Efficient cell surface expression of class II MHC molecules in the absence of associated 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.

Introduction of 65 kDa Antigen ofMycobacterium tuberculosisto Cancer Cells Enhances Anti-Tumor Effect of BCG Therapy

Bacillus Calmette Guerin (BCG) immunotherapy has anti-tumorigenic effects against bladder cancer. To improve the efficacy of BCG therapy, we introduced the gene encoding the 65 kDa heat shock protein (hsp) of Mycobacterium tuberculosis into a mouse malignant melanoma cell line (B16). An expression vector harboring the 65 kDa antigen gene was transfected into B16 using Lipofectamine, then expression of the antigen was confirmed by RT-PCR and Western blotting. Several cell lines expressing 65 kDa antigen were established (B16/65 kDa). We also established a control cell line transfected with the vector alone (B16/con). All cell lines (B16, B16/con, B16/65 kDa) were injected intraperitoneally into syngeneic mice with or without BCG prior immunization and the development of tumor ascites was examined. To analyze the mechanism of the anti-tumor effect, CD4 T cells or CD8 T cells were depleted in vivo by administering the corresponding monoclonal antibody. B16/65k Da expressed the 65 kDa hsp of M. tuberculosis. The tumor growth of B16/65 kDa was slightly retarded in naive mice, but significantly inhibited by BCG. The anti-tumor effect was totally abrogated in mice deficient in CD4 T cells, suggesting that CD4 T cells are involved in this process. The 65 kDa hsp of M. tuberculosis was expressed after gene transduction in a malignant melanoma cell line and significantly enhanced the anti-tumor effect of BCG immunotherapy. CD4 T cells play an important role in this anti-tumor effect.

HIV-derived lentiviral particles promote T-cell independent activation and differentiation of naïve cognate conventional B2-cells in vitro

In animal models, lentiviral particles (LP) were shown to be promising HIV vaccine candidates. Since little is known about the direct impact of LP on antigen-specific B cells, we incorporated Hen Egg Lysozyme (HEL) into LP (HEL-LP) derived from HIV to study their effect on HEL-specific, B cell receptor-transgenic B-cells (HEL(+)B-cells) in vitro. We observed preferential binding of HEL-LP to HEL(+)B-cells and their efficient internalization. HEL-LP were able to effectively cross-link B-cell receptors as indicated by the loss of surface CD62L. In the absence of CD4(+) T-cells, other activation events induced by LP in cognate naïve B-cells included increased expression of activation and co-stimulatory molecules as well as an enhanced proliferative response. Additionally, the B-cell phenotype shifted toward a germinal center pattern with further differentiation into memory and IgG3- and IgA-producing cells. The observed CD4(+) T-cell independent activation and differentiation may be due to LP-induced expression of CD40L by a subset of cognate B-cells. Thus, even in the absence of CD4(+) T-cells LP provide strong direct activation signals to cognate naïve B-cells, which may contribute to the strong humoral immune responses observed after LP immunization.

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.

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.

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.

Suppression of major histocompatibility complex class II-associated invariant chain enhances the potency of an HIV gp120 DNA vaccine

Summary One function of the major histocompatibility complex (MHC) class II-associated invariant chain (Ii) is to prevent MHC class II molecules from binding endogenously generated antigenic epitopes. Ii inhibition leads to MHC class II presentation of endogenous antigens by APC without interrupting MHC class I presentation. We present data that in vivo immunization of BALB/c mice with HIV gp120 cDNA plus an Ii suppressive construct significantly enhances the activation of both gp120-specific T helper (Th) cells and cytotoxic T lymphocytes (CTL). Our results support the concept that MHC class II-positive/Ii-negative (class II(+)/Ii(-)) antigen-presenting cells (APC) present endogenously synthesized vaccine antigens simultaneously by MHC class II and class I molecules, activating both CD4(+) and CD8(+) T cells. Activated CD4(+) T cells locally strengthen the response of CD8(+) CTL, thus enhancing the potency of a DNA vaccine.

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 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.

A cytosolic pathway for MHC class II–restricted antigen processing that is proteasome and TAP dependent

By convention, presentation of major histocompatibility complex (MHC) class I-restricted epitopes involves processing by cytosolic proteasomes, whereas MHC class II-restricted epitopes are generated by endosomal proteases. Here, we show that two MHC class II-restricted epitopes within influenza virus were generated by a proteasome- and TAP-dependent pathway that was accessed by exogenous virus in dendritic cells (DCs) but not cell types with less permeable endosomes. Both epitopes were presented by recycling MHC class II molecules. Challenging mice with influenza or vaccinia viruses demonstrated that a substantial portion of the MHC class II-restricted response was directed against proteasome-dependent epitopes. By complementing endosomal activities, this pathway broadens the array of MHC class II-restricted epitopes available for CD4(+) T cell activation.

