Pioneer translation products as an alternative source for MHC-I antigenic peptides

The notion that alternative peptide substrates can be processed and presented to the MHC class I pathway has opened for new aspects on how the immune system detects infected or damaged cells. Recent works show that antigenic peptides are derived from intron sequences in pre-mRNAs target for the nonsense-mediated degradation pathway. Introns are spliced out co-transcriptionally suggesting that such pioneer translation products (PTPs) are synthesized on the nascent RNAs in the nuclear compartment to ensure that the first peptides to emerge from an mRNA are destined for the class I pathway. This illustrates an independent translation event during mRNA maturation that give rise to specific peptide products with a specific function in the immune system. The characterization of the translation apparatus responsible for PTP synthesis will pave the way for understanding how PTP production is regulated in different tissues under different conditions and will help designing new vaccine strategies.

Sensing cytosolic RpsL by macrophages induces lysosomal cell death and termination of bacterial infection

The intracellular bacterial pathogen Legionella pneumophila provokes strong host responses and has proven to be a valuable model for the discovery of novel immunosurveillance pathways. Our previous work revealed that an environmental isolate of L. pneumophila induces a noncanonical form of cell death, leading to restriction of bacterial replication in primary mouse macrophages. Here we show that such restriction also occurs in infections with wild type clinical isolates. Importantly, we found that a lysine to arginine mutation at residue 88 (K88R) in the ribosome protein RpsL that not only confers bacterial resistance to streptomycin, but more importantly, severely attenuated the induction of host cell death and enabled L. pneumophila to replicate in primary mouse macrophages. Although conferring similar resistance to streptomycin, a K43N mutation in RpsL does not allow productive intracellular bacterial replication. Further analysis indicated that RpsL is capable of effectively inducing macrophage death via a pathway involved in lysosomal membrane permeabilization; the K88R mutant elicits similar responses but is less potent. Moreover, cathepsin B, a lysosomal protease that causes cell death after being released into the cytosol upon the loss of membrane integrity, is required for efficient RpsL-induced macrophage death. Furthermore, despite the critical role of cathepsin B in delaying RpsL-induced cell death, macrophages lacking cathepsin B do not support productive intracellular replication of L. pneumophila harboring wild type RpsL. This suggests the involvement of other yet unidentified components in the restriction of bacterial replication. Our results identified RpsL as a regulator in the interactions between bacteria such as L. pneumophila and primary mouse macrophages by triggering unique cellular pathways that restrict intracellular bacterial replication.

The death of the host cell during infection can be triggered by one or more microbial molecules; this “live or die” selection provides effective means for the dissection of immune recognition mechanisms as well as for the identification of the microbial molecules responsible for such responses. We found that infection of primary mouse macrophages by Legionella pneumophila strains harboring wild type RpsL, the S12 component of the bacterial ribosome, causes macrophage death by a mechanism independent of the three inflammatory caspases, caspase 1, 7 and 11. Importantly, although both confer resistance to streptomycin at indistinguishable effectiveness, the K88R, but not the K43N mutation in RpsL enables L. pneumophila to replicate in macrophages. Purified RpsL and RpsL_K43N physically delivered into macrophages cause cell death by inducing damage to lysosomal membranes and the release of cathepsins. We also found that the lysosomal protease cathepsin B is required for efficient RpsL-induced cell death but its absence is not sufficient for macrophages to support intracellular bacterial replication. Thus, RpsL functions as an immune induction molecule to trigger one or more signaling cascades that leads to lysosomal cell death as well as the termination of bacterial replication.

MAVS, cGAS, and endogenous retroviruses in T-independent B cell responses

Multivalent molecules with repetitive structures including bacterial capsular polysaccharides and viral capsids elicit antibody responses through B cell receptor (BCR) crosslinking in the absence of T cell help. We report that immunization with these T cell-independent type 2 (TI-2) antigens causes up-regulation of endogenous retrovirus (ERV) RNAs in antigen-specific mouse B cells. These RNAs are detected via a mitochondrial antiviral signaling protein (MAVS)-dependent RNA sensing pathway or reverse-transcribed and detected via the cGAS-cGAMP-STING pathway, triggering a second, sustained wave of signaling that promotes specific immunoglobulin M production. Deficiency of both MAVS and cGAS, or treatment of MAVS-deficient mice with reverse transcriptase inhibitors, dramatically inhibits TI-2 antibody responses. These findings suggest that ERV and two innate sensing pathways that detect them are integral components of the TI-2 B cell signaling apparatus.

