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ABSTRACT

Autoimmune diseases such as ankylosing spondylitis (AS) can be driven by emerging neoantigens that disrupt immune tolerance. Here, we developed a workflow to profile posttranslational modifications involved in neoantigen formation. Using mass spectrometry, we identified a panel of cysteine residues differentially modified by carboxyethylation that required 3-hydroxypropionic acid to generate neoantigens in patients with AS. The lysosomal degradation of integrin αIIb [ITGA2B (CD41)] carboxyethylated at Cys96 (ITGA2B-ceC96) generated carboxyethylated peptides that were presented by HLA-DRB1*04 to stimulate CD4+ T cell responses and induce autoantibody production. Immunization of HLA-DR4 transgenic mice with the ITGA2B-ceC96 peptide promoted colitis and vertebral bone erosion. Thus, metabolite-induced cysteine carboxyethylation can give rise to pathogenic neoantigens that lead to autoreactive CD4+ T cell responses and autoantibody production in autoimmune diseases. Description Nabbing a neoantigen Autoimmunity can be caused by neoantigens that break immune tolerance. Zhai et al. profiled protein posttranslational modifications in patients with ankylosing spondylitis, an autoimmune disease (see the Perspective by Santambrogio). They found that that a cysteine residue of integrin αIIb was carboxyethylated in a process that required the gut microbe metabolite 3-hydroxypropionic acid (3-HPA) and resulted in pathogenic neoantigens. Treatment of HLA-DR4 mice with either the modified protein or 3-HPA resulted in autoantibody production and autoimmune pathology. —STS Metabolite-induced cysteine carboxyethylation generates neoantigens that may fuel autoimmunity. INTRODUCTION Autoimmune diseases such as ankylosing spondylitis (AS) can be caused by emerging neoantigens that break immune tolerance in humans. Posttranslational modifications (PTMs) have been shown to be a critical mechanism that alters protein structure and function to generate neoantigens and induce subsequent autoimmune responses. Previous studies have confirmed that citrulline-modified peptides are a critical source of neoantigens in rheumatoid arthritis. However, the molecular mechanisms underlying neoantigen formation and pathogenic autoreactive responses for AS are largely unknown. There is an urgent need to develop a systematic approach to profiling the possible PTMs in patients with AS and identifying AS-associated PTMs responsible for autoreactive neoantigen production to better understand the etiology of autoimmune diseases. RATIONALE AS has been suggested to be an autoimmune disease because of its clear correlation with certain major histocompatibility complex (MHC) alleles, including HLA-B27. Neoantigens have been hypothesized to induce an aberrant immune response, leading to pathogenic autoreactive T cell responses and autoantibody generation in AS. Here, we developed a systematic open search approach to identify any possible amino acid residues and derivatives in the proteins that are different from the genomic coding sequences. We then applied this information to identify AS-related neoantigens with PTMs within a possible pool of PTM autoantigens and elucidate the pathogenesis of AS. RESULTS An open search approach was applied to identify any possible amino acid derivatives across the proteome of patients with AS. This approach generated a large set of noncoded amino acids representing the mass differences between the coded amino acids and actual residues. Among these, an amino acid derivative with a delta mass of 72.021 showed the greatest increase in patients with AS and resulted from a PTM called cysteine carboxyethylation. In vitro and in vivo experiments demonstrated that carboxyethylation at a cysteine residue of integrin αIIb [ITGA2B (CD41)] was catalyzed by cystathionine beta synthase (CBS) in a process that required 3-hydroxypropionic acid (3-HPA), a metabolite commonly released from gut microbes. Cysteine carboxyethylation induced the lysosomal degradation of ITGA2B and produced neoantigens that triggered MHC-II–dependent CD4+ T cell responses. Fluorescence polarization and enzyme-linked immunosorbent assay (ELISA) demonstrated that the identified carboxyethylated peptide (ITGA2B-ceC96) specifically interacted with HLA-DRA*01/HLA-DRB1*04 and was associated with autoantibody production and T cell responses in HLA-DRB1*04 patients. Additional in vitro assays showed that the neoantigen ITGA2B-ceC96 correlated with 3-HPA levels but was independent of CBS expression. HLA-DRB1 haplotype, the carboxyethylated peptide, specific autoantibodies, and 3-HPA levels in patients with AS all correlated with one another. 3-HPA–treated and ITGA2B-ceC96–immunized HLA-DR4 transgenic mice developed colitis and vertebral bone erosion. Thus, cysteine carboxyethylation induced by the metabolite 3-HPA generates a neoantigen that appears to be critical for autoimmune responses in patients with AS. CONCLUSION Cysteine carboxyethylation is an in vivo protein modification induced by the metabolite 3-HPA, which is commonly released from gut microbes. Carboxyethylated ITGA2B then induces autoantibody production and autoimmune response in AS. Our work provides a systematic workflow to identify differentially modified proteins that are important for neoantigen production in immune disorders. This approach furthers our understanding of AS pathogenesis and may aid in the development of neoantigen-based diagnosis and treatment for AS and other autoimmune diseases. Metabolite-induced cysteine carboxyethylation provokes HLA-restricted autoimmune responses in ankylosing spondylitis. 3-HPA, which is commonly obtained from food and gut microbes, induces carboxyethylation of cysteine residues in integrin αIIb (ITGA2B). Cysteine carboxyethylation requires CBS, and carboxyethylated ITGA2B (ITGA2B-ceC96) peptides are recruited to the HLA-DR4 complex and thereby stimulate CD4+ T cell responses closely related to AS.

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