Neutralizing antibody to TGFβ modulates stromal fibrosis but not regression of photoablative effect following PRK

Abstract

Purpose. Previous studies have suggested that corneal fibrosis controlled by a TGFβ-mediated cytocrine pathway underlies the development of clinical corneal haze and associated regression of photoablative effect following excimer laser PRK. Using a unique blocking antibody, we evaluated the role of TGFβ in post-PRK corneal wound healing as measured by in vivo Con-focal Microscopy Through Focusing (CMTF).Methods. Twelve rabbits received a monocular, 6-mm diameter, 9.0 D PRK myopic correction. Six animals received 50 μg of anti-TGFβ blocking antibodies applied topically 3×/day for three days post-PRK, while six animals received vehicle alone. An additional six animals served as unoperated controls. At various times during a four-month-period, animals were evaluated using CMTF, which generates a quantitative image intensity depth profile of the cornea. The location and reflectivity of corneal structures were identified from CMTF-profiles and used to determine epithelial and stromal thickness and corneal light reflectivity as an objective estimate of corneal haze. To correlate in vivo and ex vivo morphology, an additional six rabbits were analyzed at differing temporal intervals post-PRK for the expression and cytoskeletal organization of contractile micro-filaments: f-actin (stress fibers) and α-smooth muscle actin (a molecular marker for myofibroblast transformation).Results. Anti-TGFβ treated corneas showed significantly less CMTF measured light reflectivity (ANOVA, p < 0.02) following PRK compared to vehicle treated corneas with a 34% decrease at two weeks (2513 ± 758 U compared to 3810 ± 1262 U) and a 61% reduction in reflectivity at four months (447 ± 208 U compared to 1154 ± 585 U). The reduction in early development of light reflecting structures and the more rapid decline appeared related to anti-TGFβ-mediated inhibition of keratocyte activation and proliferation, myofibroblast transformation, and stromal fibrosis. Between anti-TGFβ and vehicle treated corneas, no significant differences were detected in either photoablation depth (126 ± 9 μm versus 126 ± 7 μm) or regression of photoablative effect (postoperative stromal thickening at four months: 95 ± 16 μm versus 95 ± 10 μm). Histologic examination demonstrated that regression of photoablative effect in anti-TGFβ treated corneas was related entirely to regeneration by corneal growth underlying the photoablated stromal surface. In vehicle treated corneas, fibrosis or deposition of new fibrotic tissue above the photoablated stromal surface was observed but contributed only about 25% of the total postoperative stromal thickening. No epithelial hyperplasia was detected. In unoperated control animals, a physiologic stromal thickening of 5 ± 2 μm per month (p > 0.001) was observed.Conclusions. This study confirms our earlier observations that increased corneal light reflectivity following PRK is predominantly due to: (1) distortion of the photoablated stromal surface leading to prominent reflections; and (2) increased reflections from activated and transformed keratocytes. Anti-TGFβ reduced keratocyte activation and transformation and inhibited stromal fibrosis, leading to a reduction in early light reflectivity as well as to a more rapid decline. Of greatest interest is the unexpected finding that anti-TGFβ treatment inhibited stromal fibrosis without reducing or delaying post-PRK stromal re-thickening. Based on these findings we propose that corneal thickness may be tightly and dynamically regulated by an unknown, non-TGFβ mediated pathway. We propose that anti-TGFβ treatment may be useful in reducing post-PRK corneal haze development in patients by: (1) inhibiting the recruitment of highly reflective, activated keratocytes, (2) inhibiting myofibroblast transformation, and 3) reducing stromal fibrosis. Curr. Eye Res. 17:736–747, 1998.