Adult rheumatoid arthritis is associated with MC1, a new HLA-D encoded determinant

We evaluated the association of a new HLA-D encoded determinant, MC1, with adult rheumatoid arthritis (RA). This determinant associates with DR1 and DR4 and can be defined by serological typing. We found MC1 in 83% of 80 patients with RA vs 43% of controls. Although the frequencies of DR1 and DR4 were both significantly increased in patients with RA compared with controls, MC1 had the highest relative risk (6.2) of any HLA-DR antigen tested. MC1 negative and positive populations were not significantly different in any of a variety of clinical and laboratory variables including age, sex, disease duration, age at onset, hours of morning stiffness, functional class, joint count, presence of subcutaneous nodules or bony erosions, frequency of side effects to gold or D-penicillamine, sedimentation rate, and antinuclear antibody.