PL. Distribution and surfactant association of carcinoembryonic cell adhesion molecule 6 in human lung. Am J Physiol Lung Cell Mol Physiol 302: L216 -L225, 2012. First published October 28, 2011 doi:10.1152/ajplung.00055.2011.-Carcinoembryonic cell adhesion molecule 6 (CEACAM6) is a glycosylated, glycophosphatidylinositol-anchored protein expressed in epithelial cells of various primate tissues. It binds gram-negative bacteria and is overexpressed in human cancers. CEACAM6 is associated with lamellar bodies of cultured type II cells of human fetal lung and protects surfactant function in vitro. In this study, we characterized CEACAM6 expression in vivo in human lung. CEACAM6 was present in lung lavage of premature infants at birth and increased progressively in intubated infants with lung disease. Of surfactant-associated CEACAM6, ϳ80% was the fully glycosylated, 90-kDa form that contains the glycophosphatidylinositol anchor, and the concentration (3.9% of phospholipid for adult lung) was comparable to that for surfactant proteins (SP)-A/B/C. We examined the affinity of CEACAM6 by purification of surfactant on density gradient centrifugation; concentrations of CEACAM6 and SP-B per phospholipid were unchanged, whereas levels of total protein and SP-A decreased by 60%. CEACAM6 mRNA content decreased progressively from upper trachea to peripheral fetal lung, whereas protein levels were similar in all regions of adult lung, suggesting proximal-to-distal developmental expression in lung epithelium. In adult lung, most type I cells and ϳ50% of type II cells were immunopositive. We conclude that CEACAM6 is expressed by alveolar and airway epithelial cells of human lung and is secreted into lung-lining fluid, where fully glycosylated protein binds to surfactant. Production appears to be upregulated during neonatal lung disease, perhaps related to roles of CEACAM6 in surfactant function, cell proliferation, and innate immune defense. alveolar type II cells; lung lining fluid; surfactant; airway epithelium CARCINOEMBRYONIC CELL ADHESION molecule 6 (CEACAM6) (also called NCA, NCA-50/90, and CD66c) is a member of the carcinoembryonic antigen (CEA) gene family, consisting of 29 related genes. CEA proteins function as intercellular homophilic and heterophilic adhesion molecules and have signaling properties (18). CEACAM6 contains a glycophosphatidylinositol (GPI) membrane anchor (23) and is targeted to lipid rafts in apical membranes of polarized epithelial cells (26). CEACAM6 binds a variety of gram-negative bacteria and mediates internalization and phagocytosis, participating in innate immune defense in the intestine (13). The CEACAM6 gene is not present in rodents, and its emergence in primates may represent pathogen-host coevolution, providing different protein structures with selective bacterial binding properties.Expression of CEACAM6 and closely related CEACAM5 is deregulated and overexpressed in cancers of colorectal epithelium, with surface levels inversely correlated with both the degree of colonocyte differentiat...