Cellular self-defense: how cell-autonomous immunity protects against pathogens

Pathogenicity and virulence of Pseudomonas aeruginosa: Recent advances and under-investigated topics

Pseudomonas aeruginosa is a model for the study of quorum sensing, protein secretion, and biofilm formation. Consequently, it has become one of the most intensely reviewed pathogens, with many excellent articles in the current literature focusing on these aspects of the organism's biology. Here, though, we aim to take a slightly different approach and consider some less well appreciated (but nonetheless important) factors that affect P. aeruginosa virulence. We start by reminding the reader of the global importance of P. aeruginosa infection and that the "virulome" is very niche-specific. Overlooked but obvious questions such as "what prevents secreted protein products from being digested by co-secreted proteases?" are discussed, and we suggest how the nutritional preference(s) of the organism might dictate its environmental reservoirs. Recent studies identifying host genes associated with genetic predisposition towards P. aeruginosa infection (and even infection by specific P. aeruginosa strains) and the role(s) of intracellular P. aeruginosa are introduced. We also discuss the fact that virulence is a high-risk strategy and touch on how expression of the two main classes of virulence factors is regulated. A particular focus is on recent findings highlighting how nutritional status and metabolism are as important as quorum sensing in terms of their impact on virulence, and how co-habiting microbial species at the infection site impact on P. aeruginosa virulence (and vice versa). It is our view that investigation of these issues is likely to dominate many aspects of research into this WHO-designated priority pathogen over the next decade.

Host AAA-ATPase VCP/p97 lyses ubiquitinated intracellular bacteria as an innate antimicrobial defence

Cell-autonomous immunity prevents intracellular pathogen growth through mechanisms such as ubiquitination and proteasomal targeting of bacteria for degradation. However, how the proteasome eradicates ubiquitinated bacteria has remained unclear. Here we show that host AAA-ATPase, VCP/p97, associates with diverse cytosol-exposed ubiquitinated bacteria (Streptococcus pneumoniae, Salmonella enterica serovar Typhimurium, Streptococcus pyogenes) and requires the ATPase activity in its D2 domain to reduce intracellular bacterial loads. Combining optical trap approaches along with molecular dynamic simulations, in vitro reconstitution and immunogold transmission electron microscopy, we demonstrate that p97 applies mechanical force to extract ubiquitinated surface proteins, BgaA and PspA, from S. pneumoniae cell membranes. This causes extensive membrane lysis and release of cytosolic content, thereby killing the pathogen. Further, p97 also controls S. pneumoniae proliferation in mice, ultimately protecting from fatal sepsis. Overall, we discovered a distinct innate antimicrobial function of p97 that can protect the host against lethal bacterial infections.

Base-modified nucleotides mediate immune signaling in bacteria

Signaling from pathogen sensing to effector activation is a fundamental principle of cellular immunity. Whereas cyclic (oligo)nucleotides have emerged as key signaling molecules, the existence of other messengers remains largely unexplored. In this study, we reveal a bacterial antiphage system that mediates immune signaling through nucleobase modification. Immunity is triggered by phage nucleotide kinases, which, combined with the system-encoded adenosine deaminase, produce deoxyinosine triphosphates (dITPs) as immune messengers. The dITP signal activates a downstream effector to mediate depletion of cellular nicotinamide adenine dinucleotide (oxidized form), resulting in population-level defense through the death of infected cells. To counteract immune signaling, phages deploy specialized enzymes that deplete cellular deoxyadenosine monophosphate, the precursor of dITP messengers. Our findings uncover a nucleobase modification-based antiphage signaling pathway, establishing noncanonical nucleotides as a new type of immune messengers in bacteria.

Cell-Autonomous Immunity: From Cytosolic Sensing to Self-Defense

As an evolutionarily conserved and ubiquitous mechanism of host defense, non-immune cells in vertebrates possess the intrinsic ability to autonomously detect and combat intracellular pathogens. This process, termed cell-autonomous immunity, is distinct from classical innate immunity. In this review, we comprehensively examine the defense mechanisms employed by non-immune cells in response to intracellular pathogen invasion. We provide a detailed analysis of the cytosolic sensors that recognize aberrant nucleic acids, lipopolysaccharide (LPS), and other pathogen-associated molecular patterns (PAMPs). Specifically, we elucidate the molecular mechanisms underlying key signaling pathways, including the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, the retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs)-mitochondrial antiviral signaling (MAVS) axis, and the guanylate-binding proteins (GBPs)-mediated pathway. Furthermore, we critically evaluate the involvement of these pathways in the pathogenesis of various diseases, including autoimmune disorders, inflammatory conditions, and malignancies, while highlighting their potential as therapeutic targets.

Shigella flexneri evades LPS ubiquitylation through IpaH1.4-mediated degradation of RNF213

Pathogens have evolved diverse strategies to counteract host immunity. Ubiquitylation of lipopolysaccharide (LPS) on cytosol-invading bacteria by the E3 ligase RNF213 creates 'eat me' signals for antibacterial autophagy, but whether and how cytosol-adapted bacteria avoid LPS ubiquitylation remains poorly understood. Here, we show that the enterobacterium Shigella flexneri actively antagonizes LPS ubiquitylation through IpaH1.4, a secreted effector protein with ubiquitin E3 ligase activity. IpaH1.4 binds to RNF213, ubiquitylates it and targets it for proteasomal degradation, thus counteracting host-protective LPS ubiquitylation. To understand how IpaH1.4 recognizes RNF213, we determined the cryogenic electron microscopy structure of the IpaH1.4-RNF213 complex. The specificity of the interaction is achieved through the leucine-rich repeat of IpaH1.4, which binds the RING domain of RNF213 by hijacking the conserved RING interface required for binding to ubiquitin-charged E2 enzymes. IpaH1.4 also targets other E3 ligases involved in inflammation and immunity through binding to the E2-interacting face of their RING domains, including the E3 ligase LUBAC that is required for the synthesis of M1-linked ubiquitin chains on cytosol-invading bacteria downstream of RNF213. We conclude that IpaH1.4 has evolved to antagonize multiple antibacterial and proinflammatory host E3 ligases.

The interplay between Salmonella and host: Mechanisms and strategies for bacterial survival

Salmonella, an intracellular pathogen, infects both humans and animals, causing diverse diseases such as gastroenteritis and enteric fever. The Salmonella type III secretion system (T3SS), encoded within its pathogenicity islands (SPIs), is critical for bacterial virulence by directly delivering multiple effectors into eukaryotic host cells. Salmonella utilizes these effectors to facilitate its survival and replication within the host through modulating cytoskeletal dynamics, inflammatory responses, the biogenesis of Salmonella-containing vacuole (SCV), and host cell survival. Moreover, these effectors also interfere with immune responses via inhibiting innate immunity or antigen presentation. In this review, we summarize the current progress in the survival strategies employed by Salmonella and the molecular mechanisms underlying its interactions with the host. Understanding the interplay between Salmonella and host can enhance our knowledge of the bacterium's pathogenic processes and provide new insights into how it manipulates host cellular physiological activities to ensure its survival.

