The identification and characterization of hillocks in the postnatal mouse and human airway

Adult lung regeneration has been deeply scrutinized over the past decade. In contrast, the injury resistance mechanisms of the neonatal and pediatric airway have received considerably less attention despite the manifest clinical importance. We recently reported the discovery of the airway hillock in adult murine and human airways. Adult hillocks are stratified structures with luminal squamous barrier cells overlying a dedicated basal stem cell population. Functionally, hillocks serve as an injury-resistant reservoir of dedicated hillock stem cells that can resurface and repopulate the airway epithelium after severe damage. Indeed, hillock basal stem cells undergo massive clonal expansion in the process of repopulating denuded airway epithelium. Since the postnatal lung encounters injuries that can result in airway epithelial denudation in the setting of respiratory infection or aspiration, we sought to assess whether hillocks are present in the neonatal airway. In this manuscript we identify and characterize hillocks in postnatal mouse and human pediatric airways. We show that hillocks are present in mice from postnatal day 3 onwards and that they expand in size through adulthood. The earliest hillocks are functionally immature, but they acquire their injury resistance properties over the course of postnatal maturation. By re-analyzing published pediatric scRNAseq data, we identify cells with a hillock squamous cell gene signature that is conserved across species. Finally, we identify bona fide hillocks in an 8-month-old infant and an 8-year-old child. We now wonder whether the presence and maturation of hillocks has implications for disorders of the neonatal and childhood airway. More specifically, we hypothesize that the incomplete maturation of hillocks could create a window of vulnerability in neonates who may be particularly susceptible to airway damage in the setting of infection or aspiration. Thus, there is a need to define when human hillocks first form and establish the time window during which they functionally mature as a prelude to determining whether the appearance and properties of hillocks correlate to clinical phenotypes and outcomes.