Cell surface expression of class II histocompatibility antigens occurs in the absence of the invariant chain

The transmembrane domain and luminal C-terminal region independently support invariant chain trimerization and assembly with MHCII into nonamers

Background

Invariant chain (CD74, Ii) is a multifunctional protein expressed in antigen presenting cells. It assists the ER exit of various cargos and serves as a receptor for the macrophage migration inhibitory factor. The newly translated Ii chains trimerize, a structural feature that is not readily understood in the context of its MHCII chaperoning function. Two segments of Ii, the luminal C-terminal region (TRIM) and the transmembrane domain (TM), have been shown to participate in the trimerization process but their relative importance and impact on the assembly with MHCII molecules remains debated. Here, we addressed the requirement of these domains in the trimerization of human Ii as well as in the oligomerization with MHCII molecules. We used site-directed mutagenesis to generate series of Ii and DR mutants. These were transiently transfected in HEK293T cells to test their cell surface expression and analyse their interactions by co-immunoprecipitations.

Results

Our results showed that the TRIM domain is not essential for Ii trimerization nor for intracellular trafficking with MHCII molecules. We also gathered evidence that in the absence of TM, TRIM allows the formation of multi-subunit complexes with HLA-DR. Similarly, in the absence of TRIM, Ii can assemble into high-order structures with MHCII molecules.

Conclusions

Altogether, our data show that trimerization of Ii through either TM or TRIM sustains nonameric complex formation with MHCII molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00444-6.

CD74 regulates cellularity and maturation of medullary thymic epithelial cells partially by activating the canonical NF-κB signaling pathway

Thymic epithelial cells (TECs) are indispensable for T cell development, T cell receptor (TCR) repertoire selection, and specific lineage differentiation. Medullary thymic epithelial cells (mTECs), which account for the majority of TECs in adults, are critical for thymocyte selection and self-tolerance. CD74 is a nonpolymorphic transmembrane glycoprotein of major histocompatibility complex class II (MHCII) that is expressed in TECs. However, the exact role of CD74 in regulating the development of mTEC is poorly defined. In this research, we found that loss of CD74 resulted in a significant diminution in the medulla, a selective reduction in the cell number of mature mTECs expressing CD80 molecules, which eventually led to impaired thymic CD4⁺ T cell development. Moreover, RNA-sequence analysis showed that CD74 deficiency obviously downregulated the canonical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathway in mTECs. Our results suggest that CD74 positively controls mTEC cellularity and maturation partially by activating the canonical NF-κB signaling pathway.

Partnering for the major histocompatibility complex class II and antigenic determinant requires flexibility and chaperons

Cytotoxic, or helper T cells recognize antigen via T cell receptors (TCRs) that can see their target antigen as short sequences of peptides bound to the groove of proteins of major histocompatibility complex (MHC) class I, and class II respectively. For MHC class II epitope selection from exogenous pathogens or self-antigens, participation of several accessory proteins, molecular chaperons, processing enzymes within multiple vesicular compartments is necessary. A major contributing factor is the MHC class II structure itself that uniquely offers a dynamic and flexible groove essential for epitope selection. In this review, I have taken a historical perspective focusing on the flexibility of the MHC II molecules as the driving force in determinant selection and interactions with the accessory molecules in antigen processing, HLA-DM and HLA-DO.

A step-by-step overview of the dynamic process of epitope selection by major histocompatibility complex class II for presentation to helper T cells

T cell antigen receptors (TCRs) expressed on cytotoxic or helper T cells can only see their specific target antigen as short sequences of peptides bound to the groove of proteins of major histocompatibility complex (MHC) class I, and class II respectively. In addition to the many steps, several participating proteins, and multiple cellular compartments involved in the processing of antigens, the MHC structure, with its dynamic and flexible groove, has perfectly evolved as the underlying instrument for epitope selection. In this review, I have taken a step-by-step, and rather historical, view to describe antigen processing and determinant selection, as we understand it today, all based on decades of intense research by hundreds of laboratories.

Exposing the Specific Roles of the Invariant Chain Isoforms in Shaping the MHC Class II Peptidome

The peptide repertoire (peptidome) associated with MHC class II molecules (MHCIIs) is influenced by the polymorphic nature of the peptide binding groove but also by cell-intrinsic factors. The invariant chain (Ii) chaperones MHCIIs, affecting their folding and trafficking. Recent discoveries relating to Ii functions have provided insights as to how it edits the MHCII peptidome. In humans, the Ii gene encodes four different isoforms for which structure-function analyses have highlighted common properties but also some non-redundant roles. Another layer of complexity arises from the fact that Ii heterotrimerizes, a characteristic that has the potential to affect the maturation of associated MHCIIs in many different ways, depending on the isoform combinations. Here, we emphasize the peptide editing properties of Ii and discuss the impact of the various isoforms on the MHCII peptidome.

MHC class II antigen presentation by dendritic cells regulated through endosomal sorting

For the initiation of adaptive immune responses, dendritic cells present antigenic peptides in association with major histocompatibility complex class II (MHCII) to naïve CD4(+) T lymphocytes. In this review, we discuss how antigen presentation is regulated through intracellular processing and trafficking of MHCII. Newly synthesized MHCII is chaperoned by the invariant chain to endosomes, where peptides from endocytosed pathogens can bind. In nonactivated dendritic cells, peptide-loaded MHCII is ubiquitinated and consequently sorted by the ESCRT machinery to intraluminal vesicles of multivesicular bodies, ultimately leading to lysosomal degradation. Ubiquitination of newly synthesized MHCII is blocked when dendritic cells are activated, now allowing its transfer to the cell surface. This mode of regulation for MHCII is a prime example of how molecular processing and sorting at multivesicular bodies can determine the expression of signaling receptors at the plasma membrane.

Antigen-specific blocking of CD4-specific immunological synapse formation using BPI and current therapies for autoimmune diseases

In this review, we discuss T-cell activation, etiology, and the current therapies of autoimmune diseases (i.e., MS, T1D, and RA). T-cells are activated upon interaction with antigen-presenting cells (APC) followed by a "bull's eye"-like formation of the immunological synapse (IS) at the T-cell-APC interface. Although the various disease-modifying therapies developed so far have been shown to modulate the IS and thus help in the management of these diseases, they are also known to present some undesirable side effects. In this study, we describe a novel and selective way to suppress autoimmunity by using a bifunctional peptide inhibitor (BPI). BPI uses an intercellular adhesion molecule-1 (ICAM-1)-binding peptide to target antigenic peptides (e.g., proteolipid peptide, glutamic acid decarboxylase, and type II collagen) to the APC and therefore modulate the immune response. The central hypothesis is that BPI blocks the IS formation by simultaneously binding to major histocompatibility complex-II and ICAM-1 on the APC and selectively alters the activation of T cells from T(H)1 to T(reg) and/or T(H)2 phenotypes, leading to tolerance.

