Generation of both MHC class I- and class II-restricted antigenic peptides from exogenously added ovalbumin in murine phagosomes

Pathways of MHC I cross-presentation of exogenous antigens

Phagocytes, particularly dendritic cells (DCs), generate peptide-major histocompatibility complex (MHC) I complexes from antigens they have collected from cells in tissues and report this information to CD8 T cells in a process called cross-presentation. This process allows CD8 T cells to detect, respond and eliminate abnormal cells, such as cancers or cells infected with viruses or intracellular microbes. In some settings, cross-presentation can help tolerize CD8 T cells to self-antigens. One of the principal ways that DCs acquire tissue antigens is by ingesting this material through phagocytosis. The resulting phagosomes are key hubs in the cross-presentation (XPT) process and in fact experimentally conferring the ability to phagocytize antigens can be sufficient to allow non-professional antigen presenting cells (APCs) to cross-present. Once in phagosomes, exogenous antigens can be cross-presented (XPTed) through three distinct pathways. There is a vacuolar pathway in which peptides are generated and then bind to MHC I molecules within the confines of the vacuole. Ingested exogenous antigens can also be exported from phagosomes to the cytosol upon vesicular rupture and/or possibly transport. Once in the cytosol, the antigen is degraded by the proteasome and the resulting oligopeptides can be transported to MHC I molecule in the endoplasmic reticulum (ER) (a phagosome-to-cytosol (P2C) pathway) or in phagosomes (a phagosome-to-cytosol-to-phagosome (P2C2P) pathway). Here we review how phagosomes acquire the necessary molecular components that support these three mechanisms and the contribution of these pathways. We describe what is known as well as the gaps in our understanding of these processes.

The Lack of STING Impairs the MHC-I Dependent Antigen Presentation and JAK/STAT Signaling in Murine Macrophages

STING is a transmembrane ER resident protein that was initially described as a regulator of innate immune response triggered by viral DNA and later found to be involved in a broader range of immune processes. Here, we assessed its role in the antigen presentation by generating a STING KO macrophage cell line. In the absence of STING, we observed an impaired OVA-derived SIINFEKL peptide presentation together with a decreased level of MHC-I complex on the plasma membrane, likely due to a decreased mRNA expression of β2 m light chain as no relevant alterations of the peptide-loading complex (TAPs) were found. Moreover, JAK-STAT signaling resulted in impaired STING KO cells following OVA and LPS treatments, suggesting a dampened activation of immune response. Our data revealed a new molecular role of STING in immune mechanisms that could elucidate its role in the pathogenesis of autoimmune disorders and cancer.

Genome-wide identification and characterization of transcripts translationally regulated by bacterial lipopolysaccharide in macrophage-like J774.1 cells

Although Escherichia coli LPS is known to elicit various proinflammatory responses in macrophages, its effect on the translational states of transcripts has not yet been explored on a genome-wide scale. To address this, we investigated the mRNA profiles in polysomal and free messenger ribonucleoprotein particle (mRNP) fractions of mouse macrophage-like J774.1 cells, using Affymetrix Mouse Genome 430 2.0 GeneChips. Comparison of the mRNA profiles in total cellular, polysomal, and free mRNP fractions enabled us to identify transcripts that were modulated at the translational level by LPS: among 19,791 transcripts, 115 and 418 were up- and downregulated at 1, 2, or 4 h after LPS stimulation (100 ng/ml) in a translation-dependent manner. Interestingly, gene ontology-based analysis suggested that translation-dependent downregulated genes frequently include those encoding proteins in the mitochondrial respiratory chain. In fact, the mRNA levels of some transcripts for complexes I, IV, and V in the mitochondrial respiratory chain were translationally downregulated, eventually contributing to the decline of their protein levels. Moreover, the amount of metabolically labeled cytochrome oxidase subunit Va in complex IV was decreased without any change of its mRNA level in total cellular fraction after LPS stimulation. Consistently, the total amounts and activities of complexes I and IV were attenuated by LPS stimulation, and the attenuation was independent of nitric oxide. These results demonstrated that translational suppression may play a critical role in the LPS-mediated attenuation of mitochondrial oxidative phosphorylation in a nitric oxide-independent manner in J774.1 cells.

