Next-Generation Vaccines Based on Bacille Calmette-Guérin

Abstract

Tuberculosis (TB), caused by the intracellular bacterium Mycobacterium tuberculosis (Mtb), remains a major health threat. A live, attenuated mycobacterium known as Bacille Calmette-Guérin (BCG), derived from the causative agent of cattle TB, Mycobacterium bovis, has been in clinical use as a vaccine for 90 years. The current incidence of TB demonstrates that BCG fails to protect sufficiently against pulmonary TB, the major disease manifestation and source of dissemination. The protective efficacy of BCG is on average 50% but varies substantially with geographical location and is poorer in those with previous exposure to mycobacteria. BCG can also cause adverse reactions in immunocompromised individuals. However, BCG has contributed to reduced infant TB mortality by protecting against extrapulmonary TB. In addition, BCG has been associated with reduced general childhood mortality by stimulating immune responses. In order to improve the efficacy of BCG, two major strategies have been employed. The first involves the development of recombinant live mycobacterial vaccines with improved efficacy and safety. The second strategy is to boost BCG with subunit vaccines containing Mtb antigens. This article reviews recombinant BCG strains that have been tested against TB in animal models. This includes BCG strains that have been engineered to induce increased immune responses by the insertion of genes for Mtb antigens, mammalian cytokines, or host resistance factors, the insertion of bacterial toxin-derived adjuvants, and the manipulation of bacterial genes in order to increase antigen presentation and immune activation. Subunit vaccines for boosting BCG are also briefly discussed.

Keywords: Mycobacterium bovis bacille Calmette–Guérin; mycobacteria; recombinant Mycobacterium bovis bacille Calmette–Guérin; subunit vaccine; tuberculosis; vaccine.

Figure 1

Aspects of tuberculosis (TB) immunology. DC: dendritic cell; Mϕ: macrophage; AMϕ: alveolar macrophage; T_EM: effector memory T cell; T_CM: central memory T cell; T_RM: resident memory T cell; MDSC: myeloid-derived suppressor cell; PMN: neutrophil. (1) Exposure to Mycobacterium tuberculosis (Mtb) is by the inhalation of infected aerosol droplets generated by coughing. (2) Mtb bacilli primarily live in host macrophages. Alveolar macrophages and other myeloid cells such as DCs can take up bacteria from the airways. If macrophages do not control infection, the bacteria can replicate and kill the cell. Bacteria can also infect neutrophils, which die and can be taken up by macrophages, which subsequently become infected themselves. (3) Infected DCs have delayed migration to the lymph nodes and impaired antigen presentation. In the lymph nodes, they transfer antigens to uninfected bystander DCs, which present the antigens to T cells. (4) Effector CD4⁺ and CD8⁺ T cells generated from naive T cells or from memory T cells are recruited to the lungs. (5) Effector CD4⁺ T cells produce cytokines such as IFN-γ, interleukin (IL)-17, and TNF-α, while CD8⁺ T cells can lyse infected macrophages. IFN-γ can activate bacterial killing. (6) Neutrophils, monocytes, or immature myeloid cells with suppressive functions, known as MDSCs are recruited to the lungs. Monocytes can differentiate into inflammatory macrophages or inflammatory DCs. (7) MDSCs aim to limit excessive inflammation by inhibiting T-cell proliferation and function, but they can act as a reservoir for Mtb. (8) Antibodies may also play a role in controlling infection. (9) Eventually, the immune cells form a granuloma around Mtb in an attempt to contain the bacteria. These are the typical lesions observed in the Mtb-infected lung, most commonly in the upper lobes.

Figure 2

Electron micrographs showing Bacille Calmette-Guérin (BCG) ΔureC:hly inside host macrophages. All scale bars represent 0.5 µm. Left panel: BCG ΔureC:hly within phagosomes after uptake by host cells. