Close Encounters of the Viral Kind: Cross‐Kingdom Synergies at the Host–Pathogen Interface

The synergies between viral and bacterial infections are well established. Most studies have been focused on the indirect mechanisms underlying this phenomenon, including immune modulation and alterations to the mucosal structures that promote pathogen outgrowth. A growing body of evidence implicates direct binding of virus to bacterial surfaces being an additional mechanism of synergy at the host–pathogen interface. These cross‐kingdom interactions enhance bacterial and viral adhesion and can alter tissue tropism. These bacterial–viral complexes play unique roles in pathogenesis and can alter virulence potential. The bacterial–viral complexes may also play important roles in pathogen transmission. Additionally, the complexes are recognized by the host immune system in a distinct manner, thus presenting novel routes for vaccine development. These synergies are active for multiple species in both the respiratory and gastrointestinal tract, indicating that direct interactions between bacteria and virus to modulate host interactions are used by a diverse array of species.     

Stress at the intestinal surface: catecholamines and mucosa–bacteria interactions

Psychological stress has profound effects on gastrointestinal function, and investigations over the past few decades have examined the mechanisms by which neural and hormonal stress mediators act to modulate gut motility, epithelial barrier function and inflammatory states. With its cellular diversity and large commensal bacterial population, the intestinal mucosa and its overlying mucous environment constitute a highly interactive environment for eukaryotic host cells and prokaryotic bacteria. The elaboration of stress mediators, particularly norepinephrine, at this interface influences host cells engaged in mucosal protection and the bacteria which populate the mucosal surface and gut lumen. This review will address growing evidence that norepinephrine and, in some cases, other mediators of the adaptation to stress modulate mucosal interactions with enteric bacteria. Stress-mediated changes in this delicate interplay may shift the microbial colonization patterns on the mucosal surface and alter the susceptibility of the host to infection. Moreover, changes in host-microbe interactions in the digestive tract may also influence ongoing neural activity in stress-responsive brain areas.     

Genetic Determinants of Reovirus Pathogenesis in a Murine Model of Respiratory Infection

Many viruses invade mucosal surfaces to establish infection in the host. Some viruses are restricted to mucosal surfaces, whereas others disseminate to sites of secondary replication. Studies of strain-specific differences in reovirus mucosal infection and systemic dissemination have enhanced an understanding of viral determinants and molecular mechanisms that regulate viral pathogenesis. After peroral inoculation, reovirus strain type 1 Lang replicates to high titers in the intestine and spreads systemically, whereas strain type 3 Dearing (T3D) does not. These differences segregate with the viral S1 gene segment, which encodes attachment protein σ1 and nonstructural protein σ1s. In this study, we define genetic determinants that regulate reovirus-induced pathology following intranasal inoculation and respiratory infection. We report that two laboratory isolates of T3D, T3D^C and T3D^F, differ in the capacity to replicate in the respiratory tract and spread systemically; the T3D^C isolate replicates to higher titers in the lungs and disseminates, while T3D^F does not. Two nucleotide polymorphisms in the S1 gene influence these differences, and both S1 gene products are involved. T3D^C amino acid polymorphisms in the tail and head domains of σ1 protein influence the sensitivity of virions to protease-mediated loss of infectivity. The T3D^C polymorphism at nucleotide 77, which leads to coding changes in both S1 gene products, promotes systemic dissemination from the respiratory tract. A σ1s-null virus produces lower titers in the lung after intranasal inoculation and disseminates less efficiently to sites of secondary replication. These findings provide new insights into mechanisms underlying reovirus replication in the respiratory tract and systemic spread from the lung.     

