Dendritic Cell Aggresome-Like-Induced Structure Formation and Delayed Antigen Presentation Coincide in Influenza Virus-Infected Dendritic Cells

Influenza A virus propagation requires the activation of the unfolded protein response and the accumulation of insoluble protein aggregates

Influenza A virus (IAV) employs multiple strategies to manipulate cellular mechanisms and support proper virion formation and propagation. In this study, we performed a detailed analysis of the interplay between IAV and the host cells' proteostasis throughout the entire infectious cycle. We reveal that IAV infection activates the inositol requiring enzyme 1 (IRE1) branch of the unfolded protein response, and that this activation is important for an efficient infection. We further observed the accumulation of virus-induced insoluble protein aggregates, containing both viral and host proteins, associated with a dysregulation of the host cell RNA metabolism. Our data indicate that this accumulation is important for IAV propagation and favors the final steps of the infection cycle, more specifically the virion assembly. These findings reveal additional mechanisms by which IAV disrupts host proteostasis and uncovers new cellular targets that can be explored for the development of host-directed antiviral strategies.

TLR-mediated aggresome-like induced structures comprise antimicrobial peptides and attenuate intracellular bacterial survival

Immune cells employ diverse mechanisms for host defense. Macrophages, in response to TLR activation, assemble aggresome-like induced structures (ALIS). Our group has shown TLR4-signaling transcriptionally upregulates p62/sequestome1, which assembles ALIS. We have demonstrated that TLR4-mediated autophagy is, in fact, selective-autophagy of ALIS. We hypothesize that TLR-mediated autophagy and ALIS contribute to host-defense. Here we show that ALIS are assembled in macrophages upon exposure to different bacteria. These structures are associated with pathogen-containing phagosomes. Importantly, we present evidence of increased bacterial burden, where ALIS assembly is prevented with p62-specific siRNA. We have employed 3D-super-resolution structured illumination microscopy (3D-SR-SIM) and mass-spectrometric (MS) analyses to gain insight into the assembly of ALIS. Ultra-structural analyses of known constituents of ALIS (p62, ubiquitin, LC3) reveal that ALIS are organized structures with distinct patterns of alignment. Furthermore, MS-analyses of ALIS identified, among others, several proteins of known antimicrobial properties. We have validated MS data by testing the association of some of these molecules (Bst2, IFITM2, IFITM3) with ALIS and the phagocytosed-bacteria. We surmise that AMPs enrichment in ALIS leads to their delivery to bacteria-containing phagosomes and restricts the bacteria. Our findings in this paper support hitherto unknown functions of ALIS in host-defense.

Pandemic influenza A (H1N1) virus causes abortive infection of primary human T cells

Influenza A virus still represents a noticeable epidemic risk to international public health at present, despite the extensive use of vaccines and anti-viral drugs. In the fight against pathogens, the immune defence lines consisting of diverse lymphocytes are indispensable for humans. However, the role of virus infection of lymphocytes and subsequent abnormal immune cell death remains to be explored. Different T cell subpopulations have distinct characterizations and functions, and we reveal the high heterogeneity of susceptibility to viral infection and biological responses such as apoptosis in various CD4⁺ T and CD8⁺ T cell subsets through single-cell transcriptome analyses. Effector memory CD8⁺ T cells (CD8⁺ T_EM) that mediate protective memory are identified as the most susceptible subset to pandemic influenza A virus infection among primary human T cells. Non-productive infection is established in CD8⁺ T_EM and naïve CD8⁺ T cells, which indicate the mechanism of intracellular antiviral activities for inhibition of virus replication such as abnormal viral splicing efficiency, incomplete life cycles and up-regulation of interferon-stimulated genes in human T cells. These findings provide insights into understanding lymphopenia and the infectious mechanisms of pandemic influenza A virus and broad immune host–pathogen interactional atlas in primary human T cells.

The role of dendritic cells derived osteoclasts in bone destruction diseases

The bone is previously considered as a dominant organ involved in the processes of locomotion. However, in the past two decades, a large number of studies have suggested that the skeletal system closely coordinated with the immune system so as to result in the emerging area of 'osteoimmunology'. In the evolution of many kinds of bone destruction-related diseases, osteoclasts could differentiate from dendritic cells, which contributed to increased expression of osteoclast-related membrane receptors and relatively higher activity of bone destruction, inducing severe bone destruction under inflammatory conditions. Numerous factors could influence the interaction between osteoclasts and dendritic cells, contributing to the pathogenesis of several bone diseases in the context of inflammation, including both immunocytes and a large number of cytokines. In addition, the products of osteoclasts released from bone destruction area serve as important signals for the differentiation and activation of immature dendritic cells. Therefore, the border between the dendritic cell-related immune response and osteoclast-related bone destruction has gradually unravelled. Dendritic cells and osteoclasts cooperate with each other to mediate bone destruction and bone remodelling under inflammatory conditions. In this review, we will pay attention to the interactions between dendritic cells and osteoclasts in physiological and pathological conditions to further understand the skeletal system and identify potential new therapeutic targets for the future by summarizing their significant roles and molecular mechanisms in bone destruction.

