Antimitochondrial Antibodies
(AMA)
Medical Author:
John M. Vierling, M.D., F.A.C.P.
Medical Editor:
Leslie J. Schoenfield, M.D., Ph.D.
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).
Next: How is the blood test for AMA done? »
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