Direct and phagocyte-mediated lipid peroxidation of lung surfactant by group B streptococci and other bacteria. Protective effect of antioxidants

Abstract: Respiratory distress syndrome (RDS), caused by surfactant deficiency, is a leading cause of mortality and morbidity in preterm neonates. Further, infants with systemic neonatal infection or pneumonia often have signs of RDS. As neonatal infection and RDS may occur simultaneously, infants with infections may receive exogenous surfactant as treatment. Our hypothesis was that surfactant lipids are peroxidized by reactive oxygen species from phagocytes and bacteria and that Curosurf, a surfactant preparation given to newborn infants, may be improved by the addition of antioxidants to counteract toxic free radicals. We studied superoxide production by bacteria and human neutrophils (PMN) by the nitroblue tetrazolium (NBT) test. Also, the nitric oxide (NO) production by rat alveolar macrophages (AM) was measured by Griess reaction. The lipid peroxidation (LPO) of surfactant (Curosurf) was determined as the level of malondialdehyde and 4hydroxyalkenals. The impact of free radicals on surfactant function was estimated by measuring surface tension. The ability of different antioxidants in counteracting the LPO of surfactant was estimated in vitro. In vivo, we tested the effect of GBS infection and exogenous surfactant treatment in tracheotomized, ventilated, near-term newborn rabbits. Also, the effect of vitamin E as antioxidant was investigated. Surfactant decreased the superoxide production by bacteria and PMN, as well as the NO production by AM. Surfactant was peroxidized when incubated with resting or with stimulated PMN or AM, but also with bacteria alone, and the LPO increased with time. Vitamin E supplementation decreased the LPO levels. Lung surfactant was bactericidal to group B streptococci (GBS) and Escherichia coli, but Pseudomonas aeruginosa was resistant. The biophysical activity of Curosurf was impaired by GBS-stimulated PMN, as reflected by an increased surface tension. All antioxidants (SOD, vitamin E, vitamin C, ebselen and melatonin) tested except vitamin C decreased superoxide production and surfactant peroxidation by PMN in a dose dependent manner. The best combination was that of a natural and a synthetic antioxidant (melatonin-ebselen). In the in vivo model we found that GBS infected rabbits had higher LPO of their lung lavage fluid than uninfected animals. The LPO level could be diminished by adding vitamin E to exogenous surfactant. Our findings suggest that antioxidants supplementation can improve the surfactant therapy when given to infants with GBS pneumonia.