Compartmentalization of class II antigen presentation: contribution of cytoplasmic and endosomal processing

During antigen processing, peptides are generated and displayed in the context of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) to modulate immune responses to foreign antigens and guide self-tolerance. Exogenous and cytoplasmic antigens are processed by distinct routes within APCs to yield class II ligands. Exogenous antigens are internalized, processed, and bound to class II molecules within endosomal and lysosomal compartments of APCs. Studies reviewed here demonstrate the importance of reduction in regulating exogenous antigen presentation. The differential expression of a gamma-interferon-inducible lysosomal thiol reductase in professional APCs and melanomas is discussed in the context of tumor immune evasion. Cytoplasmic autoantigens, by contrast, are degraded by the proteasome and other enzymes in the cytosol, with the resulting peptides translocating to endosomal and lysosomal compartments for intersection with class II molecules. Processing and editing of these antigenic peptides within endosomes and lysosomes may be critical in regulating their display via class II proteins. Multiple pathways may regulate the transit of cytosolic peptides to class II molecules. The role of lysosome-associated membrane protein-2a and heat-shock cognate protein 70 in promoting cytoplasmic peptide presentation by MHC class II molecules is discussed.

Roles of adaptor molecules in mast cell activation

The release of pro-inflammatory mediators from mast cells generally occurs following antigen-dependent aggregation of the high-affinity receptors for IgE (FcepsilonRI) expressed on the cell surface. Under the appropriate conditions, however, other receptors including the high-affinity receptor for IgG (FcgammaRI), Kit, the C3a complement component receptor, and adenosine receptors, can also induce or potentiate mast cell activation. In contrast, receptors such as the FcgammaRIIb low-affinity IgG receptor, and gp49b, when co-ligated with FcepsilonRI, down-regulate mast cell activation. The driving force by which the FcepsilonRI, the FcgammaRI, Kit, and potentially other receptors, lead to mast cell degranulation, arachidonic acid metabolism and cytokine gene expression, is a series of tyrosine kinase-mediated protein phosphorylation events which result in recruitment and subsequent activation of signaling enzymes. Similar processes are required by gp49b and FcgammaRIIb for the down-regulation of mast cell activation. The cellular localization and sequence of these events, the subsequent amplification and diversification of the signaling cascade, and potentially, the termination of these events, are regulated by an important group of signaling proteins termed adaptor molecules. In this chapter, we discuss the structure and properties of these molecules and how these proteins regulate the cellular processes associated with receptor-mediated mast cell activation.

Human immune response towards recombinant Helicobacter pylori urease and cellular fractions

Vaccination against Helicobacter pylori is of particular clinical interest. Recombinant urease, the major protein in H. pylori, has been used for mucosal vaccination trials in different animal models, but was found to be ineffective in humans. The current study therefore investigated the human immune response towards recombinant H. pylori urease A and B (rUreA/B) expressed in E. coli compared to different cellular fractions of H. pylori (cytosol, total, inner and outer membrane). Monocyte-derived dendritic cells (Mo-DC) were generated from monocytes isolated by magnetic antigen cell separation (MACS) from healthy volunteers and cultured in the presence of hrIL-4 and hrGM-CSF. Mo-DC were stimulated for 48h with the recombinant proteins (1 microg/ml) or cellular fractions (1-10 microg/ml) and cytokine release was determined in the culture supernatant by ELISA. rUreA and rUreB were effective in inducing IL-12 secretion (6-10 fold) and, to a much lesser extent (2 fold), IL-10 secretion from Mo-DC. Total and outer membrane preparations from H. pylori stimulated IL-12 secretion significantly, and were even more potent than intact bacteria. Mo-DCs pulsed with rUreA activated allogenic CD56+ NK-cells, as determined by TNF-alpha and IFN-gamma secretion, but not allogenic CD4+/CD45RA+ naïve T-cells. In contrast, Mo-DCs pulsed with H. pylori total membrane or outer membrane preparations activated allogenic naive T-cells in co-culture systems, as determined by increased TNF-alpha secretion. It appears that outer membrane preparations of H. pylori, but not recombinant urease are more effective in inducing a Th1 polarized response in humans in vitro.

