Introduction: Alveolar capillary dysplasia (ACD) is a rare lung developmental disorder associated with fatal outcomes in newborns. Mutations in a transcription factor involved in the development of pulmonary capillaries, the forkhead box F1 (FOXF1) gene, are causative of the disease in 60% of affected infants.

Aims and objectives: We hypothesized whether there are mechanisms beyond FOXF1 involved in the pathogenesis of ACD. To identify these, we performed three-dimensional ultrastructural imaging combined with a comprehensive molecular work-up.

Methods: Human lung tissues from ACD, non-specific pneumonia (NSIP), and healthy controls (n=12 cases, respectively) were analyzed. Morphometrical assessment of the pulmonary vasculature was performed using transmission and serial block face electron microscopy. The interacting molecules between FOXF1 and basement membrane proteins were analyzed using single gene sequencing and immunostaining. In addition, transcriptome profiling was performed using a self-designed panel of 435 genes and a biological pathway analysis tool.

Results: Pulmonary capillaries in ACD showed a unique pattern of altered, hypertrophied, multi-layered basement membranes surrounded by large collagen deposits. FN1, COL4A1, CDH5, and VIT were identified as genes potentially contributing to these alterations. In addition, ACD tissues showed a loss of bona fide lung capillaries and a replacement by COL15A1+ vessels as recently described in NSIP.

Conclusions: This study provides new insights into the ultrastructure and pathophysiology of ACD. Future studies are needed to explore the effect of FN1 knockouts and the therapeutic downregulation of FN1 overexpression in ACD.