Antigen processing for amateurs and professionals.

The initiation and propagation of immune responses is dependent on the ability of antigen-presenting cells (APCs) to convert proteins into peptides, to load them intracellularly onto major histocompatibility complex (MHC) products and then to deliver the peptide-MHC complexes to the plasma membrane. Perhaps the most effective or 'professional' of all APCs are dendritic cells (DCs). DCs express high levels of MHC molecules together with a variety of 'accessory molecules' that help render DCs more effective at stimulating T cells than any other cell type. However, much of the capacity of a DC for T-cell stimulation reflects a remarkable ability to regulate the organization and function of its endocytic and secretory pathways during its development. This review describes recent insights into the cell-biological specializations responsible for the 'professional' status of the DC in antigen processing and presentation.

Transport of peptide-MHC class II complexes in developing dendritic cells.

Major histocompatibility complex class II (MHC II) molecules capture peptides within the endocytic pathway to generate T cell receptor (TCR) ligands. Immature dendritic cells (DCs) sequester intact antigens in lysosomes, processing and converting antigens into peptide-MHC II complexes upon induction of DC maturation. The complexes then accumulate in distinctive, nonlysosomal MHC II+ vesicles that appear to migrate to the cell surface. Although the vesicles exclude soluble lysosomal contents and antigen-processing machinery, many contain MHC I and B7 costimulatory molecules. After arrival at the cell surface, the MHC and costimulatory molecules remain clustered. Thus, transport of peptide-MHC II complexes by DCs not only accomplishes transfer from late endocytic compartments to the plasma membrane, but does so in a manner that selectively concentrates TCR ligands and costimulatory molecules for T cell contact.

Transcriptomic analysis of hepatocellular carcinoma reveals molecular features of disease progression and tumor immune biology.

Hepatocellular carcinoma (HCC) develops in the context of chronic inflammatory liver disease and has an extremely poor prognosis. An immunosuppressive tumor microenvironment may contribute to therapeutic failure in metastatic HCC. Here, we identified unique molecular signatures pertaining to HCC disease progression and tumor immunity by analyzing genome-wide RNA-Seq data derived from HCC patient tumors and non-tumor cirrhotic tissues. Unsupervised clustering of gene expression data revealed a gradual suppression of local tumor immunity that coincided with disease progression, indicating an increasingly immunosuppressive tumor environment during HCC disease advancement. IHC examination of the spatial distribution of CD8+ T cells in tumors revealed distinct intra- and peri-tumoral subsets. Differential gene expression analysis revealed an 85-gene signature that was significantly upregulated in the peri-tumoral CD8+ T cell-excluded tumors. Notably, this signature was highly enriched with components of underlying extracellular matrix, fibrosis, and epithelial-mesenchymal transition (EMT). Further analysis condensed this signature to a core set of 23 genes that are associated with CD8+ T cell localization, and were prospectively validated in an independent cohort of HCC specimens. These findings suggest a potential association between elevated fibrosis, possibly modulated by TGF-ß, PDGFR, SHH or Notch pathway, and the T cell-excluded immune phenotype. Indeed, targeting fibrosis using a TGF-ß neutralizing antibody in the STAM™ model of murine HCC, we found that ameliorating the fibrotic environment could facilitate redistribution of CD8+ lymphocytes into tumors. Our results provide a strong rationale for utilizing immunotherapies in HCC earlier during treatment, potentially in combination with anti-fibrotic therapies.

Molecular cloning and sequence analysis of U3 snoRNA-associated mouse fibrillarin.

We have isolated and determined the sequence of a 1.1-kb cDNA from a murine WEHI-3 macrophage library which encodes the highly conserved, nucleolar protein, fibrillarin. The murine fibrillarin protein sequence displays 94.2% identity with human fibrillarin, 82.9% identity with amphibian fibrillarin and 74.0% identity with the yeast fibrillarin homolog, NOP1. Immunoprecipitation showed that anti-fibrillarin autoantibodies from human scleroderma sera and the monoclonal autoantibody 72B9 recognize the approx. 34-36 kDa in vitro transcribed and translated protein. Mouse fibrillarin contains a N-terminal glycine- and arginine-rich (GAR) domain which although conserved among the fibrillarins is not as strongly conserved as several regions in the carboxy tail of the protein. Specific amino acid residues in yeast NOP1 thought to be associated with the synthesis and maturation of ribosomes show strong conservation between the mouse, human, amphibian and yeast protein sequences.