Endoplasmic reticulum-associated degradation-induced dissociation of class II invariant chain complexes containing a glycosylation-deficient form of p41

The quality control system in the secretory pathway can identify and eliminate misfolded proteins through endoplasmic reticulum-associated degradation (ERAD). ERAD is thought to occur by retrotranslocation through the Sec61 complex into the cytosol and degradation by the proteasome. However, the extent of disassembly of oligomeric proteins and unfolding of polypeptide chains that is required for retrotranslocation is not fully understood. In this report we used a glycosylation mutant of the p41 isoform of invariant chain (Ii) to evaluate the ability of ERAD to discriminate between correctly folded and misfolded subunits in an oligomeric complex. We show that loss of glycosylation at position 239 of p41 does not detectably affect Ii trimerization or association with class II but does result in a defect in endoplasmic reticulum export of Ii that ultimately leads to its degradation via the ERAD pathway. Although class II associated with the mutated form of p41 is initially retained in the endoplasmic reticulum, it is subsequently released and traffics through the Golgi to the plasma membrane. ERAD-mediated degradation of the mutant p41 is dependent on mannose trimming and inhibition of mannosidase I stabilizes Ii. Interestingly, inhibition of mannosidase I also results in prolonged association between the mutant Ii and class II, indicating that complex disassembly and release of class II is linked to mannosidase-dependent ERAD targeting of the misfolded Ii. These results suggest that the ERAD machinery can induce subunit disassembly, specifically targeting misfolded subunits to degradation and sparing properly folded subunits for reassembly and/or export.

Differential requirements for endosomal reduction in the presentation of two H2-E(d)-restricted epitopes from influenza hemagglutinin

We examined the role of reduction in the presentation of two H2-E(d)-restricted epitopes (site 1 epitope (S1) and site 3 epitope (S3)) occupying distinct domains of the influenza hemagglutinin major subunit that contains four intrachain disulfide bonds and is connected to the virion by one interchain bond. S3 is situated within the stalk region that unfolds in response to mild acidification, and loads onto recycling H2-E(d) in the early endosome, while S1, located in the structurally constrained globular domain, loads onto nascent H2-E(d) in the late endosome. Predicting dependence upon reduction for either epitope seemed plausible but the results from several approaches were clear: presentation of S1 but not S3 is reduction dependent. Surprisingly, IFN-gamma-inducible lysosomal thiol reductase (GILT), the only reductase thus far known to be involved in MHC class II-restricted processing, is not necessary for the generation of S1. However, GILT is necessary for presentation of either epitope when the virus is pretreated with a reducible cross-linker. The results suggest that unfolding of the Ag, perhaps a prerequisite for proteolytic processing in many cases, proceeds either spontaneously in the early endosome or via reduction in a later endosome. They further imply mechanisms for GILT-independent reduction in the late endosome, with GILT perhaps being reserved for more intractable Ags.

MHC class II and CD80 tumor cell-based vaccines are potent activators of type 1 CD4+ T lymphocytes provided they do not coexpress invariant chain

We are developing vaccines that activate tumor-specific CD4+ T cells. The cell-based vaccines consist of MHC class I+ tumor cells that are genetically modified to express syngeneic MHC class II and costimulatory molecules. Previous studies demonstrated that treatment of mice with established tumors with these vaccines resulted in regression of solid tumors, reduction of metastatic disease, and increased survival time. Optimal vaccines will prime naïve T cells and activate T cells to tumor peptides derived from diverse subcellular compartments, since potential tumor antigens may reside in unique cellular locales. To determine if the MHC class II/costimulatory molecule vaccines fulfill these conditions, the vaccines have been tested for their ability to activate antigen-specific, naïve, transgenic CD4+ T lymphocytes. MHC class II(+)CD80+ vaccine cells were transfected with hen eggwhite lysozyme targeted to the cytosol, nuclei, mitochondria, or endoplasmic reticulum, and used as antigen-presenting cells to activate I-Ak-restricted, lysozyme-specific CD4+ 3A9 transgenic T cells. Regardless of the cellular location of lysozyme, the vaccines stimulated release of high levels of IFN-gamma and IL-2. If the vaccines coexpressed the MHC class II accessory molecule invariant chain, then IFN-gamma and IL-2 release was significantly reduced. These studies demonstrate that in the absence of invariant chain the MHC class II and CD80 tumor cell vaccines (1) function as antigen-presenting cells to activate naïve, tumor-specific CD4+ cells to endogenously synthesized tumor antigens; (2) polarize the activated CD4+ T cells toward a type 1 response; and (3) present epitopes derived from varied subcellular locales.

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.

A novel single chain I-A(b) molecule can stimulate and stain antigen-specific T cells

Multimers of soluble major histocompatibility complex class I and II molecules have proven to be useful reagents in quantifying and following specific T cell populations. This study describes the design, generation, and characterization of a novel, single chain I-A(b) molecule which utilizes a unique linker derived from the murine invariant chain. A fragment of the invariant chain, residues 58-85, binds to a region proximal to the class II peptide binding groove and stabilizes occupancy of the class II invariant chain-associated peptide. We have utilized this fragment, replacing CLIP with the Ealpha peptide sequence, to lock the attached peptide into the class II binding groove. The single chain I-A(b) molecule was recognized by a panel of conformation-sensitive, I-A(b)-specific, monoclonal antibodies. Membrane-bound and soluble forms of the single chain I-A(b) stimulated an antigen-specific T cell hybridoma, and tetramers made from soluble monomers stained these cells. The unique features of this molecule may be useful in the design of recombinant T cell receptor ligands containing peptides with low affinity for MHC.

