The limitations in our understanding of lung injury pathogenesis in preterm infants limits our ability to predict which infants may go onto to experience dysregulated inflammation and develop CLD, and also our ability to develop targeted interventions to improve outcome. Our previous results have demonstrated increased numbers of macrophages in infants with respiratory distress syndrome, a condition associated with more effective resolution of inflammation and better clinical outcome than CLD, which is characterized by chronic distal airway inflammation and poor lung function. This observation, together with the known roles of macrophages, Drostanolone Propionate suggests macrophages may regulate inflammatory responses in the preterm lung. This could derive from a number of the known roles of differentiated macrophages, alone or in combination, including the surface expression of death receptors ligands that may initiate apoptosis in vivo, the production of anti-inflammatory cytokines such as IL-10, or via efferocytosis and cell clearance. These data led us to hypothesise that the relative Ivosidenib abundance of macrophages in the preterm lung, and their differentiation status and activation phenotypes, may be associated with either the resolution of RDS or the progression to CLD. In this study, macrophages in BAL fluid samples from preterm infants retrospectively diagnosed with RDS or CLD, and from infants born at term, were phenotyped by flow cytometry, and the relationships between macrophage phenotype, disease severity and gestational age were examined. To study the relationship between gestational age and the myeloid cell populations of the lung in the days following birth, some ventilated infants underwent ongoing sampling. Day 3 was chosen for further study as it allows observations to be made on recruitment and maturation of airway cells over a biologically relevant timescale, but which precedes extubation for many of the infants in this study. On day 3, the proportion of CD14 cells was greater at low gestational ages, reaching statistical significance when analysing absolute numbers.