Secretory IgA N-glycans contribute to the protection against E. coli O55 infection of germ-free piglets

Leona Rašková Kafková ,Petra Hermanová ,Helena Tlaskalová‐Hogenová ,Zdeněk Novák ,Marek Šinkora ,Alena Fajstová ,Ivo Überall ,Luca Vannucci,Jiří Dvořák ,Dagmar Šrůtková ,Renata Štěpánková ,Kateřina Štěpánová ,Michal Křupka ,Zuzana Jirásková Zákostelská ,Jiří Městecký ,Jozef Škarda ,Diana Brokešová ,Zuzana Stehlíková ,Štěpán Coufal ,Milan Raška

doi:10.1038/s41385-020-00345-8

Abstract

Mucosal surfaces are colonized by highly diverse commensal microbiota. Coating with secretory IgA (SIgA) promotes the survival of commensal bacteria while it inhibits the invasion by pathogens. Bacterial coating could be mediated by antigen-specific SIgA recognition, polyreactivity, and/or by the SIgA-associated glycans. In contrast to many in vitro studies, only a few reported the effect of SIgA glycans in vivo. Here, we used a germ-free antibody-free newborn piglets model to compare the protective effect of SIgA, SIgA with enzymatically removed N-glycans, Fab, and Fc containing the secretory component (Fc-SC) during oral necrotoxigenic E. coli O55 challenge. SIgA, Fab, and Fc-SC were protective, whereas removal of N-glycans from SIgA reduced SIgA-mediated protection as demonstrated by piglets’ intestinal histology, clinical status, and survival. In vitro analyses indicated that deglycosylation of SIgA did not reduce agglutination of E. coli O55. These findings highlight the role of SIgA-associated N-glycans in protection. Further structural studies of SIgA-associated glycans would lead to the identification of those involved in the species-specific inhibition of attachment to corresponding epithelial cells.

Highlights

Large surfaces of mucosae, especially of the intestinal tract, are colonized by enormous numbers of highly diverse microbiota.[1]
Preparation and characterization of secretory IgA (SIgA), deglycosylated SIgA and SIgA fragments SIgA was purified by gel-permeation chromatography from a protein-enriched fraction of pooled human colostrum/milk prepared by ammonium sulfate precipitation.[37]
The composition of generated fragments was analyzed by non-reducing SDS PAGE where Fc containing the secretory component (Fc-secretory component (SC)) exhibited the molecular mass of ≈250 kDa and Fab ≈40 kDa (Fig. 1b), and by reducing SDS PAGE where Fc-SC segregated into SC (≈75 kDa), C. ramosum-cleaved α chain constant domains of IgA1 and IgA2, and J chain (≈16 kDa) (Fig. 1b, right subpanel)

Summary

Introduction

Especially of the intestinal tract, are colonized by enormous numbers of highly diverse microbiota.[1] A complex interplay of complementary mechanisms of innate and specific immunity contains this microbiota in its physiological compartment without compromising the integrity of mucosal tissues.[2] In vivo bacteria are coated with mucosal antibodies, in humans dominantly of the IgA isotype,[2,3,4,5,6] that provide protection against pathogenic microorganisms but at the same time promote successful survival and containment of endogenous commensal mucosal microbiota.[3,4,5,6] it is not known whether this coating of bacteria is exclusively dependent on the highly specific antibody activity, polyreactivity, and the interactions of IgAassociated glycans with complementary structures on microorganisms.[5,7,8,9,10,11,12,13,14,15,16] Considering the enormous numbers and highly variable species diversity along with the large number of potential antigenic determinants on bacterial structures, it is probable that all three mechanisms participate in the protective activity of secretory IgA (SIgA).[2,3,4,5,8,17,18] We reported that the binding of gram-negative bacteria to the glycan receptors on epithelial cells is inhibited by IgA-associated glycans as confirmed and extended to other species of both gram-positive and gramnegative bacteria.[10,17,19,20,21,22] In addition to the heavy (H) α chains of IgA, secretory component (SC), the extracellular part of the polymeric Ig receptor (pIgR), is heavily glycosylated and acts as a highly effective inhibitor of bacteria adherence to epithelial cells[7,10,13,14,19,20,23,24] enforcing the protective functions of SIgA.[8,25] Structural studies of human SIgA as well as monoclonal myeloma IgA proteins revealed a remarkable degree of glycan heterogeneity with respect to the composition, number, site of attachment and primary structures of the glycan sidechains.[9,19,20,26,27,28] Whether these glycan-dependent interactions are of importance in the protection in vivo has been previously demonstrated only in the murine model of protection of the respiratory tract against Shigella flexneri in which recombinant glycosylated SC bound to Shigella-specific IgA was tested.[29]

Methods

Results

Conclusion