Difficulties in the knowledge of uveitis

Arachidonic acid cascade and platelet-activating factor in the network of eye inflammatory mediators: therapeutic implications in uveitis

The cellular and biochemical events triggered by uveitis involve a complex array of cells and a heterogeneous network of mediators of intraocular inflammation. Resident cells are activated and inflammatory cells are recruited. Chemical mediators from the arachidonic acid cascade, prostaglandins, hydroxyeicosatetraenoic acids, and leukotrienes, are formed. Several of these metabolites are modulators of cellular functions, but when generated in sustained, excessive amounts, they contribute to enhanced vascular permeability and to the onset of pathophysiological responses. Another very active membrane-derived mediator is platelet-activating factor. This important mediator of immune and inflammatory responses may play a central role in uveitis due to cell priming, since interleukin-1, tumor necrosis factor, and other as yet unidentified mediators are also being generated. The concomitant accumulation of these networks of mediators in various parts of the uveal tract leads to spreading of the intraocular inflammatory response and cellular damage. At both early and late stages of uveitis, the generation of free radicals is also a major contributor to the impairment of function. Free radicals are generated in two distinct sites: in the oxidative burst of recruited white cells and in free radical formation and lipid peroxidation in resident cells. The identification of the cellular events that lead to the accumulation of networks of mediators of inflammation and their effects has important therapeutic implications in uveitis.

The role of the immune system in uveitis induced in animals

The etiological diagnosis in many cases of uveitis is hampered by the lack of insight into the pathogenesis of the disease. Animal models might provide clues for the understanding of the ocular and systemic processes leading to this invalidating disease. The immune system seems to play an important role in the pathogenesis of uveitis. Four main topics i.e. autoimmune phenomena, circulating immune complexes, effects of systemic endotoxin and bacterial or viral uveitis are reviewed. Autoimmune phenomena might cause the recurrence of uveitis after presentation of slightly altered self-antigens to the immune system. The endotoxin induced uveitis might be the initial event of recurrent episodes of uveitis caused by the deposition of immune complexes. Endotoxin might play a role in the so called HLA-B27 positive uveitis, mediated through tumour necrosis factor and interleukin-1. In this paper the characteristics of these animal models are described. Attention has been paid to those situations where several of these mechanisms combine in the pathogenesis of ocular diseases.