HLA-DP, HLA-DQ, and HLA-DR have different requirements for invariant chain and HLA-DM

The MHC is central to the adaptive immune response. The human MHC class II is encoded by three different isotypes, HLA-DR, -DQ, and -DP, each being highly polymorphic. In contrast to HLA-DR, the intracellular assembly and trafficking of HLA-DP molecules have not been studied extensively. However, different HLA-DP variants can be either protective or risk factors for infectious diseases (e.g. hepatitis B), immune dysfunction (e.g. berylliosis), and autoimmunity (e.g. myasthenia gravis). Here, we establish a system to analyze the chaperone requirements for HLA-DP and to compare the assembly and trafficking of HLA-DP, -DQ, and -DR directly. Unlike HLA-DR1, HLA-DQ5 and HLA-DP4 can form SDS-stable dimers supported by invariant chain (Ii) in the absence of HLA-DM. Uniquely, HLA-DP also forms dimers in the presence of HLA-DM alone. In model antigen-presenting cells, SDS-stable HLA-DP complexes are resistant to treatments that prevent formation of SDS-stable HLA-DR complexes. The unexpected properties of HLA-DP molecules may help explain why they bind to a more restricted range of peptides than other human MHC class II proteins and frequently present viral peptides.

Choosing and preparing antigen-presenting cells

The first issue in selecting a system for antigen-presentation experiments is to define the appropriate type of antigen-presenting cell (APC) to study. For some experiments, crude preparations such as splenocytes or peripheral blood mononuclear cells (PBMCs) may suffice to provide APC function for stimulating T cells. This unit develops approaches for preparation of more defined APC populations, including dendritic cells (DCs), macrophages, and B lymphocytes, the three types of "professional" APC. Each of these cell types exists in different stages of differentiation, maturation, and activation, or in some cases different lineages. For example, dendritic cells may be divided into subsets, including myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). Each APC type has an important antigen-presentation function, although they contribute to different aspects of the immune response. Therefore, selection of an APC type for study must include consideration of the stage or aspect of immune response that is to be modeled in the experiment.

Presenting exogenous antigen to T cells

Antigen processing and presentation experiments can be done with a wide variety of antigen-presenting cells (APCs). Most experiments will use one of the "professional" APC types: dendritic cells (DCs), macrophages, and B lymphocytes. Other types of cells may be used for antigen presentation in some circumstances. Each type of professional APC has an important antigen-presentation function, but the different APC types contribute to different aspects of the immune response. Therefore, selection of an APC type for study must include consideration of the stage or aspect of immune response that is to be modeled in the experiment. An important technical distinction for some types of experiments is whether the APCs are adherent or nonadherent, since this dictates the procedures that must be used to wash the cells as the medium is changed.

Uveal melanoma cell-based vaccines express MHC II molecules that traffic via the endocytic and secretory pathways and activate CD8+ cytotoxic, tumor-specific T cells

We are exploring cell-based vaccines as a treatment for the 50% of patients with large primary uveal melanomas who develop lethal metastatic disease. MHC II uveal melanoma vaccines are MHC class I(+) uveal melanoma cells transduced with CD80 genes and MHC II genes syngeneic to the recipient. Previous studies demonstrated that the vaccines activate tumor-specific CD4(+) T cells from patients with metastatic uveal melanoma. We have hypothesized that vaccine potency is due to the absence of the MHC II-associated invariant chain (Ii). In the absence of Ii, newly synthesized MHC II molecules traffic intracellularly via a non-traditional pathway where they encounter and bind novel tumor peptides. Using confocal microscopy, we now confirm this hypothesis and demonstrate that MHC II molecules are present in both the endosomal and secretory pathways in vaccine cells. We also demonstrate that uveal melanoma MHC II vaccines activate uveal melanoma-specific, cytolytic CD8(+) T cells that do not lyse normal fibroblasts or other tumor cells. Surprisingly, the CD8(+) T cells are cytolytic for HLA-A syngeneic and MHC I-mismatched uveal melanomas. Collectively, these studies demonstrate that MHC II uveal melanoma vaccines are potent activators of tumor-specific CD4(+) and CD8(+) T cells and suggest that the non-conventional intracellular trafficking pattern of MHC II may contribute to their enhanced immunogenicity. Since MHC I compatibility is unnecessary for the activation of cytolytic CD8(+) T cells, the vaccines could be used in uveal melanoma patients without regard to MHC I genotype.

Modulation of allergen-specific T-lymphocyte function by virus-like particles decorated with HLA class II molecules

BACKGROUND

T(H)2 lymphocytes play an important role in the induction and maintenance phase of type I allergy. Modulation of the responses of T(H)2 lymphocytes by novel forms of antigen-presenting platforms may help shape the immune response to allergen and palliate allergic diseases.

OBJECTIVE

To present HLA class II/allergen-peptide complexes on virus-like particles (VLPs) and to evaluate their potential to modulate allergen-specific T-cell responses.

METHODS

Virus-like particles that express the immunodominant T-cell epitope Art v 1(25-34) of the major mugwort pollen allergen in the context of HLA-DR1 and costimulatory molecules were produced by transfection of 293 cells. The effect of VLPs on IL-2 promoter activity, proliferation, and cytokine production of allergen-specific T cells derived from donors with and without mugwort pollen allergy was determined.

RESULTS

Flow-cytometric analyses showed that HLA class II molecules, invariant chain::Art v 1 fusion proteins, and costimulatory molecules were expressed on 293 cells. Biochemical analyses confirmed that these molecules were efficiently targeted to VLPs. The engineered VLPs activated Art v 1-specific T cells in a costimulation-dependent manner. VLPs lacking costimulators induced T-cell unresponsiveness, which was overcome by addition of exogenous IL-2. Costimulation could be provided by CD80, CD86, or CD58 and induced distinct cytokine profiles in allergen-specific T cells. Unlike the other costimulatory molecules, CD58 induced IL-10/IFN-gamma-secreting T cells.

CONCLUSION

Virus-like particles represent a novel, modular, acellular antigen-presenting system able to modulate the responses of allergen-specific T cells in a costimulator-dependent fashion. Allergen-specific VLPs show promise as tools for specific immunotherapy of allergic diseases.

