Alpha1-antitrypsin impacts innate host-pathogen interactions with Candida albicans by stimulating fungal filamentation

Our immune system possesses sophisticated mechanisms to cope with invading microorganisms, while pathogens evolve strategies to deal with threats imposed by host immunity. Human plasma protein α1-antitrypsin (AAT) exhibits pleiotropic immune-modulating properties by both preventing immunopathology and improving antimicrobial host defence. Genetic associations suggested a role for AAT in candidemia, the most frequent fungal blood stream infection in intensive care units, yet little is known about how AAT influences interactions between Candida albicans and the immune system. Here, we show that AAT differentially impacts fungal killing by innate phagocytes. We observed that AAT induces fungal transcriptional reprogramming, associated with cell wall remodelling and downregulation of filamentation repressors. At low concentrations, the cell-wall remodelling induced by AAT increased immunogenic β-glucan exposure and consequently improved fungal clearance by monocytes. Contrastingly, higher AAT concentrations led to excessive C. albicans filamentation and thus promoted fungal immune escape from monocytes and macrophages. This underscores that fungal adaptations to the host protein AAT can differentially define the outcome of encounters with innate immune cells, either contributing to improved immune recognition or fungal immune escape.

When the microbiome shapes the host: immune evolution implications for infectious disease

The microbiome includes both 'mutualist' and 'pathogen' microbes, regulated by the same innate immune architecture. A major question has therefore been: how do hosts prevent pathogenic infections while maintaining beneficial microbes? One idea suggests hosts can selectively activate innate immunity upon pathogenic infection, but not mutualist colonization. Another idea posits that hosts can selectively attack pathogens, but not mutualists. Here I review evolutionary principles of microbe recognition and immune activation, and reflect on newly observed immune effector-microbe specificity perhaps supporting the latter idea. Recent work in Drosophila has found a surprising importance for single antimicrobial peptides in combatting specific ecologically relevant microbes. The developing picture suggests these effectors have evolved for this purpose. Other defence responses like reactive oxygen species bursts can also be uniquely effective against specific microbes. Signals in other model systems including nematodes, Hydra, oysters, and mammals, suggest that effector-microbe specificity may be a fundamental principle of host-pathogen interactions. I propose this effector-microbe specificity stems from weaknesses of the microbes themselves: if microbes have intrinsic weaknesses, hosts can evolve effectors that exploit those weaknesses. I define this host-microbe relationship as 'the Achilles principle of immune evolution'. Incorporating this view helps interpret why some host-microbe interactions develop in a coevolutionary framework (e.g. Red Queen dynamics), or as a one-sided evolutionary response. This clarification should be valuable to better understand the principles behind host susceptibilities to infectious diseases. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

How infection-triggered pathobionts influence virulence evolution

Host-pathogen interactions can be influenced by the host microbiota, as the microbiota can facilitate or prevent pathogen infections. In addition, members of the microbiota can become virulent. Such pathobionts can cause co-infections when a pathogen infection alters the host immune system and triggers dysbiosis. Here we performed a theoretical investigation of how pathobiont co-infections affect the evolution of pathogen virulence. We explored the possibility that the likelihood of pathobiont co-infection depends on the evolving virulence of the pathogen. We found that, in contrast to the expectation from classical theory, increased virulence is not always selected for. For an increasing likelihood of co-infection with increasing pathogen virulence, we found scenario-specific selection for either increased or decreased virulence. Evolutionary changes, however, in pathogen virulence do not always translate into similar changes in combined virulence of the pathogen and the pathobiont. Only in one of the scenarios where pathobiont co-infection is triggered above a specific virulence level we found a reduction in combined virulence. This was not the case when the probability of pathobiont co-infection linearly increased with pathogen virulence. Taken together, our study draws attention to the possibility that host-microbiota interactions can be both the driver and the target of pathogen evolution. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

The germ theory revisited: A noncentric view on infection outcome

The germ theory states that pathogenic microorganisms are responsible for causing infectious diseases. The theory is inherently microbe-centric and does not account for variability in disease severity among individuals and asymptomatic carriership-two phenomena indicating an important role for host variability in infection outcome. The basic tenet of the germ theory was recently challenged, and a radically host-centric paradigm referred to as the "full-blown host theory" was proposed. According to this view, the pathogen is reduced to a passive environmental trigger, and the development of disease is instead due to pre-existing immunodeficiencies of the host. Here, we consider the factors that determine disease severity using established knowledge concerning evolutionary biology, microbial pathogenesis, and host-pathogen interactions. We note that the available data support a noncentric view that recognizes key roles for both the causative microbe and the host in dictating infection outcome.

Three new discovery effector proteins from Candidatus Liberibacter asiaticus psy62 inhibit plant defense through interaction with AtCAT3 and AtGAPA

KEY MESSAGE

We identify three SDEs that inhibiting host defence from Candidatus Liberibacter asiaticus psy62, which is an important supplement to the pathogenesis of HLB. Candidatus Liberibacter asiaticus (CLas) is the main pathogen of citrus Huanglongbing (HLB). 38 new possible sec-dependent effectors (SDEs) of CLas psy62 were predicted by updated predictor SignalP 5.0, which 12 new SDEs were found using alkaline phosphate assay. Among them, SDE4310, SDE4435 and SDE4955 inhibited hypersensitivity reactions (HR) in Arabidopsis thaliana (Arabidopsis, At) and Nicotiana benthamiana leaves induced by pathogens, which lead to a decrease in cell death and reactive oxygen species (ROS) accumulation. And the expression levels of SDE4310, SDE4435, and SDE4955 genes elevated significantly in mild symptom citrus leaves. When SDE4310, SDE4435 and SDE4955 were overexpressed in Arabidopsis, HR pathway key genes pathogenesis-related 2 (PR2), PR5, nonexpressor of pathogenesis-related 1 (NPR1) and isochorismate synthase 1 (ICS1) expression significantly decreased and the growth of pathogen was greatly increased relative to control with Pst DC3000/AvrRps4 treatment. Our findings also indicated that SDE4310, SDE4435 and SDE4955 interacted with AtCAT3 (catalase 3) and AtGAPA (glyceraldehyde-3-phosphate dehydrogenase A). In conclusion, our results suggest that SDE4310, SDE4435 and SDE4955 are CLas psy62 effector proteins that may have redundant functions. They inhibit ROS burst and cell death by interacting with AtCAT3 and AtGAPA to negatively regulate host defense.

A compendium of multi-omics data illuminating host responses to lethal human virus infections

Human infections caused by viral pathogens trigger a complex gamut of host responses that limit disease, resolve infection, generate immunity, and contribute to severe disease or death. Here, we present experimental methods and multi-omics data capture approaches representing the global host response to infection generated from 45 individual experiments involving human viruses from the Orthomyxoviridae, Filoviridae, Flaviviridae, and Coronaviridae families. Analogous experimental designs were implemented across human or mouse host model systems, longitudinal samples were collected over defined time courses, and global multi-omics data (transcriptomics, proteomics, metabolomics, and lipidomics) were acquired by microarray, RNA sequencing, or mass spectrometry analyses. For comparison, we have included transcriptomics datasets from cells treated with type I and type II human interferon. Raw multi-omics data and metadata were deposited in public repositories, and we provide a central location linking the raw data with experimental metadata and ready-to-use, quality-controlled, statistically processed multi-omics datasets not previously available in any public repository. This compendium of infection-induced host response data for reuse will be useful for those endeavouring to understand viral disease pathophysiology and network biology.

