Nonprecipitating IgG or IgM anti-Sm antibody: clinical significance and changes in immunoglobulin class.
OBJECTIVE
To estimate the clinical significance of IgG or IgM nonprecipitating anti-Sm antibodies and their immunoglobulin (Ig) class switch.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Tsuzaka
K
K
Department of Internal Medicine, School of Medicine, Keio University, Tokyo, Japan.
Ogasawara
T
T
Tojo
T
T
Mimori
T
T
Satoh
M
M
Homma
M
M
D000906:Antibodies / Q000032:analysis / Q000235:genetics / Q000276:immunology*; D001324:Autoantigens / Q000276:immunology*; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D015151:Immunoblotting; D005779:Immunodiffusion; D007074:Immunoglobulin G / Q000276:immunology*; D007075:Immunoglobulin M / Q000276:immunology*; D007136:Immunoglobulins / Q000145:classification*; D008180:Lupus Erythematosus, Systemic / Q000276:immunology; D008947:Mixed Connective Tissue Disease / Q000276:immunology; D011233:Precipitin Tests; D011379:Prognosis; D012313:RNA / Q000032:analysis; D017412:Ribonucleoprotein, U1 Small Nuclear / Q000276:immunology / Q000302:isolation & purification; D012261:Ribonucleoproteins / Q000276:immunology; D017411:Ribonucleoproteins, Small Nuclear; D055517:snRNP Core Proteins
D000906:Antibodies; D001324:Autoantigens; D007074:Immunoglobulin G; D007075:Immunoglobulin M; D007136:Immunoglobulins; D017412:Ribonucleoprotein, U1 Small Nuclear; D012261:Ribonucleoproteins; D017411:Ribonucleoproteins, Small Nuclear; D055517:snRNP Core Proteins; D012313:RNA