CD4 and CD8 T cell response to the rHSP60 from Klebsiella pneumoniae in peripheral blood mononuclear cells from patients with ankylosing spondylitis

OBJECTIVE

To determine the processing pathways used by peripheral blood mononuclear cells (PBMC) and present the rHSP60Kp, and the T cell subpopulations involved in the response, in patients with ankylosing spondylitis (AS) METHODS: The lymphoproliferative response to the rHSP60Kp in PBMC from 14 HLA-B27+ AS patients and 15 B27- healthy controls was assessed by 3H-TdR incorporation. The processing pathways for the rHSP60Kp were analyzed by 3H-TdR incorporation in fresh PBMC from patients using homologous PBMC preincubated with the antigen and specific inhibitors: chloroquine, N-acetyl-L-leucil-L-leucil-L-nor-leucinal (LLnL) or brefeldin A (BFA), fixed with p-formaldehyde (fixed APC). The CD4+/CD8+ T cell subpopulation activated with the antigen was determined by three colours flow cytometry in PBMC from patients.

RESULTS

Eight out of fourteen patients showed positive lymphoproliferative responses to the rHSP60Kp while none of the healthy controls responded (p < 0.012). In five patients S.I. was above 4.0. In these patients lymphoproliferation was lower when chloroquine and LLnL was used and it became negative with BFA, indicating that both pathways are used. CD4+ and CD8+ T cells populations expressed CD69 when activated by the rHSP60Kp.

CONCLUSIONS

Our results suggest that CD4 and CD8 T cells participate in the response to the rHSP60Kp in B27+ AS patients.

The impact of misfolding versus targeted degradation on the efficiency of the MHC class I-restricted antigen processing

Evidence suggests that most epitopes presented by MHC class I molecules are derived from those newly synthesized proteins that are defective due to errors during manufacture. We examined epitope production from model cytosolic and exocytic proteins modified in various ways. Substrates containing a degradation targeting sequence demonstrated very rapid turnover and enhanced epitope production, as was the case for substrate retargeted from endoplasmic reticulum to cytosol. For less radical alterations, including point mutation and deletion and elimination of glycosylation sites, despite detectable changes in folding, half-life was only moderately decreased and there were no significant increases in epitope production. Puromycin, which causes premature termination of protein synthesis, also had no impact upon epitope production. It appears that most defective proteins are not rapidly dispensed with and the targeting of most nascent proteins for Ag processing is not tied to quality control.

Presentation of cytosolic antigens via MHC class II molecules

Major histocompatibility (MHC) class II molecules function to present antigenic peptides to CD4 T lymphocytes. The pathways by which these molecules present exogenous antigens have been extensively studied. However by contrast, far less is known about the processing and trafficking of cytosolic antigens, which can also serve as an alternative source of ligands for MHC class II molecules. Self-proteins, tumor antigens, as well as viral proteins found within the cytosol of cells, can be presented via MHC class II molecules, resulting in the activation of specific CD4 T cells. Studies have begun to reveal unique steps as well as some similarities in the pathways for cytosolic and exogenous antigen presentation. Recent developments in this area are summarized here.

Co-expression and modulation of neuronal and endothelial nitric oxide synthase in human endothelial cells

