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Key Points Pathogenic bacteria evade host defences by subverting host signalling pathways in many different and sophisticated ways. An intriguing strategy used by several pathogens, mainly intracellular bacteria, is molecular or structural mimicry of host proteins. In the past few years, studies have revealed that some pathogenic bacteria secret specific proteins via their type 3 secretion systems (T3SSs) and type 4 secretion systems (T4SSs) to target host organelles. These proteins contain organelle localization signals that target the effectors, for example to the nucleus or the mitochondria. Legionella pneumophila , Chlamydia trachomatis and Burkholderia thailandensis secrete SET domain-containing proteins that encode histone methyltransferase activity to directly impose epigenetic changes on the chromatin landscape of the host cell, thus aiding bacterial intracellular replication. Listeria monocytogenes , pathogenic Escherichia coli strains and Shigella flexneri secrete specific effector proteins that change the levels of chromatin-binding proteins to indirectly alter the host chromatin to the advantage of the pathogen. Membrane dynamics, and in particular the eukaryotic secretory pathway, are targeted by bacterial pathogens to allow them to form a distinct replication-permissive vacuole inside the host cell. Legionella spp. and Brucella spp. target endoplasmic reticulum (ER)-derived vesicles and the retrograde traffic between the Golgi and the ER; Salmonella spp. and Chlamydia spp. interact with the trans -Golgi network or associated compartments. To target the ER or Golgi network, different secreted effectors evolved to specifically hijack RAB proteins and exploit phosphoinositide lipids, which are phosphorylated derivatives of phosphatidylinositol. Mitochondria are the power plants of the cell, but are also involved in essential cellular processes such as programmed cell death, calcium homeostasis, and the biosynthesis of amino acids, lipids and nucleotides. They also serve as hubs for innate immune signalling against viruses and bacteria. L. pneumophila , enteropathogenic E. coli , Vibrio cholerae and Anaplasma phagocytophilum have been reported to secrete diverse effectors that target mitochondria, mainly to inhibit inflammatory responses. Several bacterial pathogens have evolved the ability to subvert host cell functions. In this Review, Buchrieser and colleagues discuss the mechanisms used by bacteria to target eukaryotic organelles such as the nucleus, mitochondria, the endoplasmic reticulum and the Golgi apparatus, highlighting how these strategies potentiate bacterial infection. Many bacterial pathogens have evolved the ability to subvert and exploit host functions in order to enter and replicate in eukaryotic cells. For example, bacteria have developed specific mechanisms to target eukaryotic organelles such as the nucleus, the mitochondria, the endoplasmic reticulum and the Golgi apparatus. In this Review, we highlight the most recent advances in our understanding of the mechanisms that bacterial pathogens use to target these organelles. We also discuss how these strategies allow bacteria to manipulate host functions and to ultimately enable bacterial infection.

Legionella pneumophila is an intracellular bacterial pathogen that can cause a severe form of pneumonia in humans, a phenotype evolved through interactions with aquatic protozoa in the environment. Here, we show that L. pneumophila uses extracellular vesicles to translocate bacterial small RNAs (sRNAs) into host cells that act on host defence signalling pathways. The bacterial sRNA RsmY binds to the UTR of ddx58 (RIG-I encoding gene) and cRel , while tRNA-Phe binds ddx58 and irak1 collectively reducing expression of RIG-I, IRAK1 and cRel, with subsequent downregulation of IFN-β. Thus, RsmY and tRNA-Phe are bacterial trans-kingdom regulatory RNAs downregulating selected sensor and regulator proteins of the host cell innate immune response. This miRNA-like regulation of the expression of key sensors and regulators of immunity is a feature of L. pneumophila host-pathogen communication and likely represents a general mechanism employed by bacteria that interact with eukaryotic hosts. Legionella pneumophila expresses a range of bacterial determinants that mimic eukaryotic functions. Here the authors show small RNAs of L.pneumophila mimic eukaryotic microRNA and modulate the host response to infection.

Group 2 innate lymphoid cells (ILC2s) represent innate homologs of type 2 helper T cells (T H 2) that participate in immune defense and tissue homeostasis through production of type 2 cytokines. While T lymphocytes metabolically adapt to microenvironmental changes, knowledge of human ILC2 metabolism is limited, and its key regulators are unknown. Here, we show that circulating ‘naive’ ILC2s have an unexpected metabolic profile with a higher level of oxidative phosphorylation (OXPHOS) than natural killer (NK) cells. Accordingly, ILC2s are severely reduced in individuals with mitochondrial disease (MD) and impaired OXPHOS. Metabolomic and nutrient receptor analysis revealed ILC2 uptake of amino acids to sustain OXPHOS at steady state. Following activation with interleukin-33 (IL-33), ILC2s became highly proliferative, relying on glycolysis and mammalian target of rapamycin (mTOR) to produce IL-13 while continuing to fuel OXPHOS with amino acids to maintain cellular fitness and proliferation. Our results suggest that proliferation and function are metabolically uncoupled in human ILC2s, offering new strategies to target ILC2s in disease settings. ILC2 metabolism has been largely unexplored. Di Santo and colleagues examine metabolic profiles from naive and cytokine-activated ILC2s and find that IL-33-triggered ILC2s rely on distinct metabolic pathways to sustain proliferation and function.