FAT10 localises in dendritic cell aggresome-like induced structures and contributes to their disassembly

Dendritic cell (DC) aggresome-like induced structures (DALIS) are protein aggregates of polyubiquitylated proteins that form transiently during DC maturation. DALIS scatter randomly throughout the cytosol and serve as antigen storage sites synchronising DC maturation and antigen presentation. Maturation of DCs is accompanied by the induction of the ubiquitin-like modifier FAT10 (also known as UBD), which localises to aggresomes, structures that are similar to DALIS. FAT10 is conjugated to substrate proteins and serves as a signal for their rapid and irreversible degradation by the 26S proteasome similar to, yet independently of ubiquitin, thereby contributing to antigen presentation. Here, we have investigated whether FAT10 is involved in the formation and turnover of DALIS, and whether proteins accumulating in DALIS can be modified through conjunction to FAT10 (FAT10ylated). We found that FAT10 localises to DALIS in maturing DCs and that this localisation occurs independently of its conjugation to substrates. Additionally, we investigated the DALIS turnover in FAT10-deficient and -proficient DCs, and observed FAT10-mediated disassembly of DALIS. Thus, we report further evidence that FAT10 is involved in antigen processing, which may provide a functional rationale as to why FAT10 is selectively induced upon DC maturation.

Sequestration of Late Antigens Within Viral Factories Impairs MVA Vector-Induced Protective Memory CTL Responses

Cytotoxic CD8+ T cell (CTL) responses play an essential role in antiviral immunity. Here, we focused on the activation of CTL which recognize epitopes derived from viral or recombinant antigens with either early or late expression kinetics after infection with Modified Vaccinia Virus Ankara (MVA). Late antigens but not early antigens failed to efficiently stimulate murine CTL lines in vitro and were unable to activate and expand protective memory T cell responses in mice in vivo. The reduced or absent presentation of late antigens was not due to impaired antigen presentation or delayed protein synthesis, but was caused by sequestration of late antigens within viral factories (VFs). Additionally, the trapping of late antigens in VFs conflicts with antigen processing and presentation as proteasomal activity was strongly reduced or absent in VFs, suggesting inefficient antigen degradation. This study gives for the first time a mechanistic explanation for the weak immunogenicity of late viral antigens for memory CTL activation. Since MVA is preferentially used as a boost vector in heterologous prime/boost vaccinations, this is an important information for future vaccine design.

Role of the Innate Cytokine Storm Induced by the Influenza A Virus

Influenza A viruses (IAVs) can be classified into dozens of subtypes based on their hemagglutinin (HA) and neuraminidase (NA) proteins. To date, 18 HA subtypes and 11 NA subtypes of IAVs that spread in animals and humans have been found. Following infection, the IAV first induces the innate immune system, which can rapidly recruit innate immune cells and cytokines to the site of infection. Influenza-induced cytokine storms have been associated with uncontrolled proinflammatory responses, which may lead to significant immunopathy and severe disease. Cytokine storms are complicated by several types of cytokines and chemokines that have various activities. In addition to their direct effects, their crossregulation causes cytokine networks to form; these networks determine the outcome of viral infections. In this review, we focus on cytokine storms and their signaling pathways that are triggered by the different subtypes of IAV.

Cellular Proteostasis During Influenza A Virus Infection-Friend or Foe?

In order to efficiently replicate, viruses require precise interactions with host components and often hijack the host cellular machinery for their own benefit. Several mechanisms involved in protein synthesis and processing are strongly affected and manipulated by viral infections. A better understanding of the interplay between viruses and their host-cell machinery will likely contribute to the development of novel antiviral strategies. Here, we discuss the current knowledge on the interactions between influenza A virus (IAV), the causative agent for most of the annual respiratory epidemics in humans, and the host cellular proteostasis machinery during infection. We focus on the manipulative capacity of this virus to usurp the cellular protein processing mechanisms and further review the protein quality control mechanisms in the cytosol and in the endoplasmic reticulum that are affected by this virus.

Stable Isotope Labeling Reveals Novel Insights Into Ubiquitin-Mediated Protein Aggregation With Age, Calorie Restriction, and Rapamycin Treatment

Accumulation of protein aggregates with age was first described in aged human tissue over 150 years ago and has since been described in virtually every human tissue. Ubiquitin modifications are a canonical marker of insoluble protein aggregates; however, the composition of most age-related inclusions remains relatively unknown. To examine the landscape of age-related protein aggregation in vivo, we performed an antibody-based pulldown of ubiquitinated proteins coupled with metabolic labeling and mass spectrometry on young and old mice on calorie restriction (CR), rapamycin (RP)-supplemented, and control diets. We show increased abundance of many ubiquitinated proteins in old mice and greater retention of preexisting (unlabeled) ubiquitinated proteins relative to their unmodified counterparts-fitting the expected profile of age-increased accumulation of long-lived aggregating proteins. Both CR and RP profoundly affected ubiquitinome composition, half-live, and the insolubility of proteins, consistent with their ability to mobilize these age-associated accumulations. Finally, confocal microscopy confirmed the aggregation of two of the top predicted aggregating proteins, keratins 8/18 and catalase, as well as their attenuation by CR and RP. Stable-isotope labeling is a powerful tool to gain novel insights into proteostasis mechanisms, including protein aggregation, and could be used to identify novel therapeutic targets in aging and protein aggregation diseases.

