The neonatal Fc receptor (FcRn) is expressed in the bovine lung

In neonatal calves, maternal immunoglobulin (Ig) is transferred into respiratory secretion which contributes to protection against pathogens. The early predominance of IgG1 in respiratory tract secretions is progressively reduced in favor of IgA by age but in the lower, bronchoalveolar system secreted IgG remains the dominant secreted Ig even in adulthood. The trans-epithelial transport of secretory IgA into mucosal secretions is carried out by the polymeric Ig receptor. However, the mechanism by which IgG crosses epithelial cells to provide defense on mucosal surfaces is still unknown. In order to investigate the possibility that the neonatal Fc receptor, FcRn is involved in this transport we have first analyzed the localization of this receptor in the upper and lower respiratory tracts. Consistent with the in situ hybridization data, immunohistochemistry showed undetectable expression in the tracheal epithelial cells, relatively weak expression in epithelial cells of the bronchi, apparent staining those lining the bronchioli and randomly scattered signal over the alveolar tissue. The bovine FcRn may thus play a role in IgG transport across mucosal epithelial barriers as a trafficking receptor and ensure IgG predominance in the lower respiratory tract.