Cathepsin L and cystatin B gene expression discriminates immune coelomic cells in the leech Theromyzon tessulatum

Previous studies evidenced that cystatin B-like gene is specifically expressed and induced in large circulating coelomic cells following bacterial challenge in the leech Theromyzon tessulatum. In order to understand the role of that cysteine proteinase inhibitor during immune response, we investigated the existence of members of cathepsin family. We cloned a cathepsin L-like gene and studied its tissue distribution. Immunohistochemical studies using anti-cathepsin L and anti-cystatin B antibodies and ultrastructural results demonstrated the presence of three distinct coelomic cell populations: (1) the chloragocytes, which were initially defined as large coelomocytes, (2) the granular amoebocytes and (3) small coelomic cells. Among those cells, while chloragocytes contain cystatin B and cathepsin L, granular amoebocytes contain only cathepsin L and the third cell population contains neither cathepsin nor inhibitor. Finally, results evidenced that cathepsin L immunopositive granular amoebocytes are chemoattracted to the site of injury and phagocyte bacteria.

Yersinia enterocolitica: subversion of adaptive immunity and implications for vaccine development

Enteric Yersinia spp. invade Peyer's patches, disseminate to lymphoid tissues, and induce mucosal and systemic immune responses. Many virulence factors of Yersinia enterocolitica have been investigated in detail and were found to act on host cells involved in innate and adaptive immunity. Recent work explored as to whether attenuated Y. enterocolitica or recombinant components of Y. enterocolitica can be used as tools for vaccination. We and others have tested whether by means of the type three secretion system in attenuated Y. enterocolitica strains antigens might be delivered to antigen-presenting cells in order to induce CD8 and CD4 T cell responses. Alternatively, recombinant components of Y. enterocolitica such as invasin protein which binds to beta1 integrins of host cells have been tested for their ability to target antigen along with microparticles (fused to invasin) to antigen-presenting cells and to act as adjuvant. The work summarized in this article demonstrates that Y. enterocolitica and its components might be useful tools for novel vaccination strategies; in fact, invasin when fused to antigen and coated to microparticles might induce both CD4 and CD8 T cell responses. Likewise, attenuated Y. enterocolitica live carrier strains were reported to induce both CD8 and some CD4 T cell responses. However, we need to know more about how Y. enterocolitica subverts functions of antigen-presenting cells in order to design mutants with optimized antigen delivery features and deletion in those virulence factor that contribute to subversion of innate or adaptive immune responses.

LeishmaniaAntigens Are Presented to CD8+T Cells by a Transporter Associated with Antigen Processing-Independent Pathway In Vitro and In Vivo

CD8+ T cells are generated in response to Leishmania major (Lm) or Toxoplasma gondii parasitic infections, indicating that exogenously delivered Ag can be processed for presentation by MHC class I molecules. We show that presentation of Lm nucleotidase (NT)-OVA is TAP independent in vivo and in vitro, and is inhibited by chloroquine, but not by proteasome inhibitors. In contrast, the presentation of T. gondii P30-OVA relies on the TAP/proteasome pathway. Presentation of OVA- or rNT-OVA-coated beads also bypassed TAP requirement above a certain Ag threshold. TAP was also dispensable for the presentation of wild-type Lm Ags to primed CD8+ T cells in vitro. Finally, in vivo priming of CD8+ T cells involved in acquired resistance to Lm was not compromised in TAP-deficient mice. Thus, Leishmania Ags appear to be confined to an intraphagosomal processing pathway that requires higher concentrations of Ags, suggesting that these parasites may have evolved strategies to impair the efficient endoplasmic reticulum-based, TAP-dependent cross-presentation pathway to avoid or delay CD8+ T cell priming.

The Yersinia enterocolitica invasin protein promotes major histocompatibility complex class I- and class II-restricted T-cell responses

Yersinia enterocolitica invasin (Inv) protein confers internalization into and expression of proinflammatory cytokines by host cells. Both events require binding of Inv to beta1 integrins, which initiates signaling cascades including activation of focal adhesion complexes, Rac1, mitogen-activated protein kinase, and NF-kappaB. Here we tested whether Inv might be suitable as a delivery molecule and adjuvant if used as a component of a vaccine. For this purpose, hybrid proteins composed of Inv and ovalbumin (OVA) were prepared, applied as a coating to microparticles, and used for vaccination. Fusion of OVA to Inv did not significantly disturb the ability of Inv to promote host cell binding, internalization, and interleukin-8 (IL-8) secretion when applied as a coating to microparticles. The microparticles were used for vaccination of mice adoptively transferred with OVA-specific T cells from OT-1 or DO11.10 mice. Administration of OVA-Inv-coated microparticles induced OVA-specific T-cell responses. OVA-specific CD4 T cells produced both gamma interferon (IFN-gamma) and IL-4 as determined by enzyme-linked immunosorbent assay. Likewise, pronounced OVA-specific CD8 T-cell responses associated with IFN-gamma production were observed. Together, these results suggest that Inv might be an attractive tool in vaccination as it confers both host cell uptake and adjuvant activity by engagement of beta1 integrins of host cells, which leads to CD4 as well as CD8 T-cell responses.

