Studies of cryoproteins in systemic lupus erythematosus.

Cryoproteins observed in patients with systemic lupus erythematosus were shown to be of "mixed type," consisting largely of IgG and IgM. The IgM moiety possessed anti-IgG globulin reactivity and precipitated in the cold after mixing with a source of IgG. The IgM fraction of one cryoprotein precipitated only with the same patient's IgG. Antisera prepared against purified cryoproteins regularly recognized IgM, IgG, the 11 S component of C', and alpha(2)-macroglobulin. C'4 (beta(1E)) and C'3 (beta(1C)) were recognized by four and two antisera, respectively. Antisera prepared against two cryoproteins reacted (after absorption) only with the sera of origin. These "unique" antigens were associated with IgM, were destroyed by mercaptoethanol treatment, and, in one patient, were shown to disappear subsequent to clinical improvement.