Regulation of innate immune responses by autophagy-related proteins

Receptor signaling lymphocyte-activation molecule family 1 (Slamf1) regulates membrane fusion and NADPH oxidase 2 (NOX2) activity by recruiting a Beclin-1/Vps34/ultraviolet radiation resistance-associated gene (UVRAG) complex

Phagocytosis is a pivotal process by which macrophages eliminate microorganisms upon recognition by pathogen sensors. Surprisingly, the self-ligand cell surface receptor Slamf1 functions not only as a co-stimulatory molecule but also as a microbial sensor of several Gram-negative bacteria. Upon entering the phagosome of macrophages Slamf1 induces production of phosphatidylinositol 3-phosphate, which positively regulates the activity of the NOX2 enzyme and phagolysosomal maturation. Here, we report that in Escherichia coli-containing phagosomes of mouse macrophages, Slamf1 interacts with the class III PI3K Vps34 in a complex with Beclin-1 and UVRAG. Upon phagocytosis of bacteria the NOX2 activity was reduced in macrophages isolated from Beclin-1(+/-) mice compared with wild-type mice. This Slamf1/Beclin-1/Vps34/UVRAG protein complex is formed in intracellular membrane compartments as it is found without inducing phagocytosis in macrophages, human chronic lymphocytic leukemia cells, and transfectant HEK293 cells. Elimination of its cytoplasmic tail abolished the interaction of Slamf1 with the complex, but deletion or mutation of the two ITAM motifs did not. Both the BD and CCD domains of Beclin-1 were required for efficient binding to Slamf1. Because Slamf1 did not interact with Atg14L or Rubicon, which can also form a complex with Vps34 and Beclin-1, we conclude that Slamf1 recruits a subset of Vps34-associated proteins, which is involved in membrane fusion and NOX2 regulation.

Cytosolic double-stranded DNA as a damage-associated molecular pattern induces the inflammatory response in rat pancreatic stellate cells: a plausible mechanism for tissue injury-associated pancreatitis

Pancreatitis is an inflammatory disease of unknown causes. There are many triggers causing pancreatitis, such as alcohol, common bile duct stone, virus and congenital or acquired stenosis of main pancreatic duct, which often involve tissue injuries. Pancreatitis often occurs in sterile condition, where the dead/dying pancreatic parenchymal cells and the necrotic tissues derived from self-digested-pancreas were observed. However, the causal relationship between tissue injury and pancreatitis and how tissue injury could induce the inflammation of the pancreas were not elucidated fully until now. This study demonstrates that cytosolic double-stranded DNA increases the expression of several inflammatory genes (cytokines, chemokines, type I interferon, and major histocompatibility complex) in rat pancreatic stellate cells. Furthermore, these increase accompanied the multiple signal molecules genes, such as interferon regulatory factors, nuclear factor-kappa B, low-molecular-weight protein 2, and transporter associated with antigen processing 1. We suggest that this phenomenon is a plausible mechanism that might explain how cell damage of the pancreas or tissue injury triggers acute, chronic, and autoimmune pancreatitis; it is potentially relevant to host immune responses induced during alcohol consumption or other causes.

STING mediates neuronal innate immune response following Japanese encephalitis virus infection

Flavivirus-mediated inflammation causes neuronal death, but whether the infected neurons can evoke an innate immune response to elicit their own protection, is unknown. In an earlier study we have shown that neuronal RIG-I, play a significant role in inducing production and release of molecules that are related to inflammation. In this study, using a neuronal cell line, we show that RIG-I acts with STING in a concerted manner following its interaction with Japanese encephalitis viral RNA to induce a type 1 interferon response. Knock-down of STING showed that the expressions of various inflammatory signaling molecules were down-regulated along with increased intracellular viral load. Alternatively, over-expressing STING decreased intracellular viral load. Our results indicate that at the sub-cellular level, interaction between the pattern recognition receptor RIG-I and the adapter molecule STING, is a major contributor to elicit immunological responses involving the type 1 interferons in neurons following JEV infections.

