Centre for Evaluation of Vaccination (CEV)
The list of different types of HBV escape mutants is getting longer. At least theoretically, it cannot be excluded that the growing selection pressure may allow the variants to outcompete the wild-type and even reach a comparable prevalence. This consideration, combined with the manifold
known survival strategies of HBV and its additional strategic possibilities that have not properly been investigated yet, is a possible cause for concern. Therefore, there is no choice but to formulate proposals in preparation of any evolving situation.
The most urgent goal should be to gain a more complete understanding of the potential impact of escape mutants on the epidemiology and prevention of hepatitis B. This can best be achieved by increasing the activities at different levels. The present diagnostic assays should be reconsidered and their ability to detect mutants should be evaluated. An in vitro neutralisation assay to evaluate the capacity of mutants to escape immune attack would be welcome. Epidemiological studies are necessary to monitor the occurrence and geographical spread of HBV mutants, the induction of mutations after post-immunisation exposure, and the risk of transmission in susceptible and immunised individuals. Fundamental and applied research is needed to improve the present perception of the biological characteristics of HBV variation and to further explore the possibilities of new types of vaccines.
Although it has been shown that two commercial hepatitis B vaccines protected chimpanzees against infection by the prototype G145R mutant, a number of proposals have been made to improve the immunogenicity of current vaccines. Inclusion of the preS region, thus taking advantage of the characteristics of both the preS1 and preS2 proteins, is an interesting possibility. PreS1 binds directly to the hepatocyte receptor and preS2 contains the albumin receptor. The preS region contains a number of B and T cell epitopes and may be an important target for the identification and elimination of infected hepatocytes. PreS2 is highly immunogenic
because it contains a strong T cell epitope and is able to provoke an anti-HBs response in mice that fail to respond to HBsAg-based vaccines. So far, similar attempts with preS1 and/or preS2 sequences in man have not revealed much benefit compared with the standard vaccines. More recently, attempts have been made to produce mammalian or yeast cell derived recombinant preS vaccines containing both preS1 and preS2. Some have been evaluated in healthy adults and children, and proved to be highly immunogenic, but because of an imperfect design of the trials, the results were not conclusive and comparison with conventional vaccines remained difficult. It is clear that this line of research deserves further development.
Generalised incorporation of preS in hepatitis B vaccines may, however, not yield a definitive solution, because it might in the future be accompanied by the emergence of preS deletion escape mutants. It has been suggested that the design of future, effective vaccines may need to take into consideration vaccine escape HBsAg mutant sequences themselves, derived from the second a loop or from outside the determinant. Of particular importance would be G145, in combination with R145.
Other types of mutants, diagnosis escape mutants, are not only clinically important, but also cause significant obstacles in immunoassays designed for the detection of HBsAg. An unanswered question is how to develop more reliable assays for the future. Proposals in this regard have been formulated by a number of authors and involve the inclusion of variant S or preS sequences and screening for HBV DNA or antibody to HBcAg. The relevance of this consideration is obvious when it comes to planning appropriate treatment schemes for patients or guaranteeing the safety of donor blood.
It is clear that this discussion is not closed and it is undoubtedly wise to be prepared for any further evolution. The currently available vaccines are safe and effective and should be globally used in universal vaccination programmes. In terms of vaccine design, incorporation of escape mutant sequences into the present HBsAg based preparations is a possibility at present, although new mutants could emerge and undermine the value of this strategy. Apart from the mentioned experimental vaccines under investigation, potentially powerful alternatives based on blocking viral gene expression at different levels are being explored, to complement existing or future therapeutic and prophylactic strategies. Genetic antiviral strategies include the application of ribozymes (RNA enzymes), antisense oligonucleotides, and the intracellular synthesis of interfering peptides or proteins. The latter way represents a type of intracellular immunisation whereby the resulting fusion proteins yield dominant negative (DN) HBV mutants capable of significantly suppressing viral replication. This approach is especially promising because the creation of DN mutants is relatively independent from sequence variation in the viral genome and thus minimises the risk of selecting escape mutants. These and other innovative approaches to come, combined with the results of the human genome project and the exciting developments in the field of pharmacogenomics, will lead to the need to drastically redefine hitherto invariable basic concepts such as susceptibility, infection risk, herd immunity, diagnosis, treatment, prevention, and escape, and contain nothing less than the germs of a revolution in the battle against viral and other pathogens.
François G, Kew M, Van Damme P, Mphahlele MJ, Meheus A. Mutant hepatitis B viruses: a matter of academic interest only or a problem with far-reaching implications? Vaccine 2001; 19:3799-3815.