Novel recombinant BCG expressing perfringolysin O and the over-expression of key immunodominant antigens; pre-clinical characterization, safety and protection against challenge with Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis ( Mtb), the causative agent of tuberculosis (TB), has infected approximately two billion individuals worldwide with approximately 9.2 million new cases and 1.6 million deaths annually. Current efforts are focused on making better BCG priming vaccines designed to induce a comprehensive and balanced immunity followed by booster(s) targeting a specific set of relevant antigens in common with the BCG prime. We describe the generation and immunological characterization of recombinant BCG strains with properties associated with lysis of the endosome compartment and over-expression of key Mtb antigens. The endosome lysis strain, a derivative of BCG SSI-1331 (BCG 1331) expresses a mutant form of perfringolysin O (PfoA G137Q), a cytolysin normally secreted by Clostridium perfringens. Integration of the PfoA G137Q gene into the BCG genome was accomplished using an allelic exchange plasmid to replace ureC with pfoA G137Q under the control of the Ag85B promoter. The resultant BCG construct, designated AERAS-401 (BCG 1331 Δ ureC::Ω pfoA G137Q) secreted biologically active Pfo, was well tolerated with a good safety profile in immunocompromised SCID mice. A second rBCG strain, designated AFRO-1, was generated by incorporating an expression plasmid encoding three mycobacterial antigens, Ag85A, Ag85B and TB10.4, into AERAS-401. Compared to the parental BCG strain, vaccination of mice and guinea pigs with AFRO-1 resulted in enhanced immune responses. Mice vaccinated with AFRO-1 and challenged with the hypervirulent Mtb strain HN878 also survived longer than mice vaccinated with the parental BCG. Thus, we have generated improved rBCG vaccine candidates that address many of the shortcomings of the currently licensed BCG vaccine strains.