Internalization of modified lipids by CD36 and SR-A leads to hepatic inflammation and lysosomal cholesterol storage in Kupffer cells

Background & Aims

Non-alcoholic steatohepatitis (NASH) is characterized by steatosis and inflammation, which can further progress into fibrosis and cirrhosis. Recently, we demonstrated that combined deletion of the two main scavenger receptors, CD36 and macrophage scavenger receptor 1 (MSR1), which are important for modified cholesterol-rich lipoprotein uptake, reduced NASH. The individual contributions of these receptors to NASH and the intracellular mechanisms by which they contribute to inflammation have not been established. We hypothesize that CD36 and MSR1 contribute independently to the onset of inflammation in NASH, by affecting intracellular cholesterol distribution inside Kupffer cells (KCs).

Methods & Results

Ldlr^−/− mice were transplanted with wild-type (Wt), Cd36^−/− or Msr1^−/− bone marrow and fed a Western diet for 3months. Cd36^−/−- and Msr1^−/−- transplanted (tp) mice showed a similar reduction in hepatic inflammation compared to Wt-tp mice. While the total amount of cholesterol inside KCs was similar in all groups, KCs of Cd36^−/−- and Msr1^−/−-tp mice showed increased cytoplasmic cholesterol accumulation, while Wt-tp mice showed increased lysosomal cholesterol accumulation.

Conclusion

CD36 and MSR1 contribute similarly and independently to the progression of inflammation in NASH. One possible explanation for the inflammatory response related to expression of these receptors could be abnormal cholesterol trafficking in KCs. These data provide a new basis for prevention and treatment of NASH.

Modulation of pathogen recognition by autophagy

Autophagy is an ancient biological process for maintaining cellular homeostasis by degradation of long-lived cytosolic proteins and organelles. Recent studies demonstrated that autophagy is availed by immune cells to regulate innate immunity. On the one hand, cells exert direct effector function by degrading intracellular pathogens; on the other hand, autophagy modulates pathogen recognition and downstream signaling for innate immune responses. Pathogen recognition via pattern recognition receptors induces autophagy. The function of phagocytic cells is enhanced by recruitment of autophagy-related proteins. Moreover, autophagy acts as a delivery system for viral replication complexes to migrate to the endosomal compartments where virus sensing occurs. In another case, key molecules of the autophagic pathway have been found to negatively regulate immune signaling, thus preventing aberrant activation of cytokine production and consequent immune responses. In this review, we focus on the recent advances in the role of autophagy in pathogen recognition and modulation of innate immune responses.

Involvement of toll-like receptors in autoimmune sialoadenitis of the non-obese diabetic mouse

The aim of this study was to characterize the expression of Toll-like receptors (TLRs) during the development of sialoadenitis in the non-obese diabetic mouse. Submandibular glands were dissected from non-obese diabetic mice at 4, 8, 10, 12, and 16 weeks of age. The mRNA expression levels of TLR1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, MyD88, and TRIF was quantified using real-time reverse transcription polymerase chain reaction. The mRNA expression levels in 4-week-old non-obese diabetic mice were used as controls. The expression levels of TLR1, 2, 4, and 9 were significantly higher at 8, 10, 12, and 16 weeks than the levels in the controls. The expression level of TLR3 was significantly higher at 16 weeks than in the controls. A group of mice were given drinking water containing 4.75% chloroquine starting at 4 weeks. Chloroquine caused a significant decrease in the expression of TLR1, 2, 3, 4, and 9 at 16 weeks compared with control mice who did not receive chloroquine. The areas of lymphocyte infiltration seen on serial sections of submandibular glands in the mice receiving chloroquine were significantly smaller than the areas of infiltration in control glands. Increased expression of Toll-like receptors may be involved in the development and/or progression of sialoadenitis in the non-obese diabetic mouse. Toll-like receptors may be a therapeutic target for autoimmune sialoadenitis.