Self-encounters of the third kind: lymph node stroma promotes tolerance to peripheral tissue antigens.

Multiple mechanisms have evolved to maintain tolerance among CD8(+) T cells to innocuous antigens that arise in cutaneous and mucosal tissues. In the thymus, medullary thymic epithelial cells directly present peripheral tissue antigens (PTAs) and incite the deletion of self-reactive thymocytes. Cross-presentation of PTAs by functionally immature, CD8alpha(+) dendritic cells can lead to the deletion of self-reactive CD8(+) T cells in secondary lymphoid organs. A third mechanism of deletional tolerance has recently been uncovered in which lymph node-resident stromal cells of non-hematopoietic origin present endogenously expressed PTAs to circulating CD8(+) T cells. Emerging data suggest that lymph node stroma is a unique niche for controlling self-reactive T cells.

Genetic susceptibility to silver-induced anti-fibrillarin autoantibodies in mice.

Similar to mercuric chloride, silver nitrate has recently been shown to induce IgG autoantibodies targeting the nucleolar 34-kDa protein fibrillarin i SJL (H-2s) mice. In the present study we show that the autoimmunogenic effect of silver is dependent on intact T-cell function since SJL/N mice homozygous for the nude mutation (athymic), in contrast to the functionally T-cell-intact SJL/N-nu/+ littermates, did not develop anti-nucleolar/anti-fibrillarin autoantibodies (ANoA/AFA). The genetic susceptibility for silver-induced AFA was localized to the H-2A locus using congenic and intra-H-2-recombinant strains. However, background (non-H-2) genetic factors substantially influenced both the response rate and the titer of ANoA/AFA attained. Strains bearing H-2As on the SJL and A backgrounds (SJL, A.SW, A.TH) showed 100% response rate and high ANoA titers (3750 +/- 246, mean reciprocal titer +/- SEM), whereas H-2As mice on the B10 background (B10.S) showed 60% response rate and significantly lower ANoA titers (1170 +/- 305) in the responding mice. Expression of H-2E [B10.S(9R) mice] further reduced the response rate (22%) and the ANoA titer (640 +/- 0). A suppressive effect on the B10 background has previously been observed in mercury treatment, but the effect was stronger in silver-treated mice. Two major differences were noted between silver- and mercury-induced murine autoimmunity. First, silver-treated mice did not show elevated titers of other autoantibody specificities, specifically not of antichromatin and anti-histone antibodies, which develop in mercury-treated SJL mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the autoantibody response to fibrillarin in human disease and murine models of autoimmunity.

Fibrillarin, a component of the U3 RNP particle, is a target for the spontaneously arising autoantibodies in human scleroderma and a monoclonal autoantibody (72B9) derived from the autoimmune mouse strain (NZB x NZW) F1. Autoantibodies against fibrillarin can also be induced in H-2s mice by treatment with mercuric chloride (HgCl2). The objective of this study was to compare the spontaneously occurring anti-fibrillarin autoantibody response with the autoantibody response induced by HgCl2 treatment. Immunofluorescence microscopy on human HEp2, mouse 3T3, and Xenopus XIK-2 cells, immunoblotting with use of nuclear extract from human MOLT-4, mouse 3T3, and Xenopus XIK-2 cells, and immunoprecipitation with use of in vitro translation products of RNA transcripts from yeast fibrillarin cDNA were used in this analysis. Both spontaneous and induced autoantibodies displayed common reactivity in that, irrespective of the antigenic source, they gave the same nucleolar immunofluorescence pattern and a restricted immunoblotting reactivity targeting predominantly the 34-kDa protein fibrillarin. Immunoprecipitation of N- and C-terminal truncated fibrillarin constructs also demonstrated a common pattern of reactivity. All Abs precipitated a fragment comprising amino acids 1-312 but not a smaller fragment containing amino acids 1-257. The majority of sera could not precipitate an N-terminal truncated molecule consisting of amino acids 157-327. These immunoprecipitation experiments support recognition of a common epitope requiring both N- and C-regions, which may be exemplified by the reactivity of murine monoclonal anti-fibrillarin autoantibody 72B9. Our results indicate that spontaneous human and toxin-induced murine autoantibodies to fibrillarin share common reactivity against this highly conserved nucleolar protein.