Targeted in vivo inhibition of specific protein-protein interactions using recombinant antibodies

With the growing availability of genomic sequence information, there is an increasing need for gene function analysis. Antibody-mediated "silencing" represents an intriguing alternative for the precise inhibition of a particular function of biomolecules. Here, we describe a method for selecting recombinant antibodies with a specific purpose in mind, which is to inhibit intrinsic protein-protein interactions in the cytosol of plant cells. Experimental procedures were designed for conveniently evaluating desired properties of recombinant antibodies in consecutive steps. Our selection method was successfully used to develop a recombinant antibody inhibiting the interaction of ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 3 with such of its upstream interaction partners as the receiver domain of CYTOKININ INDEPENDENT HISTIDINE KINASE 1. The specific down-regulation of the cytokinin signaling pathway in vivo demonstrates the validity of our approach. This selection method can serve as a prototype for developing unique recombinant antibodies able to interfere with virtually any biomolecule in the living cell.

Cytosolic DNA sensing via the stimulator of interferon genes adaptor: Yin and Yang of immune responses to DNA

DNA is immunogenic and many cells express cytosolic DNA sensors that activate the stimulator of interferon genes (STING) adaptor to trigger interferon type I (IFN-β) release, a potent immune activator. DNA sensing to induce IFN-β triggers host immunity to pathogens but constitutive DNA sensing can induce sustained IFN-β release that incites autoimmunity. Here, we focus on cytosolic DNA sensing via the STING/IFN-β pathway that regulates immune responses. Recent studies reveal that cytosolic DNA sensing via the STING/IFN-β pathway induces indoleamine 2,3 dioxygenase (IDO), which catabolizes tryptophan to suppress effector and helper T-cell responses and activate Foxp3-lineage CD4(+) regulatory T (Treg) cells. During homeostasis, and in some inflammatory settings, specialized innate immune cells in the spleen and lymph nodes may ingest and sense cytosolic DNA to reinforce tolerance that prevents autoimmunity. However, malignancies and pathogens may exploit DNA-induced regulatory responses to suppress natural and vaccine-induced immunity to malignant and infected cells. In this review, we discuss the biologic significance of regulatory responses to DNA and novel approaches to exploit DNA-induced immune responses for therapeutic benefit. The ability of DNA to drive tolerogenic or immunogenic responses highlights the need to evaluate immune responses to DNA in physiologic settings relevant to disease progression or therapy.

Distinctive subcellular inhibition of cytokine-induced SRC by salubrinal and fluid flow

A non-receptor protein kinase Src plays a crucial role in fundamental cell functions such as proliferation, migration, and differentiation. While inhibition of Src is reported to contribute to chondrocyte homeostasis, its regulation at a subcellular level by chemical inhibitors and mechanical stimulation has not been fully understood. In response to inflammatory cytokines and stress to the endoplasmic reticulum (ER) that increase proteolytic activities in chondrocytes, we addressed two questions: Do cytokines such as interleukin 1 beta (IL1β) and tumor necrosis factor alpha (TNFα) induce location-dependent Src activation? Can cytokine-induced Src activation be suppressed by chemically alleviating ER stress or by applying fluid flow? Using live cell imaging with two Src biosensors (i.e., cytosolic, and plasma membrane-bound biosensors) for a fluorescence resonance energy transfer (FRET) technique, we determined cytosolic Src activity as well as membrane-bound Src activity in C28/I2 human chondrocytes. In response to TNFα and IL1β, both cytosolic and plasma membrane-bound Src proteins were activated, but activation in the cytosol occurred earlier than that in the plasma membrane. Treatment with salubrinal or guanabenz, two chemical agents that attenuate ER stress, significantly decreased cytokine-induced Src activities in the cytosol, but not in the plasma membrane. In contrast, fluid flow reduced Src activities in the plasma membrane, but not in the cytosol. Collectively, the results demonstrate that Src activity is differentially regulated by salubrinal/guanabenz and fluid flow in the cytosol and plasma membrane.

Rad50-CARD9 interactions link cytosolic DNA sensing to IL-1β production

Double-stranded DNA (dsDNA) in the cytoplasm triggers the production of interleukin 1β (IL-1β) as an antiviral host response, and deregulation of the pathways involved can promote inflammatory disease. Here we report a direct cytosolic interaction between the DNA-damage sensor Rad50 and the innate immune system adaptor CARD9. Transfection of dendritic cells with dsDNA or infection of dendritic cells with a DNA virus induced the formation of dsDNA-Rad50-CARD9 signaling complexes for activation of the transcription factor NF-κB and the generation of pro-IL-1β. Primary cells conditionally deficient in Rad50 or lacking CARD9 consequently exhibited defective DNA-induced production of IL-1β, and Card9(-/-) mice had impaired inflammatory responses after infection with a DNA virus in vivo. Our results define a cytosolic DNA-recognition pathway for inflammation and a physical and functional connection between a conserved DNA-damage sensor and the innate immune response to pathogens.

Fusobacterium nucleatum activates the immune response through retinoic acid-inducible gene I

Retinoic acid-inducible gene I (RIG-I) is a cytosolic pattern recognition receptor involved in the sensing of RNA viruses and the initiation of antiviral responses. Fusobacterium nucleatum, a Gram-negative anaerobic bacterium associated with periodontal disease, is capable of invading cells. We hypothesized that F. nucleatum's ability to invade cells allows the microorganism to activate the immune response through RIG-I. Bacterial invasion was found to be necessary for F. nucleatum-induced nuclear factor kappa B (NF-κB) activation. Following invasion of the human periodontal ligament fibroblast (PDLF), F. nucleatum was located in the cytosol. F. nucleatum infection led to an 80-fold increase in RIG-I expression. Silencing RIG-I in PDLF by siRNA led to a significant decrease of NF-κB activation and expression of proinflammatory genes. Additionally, F. nucleatum was able to secrete nucleic acids, and introduction of F. nucleatum RNA into PDLF led to a RIG-I-dependent activation of NF-κB. Our findings showed RIG-I to be involved in the recognition of F. nucleatum. The function of RIG-I is likely to be broad and not limited to sensing of viruses only. Hence, this receptor may play an important role in detecting invasive forms of oral pathogens and contribute to inflammation in periodontal tissues.

Insights into phagocytosis-coupled activation of pattern recognition receptors and inflammasomes

A decade of work shows that the core function of phagocytosis in engulfment and destruction of microorganisms is only a small facet of the full spectrum of roles for phagocytosis in the immune system. The regulation of phagocytosis and its outcomes by inflammatory pattern recognition receptors (PRRs) is now followed by new studies strengthening this concept and adding further complexity to the relationship between phagocytosis and innate immune signaling. Phagocytosis forms the platform for activation of distinct members of the Toll-like receptor family, and even dictates their signaling outcomes. In many cases, phagocytosis is a necessary precedent to the activation of cytosolic PRRs and assembly of canonical and non-canonical inflammasomes, leading to strong pro-inflammatory responses and inflammatory cell death.

Hijacking multivesicular bodies enables long-term and exosome-mediated long-distance action of anthrax toxin

Anthrax lethal toxin is a classical AB toxin comprised of two components: protective antigen (PA) and lethal factor (LF). Here, we show that following assembly and endocytosis, PA forms a channel that translocates LF, not only into the cytosol, but also into the lumen of endosomal intraluminal vesicles (ILVs). These ILVs can fuse and release LF into the cytosol, where LF can proteolyze and disable host targets. We find that LF can persist in ILVs for days, fully sheltered from proteolytic degradation, both in vitro and in vivo. During this time, ILV-localized LF can be transmitted to daughter cells upon cell division. In addition, LF-containing ILVs can be delivered to the extracellular medium as exosomes. These can deliver LF to the cytosol of naive cells in a manner that is independent of the typical anthrax toxin receptor-mediated trafficking pathway, while being sheltered from neutralizing extracellular factors of the immune system.

