Dissecting variation in host responsiveness to a recombinant vaccine designed to control teladorsagiosis in sheep

Delivering food security globally requires that a sustainable, safe and secure supply of quality food be provided by making the most efficient use of inputs. Infectious diseases substantially affect the efficiency of livestock production and parasitic roundworms are major culprits in this arena. Indeed, worm infections are the most costly endemic disease affecting sheep in the UK, with Teladorsagia circumcincta being the dominant worm present in this country. Estimated costs of roundworm infections to the UK sheep industry run at around £80 million per annum.

Dewormers (known as anthelmintics) have been used for over four decades to control worm infections in sheep throughout the world, but resistance to the three traditional drug classes is common. Worryingly, T. circumcincta resistance has already been reported against one of the two new classes of dewormer, even although this product was only launched two years ago. Historically, much resource and research has been directed at selective breeding of sheep for resistance to worms, but there have been few outputs from this work that have made it to practical application on farms in the UK.

An obvious alternative approach to breeding sheep for resistance to worms is to induce resistance to infection in sheep by vaccinating them to specifically stimulate immune protection against worm challenge. This will have the effect of negating the impact of the worms on sheep growth and productivity and importantly, will reduce worm transmission via the environment. Vaccines also hold the advantage over anthelmintic methods of control in that they are not associated with chemical residues in meat, have no environmental impact and are likely to require fewer administration applications over the lifetime of the sheep.

Recently, this research group have had singular success in the discovery of an effective recombinant (sub-unit) vaccine for control of T. circumcincta. This vaccine (known as 'CircVax') comprises 8 recombinant (synthetic) proteins and induces significant, albeit variable, levels of immunity against worm challenge in lambs. The next step in development of this vaccine is to understand the underlying local immune mechanism(s) involved in this variation in sheep responsiveness to the vaccine and to examine if this is affected by the age of the animal. Such a comprehensive understanding of variation in individual responses during vaccination and subsequent worm challenge is essential to optimize the vaccine further (for example, in adjuvant selection) as a tool for integrated worm control in sheep flocks.

The knowledge gained in this project will help determine if the effect of the vaccine can be improved in very young lambs and if responses in older animals can be optimised further. Each of these parameters could be addressed by using more appropriate adjuvants or delivery systems. In addition, if the results point to an inherent inability of the immune system of the very young lamb to control T. circumcincta, the derived information can be exploited in the deployment of the vaccine in a holistic, integrated control strategy that addresses the minimization of selection pressure for drug resistance in the worm.

In summary, the outputs will provide significant steps to developing a commercially relevant vaccine for use by farmers to mitigate the effects of this important parasitic infection of sheep and, as such, fall squarely within the priorities of the BBSRC Animal Health Research Club.

The overall objective is to understand and overcome variation in host immune responsiveness to a promising Teladorsagia circumcincta sub-unit vaccine prototype, CircVax.

Here, the applicants will analyse parasitological outputs from vaccinated, then challenged, lambs and assess temporal parameters in vaccine responsiveness using recent technological advances in transcriptomics (RNAseq, nextSeq 500 Sequencing) and ovine genomic and immunological tools (http://www.sheephapmap.org/news/OARv2p0.php, http://www.immunologicaltoolbox.co.uk). Using multivariate (general linear mixed models) and network analyses (BioLayoutExpress 3D, Qiagen's Ingenuity Pathway Analysis) to assess relationships between immune gene expression and phenotypic characteristics, and between parasitological outcomes and immune variables and, importantly, how these are affected by lamb age, the project will generate a deep understanding of how sheep respond optimally to CircVax vaccination.

The resultant data will be exploited to develop an understanding of how the vaccine induces immunity against worm challenge and to identify what pathways are different in animals that are not protected by the vaccine. This knowledge, in turn, will be used to optimise the vaccine's effect in the field through optimal delivery(for exmaple, via adjuvant selection) and/or strategic use (for example, in integrated control programmes exploiting the vaccine with minimal use of effective anthelmintics in sheep of different ages).

It is estimated that the direct global cost of parasitic worms of livestock is ~£1,200M per annum. There are approximately 101 M sheep and 12 M goats in the EU, with the UK having the largest sheep industry in Europe. In the UK, the brown stomach worm, Teladorsagia circumcincta, is the major contributor to parasitic gastroenteritis, estimated to cost the UK sheep industry >£80M each year. Helminths such as T. circumcincta impact hugely on the welfare and productivity of sheep. They do this by affecting growth rate, fertility, meat quality, and wool/milk production and, in heavy infections, by causing clinical disease. The research in this project will impact upon the effect of these worms by furthering development of a much-needed alternative control option; an anti-nematode vaccine known as CircVax.

Chemotherapeutic approaches have been the cornerstone of sheep nematode control for decades; however, resistance to the traditional three broad-spectrum anthelmintic classes is common in T. circumcincta, with resistance already reported to a member of a new class of anthelmintic, monepantel, which was only launched 2 years ago. For these reasons, alternative control methods are sought for sustainable control in the long term. Control by vaccination is the most obvious alternative, with the added value that vaccines have no environmental impact, do not leave chemical residues in food and are likely to require fewer applications over an animal's lifetime. Over the last 8 years, the applicants have developed, designed and tested a recombinant sub-unit T. circumcincta vaccine. Immunisation with this vaccine had a significant effect on worm burden and egg excretion in parasite-challenged lambs >6 months-old. Here, the applicants aim to develop this prototype vaccine further by investigating variation in sheep responsiveness to the vaccine; particularly how immunological unresponsiveness may be overcome in very young lambs. A comprehensive understanding of such responses during vaccination and subsequent parasite challenge is essential to develop this vaccine as a tool for integrated control that will be taken up on UK sheep farms and beyond. The aims are to establish what elements of the local response correlate best with efficacy amongst vaccinates and how variability in vaccine efficacy is affected by lamb age. This knowledge will determine whether or not vaccine efficacy can be improved in young lambs and whether variability in responsiveness in older animals can be addressed by exploiting appropriate adjuvants or delivery systems. Moreover, if the results point to an inherent, and insurmountable, inability to control T. circumcincta in the very young, the information will be used to deploy the vaccine in older animals as part of an integrated strategy to minimise selection pressure for anthelmintic resistance, in particular aiming to preserve use of the Class IV and V anthelmintics.

The outputs of this project will provide significant steps to developing a commercially-relevant subunit nematode vaccine (the 1st of its kind) for use by farmers to mitigate the effects of this important parasite. In so doing, the outcomes address a priority of the Animal Health Research Club ('Understanding variation in vaccine responsiveness and immune-competence at different developmental stages and its impact on disease outcomes'). Industrial stakeholders will benefit through generation of revenue from vaccine sales, as well as from availability of an option that will help prolong efficacy of the remaining effective anthelmintics. In addition, human health policymakers can exploit the data if defined strategies for delivery against ruminant gastrointestinal nematodes can be shown to improve efficacy. This is because there are many similarities (such as host niche, distribution pattern, primary candidate antigens) between the T. circumcincta vaccine and the subunit vaccine currently under development for control of human hookworm.