The pulmonary surfactant system of the lung is a lipid and protein complex that plays a central role in regulating the surface tension of the alveolar compartment and controlling innate immunity. The major lipid component of surfactant, dipalmitoyl-phosphatidylcholine, is the molecule responsible for the reduction of alveolar surface tension. Surfactant proteins A and D (SP-A and SP-D) are now understood to be important regulatory molecules involved in recognizing pathogens, facilitating their phagocytosis and regulating inflammation. We have recently discovered that two of the minor components of pulmonary surfactant, the anionic phospholipids phosphatidylglycerol (PtdGro) and phosphatidylinositol (PtdIns), are also potent regulators of inflammation and infection within the lung. These lipids are effective inhibitors of the activation of multiple Toll-like receptors (TLRs), which serve as sensors for microbial invasion and elicit inflammatory responses. The actions of PtdGro and PtdIns within surfactant appear to play a protective role in the lung by suppressing unnecessary tissue inflammation that would otherwise occur in response to non-infectious stimuli such as bacterial lipopolysaccharide present on ultrafine particles inhaled into the lung on a daily basis. We hypothesize that the role of PtdGro and PtdIns is to set a high threshold for the engagement of inflammation thereby preventing the lung from being chronically inflamed by casual exposure to airborne microbial products. In addition to suppressing inflammation, PtdGro and PtdIns also potently suppress infection of lung cells by respiratory syncytial virus (RSV) and influenza A virus (IFA). RSV is a major cause of hospitalization in the first two years of life. RSV and IFA also act as serious exacerbants of asthma and chronic obstructive pulmonary disease. The focus of this proposal is to understand how the anionic surfactant lipids exert their action. We plan to elucidate the molecular basis of the actions of the lipids by: 1) determining the structural basis of suppression of TLRs and viral infection, by using cell culture models of inflammation and infection;2) determining the efficacy of action of the anionic lipid in vivo in the context microbial ligand challenges to the lungs of mice;3) quantifying the action of the lipid antagonists against RSV and IFA in vitro and in vivo;4) defining the molecular mechanisms by which the anionic lipids suppress RSV and IFA infection. The work described in this proposal has direct relevance to treatment of bacterial and respiratory viral infections in children and adults, and important implications for treatment of individuals with asthma and chronic obstructive pulmonary disease.
This work focuses on the newly discovered function of lipids found within the gas exchange regions of the lung. These lipids are potent suppressors of inflammation in the lung, and protect the organ from inadvertent activation of inflammatory processes. These lipids also have unexpected anti-viral activity against respiratory syncytial virus, which causes serious disease in newborns and individuals with asthma and chronic obstructive pulmonary disease.
