Recognition of Gram-positive Intestinal Bacteria by Hybridoma- and Colostrum-derived Secretory Immunoglobulin A Is Mediated by Carbohydrates

Amandine Mathias,Blaise Corthésy

doi:10.1074/jbc.m110.209015

Abstract

Humans live in symbiosis with 10(14) commensal bacteria among which >99% resides in their gastrointestinal tract. The molecular bases pertaining to the interaction between mucosal secretory IgA (SIgA) and bacteria residing in the intestine are not known. Previous studies have demonstrated that commensals are naturally coated by SIgA in the gut lumen. Thus, understanding how natural SIgA interacts with commensal bacteria can provide new clues on its multiple functions at mucosal surfaces. Using fluorescently labeled, nonspecific SIgA or secretory component (SC), we visualized by confocal microscopy the interaction with various commensal bacteria, including Lactobacillus, Bifidobacteria, Escherichia coli, and Bacteroides strains. These experiments revealed that the interaction between SIgA and commensal bacteria involves Fab- and Fc-independent structural motifs, featuring SC as a crucial partner. Removal of glycans present on free SC or bound in SIgA resulted in a drastic drop in the interaction with gram-positive bacteria, indicating the essential role of carbohydrates in the process. In contrast, poor binding of gram-positive bacteria by control IgG was observed. The interaction with gram-negative bacteria was preserved whatever the molecular form of protein partner used, suggesting the involvement of different binding motifs. Purified SIgA and SC from either mouse hybridoma cells or human colostrum exhibited identical patterns of recognition for gram-positive bacteria, emphasizing conserved plasticity between species. Thus, sugar-mediated binding of commensals by SIgA highlights the currently underappreciated role of glycans in mediating the interaction between a highly diverse microbiota and the mucosal immune system.

Highlights

Secretory IgA (SIgA)2 produced by plasma cells in the lamina propria represents the major immunoglobulin found at mucosal surfaces
We made the assumption that nonspecific SIgA serving as substitute of natural SIgA can bind to commensals in a Fab-independent fashion
Contrary to what was observed with commensal bacteria of the Lactobacillus and Bifidobacterium genera, we found that the commensal Gram-negative nonpathogenic strains E. coli D2241 and Nissle and of the Bacteroides phylum were recognized by IgGC20 to the same extend as with either SIgA or Mouse SC (mSC)

Summary

Introduction

Secretory IgA (SIgA)2 produced by plasma cells in the lamina propria represents the major immunoglobulin found at mucosal surfaces. Bifidobacterium, interaction of the three tested Gram-negative bacteria with deglycosylated or native proteins resulted in similar high level binding, indicative of a selective role of carbohydrates in the binding of Gram-positive bacteria only (data not shown).

Results

Conclusion