Macrophages increase in corneal epithelium and stroma after photorefractive keratectomy-like corneal abrasion and amplify ocular pain responses in rats.

In a preclinical rat model that mimics initial stages of photorefractive keratectomy (PRK) we examined changes in the localization of CD68+ macrophages and in sensory nerves in cornea. Rats received a PRK-like corneal abrasion, then behavior and morphology were assessed 24 h, 72 h, and 1 week later. Corneal tissues were examined for changes in β-tubulin-labeled nerves and CD68+ macrophages at these time points. We found an increase in CD68+ macrophages in both epithelium and stroma that peaked 72 h after abrasion. We noted sex differences, with female rats showing slower epithelial healing and greater CD68+ labeling in peripheral cornea. CD68+ macrophages infrequently associated with intact corneal nerves and only occasionally associated with nerve fragments; thus, CD68+ macrophages do not appear to directly chaperone nerves as they reinnervate the epithelium after abrasion. Corneal pain was assessed using orbital tightening (OT) in unrestrained rats. Pain peaked at 24 h after corneal abrasion, was elevated at 72 h, and returned to baseline levels by 1 week. Acute OT behavior correlated with CD68+ density, suggesting a relationship between the intensity of acute pain responses and immune cell activation. We used subconjunctival liposomal clodronate to deplete macrophages prior to corneal abrasion and found a modest attenuation in acute pain responses, as well as an uncoupling of the relationship between acute pain and the prevalence of CD68+ macrophages at 72 h. These results suggest that CD68+ macrophages contribute to acute pain after corneal abrasion, but they do not typically make direct contact with corneal nerves.