Crystal structures reveal nucleotide-induced conformational changes in G motifs and distal regions in human guanylate-binding protein 2

Guanylate-binding proteins (GBPs) are interferon-inducible GTPases that confer protective immunity against a variety of intracellular pathogens. GBP2 is one of the two highly inducible GBPs, yet the precise mechanisms underlying the activation and regulation of GBP2, in particular the nucleotide-induced conformational changes in GBP2, remain poorly understood. In this study, we elucidate the structural plasticity of GBP2 upon nucleotide binding through crystallographic analysis. By determining the crystal structures of GBP2 G domain (GBP2GD) in complex with GDP and nucleotide-free full-length GBP2 with K51A mutation (GBP2K51A), we unveil distinct conformational states adopted by the nucleotide-binding pocket and distal regions of the protein. Comparison between the nucleotide-free full-length GBP2K51A structure with homologous structures reveals notable movement in the C-terminal helical region, along with conformational changes in the G domain. Through comparative analysis, we identify subtle but critical differences in the nucleotide-bound states of GBP2, providing insights into the molecular basis of its dimer-monomer transition and enzymatic activity. These findings pave the way for future investigations aimed at elucidating the precise molecular mechanisms underlying GBP2's role in the immune response and open avenues for exploring how the unique functions of GBPs could be leveraged to combat pathogen invasion.

mGBP2 engages Galectin-9 for immunity against Toxoplasma gondii

Guanylate binding proteins (GBPs) are large interferon-inducible GTPases, executing essential host defense activities against Toxoplasma gondii, an invasive intracellular apicomplexan protozoan parasite of global importance. T. gondii establishes a parasitophorous vacuole (PV) which shields the parasite from the host's intracellular defense mechanisms. Murine GBPs (mGBPs) recognize T. gondii PVs and assemble into supramolecular mGBP homo- and heterocomplexes that are required for the disruption of the membrane of PVs eventually resulting in the cell-autonomous immune control of vacuole-resident pathogens. We have previously shown that mGBP2 plays an important role in T. gondii immune control. Here, to unravel mGBP2 functions, we report Galectin-9 (Gal9) as a critical mGBP2 interaction partner engaged for immunity to T. gondii. Interestingly, Gal9 also accumulates and colocalizes with mGBP2 at the T. gondii PV. Furthermore, we could prove the requirement of Gal9 for growth control of T. gondii by CRISPR/Cas9 mediated gene editing. These discoveries clearly indicate that Gal9 is a crucial factor for the mGBP2-coordinated cell-autonomous host defense mechanism against T. gondii.

Macrophages orchestrate elimination of Shigella from the intestinal epithelial cell niche via TLR-induced IL-12 and IFN-γ

Bacteria of the genus Shigella replicate in intestinal epithelial cells and cause shigellosis, a severe diarrheal disease that resolves spontaneously in most healthy individuals. During shigellosis, neutrophils are abundantly recruited to the gut, and have long been thought to be central to Shigella control and pathogenesis. However, how shigellosis resolves remains poorly understood due to the longstanding lack of a tractable and physiological animal model. Here, using our newly developed Nlrc4 -/- Casp11 -/- mouse model of shigellosis, we unexpectedly find no major role for neutrophils in limiting Shigella or in disease pathogenesis. Instead, we uncover an essential role for macrophages in the host control of Shigella . Macrophages respond to Shigella via TLRs to produce IL-12, which then induces IFN-γ, a cytokine that is essential to control Shigella replication in intestinal epithelial cells. Collectively, our findings reshape our understanding of the innate immune response to Shigella .

Interplay of swine acute diarrhoea syndrome coronavirus and the host intrinsic and innate immunity

Swine acute diarrhoea syndrome coronavirus (SADS-CoV), a novel HKU2-related coronavirus of bat origin, is a newly emerged swine enteropathogenic coronavirus that causes severe diarrhoea in piglets. SADS-CoV has a broad cell tropism with the capability to infect a wide variety of cells from human and diverse animals, which implicates its ability to hold high risks of cross-species transmission. The intracellular antiviral immunity, comprised of the intrinsic and innate immunity, represents the first line of host defence against viral infection prior to the onset of adaptive immunity. To date, there are no vaccines and drugs approved to prevent or treat SADS-CoV infection. Understanding of the mutual relationship between SADS-CoV infection and host immunity is crucial for the development of novel vaccines and drugs against SADS-CoV. Here, we review recent advancements in our understanding of the interplay between SADS-CoV infection and the host intrinsic and innate immunity. The extensive and in-depth investigation on their interactive relationship will contribute to the identification of new targets for developing intervention strategies to control SADS-CoV infection.

Structural basis of antimicrobial membrane coat assembly by human GBP1

Guanylate-binding proteins (GBPs) are interferon-inducible guanosine triphosphate hydrolases (GTPases) mediating host defense against intracellular pathogens. Their antimicrobial activity hinges on their ability to self-associate and coat pathogen-associated compartments or cytosolic bacteria. Coat formation depends on GTPase activity but how nucleotide binding and hydrolysis prime coat formation remains unclear. Here, we report the cryo-electron microscopy structure of the full-length human GBP1 dimer in its guanine nucleotide-bound state and describe the molecular ultrastructure of the GBP1 coat on liposomes and bacterial lipopolysaccharide membranes. Conformational changes of the middle and GTPase effector domains expose the isoprenylated C terminus for membrane association. The α-helical middle domains form a parallel, crossover arrangement essential for coat formation and position the extended effector domain for intercalation into the lipopolysaccharide layer of gram-negative membranes. Nucleotide binding and hydrolysis create oligomeric scaffolds with contractile abilities that promote membrane extrusion and fragmentation. Our data offer a structural and mechanistic framework for understanding GBP1 effector functions in intracellular immunity.

Early host defense against virus infections

Early host defense eliminates many viruses before infections are established while clearing others so they remain subclinical or cause only mild disease. The field of immunology has been shaped by broad concepts, including the pattern recognition theory that currently dominates innate immunology. Focusing on early host responses to virus infections, we analyze the literature to build a working hypothesis for the principles that govern the early line of cellular antiviral defense. Aiming to ultimately arrive at a criteria-based theory with strong explanatory power, we propose that both controlling infection and limiting inflammation are key drivers for the early cellular antiviral response. This response, which we suggest is exerted by a set of "microbe- and inflammation-restricting mechanisms," directly restrict viral replication while also counteracting inflammation. Exploring the mechanisms and physiological importance of the early layer of cellular antiviral defense may open further lines of research in immunology.