Natural history of Helicobacter pylori VacA toxin in human gastric epithelium in vivo: vacuoles and beyond

Uptake, intracellular trafficking and pathologic effects of VacA toxin from Helicobacter pylori have been widely investigated in vitro. However, no systematic analysis investigated VacA intracellular distribution and fate in H. pylori-infected human gastric epithelium in vivo, using ultrastructural immunocytochemistry that combines precise toxin localization with analysis of the overall cell ultrastructure and intercompartimental/interorganellar relationships. By immunogold procedure, in this study we investigated gastric biopsies taken from dyspeptic patients to characterize the overall toxin’s journey inside human gastric epithelial cells in vivo. Endocytic pits were found to take up VacA at sites of bacterial adhesion, leading to a population of peripheral endosomes, which in deeper (juxtanuclear) cytoplasm enlarged and fused each other to form large VacA-containing vacuoles (VCVs). These directly opened into endoplasmic reticulum (ER) cisternae, which in turn enveloped mitochondria and contacted the Golgi apparatus. In all such organelles we found toxin molecules, often coupled with structural damage. These findings suggest direct toxin transfer from VCVs to other target organelles such as ER/Golgi and mitochondria. VacA-induced cytotoxic changes were associated with the appearance of auto(phago)lysosomes containing VacA, polyubiquitinated proteins, p62/SQSTM1 protein, cathepsin D, damaged mitochondria and bacterial remnants, thus leading to persistent cell accumulation of degradative products.

Dendritic cell maturation and cross-presentation: timing matters!

As a population, dendritic cells (DCs) appear to be the best cross‐presenters of internalized antigens on major histocompatibility complex class I molecules in the mouse. To do this, DCs have developed a number of unique and dedicated means to control their endocytic and phagocytic pathways: among them, the capacity to limit acidification of their phagosomes, to prevent proteolytic degradation, to delay fusion of phagosomes to lysosomes, to recruit ER proteins to phagosomes, and to export phagocytosed antigens to the cytosol. The regulation of phagocytic functions, and thereby of antigen processing and presentation by innate signaling, represents a critical level of integration of adaptive and innate immune responses. Understanding how innate signals control antigen cross‐presentation is critical to define effective vaccination strategies for CD8⁺ T‐cell responses.

Visualization of early influenza A virus trafficking in human dendritic cells using STED microscopy

Influenza A viruses (IAV) primarily target respiratory epithelial cells, but can also replicate in immune cells, including human dendritic cells (DCs). Super-resolution microscopy provides a novel method of visualizing viral trafficking by overcoming the resolution limit imposed by conventional light microscopy, without the laborious sample preparation of electron microscopy. Using three-color Stimulated Emission Depletion (STED) microscopy, we visualized input IAV nucleoprotein (NP), early and late endosomal compartments (EEA1 and LAMP1 respectively), and HLA-DR (DC membrane/cytosol) by immunofluorescence in human DCs. Surface bound IAV were internalized within 5 min of infection. The association of virus particles with early endosomes peaked at 5 min when 50% of NP+ signals were also EEA1+. Peak association with late endosomes occurred at 15 min when 60% of NP+ signals were LAMP1+. At 30 min of infection, the majority of NP signals were in the nucleus. Our findings illustrate that early IAV trafficking in human DCs proceeds via the classical endocytic pathway.

Multiple functions of the E3 ubiquitin ligase CHIP in immunity

The carboxyl terminal of Hsp70-interacting protein (CHIP) is an E3 ubiquitin ligase that plays a pivotal role in the protein quality control system by shifting the balance of the folding-refolding machinery toward the degradative pathway. However, the precise mechanisms by which nonnative proteins are selected for degradation by CHIP either directly or indirectly via chaperone Hsp70 or Hsp90 are still not clear. In this review, we aim to provide a comprehensive model of the mechanism by which CHIP degrades its substrate in a chaperone-dependent or direct manner. In addition, through tight regulation of the protein level of its substrates, CHIP plays important roles in many physiological and pathological conditions, including cancers, neurological disorders, cardiac diseases, bone metabolism, immunity, and so on. Nonetheless, the precise mechanisms underlying the regulation of the immune system by CHIP are still poorly understood despite accumulating developments in our understanding of the regulatory roles of CHIP in both innate and adaptive immune responses. In this review, we also aim to provide a view of CHIP-mediated regulation of immune responses and the signaling pathways involved in the model described. Finally, we discuss the roles of CHIP in immune-related diseases.

