Antibody Transcytosis and Neutralizing Activity in Respiratory Epithelial Cells
The neutralizing activity present in human serum is considered a correlate of protection against SARS-CoV-2 infection and disease but the mechanisms by which serum antibodies are transported into the lumen of the respiratory tract, where they are required to interact with virus particles and infected cells remain incompletely understood. The transcytosis and neutralizing activity of serum-derived IgG and IgA antibodies was investigated using an in vitro SARS-CoV-2 infection model with primary differentiated human nasal and basal epithelial cells (hNECs and hBECs) cultures. Expression of the antibody transport receptors neonatal Fc receptor (FcRn) and polymeric immunoglobulin receptor (pIgR) in hNECs cultures was confirmed by qPCR, immunofluorescence microscopy, and flow cytometry. Both receptors were expressed throughout the epithelial cultures, with enriched expression observed in ciliated cells compared with goblet and basal cells. Purified IgG and IgA isolated from convalescent plasma demonstrated specificity for SARS-CoV-2 spike protein and inhibited ACE2-Spike interactions, although activity was reduced against later variants. Purified IgG contained higher anti-spike antibody titers than purified IgA. Functional neutralization assays showed that transcytosed IgG and IgA significantly reduced SARS-CoV-2 infection compared with untreated controls. However, serial dilution studies demonstrated that IgG-mediated neutralization was more potent than IgA-mediated neutralization. Similar results were determined for influenza A virus H3N2 subtype. The transcytosis of IgG was more efficient in hBEC cultures while IgA transcytosis was higher in hNEC cultures, reflecting the levels of the corresponding transport proteins. Together, these findings demonstrate that serum-derived IgG and IgA can undergo transepithelial transport across human nasal epithelium while retaining SARS-CoV-2 or influenza A virus neutralizing activity in vitro. These results suggest that FcRn- and pIgR-mediated antibody transport may contribute to mucosal protection following vaccination or infection and may help identify antibody responses associated with protection against SARS-CoV-2. ImportanceSerum antibody levels are considered correlates of protection for SARS-CoV-2 and Influenza A virus but its unclear how those antibodies are transported to the apical surface of respiratory epithelial cells, where they must be present for optimal activity. We show that IgG and IgA specific for SARS-CoV-2 or influenza A virus is transcytosed across respiratory epithelial cell cultures, the efficiency of which reflects the level of FcRn or pIgR expression levels, suggesting that cells of the upper and lower respiratory tract transport different antibodies from the blood.