Uderstanding the basis of strain-restricted immunity to theileria parva

Over 20% of people in sub-Saharan Africa are classified as living below the poverty line and many of these people reside in rural communities that are heavily dependent on livestock for their livelihoods. Infectious diseases have a major impact on livestock production in these regions. One of the most important diseases of cattle is East Coast fever (ECF) caused by a tick-borne parasite Theileria parva. This disease is present in 16 countries in eastern and southern Africa and it is the most economically important cattle disease in 11 of these countries. Estimates of economic losses due to ECF exceed 300 million US dollars annually. Imported breeds of cattle, which are increasingly being used to satisfy demands for milk production, are particularly susceptible.
Although treatment of infected animals with anti-parasitic drugs and application of chemicals to prevent tick infestation can be used to control ECF, these methods are difficult to apply and generally are unaffordable by poor livestock keepers. Vaccination, therefore offers a more sustainable means of controlling the disease.
Cattle can be immunised against the disease by infection with live parasites and simultaneous drug treatment, but the immunity induced by one parasite strain is not effective against all other strains. Use of a mixture of 3 parasite strains for vaccination has been found to give broad protection. This vaccine has been used locally with some success, but widespread application has been hindered by difficulties in production and distribution. The antigenic composition of the vaccine is poorly defined and there remain questions as to whether the content of parasite strains is optimal for obtaining robust immunity. More recently, antigens recognised by the protective cellular immune responses have been identified, with the aim of investigating their potential for developing a subunit vaccine. Preliminary evidence indicates that several of these antigens vary between parasite strains. Hence, understanding antigenic variability in T. parva is a key issue both for improving the current live vaccine and for the design of a sub-unit vaccine.
The overall aim of the current project is to understand the antigenic basis of strain restricted immunity to T. parva, in order to develop improved vaccines against ECF.
The key elements of the proposed research are:
• An analysis of T. parva isolates from naturally infected cattle in different regions of Kenya, to determine the nature and extent of antigenic diversity in the parasite population.
• An analysis of antigenic diversity in the 3 parasite isolates that make up the current live vaccine.
• A detailed study of how variation in the antigens affects the ability of the cellular immune response to recognise different parasite strains.
• An analysis of the specificity of cellular immune responses in cattle immunised sequentially and simultaneous with different parasite strains, to understand how these immunisation procedures affect the fine specificity of the immune response.
• Analysis of immune Friesian cattle of different MHC genotypes for their ability to recognise currently defined T. parva antigens, in order to identify priorities for further antigen screening.
It is anticipated that the results will provide methods for improving quality control of the current live vaccine, identify parasite strains that could be incorporated into an improved second generation live vaccine and indicate which and how many antigens will need to be incorporated into a subunit vaccine.
Realisation of the project objectives will ultimately result in improved vaccines for ECF leading to better control of the disease. This will help to secure the assets and increase the incomes of smallholder and pastoral livestock keepers in affected regions. Improved disease control will result in increased production of meat and milk and reduce the levels of poverty in these vulnerable communities.

Over 20% of people in sub-Saharan Africa are classified as living below the poverty line and many of these people reside in rural communities that are heavily dependent on livestock for their livelihoods. Infectious diseases have a major impact on livestock production in these regions. One of the most important diseases of cattle is East Coast fever (ECF) caused by a tick-borne parasite Theileria parva.
Cattle can be immunised against the disease by infection with live parasites and simultaneous drug treatment, but the immunity induced by one parasite strain is not effective against all other strains. Use of a mixture of 3 parasite strains for vaccination has been found to give broad protection. This vaccine has been used locally with some success, but widespread application has been hindered by difficulties in production and distribution. The antigenic composition of the vaccine is poorly defined and there remain questions as to whether the content of parasite strains is optimal for obtaining robust immunity. More recently, antigens recognised by the protective cellular immune responses have been identified, with the aim of investigating their potential for developing a subunit vaccine.
The overall aim of the current project is to understand the antigenic basis of strain restricted immunity to T. parva, in order to develop improved vaccines against ECF. This will involve both experimental studies to investigate the effects of immunisation protocol on strain specificity of immunity and analyses of parasite populations in the field and the current live vaccine by deep DNA sequencing methods to determine the extent of antigenic diversity.

- Polymorphism of antigens recognised by CD8 T cells is clearly associated with escape from recognition by CD8 T cells.
- A deep-sequencing method was developed for analysing the sequence diversity of antigen-encoding genes
- Buffalo T. parva parasites showed enormous genetic diversity both at the population and individual animal level.
- However at the translated amino acid level, the antigen genes seqregated into those that show extensive diversity and those that are highly conserved. Preliminary data indicated that the former tend to be highly dominant antigens.
- Cattle T. parva show much less sequence diversity than those from buffalo, supporting the idea that they represent a subset of the population that has adapted to tick transmission between cattle.
- The live T. parva vaccine shows very restricted diversity, even compared to the cattle T. parva population. It also contains minor constituents that are at risk of being lost during passage of the vaccine parasite stocks.