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Phospholipase A₂ Activity in Normal and Staphylococcus aureus-Infected Rabbit Eyes

¹From the Department of Microbiology, Immunology, and Parasitology, Louisiana State University Health Sciences Center, New Orleans, Louisiana; and the ²Department of Ophthalmology, LSU Eye Center, New Orleans, Louisiana.

PURPOSE. To quantify phospholipase A₂ (PLA₂) activity in normal rabbit eyes and in eyes with Staphylococcus aureus keratitis.

METHODS. PLA₂ was assayed by the killing of S. aureus at 33°C or by the release of arachidonic acid from S. aureus labeled with radioactive oleic acid. Rabbit corneas were intrastromally injected with 100 log phase colony-forming units (CFU) of S. aureus 8325-4. The activity of myeloperoxidase (MPO) and PLA₂ were quantified in ocular tissues.

RESULTS. The PLA₂-mediated killing of S. aureus by normal rabbit tears decreased by more than 70% as the rabbits aged from 10 to 28 weeks and by nearly 50% from early morning to afternoon. In rabbits with S. aureus keratitis, the activity of PLA₂ and MPO increased proportionally with time from 5 to 25 hours postinfection (PI), as measured in ocular tissues. PLA₂ activity increased fivefold in tears from infected eyes collected at 25 hours PI compared with normal tears (P ≤ 0.0001), whereas a ninefold increase was found in aqueous humor of infected eyes at 25 hours PI (P ≤ 0.0001). Infected eyes demonstrated a significant increase in MPO activity compared with uninfected eyes beginning at 10 hours PI for the aqueous humor (P = 0.03), at 16 hours PI for the tear film (P = 0.0024) and at 22 hours PI for the corneal homogenate (P = 0.0007).

CONCLUSIONS. The decrease in PLA₂ activity in the rabbit eye with age or after sleep and its increase during sleep or with the progression of infection are consistent with its role as an innate host defense factor.

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