Network model of immune responses reveals key effectors to single and co-infection dynamics by a respiratory bacterium and a gastrointestinal helminth

Co-infections alter the host immune response but how the systemic and local processes at the site of infection interact is still unclear. The majority of studies on co-infections concentrate on one of the infecting species, an immune function or group of cells and often focus on the initial phase of the infection. Here, we used a combination of experiments and mathematical modelling to investigate the network of immune responses against single and co-infections with the respiratory bacterium Bordetella bronchiseptica and the gastrointestinal helminth Trichostrongylus retortaeformis. Our goal was to identify representative mediators and functions that could capture the essence of the host immune response as a whole, and to assess how their relative contribution dynamically changed over time and between single and co-infected individuals. Network-based discrete dynamic models of single infections were built using current knowledge of bacterial and helminth immunology; the two single infection models were combined into a co-infection model that was then verified by our empirical findings. Simulations showed that a T helper cell mediated antibody and neutrophil response led to phagocytosis and clearance of B. bronchiseptica from the lungs. This was consistent in single and co-infection with no significant delay induced by the helminth. In contrast, T. retortaeformis intensity decreased faster when co-infected with the bacterium. Simulations suggested that the robust recruitment of neutrophils in the co-infection, added to the activation of IgG and eosinophil driven reduction of larvae, which also played an important role in single infection, contributed to this fast clearance. Perturbation analysis of the models, through the knockout of individual nodes (immune cells), identified the cells critical to parasite persistence and clearance both in single and co-infections. Our integrated approach captured the within-host immuno-dynamics of bacteria-helminth infection and identified key components that can be crucial for explaining individual variability between single and co-infections in natural populations

Severity of bovine tuberculosis is associated with co-infection with common pathogens in wild boar

Co-infections with parasites or viruses drive tuberculosis dynamics in humans, but little is known about their effects in other non-human hosts. This work aims to investigate the relationship between Mycobacterium bovis infection and other pathogens in wild boar (Sus scrofa), a recognized reservoir of bovine tuberculosis (bTB) in Mediterranean ecosystems. For this purpose, it has been assessed whether contacts with common concomitant pathogens are associated with the development of severe bTB lesions in 165 wild boar from mid-western Spain. The presence of bTB lesions affecting only one anatomic location (cervical lymph nodes), or more severe patterns affecting more than one location (mainly cervical lymph nodes and lungs), was assessed in infected animals. In addition, the existence of contacts with other pathogens such as porcine circovirus type 2 (PCV2), Aujeszky's disease virus (ADV), swine influenza virus, porcine reproductive and respiratory syndrome virus, Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Haemophilus parasuis and Metastrongylus spp, was evaluated by means of serological, microbiological and parasitological techniques. The existence of contacts with a structured community of pathogens in wild boar infected by M. bovis was statistically investigated by null models. Association between this community of pathogens and bTB severity was examined using a Partial Least Squares regression approach. Results showed that adult wild boar infected by M. bovis had contacted with some specific, non-random pathogen combinations. Contact with PCV2, ADV and infection by Metastrongylus spp, was positively correlated to tuberculosis severity. Therefore, measures against these concomitant pathogens such as vaccination or deworming, might be useful in tuberculosis control programmes in the wild boar. However, given the unexpected consequences of altering any community of organisms, further research should evaluate the impact of such measures under controlled conditions. Furthermore, more research including other important pathogens, such as gastro-intestinal nematodes, will be necessary to complete this picture

The mucus barrier : immune defence against gastrointestinal nematodes

Trichuriasis, caused by the intestinal nematode Trichuris, is a disease that affects up to a billion people worldwide. To date, most of our knowledge of this disease comes from the mouse model, Trichuris muris, which has been successfully used to dissect the immune-mediated effector mechanisms that elicit the expulsion of the nematode. Numerous studies have shown a temporal association between intestinal nematode expulsion and goblet cell hyperplasia; however their precise role in response to nematode infection remains elusive. Goblet cells found at mucosal surfaces secrete many constituent components of the mucus barrier, including the gel-forming mucins (Muc2 in the intestine); mucins are large multifunctional glycoproteins that provide the structural framework of the barrier. The studies presented in this thesis demonstrate that the mucosal barrier and in particular its mucin components, changes in response to acute and chronic T.muris infection. In animals resistant to chronic T. muris infection, IL-13-mediated increase in Muc2 production and secretion was observed at the site of infection. Critically, expulsion of the nematode was significantly delayed in the absence of Muc2. Further investigation subsequently showed that Muc5ac, a mucin normally expressed in non-intestinal mucosa was, in fact, expressed in the intestine following nematode infection and was associated with nematode expulsion in the resistant mice. Moreover, mice deficient in Muc5ac were susceptible to chronic infection, despite a strong underlying TH2-type immune response which is essential to eliminate the nematodes, suggesting that Muc5ac acts as a critical effector molecule. Several qualitative changes in the mucins were also noted during resistance: mucins were more highly charged and more sulphated during nematode expulsion. Overall, the changes within the mucus barrier during resistance result in altering the rheological properties of the mucus layer making it less porous and mucins were shown to directly 'damage' the nematodes during nematode expulsion as reflected by a significant reduction in ATP levels. Chronic infection was accompanied by decreased levels of low charged and highly sialylated Muc2. Additionally, we demonstrated that the excretory secretory products released by the nematode consist of serine proteases capable of depolymerising the Muc2 mucin network, which may be part of the nematodes regime to improve its niche and/or aid movement through the mucus layer. Overall, this resulted in a porous mucus layer and a favourable environment for the parasite.Data is presented to show that the intestinal mucus barrier and its constituent mucins are an integral part of the co-ordinated expulsion mechanisms that occur in animals resistant to T.muris infection and we identify a mechanism whereby the nematode exerts its effects on the mucin environment to promote its survival within the host.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The role of endemic infection in disease emergence

