Live antigen carriers as tools for improved anti-tuberculosis vaccines

Recombinant (r) Mycobacterium bovis BCG strains have been constructed which secrete biologically active listeriolysin (Hly) fusion protein of Listeria monocytogenes. In human and murine macrophage-like cell lines, intracellular persistence of these r-BCG strains was reduced as compared to the parental BCG strain. By immunogold labelling Hly was detected in membrane structures and within the phagosomal space of macrophages. Hly fusions consistently co-localized with a lysosome-associated membrane glycoprotein (LAMP-1) suggesting that membrane attack conformation of Hly was not altered. Although r-BCG microorganisms apparently did not egress into the cytoplasmic compartment of host cells, they both improved major histocompatibility complex class I presentation of co-phagocytosed soluble ovalbumin as compared with wild-type BCG microbes. These data suggest that Hly secretion endows BCG with an improved capacity to stimulate CD8 T cells. Because CD8 T cells play a major role in protection against tuberculosis such Hly-secreting r-BCG constructs are anti-tuberculosis vaccine candidates. In addition, we report on our r-Salmonella typhimurium expression system combined with the HlyB/HlyD/ TolC export machinery for delivering the prominent mycobacterial antigen Ag85B for immune recognition.