Indole N-Linked Hydroperoxyl Adduct of Protein-Derived Cofactor Modulating Catalase-Peroxidase Functions

Bifunctional catalase-peroxidase (KatG) features a posttranslational methionine-tyrosine-tryptophan (MYW) crosslinked cofactor crucial for its catalase function, enabling pathogens to neutralize hydrogen peroxide during infection. We discovered the presence of indole nitrogen-linked hydroperoxyl adduct (MYW-OOH) in Mycobacterium tuberculosis KatG in the solution state under ambient conditions, suggesting its natural occurrence. By isolating predominantly MYW-OOH-containing KatG protein, we investigated the chemical stability and functional impact of MYW-OOH. We discovered that MYW-OOH inhibits catalase activity, presenting a unique temporary lock. Exposure to peroxide or increased temperature removes the hydroperoxyl adduct from the protein cofactor, converting MYW-OOH to MYW and restoring the detoxifying ability of the enzyme against hydrogen peroxide. Thus, the N-linked hydroperoxyl group is releasable. KatG with MYW-OOH represents a catalase dormant, but primed, state of the enzyme. These findings provide insight into chemical strategies targeting the bifunctional enzyme KatG in pathogens, highlighting the role of N-linked hydroperoxyl modifications in enzymatic function.

A personal view on developmental and comparative immunology: What, how and why?

What are the future directions of the fields of developmental and comparative immunology? In thinking through this question as I write, I find myself marvelling at the very long ways that we have come since I began as a PhD student some 50 years ago. I think that we cannot know what technical and theoretical advances will emerge in the future, nor will our initial aims survive the realities of what appears in our sights, often from unexpected directions. I feel that we should not allow what we already know about some well-studied systems to blind us to the wide range of possibilities, and that remaining a humble seeker helps the uptake of new realities. Finally, it would be good to try answering the whole range of questions about developmental and comparative immunology, from what to how to why.

Type I interferon signaling and peroxisomal dysfunction contribute to enhanced inflammatory cytokine production in IRGM1-deficient macrophages

The human IRGM gene has been linked to inflammatory diseases including sepsis and Crohn's disease. Decreased expression of human IRGM, or the mouse orthologues Irgm1 and Irgm2, leads to increased production of a number of inflammatory chemokines and cytokines in vivo and/or in cultured macrophages. Prior work has indicated that increased cytokine production is instigated by metabolic alterations and changes in mitochondrial homeostasis; however, a comprehensive mechanism has not been elucidated. In the studies presented here, RNA deep sequencing and quantitative PCR were used to show that increases in cytokine production, as well as most changes in the transcriptional profile of Irgm1-/- bone marrow-derived macrophages (BMM), are dependent on increased type I IFN production seen in those cells. Metabolic alterations that drive increased cytokines in Irgm1-/- BMM - specifically increases in glycolysis and increased accumulation of acyl-carnitines - were unaffected by quenching type I IFN signaling. Dysregulation of peroxisomal homeostasis was identified as a novel upstream pathway that governs type I IFN production and inflammatory cytokine production. Collectively, these results enhance our understanding of the complex biochemical changes that are triggered by lack of Irgm1 and contribute to inflammatory disease seen with Irgm1-deficiency.

Arrest and Attack: Microtubule-Targeting Agents and Oncolytic Viruses Employ Complementary Mechanisms to Enhance Anti-Tumor Therapy Efficacy

Oncolytic viruses (OVs) are promising cancer immunotherapy agents that stimulate anti-tumor immunity through the preferential infection and killing of tumor cells. OVs are currently under limited clinical usage, due in part to their restricted efficacy as monotherapies. Current efforts for enhancement of the therapeutic potency of OVs involve their combination with other therapy modalities, aiming at the concomitant exploitation of complementary tumor weaknesses. In this context, microtubule-targeting agents (MTAs) pose as an enticing option, as they perturb microtubule dynamics and function, induce cell-cycle arrest, and cause mitotic cell death. MTAs induce therapeutic benefit through cancer-cell-autonomous and non-cell-autonomous mechanisms and are a main component of the standard of care for different malignancies. However, off-target effects and acquired resistance involving distinct cellular and molecular mechanisms may limit the overall efficacy of MTA-based therapy. When combined, OVs and MTAs may enhance therapeutic efficacy through increases in OV infection and immunogenic cell death and a decreased probability of acquired resistance. In this review, we introduce OVs and MTAs, describe molecular features of their activity in cancer cells, and discuss studies and clinical trials in which the combination has been tested.

Decoding Toxoplasma gondii virulence: the mechanisms of IRG protein inactivation

Toxoplasmosis is a common parasitic zoonosis that can be life-threatening in immunocompromised patients. About one-third of the human population is infected with Toxoplasma gondii. Primary infection triggers an innate immune response wherein IFN-γ-induced host cell GTPases, namely IRG and GBP proteins, serve as a vital component for host cell resistance. In the past decades, interest in elucidating the function of these GTPase families in controlling various intracellular pathogens has emerged. Numerous T. gondii effectors were identified to inactivate particular IRG proteins. T. gondii is re-optimizing its effectors to combat IRG function and in this way secures transmission. We discuss the IRG-specific effectors employed by the parasite in murine infections, contributing to a better understanding of T. gondii virulence.

An Intrinsic Host Defense against HSV-1 Relies on the Activation of Xenophagy with the Active Clearance of Autophagic Receptors

Autophagy engulfs cellular components in double-membrane-bound autophagosomes for clearance and recycling after fusion with lysosomes. Thus, autophagy is a key process for maintaining proteostasis and a powerful cell-intrinsic host defense mechanism, protecting cells against pathogens by targeting them through a specific form of selective autophagy known as xenophagy. In this context, ubiquitination acts as a signal of recognition of the cargoes for autophagic receptors, which direct them towards autophagosomes for subsequent breakdown. Nevertheless, autophagy can carry out a dual role since numerous viruses including members of the Orthoherpesviridae family can either inhibit or exploit autophagy for its own benefit and to replicate within host cells. There is growing evidence that Herpes simplex virus type 1 (HSV-1), a highly prevalent human pathogen that infects epidermal keratinocytes and sensitive neurons, is capable of negatively modulating autophagy. Since the effects of HSV-1 infection on autophagic receptors have been poorly explored, this study aims to understand the consequences of HSV-1 productive infection on the levels of the major autophagic receptors involved in xenophagy, key proteins in the recruitment of intracellular pathogens into autophagosomes. We found that productive HSV-1 infection in human neuroglioma cells and keratinocytes causes a reduction in the total levels of Ub conjugates and decreases protein levels of autophagic receptors, including SQSTM1/p62, OPTN1, NBR1, and NDP52, a phenotype that is also accompanied by reduced levels of LC3-I and LC3-II, which interact directly with autophagic receptors. Mechanistically, we show these phenotypes are the result of xenophagy activation in the early stages of productive HSV-1 infection to limit virus replication, thereby reducing progeny HSV-1 yield. Additionally, we found that the removal of the tegument HSV-1 protein US11, a recognized viral factor that counteracts autophagy in host cells, enhances the clearance of autophagic receptors, with a significant reduction in the progeny HSV-1 yield. Moreover, the removal of US11 increases the ubiquitination of SQSTM1/p62, indicating that US11 slows down the autophagy turnover of autophagy receptors. Overall, our findings suggest that xenophagy is a potent host defense against HSV-1 replication and reveals the role of the autophagic receptors in the delivery of HSV-1 to clearance via xenophagy.

