Primary Biliary Cirrhosis (cont.)
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What are antimitochondrial antibodies (AMA)?
Between 95 and 98% of patients with PBC have autoantibodies 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 bind to protein antigens that are contained in multienzyme
complexes (packages of enzymes) within the inner lining of the mitochondria. The
multienzyme complexes produce key chemical reactions necessary for life. These
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 will be discussed below,
in a diagnostic test for detection of AMA. 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 the AMA react with the bile ducts?
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 cell lining of the bile
ducts. So, investigators prepared antibodies to PDC-E2. As expected, they found
that these antibodies bound to the mitochondria within 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.
Indeed, these cells are the main targets of destruction in PBC.
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, which will be discussed later.)
These observations led to a speculation that the antibodies were actually
reacting with an antigen from an infectious agent. The idea was that the
infectious agent was present in the biliary epithelial cells of patients with
PBC and that the agent could also infect the biliary cells of a transplanted
liver. (See the section below on the role of infection).
Next: What causes destruction of the bile ducts in PBC? »
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