1Centre for Molecular Therapeutics, Australian Institute of Tropical Health and Medicine, James Cook University, Australia; 2 College of Medicine and Dentistry, James Cook University, Cairns & Townsville, Queensland, Australia; 3 Biochemistry Research Laboratory, Central India Institute of Medical Sciences, Bajaj Nagar Nagpur, India; 4Centre for Tropical Bioinformatics and Molecular Biology, Australian Institute of Tropical Health and Medicine, James Cook University, Cairns, Queensland, Australia
5Institut Pasteur, Unit for Integrated Mycobacterial; Pathogenomics, Paris, France; 6Science and Technology, University of New England, Armidale, New South Wales, Australia; 7College of Public Health, Medical and Veterinary Sciences, James Cook University, Townsville, Queensland, Australia
BCG affords variable protection against TB but remains the only licensed vaccine. The replacement of BCG with a more effective vaccine is considered to be crucial for achieving TB elimination.
About 20 years ago it was shown that the incorporation of the Mycobacterium tuberculosis secretion system ESX-1 into BCG improves immunogenicity and protection against TB in animal models. However, the resulting strain, BCG::ESX1Mtb, was deemed unsafe as a human vaccine, due to prolonged persistence, increased virulence in immunocompromised mice and interference with TB diagnostics.
In addition to genetic modifications, it is now clear that heterologous routes of vaccine delivery and booster vaccination also need to be considered for BCG improvement. We used 24 vaccination regimens, comprised of three BCG strains and eight combinations of delivery, to systematically rank vaccination strategies in a mouse model of TB. By integrating multiple measurements of safety, immunogenicity and efficacy into an empirical mathematical model, we demonstrate that mucosal delivery and expression of ESX-1-associated antigens is the most effective way to enhance BCG performance.
Furthermore, using a diet-induced model of type 2 diabetes, a major risk factor for TB, we show that mucosal delivery of ESX-1 expressing BCG strains is safe and confers near-sterile immunity against TB in the context of type 2 diabetes.
To overcome the detrimental safety concerns previously associated with the incorporation of ESX-1 into BCG, we also generated a novel recombinant BCG strain that uncouples ESX-1-dependent virulence from immunogenicity and safety. We demonstrate that this strain improves protection against murine TB, but is associated with reduced persistence in immunocompetent mice and reduced virulence in immunocompromised mice compared to BCG::ESX1Mtb.
Collectively, our results highlight that the pre-clinical and clinical evaluation of ‘virulent’ BCG strains should be reconsidered.