Despite originally identified in neurones, the neuronal type of nitric oxide synthase (nNOS) is present also in cardiac and skeletal myocytes. Whether nNOS is functionally expressed in human endothelial cells--as the endothelial enzyme (eNOS)--is unknown. Human umbilical vein endothelial cells (HUVEC) were studied under control culture conditions and after 48 h treatment with cytomix (human tumour necrosis factor-alpha, interferon-gamma and E. coli endotoxin). We tested: (i) localisation and expression of nNOS and eNOS proteins by immunostaining and immunoblotting; (ii) activity of nNOS and eNOS by measuring L-arginine to L-citrulline conversion with 1-(2-trifluoromethylphenyl)imidazole (TRIM), a specific nNOS antagonist, in sub-cellular fractions; (iii) intracellular cGMP levels, as a marker for nitric oxide production, after TRIM pre-treatment, by radioimmunoassay. nNOS protein was expressed in the cytosolic fraction and immunolocalised in cultured HUVEC, and co-localised with the eNOS protein in frozen sections of the human umbilical cord. nNOS protein contributed to total L-citrulline production as TRIM selectively and dose-dependently reduced L-citrulline synthesis in the cytosolic but not particulate fraction of HUVEC. Similarly, TRIM reduced intracellular cGMP content both at baseline and after stimulation with a calcium ionophore. Cytomix down-regulated the expression and function of both nNOS and eNOS while no inducible NOS (iNOS) was detected. In conclusion, a functional neuronal type of NOS is co-expressed with the endothelial NOS type in HUVEC, suggesting a possible role for nNOS in regulation of blood flow.

Autoantibodies against pituitary proteins in patients with adrenocorticotropin-deficiency

BACKGROUND

An autoimmune cause of adrenocorticotropin (ACTH)-deficiency is presented, as it is known to be a characteristic feature of lymphocytic hypophysitis, a disease of the pituitary gland considered to be autoimmune.

MATERIALS AND METHODS

The aim of this study was twofold: (1) to evaluate the occurrence of pituitary autoantibodies and (2) to correlate it to clinical and immunological features in a large group of patients with ACTH-deficiency of possible autoimmune aetiology. Sixty-five patients with ACTH-deficiency and 57 healthy subjects participated in the study. Pituitary autoantibodies were measured by an immunoblotting assay with human pituitary cytosol as antigen.

RESULTS

Autoantibodies to a novel 36-kDa pituitary autoantigen were seen in sera from 18.5% (12/65) patients and only 3.5% (2/57) of control subjects (P = 0.0214). When taking only those subjects with strong immunoreactivity into account, the significance was lost; P = 0.3642. Immunoreactivity to a 49-kDa pituitary autoantigen was observed in 21.5% (14/65) of ACTH-deficient patients compared with 8.8% (5/57) of control subjects (P = 0.0910). This 49-kDa pituitary autoantigen has recently been identified as neurone-specific enolase and a candidate marker for neuroendocrine autoimmunity. Clinical parameters in patients with positive versus those with negative pituitary immunoreactivity did not differ. However, autoantibodies to thyroglobulin were positively correlated to immunoreactivity against the 36-kDa pituitary autoantigen (P = 0.014).

CONCLUSIONS

Our findings of pituitary autoantibodies in patients' sera support the theory that an autoimmune destruction of corticotrophs may be the underlying cause of hormonal deficit in some patients with ACTH-deficiency.

Immune Activation of Type I IFNs by Listeria monocytogenes Occurs Independently of TLR4, TLR2, and Receptor Interacting Protein 2 but Involves TANK-Binding Kinase 1

Type I IFNs are well established antiviral cytokines that have also been shown to be induced by bacteria. However, the signaling mechanisms regulating the activation of these cytokines during bacterial infections remain poorly defined. We show that although Gram-negative bacteria can activate the type I IFN pathway through TLR4, the intracellular Gram-positive bacterium Listeria monocytogenes (LM) can do so independently of TLR4 and TLR2. Furthermore, experiments using genetic mutants and chemical inhibitors suggest that LM-induced type I IFN activation occurs by an intracellular pathway involving the serine-threonine kinase TNFR-associated NF-kappaB kinase (TANK)-binding kinase 1 (TBK1). Interestingly, receptor-interacting protein 2, a component of the recently discovered nucleotide-binding oligomerization domain-dependent intracellular detection pathway, was not involved. Taken together, our data describe a novel signal transduction pathway involving TBK1 that is used by LM to activate type I IFNs. Additionally, we provide evidence that both the LM- and TLR-dependent pathways converge at TBK1 to activate type I IFNs, highlighting the central role of this molecule in modulating type I IFNs in host defense and disease.