Autophagy in innate recognition of pathogens and adaptive immunity

Autophagy is a specialized cellular pathway involved in maintaining homeostasis by degrading long-lived cellular proteins and organelles. Recent studies have demonstrated that autophagy is utilized by immune systems to protect host cells from invading pathogens and regulate uncontrolled immune responses. During pathogen recognition, induction of autophagy by pattern recognition receptors leads to the promotion or inhibition of consequent signaling pathways. Furthermore, autophagy plays a role in the delivery of pathogen signatures in order to promote the recognition thereof by pattern recognition receptors. In addition to innate recognition, autophagy has been shown to facilitate MHC class II presentation of intracellular antigens to activate CD4 T cells. In this review, we describe the roles of autophagy in innate recognition of pathogens and adaptive immunity, such as antigen presentation, as well as the clinical relevance of autophagy in the treatment of human diseases.

Autophagy regulation in macrophages and neutrophils

Autophagy is a conserved proteolytic mechanism that degrades cytoplasmic material including cell organelles. Accumulating evidence exists that autophagy also plays a major role in immunity and inflammation. Specifically, it appears that autophagy protects against infections and inflammation. Here, we review recent work performed in macrophages and neutrophils, which both represent critical phagocytes in mammalians.

Novel approach to bile duct damage in primary biliary cirrhosis: participation of cellular senescence and autophagy

Primary biliary cirrhosis (PBC) is characterized by antimitochondrial autoantibodies (AMAs) in patients' sera and histologically by chronic nonsuppurative destructive cholangitis in small bile ducts, eventually followed by extensive bile duct loss and biliary cirrhosis. The autoimmune-mediated pathogenesis of bile duct lesions, including the significance of AMAs, triggers of the autoimmune process, and so on remain unclear. We have reported that cellular senescence in biliary epithelial cells (BECs) may be involved in bile duct lesions and that autophagy may precede the process of biliary epithelial senescence in PBC. Interestingly, BECs in damaged bile ducts show characteristicsof cellular senescence and autophagy in PBC. A suspected causative factor of biliary epithelial senescence is oxidative stress. Furthermore, senescent BECs may modulate the microenvironment around bile ducts by expressing various chemokines and cytokines called senescence-associated secretory phenotypes and contribute to the pathogenesis in PBC.

Oxidative stress and inflammation: essential partners in alcoholic liver disease

Alcoholic liver disease (ALD) is a multifaceted disease that is characterized by hepatic steatosis or fat deposition and hepatitis or inflammation. Over the past decade, multiple lines of evidence have emerged on the mechanisms associated with ALD. The key mechanisms identified so far are sensitization to gut-derived endotoxin/lipopolysaccharide resulting in proinflammatory cytokine production and cellular stress due to oxidative processes, contributing to the development and progression of disease. While oxidative stress and inflammatory responses are studied independently in ALD, mechanisms linking these two processes play a major role in pathogenesis of disease. Here we review major players of oxidative stress and inflammation and highlight signaling intermediates regulated by oxidative stress that provokes proinflammatory responses in alcoholic liver disease.

When Cells Suffocate: Autophagy in Cancer and Immune Cells under Low Oxygen

Hypoxia is a signature feature of growing tumors. This cellular state creates an inhospitable condition that impedes the growth and function of all cells within the immediate and surrounding tumor microenvironment. To adapt to hypoxia, cells activate autophagy and undergo a metabolic shift increasing the cellular dependency on anaerobic metabolism. Autophagy upregulation in cancer cells liberates nutrients, decreases the buildup of reactive oxygen species, and aids in the clearance of misfolded proteins. Together, these features impart a survival advantage for cancer cells in the tumor microenvironment. This observation has led to intense research efforts focused on developing autophagy-modulating drugs for cancer patient treatment. However, other cells that infiltrate the tumor environment such as immune cells also encounter hypoxia likely resulting in hypoxia-induced autophagy. In light of the fact that autophagy is crucial for immune cell proliferation as well as their effector functions such as antigen presentation and T cell-mediated killing of tumor cells, anticancer treatment strategies based on autophagy modulation will need to consider the impact of autophagy on the immune system.