Different Polyubiquitinated Bodies in Human Dendritic Cells: IL-4 Causes PaCS During Differentiation while LPS or IFNα Induces DALIS During Maturation

Two types of polyubiquitin-reactive cytoplasmic bodies, particulate cytoplasmic structures (PaCS) and dendritic cell (DC) aggresome-like induced structures (DALIS), were analyzed by electron microscopy, immunocytochemistry, immunoblotting, and flow cytometry in DC obtained from human blood monocytes incubated with GM-CSF plus IL-4 (IL4-DC), GM-CSF plus IFNα (IFN-DC), or GM-CSF alone (GM-DC), with or without LPS maturation. PaCS developed as monomorphic aggregates of proteasome-reactive barrel-like particles only in ribosomes-rich cytoplasmic areas of differentiating IL4-DC. In contrast, DALIS formed as vesicular bodies storing K63-linked ubiquitinated proteins by coalescence of increased endosomal structures, in IFN-DC or after LPS maturation of GM-DC. DALIS-forming cells showed incomplete morphological and functional DC-type differentiation when compared to PaCS-forming IL4-DC. PaCS and DALIS may have different function as well as different origin and cytochemistry. DALIS may be a transient accumulation site of potentially antigenic polyubiquitinated proteins during their processing and presentation. PaCS are found under physiologic or pathologic conditions associated with increased/deranged protein synthesis and increased ubiquitin-proteasome activity. Given its high heat-shock protein content PaCS may work as a quality control structure for newly synthesized, cytosolic proteins. This comparative analysis suggests that PaCS and DALIS have distinctive roles in DC.

Intranasal administration of poly-gamma glutamate induced antiviral activity and protective immune responses against H1N1 influenza A virus infection

BACKGROUND

The global outbreak of a novel swine-origin strain of the 2009 H1N1 influenza A virus and the sudden, worldwide increase in oseltamivir-resistant H1N1 influenza A viruses highlight the urgent need for novel antiviral therapy.

METHODS

Here, we investigated the antiviral efficacy of poly-gamma glutamate (γ-PGA), a safe and edible biomaterial that is naturally synthesized by Bacillus subtilis, against A/Puerto Rico/8/1934 (PR8) and A/California/04/2009 (CA04) H1N1 influenza A virus infections in C57BL/6 mice.

RESULTS

Intranasal administration of γ-PGA for 5 days post-infection improved survival, increased production of antiviral cytokines including interferon-beta (IFN-β) and interleukin-12 (IL-12), and enhanced activation of natural killer (NK) cells and influenza antigen-specific cytotoxic T lymphocytes (CTL) activity.

CONCLUSIONS

These results suggest that γ-PGA protects mice against H1N1 influenza A virus by enhancing antiviral immune responses.

Artificial Loading of ASC Specks with Cytosolic Antigens

Inflammasome complexes form upon interaction of Nod Like Receptor (NLR) proteins with pathogen associated molecular patterns (PAPMS) inside the cytosol. Stimulation of a subset of inflammasome receptors including NLRP3, NLRC4 and AIM2 triggers formation of the micrometer-sized spherical supramolecular complex called the ASC speck. The ASC speck is thought to be the platform of inflammasome activity, but the reason why a supramolecular complex is preferred against oligomeric platforms remains elusive. We observed that a set of cytosolic proteins, including the model antigen ovalbumin, tend to co-aggregate on the ASC speck. We suggest that co-aggregation of antigenic proteins on the ASC speck during intracellular infection might be instrumental in antigen presentation.

The impact of macroautophagy on CD8(+) T-cell-mediated antiviral immunity

Macroautophagy is a catabolic recycling pathway, which can be induced by various stress stimuli. Viruses are able to manipulate autophagy in the cells that they infect. The impact of autophagy on the innate immune response to viruses and its stimulatory role in antigen presentation to CD4(+) T cells are well documented. Herein, we present the impact of autophagy on the activation of cytotoxic T lymphocyte (CTL)-mediated antiviral immune responses, which are required for the eradication or control of multiple viruses. We first discuss the general mechanisms by which viruses can either induce or block autophagy in cells. We then explore the cross-talk between autophagy and innate immune processes, which are both first line defenses against viruses; and constitute crucial steps for the initiation of potent adaptive immune responses. We describe the impact of autophagy on the presentation of viral peptide antigens on class I major histocompatibility complex (MHC I), a prerequisite for the priming of CTL responses. In sum, our review highlights the interplay between viruses and three integrated host response pathways - autophagy, innate and adaptive immunity - providing a framework for future mechanistic and pathogenesis-based research.

