Neutralization of Apx toxicity as an alternative to antibiotics for control of contagious porcine pleuropneumonia
A new MRC grant has been awarded to Professors Andrew Rycroft and Brian Catchpole.
The grant has been awarded under the programme "Tackling Antimicrobial Resistance: Theme 2". The project is called “Neutralization of Apx toxicity as an alternative to antibiotics for control of contagious porcine pleuropneumonia” and builds on the understanding gained over many years of working with this respiratory disease in pigs. It is aimed at improving the immunological protection necessary to replace prescription antibiotics used in pig farms.
Contagious pleuropneumonia is a severe acute disease of the pig. It kills many growing pigs and causes lifelong damage to the lungs of those that survive. This impacts on the profitability of pig farms and so they regularly use antibiotics to control this disease. This is a problem throughout the world and is one of the primary reasons for using prescription antibiotics in pigs throughout Europe. In the UK, there is no effective vaccine because these have failed to protect pigs from disease, or the vaccine was itself toxic. Efforts to make on-farm vaccines (emergency vaccines) have not solved the problem because they are not effective and antibiotics in feed, in water and by injection, are used in an attempt to avoid catastrophic losses.
The disease is caused by a bacterial pathogen, Actinobacillus pleuroneumoniae. This organism produces two of three different protein toxins (ApxI, II and III). Production of these toxins by the pathogen is key to the disease process. Pigs that recover from disease have antibodies which neutralize the toxins and evidence suggests this is crucial in protecting pigs from the disease. This needs to be replicated in a successful vaccine. However, simply using the toxin(s) as a vaccine does not protect pigs. Despite stimulating production of antibodies these cannot neutralise the toxin. It appears that these bacteria have evolved to synthesise toxin molecules with irrelevant but highly immunogenic regions to distract the immune response to the wrong part of the toxin. Thus the antibody response is ineffective allowing the bacteria to spread and the disease continue.
In this project we will eliminate those parts of the toxin that are distracting the immune response and which appear to be causing failure of the toxin molecules to generate a neutralizing response when used as a vaccine. These small fragments will be joined to a carrier protein, modified diphtheria toxin. This will enhance the immune response and make the antigen large enough to be recognised by the pigs' immune system as a vaccine antigen.
We will immunize pigs with these modified toxins and measure the immune response and the neutralizing effect against the active toxins. To improve the method of testing the toxins and the effect of neutralizing antibody, we will also develop and test a pig model of dermal oedema using the Apx toxins. This will be used to indicate the best vaccine antigens for use in the pig model of pleuropneumonia. We will then proceed to immunize pigs and test the efficacy of the vaccination by experimental challenge of the pigs with the virulent pathogen. If our hypothesis is correct, and the immune response to the toxin fragment is effective, this could be the step needed for production of an effective vaccination against pleuropneumonia and the opportunity, finally, to offer the pig industry analternative to antibiotics which would markedly reduce the quantity of these drugs used in controlling pig respiratory disease.