The role of posttranslational modifications in generating neo-epitopes that bind to rheumatoid arthritis-associated HLA-DR alleles and promote autoimmune T cell responses

Abstract

While antibodies to citrullinated proteins have become a diagnostic hallmark in rheumatoid arthritis (RA), we still do not understand how the autoimmune T cell response is influenced by these citrullinated proteins. To investigate the role of citrullinated antigens in HLA-DR1- and DR4-restricted T cell responses, we utilized mouse models that express these MHC-II alleles to determine the relationship between citrullinated peptide affinity for these DR molecules and the ability of these peptides to induce a T cell response. Using a set of peptides from proteins thought to be targeted by the autoimmune T cell responses in RA, aggrecan, vimentin, fibrinogen, and type II collagen, we found that while citrullination can enhance the binding affinity for these DR alleles, it does not always do so, even when in the critical P4 position. Moreover, if peptide citrullination does enhance HLA-DR binding affinity, it does not necessarily predict the generation of a T cell response. Conversely, citrullinated peptides can stimulate T cells without changing the peptide binding affinity for HLA-DR1 or DR4. Furthermore, citrullination of an autoantigen, type II collagen, which enhances binding affinity to HLA-DR1 did not enhance the severity of autoimmune arthritis in HLA-DR1 transgenic mice. Additional analysis of clonal T cell populations stimulated by these peptides indicated cross recognition of citrullinated and wild type peptides can occur in some instances, while in others cases the citrullination generates a novel T cell epitope. Finally, cytokine profiles of the wild type and citrullinated peptide stimulated T cells unveiled a significant disconnect between proliferation and cytokine production. Altogether, these data demonstrate the lack of support for a simplified model with universal correlation between affinity for HLA-DR alleles, immunogenicity and arthritogenicity of citrullinated peptides. Additionally they highlight the complexity of both T cell receptor recognition of citrulline as well as its potential conformational effects on the peptide:HLA-DR complex as recognized by a self-reactive cell receptor.

Emerging Concepts in Immune Thrombotic Thrombocytopenic Purpura

“…This raises the possibility that citrullination of ADAMTS13 is another contributing factor for the loss of tolerance towards ADAMTS13, leading to iTTP. Citrullination was earlier described as a fundamental process in driving the autoimmune processes in rheumatoid arthritis and other inflammatory conditions ( 50 ), and was found to be capable of altering the specificity of TCRs towards T-cell epitopes, even though it did not impact HLA binding ( 51 ). Citrullination through peptidyl-arginine deiminase 4 (PAD4) drives the formation of neutrophil extracellular traps (NETs), which are normally triggered by infectious agents and contribute towards thrombosis by several mechanisms, including oxidation of ADAMTS13 methionines and possibly citrullination of ADAMTS13 through PAD4 ( 52 ).…”

HLA-G and the MHC Cusp Theory

“…Although it has been demonstrated that some citrullinated proteins have a higher binding affinity to the SE than their native counterparts (87), that observation is not universal and does not necessarily govern T cell response (88). Thus, it has been argued that presentation of citrullinated peptides is unlikely to be the sole link between the SE and generation of ACPA (88,89). It has also been hypothesized that cigarette smoke facilitates protein citrullination in the lungs of susceptible individuals, resulting in generation of autoantibodies directed against these citrullinated proteins (90, 91).…”

“…The increased risk of RA development in smokers has been established for years (84), especially amongst individuals carrying particular HLA-DRB1 alleles encoding the SE (85,86). Although it has been demonstrated that some citrullinated proteins have a higher binding affinity to the SE than their native counterparts (87), that observation is not universal and does not necessarily govern T cell response (88). Thus, it has been argued that presentation of citrullinated peptides is unlikely to be the sole link between the SE and generation of ACPA (88,89).…”

Citrullination and PAD Enzyme Biology in Type 1 Diabetes – Regulators of Inflammation, Autoimmunity, and Pathology

“…In both of the collagen-induced arthritis (CIA) and experimental autoimmune encephalomyelitis (EAE) mouse models, citrullination of joint and brain proteins creates neoantigens that become additional targets in epitope spreading of autoimmune responses (172). Citrullination of aggrecan, vimentin, fibrinogen, and type II collagen, known target proteins in RA, initiates epitope spreading by promoting T-cell responses to both citrullinated peptides and the corresponding control peptides (176). In addition, citrullination-induced conformational changes of HSP90 protein unmasks cryptic epitopes to bypass B-cell tolerance in RA (177).…”