The 'ins and outs' of Brucella intracellular journey

Members of the genus Brucella are the causative agents of brucellosis, a worldwide zoonosis affecting wild and domestic animals and humans. These facultative intracellular pathogens cause long-lasting chronic infections by evolving sophisticated strategies to counteract, evade, or subvert host bactericidal mechanisms in order to establish a secure replicative niche necessary for their survival. In this review, we present recent findings on selected Brucella effectors to illustrate how this pathogen modulates host cell signaling pathways to gain control of the vacuole, promote the formation of a safe intracellular replication niche, alter host cell metabolism to its advantage, and exploit various cellular pathways to ensure egress from the infected cell.

Pathogen evolution following spillover from a resident to a migrant host population depends on interactions between host pace of life and tolerance to infection

Changes to migration routes and phenology create novel contact patterns among hosts and pathogens. These novel contact patterns can lead to pathogens spilling over between resident and migrant populations. Predicting the consequences of such pathogen spillover events requires understanding how pathogen evolution depends on host movement behaviour. Following spillover, pathogens may evolve changes in their transmission rate and virulence phenotypes because different strategies are favoured by resident and migrant host populations. There is conflict in current theoretical predictions about what those differences might be. Some theory predicts lower pathogen virulence and transmission rates in migrant populations because migrants have lower tolerance to infection. Other theoretical work predicts higher pathogen virulence and transmission rates in migrants because migrants have more contacts with susceptible hosts. We aim to understand how differences in tolerance to infection and host pace of life act together to determine the direction of pathogen evolution following pathogen spillover from a resident to a migrant population. We constructed a spatially implicit model in which we investigate how pathogen strategy changes following the addition of a migrant population. We investigate how differences in tolerance to infection and pace of life between residents and migrants determine the effect of spillover on pathogen evolution and host population size. When the paces of life of the migrant and resident hosts are equal, larger costs of infection in the migrants lead to lower pathogen transmission rate and virulence following spillover. When the tolerance to infection in migrant and resident populations is equal, faster migrant paces of life lead to increased transmission rate and virulence following spillover. However, the opposite can also occur: when the migrant population has lower tolerance to infection, faster migrant paces of life can lead to decreases in transmission rate and virulence. Predicting the outcomes of pathogen spillover requires accounting for both differences in tolerance to infection and pace of life between populations. It is also important to consider how movement patterns of populations affect host contact opportunities for pathogens. These results have implications for wildlife conservation, agriculture and human health.

The impact of volatiles on tick-host interaction and vector competence

Ticks are obligatory hematophagous arachnids, serving as vectors for a wide array of pathogens that can be transmitted to humans or animals. The ability of tick-borne pathogens to maintain within natural reservoirs is intricately influenced by the attractiveness of ticks to their animal hosts, including humans. However, the complex dynamics of tick behavior and host-seeking strategies remain understudied. This review aims to summarize the impact of volatiles or odors on tick behavior and vector competence. Our literature review has identified a selection of compounds, such as 1-octen-3-ol, hexanal, heptanal, nonanal, 6-methyl-5-hepten-2-one, acetone, and octanal, as having the potential to impact both ticks' and mosquitos' behaviors. In addition, carbon dioxide (CO2) is a universal attractant for hematophagous arthropods. Moreover, we have gathered some clues indicating that volatiles emitted by infected animal hosts might play a role in the transmission of tick-borne pathogens. Nonetheless, our understanding of this phenomenon remains largely inadequate, particularly with regarding to whether the tick microbiome or the skin microbiota of the feeding mammals, including humans, can actively modulate tick-host-seeking behavior. Further investigations in this emerging field hold immense promise for the development of innovative strategies aimed at controlling vectors and curtailing the spread of tick-borne diseases.

Unraveling pathogen deceptive disguise: from modules to mimicry

Pathogens rely on their effector proteins to colonize host plants. These effectors have diverse functions. A recent study by Li et al. highlights the significance of protein modularity in generating functional diversity among Phytophthora effectors. It underscores the sophisticated tactics that phytopathogens adopt to alter host cellular processes.

Mapping of the influenza A virus genome RNA structure and interactions reveals essential elements of viral replication

Influenza A virus (IAV) represents a constant public health threat. The single-stranded, segmented RNA genome of IAV is replicated in host cell nuclei as a series of 8 ribonucleoprotein complexes (vRNPs) with RNA structures known to exert essential function to support viral replication. Here, we investigate RNA secondary structures and RNA interactions networks of the IAV genome and construct an in vivo structure model for each of the 8 IAV genome segments. Our analyses reveal an overall in vivo and in virio resemblance of the IAV genome conformation but also wide disparities among long-range and intersegment interactions. Moreover, we identify a long-range RNA interaction that exerts an essential role in genome packaging. Disrupting this structure displays reduced infectivity, attenuating virus pathogenicity in mice. Our findings characterize the in vivo RNA structural landscape of the IAV genome and reveal viral RNA structures that can be targeted to develop antiviral interventions.

Metabolomics approach to understand molecular mechanisms involved in fungal pathogen-citrus pathosystems

Citrus is a crucial crop with a significant economic impact globally. However, postharvest decay caused by fungal pathogens poses a considerable threat, leading to substantial financial losses. Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii and Phyllosticta citricarpa are the main fungal pathogens, causing green mold, blue mold, sour rot and citrus black spot diseases, respectively. The use of chemical fungicides as a control strategy in citrus raises concerns about food and environmental safety. Therefore, understanding the molecular basis of host-pathogen interactions is essential to find safer alternatives. This review highlights the potential of the metabolomics approach in the search for bioactive compounds involved in the pathogen-citrus interaction, and how the integration of metabolomics and genomics contributes to the understanding of secondary metabolites associated with fungal virulence and the fungal infection mechanisms. Our goal is to provide a pipeline combining metabolomics and genomics that can effectively guide researchers to perform studies aiming to contribute to the understanding of the fundamental chemical and biochemical aspects of pathogen-host interactions, in order to effectively develop new alternatives for fungal diseases in citrus cultivation. We intend to inspire the scientific community to question unexplored biological systems, and to employ diverse analytical approaches and metabolomics techniques to address outstanding questions about the non-studied pathosystems from a chemical biology perspective.

Pathogen evolution: Protective microbes act as a double-edged sword

Vaccines and infection can sometimes cause incomplete immunity, which allows for pathogen re-infection with decreased disease severity but also contributes to the evolution of pathogen virulence. A new study demonstrates that incomplete immunity from resident protective microbes results in similar evolutionary trajectories.

