Reversibility of pulmonary abnormalities by conditional replacement of surfactant protein D (SP-D) in vivo

Surfactant protein D (SP-D) gene-targeted mice develop severe pulmonary disease associated with emphysema, pulmonary lipidosis, and foamy macrophage infiltrations. To determine the potential reversibility of these abnormalities, transgenic mice were developed in which SP-D was conditionally replaced in the respiratory epithelium of SP-D(-/-) mice. SP-D was not detected in the absence of doxycycline. Treatment with doxycycline after birth restored pulmonary SP-D concentrations and corrected pulmonary pathology at adulthood. When SP-D was replaced in adult SP-D(-/-) mice, alveolar SP-D was restored within 3 days, pulmonary lipid abnormalities were corrected, but emphysema persisted. In corrected adult SP-D(-/-) mice, loss of SP-D caused focal emphysema and pulmonary inflammation but did not cause phospholipid abnormalities characteristic of SP-D(-/-) mice. Thus, abnormalities in surfactant phospholipid homeostasis and alveolar macrophage abnormalities were readily corrected by restoration of SP-D. However, once established, emphysema was not reversed by SP-D. SP-D-dependent processes regulating surfactant lipid homeostasis were disassociated from those mediating emphysema.