The proteolytic activity of plasmin is considered to contribute to tissue remodeling in the wound healing process
. Activation of plasminogen by a plasminogen activator is essential in the initiation of fibrinolysis, and there are two types of plasminogen activators; i.e., tissue-type plasminogen activator (t-PA) and u-PA.
u-PA is expressed at the leading edge of the migrating corneal epithelium, whereas t-PA is expressed at a low level during corneal epithelial wound healing
In our case, a large number of migrating inflammatory cells and corneal fibroblasts were observed in the wound edge of the ulcer (Figure
2). This probably indicates repeated and prolonged wound healing reactions. Steroid eye drops are known to delay wound healing and are a factor complicating corneal ulceration. In the present case, long-term steroid treatment may increase the risk of opportunistic corneal infection and delay wound healing. As a result, perforation developed in the central cornea.
Immunohistochemical examination revealed the expression of u-PA in concordance with CD68-positive cells (Figure
4A). At the same time, u-PA was also observed to be partially expressed in α-SMA positive cells (Figure
4B). We suggest that u-PA is mainly produced by CD68-positive cells and also by corneal fibroblasts.
In addition, the expression of u-PAR was observed in the same cells in which u-PA was expressed (Figure
3). This clearly indicated that the infiltrating cells were the u-PA/u-PAR complex. u-PAR expression has been demonstrated in many cell types including macrophages and corneal fibroblasts
[12, 13]; however, the role of u-PA/u-PAR interaction during ocular inflammation is not clearly understood. Berstein et al. demonstrated that non-cleaved u-PAR (D1D2D3) is present in corneal fibroblasts and cleaved u-PAR (D2D3) is increased in myofibroblasts
. The antibody used in our study was made for a C-terminal peptide present both on the intact and cleaved form of the receptor. Therefore, we could not differentiate whether the molecules that reacted to the antibody were non-cleaved or cleaved u-PAR (Figure
3). We hypothesize that u-PAR-bearing cells promote cellular infiltration into the ulcer by binding u-PA and by local degradation of the peripheral corneal stroma.
On the other hand, although expression of α2AP in the cornea has been previously reported
, the function of α2AP in the cornea has not been elucidated. In our case, interestingly, α2AP expression was observed mostly in the corneal stromal opacity area presumably affected with fibrosis and was barely seen in the ulceration area. Double staining results revealed that α2AP was expressed in α-SMA-positive cells and no α2AP expression was observed in CD68-positive cells (Figures
5A and B).
Many reports have suggested that activated corneal fibroblasts produce ECM and are involved in tissue repair and fibrosis
[1, 16, 17]. The double staining results could be explained by at least two mechanisms: (1) α-SMA-positive fibroblasts produce and secrete α2AP, or (2) α-SMA-positive fibroblasts bind to α2AP. The concordance between α2AP and α-SMA expression may indicate that α2AP is somehow functioning with α-SMA-positive fibroblasts in the fibrotic response.
Kanno et al.
 demonstrated a new action for α2AP using α2AP knock-out mice where α2AP was associated with tissue fibrosis. This may also be the same mechanism in corneal fibrosis.
In the present case, the wound healing process became prolonged because of the repeated passages of corneal melting and remodeling at the ulcerated area, eventually leading to the development of corneal opacity and perforation. Further investigations are necessary to elucidate the role of u-PA, u-PAR and α2AP in corneal ulcers. These molecules are believed to play a crucial role in inflammatory cell recruitment, ECM synthesis and degradation during corneal wound healing.
To our knowledge, we have identified, for the first time, the expression of the u-PA/u-PAR complex and α2AP in a patient with a corneal ulcer.
Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the series editor of this journal.