Bronchopulmonary dysplasia (BPD), a neonatal chronic lung disease, is characterised by inflammation, matrix remodeling and lung growth arrest. The lack of therapies emphasizes the need to define new therapeutic strategies to prevent acute injury and promote lung regeneration. Since netrin-1 has anti-inflammatory and stemness-promoting function, we aim to identify novel netrin-1 targeting agonists and antagonists to promote alveolarization by preventing inflammation and matrix remodeling in neonatal chronic lung disease.

Preterm infants, whose lungs are incompletely developed, are at high risk for respiratory failure. Oxygen (O₂) and mechanical ventilation (MV) offer life-saving treatments for respiratory distress; such treatment, however, leads to bronchopulmonary dysplasia (BPD), a neonatal chronic lung disease. Despite the advances in perinatal care, BPD is the most common complication of prematurity. Lungs of infants afflicted with BPD are characterized by impaired formation of alveoli and pulmonary micro-vessels as well as perturbed matrix remodeling, yielding structural changes resembling emphysema or chronic obstructive lung disease (COPD).

These structural and functional sequelae of prematurity persist beyond infancy, suggesting a reduced regenerative capacity of lungs with BPD. Inflammation and perturbed matrix remodeling favoring fibrosis are intimately linked to the pathogenesis of BPD. The high incidence of BPD and the lack of pharmaceutical therapies emphasize the need of unravelling the early molecular origins that cause the initial lung injury to identify novel targets in BPD to inhibit inflammation, to preserve lung matrix and thereby lung growth. 

Molecular mechanisms of experimental and clinical BPD

The pathogenesis of BPD includes inflammation, matrix remodeling and reduced regeneration, but the mechanisms are not yet known. Our research group has established two animal models of neonatal lung growth arrest, hyperoxia- and ventilation-induced lung injury. Neonatal lung growth arrest was related to an infiltration of immune cells in the lung, to increased protease activity and elevated inflammatory cytokines, e.g. IL6.  Moreover, we found that Krüppel-like factor 4 (Klf4), a zinc-finger transcription factor that plays a major role in regulating cell pluripotency, cell survival and cell differentiation as well as inflammation, is markedly reduced in neonatal lungs with growth arrest.

We studied the role of Klf4 in vivo and in vitro and identified Klf4 as a key regulator of alveolar epithelial cells type II (ATII), the alveolar progenitor cells that promote alveolar regeneration. Our translational studies showed that ATII-specific ablation of Klf4 during alveolarization promotes ATII differentiation and enables lung growth. These findings highlight Klf4 as a novel target to promote lung growth and regeneration.

Blocking inflammation to enable lung growth

The pathogenesis of BPD includes inflammation, but the mechanisms are not yet known. Macrophages are key constituents of lung inflammation, secreting various cytokines, such as Interleukin 6 (IL6), CXCL10 and MMP12, regulator of cell homeostasis, matrix remodeling, immune response and tissue regeneration.Interestingly,Klf4 has been identified as a key regulator of macrophage differentiation, maintaining anti-inflammatory M2 phenotype; in contrast, deletion of Klf4 favors inflammatory M1. Infants evolving BPD, show an elevation of M1 markers, whereas M2 markers are unchanged or even reduced.

Similarly, in models of neonatal lung growth arrest a macrophage influx is linked to increased protease activity and macrophage-related cytokines, e.g. IL6. We next tested if blockade of these cytokines enables lung formation after hyperoxia. Indeed, IL6 and MMP12 null mice were partly protected from hyperoxia-induced lung injury. In particular, loss of IL6 preserved survival and homeostasis of ATII, alveolar formation, and ultimately lung function, suggesting IL6 as a novel therapeutical target to treat BPD (Figure 1).