Smith (Sm) Antigen

Sm antigen is a non-histone nuclear protein composed of several polypeptides of differing molecular weights. They include B (26 kD), B'(27 k D), and D (13 kD). The principle reactivity has been shown to reside in the B, B’, and D polypeptides (3). Anti-Sm autoantibodies were described originally as precipitating autoantibodies in sera of patients with Systemic Lupus Erythematosus (1). Anti-Sm antibodies are also usually accompanied by antinuclear ribonucleoprotein (nRNP) antibodies (2). The U1 RNP particle has both Sm and RNP binding specificities. The difference is that the RNP particles bound by U2, U4/6 and U5 are bound by anti-Sm autoantibodies, but not by anti-nRNP autoantibodies. Autoantibodies against the Sm antigen precipitate the U1, U2, U4/6, and U5, small nuclear RNAs (4). The Sm antigen is involved in normal post-transcriptional, premessenger RNA processing to excise introns (5). It has been demonstrated that the Sm antigenicity is both RNase and DNase resistant and partially resistant to tryptic digestion (6). Autoantibodies to Sm antigen have been observed in 15 to 30% of SLE sera as a diagnostic marker. (7) It is thought that IgG anti-Sm correlates with Lupus disease activity and is a useful variable in predicting exacerbation and prognosis of SLE. (8) It has been reported that IgG anti-Sm is specifically detected in patients with SLE. IgM anti-Sm has rarely been detected in SLE and was recognized in other diseases (2). It has also been reported in a study of clinical significance, that the frequency of lung fibrosis and pericarditis was significantly higher in patients with IgG, IgA, and/or IgM anti-Sm (2).

1. Tan EM, Kunkel HG. “Characteristics of a Soluble Nuclear Antigen Precipitating with Sera of Patients with Systemic Lupus Erythematosus.” J Immunol Vol 96: 1966, pg 464-471.
2. Yasuma M, Takasaki Y, Matsumoto K, et al. “Clinical Significance of IgG Anti-Sm Antibodies in Patients with Systemic Lupus Erythematosus.” The J of Rheumatology Vol 17: 1990, pg 469-475.
3. James JA, Harley JB. “Sequential Epitopes of an Sm B/B’ Protein” American College of Rheumatology 55th Annual Scientific Meeting, Boston, MA 1991.
4. Lerner MR, & Steitz JA “Antibodies to Small Nuclear RNAs Complexed with Proteins are Produced by Patients with Systemic Lupus Erythematosus” Proc Natl Acad Sci Vol 76: 1979, pg 5495-5499.
5. Yang VW, Lerner MR, Steitz JA & Flint SJ. “A Small Nuclear Ribonucleoprotein is Required for Splicing of Adenoviral Early RNA Sequences.” Proc Natl Acad Sci USA, Vol 78: 1981, pg 1371-1375.
6. Schrier WH, Reddy R and Busch H. “Identification of An Antigenic Protein Recognized by Anti-Sm Autoantibody.” Cell Biology International Reports Vol 6: 1982.
7. Tan EM. “Special Antibodies for the Study of Systemic Lupus Erythematosus.” Arthritis Rheum Vol 25: 1982, pg 753-756.
8. Pollard KM, Tan EM. “Purification of the Sm Nuclear Autoantigen, Detection and Clinical Significance of IgM Antibody.” Clin Exp Immunol Vol 60: 1985, pg 586-596