Premature birth is a health concern worldwide as it is associated with increased morbidity and mortality of the newborn. In the most developed countries, improved neonatal care has allowed for the survival of ever smaller premature infants and raised the survival rates of infants born very prematurely i.e. infants born before completion of the 32nd week of gestation. This has caused the incidence and mortality rates of necrotizing enterocolitis (NEC) – the most serious acquired gastrointestinal disease in preterm neonates and a leading cause of death in the neonatal intensive care unit (NICU) – to remain constant with an incidence of ~1 in 1000 live births. Nutritional
interventions and feeding strategies are central for the critical care of preterm infants in general, and for the prevention and management of NEC in particular, and include parenteral nutrition (PN), minimal enteral nutrition (MEN), and slow advancement of enteral feeds. Human breast milk is the gold standard of enteral nutrition (EN) of the newborn and protects against NEC in premature infants, but is often unavailable or limited after preterm birth. Innovative nutrition strategies and novel sources of the first EN are therefore in demand.
Amniotic Fluid (AF) is the natural source of fetal EN throughout gestation in ammals. Fetal AF swallowing stimulates somatic and gastrointestinal growth during fetal development, and modulates the development of the intestinal mucosa. In addition, AF protects the fetus against infections and maintains homeostasis in the amnion sac through anti-inflammatory mechanisms. The growth promoting and protective effects of AF in utero are ascribed to bioactive proteins including growth factors, anti-inflammatory cytokines, and antimicrobial peptides, which may exert similar beneficial effects ex utero as first EN and protect against NEC in preterm infants.
The primary objective of this thesis was to evaluate the applicability of AF as a novel sourceof first EN in premature infants and thereby to address the main hypothesis: “Enteral feeding with AF reduces the incidence and severity of NEC by improving gastrointestinal structure and function in preterm pigs”. Accordingly, the aim was to test the effects of enteral administration of AF as MEN during PN, as a supplement to a suboptimal enteral diet, or both in a preterm pig model of NEC. To evaluate the effects of AF, NEC sensitivity, intestinal digestive and absorptive capacities, intestinal permeability, mucosal structure, and inflammatory markers were analyzed. Enteral administration of AF reduced the incidence and severity of NEC in preterm pigs
when fed continuously during a period of PN and throughout the transition to full enteral feeding by modulating intestinal expression of inflammatory pathways. Enteral AF intake as MEN during a short period of PN or during full enteral formula feeding, respectively, did not reduce the susceptibility to NEC in preterm pigs. However, MEN with AF during a short period PN did improve subclinical parameters and reduced intestinal pro-inflammatory cytokine levels. Finally, administration of AF from another species increased NEC sensitivity.
The second objective was to characterize the innate host defense and inflammatory response of the small intestine in newborn pigs of advancing gestational age in order to address the second hypothesis: “The expression of innate host defense genes and the inflammatory responses of the newborn pig intestine are associated with gestational ages of high susceptibility to NEC in preterm pigs”. Accordingly, the aim was to analyze the expression of host defense genes involved in barrier function and innate immunity of the small intestine, and to establish an experimental model to investigate the inflammatory responses of the small intestine to microbial-associated
molecular patterns (MAMPs) ex vivo. The expression of certain host defense genes was affected by gestational age in the newborn pig intestine: The expression of IAP, LBP, and MUC2 increased, whereas the expression of TLR4, MD2, and MUC1 decreased with increasing gestational age. Furthermore, the expression of IAP, LBP, and MUC1 was associated with gestational ages of high NEC susceptibility and may be central to the pathogenesis of NEC in preterm pigs. In addition, the inflammatory response of small intestinal explants to MAMPs, quantified as the secretion of pro-inflammatory cytokines IL-6 and IL-8, was generally enhanced in the intestine of preterm vs. term pigs.
The results presented in this thesis provide some support for the use of AF as the first enteral diet in the immediate postnatal period after preterm birth. Although additional experimental studies and clinical trials are required to further test tolerability and efficacy, and to refine the feeding protocol for postnatal AF administration in premature infants, AF may be considered as a potential novel modality in the critical care in the NICUs during the first days after preterm birth. Furthermore, the results provide novel insight into the developmental regulation of host defense genes and the inflammatory responses of the premature intestine, which may predispose experimental NEC in the preterm pig.