Sorting of MHC class II molecules into exosomes through a ubiquitin-independent pathway

Major histocompatibility complex class II (MHC-II) molecules accumulate in exocytic vesicles, called exosomes, which are secreted by antigen presenting cells. These vesicles are released following the fusion of multivesicular bodies (MVBs) with the plasma membrane. The molecular mechanisms regulating cargo selection remain to be fully characterized. As ubiquitination of the MHC-II beta-chain cytoplasmic tail has recently been demonstrated in various cell types, we sought to determine if this post-translational modification is required for the incorporation of MHC-II molecules into exosomes. First, we stably transfected HeLa cells with a chimeric HLA-DR molecule in which the beta-chain cytoplasmic tail is replaced by ubiquitin. Western blot analysis did not indicate preferential shedding of these chimeric molecules into exosomes. Next, we forced the ubiquitination of MHC-II in class II transactivator (CIITA)-expressing HeLa and HEK293 cells by transfecting the MARCH8 E3 ubiquitin ligase. Despite the almost complete downregulation of MHC-II from the plasma membrane, these molecules were not enriched in exosomes. Finally, site-directed mutagenesis of all cytoplasmic lysine residues on HLA-DR did not prevent inclusion into these vesicles. Taken together, these results demonstrate that ubiquitination of MHC-II is not a prerequisite for incorporation into exosomes.

Choosing and preparing antigen-presenting cells

The first issue in many antigen-processing experiments is to choose and obtain appropriate antigen-presenting cells (APCs). Macrophages have characteristics that dictate their use in certain antigen-processing experiments, so this unit describes preparation of activated macrophages, which involves eliciting peritoneal macrophages from mice by infection with Listeria monocytogenes. This method is often preferred because the resulting macrophages are more consistently activated to a high degree, but macrophages can also be obtained (as described here) by sequential injection of L. moncytogenes and proteose peptone, by induction with concanavalin A (Con A), or by isolation from bone marrow. Lipopolysaccharide-stimulated B lymphoblasts can also be used as APCs. Protocols are included for preparation of L. moncytogenes stocks and preparation of L929-conditioned medium which is used for culturing bone marrow-derived macrophages.

Tumor cells transduced with the MHC class II Transactivator and CD80 activate tumor-specific CD4+ T cells whether or not they are silenced for invariant chain

The specificity and potency of the immune system make immunotherapy a potential strategy for the treatment of cancer. To exploit this potential, we have developed cell-based cancer vaccines consisting of tumor cells expressing syngeneic MHC class II and costimulatory molecules. The vaccines mediate tumor regression in mice and activate human CD4+ T cells in vitro. Previous vaccines were generated by transducing MHC II negative tumor cells with a single HLA-DR allele. Because expression of multiple MHC II alleles would facilitate presentation of a broader repertoire of tumor antigens, we have now transduced tumor cells with the MHC class II transactivator (CIITA), a regulatory gene that coordinately increases expression of all MHC II alleles. Previous studies in mice indicated that coexpression of the MHC II accessory molecule invariant chain (Ii) inhibited presentation of endogenously synthesized tumor antigens and reduced vaccine efficacy. To determine if Ii expression affects presentation of MHC class II-restricted endogenously synthesized tumor antigens in human tumor cells, HLA-DR-MCF10 breast cancer cells were transduced with the CIITA, CD80 costimulatory molecule gene, and with or without small interfering RNAs (siRNA) specific for Ii. Ii expression is silenced >95% in CIITA/CD80/siRNA transductants; down-regulation of Ii does not affect HLA-DR expression or stability; and Ii(+) and Ii(-) transductants activate human CD4+ T cells to DRB1*0701-restricted HER-2/neu epitopes. Therefore, tumor cells transduced with the CIITA, CD80, and with or without Ii siRNA present endogenously synthesized tumor antigens and are potential vaccines for activating tumor-specific CD4+ T cells.

A three-amino-acid-long HLA-DRbeta cytoplasmic tail is sufficient to overcome ER retention of invariant-chain p35

The p35 isoform of the human invariant chain (Iip35) contains an N-terminal RXR endoplasmic-reticulum (ER) retention signal that becomes nonfunctional only after assembly with MHC-class-II molecules. We have previously shown that the MHC-class-II beta-chain cytoplasmic tail is crucial for the maturation of class-II/Iip35 complexes. In order to shed some light on the molecular determinants involved in shielding the RXR motif, we performed site-directed mutagenesis of the DRbeta chain and Ii cytoplasmic domains. Chimeric beta chains with irrelevant cytoplasmic tails allowed the efficient transport of Iip35 out of the ER in transiently transfected HEK 293T cells. An alanine scan of the cytoplasmic tail of HLA-DRbeta confirmed that no specific motif is required to overcome ER retention. Surprisingly, a beta chain with a three-amino-acid-long cytoplasmic tail (Tyr-Phe-Arg) was sufficient to overcome the Iip35 RXR motif. Moreover, replacement of residues F231 and R232 with alanines created a cytoplasmic tail (Tyr-Ala-Ala) that allowed ER egress. Given the limited length of this tail, steric hindrance would only be possible if the Ii ER retention motif was close to the membrane in the first place. However, this is not likely because an Ii molecule with an internal cytoplasmic deletion bringing the RXR motif closer to the membrane is not retained in the ER, even in the absence of class-II molecules. These results suggest that MHC-class-II molecules overcome ER retention and prevent COPI binding to the Iip35 RXR motif through a mechanism distinct from steric hindrance by its beta chain.

A defective viral superantigen-presenting phenotype in HLA-DR transfectants is corrected by CIITA

Activation of T lymphocytes by mouse mammary tumor virus superantigen (vSAg) requires binding to MHC class II molecules. The subcellular location where functional interactions occur between MHC class II molecules and vSAgs is still a matter of debate. To gain further insight into this issue, we have used human epithelial HeLa cells expressing HLA-DR1. Surprisingly, the human cells were unable to present transfected vSAg7 or vSAg9 to a series of murine T cell hybridomas. The defect is not related to a lack of vSAg processing, because these cells can indirectly activate T cells after coculture in the presence of B lymphocytes. However, after IFN-gamma treatment, the HeLa DR1(+) cells became apt at directly presenting the vSAg. Furthermore, transfection of CIITA was sufficient to restore presentation. Reconstitution experiments demonstrated the necessity of coexpressing HLA-DM and invariant chain (Ii) for efficient vSAg presentation. Interestingly, inclusion of a dileucine motif in the DRbeta cytoplasmic tail bypassed the need for HLA-DM expression and allowed the efficient presentation of vSAg7 in the presence of Ii. A similar trafficking signal was included in vSAg7 by replacing its cytoplasmic tail with the one of Ii. However, sorting of this chimeric Ii/vSAg molecule to the endocytic pathway completely abolished both its indirect and direct presentation. Together, our results suggest that functional vSAgs-DR complexes form after the very late stages of class II maturation, most probably at the cell surface.