In silico agent-based modeling approach to characterize multiple in vitro tuberculosis infection models

In vitro models of Mycobacterium tuberculosis (Mtb) infection are a valuable tool for examining host-pathogen interactions and screening drugs. With the development of more complex in vitro models, there is a need for tools to help analyze and integrate data from these models. To this end, we introduce an agent-based model (ABM) representation of the interactions between immune cells and bacteria in an in vitro setting. This in silico model was used to simulate both traditional and spheroid cell culture models by changing the movement rules and initial spatial layout of the cells in accordance with the respective in vitro models. The traditional and spheroid simulations were calibrated to published experimental data in a paired manner, by using the same parameters in both simulations. Within the calibrated simulations, heterogeneous outputs are seen for bacterial count and T cell infiltration into the macrophage core of the spheroid. The simulations also predict that equivalent numbers of activated macrophages do not necessarily result in similar bacterial reductions; that host immune responses can control bacterial growth in both spheroid structure dependent and independent manners; that STAT1 activation is the limiting step in macrophage activation in spheroids; and that drug screening and macrophage activation studies could have different outcomes depending on the in vitro culture used. Future model iterations will be guided by the limitations of the current model, specifically which parts of the output space were harder to reach. This ABM can be used to represent more in vitro Mtb infection models due to its flexible structure, thereby accelerating in vitro discoveries.

A fungal core effector exploits the OsPUX8B.2-OsCDC48-6 module to suppress plant immunity

Proteins containing a ubiquitin regulatory X (UBX) domain are cofactors of Cell Division Cycle 48 (CDC48) and function in protein quality control. However, whether and how UBX-containing proteins participate in host-microbe interactions remain unclear. Here we show that MoNLE1, an effector from the fungal pathogen Magnaporthe oryzae, is a core virulence factor that suppresses rice immunity by specifically interfering with OsPUX8B.2. The UBX domain of OsPUX8B.2 is required for its binding to OsATG8 and OsCDC48-6 and controls its 26 S proteasome-dependent stability. OsPUX8B.2 and OsCDC48-6 positively regulate plant immunity against blast fungus, while the high-temperature tolerance heat-shock protein OsBHT, a putative cytoplasmic substrate of OsPUX8B.2-OsCDC48-6, negatively regulates defense against blast infection. MoNLE1 promotes the nuclear migration and degradation of OsPUX8B.2 and disturbs its association with OsBHT. Given the high conservation of MoNLE1 among fungal isolates, plants with broad and durable blast resistance might be generated by engineering intracellular proteins resistant to MoNLE1.

Activation of protein kinase receptor (PKR) plays a pro-viral role in mammarenavirus-infected cells

Many viruses, including mammarenaviruses, have evolved mechanisms to counteract different components of the host cell innate immunity, which is required to facilitate robust virus multiplication. The double-stranded RNA (dsRNA) sensor protein kinase receptor (PKR) pathway plays a critical role in the cell anti-viral response. Whether PKR can restrict the multiplication of the Old World mammarenavirus lymphocytic choriomeningitis virus (LCMV) and the mechanisms by which LCMV may counteract the anti-viral functions of PKR have not yet been investigated. Here we present evidence that LCMV infection results in very limited levels of PKR activation, but LCMV multiplication is enhanced in the absence of PKR. In contrast, infection with a recombinant LCMV with a mutation affecting the 3'-5' exonuclease (ExoN) activity of the viral nucleoprotein resulted in robust PKR activation in the absence of detectable levels of dsRNA, which was associated with severely restricted virus multiplication that was alleviated in the absence of PKR. However, pharmacological inhibition of PKR activation resulted in reduced levels of LCMV multiplication. These findings uncovered a complex role of the PKR pathway in LCMV-infected cells involving both pro- and anti-viral activities.IMPORTANCEAs with many other viruses, the prototypic Old World mammarenavirus LCMV can interfere with the host cell innate immune response to infection, which includes the dsRNA sensor PKR pathway. A detailed understanding of LCMV-PKR interactions can provide novel insights about mammarenavirus-host cell interactions and facilitate the development of effective anti-viral strategies against human pathogenic mammarenaviruses. In the present work, we present evidence that LCMV multiplication is enhanced in PKR-deficient cells, but pharmacological inhibition of PKR activation unexpectedly resulted in severely restricted propagation of LCMV. Likewise, we document a robust PKR activation in LCMV-infected cells in the absence of detectable levels of dsRNA. Our findings have revealed a complex role of the PKR pathway during LCMV infection and uncovered the activation of PKR as a druggable target for the development of anti-viral drugs against human pathogenic mammarenaviruses.

An ELISA-based method for Galleria mellonella apolipophorin-III quantification

Mammalian models, such as murine, are used widely in pathophysiological studies because they have a high degree of similarity in body temperature, metabolism, and immune response with humans. However, non-vertebrate animal models have emerged as alternative models to study the host-pathogen interaction with minimal ethical concerns. Galleria mellonella is an alternative model that has proved useful in studying the interaction of the host with either bacteria or fungi, performing drug testing, and assessing the immunological response to different microorganisms. The G. mellonella immune response includes cellular and humoral components with structural and functional similarities to the immune effectors found in higher vertebrates, such as humans. An important humoral effector stimulated during infections is apolipophorin III (apoLp-III), an opsonin characterized by its lipid and carbohydrate-binding properties that participate in lipid transport, as well as immunomodulatory activity. Despite some parameters, such as the measurement of phenoloxidase activity, melanin production, hemocytes counting, and expression of antimicrobial peptides genes are already used to assess the G. mellonella immune response to pathogens with different virulence degrees, the apoLp-III quantification remains to be a parameter to assess the immune response in this invertebrate. Here, we propose an immunological tool based on an enzyme-linked immunosorbent assay that allows apoLp-III quantification in the hemolymph of larvae challenged with pathogenic agents. We tested the system with hemolymph coming from larvae infected with Escherichia coli, Candida albicans, Sporothrix schenckii, Sporothrix globosa, and Sporothrix brasiliensis. The results revealed significantly higher concentrations of apoLp-III when each microbial species was inoculated, in comparison with untouched larvae, or inoculated with phosphate-buffered saline. We also demonstrated that the apoLp-III levels correlated with the strains' virulence, which was already reported. To our knowledge, this is one of the first attempts to quantify apoLp-III, using a quick and easy-to-use serological technique.

Cell death induction facilitates egress of Coxiella burnetii from infected host cells at late stages of infection

Intracellular bacteria have evolved mechanisms to invade host cells, establish an intracellular niche that allows survival and replication, produce progeny, and exit the host cell after completion of the replication cycle to infect new target cells. Bacteria exit their host cell by (i) initiation of apoptosis, (ii) lytic cell death, and (iii) exocytosis. While bacterial egress is essential for bacterial spreading and, thus, pathogenesis, we currently lack information about egress mechanisms for the obligate intracellular pathogen C. burnetii, the causative agent of the zoonosis Q fever. Here, we demonstrate that C. burnetii inhibits host cell apoptosis early during infection, but induces and/or increases apoptosis at later stages of infection. Only at later stages of infection did we observe C. burnetii egress, which depends on previously established large bacteria-filled vacuoles and a functional intrinsic apoptotic cascade. The released bacteria are not enclosed by a host cell membrane and can infect and replicate in new target cells. In summary, our data argue that C. burnetii egress in a non-synchronous way at late stages of infection. Apoptosis-induction is important for C. burnetii egress, but other pathways most likely contribute.