PaCS is a novel cytoplasmic structure containing functional proteasome and inducible by cytokines/trophic factors

A variety of ubiquitinated protein-containing cytoplasmic structures has been reported, from aggresomes to aggresome-like induced structures/sequestosomes or particle-rich cytoplasmic structures (PaCSs) that we recently observed in some human diseases. Nevertheless, the morphological and cytochemical patterns of the different structures remain largely unknown thus jeopardizing their univocal identification. Here, we show that PaCSs resulted from proteasome and polyubiquitinated protein accumulation into well-demarcated, membrane-free, cytoskeleton-poor areas enriched in glycogen and glycosaminoglycans. A major requirement for PaCS detection by either electron or confocal microscopy was the addition of osmium to aldehyde fixatives. However, by analyzing living cells, we found that proteasome chymotrypsin-like activity concentrated in well-defined cytoplasmic structures identified as PaCSs by ultrastructural morphology and immunocytochemistry of the same cells. PaCSs differed ultrastructurally and cytochemically from sequestosomes which may coexist with PaCSs. In human dendritic or natural killer cells, PaCSs were induced in vitro by cytokines/trophic factors during differentiation/activation from blood progenitors. Our results provide evidence that PaCS is indeed a novel distinctive cytoplasmic structure which may play a critical role in the ubiquitin–proteasome system response to immune, infectious or proneoplastic stimuli.

Immunomodulaton and attenuation of lethal influenza A virus infection by oral administration with KIOM-C

Herbal medicine is used to treat many conditions such as asthma, eczema, premenstrual syndrome, rheumatoid arthritis, migraine, headaches, menopausal symptoms, chronic fatigue, irritable bowel syndrome, cancer, and viral infections such as influenza. In this study, we investigated the antiviral effect of KIOM-C for the treatment of influenza A virus infection. Our results show that oral administration of KIOM-C conferred a survival benefit to mice infected with the 2009 pandemic H1N1 [A(H1N1)pdm09] virus, and resulted in a 10- to 100-fold attenuation of viral replication in ferrets in a dose-dependent manner. Additionally, oral administration of KIOM-C increased the production of antiviral cytokines, including IFN-γ and TNF-α, and decreased levels of pro-inflammatory cytokines (IL-6) and chemokines (KC, MCP-1) in the Bronchoalveolar lavage fluid (BALF) of A(H1N1)pdm-infected mice. These results indicate that KIOM-C can promote clearance of influenza virus in the respiratory tracts of mice and ferrets by modulating cytokine production in hosts. Taken together, our results suggest that KIOM-C is a potential therapeutic compound mixture for the treatment of influenza virus infection in humans.

Endosome-mediated autophagy: an unconventional MIIC-driven autophagic pathway operational in dendritic cells

Activation of TLR signaling has been shown to induce autophagy in antigen-presenting cells (APCs). Using high-resolution microscopy approaches, we show that in LPS-stimulated dendritic cells (DCs), autophagosomes emerge from MHC class II compartments (MIICs) and harbor both the molecular machinery for antigen processing and the autophagosome markers LC3 and ATG16L1. This ENdosome-Mediated Autophagy (ENMA) appears to be the major type of autophagy in DCs, as similar structures were observed upon established autophagy-inducing conditions (nutrient deprivation, rapamycin) and under basal conditions in the presence of bafilomycin A1. Autophagosome formation was not significantly affected in DCs expressing ATG4B (C74A) mutant and atg4b (-/-) bone marrow DCs, but the degradation of the autophagy substrate SQSTM1/p62 was largely impaired. Furthermore, we demonstrate that the previously described DC aggresome-like LPS-induced structures (DALIS) contain vesicular membranes, and in addition to SQSTM1 and ubiquitin, they are positive for LC3. LC3 localization on DALIS is independent of its lipidation. MIIC-driven autophagosomes preferentially engulf the LPS-induced SQSTM1-positive DALIS, which become later degraded in autolysosomes. DALIS-associated membranes also contain ATG16L1, ATG9 and the Q-SNARE VTI1B, suggesting that they may represent (at least in part) a membrane reservoir for autophagosome expansion. We propose that ENMA constitutes an unconventional, APC-specific type of autophagy, which mediates the processing and presentation of cytosolic antigens by MHC class II machinery, and/or the selective clearance of toxic by-products of elevated ROS/RNS production in activated DCs, thereby promoting their survival.

Cellular strategies for regulating functional and nonfunctional protein aggregation

Growing evidence suggests that aggregation-prone proteins are both harmful and functional for a cell. How do cellular systems balance the detrimental and beneficial effect of protein aggregation? We reveal that aggregation-prone proteins are subject to differential transcriptional, translational, and degradation control compared to nonaggregation-prone proteins, which leads to their decreased synthesis, low abundance, and high turnover. Genetic modulators that enhance the aggregation phenotype are enriched in genes that influence expression homeostasis. Moreover, genes encoding aggregation-prone proteins are more likely to be harmful when overexpressed. The trends are evolutionarily conserved and suggest a strategy whereby cellular mechanisms specifically modulate the availability of aggregation-prone proteins to (1) keep concentrations below the critical ones required for aggregation and (2) shift the equilibrium between the monomeric and oligomeric/aggregate form, as explained by Le Chatelier’s principle. This strategy may prevent formation of undesirable aggregates and keep functional assemblies/aggregates under control.

Abortive replication of influenza virus in mouse dendritic cells

The interaction between influenza virus and dendritic cells (DCs) remains poorly defined and controversial. Here we show that influenza virus replication in mouse bone marrow-derived DCs is abortive, despite viral genome transcription and replication occurring for each gene segment and viral hemagglutinin and nucleoprotein, at least, being produced. Electron microscopy reveals that virus assembly, rather than release of virus from the cell surface, is defective.

