Strong association of dermatomyositis‐specific Mi‐2 autoantibodies with a tryptophan at position 9 of the HLA‐DRβ chain

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Abstract
Objective. To characterize the clinical and immunogenetic features of patients with Mi-2 autoantibodies. Methods. Eighteen adult white patients with Mi-2 antibodies were clinically characterized and compared with 41 Mi-2–negative dermatomyositis (DM) patients. HLA class I and class II typing for DRB alleles was done by microcytotoxicity assay and for DQA and DQB alleles by polymerase chain reaction–based oligotyping. Results. Seventeen of the 18 Mi-2–positive patients had DM. Symptoms of scleroderma, lung involvement, and arthritis were less common in this group than in the Mi-2–negative DM patients; the V-sign rash and nailfold involvement were found more frequently. Mi-2 antibodies were strongly associated with HLA-DR7 (88% versus 24% in healthy controls), HLA–DQA1*0201 (86% versus 23%), and DR7 “homozygosity” (31% versus 0%). A tryptophan residue at position 9 of the HLA–DRβ chain was present in all Mi-2–positive patients (100% versus 62%; homozygous in 81% versus 15%). Conclusion. Our results reemphasize the specificity of Mi-2 antibodies for DM, and extend previous reports that Mi-2 antibody production is associated with certain HLA class II antigens. We propose β9-Trp as a candidate epitope on the HLA–DRβ chain as a prerequisite for this type of autoimmune response.