The development of various types of virus-like particles (VLPs) has accelerated over the past two decades as the importance of VLPs for generating nextgeneration vaccines has been appreciated. Yeast has advantages such as scalable fermentation, low risk of contamination by adventitious agents, low production costs and the ability to produce VLPs with reliable qualities. It is generally recognized that yeast is suitable for producing VLPs that have simple structures and are produced intracellularly. However, recently there has been much effort to extend its applicability, and there is now evidence that it can be used as an expression platform for the productions of VLPs not only of nonenveloped viruses but also of enveloped viruses. Moreover, evidences indicated that yeast allows secretory VLP productions. Meanwhile, it has become evident that the quality and quantity of yeast-derived VLPs are influenced by the choice of plasmid and promoter, the ratio of the structural proteins produced. Here, we review the characteristics of the yeast expression system in terms of the production of VLP and compare it with other expression systems. We also consider strategies for VLP production in yeast and factors that need to be taken into account.

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Significance and impact of the study: Provoking sufficient antibody responses by oral immunization has been an enormous challenge because of the harsh conditions of the gastrointestinal (GI) tract. Immunization strategies using purified antigen to make oral vaccines are incapable of commercialization because excessive amount of antigen is required to provoke antibody responses. Therefore, resolving the problems concerning the cost and effectiveness of oral vaccines is a high priority. Our results suggest that recombinant yeast has great potential for inducing antigen-specific immune responses by oral immunization. We believe that oral immunization using recombinant yeast can be a breakthrough technology. AbstractWeak antibody responses to protein antigens after oral immunization remain a serious problem. Yeasts have a rigid cell wall and are inherently resistant to harsh conditions, suggesting that recombinant antigens made in yeast could have a greater chance of making contact with the immune cells of the gastrointestinal (GI) tract in intact form. We compared antibody responses to oral immunization with purified recombinant antigen, used in the conventional manner, and responses to whole recombinant yeast producing the antigen intracellularly. Recombinant capsid protein (CP) of red-spotted grouper necrosis virus (RGNNV) was used as model antigen and Saccharomyces cerevisiae as host. The purified CP was obtained from the S. cerevisiae producing the RGNNV CP. Whole recombinant yeast producing RGNNV CP provoked 9-27 times higher anti-RGNNV CP IgG titres than purified RGNNV CP. Moreover, sera from mice immunized with the recombinant yeast had neutralizing activity against RGNNV, while those from mice immunized with purified CP did not. These results show that whole recombinant yeast is a promising platform for antigen delivery by oral immunization.

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