The DHX33 RNA helicase senses cytosolic RNA and activates the NLRP3 inflammasome

The NLRP3 inflammasome plays a major role in innate immune responses by activating caspase-1, resulting in secretion of interleukin-18 (IL-18) and IL-1β. Although cytosolic double-stranded RNA (dsRNA) and bacterial RNA are known to activate the NLRP3 inflammasome, the upstream sensor is unknown. We investigated the potential function of DExD/H-box RNA helicase family members (previously shown to sense cytosolic DNA and RNA to induce type 1 interferon responses) in RNA-induced NLRP3 inflammasome activation. Among the helicase family members tested, we found that targeting of DHX33 expression by short hairpin RNA efficiently blocked the activation of caspase-1 and secretion of IL-18 and IL-1β in human macrophages that were activated by cytosolic poly I:C, reoviral RNA, or bacterial RNA. DHX33 bound dsRNA via the helicase C domain. DHX33 interacted with NLRP3 and formed the inflammasome complex following stimulation with RNA. We therefore identified DHX33 as a cytosolic RNA sensor that activates the NLRP3 inflammasome.

Targeting the cytosolic innate immune receptors RIG-I and MDA5 effectively counteracts cancer cell heterogeneity in glioblastoma

Cellular heterogeneity, for example, the intratumoral coexistence of cancer cells with and without stem cell characteristics, represents a potential root of therapeutic resistance and a significant challenge for modern drug development in glioblastoma (GBM). We propose here that activation of the innate immune system by stimulation of innate immune receptors involved in antiviral and antitumor responses can similarly target different malignant populations of glioma cells. We used short-term expanded patient-specific primary human GBM cells to study the stimulation of the cytosolic nucleic acid receptors melanoma differentiation-associated gene 5 (MDA5) and retinoic acid-inducible gene I (RIG-I). Specifically, we analyzed cells from the tumor core versus "residual GBM cells" derived from the tumor resection margin as well as stem cell-enriched primary cultures versus specimens without stem cell properties. A portfolio of human, nontumor neural cells was used as a control for these studies. The expression of RIG-I and MDA5 could be induced in all of these cells. Receptor stimulation with their respective ligands, p(I:C) and 3pRNA, led to in vitro evidence for an effective activation of the innate immune system. Most intriguingly, all investigated cancer cell populations additionally responded with a pronounced induction of apoptotic signaling cascades revealing a second, direct mechanism of antitumor activity. By contrast, p(I:C) and 3pRNA induced only little toxicity in human nonmalignant neural cells. Granted that the challenge of effective central nervous system (CNS) delivery can be overcome, targeting of RIG-I and MDA5 could thus become a quintessential strategy to encounter heterogeneous cancers in the sophisticated environments of the brain.

Diversity of natural self-derived ligands presented by different HLA class I molecules in transporter antigen processing-deficient cells

The transporter associated with antigen processing (TAP) translocates the cytosol-derived proteolytic peptides to the endoplasmic reticulum lumen where they complex with nascent human leukocyte antigen (HLA) class I molecules. Non-functional TAP complexes and viral or tumoral blocking of these transporters leads to reduced HLA class I surface expression and a drastic change in the available peptide repertoire. Using mass spectrometry to analyze complex human leukocyte antigen HLA-bound peptide pools isolated from large numbers of TAP-deficient cells, we identified 334 TAP-independent ligands naturally presented by four different HLA-A, -B, and -C class I molecules with very different TAP dependency from the same cell line. The repertoire of TAP-independent peptides examined favored increased peptide lengths and a lack of strict binding motifs for all four HLA class I molecules studied. The TAP-independent peptidome arose from 182 parental proteins, the majority of which yielded one HLA ligand. In contrast, TAP-independent antigen processing of very few cellular proteins generated multiple HLA ligands. Comparison between TAP-independent peptidome and proteome of several subcellular locations suggests that the secretory vesicle-like organelles could be a relevant source of parental proteins for TAP-independent HLA ligands. Finally, a predominant endoproteolytic peptidase specificity for Arg/Lys or Leu/Phe residues in the P(1) position of the scissile bond was found for the TAP-independent ligands. These data draw a new and intricate picture of TAP-independent pathways.