Legionella pneumophila, the causative agent of Legionnaires’ disease, a severe pneumonia, injects via a type 4 secretion system (T4SS) more than 300 proteins into macrophages, its main host cell in humans. Certain of these proteins are implicated in reprogramming the metabolism of infected cells by reducing mitochondrial oxidative phosphorylation (OXPHOS) early after infection. Here. we show that despite reduced OXPHOS, the mitochondrial membrane potential (Δ ψ m ) is maintained during infection of primary human monocyte-derived macrophages (hMDMs). We reveal that L. pneumophila reverses the ATP-synthase activity of the mitochondrial F O F 1 -ATPase to ATP-hydrolase activity in a T4SS-dependent manner, which leads to a conservation of the Δ ψ m , preserves mitochondrial polarization, and prevents macrophage cell death. Analyses of T4SS effectors known to target mitochondrial functions revealed that Lp Spl is partially involved in conserving the Δ ψ m , but not LncP and MitF. The inhibition of the L. pneumophila -induced ‘reverse mode’ of the F O F 1 -ATPase collapsed the Δ ψ m and caused cell death in infected cells. Single-cell analyses suggested that bacterial replication occurs preferentially in hMDMs that conserved the Δ ψ m and showed delayed cell death. This direct manipulation of the mode of activity of the F O F 1 -ATPase is a newly identified feature of L. pneumophila allowing to delay host cell death and thereby to preserve the bacterial replication niche during infection.

Accurately tracking dynamic state transitions is crucial for modeling and predicting biological outcomes, as it captures heterogeneity of cellular responses. To build a model to predict bacterial infection in single cells, we have monitored in parallel infection progression and metabolic parameters in thousands of human primary macrophages infected with the intracellular pathogen Legionella pneumophila . By combining live-cell imaging with a tool for classifying cells based on infection outcomes, we were able to trace the specific evolution of metabolic parameters linked to distinct outcomes, such as bacterial replication or cell death. Our findings revealed that early changes in mitochondrial membrane potential (Δ ψ m) and in the production of mitochondrial Reactive Oxygen Species (mROS) are associated with macrophages that will later support bacterial growth. We used these data to train an explainable machine-learning model and achieved 83% accuracy in predicting L. pneumophila replication in single, infected cells before bacterial replication starts. Our results highlight backtracking as a valuable tool to gain new insights in host-pathogen interactions and identify early mitochondrial alterations as key predictive markers of success of bacterial infection. Computational models can help to explain the dynamics of cellular infection with pathogens. Here the authors use computational models to assess the single cell infection parameters of human macrophage infection with Legionella pneumophila and the effects on immunometabolism at a single cell and population level.

In this volume expert authors critically review the most important current research in this exciting field. Topics include: the seven most important bacterial secretion systems; within-host envelope remodelling; subversion of macrophages; pathogen manipulation of host autophagy; mechanisms involved in sensing and restriction of bacterial replication; mechanisms of evasion by Salmonella; evasion strategies of mycobacteria; and role of Cyclic di-GMP in virulence and evasion of plant immune systems. This text is essential reading for everyone involved in bacterial pathogenesis research and an invaluable reference work for those working in fields as diverse as medicine, biotechnology, agriculture, food and industry. A recommended acquisition for all microbiology laboratories.