Host sialoglycans and bacterial sialidases: a mucosal perspective

Sialic acids are nine-carbon-backbone sugars that occupy outermost positions on vertebrate cells and secreted sialoglycoproteins. These negatively charged hydrophilic carbohydrates have a variety of biological, biophysical and immunological functions. Mucosal surfaces and secretions of the mouth, airway, gut and vagina are especially sialoglycan-rich. Given their prominent positions and important functions, a variety of microbial strategies have targeted host sialic acids for adherence, mimicry and/or degradation. Here we review the roles of bacterial sialidases (neuraminidases) during colonization and pathogenesis of mammalian mucosal surfaces. Evidence is presented to support the myriad roles of mucosal sialoglycans in protecting the host from bacterial infection. In opposition, many bacteria hydrolyse sialic acids during associations with the gastrointestinal, oral, respiratory and reproductive tracts. Sialidases promote bacterial survival in mucosal niche environments in several ways, including: (i) nutritional benefits of sialic acid catabolism, (ii) unmasking of cryptic host ligands used for adherence, (iii) participation in biofilm formation and (iv) modulation of immune function. Bacterial sialidases are among the best-studied enzymes involved in pathogenesis and may also drive commensal and/or symbiotic host associations. Future studies should continue to define host substrates of bacterial sialidases and the mechanisms of their pathologic, commensal and symbiotic interactions with the mammalian host.     

The zebrafish as a model for gastrointestinal tract–microbe interactions

The zebrafish (Danio rerio) has become a widely used vertebrate model for bacterial, fungal, viral, and protozoan infections. Due to its genetic tractability, large clutch sizes, ease of manipulation, and optical transparency during early life stages, it is a particularly useful model to address questions about the cellular microbiology of host–microbe interactions. Although its use as a model for systemic infections, as well as infections localised to the hindbrain and swimbladder having been thoroughly reviewed, studies focusing on host–microbe interactions in the zebrafish gastrointestinal tract have been neglected. Here, we summarise recent findings regarding the developmental and immune biology of the gastrointestinal tract, drawing parallels to mammalian systems. We discuss the use of adult and larval zebrafish as models for gastrointestinal infections, and more generally, for studies of host–microbe interactions in the gut.     

Host resistance to mucosal gram-negative infection. Susceptibility of lipopolysaccharide nonresponder mice

The effect of Lps on the resistance of mice to gram-negative infection was compared in two genetically different backgrounds; C3H and C57BL. To mimic the natural sequence of pathogenetic events, infection was induced via a mucosal surface (intravesically), with Escherichia coli which remained at the mucosal site and Salmonella typhimurium which invaded to e.g., livers and spleens. Susceptibility was assessed as the bacterial persistence in kidneys, bladders, livers, and spleens at various times after infection. The initial clearance of both bacterial species from the mucosal site was significantly impaired in Lpsd mice both in the C3H and C57BL backgrounds. In the C57BL mice, additional unknown determinants conferred increased resistance to mucosal infection compared to the C3H mouse. For S. typhimurium, these resistance factors and alleles at the Lps locus dominated over Ity as determinants of resistance to mucosal infection. The Itys genotype conferred a significant increase in the susceptibility only to systemic infection, especially in the Lpsd, Itys mice. These results demonstrate an important difference between the genetic determinants of host resistance at mucosal and systemic sites, and emphasize the role of LPS induced host defense mechanisms for bacterial clearance from mucosal surfaces.     

Review: Microbial endocrinology: intersection of microbiology and neurobiology matters to swine health from infection to behavior

