A. M. Pugh1, T. C. Rice1, A. P. Seitz1, B. E. Whitacre1, M. J. Edwards1, E. Gulbins1,2, C. C. Caldwell1 1University Of Cincinnati,Division Of Research, Department Of Surgery,Cincinnati, OH, USA 2University Of Duisburg-Essen,Department Of Molecular Biology,Essen, , Germany
Introduction:
Pulmonary infection by Pseudomonas aeruginosa (PA) occurs in a high percentage of injured, septic, and aged patients with increased mortality rates. Due to increased susceptibility of this population, there is a need to identify innate defense mechanisms against bacterial infection that may be defective and evaluate for possible novel therapies. Microparticles (MPs) are small vesicles derived from the cell membrane and are known to play a significant role in the immune response. MPs are also a source of sphingosine (SPH), a long chain base that has previously been demonstrated to have antimicrobial properties. However, the role of SPH-containing MPs in the defense against pulmonary infection in patients has not been elucidated. We hypothesize that 1) decreased MP SPH content increases susceptibility to infection and 2) MP treatment will prevent pulmonary infection with PA.
Methods:
Similar to what is observed clinically, we demonstrate that injured, septic, or aged mice (susceptible mice) with pulmonary PA infection have increased mortality as compared to infected sham mice. To determine underlying mechanisms of this mortality, we observed a reduction in bronchoalveolar lavage (BAL) fluid MP SPH content in each of the susceptible mouse models. To test our hypothesis, SPH, MPs, and SPH enriched MPs were used as treatments against PA in vitro and in vivo bacterial killing assays.
Results:
When SPH, MPs, and SPH enriched MPs were applied to PA in vitro cultures, each were shown to directly reduce bacterial growth. Next, susceptible mice were given exogenous MPs from healthy donors, which significantly improved survival and/or bacterial load compared to untreated infected mice. Upon testing the impact of SPH treatment alone, we also observed significant improvement when susceptible mice were treated with aerosolized SPH. Overall, we demonstrated that treatment with SPH or SPH containing MPs improves survival and rescues susceptible mice from pulmonary PA infection.
Conclusion:
The data indicates that MPs and SPH are important in the innate antimicrobial defense of the pulmonary system, specifically the upper airways. We suggest that healthy tracheal epithelial cells release SPH-containing MPs that eliminate invading bacteria and pathogens. Although the mechanism is unknown, we hypothesize that the release of these MPs decreases in injured, septic, and aged patients therefore increasing their susceptibility to PA infection. Potential treatment with SPH or SPH-enriched MPs could restore the innate immune response to PA and decrease the mortality in susceptible patients.