Innate immunity is comprised of cellular and humoral factors that provide rapid protection against microbial invasion. The past 2 decades have seen a rapid expansion of our understanding of the innate immune system on a molecular level. Host receptors for microbes and their surface components have been defined, intracellular signaling pathways have been elucidated, and effector molecules have been isolated and characterized (1). Despite its meaning (i.e. “present at birth”) and its importance in the context of impairment in acquired immunity at birth, relatively few studies have addressed innate immunity in newborns.
A key cellular effector of innate immunity is the neutrophil whose cytoplasmic granules are replete with antimicrobial proteins and peptides (2–4). Among the antimicrobial proteins of humans is the bactericidal/permeability-increasing protein (BPI), a cationic ∼50 kD protein found in the primary (azurophilic) granules of neutrophils that was discovered and isolated by Elsbach and Weiss working at New York University in the late 1970s (5, 6). BPI possesses high affinity toward the lipid A region of the lipopolysaccharides (LPS or “endotoxin”) that comprise the outer leaflet of the Gram-negative bacterial outer membrane (7). Binding of BPI to the lipid A moiety of LPS targets BPI's cytotoxic activity (manifest against many, but not all, species of Gram-negative bacteria (8, 9);), opsonizes such bacteria for enhanced phagocytosis (10); and neutralizes the inflammatory (endotoxic) effects of LPS (11).
BPI's endotoxin-neutralizing activity, based on BPI's ability to bind LPS thereby preventing interaction of LPS with host receptors, is particularly potent (at nanomolecular levels), is evident against both isolated LPS and whole Gram-negative bacteria, and is manifest in biologic fluids, including whole blood (8). Of note, BPI's endotoxin-neutralizing activity is opposite to that of its structural homolog, LPS-binding protein (LBP), a liver-derived plasma constituent that enhances endotoxin's activity by delivering LPS to its cellular receptors (12). It is this potent endotoxin neutralizing activity of BPI that has rendered it an attractive candidate for pharmaceutical development as a novel anti-infective agent (13). Recombinant N-terminal fragments of BPI (such as rBPI21 and rBPI23) have demonstrated safety without immunogenicity in phase I human trials (14) and neutralized endotoxin-induced cytokine responses and physiologic changes in human volunteers (15, 16). In a recently completed phase III trial in children with fulminant meningococcal sepsis, administration of rBPI21 was associated with improved clinical outcomes (17). However, the mortality rate in this study was substantially lower than expected and, thus, the trial was underpowered to detect significant differences in survival, the primary outcome variable. As a result, approval of rBPI21 as a novel drug awaits “further information”(18).
Although impairment in the antibacterial function of newborn neutrophils has been noted for some time (19), it is only recently that BPI has been studied in newborns. The genesis of work on BPI in newborns can be traced to a study by Qing et al. demonstrating that human cord blood-derived neutrophils lack a ∼50 kD membrane protein that binds the lipid A region of LPS (20). The localization (BPI can be found on the cell membrane (21);), molecular weight, and lipid A binding properties of this deficient membrane protein were similar to those of BPI (and to that of the LPS receptor, CD14), prompting our laboratory to directly measure intracellular neutrophil BPI content of adults and newborns. Employing detergent extraction of neutrophils and BPI-specific Western Blotting, we found that the neutrophils of newborns have, on average, 3- to 4-fold less intracellular BPI than neutrophils of adults (22). This diminished BPI level correlated with diminished activity of neutrophil sulfuric acid extracts against the BPI-sensitive bacterium E. coli K1/r. Deficiency of BPI was apparently selective as the relative levels of two other primary granule constituents, myeloperoxidase (MPO) and the defensin peptides, were indistinguishable.
Approaching the question from the standpoint of neutrophil degranulation, Nupponen and colleagues have now studied the ability of neutrophils from newborns and adults to release BPI to the extracellular space (23). They find that when stimulated with the secretagogue phorbol myristate acetate (PMA), the neutrophils of preterm newborns release significantly less BPI per cell than adult neutrophils. No differences were found in the ability of newborn and adult neutrophils to release MPO, suggesting a selective deficiency in BPI release by the neutrophils of preterm infants.
In an apparent discordance with our study demonstrating lower intracellular pools of BPI in neutrophils of full term newborns relative to those of adults (22), Nupponen et al. find no difference in the amounts of BPI released from the neutrophils of full term newborns and those of adults. However, our study examined intracellular BPI content whereas Nupponen's focused on BPI released by PMA stimulation. Thus, the use of these distinct methodologies may have provided somewhat different information. Of note, BPI possesses a highly hydrophobic C-terminal half that is apparently tightly associated with the primary granule membrane (24). Consistent with this, the majority of cell-associated BPI remains intracellular even in the presence of a strong secretagogue (25) (and unpublished observations). One possible explanation for the apparent discrepancy between our studies could therefore be the presence of distinct sub-populations of intracellular BPI subject to different degrees of extracellular release. Regardless of these differences, what is most consistent and remarkable among the Nupponen study, our study, and, most likely, the Qing study is the consistently lower BPI content in the neutrophils of (preterm) newborns when compared with adults. Taken together, these three studies suggest an age-dependent maturation in the ability of human neutrophils to mobilize BPI to sites of infection.