The Hsc/Hsp70 co-chaperone network controls antigen aggregation and presentation during maturation of professional antigen presenting cells

The maturation of mouse macrophages and dendritic cells involves the transient deposition of ubiquitylated proteins in the form of dendritic cell aggresome-like induced structures (DALIS). Transient DALIS formation was used here as a paradigm to study how mammalian cells influence the formation and disassembly of protein aggregates through alterations of their proteostasis machinery. Co-chaperones that modulate the interplay of Hsc70 and Hsp70 with the ubiquitin-proteasome system (UPS) and the autophagosome-lysosome pathway emerged as key regulators of this process. The chaperone-associated ubiquitin ligase CHIP and the ubiquitin-domain protein BAG-1 are essential for DALIS formation in mouse macrophages and bone-marrow derived dendritic cells (BMDCs). CHIP also cooperates with BAG-3 and the autophagic ubiquitin adaptor p62 in the clearance of DALIS through chaperone-assisted selective autophagy (CASA). On the other hand, the co-chaperone HspBP1 inhibits the activity of CHIP and thereby attenuates antigen sequestration. Through a modulation of DALIS formation CHIP, BAG-1 and HspBP1 alter MHC class I mediated antigen presentation in mouse BMDCs. Our data show that the Hsc/Hsp70 co-chaperone network controls transient protein aggregation during maturation of professional antigen presenting cells and in this way regulates the immune response. Similar mechanisms may modulate the formation of aggresomes and aggresome-like induced structures (ALIS) in other mammalian cell types.

Infection of HLA-DR1 transgenic mice with a human isolate of influenza a virus (H1N1) primes a diverse CD4 T-cell repertoire that includes CD4 T cells with heterosubtypic cross-reactivity to avian (H5N1) influenza virus

The specificity of the CD4 T-cell immune response to influenza virus is influenced by the genetic complexity of the virus and periodic encounters with variant subtypes and strains. In order to understand what controls CD4 T-cell reactivity to influenza virus proteins and how the influenza virus-specific memory compartment is shaped over time, it is first necessary to understand the diversity of the primary CD4 T-cell response. In the study reported here, we have used an unbiased approach to evaluate the peptide specificity of CD4 T cells elicited after live influenza virus infection. We have focused on four viral proteins that have distinct intracellular distributions in infected cells, hemagglutinin (HA), neuraminidase (NA), nucleoprotein, and the NS1 protein, which is expressed in infected cells but excluded from virion particles. Our studies revealed an extensive diversity of influenza virus-specific CD4 T cells that includes T cells for each viral protein and for the unexpected immunogenicity of the NS1 protein. Due to the recent concern about pandemic avian influenza virus and because CD4 T cells specific for HA and NA may be particularly useful for promoting the production of neutralizing antibody to influenza virus, we have also evaluated the ability of HA- and NA-specific CD4 T cells elicited by a circulating H1N1 strain to cross-react with related sequences found in an avian H5N1 virus and find substantial cross-reactivity, suggesting that seasonal vaccines may help promote protection against avian influenza virus.

Modulation of ubiquitin dynamics and suppression of DALIS formation by the Legionella pneumophila Dot/Icm system

Legionella pneumophila is an intracellular pathogen that uses effector proteins translocated by the Dot/Icm type IV secretion system to modulate host cellular processes. Here we investigate the dynamics of subcellular structures containing ubiquitin during L. pneumophila infection of phagocytic host cells. The Dot/Icm system mediated the formation of K48 and K63 poly-ubiquitin conjugates to proteins associated with L. pneumophila-containing vacuoles in macrophages and dendritic cells, suggesting that regulatory events and degradative events involving ubiquitin are regulated by bacterial effectors during infection. Stimulation of TLR2 on the surface of macrophages and dendritic cells by L. pneumophila-derived molecules resulted in the production of ubiquitin-rich dendritic cell aggresome-like structures (DALIS). Cells infected by L. pneumophila with a functional Dot/Icm system, however, failed to produce DALIS. Suppression of DALIS formation did not affect the accumulation of ubiquitinated proteins on vacuoles containing L. pneumophila. Examining other species of Legionella revealed that Legionella jordanis was unable to suppress DALIS formation after creating a ubiquitin-decorated vacuole. Thus, the L. pneumophila Dot/Icm system has the ability to modulate host processes to promote K48 and K63 ubiquitin conjugates on proteins at the vacuole membrane, and independently suppress cellular events required for the formation of DALIS.

