immunologic principles and immunotherapeutic approaches in ovarian cancer

Ovarian cancer is an immunogenic tumor, and numerous antigens have been identified in recent years. Several of these antigens are important in regulating tumor growth and may be ideal targets for the development of immune-based strategies. In the absence of immunologic intervention, tumors evade the immune system by several mechanisms, most notably tolerance and immunosuppression. As understanding of the immune response improves, strategies are being designed to circumvent T-cell tolerance to self-antigens through modulation of APC function. In addition, techniques are being developed to identify reverse ovarian cancer-induced immune evasion tactics. The type of the immune-based therapy to apply varies with disease burden. It is hoped that discoveries at the bench along with lessons learned in prior clinical trials soon will allow clinicians to develop rationally based immunologic strategies to treat and prevent ovarian cancer.

Composition of MHC class II-enriched lipid microdomains is modified during maturation of primary dendritic cells

Dendritic cells (DCs) are the most potent antigen presenting cells. Major histocompatibility complex (MHC) class II molecule expression changes with maturation; immature DCs concentrate MHC class II molecules intracellularly, whereas maturation increases surface expression of MHC class II and costimulatory molecules to optimize antigen presentation. Signal transduction via MHC class II molecules localized in lipid microdomains has been described in B lymphocytes and in the THP-1 monocyte cell line. We have characterized MHC class II molecules throughout human DC maturation with particular attention to their localization in lipid-rich microdomains. Only immature DCs expressed empty MHC class II molecules, and maturation increased the level of peptide-bound heterodimers. Ligand binding to surface human leukocyte antigen (HLA)-DR induced rapid internalization in immature DCs. The proportion of cell-surface detergent-insoluble glycosphingolipid-enriched microdomain-clustered HLA-DR was higher in immature DCs despite the higher surface expression of HLA-DR in mature DCs. Constituents of HLA-DR containing microdomains included the src kinase Lyn and the cytoskeletal protein tubulin in immature DCs. Maturation modified the composition of the HLA-DR-containing microdomains to include protein kinase C (PKC)-delta, Lyn, and the cytoskeletal protein actin, accompanied by the loss of tubulin. Signaling via HLA-DR redistributed HLA-DR and -DM and PKC-delta as well as enriching the actin content of mature DC microdomains. The increased expression of HLA-DR as a result of DC maturation was therefore accompanied by modification of the spatial organization of HLA-DR. Such regulation could contribute to the distinct responses induced by ligand binding to MHC class II molecules in immature versus mature DCs.

Against the self: dendritic cells versus cancer

The role of host defense in cancer is highly variable. Although there are cases where spontaneous cures of cancer appear to be mediated by immunologic mechanisms, malignant disease generally progresses even in patients where tumor-specific immunity can be demonstrated. It is apparent that there are complex interactions between tumor cells and dendritic cells, the dominant antigen-presenting cells of the immune system. Through their inhibitory actions upon dendritic cells, tumor cells can negatively regulate priming of tumor-specific immunity. Recent work has also shown that dendritic cells have direct cytotoxic effects upon tumor cells. These interactions may impact on the efficacy of current strategies using dendritic cell-based vaccines for tumor immunotherapy.

Cutting edge: link between innate and adaptive immunity: Toll-like receptor 2 internalizes antigen for presentation to CD4+ T cells and could be an efficient vaccine target

An ideal vaccine for induction of CD4(+) T cell responses should induce local inflammation, maturation of APC, and peptide loading of MHC class II molecules. Ligation of Toll-like receptor (TLR) 2 provides the first two of these three criteria. We have studied whether targeting of TLR2 results in loading of MHC class II molecules and enhancement of CD4(+) T cell responses. To dissociate MHC class II presentation from APC maturation, we have used an antagonistic, mouse anti-human TLR2 mAb (TL2.1) as ligand and measured proliferation of a mouse Ckappa-specific human CD4(+) T cell clone. TL2.1 mAb was 100-1000 times more efficiently presented by APC compared with isotype-matched control mAb. Moreover, TL2.1 mAb was internalized into endosomes and processed by the conventional MHC class II pathway. This novel function of TLR2 represents a link between innate and adaptive immunity and indicates that TLR2 could be a promising target for vaccines.

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.

