THE ROLE OF APOPTOSIS IN THE PATHOGENESIS OF CANINE KERATOCONJUNCTIVITIS SICCA - THE EFFECT OF TOPICAL CYCLOSPORINE-A THERAPY

Purpose. The exact etiology of dry eye is unknown but is believed to b e multifactorial. Apoptosis has been implicated in the pathogenesis of autoimmune diseases such as Sjogren's syndrome (SS). This study attem pted to gain a better understanding of the role of apoptosis and its r egulation in the pathophysiology of dry eye. The therapeutic effect of immunomodulatory agents such as cyclosporin A (CsA) in the treatment of dry eye, particularly its impact on the level of apoptosis in the t arget tissues, is also investigated. Methods, A colony of dogs with sp ontaneous chronic idiopathic keratoconjunctivitis sicca (KCS) was main tained. Nictitans lacrimal gland (NLG), an accessory lacrimal gland, a nd conjunctival biopsies of the KCS and normal dogs were obtained befo re and after 12 weeks of treatment with 0.2% topical CsA ophthalmic em ulsion b.i.d. (Allergan, Inc., Irvine, CA, U.S.A.). Tissues were prepa red for the terminal deoxynucleotidyl transferase-mediated dUTP-digoxi genin nick end labeling (TUNEL) apoptosis assay and immunohistochemica l analysis for various apoptosis mediators. Results. The TUNEL assay d emonstrated that (i) the normal NLG and conjunctival epithelial cells exhibited a limited level of apoptosis; (ii) in KCS dogs, lacrimal aci nar and conjunctival epithelial cells underwent an increased apoptosis , whereas the lymphocytes had a significantly lower level of apoptosis compared to those of the normal dogs, (iii:) after topical CsA, apopt osis was induced in the lymphocytes and suppressed in the acinar and c onjunctival epithelial cells in KCS dogs. Immunohistochemistry reveale d that p53, fas, and fast, but not bcl-2 were highly expressed in the target tissues of KCS dogs. The immunoreactivity of p53 was significan tly decreased, whereas the bcl-2 level was increased after CsA adminis tration. Conclusions, The induction of epithelial cell apoptosis and t he suppression of lymphocytic apoptosis in the NLG and ocular-surface tissues, such as conjunctiva of KCS dogs, indicates the important role of this phenomenon in the etiology of dry eye. Topical CsA appears to facilitate lymphocytic apoptosis and suppress epithelial cell apoptos is in the KCS dog. The differential expression of various apoptotic me diators after topical treatment implicates CsA in facilitating the ree stablishment of the normal apoptotic balance,suggesting additional mec hanisms by which CsA is therapeutic for dry-eye syndrome.