An inducible expression system for high-level expression of recombinant proteins in slow growing mycobacteria.

A novel protein expression vector utilising the inducible hspX promoter of Mycobacterium tuberculosis was constructed and evaluated in this study. High-level induction of three mycobacterial antigens, comprising up to 9% of bacterial sonicate, was demonstrated in recombinant Mycobacterium bovis BCG when grown under low-oxygen tension, which serves to enhance hspX promoter activity. Recombinant proteins were efficiently purified from bacterial lysates in a soluble form by virtue of a C-terminal 6-histidine tag. Purification of the immunodominant M. tuberculosis Ag85B antigen using this system resulted in a recombinant protein that stimulated significant IFN-γ release from Ag85B-reactive T cells generated after vaccination of mice with an Ag85B-expressing vaccine. Further, the M. tuberculosis L-alanine dehydrogenase (Ald) protein purified from recombinant BCG displayed strong enzymatic activity in recombinant form. This study demonstrated that high levels of native-like recombinant mycobacterial proteins can be produced in mycobacterial hosts, and this may aid the analysis of mycobacterial protein function and the development of new treatments.

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:P(hspX)-GFP, in which P(hspX) controlled GFP expression. Vaccination of mice with BCG:P(hspX)-GFP led to rapid in vivo induction of GFP associated with an influx of GFP(+) DCs at the infection site. P(hspX)-driven antigen expression resulted in an improved capacity of DCs to prime antigen-specific T cells, as infection of DCs with BCG:P(hspX)-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:P(hspX)-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.

Modulation of pulmonary DC function by vaccine-encoded GM-CSF enhances protective immunity against Mycobacterium tuberculosis infection.

The rational design of new vaccines engineered to target key components of the host immune response is crucial to aid control of important infectious diseases such as tuberculosis. In this report, we determined whether modifying the function of pulmonary APC could improve protection against infection with Mycobacterium tuberculosis. Targeted delivery to the lung of the cytokine GM-CSF, expressed by the Mycobacterium bovis BCG vaccine strain, increased pulmonary DC numbers and secretion of the immunoregulatory cytokine IL-12, compared with parental BCG immunization. This impact on APC number by BCG:GM-CSF resulted in accelerated priming of antigen-specific CD4+ T cells in the mediastinal lymph nodes and increased migration of activated CD4+ T cells into the lung. i.n. administration of BCG:GM-CSF resulted in significantly increased protection against M. tuberculosis infection compared with mice vaccinated with BCG alone. BCG:GM-CSF exhibited an improved safety profile, as immunodeficient RAG1-/- mice vaccinated i.n. with BCG:GM-CSF survived significantly longer than control BCG-vaccinated mice. These data demonstrate that manipulating immune cells in the lung by BCG-based delivery of GM-CSF can assist the development of protective mucosal immunity against pulmonary bacterial infection.