The functional role of Daphnia in the host-pathogen interaction of crayfish and the crayfish plague disease agent (Aphanomyces astaci)

Pathogen spores have been recognized as prey with implications for resource dynamics, energy transfer and disease transmission. In aquatic ecosystems, filter-feeders are able to consume such motile forms of pathogens that can cause severe disease in susceptible hosts. The interactions between European crayfish and the crayfish plague pathogen Aphanomyces astaci are of particular conservation interest. In this study, we aim to evaluate the ecological interactions between Ap. astaci, its host Astacus astacus and individuals of the genus Daphnia, filter-feeding planktonic crustaceans. Our focus was on the consumption of the motile zoospores by Daphnia individuals, but we also considered the potential of Daphnia as non-target hosts. We conducted a series of infection and life-history experiments with Ap. astaci, three Daphnia species (D. magna, D. galeata, and D. pulex) and the noble crayfish As. astacus. We did not observe any lethal effects in the infection experiments involving Ap. astaci and Daphnia. Only D. pulex showed differences in some life-history traits. The feeding experiment using the motile zoospores of Ap. astaci as alternative food source or as supplement to different amounts of algal food revealed their nutritional value: D. magna individuals survived, grew, and reproduced on a zoospore diet alone. When zoospores were supplemented to the regular algal diet, all life-history parameters have been significantly improved. However, this successful consumption of zoospores did not result in a reduced mortality of the susceptible crayfish As. astacus during the infection experiment. Nevertheless, the pathogen load of Ap. astaci in the tissues of As. astacus was significantly reduced as a consequence of the feeding activity of Daphnia. Our results indicate that an abundant filter-feeding community can reduce the amount of infective zoospores in the water body and thus be beneficial to susceptible crayfish hosts, potentially acting as a general buffer against zoospore-transmitted diseases in lentic waters.

Pseudomonas aeruginosa outer membrane vesicle-packed sRNAs can enter host cells and regulate innate immune responses

Bacterial outer membrane vesicles (OMVs) can package and deliver virulence factors into host cells, which is an important mechanism mediating host-pathogen interactions. It has been reported that small RNAs (sRNAs) can be packed into OMVs with varying relative abundance, which might affect the function and/or stability of host mRNAs. In this study, we used OptiPrep density gradient ultra-high-speed centrifugation to purify OMVs from Pseudomonas aeruginosa. Next, the sequences and abundance of sRNAs were detected by using Small RNA-Seq. In particular, sRNA4518698, sRNA2316613 and sRNA809738 were the three most abundant sRNAs in OMVs, which are all fragments of P. aeruginosa non-coding RNAs. sRNAs were shielded within the interior of OMVs and remained resistant to external RNase cleavage. The miRanda and RNAhybrid analysis demonstrated that those sRNAs could target a large number of host mRNAs, which were enriched in host immune responses by the functions of GO and KEGG enrichment. Experimentally, we demonstrated that the transfection of synthetic sRNA4518698, sRNA2316613, or sRNA809738 could reduce the expression of innate immune response genes in RAW264.7 cells. Together, we demonstrated that P. aeruginosa OMVs sRNAs can regulate innate immune responses. This study uncovered a mechanism in which the OMVs regulate host responses by transferring bacterial sRNAs.

Border Control: Manipulation of the Host-Pathogen Interface by Perihaustorial Oomycete Effectors

Filamentous plant pathogens, including fungi and oomycetes, cause some of the most devastating plant diseases. These organisms serve as ideal models for understanding the intricate molecular interplay between plants and the invading pathogens. Filamentous pathogens secrete effector proteins via haustoria, specialized structures for infection and nutrient uptake, to suppress the plant immune response and to reprogram plant metabolism. Recent advances in cell biology have provided crucial insights into the biogenesis of the extrahaustorial membrane and the redirection of host endomembrane trafficking toward this interface. Functional studies have shown that an increasing number of oomycete effectors accumulate at the perihaustorial interface to subvert plant focal immune responses, with a particular convergence on targets involved in host endomembrane trafficking. In this review, we summarize the diverse mechanisms of perihaustorial effectors from oomycetes and pinpoint pressing questions regarding their role in manipulating host defense and metabolism at the haustorial interface. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

"We've got to get out"-Strategies of human pathogenic fungi to escape from phagocytes

Human fungal pathogens are a deadly and underappreciated risk to global health that most severely affect immunocompromised individuals. A virulence attribute shared by some of the most clinically relevant fungal species is their ability to survive inside macrophages and escape from these immune cells. In this review, we discuss the mechanisms behind intracellular survival and elaborate how escape is mediated by lytic and non-lytic pathways as well as strategies to induce programmed host cell death. We also discuss persistence as an alternative to rapid host cell exit. In the end, we address the consequences of fungal escape for the host immune response and provide future perspectives for research and development of targeted therapies.

Host cell environments and antibiotic efficacy in tuberculosis

The aetiologic agent of tuberculosis (TB), Mycobacterium tuberculosis (Mtb), can survive, persist, and proliferate in a variety of heterogeneous subcellular compartments. Therefore, TB chemotherapy requires antibiotics crossing multiple biological membranes to reach distinct subcellular compartments and target these bacterial populations. These compartments are also dynamic, and our understanding of intracellular pharmacokinetics (PK) often represents a challenge for antitubercular drug development. In recent years, the development of high-resolution imaging approaches in the context of host-pathogen interactions has revealed the intracellular distribution of antibiotics at a new level, yielding discoveries with important clinical implications. In this review, we describe the current knowledge regarding cellular PK of antibiotics and the complexity of drug distribution within the context of TB. We also discuss the recent advances in quantitative imaging and highlight their applications for drug development in the context of how intracellular environments and microbial localisation affect TB treatment efficacy.

Extracellular vimentin as a modulator of the immune response and an important player during infectious diseases

Vimentin, an intermediate filament protein primarily recognized for its intracellular role in maintaining cellular structure, has recently garnered increased attention and emerged as a pivotal extracellular player in immune regulation and host-pathogen interactions. While the functions of extracellular vimentin were initially overshadowed by its cytoskeletal role, accumulating evidence now highlights its significance in diverse physiological and pathological events. This review explores the multifaceted role of extracellular vimentin in modulating immune responses and orchestrating interactions between host cells and pathogens. It delves into the mechanisms underlying vimentin's release into the extracellular milieu, elucidating its unconventional secretion pathways and identifying critical molecular triggers. In addition, the future perspectives of using extracellular vimentin in diagnostics and as a target protein in the treatment of diseases are discussed.