T cell receptor-induced lipid raft recruitment of the I kappa B kinase complex is necessary and sufficient for NF-kappa B activation occurring in the cytosol

TCR-induced NF-kappa B activation is necessary for the innate immune response and involves induced lipid raft recruitment of the I kappa B kinase (IKK) complex. In this study, we systematically investigated lipid raft recruitment of members of the NF-kappa B activation pathway in human T cells. All upstream components leading to IKK activation were found constitutively or inducibly in lipid rafts, while the NF-kappa B/I kappa B complex and phosphorylated forms of IKK alpha/beta, I kappa B alpha and p65 are exclusively found in the cytosolic fraction. Disruption of raft organization precluded NF-kappaB activation induced by T cell costimulation, but IL-1-triggered NF-kappa B activation remained unaffected. Targeting of the IKK complex to lipid rafts caused constitutive IKK activation and NF-kappa B DNA binding, which was further triggered upon T cell costimulation. Various experimental approaches revealed that costimulation-induced IKK alpha/beta activation loop phosphorylation is independent from IKK beta-mediated transautophosphorylation, but rather involves phosphorylation by the IKK-interacting protein NIK and its upstream activator COT.

Processing and presentation of HLA class I and II epitopes by dendritic cells after transfection with in vitro-transcribed MUC1 RNA

RNA transfection of dendritic cells (DCs) was shown to be highly efficient in eliciting CD8+ and CD4+ T-cell responses. However, antigen presentation pathways involved in generation of human leukocyte antigen (HLA) class I and class II peptides have remained elusive. To analyze this we incubated mucin 1 (MUC1) RNA-transfected DCs with compounds known to inhibit HLA class I presentation and used these cells in chromium 51 (51Cr)-release assays. As effectors, we used cytotoxic T lymphocyte (CTL) lines specific for the MUC1 peptides M1.1 and M1.2. We observed that the presentation of HLA-A*02 epitopes is inhibited by brefeldin A and lactacystin. To determine the requirement of a functional transporter associated with antigen processing (TAP), we cotransfected DCs with MUC1 and infected cell peptide 47 (ICP47) RNA. ICP47 could only inhibit the presentation of the M1.1 but not the M1.2 peptide, indicating that this epitope derived from the signal sequence is presented independently of TAP. Cocultivation of MUC1 RNA-transfected DCs with MUC1-specific CD4+ T lymphocytes revealed that the presentation of HLA class II peptides is sensitive to proteasomal inhibitors and brefeldin A. Furthermore, the presentation pathway requires lysosomal and endosomal processing and is mediated by autophagy. Our results demonstrate that the efficient presentation of cytosolic proteins on major histocompatibility complex (MHC) class II combines the proteolytic and lysosomal pathways.

Differential Cytosolic Delivery and Presentation of Antigen by Listeriolysin O-Liposomes to Macrophages and Dendritic Cells

Delivery of antigenic protein to the cytosol of antigen-presenting cells (APCs), such as macrophages (MPhi) and dendritic cells (DCs), is required for an efficient CD8 T-cell-mediated immune response. We have previously shown that co-encapsulation of antigenic protein inside pH-sensitive liposomes with listeriolysin O (LLO), a pore-forming protein of Listeria monocytogenes, generates efficient major histocompatibility complex class I (MHC I)-restricted immune responses both in vitro and in vivo. In this study, we sought to analyze the relative efficiency of LLO-mediated cytosolic delivery of liposomal antigen in two important APCs, macrophages and dendritic cells, by examining the sequential steps involved in antigen presentation to T-cells in cultured mouse bone marrow-derived MPhis (BMMPhis) and DCs (BMDCs). BMMPhis overall presented liposomal antigen better than BMDCs at a given concentration of liposomal antigen incubated with cells, and the trend was also observed after the presentation was normalized by the uptake of antigen. When soluble antigen was directly introduced into the cytosol, however, BMDCs presented the antigen more efficiently than BMMPhis. In addition, when the APCs were externally loaded with the antigenic peptide of the protein, BMDCs displayed a higher level of cell surface MHC I-peptide complexes and presented the peptide more efficiently than BMMPhis. These results combined together suggest that LLO-mediated release of liposomal antigen from the endosomal/lysosomal compartment may be more pronounced in BMMPhis than in BMDCs, and further implicates differential activity of LLO and varying efficiency of LLO-mediated endosomal escape in different antigen-presenting cell types.

