Past project's Collaborator
Amandine MATHIAS (to December 2011)
Humans live in symbiosis with billions of commensal bacteria among which more than 90 percents reside in their gastrointestinal tract. The protective role of secretory immunoglobulins A (SIgA) has been well established in pathogenic contexts such as shigellosis. It has been also demonstrated that commensals are coated by SIgA in the gut lumen. Nevertheless, the precise mechanisms underlying the relation between intestinal microbiota and the immune system, in particular SIgA, remain enigmatic. Cross-talk between cellular and molecular partners including SIgA, is an essential component of the complex function of the epithelium.
In this context, using different nonspecific fluorescently-labeled SIgA, we have visualized, by confocal laser scanning microscopy, immune complexes with commensal bacteria such as Lactobacillus, Bifidobacteria or Escherichia coli strains. This demonstrated that SIgA interacted with commensal bacteria in a Fab-independent manner. As both polymeric IgA and secretory component (SC) in SIgA are heavily glycosylated, we sought to analyze the impact of carbohydrates in the formation of immune complexes. We found that deglycosylation led to a drastic drop in the interaction between SIgA and bacteria, and that removal of glycan residues on SC was particularly involved in the process.
Thus, coating of commensals by SIgA represents a novel role of SIgA in sensing and controlling the resident microbiota. Polarized intestinal epithelial cell (IEC) lines have been frequently used to examine the consequences of the interaction with bacteria and proteins. Nevertheless, very few is known about the consequences of SIgA and commensal bacteria either on epithelial responses or on their role during pathogenic infections. In this context, we have used polarized enterocyte-like Caco-2 cell monolayers to analyze the outcome of the interaction between IECs, microorganisms and SIgA based-immune complexes for the invasive bacterium Shigella flexneri. This in vitro model was used in order to analyze the direct effect of invasive bacteria on the cellular morphology. In association with transepithelial electrical resistance measurement, laser scanning confocal microscopy allowed us to visualize the conserved integrity in presence of SIgA (Figure), demonstrating the protective effect of SIgA against the invasive bacterium.