Pathogenic Rickettsia spp. as emerging models for bacterial biology

Our understanding of free-living bacterial models like Escherichia coli far outpaces that of obligate intracellular bacteria, which cannot be cultured axenically. All obligate intracellular bacteria are host-associated, and many cause serious human diseases. Their constant exposure to the distinct biochemical niche of the host has driven the evolution of numerous specialized bacteriological and genetic adaptations, as well as innovative molecular mechanisms of infection. Here, we review the history and use of pathogenic Rickettsia species, which cause an array of vector-borne vascular illnesses, as model systems to probe microbial biology. Although many challenges remain in our studies of these organisms, the rich pathogenic and biological diversity of Rickettsia spp. constitutes a unique backdrop to investigate how microbes survive and thrive in host and vector cells. We take a bacterial-focused perspective and highlight emerging insights that relate to new host-pathogen interactions, bacterial physiology, and evolution. The transformation of Rickettsia spp. from pathogens to models demonstrates how recalcitrant microbes may be leveraged in the lab to tap unmined bacterial diversity for new discoveries. Rickettsia spp. hold great promise as model systems not only to understand other obligate intracellular pathogens but also to discover new biology across and beyond bacteria.

Alternative translation contributes to the generation of a cytoplasmic subpopulation of the Junín virus nucleoprotein that inhibits caspase activation and innate immunity

The highly pathogenic arenavirus, Junín virus (JUNV), expresses three truncated alternative isoforms of its nucleoprotein (NP), i.e., NP53kD, NP47kD, and NP40kD. While both NP47kD and NP40kD have been previously shown to be products of caspase cleavage, here, we show that expression of the third isoform NP53kD is due to alternative in-frame translation from M80. Based on this information, we were able to generate recombinant JUNVs lacking each of these isoforms. Infection with these mutants revealed that, while all three isoforms contribute to the efficient control of caspase activation, NP40kD plays the predominant role. In contrast to full-length NP (i.e., NP65kD), which is localized to inclusion bodies, where viral RNA synthesis takes place, the loss of portions of the N-terminal coiled-coil region in these isoforms leads to a diffuse cytoplasmic distribution and a loss of function in viral RNA synthesis. Nonetheless, NP53kD, NP47kD, and NP40kD all retain robust interferon antagonistic and 3'-5' exonuclease activities. We suggest that the altered localization of these NP isoforms allows them to be more efficiently targeted by activated caspases for cleavage as decoy substrates, and to be better positioned to degrade viral double-stranded (ds)RNA species that accumulate in the cytoplasm during virus infection and/or interact with cytosolic RNA sensors, thereby limiting dsRNA-mediated innate immune responses. Taken together, this work provides insight into the mechanism by which JUNV leverages apoptosis during infection to generate biologically distinct pools of NP and contributes to our understanding of the expression and biological relevance of alternative protein isoforms during virus infection.IMPORTANCEA limited coding capacity means that RNA viruses need strategies to diversify their proteome. The nucleoprotein (NP) of the highly pathogenic arenavirus Junín virus (JUNV) produces three N-terminally truncated isoforms: two (NP47kD and NP40kD) are known to be produced by caspase cleavage, while, here, we show that NP53kD is produced by alternative translation initiation. Recombinant JUNVs lacking individual NP isoforms revealed that all three isoforms contribute to inhibiting caspase activation during infection, but cleavage to generate NP40kD makes the biggest contribution. Importantly, all three isoforms retain their ability to digest double-stranded (ds)RNA and inhibit interferon promoter activation but have a diffuse cytoplasmic distribution. Given the cytoplasmic localization of both aberrant viral dsRNAs, as well as dsRNA sensors and many other cellular components of innate immune activation pathways, we suggest that the generation of NP isoforms not only contributes to evasion of apoptosis but also robust control of the antiviral response.

Enterovirus D68 3C protease antagonizes type I interferon signaling by cleaving signal transducer and activator of transcription 1

Following the successful control of poliovirus, the re-emergence of respiratory enterovirus D68 (EV-D68), a prominent non-polio enterovirus, has become a serious public health concern worldwide. Host innate immune responses are the primary defense against EV-D68 invasion; however, the mechanism underlying viral evasion of the antiviral activity of interferons (IFN) remains unclear. In this study, we found that EV-D68 inhibited type I IFN signaling by cleaving signal transducer and activator of transcription 1 (STAT1), a crucial factor in cellular responses to interferons and other cytokines. We observed that the prototype and circulating EV-D68 strains conserved their ability to induce STAT1 cleavage and attenuate IFN signal transduction. Further investigation revealed that EV-D68 3C protease cleaves STAT1 at the 131Q residue. Interestingly, not all enterovirus-encoded 3C proteases exhibited this ability. EV-D68 and poliovirus 3C proteases efficiently induced STAT1 cleavage; whereas, 3C proteases from EV-A71, coxsackievirus A16, and echoviruses did not. STAT1 cleavage also abolished the nuclear translocation capacity of STAT1 in response to IFN stimulation to activate downstream signaling elements. Overall, these results suggest that STAT1, targeted by viral protease 3C, is utilized by EV-D68 to subvert the host's innate immune response.IMPORTANCEEnterovirus D68 (EV-D68) has significantly transformed over the past decade, evolving from a rare pathogen to a potential pandemic pathogen. The interferon (IFN) signaling pathway is an important defense mechanism and therapeutic target for the host to resist viral invasion. Previous studies have reported that the EV-D68 virus blocks or weakens immune recognition and IFN production in host cells through diverse strategies; however, the mechanisms of EV-D68 resistance to IFN signaling have not been fully elucidated. Our study revealed that EV-D68 relies on its own encoded protease, 3C, to directly cleave signal transducer and activator of transcription 1 (STAT1), a pivotal transduction component in the IFN signaling pathway, disrupting the IFN-mediated antiviral response. Previous studies on human enteroviruses have not documented direct cleavage of the STAT1 protein to evade cellular immune defenses. However, not all enteroviral 3C proteins can cleave STAT1. These findings highlight the diverse evolutionary strategies different human enteroviruses employ to evade host immunity.

Ceramide promotes lytic reactivation of Epstein-Barr virus in gastric carcinoma

Epstein-Barr virus (EBV) has a lifelong latency period after initial infection. Rarely, however, when the EBV immediate early gene BZLF1 is expressed by a specific stimulus, the virus switches to the lytic cycle to produce progeny viruses. We found that EBV infection reduced levels of various ceramide species in gastric cancer cells. As ceramide is a bioactive lipid implicated in the infection of various viruses, we assessed the effect of ceramide on the EBV lytic cycle. Treatment with C6-ceramide (C6-Cer) induced an increase in the endogenous ceramide pool and increased production of the viral product as well as BZLF1 expression. Treatment with the ceramidase inhibitor ceranib-2 induced EBV lytic replication with an increase in the endogenous ceramide pool. The glucosylceramide synthase inhibitor Genz-123346 inhibited C6-Cer-induced lytic replication. C6-Cer induced extracellular signal-regulated kinase 1/2 (ERK1/2) and CREB phosphorylation, c-JUN expression, and accumulation of the autophagosome marker LC3B. Treatment with MEK1/2 inhibitor U0126, siERK1&2, or siCREB suppressed C6-Cer-induced EBV lytic replication and autophagy initiation. In contrast, siJUN transfection had no impact on BZLF1 expression. The use of 3-methyladenine (3-MA), an inhibitor targeting class III phosphoinositide 3-kinases (PI3Ks) to inhibit autophagy initiation, resulted in reduced beclin-1 expression, along with suppressed C6-Cer-induced BZLF1 expression and LC3B accumulation. Chloroquine, an inhibitor of autophagosome-lysosome fusion, increased BZLF1 protein intensity and LC3B accumulation. However, siLC3B transfection had minimal effect on BZLF1 expression. The results suggest the significance of ceramide-related sphingolipid metabolism in controlling EBV latency, highlighting the potential use of drugs targeting sphingolipid metabolism for treating EBV-positive gastric cancer.IMPORTANCEEpstein-Barr virus remains dormant in the host cell but occasionally switches to the lytic cycle when stimulated. However, the exact molecular mechanism of this lytic induction is not well understood. In this study, we demonstrate that Epstein-Barr virus infection leads to a reduction in ceramide levels. Additionally, the restoration of ceramide levels triggers lytic replication of Epstein-Barr virus with increase in phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and CREB. Our study suggests that the Epstein-Barr virus can inhibit lytic replication and remain latent through reduction of host cell ceramide levels. This study reports the regulation of lytic replication by ceramide in Epstein-Barr virus-positive gastric cancer.