Cyclic GMP-AMP synthase is a cytosolic DNA sensor that activates the type I interferon pathway

The presence of DNA in the cytoplasm of mammalian cells is a danger signal that triggers host immune responses such as the production of type I interferons. Cytosolic DNA induces interferons through the production of cyclic guanosine monophosphate-adenosine monophosphate (cyclic GMP-AMP, or cGAMP), which binds to and activates the adaptor protein STING. Through biochemical fractionation and quantitative mass spectrometry, we identified a cGAMP synthase (cGAS), which belongs to the nucleotidyltransferase family. Overexpression of cGAS activated the transcription factor IRF3 and induced interferon-β in a STING-dependent manner. Knockdown of cGAS inhibited IRF3 activation and interferon-β induction by DNA transfection or DNA virus infection. cGAS bound to DNA in the cytoplasm and catalyzed cGAMP synthesis. These results indicate that cGAS is a cytosolic DNA sensor that induces interferons by producing the second messenger cGAMP.

Molecular interface of S100A8 with cytochrome b558 and NADPH oxidase activation

S100A8 and S100A9 are two calcium binding Myeloid Related Proteins, and important mediators of inflammatory diseases. They were recently introduced as partners for phagocyte NADPH oxidase regulation. However, the precise mechanism of their interaction remains elusive. We had for aim (i) to evaluate the impact of S100 proteins on NADPH oxidase activity; (ii) to characterize molecular interaction of either S100A8, S100A9, or S100A8/S100A9 heterocomplex with cytochrome b₅₅₈; and (iii) to determine the S100A8 consensus site involved in cytochrome b₅₅₈/S100 interface. Recombinant full length or S100A9-A8 truncated chimera proteins and ExoS-S100 fusion proteins were expressed in E. coli and in P. aeruginosa respectively. Our results showed that S100A8 is the functional partner for NADPH oxidase activation contrary to S100A9, however, the loading with calcium and a combination with phosphorylated S100A9 are essential in vivo. Endogenous S100A9 and S100A8 colocalize in differentiated and PMA stimulated PLB985 cells, with Nox2/gp91^phox and p22^phox. Recombinant S100A8, loaded with calcium and fused with the first 129 or 54 N-terminal amino acid residues of the P. aeruginosa ExoS toxin, induced a similar oxidase activation in vitro, to the one observed with S100A8 in the presence of S100A9 in vivo. This suggests that S100A8 is the essential component of the S100A9/S100A8 heterocomplex for oxidase activation. In this context, recombinant full-length rS100A9-A8 and rS100A9-A8 truncated 90 chimera proteins as opposed to rS100A9-A8 truncated 86 and rS100A9-A8 truncated 57 chimeras, activate the NADPH oxidase function of purified cytochrome b₅₅₈ suggesting that the C-terminal region of S100A8 is directly involved in the molecular interface with the hemoprotein. The data point to four strategic ⁸⁷HEES⁹⁰ amino acid residues of the S100A8 C-terminal sequence that are involved directly in the molecular interaction with cytochrome b₅₅₈ and then in the phagocyte NADPH oxidase activation.

The orchestration of cellular and humoral responses is facilitated by divergent intracellular antigen trafficking in nanoparticle-based therapeutic vaccine

Therapeutic vaccination for the treatment of chronic hepatitis B is promising but has so far shown limited clinical efficacy. Herein, we employ polylactide nanoparticles (NPs) as the vaccine adjuvant and systematically explore their effect on activation of specific immunity and the underlying theoretical mechanisms. In vitro studies show that hepatitis B surface antigen (HBsAg) accumulates in antigen-presenting cells (APCs) to a larger content (270%) with the assistant of NP in comparison with the pure-antigen group. Besides the elevated costimulators (CD80/86) and increased major histocompatibility complex (MHC) II molecules, the MHC I molecules are also found upregulated. This result is mostly owing to the divergent antigen trafficking ways of NP-antigen in APCs, especially for the escape of exogenous HBsAg from the lysosomes to the cytosol. Interestingly, the MHC I level is downregulated in alum-antigen group, indicating a possible reason for its inefficiency in priming cellular response. Further in vivo experiments establish that NP-antigen group indeed enhances the CD8(+) CTL cytotoxicity and IFN-γ cytokine secretion. Meanwhile, specific antibody titer is also upregulated, and even surpasses that of the commercialized alum-antigen. All these results strongly support that NP-based antigen promotes an orchestration of cellular and humoral immune response, exhibiting favorable intrinsic properties to be applied in therapeutic vaccines.

