Antibodies to Mi-1 in SLE: relationship to other precipitins and reaction with bovine immunoglobulin.

Precipitating antibodies to Mi-1, a protein antigen purified from calf thymus nuclear extract, have been reported in a small percentage of patients with dermatomyositis (DM) but not in patients with other connective tissue diseases or controls. Mi-1 shares certain characteristics with immunoglobulin, including the ability to react with rabbit anti-bovine immunoglobulin G (BIgG). A more extensive survey for anti-Mi-1 has been performed, involving 51 normal controls, 67 patients with polymyositis (PM) or DM, 47 patients with rheumatoid arthritis (RA), 41 SLE patients without any other precipitating antibodies to saline extractable tissue antigens on routine testing, and 247 patients, many with SLE, known to have other precipitins. Anti-Mi-1 was found in 13 patients from the latter group (5.25%) and two patients with PM or DM (3.0%), and was not found in other groups. It was found in seven of 95 (7.4%) with anti-nRNP, three of 32 (9.4%) with anti-Sm (with or without anti-nRNP) and three of 120 (2.5%) with anti-Ro (SSA). Anti-Mi-1 cross-reacts directly with BIgG but differs from ordinary rheumatoid factor in its frequent reaction with Fab2 fragments and restricted species specificity. Thus, anti-Mi-1 can be found in patients with diseases other than DM, including SLE, most often in patients with antibodies to nRNP.