Within our body, our intestinal cells provide a valuable layer, which is responsible for the nutrient uptake needed to fuel our bodies. However, what many people do not appreciate is the important role that our intestinal cells play in protecting our body from foreign pathogens. Unlike immune cells which are programed to fight and respond to the presence of foreign invaders, our intestinal cells are in constant contact with the commensal microbiota and therefore must have a fine balance between tolerating and responding to incoming pathogens (Figure 1). One way that intestinal cells accomplish this is through cellular polarization (creating functional differences between the top side and the bottom side of the cells) and the formation of a tight barrier. Intestinal cell polarization arises from a complex signaling pathway which creates an apical membrane facing the microbial containing lumen of the gut and the basolateral membrane facing the tissue side (lamina propria) (Figure 1). The barrier is formed by tight protein interactions between neighboring cells called tight and adherens junctions (Figure 1).
Figure 1: Complexity of the human intestinal tract and the polarized nature of intestinal epithelial cells
As the cells have created a complex structure, we wanted to address whether this polarized nature would impact how enteric viruses were able to infect intestinal cells. For this work we used human colon carcinoma cells, primary human mini-gut organoids and mouse intestinal organoids that we seeded on transwell inserts allowing us to access their apical and basolateral sides independently (Figure 2).
Figure 2: Transwell inserts of colon carcinoma cells or human intestinal organoids allow us to access both their apical and basolateral sides for infections/stimulations
We started the work focusing on enteric virus uptake into cells and determining whether viruses could be taken up in a similar manner from the apical and basolateral sides. Initial experiments showed no differences between apical and basolateral infection and that viruses entered cells and started replicating in a similar manner. However, one day we weren’t able to finish our experiment and ended up letting the virus infection continue for multiple days. Interestingly, our virus infection now started to look very different and started to show that apical infection allowed for a large virus infection when the viruses started spreading within the monolayer. Instead of ignoring this strange phenotype we decided to push further and see what was causing the difference in virus propagation. After many years of work, we found that intestinal cells would respond to Toll-like receptor (TLR) 3 stimuli (both viruses and additional agonist) in a polarized manner by producing a large amount of the antiviral compound type III interferon following stimulation from the basolateral side while apical infection/stimulation lead to an acute and low interferon induction. Through microscopic visualization and genetic manipulations we found that this was due to the polarized distribution of the TLR3 receptor to the basolateral side of intestinal cells. Together we believe that this work shows a mechanism that cells use to maintain homeostasis by partially tolerating the presence of the commensal bacteria at the apical side while being posed to respond in a strong manner when stimuli come from the normally sterile basolateral side (Figure 3).
In the end this work has helped to shed light on how intestinal cells achieve a homeostatic state and reinforced the old mantra: New ideas in science do not start with the observation of “Eureka” but of “hmm that’s strange”.
More details can be found in the original paper:https://www.nature.com/articles/s41564-019-0594-3
Figure 3: Within a healthy gut the commensals and pathogens are normally exposed to the apical side. Intestinal cells then respond to apical stimulation/infection in an acute manner to limit tissue inflammation. However, when pathogens pass to the underlying basolateral side, intestinal cells respond in a strong and prolonged manner to protect the tissue.