Antigen Binding to Secretory Immunoglobulin A Results in Decreased Sensitivity to Intestinal Proteases and Increased Binding to Cellular Fc Receptors

Mélanie Duc,Finn-Eirik Johansen,Blaise Corthésy

doi:10.1074/jbc.m109.059220

Abstract

In intestinal secretions, secretory IgA (SIgA) plays an important sentinel and protective role in the recognition and clearance of enteric pathogens. In addition to serving as a first line of defense, SIgA and SIgA x antigen immune complexes are selectively transported across Peyer's patches to underlying dendritic cells in the mucosa-associated lymphoid tissue, contributing to immune surveillance and immunomodulation. To explain the unexpected transport of immune complexes in face of the large excess of free SIgA in secretions, we postulated that SIgA experiences structural modifications upon antigen binding. To address this issue, we associated specific polymeric IgA and SIgA with antigens of various sizes and complexity (protein toxin, virus, bacterium). Compared with free antibody, we found modified sensitivity of the three antigens assayed after exposure to proteases from intestinal washes. Antigen binding further impacted on the immunoreactivity toward polyclonal antisera specific for the heavy and light chains of the antibody, as a function of the antigen size. These conformational changes promoted binding of the SIgA-based immune complex compared with the free antibody to cellular receptors (Fc alphaRI and polymeric immunoglobulin receptor) expressed on the surface of premyelocytic and epithelial cell lines. These data reveal that antigen recognition by SIgA triggers structural changes that confer to the antibody enhanced receptor binding properties. This identifies immune complexes as particular structural entities integrating the presence of bound antigens and adds to the known function of immune exclusion and mucus anchoring by SIgA.

Highlights

A combination of equimolar amounts of purified polymeric IgA (pIgA) and human SC (hSC) (IgAPCG-4) or Mouse secretory component (mSC) (IgA7D9 and IgAC5) for 1 h at room temperature resulted in the formation of covalent heterodimer complexes, as indicated by the detection of shifted secretory component (SC) to the position of pIgA when the protein was analyzed under nonreducing conditions
When loaded onto a molecular sieving column run in phosphate-buffered saline (PBS), co-elution of SC and pIgA confirmed that the two proteins were associated in the form of covalent and noncovalent secretory IgA (SIgA) complexes (15)
The data reported demonstrate that, upon Ag binding, pIgA and SIgA experience structural changes reflected in the differential sensitivity to intestinal proteases and immunoreactivity toward ␣ and ␬ chain-specific antisera

Summary

Introduction

These conformational changes promoted binding of the SIgA-based immune complex compared with the free antibody to cellular receptors (Fc␣RI and polymeric immunoglobulin receptor) expressed on the surface of premyelocytic and epithelial cell lines. We found that Ag-driven conformational changes can be evidenced by examining the differential sensitivity to intestinal proteases and immunoreactivity of IC compared with free Ab. Increased selective binding to known cellular receptors (Fc␣RI, pIgR) further revealed major changes in IC compared with free Ab. The data support the notion that binding to Ag of various sizes affects the structure of pIgA and meric Ig receptor; SC, secretory component; hSC, human SC; mSC, mouse SC; SIgA, secretory IgA.

Results

Conclusion