Our previous studies showed that
surfactant protein D (
SP-D) is present in human tear fluid and that it can protect corneal epithelial cells against bacterial invasion. Here we developed a novel null-
infection model to test the hypothesis that
SP-D contributes to the clearance of viable Pseudomonas aeruginosa from the healthy ocular surface in vivo. Healthy corneas of Black Swiss mice were inoculated with 10(7) or 10(9) CFU of invasive (PAO1) or cytotoxic (6206) P. aeruginosa. Viable counts were performed on tear fluid collected at time points ranging from 3 to 14 h postinoculation. Healthy ocular surfaces cleared both P. aeruginosa strains efficiently, even when 10(9) CFU was used: e.g., <0.01% of the original inoculum was recoverable after 3 h. Preexposure of eyes to bacteria did not enhance clearance. Clearance of strain 6206 (low
protease producer), but not strain PAO1 (high
protease producer), was delayed in
SP-D gene-targeted (
SP-D(-/-)) knockout mice. A
protease mutant of PAO1 (PAO1
lasA lasB aprA) was cleared more efficiently than wild-type PAO1, but this difference was negligible in
SP-D(-/-) mice, which were less able to clear the
protease mutant. Experiments to study mechanisms for these differences revealed that purified
elastase could degrade tear fluid
SP-D in vivo. Together, these data show that
SP-D can contribute to the clearance of P. aeruginosa from the healthy ocular surface and that
proteases can compromise that clearance. The data also suggest that
SP-D degradation in vivo is a mechanism by which P. aeruginosa
proteases could contribute to virulence.