Estimates of differential toxin expression governing heterogeneous intracellular lifespans of Streptococcus pneumoniae

Following invasion of the host cell, pore-forming toxins secreted by pathogens compromise vacuole integrity and expose the microbe to diverse intracellular defence mechanisms. However, the quantitative correlation between toxin expression levels and consequent pore dynamics, fostering the intracellular life of pathogens, remains largely unexplored. In this study, using Streptococcus pneumoniae and its secreted pore-forming toxin pneumolysin (Ply) as a model system, we explored various facets of host-pathogen interactions in the host cytosol. Using time-lapse fluorescence imaging, we monitored pore formation dynamics and lifespans of different pneumococcal subpopulations inside host cells. Based on experimental histograms of various event timescales such as pore formation time, vacuolar death or cytosolic escape time and total degradation time, we developed a mathematical model based on first-passage processes that could correlate the event timescales to intravacuolar toxin accumulation. This allowed us to estimate Ply production rate, burst size and threshold Ply quantities that trigger these outcomes. Collectively, we present a general method that illustrates a correlation between toxin expression levels and pore dynamics, dictating intracellular lifespans of pathogens.

Lumpy skin disease virus ORF127 protein suppresses type I interferon responses by inhibiting K63-linked ubiquitination of tank binding kinase 1

Lumpy skin disease (LSD) is a severe animal infectious disease caused by lumpy skin disease virus (LSDV), inducing extensive nodules on the cattle mucosa or the scarfskin. LSDV genome encodes multiple proteins to evade host innate immune response. However, the underlying molecular mechanisms are poorly understood. In this study, we found that LSDV could suppress the expression of IFN-β and interferon-stimulated genes (ISGs) in MDBK cells during the early stage of infection. Subsequently, an unbiased screen was performed to screen the LSDV genes with inhibitory effects on the type I interferon (IFN-I) production. ORF127 protein was identified as one of the strongest inhibitory effectors on the expression of IFN-β and ISGs, meanwhile, the 1-43 aa of N-terminal of ORF127 played a vital role in suppressing the expression of IFN-β. Overexpression of ORF127 could significantly promote LSDV replication through inhibiting the production of IFN-β and ISGs in MDBK cells. Mechanism study showed that ORF127 specifically interacted with TBK1 and decreased the K63-linked polyubiquitination of TBK1 which suppressed the phosphorylation of TBK1 and ultimately decreased the production of IFN-β. In addition, truncation mutation analysis indicated that the 1-43 aa of N-terminal of ORF127 protein was the key structural domain for its interaction with TBK1. In short, these results validated that ORF127 played a negative role in regulating IFN-β expression through cGAS-STING signaling pathway. Taken together, this study clarified the molecular mechanism of ORF127 gene antagonizing IFN-I-mediated antiviral, which will helpfully provide new strategies for the treatment and prevention of LSD.

Darwin review: the evolution of virulence in human pathogens

By definition, all pathogens cause some level of harm to their hosts. If this harm occurs while the pathogen is transmitting, it can negatively affect the pathogen's fitness by shortening the duration over which transmission can occur. However, many of the factors that increase virulence (i.e. harm to host) also promote transmission, driving the pathogen population towards an optimal state of intermediate virulence. A wider spectrum of virulence may be maintained among pathogen populations which are structured into multiple discrete strains though direct resource and immune-mediated competition. These various evolutionary outcomes, and the effects of medical and public health interventions, are best understood within a framework that recognizes the complex relationship between transmission and virulence in the context of the antigenic diversity of the pathogen population.

Quantifying how much host, pathogen, and other factors affect human protective adaptive immune responses

Recognizing the "essential" factors that contribute to a clinical outcome is critical for designing appropriate therapies and prioritizing limited medical resources. Demonstrating a high correlation between a factor and an outcome does not necessarily imply an essential role of the factor to the outcome. Human protective adaptive immune responses to pathogens vary among (and perhaps within) pathogenic strains, human individual hosts, and in response to other factors. Which of these has an "essential" role? We offer three statistical approaches that predict the presence of newly contributing factor(s) and then quantify the influence of host, pathogen, and the new factors on immune responses. We illustrate these approaches using previous data from the protective adaptive immune response (cellular and humoral) by human hosts to various strains of the same pathogenic bacterial species. Taylor's law predicts the existence of other factors potentially contributing to the human protective adaptive immune response in addition to inter-individual host and intra-bacterial species inter-strain variability. A mixed linear model measures the relative contribution of the known variables, individual human hosts and bacterial strains, and estimates the summed contributions of the newly predicted but unknown factors to the combined adaptive immune response. A principal component analysis predicts the presence of sub-variables (currently not defined) within bacterial strains and individuals that may contribute to the combined immune response. These observations have statistical, biological, clinical, and therapeutic implications.

Beyond Impairment of Virion Infectivity: New Activities of the Anti-HIV Host Cell Factor SERINC5

Members of the serine incorporator (SERINC) protein family exert broad antiviral activity, and many viruses encode SERINC antagonists to circumvent these restrictions. Significant new insight was recently gained into the mechanisms that mediate restriction and antagonism. In this review, we summarize our current understanding of the mode of action and relevance of SERINC proteins in HIV-1 infection. Particular focus will be placed on recent findings that provided important new mechanistic insights into the restriction of HIV-1 virion infectivity, including the discovery of SERINC's lipid scramblase activity and its antagonism by the HIV-1 pathogenesis factor Nef. We also discuss the identification and implications of several additional antiviral activities by which SERINC proteins enhance pro-inflammatory signaling and reduce viral gene expression in myeloid cells. SERINC proteins emerge as versatile and multifunctional regulators of cell-intrinsic immunity against HIV-1 infection.