Human and animal populations are confronted by emerging microparasitic infections which pose a major threat to public health and the global economy. In natural conditions, emerging microparasites will encounter host populations that are already infected with common endemic macroparasites. Interspecific interactions between coinfecting parasites may alter the host immune response, the emerging parasite infection dynamics, the disease outcome and the efficacy of parasite control strategies. This thesis explores the role of macroparasites as potential suppressors or promoters of microparasite disease emergence. The potential impact endemic infections may have on disease emergence were explored experimentally using the model German cockroach host Blattella germanica, its endemic gut macroparasite Gregarina blattarum and the virulent microparasite Steinernema carpocapsae. First the effect of a hosts’ endemic parasite burden on the immune response and secondly, susceptibility to infection were investigated (Chapter 2). These experiments revealed that the host immune response was altered by the endemic parasite burden but this had no effect on susceptibility to infection with the emerging microparasite. The impact of host endemic parasite burden on the quality and quantity of the emerging parasite transmission stages was then explored. Here, coinfection resulted in a reduced output of the epidemic parasite transmission stages compared to a single infection. Further, endemic parasites had a non-linear effect on the quality of the transmission stages of the emerging microparasite measured as lipid energy reserves (Chapter 3). Finally, the fitness cost of coinfection on the between-hosts transmission of the emerging parasite was explored. Experimental findings revealed that the disease spread of the microparasite within the host population was altered by the endemic parasite (Chapter 4). The findings from this thesis demonstrate the importance of considering macro- and microparasite coinfections, and that this, in turn, is pivotal to improving control strategies and ability to accurately predict epidemic outbreaks

Virus host shifts in Drosophila: The influences of virus genotype and coinfection on susceptibility within and across host species

Virus host shifts are a major source of outbreaks and emerging infectious diseases, and continue to cause considerable damage to public health, society, and the global economy. Predicting and preventing future virus host shifts has become a primary goal of infectious disease research, and multiple tools and approaches are being developed to work towards this goal. In this thesis, I examine three key aspects of infection that have implications for our wider understanding of virus host shifts and their predictability in natural systems: whether the outcome of infections across species is correlated between related viruses, whether the presence of a coinfecting virus can alter the outcomes of cross-species transmission, and the influence of host genetics and immunity on the outcomes of coinfection. These experiments make use of a large and evolutionarily diverse panel of Drosphilidae host species, and infections with two insect Cripaviruses: Drosophila C virus (DCV) and Cricket Paralysis virus (CrPV), with the outcomes of infection quantified throughout as viral loads via qRT-PCR. In Chapter Two, phylogenetic generalised linear mixed models are applied to data on the outcome of single infections with three isolates of DCV (DCV-C, DCV-EB, DCV-M) and one isolate of CrPV, to look for correlations in viral load across host species. Strong positive corrections were found between DCV isolates and weaker positive correlations between DCV and CrPV, with evidence of host species by virus interactions on the outcome of infection. Of the four viruses tested, the most closely related isolates tended to be the most strongly correlated, with correlation strength deteriorating with the evolutionary distance between isolates, although we lacked the diversity or sample size of viruses to properly determine any effect of evolutionary distance on correlation strength. Together, this suggests that hosts susceptible to one virus are also susceptible to closely related viruses, and that knowledge of one virus may be extrapolated to closely related viruses, at least within the range of evolutionary divergence tested here. In the remainder of this thesis, I examine the outcome of coinfection with DCV-C and CrPV across host species (Chapter Three) and across genotypes and immune mutants of Drosophila melanogaster (Chapter Four). These chapters aim to assess the potential for coinfection to alter the outcomes of cross-species transmission – and so interfere with predictions of virus host shifts – and the potential influence of host genetics and immunity on the outcome of coinfection. Chapter Three finds little evidence of systematic changes in the outcome of single and coinfection for both viruses across species, suggesting that coinfection may not be a required consideration in predictive models of every host-virus system. Effects of coinfection were found in a subset of species but were not recapitulated in a follow-up experiment looking at tissue tropism during coinfection on a subset of host species. Together, this suggests that any effects of coinfection across species with DCV and CrPV are due to stochastic effects within individual hosts. Chapter Four finds small but credible effects of coinfection across genotypes of D. melanogaster, but these effects showed little host genetic basis or effect on the genetic basis of susceptibility to each virus separately. Mutations in several immune genes caused virus-specific changes in viral load between single and coinfection, suggesting that coinfection interactions between viruses can be moderated by the host immune response. This thesis has aimed to explore several fundamental features of cross-species transmission that are relevant to our understanding – and ability to predict – virus host shifts. Both the finding that correlations exist between viruses and the approach used to characterise coinfection across and within host species would now benefit from an increased diversity of experimental pathogens, to better investigate the influence of virus evolutionary relationships on the outcomes of virus host shifts and present a broader understanding of the potential impact of coinfection on the outcomes of cross-species transmission.Natural Environment Research Council (NERC