Myeloperoxidase antibodies are specific proteins produced by the immune system that target an enzyme called myeloperoxidase (MPO). These antibodies are of interest in medicine because their presence often indicates an autoimmune condition where the body’s immune system mistakenly attacks its own tissues. Their detection helps in understanding and diagnosing certain inflammatory diseases.
Understanding Myeloperoxidase Antibodies
Myeloperoxidase (MPO) is an enzyme primarily found in neutrophils, which are a type of white blood cell, and to a lesser extent in monocytes. Neutrophils play a significant role in the body’s immune response, acting as a first line of defense against infections. Within neutrophils, MPO is stored in specialized compartments called azurophilic granules. When activated, neutrophils release MPO, which helps to generate powerful antimicrobial substances, like hypochlorous acid, to destroy invading pathogens.
An antibody is a Y-shaped protein created by the immune system to identify and neutralize foreign invaders. Normally, antibodies recognize and bind to specific targets called antigens. However, the immune system can sometimes mistakenly produce antibodies against the body’s own tissues; these are called autoantibodies. Myeloperoxidase antibodies (MPO-Abs), also known as anti-myeloperoxidase antibodies, are a specific type of autoantibody that targets the MPO enzyme. Their presence means the immune system has developed a misguided response against this self-component, which can lead to inflammation and damage.
Role in Autoimmune Conditions
Myeloperoxidase antibodies are clinically significant because they are closely associated with a group of autoimmune conditions known as ANCA-associated vasculitis (AAV). These conditions involve inflammation of small blood vessels throughout the body. The presence of MPO antibodies is a hallmark of specific AAV subtypes, particularly microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA), previously known as Churg-Strauss syndrome.
In MPA, MPO antibodies are frequently found, and the condition primarily affects small blood vessels, often leading to damage in organs such as the kidneys and lungs. EGPA is a rarer form of AAV characterized by asthma and a high number of eosinophils, another type of white blood cell, in addition to vasculitis. About 30% of EGPA patients test positive for ANCA, mostly MPO-ANCA. MPO-ANCA positive EGPA patients may show a higher prevalence of arthritis and glomerulonephritis.
MPO antibodies contribute to these diseases by interacting with neutrophils. MPO-ANCA can activate neutrophils, leading to the release of toxic substances and reactive oxygen species. This process causes inflammation and damage to the walls of small blood vessels. This misdirected immune activity leads to the organ damage seen in AAV.
Detecting Myeloperoxidase Antibodies
Detecting myeloperoxidase antibodies involves a blood test ordered by a healthcare provider. This test measures the presence and levels of MPO antibodies. Healthcare providers typically order this test when a patient presents with symptoms that suggest ANCA-associated vasculitis, such as kidney disease or lung involvement. Early symptoms can also be non-specific, including fever, muscle aches, weight loss, or joint pain.
The most common laboratory methods used to detect MPO antibodies include Enzyme-Linked Immunosorbent Assay (ELISA) and indirect immunofluorescence (IIF). ELISA is an immunoassay that directly measures the antibodies, while IIF involves observing fluorescent patterns on neutrophil cells to indicate the presence of autoantibodies. These tests help to diagnose AAV and to differentiate it from other conditions that might present with similar symptoms.
The MPO antibody test is often performed alongside other tests, such as those for proteinase 3 (PR3) antibodies, as both are types of ANCA that can indicate vasculitis. The results of these tests, combined with a patient’s clinical presentation, guide the diagnostic process. No special preparation is typically required for the blood draw.
Interpreting Your MPO Antibody Test Results
Interpreting myeloperoxidase antibody test results requires careful consideration by a healthcare professional, taking into account a patient’s overall health, symptoms, and other diagnostic findings. A positive MPO antibody test, especially when combined with clinical symptoms, strongly suggests ANCA-associated vasculitis, most commonly microscopic polyangiitis (MPA) or eosinophilic granulomatosis with polyangiitis (EGPA).
It is important to note that a positive result alone is not definitive for diagnosis; other conditions, including certain infections, malignancies, or other autoimmune diseases, can sometimes lead to false positives. Therefore, the test results must be interpreted within the full clinical context, often alongside a tissue biopsy for confirmation.
Conversely, a negative MPO antibody test makes ANCA-associated vasculitis less likely but does not entirely rule out the disease. Some individuals with vasculitis may not have detectable MPO antibodies, or other types of ANCA may be present.
The level of MPO antibodies can sometimes correlate with disease activity, with rising levels potentially indicating a flare-up and declining levels suggesting a response to treatment. However, monitoring MPO antibody levels should not be the sole factor in evaluating treatment response or predicting disease flares. The healthcare provider will use these results as one piece of information to help guide treatment decisions and monitor the condition over time.