CFTR-rich ionocytes mediate chloride absorption across airway epithelia

The volume and composition of a thin layer of liquid covering the airway surface defend the lung from inhaled pathogens and debris. Airway epithelia secrete Cl(–) into the airway surface liquid through cystic fibrosis transmembrane conductance regulator (CFTR) channels, thereby increasing the volume of airway surface liquid. The discovery that pulmonary ionocytes contain high levels of CFTR led us to predict that ionocytes drive secretion. However, we found the opposite. Elevating ionocyte abundance increased liquid absorption, whereas reducing ionocyte abundance increased secretion. In contrast to other airway epithelial cells, ionocytes contained barttin/Cl(–) channels in their basolateral membrane. Disrupting barttin/Cl(–) channel function impaired liquid absorption, and overexpressing barttin/Cl(–) channels increased absorption. Together, apical CFTR and basolateral barttin/Cl(–) channels provide an electrically conductive pathway for Cl(–) flow through ionocytes, and the transepithelial voltage generated by apical Na(+) channels drives absorption. These findings indicate that ionocytes mediate liquid absorption, and secretory cells mediate liquid secretion. Segregating these counteracting activities to distinct cell types enables epithelia to precisely control the airway surface. Moreover, the divergent role of CFTR in ionocytes and secretory cells suggests that cystic fibrosis disrupts both liquid secretion and absorption.