From birth to slaughter, pigs are in constant interaction with microorganisms. Exposure of the skin, gastrointestinal and respiratory tracts, and other systems allows microorganisms to affect the developmental trajectory and function of porcine physiology as well as impact behavior. These routes of communication are bi-directional, allowing the swine host to likewise influence microbial survival, function and community composition. Microbial endocrinology is the study of the bi-directional dialogue between host and microbe. Indeed, the landmark discovery of host neuroendocrine systems as hubs of host–microbe communication revealed neurochemicals act as an inter-kingdom evolutionary-based language between microorganism and host. Several such neurochemicals are stress catecholamines, which have been shown to drastically increase host susceptibility to infection and augment virulence of important swine pathogens, including Clostridium perfringens. Catecholamines, the production of which increase in response to stress, reach the epithelium of multiple tissues, including the gastrointestinal tract and lung, where they initiate diverse responses by members of the microbiome as well as transient microorganisms, including pathogens and opportunistic pathogens. Multiple laboratories have confirmed the evolutionary role of microbial endocrinology in infectious disease pathogenesis extending from animals to even plants. More recent investigations have now shown that microbial endocrinology also plays a role in animal behavior through the microbiota–gut–brain axis. As stress and disease are ever-present, intersecting concerns during each stage of swine production, novel strategies utilizing a microbial endocrinology-based approach will likely prove invaluable to the swine industry.     

Lysogeny in the oceans: Lessons from cultivated model systems and a reanalysis of its prevalence

In the oceans, viruses that infect bacteria (phages) influence a variety of microbially mediated processes that drive global biogeochemical cycles. The nature of their influence is dependent upon infection mode, be it lytic or lysogenic. Temperate phages are predicted to be prevalent in marine systems where they are expected to execute both types of infection modes. Understanding the range and outcomes of temperate phage–host interactions is fundamental for evaluating their ecological impact. Here, we (i) review phage-mediated rewiring of host metabolism, with a focus on marine systems, (ii) consider the range and nature of temperate phage–host interactions, and (iii) draw on studies of cultivated model systems to examine the consequences of lysogeny among several dominant marine bacterial lineages. We also readdress the prevalence of lysogeny among marine bacteria by probing a collection of 1239 publicly available bacterial genomes, representing cultured and uncultivated strains, for evidence of complete prophages. Our conservative analysis, anticipated to underestimate true prevalence, predicts 18% of the genomes examined contain at least one prophage, the majority (97%) were found within genomes of cultured isolates. These results highlight the need for cultivation of additional model systems to better capture the diversity of temperate phage–host interactions in the oceans.     

Immuno-epidemiology of chronic bacterial and helminth co-infections: observations from the field and evidence from the laboratory

Co-infections can alter the host immune responses and modify the intensity and dynamics of concurrent parasitic species. The extent of this effect depends on the properties of the system and the mechanisms of host-parasite and parasite-parasite interactions. We examined the immuno-epidemiology of a chronic co-infection to reveal the immune mediated relationships between two parasites colonising independent organs, and the within-host molecular processes influencing the dynamics of infection at the host population level. The respiratory bacterium, Bordetella bronchiseptica, and the gastrointestinal helminth, Graphidium strigosum, were studied in the European rabbit (Oryctolagus cuniculus), using long-term field data and a laboratory experiment. We found that 65% of the rabbit population was co-infected with the two parasites; prevalence and intensity of co-infection increased with rabbit age and exhibited a strong seasonal pattern with the lowest values recorded during host breeding (from April to July) and the highest in the winter months. Laboratory infections showed no significant immune-mediated effects of the helminth on bacterial intensity in the lower respiratory tract but a higher abundance was observed in the nasal cavity during the chronic phase of the infection, compared with single bacterial infections. In contrast, B. bronchiseptica enhanced helminth intensity and this was consistent throughout the 4-month trial. These patterns were associated with changes in the immune profiles between singly and co-infected individuals for both parasites. This study confirmed the general observation that co-infections alter the host immune responses but also highlighted the often ignored role of bacterial infection in helminth dynamics. Additionally, we showed that G. strigosum had contrasting effects on B. bronchiseptica colonising different parts of the respiratory tract. At the host population level our findings suggest that B. bronchiseptica facilitates G. strigosum infection, and re-infection with G. strigosum assists in maintaining bacterial infection in the upper respiratory tract and thus long-term persistence.     