Examination of human tissue cytosols for expression of sulfotransferase isoform 1A2 (SULT1A2) using a SULT1A2-specific antibody

Sulfotransferase isoform 1A2 (SULT1A2) is a member of the cytosolic sulfotransferase family of phase II detoxification enzymes. Studies with recombinant enzymes have shown that SULT1A2 can catalyze the bioactivation of several procarcinogens, indicating a potential role in chemical carcinogenesis. However, previous studies have suggested that the SULT1A2 transcript has a splicing defect that might prevent it from becoming translated into protein; therefore, we sought to determine the expression of SULT1A2 in tissues. An antibody directed against a region of human SULT1A2 that differs from other known sulfotransferase isoforms was developed and used to screen a large number of cytosolic fractions from various tissues. Although the SULT1A2 antibody recognized recombinant SULT1A2 and did not cross-react with other SULT isoforms, the expression of SULT1A2 was not detected in any tissue examined. These studies suggest that if SULT1A2 is expressed as protein, the levels are very low and that SULT1A2 probably does not play a physiological role in chemical carcinogenesis.

Induction of antigen cross-presentation by Toll-like receptors

Cross-presentation is the pathway by which exogenous antigens are routed for presentation on MHC class I for activation of CD8(+) T cells. This pathway is important for the development of CD8(+) cytotoxic T lymphocyte responses against tumors and infectious pathogens that do not directly infect APC. We review studies showing that certain Toll-like receptors mediate cross-presentation by dendritic cells, initiating cytosolic processing of antigen after inducing dendritic cell maturation. The implications of these studies for understanding CD8(+) T cell activation and implementing novel vaccine strategies is considered.

Bacterial heat shock proteins promote CD91-dependent class I MHC cross-presentation of chaperoned peptide to CD8+ T cells by cytosolic mechanisms in dendritic cells versus vacuolar mechanisms in macrophages

APCs process mammalian heat shock protein (HSP):peptide complexes to present HSP-chaperoned peptides on class I MHC (MHC-I) molecules to CD8(+) T cells. HSPs are also expressed in prokaryotes and chaperone microbial peptides, but the ability of prokaryotic HSPs to contribute chaperoned peptides for Ag presentation is unknown. Our studies revealed that exogenous bacterial HSPs (Escherichia coli DnaK and Mycobacterium tuberculosis HSP70) delivered an extended OVA peptide for processing and MHC-I presentation by both murine macrophages and dendritic cells. HSP-enhanced MHC-I peptide presentation occurred only if peptide was complexed to the prokaryotic HSP and was dependent on CD91, establishing CD91 as a receptor for prokaryotic as well as mammalian HSPs. Inhibition of cytosolic processing mechanisms (e.g., by transporter for Ag presentation deficiency or brefeldin A) blocked HSP-enhanced peptide presentation in dendritic cells but not macrophages. Thus, prokaryotic HSPs deliver chaperoned peptide for alternate MHC-I Ag processing and cross-presentation via cytosolic mechanisms in dendritic cells and vacuolar mechanisms in macrophages. Prokaryotic HSPs are a potential source of microbial peptide Ags during phagocytic processing of bacteria during infection and could potentially be incorporated in vaccines to enhance presentation of peptides to CD8(+) T cells.

