New Approaches to Passive and Active Immunoprophylaxis

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Vaccines remain one of the most cost-beneficial medical interventions. The Hepatitis Virus Section (HVS) has been instrumental in the development of licensed vaccines for the prevention of hepatitis A and hepatitis B. When appropriately administered as part of an integrated public health program, they can have a very large impact on the incidence of these two diseases worldwide. Other hepatitis viruses have worldwide importance as pathogens. Among these are hepatitis C virus and hepatitis E virus (HEV). The HVS has developed a candidate hepatitis E vaccine that is highly promising and that will enter clinical trials during 1998. This vaccine consists of a recombinant protein derived from the genome of HEV and expressed in insect cells via a baculovirus vector. The protein, when combined with alum adjuvant, is highly immunogenic and elicits antibodies that protect non-human primates against challenge with virulent HEV.Although inactivated HAV vaccines are licensed and commercially available, there remains a need for a live attenuated HAV vaccine for administration in developing countries where the cost and logistics of administering an inactivated vaccine are prohibitive. The HVS in collaboration with SmithKline Beecham, Rixensart, Belgium, has developed several candidate live attenuated HAV vaccines. These include: (a) human strains of HAV that have been attenuated by adaptation to growth in monkey kidney cells, and (b) attenuated candidate vaccines that consist of chimeras of a human HAV strain and a simian HAV strain. These candidate vaccines are attenuated but immunogenic in non-human primates (chimpanzees and tamarins) and they are currently being tested for their ability to infect non-human primates by the oral route, a valuable attribute for a candidate live attenuated vaccine.In addition, the HVS is attempting to improve the replication of HAV in vaccine-suitable cell culture systems. A candidate vaccine strain that replicated more efficiently in cell culture could be manufactured less expensively, resulting in a vaccine that could be made available to economically disadvantaged countries.A recent advance in vaccine development was the discovery that "naked" viral DNA can serve as a vaccine when administered to animals. The DNA molecules are taken up by cells, where proteins encoded by them are expressed in relatively native form. Preliminary evidence suggests that DNA vaccines may have distinct advantages over protein vaccines, especially when multivalent vaccines are necessary. The HVS is studying the technology of DNA vaccines with a model system based upon hepatitis B virus (HBV) vaccine, a vaccine with which the HYS has had extensive experience. We have shown previously that DNA vaccines are potentially superior to, for example, synthetic peptide vaccines and live virus-vectored vaccines. We are currently testing the efficacy of an immunostimulant (CpG) as an adjuvant for DNA vaccines, as well as protein vaccines. These are being compared with licensed protein-based hepatitis B vaccine.10 - 70In addition, the utility of DNA vaccines for the control of hepatitis C virus (HCV) is being explored. New constructs of DNA vaccine vectors, containing modified HCV envelope proteins that are expressed on the surface of cells rather than being retained within the cell or secreted, have been developed and tested in mice. Preliminary results from mice indicate that cell surface expression of the second envelope protein of HCV (E2) is more efficient in stimulating an immune response in mice than other methods of expression. These studies are now being extended to non-human primates (rhesus monkeys). Should the DNA vaccine prove to be highly immunogenic in this species, we will proceed to chimpanzees, which can be challenged with live HCV.In addition to studies of active immunoprophylaxis, the HVS is exploring passive immunoprophylaxis for certain hepatitis viruses. Bone marrow aspirates have been obtained from chimpanzees that were experimentally infected with each of the five recognized human hepatitis viruses. Such bone marrow samples should contain cells that represent the entire immunological experience of the chimpanzee with the hepatitis viruses.Combinatorial phage Fab display libraries have been prepared from these bone marrow cells and the search for specific monoclonal antibodies initiated. So far, two chimpanzee monoclonal antibodies to hepatitis E virus have been identified. In addition, monoclonal antibodies to hepatitis B surface antigen have been detected in preliminary screening experiments. Monoclonal antibodies to HAV, HCV, and HDV will be sought. The advantage of using chimpanzee bone marrow cells for the establishment of combinatorial libraries is that chimpanzee immunoglobulins are virtually identical to human immunoglobulins, thus allowing useful or unique monoclonal antibodies to be administered to humans in an immunoprophylactic or immunotherapeutic regimen.