Why should the expression of BPI be developmentally regulated? One possibility is that the need to quench endotoxin activity may vary with age, perhaps due to a similar age-dependent variation in the sensitivity of humans to the inflammatory effects of endotoxin (26). Little is known about the regulation of BPI gene expression. Of note, a recent study demonstrates novel expression of BPI by human mucosal epithelial cells that is inducible by lipoxins (endogenous anti-inflammatory lipids induced by aspirin in vivo) (27). Whether endogenous lipid mediators may play a role in the developmental expression of BPI is another question that can now be addressed.
Lastly, these emerging data may have important clinical consequences. For example, the ability of recombinant proteins such as rBPI21 to enhance antibacterial activity of human cord blood and to block bacterial endotoxic activity in cord blood (viz., tumor necrosis factor release (9);) raises the possibility that supplementing the relatively low endogenous BPI stores of newborns (e.g. by i.v. administration of rBPI21) may provide clinical benefit. A population that might particularly benefit from such intervention would be very low birthweight premature infants who are at high risk for Gram-negative sepsis (28, 29) and/or other conditions associated with endotoxemia, such as necrotizing enterocolitis (30–32).
References
Hoffman J, Kafatos F, Janeway C, Ezekowitz R 1999 Phylogenetic perspectives in innate immunity. Science 284: 1313–1318
Ganz T, Lehrer RI 1999 Antibiotic peptides from higher eukaryotes: biology applications. Mol Med Today 5: 292–297
Levy O 2000 Antibiotic proteins peptides of blood: Templates for novel antimicrobial agents. Blood 96: 2664–2672
Hancock R 2001 Cationic peptides: effectors in innate immunity novel antimicrobials. Lancet Infect Dis 1: 156–164
Weiss J, Elsbach P, Olsson I, Odeberg H 1978 Purification characterization of a potent bactericidal membrane active protein from the granules of human polymorphonuclear leukocytes. J Biol Chem 253: 2664–2672
Levy O, Elsbach P 2001 Bactericidal/Permeability-Increasing Protein in host defense its efficacy in the treatment of bacterial sepsis. Curr Infect Dis Rep 3: 407–412
Gazzano-Santoro H, Parent JB, Grinna L, Horwitz A, Parsons T, Theofan G, Elsbach P, Weiss J, Conlon PJ 1992 High-affinity binding of the bactericidal/permeability-increasing protein a recombinant amino-terminal fragment to the lipid A region of lipopolysaccharide. Infect Immun 60: 4754–4761
Weiss J, Elsbach P, Shu C, Castillo J, Grinna L, Horwitz A, Theofan G 1992 Human bactericidal/permeability-increasing protein a recombinant NH2-terminal fragment cause killing of serum-resistant gram-negative bacteria in whole blood inhibit tumor necrosis factor release induced by the bacteria. J Clin Invest 90: 1122–1130
Levy O, Sisson R, Kenyon J, Eichenwald E, Macone A, Goldmann D 2000 Enhancement of neonatal innate defense: Effects of adding an N-terminal recombinant fragment of bactericidal/permeability-increasing protein (rBPI21) on growth TNF-inducing activity of Gram-negative bacteria tested in neonatal cord blood ex vivo. Infect Immun 68: 5120–5125
Iovine NM, Elsbach P, Weiss J 1997 An opsonic function of the neutrophil bactericidal/permeability-increasing protein depends on both its N- C-terminal domains. Proc Natl Acad Sci USA 94: 10973–10978
Marra MN, Wilde CG, Griffith JE, Snable JL, Scott RW 1990 Bactericidal/permeability-increasing protein has endotoxin-neutralizing activity. J Immunol 144: 662–666
Mathison JC, Tobias PS, Wolfson E, Ulevitch RJ 1992 Plasma lipopolysaccharide (LPS)-binding protein. A key component in macrophage recognition of gram-negative LPS. J Immunol 149: 200–206
Levy O 2002 Therapeutic potential of the bactericidal/permeability-increasing protein. Expert Opinion in Investigational Drugs 11: 159–167
Bauer RJ, White ML, Wedel N, Nelson BJ, Friedmann N, Cohen A, Hustinx WN, Kung AH 1996 A phase I safety pharmacokinetic study of a recombinant amino terminal fragment of bactericidal/permeability-increasing protein in healthy male volunteers. Shock 5: 91–96