Cathepsin-dependent apoptosis triggered by supraoptimal activation of T lymphocytes: a possible mechanism of high dose tolerance

High doses of Ag can paradoxically suppress immune responses in vivo. This Ag-specific unresponsiveness (termed high dose tolerance) involves extrathymic mechanisms in mature T lymphocytes. To investigate these mechanisms, we used the in vitro model of PBL activated with anti-CD3 or PHA. In these conditions, increasing mitogen concentrations resulted in a reduction of the proliferative response, associated with an increased percentage of apoptotic cells. Apoptosis did not require prior exposure to IL-2, it was not the consequence of CD178/CD95 or TNF/TNFR interactions, and was therefore clearly distinct from activation-induced cell death. Although the pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk) decreased DNA fragmentation, cytochrome c release and caspase-9 and caspase-3 activation were not implicated, suggesting that this apoptosis did not primarily involve the intrinsic mitochondrial pathway. E64d, a cysteine protease inhibitor, as well as specific inhibitors of cathepsin B and cathepsin L conferred protection. We further demonstrated that cathepsin B and cathepsin L were released from the lysosomes and catalytically active in the cytosol. Release of cathepsin B and cathepsin L was the consequence of lysosomal membrane permeabilization without complete disruption of the cytosol-lysosome pH gradient. These results demonstrate a role for cathepsins in supraoptimal activation-induced apoptosis in vitro and suggest their possible participation in high dose tolerance in vivo.

Recognition of Bacteria in the Cytosol of Mammalian Cells by the Ubiquitin System

Recent studies have suggested the existence of innate host surveillance systems for the detection of bacteria in the cytosol of mammalian cells. The molecular details of how bacteria are recognized in the cytosol, however, remain unclear. Here we examined the fate of Salmonella typhimurium, a gram-negative bacterial pathogen that can infect a variety of hosts, in the cytosol of mammalian cells. These bacteria typically occupy a membrane bound compartment, the Salmonella-containing vacuole (SCV), in host cells. We show that some wild-type bacteria escape invasion vacuoles and are released into the cytosol. Subsequently, polyubiquitinated proteins accumulate on the bacterial surface, a response that was witnessed in several cell types. In macrophages but not epithelial cells, the proteasome was observed to undergo a dramatic subcellular relocalization and become associated with the surface of bacteria in the cytosol. Proteasome inhibition promoted replication of S. typhimurium in the cytosol of both cell types, in part through destabilization of the SCV. Surprisingly, the cytosol-adapted pathogen Listeria monocytogenes avoided recognition by the ubiquitin system by using actin-based motility. Our findings indicate that the ubiquitin system plays a major role in the recognition of bacterial pathogens in the cytosol of mammalian cells.

Characterization of M. Tuberculosis-derived IL-12-inducing material by alveolar macrophages

We have investigated the substance derived from Mycobacterium tuberculosis (Mtb) that induces interleukin (IL)-12 production by alveolar macrophages (AMs) in vitro. The cytosol fraction of live Mtb H37Rv induced IL-12 production by AMs in a dose-dependent manner. The addition of interferon-gamma (IFN-gamma) augmented IL-12 production. IL-12-inducing activity by AMs (termed as surely active keeping rescue antigen, SAKRA) was purified by gel filtration and ion exchange column chromatography, and the molecular weight of SAKRA was estimated by gel filtration to be more than 700 kDa. SDS-polyacrylamide gel electrophoresis (PAGE) and Western blotting of SAKRA using rabbit anti-SAKRA antibody suggested that SAKRA is composed with several low molecular weight proteins. Amino acids sequence analysis of several bands after SDS-PAGE suggested that SAKRA is a part of ribosomes. RT-PCR showed that SAKRA induced not only expression of IL-12 p40 mRNA, but expression of tumor necrosis factor (TNF)-alpha and inducible nitric oxide synthase (iNOS) mRNA at least 6 h after stimulation, suggesting that SAKRA activates the bactericidal activity of macrophages. To investigate the potential use of SAKRA as a vaccine against tuberculosis, SAKRA was administered to BALB/c mouse that had been immunized with BCG for 18 months, and mouse were infected with Mtb H37Rv via a respiratory route. Replication of Mtb in lungs and spleens was examined 6 weeks after infection. Administration of SAKRA to BCG-vaccinated mice significantly reduced the numbers of Mtb in lungs and spleens as compared with BCG-vaccinated control mice. Taken together, these results suggest that SAKRA is one of the Mtb-derived immunomodulatory substances which induce IL-12 production during infection and also increases mycobactericidal activities of macrophages, and that SAKRA may be a promising new vaccine candidate against tuberculosis.

