Neonatal neutrophil host defense. Prospects for immunologic enhancement during neonatal sepsis.

Neonatal host defense simulates a clinical state of immunodeficiency that predisposes the preterm and term newborn to overwhelming bacterial sepsis. There are various immunologic components that are deficient in the newborn and new methods to enhance their function. Defects in both the quantitative and qualitative aspects of the neonatal phagocyte contribute substantially to the immaturity of neonates' immune systems. The neonate lacks an adequate number of granulocyte bone marrow progenitor cells, and has a decreased neutrophil storage pool and an increased tendency to peripheral neutropenia during neonatal sepsis. Additionally, the neonatal granulocyte demonstrates altered physiologic function compared with that found in the adult with respect to chemotaxis, phagocytosis, oxidative metabolism, and bacterial killing. Some recent clinical studies have suggested the benefit of using adult neutrophil transfusions as adjuvant treatment during neonatal bacterial sepsis, yet other studies have found the use of polymorphonuclear neutrophil leukocyte transfusions to be inconclusive. Reduced circulating immunoglobulins and impaired production of specific antibody have also led to recent trials in the use of prophylactic intravenous immunoglobulin in preterm infants predisposed to sepsis. Recently, hematopoietic colony-stimulating factors have been demonstrated to improve in vitro neonatal neutrophil physiologic activity. Future therapy of neonatal sepsis will depend on new nontoxic methods for enhancing neonatal host defense.