Presentation by recycling MHC class II molecules of an influenza hemagglutinin-derived epitope that is revealed in the early endosome by acidification

We investigated the roles of nascent and recycling MHC class II molecules (MHC II) in the presentation of two well-defined I-E(d)-restricted epitopes that are within distinct regions of the influenza virus hemagglutinin (HA) protein. The site 3 epitope (S3; residues 302-313) lies in the stalk region that unfolds in response to mild acidification, while the site 1 epitope (S1; residues 107-119) is situated in the stable globular domain. In a murine B lymphoma cell line and an I-E(d)-transfected fibroblast cell line, presentation from inactivated virus of S3 is inhibited by primaquine, a compound that prevents recycling of cell surface proteins, including MHC II, while S1 presentation is unaffected. In contrast, brefeldin A, an agent that inhibits exit of proteins from the endoplasmic reticulum, selectively inhibited S1 presentation without affecting S3 presentation, suggesting that S1 presentation requires nascent MHC II. The use of agents that perturb endosomal function revealed a requirement for acidification of internalized viral particles for presentation of both epitopes. Notably, all compounds tested had similar effects on presentation of the two epitopes derived from endogenously synthesized HA. Thus, recycling I-E(d) molecules appear to be crucial for capturing and presenting an epitope that is revealed in mild acidic conditions following the uptake of virions or the synthesis of Ag, while nascent I-E(d) molecules are required for presentation of a second epitope located in a structurally constrained region of the same polypeptide. Viral glycoproteins, such as HA, may have been a major impetus for the evolutionary establishment of this recycling pathway.

Live-cell dynamics and the role of costimulation in immunological synapse formation

Using transfected fibroblasts expressing both wild-type I-E(k) and green fluorescent protein-tagged I-E(k) with covalently attached antigenic peptide, we have monitored movement of specific MHC:peptide complexes during CD4(+) T cell-APC interactions by live-cell video microscopy. Ag recognition occurs within 30 s of T cell-APC contact, as shown by a sharp increase in cytoplasmic calcium ion concentration. Within 1 min, small MHC:peptide clusters form in the contact zone that coalesce into an immunological synapse over 3-20 min. When T cells conjugated to APC move across the APC surface, they appear to drag the synapse with them. This system was used to examine the role of costimulation in the formation of the immunological synapse. Blocking CD80/CD28 or ICAM-1/LFA-1 interactions alters synapse morphology and reduces the area and density of accumulated complexes. These reductions correlate with reduced T cell proliferation, while CD69 and CD25 expression and TCR down-modulation remain unaffected. Thus, costimulation is essential for normal mature immunological synapse formation.

Differences in in vitro invasive capacity induced by differences in Ki-Ras protein mutations

The p21 proteins encoded by N-, Ki-, and H-ras are small guanine nucleotide-binding proteins that act as switches in several signal transduction pathways. Recently, evidence has been accumulating to suggest that valine-12 mutation in the Ki-Ras protein is associated with lung and colorectal tumours that are more aggressive than those carrying aspartate-12 mutation. The purpose of this study was to determine whether cells transfected with different Ki-ras codon-12 mutants have different biological behaviours in vitro that could reflect the differences in behaviour in vivo. For that reason, Rat-1 fibroblasts transfected with the valine-12 or aspartate-12 mutant or the wild-type Ki-ras gene were assessed in terms of in vitro invasion, transformation, and VEGF production. Both mutants demonstrated equal abilities to transform Rat-1 cells and induce VEGF production, while cells transfected with wild-type Ki-Ras failed to do so. Most significantly, the valine-12 mutants demonstrated a greater ability to invade Matrigel than cells expressing the aspartate-12 mutant or wild-type Ki-Ras proteins. This study complements previous experimental data that specific Ras mutations differ in their effects in vivo and shows, for the first time, a significant difference in Matrigel invasion in vitro. The precise mechanisms behind these biological differences in vivo and in vitro should now be investigated.

Trace amines: identification of a family of mammalian G protein-coupled receptors

Tyramine, beta-phenylethylamine, tryptamine, and octopamine are biogenic amines present in trace levels in mammalian nervous systems. Although some "trace amines" have clearly defined roles as neurotransmitters in invertebrates, the extent to which they function as true neurotransmitters in vertebrates has remained speculative. Using a degenerate PCR approach, we have identified 15 G protein-coupled receptors (GPCR) from human and rodent tissues. Together with the orphan receptor PNR, these receptors form a subfamily of rhodopsin GPCRs distinct from, but related to the classical biogenic amine receptors. We have demonstrated that two of these receptors bind and/or are activated by trace amines. The cloning of mammalian GPCRs for trace amines supports a role for trace amines as neurotransmitters in vertebrates. Three of the four human receptors from this family are present in the amygdala, possibly linking trace amine receptors to affective disorders. The identification of this family of receptors should rekindle the investigation of the roles of trace amines in mammalian nervous systems and may potentially lead to the development of novel therapeutics for a variety of indications.

Secreted type of modified interleukin-18 gene transduced into mouse renal cell carcinoma cells induces systemic tumor immunity

PURPOSE

Interleukin (IL)-18 is a novel cytokine that has been identified as a strong interferon-gamma inducer. IL-18 has bioactivity similar to that of IL-12 and demonstrates antitumor effects. Since IL-18 does not have a signal sequence, we constructed the gene, consisting of the signal sequences of interferon-beta and mature IL-18 complementary (c) DNA. The modified gene was subsequently transduced into a mouse renal cell carcinoma cell line to induce IL-18 secretion from tumor cells to establish whether this IL-18 secreting tumor cell line may induce systemic tumor immunity.

MATERIALS AND METHODS

Modified IL-18 cDNA consisting of the signal sequences of interferon-beta and mature type of IL-18 cDNA was constructed by the overlap extension method. The modified and original IL-18 cDNA was transduced into the RenCa mouse renal cell carcinoma cell line. Expression of IL-18 messenger RNA and concentration of IL-18 in the culture supernatant were determined. Direct antitumor and tumor vaccine effects were investigated in syngeneic Balb/c mice. To determine the mechanism of the antitumor effect immunodeficient mice were challenged with IL-18 secreting RenCa cells.

