Markers of autoimmune connective tissue diseases (CTDs)

Autoimmune Diseases Overview

Autoimmune diseases represent a broad category of conditions characterized by diverse clinical manifestations. They arise when the body's immune system mistakenly attacks its own healthy tissues. This occurs due to the abnormal production of autoantibodies (antibodies targeting self-antigens) or the proliferation of autoaggressive immune cells (like T-killer cells) directed against normal body components. This misdirected immune attack leads to inflammation, tissue damage, and organ dysfunction.

Causes and Development of Autoimmune Diseases

The precise causes of most autoimmune diseases are complex and often involve a combination of genetic predisposition and environmental triggers. Proposed mechanisms include:

1. Molecular Mimicry: Infections with microorganisms (bacteria, viruses) whose antigens resemble the body's own tissue antigens. The immune response generated against the microbe cross-reacts with self-tissues. Examples include autoimmune glomerulonephritis following streptococcal infections or reactive arthritis after certain bacterial infections.
2. Tissue Alteration/Damage: Injury, necrosis (tissue death), or inflammation can alter self-antigens, making them appear foreign to the immune system and triggering an autoimmune response. Chronic active hepatitis developing after hepatitis B infection is an example.
3. Loss of Immune Privilege/Barrier Integrity: Some tissues (like the eye, testes, central nervous system, thyroid) are normally shielded from the immune system. Damage to these barriers can expose previously hidden antigens, leading to autoimmunity (e.g., autoimmune thyroiditis after thyroid injury).
4. Immune Dysregulation: Defects in the mechanisms that normally maintain self-tolerance, such as impaired function of regulatory T cells (T-suppressors) or failure of the thymus to eliminate autoaggressive lymphocytes during their development. Genetic factors often play a role here.

The exact mechanisms underlying diseases like systemic scleroderma, polyarteritis nodosa, or acquired hemolytic anemia remain less clear. Autoimmune responses can involve different components of the immune system, including autoantibodies (predominant in some blood disorders) and immune lymphocytes (central in delayed-type hypersensitivity reactions seen in many connective tissue diseases).

Autoantigens can be diverse, including proteins, nucleic acids (like DNA), phospholipids, carbohydrates, and even other immunoglobulins (e.g., rheumatoid factor, which is an autoantibody against IgG).

It's noteworthy that low levels of natural autoantibodies (often IgM class) are normally present in the blood and may play physiological roles, such as clearing cellular debris or stimulating tissue regeneration, without causing disease.

Types of Autoimmune Diseases

Autoimmune diseases can be broadly classified:

1. Organ-Specific: Primarily affecting a single organ or gland. Examples include Hashimoto's thyroiditis, Graves' disease (thyrotoxicosis), pernicious anemia (stomach), Addison's disease (adrenal glands), and type 1 diabetes mellitus (pancreas).
2. Systemic (Non-Organ-Specific): Affecting multiple organs and tissues throughout the body. This group largely comprises the rheumatic or connective tissue diseases. Examples include Systemic Lupus Erythematosus (SLE), Rheumatoid Arthritis (RA), Systemic Sclerosis (Scleroderma), Sjögren's Syndrome, Polymyositis/Dermatomyositis.

Connective Tissue Diseases (CTDs) / Rheumatic Diseases

Systemic autoimmune diseases primarily affecting connective tissues are often referred to as Connective Tissue Diseases (CTDs) or Rheumatic Diseases. This group includes:

• Systemic Lupus Erythematosus (SLE)
• Systemic Sclerosis (Scleroderma)
• Sjögren's Syndrome
• Mixed Connective Tissue Disease (MCTD)
• Polymyositis / Dermatomyositis
• Rheumatoid Arthritis (RA)
• Myasthenia Gravis (often considered autoimmune, affects neuromuscular junction)
• Multiple Sclerosis (autoimmune disease affecting central nervous system myelin)
• Antiphospholipid Syndrome (APS) (sometimes associated with other CTDs)
• Systemic Vasculitides (inflammation of blood vessels, e.g., Polyarteritis Nodosa)