Leptospira interrogans are pathogenic bacteria responsible for leptospirosis, a zoonosis impacting 1 million people per year worldwide. Leptospires can infect all vertebrates, but not all hosts develop similar symptoms. Human and cattle may suffer from mild to acute illnesses and are therefore considered as sensitive to leptospirosis. In contrast, mice and rats remain asymptomatic upon infection, although they get chronically colonized in their kidneys. Upon infection, leptospires are stealth pathogens that partially escape the recognition by the host innate immune system. Although leptospires are mainly extracellular bacteria, it was suggested that they could also replicate within macrophages. However, contradictory data in the current literature led us to reevaluate these findings. Using a gentamicin–protection assay coupled to high-content (HC) microscopy, we observed that leptospires were internalized in vivo upon peritoneal infection of C57BL/6J mice. Additionally, three different serotypes of pathogenic L. interrogans and the saprophytic L. biflexa actively infected both human (PMA differentiated) THP1 and mouse RAW264.7 macrophage cell lines. Next, we assessed the intracellular fate of leptospires using bioluminescent strains, and we observed a drastic reduction in the leptospiral intracellular load between 3 h and 6 h post-infection, suggesting that leptospires do not replicate within these cells. Surprisingly, the classical macrophage microbicidal mechanisms (phagocytosis, autophagy, TLR–mediated ROS, and RNS production) were not responsible for the observed decrease. Finally, we demonstrated that the reduction in the intracellular load was associated with an increase of the bacteria in the supernatant, suggesting that leptospires exit both human and murine macrophages. Overall, our study reevaluated the intracellular fate of leptospires and favors an active entrance followed by a rapid exit, suggesting that leptospires do not have an intracellular lifestyle in macrophages.

A subset of neurons in the normal vertebrate nervous system contains double the normal amount of DNA in their nuclei. These neurons are all thought to derive from aberrant mitoses in neuronal precursor cells. Here we show that endogenous NGF induces DNA replication in a subpopulation of differentiating chick retinal ganglion cells that express both the neurotrophin receptor p75 and the E2F1 transcription factor, but that lack the retinoblastoma protein. Many of these neurons avoid G2/M transition and remain alive in the retina as tetraploid cells with large cell somas and extensive dendritic trees, and most of them express β2 nicotinic acetylcholine receptor subunits, a specific marker of retinal ganglion cells innervating lamina F in the stratum-griseum-et-fibrosum-superficiale of the tectal cortex. Tetraploid neurons were also observed in the adult mouse retina. Thus, a developmental program leading to somatic tetraploidy in specific retinal neurons exists in vertebrates. This program might occur in other vertebrate neurons during normal or pathological situations.

Retinal ganglion cells (RGCs) are neurons that relay visual signals from the retina to the brain. The RGC cell bodies reside in the retina and their fibers form the optic nerve. Full transection (axotomy) of the optic nerve is an extra-retinal injury model of RGC degeneration. Optic nerve transection permits time-kinetic studies of neurodegenerative mechanisms in neurons and resident glia of the retina, the early events of which are reported here. One day after injury, and before atrophy of RGC cell bodies was apparent, glia had increased levels of phospho-Akt, phospho-S6, and phospho-ERK1/2; however, these signals were not detected in injured RGCs. Three days after injury there were increased levels of phospho-Rb and cyclin A proteins detected in RGCs, whereas these signals were not detected in glia. DNA hyperploidy was also detected in RGCs, indicative of cell cycle re-entry by these post-mitotic neurons. These events culminated in RGC death, which is delayed by pharmacological inhibition of the MAPK/ERK pathway. Our data show that a remote injury to RGC axons rapidly conveys a signal that activates retinal glia, followed by RGC cell cycle re-entry, DNA hyperploidy, and neuronal death that is delayed by preventing glial MAPK/ERK activation. These results demonstrate that complex and variable neuro-glia interactions regulate healthy and injured states in the adult mammalian retina.

Studies here respond to two long-standing questions: Are human \"pre/pro-B\" CD34⁺CD10⁻CD19⁺ and \"common lymphoid progenitor (CLP)/early-B\" CD34⁺CD10⁺CD19⁻ alternate precursors to \"pro-B\" CD34⁺CD19⁺CD10⁺ cells, and do the pro-B cells that arise from these progenitors belong to the same or distinct B-cell development pathways? Using flow cytometry, gene expression profiling, and Ig VH-D-JH sequencing, we monitor the initial 10 generations of development of sorted cord blood CD34highLineage⁻ pluripotential progenitors growing in bone marrow S17 stroma cocultures. We show that (i) multipotent progenitors (CD34⁺CD45RA⁺CD10⁻CD19⁻) directly generate an initial wave of Pax5⁺TdT⁻ \"unilineage\" pre/pro-B cells and a later wave of \"multilineage\" CLP/early-B cells and (ii) the cells generated in these successive stages act as precursors for distinct pro-B cells through two independent layered pathways. Studies by others have tracked the origin of B-lineage leukemias in elderly mice to the mouse B-1a pre/pro-B lineage, which lacks the TdT activity that diversifies the VH-D-JH Ig heavy chain joints found in the early-B or B-2 lineage. Here, we show a similar divergence in human B-cell development pathways between the Pax5⁺TdT⁻ pre/pro-B differentiation pathway that gives rise to infant B-lineage leukemias and the early-B pathway.