HISTOPATHOLOGICAL STUDIES OF STAPHYLOCOCCAL ALPHA-TOXIN - EFFECTS ON RABBIT CORNEAS

Purpose. Previous studies from this laboratory have demonstrated, in a rabbit model of keratitis, a relationship between the corneal virulen ce of Staphylococcus aureus and the alpha-toxin activity of the infect ing bacteria. This study is a histopathological characterization of th e action of purified alpha-toxin on corneal tissue. Methods. Alpha-tox in was purified by isoelectric focusing and intrastromally injected in to rabbit corneas (2 mu g per cornea). A kinetic analysis of toxin eff ect was performed following injection, Normal corneas and corneas inje cted with phosphate buffered saline (PBS) or heat-inactivated alpha-to xin in PBS served as controls. Eyes were examined from 0 to 4 h by sli t lamp examination (SLE) and scored on the basis of seven ocular param eters. Corneal tissue was removed and examined for histopathological c hanges. Results. From 0.5 to 4 h post-injection, alpha-toxin injection induced a significant increase in the SLE score relative to untreated eyes or eyes injected with PBS (P < 0.0001). Histolopathological exam ination of corneas one-half h after alpha-toxin injection revealed ede ma of the central cornea and death of epithelial cells by both necrosi s and apoptosis. Later times showed continued edema and loss of appare ntly normal epithelial cells. Development of polymorphonuclear (PMN) l eukocyte infiltration from the tear film into the central cornea and f rom limbal vessels into the peripheral cornea was observed. Conclusion s. Purified alpha-toxin mediates cell death by necrosis and apoptosis, sloughing of viable corneal epithelial cells, severe corneal edema, a nd PMN migration into the cornea from both the tear film and limbal ve ssels. The pathologic changes revealed by histological studies of corn eas injected with purified alpha-toxin included death of cells by necr osis and apoptosis as well as overall changes analogous to that seen b y SLE of eyes infected with wild-type, but not alpha-toxin-deficient s trains of Staphylococcus aureus.