Postnatal maturation of pulmonary antimicrobial defense mechanisms in conventional and germ-free lambs

The phagocytic and bactericidal capacities of ovine bronchoalveolar lavage (BAL) cells were investigated as a function of postnatal age. In addition, age-related changes in the elaboration by alveolar macrophages of chemotaxins for neutrophils, concentrations of BAL fluid and serum immunoglobulins, and serum opsonic capacity were determined. BAL cells exhibited major changes in morphology, composition, and in vitro proliferation during the 1st postnatal wk. Studies in germ-free lambs indicated that the antigenic burden of the ambient environment markedly influenced the concentration of BAL neutrophils but had no effect on the influx, phagocytic, and proliferative activities of alveolar macrophages. Phagocytic and bactericidal functions of BAL cells improved rapidly during the 1st postnatal wk, then declined, and did not reattain adult levels until day 180. The capacity of alveolar macrophages to elaborate chemotaxins for neutrophils was deficient at day 8, but not at subsequent ages. The concentration of BAL IgG1 increased until day 8, fell at day 21, and then continued to increase gradually. IgA was not detected in BAL until day 21 and increased rapidly thereafter. Serum opsonic capacity at days 1 and 4 was comparable to that of adult serum, but sera from days 8 to 42 showed a marked reduction in opsonic capacity. Pulmonary antimicrobial defenses in neonatal sheep were thus found to be deficient to some degree throughout the first 3 months of life. It was not until day 180 that the parameters investigated in this study approximated those of adult sheep.