Adverse immunological effects and autoimmunity induced by dental amalgam and alloy in mice.

Dental amalgam fillings are the most important source of mercury exposure in the general population, but their potential to cause systemic health consequences is disputed. In this study, inbred mice genetically susceptible to mercury-induced immune aberrations were used to examine whether dental amalgam may interfere with the immune system and cause autoimmunity. Female SJL/N mice were implanted in the peritoneal cavity with 8-100 mg silver amalgam or silver alloy for 10 weeks or 6 months. Chronic hyperimmunoglobulinemia, serum IgG autoantibodies targeting the nucleolar protein fibrillarin, and systemic immune-complex deposits developed in a time- and dose-dependent manner after implantation of amalgam or alloy. Splenocytes from mice implanted with amalgam or alloy showed an increased expression of class II molecules. The functional capacity of splenic T and B cells was affected in a dose-dependent way: 10 weeks of low-dose and 6 months of high-dose amalgam implantation strongly increased mitogen-induced T and B cell proliferation, whereas 10 weeks of high-dose implantation decreased the proliferation. Not only mercury but also silver accumulated in the spleen and kidneys after amalgam implantation. In conclusion, dental amalgam implantation in a physiological body milieu causes chronic stimulation of the immune system with induction of systemic autoimmunity in genetically sensitive mice. Implantation of silver alloy not containing mercury also induced autoimmunity, suggesting that other elements, especially silver, have the potential to induce autoimmunity in genetically susceptible vertebrates. Accumulation of heavy metals, from dental amalgam and other sources, may lower the threshold of an individual metal to elicit immunological aberrations. We hypothesize that under appropriate conditions of genetic susceptibility and adequate body burden, heavy metal exposure from dental amalgam may contribute to immunological aberrations, which could lead to overt autoimmunity.

Murine genotype influences the specificity, magnitude and persistence of murine mercury-induced autoimmunity.

Genetic factors are major contributors in determining the susceptibility to systemic autoimmune diseases. We studied the influence of genotype on systemic autoimmunity by treating female mice of the H-2s strains SJL/N, SJL/J, A.SW, and B10.S with mercuric chloride (HgCl2) for 10 weeks and then following autoantibody and tissue immune deposits during the subsequent 12 months. All strains developed antinucleolar antibodies (ANoA) of the IgG class which reacted in immunoblotting with a 34-kDa nucleolar protein identified as fibrillarin. The titre of ANoA attained after 10 weeks' treatment varied from 1:1,280 to 1:20,480 in the order: A.SW > SJL > > B10.S. Following cessation of HgCl2 treatment ANoA and antifibrillarin antibodies (AFA) persisted for up to 12 months, although some B10.S mice showed pronounced reduction not only of their autoantibody titres, but also systemic immune deposits when compared to other H-2s strains. A second set of autoantibodies targeted chromatin and in some mice specifically histones, and were distinguished from the ANoA by a rapid decline after treatment and a susceptibility linked to the non-H-2 genes of the SJL. Tissue levels of mercury remained elevated above untreated controls throughout the study period, suggesting that the mercury detected in lymphoid tissues may provide stimulation of lymphoid cells specific for fibrillarin for a considerable period after exposure has ceased. We conclude that H-2 as well as non-H-2 genetic factors distinctly influence not only the susceptibility to induction of autoimmunity, but also the specificity and magnitude of the response.

Fibrillarin and other snoRNP proteins are targets of autoantibodies in xenobiotic-induced autoimmunity.

