Yeast species such as Saccharomyces cerevisiae are useful tools for the high yield production of recombinant proteins and have a Generally Regarded As Safe (GRAS) status. They are capable of performing several complex post-translational modifications that are not achieved in many other expression systems, and are easily grown to very high densities producing large quantities of stable particles.
Recently, the idea of using S. cerevisiae as a delivery vehicle for cancer, viral, and bacterial vaccines has been explored, inducing robust humoral and cellular immune responses. In addition to using yeast to produce a vaccine antigen of interest, the yeast cell itself has been shown to have adjuvant-like properties and has the potential to activate both inflammatory and phagocytic receptors expressed on antigen-presenting cells. Our own data demonstrates that freeze-drying recombinant yeast cultures expressing viral protein at their surface renders the recombinant yeast completely non-viable (unpublished). However, the freeze-drying process does not alter conformation of these proteins, as surface expression is equal in live and freeze-dried yeast as quantified by flow cytometry and Western blotting.
This has interesting implications in vaccine design as a non-viable S. cerevisiae is not categorised as a genetically modified organism (GMO) and such a killed vaccine would not be subject to GMO regulations. Additionally there would be no need for refrigeration of the freeze-dried yeast, reducing transport and storage costs. Vaccines based upon S. cerevisiae are likely to be particularly valuable against diseases of farmed poultry, where safety, scalability, stability, delivery and cost are crucial. In one example modern poultry production relies on effective control of Eimeria, but current approaches using drugs and live vaccines require improvement. Eimeria parasites cause coccidiosis, a serious intestinal disease in many livestock species, most notably chickens. These parasites are highly prevalent so the majority of chicken flocks in the world are exposed and, in the absence of life-long effective control, many chickens become infected.
The global economic impact of coccidiosis is estimated to exceed £2 billion per annum due to production losses and costs associated with prevention and treatment. Economic losses are attributed largely to three species (Eimeria tenella, E. maxima and E. acervulina). Prophylactic anticoccidial drugs (chemicals and ionophores) are crucial, but multi-drug resistant parasites are now widespread and no new drugs have become available in the last 20 years. Live parasite vaccines are widely used to protect breeding and egg-laying birds. Uptake for broilers is limited. Wild-type parasite (first generation) vaccine formulations such as Coccivac® are still common in North America and Asia, but these have been replaced by safer attenuated (second generation) vaccines such as Paracox® and Hipracox® in much of the world. Both wild-type and attenuated vaccine strains must be propagated in chickens and the low reproductive index of the attenuated ‘lines’ makes production costs high and limits the number of doses produced.
Recognition of the limitations associated with live anticoccidial vaccines has promoted development of a third generation of subunit or recombinant vaccines. However, despite significant attempts for more than 30 years a subunit or recombinant vaccine remains elusive. Many candidate antigens have been proposed, several of which have been described as capable of inducing significant reductions in parasite replication (determined as oocyst excretion), pathology (lesion score) and/or improved weight gain. In recent years a panel of coccidial antigens have been identified as vaccine candidates, each individually capable of inducing up to a 65% reduction in oocyst output. Now, the focus is shifting from antigen discovery to antigen formulation and delivery with yeast being a leading option, especially given the importance of T-cell mediated responses in anticoccidial immunity.