The pristine unused pulmonary surfactant isolated from human amniotic fluid forms highly condensed interfacial films

Pulmonary surfactant is a lipid/protein complex that coats the alveolar air-liquid interface to minimize surface tension facilitating breathing mechanics. Native surfactant (NS) is typically obtained from lavages of animal lungs, where it has gone through structural alterations as a result of exposure to respiratory dynamics and highly oxidative environments. We have studied here the structure of interfacial films formed by human amniotic fluid surfactant (AFS), thought to maintain the structural and functional features of a fully operative still non-used surfactant, as it has not been subjected to breathing dynamics yet. The results show that AFS adsorbs better at the interface, to form films supporting higher compression rates, than NS upon spreading at comparable concentrations and amounts. Films formed by AFS exhibit condensed regions excluding fluorescently labeled lipids from the mere adsorption, while films formed by NS only showed segregation of ordered-like domains once subjected to compression-expansion dynamics. Finally, AFS films were consistent with the presence of solid-like highly ordered phases, while NS consisted under comparable conditions of a coexistence of liquid-disordered/liquid-ordered fluid phases. This indicates that operative surfactant films formed by freshly secreted surfactant could be much more condensed than previously supposed, likely providing maximal stability under breathing mechanics.