Phagocytosis is an important element in host defense, and deficiencies may be reflected in decreased immune competence in neonates. In the present study, we analyzed phagocytosis in monocytes and granulocytes from term and preterm cord blood and adult peripheral blood. Heparinized whole blood was incubated(37°C) with formalin-fixed propidium iodide-labeled Staph. aureus (S.a.). RBC were then lysed and samples analyzed by flow cytometry. Internalization of bacteria was confirmed using fluorescent confocal microscopy. Time course analysis indicated that the initial relative rate of phagocytosis observed in full term cord blood monocytes was 57% lower than adult monocytes. Furthermore, phagocytosis of S.a. by full term cord blood monocytes reached maximal levels within 15 min, while phagocytosis in adult monocytes plateaued at approximately 25 min. The relative quantities of bacteria internalized by monocytes were also determined after 20 min incubation with S.a. Cord blood monocytes from both term and preterm (25-32 wk gestational age) neonates contained 61% less S.a. than adult monocytes. Monocytes from preterm infants did not differ significantly from full term. Full term cord blood granulocytes exhibited maximal rates of ingestion which were comparable to adult cells, but 47% less maximal internalization at 20 min incubation. Preterm cord granulocytes exhibited a trend toward diminished internalization relative to full term cells. These data suggest that monocytes and granulocytes from term and preterm neonates exhibit diminished capacity to phagocytize S.a. relative to adult cells.