Tumor and CD4 T-cell interactions: tumor escape as result of reciprocal inactivation

This paper addresses the capacity of naive, effector, and memory CD4 T cells to control growth of a major histocompatibility complex (MHC) class II-positive B-cell lymphoma in vivo. To assess the role of T cells on their own without contributions by B cells, antibodies, or natural killer (NK) cells, we generated pure effector or memory CD4 T cells in Rag-/-gc-/- mice deficient in endogenous lymphocytes and NK cells. Lymphoma cells expressing a model antigen were injected into mice with T cells of cognate specificity that were either naive or in effector or resting memory state. Naive T cells were unable to prevent tumor growth, probably due to delay of efficient cross-presentation by dendritic cells. However, both effector and memory T cells, dependent on the amount of antigen available, controlled the tumor for a considerable period of time without the need for dendritic cell stimulation. Nevertheless, the tumor eventually grew uncontrolled in all cases. This was not because of a defect in T-cell homing to the tumor site or loss of MHC class II or costimulatory molecules by the tumor, but reflected mutual paralysis of T-cell responsiveness and antigen processing by tumor cells.

Multiple mechanisms of reduced major histocompatibility complex class II expression in endotoxin tolerance

Patients after polytrauma, burns, or septic shock frequently develop a life-threatening immunodeficiency. This state is associated with specific functional alterations of monocytic cells. We previously proposed endotoxin tolerance, the monocyte state after acute response to lipopolysaccharide, as a respective model system. One major feature in both the clinical situation and the in vitro model is the dramatic down-regulation of monocyte major histocompatibility complex (MHC) class II surface expression, which is associated with impaired antigen presentation capacity. This study focused on the mechanisms behind reduced MHC class II expression in endotoxin tolerance. Endotoxin priming provoked a decrease of monocyte intracellular MHC class II. It also led to a reduced expression of the chaperonic invariant chain and to an inhibited synthesis of the major lysosomal enzyme for final cleavage of the invariant chain going along with a relative accumulation of p10. The expression of HLA-DM necessary for loading MHC class II with antigenic peptide was also decreased. Additionally, reduced export of MHC class II alphabeta complexes to the cell surface was observed. The down-regulation of HLA-DR, invariant chain, and HLA-DM was regulated at the mRNA level and may be the consequence of reduced class II transactivator expression observed in this study. The simultaneous interference at different regulatory levels may explain the uniquely strong and long lasting MHC class II down-modulating effect of endotoxin priming compared with transforming growth factor-beta and interleukin-10. These results not only contribute to a better understanding of experimental endotoxin tolerance but may also give rise to new therapeutics for temporary immunodeficiency and, conversely, for MHC class II-dependent diseases such as autoimmunity and transplant rejection.

Antigen processing and presentation in human muscle: cathepsin S is critical for MHC class II expression and upregulated in inflammatory myopathies

The immunological properties of muscle cells are of critical importance for both the pathogenesis of inflammatory muscle disorders as well as for understanding and controlling novel therapeutic strategies. Muscle cells can present antigens to both CD4 and CD8 cells. However, the cellular biochemistry of antigen processing and presentation by muscle cells is not clear. Cathepsins play a central role in the generation of antigenic peptide and control transport and maturation of MHC class II molecules. To further elucidate the molecular basis for the MHC class II-mediated antigen presentation by muscle cells, we here analyzed cultured human myoblasts and biopsies from inflammatory myopathies with respect to the expression and function of the constituents of the MHC class II antigen presentation machinery. We identified cathepsin S (CatS) as the dominant endocytic protease that is specifically upregulated under inflammatory conditions to significant mRNA levels, synchronously with HLA-DR, -DM and the class II invariant chain (Ii), both in muscle biopsies from affected individuals with inflammatory myopathies and in human myoblasts cultured in the presence of IFN-gamma. This led to translation of the mature CatS polypeptide that was enzymatically active in human myoblasts under inflammatory conditions. By contrast, expression of CatL and CatB was unaffected by IFN-gamma at both the expression and activity levels. CatS activity is required for efficient surface display of MHC class II in this cell type: functional inhibition of CatS using a CatS-selective inhibitor reduced the levels of surface class II alphabeta:peptide complexes on stimulated myoblasts by almost 50%. Surprisingly, and in contrast to B cells and dendritic cells, this was not due to inefficient processing of Ii in the absence of CatS, which was unaffected by the elimination of CatS activity. We therefore conclude that CatS is involved in the regulation of class II expression in human myoblasts independently from Ii processing.

