Effects of the strictly enteric helminth, Heligmosomoides polygyrus, on respiratory syncytial virus (RSV) infection

RSV is the most common cause of infant bronchiolitis, leading to morbidity and mortality in both infants and the elderly. The relationship between RSV and asthma development further highlights the need to fully understand the immune responses involved in order to develop effective vaccines and therapeutics to aid prevention and treatment of RSV infection respectively. Helminths have long been studied both as a major pathogen of humans, infecting approximately 3 billion people worldwide, and also their ability to modulate the host immune response to allow survival and chronic infection to ensue. Specifically, helminth infections are thought to modulate the host immune response through regulatory mechanisms which are not fully understood. This not only confers protection and survival of the parasites themselves, but also modulates the immune response to unrelated antigens and pathogens. In this thesis, the potential role of a strictly enteric helminth infection, with Heligmosomoides polygyrus, in the modulation of respiratory syncytial virus (RSV) infection was investigated and the associated immune mechanisms were investigated. Firstly, the effects of prior H. polygyrus infection on RSV infection and immune responses in the lung were analysed. H. polygyrus significantly reduced the number of natural killer cells, CD8+ T cells, B cells and conventional dendritic cells in the lung following RSV infection. Co-infection also reduced the production of pro-inflammatory cytokines IL-6 and TNF-α in the lungs. All of these reductions were associated with significantly lower viral titres on day 4 of RSV infection. Interestingly, this attenuation of immune responses and viral titres, correlated with reduced severity of clinical disease, as assessed by weight loss and lung function. H. polygyrus excretory secretory product (HES) was not found to be the immune-modulatory factor in this system, as HES failed to suppress viral titres and reduce immune cell responses to RSV infection. However, irradiated larvae with stunted maturation to adult worms, revealed that larval stages were sufficient to suppress viral titres. Next, the role of type 2 signalling for H. polygyrus effects on RSV infection were examined, using IL-4Rα-/- mice. H. polygyrus infection maintained the ability to attenuate RSV infection and subsequent immune responses in IL-4Rα-/- mice. Furthermore, the presence of the adaptive immune response was not required for H. polygyrus-induced attenuation of RSV infection, as demonstrated in recombinase-activating gene (RAG-/-) deficient mice. H. polygyrus induces innate type 2 immune responses indicating the release of the innate alarmin, IL-33, in the lung and consequently an accumulation of group 2 innate lymphoid cells (ILC2). Their contribution to H. polygyrus effects remain to be fully elucidated. Finally, the role of antiviral responses was explored in H. polygyrus and RSV co-infection. H. polygyrus infection alone induced expression of antiviral genes, IFN-β, OAS1A, Viperin and the antimicrobial peptide CRAMP, in both the duodenum and the lung. Expression of these genes was still higher in the lung 1 hour after RSV in H. polygyrus co-infected mice compared to controls without co-infection. The importance of type I IFN signalling pathway was demonstrated using mice deficient in the type I IFN receptor in H. polygyrus co-infection, which failed to suppress RSV titres and subsequent lung immune cell infiltration. These data highlight the ability of the strictly enteric helminth H. polygyrus to attenuate RSV infection and subsequent immune responses in the lung through the potentiation of type I IFN signalling and consequent upregulation of antiviral immune responses in the lung