Correlation of dendritic cell maturation and the formation of aggregates of poly-ubiquitinated proteins in the cytosol

Dendritic cells (DCs) are the most powerful antigen presenting cells (APCs) in the immune system. Therefore, they are able to take up antigen by phagocytosis, macropinocytosis or endocytosis, process it in the cytosol and present it to naive T cells. It is known that presentation of the immunodominant influenza virus nucleoprotein-derived CTL epitope is delayed in bone marrow-derived DCs (BMDCs) compared to non-professional APCs. This delay coincided with the formation of transient aggregations of ubiquitinated proteins (DALIS, dendritic cell aggresome-like induced structures), which contain probably defective ribosomal products (DRiPs). DRiPs appear in the cytosol of maturing DCs and macrophages. Normally, DRiPs are degraded rapidly by proteasomes. However, their storage in DALIS delays their degradation. So, it is hypothesized that DALIS can function as antigen depots allowing DCs to coordinate maturation and antigen presentation during their migration to the lymph nodes. Upon inhibition of several pathways among the in signal transduction pathways of DCs, like the phosphatidylinositol 3-kinase (PI3-K) or the mammalian target of Rapamycin (mTOR), the cells show a rendered maturation profile. The formation of DALIS is inhibited in these cells which can be expected to influence antigen processing and presentation.

Adjuvant potential of aggregate-forming polyglutamine domains

Aggregation may significantly affect the fate of a polypeptide, including its susceptibility to proteasome-dependent or autophagic degradation, its interaction with chaperones, etc. Since all these factors may affect the antigenicity of a polypeptide, we hypothesized that stimulating aggregation of an antigenic protein by its fusion to polyQ domain may enhance its antigenic potential. This hypothesis was tested with the weakly immunogenic model antigen GFP, which was fused to either long polyQ domain that triggers protein aggregation (103Q), or short polyQ domain that does not promote aggregation (25Q). Plasmids encoding control pGFP or soluble 25Q-GFP generated a very weak antibody response, while a significant increase in anti-GFP antibody titer was seen in groups immunized with DNA encoding aggregating 103Q-GFP. Similarly, fusion with 103Q strongly enhanced anti-GFP CTL activity, compared to fusion with 25Q. No apparent toxicity was observed after immunization with polyQ-GFP fusions. These data suggest that fusion of an antigen with expanded polyQ domains could have a significant adjuvant potential.

Regulation of translation is required for dendritic cell function and survival during activation

In response to inflammatory stimulation, dendritic cells (DCs) have a remarkable pattern of differentiation (maturation) that exhibits specific mechanisms to control antigen processing and presentation. Here, we show that in response to lipopolysaccharides, protein synthesis is rapidly enhanced in DCs. This enhancement occurs via a PI3K-dependent signaling pathway and is key for DC activation. In addition, we show that later on, in a manner similar to viral or apoptotic stress, DC activation leads to the phosphorylation and proteolysis of important translation initiation factors, thus inhibiting cap-dependent translation. This inhibition correlates with major changes in the origin of the peptides presented by MHC class I and the ability of mature DCs to prevent cell death. Our observations have important implications in linking translation regulation with DC function and survival during the immune response.

Viruses and dendritic cells: enemy mine

Dendritic cells (DCs) act not only as sentinels for detection of, but also as target cells for viruses, and this can be important for viral transport and spread. All subsets of DCs are equipped with a battery of receptors recognizing virus‐associated molecular signatures, and recognition of those launches a maturation programme that results in substantial alterations of morphology, motility and the DCs' interactive properties with the extracellular matrix and scanning T cells. In addition to being sensed, viruses are internalized into DCs and, for the major proportion, processed into peptides that are subsequently presented by major histocompatibility complex (MHC) molecules. Transmission of virus to T cells can occur after completion of their replication cycle if the intracellular milieu of the DC permits that. Alternatively, viruses can remain protected from degradation following entrapment by pattern recognition receptors in intracellular compartments, also referred to as virosomes, which translocate towards the DC/T cell interface. Most likely, transfer of virus to T cells occurs in these junctions, referred to as infectious synapses. In addition to promoting DC maturation, many viruses are able to downmodulate DC development and functions in order to evade immune recognition or to induce a generalized immunosuppression.

Ubiquitinated-protein aggregates form in pancreatic beta-cells during diabetes-induced oxidative stress and are regulated by autophagy

Diabetes-induced oxidative stress can lead to protein misfolding and degradation by the ubiquitin-proteasome system. This study examined protein ubiquitination in pancreatic sections from Zucker diabetic fatty rats. We observed large aggregates of ubiquitinated proteins (Ub-proteins) in insulin-expressing beta-cells and surrounding acinar cells. The formation of these aggregates was also observed in INS1 832/13 beta-cells after exposure to high glucose (30 mmol/l) for 8-72 h, allowing us to further characterize this phenotype. Oxidative stress induced by aminotriazole (ATZ) was sufficient to stimulate Ub-protein aggregate formation. Furthermore, the addition of the antioxidants N-acetyl cysteine (NAC) and taurine resulted in a significant decrease in formation of Ub-protein aggregates in high glucose. Puromycin, which induces defective ribosomal product (DRiP) formation was sufficient to induce Ub-protein aggregates in INS1 832/13 cells. However, cycloheximide (which blocks translation) did not impair Ub-protein aggregate formation at high glucose levels, suggesting that long-lived proteins are targeted to these structures. Clearance of Ub-protein aggregates was observed during recovery in normal medium (11 mmol/l glucose). Despite the fact that 20S proteasome was localized to Ub-protein aggregates, epoxomicin treatment did not affect clearance, indicating that the proteasome does not degrade proteins localized to these structures. The autophagy inhibitor 3MA blocked aggregate clearance during recovery and was sufficient to induce their formation in normal medium. Together, these findings demonstrate that diabetes-induced oxidative stress induces ubiquitination and storage of proteins into cytoplasmic aggregates that do not colocalize with insulin. Autophagy, not the proteasome, plays a key role in regulating their formation and degradation. To our knowledge, this is the first demonstration that autophagy acts as a defense to cellular damage incurred during diabetes.