von der Mohlen M, van Deventer S, Levi M, et al 1995 Inhibition of endotoxin-induced cytokine release neutrophil activation in humans by use of recombinant bactericidal/permeability-increasing protein. J Infect Dis 172: 144–151
von der Mohlen M, van Deventer S, Levi M, van den Ende B, Wedel N, Nelson B, Friedmann N, ten Cate J 1995 Inhibition of endotoxin-induced activation of the coagulation fibrinolytic pathways using a recombinant endotoxin-binding protein (rBPI23). Blood 85: 3437–3443
Levin M, Quint P, Goldstein B, Barton P, Bradley J, Shemie S, Yeh T, Kim S, Cafaro D, Scannon P, Giroir B 2000 Recombinant bactericidal/permeability-increasing protein (rBPI21) as adjunctive treatment for children with severe meningococcal sepsis: a randomised trial. Lancet 356: 961–967
Giroir BP, Scannon PJ, Levin M 2001 Bactericidal/Permeability-Increasing Protein (BPI)- Lessons learned from the phase III, randomised, clinical trial of rBPI21 for adjunctive treatment of children with severe meningococcal sepsis. Crit Care Med 29: 5130–5135
Wright WJ, Ank B, Herbert J, Stiehm E 1975 Decreased bactericidal activity of leukocytes of stressed newborn infants. Pediatrics 56: 579–584
Qing G, Howlett S, Bortolussi R 1996 Lipopolysaccharide binding proteins on polymorphonuclear leukocytes: comparison of adult neonatal cells. Infect Immun 64: 4638–4642
Weersink AJ, van Kessel KP, van den Tol ME, van Strijp JA, Torensma R, Verhoef J, Elsbach P, Weiss J 1993 Human granulocytes express a 55-kDa lipopolysaccharide-binding protein on the cell surface that is identical to the bactericidal/permeability-increasing protein. J Immunol 150: 253–263
Levy O, Martin S, Eichenwald E, Ganz T, Valore E, Carroll S, Lee K, Goldmann D, Thorne G 1999 Impaired innate immunity in the newborn: newborn neutrophils are deficient in bactericidal/permeability-increasing protein (BPI). Pediatrics 104: 1327–1333
Nupponen I, Turunen R, Nevalainen T, Peuravuori H, Pohjavuori M, Repo H, Andersson S 2002 Extracellular release of bactericidal/permeability-increasing protein in newborn infants. Pediatr Res 51: 670–674
Gray PW, Flaggs G, Leong SR, Gumina RJ, Weiss J, Ooi CE, Elsbach P 1989 Cloning of the cDNA of a human neutrophil bactericidal protein. Structural functional correlations. J Biol Chem 264: 9505–9509
Weiss J, Olsson I 1987 Cellular subcellular localization of the bactericidal/permeability-increasing protein of neutrophils. Blood 69: 652–659
Qing G, Rajaraman K, Bortolussi R 1995 Diminished priming of neonatal polymorphonuclear leukocytes by lipopolysaccharide is associated with reduced CD14 expression. Infect Immun 63: 248–252
Caney G, Levy O, Sisson R, Narravula-Alipati S, Serhan C, Colgan S 2002 Lipid mediator-induced expression of bactericidal/permeability-increasing protein (BPI) in human mucosal epithelia. Proc Nat Acad Sci USA 99: 3902–3907
Beck-Sague C, Azimi P, Fonseca S, Baltimore R, Powell D, Bland L, Ardino M, McAllister S, Huberman R, Sinkowitz R 1994 Bloodstream infections in neonatal intensive care unit patients: results of a multicenter study. Pediatr Infect Dis J 13: 1110–1116
Stoll B, Gordon T, Korones S, Shankkaran S, Tyson J, Bauer C, Fanaroff A, Lemons J, Donovan E, Oh W, Stevenson D, Ehrenkranz R, Papile L, Verter J, Wright L 1996 Late-onset sepsis in very low birthweight neonates: a report from the National Institute of Child Health Human Development Neonatal Research Network. J Pediatr 129: 63–71
Caplan MS, Hsueh W 1990 Necrotizing enterocolitis: role of platelet activating factor, endotoxin, tumor necrosis factor. J Pediatr 117: S47–51
Scheifele DW, Olsen EM, Pendray MR 1985 Endotoxinemia thrombocytopenia during neonatal necrotizing enterocolitis. Am J Clin Pathol 83: 227–229
Duffy L, Zielezny M, Carrion V, Griffiths E, Dryja D, Hilty M, Rook C, Morin F 1997 Concordance of bacterial cultures with endotoxin interleukin-6 in necrotizing enterocolitis. Dig Dis Sci 42: 359–365
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Levy, O. Impaired Innate Immunity at Birth: Deficiency of Bactericidal/Permeability-Increasing Protein (BPI) in the Neutrophils of Newborns. Pediatr Res 51, 667–669 (2002). https://doi.org/10.1203/00006450-200206000-00001
Issue Date:
DOI: https://doi.org/10.1203/00006450-200206000-00001