LncRNA Nostrill promotes interferon-γ-stimulated gene transcription and facilitates intestinal epithelial cell-intrinsic anti-Cryptosporidium defense

Intestinal epithelial cells possess the requisite molecular machinery to initiate cell-intrinsic defensive responses against intracellular pathogens, including intracellular parasites. Interferons(IFNs) have been identified as cornerstones of epithelial cell-intrinsic defense against such pathogens in the gastrointestinal tract. Long non-coding RNAs (lncRNAs) are RNA transcripts (>200 nt) not translated into protein and represent a critical regulatory component of mucosal defense. We report here that lncRNA Nostrill facilitates IFN-γ-stimulated intestinal epithelial cell-intrinsic defense against infection by Cryptosporidium, an important opportunistic pathogen in AIDS patients and a common cause of diarrhea in young children. Nostrill promotes transcription of a panel of genes controlled by IFN-γ through facilitating Stat1 chromatin recruitment and thus, enhances expression of several genes associated with cell-intrinsic defense in intestinal epithelial cells in response to IFN-γ stimulation, including Igtp, iNos, and Gadd45g. Induction of Nostrill enhances IFN-γ-stimulated intestinal epithelial defense against Cryptosporidium infection, which is associated with an enhanced autophagy in intestinal epithelial cells. Our findings reveal that Nostrill enhances the transcription of a set of genes regulated by IFN-γ in intestinal epithelial cells. Moreover, induction of Nostrill facilitates the IFN-γ-mediated epithelial cell-intrinsic defense against cryptosporidial infections.

Insights into Chlamydia Development and Host Cells Response

Chlamydia infections commonly afflict both humans and animals, resulting in significant morbidity and imposing a substantial socioeconomic burden worldwide. As an obligate intracellular pathogen, Chlamydia interacts with other cell organelles to obtain necessary nutrients and establishes an intracellular niche for the development of a biphasic intracellular cycle. Eventually, the host cells undergo lysis or extrusion, releasing infectious elementary bodies and facilitating the spread of infection. This review provides insights into Chlamydia development and host cell responses, summarizing the latest research on the biphasic developmental cycle, nutrient acquisition, intracellular metabolism, host cell fates following Chlamydia invasion, prevalent diseases associated with Chlamydia infection, treatment options, and vaccine prevention strategies. A comprehensive understanding of these mechanisms will contribute to a deeper comprehension of the intricate equilibrium between Chlamydia within host cells and the progression of human disease.

Innate immune responses and monocyte-derived phagocyte recruitment in protective immunity to pathogenic bacteria: insights from Legionella pneumophila

Legionella species are Gram-negative intracellular bacteria that evolved in soil and freshwater environments, where they infect and replicate within various unicellular protozoa. The primary virulence factor of Legionella is the expression of a type IV secretion system (T4SS), which contributes to the translocation of effector proteins that subvert biological processes of the host cells. Because of its evolution in unicellular organisms, T4SS effector proteins are not adapted to subvert specific mammalian signaling pathways and immunity. Consequently, Legionella pneumophila has emerged as an interesting infection model for investigating immune responses against pathogenic bacteria in multicellular organisms. This review highlights recent advances in our understanding of mammalian innate immunity derived from studies involving L. pneumophila. This includes recent insights into inflammasome-mediated mechanisms restricting bacterial replication in macrophages, mechanisms inducing cell death in response to infection, induction of effector-triggered immunity, activation of specific pulmonary cell types in mammalian lungs, and the protective role of recruiting monocyte-derived cells to infected lungs.

Mechanisms of host adaptation by bacterial pathogens

The emergence of new infectious diseases poses a major threat to humans, animals, and broader ecosystems. Defining factors that govern the ability of pathogens to adapt to new host species is therefore a crucial research imperative. Pathogenic bacteria are of particular concern, given dwindling treatment options amid the continued expansion of antimicrobial resistance. In this review, we summarize recent advancements in the understanding of bacterial host species adaptation, with an emphasis on pathogens of humans and related mammals. We focus particularly on molecular mechanisms underlying key steps of bacterial host adaptation including colonization, nutrient acquisition, and immune evasion, as well as suggest key areas for future investigation. By developing a greater understanding of the mechanisms of host adaptation in pathogenic bacteria, we may uncover new strategies to target these microbes for the treatment and prevention of infectious diseases in humans, animals, and the broader environment.

Bacterial esterases reverse lipopolysaccharide ubiquitylation to block host immunity

Aspects of how Burkholderia escape the host's intrinsic immune response to replicate in the cell cytosol remain enigmatic. Here, we show that Burkholderia has evolved two mechanisms to block the activity of Ring finger protein 213 (RNF213)-mediated non-canonical ubiquitylation of bacterial lipopolysaccharide (LPS), thereby preventing the initiation of antibacterial autophagy. First, Burkholderia's polysaccharide capsule blocks RNF213 association with bacteria and second, the Burkholderia deubiquitylase (DUB), TssM, directly reverses the activity of RNF213 through a previously unrecognized esterase activity. Structural analysis provides insight into the molecular basis of TssM esterase activity, allowing it to be uncoupled from its isopeptidase function. Furthermore, a putative TssM homolog also displays esterase activity and removes ubiquitin from LPS, establishing this as a virulence mechanism. Of note, we also find that additional immune-evasion mechanisms exist, revealing that overcoming this arm of the host's immune response is critical to the pathogen.

TRIM21 and Fc-engineered antibodies: decoding its complex antibody binding mode with implications for viral neutralization

TRIM21 is a pivotal effector in the immune system, orchestrating antibody-mediated responses and modulating immune signaling. In this comprehensive study, we focus on the interaction of TRIM21 with Fc engineered antibodies and subsequent implications for viral neutralization. Through a series of analytical techniques, including biosensor assays, mass photometry, and electron microscopy, along with structure predictions, we unravel the intricate mechanisms governing the interplay between TRIM21 and antibodies. Our investigations reveal that the TRIM21 capacity to recognize, bind, and facilitate the proteasomal degradation of antibody-coated viruses is critically dependent on the affinity and avidity interplay of its interactions with antibody Fc regions. We suggest a novel binding mechanism, where TRIM21 binding to one Fc site results in the detachment of PRYSPRY from the coiled-coil domain, enhancing mobility due to its flexible linker, thereby facilitating the engagement of the second site, resulting in avidity due to bivalent engagement. These findings shed light on the dual role of TRIM21 in antiviral immunity, both in recognizing and directing viruses for intracellular degradation, and demonstrate its potential for therapeutic exploitation. The study advances our understanding of intracellular immune responses and opens new avenues for the development of antiviral strategies and innovation in tailored effector functions designed to leverage TRIM21s unique binding mode.

