Recruitment of antigen-nonspecific cells plays a pivotal role in the pathogenesis of a T cell-mediated organ-specific autoimmune disease, experimental autoimmune uveoretinitis.
Experimental autoimmune uveoretinitis (EAU) is a prototypic T cell-mediated autoimmune disease, whose target tissue is the neural retina, that is used as a model for a number of human blinding ocular diseases of a presumed autoimmune nature. EAU in rats can be induced by adoptive transfer of small numbers of retinal antigen-specific CD4+ T cell lines. Although recruitment mechanisms were assumed to play a role in the immunopathogenesis of uveitis, there is no direct evidence that would permit assessment of the importance of recruited non-antigen-specific T cells in retinal autoimmunity. In the present study, we addressed this question by using congenitally athymic Lewis rats (LEW.rnu/rnu), that are deficient in functional endogenous T cells, but are otherwise syngeneic with the euthymic Lewis rats that develop characteristically severe EAU. The uveitogenic stimulus was delivered in the form of phenotypically and functionally homogeneous pathogenic T cell lines, specific to the major pathogenic epitope of either the intracellular photoreceptor protein, S-Ag, or the extracellular photoreceptor matrix protein, IRBP. Depending on the T cell line used, EAU in athymic rats was either drastically reduced in severity (IRBP), or essentially absent (S-Ag). Susceptibility was restored when the athymic animals were reconstituted with immunocompetent T cells from syngeneic euthymic donors. While the intraocular infiltrate in euthymic rats was predominantly lymphocytic, with smaller numbers of monocyte/macrophages and even fewer neutrophils, the sparse infiltrate in athymics was largely monocytic, and with a relatively high proportion of neutrophils and eosinophils. Reconstituted animals had an intermediate histological picture with respect to the infiltrating cell types and disease severity. Our data are consistent with the interpretation that recruitment of naive T cells constitutes an amplification mechanism that is central to the expression and pathogenesis of uveitis. The extent of dependence on this phenomenon appears to be influenced by the antigenic specificity of the T cell line, and could be connected to the 'accessibility' of the target antigen in vivo.