Impact of the Hypoxic Gut Microenvironment on Host Enteric Pathogen Interactions

Intestinal epithelial cells lining the surface of our gastro-intestinal tract form a tight monolayer which serves as the primary barrier separating us from the luminal content of the gut. The lumen of the gut is home to a very complex commensal bacterial microbiota but also contains potential enteric viral (e.g. Rotavirus, Norovirus), bacterial (e.g Salmonella) and eukaryotic unicellular pathogens (e.g. Cryptosporidium). An important biochemical feature of our gastro-intestinal tract is the near absence of oxygen in the lumen of the gut known as the hypoxia environment. While it well established that hypoxia is critical to maintain a healthy microbiota, how the low oxygen environment impacts the response of intestinal epithelial cells to enteric pathogens remains unexplored. Here, we employed both intestinal cell lines and human intestinal organoids to monitor how hypoxia impacts the intrinsic immune response generated by intestinal epithelial cells upon enteric virus infection. We found that hypoxia decreased the induction of both type I and III interferons which are key cytokines controlling virus infection, replication, and spread. We could show that under low oxygen conditions, intestinal epithelial cells are more susceptible to viral infection. Under hypoxia, enteric virus replication is significantly increased, and cells generate more de-novo infectious progeny virus particles compared to intestinal epithelial cells grown in normal levels of oxygen (normoxia). Interestingly, we observed the same inhibition of interferon expression in non-infected cells showing that hypoxia dampens the basal immune response of cells directly impacting their steady state susceptibility to virus infection. We identified the mechanisms leading to this hypoxia-mediated inhibition of immune response and impairing the activity or expression of the key proteins regulating these processes restore the proper immune response and control of enteric virus infection by intestinal epithelial cells under hypoxia. Importantly, we could show that this hypoxia-mediated inhibition of immune response is not limited to enteric viruses but also applies to enteric bacterial pathogens (Salmonella) and eukaryotic unicellular enteric pathogens (Cryptosporidium).

Our results demonstrate that hypoxia plays a key role in regulating the response of intestinal epithelial cells to enteric pathogens. While this hypoxia-mediated down regulation of immune response appears at first to be bad for the host to as it may favor enteric pathogen infection, we suggest that it participates in maintaining gut homeostasis. Indeed, oxygen levels vary along the crypt/villi structures formed by the epithelium with the tip of the villi being located under hypoxia while the crypts structures are located under normoxic conditions of oxygen. As such, we propose that along the crypt/villi structure a gradient of immune response is established to allow efficient protection of the stem cells located in the crypts (normoxia) and to blunt the response against the microbiome at the tip of the villi.

Learning objectives:

1. Define the intrinsic innate immune response generate by intestinal epithelial cells to fight enteric viruses.

2. Explain the difference between type I and type III interferons.

3. Discuss how low oxygen levels impair intrinsic innate immune response and how this, in turn, favors pathogen infection.