MicroMeeting: Who's talking to whom? Epithelial–bacterial pathogen interactions

Our perception that host-bacterial interactions lead to disease comes from rare, unsuccessful interactions resulting in the development of detectable symptoms. In contrast, the majority of host-bacterial interactions go unnoticed as the host and bacteria perceive each other to be no threat. In July 2004, a focused international symposium on epithelial-bacterial pathogen interactions was held in Newcastle upon Tyne (UK). The symposium concentrated on recent advances in our understanding of bacterial interactions at respiratory and gastrointestinal mucosal epithelial layers. For the host these epithelial tissues represent a first line of defence against invading bacterial pathogens. Through the discovery that the innate immune system plays a pivotal role during host-bacterial interactions, it has become clear that epithelia are being utilized by the host to monitor or communicate with both pathogenic and commensal bacteria. Interest in understanding the bacterial perspective of these interactions has lead researchers to realize that the bacteria utilize the same factors associated with disease to establish successful long-term interactions. Here we discuss several common themes and concepts that emerged from recent studies that have allowed physiologists and microbiologists to interact at a common interface similar to their counterparts -- epithelia and bacterial pathogens. These studies highlight the need for further multidisciplinary studies into how the host differentiates between pathogenic and commensal bacteria.     

Modulation of the host immune response by respiratory syncytial virus proteins

Respiratory syncytial virus (RSV) causes severe respiratory disease in both the very young and the elderly. Nearly all individuals become infected in early childhood, and reinfections with the virus are common throughout life. Despite its clinical impact, there remains no licensed RSV vaccine. RSV infection in the respiratory tract induces an inflammatory response by the host to facilitate efficient clearance of the virus. However, the host immune response also contributes to the respiratory disease observed following an RSV infection. RSV has evolved several mechanisms to evade the host immune response and promote virus replication through interactions between RSV proteins and immune components. In contrast, some RSV proteins also play critical roles in activating, rather than suppressing, host immunity. In this review, we discuss the interactions between individual RSV proteins and host factors that modulate the immune response and the implications of these interactions for the course of an RSV infection.     

Open Modification Searching of SARS-CoV-2–Human Protein Interaction Data Reveals Novel Viral Modification Sites

The outbreak of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the coronavirus 2019 disease, has led to an ongoing global pandemic since 2019. Mass spectrometry can be used to understand the molecular mechanisms of viral infection by SARS-CoV-2, for example, by determining virus–host protein–protein interactions through which SARS-CoV-2 hijacks its human hosts during infection, and to study the role of post-translational modifications. We have reanalyzed public affinity purification–mass spectrometry data using open modification searching to investigate the presence of post-translational modifications in the context of the SARS-CoV-2 virus–host protein–protein interaction network. Based on an over twofold increase in identified spectra, our detected protein interactions show a high overlap with independent mass spectrometry-based SARS-CoV-2 studies and virus–host interactions for alternative viruses, as well as previously unknown protein interactions. In addition, we identified several novel modification sites on SARS-CoV-2 proteins that we investigated in relation to their interactions with host proteins. A detailed analysis of relevant modifications, including phosphorylation, ubiquitination, and S-nitrosylation, provides important hypotheses about the functional role of these modifications during viral infection by SARS-CoV-2.     

Commensal microbial regulation of natural killer T cells at the frontiers of the mucosal immune system

The commensal microbiota co-exists in a mutualistic relationship with its human host. Commensal microbes play critical roles in the regulation of host metabolism and immunity, while microbial colonization, conversely, is under control of host immunity and metabolic pathways. These interactions are of central importance to the maintenance of homeostasis at mucosal surfaces and their perturbation can provide the basis for atopic and chronic inflammatory diseases such as asthma and inflammatory bowel disease (IBD). Recent evidence has revealed that natural killer T (NKT) cells, a subgroup of T cells which recognizes self and microbial lipid antigens presented by CD1d, are key mediators of host-microbial interactions. Mucosal and systemic NKT cell development is under control of the commensal microbiota, while CD1d regulates microbial colonization and influences the composition of the intestinal microbiota. Here, we outline the mechanisms of bidirectional cross-talk between the microbiota and CD1d-restricted NKT cells and discuss how a perturbation of these processes can contribute to the pathogenesis of immune-mediated disorders at mucosal surfaces.     

