Pulmonary Surfactant Inhibits Priming of Rabbit Alveolar Macrophage: Evidence that Surfactant Suppresses the Oxidative Burst of Alveolar Macrophage in Infant Rabbits

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Abstract
We have investigated the possible role of pulmonary surfactant in mediating a deficiency in alveolar macrophages (Am) from infant animals to produce chemiluminescent (CL) responses elicited by phorbol myristate acetate (PMA) or opsonized zymosan (Op-Zym). Freshly harvested AM from rabbits 7 to 28 days of age produced much reduced PMA-elicited CL responses compared to 5- to 6-month-old rabbits. Initially we observed that AM from both infant and adult rabbits exhibited increasing PMA- or Op-Zym-elicited CL responses in a time-dependent manner when they were incubated in RPMI 1640 medium without serum for more than 3 h. In addition, AM from infant rabbits acquired the capacity to become primed with macrophage activation factor (MAF) containing supernatants or fetal bovine serum (FBS) during an 18-h incubation period that was basically equivalent to that of AM from adult rabbits. Our studies reveal that natural and artificial surfactant inhibited "spontaneous" priming as well as priming of Am induced by MAF-containing supernatant or FBS for producing PMA- or OP-Zym-elicited CL responses from both normal infant and adult animals. In addition, these surfactant preparations also suppressed the response of AM from adult BCG-sensitized rabbits during the course of an 18-h incubation period. It is especially noteworthy that AM from infant rabbits appeared to be more susceptible to the inhibitory activity of surfactant than Am from adult animals. Our data indicate that freshly harvested AM from infant rabbits are under greater suppression than AM from adult rabbits in terms of their capacities to produce oxidative responses. We conclude that surfactant is at least partly responsible for this marked suppression of infant AM.