Maturation of Human Neutrophil Phagosomes Includes Incorporation of Molecular Chaperones and Endoplasmic Reticulum Quality Control Machinery

A Human neutrophils are an essential component of the innate immune response. Although significant progress has been made toward understanding mechanisms of phagocytosis and microbicidal activity, a comprehensive analysis of proteins comprising neutrophil phagosomes has not been conducted. To that end, we used subcellular proteomics to identify proteins associated with human neutrophil phagosomes following receptor-mediated phagocytosis. Proteins (n = 411 spots) resolved from neutrophil phagosome fractions were identified by MALDI-TOF MS and/or LC-MS/MS analysis. Those associated with phagocytic vacuoles originated from multiple subcellular compartments, including the cytosol, plasma membrane, specific and azurophilic granules, and cytoskeleton. Unexpectedly several enzymes typically associated with mitochondria were identified in phagosome fractions. Furthermore proteins characteristic of the endoplasmic reticulum, including 11 molecular chaperones, were resolved from phagosome preparations. Confocal microscopy confirmed that proteins representing these major subcellular compartments were enriched on phagosomes of intact neutrophils. Notably calnexin and glucose-regulated protein 78 co-localized with gp91(phox) in human neutrophils and were thus likely delivered to phagosomes by fusion of specific granules. We conclude that neutrophil phagosomes have heretofore unrecognized complexity and function, which includes potential for antigen processing events.

RNAi-mediated gene silencing to assess the role of synaptobrevin and cystatin in tick blood feeding

In addition to being the conduit for pathogens into hosts, tick saliva contains a broad array of secretory products that facilitate prolonged tick attachment and blood feeding. Proteins found in tick saliva modulate host hemostasis and immune responses. However, it is not clear whether ticks manipulate the immune responses of their hosts by disrupting the antigen-processing pathways of the hosts. Protein secretion into tick saliva from the salivary glands is due to exocytosis of vesicular membrane-bound granular material regulated by SNARE complex proteins. Proteins associated with vesicles (v-SNAREs) are essential components of the exocytotic process. In this study, we assessed the functional significance of synaptobrevin, a SNARE protein, and cystatin, a cysteine protease inhibitor to blood feeding success, in the lone star tick, Amblyomma americanum, using in vivo RNA interference. In separate experiments, tick salivary cystatin and synpatobrevin genes were silenced by injecting adult ticks with 500 ng of dsRNA complementing each gene sequence. Silencing was demonstrated by reduced transcript in midguts and salivary glands. Additionally, disrupting expression of cystatin and synaptobrevin by RNAi reduced the ability of ticks to feed successfully, as demonstrated by feeding inhibition and reduced engorgement weights. Moreover, normal ticks exposed to a rabbit previously exposed to cystatin-silenced ticks exhibited significant resistance to tick feeding. Based on these findings, ticks appear to skillfully evade the host immune system by secreting cystatin, which disrupts normal antigen processing in antigen-presenting cells of hosts.

Malondialdehyde-acetaldehyde haptenated protein binds macrophage scavenger receptor(s) and induces lysosomal damage