Rabbit intestinal xenograft model for human Encephalitozoon infections in mice.

BACKGROUND AND PURPOSE: The gastrointestinal tract is a common portal of entry for Encephalitozoon cuniculi, one of several microsporidial organisms emerging as opportunistic pathogens in immunocompromised humans. Although most human microsporidial pathogens can be propagated in vitro and in a variety of laboratory animals, an experimental animal system to specifically study intestinal uptake and systemic spread of these organisms does not exist. METHODS: Paired segments of near-term fetal rabbit small intestine were implanted subcutaneously into 25 athymic nude or 10 severe combined immune deficient mice. Five weeks after surgery, 65 xenografts were inoculated intraluminally with E. cuniculi (n = 14), E. intestinalis (n = 27), E. hellem (n = 20), or RK-13 cells (n = 2), or were left uninoculated (n = 2). RESULTS: Intestinal xenograft infection with E. cuniculi (n = 11), E. intestinalis (n = 17), and E. hellem (n = 18) was determined by light microscopy; control xenografts remained uninfected. Extraintestinal infection with E. cuniculi developed in host mouse brain, respiratory tract, spleen, salivary glands, and gastrointestinal tract (3 of 3 mice), and infection with E. intestinalis developed in the liver (8 of 15 mice). CONCLUSION: Intestinal xenografts provide a unique, sterile, and biologically relevant animal model system for studying host enterocyte/parasite interactions, mechanisms of microsporidial pathogenicity, antimicrosporidial chemotherapeutic agents, and immune effector mechanisms. This model provides evidence for persistent graft infection with three Encephalitozoon spp., and for intestinal spread of E. cuniculi and E. intestinalis from infected enterocytes in immunoincompetent mice.     

STAT3: a critical component in the response to Helicobacter pylori infection

STAT3 imparts a profound influence on both the epithelial and immune components of the gastric mucosa, and through regulation of key intracellular signal transduction events, is well placed to control inflammatory and oncogenic outcomes in the context of Helicobacter (H.) pylori infection. Here we review the roles of STAT3 in the host immune response to H. pylori infection, from both gastric mucosal and systemic perspectives, as well as alluding more specifically to STAT3‐dependent mechanisms that might be exploited as drug targets.     

Integrative neuroimmunomodulation of gastrointestinal function during enteric parasitism.

Enteric helminths have a significant impact on the structure, function, and neural control of the gastrointestinal (GI) tract of the host. Interactions between the host's nervous and immune systems redirect activity in neuronal circuits intrinsic to the gut into an alternative repertoire of defensive and adaptive motor programs. Gut inflammation and activation of the enteric neuroimmune axis play integral roles in the dynamic interaction between host and parasite that occurs at the mucosal surface. Three inter-related themes are stressed in this review to underscore the pivotal role that neural control mechanisms play in the host's GI tract functional responses to enteric parasitism. First, we address the discovery that signaling molecules of both parasite and host origin can reorient the dynamic ecology of enteric host-parasite interactions. Second, we explore what has been learned from investigations of altered gut propulsive and secretomotor reflex activities that occur during enteric parasitic infections and the emerging picture derived from these studies that elucidates how nerves help facilitate and orchestrate functional reorganization of the parasitized gut. Third, we provide an overview of the direct impact that enteric parasitism has on nerve cell function and neurotransmission pathways in both the enteric and central nervous systems of the host. In summary, this review highlights and clarifies the complex mechanisms underlying integrative neuroimmunophysiological responses to the presence of both invasive and noninvasive enteric helminths and identifies directions for future research investigations in this highly important but understudied area.     

