The use of attenuated bacterial strains has long been recognised as a means to protect against severe disease states caused by infection with the progenitor strain. However, there are inherent problems associated with the isolation of attenuated strains that stimulate an effective immune response in the absence of adverse clinical symptoms. The administration of purified antigenic material circumvents these problems, yet bears significant industrial and administrative costs, particularly within under-developed countries. Thus, an attenuated bacterial strain that expresses recombinant antigens to elicit a specific immune response presents an opportunity to combine these two approaches.
Previously, we used the Type V bacterial secretion system to facilitate the expression and accumulation of natively folded recombinant proteins into cell culture supernatants. Here, we report the use of this secretion platform in Salmonella enterica serovar Typhimurium strain SL3261 to deliver the Mycobacterium tuberculosis antigen ESAT-6, within a murine model of Salmonella spp. infection. Using this model, we were able to examine cellular and humoral responses generated by infection with Salmonella Typhimurium expressing either cytosolic, surface bound or secreted ESAT-6 antigen. Cells from spleens of infected mice were stained with tetrameric ESAT-6 peptide-MHC complexes to reveal a population of ESAT-6 specific, CD4+ memory T-cells, after bacterial clearance. Furthermore, we show the presence of serum ESAT-6 specific IgG, during the course of Salmonella Typhimurium infection.