RESULTS

Although the modified and original IL-18 genes transduced RenCa showed almost the same level of IL-18 messenger RNA expression, only RenCa cells transduced with modified IL-18 gene secreted IL-18 into the culture supernatant and were completely rejected when transplanted into syngeneic mice. T lymphocytes were involved in this antitumor effect. Moreover, the modified IL-18 transduced RenCa induced tumor vaccine effect against parental RenCa cells injected at a distant site.

CONCLUSIONS

Immune gene therapy using modified IL-18 cDNA appears to be effective and IL-18 secreting cancer cells may be a candidate for tumor vaccine therapy.

SECRETED TYPE OF MODIFIED INTERLEUKIN-18 GENE TRANSDUCED INTO MOUSE RENAL CELL CARCINOMA CELLS INDUCES SYSTEMIC TUMOR IMMUNITY

PURPOSE

Interleukin (IL)-18 is a novel cytokine that has been identified as a strong interferon-gamma inducer. IL-18 has bioactivity similar to that of IL-12 and demonstrates antitumor effects. Since IL-18 does not have a signal sequence, we constructed the gene, consisting of the signal sequences of interferon-beta and mature IL-18 complementary (c) DNA. The modified gene was subsequently transduced into a mouse renal cell carcinoma cell line to induce IL-18 secretion from tumor cells to establish whether this IL-18 secreting tumor cell line may induce systemic tumor immunity.

MATERIALS AND METHODS

Modified IL-18 cDNA consisting of the signal sequences of interferon-beta and mature type of IL-18 cDNA was constructed by the overlap extension method. The modified and original IL-18 cDNA was transduced into the RenCa mouse renal cell carcinoma cell line. Expression of IL-18 messenger RNA and concentration of IL-18 in the culture supernatant were determined. Direct antitumor and tumor vaccine effects were investigated in syngeneic Balb/c mice. To determine the mechanism of the antitumor effect immunodeficient mice were challenged with IL-18 secreting RenCa cells.

RESULTS

Although the modified and original IL-18 genes transduced RenCa showed almost the same level of IL-18 messenger RNA expression, only RenCa cells transduced with modified IL-18 gene secreted IL-18 into the culture supernatant and were completely rejected when transplanted into syngeneic mice. T lymphocytes were involved in this antitumor effect. Moreover, the modified IL-18 transduced RenCa induced tumor vaccine effect against parental RenCa cells injected at a distant site.

CONCLUSIONS

Immune gene therapy using modified IL-18 cDNA appears to be effective and IL-18 secreting cancer cells may be a candidate for tumor vaccine therapy.

Regulation of MUC1 expression in human mammary cell lines by the c-ErbB2 and ras signaling pathways

The MUC1 protein is a highly O-glycosylated transmembrane molecule that is expressed at the luminal surface of most glandular epithelial cells and is upregulated in carcinomas. Here, we report the effect of the activation of the c-ErbB2 --> Ras pathway on the expression of the MUC1 gene in the nontumorigenic mammary cell lines MTSV1-7 and HB2 and in the malignant cell lines T47D and ZR75. Endogenous levels of MUC1 mRNA and protein in HB2 clones permanently overexpressing c-ErbB2 or V12-H-Ras were markedly reduced compared with levels in the parental cell lines. Furthermore, in transient transfection assays, the transcription of a CAT reporter construct driven by the MUC1 promoter was inhibited when cotransfected with a c-ErbB2 or a V12-H-ras expressing vector. Transient transfections using mutant forms of the ras oncogene, and the inhibitor chemical wortmannin, indicated that the pathway activated by c-ErbB2 proceeds via activation of Ras and that the Raf and phosphoinositide 3-kinase pathways are involved. Finally, cotransfection assays using a reporter gene driven by the MUC1 promoter carrying abolishing mutations in some of the cis-acting elements showed that a GC box at -99/-91 is crucial for responsiveness to c-ErbB2 inhibition of transcription.

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.

Antitumor effects on mouse melanoma elicited by local secretion of interleukin-12 and their enhancement by treatment with interleukin-18

To investigate the mechanism of the antitumor effect of locally secreted interleukin-12 (IL-12), we introduced the IL-12 p35 and p40 cDNAs into mouse B16 melanoma cells. IL-12 gene-transfected B16 melanoma (B16/IL12) showed marked retardation of tumor growth when implanted subcutaneously into syngeneic mice. In these mice, depletion of not only Natural Killer (NK) cells but also CD8+ T cells diminished the antitumor effect of locally secreted IL-12. Immunohistochemical analysis showed that NK cells and macrophages accumulated more densely at the center and periphery of B16/IL12 tumors than that of parental B16 tumors, whereas CD4+ T cells and CD8+ T cells accumulated sparsely only at the periphery of both transfected and untransfected tumors. Systemic treatment with interleukin-18 (IL-18) markedly inhibited the growth of B16/IL12 but did not influence the tumor growth of parental B16 cells in vivo. These results suggest that local IL-12 secretion can retard the growth of B16 melanoma mediated primarily by NK cells and indirectly by CD8+ T cells and that its antitumor effect is augmented by systemic treatment with the novel cytokine IL-18.

Transfection of nonmelanocytic cells with tyrosinase gene constructs for survival studies

To better understand the role of melanin in the response of cells to radiation, the vector pcTYR containing the tyrosinase cDNA and a control vector pcTYW with no tyrosinase cDNA were transfected and expressed in nonpigmented CHOK1-A(L) 1282B5 cells. A pigmented clone was selected from the pcTYR transfectants and an antibiotic-resistant clone was selected from the controls. Melanin was assessed qualitatively by electron paramagnetic resonance (EPR) and quantitatively by a 14C-based assay. The EPR signal detectable in pcTYR-containing cells was at least twice that of pcTYW and parental CHOK1-A(L) cells and the tyrosinase activity was found to be at least six times greater. Melanin was classified to be eumelanin. Survivals of the transfectants were compared to those of the parent cells after irradiation by UVC from a germicidal lamp, UVB from TL01 lamps, UVA from Alisun lamps, UVB/UVA from FS20 lamps, and by gamma-rays from a 137Cs source. Compared to the pcTYW-containing cells, the pigmented cells were more sensitive to killing by UVC, and resistant to killing by UVA and gamma-rays. There were no significant differences in survival after the other irradiations. These results suggest that the pigment synthesized by the activity of tyrosinase alone, unmodified by the activities of TRP1 and TRP2, is protective against the types of reactive oxygen species produced by UVA and gamma-rays but not protective against lethal damage from photons in the UVB range and sensitizes to UVC photons.

Scavenger receptor class B, type I is expressed in porcine brain capillary endothelial cells and contributes to selective uptake of HDL-associated vitamin E

It is clearly established that an efficient supply to the brain of alpha-tocopherol (alphaTocH), the most biologically active member of the vitamin E family, is of the utmost importance for proper neurological functioning. Although the mechanism of uptake of alphaTocH into cells constituting the blood-brain barrier (BBB) is obscure, we previously demonstrated that high-density lipoprotein (HDL) plays a major role in the supply of alphaTocH to porcine brain capillary endothelial cells (pBCECs). Here we studied whether a porcine analogue of human and rodent scavenger receptor class B, type I mediates selective (without concomitant lipoprotein particle internalization) uptake of HDL-associated alphaTocH in a similar manner to that described for HDL-associated cholesteryl esters (CEs). In agreement with this hypothesis we observed that a major proportion of alphaTocH uptake by pBCECs occurred by selective uptake, exceeding HDL3 holoparticle uptake by up to 13-fold. The observation that selective uptake of HDL-associated CE exceeded HDL3 holoparticle up to fourfold suggested that a porcine analogue of SR-BI (pSR-BI) may be involved in lipid uptake at the BBB. In line with the observation of selective lipid uptake, RT-PCR and northern and western blot analyses revealed the presence of pSR-BI in cells constituting the BBB. Adenovirus-mediated overexpression of the human analogue of SR-BI (hSR-BI) in pBCECs resulted in a fourfold increase in selective HDL-associated alphaTocH uptake. In accordance with the proposed function of SR-BI, selective HDL-CE uptake was increased sixfold in Chinese hamster ovary cells stably transfected with murine SR-BI (mSR-BI). Most importantly stable mSR-BI overexpression mediated a twofold increase in HDL-associated [14C]alphaTocH selective uptake in comparison with control cells. In line with tracer experiments, mass transfer studies with unlabelled lipoproteins revealed that mSR-BI overexpression resulted in a twofold increase in endogenous HDL3-associated alphaTocH uptake. The results of this study indicate that SR-BI promotes the uptake of HDL-associated alphaTocH into cells constituting the BBB and plays an important role during the supply of the CNS with this indispensable micronutrient.

Identification and characterization of two neuromedin U receptors differentially expressed in peripheral tissues and the central nervous system

Two structurally related, G-protein-coupled receptors were identified as receptors for the neuropeptide, neuromedin U. This peptide is found in highest levels in the gut and genitourinary system where it potently contracts smooth muscle but is also expressed in the spinal cord and discrete regions of the brain. Binding sites for neuromedin U have been characterized in rat uterus, however, little is known about the activity of this peptide in the regions of the central nervous system where it is expressed. The receptors characterized in this report are activated by neuromedin U at nanomolar potency in heterologous expression systems and bind radiolabeled neuromedin U with high affinity. Localization of the receptor RNA by quantitative reverse transcription-polymerase chain reaction in a variety of human tissues shows distinct expression patterns for the two receptors. NMU1 is expressed predominantly in peripheral tissues, whereas NMU2 is more highly expressed in the central nervous system. Identification of neuromedin U receptor subtypes will greatly aid in the determination of the physiological roles of this peptide.

Mutation of Ha-Ras C terminus changes effector pathway utilization

In PC12 cells, Ha-Ras modulates multiple effector proteins that induce neuronal differentiation. To regulate these pathways Ha-Ras must be located at the plasma membrane, a process normally requiring attachment of farnesyl and palmitate lipids to the C terminus. Ext61L, a constitutively activated and palmitoylated Ha-Ras that lacks a farnesyl group, induced neurites with more actin cytoskeletal changes and lamellipodia than were induced by farnesylated Ha-Ras61L. Ext61L-triggered neurite outgrowth was prevented easily by co-expressing inhibitory Rho, Cdc42, or p21-activated kinase but required increased amounts of inhibitory Rac. Compared with Ha-Ras61L, Ext61L caused 2-fold greater Rac GTP binding and phosphatidylinositol 3-kinase activity in membranes, a hyperactivation that explained the numerous lamellipodia and ineffectiveness of Rac(N17). In contrast, Ext61L activated B-Raf kinase and ERK phosphorylation more poorly than Ha-Ras61L. Thus, accentuated differentiation by Ext61L apparently results from heightened activation of one Ras effector (phosphatidylinositol 3-kinase) and suboptimal activation of another (B-Raf). This surprising unbalanced effector activation, without changes in the designated Ras effector domain, indicates the Ext61L C-terminal alternations are a new way to influence Ha-Ras-effector utilization and suggest a broader role of the lipidated C terminus in Ha-Ras biological functions.

