Yeasts are distinguished by a growing track record as expression platforms for the production of pharmaceuticals. Commercially
available, yeast-derived, recombinant pharmaceuticals include, among others, insulin, the anti-coagulant hirudin, interferon-alpha-2a,
and various vaccines against the hepatitis B virus and papillomavirus infections. The vaccines are produced in either baker's
yeast (Saccharomyces cerevisiae), or the methylotrophic species Hansenula polymorpha and Pichia pastoris. In this article, we focus on a production process for hepatitis B vaccines in methylotrophs. Methylotrophs provide highly
balanced production of both the membrane and the protein component of a recombinant viral particle. A brief outlook is given
for the development of yeast strains designed for the production of other vaccine candidates.
The advent of gene technology has provided new and powerful methods for the safe, efficient production of pharmaceuticals,
with the bacterium Escherichia coli, mammalian cells, and various yeasts as preferred platforms for the production of such recombinant compounds.1 Early examples include human growth hormone2 and insulin3 produced in recombinant strains of E. coli. Among the most important available recombinant pharmaceuticals are yeast-derived vaccines against papillomavirus4 and hepatitis B infections. Hepatitis B vaccines are based on particles containing the hepatitis B surface antigen (HBsAg)
inserted into host-derived membranes.5–7 The success of current vaccination programs against hepatitis B is a result of the development of effective, yeast-derived
recombinant hepatitis B surface proteins. Initially, the production of such vaccines was restricted to baker's yeast, S. cerevisiae, but with improvements in biotechnological methods, alternative yeast expression systems have been identified and developed.
In particular, the methylotrophic yeast H. polymorpha
has been found to exhibit many superior expression characteristics, and is currently being used in the production of several
vaccines against different subtypes of the hepatitis B virus.7,15
In this article, we briefly describe the hepatitis B virus, its subtypes, and the disease it causes. Subsequently, recombinant
protein production is discussed, focusing in particular on the application of the H. polymorpha expression system. We describe how a heterologous H. polymorpha strain expressing HBsAg is constructed and how efficient vaccine production is developed based on such recombinant strains.
Finally, we will provide an outlook for alternative vaccine strategies and the development of alternative vaccine candidates.
Virus and Disease Characteristics
Hepatitis B virus (HBV) was identified as the causative agent of serum hepatitis in the 1970s16 after B. Blumberg discovered the Australia antigen.17 Blumberg first recognized this antigen as a serum protein specific for aborigines in Australia. It was only later that the
infectious nature of the Australia antigen was identified; it turned out to be the surface protein of HBV that is secreted
into the bloodstream of infected patients in large excess over viral particles.18 HBV was found to be endemic in many parts of the world, with more than 2 billion people having had contact with the virus
and more than 350 million chronic carriers of the virus.19