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I.R. Zolfaghar, D.J. Evans, R. Ronaghi, S.M. J. Fleiszig; Type III–Secreted Effectors of Pseudomonas Aeruginosa Modulate Phagocyte Infiltration of Infected Corneas . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1169.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Pseudomonas aeruginosa keratitis is a sight–threatening infection. We previously showed that the ExsA–regulated Type III secretion System (TTSS) of P. aeruginosa contributes to virulence in two different murine models of keratitis, and that two TTSS effectors, ExoU or ExoT, were each sufficient to promote bacterial colonization of the cornea. This functional redundancy was surprising because ExoU and ExoT exert different toxic effects on mammalian cells in–vitro. The theoretical model we are testing is that both ExoU and ExoT can protect bacteria against phagocytosis by infiltrating phagocytes in–vivo. The aim of this study was to determine their effects on phagocyte infiltration. Methods: Corneas of C57BL/6 female mice were scarified, allowed to heal for 6 h, then inoculated with 106 cfu of wild–type P. aeruginosa strain PA103 (Wt), PA103exoUTn5::Tc (exoU mutant), PA103exoT::Tc (exoT mutant) or PA103exoUexoT::Tc (exoU/exoT double mutant). Resulting pathology was assessed and photographed at 24 and 48 h. Bacterial colonization levels were determined by viable counts of corneal homogenates at 48 h. Other infected corneas were examined histologically. Results: Wt P. aeruginosa caused peripheral "ring" infiltration, a common characteristic of human keratitis, coinciding with large numbers of phagocytes in the corneal periphery, and their absence from the central cornea where large numbers of bacteria were detected. Ring infiltration was absent in infections caused by the double mutant that lacked both toxins, i.e. phagocytes infiltrated the central cornea. This coincided with a ∼1000–fold reduction in bacterial corneal colonization as compared to Wt. Eyes infected with single mutants of exoU or exoT showed that ExoU expression was required for the formation of the ring infiltrate. Although expression of ExoT alone did not suppress phagocyte infiltration of the central cornea, it did allow bacteria to colonize the cornea at levels similar to those noted with ExoU–expressing bacteria. Conclusions: ExoU and ExoT may promote P. aeruginosa corneal colonization by different mechanisms. ExoU suppressed phagocyte infiltration of infected central corneas in–vivo, while ExoT promoted bacterial colonization without suppressing the entry of phagocytes into the central cornea. Whether the contribution of ExoT to colonization involves inhibition of the phagocytic activity of infiltrating cells in–vivo is yet to be determined.
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