Description of the vaccine
rBCG30 (Tice® strain) is a recombinant BCG overexpressing the 30 kDa major secretory protein of Mycobacterium tuberculosis (1). This protein, also known as alpha antigen and Antigen 85B, is a mycolyl transferase and the most abundant protein secreted by M. tuberculosis into broth culture (2). The protein is also among the most abundant proteins of all M. tuberculosis proteins produced by the pathogen in infected human macrophages (3), and the protein is abundantly released into the M. tuberculosis phagosome in these macrophages (4). rBCG30 was constructed by inserting the plasmid pMTB30 into BCG (1). This plasmid, derived from the E.coli/Mycobacterium shuttle plasmid pSMT3, contains the full-length M. tuberculosis 30 kDa protein gene including the sequence encoding the leader peptide and the flanking 5' and 3' regions (1, 5). rBCG30 secretes approximately 5-fold more 30 kDa protein than the parental BCG strain (1, 5), which expresses and secretes a highly homologous protein that differs from the M. tuberculosis protein by only two contiguous amino acids. rBCG30 secretes the protein in its native form, something that requires a mycobacterial host (6). The plasmid pMTB30 also contains a gene providing resistance to hygromycin. Even in the absence of the selectable marker hygromycin, rBCG30 stably maintains the plasmid in broth culture for at least one year and in vivo in guinea pigs (1, 5). The retention of the plasmid by rBCG30 in the absence of selective pressure allows continued high expression of the 30 kDa protein by the vaccine in the immunized host.
rBCG30 was developed at the University of California—Los Angeles by a team of investigators in the laboratory of Marcus A. Horwitz with funding from the U.S. National Institutes of Health. Other members of the development team included Günter Harth, Saša Masleša-Galić and Barbara Jane Dillon.