Purpose: To study Bcl-2 expression and apoptotic cell shedding of the rabbit corneal epithelium during extended wear of low and hyper Dk rigid gas permeable (RGP) contact lenses.
Methods: Rabbits were fit with either a low or a hyper Dk RGP lens (Dk/Ltotal= 10 and 97). The rabbits wore the lenses for either 24 hours, 3 days, or 1 week at which point they were humanely sacrificed. Immunocytochemistry and western blot analyses were performed to detect Bcl-2 in the corneal epithelium; TUNEL assay (TdT-mediated dUTP nick-end labeling) was used to identify apoptotic epithelial cells.
Results: 1) Immunocytochemistry: In the normal cornea, antibodies to Bcl-2 uniformly stained nuclei of all epithelial cell layers. Occasional surface epithelial cells, however, showed no anti-Bcl-2 nuclear staining; concomitant TUNEL assay revealed that all TUNEL-labeled-surface cells were Bcl-2 negative. By contrast, RGP contact lens wear, regardless of test lens oxygen transmissibility or lens wearing interval, significantly decreased both the total number of Bcl-2 negative and TUNEL-labeled cells on the epithelial surface (P < 0.05). In addition, contact lens wear was associated with labeling of keratocytes with TUNEL assay in the anterior stroma. 2) Western blot analysis: Total epithelial layer Bcl-2 expression was markedly decreased in the low Dk lens test group but was similar to control values in the hyper Dk lens test group.
Conclusion: Bcl-2 protein seems to play an important role in the regulation of apoptotic cell shedding in the normal rabbit corneal epithelium. The identical staining pattern was seen in previous studies of the normal human cornea. RGP contact lens wear, however, appears to block the changes in Bcl-2 protein prior to apoptotic surface cell shedding, suggesting a lens-related anti-apoptotic effect. Taken together, these findings may explain why contact lens wear reduces surface cell exfoliation as previously reported in human studies.