L-Arginine-Threonine-Arginine (RTR) Tetramer Peptide Inhibits Ulceration in the Alkali-injured Rabbit Cornea

Abstract

Proline-glycine-proline (PGP) peptides have been identified as inflammatory mediators initiating neutrophil invasion into alkali-injured cornea. The complementary peptide, arginine-threonine-arginine (RTR), has been shown to bind to the PGP sequence and impede neutrophil infiltration. A prior study showed that L-RTR tetramer and D-RTR tetramer, used alternately (14 times a day), resulted in significantly reduced incidences of corneal ulceration and severity. The purpose of this experiment is to determine the effectiveness of both tetramers, used separately, compared with control. Rabbit corneas were exposed to 1 N NaOH for 35 seconds. Sixteen animals were randomly assigned to each of 3 groups: 1) phosphate-buffered saline (PBS), 2) 1.5 mM L-RTR, or 3) 800 microM D-RTR. One drop of each was administered hourly (14 times a day) for 36 days. Additional studies were done to assess neutrophil infiltration into corneas with and without RTR treatment. The severity of corneal ulceration in both RTR groups was statistically significantly different from the 21st day of the experiment to the end. As a result of ulcers healing in the L-RTR group, there was a statistically significant reduction in the number of ulcers beginning on day 22 versus control. Although there was healing in the D-RTR group, the incidence of ulcers was not significantly different from control or L-RTR. Morphometric analysis revealed decreased neutrophil (PMN) invasion with RTR treatment compared with PBS control. Binding of the PGP molecules by RTR tetramer seems to deprive the cornea of this neutrophilic chemotactic stimulus, leading to a reduction in the severity and incidence of corneal ulceration.