Characterization of the antigenicity of the formiminotransferase-cyclodeaminase in type 2 autoimmune hepatitis

Human formiminotransferase-cyclodeaminase (hFTCD) is the autoantigen recognized by anti-liver cytosol type 1 (LC1) autoantibodies in type 2 autoimmune hepatitis (AIH) patients. In rats, this octameric protein is localized on the Golgi apparatus and binds brain microtubules (MTs) and vimentin. Subcellular localization of human formiminotransferase-cyclodeaminase and its implication in the pathogenesis of autoimmune hepatitis are unknown. Localization of the human formiminotransferase-cyclodeaminase in human hepatocytes was done using indirect immunofluorescence and subcellular fractionations followed by in vitro binding techniques. The formiminotransferase-cyclodeaminase antigen at two distinct locations in hepatocytes, free in the cytosol and associated with the Golgi membranes are recognized by anti-liver cytosol type 1 autoantibodies. The human formiminotransferase-cyclodeaminase binds reversibly to the Golgi membranes and this complex formation is increased by anti-liver cytosol type 1 autoantibodies. Finally, human formiminotransferase-cyclodeaminase does not interact with liver-specific cytoskeleton proteins. Anti-liver cytosol type 1 autoantibodies are directed against the mature high molecular form of human formiminotransferase-cyclodeaminase. Therefore, the subcellular location of the protein may influence the production of autoantibodies and their role in the pathogenesis of type 2 autoimmune hepatitis. This antigen-driven response does not appear to be facilitated or enhanced by a possible interaction between human formiminotransferase-cyclodeaminase and hepatocyte cytoskeleton proteins.

Cutting edge: efficient MHC class I cross-presentation during early vaccinia infection requires the transfer of proteasomal intermediates between antigen donor and presenting cells

Priming of CD8(+) T cells requires presentation of short peptides bound to MHC class I molecules of professional APCs. Cross-presentation is a mechanism whereby professional APC present on their own MHC class I molecules peptides derived from degradation of Ags synthesized by other Ag "donor cells." The mechanism of cross-presentation is poorly understood, and the nature of the transferred Ag is unknown. In this report, we demonstrate that the bulk of a cross-presented Ag transferred from donor cells recently infected with vaccinia virus are proteasomal products that are susceptible to peptidases within the donor cell cytosol and not full-length proteins or mature epitopes either free or bound to chaperones.

Retrotranslocation of MHC class I heavy chain from the endoplasmic reticulum to the cytosol is dependent on ATP supply to the ER lumen

MHC class I heavy chains (HC) that fail to acquire a mature conformation in the endoplasmic reticulum (ER) as a result of defective folding or assembly with beta2-microglobulin, or lack of appropriate peptide cargo are retrotranslocated through the Sec61 channel to the cytosol for degradation by proteasomes. The mechanisms involved in ER retrotranslocation of HC are as yet incompletely understood. Using a microsomal system, we characterized the molecular requirements for the release of HC into the soluble fraction. Extraction of ubiquitinated HC was facilitated by cytosol, or by addition of proteins that stabilized the membrane association of the cytoplasmic ATPase p97. Functional proteasomes were not needed for HC mobilization. ATP supply to the ER lumen was found to be an essential factor since an inhibitor of the ATP importing pump in the ER membrane blocked HC release. Also non-hydrolyzable ATP analogs delivered to the ER lumen facilitated HC export suggesting that ATP binding by ER chaperones rather than ATP hydrolysis is involved.