Laboratory Testing for CTDs

Laboratory tests play a crucial role in the diagnosis, classification, monitoring, and management of CTDs. Key categories of tests include:

• Autoantibody Tests: Detecting specific antibodies directed against self-components (e.g., ANA, ENA panel, RF, anti-CCP).
• Inflammatory Markers: Measuring general markers of inflammation (e.g., ESR, CRP).
• Immunoglobulins: Assessing levels of different antibody classes (IgG, IgM, IgA).
• Complement System Components: Measuring levels of complement proteins (e.g., C3, C4), which can be consumed during autoimmune processes.
• Circulating Immune Complexes (CIC): Detecting complexes formed by antigens and antibodies.
• Markers of Endothelial Dysfunction/Damage: Assessing blood vessel involvement.
• Genetic Markers: Identifying specific genes associated with increased risk (e.g., HLA types).
• Markers of Organ Involvement: Tests related to kidney function, liver function, muscle enzymes, etc., depending on the suspected disease and complications.
• Markers of Bone Metabolism: Relevant in conditions affecting bone and joints like RA.

Antinuclear Antibodies (ANA) & ENA Panel

Antinuclear Antibodies (ANA) are a group of autoantibodies directed against components within the cell nucleus. A positive ANA test is common in many systemic autoimmune diseases, especially SLE (highly sensitive, but not specific). Testing is typically done by immunofluorescence (IFA), reporting a titer and pattern (e.g., homogenous, speckled, nucleolar), or by solid-phase immunoassays.

If the ANA test is positive, further testing for specific antibodies against Extractable Nuclear Antigens (ENA panel) is often performed. These include antibodies like anti-dsDNA, anti-Sm (highly specific for SLE), anti-RNP (associated with MCTD), anti-SS-A/Ro, anti-SS-B/La (associated with Sjögren's syndrome and SLE), anti-Scl-70 (associated with diffuse scleroderma), anti-Jo-1 (associated with polymyositis), and anti-centromere (associated with limited scleroderma/CREST syndrome).

Rheumatoid Factor (RF)

Rheumatoid Factor (RF) refers to autoantibodies (most commonly IgM, but also IgG or IgA) directed against the Fc portion (the constant region) of IgG antibodies. RF forms immune complexes that contribute to inflammation.

While classically associated with Rheumatoid Arthritis (RA) (found in 75-80% of RA patients), RF is not specific to RA. It can also be positive in other autoimmune diseases like Sjögren's syndrome, scleroderma, and dermatomyositis, as well as in chronic infections (like hepatitis C, endocarditis), certain cancers (lymphoproliferative diseases), and even in a percentage of healthy older individuals.

In RA, the presence of RF (seropositive RA) is often associated with more severe joint damage, erosions, and extra-articular manifestations (systemic involvement).

Anti-Cyclic Citrullinated Peptide (Anti-CCP) Antibodies

Antibodies to Cyclic Citrullinated Peptide (Anti-CCP), also known as ACPA (Anti-Citrullinated Protein Antibodies), are autoantibodies targeting proteins or peptides containing citrulline. Citrulline is an amino acid formed by the post-translational modification (deimination) of arginine residues, a process that occurs during inflammation.

Anti-CCP antibodies are highly specific for Rheumatoid Arthritis (RA), with a specificity often exceeding 95%. They are found in about 70-80% of RA patients.

Key features of anti-CCP testing:

• High specificity for RA.
• Can be detected very early in the disease course, sometimes even before symptoms appear.
• Presence is associated with a higher likelihood of developing erosive joint disease and a more severe disease course.
• Helps differentiate RA from other forms of arthritis.
• Can be positive in some RF-negative RA patients.

Anti-Modified Citrullinated Vimentin (Anti-MCV) Antibodies

Vimentin is an intermediate filament protein found in the cytoskeleton of various mesenchymal cells, including fibroblasts and endothelial cells. In inflamed joints, vimentin can undergo citrullination.

Antibodies to Modified Citrullinated Vimentin (Anti-MCV) target these citrullinated forms of vimentin. Anti-MCV testing is another marker used in the diagnosis of Rheumatoid Arthritis (RA).

