Phosphatidylinositol Inhibits Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is a problematic pathogen for all children before age 2 and is the major cause for hospitalization in newborns (1). Immunity to the virus is incomplete, and there is now clear evidence that RSV infections in adults with chronic lung diseases such as asthma and chronic obstructive pulmonary disease contribute significantly to exacerbation of these underlying conditions (2–6). Infection with RSV can also be fatal in immunocompromised individuals such as bone marrow transplant patients and those with human immunodeficiency virus-acquired immunodeficiency syndrome (7). Worldwide, RSV is estimated to infect 34 million individuals annually and results in high mortality in children during the first 2 years of life (8). The virus is second only to malaria in causing death in early childhood in undeveloped countries (9). Currently, there is no vaccine for the virus, and there are no beneficial medications for treatment of infected individuals. Pulmonary surfactant is a lipid and protein complex secreted at the air-liquid interface of the alveolar compartment of the lung that plays important roles in reducing alveolar surface tension and regulating innate immunity (10). Recently, several studies have identified significant antiviral activity exerted by the minor anionic pulmonary surfactant phospholipid 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG) against respiratory viruses, including multiple strains of RSV and influenza A viruses (11–13). The concentration of POPG and other phosphatidylglycerol (PG) species in the surfactant complex is remarkably high (∼10 mg/ml, 12.5 mM) and greatly exceeds that described for any other tissues. In addition to inhibiting viral infection, POPG also antagonizes the activation of Toll-like receptors (TLRs) 1, 2, 4, and 6 (14, 15). Pulmonary surfactant also contains a second minor anionic phospholipid, phosphatidylinositol (PI), at concentrations of ∼1 mg/ml (∼1.15 mM) (16). Previous work has also identified PI as a regulator of TLRs 1, 2, 4, and 6 (14, 15, 17, 18). The antagonism of TLR4 activation by PI and POPG appears to occur by partially overlapping mechanisms involving the inhibition of CD14 recognition of bacterial lipopolysaccharide (14). These previous observations led us to test whether PI could also inhibit RSV infection in vitro and in vivo. The purpose of this study was to determine whether PI could 1) antagonize RSV-induced inflammatory cytokine production from epithelial cells, 2) inhibit RSV infection in vitro, 3) directly bind to the virus and disrupt cell surface attachment, 4) block RSV infection in mice, and 5) provide prophylaxis against RSV infection. Our findings demonstrate that POPG and PI have overlapping antiviral functions in the lung and suggest that PI or structurally related molecules could be useful in preventing and treating RSV infection.