Bacteria modulate the CD8+ T cell epitope repertoire of host cytosol-exposed proteins to manipulate the host immune response

The main adaptive immune response to bacteria is mediated by B cells and CD4+ T-cells. However, some bacterial proteins reach the cytosol of host cells and are exposed to the host CD8+ T-cells response. Both gram-negative and gram-positive bacteria can translocate proteins to the cytosol through type III and IV secretion and ESX-1 systems, respectively. The translocated proteins are often essential for the bacterium survival. Once injected, these proteins can be degraded and presented on MHC-I molecules to CD8+ T-cells. The CD8+ T-cells, in turn, can induce cell death and destroy the bacteria's habitat. In viruses, escape mutations arise to avoid this detection. The accumulation of escape mutations in bacteria has never been systematically studied. We show for the first time that such mutations are systematically present in most bacteria tested. We combine multiple bioinformatic algorithms to compute CD8+ T-cell epitope libraries of bacteria with secretion systems that translocate proteins to the host cytosol. In all bacteria tested, proteins not translocated to the cytosol show no escape mutations in their CD8+ T-cell epitopes. However, proteins translocated to the cytosol show clear escape mutations and have low epitope densities for most tested HLA alleles. The low epitope densities suggest that bacteria, like viruses, are evolutionarily selected to ensure their survival in the presence of CD8+ T-cells. In contrast with most other translocated proteins examined, Pseudomonas aeruginosa's ExoU, which ultimately induces host cell death, was found to have high epitope density. This finding suggests a novel mechanism for the manipulation of CD8+ T-cells by pathogens. The ExoU effector may have evolved to maintain high epitope density enabling it to efficiently induce CD8+ T-cell mediated cell death. These results were tested using multiple epitope prediction algorithms, and were found to be consistent for most proteins tested.

Provision of continuous maturation signaling to dendritic cells by RIG-I-stimulating cytosolic RNA synthesis of Sendai virus

Dendritic cell (DC)-based immunotherapy has potential for treating infections and malignant tumors, but the functional capacity of DC must be assessed in detail, especially maturation and Ag-specific CTL priming. Recent reports suggest that DC that are provided with continuous maturation signals in vivo after transfer into patients are required to elicit the full DC functions. We demonstrate in this study that the rSendai virus vector (SeV) is a novel and ideal stimulant, providing DC with a continuous maturation signal via viral RNA synthesis in the cytosol, resulting in full maturation of monocyte-derived DC(s). Both RIG-I-dependent cytokine production and CD4 T cell responses to SeV-derived helper Ags are indispensable for overcoming regulatory T cell suppression to prime melanoma Ag recognized by T cell-1-specific CTL in the regulatory T cell abundant setting. DC stimulated via cytokine receptors, or TLRs, do not show these functional features. Therefore, SeV-infected DC have the potential for DC-directed immunotherapy.

The role of lysozyme and complement in the antibacterial activity of zebrafish (Danio rerio) egg cytosol

Most fish embryos develop in an aquatic environment full of potential pathogens. How fish embryos survive pathogenic attacks before the maturation of their own immunocompetence remains poorly understood. Here we demonstrated that bacteriolytic mechanism contributed to the antibacterial activity of zebrafish egg cytosol. In addition, the maternal lysozyme in the egg cytosol plays a key role in the bacteriolytic activity of the zebrafish eggs, which can be significantly stimulated by the cooperation with complement. This is the first report providing the evidence for the functional role of the maternal lysozyme and its cooperation with complement in fish eggs.

Identification of key cytosolic kinases containing evolutionarily conserved kinase tyrosine-based inhibitory motifs (KTIMs)

We previously reported that SHP-1 regulates IRAK-1 activity by binding to an ITIM-like motif found within its kinase domain, which we named kinase tyrosine-based inhibitory motif (KTIM). Herein, we further investigated the presence, number, location, and evolutionary time of emergence of potential KTIMs in many cytosolic kinases, all known to play important roles in the signalling and function of immune cells. We unveil that several kinases contain potential KTIMs, mostly located within their kinase domain and appearing predominantly at the level of early vertebrates becoming highly conserved thereafter. Regarding the KTIMs that were found conserved in both vertebrates and invertebrates, we provide experimental data suggesting that such motifs may have constituted readily available sites that performed new regulatory functions as soon as their binding partners (e.g. SHP-1) appeared in vertebrates. We thus propose KTIMs as novel regulatory motifs in kinases that function through binding to SH2 domain-containing proteins such as SHP-1.

