Enteral docosahexaenoic acid supplementation to attenuate inflammation in the preterm infant

Abstract

Preterm infants have an underdeveloped immune system and as such they are predisposed to developing unregulated inflammatory responses that are associated with disease in the postnatal period. Docosahexaenoic acid (DHA) is an omega-3 long-chain polyunsaturated fatty acid (LCPUFA) with known immunomodulatory properties, however the effect of dietary DHA on the regulation of immune responses in preterm infants is largely unknown. This thesis employs a multi-system approach to address questions related to the efficacy of omega-3 DHA to regulate inflammation in preterm infants and in human type II alveolar epithelial cells (AEC). The N3RO randomised controlled trial (RCT) provided the opportunity to carry out a single-centre nested study to examine the effect of supplemental DHA in preterm infants on pro-inflammatory and regulatory biomarkers in blood and levels of a common bacterial pathogen in the gastrointestinal tract. The aim of the N3RO RCT was to assess the efficacy of an enteral DHA emulsion to reduce bronchopulmonary dysplasia (BPD) in preterm infants < 29 weeks gestation compared to a standard soy emulsion without DHA. Prior to analysis of biological samples from preterm infants, the immune response to enteral DHA and soy emulsions in human type II AECs, one of the primary cell types affected in respiratory disorders, was assessed in vitro. The enteral emulsions assessed in the N3RO RCT were tested in conjunction with other commercially available parenteral lipid emulsions. Omega-3 DHA in both enteral and parenteral emulsions significantly reduced pro-inflammatory cytokines (IL-1β, IL-8 and IFNγ) when compared to soy-based emulsions. There are very few studies that have assessed what, if any, targets DHA interacts with to exert an immunomodulatory effect in preterm infants. Inflammatory cytokines are known to play a crucial role in the progression of airway inflammation, epithelial and vascular damage and subsequent development of BPD. Such inflammatory mediators are also involved in the development of other neonatal inflammatory disorders such as sepsis, necrotising enterocolitis and retinopathy of prematurity. A total of 144 blood samples were collected from 51 preterm infants enrolled in the nested study. Supplemental DHA did not reduce pro-inflammatory cytokine levels in plasma or whole blood culture supernatants (after a 24 hour incubation with E. coli lipopolysaccharide). Inflammatory mediators in the gut environment can influence initial colonisation and resulting abundance of both commensal and pathogenic bacteria. Staphylococcus is among the first colonisers of the respiratory and gastrointestinal tracts and it is one of the most important pathogens in the neonatal intensive care unit. Colonisation by methicillin-resistant bacteria including Staphylococcus in preterm infants also causes significant morbidity and mortality in the neonatal intensive care unit. In the neonatal period, diet has a significant effect on microbial colonisation of the gut, however the effect of supplemental omega-3 LCPUFA on Staphylococcus colonisation in preterm infants is unknown. A total of 220 stool samples were collected from 41 preterm infants enrolled in the nested study. Levels of Staphylococcus and bacteria carrying the gene coding for methicillin-resistance (mecA) decreased significantly over time in both groups, but DHA did not have an effect on abundance. The original contribution this thesis makes to the knowledge base is that supplementing preterm infants < 29 weeks gestation enterally with 60 mg/kg/day of DHA does not affect circulating levels of pro-inflammatory or regulatory cytokines, the immune response to an infectious stimuli nor does it influence Staphylococcus and mecA+ bacteria in the gut. This thesis contributes important information regarding the use of DHA at supplemental levels in nutrition regimens for preterm infants.