While generally considered less specific than anti-CCP, some studies suggest anti-MCV may have similar or slightly higher sensitivity, particularly in early RA. It may also be positive in some RA patients who are negative for both RF and anti-CCP.

Circulating Immune Complexes (CIC)

Circulating Immune Complexes (CIC) are formed when antibodies bind to antigens (either self-antigens or foreign antigens like microbes). Normally, these complexes are efficiently cleared from circulation by the reticuloendothelial system (macrophages in the liver and spleen).

In certain conditions, particularly autoimmune diseases like SLE, vasculitis, and some infections, CICs may form in excess or be cleared inefficiently. These complexes can deposit in tissues (like blood vessel walls, kidneys, joints), activate the complement system, and trigger inflammation and tissue damage.

Measuring CIC levels in serum can be used as a marker of disease activity in some autoimmune conditions, reflecting ongoing immune system activation and potential for tissue deposition. However, CIC tests often lack specificity and are not routinely used for diagnosis in all CTDs.

Frequently Asked Questions (FAQ)

General Interpretation Considerations

It is vital to interpret autoantibody test results in the context of the patient's clinical presentation (symptoms, signs, history).

• Non-Specificity: Many autoantibodies (like ANA and RF) are not specific to a single disease and can be found in multiple autoimmune conditions, infections, or even healthy individuals (especially ANA at low titers).
• Diagnostic Value: Some antibodies are highly specific and strongly support a diagnosis (e.g., anti-Sm for SLE, anti-CCP for RA, anti-Scl-70 for diffuse scleroderma).
• Monitoring: Changes in antibody levels (titers) over time can sometimes correlate with disease activity or response to therapy (e.g., anti-dsDNA in SLE).
• Prognosis: The presence of certain autoantibodies can have prognostic implications (e.g., RF and anti-CCP positivity predicting more erosive RA).

Antibody Frequencies in Specific Diseases (Tables)

The following tables summarize the approximate frequency (%) of detecting certain autoantibodies in various systemic autoimmune diseases. Note that these frequencies can vary depending on the patient population and testing methods used.

The Blood Test Procedure

Testing for these autoimmune markers involves a standard blood draw:

• Preparation: Generally, no specific preparation like fasting is required.
• Collection: Blood is drawn from a vein, usually in the arm.
• Processing: The blood sample (serum) is sent to a laboratory where specialized tests (e.g., IFA for ANA, ELISA or other immunoassays for specific antibodies like RF, anti-CCP, ENA) are performed.

References

1. American College of Rheumatology (ACR). (n.d.). Glossary. Retrieved from https://www.rheumatology.org/I-Am-A/Patient-Caregiver/Glossary (Provides definitions for many terms)
2. Lab Tests Online. (n.d.). Autoantibodies. Retrieved from https://labtestsonline.org/conditions/autoimmune-disorders (Explains various autoantibody tests)
3. Kumar, V., Abbas, A. K., & Aster, J. C. (Eds.). (2021). *Robbins & Cotran Pathologic Basis of Disease* (10th ed.). Elsevier. (Chapter 6: Diseases of the Immune System)
4. Firestein, G. S., Budd, R. C., Gabriel, S. E., McInnes, I. B., & O'Dell, J. R. (Eds.). (2020). *Kelley & Firestein's Textbook of Rheumatology* (11th ed.). Elsevier. [Note: Comprehensive rheumatology textbook]
5. Tan, E. M. (1989). Antinuclear antibodies: diagnostic markers for autoimmune diseases and probes for cell biology. *Advances in Immunology*, 44, 93–151. https://doi.org/10.1016/s0065-2776(08)60641-4 (Classic reference on ANA)
6. Schellekens, G. A., Visser, H., de Jong, B. A., van den Hoogen, F. H., Hazes, J. M., Breedveld, F. C., & van Venrooij, W. J. (2000). The diagnostic properties of rheumatoid arthritis antibodies recognizing a cyclic citrullinated peptide. *Arthritis and Rheumatism*, 43(1), 155–163. (Key paper on Anti-CCP)