Nonprecipitating IgG or IgM anti-Sm antibody: clinical significance and changes in immunoglobulin class

To estimate the clinical significance of IgG or IgM nonprecipitating anti-Sm antibodies and their immunoglobulin (Ig) class switch.

Ninety-one sera which were positive for anti-nRNP antibodies by double immunodiffusion (DID) were obtained from patients with various rheumatic diseases. Anti-Sm antibody was detected by RNA-immunoprecipitation (RNA-IP). IgG or IgM anti-Sm Ig class was measured by enzyme linked immunosorbent assay using affinity purified D polypeptide from HeLa cells. One hundred sixteen clinical manifestations were compared among these groups of patients.

By RNA-IP and DID, 91 were divided into 43 with nonprecipitating anti-Sm which were not detected by DID but by RNA-IP, 17 no anti-Sm, and 31 precipitating anti-Sm sera. In 43 patients with nonprecipitating anti-Sm, renal involvement was more common than in no anti-Sm patients. Besides, systemic lupus erythematosus (SLE)-type manifestations were less common than in precipitating anti-Sm patients. In these 43 patients, IgM anti-Sm class was common (13/43). Decrease in the ratio of IgM to IgG anti-Sm antibody was observed in proportion to the years of observation in these patients. IgM anti-Sm predominated in these patients with mixed connective tissue disease (MCTD)-type manifestations while IgG anti-Sm did in those with SLE-type features.

Patients with nonprecipitating anti-Sm who have IgM anti-Sm at early stages of disease are more likely to have MCTD-type manifestations and later may shift to SLE-type features. These may reflect the anti-Sm Ig class switches from IgM to IgG.