Calcium and Its Role in the Nuclear Translocation and Activation of Cytosolic Phospholipase A2in Cells Rendered Sensitive to TNF-Induced Apoptosis by Cycloheximide

In these experiments, we investigated the role of calcium as a second messenger in the apoptotic activation of cytosolic phospholipase A(2) (cPLA(2)). As our model, we used a murine fibroblast cell line (C3HA) that was induced to undergo apoptosis by a combination of TNF and cycloheximide. Using fura 2 Ca(2+) imaging, we found strong evidence for an intracellular calcium response after 1 h of treatment, which correlated with the onset of phosphatidylserine externalization, but preceded effector procaspase processing by several hours. The response was strongest in the perinuclear region, where mean levels rose 83% (144 +/- 14 nM in untreated cells vs 264 +/- 39 nM in treated), while cells displaying morphological evidence of apoptosis had the highest levels of calcium (250-1000 nM). Verapamil blocked this response, indicating an extracellular source for the calcium. Fluorescence microscopy revealed a pattern of nuclear translocation of cPLA(2) during apoptosis, which was also blocked by verapamil, indicating an important role for calcium in this process. In addition, we found that verapamil prevented the release of [(3)H]arachidonic acid from C3HA cells induced to undergo apoptosis by the chemotherapeutic agents vinblastine, melphalan, and cis-platinum. Together, these data suggest that calcium is important for cPLA(2) activation by diverse apoptotic stimuli.

Effects of anti-endothelial cell antibodies in leprosy and malaria

As a result of damaging endothelial cells (ECs), Mycobacterium leprae triggers the production of antibodies (Abs). These anti-EC Abs (AECAs) can be divided into two types. The first type nonspecifically reacts with components of the cytosol (CY) and can be detected by enzyme-linked immunosorbent assay (ELISA). The second specifically reacts with the EC membrane (MB) and requires fluorescence-activated cell sorter (FACS) analysis to be detected. The presence of both types of AECAs was determined in 68 leprosy patients. The ELISA was positive for 35 of them but also for 30 of 34 malaria patients and 17 of 50 healthy African controls. However, whereas FACS analysis showed MB reactivity in only three malaria patients and four controls, this reactivity was found in 27 leprosy patients, more of those having the lepromatous than the tuberculoid form. Specificity for MB, which we failed to absorb by incubation with CY lysates, predominated over that for CY in leprosy, unlike malaria, where the EC reactivity was restricted to the CY. Western blot analysis and two-dimensional electrophoresis revealed that calreticulin, vimentin, tubulin, and heat shock protein 70 were targeted by AECAs from leprosy patients, but other proteins remained unidentified. These auto-Abs, but not those from malaria patients, did activate ECs, as indicated by the E-selectin and intercellular adhesion molecule 1 upregulation, and/or induced them into apoptosis, as documented by four different methods. Our findings suggest that, in some but not all leprosy patients, AECAs may play a role in pathogenesis.

Cytosolic processing of proteasomal cleavage products can enhance the presentation efficiency of MHC-1 epitopes

The vertebrate immune system is able to detect abnormal body cells by the specific repertoire of 8 - 12 residues long peptides (= epitopes or peptide antigens) presented at the cell surface by the MHC-1 molecule complex. The generation of an epitope starts with the degradation of endogenous proteins into primary oligomeric fragments by cytosolic proteases, predominantly the proteasome. These primary fragments may be further attacked by various amino peptidases resident in the cytosol or, alternatively, may escape from this attack by entering the endoplasmic reticulum (ER) by the transporter associated with antigen presentation (TAP). To study the possible consequences of this scenario for the efficiency of antigen presentation we have applied kinetic modelling. The mathematical model comprises the generation of primary oligomeric fragments containing the definitive epitope, the successive N-terminal shortening of these primary fragments by cytosolic amino peptidases and the TAP-mediated transport of cytosolic peptides into the ER. Because the number of peptide molecules may become very small we have performed deterministic and stochastic simulations of the kinetic model. Our simulations show that cytosolic N-terminal trimming of primary fragments may drastically increase loading epitope precursors into the ER. In particular, a primary fragment generated with a low rate of TAP transport into the ER may nevertheless become a potent epitope precursor if at least one of its N-terminal trimming products will be efficiently transported.