Impaired nitric oxide synthase-2 signaling pathway in cystic fibrosis airway epithelium

Cystic fibrosis (CF) airway epithelial cells are more susceptible to viral infection due to impairment of the innate host defense pathway of nitric oxide (NO). NO synthase-2 (NOS2) expression is absent, and signal transducer and activator of transcription (STAT) 1 activation is reduced in CF. We hypothesized that the IFN-γ signaling pathway, which leads to NOS2 gene induction in CF airway epithelial cells, is defective. In contrast to a lack of NOS2 induction, the major histocompatibility complex class 2, an IFN-γ-regulated delayed-responsive gene, is similarly induced in CF and non-CF airway epithelial (NL) cells, suggesting an NOS2-specific defect in the IFN-γ signaling pathway. STAT1 and activator protein-1, both required for NOS2 gene expression, interact normally in CF cells. Protein inhibitor of activated STAT1 is not increased in CF cells. IFN-γ induces NOS2 expression in airway epithelial cells through an autocrine mechanism involving synthesis and secretion of IFN-γ-inducible mediator(s), which activates STAT1. Here, CF cells secrete IFN-γ-inducible factor(s), which stimulate NOS2 expression in NL cells, but not in CF cells. In contrast, IFN-γ-inducible factor(s) similarly inhibit virus in CF and NL cells. Thus autocrine activation of NOS2 is defective in CF cells, but IFN-γ induction of antiviral host defense is intact.

Delineation of the HLA-DR Region and the Residues Involved in the Association with the Cytoskeleton

Whereas the association of major histocompatibility complex (MHC) class II molecules with the cytoskeleton and their recruitment into lipid rafts play a critical role during cognate T/antigen-presenting cell interactions, MHC class II-induced signals, regions, and residues involved in their association and recruitment have not yet been fully deciphered. In this study, we show that oligomerization of HLA-DR molecules induces their association with the cytoskeleton and their recruitment into lipid rafts. The association of oligomerized HLA-DR molecules with the cytoskeleton and their recruitment into lipid rafts occur independently. Furthermore, the association with the cytoskeleton is HLA-DR-specific, since oligomerization of HLA-DP triggers its recruitment only into lipid rafts. HLA-DR molecules devoid of both alpha and beta cytoplasmic tails did not associate with the cytoskeleton, but their recruitment into lipid rafts was unimpeded. Deletion of either the alpha or beta cytoplasmic tail did not affect the association of HLA-DR with the cytoskeleton and/or recruitment into lipid rafts. HLA-DR molecules that were devoid of the alpha cytoplasmic chain and that had their beta cytoplasmic chain replaced with the HLA-DP beta chain or with a beta chain in which the residues at positions Gly(226)-His(227)-Ser(228) were substituted by alanine no longer associated with the cytoskeleton. They were, however, still recruited into lipid rafts. Together, these results support the involvement of different regions of the cytoplasmic tails in the association and the recruitment of HLA-DR into different compartments. The differential behavior of HLA-DP and -DR with respect to their association with the cytoskeleton may explain the previously described difference in their transduced signals.

Inhibition of invariant chain expression in dendritic cells presenting endogenous antigens stimulates CD4+ T-cell responses and tumor immunity

Induction of potent and sustained antiviral or antitumor immunity is dependent on the efficient activation of CD8+ and CD4+ T cells. While dendritic cells constitute a powerful platform for stimulating cellular immunity, presentation of endogenous antigens by dendritic cells transfected with nucleic acid-encoded antigens favors the stimulation of CD8+ T cells over that of CD4+ T cells. A short incubation of mRNA-transfected dendritic cells with antisense oligonucleotides directed against the invariant chain enhances the presentation of mRNA-encoded class II epitopes and activation of CD4+ T-cell responses in vitro and in vivo. Immunization of mice with the antisense oligonucleotide-treated dendritic cells stimulates a more potent and longer lasting CD8+ cytotoxic T-cell (CTL) response and enhances the antitumor efficacy of dendritic cell-based tumor vaccination protocols. Transient inhibition of invariant chain expression represents a simple and general method to enhance the stimulation of CD4+ T-cell responses from endogenous antigens.

Inhibition of HLA-DR assembly, transport, and loading by human cytomegalovirus glycoprotein US3: a novel mechanism for evading major histocompatibility complex class II antigen presentation

Human cytomegalovirus (HCMV) establishes persistent lifelong infections and replicates slowly. To withstand robust immunity, HCMV utilizes numerous immune evasion strategies. The HCMV gene cassette encoding US2 to US11 encodes four homologous glycoproteins, US2, US3, US6, and US11, that inhibit the major histocompatibility complex class I (MHC-I) antigen presentation pathway, probably inhibiting recognition by CD8(+) T lymphocytes. US2 also inhibits the MHC-II antigen presentation pathway, causing degradation of human leukocyte antigen (HLA)-DR-alpha and -DM-alpha and preventing recognition by CD4(+) T cells. We investigated the effects of seven of the US2 to US11 glycoproteins on the MHC-II pathway. Each of the glycoproteins was expressed by using replication-defective adenovirus vectors. In addition to US2, US3 inhibited recognition of antigen by CD4(+) T cells by a novel mechanism. US3 bound to class II alpha/beta complexes in the endoplasmic reticulum (ER), reducing their association with Ii. Class II molecules moved normally from the ER to the Golgi apparatus in US3-expressing cells but were not sorted efficiently to the class II loading compartment. As a consequence, formation of peptide-loaded class II complexes was reduced. We concluded that US3 and US2 can collaborate to inhibit class II-mediated presentation of endogenous HCMV antigens to CD4(+) T cells, allowing virus-infected cells to resist recognition by CD4(+) T cells.

Role of Molecular Typing in Live Related Donor Renal Transplantation

In renal transplantation, a good HLA-DR match is associated with higher success rate of graft outcome. It is particularly so In high risk recipients. Serological HLA-DR typing is not always easy due to a number of technical problems. In view of this, a comparison of serological and molecular typing was done in our institutions. A total of 64 live related donor patients of renal transplantation were studied. Serological typing was done by conventional methods. Molecular HLA class II typing was done by polymerase chain reaction (PCR) based sequence specific oligonucleotide probe (SSOP) hybridization technique. An overall discrepancy of 19.5% was observed in the DR typing obtained by serology and PCR-SSOP of all the recipients and donors. 14.5% of cases showed discrepancy in the results of only one DR antigen. Serological typing failure was seen in 10.9% of total cases. In 19.5% cases, only one DR antigen was assigned by PCR-SSOP as compared to two antigens by serological methods. Maximum number of discrepancies were seen in DR 2 antigens. There was no appreciable difference of graft survival shown in the patients typed by both methods. However, higher incidence of acute graft rejection episodes were seen in patients with 1 antigen mismatch as compared to zero mismatch. It is concluded that HLA-DR typing should be carried out by molecular methods as these have been found to be more specific and accurate.

Beryllium binding to HLA-DP molecule carrying the marker of susceptibility to berylliosis glutamate beta 69

Berylliosis is a chronic granulomatous disorder caused by inhalation of Be dusts that is driven by the accumulation of Be-specific CD4+ Th1-cells at disease sites. Susceptibility to berylliosis has been associated with the supratypic variant of HLA-DP gene coding for glutamate at position beta69 (HLA-DPbetaGlu69). The aim of this study was to test the hypothesis that the HLA-DPbetaGlu69 residue plays a role in the interaction with Be. To this end, soluble HLA-DP2 molecule (carrying betaGlu69) and its mutated form carrying lysine at position beta69 (HLA-DP2Lys69) were produced in Drosophila melanogaster and then used in a Be binding assays. BeSO4 (1-1000 microM) was used to compete for the binding of the biotinilated invariant chain-derived peptide CLIP (50 microM). BeSO4 was capable of compete out biotin-CLIP binding from the HLA-DP2 (IC50%: 4.5 microM of BeSO4 at pH 5.0 and 5.5 microM of BeSO4 at pH 7.5), but not from the HLA-DP2Lys69 molecule (IC50%: 480 microM of BeSO4 at pH 5.0 and 220 microM of BeSO4 at pH 7.5). Moreover, the binding of NFLD.M60, a MoAb recognizing an epitope in the HLA-DP peptide binding region, to the HLA-DP2, but not to the HLA-DP2Lys69 soluble molecules was inhibited BeSO4. NFLD.M60 binding to HLA-DP2, but not to HLA-DP2Lys69 stably transfected murine cells was also inhibited by Be both at pH 5.0 and at pH 7.5. The data indicate a direct interaction of Be with the HLA-DPGlu69 molecule, in the absence of antigen processing.

Mutation of the invariant chain transmembrane region inhibits II degradation, prolongs association with MHC class II, and selectively disrupts antigen presentation

The invariant chain (Ii) is a key player in regulating the MHC Class II antigen presentation pathway. Here we used site-directed mutagenesis to identify functionally important regions of the invariant chain in regulating antigen presentation function in transfected cells. Mutation of Ii residues 42-53 caused a defect in the presentation of the ovalbumin 247-265/A(k) epitope, but not in the inhibition of presentation of two hen egg lysozyme epitopes, HEL34-45/A(k) and HEL74-88/A(b), from endogenously expressed antigens. The mutation did not prevent ER translocation, trimerization, or association with MHC Class II molecules and had no obvious effect on endosomal targeting of Ii. It did, however, increase the half-life of the invariant chain, suggesting that sequences in this region influence the degradation of the invariant chain and as a consequence its function in antigen presentation.

Introducing endogenous antigens into the major histocompatibility complex (MHC) class II presentation pathway. Both Ii mediated inhibition and enhancement of endogenous peptide/MHC class II presentation require the same Ii domains

The invariant chain (Ii) plays a key role in regulating the antigen presentation function of major histocompatibility complex (MHC) class II molecules. Ii also influences the presentation of usually excluded endogenously synthesized proteins into the MHC class II presentation pathway. To evaluate the role of Ii in the generation of peptide-MHC class II complexes derived from endogenously synthesized proteins, we tested mutant Ii constructs in two model systems. Co-expression of wild-type Ii inhibits the presentation of hen-egg lysozyme (HEL) 35-45/Ak complex, but enhances the presentation of ovalbumin (OVA) 247-265/Ak complex from endogenously synthesized HEL or OVA precursors. The differential sensitivity of these antigens to chloroquine was consistent with their being processed in distinct compartments. Nevertheless, with a panel of Ii deletion constructs we show here that both the Ii-mediated inhibition and enhancement functions require the endosomal targeting and CLIP residues. Surprisingly, the Ii mutant lacking the endoplasmic reticulum lumenal residues 126-215, despite apparently lower expression, was at least as effective as full-length Ii in antigen presentation assays. Thus, alternative pathways exist for processing endogenously expressed antigens, and Ii-mediated inhibition and enhancement of peptide/MHC class II expression depend upon the same regions, with neither requiring the 89 C-terminal, lumenal Ii residues.

Functional characterization of a lysosomal sorting motif in the cytoplasmic tail of HLA-DObeta

HLA-DO is an intracellular non-classical class II major histocompatibility complex molecule expressed in the endocytic pathway of B lymphocytes, which regulates the loading of antigenic peptides onto classical class II molecules such as HLA-DR. The activity of HLA-DO is mediated through its interaction with the peptide editor HLA-DM. Here, our results demonstrate that although HLA-DO is absolutely dependent on its association with DM to egress the endoplasmic reticulum, the cytoplasmic portion of its beta chain encodes a functional lysosomal sorting signal. By confocal microscopy and flow cytometry analysis, we show that reporter transmembrane molecules fused to the cytoplasmic tail of HLA-DObeta accumulated in Lamp-1(+) vesicles of transfected HeLa cells. Mutagenesis of a leucine-leucine motif abrogated lysosomal accumulation and resulted in cell surface redistribution of reporter molecules. Finally, we show that mutation of the di-leucine sequence in DObeta did not alter its lysosomal sorting when associated with DM molecules. Taken together, these results demonstrate that lysosomal expression of the DO-DM complex is mediated primarily by the tyrosine-based motif of HLA-DM and suggest that the DObeta-encoded motif is involved in the fine-tuning of the intracellular sorting.

Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed antigens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and associated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cells are thus potentially important for antigen presentation. Here we review some cell biological aspects of polarized sorting of MHC class II and the associated invariant chain and the signals that are involved in the sorting process to the basolateral domain. The molecules involved in sorting and loading of peptide may modulate antigen presentation, and in particular we discuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.

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

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

Xenogeneic and allogeneic anti-MHC immune responses induced by plasmid DNA immunization

Major histocompatibility complex (MHC) proteins are known to be incorporated into the human immunodeficiency virus (HIV-1) envelope as the virion buds from the host cell surface. Studies using simian immunodeficiency virus (SIV) infection of macaques have demonstrated that immunization with uninfected human cells or purified HLA proteins can provide protection from challenge with live SIV when it is grown in human cells expressing the same MHC alleles. Thus the induction of anti-MHC immune responses represents an important option to consider with respect to vaccine design for SIV and HIV. Here we examine plasmid DNA immunization strategies as an alternative to cellular or protein immunogens for the induction of xenogeneic and allogeneic immune responses in C57BL/6 mice and in an HLA transgenic mouse model system, respectively. We compared the immunogenicity of HLA-A2- and HLA-B27-expressing splenocytes with the corresponding plasmid DNA immunogens. Results from the transgenic mouse experiments indicate that plasmid DNA immunization with both class I and class II MHC-encoding vectors can elicit antibody responses recognizing conformationally intact MHC molecules. Our data also show that immunization with class I MHC-encoding DNA immunogens can elicit cytotoxic T-lymphocyte responses, demonstrating the potential to mobilize both antibody and cell-mediated anti-MHC immune responses in the context of this approach to HIV-1 vaccine design.

