An immunologically privileged retinal antigen elicits tolerance: major role for central selection mechanisms

Mouse lysozyme-M knockout mice reveal how the self-determinant hierarchy shapes the T cell repertoire against this circulating self antigen in wild-type mice

We have studied T cell tolerance to defined determinants within ML-M using wild-type (WT; ML-M(+/+)) and LysMcre (ML-M(-/-)) C3H (H-2(k)) mice to determine the relative contribution of ML-M-derived epitopes vs those from other self Ags in selection of the ML-M-specific T cell repertoire. ML-M was totally nonimmunogenic in WT mice, but was rendered immunogenic in LysMcre mice. Most of the response to ML-M in LysMcre mice was directed to the immunodominant determinant region 105-119. This determinant is spontaneously displayed (without adding exogenous ML-M) by macrophages of WT, but not LysMcre, mice and is stimulatory for peptide 105-119 (p105-119)-primed T cells. Moreover, neonatal tolerization of LysMcre mice with p105-119 or ML-M abrogated the T cell response to subsequent challenge with ML-M or p105-119. Furthermore, p95-109 and p110-125 of ML-M were immunogenic in LysMcre mice, but not in WT mice, thereby representing subdominant, tolerance-inducing epitopes of ML-M. As expected, the T cell repertoire to cryptic ML determinants in WT mice was also intact in LysMcre mice. Furthermore, the pattern of response to the related homologue of ML-M, hen eggwhite lysozyme, was similar in these two groups of mice. Thus, several codominant T cell determinants within ML-M contribute significantly to tolerance induction, and the anti-cryptic T cell repertoire to ML-M was positively selected on non-ML-M self ligands. These results reveal that the induction of self tolerance to a multideterminant protein follows the quantitative hierarchy of self-determinant expression and are of relevance in understanding the pathogenesis of autoimmunity.

Self-representation in the thymus: an extended view

The thymus has been viewed as the main site of tolerance induction to self-antigens that are specifically expressed by thymic cells and abundant blood-borne self-antigens, whereas tolerance to tissue-restricted self-antigens has been ascribed to extrathymic (peripheral) tolerance mechanisms. However, the phenomenon of promiscuous expression of tissue-restricted self-antigens by medullary thymic epithelial cells has led to a reassessment of the role of central T-cell tolerance in preventing organ-specific autoimmunity. Recent evidence indicates that both genetic and epigenetic mechanisms account for this unorthodox mode of gene expression. As we discuss here, these new insights have implications for our understanding of self-tolerance in humans, its breakdown in autoimmune diseases and the significance of this tolerance mode in vertebrate evolution.

Interphotoreceptor retinoid-binding protein (IRBP)-deficient C57BL/6 mice have enhanced immunological and immunopathogenic responses to IRBP and an altered recognition of IRBP epitopes

Experimental autoimmune uveitis (EAU) and pinealitis (EAP) can be induced in susceptible mice by immunization with immunologically privileged retinal antigens. In the present study, we analyzed the immunologic and immunopathologic responses of mice deficient in the retinal autoantigen interphotoreceptor retinoid-binding protein (IRBP). The consequences of IRBP deficiency on the T-cell repertoire were also investigated. IRBP+/+, IRBP+/- and IRBP-/- mice on the C57BL/6 background were immunized with IRBP or with a pathogenic epitope, IRBP(1-20) peptide in adjuvant, and were evaluated for disease severity and immunological responses. C57BL/6 IRBP-/- mice were completely resistant to EAU and EAP, and had enhanced immunological responses to IRBP and to its pathogenic peptide 1-20, as compared to their IRBP+/+ counterparts. IRBP-/- mice exhibited an altered IRBP epitope recognition. T cell epitope mapping revealed a response to IRBP peptide 271-290 in IRBP-/- mice, that was absent in the wild type. Primed T cells of IRBP-/- mice transferred an exacerbated form of EAU to nai;ve wild type recipients. A gene-dose effect was evident in that C57BL/6 IRBP+/- mice, exhibited intermediate immunological responses and lower disease scores compared to wild type. We conclude that expression of IRBP in target tissues is a necessary prerequisite for disease induction, excluding other retinoid-binding or vision-related proteins as surrogate targets. Furthermore, endogenous expression of IRBP is directly responsible for lowering the threshold of susceptibility to uveitic disease.

Reviving suppression?

Although the field of regulatory T cells has experienced a resurgence in recent years, the term 'suppressor T cell' has remained taboo. The explanation for this contradictory reaction to similar phenomena can be traced to the historical discovery and definition of each cell type.