Orf virus-encoded interleukin-10 inhibits maturation, antigen presentation and migration of murine dendritic cells

Orf virus (ORFV) belongs to the genus Parapoxvirus and induces cutaneous pustular lesions in sheep, goats and humans. ORFV is unusual in that it has the ability to reinfect its host and this suggests that the generation of immunological memory has been impaired, thus exposing the host to subsequent infection. The discovery that ORFV encodes an IL-10-like virokine raises the question of whether this factor adversely affects the cells that initiate the acquired immune response. We examined the effect of ORFV-IL-10 on immature murine bone marrow-derived dendritic cells (BMDC). Immature BMDC are activated on exposure to antigen and undergo maturation. This process is characterized by increased expression of CD80, CD86 and MHC class II and reduced antigen uptake. We found that the maturation of BMDC is impaired in cells treated with ORFV-IL-10 prior to antigen exposure and this was exemplified by the reduced expression of the cell-surface markers described above. We have also shown that the activation of a haemagglutinin peptide (HAT)-specific T cell hybridoma by dendritic cell-mediated presentation of HAT and heat-inactivated influenza virus AP8/34 was markedly reduced following exposure to ORFV-IL-10. Finally, we examined the effect of ORFV-IL-10 on Langerhans' cell (LC) migration using cultured murine skin explant tissue and showed that this virokine impaired the spontaneous migration of LC from the epidermis and induced changes in LC morphology. Our findings suggest that ORFV-IL-10 has the capacity to impair the initiation of an acquired immune response and hence inhibit the generation of immunological memory necessary for immunity on subsequent exposure.

Interferon-producing cells fail to induce proliferation of naive T cells but can promote expansion and T helper 1 differentiation of antigen-experienced unpolarized T cells

Interferon-producing cells (IPCs) secrete high levels of type I interferon in response to certain viruses. The lack of lineage markers, the expression of major histocompatibility complex (MHC) class II and the capacity to stimulate allogeneic T cells have led these cells to be classified as a subset of dendritic cells (DCs), called plasmacytoid DCs (PDCs). However, the role of IPCs/PDCs in initiating primary immune responses remains elusive. Here we examined the antigen presenting capacity of murine IPCs in antigen specific systems. While CD8alpha+ and CD11b+ DCs induced logarithmic expansion of naive CD4 and CD8 T cells, without conferring T helper commitment at a first encounter, primary IPCs lacked the ability to stimulate naive T cells. However, when antigen-experienced, nonpolarized T cells expanded by classical DC subsets, were restimulated by IPCs, they proliferated and produced high amounts of IFN-gamma. These data indicate that IPCs can effectively stimulate preactivated or memory-type T cells and exert an immune-regulatory role. They also suggest that expansion of naive T cells and acquisition of effector function during antigen-specific T cell responses may involve different antigen-presenting cell (APC) types. Independent and coordinated control of T cell proliferation and differentiation would provide the immune system with greater flexibility in regulating immune responses.

Fever-like thermal conditions regulate the activation of maturing dendritic cells

Fever is one of the most frequent clinical signs encountered in pathology, especially with respect to infectious diseases. It is currently thought that the role of fever on immunity is limited to activation of innate immunity; however, its relevance to activation of adaptive immunity remains unclear. Dendritic cells (DCs) that behave as sentinels of the immune system provide an important bridge between innate and adaptive immunity. To highlight the role of fever on adaptive immunity, we exposed murine bone marrow-derived lipopolysaccharide (LPS)- or live bacteria-maturing DCs over a 3-h period to 37 degrees C or to fever-like thermal conditions (39 degrees C or 40 degrees C). At these three temperatures, we measured the kinetics of cytokine production and the ability of DCs to induce an allogeneic mixed lymphocyte reaction. Our results show that short exposure of DCs to temperatures of 39 degrees C or 40 degrees C differentially increased the secretion of interleukin (IL)-12p70 and decreased the secretion of IL-10 and tumor necrosis factor alpha by maturing DCs. These fever-like conditions induced a regulation of cytokine production at the single-cell level. In addition, short-term exposed LPS-maturing DCs to 39 degrees C induced a stronger reaction with allogeneic CD4(+) T cells than maturing DCs incubated at 37 degrees C. These results provide evidence that temperature regulates cytokine secretion and DC functions, both of which are of particular importance in bacterial diseases.

