Objective To investigate the reactivity of monoclonal anti–citrullinated protein antibody (ACPA) obtained from peripheral blood B cells of rheumatoid arthritis (RA) patients with human autoantigens as well as environmental proteins by determining the essential epitope for the ACPA. Methods A human monoclonal ACPA (cyclic citrullinated peptide antibody 1 [CCP‐Ab1]) was obtained by screening peripheral blood lymphocytes from 31 patients with RA using a novel monoclonal antibody–secreting cell (ASC) screening system, the immunospot‐array assay on a chip. The essential epitope for CCP‐Ab1 was determined using epitope mapping. Then, human, microbial, and plant proteins that share the essential epitope identified were searched using BLAST. Finally, representative proteins identified by the search were produced in vitro, and their reactivity with CCP‐Ab1 was examined. Results CCP‐Ab1 bound CCP in a citrulline‐indispensable manner. In CCP, the 6 amino acid residues required for CCP‐Ab1 binding were identified. In the BLAST search, 38 human, 56 viral, 1,383 fungal, 547 bacterial, and 1,072 plant proteins were found to share the essential epitope, and CCP‐Ab1 reacted with all of the recombinant citrullinated proteins tested, which included the various environmental factors, such as various plant proteins that are part of the daily diet. Conclusion Our findings demonstrate, for the first time, that a monoclonal ACPA (CCP‐Ab1) derived from RA patients cross‐reacts not only with various autoantigens but also with numerous plant and microbial proteins. We propose that countless environmental factors, including microbes and diet, may trigger the generation of ACPAs that then cross‐react with various citrullinated human autoantigens through molecular mimicry to induce RA.

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Anti-Ro52 autoantibodies (Ro52-autoAbs) appear in the sera of connective tissue disease (CTD) patients with interstitial lung disease (ILD). Studies using patient sera have shown a correlation between the generation of Ro52-autoAbs and the clinical morbidity and severity of CTD with ILD. In this study, we used a single B-cell manipulating technology and obtained 12 different monoclonal Ro52-autoAbs (mRo52-autoAbs) from the selected four patients suffering from severe ILD with a high titer of Ro52-autoAbs in their sera. Western blot analysis revealed that 11 of 12 mRo52-autoAbs bound to the coiled-coil domain of Ro52. Competitive ELISA demonstrated that mRo52-autoAbs competed with each other to bind to Ro52. Epitope mapping showed that two of them specifically bound to a peptide (PEP08) in the coiled-coil domain. We then examined the titer of Ro52-autoAbs in the sera of 192 CTD patients and assessed the relationship between the serum levels of Ro52-autoAbs that were reactive to PEP08 peptide and the clinical morbidity and severity of ILD. Statistical analysis revealed that the production of PEP08-reactive Ro52-autoAbs correlated with the morbidity and severity of ILD in CTD. Assessment of the production of PEP08-reactive Ro52-autoAbs in autoimmune diseases is useful for predicting the clinical morbidity of ILD.

Objective In plasma from a patient with rheumatoid arthritis (RA), we previously isolated a human monoclonal anti–citrullinated protein antibody (ACPA), CCP‐Ab1, that recognizes various citrullinated antigens. In this study, we aimed to explore the physiologic target of CCP‐Ab1 and the role of molecular evolution, through affinity maturation, of this ACPA in the onset and the exacerbation of RA. Methods The target protein of CCP‐Ab1 was identified in the plasma of a patient with RA and purified under native conditions. Germline‐reverted (GL‐rev) CCP‐Ab1 was generated, and its reactivity was compared to that of mature CCP‐Ab1. The functions of CCP‐Ab1 and GL‐rev CCP‐Ab1 in the onset or exacerbation of autoimmune arthritis were analyzed using autoimmune arthritis–prone SKG mice. Results CCP‐Ab1 bound citrullinated fibrinogen under native conditions. In cultures with GL‐rev CCP‐Ab1, the binding affinity to citrullinated fibrinogen was drastically reduced (P < 0.05). The elements implicated in GL‐rev CCP‐Ab1 binding to a citrullinated peptide, cfc1‐cyc, were almost identical to those implicated in CCP‐Ab1 binding. In arthritis‐prone SKG mice, CCP‐Ab1, but not GL‐rev CCP‐Ab1, induced significant exacerbation of experimental arthritis (P < 0.05). Increased production of interleukin‐6, both in the joint tissue and in the serum, was observed in SKG mice treated with CCP‐Ab1 compared to those treated with GL‐rev CCP‐Ab1 (P < 0.05). Furthermore, the immune complex formed by CCP‐Ab1 and fibrinogen was detected at higher concentrations in the synovial tissue of SKG mice administered CCP‐Ab1 (P < 0.05 versus control treatment groups). Conclusion These data show that germline‐encoded CCP‐Ab1, which binds weakly to citrullinated fibrinogen, undergoes hypermutation through the activation of naive B cells by citrullinated peptides/proteins, thereby stimulating high reactivity to citrullinated fibrinogen. These findings deepen our understanding of the role of molecular evolution of ACPAs in the onset and exacerbation of RA.

Cytophagic histiocytic panniculitis is a chronic histiocytic disease of the subcutaneous adipose tissue characterised by lobular panniculitis with histiocytes containing blood cell fragments. It is also associated with marked systemic features such as fever, pancytopenia, hepatosplenomegaly, liver abnormalities and coagulopathy. We report a case of cytophagic histiocytic panniculitis in a 74-year-old man successfully treated using combination therapy with prednisolone and cyclosporine A.