Dendritic cells permit identification of genes encoding MHC class II-restricted epitopes of transplantation antigens

Minor or histocompatibility (H) antigens are recognized by CD4+ and CD8+ T lymphocytes as short polymorphic peptides associated with MHC molecules. They are the targets of graft versus host and graft versus leukemia responses following bone marrow transplantation between HLA-identical siblings. Several genes encoding class I-restricted minor H epitopes have been identified, but approaches used for these have proved difficult to adapt for cloning class II-restricted minor H genes. We have combined the unique antigen-presenting properties of dendritic cells and high levels of episomal expression following transfection of COS cells to identify a Y chromosome gene encoding two HY peptide epitopes, HYAb and HYEk.

Glucose-stimulated preproinsulin gene expression and nuclear trans-location of pancreatic duodenum homeobox-1 require activation of phosphatidylinositol 3-kinase but not p38 MAPK/SAPK2

Exposure of islet beta-cells to elevated glucose concentrations (30 versus 3 mm) prompts enhanced preproinsulin (PPI) gene transcription and the trans-location to the nucleoplasm of pancreatic duodenum homeobox-1 (PDX-1; Rafiq, I., Kennedy, H., and Rutter, G. A. (1998) J. Biol. Chem. 273, 23241-23247). Here, we show that in MIN6 beta-cells, over-expression of p110.CAAX, a constitutively active form of phosphatidylinositol 3-kinase (PI3K) mimicked the activatory effects of glucose on PPI promoter activity, whereas Deltap85, a dominant negative form of the p85 subunit lacking the p110-binding domain, and the PI3K inhibitor LY 294002, blocked these effects. Similarly, glucose-stimulated nuclear trans-location of endogenous PDX-1 was blocked by Deltap85 expression, and wortmannin or LY 294002 blocked the trans-location from the nuclear membrane to the nucleoplasm of epitope-tagged PDX-1.c-myc. By contrast, SB 203580, an inhibitor of stress-activated protein kinase-2 (SAPK2)/p38 MAP kinase, had no effect on any of the above parameters, and PPI promoter activity and PDX-1.c-myc localization were unaffected by over-expression of the upstream kinase MKK6 (MAP kinase kinase-6) or wild-type p38/SAPK2, respectively. Furthermore, no change in the activity of extracted p38/SAPK2 could be detected after incubation of cells at either 3 or 30 mm glucose. These data suggest that stimulation of PI3K is necessary and sufficient for the effects of glucose on PPI gene transcription, acting via a downstream signaling pathway that does not involve p38/SAPK2.

Intracellular targeting and retention of the glucose transporter GLUT4 by the perinuclear storage compartment involves distinct carboxyl-tail motifs

The mechanisms by which the insulin-sensitive glucose transporter, GLUT4, is targeted and retained in a storage compartment near to the Golgi complex are poorly understood. Here we report that removal of the carboxyl-terminal acidic Pro(505)AspGluAsnAsp(509) sequence prevents the storage of GLUT4 in the VAMP-2 positive compartment adjacent to the Golgi complex (GSC), and results in its targeting to GLUT4-positive vesicles and Rab7-positive late endosomes. Storage of the truncated GLUT4 in the GSC is restored by substitution of Phe for the Tyr(502) residue adjacent to Pro(505) or by treatment of cells with the tyrosine kinase inhibitor genistein. Ablation of the Leu(489)Leu(490)-based motif prevents the targeting of GLUT4delta5 to GLUT4-positive-vesicles and late endosomes as well as the retention of GLUT4delta5Phe(502)by the GSC. These results are consisting with a model of GLUT4 transport in which the targeting of the protein from the TGN to the GSC is mediated by the Leu(489)Leu(490)-based motif and its release from the GSC involves Tyr(502)and the adjacent carboxyl-terminal Pro(505)AspGluAsnAsp(509) sequence.

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.

A Role for the Transmembrane Domain in the Trimerization of the MHC Class II-Associated Invariant Chain

MHC class II and invariant chain (Ii) associate early in biosynthesis to form a nonameric complex. Ii first assembles into a trimer and then associates with three class II αβ heterodimers. Although the membrane-proximal region of the Ii luminal domain is structurally disordered, the C-terminal segment of the luminal domain is largely α-helical and contains a major interaction site for the Ii trimer. In this study, we show that the Ii transmembrane domain plays an important role in the formation of Ii trimers. The Ii transmembrane domain contains an unusual patch of hydrophilic residues near the luminal interface. Substitution of these polar residues with nonpolar amino acids resulted in a decrease in the efficiency of Ii trimerization and subsequent class II association. Moreover, N-terminal fragments of Ii were found to trimerize independently of the luminal α-helical domain. Progressive C-terminal truncations mapped a homotypic association site to the first 80 aa of Ii. Together, these results implicate the Ii transmembrane domain as a site of trimer interaction that can play an important role in the initiation of trimer formation.

Mice lacking all conventional MHC class II genes

MHC class II (MHC-II) molecules play a central role in the selection of the T cell repertoire, in the establishment and regulation of the adaptive immune response, and in autoimmune deviation. We have generated knockout mice lacking all four of the classical murine MHC-II genes (MHCII(Delta/Delta) mice), via a large (80-kilobase) deletion of the entire class II region that was engineered by homologous recombination and Cre recombinase-mediated excision. These mice feature immune system perturbations like those of Aalpha and Abeta knockout animals, notably a dearth of CD4(+) lymphocytes in the thymus and spleen. No new anatomical or physiological abnormalities were observed in MHCII(Delta/Delta) mice. Because these animals are devoid of all classical MHC-II chains, even unpaired chains, they make excellent recipients for MHC-II transgenes from other species, avoiding the problem of interspecies cross-pairing of MHC-II chains. Therefore, they should be invaluable for engineering "humanized" mouse models of human MHC-II-associated autoimmune disorders.