Generating CTLs against the subdominant Epstein-Barr virus LMP1 antigen for the adoptive immunotherapy of EBV-associated malignancies

The Epstein-Barr virus (EBV)-encoded LMP1 protein is expressed in EBV-positive Hodgkin disease and is a potential target for cytotoxic T-lymphocyte (CTL) therapy. However, the LMP1-specific CTL frequency is low, and so far the generation of LMP1-specific CTLs has required T-cell cloning. The toxicity of LMP1 has prevented the use of dendritic cells (DCs) for CTL stimulation, and we reasoned that an inactive, nontoxic LMP1 mutant (DeltaLMP1) could be expressed in DCs and would enable the activation and expansion of polyclonal LMP1-specific CTLs. Recombinant adenoviral vectors expressing LMP1 or DeltaLMP1 were tested for their ability to transduce DCs. LMP1 expression was toxic within 48 hours whereas high levels of DeltaLMP1 expression were achieved with minimal toxicity. DeltaLMP1-expressing DCs were able to reactivate and expand LMP1-specific CTLs from 3 healthy EBV-seropositive donors. LMP1-specific T cells were detected by interferon-gamma (IFN-gamma) enzyme-linked immunospot assay (ELISPOT) assays using the HLA-A2-restricted LMP1 peptide, YLQQNWWTL (YLQ). YLQ-specific T cells were undetectable (less than 0.001%) in donor peripheral blood mononuclear cells (PBMCs); however, after stimulation the frequency increased to 0.5% to 3.8%. Lysis of autologous target cells by CTLs was dependent on the level of LMP1 expression. In contrast, the frequency of YLQ-specific CTLs in EBV-specific CTLs reactivated and expanded using lymphoblastoid cell lines was low and no LMP1-specific cytotoxic activity was observed. Thus, DeltaLMP1 expression in DCs is nontoxic and enables the generation of LMP1-specific CTLs for future adoptive immunotherapy protocols for patients with LMP1-positive malignancies such as EBV-positive Hodgkin disease. Targeting LMP1 in these malignancies may improve the efficacy of current adoptive immunotherapy approaches.

Activation of lysosomal function during dendritic cell maturation

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

Activity and subcellular distribution of cathepsins in primary human monocytes

Cathepsins (Cat) in antigen presenting cells (APC) control antigen processing as well as major histocompatibility complex class II transport and function. The set of active Cat and the subcellular architecture of the class II antigen presentation compartment are largely unknown in primary human APC, including peripheral blood monocytes. We used novel chemical tools to visualize Cat in an activity-dependent manner. Primary human monocytes contained active CatS, -B, and -H, while CatL was absent. Expression and activity patterns of Cat in human myelo-monocytoid cell lines were distinct from those found in primary cells. On a subcellular scale, the bulk of active Cat was concentrated in lysosomes in primary monocytes. In late endosomes, only active CatS was found in sizable amounts, colocalizing with C-terminal processing of the class II invariant chain and with cystatin C, the major endogenous Cat inhibitor. Late endosomes of human peripheral blood monocytes contain a well-controlled proteolytic machinery distinct from lysosomes, which is likely to play a key role in class II function.

Differential regulation of cathepsin S and cathepsin L in interferon gamma-treated macrophages

Cathepsin S (catS) and cathepsin L (catL) mediate late stages of invariant chain (Ii) degradation in discrete antigen-presenting cell types. Macrophages (Mphis) are unique in that they express both proteases and here we sought to determine the relative contribution of each enzyme. We observe that catL plays no significant role in Ii cleavage in interferon (IFN)-gamma-stimulated Mphis. In addition, our studies show that the level of catL activity is significantly decreased in Mphis cultured in the presence of IFN-gamma whereas catS activity increases. The decrease in catL activity upon cytokine treatment occurs despite the persistence of high levels of mature catL protein, suggesting that a specific inhibitor of the enzyme is up-regulated in IFN-gamma-stimulated peritoneal Mphis. Similar inhibition of activity is observed in dendritic cells engineered to overexpress catL. Such enzymatic inhibition in Mphis exhibits only partial dependence upon Ii and therefore, other mechanisms of catL inhibition are regulated by IFN-gamma. Thus, during a T helper cell type 1 immune response catL inhibition in Mphis results in preferential usage of catS, such that major histocompatibility complex class II presentation by all bone marrow-derived antigen-presenting cell is regulated by catS.

