Macromolecular Transport across Nasal and Respiratory Epithelia

Abstract

The airway and alveolar epithelia in conjunction with the mucociliary escalator act as a barrier to prevent the translocation of inhaled macromolecules, e.g., protein and peptide antigens, across pulmonary surfaces. Nevertheless, some exogenous macromolecules appear to cross the epithelia antigenically intact through either the paracellular or the transcellular route. Endogenous macromolecules including albumin, immunoglobulin, lactoferrin, α2-macroglobin, and α1antitrypsin have been measured in either bronchoalveolar lavage fluid or the liquid that lines airway surfaces, airway surface liquid (Mentz et al., 1984; Bignon et al., 1976). These proteins may have specialized methods for translocation into and out of the airway surface liquid and may also play significant roles in fluid homeostasis, particularly in the alveolar region. Yet the knowledge of the mechanisms responsible for the maintenance of protein gradients across respiratory epithelia is primitive in relation to similar knowledge regarding ion gradients.