Unveiling the Ro60-Ro52 complex

The coexistence within a subcellular complex of inter-cellular proteins Ro60, responsible for preserving ncRNA quality, and Ro52, involved in intracellular proteolysis, has been a subject of ongoing debate. Employing molecular docking in tandem with experimental methods like Quartz Crystal Microbalance with Dissipation (QCM-D), Proximity Ligation Assay (PLA), and Indirect Immunofluorescence (IIF), we reveal the presence of Ro60 associating with Ro52 within the cytoplasm. This result unveils the formation of a weak transient complex with a Ka ≈ (3.7 ± 0.3) x 106 M-1, where the toroid-shaped Ro60 structure interacts with the Ro52's Fc receptor, aligning horizontally within the PRY-SPRY domains of the Ro52's homodimer. The stability of this complex relies on the interaction between Ro52 chain A and specific Ro60 residues, such as K133, W177, or L185, vital in the Ro60-YRNA bond. These findings bridge the role of Ro60 in YRNA management with Ro52's function in intracellular proteolysis, emphasizing the potential impact of transient complexes on cellular pathways. See also the graphical abstract(Fig. 1).

A nox2/cybb zebrafish mutant with defective myeloid cell reactive oxygen species production displays normal initial neutrophil recruitment to sterile tail injuries

Reactive oxygen species are important effectors and modifiers of the acute inflammatory response, recruiting phagocytes including neutrophils to sites of tissue injury. In turn, phagocytes such as neutrophils are both consumers and producers of reactive oxygen species. Phagocytes including neutrophils generate reactive oxygen species in an oxidative burst through the activity of a multimeric phagocytic nicotinamide adenine dinucleotide phosphate oxidase complex. Mutations in the NOX2/CYBB (previously gp91phox) nicotinamide adenine dinucleotide phosphate oxidase subunit are the commonest cause of chronic granulomatous disease, a disease characterized by infection susceptibility and an inflammatory phenotype. To model chronic granulomatous disease, we made a nox2/cybb zebrafish (Danio rerio) mutant and demonstrated it to have severely impaired myeloid cell reactive oxygen species production. Reduced early survival of nox2 mutant embryos indicated an essential requirement for nox2 during early development. In nox2/cybb zebrafish mutants, the dynamics of initial neutrophil recruitment to both mild and severe surgical tailfin wounds was normal, suggesting that excessive neutrophil recruitment at the initiation of inflammation is not the primary cause of the "sterile" inflammatory phenotype of chronic granulomatous disease patients. This nox2 zebrafish mutant adds to existing in vivo models for studying reactive oxygen species function in myeloid cells including neutrophils in development and disease.

Vaccinia virus subverts xenophagy through phosphorylation and nuclear targeting of p62

Autophagy is an essential degradation program required for cell homeostasis. Among its functions is the engulfment and destruction of cytosolic pathogens, termed xenophagy. Not surprisingly, many pathogens use various strategies to circumvent or co-opt autophagic degradation. For poxviruses, it is known that infection activates autophagy, which however is not required for successful replication. Even though these complex viruses replicate exclusively in the cytoplasm, autophagy-mediated control of poxvirus infection has not been extensively explored. Using the prototypic poxvirus, vaccinia virus (VACV), we show that overexpression of the xenophagy receptors p62, NDP52, and Tax1Bp1 restricts poxvirus infection. While NDP52 and Tax1Bp1 were degraded, p62 initially targeted cytoplasmic virions before being shunted to the nucleus. Nuclear translocation of p62 was dependent upon p62 NLS2 and correlated with VACV kinase mediated phosphorylation of p62 T269/S272. This suggests that VACV targets p62 during the early stages of infection to avoid destruction and further implies that poxviruses exhibit multi-layered control of autophagy to facilitate cytoplasmic replication.

Logistics of defense: The contribution of endomembranes to plant innate immunity

Phytopathogens cause plant diseases that threaten food security. Unlike mammals, plants lack an adaptive immune system and rely on their innate immune system to recognize and respond to pathogens. Plant response to a pathogen attack requires precise coordination of intracellular traffic and signaling. Spatial and/or temporal defects in coordinating signals and cargo can lead to detrimental effects on cell development. The role of intracellular traffic comes into a critical focus when the cell sustains biotic stress. In this review, we discuss the current understanding of the post-immune activation logistics of plant defense. Specifically, we focus on packaging and shipping of defense-related cargo, rerouting of intracellular traffic, the players enabling defense-related traffic, and pathogen-mediated subversion of these pathways. We highlight the roles of the cytoskeleton, cytoskeleton-organelle bridging proteins, and secretory vesicles in maintaining pathways of exocytic defense, acting as sentinels during pathogen attack, and the necessary elements for building the cell wall as a barrier to pathogens. We also identify points of convergence between mammalian and plant trafficking pathways during defense and highlight plant unique responses to illustrate evolutionary adaptations that plants have undergone to resist biotic stress.

The Balance between Protealysin and Its Substrate, the Outer Membrane Protein OmpX, Regulates Serratia proteamaculans Invasion

Serratia are opportunistic bacteria, causing infections in plants, insects, animals and humans under certain conditions. The development of bacterial infection in the human body involves several stages of host-pathogen interaction, including entry into non-phagocytic cells to evade host immune cells. The facultative pathogen Serratia proteamaculans is capable of penetrating eukaryotic cells. These bacteria synthesize an actin-specific metalloprotease named protealysin. After transformation with a plasmid carrying the protealysin gene, noninvasive E. coli penetrate eukaryotic cells. This suggests that protealysin may play a key role in S. proteamaculans invasion. This review addresses the mechanisms underlying protealysin's involvement in bacterial invasion, highlighting the main findings as follows. Protealysin can be delivered into the eukaryotic cell by the type VI secretion system and/or by bacterial outer membrane vesicles. By cleaving actin in the host cell, protealysin can mediate the reversible actin rearrangements required for bacterial invasion. However, inactivation of the protealysin gene leads to an increase, rather than decrease, in the intensity of S. proteamaculans invasion. This indicates the presence of virulence factors among bacterial protealysin substrates. Indeed, protealysin cleaves the virulence factors, including the bacterial surface protein OmpX. OmpX increases the expression of the EGFR and β1 integrin, which are involved in S. proteamaculans invasion. It has been shown that an increase in the invasion of genetically modified S. proteamaculans may be the result of the accumulation of full-length OmpX on the bacterial surface, which is not cleaved by protealysin. Thus, the intensity of the S. proteamaculans invasion is determined by the balance between the active protealysin and its substrate OmpX.

Hepatocytes and the art of killing Plasmodium softly

The Plasmodium parasites that cause malaria undergo asymptomatic development in the parenchymal cells of the liver, the hepatocytes, prior to infecting erythrocytes and causing clinical disease. Traditionally, hepatocytes have been perceived as passive bystanders that allow hepatotropic pathogens such as Plasmodium to develop relatively unchallenged. However, now there is emerging evidence suggesting that hepatocytes can mount robust cell-autonomous immune responses that target Plasmodium, limiting its progression to the blood and reducing the incidence and severity of clinical malaria. Here we discuss our current understanding of hepatocyte cell-intrinsic immune responses that target Plasmodium and how these pathways impact malaria.