BALB/c Invariant Chain Mutant Mice Display Relatively Efficient Maturation of CD4+ T Cells in the Periphery and Secondary Proliferative Responses Elicited upon Peptide Challenge

Allelic differences are known to influence many important aspects of class II biosynthesis, including subunit assembly, Ii chain associations, and DM-mediated peptide loading. Mutant mouse strains lacking Ii chain expression have been previously studied on mixed genetic backgrounds. The present experiments describe cellular and functional characteristics of congenic BALB/c Ii chain mutants. As expected, class II surface expression was markedly decreased, but in contrast to I-Ad-transfected cell lines, serological analysis of BALB/c Ii chain-deficient spleen cells gave no evidence for discordant expression of class II conformational epitopes. Thus, we conclude that properly folded class II molecules are exported via the Ii chain-independent pathway. Functional assays demonstrate consistently superior peptide-loading capabilities, suggesting that these I-Ad molecules are empty or occupied by an easily displaced peptide(s). Defective B cell development was observed for three mutant strains established on diverse genetic backgrounds. Ii chain function is also essential for optimal class II surface expression by mature splenic dendritic cells. Surprisingly, we observe in BALB/c Ii chain mutants, relatively efficient maturation of CD4+ T cells in the periphery and secondary proliferative responses elicited upon peptide challenge. The milder phenotype displayed by BALB/c Ii chain mutants in comparison with class II functional defects previously described for mouse strains lacking Ii chain is likely to have an effect on disease susceptibility.

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.

SHIP recruitment attenuates Fc gamma RIIB-induced B cell apoptosis

Fc gammaRIIB is an inhibitory receptor that terminates activation signals initiated by antigen cross-linking of the BCR through the recruitment of SHIP. Fc gammaRIIB can also signal independently of BCR coligation to directly mediate an apoptotic response, requiring only an intact transmembrane domain. Failure to recruit SHIP, either by deletion of SHIP or mutation of Fc gammaRIIB, results in enhanced Fc gammaRIIB-triggered apoptosis. Thus, in the germinal center, where ICs are retained by FDCs, Fc gammaRIIB may be an active determinant in the negative selection of B cells whose BCRs have reduced affinity for antigen as a result of somatic hypermutation. Selection of B cells may represent the sum of opposing signals generated by the interaction of ICs with the BCR and Fc gammaRIIB through pathways modulated by SHIP.

A mutational analysis of the Abetaz/Aalphad major histocompatibility complex class II molecule that restricts autoreactive T cells in (NZBxNZW)F1 mice. The critical influence of alanine at position 69 in the Aalphad chain

Autoimmune symptoms of (NZBxNZW)F1 (H-2d/z) mice are reported to be critically related to the heterozygosity at the H-2 complex of the murine major histocompatibility complex (MHC). We previously showed that several Abetaz/Aalphad MHC class II molecule-restricted autoreactive T-cell clones from B/WF1 mice were pathogenic upon transfer to preautoimmune B/WF1 mice. In this study, to identify the crucial amino acid residues in Abetaz/Aalphad molecules for T-cell activation, we generated a panel of transfectant cell lines. These transfectant cell lines express the Abetaz/Aalphad MHC molecules with a mutation at each residue alpha11, alpha28, alpha57, alpha69, alpha70, alpha76 of Aalphad chain and beta86 of Abetaz chain. Replacing alpha69 alanine with threonine, valine or serine completely eliminated the ability to stimulate autoreactive T-cell clones without affecting the ability to present foreign antigen keyhole limpet haemocyanin (KLH) or L-plastin peptide to specific T-cell clones. Replacing beta86 valine with aspartic acid resulted in a decrease in the stimulation for antigen-reactive as well as autoreactive T-cell clones. Substitutions at other residues had minimal or no effect on the stimulation of either auto- or antigen-reactive T-cell clones. These results suggest that alanine at residue 69 of the Aalphad chain is critical for the activation of autoreactive Abetaz/Aalphad-restricted T-cell clones. Possible explanations for this are discussed.

TCR, LFA-1, and CD28 Play Unique and Complementary Roles in Signaling T Cell Cytoskeletal Reorganization

T cells interacting with APCs undergo rearrangement of surface receptors and cytoskeletal elements to face the zone of contact with the APC. This polarization process is thought to affect T cell signaling by organizing a specialized domain on the T cell surface and to direct T cell effector function toward the appropriate APC. We have investigated the contribution of TCR, CD28, and LFA-1 signaling to T cell cytoskeletal polarization by assaying the response of an Ag-specific Th1 clone toward a panel of transfected APCs expressing MHC class II alone or in combination with ICAM-1 or B7-1. We show that polarization of talin, an actin-binding protein, occurs in response to integrin engagement. In contrast, reorientation of the T cell microtubule-organizing center (MTOC) is dependent on and directed toward the site of TCR signaling, regardless of whether integrins or costimulatory molecules are engaged. MTOC reorientation in response to peptide-MHC complexes is sensitive to the phosphatidylinositol 3-kinase inhibitor wortmannin. CD28 coengagement overcomes this sensitivity, as does activation via Ab cross-linking of the TCR or via covalent peptide-MHC complexes, suggesting that phosphatidylinositol 3-kinase is not required per se but rather plays a role in signal amplification. Engagement of TCR in trans with LFA-1 results in separation of MTOC reorientation and cortical cytoskeletal polarization events, indicating that the two processes are not directly mechanistically linked. These studies show that T cells mobilize individual cytoskeletal components in response to distinct and specific cell surface interactions.