Autophagy as an innate immunity paradigm: expanding the scope and repertoire of pattern recognition receptors

Autophagy is rapidly developing into a new immunological paradigm. The latest links now include overlaps between autophagy and innate immune signaling via TBK-1 and IKKα/β, and the role of autophagy in inflammation directed by the inflammasome. Autophagy's innate immunity connections include responses to pathogen and damage-associated molecular patterns including alarmins such as HMGB1 and IL-1β, Toll-like receptors, Nod-like receptors including NLRC4, NLRP3 and NLRP4, and RIG-I-like receptors. Autophagic adaptors referred to as SLRs (sequestosome 1/p62-like receptors) are themselves a category of pattern recognition receptors. SLRs empower autophagy to eliminate intracellular microbes by direct capture and by facilitating generation and delivery of antimicrobial peptides, and also serve as inflammatory signaling platforms. SLRs contribute to autophagic control of intracellular microbes, including Mycobacterium tuberculosis, Salmonella, Listeria, Shigella, HIV-1 and Sindbis virus, but act as double-edged sword and contribute to inflammation and cell death. Autophagy roles in innate immunity continue to expand vertically and laterally, and now include antimicrobial function downstream of vitamin D3 action in tuberculosis and AIDS. Recent data expand the connections between immunity-related GTPases and autophagy to include not only IRGM but also several members of the Gbp (guanlyate-binding proteins) family. The efficacy with which autophagy handles microbes, microbial products and sterile endogenous irritants governs whether the outcome will be with suppression of or with excess inflammation, the latter reflected in human diseases that have strong inflammatory components including tuberculosis and Crohn's disease.

Manipulation of autophagy by bacteria for their own benefit

Autophagy is the host innate immune system's first line of defense against microbial intruders. When the innate defense system recognizes invading bacterial pathogens and their infection processes, autophagic proteins act as cytosolic sensors that allow the autophagic pathway to be rapidly activated. However, many intracellular bacterial pathogens deploy highly evolved mechanisms to evade autophagic recognition, manipulate the autophagic pathway, and remodel the autophagosomal compartment for their own benefit. Here current topics regarding the recognition of invasive bacteria by the cytosolic innate immune system are highlighted, including autophagy and the mechanisms that enable bacteria to evade autophagy. Also highlighted are some selective examples of bacterial activities that manipulate the autophagic pathways for their own benefit.

MyD88 interacts with interferon regulatory factor (IRF) 3 and IRF7 in Atlantic salmon (Salmo salar): transgenic SsMyD88 modulates the IRF-induced type I interferon response and accumulates in aggresomes

MyD88 is an intracellular adaptor protein that transmits signals downstream of immune receptors such as the IL-1 receptor and the majority of the known mammalian toll-like receptors. Homologs of MyD88 have been identified in many vertebrate species; however, the adaptor has been studied mostly in mammals, and little is known about its function in lower vertebrates. The results presented in the current paper demonstrate, for the first time, that the teleost MyD88, through its Toll/Interleukin-1 receptor domain, interacts with SsIRF3 and two SsIRF7 paralogs: transcription factors that are critically involved in the virus-induced IFN responses. The data further highlight the potential of transgenic SsMyD88 to modulate the IRF-induced type I IFN response as the adaptor synergized with SsIRF3 to activate IRF-E/IFN-stimulated response element-containing reporter gene constructs and endogenous myxovirus resistance homolog expression. Microscopy analyses demonstrated that, similar to mammalian MyD88, both endogenous and transgenic SsMyD88 accumulated in intracellular aggregates. However, unlike the endogenous SsMyD88 clusters, which co-localized with endocytosed CpGs and probably represented myddosomes, overexpressed SsMyD88 accumulated in aggresomes. Although these structures accumulated ubiquitinated proteins, they did not associate with the autophagosome markers p62 and light chain 3-like protein, indicating that they are most likely classical aggresomes rather than aggresome-like induced structures, aggregates of ubiquitinated proteins induced by toll-like receptor/MyD88 signaling in antigen-presenting cells. The significance of the accumulation of transgenic MyD88 in aggresomes is currently unknown; nevertheless it is tempting to speculate that it might represent a defense mechanism against the potentially harmful effects of excessive MyD88 signaling.

