Evaluation of the immunogenicity of a promising vaccine regimen to identify immune correlates of protection

A vaccine against tuberculosis (TB), a disease resulting after infection with Mycobacterium tuberculosis (M.tb), is urgently needed to prevent more than a million deaths per year. Bacillus Calmette–Guérin (BCG) is the only available vaccine against TB but its efficacy is variable throughout the world. A promising vaccination regimen composed of BCG prime, followed by vaccination with a chimpanzee adenoviral vector (ChAdOx1) and a modified vaccinia Ankara virus (MVA), both expressing Ag85A, was identified in our laboratory. This regimen significantly improved BCG efficacy in animal studies. Antibody, effector and memory T cell responses induced by BCG-ChAdOx1.85A-MVA85A (B-C-M), were evaluated, to identify immune correlates of protection. This regimen induced Ag85A-specific IgG and IgA responses as well as cytokine responses in CD4+ and CD8+ T cells, both in the systemic and pulmonary system. To discriminate lymphocytes in the vasculature from lymphocytes in the lung parenchyma, a fluorochrome-conjugated antibody was injected intravascularly before harvesting the lungs. Lung parenchymal Ag85A-specific memory CD4+ T cells that were CXCR3+ KLRG1-, significantly increased in B-C-M immunised mice compared to BCG alone at 4, 8 and 20 weeks post vaccination but the cell number decreased at the latter time point. These memory cells associated with protection against M.tb challenge, which was observed at 4 and 8 weeks but not when mice were challenged 20 weeks post vaccination. These putative T cells may have an important role against M.tb infection and could be a target for designing a protective vaccine. In parallel, six novel M.tb antigens were selected and expressed individually in a ChAdOx1 vector. The immunogenicity and protective efficacy of these vaccines were evaluated. Three antigens were not immunogenic when expressed in viral vectors and DNA. The immunogenic antigens induced cytokine production but failed to confer protection when delivered alone or with B-C-M regimen.