Invariant Chain Can Bind MHC Class II at a Site Other Than the Peptide Binding Groove

Invariant chain binds to class II molecules and guides them to the cell surface via the endosomes. Class II-associated invariant chain peptide (CLIP), a conserved sequence in an unstructured region of invariant chain, binds in the peptide binding groove of class II and is thought to be the major contributor to the interaction between invariant chain and class II molecules. However, other interaction sites between the two proteins may exist. The published data on this subject are conflicting. We have studied the ability of invariant chain to interact with a class II molecule in which the peptide binding groove of the protein is already occupied by a covalently attached peptide. Precipitation of these class II/peptide complexes with an Ab specific for this particular combination also precipitates invariant chain. This binding between class II/peptide and invariant chain is weak, and coprecipitation is only apparent in mild detergents. Thus, when the class II peptide binding groove is occluded by peptide and is not free to interact with CLIP, invariant chain can still bind the class II molecule at other lower affinity sites.

Modulation of cellular tryptophan metabolism in human fibroblasts by transforming growth factor-beta: selective inhibition of indoleamine 2,3-dioxygenase and tryptophanyl-tRNA synthetase gene expression

Alterations in the rate of cellular tryptophan metabolism are involved in mediating important biological activities associated with cytokines and growth factors. Indoleamine 2,3-dioxygenase (IDO) and tryptophanyl-tRNA synthetase are enzymes of tryptophan metabolism whose expression in a variety of cells and tissues is highly inducible by interferon-gamma (IFN-gamma). Transforming growth factor-beta (TGF-beta) antagonizes many cellular responses to IFN-gamma. The interaction of these two cytokines plays an important role in maintaining homeostasis during inflammation and repair. In human skin and synovial fibroblasts in vitro, TGF-beta caused time- and dose-dependent abrogation of IFN-gamma-stimulated expression of IDO and tryptophanyl-tRNA synthetase mRNAs. The inhibition was selective and did not appear to be due to down-regulation of IFN-gamma signaling by TGF-beta. In parallel with its effect on IDO mRNA expression, TGF-beta caused a marked reduction in intracellular IDO protein levels and abrogated IDO activity and tryptophan catabolism in these cells induced by IFN-gamma. IFN-gamma caused a rapid and striking increase in the amount of IDO heterogeneous nuclear pre-mRNA and induced transcription of the IDO gene, as demonstrated by transient transfection assays. TGF-beta partially reversed this stimulation. IFN regulatory factor (IRF)-1 and stat1 are cellular intermediates in IFN signaling. Both are implicated in activation of IDO transcription in response to IFN-gamma. The stimulation by IFN-gamma of IRF-1 protein and mRNA expression was not prevented by treatment of fibroblasts with TGF-beta. Furthermore, gel mobility shift assays indicated that TGF-beta did not inhibit the induction of stat1 and IRF-1 binding activity to their cognate DNA recognition sites in the IDO gene promoter. In contrast, the stability of IDO mRNA transcripts was reduced in fibroblasts treated with TGF-beta, as shown by determination of mRNA half-lives following blockade of transcription with 5,6-dichlorobenzimidazole riboside. The findings indicate that TGF-beta prevents the induction of IDO and tryptophanyl-tRNA synthetase gene expression in fibroblasts. The repression of IDO expression by TGF-beta is mediated at both transcriptional and posttranscriptional levels. These results implicate TGF-beta in the negative regulation of tryptophan metabolism, provide evidence for the molecular basis of this regulation, and indicate that cellular tryptophan metabolism is under tight immunological control.

Retinoblastoma protein expression facilitates chromatin remodeling at the HLA-DRA promoter

The major histocompatibility complex (MHC) class II genes encode a series of heterodimeric cell surface glycoproteins that bind peptide antigen. The MHC class II/peptide complex is bound by the T-cell receptor of CD4(+) T cells, thereby stimulating an immune response. The MHC class II genes are coordinately regulated by conserved promoter elements and are inducible by IFN-gamma. Furthermore, IFN-gamma induction of the MHC class II genes in solid human tumor lines requires retinoblastoma protein (Rb). In vivo footprinting analyses of the HLA-DRA gene, which encodes the heavy chain subunit of the human MHC class II molecule, HLA-DR, revealed that Rb facilitates occupancy of multiple HLA-DRA promoter elements. Detecting the effect of Rb on HLA-DRA promoter occupancy in vivo required IFN-gamma treatment. However, use of a variation on the in vivo footprinting technique, nuclei footprinting, which assays for promoter occupancy in isolated nuclei, revealed that expression of Rb facilitates promoter occupancy even in the absence of IFN-gamma. These results indicate that expression of Rb leads to modification of the chromatin environment of the HLA-DRA promoter independently of transcription.

Major histocompatibility complex class II-dependent unfolding, transport, and degradation of endogenous proteins

We have analyzed the ability of major histocompatibility (MHC) class II molecules to capture proteins in the biosynthetic pathway and whether this may be associated with MHC class II-dependent antigen processing. When coexpressed with HLA-DR 4 molecules in HeLa cells, influenza hemagglutinin was inhibited from folding and trimerization in the biosynthetic pathway, targeted to endosomal compartments, and rapidly degraded. Due to the interaction with MHC class II molecules, therefore, unfolded forms of hemagglutinin were bypassing the quality control mechanism of the secretory pathway. More important, however, the transport, endocytosis, and rapid degradation of unfolded hemagglutinin in the presence of MHC class II molecules suggest that proteins captured in the endoplasmic reticulum by class II molecules may become substrates for antigen processing and presentation to CD4-positive T cells. In insect cells we show that this phenomenon is not restricted to a few proteins such as hemagglutinin. A highly heterogeneous mixture of proteins from the endoplasmic reticulum including coexpressed hemagglutinin can form stable complexes with soluble HLA-DR alpha and beta chains that were transported into the supernatant. This mechanism may gain biological significance in abnormal situations associated with accumulation of unfolded or malfolded proteins in the endoplasmic reticulum, for example during viral infections.

Major histocompatibility complex class II-transfected tumor cells present endogenous antigen and are potent inducers of tumor-specific immunity

We have developed an immunotherapy in which tumor cells transfected with syngeneic major histocompatibility complex (MHC) class II genes are cell-based vaccines for the treatment of established tumor and metastatic disease. If this strategy is to be used clinically, convenient methods for generating class II+ tumor cells are necessary. Interferon-gamma treatment or transduction of the class II transactivator (CIITA) gene induces class II expression but also up-regulates the class II-associated accessory molecules, invariant chain (Ii) and DM. To determine if interferon-gamma treatment and CIITA transduction are potential immunotherapies, we assessed the tumorigenicity of sarcoma cells expressing combinations of class II, Ii, and DM. Since we hypothesized that class II-transfected tumor cells not coexpressing Ii and DM present endogenously encoded tumor peptides, we have assessed the transfectants for antigen presentation activity to MHC class II-restricted antigen-specific CD4(+) T cells. Tumor challenge studies demonstrate that tumor cells expressing class II without coexpression of Ii or Ii plus DM are highly immunogenic and preferentially present endogenous antigens, while tumors coexpressing class II with Ii or Ii plus DM are not effective immunogens. Because tumor rejection correlates with expression of class II without coexpression of Ii and DM, the most efficacious vaccines will express MHC class II without coexpression of Ii and DM and will preferentially present endogenous antigen.

