Impaired Innate Immunity in the Newborn: Newborn Neutrophils Are Deficient in Bactericidal/Permeability-Increasing Protein

Abstract

Objective. The mechanisms by which newborns are at increased risk for invasive bacterial infections have been incompletely defined. A central element of innate immunity to bacterial infection is the neutrophil—a cell that contains cytoplasmic granules replete with antibiotic proteins and peptides. The activity of adult neutrophils against Gram-negative bacteria is believed to depend to a significant degree on the presence in neutrophil primary (azurophilic) granules of the 55-kDa bactericidal/permeability-increasing protein (BPI), which binds with high affinity to bacterial lipopolysaccharides and kills Gram-negative bacteria. In light of the importance of BPI to antibacterial host defense and to investigate possible factors underlying the risk of neonatal bacterial infections, we determined the relative content of BPI in the neutrophils of adults and newborns. Design. The cellular content of BPI was determined by Western blotting of neutrophils derived from full-term newborn cord blood (n = 21; mean gestational age: 38.6 weeks) and from adult peripheral blood (n = 22; mean age: 29 years). Extracellular levels of BPI in adult and newborn plasma were assessed by enzyme-linked immunosorbent assay. Neutrophil content of other azurophil granule markers also was assessed: myeloperoxidase by Western blotting and defensin peptides by acid-urea polyacrylamide gel electrophoresis and Coomassie staining. Acid extracts of newborn and adult neutrophils were analyzed for antibacterial activity against serum-resistant encapsulated isolate Escherichia coliK1/r. Results. The neutrophils of newborns contain at least threefold to fourfold less BPI per cell than adult neutrophils (67 ± 13 ng per 106 cells vs 234 ± 27 ng per 106 cells). The relative BPI-deficiency of newborn neutrophils apparently was not attributable to perinatal stress-related degranulation of intracellular BPI stores because: 1) newborn and adult neutrophils contained nearly identical amounts of 2 microbicidal constituents derived from the same primary (azurophil) granule compartment as BPI (the enzyme myeloperoxidase as well as defensin peptides), and 2) levels of extracellular BPI in newborn plasma, measured by enzyme-linked immunosorbent assay, represent only ∼2% of cellular BPI content. As predicted by their lower BPI content, newborn neutrophil acid extracts demonstrated significantly lower antibacterial activity against E coli K1/r than did adult neutrophil acid extracts. Conclusion. These data suggest that the neutrophils of newborns are selectively deficient in BPI, a central effector of antibacterial activity against Gram-negative bacteria. BPI deficiency correlates with decreased antibacterial activity of newborn neutrophil extracts against serum-resistant E coli and could contribute to the increased incidence of Gram-negative sepsis among newborns relative to healthy adults. neonatal sepsis, Gram-negative bacteria, endotoxin, neutrophil, polymorphonuclear leukocyte, innate immunity, bactericidal/permeability-increasing protein, defensin, myeloperoxidase.