Biology of peripheral ulcerative keratitis.

Peripheral ulcerative keratitis (PUK) is a progressive peripheral thinning of the corneal stroma caused by proinflammatory mediators' release from corneal limbal vasculitis. The clinical presentation is an epithelial defect with a crescent-shaped stromal inflammation. Its exact pathophysiologic mechanisms of PUK remain partially understood, but the overall understanding of the fundamental processes that mediate and effect corneal immunity has continued to expand over the past 25 years. The unique anatomical and physiological characteristics of the periphery in relation to collagen bundles and peripheral corneal vascular arch contribute to the occurrence of this type of ulcer in this region, in addition to the concentration of complement and immunoglobulins. There is a relevant participation of the adjacent conjunctiva. Both cell-mediated immunity and humoral immunity are implicated in the pathogenesis of PUK, and the postulated mechanisms are autoimmune reactions to corneal antigens, deposition of circulating immune complexes and hypersensitivity reactions to foreign antigens. These immunocomplexes are deposited in limbic vessels resulting in the activation of the classical pathway of the complement system and, consequently, in the chemotaxis of inflammatory cells and in the release of several pro-inflammatory cytokines, which allow the production and release of matrix metalloproteinases. The release of inflammatory cytokines by infiltrating cells may induce keratocyte activation, which could then generate more release of a variety of cytokines, such as the neutrophil calgranulin C, thus facilitating an autoimmune response to the protein and precipitating an antibody- and cell-mediated hyperimmune reaction in the peripheral cornea.