Anti-myeloperoxidase (anti-MPO) antibodies are specific markers within the human immune system, often signaling the presence of certain autoimmune conditions. These antibodies are produced when the body’s defense mechanisms mistakenly target its own components, leading to an immune response against itself. They are a particular type of autoantibody that can be detected through specialized blood tests.
Understanding Myeloperoxidase (MPO)
Myeloperoxidase (MPO) is an enzyme primarily found within specific white blood cells, particularly neutrophils and, to a lesser extent, monocytes, which are crucial components of the innate immune system representing a primary defense against invading pathogens. MPO is stored in specialized compartments called azurophilic granules within neutrophils. The main function of MPO involves its role in fighting infections; when neutrophils encounter bacteria or fungi, MPO catalyzes a reaction that produces potent antimicrobial agents, such as hypochlorous acid, a strong oxidant that helps destroy and clear pathogens. While essential for defense, the uncontrolled release of MPO and its products can also cause tissue damage.
Anti-MPO Antibodies Explained
Anti-MPO antibodies are a type of autoantibody, meaning they are antibodies produced by the immune system that target the body’s own proteins, specifically recognizing and binding to the myeloperoxidase enzyme. The formation of autoantibodies like anti-MPO suggests an autoimmune process, where the immune system loses its ability to distinguish between “self” and “non-self,” which can lead to an inflammatory response. When anti-MPO antibodies bind to MPO, particularly on the surface of activated neutrophils, they can trigger these white blood cells to release their harmful contents, contributing to inflammation and damage within the body’s tissues. Most anti-MPO antibodies detected in clinical settings are of the IgG class.
Conditions Associated with Anti-MPO Antibodies
Anti-MPO antibodies are most significantly associated with a group of autoimmune diseases known as ANCA-associated vasculitis (AAV). AAV comprises conditions characterized by inflammation of small blood vessels throughout the body. This inflammation can lead to damage in various organs, including the kidneys, lungs, nerves, and skin. Microscopic polyangiitis (MPA) is the condition most strongly linked to the presence of anti-MPO antibodies. In MPA, anti-MPO antibodies are found in a substantial majority of patients. Another AAV subtype, eosinophilic granulomatosis with polyangiitis (EGPA), also shows this association, though less frequently than MPA. While primarily indicators of AAV, anti-MPO antibodies can occasionally be detected in other autoimmune conditions. For example, low levels might be found in some patients with chronic bacterial infections, colitis, liver disease, or systemic lupus erythematosus (SLE). However, their detection often prompts investigation for AAV due to the strong and consistent association.
Testing for Anti-MPO Antibodies and Interpreting Results
Detecting anti-MPO antibodies involves blood tests. Common methods are Enzyme-Linked Immunosorbent Assay (ELISA) and Indirect Immunofluorescence (IIF). ELISA measures anti-MPO antibody levels, providing a quantitative result, while IIF identifies autoantibodies by observing a characteristic staining pattern (perinuclear ANCA or p-ANCA) on neutrophils. A positive anti-MPO antibody test result suggests an autoimmune process and indicates conditions like ANCA-associated vasculitis, particularly microscopic polyangiitis or eosinophilic granulomatosis with polyangiitis. However, these results are interpreted with a patient’s clinical symptoms, physical examination findings, and other laboratory tests or biopsies. A negative result does not rule out an autoimmune disease, as some individuals may not have detectable anti-MPO antibodies. Monitoring anti-MPO antibody levels can also track disease activity and assess treatment response.