Roles of Adherence and Matrix Metalloproteinases in Growth Patterns of Fungal Pathogens in Cornea

Dear Editor,

We thank Dr. Garg and his colleagues for their valuable comments and would like to have a discussion with these scientists and ophthalmologists who are engaged in the relevant investigations on fungal keratitis.

Although the use of corticosteroids can facilitate infections, it inevitably affects the invasive process of fungal pathogens in corneas. In our opinion, the key to establishing an animal model of fungal keratitis is mimicking the natural development of the disease in human eyes. After repeated experimental comparison with the models previously reported, we found it an ideal approach to inject an inoculum into the space between the sutured graft and the recipient bed after debriding the corneal epithelium. In this model, donor graft can prevent the inoculum from tear flush, so that the conidia can adhere to the stroma for a longer time. The space between the graft and the recipient bed can offer stable temperature and moisture favoring fungal growth. Moreover, because it was not easy to achieve sustained fungal infection in rabbit corneas with no use of corticosteroids,¹ the inocula containing a relatively large number of conidia were prepared. Therefore, persistent fungal keratitis could be successfully developed in our rabbit model in the absence of corticosteroids. In addition, awareness of fungal growth patterns may benefit the clinical treatment of fungal keratitis.², ³ This model, which produces infection from the corneal surface, brings convenience to the observation on the invasive patterns of fungal pathogens in corneas.

In our study, the corneal infections did not heal on day 8. The number of inflammatory cells and hyphae gradually decreased from day 3 to day 8 in all the inoculated groups to various degree,⁴which was consistent with the corresponding clinical score and the levels of MMP-9.

With regard to the virulence of Fusarium solani, the leading cause of the contradictory findings between our study and some previous reports⁵, ⁶ may be the difference of the specific F. solani strains involved. Individual strains of the same F. solani species may show different pathogenicity. SC5314 and VE175 are two Candida albicans strains. O'Day et al.⁷ reported that strain SC5314 was significantly more virulent than strain VE175 as measured by invasive depth of fungal filaments into corneas and clinical evaluation of infection. Moreover, the susceptibility of fungal pathogens may vary with the host species. In our recent study on a mouse model of fungal keratitis, the pathogenicity of Aspergillus fumigatus and F. solani (same strains as in our rabbit model) differed from the results in the rabbit model. Therefore, it is possible that F. solani in human corneas produces an equally or more serious keratitis compared with A. fumigatus. The different findings on the patterns of fungal growth in corneal stroma may also be related to the above factors as well as the duration of the disease and the therapy involved in clinical practice. Further investigations are needed on the fungal susceptibility to animal and human corneas and the virulence of various fungal pathogens.