Plant-derived recombinant immune complexes as self-adjuvanting TB immunogens for mucosal boosting of BCG

Progress with protein-based tuberculosis (TB) vaccines has been limited by poor availability of adjuvants suitable for human application. Here, we developed and tested a novel approach to molecular engineering of adjuvanticity that circumvents the need for exogenous adjuvants. Thus, we generated and expressed in transgenic tobacco plants the recombinant immune complexes (RICs) incorporating the early secreted Ag85B and the latency-associated Acr antigen of Mycobacterium tuberculosis, genetically fused as a single polypeptide to the heavy chain of a monoclonal antibody to Acr. The RICs were formed by virtue of the antibody binding to Acr from adjacent molecules, thus allowing self-polymerization of the complexes. TB-RICs were purified from the plant extracts and shown to be biologically active by demonstrating that they could bind to C1q component of the complement and also to the surface of antigen-presenting cells. Mice immunized with BCG and then boosted with two intranasal immunizations with TB-RICs developed antigen-specific serum IgG antibody responses with mean end-point titres of 1 : 8100 (Acr) and 1 : 24 300 (Ag85B) and their splenocytes responded to in vitro stimulation by producing interferon gamma. 25% of CD4+ proliferating cells simultaneously produced IFN-γ, IL-2 and TNF-α, a phenotype that has been linked with protective immune responses in TB. Importantly, mucosal boosting of BCG-immunized mice with TB-RICs led to a reduced M. tuberculosis infection in their lungs from log10 mean = 5.69 ± 0.1 to 5.04 ± 0.2, which was statistically significant. We therefore propose that the plant-expressed TB-RICs represent a novel molecular platform for developing self-adjuvanting mucosal vaccines.