Regulation of Toll-like receptor signaling by NDP52-mediated selective autophagy is normally inactivated by A20

Toll-like receptor (TLR) signaling is linked to autophagy that facilitates elimination of intracellular pathogens. However, it is largely unknown whether autophagy controls TLR signaling. Here, we report that poly(I:C) stimulation induces selective autophagic degradation of the TLR adaptor molecule TRIF and the signaling molecule TRAF6, which is revealed by gene silencing of the ubiquitin-editing enzyme A20. This type of autophagy induced formation of autophagosomes and could be suppressed by an autophagy inhibitor and lysosomal inhibitors. However, this autophagy was not associated with canonical autophagic processes, including involvement of Beclin-1 and conversion of LC3-I to LC3-II. Through screening of TRIF-interacting ‘autophagy receptors’ in human cells, we identified that NDP52 mediated the selective autophagic degradation of TRIF and TRAF6 but not TRAF3. NDP52 was polyubiquitinated by TRAF6 and was involved in aggregation of TRAF6, which may result in the selective degradation. Intriguingly, only under the condition of A20 silencing, NDP52 could effectively suppress poly(I:C)-induced proinflammatory gene expression. Thus, this study clarifies a selective autophagic mechanism mediated by NDP52 that works downstream of TRIF–TRAF6. Furthermore, although A20 is known as a signaling fine-tuner to prevent excess TLR signaling, it paradoxically downregulates the fine-tuning effect of NDP52 on TLR signaling.

Surrogate markers of infection: interrogation of the immune system

Infectious diseases remain the greatest causes of morbidity and mortality in global terms. As much of the burden occurs in the developing world, limited access to diagnostic testing has hampered the diagnosis and treatment of these conditions, while, in the developed world, the cost of managing infectious diseases remains considerable. Despite the size of the problem there remains an ongoing need for tests that improve diagnostic sensitivity and specificity, provide more rapid diagnoses, are available for point-of-care testing in remote regions, and can help inform therapeutic decision-making by identifying resistance patterns or patient outcomes. This article discusses the background to biomarker development for infectious diseases, some current assays that are providing useful information regarding the host's response to infection (using examples such as Cytomegalovirus and Mycobacterium tuberculosis), as well as likely future technologies and their limitations.

Mitochondria and the autophagy-inflammation-cell death axis in organismal aging

Alterations of mitochondrial functions are linked to multiple degenerative or acute diseases. As mitochondria age in our cells, they become progressively inefficient and potentially toxic, and acute damage can trigger the permeabilization of mitochondrial membranes to initiate apoptosis or necrosis. Moreover, mitochondria have an important role in pro-inflammatory signaling. Autophagic turnover of cellular constituents, be it general or specific for mitochondria (mitophagy), eliminates dysfunctional or damaged mitochondria, thus counteracting degeneration, dampening inflammation, and preventing unwarranted cell loss. Decreased expression of genes that regulate autophagy or mitophagy can cause degenerative diseases in which deficient quality control results in inflammation and the death of cell populations. Thus, a combination of mitochondrial dysfunction and insufficient autophagy may contribute to multiple aging-associated pathologies.

Combination therapy utilizing shRNA knockdown and an optimized resistant transgene for rescue of diseases caused by misfolded proteins