Interferon-gamma regulates collagen and fibronectin gene expression by transcriptional and post-transcriptional mechanisms

Interferon-gamma (IFN-gamma) regulates the expression of collagen and fibronectin genes by molecular mechanisms not completely understood. We investigated the effects of IFN-gamma on the expression of the genes encoding alpha 1 (I) procollagen (COL1A1), alpha 1 (III) procollagen (COL3A1), and fibronectin (FN) in cultured normal human lung fibroblasts. Labeled newly synthesized proteins were analysed by electrophoresis, mRNA levels and stability by Northern hybridizations, and transcription rates by in vitro assays. IFN-gamma caused a reduction in the production of alpha 1 (I) and alpha 1 (III) procollagens and of fibronectin. The reduction in the production of procollagen chains was shown to result from a combination of IFN-gamma-induced inhibition of the transcription rates of the COL1A1 and COL3A1 genes and destabilization of the corresponding transcripts. IFN-gamma increased the transcription rate of FN, but also decreased the stability of the corresponding transcripts. The net results indicate that the regulation of the expression of extracellular matrix genes by IFN-gamma is a complex process that involves changes in gene transcription rates, alterations in mRNAs stability, and possibly, modulation of the rates of translation.

Effects of cytokines combined with high-dose gamma irradiation on the expression of major histocompatibility complex molecules and intercellular adhesion molecule-1 in human ovarian cancers

Tumor cells from 7 freshly isolated human ovarian tumors and 2 continuous human ovarian cancer cell lines were analyzed for their surface expression of MHC class-1, class 11 and ICAM-1 surface antigens before and after exposure to gamma-irradiation and/or the cytokines TNF-alpha plus IFN-gamma. All 7 fresh tumors expressed high levels of MHC class 1 and 1CAM-1 antigens, and levels were markedly up-regulated after exposure to TNF-alpha plus IFN-gamma Similarly, class-11 antigens were either induced (3 out of 7 tumors) or significantly up-regulated by TNF-alpha plus IFN-gamma. Exposure to high doses of gamma-irradiation also increased the expression of MHC class-1 and ICAM-1 antigens, albeit to a modest degree. MHC class 1 and ICAM-1 antigens expression was much lower on continuous human ovarian cell lines than on the fresh tumors. Exposure of these cells to TNF-alpha plus IFN-gamma markedly up-regulated antigen expression to levels comparable to those expressed on the freshly isolated tumors. With the established ovarian cell lines, removal of cytokines caused a rapid down-regulation of antigen expression to basal levels within 6 days, while in the fresh tumors a low level of up-regulation was still present at this time. In contrast, exposure to cytokines followed by high-dose gamma-irradiation resulted in a highly significant and long-lasting expression of each surface antigen which was either up-regulated or induced by the cytokines. These data indicated that the combination of these modalities may be beneficial in generating optimal antigen expression for use of tumor cells in vaccine studies.

Inhibition of invariant chain (Ii)-calnexin interaction results in enhanced degradation of Ii but does not prevent the assembly of alpha beta Ii complexes

Calnexin is a resident protein of the endoplasmic reticulum (ER) that associates with nascent protein chains. Among the newly synthesized integral membrane proteins known to bind to calnexin is invariant chain (Ii), and Ii release from calnexin coincides with proper assembly with major histocompatibility complex (MHC) class II heterodimers. Although calnexin association with several membrane glycoproteins depends on interactions involving N-linked glycans, we previously reported that a truncation mutant of mouse Ii (mIi1-107) lacking both N-glycosylation sites was highly effective in associating with MHC class II heterodimers and escorting these dimers through the secretory pathway. This could indicate that calnexin, despite binding to both Ii and class II, is not necessary for the proper interaction of these proteins, or that in contrast to most membrane glycoproteins, the N-linked glycans of Ii are not critical to its interaction with this chaperone. To examine this issue, we have directly explored the binding of calnexin to both Ii truncation mutants lacking the typical sites of N-glycosylation or Ii produced in cells treated with tunicamycin to prevent glycan addition. These experiments revealed that either method of eliminating N-linked carbohydrates on Ii also inhibited association with calnexin. A lumenally truncated form of Ii (mIi1-131) that still has N-linked carbohydrates showed a decreased affinity for calnexin compared with intact Ii, however, indicating that calnexin-Ii binding is not determined solely by the sugar moieties. All forms of Ii lacking N-linked sugars and showing defective association with calnexin also had enhanced rates of preendosomal degradation. Despite this effect on degradation rate, tunicamycin treatment did not inhibit the association of class II with glycan-free Ii. These data support the view that calnexin is not an absolute requirement for the proper assembly of class II-Ii nonamers, but rather acts primarily to retain Ii in the ER and to inhibit its degradation. These two properties of calnexin-Ii interaction may help ensure that sufficient intact Ii is available for efficient inactivation of the binding sites of newly synthesized class II molecules, while limiting the ability of excess free Ii to alter the transport properties of the early endocytic pathway.

Potent effects of low levels of MHC class II-associated invariant chain on CD4+ T cell development

Invariant chain (Ii)-negative mice exhibit defects in MHC class II assembly and transport that results in reduced levels of surface class II, altered antigen presentation, and inefficient positive selection of CD4+ T cells. Many CD4+ T cells that do mature in Ii-negative mice express a cell surface phenotype consistent with aberrant positive selection or peripheral activation. Reconstitution of these mice with low levels of either the p31 or p41 form of Ii does not restore transport of the bulk of class II or class II surface expression, but surprisingly does restore positive selection as measured by numbers and surface phenotype of CD4+ T cells. Thus, an Ii-dependent process, independent of effects on class II surface density, appears to be required for normal positive selection of CD4+ T cells.

HLA-DR polymorphism affects the interaction with CD4

Major histocompatibility complex (MHC) class II molecules are highly polymorphic and bind peptides for presentation to CD4+ T cells. Functional and adhesion assays have shown that CD4 interacts with MHC class II molecules, leading to enhanced responses of CD4+ T cells after the activation of the CD4-associated tyrosine kinase p56lck. We have addressed the possible contribution of allelic polymorphism in the interaction between CD4 and MHC class II molecules. Using mouse DAP-3-transfected cells expressing different isotypes and allelic forms of the HLA-DR molecule, we have shown in a functional assay that a hierarchy exists in the ability of class II molecules to interact with CD4. Also, the study of DR4 subtypes minimized the potential contribution of polymorphic residues of the peptide-binding groove in the interaction with CD4. Chimeras between the DR4 or DR1 molecules, which interact efficiently with CD4, and DRw53, which interacts poorly, allowed the mapping of polymorphic residues between positions beta 180 and 189 that can exert a dramatic influence on the interaction with CD4.

