Interfacial Delivery of Surfactant Protein SP-D Through Its Association with Pulmonary Surfactant

Surfactant protein D (SP-D) is a glycoprotein part of the pulmonary surfactant (PS) system that carries out important roles in the innate immune defense of the lungs and participates in surfactant homeostasis. Pulmonary surfactant (PS) is a lipid-protein complex, synthesized and secreted to the alveolar space by type II pneumocytes. It spontaneously adsorbs and spreads over the air-liquid interface, reducing the surface tension and enabling the process of breathing. PS has been recently proposed as a drug delivery carrier due to its unique surface-active properties and its peculiar composition. The anti-inflammatory and immunomodulatory features of SP-D make it a good candidate as a therapy to alleviate lung injuries associated with inflammation or infectious processes. In this work, we used a recombinant human SP-D (rhSP-D) and in vitro surface balances to evaluate the possibility of the protein to be transported through the air-liquid interface using PS as a carrier. We have analyzed the interfacial properties of the protein alone or in combination with PS by using a custom-built double-surface balance setup. rhSP-D showed low adsorption and spreading capabilities by itself, but this was improved upon combination with PS or by the mere presence of a surfactant film at the air-liquid interface. The interaction of the protein with surfactant membranes was investigated by using different PS/rhSP-D preparations and modes of combination. The transport of SP-D over long distances by its association with PS membranes suggests the possibility to formulate SP-D/PS combinations to ameliorate different lung pathologies.