The challenge of maturing the preterm lungs so they can function outside the womb while attempting to maintain the pregnancy inside it has existed since Dr. Mary Ellen Avery first described surfactant deficiency as the cause of hyaline membrane disease in 1959.1 Antenatal corticosteroids, currently the most effective stimulate for fetal maturation, were first reported by Sir “Mont” Liggins in 1972, but some of the corticosteroid effects are transient and corticosteroids may have long-term effects on infants that do not deliver preterm.2,3,4 In their current manuscript, “Combined Transamniotic Delivery of Surfactant Proteins B and C mRNA Enhances Preterm Fetal Surfactant Production is a Rodent Model”, Moskowitzova et al. attempt to provide more localized lung maturation through the application of human surfactant mRNA into the amniotic sacs of fetal rats. The research team performed fetal surgeries at E17 to inject encapsulated human mRNA into the amniotic sacs, then collected the pups between E18 to E21 (term) to evaluate for lung maturation. They demonstrated, in a lab not far from Dr. Avery’s original lab at Harvard, a transient improvement in surfactant proteins and phosphatidyl-choline (PC) preterm rat lungs.

The study compares the effects of modified mRNA for human surfactant protein B (SP-B), surfactant protein C (SP-C), or the combination SP-B and SP-C to nanoparticle vehicle alone on lung maturation. The combination seems to affect the SP-B protein levels and not the SP-C protein levels, but all the mRNA combinations have some effect on the amniotic fluid PC levels. There is not a clear explanation for the benefits of combined mRNA for SP-B and SP-C on lung maturation, but minimal effect of SP-C or SP-B mRNA when given alone. It may be that the higher concentration of the mRNA in the combined groups (2 ÎĽg/injection) caused more of an inflammatory response than a single mRNA alone (1 ÎĽg/injection). Lipopolysaccharide (LPS) has been utilized in preterm sheep models to mimic chorioamnionitis, and the lung inflammation present from the LPS causes increases in surfactant proteins and phospholipids.5 Even lung inflammation from a very brief mechanical stretch can mature the fetal lung and increase surfactant proteins.6 It is unclear if inflammation from the mRNA might be a cause for lung maturation since lung inflammation was not assessed, and the use of mRNA that did not code for surfactant proteins was not utilized as a second control. The cross-reactivity of ELISA for the human surfactant protein and rat surfactant proteins also makes interpretation of the results more difficult. The fetal rat type 2 cells may be translating the human mRNA into stable protein, or the processing of the human mRNA may be priming the cells to produce additional rat mRNA. The increase in phospholipids (PC) found in the amniotic fluid is explained by authors as likely due to priming of the type 2 cells by the mRNA. This could be correct, but direct translation by type 2 cells would require the encapsulated mRNA to migrate down the airways against both a flow and pressure gradient created by the fetal lung fluid and fetal breathing that typically moves from the distal lungs up into the larynx. Although we do not fully understand the mechanism of the increases in protein and phospholipids, it does appear that the rats had some transient increases after transamniotic administration of surfactant mRNA. The effects are short-lived and only occur at some of the time points, so the dosage and encapsulation ratios would need to be further optimized.

The clinical utility of transamniotic mRNA delivery may be limited by the reasons mothers present with preterm labor or are inducted for preterm births. A high percentage of women who deliver preterm have either experienced preterm rupture of membrane, preterm contractions, or chorioamnionitis. One of the goals of the obstetrical team is to calm down the uterus to stop contractions, and the introduction of a needle into the uterus may lead to worsening contractions. Infants with preterm rupture of membranes may not have enough amniotic fluid to carry the mRNA, and decreased amniotic fluid would increase the efflux of fetal lung fluid out of the lung. The mother with pre-eclampsia who requires induction for hypertension may be a good candidate for lung-specific treatments since their myometrial is typically quiescent. Since all new therapies would have to be tested against the current standards of antenatal steroids, large studies of new interventions will be needed to demonstrate additional clinical benefits, but this will affect all new therapeutics for lung maturation.

The benefit of targeting the premature lung with surfactant mRNA would be to avoid the systemic effects of other treatments, but it may be that the systemic effects are also necessary for survival at the limits of prematurity. Antenatal corticosteroids mature the lungs, but they also decrease the rates of intraventricular hemorrhage, necrotizing enterocolitis, and may decrease severe retinopathy of prematurity.7 This maturational effect on all organ systems is beneficial to the extreme preterm infant, but could have negative effects on the infants exposed to corticosteroids that delivered at term.2 When Sir Liggins serendipitously discovered the effects of corticosteroids on lungs in preterm sheep, he was testing a steroid combination used for orthopedic treatments in adults, and we continue to use a similar formulation nearly 50 years later. Dr. Alan Jobe, one of the pioneers in surfactant replacement therapy for RDS, has spent another large portion of his career trying to determine the mechanisms for the lung maturation from corticosteroid.2,8 The antenatal steroids, long before the appearance of surfactant within the airways, turn on sodium channels to pump chloride out of the lungs, and cause thinning of the septum between the airspaces and developing capillary system. These changes in lung structure and lung fluid balance may provide additional benefits to the infant during the resuscitation and stabilization and reduce injury at birth.9 Dr. Jobe has also studied the effects of different corticosteroids, different dosing regiments, and safe use of corticosteroids in the developing world in a quest to improve upon the doses trialed by Liggins. Since not all women respond to antenatal steroids, additional mechanisms or therapies for maturing the lungs could be useful. Dr. Jobe has always taught me to continue to look for new solutions and mechanisms for improving the respiratory health of our smallest infants, and the current study on transamniotic surfactant mRNA explores a new molecular pathway.

Since there is likely lung injury occurring with mechanical ventilation of the most preterm infants prior to the administration of exogenous surfactant, the desire to further develop therapies that mature the lungs prior to delivery still exists. Antenatal steroids, for all the benefits for the lung and other organ systems, do not work in all women and may have long-term effects in infants that subsequently deliver at term. The transamniotic delivery of surfactant mRNA, when given in combination of both SP-B and SP-C, appears to cause some transient increases in lung surfactants, though the mechanism and clinical applications need further development. The observations by Dr. Avery about surfactant deficiency causing hyaline membrane disease eventually led to the discoveries of Sir Mont Liggins on the use of antenatal steroids and the development of surfactant replacement therapies, both of which Dr. Jobe has helped refine. It is context of a continued pursuit of both understanding surfactant biology and developing therapeutics that prepare the preterm fetus for extra uterine life that the current manuscript should be placed.