Prediction of motif-mediated viral mimicry through the integration of host-pathogen interactions

One of the mechanisms viruses use in hijacking host cellular machinery is mimicking Short Linear Motifs (SLiMs) in host proteins to maintain their life cycle inside host cells. In the face of the escalating volume of virus-host protein-protein interactions (vhPPIs) documented in databases; the accurate prediction of molecular mimicry remains a formidable challenge due to the inherent degeneracy of SLiMs. Consequently, there is a pressing need for computational methodologies to predict new instances of viral mimicry. Our present study introduces a DMI-de-novo pipeline, revealing that vhPPIs catalogued in the VirHostNet3.0 database effectively capture domain-motif interactions (DMIs). Notably, both affinity purification coupled mass spectrometry and yeast two-hybrid assays emerged as good approaches for delineating DMIs. Furthermore, we have identified new vhPPIs mediated by SLiMs across different viruses. Importantly, the de-novo prediction strategy facilitated the recognition of several potential mimicry candidates implicated in the subversion of host cellular proteins. The insights gleaned from this research not only enhance our comprehension of the mechanisms by which viruses co-opt host cellular machinery but also pave the way for the development of novel therapeutic interventions.

Microtubules provide force to promote membrane uncoating in vacuolar escape for a cyto-invasive bacterial pathogen

Intracellular bacterial pathogens gain entry to mammalian cells inside a vacuole derived from the host membrane. Some of them escape the bacteria-containing vacuole (BCV) and colonize the cytosol. Bacteria replicating within BCVs coopt the microtubule network to position it within infected cells, whereas the role of microtubules for cyto-invasive pathogens remains obscure. Here, we show that the microtubule motor cytoplasmic dynein-1 and specific activating adaptors are hijacked by the enterobacterium Shigella flexneri. These host proteins were found on infection-associated macropinosomes (IAMs) formed during Shigella internalization. We identified Rab8 and Rab13 as mediators of dynein recruitment and discovered that the Shigella effector protein IpaH7.8 promotes Rab13 retention on moving BCV membrane remnants, thereby facilitating membrane uncoating of the Shigella-containing vacuole. Moreover, the efficient unpeeling of BCV remnants contributes to a successful intercellular spread. Taken together, our work demonstrates how a bacterial pathogen subverts the intracellular transport machinery to secure a cytosolic niche.

The intersection between host-pathogen interactions and metabolism during Vibrio cholerae infection

Vibrio cholerae (V. cholerae), the etiological agent of cholera, uses cholera toxin (CT) to cause severe diarrheal disease. Cholera is still a significant cause of mortality worldwide with about half of all cholera cases and deaths occurring in children under five. Owing to the lack of cost-effective vaccination and poor vaccine efficacy in children, there is a need for alternative preventative and therapeutic strategies. Recent advances in our knowledge of the interplay between CT-induced disease and host-pathogen metabolism have opened the door for investigating how modulation of intestinal metabolism by V. cholerae during disease impacts host intestinal immunity, the gut microbiota, and pathogen-phage interactions. In this review article, we examine recent progress in our understanding of host-pathogen interactions during V. cholerae infection and discuss future work deciphering how modulation of gut metabolism during cholera intersects these processes to enable successful fecal-oral transmission of the pathogen.

Shared infection strategy of a fungal pathogen across diverse lineages of land plants, the Fusarium example

Plants engage with a wide variety of microorganisms either in parasitic or mutualistic relationships, which have helped them to adapt to terrestrial ecosystems. Microbial interactions have driven plant evolution and led to the emergence of complex interaction outcomes via suppression of host defenses by evolving pathogens. The evolution of plant-microbe interactions is shaped by conserved host and pathogen gene modules and fast-paced lineage-specific adaptability which determines the interaction outcome. Recent findings from different microbes ranging from bacteria, oomycetes, and fungi suggest recurrent concepts in establishing interactions with evolutionarily distant plant hosts, but also clade-specific adaptation that ultimately contributes to pathogenicity. Here, we revisit some of the latest features that illustrate shared colonization strategies of the fungal pathogen Fusarium oxysporum on distant plant lineages and lineage-specific adaptability of mini-chromosomal units encoding effectors, for shaping host-specific pathogenicity in angiosperms.

In Vitro Models for Investigating Intestinal Host-Pathogen Interactions

Infectious diseases are increasingly recognized as a major threat worldwide due to the rise of antimicrobial resistance and the emergence of novel pathogens. In vitro models that can adequately mimic in vivo gastrointestinal physiology are in high demand to elucidate mechanisms behind pathogen infectivity, and to aid the design of effective preventive and therapeutic interventions. There exists a trade-off between simple and high throughput models and those that are more complex and physiologically relevant. The complexity of the model used shall be guided by the biological question to be addressed. This review provides an overview of the structure and function of the intestine and the models that are developed to emulate this. Conventional models are discussed in addition to emerging models which employ engineering principles to equip them with necessary advanced monitoring capabilities for intestinal host-pathogen interrogation. Limitations of current models and future perspectives on the field are presented.

mSphere of Influence: How host genetics impact microbial pathogenesis and treatment of infectious disease

Emily Rosowski works in the field of host-pathogen interactions, studying how host innate immune mechanisms control pathogens. In this mSphere of Influence article, she reflects on how "Host genotype-specific therapies can optimize the inflammatory response to mycobacterial infections" by D. M. Tobin, F. J. Roca, S. F. Oh, R. McFarland, et al. (Cell 148:434-446, 2012, https://doi.org/10.1016/j.cell.2011.12.023) made an impact on her by investigating how differences in host genetics can affect modes of microbial pathogenesis and inform treatments for infectious disease.

Host Subcellular Organelles: Targets of Viral Manipulation

Viruses have evolved sophisticated mechanisms to manipulate host cell processes and utilize intracellular organelles to facilitate their replication. These complex interactions between viruses and cellular organelles allow them to hijack the cellular machinery and impair homeostasis. Moreover, viral infection alters the cell membrane's structure and composition and induces vesicle formation to facilitate intracellular trafficking of viral components. However, the research focus has predominantly been on the immune response elicited by viruses, often overlooking the significant alterations that viruses induce in cellular organelles. Gaining a deeper understanding of these virus-induced cellular changes is crucial for elucidating the full life cycle of viruses and developing potent antiviral therapies. Exploring virus-induced cellular changes could substantially improve our understanding of viral infection mechanisms.

Unveiling the role of host kinases at different steps of influenza A virus life cycle

Influenza viruses remain a major public health concern causing contagious respiratory illnesses that result in around 290,000-650,000 global deaths every year. Their ability to constantly evolve through antigenic shifts and drifts leads to the emergence of newer strains and resistance to existing drugs and vaccines. To combat this, there is a critical need for novel antiviral drugs through the introduction of host-targeted therapeutics. Influenza viruses encode only 14 gene products that get extensively modified through phosphorylation by a diverse array of host kinases. Reversible phosphorylation at serine, threonine, or tyrosine residues dynamically regulates the structure, function, and subcellular localization of viral proteins at different stages of their life cycle. In addition, kinases influence a plethora of signaling pathways that also regulate virus propagation by modulating the host cell environment thus establishing a critical virus-host relationship that is indispensable for executing successful infection. This dependence on host kinases opens up exciting possibilities for developing kinase inhibitors as next-generation anti-influenza therapy. To fully capitalize on this potential, extensive mapping of the influenza virus-host kinase interaction network is essential. The key focus of this review is to outline the molecular mechanisms by which host kinases regulate different steps of the influenza A virus life cycle, starting from attachment-entry to assembly-budding. By assessing the contributions of different host kinases and their specific phosphorylation events during the virus life cycle, we aim to develop a holistic overview of the virus-host kinase interaction network that may shed light on potential targets for novel antiviral interventions.

