Introduction:
Systemic lupus erythematosus (SLE) is a severe autoimmune disorder affecting multiple organs with circulating autoantibodies (antibodies that generate an immune response against self). SLE has diverse and dynamic symptoms manifested by flare-ups. A recent Indian study reported the mean age of onset of about 24 years with a female predilection. Among the various circulating autoantibodies in SLE [antinuclear antibody (ANA), anti-Smith (anti-Sm), anti-histone, anti-ribonucleoprotein (anti-RNP)], anti-double-stranded DNA (anti-dsDNA, antibodies that react with deoxyribonucleic acid, DNA) antibodies play an important role in the diagnosis, classification, and management of SLE.
How Do Anti-dsDNA Antibodies Originate?
Under usual circumstances, dsDNA (double-stranded DNA is a form of genetic material) is not accessible to the immune system as the nucleus in the cell contains it. However, dsDNA can be released through apoptosis (programmed cell death) after exposure to infection, radiation, and drugs. Then, the released dsDNA can be recognized by antibodies.
Anti-dsDNA antibodies can be different in origin, including IgA, IgE, IgG, and IgM (all are classes of antibodies). However, all of them do not contribute to tissue injuries in SLE. It is because only IgG and IgA correlate with disease activity in humans. Further, most disease-causing antibodies belong to the IgG class in SLE patients. Also, the IgE subclass of anti-DNA antibodies is implicated in lupus nephritis (LN, kidney inflammation caused as a complication of SLE).
Many autoimmune disorders result in autoantibody generation, but anti-dsDNA antibodies are particular to SLE. This is because less than 0.5 percent of patients with other autoimmune diseases have anti-dsDNA antibodies.
What Are the Various Anti-dsDNA Tests for Diagnosing SLE?
There are various tests for anti-dsDNA antibody detection, such as the Farr radioimmunoassay (FARR-RIA), Crithidia luciliae indirect immunofluorescence test (CLIFT), and enzyme-linked immunosorbent assays (ELISA). All of them are explained below.
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Farr Radioimmunoassay: Farr radioimmunoassay uses radiolabeled molecules (molecules combined with a radioactive substance) and is a precise method for detecting dsDNA autoantibodies in SLE. Notable elevations in dsDNA autoantibodies confirm SLE diagnosis. Also, serial elevated values of dsDNA autoantibodies are useful for predicting SLE activity. However, the absence of dsDNA autoantibodies does not exclude the diagnosis of SLE. Its advantages are high specificity (the ability to identify an individual without the disease) and qualitative analysis. On the other hand, the disadvantages are that it lacks sensitivity (the ability to identify an individual without disease), uses radioactive material, and is time-consuming.
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Crithidia Luciliae Indirect Immunofluorescence Test: Indirect immunofluorescence detects circulating autoantibodies in a patient’s serum. Different staining patterns will aid in diagnosing the autoimmune disorder. In the CLIFT, the immunofluorescence patterns reveal the absence or presence of dsDNA autoantibodies in SLE. The source in CLIFT is the DNA of the parasite Crithidia luciliae. The test detects specific anti-dsDNA antibodies and has high sensitivity and specificity. Also, it is a qualitative test similar to the Farr radioimmunoassay. However, it lacks the accurate serum concentration of the autoantibodies.
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Enzyme-Linked Immunosorbent Assay: If a patient has a positive ANA test and symptoms of SLE, an enzyme-linked immunosorbent assay (ELISA) test is highly recommended. In ELISA, antibodies and color modifications are used to identify foreign substances. It involves the collection of the blood sample that will be added to the test apparatus containing antigens. If the blood comprises antibodies to the specific antigens, it leads to an antigen-antibody complex formation. Next, an enzyme is added to the complex to look for color changes. If there is a color change, the patient has a positive result for anti-dsDNA. The advantages are that it detects both low and high-affinity autoantibodies, has high sensitivity, and is a quantitative assay. However, it lacks specificity.
These tests correlate with each other when applied to populations of patients, but there are substantial discrepancies when applied to individual patients. It is because each of these may detect different anti-DNA antibodies due to affinities of the targeted antibodies for dsDNA and substrate differences. The Farr radioimmunoassay and CLIFT are well-demonstrated for diagnosis and the disease outcome. On the other hand, ELISA is closely associated with SLE activity. However, the substances used in these tests limit their utilization. Therefore, researchers are developing new assays with high sensitivity and specificity. Although the association between high levels of anti-DNA antibodies and disease activity is widely appreciated, a single assay cannot reliably evaluate SLE disease activity. This is because the current assays are not sufficient for detecting all the antibodies in a patient’s serum. Hence, two assays should be combined for better evaluation and higher accuracy.
What Is the Significance of Anti-dsDNA Tests and Anti-dsDNA Antibodies?
One must note that the particular symptoms of the patient, the medical history, and the irregular occurrence of autoimmune disorders can affect the anti-dsDNA test results. This is because it may take months or years to identify the DNA particular to SLE. Further, in infections, drug intake, or genetic disorders, only subpopulations of anti-dsDNA antibodies may be unique to SLE. Studies show that only 50 to 70 percent of patients are detected with anti-ds DNA.
The anti-dsDNA antibodies usually appear before the clinical onset of SLE. Hence, tests for anti-dsDNA antibodies can be positive about two years before clinical diagnosis of SLE. Therefore, a rise in their levels can predict a severe SLE flare-up in the next six months. Further, their presence for several years before the clinical manifestations suggests their involvement in the progression of SLE to clinical disease. The most affected parts of the body in SLE are joints, skin, central nervous system (CNS), lungs, kidneys, and blood vessels. Although disease activity does not always correlate with altered anti-dsDNA antibodies, LN is the most strongly associated clinical feature.
Conclusion:
Anti-dsDNA antibodies constitute a potent parameter for diagnosing SLE. They damage the kidneys, skin, and brain in SLE. The production of anti-dsDNA antibodies results from the failure of removal of released nuclear materials from the cell after damage. One must note that although any of the available methods can detect anti-dsDNA antibodies, the antidsDNA antibodies are not distinctive for SLE. Further, changes in antibody levels measured by these assays are related to disease exacerbations. Hence, newer assays are required to assess the overall role of anti-dsDNA antibodies in SLE.
