Lung Surfactant Lipids Provide Immune Protection Against Haemophilus influenzae Respiratory Infection

Abstract

Non-typeable Haemophilus influenzae (NTHi) causes persistent respiratory infections in patients with chronic obstructive pulmonary disease (COPD), probably linked to its capacity to invade and reside within pneumocytes. In the alveolar fluid, NTHi is in contact with pulmonary surfactant, a lipoprotein complex that protects the lung against alveolar collapse and constitutes the front line of defense against inhaled pathogens and toxins. Decreased levels of surfactant phospholipids have been reported in smokers and patients with COPD. The objective of this study was to investigate the effect of surfactant phospholipids on the host-pathogen interaction between NTHi and pneumocytes. For this purpose, we used two types of surfactant lipid vesicles present in the alveolar fluid: (i) multilamellar vesicles (MLVs, > 1 μm diameter), which constitute the tensioactive material of surfactant, and (ii) small unilamellar vesicles (SUVs, 0.1 μm diameter), which are generated after inspiration/expiration cycles, and are endocytosed by pneumocytes for their degradation and/or recycling. Results indicated that extracellular pulmonary surfactant binds to NTHi, preventing NTHi self-aggregation and inhibiting adhesion of NTHi to pneumocytes and, consequently, inhibiting NTHi invasion. In contrast, endocytosed surfactant lipids, mainly via the scavenger receptor SR-BI, did not affect NTHi adhesion but inhibited NTHi invasion by blocking bacterial uptake in pneumocytes. This blockade was made possible by inhibiting Akt phosphorylation and Rac1 GTPase activation, which are signaling pathways involved in NTHi internalization. Administration of the hydrophobic fraction of lung surfactant in vivo accelerated bacterial clearance in a mouse model of NTHi pulmonary infection, supporting the notion that the lipid component of lung surfactant protects against NTHi infection. These results suggest that alterations in surfactant lipid levels in COPD patients may increase susceptibility to infection by this pathogen.

Haemophilus influenzae no tipable (NTHi) es la causante mayoritaria de infección respiratoria en pacientes con enfermedad pulmonar obstructiva crónica. El posible papel protector de los lípidos del surfactante pulmonar en la infección respiratoria por NTHi era hasta ahora desconocido. Nuestros resultados indican que los lípidos del surfactante tienen un efecto protector ante la infección por NTHi actuando intra- y extracelularmente. En el espacio extracelular, los lípidos interfieren en la interacción huésped-patógeno al impedir la agregación bacteriana e inhibir la adhesión de NTHi al epitelio alveolar. Por otra parte, los lípidos internalizados en el epitelio bloquean los mecanismos de señalización disparados por la bacteria que permiten su internalización. Consistentemente, la instilación del surfactante pulmonar en ratones con infección respiratoria por NTHi acelera el aclaramiento bacteriano. Estos resultados indican la potencial utilidad del surfactante pulmonar como terapia preventiva en pacientes con enfermedades respiratorias crónicas susceptibles a infecciones con NTHi y sugieren que la utilización de nanopartículas formadas por lípidos del surfactante puede ser beneficiosa en la administración intranasal de fármacos