THE CELLULAR IMMUNE RESPONSE OF DAPHNIA MAGNA UNDER HOST–PARASITE GENETIC VARIATION AND VARIATION IN INITIAL DOSE

In invertebrate–parasite systems, the likelihood of infection following parasite exposure is often dependent on the specific combination of host and parasite genotypes (termed genetic specificity). Genetic specificity can maintain diversity in host and parasite populations and is a major component of the Red Queen hypothesis. However, invertebrate immune systems are thought to only distinguish between broad classes of parasite. Using a natural host–parasite system with a well‐established pattern of genetic specificity, the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa, we found that only hosts from susceptible host–parasite genetic combinations mounted a cellular response following exposure to the parasite. These data are compatible with the hypothesis that genetic specificity is attributable to barrier defenses at the site of infection (the gut), and that the systemic immune response is general, reporting the number of parasite spores entering the hemocoel. Further supporting this, we found that larger cellular responses occurred at higher initial parasite doses. By studying the natural infection route, where parasites must pass barrier defenses before interacting with systemic immune responses, these data shed light on which components of invertebrate defense underlie genetic specificity.     

Animal models of mucosal Candida infection

Rodent models of oral, vaginal and gastrointestinal Candida infection are described and discussed in terms of their scientific merits. The common feature of all experimental mucosal Candida infections is the need for some level of host immunocompromise or exogenous treatment to ensure quantitatively reproducible disease. A growing literature describes the contributions of such candidiasis models to our understanding of certain aspects of fungal virulence and host response to mucosal Candida albicans challenge. Evidence to date shows that T-lymphocyte responses dominate host immune defences to oral and gastrointestinal challenge, while other, highly compartmentalized responses defend vaginal surfaces. By contrast the study of C. albicans virulence factors in mucosal infection models has only begun to unravel the complex of attributes required to define the difference between strongly and weakly muco-invasive strains.     

Connexins in respiratory and gastrointestinal mucosal immunity

The mucosal lining forms the physical and chemical barrier that protects against pathogens and hostile particles and harbors its own population of bacteria, fungi and archea, known as the microbiota. The immune system controls tolerance of this population of microorganisms that have proven to be beneficial for its host. Keeping its physical integrity and a correct balance with the microbiota, the mucosa preserves its homeostasis and its protective function and maintains host’s health. However, in some conditions, pathogens may succeed in breaching mucosal homeostasis and successfully infecting the host. In this review we will discuss the role the mucosa plays in the defense against bacterial pathogens by considering the gap junction protein connexins. We will detail their implication in mucosal homeostasis and upon infection with bacteria in the respiratory and the gastrointestinal tracts.     

Isolation of Lymphocytes from Mouse Genital Tract Mucosa

Mucosal surfaces, including in the gastrointestinal, urogenital, and respiratory tracts, provide portals of entry for pathogens, such as viruses and bacteria ¹. Mucosae are also inductive sites in the host to generate immunity against pathogens, such as the Peyers patches in the intestinal tract and the nasal-associated lymphoreticular tissue in the respiratory tract. This unique feature brings mucosal immunity as a crucial player of the host defense system. Many studies have been focused on gastrointestinal and respiratory mucosal sites. However, there has been little investigation of reproductive mucosal sites. The genital tract mucosa is the primary infection site for sexually transmitted diseases (STD), including bacterial and viral infections. STDs are one of the most critical health challenges facing the world today. Centers for Disease Control and Prevention estimates that there are 19 million new infectious every year in the United States. STDs cost the U.S. health care system $17 billion every year ², and cost individuals even more in immediate and life-long health consequences. In order to confront this challenge, a greater understanding of reproductive mucosal immunity is needed and isolating lymphocytes is an essential component of these studies. Here, we present a method to reproducibly isolate lymphocytes from murine female genital tracts for immunological studies that can be modified for adaption to other species. The method described below is based on one mouse.