There is currently no cure for Johne disease and hygienic measures or culling procedures are not sufficient to prevent its spread.9
Moreover, the available vaccines against Johne disease do not fully protect animals from infection, but rather reduce clinical symptoms and limit shedding of MAP in feces.9
While several whole cell vaccines have been developed, based on heat killed or live attenuated strains of MAP, issues related to their use prevents their widespread application in control strategies.9,10
The associated drawbacks with these vaccines include (1) the presence of local granulomatous lesions at the site of injection, (2) interference with current serodiagnostic tests for MAP and (3) failure to fully protect animals from subsequent exposure to MAP. Administration of vaccines developed from whole organisms also increases the likelihood of interference with bovine tuberculosis screening tests.11,12
Sequencing of the entire genome of MAP strain K-10 in 2005 has permitted the identification of immunodominant protein antigens, which induce strong humoral or cell mediated immune responses, for subsequent inclusion in diagnostic tests or subunit based vaccines.13,14
Recent subunit vaccines appear to overcome many of the issues associated with whole cell strategies but nevertheless they have failed to fully protect against MAP in experimental infection models.9
Huygen and Rosseels recently carried out a definitive review, entitled “Vaccination against Paratuberculosis”, which describes in detail the past and current approaches to vaccine design for paratuberculosis as well as immunodominant antigens identified prior to its publication.9
Next generation animal vaccines will be complex molecular entities with multiple components tailored to generate the most potent and effective immune response. Accordingly, through comparative bioinformatic analysis of the completed MAP genome, a defined set of MAP genes encoding potentially immunodominant secreted, cytosolic and surface expressed antigens have been assembled within our laboratory. The challenge ahead is to effectively deliver these antigens in a manner that will stimulate the appropriate immune responses, a task which will require an efficient and controllable vaccine strategy. Building on the experience of previous vaccine strategies will be crucial.
Vaccine developments against other intracellular pathogens, similar to MAP, have used attenuated strains of the intracellular pathogen L. monocytogenes
as effective vaccine carriers to the immune system.15
These strategies are based on observations that the pathogen has both a phagosomal and cytosolic phase, facilitating the stimulation of CD4+
immune responses respectively.15,16
The infection strategy of L. monocytogenes
has notable similarities to that employed by MAP but despite this, its potential as a MAP vaccine carrier has not been investigated. It is likely that this is due to concerns over the use of attenuated pathogens in vaccine development which comes with the possibility of reversion to a virulent phenotype.17
Alternatively, probiotic or GRAS (generally regarded as safe) bacterial strains have also been assessed for applications in vaccine delivery strategies.18,19
In this field however, their potential is hampered by fragility and sensitivity toward stresses associated with formulation and environmental conditions in the gut including the presence of bile, high osmolarity, low iron or acidic environments.20
The advent of patho-biotechnology, which exploits inherent mechanisms from pathogenic bacteria, has brought about novel strategies to improve the potential of these probiotic or GRAS strains for use in clinical, drug, and vaccine delivery applications.17,21
The main purpose of this review is to propose and evaluate the development of a novel oral subunit-vaccine against MAP and Johne disease which will utilize a patho-biotechnological approach. A Lactobacillus strain with GRAS status, harbouring an inducible expression vector encoding immunodominant MAP antigens, is to be equipped with patho-genetic elements derived from L. monocytogenes which will allow the strain to access appropriate antigen presentation pathways in order to stimulate a strong immune response. After vaccination with MAP antigens via this novel delivery platform, levels of protective efficacy will be assessed in a murine model of infection.
A successful vaccine strategy against Johne disease will require a detailed understanding of the causative agent, its mode of transmission and the associated host immune responses. The following sections will attempt to describe these aspects of MAP infection while simultaneously comparing L. monocytogenes infection, to illustrate their combined potential as part of a patho-biotechnological approach to vaccine design for Johne disease. The patho-biotechnological methodologies which have led to the present vaccine development will be discussed. The biological containment strategies to prevent the dissemination and proliferation of the vaccine strain within the environment are also outlined.