Major histocompatibility class II peptide occupancy, antigen presentation, and CD4+ T cell function in mice lacking the p41 isoform of invariant chain

We used a "hit and run" gene targeting strategy to generate mice expressing only the p31 isoform of the conserved invariant (Ii) chain associated with major histocompatibility complex (MHC) class II molecules. Spleen cells from these mice appear indistinguishable from wild type with respect to class II subunit assembly, transport, peptide acquisition, surface expression, and the ability to present intact protein antigens. Moreover, these mutant mice have normal numbers of thymic and peripheral CD4+ T cells, and intact CD4+ T-dependent proliferative responses towards a soluble antigen. In short, MHC class II expression and function are surprisingly unaffected in mice lacking p41 invariant chain, implying that the p31 and p41 isoforms may be functionally redundant in the intact animal.

Inhibition of type I collagen mRNA expression independent of tryptophan depletion in interferon-gamma-treated human dermal fibroblasts

Interferon-gamma (IFN-gamma) is a pleiotropic cytokine that modulates type I collagen synthesis. In addition, IFN-gamma also exerts potent effects on cellular tryptophan levels by inducing the expression of indoleamine 2,3-dioxygenase (IDO) and tryptophanyl-tRNA synthetase. Because recent evidence indicates that IDO-mediated oxidative tryptophan catabolism is important in cellular responses to IFN-gamma, we investigated the role of IDO in the IFN-gamma-induced modulation of type I collagen gene expression. IFN-gamma ( > or = 50 U/ml) stimulated IDO expression in human dermal fibroblasts in vitro, resulting in a > 90% depletion of tryptophan in the culture media following incubation for 48 h. Higher concentrations of IFN-gamma ( > or = 500 U/ml) caused a marked decrease in type I collagen mRNA levels. Time-course studies indicated that maximal induction of IDO mRNA expression in IFN-gamma-treated fibroblast cultures (24 h) preceded the maximal decrease in collagen mRNA (96 h). Type I collagen mRNA levels were also markedly and selectively decreased in fibroblasts maintained in tryptophan-depleted cultures. Addition of exogenous tryptophan (up to 2500 microM) to IFN-gamma-treated fibroblasts restored "normal" concentrations of tryptophan in the culture media, but did not abrogate the IFN-gamma-induced decrease in collagen mRNA. Addition of the tryptophan metabolite kynurenine, in concentrations similar to those generated in fibroblast cultures following IFN-gamma treatment for 48 h, had no significant effect on type I collagen mRNA levels. These results indicate that although IFN-gamma causes activation of IDO and enhanced tryptophan catabolism in fibroblast cultures, neither the ensuing tryptophan starvation nor the accumulation of kynurenine in the culture media can fully account for the inhibitory effects of IFN-gamma on type I collagen mRNA expression.

Expression of endogenous peptide-major histocompatibility complex class II complexes derived from invariant chain-antigen fusion proteins

CD4+ T cells recognize major histocompatibility complex (MHC) class II-bound peptides that are primarily obtained from extracellular sources. Endogenously synthesized proteins that readily enter the MHC class I presentation pathway are generally excluded from the MHC class II presentation pathway. We show here that endogenously synthesized ovalbumin or hen egg lysozyme can be efficiently presented as peptide-MHC class II complexes when they are expressed as fusion proteins with the invariant chain (Ii). Similar to the wild-type Ii, the Ii-antigen fusion proteins were associated intracellularly with MHC molecules. Most efficient expression of endogenous peptide-MHC complex was obtained with fusion proteins that contained the endosomal targeting signal within the N-terminal cytoplasmic Ii residues but did not require the luminal residues of Ii that are known to bind MHC molecules. These results suggest that signals within the Ii can allow endogenously synthesized proteins to efficiently enter the MHC class II presentation pathway. They also suggest a strategy for identifying unknown antigens presented by MHC class II molecules.

Inhibition of collagenase and stromelysin gene expression by interferon-gamma in human dermal fibroblasts is mediated in part via induction of tryptophan degradation

The expression of the matrix-degrading enzymes collagenase and stromelysin is modulated by a variety of biologic and pharmacologic agents. IFN-gamma has potent effects on metalloproteinase production and therefore may play an important role in preventing excessive connective tissue degradation during inflammation and repair. We investigated the mechanisms of collagenase and stromelysin regulation by IFN-gamma in human dermal fibroblasts. IFN-gamma (300 U/ml) prevented the stimulation of metalloproteinase gene expression by IL-1 beta. In addition, incubation of fibroblasts with IFN-gamma resulted in a marked increase in cellular indoleamine 2,3-dioxygenase (IDO) mRNA, a > 90% depletion of tryptophan, and a corresponding > 30-fold increase in the tryptophan metabolite kynurenine in the culture media. Reducing the concentration of tryptophan from 25 microM to 0 markedly diminished the ability of fibroblasts to increase collagenase and stromelysin mRNA and collagenase production in response to IL-1 beta. Addition of exogenous tryptophan (25-50 micrograms/ml) to cultures that had been tryptophan depleted by pretreatment with IFN-gamma for 48 h restored the fibroblast response to IL-1 beta or PMA, but had no effect on IFN-gamma-induced HLA-DR alpha chain mRNA expression. These results indicate that inhibition of collagenase and stromelysin gene expression by IFN-gamma in fibroblasts is associated with activation of IDO and enhanced cellular tryptophan metabolism. Tryptophan degradation and ensuing tryptophan depletion may account, at least in part, for the inhibitory effect of IFN-gamma on metalloproteinase production in dermal fibroblasts.

Allelic differences affecting invariant chain dependency of MHC class II subunit assembly

The conserved invariant chain associates with highly polymorphic alpha and beta subunits guiding class II transport through the secretory pathway. Early associations of these three polypeptides inside antigen-presenting cells are poorly understood. The present experiments provide a detailed picture of the structure and fate of class II alpha and beta subunits in invariant chain mutants possessing different MHC haplotypes. In the absence of invariant chain, A alpha bA beta b is predominantly expressed as free A alpha b and A beta b chains by both splenocytes and activated LPS/IL-4 blasts, confirming that A alpha bA beta b assembly is strongly dependent on invariant chain coexpression. A quite different situation exists with respect to other allelic products. In the absence of invariant chain, A alpha kA beta k, E alpha kE beta k, and A alpha dA beta d molecules assemble efficiently and are conformationally similar to mature wild-type heterodimers. The contribution of invariant chain to subunit assembly thus differs for allelic variants, suggesting that sequential associations of alpha, beta, and invariant chain may be affected by polymorphic differences.