How to assess the effectiveness of nasal influenza vaccines? Role and measurement of sIgA in mucosal secretions

Abstract

Secretory IgAs (sIgA) constitute the principal isotype of antibodies present in nasal and mucosal secretions. They are secreted by plasma cells adjacent to the mucosal epithelial cells, the site where infection occurs, and are the main humoral mediator of mucosal immunity. Mucosally delivered vaccines, such as live attenuated influenza vaccine (LAIV), are able to mimic natural infection without causing disease or virus transmission and mainly elicit a local immune response. The measurement of sIgA concentrations in nasal swab/wash and saliva samples is therefore a valuable tool for evaluating their role in the effectiveness of such vaccines. Here, we describe two standardized assays (enzyme-linked immunosorbent assay and microneutralization) available for the quantification of sIgA and discuss the advantages and limitations of their use.

Keywords: enzyme-linked immunosorbent assay; influenza vaccines; influenza virus; mucosal immunity; secretory IgAs.

Figure 1

Simplified scheme of immune responses following influenza virus infection of the upper respiratory tract with focus on induction and mode of action of secreted IgA (sIgA). Abbreviations: B cell, B lymphocyte; DCs, dendritic cells; dIgA, dimeric IgA; IgA, immunoglobulin A; IgG, immunoglobulin G; MALT, mucosa‐associated lymphoid tissue; PCs, plasma cells; pIgR, polymeric immunoglobulin receptor (pIgR); T cell, T lymphocyte. Influenza viruses infect epithelial cells of the mucosa and induce mucosal immune responses. Mucosal immune system consists of two sites. Inductive site (MALT) for antigen uptake by DCs and priming of T and B cells for IgA antibody production. Effector site with IgA‐secreting PCs. DCs take up exogenous virus antigens (from virus particles or apoptotic infected cells) by endocytosis and activate naïve T and B cells. PCs secrete IgA antibodies. IgG antibodies transude from the serum to the mucus by diffusion and provide protection against homologous influenza viruses. dIgA are actively transcytosed across epithelial cells via pIgR and provide protection against homologous and heterologous influenza viruses. dIgA can bind to newly synthesized viral proteins within virus‐infected epithelial cells and prevent virus

Figure 2

Principles of ELISA assays for the determination and standardization of the IgA content in test samples. Test samples include nasal washes, swabs, or saliva samples. Method 1: Standardization of antigen‐specific IgA antibodies against total IgA (left side, yellow arrows). Method 2: Standardization of antigen‐specific IgA antibodies against total protein (right side, blue and red arrows) Variation 1: high total protein content (>1 mg/mL; middle right side, blue arrows). Variation 2: low protein content (<1 mg/mL) or big differences of the total protein content of different test samples (rightmost side, red arrows)