Molecular mechanisms of sensitization to allergic asthma and therapeutic innovation – Innovasthma

Asthma is a highly prevalent disease with increasing socioeconomic burden, whose most frequent allergic cause is sensitization to house dust mite (HDM). High-throughput sequencing of HDM extracts has revealed the presence of Gram-negative bacteria that may likely arise from their gastrointestinal tract. Furthermore, clinical studies have highlighted a relationship between asthma severity and Gram-negative bacteria in the environment. Interestingly, GWAS studies have shown a link between allergic asthma and polymorphisms within the Nucleotide-binding Oligomerization Domain protein 1 (NOD1)-encoding gene that is primarily involved in sensing of Gram-negative bacteria through bacterial peptidoglycan recognition (Chamaillard et al., 2003) and in mucosal homeostasis towards the gut microbiota (Bouskra et al, Nature 2008).
Our previous findings led us to demonstrate that a Nod1 agonist used as a systemic adjuvant exacerbates ovalbumin-induced Th2-mediated allergic asthma through dendritic cell activation (Ait Yahia et al., 2014) and more recently that HDM-induced asthma is improved in Nod1-deficient mice. Furthermore, HDM is able to activate HEK293 cells transfected with a reporter of either human or murine NOD1 activity. Collectively, these results suggested us that activation of Nod1 by HDM-derived Gram negative bacteria may control adaptive immune responses and that the asthma associated Nod1 variant may enhance host responsiveness to Nod1 agonists that are present in HDM.

The working hypothesis underlying the InnovAsthma project is that the pro-allergenic effect of Nod1-mediated sensing of HDM may depend upon direct recognition of HDM-derived Gram negative bacteria. To this end, our consortium will address four complementary tasks through a multidisciplinary approach gathering expertise in allergology, immunology, bacteriology, and genetically modified mice.

• The first objective consists in defining how Nod1 sensing of HDM may promote allergic asthma. A direct effect of HDM will be evaluated on murine and human dendritic and epithelial cell lines (where NOD1 is primarily expressed) knocked down or not for NOD1. The role of endotoxin depletion (resin column) and peptidoglycan inactivation (acid hydrolysis) of the HDM extract will be evaluated in vitro in the reporter cell lines as well as in vivo on the parameters of the HDM-induced asthma model.
• The second objective will be to identify the cell type through which Nod1 acts to coordinate HDM-driven allergic response. We will focus on the role of either dendritic cells or epithelial cells, according to the results obtained in vitro in the first part. To specifically invalidate Nod1 expression in these cells, either dendritic cell specific (Zbtb-46) or lung epithelial specific (Aqp5) Cre mice will be crossed with newly generated Nod1 floxed mice.
• The third part will consist in assessing the preclinical relevance of these observations in a humanized mouse model that is expressing either human Nod1 orthologue (Zarantonelli et al., 2013) or its variant that is predisposing to asthma.
• The fourth part will pre-clinically evaluate the therapeutic potential of recently developed novel inhibitors of the Nod1 signalling pathway in the HDM-induced asthma model, as well as on human dendritic or epithelial cells obtained from asthmatic patients.
In conclusion, our consortium will unveil how allergic response to HDM is regulated by Nod1, and the InnovAsthma project should pave the way towards new therapeutic strategies in order to decrease the economic consequences and the burden of asthma.