ANTI-ENA ANTIBODIES: RNP, Sm, SSA, SSB

CLINICAL UTILITY:

Antibodies to extractable nuclear antigens are characteristic of some rheumatic diseases. In the Anti-ENA antibody test at University of Washington several of these antibodies are identified: Anti-Sm, Anti-RNP, anti-SSA, and Anti-SSB. Anti-Sm is found almost exclusively in patients with naturally-occurring SLE, and is usually absent in other rheumatic diseases, including drug-induced lupus. (Sm stands for Smith, the name of the patient in whom the antibody first was identified.) The antibody is found in approximately 30% of SLE patients.

Anti-RNP (ribonucleoprotein) antibodies appear in patients with a variety of rheumatic diseases, including SLE. The greatest frequency (95-100%) is in mixed connective tissue disease (MCTD), which is characterized by high titers of anti-RNP antibody. The titers by passive hemagglutination usually exceed 1:10,000 (equivalent to approximately 1:16 by counter immunoelectrophoresis, the method used in this lab.) Lower frequency of the antibody, and lower titers, occur in SLE, rheumatoid arthritis (RA), progressive systemic sclerosis (PSS) and Sjogren’s syndrome (SS). The RNP antigen is sometimes referred to as U1-RNP or ribonuclease-sensitive ENA. Some SLE patients have both anti-Sm and anti-RNP antibodies. Anti-SSB antibodies occur in approximately 50% of patients with primary Sjogren’s syndrome, a disease involving the sicca complex (drying of secretions). In the secondary form of Sjogren’s syndrome associated with rheumatoid arthritis, anti-SSB antibodies seldom appear. The antibody is found in approximately 15% of SLE patients. For discussion of anti-SSA antibodies, see page 3-10. Antibodies to the SSA antigen (also known as Ro antigen), appear in 60-70% of patients with primary Sjogren’s Syndrome (SS), in 30-40% of patients with SLE, in other connective tissue diseases, and in babies with neonatal lupus erythematosus. On rodent tissue, the substrate traditionally used for ANA testing, the SSA (Ro) antigen is predominantly cytoplasmic, and not located in the nucleus. Therefore, patients with antibody to SSA may be ANA negative on routine testing. These ANA negative patients represent about 1-5% of SLE cases. Approximately 62% of ANA negative lupus patients have antibodies to SSA, and frequently have subacute cutaneous lupus erythematosus, a variant of SLE characterized by subacute or chronic photosensitive rashes.

Another disease associated with anti-SSA antibodies is neonatal lupus, a condition found occasionally in the infants of mothers with SLE. In rare instances neonatal lupus is manifest as congenital heart block. Transplacental passage of maternal antibodies seems to lead to the transient photosensitive rash or the congenital heart block in these infants.

METHOD DESCRIPTION:

Counterimmunoelectrophoresis (CIE) is the method used to detect antibodies to the saline-extractable nuclear antigens. Anti- Sm, anti-RNP and anti-SSB are the antibodies identified in this laboratory. If the antibody is one of numerous other salineextractable antibodies, the result will be positive for the screen, but negative for the above three antibodies, with comment: "Unidentified antibody present, suggest ordering anti-Jo-1 (myositis precipitin) if clinically indicated." If the precipitin line is too weak to identify conclusively, the result will be reported as such. For titers of anti-RNP antibodies, dilutions of patient serum (1:4 to 1:1024) are tested by CIE, as outlined above. The highest titer still showing a precipitin line is the reported result.

Double diffusion and CIE are the methods used to identify anti-SSA antibodies. Human spleen extract containing the SSA antigen reacts with patient sera, causing precipitin lines to occur if antigen-antibody complexes form. Results are reported as positive or negative.

REFERENCE RANGE:

Negative.

SPECIMEN REQUIREMENTS:

1.0 ml serum. Freeze.