Opsonization of apoptotic cells by autologous iC3b facilitates clearance by immature dendritic cells, down-regulates DR and CD86, and up-regulates CC chemokine receptor 7

Immature dendritic cells (iDCs) do not mature after uptake of apoptotic cells and may play a role in the induction of peripheral tolerance to self antigens derived from apoptotic material. The integrins, alphavbeta3, alphavbeta5, and the scavenger receptor, CD36, have been shown to mediate uptake of apoptotic cells by iDCs. However, it is not known whether the complement system, also takes part in this process. In this study we investigated the ability of iDCs to bind to apoptotic cells opsonized by iC3b. Monocyte-derived dendritic cells were offered apoptotic Jurkat cells opsonized by autologous iC3b and labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanineperchlorate. A significant increase (P < 0.001) in the amount of cleared apoptotic cells was seen at low ratios. Despite increased efficiency of uptake, interaction between iC3b-opsonized apoptotic cells and iDCs down-regulated the expression of major histocompatibility complex class II, CD86, CC chemokine receptor (CCR)2, CCR5, and beta2-integrins (P < 0.001), and up-regulated expression of CCR7 (P < 0.001). In addition, iDC maturation responses to CD40L and lipopolysaccharide were significantly inhibited. We conclude that opsonization of apoptotic cells by iC3b induces tolerant iDCs that are able to migrate to lymph nodes.

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.

Inflammatory stimuli recruit cathepsin activity to late endosomal compartments in human dendritic cells

Proteolysis by endocytic cysteine proteases is a central element of the antigen-presentation machinery in dendritic cells (DC). It controls the generation of immunogenic peptides, guides the transit of both MHC class II and MHC-like molecules through the endocytic compartment and converts class II into a peptide-receptive state - features closely linked to DC maturation. Differential activity of endocytic proteases, in particular cathepsins, in subcellular compartments has been implicated as a key regulatory element in controlling this machinery in murine DC. We analyzed the expression and subcellular distribution of the major endocytic cysteine proteases (cathepsins S, B, L and H) along with their major endogenous inhibitor, Cystatin C, in resting and stimulated human DC. Although the majority of cathepsin activity was restricted to lysosomes in resting DC, cathepsins selectively accumulated in late endosomes after LPS-induced stimulation. Surprisingly, expression and distribution of Cystatin C was unaffected by DC maturation. Thus, late endosomes represent a specialized compartment where proteolytic activity is developmentally regulated in DC. This could facilitate the conversion of exogenous protein into MHC class II-peptide complexes.

Caspase-mediated death of newly formed neurons in the adult rat dentate gyrus following status epilepticus

A large proportion of cells that proliferate in the adult dentate gyrus under normal conditions or in response to brain insults exhibit only short-term survival. Here, we sought to determine which cell death pathways are involved in the degeneration of newly formed neurons in the rat dentate gyrus following 2 h of electrically induced status epilepticus. We investigated the role of three families of cysteine proteases, caspases, calpains, and cathepsins, which can all participate in apoptotic cell death. Status epilepticus increased the number of bromodeoxyuridine (BrdU)-positive proliferated cells in the subgranular zone of the dentate gyrus. At the time of maximum cell proliferation, immunohistochemical analyses revealed protein expression of active caspase-cleaved poly (ADP-ribose) polymerase (PARP) in approximately 66% of the BrdU-positive cells, while none of them expressed cathepsin B or the 150-kDa calpain-produced fodrin breakdown product. To evaluate the importance of cysteine proteases in regulating survival of the newly formed neurons, we administered intracerebroventricular infusions of a caspase inhibitor cocktail (zVAD-fmk, zDEVD-fmk and zLEHD-fmk) over a 2-week period, sufficient to allow for neuronal differentiation, starting 1 week after the epileptic insult. Increased numbers of cells double-labelled with BrdU and neuron-specific nuclear protein (NeuN) marker were detected in the subgranular zone and granule cell layer of the caspase inhibitor-treated rats. Our data indicate that caspase-mediated cell death pathways are active in progenitor cell progeny generated by status epilepticus and compromise survival during neuronal differentiation.

