What are antimitochondrial antibodies (AMA)?

Between 95 and 98% of patients with primary biliary cirrhosis (PBC) have autoantibodies (antibodies to self) in their blood that react with the inner lining of mitochondria. These autoantibodies are called antimitochondrial antibodies (AMA). Mitochondria are the energy factories present inside all of our cells, not just the cells of the liver or bile ducts. (The mitochondria use the oxygen carried in the blood from the lungs as a fuel to generate energy.) AMA actually bind to protein antigens that are contained in multienzyme complexes (packages of enzymes) within the inner lining of the mitochondria. These multienzyme complexes produce key chemical reactions necessary for life. The complexes are referred to as multienzyme because they are made up of multiple enzyme units.

AMA specifically react against a component of this multienzyme complex called E2. In PBC, AMA preferentially react with the E2 component of one of the multienzymes that is called the pyruvate dehydrogenase complex (PDC). Accordingly, the antigen is designated as PDC-E2. The practical importance of all of this is that the PDC-E2 antigen is now used, as discussed below, in a diagnostic test for PBC. The PDC-E2 antigen is also referred to as M2, a term introduced to designate it as the second mitochondrial antigen discovered by researchers interested in PBC.

Do AMA cause the destruction of the bile ducts in PBC?

In as much as the bile ducts are the main targets of destruction in PBC, the question was asked whether the AMA reacts with the epithelial cells that line the bile ducts. So, investigators prepared antibodies to PDC-E2. As expected, they found that these antibodies bound to the mitochondria within the cells. But, sure enough, recent information suggests that these AMA autoantibodies also bind to PDC-E2 that lies outside the mitochondria, yet within the epithelial cells lining the bile ducts.

This accumulation of PDC-E2 within the biliary epithelial cells is observed exclusively in the livers of patients with PBC, and not in normal livers or in livers from patients with any other types of liver disease. Interestingly, it was also observed in the livers of those two to five percent of PBC patients who do not have AMA in their blood (AMA-negative PBC). Furthermore, intense binding of these antibodies to biliary epithelial cells was also found to be the earliest indication of recurrence of PBC in a transplanted liver. (PBC is sometimes treated by liver transplantation.)

Nevertheless, no evidence exists that the AMA itself causes the destruction of the biliary epithelial cells lining the small bile ducts. Neither the presence nor the amount (titer) of AMA in the blood appears to be related to the inflammatory destruction of the bile ducts. Indeed, immunization of animals with PDC-E2 antigen results in production of AMA without any liver or bile duct damage (pathology).

How is the blood test for AMA done?

The most economical test for AMA applies diluted samples of a patient's serum onto tissue sections from rat stomach or kidney in the laboratory. (Remember that mitochondria are present in all cells, not just the cells of the liver and bile ducts.) Serum antibodies that attach (bind) to mitochondrial membranes within the tissue cells can then be observed with a microscope. The most dilute sample of serum showing this binding reaction is reported, using the term titer. The titer indicates the most dilute serum sample that reacts with the tissue mitochondria. A higher titer means there is a greater amount of AMA in the serum.

The antigen recognized by AMA in patients with PBC is now known to be PDC-E2 and is also often referred to as the M2 antigen. So, newly developed tests for antibodies that bind to PDC-E2 are more specific and are now available to confirm the diagnosis of PBC.

What is the value of the AMA blood test?

AMA are detectable in the serum in 95 to 98% of patients with PBC. So, AMA are tremendously important as a diagnostic marker in patients with PBC. The AMA titers in PBC are almost universally greater than or equal to 1 to 40. This means that a serum sample diluted with 40 times its original volume still contains enough antimitochondrial antibodies to be detected in the binding reaction. A positive AMA with a titer of at least 1:40 in an adult with an elevated alkaline phosphatase in the blood is highly specific for a diagnosis of PBC.

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