Alteration of a model antigen by Au(III) leads to T cell sensitization to cryptic peptides

Abstract

Certain metal ions are known to be potent sensitizers, but the self proteins modified by metal ions and the self peptides recognized by 'metal-specific' T cells are unknown. In humans and mice treatment with gold anti-rheumatic drugs, containing Au(I), may lead to allergic and autoimmune side effects. Human and murine T cells do not react to Au(I), however, but to the reactive metabolite Au(III). Here we show that alteration by Au(III) of a model antigen, bovine ribonuclease (RNase)A, results in T cell sensitization to cryptic peptides of this protein. Upon immunization of mice with Au(III)-pretreated RNase [RNase/Au(III)], CD4+ T cell hybridomas specific for RNase/Au(III) were obtained in addition to those recognizing the immunodominant peptide RNase 74-88; the latter also were obtained after immunization with native RNase. RNase/Au(III)-specific T cell hybridomas reacted against RNase/Au(III) and RNase denatured by S-sulfonation of cysteine residues, but not against native RNase, or RNase pretreated with Au(I), A1(III), Cu(II), Fe(II), Fe(III), Ni(II), Mn(II), or Zn(II). Using a panel of overlapping, synthetic RNase peptides which were devoid of gold or gold-induced modifications, epitope mapping revealed that RNase/Au(III)-specific T cell hybridomas recognized the cryptic peptides 7-21 and 94-108, respectively. Comparison of the proliferative response of bulk CD4+ T cells, prepared from splenocytes after immunization with either RNase/Au(III) or native RNase, revealed that Au(III) pretreatment of RNase led to a markedly enhanced response to the two cryptic peptides while it did not influence the response to the immunodominant peptide. The cryptic peptides were also presented after preincubation of bone marrow-derived macrophages with RNase and Au(I), but not with RNase alone, suggesting that oxidation of Au(I) to Au(III) and subsequent protein alteration by Au(III) can happen in mononuclear phagocytes. We conclude that Au(III) alteration of proteins alters antigen processing and, thus leads to presentation of cryptic peptides. This mechanism may shed light on the development of allergic and autoimmune side effects of Au(I) anti-rheumatic drugs. In addition, it might provide a general mechanism of how metal ions act as T cell sensitizers.

Proteolytic Cleavage of a Self-Antigen Following Xenobiotic-Induced Cell Death Produces a Fragment with Novel Immunogenic Properties

“…As mercury-protein interaction has been shown to alter protease sensitivity (16), it is possible that mercury modification of fibrillarin may influence Ag processing, leading to the production of cryptic T cell determinants. Abnormal Ag processing of metal-modified protein is supported by recent studies that have identified cryptic peptides that stimulate IL-2 production by T cell hybridomas obtained from mice immunized with bovine RNase A complexed to gold (17,18). The cryptic nature of these determinants has been suggested by studies showing that T cell hybridomas from mice immunized with bovine RNase alone do not respond to the same peptides (17,18).…”

Gold drugs: Mechanism of action and toxicity

“…with Au(III) results in T cell sensitization in mice to so-called 'cryptic peprides'j derived from this protein while T cells from mice injected with RNase alone are sensitized only to a common, the immunodominant, peptide derived from RNase (38). The scission and presentation of cryptic peptides may arise from gold(III)-induced oxidation of methionine residues and/or gold(III)-induced conformational changes in the antigenic protein leading to a different intracellular processing of the protein and therefore to the formation of different antigenic peptides.…”

“…The scission and presentation of cryptic peptides may arise from gold(III)-induced oxidation of methionine residues and/or gold(III)-induced conformational changes in the antigenic protein leading to a different intracellular processing of the protein and therefore to the formation of different antigenic peptides. The novel aspect of these data is that the metal is not involved in the recognition process between MHC class II protein, peptide and T cell, but affects the prior processing step of a protein altered by the effect of the metal (gold) (38), (Figure 2b It has recently been suggested that gold(III) produced from gold (I) drugs might be involved in the toxic sideeffects encountered in chrysotherapy. The chronic treatment of mice with gold(I) drugs results in the production ofT cells that are stimulated not by gold(I) but by gold(III) (36) and patients with gold-induced dermatitis show significant lymphocyte proliferation in response to gold(III) but not to gold (I) [37].…”

“…In contrast, cryptic peptides are peptides that do not elicit a T cell response under normal conditions, but induce a response when their presentation is up-regulated for a particular reason (38). [49] and [Au(Gfy-Gfy-L-His-H-JiCf-H 20 [50] 3 RSSR + 10 KAuBrq + 18 H 20 -6 RS0 3 -…”

“…Gold-specific T cell clones that proliferate when exposed to gold(l) and gold(III) compounds in vitro have been isolated from patients who have developed such hypersensitivity reactions (2I). Two recent reviews summarise the current view of the effect of gold on T cell recognition (22,24).…”