Surfactant catabolism by alveolar macrophages plays a small but critical part in surfactant recycling and metabolism. Upon ligand binding, granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR), a heterodimer of alpha (CSF2RA) and beta (CSF2RB) subunits, initiates a signalling process that not only induces proliferation, differentiation and functional activation of hematopoietic cells but can also determine surfactant uptake into alveolar macrophages and its degradation via clathrin-coated vesicles. Defects in human CSF2RA can cause pulmonary surfactant metabolism dysfunction 4 (SMDP4; MIM:300770, aka congenital pulmonary alveolar proteinosis, (PAP)), a rare lung disorder due to impaired surfactant homeostasis characterised by alveoli filling with floccular material. Cellular responses to the misfolded pro-SFTPC products include ER stress, the activation of reactive oxygen species and autophagy. Excessive lipoprotein accumulation in the alveoli results in a form of respiratory distress syndrome in premature infants (RDS; MIM:267450) (Whitsett et al. 2015).