Delivery of cytosolic components by autophagic adaptor protein p62 endows autophagosomes with unique antimicrobial properties

Autophagy allows cells to self-digest portions of their own cytoplasm for a multitude of physiological purposes, including innate and adaptive immunity functions. In one of its innate immunity manifestations, autophagy, is known to contribute to the killing of intracellular microbes, including Mycobacterium tuberculosis, although the molecular mechanisms have been unclear. Here, we delineated sequential steps of the autophagic pathway necessary to control intracellular M. tuberculosis and found that in addition to autophagy initiation and maturation, an accessory autophagy-targeting molecule p62 (A170 or SQSTM1) was required for mycobactericidal activity. The p62 adaptor protein delivered specific ribosomal and bulk ubiquitinated cytosolic proteins to autolysosomes where they were proteolytically converted into products capable of killing M. tuberculosis. Thus, p62 brings cytosolic proteins to autolysosomes where they are processed from innocuous precursors into neo-antimicrobial peptides, explaining in part the unique bactericidal properties of autophagic organelles.

TRAF6 establishes innate immune responses by activating NF-kappaB and IRF7 upon sensing cytosolic viral RNA and DNA

Background

In response to viral infection, the innate immune system recognizes viral nucleic acids and then induces production of proinflammatory cytokines and type I interferons (IFNs). Toll-like receptor 7 (TLR7) and TLR9 detect viral RNA and DNA, respectively, in endosomal compartments, leading to the activation of nuclear factor κB (NF-κB) and IFN regulatory factors (IRFs) in plasmacytoid dendritic cells. During such TLR signaling, TNF receptor-associated factor 6 (TRAF6) is essential for the activation of NF-κB and the production of type I IFN. In contrast, RIG-like helicases (RLHs), cytosolic RNA sensors, are indispensable for antiviral responses in conventional dendritic cells, macrophages, and fibroblasts. However, the contribution of TRAF6 to the detection of cytosolic viral nucleic acids has been controversial, and the involvement of TRAF6 in IRF activation has not been adequately addressed.

Principal Findings

Here we first show that TRAF6 plays a critical role in RLH signaling. The absence of TRAF6 resulted in enhanced viral replication and a significant reduction in the production of IL-6 and type I IFNs after infection with RNA virus. Activation of NF-κB and IRF7, but not that of IRF3, was significantly impaired during RLH signaling in the absence of TRAF6. TGFβ-activated kinase 1 (TAK1) and MEKK3, whose activation by TRAF6 during TLR signaling is involved in NF-κB activation, were not essential for RLH-mediated NF-κB activation. We also demonstrate that TRAF6-deficiency impaired cytosolic DNA-induced antiviral responses, and this impairment was due to defective activation of NF-κB and IRF7.

Conclusions/Significance

Thus, TRAF6 mediates antiviral responses triggered by cytosolic viral DNA and RNA in a way that differs from that associated with TLR signaling. Given its essential role in signaling by various receptors involved in the acquired immune system, TRAF6 represents a key molecule in innate and antigen-specific immune responses against viral infection.

Involvement of polyamine oxidase in abscisic acid-induced cytosolic antioxidant defense in leaves of maize

Using pharmacological and biochemical approaches, the role of maize polyamine oxidase (MPAO) in abscisic acid (ABA)-induced antioxidant defense in leaves of maize (Zea mays L.) plants was investigated. Exogenous ABA treatment enhanced the expression of the MPAO gene and the activities of apoplastic MPAO. Pretreatment with two different inhibitors for apoplastic MPAO partly reduced hydrogen peroxide (H2O2) accumulation induced by ABA and blocked the ABA-induced expression of the antioxidant genes superoxide dismutase 4 and cytosolic ascorbate peroxidase and the activities of the cytosolic antioxidant enzymes. Treatment with spermidine, the optimum substrate of MPAO, also induced the expression and the activities of the antioxidant enzymes, and the upregulation of the antioxidant enzymes was prevented by two inhibitors of MPAO and two scavengers of H2O2. These results suggest that MPAO contributes to ABA-induced cytosolic antioxidant defense through H2O2, a Spd catabolic product.