New concepts in collectin-mediated host defense at the air-liquid interface of the lung

Surfactant proteins A (SP-A) and D are collectins that play key roles in innate immune defense of the lung. Recent studies suggest that these proteins have a direct effect on the growth and viability of Gram-negative bacteria, Mycoplasma pneumoniae and Histoplasma capsulatum.

The researchers examined membrane permeabilization by BAL fluid of mice that were sufficient or deficient in surfactant proteins, of rough and smooth LPS-containing membranes, and of genetically altered bacteria. The permeability assay is based on the ability of agents to promote access of impermeable dyes propidium iodide and ELF97 to intracellular targets.

The permeabilizing activity of concentrated BAL material from SP-A+/+ mice was substantially greater than that from SP-A-/- animals, and was sensitive to hyperoxic exposure. Oxidation of the collectins in vitro also blocked their antimicrobial effects. Rough but not smooth LPS conferred susceptibility of model bacterial membranes to permeabilization by the collectins. The screening of genetically engineered SP-A mice with a Pseudomonas aeruginosa signature tagged mutagenesis library has revealed several pathways which are required for resistance to the permeabilizing effects of SP-A, including those required for salicylate biosynthesis and flagellar function.

The researchers conclude that the pulmonary collectins directly permeabilize bacteria in an LPS-dependent and rough LPS-specific manner. Oxidative damage blocks the permeabilizing activity of alveolar lining fluid and purified proteins. The mechanism(s) of permeabilization is not known, but the crystal structure of SP-A reveals a hydrophobic cleft lined by charged residues which may play a role in membrane perturbation.