Microglia express CCR5, CXCR4, and CCR3, but of these, CCR5 is the principal coreceptor for human immunodeficiency virus type 1 dementia isolates

Microglia are the main human immunodeficiency virus (HIV) reservoir in the central nervous system and most likely play a major role in the development of HIV dementia (HIVD). To characterize human adult microglial chemokine receptors, we analyzed the expression and calcium signaling of CCR5, CCR3, and CXCR4 and their roles in HIV entry. Microglia expressed higher levels of CCR5 than of either CCR3 or CXCR4. Of these three chemokine receptors, only CCR5 and CXCR4 were able to transduce a signal in microglia in response to their respective ligands, MIP-1beta and SDF-1alpha, as recorded by single-cell calcium flux experiments. We also found that CCR5 is the predominant coreceptor used for infection of human adult microglia by the HIV type 1 dementia isolates HIV-1DS-br, HIV-1RC-br, and HIV-1YU-2, since the anti-CCR5 antibody 2D7 was able to dramatically inhibit microglial infection by both wild-type and single-round luciferase pseudotype reporter viruses. Anti-CCR3 (7B11) and anti-CXCR4 (12G5) antibodies had little or no effect on infection. Last, we found that virus pseudotyped with the DS-br and RC-br envelopes can infect cells transfected with CD4 in conjunction with the G-protein-coupled receptors APJ, CCR8, and GPR15, which have been previously implicated in HIV entry.

Identification of a receptor mediating absorption of dietary cholesterol in the intestine

Here we show that scavenger receptor class B type I is present in the small-intestine brush border membrane where it facilitates the uptake of dietary cholesterol from either bile salt micelles or phospholipid vesicles. This receptor can also function as a port for several additional classes of lipids, including cholesteryl esters, triacylglycerols, and phospholipids. It is the first receptor demonstrated to be involved in the absorption of dietary lipids in the intestine. In liver and steroidogenic tissues, the physiological ligand of this receptor is high-density lipoprotein. We show that binding of high-density lipoprotein and apolipoprotein A-I to the brush border membrane-resident receptor inhibits uptake of cholesterol (sterol) into the brush border membrane from lipid donor particles. This finding lends further support to the conclusion that scavenger receptor BI catalyzes intestinal cholesterol uptake. Our findings suggest new therapeutic approaches for limiting the absorption of dietary cholesterol and reducing hypercholesterolemia and the risk of atherosclerosis.

Peptide-binding motifs of the mixed haplotype Abetaz/Aalphad major histocompatibility complex class II molecule: a restriction element for auto-reactive T cells in (NZBxNZW)F1 mice

We previously showed that the mixed haplotype Abetaz/Aalphad major histocompatibility complex (MHC) class II molecules function as restricting element for autoreactive T-cell clones derived from autoimmune prone (NZBxNZW)F1 (B/WF1) mice. Subsequent analysis revealed that some of these Abetaz/Aalphad-restricted autoreactive T-cell clones were pathogenic upon transfer to pre-autoimmune B/WF1 mice. In this paper, we analysed the peptide-binding motif of Abetaz/Aalphad class II molecules. Amino acid-sequencing analysis of peptides eluted from purified Abetaz/Aalphad molecules revealed several sequences, including one that corresponds to murine l-plastin 588-601. Synthetic 18-mer l-plastin 588-605 peptide (SMARKIGARVYALPEDLV, as described by the amino acid single letter code) was demonstrated to bind to Abetaz/Aalphad MHC class II molecules on transfectant B lymphoma cells (TAbetaz). A competitive binding inhibition assay using truncation peptides revealed the core sequence for binding resides in 591Arg to 601Pro. Binding inhibition assay using substitution peptides, each having substitution to the other 19 residues at positions from 590Ala to 601Pro, revealed four major anchor sites 592Lys (p1), 594Gly (p3), 595Ala (p4), 597Val (p6) and one minor anchor site 600Leu (p9). Positively charged residues are not allowed at p3 and negatively charged residues are not allowed at p4 and p6. Relatively large hydrophobic residues (Leu, Ile) are not tolerated at p3 and p4. Met and Trp are not tolerated at p6. Based on these findings, the characteristics of peptides recognized by autoreactive T cells in B/WF1 mice are discussed.

The functional role of B7 molecules on the induction of thymocyte activation and apoptosis

To evaluate the role of B7 on thymocyte activation and apoptosis, we took advantage of TCR transgenic mice in which the majority of thymocytes express a uniform TCR that is specific for ovalbumin. We also prepared Chinese hamster ovary (CHO) cells expressing B7 and appropriate class II molecules. We found that the apoptosis of double-positive thymocytes by TCR-mediated signaling, which presumably represents negative selection, requires a costimulatory signal provided by B7-1 or B7-2. The requirement of B7-1 costimulation for the apoptosis of thymocytes does not change in either low or high antigenic peptide loading. We also demonstrated that two signals through TCR and CD28 augmented the proliferation of thymocytes, and the requirement of CD28-mediated signal by B7-1 or B7-2 for thymocyte proliferation became less evident when high doses of antigenic peptide were loaded, indicating that the intensity of TCR-mediated signal determines the requirement of B7-mediated second signal for thymocyte proliferation.