Collagenolytic proteinases in keratoconus

Purpose: To study the proteolytic phenomena contributing to the pathogenesis of keratoconus, corneal enzymes with potential to cleave fibrillar collagen were studied. Methods: Immunohistochemical labeling was undertaken of conventional and novel mammalian collagenases (MMP-1, -2, -8, -13, and -14) of the matrix metalloproteinase (MMP) family and other collagenolytic proteinases of the serine (human trypsin-2) and cysteine (cathepsin K) endoproteinase families. The results were analyzed using a semiquantitative scoring system. Results: Labeling of MMP-8 was moderate in healthy controls but weak in keratoconus. Moderate MMP-2 and weak MMP-14 expressions were similar in controls and keratoconus. MMP-1 was slightly overexpressed in keratoconus. In contrast, MMP-13 was weak in controls compared to moderate in keratoconus and human trypsin-2 and cathepsin K were moderate in controls and strong in keratoconus. Conclusions: The collagenolytic milieu of human cornea is more complex than expected. Mesenchymal isoform of MMP-8 (ie, collagenase-2) participates in normal tissue remodeling, which may be impaired in keratoconus. MMP-2 (gelatinase A with interstitial collagenase activity) and MMP-14 (membrane-type MMP type I with collagenolytic potential) seem to be constitutively expressed and probably play a role in normal corneal remodeling. The most prominent changes in keratoconic cornea were observed in collagenase MMP-13 (ie, collagenase-3), and particularly, in cathepsin K and human trypsin-2, which were strongly expressed in keratoconus suggesting a role in intra- and extracellular pathologic collagen destruction, respectively. This may contribute to stromal thinning characteristic for keratoconus.