There is evidence that the chemical modification of proteins (haptens) with malondialdehyde-acetaldehyde (MAA) and the immune response to these haptenated proteins is associated with the initiation and/or progression of alcohol liver disease. Experimentally, proteins modified with MAA induce antibody and T cell responses, which are mediated by scavenger receptor(s). Moreover, macrophages have been shown to play an important role in processing and presenting MAA-haptenated proteins in vitro. In vitro, MAA-modified proteins have been shown to induce both apoptosis and necrosis in a dose- and cell-type-dependent manner. Natural ligands modified by oxidative stress, such as oxidized LDL, similarly initiate not only antibody responses, but also cause cell death by disrupting lysosomes after binding to scavenger receptors and internalization. We therefore investigated the binding, internalization, and lysosomal integrity in a macrophage cell line to a MAA-haptenated protein. We demonstrate for the first time that MAA-haptenated proteins are preferentially bound by scavenger receptors on macrophages, which internalize the ligands and shuttle them to lysosomes. Moreover, MAA-haptenated proteins are demonstrated to be associated with a rapid dose-dependent disruption in lysosomal integrity, resulting in leakage and caspase activation. Similarly, as hen egg lysozyme (HEL)-MAA concentrations increased (>31.3 microg/ml), increased levels of apoptosis and a G1/S cell cycle checkpoint inhibition were identified. This study identifies mechanisms by which MAA-haptenated proteins are taken up by a representative antigen-presenting cell and may delineate steps by which MAA-haptenated proteins induce cell death and induce their immunogenicity to the carrier protein.

Transcriptomic analysis in the leech Theromyzon tessulatum: involvement of cystatin B in innate immunity

At the present time, there is little information on mechanisms of innate immunity in invertebrate groups other than insects, especially annelids. In the present study, we have performed a transcriptomic study of the immune response in the leech Theromyzon tessulatum after bacterial challenge, by a combination of differential display RT (reverse transcriptase)-PCR and cDNA microarrays. The results show relevant modulations concerning several known and unknown genes. Indeed, threonine deaminase, malate dehydrogenase, cystatin B, polyadenylate-binding protein and alpha-tubulin-like genes are up-regulated after immunostimulation. We focused on cystatin B (stefin B), which is an inhibitor of cysteine proteinases involved in the vertebrate immune response. We have cloned the full-length cDNA and named the T. tessulatum gene as Tt-cysb. Main structural features of cystatins were identified in the derived amino acid sequence of Tt-cysb cDNA; namely, a glycine residue in the N-terminus and a consensus sequence of Gln-Xaa-Val-Xaa-Gly (QXVXG) corresponding to the catalytic site. Moreover, Tt-cysb is the first cystatin B gene characterized in invertebrates. We have determined by in situ hybridization and immunocytochemistry that Tt-cysb is only expressed in large coelomic cells. In addition, this analysis confirmed that Tt-cysb is up-regulated after bacterial challenge, and that increased expression occurs only in coelomic cells. These data demonstrate that the innate immune response in the leech involves a cysteine proteinase inhibitor that is not found in ecdysozoan models, such as Drosophila melanogaster or Caenorhabditis elegans, and so underlines the great need for information about innate immunity mechanisms in different invertebrate groups.

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

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

Phagosomes are competent organelles for antigen cross-presentation

The ability to process microbial antigens and present them at the surface of cells is an important aspect of our innate ability to clear infections. It is generally accepted that antigens in the cytoplasm are loaded in the endoplasmic reticulum and presented at the cell surface on major histocompatibility complex (MHC) class I molecules, whereas peptides present in endo/phagocytic compartments are presented on MHC class II molecules. Despite the apparent segregation of the class I and class II pathways, antigens from intracellular pathogens including mycobacteria, Escherichia coli, Salmonella typhimurium, Brucella abortus and Leishmania, have been shown to elicit an MHC class-I-dependent CD8+ T-cell response, a process referred to as cross-presentation. The cellular mechanisms allowing the cross-presentation pathway are poorly understood. Here we show that phagosomes display the elements and properties needed to be self-sufficient for the cross-presentation of exogenous antigens, a newly ascribed function linked to phagocytosis mediated by the endoplasmic reticulum.

Early intracellular events during internalization of Listeria monocytogenes by J774 cells

The gram-positive bacillus Listeria monocytogenes gains entry into host cells through a phagosome membrane that forms around entering bacteria. During the early stages of internalization the invading bacteria appear to modify the protein composition of the forming phagosome membrane in J774 cells. MHC class II molecules on the cell surface and exposed surface molecules available for biotinylation are excluded from the bacteria-host cell membrane interface and from the forming phagosome. This exclusion of MHC class II molecules from the early phagosome may partially help to explain previous reports suggesting that L. monocytogenes is able to interfere with antigen presentation. Inside the host cell, MHC class II molecules are delivered to the phagosome membrane. This is followed by delivery of LAMP 1, a marker of late endocytic compartments, and fusion with low-pH compartments. The bacteria then escape into the cell cytoplasm, possibly assisted by rapid delivery of this low-pH environment.