Plumbing the sources of endogenous MHC class I peptide ligands

From fish to fowl to pharaohs, nearly all cells in jawed vertebrates constitutively process and present peptides derived from endogenously synthesized polypeptides. Such peptides, snug in the binding groove of cell surface MHC class I molecules, enable CD8(+) T cell mediated immunosurveillance of viruses, other intracellular pathogens, and spontaneously arising tumors. The MHC class I system also plays an important role in olfactory-based vertebrate mate selection and perhaps even in preventing direct transmission of tumors between individuals. Recent findings indicate that MHC class I bound peptides are generated at higher efficiency from rapidly degraded polypeptides (including defective ribosomal products) than from old proteins. Intimately linking translation and antigen presentation makes perfect sense for immunosurveillance of acute virus infections, in which speed is of the essence to minimize viral replication, pathogenesis and transmission. The intriguing question of how translation is linked to presentation has prompted the immunoribosome hypothesis of immunosurveillance, which posits that MHC class I peptide ligands are preferentially generated from a subset of translation products.

Autophagy: Eating for Good Health

A renaissance in the autophagy field has illuminated many areas of biology, and infectious disease is no exception. By identifying key components of this broadly conserved membrane traffic pathway, yeast geneticists generated tools for microbiologists and immunologists to explore whether autophagy contributes to host defenses. As a result, autophagy is now recognized to be another barrier confronted by microbes that invade eukaryotic cells. Mounting evidence also indicates that autophagy equips cells to deliver cytosolic Ags to the MHC class II pathway. By applying knowledge of the autophagy machinery and exploiting microbes as genetic probes, experimentalists can now examine in detail how this ancient membrane traffic pathway contributes to these and other mechanisms critical for infection and immunity.

Cytomegalovirus encodes a positive regulator of antigen presentation

Murine cytomegalovirus encodes three regulators of antigen presentation to antiviral CD8 T cells. According to current paradigms, all three regulators are committed to the inhibition of the presentation of antigenic peptides. Whereas m152/gp40 catalyzes the retention of peptide-loaded major histocompatibility complex (MHC) class I molecules in a cis-Golgi compartment, m06/gp48 binds stably to class I molecules and directs them into the cellular cargo-sorting pathway of lysosomal degradation. Regulator m04/gp34 also binds stably to class I molecules, but unlike m152 and m06, it does not downmodulate MHC class I cell surface expression. It has entered the literature as a direct inhibitor of T-cell recognition of the MHC-peptide complex at the cell surface. In this work, we have studied the presentation of antigenic viral peptides in cells infected with a comprehensive set of mutant viruses expressing the three regulators separately as well as in all possible combinations. The results redefine m04 as a positive regulator dedicated to the facilitation of antigen presentation. When expressed alone, it did not inhibit T-cell recognition, and when expressed in the presence of m152, it restored antigen presentation by antagonizing the inhibitory function of m152. Its intrinsic positive function, however, was antagonized and even slightly overcompensated for by the negative regulator m06. In an adoptive cell transfer model, the opposing forces of the three regulators were found to govern immune surveillance in the infected host. While negative regulators, also known as immunoevasins, are common, the existence of a positive regulator is without precedent and indicates an intriguing genetic potential of this virus to influence antigen presentation.

Tight Linkage between Translation and MHC Class I Peptide Ligand Generation Implies Specialized Antigen Processing for Defective Ribosomal Products

There is mounting evidence that MHC class I peptide ligands are predominantly generated from defective ribosomal products and other classes of polypeptides degraded rapidly (t1/2 < 10 min) following their synthesis. The most direct evidence supporting this conclusion is the rapid inhibition of peptide ligand generation following cycloheximide-mediated inhibition of protein synthesis. In this study, we show that this linkage is due to depleting the pool of rapidly degraded proteins, and not to interference with other protein synthesis-dependent processes. Our findings indicate that in the model systems used in this study, MHC class I peptides are preferentially generated from rapidly degraded polypeptides relative to slowly degraded proteins. This conclusion is supported by the properties of peptide presentation from slowly degraded (t1/2 = 4 h) defective ribosomal products generated artificially by incorporation of the amino acid analog canavanine into a model viral Ag. We propose that specialized machinery exists to link protein synthesis with class I peptide ligand generation to enable the rapid detection of viral gene expression.