Lipopolysaccharide binding proteins on polymorphonuclear leukocytes: comparison of adult and neonatal cells

A Polymorphism of Bactericidal/Permeability-Increasing Protein Affects Its Neutralization Efficiency towards Lipopolysaccharide

Gram-negative sepsis driven by lipopolysaccharide (LPS) has detrimental outcomes, especially in neonates. The neutrophil-derived bactericidal/permeability-increasing protein (BPI) potently neutralizes LPS. Interestingly, polymorphism of the BPI gene at position 645 (rs4358188) corresponds to a favorable survival rate of these patients in the presence of at least one allele 645 A as opposed to 645 G. When we exploited the existing X-ray crystal structure, the corresponding amino acid at position 216 was revealed as surface exposed and proximal to the lipid-binding pocket in the N-terminal domain of BPI. Our further analysis predicted a shift in surface electrostatics by a positively charged lysine (BPI_216K) exchanging a negatively charged glutamic acid (BPI_216E). To investigate differences in interaction with LPS, we expressed both BPI variants recombinantly. The amino acid exchange neither affected affinity towards LPS nor altered bactericidal activity. However, when stimulating human peripheral blood mononuclear cells, BPI_216K exhibited a superior LPS-neutralizing capacity (IC₅₀ 12.0 ± 2.5 pM) as compared to BPI_216E (IC₅₀ 152.9 ± 113.4 pM, p = 0.0081) in respect to IL-6 secretion. In conclusion, we provide a functional correlate to a favorable outcome of sepsis in the presence of BPI_216K.

Killing three birds with one BPI: Bactericidal, opsonic, and anti-inflammatory functions

Bactericidal/permeability-increasing protein (BPI) is an anti-microbial protein predominantly expressed in azurophilic granules of neutrophils. BPI has been shown to mediate cytocidal and opsonic activity against Gram-negative bacteria, while also blunting inflammatory activity of lipopolysaccharide (LPS). Despite awareness of these functions in vitro, the magnitude of the contribution of BPI to innate immunity remains unclear, and the nature of the functional role of BPI in vivo has been submitted to limited investigation. Understanding this role takes on particular interest with the recognition that autoimmunity to BPI is tightly linked to a specific infectious trigger like Pseudomonas aeruginosa in chronic lung infection. This has led to the notion that anti-BPI autoantibodies compromise the activity of BPI in innate immunity against P. aeruginosa, which is primarily mediated by neutrophils. In this review, we explore the three main mechanisms in bactericidal, opsonic, and anti-inflammatory of BPI. We address the etiology and the effects of BPI autoreactivity on BPI function. We explore BPI polymorphism and its link to multiple diseases. We summarize BPI therapeutic potential in both animal models and human studies, as well as offer therapeutic approaches to designing a sustainable and promising BPI molecule.

Role of intestinal mucosal integrity in HIV transmission to infants through breast-feeding: the BAN study

BACKGROUND

Increased intestinal permeability may be one of the mechanisms of transmission of human immunodeficiency virus (HIV) to infants through breast-feeding. Intestinal permeability correlates with microbial translocation, which can be measured through quantification of bacterial lipopolysaccharide (LPS).

METHODS

We evaluated levels of plasma LPS (by the Limulus amebocyte lysate assay) and immune activation markers in serial specimens from infants exposed to but uninfected with HIV and infants infected with HIV from the Breastfeeding, Antiretrovirals, and Nutrition (BAN) study.

RESULTS

Plasma LPS levels increased after infants in the BAN study were weaned from the breast, at 24 weeks of age. Cotrimoxazole prophylaxis was associated with higher plasma LPS levels (P = .004). Infants with HIV infection had higher LPS levels, compared with uninfected infants (P = .004). Higher preinfection plasma LPS levels were a significant predictor of infant HIV infection through breast-feeding (hazard ratio = 1.60 for every unit increase in plasma LPS level; P = .01) and of lower infant length-for-age z scores (P = .02).

CONCLUSIONS

These findings suggest that disruption in intestinal integrity is a mechanism of HIV transmission to infants through breast-feeding. Weaning from breast milk and use of antibiotic prophylaxis was associated with increased levels of microbial translocation, which could facilitate HIV entry through the intestine. Complementary approaches to enhance intestinal mucosal integrity in the infant may further reduce breast-feeding transmission of HIV.

