Role of secretory IgA in the mucosal sensing of commensal bacteria

Abstract

While the gut epithelium represents the largest mucosal tissue, the mechanisms underlying the interaction between intestinal bacteria and the host epithelium lead to multiple outcomes that remain poorly understood at the molecular level. Deciphering such events may provide valuable information as to the mode of action of commensal and probiotic microorganisms in the gastrointestinal environment. Potential roles of such microorganisms along the privileged target represented by the intestinal immune system include maturation processes prior, during and after weaning, and the reduction of inflammatory reactions in pathogenic conditions. As commensal bacteria are naturally coated by natural and antigen-specific SIgA in the gut lumen, understanding the consequences of such an interaction may provide new clues on how the antibody contributes to homeostasis at mucosal surfaces. This review discusses several aspects of the role of SIgA in the essential communication existing between the host epithelium and members of its microbiota.

Keywords: DC, dendritic cell; IEC, intestinal epithelial cell; SC, secretory component; SED, subepithelial dome; SIgA, secretory IgA; dendritic cells; gut microbiota; intestinal epithelial cells; mucosal homeostasis; pIgR, polymeric Ig receptor; secretory IgA.

Figure 1.

Interactions between (A) SIgA or (B) transferrin (Tf) and transferrin receptor (CD71), expressed by polarized Caco-2 epithelial cell monolayers. (A) Laser scanning confocal microscope analysis showing apical colocalization of fluorescently labeled SIgA (green) and CD71 (detected in red) in polarized Caco-2 cells as revealed by the appearance of yellow spots. (B) Physical interaction revealed by yellow dots at the apical surface between Tf and CD71. For both sets of images, cell nuclei are counterstained using 4′,6′-diamidino-2-phenylindole (DAPI, gray). SIgA or Tf were incubated for 1 hour at 37°C. The top view of 3D reconstructions from Z-stack acquisitions are depicted. Results are representative of 3 experiments performed in triplicate Transwell filters carrying polarized Caco-2 cell monolayers. White bar, 20 μm.

Figure 2.

Preassociation with SIgA monoclonal antibody promotes entry of commensal bacteria into Peyer's patches (PP) and subsequent uptake by DCs in the SED region. (A) Representative picture of the tracking of FITC-labeled BL (green rods), administered alone or in complex with non-specific SIgA, 2 hours after injection in a ligated ileal loop comprising a PP. More elevated number of bacteria could be detected in the subepithelial dome (SED) region of PPs when administered in association with SIgA (right panel), with most of them observed in close contact with red-labeled DCs, resulting in the appearance of yellow spots (arrows). Cell nuclei are counterstained with 4′,6′-diamidino-2-phenylindole (DAPI, blue). L, lumen; FAE, follicule associated epithelium. (B) Quantification of FITC-labeled BL in the FAE and SED region of 6 individual sections obtained from 3 independent experiments. Numbers are median ± SE (error bars) and statistical differences calcuated with the Student's t test comparing the 2 experimental conditions at the same time-point are indicated by their respective p value.

Figure 3.

In vivo coating of BL by engodenous intestinal SIgA and subsequent entry into a Peyer's patch. (A) Representative image of the visualization of FITC-labeled BL (green rods) administered as such in a mouse ligated ileal loop containing a Peyer's patch. Cell nuclei are stained in blue (DAPI). (B) Three-dimensional reconstructed image from the magnified area (yellow square) depicted in panel (A). Magnification of the subepithelial dome (SED) region demonstrates surface (yellow) and internal (green) co-localization with red-labeled DCs. Bacteria interacting with DCs are associated with endogenous SIgA (arrows in magnified insets 1 and 2). In the insets, the red channel was not activated to allow visualization of coating SIgA on the bacterial surface (pink). L, lumen; V, villus.

Figure 4.

Presence of SIgA during the pre-weaning period promotes early-life immune maturation. Mouse pups from germ-free dams were daily supplemented for the last 14 d of the suckling period with either a saline solution (PBS, control for absence of SIgA), 10⁴ CFU of the commensal bacteria BL known to promote immune maturation in humans, 10⁴ CFU of BL associated with 500 pg of non-specific murine SIgA monoclonal antibody, or the equivalent amount of the same SIgA alone. Germ-free dams have an almost completely SIgA-deprived milk. Concomitantly to supplementation, pups were exposed to conventional mouse fecal bacteria to induce natural microbial colonization. Monitoring of mucosal immune maturation was assessed through quantification of the number IgA secreting cells in Peyer's patches (PPs, top panels) and mesenteric lymph nodes (MLNs, bottom panels), as measured by the ELISPOT technique (median ± SEM). IgA production was assessed at the end of the supplementation period (weaning, day 21 of age, left panels) and 6 weeks later (early adulthood, day 63 of age, right panels). Statistical significance (Wilcoxon test) is indicated by the letters: a = P < 0.001 vs. PBS control; b = P ≤ 0.002 vs. BL alone; c = P < 0.001 vs. SIgA alone. n = 10 to 11 pups per experimental group for each organ and time point investigated.