Intracellular infection-responsive macrophage-targeted nanoparticles for synergistic antibiotic immunotherapy of bacterial infection

Intracellular bacteria are considered to play a key role in the failure of bacterial infection therapy and increase of antibiotic resistance. Nanotechnology-based drug delivery carriers have been receiving increasing attention for improving the intracellular antibacterial activity of antibiotics, but are accompanied by disadvantages such as complex preparation procedures, lack of active targeting, and monotherapy, necessitating further design improvements. Herein, nanoparticles targeting bacteria-infected macrophages are fabricated to eliminate intracellular bacterial infections via antibiotic release and upregulation of intracellular reactive oxygen species (ROS) levels and proinflammatory responses. These nanoparticles were formed through the reaction of the amino group on selenocystamine dihydrochloride and the aldehyde group on oxidized dextran (ox-Dex), which encapsulates vancomycin (Van) through hydrophobic interactions. These nanoparticles could undergo targeted uptake by macrophages via endocytosis and respond to the bacteria-infected intracellular microenvironment (ROS and glutathione (GSH)) for controlled release of antibiotics. Furthermore, these nanoparticles could consume intracellular GSH and promote a significant increase in the level of ROS in macrophages, subsequently up-regulating the proinflammatory response to reinforce antibacterial activity. These nanoparticles can accelerate bacteria-infected wound healing. In this work, nanoparticles were fabricated for bacteria-infected macrophage-targeted and microenvironment-responsive antibiotic delivery, cellular ROS generation, and proinflammatory up-regulation activity to eliminate intracellular bacteria, which opens up a new possibility for multifunctional drug delivery against intracellular infection.

Integrative bioinformatics and experimental validation of hub genetic markers in acne vulgaris: Toward personalized diagnostic and therapeutic strategies

BACKGROUND

Acne vulgaris is a widespread chronic inflammatory dermatological condition. The precise molecular and genetic mechanisms of its pathogenesis remain incompletely understood. This research synthesizes existing databases, targeting a comprehensive exploration of core genetic markers.

METHODS

Gene expression datasets (GSE6475, GSE108110, and GSE53795) were retrieved from the GEO. Differentially expressed genes (DEGs) were identified using the limma package. Enrichment analyses were conducted using GSVA for pathway assessment and clusterProfiler for GO and KEGG analyses. PPI networks and immune cell infiltration were analyzed using the STRING database and ssGSEA, respectively. We investigated the correlation between hub gene biomarkers and immune cell infiltration using Spearman's rank analysis. ROC curve analysis validated the hub genes' diagnostic accuracy. miRNet, TarBase v8.0, and ChEA3 identified miRNA/transcription factor-gene interactions, while DrugBank delineated drug-gene interactions. Experiments utilized HaCaT cells stimulated with Propionibacterium acnes, treated with retinoic acid and methotrexate, and evaluated using RT-qPCR, ELISA, western blot, lentiviral transduction, CCK-8, wound-healing, and transwell assays.

RESULTS

There were 104 genes with consistent differences across the three datasets of paired acne and normal skin. Functional analyses emphasized the significant enrichment of these DEGs in immune-related pathways. PPI network analysis pinpointed hub genes PTPRC, CXCL8, ITGB2, and MMP9 as central players in acne pathogenesis. Elevated levels of specific immune cell infiltration in acne lesions corroborated the inflammatory nature of the disease. ROC curve analysis identified the acne diagnostic potential of four hub genes. Key miRNAs, particularly hsa-mir-124-3p, and central transcription factors like TFEC were noted as significant regulators. In vitro validation using HaCaT cells confirmed the upregulation of hub genes following Propionibacterium acnes exposure, while CXCL8 knockdown reduced pro-inflammatory cytokines, cell proliferation, and migration. DrugBank insights led to the exploration of retinoic acid and methotrexate, both of which mitigated gene expression upsurge and inflammatory mediator secretion.

CONCLUSION

This comprehensive study elucidated pivotal genes associated with acne pathogenesis, notably PTPRC, CXCL8, ITGB2, and MMP9. The findings underscore potential biomarkers, therapeutic targets, and the therapeutic potential of agents like retinoic acid and methotrexate. The congruence between bioinformatics and experimental validations suggests promising avenues for personalized acne treatments.

Pin1-Catalyzed Conformation Changes Regulate Protein Ubiquitination and Degradation

The unique prolyl isomerase Pin1 binds to and catalyzes cis-trans conformational changes of specific Ser/Thr-Pro motifs after phosphorylation, thereby playing a pivotal role in regulating the structure and function of its protein substrates. In particular, Pin1 activity regulates the affinity of a substrate for E3 ubiquitin ligases, thereby modulating the turnover of a subset of proteins and coordinating their activities after phosphorylation in both physiological and disease states. In this review, we highlight recent advancements in Pin1-regulated ubiquitination in the context of cancer and neurodegenerative disease. Specifically, Pin1 promotes cancer progression by increasing the stabilities of numerous oncoproteins and decreasing the stabilities of many tumor suppressors. Meanwhile, Pin1 plays a critical role in different neurodegenerative disorders via the regulation of protein turnover. Finally, we propose a novel therapeutic approach wherein the ubiquitin-proteasome system can be leveraged for therapy by targeting pathogenic intracellular targets for TRIM21-dependent degradation using stereospecific antibodies.

Interferon- and infectious diseases: Lessons and prospects

Infectious diseases continue to claim many lives. Prevention of morbidity and mortality from these diseases would benefit not just from new medicines and vaccines but also from a better understanding of what constitutes protective immunity. Among the major immune signals that mobilize host defense against infection is interferon-γ (IFN-γ), a protein secreted by lymphocytes. Forty years ago, IFN-γ was identified as a macrophage-activating factor, and, in recent years, there has been a resurgent interest in IFN-γ biology and its role in human defense. Here we assess the current understanding of IFN-γ, revisit its designation as an "interferon," and weigh its prospects as a therapeutic against globally pervasive microbial pathogens.

DAMPs and DAMP-sensing receptors in inflammation and diseases

Damage-associated molecular patterns (DAMPs) are endogenous danger molecules produced in cellular damage or stress, and they can activate the innate immune system. DAMPs contain multiple types of molecules, including nucleic acids, proteins, ions, glycans, and metabolites. Although these endogenous molecules do not trigger immune response under steady-state condition, they may undergo changes in distribution, physical or chemical property, or concentration upon cellular damage or stress, and then they become DAMPs that can be sensed by innate immune receptors to induce inflammatory response. Thus, DAMPs play an important role in inflammation and inflammatory diseases. In this review, we summarize the conversion of homeostatic molecules into DAMPs; the diverse nature and classification, cellular origin, and sensing of DAMPs; and their role in inflammation and related diseases. Furthermore, we discuss the clinical strategies to treat DAMP-associated diseases via targeting DAMP-sensing receptors.