Reduced frequency of a CD14+ CD16+ monocyte subset with high Toll-like receptor 4 expression in cord blood compared to adult blood contributes to lipopolysaccharide hyporesponsiveness in newborns

The human innate immune response to pathogens is not fully effective and mature until well into childhood, as exemplified by various responses to Toll-like receptor (TLR) agonists in newborns compared to adults. To better understand the mechanistic basis for this age-related difference in innate immunity, we compared tumor necrosis factor alpha (TNF-α) production by monocytes from cord blood (CB) and adult blood (AB) in response to LAM (lipoarabinomannan from Mycobacterium tuberculosis, a TLR2 ligand) and LPS (lipopolysaccharide from Escherichia coli, a TLR4 ligand). LPS or LAM-induced TNF-α production was 5 to 18 times higher in AB than in CB monocytes, whereas interleukin-1α (IL-1α) stimulated similar levels of TNF-α in both groups, suggesting that decreased responses to LPS or LAM in CB are unlikely to be due to differences in the MyD88-dependent signaling pathway. This impaired signaling was attributable, in part, to lower functional TLR4 expression, especially on CD14(+) CD16(+) monocytes, which are the primary cell subset for LPS-induced TNF-α production. Importantly, the frequency of CD14(+) CD16(+) monocytes in CB was 2.5-fold lower than in AB (P < 0.01). CB from Kenyan newborns sensitized to parasite antigens in utero had more CD14(+) CD16(+) monocytes (P = 0.02) and produced higher levels of TNF-α in response to LPS (P = 0.004) than CB from unsensitized Kenyan or North American newborns. Thus, a reduced CD14(+) CD16(+) activated/differentiated monocyte subset and a correspondingly lower level of functional TLR4 on monocytes contributes to the relatively low TNF-α response to LPS observed in immunologically naive newborns compared to the response in adults.

Deficient expression of bactericidal/permeability-increasing protein in immunocompromised hosts: translational potential of replacement therapy

BPI (bactericidal/permeability-increasing protein) is a 55 kDa anti-infective molecule expressed in neutrophil and eosinophil granules and on some epithelial cells. BPI's high affinity for the lipid A region of endotoxin targets its opsonizing, microbicidal and endotoxin-neutralizing activities towards Gram-negative bacteria. Several immunocompromised patient populations demonstrate BPI deficiency, including newborns, those with anti-neutrophil cytoplasmic antibodies (as in cystic fibrosis and HIV infection) and those exposed to radiochemotherapy. BPI may be replenished by administering agents that induce its expression or by administration of recombinant BPI congeners, potentially shielding BPI-deficient individuals against Gram-negative bacterial infection, endotoxemia and its toxic sequelae.

Microbial translocation induces persistent macrophage activation unrelated to HIV-1 levels or T-cell activation following therapy

OBJECTIVE

HIV-1 replication and microbial translocation occur concomitant with systemic immune activation. This study delineates mechanisms of immune activation and CD4 T-cell decline in pediatric HIV-1 infection.

DESIGN

Cross-sectional and longitudinal cellular and soluble plasma markers for inflammation were evaluated in 14 healthy and 33 perinatally HIV-1-infected pediatric study volunteers prior to and over 96 weeks of protease-inhibitor-containing combination antiretroviral therapy (ART). All HIV-1-infected patients reconstituted CD4 T cells either with suppression of viremia or rebound of drug-resistant virus.

METHODS

Systemic immune activation was determined by polychromatic flow cytometry of blood lymphocytes and ELISA for plasma soluble CD27, soluble CD14, and tumor necrosis factor. Microbial translocation was evaluated by limulus amebocyte lysate assay to detect bacterial lipopolysaccharide (LPS) and ELISA for antiendotoxin core antigen immunoglobulin M (IgM) antibodies. Immune activation markers were compared with viral load, CD4 cell percentage, and LPS by regression models. Comparisons between healthy and HIV-1-infected or between different viral outcome groups were performed by nonparametric rank sum.

RESULTS

Microbial translocation was detected in healthy infants but resolved with age (P < 0.05). LPS and soluble CD14 levels were elevated in all HIV-1-infected patients (P < 0.05 and P < 0.0001, respectively) and persisted even if CD4 T cells were fully reconstituted, virus optimally suppressed, and lymphocyte activation resolved by ART. Children with CD4 T-cell reconstitution but viral rebound following ART continued to display high levels of soluble CD27.

CONCLUSION

Microbial translocation in pediatric HIV-1 infection is associated with persistent monocyte/macrophage activation independent of viral replication or T-cell activation.

Intrauterine neutrophil activation is associated with pulmonary haemorrhage in preterm infants

BACKGROUND

Clinical and experimental studies showing lung damage associated with biologically active neutrophil derivatives suggests the possibility that intrauterine neutrophil activation may predispose to the development of pulmonary haemorrhage in extremely low birthweight infants early after birth.

OBJECTIVES

To assess neutrophil functional activity in cord blood from extremely low birthweight infants who subsequently developed severe pulmonary haemorrhage.

METHODS

Neutrophil functional activity was evaluated in the cord blood of preterm neonates (gestational age <28 weeks and weight <800 g) who developed pulmonary haemorrhage (n = 6) and in controls who did not (n = 6). Infants with congenital abnormalities and noteworthy maternal complications such as diabetes, pre-eclampsia, haemorrhagic disorders, and connective tissue disease were excluded. Neutrophils isolated from cord blood immediately after delivery were tested to evaluate neutrophil chemotaxis, random motility, and chemiluminescence. Standard methodology was used.

RESULTS

Chemotaxis and chemiluminescence of neutrophils from cord blood of infants with pulmonary haemorrhage was higher than in the controls (34.8 (2.0) v 28.6 (0.8) microm, p<0.001, and 95.8 (8.5) v 73.2 (3.6) cpm x10(3), p<0.001 respectively). Random motility of cord blood neutrophils was comparable in the two groups (28.6 (1.2) v 25.3 (1.2) microm, p<0.09).