Distinct recognition by two subsets of T cells of an MHC class II-peptide complex

We examine here the nature of the differential recognition by CD4+ T cells of a single peptide from hen-egg white lysozyme (HEL) presented by I-A(k) class II MHC molecules. Two subsets of T cells (called A and B) interact with the same peptide, each in unique ways that reflect the nature of the complex of peptide and MHC. We show that the A and B set of T cells can be distinguished by their functional interaction with the three T cell receptor (TCR) contact residues of the bound peptide. The dominant peptide of HEL selected from processing is bound in a single register where a critical TCR contact residue is situated about the middle of the core segment of the peptide: all T cells establish functional contact with it. Three sets of T cells, however, can be distinguished by their differential recognition of two TCR contacts situated at the amino and carboxyl sides of the central TCR contact residue. Type A T cells, the conventional cells that see the peptide after processing of HEL, need to recognize all three TCR contact residues. In contrast, the type B T cells recognize the peptide given exogenously, but not when processed: these T cells recognize either one of the peripheral TCR contact residues, indicating a much more flexible interaction of peptide with I-A(k) molecules. We discuss the mode of generation of the various T cells and their biological relevance.

Antigen presentation and T cell stimulation by dendritic cells

Dendritic cells take up antigens in peripheral tissues, process them into proteolytic peptides, and load these peptides onto major histocompatibility complex (MHC) class I and II molecules. Dendritic cells then migrate to secondary lymphoid organs and become competent to present antigens to T lymphocytes, thus initiating antigen-specific immune responses, or immunological tolerance. Antigen presentation in dendritic cells is finely regulated: antigen uptake, intracellular transport and degradation, and the traffic of MHC molecules are different in dendritic cells as compared to other antigen-presenting cells. These specializations account for dendritic cells' unique role in the initiation of immune responses and the induction of tolerance.

Regulation of major histocompatibility complex class II synthesis by interleukin-10

We have shown previously that interleukin-10 (IL-10) blocks the development and T-cell stimulatory capacity of human monocyte-derived dendritic cells, without apparently down-regulating the surface expression of co-stimulatory molecules or human leucocyte antigen (HLA) molecules. In the majority of donors (60%), the cell surface levels of HLA-DR actually increased upon IL-10 treatment. Here we have shown that IL-10 does not regulate HLA-DR transcription as assessed by polymerase chain reation. Epifluorescence microscopy analysis showed that IL-10 primarily increased the intracellular pool of HLA-DR. In fact, IL-10 directly increased HLA-DR protein synthesis. However, IL-10 did not significantly alter the synthesis of invariant chain (Ii), which plays a crucial role in the assembly, transport and loading of newly formed HLA class II molecules, nor the amount of Ii reaching the cell-surface. In contrast, IL-10 increased the amount of HLA-DR-bound Iip33 shortly after the HLA-DR complex assembly. We postulate that, upon IL-10 treatment, immature Ii-associated HLA II molecules can still transit to the cell surface as they do in immature dendritic cells and recycle to the intracellular space, where they accumulate. A higher proportion of Ii-associated HLA-DR, coupled to increased membrane recycling, may contribute to the lower T-cell stimulatory capacity of IL-10-treated dendritic cells.

IFN regulatory factor-1 regulates IFN-gamma-dependent cathepsin S expression

Cathepsin S is a cysteine protease with potent endoproteolytic activity and a broad pH profile. Cathepsin S activity is essential for complete processing of the MHC class II-associated invariant chain within B cells and dendritic cells, and may also be important in extracellular matrix degradation in atherosclerosis and emphysema. Unique among cysteine proteases, cathepsin S activity is up-regulated by IFN-gamma. Given its importance, we sought to elucidate the pathway by which IFN-gamma increases cathepsin S expression. Our data demonstrate that the cathepsin S promoter contains an IFN-stimulated response element (ISRE) that is critical for IFN-gamma-induced gene transcription in a cell line derived from type II alveolar epithelial (A549) cells. IFN response factor (IRF)-2 derived from A549 nuclear extracts associates with the ISRE oligonucleotide in gel shift assays, but is quickly replaced by IRF-1 following stimulation with IFN-gamma. The time course of IRF-1/ISRE complex formation correlates with increased levels of IRF-1 protein and cathepsin S mRNA. Overexpression of IRF-1, but not IRF-2, markedly augments cathepsin S promoter activity in A549 cells. Furthermore, overexpression of IRF-1 increases endogenous cathepsin S mRNA levels in 293T epithelial cells. Finally, freshly isolated bone marrow cells from IRF-1(-/-) mice fail to up-regulate cathepsin S activity in response to IFN-gamma. Thus, IRF-1 is the critical transcriptional mediator of IFN-gamma-dependent cathepsin S activation. These data elucidate a new pathway by which IRF-1 may affect MHC class II processing and presentation.

