In cystic fibrosis (CF), the condition limiting the prognosis of affected children is the chronic obstructive lung disease accompanied by chronic and persistent infection with mostly mucoid strains of Pseudomonas aeruginosa. The majority of CF patients have antineutrophil cytoplasmic antibodies (ANCA) primarily directed against the bactericidal permeability-increasing protein (BPI) potentially interfering with antimicrobial effects of BPI. We analyzed the expression of BPI in the airways of patients with CF. In their sputum samples or bronchoalveolar lavage specimens, nearly all patients expressed BPI mRNA and protein, which were mainly products of neutrophil granulocytes as revealed by intracellular staining and subsequent flow cytometry. Repeated measurements revealed consistent individual BPI expression levels during several months quantitatively correlating with interleukin-8. In vitro, P. aeruginosa isolates from CF patients initiated the rapid release of BPI occurring independently of protein de novo syntheses. Furthermore, purified natural BPI as well as a 27-mer BPI-derived peptide displayed antimicrobial activity against even patient-derived mucoid P. aeruginosa strains and bacteria resistant against all antibiotics tested. Thus, BPI that is functionally active against mucoid P. aeruginosa strains is expressed in the airways of CF patients but may be hampered by autoantibodies, resulting in chronic infection.

Recognition of LPS by TLR4 initiates inflammatory responses inducing potent antimicrobial immunity. However, uncontrolled inflammatory responses can be detrimental. To prevent the development of septic shock during an infection with Gram-negative bacteria, the immune system has developed mechanisms to neutralize LPS by specialized proteins. In this study, we report the recombinant expression and functional characterization of the mouse homolog of human bactericidal/permeability-increasing protein (BPI). Purified recombinant mouse BPI was able to neutralize LPS-mediated activation of macrophages and to block LPS-dependent maturation of dendritic cells. Recombinant mouse BPI neutralized the capacity of Gram-negative bacteria to activate immune cells, but did not influence the stimulatory properties of Gram-positive bacteria. Unlike human BPI, mouse BPI failed to kill or inhibit the growth of Pseudomonas aeruginosa. Together, these data demonstrate that murine BPI is a potent LPS-neutralizing protein that may limit innate immune responses during Gram-negative infections.

Lipoteichoic acid (LTA) is a major immunostimulatory molecule in the cell wall of Gram-positive bacteria. Adhesion of LTA to a polystyrene surface drastically increased its immunostimulatory potency in human whole blood in comparison to soluble LTA, although only 1% of the LTA had bound, as determined using rhodamine-labelled LTA. The release of the proinflammatory cytokines IL-1beta, TNF and IL-6 and the chemokines IL-8 and G-CSF was increased 2- to 10-fold, but IL-10 release was unaltered. This presentation effect was not shared by lipopolysaccharide (LPS) or other toll-like receptor 2 agonists and was less pronounced in polypropylene vessels. LTA did not induce cytokine release in silicone-coated borosilicate vessels, but covalent coupling of LTA to polystyrene beads restored cytokine induction in these vessels, indicating that presentation of LTA on a surface is in fact essential for its immunostimulatory potency. This novel aspect of presentation as a factor in the recognition of LTA may reflect the physiological situation in the bacterial cell wall, where LTA is anchored in the bacterial membrane and projects through the peptidoglycan. In practical terms, contamination of medical devices with components of Gram-positive bacteria may pose an underestimated inflammatory risk.