CONCLUSIONS

Intrauterine induction of chemotactic activity and metabolic oxygenation of neutrophils may predispose to the development of pulmonary haemorrhage in extremely low birthweight infants with respiratory distress early after birth.

First line of defense in early human life

Innate antimicrobial peptides are considered to play an important role in host defense against microbial invasion. They are expressed in a wide variety of organisms. In the case of human beings, defensins and the cathelicidin LL-37 appear to be the major microbicidal peptides. With respect to human neonates, only few investigations have been performed in this context, revealing the presence of alpha-defensins and LL-37 in neutrophils and vernix caseosa. In addition, beta-defensins are present in tracheal aspirates and breast milk, whereas LL-37 has been detected in the skin of the newborn baby. During recent years, immunomodulatory activities such as chemotaxis have emerged as important functions of antimicrobial peptides. Thus, these innate effectors may work synergistically to provide a first line of defense against infection, as well as to promote interactions between the innate and adaptive immunity in newborn infants.

Activation of extracellular signal-related protein kinases 1 and 2 of the mitogen-activated protein kinase family by lipopolysaccharide requires plasma in neutrophils from adults and newborns

Neutrophils exposed to low concentrations of gram-negative lipopolysaccharide (LPS) become primed and have an increased oxidative response to a second stimulus (e.g., formyl-methionyl-leucyl-phenylalanine [fMLP]). In studies aimed at understanding newborn sepsis, we have shown that neutrophils of newborns are not primed in response to LPS. To further understand the processes involved in LPS-mediated priming of neutrophils, we explored the role of extracellular signal-related protein kinases (ERK 1 and 2) of the mitogen-activated protein kinase family. We found that LPS activated ERK 1 and 2 in cells of both adults and newborns and that activation was plasma dependent (maximal at > or =5%) through LPS-binding protein. Although fibronectin in plasma is required for LPS-mediated priming of neutrophils of adults assessed by fMLP-triggered oxidative burst, it was not required for LPS-mediated activation of ERK 1 and 2. LPS-mediated activation was dose and time dependent; maximal activation occurred with approximately 5 ng of LPS per ml and at 10 to 40 min. We used the inhibitor PD 98059 to study the role of ERK 1 and 2 in the LPS-primed fMLP-triggered oxidative burst. While Western blotting showed that 100 microM PD 98059 completely inhibited LPS-mediated ERK activation, oxidative response to fMLP by a chemiluminescence assay revealed that the same concentration inhibited the LPS-primed oxidative burst by only 40%. We conclude that in neutrophils, LPS-mediated activation of ERK 1 and 2 requires plasma and that this activation is not dependent on fibronectin. In addition, we found that the ERK pathway is not responsible for the lack of LPS priming in neutrophils of newborns but may be required for 40% of the LPS-primed fMLP-triggered oxidative burst in cells of adults.

Impaired innate immunity in the newborn: newborn neutrophils are deficient in bactericidal/permeability-increasing protein

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 coli K1/r.

RESULTS

The neutrophils of newborns contain at least threefold to fourfold less BPI per cell than adult neutrophils (67 +/- 13 ng per 10(6) cells vs 234 +/- 27 ng per 10(6) 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 approximately 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.

Fibronectin Enhances In Vitro Lipopolysaccharide Priming of Polymorphonuclear Leukocytes

We investigated the role of humoral factors in lipopolysaccharide (LPS) priming of polymorphonuclear leukocytes (PMN) using cells isolated from adults and from neonates. Plasma from newborn infants had decreased priming activity of adult plasma when mixed with LPS in studies measuring oxidative radical production of PMN after stimulation with a formyl bacterial oligopeptide (fMLP). This marked difference was not caused by LPS binding protein (LBP) because the LBP concentration in newborn and adult plasma were similar (138.4 ± 12.9 U for adults, and 126.9 ± 12.1 U for neonates, P = .53). Therefore, we attempted to identify other plasma factors that may contribute to LPS priming of PMN. We identified an LPS priming factor for PMN that is present in plasma, heat stable (56°C for 30 minutes), enhanced by heparin, and concentrated in cold precipitates of plasma. Because these properties resemble those of plasma fibronectin, we assessed the role of fibronectin in LPS priming of PMN. Although fibronectin in phosphate-buffered saline (PBS) had little effect on LPS priming of PMN, fibronectin in combination with other plasma factors appeared to play a role in LPS priming of PMN because (1) removing fibronectin from adult plasma dramatically decreased LPS priming activity from plasma (P < .005), (2) addition of fibronectin to fibronectin-depleted plasma restored its LPS plasma priming activity (P < .05), and (3) neutralizing fibronectin with antibody decreased the LPS priming activity of plasma (60.3 ± 1.3 v 30.2 ± 2.2, P < .01). Thus, plasma fibronectin plays a role in LPS priming of PMN in the presence of other factors in plasma.