Optimising response to oral yeast-based vaccines against coccidiosis in chickens

Humankind is farming more poultry than ever before, with an increasing reliance on poultry meat and eggs for provision of dietary protein for human consumption. Concurrently, we are attempting to reduce routine reliance on drugs to control pathogens, replacing them with vaccines or other strategies. For species such as chickens, these vaccines have to be cheap, reliable, easy to administer and (ideally) use existing distribution pathways in such a way that they are accessible to industry as well as backyard farmers in low- and medium-income countries. Such vaccines should be thermostable and not rely on a constant cool-chain. Unfortunately, scalable and cost-effective vaccines are not available for many pathogens such as Eimeria, cause of the disease coccidiosis in chickens. We and others have recently estimated that coccidiosis costs the UK poultry industry ~£100 million every year, exceeding £10 billion globally. Approximately 40% of the drugs used in British livestock production are required to control these parasites. Live vaccines are available, but represent a four-fold higher cost to the producer and cannot be produced at sufficient scale to replace drugs. A robust, cost-effective alternative is required.

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 (e.g. United States Food and Drug Agency). They are capable of performing several complex protein modifications that are not achieved in many other expression systems, and are easily grown to very high densities producing large quantities of stable particles. 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. Recently, we have developed a yeast-delivery platform in which antigens from Eimeria can be expressed in a stable, non-secreted form, with the yeast itself acting as transport system and adjuvant. We have tested this yeast to vaccinate against one form of coccidiosis, inducing control of parasite replication following low-level challenge of chickens at levels better than achieved using other vaccine delivery systems. Here, we propose to develop the yeast delivery system to improve protection against high-level challenge and expand its range to protect against two additional forms of coccidiosis. Combined, these three forms of disease cause the overwhelming majority of the burden of coccidiosis in Europe, North and South America, Asia and Africa. The efficacy of these new vaccines will be tested under commercial conditions, assessing value in terms of farm-level performance and chicken welfare (freedom from disease).

Importantly, heat inactivation of the yeast cells prior to vaccination means that the vaccine is not categorised as a genetically modified organism (GMO) at the time of distribution or administration, and would therefore not be subject to GMO regulations. Additional use of freeze-drying to preserve the heat-killed yeast removes the requirement for a cold chain, reducing transport and storage costs. We will characterise immune responses induced following vaccination and parasite challenge to assess the likely utility of the killed-yeast approach to vaccinate against other important pathogens of poultry. 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 the work proposed here, we intend to improve our application of Saccharomyces cerevisiae as an oral vaccine vector platform for use with poultry, testing alternative intra-cellular compartments for antigen expression and the value of immune targeting epitopes. We will build on our recent studies demonstrating that heat-killed, freeze dried yeast can be used for oral delivery of anticoccidial vaccine candidates to chickens, inducing immune responses that limit Eimeria tenella replication following low-dose challenge at a level equivalent to modern ionophore chemoprophylaxis. Each new yeast-vectored vaccine will be tested for protective capacity against high-level E. tenella challenge, assessing traits that are relevant to production and welfare including body weight gain (BWG) and lesion score (LS). The optimised yeast vector will subsequently be used to express validated vaccine antigen homologues for E. acervulina and E. maxima, addressing the three most economically important Eimeria species that infect chickens. The resulting multi-valent vaccine formulation will be tested under field-type conditions, measuring parasite replication, BWG, LS and feed conversion ratio (FCR). Local immune cell phenotypes induced by vaccination with existing and new yeast-vectored vaccines will be defined, assessing their potential as correlates of protection for vaccine development against E. tenella and other enteric pathogens.
Major outputs will include understanding of the breadth of immune responses, systemic and local, stimulated by an oral yeast-vectored vaccine with different yeast cell expression profiles, the influence of immune targeting epitopes, and their interaction with the well-established anticoccidial vaccine candidates apical membrane antigen 1 (AMA1) and immune mapped protein 1 (IMP1). Consideration of immune responses in different chicken body compartments will be used to indicate the range of pathogens that may be rationally targeted in future applications.