A role for cathepsin L and cathepsin S in peptide generation for MHC class II presentation

The enzymes that degrade proteins to peptides for presentation on MHC class II molecules are poorly understood. The cysteinal lysosomal proteases, cathepsin L (CL) and cathepsin S (CS), have been shown to process invariant chain, thereby facilitating MHC class II maturation. However, their role in Ag processing is not established. To examine this issue, we generated embryonic fibroblast lines that express CL, CS, or neither. Expression of CL or CS mediates efficient degradation of invariant chain as expected. Ag presentation was evaluated using T cell hybridoma assays as well as mass spectroscopic analysis of peptides eluted from MHC class II molecules. Interestingly, we found that the majority of peptides are presented regardless of CL or CS expression, although these proteases often alter the relative levels of the peptides. However, for a subset of Ags, epitope generation is critically regulated by CL or CS. This result suggests that these cysteinal proteases participate in Ag processing and generate qualitative and quantitative differences in the peptide repertoires displayed by MHC class II molecules.

Visualization of the ER-to-cytosol dislocation reaction of a type I membrane protein

The human cytomegalovirus gene products US2 and US11 induce proteasomal degradation of MHC class I heavy chains. We have generated an enhanced green fluorescent protein-class I heavy chain (EGFP-HC) chimeric molecule to study its dislocation and degradation in US2- and US11-expressing cells. The EGFP-HC fusion is stable in control cells, but is degraded rapidly in US2- or US11-expressing cells. Proteasome inhibitors induce in a time-dependent manner the accumulation of EGFP-HC molecules in US2- and US11-expressing cells, as assessed biochemically and by cytofluorimetry of intact cells. Pulse-chase analysis and subcellular fractionation show that EGFP-HC proteins are dislocated from the endoplasmic reticulum and can be recovered as deglycosylated fluorescent intermediates in the cytosol. These results raise the possibility that dislocation of glycoproteins from the ER may not require their full unfolding.

Membrane-specific, host-derived factors are required for US2- and US11-mediated degradation of major histocompatibility complex class I molecules

Human cytomegalovirus encodes two glycoproteins, US2 and US11, that target major histocompatibility complex (MHC) class I heavy chains for proteasomal degradation. We have developed a mRNA-dependent cell-free system that recapitulates US2- and US11-mediated degradation of MHC class I heavy chains. Microsomes support the degradation of MHC class I heavy chains in the presence of US2 or US11 in a cytosol-dependent manner. In vitro, the glycosylated heavy chain is exported from the microsomes. A deglycosylated breakdown intermediate of the heavy chain identical to that generated in intact cells accumulates in soluble form in the presence of proteasome inhibitors. Microsomes derived from the U373 astrocytoma cell line are far more effective than canine-derived membranes in supporting this US2- or US11-dependent reaction. In contrast, the HIV-encoded Vpu membrane protein can cause the destruction of CD4 from either human- or canine-derived membranes. Using the in vitro system, we show that a truncation mutant of US2 that lacks the cytosolic domain is unable to catalyze degradation, whereas a similar truncation of US11 continues to catalyze degradation of class I heavy chains. Therefore, US2 requires both transmembrane and cytosolic interactions to trigger dislocation of heavy chains, whereas US11 relies on the transmembrane domain to target heavy chains. US2 and US11 thus utilize different targeting mechanisms for class I degradation.

