Airway surfactant, a primary defense barrier: Mechanical and immunological aspects

Epidemiologic studies have shown strong associations between mortality and morbidity from respiratory and cardiac causes and exposure to fine (PM(10)), but not coarse, particulates. A plausible mechanistic explanation for these associations is lacking. It has been shown that particles may be retained for an extended period of time in the airways, and that their clearance is inversely proportional to particle size. Such particles are localized in close association with the airway epithelium, and if they consist of low surface energy material, will be coated with an osmiophilic layer, consistent with surfactant. Particles are displaced into this position by surface and line tension forces exerted by the surfactant film at the air-aqueous interface. Particle displacement due to line tension is much greater for smaller particles in the micrometer range. The surface forces acting on the particles leave deep indentations on the epithelial cells. During the displacement process they may come into contact with airway macrophages in the mucous layer and/or dendritic cells situated in the airway epithelium. The smallest particles may even penetrate the mucosa to enter the interstitial compartment. In addition to altering the physical properties of particles, surfactant coatings reduce particle toxicity and enhance phagocytosis by opsonization. We speculate that surfactant acts as a primary defense barrier and plays a role in antigen presentation and elimination at the air-mucus interface of the airways.