AN6520 Ag: an antigen purified from liver with non-A, non-B hepatitis

Development of monoclonal antibodies to human microsomal epoxide hydrolase and analysis of "preneoplastic antigen"-like molecules

Microsomal epoxide hydrolase (mEH) is a drug metabolizing enzyme which resides on the endoplasmic reticulum (ER) membrane and catalyzes the hydration of reactive epoxide intermediates that are formed by cytochrome P450s. mEH is also thought to have a role in bile acid transport on the plasma membrane of hepatocytes. It is speculated that efficient execution of such multiple functions is secured by its orientation and association with cytochrome P450 enzymes on the ER membrane and formation of a multiple transport system on the plasma membrane. In certain disease status, mEH loses its association with the membrane and can be detected as distinct antigens in the cytosol of preneoplastic foci of liver (preneoplastic antigen), in the serum in association with hepatitis C virus infection (AN antigen), or in some brain tumors. To analyze the antigenic structures of mEH in physiological and pathological conditions, we developed monoclonal antibodies against different portions of mEH. Five different kinds of antibodies were obtained: three, anti-N-terminal portions; one anti-C-terminal; and one, anti-conformational epitope. By combining these antibodies, we developed antigen detection methods which are specific to either the membrane-bound form or the linearized form of mEH. These methods detected mEH in the culture medium released from a hepatocellular carcinoma cell line and a glioblastoma cell line, which was found to be a multimolecular complex with a unique antigenic structure different from that of the membrane-bound form of mEH. These antibodies and antigen detection methods may be useful to study pathological changes of mEH in various human diseases.

Autoantibody response to microsomal epoxide hydrolase in hepatitis C and A

Autoimmune responses were observed in a large proportion of hepatitis C cases and are suspected to be part of viral pathogenesis. The AN6520 antigen (AN-Ag) is a normal cellular protein mainly expressed in liver that was found associated with non-A, non-B hepatitis. To elucidate its pathogenic role in hepatitis C, we developed an IgM capture assay using purified AN-Ag and confirmed that the antibody response to AN-Ag is associated almost exclusively with hepatitis C cases (29%). Screening of a human liver expression library revealed that AN-Ag is mainly the microsomal epoxide hydrolase (mEH), a drug-metabolizing enzyme that plays an important role in the metabolism of some mutagenic and carcinogenic epoxides. Using the purified recombinant human mEH as an antigen, we now found that antibodies against this protein are associated with nearly 82% of hepatitis C virus infections and surprisingly with 46% of patients with hepatitis A. The appearance of AN-Ag/mEH in the incubation period of hepatitis C as previously reported and the antibody responses shown here indicate that this enzyme may be a marker for or even a cause of some of the pathology associated with hepatitis C and A.

Non-A, non-B hepatitis related AN6520 Ag is a normal cellular protein mainly expressed in liver. II

Detection of AN6520 Ag/Ab in human sera had indicated a close association with non-A, non-B hepatitis (NANBH). In this study, we investigated the immunochemical nature of AN6520 Ag and measured the amounts in various human and chimpanzee organs in order to clarify the association with NANBH. AN6520 Ag was found to be composed of polypeptide(s) with an apparent molecular weight of 45,000 daltons (45 kD), which are noncovalently linked together. Human antibodies in convalescent sera from NANBH patients as well as monoclonal antibodies were found to recognize only the high-order structure of the antigen, whereas rabbit antibody recognized both the high-order structure and the reduced form of 45 kD polypeptide(s). AN6520 Ag could be detected in most of the livers tested including those without any liver damage and fetal livers; their amounts varied considerably from each other. The antigen could be detected also in organs other than liver, but in contrast to liver, the amounts were small and did not vary as much between individuals. From the data of immunoblotting using rabbit antibody, our observed variation of antigen content in liver was considered to be due to the difference in expression of 45 kD polypeptide(s). Although no specific relationship was found between the amount of the antigen in liver and NANBH, the antigen was found to increase several times in livers of chimpanzees after the inoculation of NANBH virus. These data suggest that AN6520 Ag is a normal cellular protein existing mainly in liver and that its quantity may vary under some conditions such as NANBH.

Detection of an antigen (AN6520), possibly related to non-A, non-B hepatitis, by monoclonal antibodies. I

The antibody to AN6520 antigen, which was isolated from the liver of a patient with non-A, non-B hepatitis (NANBH), has been detected frequently in convalescent sera from patients with NANBH by the passive hemagglutination (PHA) test. In a further study, we established hybridoma cells secreting antibodies against AN6520 antigen and obtained ascitic fluids with PHA titers ranging from 1:10(5) to 1:10(7). In immunodiffusion with AN6520 antigen, all monoclonal antibodies were found to form an identical precipitin line. These lines were also identical to those formed by rabbit antiserum against AN6520 antigen and by convalescent sera from patients with NANBH. With one of the monoclonal antibodies, 1-F12, solid-phase radioimmunoassay (SP-RIA) for detecting AN6520 antigen was developed as well as blocking RIA for anti-AN6520 antibody detection. The antigen assay was 50 times more sensitive than the reverse passive hemagglutination (R-PHA) test, with a sensitivity threshold of the 1 ng/ml of antigen solution; the antibody assay was 10 times more sensitive than PHA. The results with this blocking RIA were mostly in agreement with the data obtained by PHA. Furthermore, the antigen in human sera, which had never been detected by R-PHA test, could be detected by SP-RIA.