Direct Salmonella injection into enteroid cells allows the study of host-pathogen interactions in the cytosol with high spatiotemporal resolution

Intestinal epithelial cells (IECs) play pivotal roles in nutrient uptake and in the protection against gut microorganisms. However, certain enteric pathogens, such as Salmonella enterica serovar Typhimurium (S. Tm), can invade IECs by employing flagella and type III secretion systems (T3SSs) with cognate effector proteins and exploit IECs as a replicative niche. Detection of flagella or T3SS proteins by IECs results in rapid host cell responses, i.e., the activation of inflammasomes. Here, we introduce a single-cell manipulation technology based on fluidic force microscopy (FluidFM) that enables direct bacteria delivery into the cytosol of single IECs within a murine enteroid monolayer. This approach allows to specifically study pathogen-host cell interactions in the cytosol uncoupled from preceding events such as docking, initiation of uptake, or vacuole escape. Consistent with current understanding, we show using a live-cell inflammasome reporter that exposure of the IEC cytosol to S. Tm induces NAIP/NLRC4 inflammasomes via its known ligands flagellin and T3SS rod and needle. Injected S. Tm mutants devoid of these invasion-relevant ligands were able to grow in the cytosol of IECs despite the absence of T3SS functions, suggesting that, in the absence of NAIP/NLRC4 inflammasome activation and the ensuing cell death, no effector-mediated host cell manipulation is required to render the epithelial cytosol growth-permissive for S. Tm. Overall, the experimental system to introduce S. Tm into single enteroid cells enables investigations into the molecular basis governing host-pathogen interactions in the cytosol with high spatiotemporal resolution.

Native architecture of a human GBP1 defense complex for cell-autonomous immunity to infection

All living organisms deploy cell-autonomous defenses to combat infection. In plants and animals, large supramolecular complexes often activate immune proteins for protection. In this work, we resolved the native structure of a massive host-defense complex that polymerizes 30,000 guanylate-binding proteins (GBPs) over the surface of gram-negative bacteria inside human cells. Construction of this giant nanomachine took several minutes and remained stable for hours, required guanosine triphosphate hydrolysis, and recruited four GBPs plus caspase-4 and Gasdermin D as a cytokine and cell death immune signaling platform. Cryo-electron tomography suggests that GBP1 can adopt an extended conformation for bacterial membrane insertion to establish this platform, triggering lipopolysaccharide release that activated coassembled caspase-4. Our "open conformer" model provides a dynamic view into how the human GBP1 defense complex mobilizes innate immunity to infection.

Restriction and evasion: a review of IFNγ-mediated cell-autonomous defense pathways during genital Chlamydia infection

Chlamydia trachomatis is the most common cause of bacterial sexually transmitted infection (STI) in the USA. As an STI, C. trachomatis infections can cause inflammatory damage to the female reproductive tract and downstream sequelae including infertility. No vaccine currently exists to C. trachomatis, which evades sterilizing immune responses in its human host. A better understanding of this evasion will greatly benefit the production of anti-Chlamydia therapeutics and vaccination strategies. This minireview will discuss a single branch of the immune system, which activates in response to genital Chlamydia infection: so-called "cell-autonomous immunity" activated by the cytokine interferon-gamma. We will also discuss the mechanisms by which human and mouse-adapted Chlamydia species evade cell-autonomous immune responses in their native hosts. This minireview will examine five pathways of host defense and their evasion: (i) depletion of tryptophan and other nutrients, (ii) immunity-related GTPase-mediated defense, (iii) production of nitric oxide, (iv) IFNγ-induced cell death, and (v) RNF213-mediated destruction of inclusions.

Pregnancy Protects against Abnormal Gut Permeability Promoted via the Consumption of a High-Fat Diet in Mice

The consumption of large amounts of dietary fats and pregnancy are independent factors that can promote changes in gut permeability and the gut microbiome landscape. However, there is limited evidence regarding the impact of pregnancy on the regulation of such parameters in females fed a high-fat diet. Here, gut permeability and microbiome landscape were evaluated in a mouse model of diet-induced obesity in pregnancy. The results show that pregnancy protected against the harmful effects of the consumption of a high-fat diet as a disruptor of gut permeability; thus, there was a two-fold reduction in FITC-dextran passage to the bloodstream compared to non-pregnant mice fed a high-fat diet (p < 0.01). This was accompanied by an increased expression of gut barrier-related transcripts, particularly in the ileum. In addition, the beneficial effect of pregnancy on female mice fed the high-fat diet was accompanied by a reduced presence of bacteria belonging to the genus Clostridia, and by increased Lactobacillus murinus in the gut (p < 0.05). Thus, this study advances the understanding of how pregnancy can act during a short window of time, protecting against the harmful effects of the consumption of a high-fat diet by promoting an increased expression of transcripts encoding proteins involved in the regulation of gut permeability, particularly in the ileum, and promoting changes in the gut microbiome.

Programmed Cell Death in Unicellular Versus Multicellular Organisms

Programmed cell death (self-induced) is intrinsic to all cellular life forms, including unicellular organisms. However, cell death research has focused on animal models to understand cancer, degenerative disorders, and developmental processes. Recently delineated suicidal death mechanisms in bacteria and fungi have revealed ancient origins of animal cell death that are intertwined with immune mechanisms, allaying earlier doubts that self-inflicted cell death pathways exist in microorganisms. Approximately 20 mammalian death pathways have been partially characterized over the last 35 years. By contrast, more than 100 death mechanisms have been identified in bacteria and a few fungi in recent years. However, cell death is nearly unstudied in most human pathogenic microbes that cause major public health burdens. Here, we consider how the current understanding of programmed cell death arose through animal studies and how recently uncovered microbial cell death mechanisms in fungi and bacteria resemble and differ from mechanisms of mammalian cell death.

Japanese Encephalitis virus infection in astrocytes modulate microglial function: Correlation with inflammation and oxidative stress

BACKGROUND

Japanese Encephalitis Virus (JEV) is a neurotropic virus which has the propensity to infect neuronal and glial cells of the brain. Astrocyte-microglia crosstalk leading to the secretion of various factors plays a major role in controlling encephalitis in brain. This study focused on understanding the role of astrocytic mediators that further shaped the microglial response towards JEV infection.

METHODS

After establishing JEV infection in C8D1A (mouse astrocyte cell line) and primary astrocyte enriched cultures (PAEC), astrocyte supernatant was used for preparation of conditioned media. Astrocyte supernatant was treated with UV to inactivate JEV and the supernatant was added to N9 culture media in ratio 1:1 for preparation of conditioned media. N9 microglial cells post treatment with astrocyte conditioned media and JEV infection were checked for expression of various inflammatory genes by qRT-PCR, levels of secreted cytokines in N9 cell supernatant were checked by cytometric bead array. N9 cell lysates were checked for expression of proteins - pNF-κβ, IBA-1, NS3 and RIG-I by western blotting. Viral titers were measured in N9 supernatant by plaque assays. Immunocytochemistry experiments were done to quantify the number of infected microglial cells after astrocyte conditioned medium treatment. Expression of different antioxidant enzymes was checked in N9 cells by western blotting, levels of reactive oxygen species (ROS) was detected by fluorimetry using DCFDA dye.

