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LKM antibody: getting in some target practice
  1. D VERGANI
  1. Institute of Hepatology
  2. University College London Medical School
  3. 69–75 Chenies Mews
  4. London WC1E 6HX, UK
  5. Email: D.Vergani{at}ucl.ac.uk

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    See article on page 553

    In the current issue of the journal (see page 553), Muratoriet al provide convincing evidence that cytochrome P4502D6 (CYP2D6) is present on the liver cell plasma membrane. This finding has important implications because CYP2D6 is the main target of liver kidney microsomal antibody type 1 (LKM1). Not only is LKM1 the serological hallmark of autoimmune hepatitis type 2 but it is also found in up to 10% of patients with chronic hepatitis C virus (HCV) infection where it appears to single out patients experiencing serious side effects while receiving interferon treatment.1

    The issue as to whether CYP2D6 is present on the plasma membrane was critically reviewed in 19932 but was not resolved in view of the contrasting evidence at the time. Muratoriet al have settled this matter by incubating LKM1 containing sera with live hepatocytes which can offer the external aspect of the plasma membrane to the antibody. The presence of hepatocyte bound antibody is then determined by addition of a second antibody emitting a green fluorescent signal. Lastly and crucially, an antibody specific for CYP2D6, and emitting a red fluorescent signal, is used to counterstain the hepatocytes. With this manoeuvre when “green” and “red” antibodies recognise the same structure, a yellow signal is generated. And, as the reader can verify on page 000, yellow staining results from the experiment described above demonstrating elegantly that: (1) LKM1 positive sera react with the outer aspect of the plasma membrane; and (2) the target they recognise is CYP2D6.

    In common with other liver cytochromes, CYP2D6 is mainly localised in the endoplasmic reticulum and sediments in the so-called “microsomal” fraction following differential ultracentrifugation of a liver homogenate. The microsomal fraction was used in the original absorption experiments3 aimed at identifying the antigen targeted by LKM1 (hence the L and M of the acronym). The ability of cytochromes to migrate to the plasma membrane has been documented by the studies of Robin et al.4There is an extensive flow of vesicles from the endoplasmic reticulum, in which CYPs are anchored through their N-terminal end, to the Golgi apparatus, and then along the microtubules to the plasma membrane.5 Pulse chase labelling experiments show that the bulk of CYPs reside in the endoplasmic reticulum where the label persists for several hours, while about 10% is present in the plasma membrane where the label lasts for minutes, indicating fast turnover of the enzymes in this location, the probable result of rapid endocytotic membrane internalisation.4

    The presence of an autoantibody target on the plasma membrane has pathogenic implications. LKM1 now ceases to be just a marker of disease1 and acquires damaging connotations. Thus the association of LKM1 with a particularly aggressive form of autoimmune hepatitis,6 and with the severity of liver injury in LKM1 positive chronic HCV patients,7 is now seen in a new light. As indeed is the fact that LKM1 singles out among HCV positive patients a group prone to develop severe side effects during interferon treatment.1

    The question as to how CYP2D6 becomes an autoantigen remains unresolved. CYPs represent an enzymatic system evolved as a defence mechanism against xenobiotics, such as drugs, metals, industrial and naturally occurring chemicals. In this capacity CYP2D6 metabolises debrisoquine and several other drugs. The detoxifying function of cytochromes may provide a clue as to how they can become autoantigens.8 In the line of service, these enzymes generate reactive metabolites capable of altering the shape of the molecule to a degree not tolerated by the immune system. The immune response may then also turn against the native form of the enzyme. In an alternative scenario, CYPs may become autoantigens as a consequence of molecular mimicry and cross reactive immunity.9 CYP2D6 shares sequences in common with viruses such as herpes viruses, including CMV, and with HCV, hence the suggestion by Manns and colleagues10 that cross reactive immunity explains the presence of LKM1 in HCV infection. The question of whether CYP2D6 becomes an autoantigen for its detoxifying functions or because it is targeted by a “friendly immunological fire”, a fire originally intended at eliminating a pathogen that too closely resembles sequences of the enzyme, will be addressed in the months and years to come.

    See article on page 553

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