Host factor MxA restricts Dabie bandavirus infection by targeting the viral NP protein to inhibit NP-RdRp interaction and ribonucleoprotein activity

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with high case mortality rates, which is caused by Dabie bandavirus (DBV), a novel pathogen also termed as SFTS virus (SFTSV). Currently, no specific therapeutic drugs or vaccines are available for SFTS. Myxovirus resistance protein A (MxA) has been shown to inhibit multiple viral pathogens; however, the role of MxA in DBV infection is unknown. Here, we demonstrated that DBV stimulates MxA expression which, in turn, restricts DBV infection. Mechanistic target analysis revealed that MxA specifically interacts with the viral nucleocapsid protein (NP) in a manner independent of RNA. Minigenome reporter assay showed that in agreement with its targeting of NP, MxA inhibits DBV ribonucleoprotein (RNP) activity. In detail, MxA interacts with the NP N-terminal and disrupts the interaction of NP with the viral RNA-dependent RNA polymerase (RdRp) but not NP multimerization, the critical activities of NP for RNP formation and function. Furthermore, MxA N-terminal domain was identified as the functional domain inhibiting DBV infection, and, consistently, then was shown to interact with NP and obstruct the NP-RdRp interaction. Additionally, threonine 103 within the N-terminal domain is important for MxA inhibition to DBV, and its mutation (T103A) attenuates MxA binding to NP and obstruction of the NP-RdRp interaction. This study uncovers MxA inhibition of DBV with a series of functional and mechanistical analyses, providing insights into the virus-host interactions and probably helping inform the development of antiviral agents in the future.IMPORTANCEDBV/SFTSV is an emerging high-pathogenic virus. Since its first identification in China in 2009, cases of DBV infection have been reported in many other countries, posing a significant threat to public health. Uncovering the mechanisms of DBV-host interactions is necessary to understand the viral pathogenesis and host response and may advance the development of antiviral therapeutics. Here, we found that host factor MxA whose expression is induced by DBV restricts the virus infection. Mechanistically, MxA specifically interacts with the viral NP and blocks the NP-RdRp interaction, inhibiting the viral RNP activity. Further studies identified the key domain and amino acid residue required for MxA inhibition to DBV. Consistently, they were then shown to be important for MxA targeting of NP and obstruction of the NP-RdRp association. These findings unravel the restrictive role of MxA in DBV infection and the underlying mechanism, expanding our knowledge of the virus-host interactions.

Color-Flu Fluorescent Reporter Influenza A Viruses Allow for In Vivo Studies of Innate Immune Function in Zebrafish

Influenza virus infection can cause severe respiratory disease and is estimated to cause millions of illnesses annually. Studies on the contribution of the innate immune response to influenza A virus (IAV) to viral pathogenesis may yield new antiviral strategies. Zebrafish larvae are useful models for studying the innate immune response to pathogens, including IAV, in vivo. Here, we demonstrate how Color-flu, four fluorescent IAV strains originally developed for mice, can be used to study the host response to infection by simultaneously monitoring infected cells, neutrophils, and macrophages in vivo. Using this model, we show how the angiotensin-converting enzyme inhibitor, ramipril, and mitophagy inhibitor, MDIVI-1, improved survival, decreased viral burden, and improved the respiratory burst response to IAV infection. The Color-flu zebrafish larvae model of IAV infection is complementary to other models where the dynamics of infection and the response of innate immune cells can be visualized in a transparent host in vivo.

The uncharted territory of host-pathogen interaction in tuberculosis

Mycobacterium tuberculosis (M.tb) effectively manipulates the host processes to establish the deadly respiratory disease, Tuberculosis (TB). M.tb has developed key mechanisms to disrupt the host cell health to combat immune responses and replicate efficaciously. M.tb antigens such as ESAT-6, 19kDa lipoprotein, Hip1, and Hsp70 destroy the integrity of cell organelles (Mitochondria, Endoplasmic Reticulum, Nucleus, Phagosomes) or delay innate/adaptive cell responses. This is followed by the induction of cellular stress responses in the host. Such cells can either undergo various cell death processes such as apoptosis or necrosis, or mount effective immune responses to clear the invading pathogen. Further, to combat the infection progression, the host secretes extracellular vesicles such as exosomes to initiate immune signaling. The exosomes can contain M.tb as well as host cell-derived peptides that can act as a double-edged sword in the immune signaling event. The host-symbiont microbiota produces various metabolites that are beneficial for maintaining healthy tissue microenvironment. In juxtaposition to the above-mentioned mechanisms, M.tb dysregulates the gut and respiratory microbiome to support its replication and dissemination process. The above-mentioned interconnected host cellular processes of Immunometabolism, Cellular stress, Host Microbiome, and Extracellular vesicles are less explored in the realm of exploration of novel Host-directed therapies for TB. Therefore, this review highlights the intertwined host cellular processes to control M.tb survival and showcases the important factors that can be targeted for designing efficacious therapy.

Impact of surface receptors TLR2, CR3, and FcγRIII on Rhodococcus equi phagocytosis and intracellular survival in macrophages

The virulence-associated protein A (VapA) produced by virulent Rhodococcus equi allows it to replicate in macrophages and cause pneumonia in foals. It is unknown how VapA interacts with mammalian cell receptors, but intracellular replication of avirulent R. equi lacking vapA can be restored by supplementation with recombinant VapA (rVapA). Our objectives were to determine whether the absence of the surface receptors Toll-like receptor 2 (TLR2), complement receptor 3 (CR3), or Fc gamma receptor III (FcγRIII) impacts R. equi phagocytosis and intracellular replication in macrophages, and whether rVapA restoration of virulence in R. equi is dependent upon these receptors. Wild-type (WT) murine macrophages with TLR2, CR3, or FcγRIII blocked or knocked out (KO) were infected with virulent or avirulent R. equi, with or without rVapA supplementation. Quantitative bacterial culture and immunofluorescence imaging were performed. Phagocytosis of R. equi was not affected by blockade or KO of TLR2 or CR3. Intracellular replication of virulent R. equi was not affected by TLR2, CR3, or FcγRIII blockade or KO; however, avirulent R. equi replicated in TLR2-/- and CR3-/- macrophages but not in WT and FcγRIII-/-. rVapA supplementation did not affect avirulent R. equi phagocytosis but promoted intracellular replication in WT and all KO cells. By demonstrating that TLR2 and CR3 limit replication of avirulent but not virulent R. equi and that VapA-mediated virulence is independent of TLR2, CR3, or FcγRIII, our study provides novel insights into the role of these specific surface receptors in determining the entry and intracellular fate of R. equi.