Molecular knockdown of disease proteins and restoration of wild-type activity represent a promising but challenging strategy for the treatment of diseases that result from the accumulation of misfolded proteins (i.e., Huntington disease, amyotrophic lateral sclerosis, and α-1 antitrypsin deficiency). In this study we used alpha-1 antitrypsin (AAT) deficiency with the piZZ mutant phenotype as a model system to evaluate the efficiency of gene-delivery approaches that both silence the piZZ transcript (e.g., shRNA) and restore circulating wild-type AAT expression from resistant codon-optimized AAT (AAT-opt) transgene cassette using adeno-associated virus (AAV) vector delivery. After systemic injection of a self-complimentary AAV serotype 8 (scAAV8) vector encoding shRNA in piZZ transgenic mice, both mutant AAT mRNA in the liver and defected serum protein level were inhibited by 95%, whereas liver pathology, as monitored by dPAS and fibrosis staining, reversed. To restore blood AAT levels in AAV8/shRNA-treated mice, several strategies to restore functional AAT levels were tested, including using AAV AAT-opt transgene cassettes targeted to muscle and liver, or combination vectors carrying piZZ shRNA and AAT-opt transgenes separately, or a single bicistronic AAV vector. With these molecular approaches, we observed over 90% knockdown of mutant AAT with a 13- to 30-fold increase of circulating wild-type AAT protein from the shRNA-resistant AAT-opt cassette. The molecular approaches applied in this study can simultaneously prevent liver pathology and restore blood AAT concentration in AAT deficiencies. Based on these observations, similar gene-therapy strategies could be considered for any diseases caused by accumulation of misfolded proteins.

Early versus late parenteral nutrition in critically ill adults

BACKGROUND

Controversy exists about the timing of the initiation of parenteral nutrition in critically ill adults in whom caloric targets cannot be met by enteral nutrition alone.

METHODS

In this randomized, multicenter trial, we compared early initiation of parenteral nutrition (European guidelines) with late initiation (American and Canadian guidelines) in adults in the intensive care unit (ICU) to supplement insufficient enteral nutrition. In 2312 patients, parenteral nutrition was initiated within 48 hours after ICU admission (early-initiation group), whereas in 2328 patients, parenteral nutrition was not initiated before day 8 (late-initiation group). A protocol for the early initiation of enteral nutrition was applied to both groups, and insulin was infused to achieve normoglycemia.

RESULTS

Patients in the late-initiation group had a relative increase of 6.3% in the likelihood of being discharged alive earlier from the ICU (hazard ratio, 1.06; 95% confidence interval [CI], 1.00 to 1.13; P=0.04) and from the hospital (hazard ratio, 1.06; 95% CI, 1.00 to 1.13; P=0.04), without evidence of decreased functional status at hospital discharge. Rates of death in the ICU and in the hospital and rates of survival at 90 days were similar in the two groups. Patients in the late-initiation group, as compared with the early-initiation group, had fewer ICU infections (22.8% vs. 26.2%, P=0.008) and a lower incidence of cholestasis (P<0.001). The late-initiation group had a relative reduction of 9.7% in the proportion of patients requiring more than 2 days of mechanical ventilation (P=0.006), a median reduction of 3 days in the duration of renal-replacement therapy (P=0.008), and a mean reduction in health care costs of €1,110 (about $1,600) (P=0.04).

CONCLUSIONS

Late initiation of parenteral nutrition was associated with faster recovery and fewer complications, as compared with early initiation. (Funded by the Methusalem program of the Flemish government and others; EPaNIC ClinicalTrials.gov number, NCT00512122.).

Impact of the autophagy machinery on hepatitis C virus infection

Autophagy is a cellular process that catabolizes cytoplasmic components and maintains energy homeostasis. As a stress response, the autophagy machinery interconnects a wide range of cellular pathways, enhancing the spread of certain pathogens while limiting others, and has become a highly active research area over the past several years. Independent laboratories have recently reported that autophagy vesicles accumulate in hepatitis C virus (HCV) infected cells and that autophagy proteins can function as proviral factors required for HCV replication. In this review, we summarize what is currently known about the interplay between autophagy and HCV and the possible mechanisms whereby autophagy proteins might favor HCV propagation.

Functions of autophagy in hepatic and pancreatic physiology and disease

Autophagy is a lysosomal pathway that degrades and recycles intracellular organelles and proteins to maintain energy homeostasis during times of nutrient deprivation and to remove damaged cell components. Recent studies have identified new functions for autophagy under basal and stressed conditions. In the liver and pancreas, autophagy performs the standard functions of degrading mitochondria and aggregated proteins and regulating cell death. In addition, autophagy functions in these organs to regulate lipid accumulation in hepatic steatosis, trypsinogen activation in pancreatitis, and hepatitis virus replication. This review discusses the effects of autophagy on hepatic and pancreatic physiology and the contribution of this degradative process to diseases of these organs. The discovery of novel functions for this lysosomal pathway has increased our understanding of the pathophysiology of diseases in the liver and pancreas and suggested new possibilities for their treatment.

Antiviral protein Viperin promotes Toll-like receptor 7- and Toll-like receptor 9-mediated type I interferon production in plasmacytoid dendritic cells

Toll-like receptor 7 (TLR7) and TLR9 sense viral nucleic acids and induce production of type I interferon (IFN) by plasmacytoid dendritic cells (pDCs) to protect the host from virus infection. We showed that the IFN-inducible antiviral protein Viperin promoted TLR7- and TLR9-mediated production of type I IFN by pDCs. Viperin expression was potently induced after TLR7 or TLR9 stimulation and Viperin localized to the cytoplasmic lipid-enriched compartments, lipid bodies, in pDCs. Viperin interacted with the signal mediators IRAK1 and TRAF6 to recruit them to the lipid bodies and facilitated K63-linked ubiquitination of IRAK1 to induce the nuclear translocation of transcription factor IRF7. Loss of Viperin reduced TLR7- and TLR9-mediated production of type I IFN by pDCs. However, Viperin was dispensable for the production of type I IFN induced by intracellular nucleic acids. Thus, Viperin mediates its antiviral function via the regulation of the TLR7 and TLR9-IRAK1 signaling axis in pDCs.

Autophagy in nonalcoholic steatohepatitis

Autophagy is a critical pathway for the degradation of intracellular components by lysosomes. Established functions for both macroautophagy and chaperone-mediated autophagy in hepatic lipid metabolism, insulin sensitivity and cellular injury suggest a number of potential mechanistic roles for autophagy in nonalcoholic steatohepatitis (NASH). Decreased autophagic function in particular may promote the initial development of hepatic steatosis and progression of steatosis to liver injury. Additional functions of autophagy in immune responses and carcinogenesis may also contribute to the development of NASH and its complications. The impairment in autophagy that occurs with cellular lipid accumulation, obesity and aging may therefore have an important impact on this disease, and agents to augment hepatic autophagy have therapeutic potential in NASH.

Human cytomegalovirus decreases constitutive transcription of MHC class II genes in mature Langerhans cells by reducing CIITA transcript levels

Human cytomegalovirus (HCMV) productively infects CD34(+) progenitor-derived, mature Langerhans-type dendritic cells (matLC) and reduces surface expression of MHC class II complexes (MHC II) by increasing intracellular retention of these molecules. To determine whether HCMV also inhibits MHC II expression by other mechanisms, we assessed mRNA levels of the class II transcriptional regulator, CIITA, and several of its target genes in infected matLC. Levels of CIITA, HLA-DRA (DRA) and DRB transcripts, and new DR protein synthesis were compared in mock-infected and HCMV-infected cells by quantitative PCR and pulse-chase immunoprecipitation analyses, respectively. CIITA mRNA levels were significantly lower in HCMV-infected matLC as compared to mock-infected cells. When assessed in the presence of Actinomycin D, the stability of CIITA transcripts was not diminished by HCMV. Analysis of promoter-specific CIITA isoforms revealed that types I, III and IV all were decreased by HCMV, a result that differs from changes after incubation of these cells with lipopolysaccharide (LPS). Exposure to UV-inactivated virus failed to reduce CIITA mRNA levels, implicating de novo viral gene expression in this effect. HCMV-infected matLC also expressed lower levels of DR transcripts and reduced DR protein synthesis rates compared to mock-infected matLC. In summary, we demonstrate that HCMV infection of a human dendritic cell subset inhibits constitutive CIITA expression, most likely at the transcriptional level, resulting in reduced MHC II biosynthesis. We suggest this represents a new mechanism of modulation of mature LC by HCMV.

