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We compared the oxidative metabolism of alveolar macrophages (AM) from adult and neonatal (1- and 7-day-old) rabbits before and after their in vitro exposure to type Ia group B streptococci (GBS) opsonized with immune rabbit serum. Nonstimulated AM from 1-day-old, 7-day-old, or adult rabbits consumed O2 at a rate of 17 to 20 nmol/10(6) AM per 10 min under basal conditions and released minimal amounts of superoxide (O2-) into the medium. Approximately 80% of this basal respiration was of mitochondrial origin, based on its inhibition by NaCN. Exposure to GBS opsonized with immune rabbit serum stimulated O2 consumption approximately half as effectively in the neonatal AM as in the adult AM. Little O2- was released into the medium unless the cells were pretreated with dihydrocytochalasin B. Under such conditions, 1-day-old, 7-day-old, and adult AM released 3.6, 5.3, and 13.9 nmol of O2-/10(6) AM per 10 min, respectively. The uptake of opsonized GBS by 1-day-old AM was not affected by 1 mM NaCN, whereas phagocytosis by adult AM was substantially reduced under these conditions. Overall, our findings suggest that neonatal AM have less-well-developed postphagocytic oxidative metabolic responses and release less superoxide after exposure to opsonized GBS than do adult AM. They also demonstrate that the energy for phagocytosis is derived principally from mitochondrial metabolism in adult AM but not in neonatal AM. We conclude that metabolic differences between neonatal and adult AM may contribute to neonatal pulmonary susceptibility to GBS infections and account, in part, for the ability of GBS to succeed as neonatal pulmonary pathogens.

Oxidative metabolism of neonatal and adult rabbit lung macrophages stimulated with opsonized group B streptococci