Trafficking of MHC class II molecules in the late secretory pathway

Antigen presenting cells (APCs) alert the immune system to attack by extracellular organisms; APCs achieve this via internalization, degradation, and display of antigenic fragments on the cell surface by MHC class II molecules. These class II molecules bind to an accessory protein, termed the invariant chain, that ensures proper folding of the molecules. Invariant-chain binding also directs class II molecules to lysosomes, which are probably the most important sites for antigen loading. Endosomes are intermediates in the transport of class-II-invariant chain complexes to antigen-processing compartments, whereas trafficking of class II-peptide complexes to the membrane (and beyond) is less-well understood. Unlike other APCs, dendritic cells alter their capacity to present peptides via MHC class II molecules during differentiation, revealing a complex level of regulated antigen-presentation by this APC subtype.

A closer look at proteolysis and MHC-class-II-restricted antigen presentation

Antigen presentation by MHC class II molecules relies on the action of endocytic proteases, which are differentially expressed in antigen-presenting cells and are regulated by different components of the immune system. Endocytic enzymes process and convert exogenous antigens into peptidic determinants capable of interaction with MHC class II molecules. Chemical and genetic tools have recently been developed to study the role of lysosomal proteases in antigen presentation.

Repetitive injections of dendritic cells matured with tumor necrosis factor alpha induce antigen-specific protection of mice from autoimmunity

Mature dendritic cells (DCs) are believed to induce T cell immunity, whereas immature DCs induce T cell tolerance. Here we describe that injections of DCs matured with tumor necrosis factor (TNF)-alpha (TNF/DCs) induce antigen-specific protection from experimental autoimmune encephalomyelitis (EAE) in mice. Maturation by TNF-alpha induced high levels of major histocompatibility complex class II and costimulatory molecules on DCs, but they remained weak producers of proinflammatory cytokines. One injection of such TNF/DCs pulsed with auto-antigenic peptide ameliorated the disease score of EAE. This could not be observed with immature DCs or DCs matured with lipopolysaccharide (LPS) plus anti-CD40. Three consecutive injections of peptide-pulsed TNF/DCs derived from wild-type led to the induction of peptide-specific predominantly interleukin (IL)-10-producing CD4(+) T cells and complete protection from EAE. Blocking of IL-10 in vivo could only partially restore the susceptibility to EAE, suggesting an important but not exclusive role of IL-10 for EAE prevention. Notably, the protection was peptide specific, as TNF/DCs pulsed with unrelated peptide could not prevent EAE. In conclusion, this study describes that stimulation by TNF-alpha results in incompletely matured DCs (semi-mature DCs) which induce peptide-specific IL-10-producing T cells in vivo and prevent EAE.

Proteolysis and antigen presentation by MHC class II molecules

Proteolysis is the primary mechanism used by all cells not only to dispose of unwanted proteins but also to regulate protein function and maintain cellular homeostasis. Proteases that reside in the endocytic pathway are the principal actors of terminal protein degradation. The proteases contained in the endocytic pathway are classified into four major groups based on the active-site amino acid used by the enzyme to hydrolyze amide bonds of proteins: cysteine, aspartyl, serine, and metalloproteases. The presentation of peptide antigens by major histocompatibility complex (MHC) class II molecules is strictly dependent on the action of proteases. Class II molecules scour the endocytic pathway for antigenic peptides to bind and present at the cell surface for recognition by CD4+ T cells. The specialized cell types that support antigen presentation by class II molecules are commonly referred to as professional antigen presenting cells (APCs), which include bone marrow-derived B lymphocytes, dendritic cells (DCs), and macrophages. In addition, the expression of certain endocytic proteases is regulated either at the level of gene transcription or enzyme maturation and their activity is controlled by the presence of endogenous protease inhibitors.

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.

MHC class II expression is regulated in dendritic cells independently of invariant chain degradation

We have investigated the mechanisms that control MHC class II (MHC II) expression in immature and activated dendritic cells (DC) grown from spleen and bone marrow precursors. Degradation of the MHC II chaperone invariant chain (Ii), acquisition of peptide cargo by MHC II, and delivery of MHC II-peptide complexes to the cell surface proceeded similarly in both immature and activated DC. However, immature DC reendocytosed and then degraded the MHC II-peptide complexes much faster than the activated DC. MHC II expression in DC is therefore not controlled by the activity of the protease(s) that degrade Ii, but by the rate of endocytosis of peptide-loaded MHC II. Late after activation, DC downregulated MHC II synthesis both in vitro and in vivo.