Autophagy basics

Autophagy (macroautophagy) is a dynamic process for degradation of cytosolic components. Autophagy has intracellular anti-viral and anti-bacterial functions, and plays a role in the initiation of innate and adaptive immune system responses to viral and bacterial infections. Some viruses encode virulence factors for blocking autophagy, whereas others utilize some autophagy components for their intracellular growth or cellular budding. The "core" autophagy-related (Atg) complexes in mammals are ULK1 protein kinase, Atg9-WIPI-1 and Vps34-beclin1 class III PI3-kinase complexes, and the Atg12 and LC3 conjugation systems. In addition, PI(3)-binding proteins, PI3-phosphatases, and Rab proteins contribute to autophagy. The autophagy process consists of continuous dynamic membrane formation and fusion. In this review, the relationships between these Atg complexes and each process are described. Finally, the critical points for monitoring autophagy, including the use of GFP-LC3 and GFP-Atg5, are discussed.

Autophagy in immunity and inflammation

Autophagy is an essential, homeostatic process by which cells break down their own components. Perhaps the most primordial function of this lysosomal degradation pathway is adaptation to nutrient deprivation. However, in complex multicellular organisms, the core molecular machinery of autophagy - the 'autophagy proteins' - orchestrates diverse aspects of cellular and organismal responses to other dangerous stimuli such as infection. Recent developments reveal a crucial role for the autophagy pathway and proteins in immunity and inflammation. They balance the beneficial and detrimental effects of immunity and inflammation, and thereby may protect against infectious, autoimmune and inflammatory diseases.

Early induction of autophagy in human fibroblasts after infection with human cytomegalovirus or herpes simplex virus 1

The infection of human fetal foreskin fibroblasts (HFFF2) with human cytomegalovirus (HCMV) resulted in the induction of autophagy. This was demonstrated by the increased lipidation of microtubule-associated protein 1 light chain 3 (LC3), a hallmark of autophagy, and by the visualization of characteristic vesicles within infected cells. The response was detected first at 2 h postinfection and persisted for at least 3 days. De novo protein synthesis was not required for the effect, since HCMV that was irradiated with UV light also elicited the response, and furthermore the continuous presence of cycloheximide did not prevent induction. Infection with herpes simplex virus type 1 (HSV-1) under conditions that inhibited viral gene expression provoked autophagy, whereas UV-irradiated respiratory syncytial virus did not. The induction of autophagy occurred when cells were infected with HCMV or HSV-1 that was gradient purified, but HCMV dense bodies and HSV-1 light particles, each of which lack nucleocapsids and genomes, were inactive. The depletion of regulatory proteins Atg5 and Atg7, which are required for autophagy, reduced LC3 modification in response to infection but did not result in any detectable difference in viral or cellular gene expression at early times after infection. The electroporation of DNA into HFFF2 cultures induced the lipidation of LC3 but double-stranded RNA did not, even though both agents stimulated an innate immune response. The results show a novel, early cellular response to the presence of the incoming virion and additionally demonstrate that autophagy can be induced by the presence of foreign DNA within cells.

Nrf2-mediated induction of p62 controls Toll-like receptor-4-driven aggresome-like induced structure formation and autophagic degradation

Toll-like receptors (TLRs) play a crucial role in several innate immune responses by regulating autophagy, but little is known about how TLR signaling controls autophagy. Here we demonstrate that p62/SQSTM1 is required for TLR4-mediated autophagy, which we show as selective autophagy of aggresome-like induced structures (ALIS). Treatment with LPS or Escherichia coli induced LC3(+) dot-like structures, and their assembly, but not lysosomal degradation, occurred independently of classic autophagic machinery. Microscopic and ultrastructural analyses showed that p62 is a component of the induced LC3(+) dots and these TLR4-induced p62(+) structures resemble ALIS. The levels of p62 mRNA and protein were increased in TLR4-activated cells and knockdown of p62 suppressed the ALIS formation and LC3-II conversion. The accumulation of p62 and ALIS required activation of Nrf2 by reactive oxygen species-p38 axis-dependent TLR4/MyD88 signaling, suggesting a link between innate immune and oxidative-stress responses. These findings indicate that TLR4-driven induction of p62 plays an essential role in the formation and the autophagic degradation of ALIS, which might be critical for regulating host defense.