What is the clinical association of antibodies to the different U1 antigens?
Anti-U1 RNP antibodies are found in about 30 to 40 % of patients with SLE. These antibodies may occur alone but are often present in conjunction with other specificities. Although these antibodies may occur alone in SLE, the converse finding, patients with anti-Sm who lack anti-RNP, is unusual.
The major clinical association of anti-U1 RNP antibodies is with mixed connective tissue disease (MCTD) where they typically occur in high titers and are not associated with other specificities. Indeed, this illness is defined by the presence of these antibodies.
Anti-U1 RNP antibodies may also occur in a small fraction of patients with Sjögren¿s syndrome, rheumatoid arthritis, scleroderma, and polymyositis.
Longitudinal studies have indicated that anti-U1 RNP antibody titers vary over time, but it is uncertain whether these levels reflect underlying disease activity.
Different antibody populations
The three unique proteins of U1 snRNP, 70 kD, A, and C, do not share known cross-reactive epitopes, and are recognized by at least three separate antibody populations, which may occur together or singly in a given patient. In other words, all three antibodies contribute to the anti-U1 RNP response.
When patient sera are grouped and examined irrespective of disease, anti-70kD, anti-A and anti-C antibodies appear to have similar prevalences.
Anti-70 kD U1 RNP
In general, anti-70 kD antibodies are found in around 12 % of patients with SLE.
As few as 8% to 21%, up to 85% of preselected anti-U1 positive SLE patients will have anti-70 kD antibodies, depending on the sensitivity of the assay. Anti-70 kD occur in 75 to 95 % of patients with MCTD (who are, per definition, preselected as anti-U1 RNP positive).
When patients with MCTD and SLE are grouped together, anti-70 kD antibodies appear to correlate with myositis, esophageal hypomotility, Raynaud¿s phenomenon, lack of nephritis, and the HLA-DR4 phenotype.
Among patients selected because they have SLE, anti-A antibodies appear to be twice as common as anti-70 kD antibodies, appearing in approximately 23% of such patients overall, or in approximately 75% of preselected anti-U1 positive SLE patients.
Presently, it is unclear, whether anti-A antibodies are associated with specific disease manifestations.
Clinical associations of these antibodies have not been recognized as yet.