Exposure of SJL/J mice to mercury induces an anti-nucleolar autoantibody response. The predominant target is fibrillarin, a 34-kDa component of the small nucleolar ribonucleoprotein particles (snoRNP), but other proteins are also recognized. To characterize these proteins, monoclonal IgG anti-nucleolar antibodies were produced from HgC12-treated SJL/J mice. One monoclonal, 17C12, recognized fibrillarin, while two others, 7G3 and 6G10, were found to immunoprecipitate snoRNP particles but not fibrillarin. Antibody 6G10 gave a nucleolar immunofluorescence pattern in human, murine, and amphibian cells, but was negative in immunoblot. The 7G3 monoclone reacted with a 60-kDa protein conserved in human and murine, but not amphibian, cell lines. The 7G3 and 6G10 antigens and fibrillarin colocalized to the nucleolus and Cajal bodies in interphase cells and decorated metaphase chromosomes. These studies suggest that the mercury-induced anti-nucleolar antibody response targets other protein components of the snoRNP particles in addition to fibrillarin.

Selective induction of anti-fibrillarin autoantibodies by silver nitrate in mice.

Female SJL (H-2s) mice developed serum IgG anti-nucleolar antibodies (ANoA) after 5 weeks treatment with 0.05% or 0.01% silver nitrate (AgNO3) in drinking water. Five more weeks of treatment increased the ANoA titre to 3410 +/- 853 and 640 +/- 175 (reciprocal mean +/- s.e.m.), respectively. Controls receiving ordinary tap water and mice given 0.002% AgNO3 showed no antinucleolar antibodies. The high-titre ANoA targeted a 34-kD nucleolar protein identified as fibrillarin, the major autoantigen in murine mercury-induced autoimmunity and in a fraction of patients with systemic scleroderma. Serum autoantibodies to chromatin or histones, kidney, spleen, stomach, thyroid, or skin antigens (except the nucleolus) were not found in any of the mice. There was no consistent significant increase of serum IgG1, IgG2a, IgG2b, or IgG3 concentrations after AgNO3 treatment compared with controls. Mice treated with 0.05% AgNO3 for 10 weeks showed a slight decrease in serum IgG1, IgG2b and IgG3 concentrations. These mice also showed a small but statistically significant increase in renal, mesangial IgM deposits, which was not accompanied by any increase in C3c deposits, whereas mice given lower doses of silver nitrate showed no significant increase in mesangial immunoglobulin immune deposits. Systemic vessel wall immune deposits were not found in any of the mice. In mice given 0.05% silver nitrate, the kidney showed the highest concentration of silver (12.2 +/- 0.09 micrograms Ag/g wet weight; mean +/- s.e.m.), followed by the spleen (8.7 +/- 1.3), and the liver (3.9 +/- 0.4). Treatment with 0.01% silver nitrate caused a different distribution of silver, with the highest concentration in the spleen (2.1 +/- 0.16 micrograms Ag/g), followed by the kidney (0.63 +/- 0.037), and the liver (< 0.29 micrograms Ag/g; mean). Silver seems to be a more specific inducer of antinucleolar/anti-fibrillarin autoantibodies than mercury and gold, lacks the general immune stimulating potential of mercury, and has only a weak tendency to induce renal immune deposits. These observations suggest that the autoimmune sequelae induced in mice by metals is dependent, not only upon the genetic haplotype of the murine strain, but also on the metal under investigation.

Developmental regulation of MHC class II transport in mouse dendritic cells.

Dendritic cells (DCs) have the unique capacity to initiate primary and secondary immune responses. They acquire antigens in peripheral tissues and migrate to lymphoid organs where they present processed peptides to T cells. DCs must therefore exist in distinct functional states, an idea that is supported by observations that they downregulate endocytosis and upregulate surface molecules of the class II major histocompatibility complex (MHC) upon maturation. Here we investigate the features of DC maturation by reconstituting the terminal differentiation of mouse DCs in vitro and in situ. We find that early DCs, corresponding to those found in peripheral tissues, exhibit a phenotype in which most class II molecules are intracellular and localized to lysosomes. Upon maturation, these cells give rise to a new intermediate phenotype in which intracellular class II molecules are found in peripheral non-lysosomal vesicles, similar to the specialized CIIV population seen in B cells. The intermediate cells then differentiate into late DCs which express almost all of their class II molecules on the plasma membrane. These variations in class II compartmentalization are accompanied by dramatic alterations in the intracellular transport of the new class II molecules and in antigen presentation. We found that although early DCs could not present antigen immediately after uptake, efficient presentation of the previously internalized antigen occurred after maturation, 24-48 hours later. By regulating class II transport and compartmentalization, DCs are able to delay antigen display, a property crucial to their role in immune surveillance.