Various types of vaccines have been developed against pathogens. When a humoral immune response is able to confer protection, subunit or killed vaccines are efficient. However, in the case of tuberculosis and certain other infectious diseases, killed pathogens are not protective.
The vaccine currently used to protect against tuberculosis, Mycobacterium bovis BCG (Bacille Calmette-Guerin) or "BCG", is the unique live bacterial vaccine in use. M. bovis BCG and all other mycobacteria survive in macrophages which are antigen-presenting cells and initiate the humoral and T-cell mediated immune response (EDWARDS and KIRKPATRICK, 1986). This might explain the stimulant activities of this vaccine. M. bovis BCG offers many advantages for development of a recombinant polyvalent vaccine vector expressing antigens from a wide variety of pathogens, particularly those in which cell-mediated immunity is important for protection (BLOOM, 1986; JACOBS et al, 1988). It is an attenuated M. bovis strain which has been used without major side effects to vaccinate more than two billion people, it is produced at low cost, and it can be given at birth as a single dose and is then able to confer long term immunity. It also has stimulant activities and has been used as an adjuvant in various protocols of immunization.
The recent development of genetic tools for transforming mycobacteria has enabled the cloning of foreign genes in both fast-growing (M. smegmatis) and slow-growing (M. bovis BCG) strains. Several phasmid-and plasmid-based vectors have been reported, for example in PCT publication WO88/06626. Starting from pAL5000, a plasmid from M. fortuitum whose entire nucleotide sequence has been determined (RAUZIER et al, 1988), various E. coli-mycobacteria shuttle plasmids have been constructed which stably replicate in mycobacteria, including a "mini" mycobacterial replicon, pRR3 (RANES et al, 1990). Foreign genes have been cloned on these vectors and on other integrative vectors described in STOVER et al (1991) and shown to be expressed in mycobacteria using their own control elements or as fused genes (MATSUO et al, 1990).