RESULTS

N9 microglial cells post treatment with JEV-infected astrocyte conditioned media and JEV infection were activated, showed an upsurge in expression of inflammatory genes and cytokines both at the transcript and protein levels. These N9 cells showed a decrease in quantity of viral titers and associated viral proteins in comparison to control cells (not treated with conditioned media but infected with JEV). Also, N9 cells upon conditioned media treatment and JEV infection were more prone to undergo oxidative stress as observed by the decreased expression of antioxidant enzymes SOD-1, TRX-1 and increased secretion of reactive oxygen species (ROS).

CONCLUSION

Astrocytic mediators like TNF-α, MCP-1 and IL-6 influence microglial response towards JEV infection by promoting inflammation and oxidative stress in them. As a result of increased microglial inflammation and secretion of ROS, viral replication is lessened in conditioned media treated and JEV infected microglial cells as compared to control cells with no conditioned media treatment but only JEV infection.

Parkinson's disease kinase LRRK2 coordinates a cell-intrinsic itaconate-dependent defence pathway against intracellular Salmonella

Cell-intrinsic defences constitute the first line of defence against intracellular pathogens. The guanosine triphosphatase RAB32 orchestrates one such defence response against the bacterial pathogen Salmonella, through delivery of antimicrobial itaconate. Here we show that the Parkinson's disease-associated leucine-rich repeat kinase 2 (LRRK2) orchestrates this defence response by scaffolding a complex between RAB32 and aconitate decarboxylase 1, which synthesizes itaconate from mitochondrial precursors. Itaconate delivery to Salmonella-containing vacuoles was impaired and Salmonella replication increased in LRRK2-deficient cells. Loss of LRRK2 also restored virulence of a Salmonella mutant defective in neutralizing this RAB32-dependent host defence pathway in mice. Cryo-electron tomography revealed tether formation between Salmonella-containing vacuoles and host mitochondria upon Salmonella infection, which was significantly impaired in LRRK2-deficient cells. This positions LRRK2 centrally within a host defence mechanism, which may have favoured selection of a common familial Parkinson's disease mutant allele in the human population.

Crossing the Gateless Barriers: Factors Involved in the Movement of Circulative Bacteria Within Their Insect Vectors

Plant bacterial pathogens transmitted by hemipteran vectors pose a large threat to the agricultural industry worldwide. Although virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.

Ubiquitination and cell-autonomous immunity

Cell-autonomous immunity is the first line of defense by which cells recognize and contribute to eliminating invasive pathogens. It is composed of immune signaling networks that sense microbial pathogens, promote pathogen restriction, and stimulate their elimination, including host cell death. Ubiquitination is a pivotal orchestrator of these pathways, by changing the activity of signal transducers and effector proteins in an efficient way. In this review, we will focus on how ubiquitin connects the pathways that sense pathogens to the cellular responses to invaders and shed light on how ubiquitination impacts the microenvironment around the infected cell, stimulating the appropriate immune response. Finally, we discuss therapeutic options directed at favoring cell-autonomous immune responses to infection.

Atg8ylation as a host-protective mechanism against Mycobacterium tuberculosis

Nearly two decades have passed since the first report on autophagy acting as a cell-autonomous defense against Mycobacterium tuberculosis. This helped usher a new area of research within the field of host-pathogen interactions and led to the recognition of autophagy as an immunological mechanism. Interest grew in the fundamental mechanisms of antimicrobial autophagy and in the prophylactic and therapeutic potential for tuberculosis. However, puzzling in vivo data have begun to emerge in murine models of M. tuberculosis infection. The control of infection in mice affirmed the effects of certain autophagy genes, specifically ATG5, but not of other ATGs. Recent studies with a more complete inactivation of ATG genes now show that multiple ATG genes are indeed necessary for protection against M. tuberculosis. These particular ATG genes are involved in the process of membrane atg8ylation. Atg8ylation in mammalian cells is a broad response to membrane stress, damage and remodeling of which canonical autophagy is one of the multiple downstream outputs. The current developments clarify the controversies and open new avenues for both fundamental and translational studies.

TECPR1 conjugates LC3 to damaged endomembranes upon detection of sphingomyelin exposure

Invasive bacteria enter the cytosol of host cells through initial uptake into bacteria-containing vacuoles (BCVs) and subsequent rupture of the BCV membrane, thereby exposing to the cytosol intraluminal, otherwise shielded danger signals such as glycans and sphingomyelin. The detection of glycans by galectin-8 triggers anti-bacterial autophagy, but how cells sense and respond to cytosolically exposed sphingomyelin remains unknown. Here, we identify TECPR1 (tectonin beta-propeller repeat containing 1) as a receptor for cytosolically exposed sphingomyelin, which recruits ATG5 into an E3 ligase complex that mediates lipid conjugation of LC3 independently of ATG16L1. TECPR1 binds sphingomyelin through its N-terminal DysF domain (N'DysF), a feature not shared by other mammalian DysF domains. Solving the crystal structure of N'DysF, we identified key residues required for the interaction, including a solvent-exposed tryptophan (W154) essential for binding to sphingomyelin-positive membranes and the conjugation of LC3 to lipids. Specificity of the ATG5/ATG12-E3 ligase responsible for the conjugation of LC3 is therefore conferred by interchangeable receptor subunits, that is, the canonical ATG16L1 and the sphingomyelin-specific TECPR1, in an arrangement reminiscent of certain multi-subunit ubiquitin E3 ligases.

Dedicated bacterial esterases reverse lipopolysaccharide ubiquitylation to block immune sensing

Pathogenic bacteria have evolved diverse mechanisms to counteract cell-autonomous immunity, which otherwise guards both immune and non-immune cells from the onset of an infection1,2. The versatile immunity protein Ring finger protein 213 (RNF213)3-6 mediates the non-canonical ester-linked ubiquitylation of lipopolysaccharide (LPS), marking bacteria that sporadically enter the cytosol for destruction by antibacterial autophagy4. However, whether cytosol-adapted pathogens are ubiquitylated on their LPS and whether they escape RNF213-mediated immunity, remains unknown. Here we show that Burkholderia deubiquitylase (DUB), TssM7-9, is a potent esterase that directly reverses the ubiquitylation of LPS. Without TssM, cytosolic Burkholderia became coated in polyubiquitin and autophagy receptors in an RNF213-dependent fashion. Whereas the expression of TssM was sufficient to enable the replication of the non-cytosol adapted pathogen Salmonella, we demonstrate that Burkholderia has evolved a multi-layered defence system to proliferate in the host cell cytosol, including a block in antibacterial autophagy10-12. Structural analysis provided insight into the molecular basis of TssM esterase activity, allowing it to be uncoupled from isopeptidase function. TssM homologs conserved in another Gram-negative pathogen also reversed non-canonical LPS ubiquitylation, establishing esterase activity as a bacterial virulence mechanism to subvert host cell-autonomous immunity.