Targeted induction of antigen expression within dendritic cells modulates antigen-specific immunity afforded by recombinant BCG

The targeted modulation of antigen expression by recombinant vaccine vehicles would significantly aid development of effective immunotherapeutic strategies. In this report we demonstrate that the Mycobacterium tuberculosis hspX promoter can be used to regulate in vivo induction of antigens expressed by recombinant Bacille Calmette Guérin (rBCG). HspX promoter induction occurred rapidly upon entry of rBCG into cultured dendritic cells (DCs), as evidenced by GFP levels in DCs when infected with BCG:PhspX-GFP, in which PhspX controlled GFP expression. Vaccination of mice with BCG:PhspX-GFP led to rapid in vivo induction of GFP associated with an influx of GFP+ DCs at the infection site. PhspX-driven antigen expression resulted in an improved capacity of DCs to prime antigen-specific T cells, as infection of DCs with BCG:PhspX-85B, where the hspX promoter controls expression of M. tuberculosis Ag85B, led to enhanced proliferation of Ag85B-reactive CD4+ T cells compared to BCG overexpressing Ag85B using the strong Mycobacterium bovis hsp60 promoter. This enhancement of rBCG-induced immunity was also evident in vivo; mice vaccinated with BCG:PhspX-85B displayed markedly improved generation of Ag85B-reactive IFN-γ-secreting T cells compared to control BCG-vaccinated mice, which was most pronounced at extended times points post-vaccination. These data reveal a novel strategy to enhance the development and maintenance of vaccine-specific T cell responses.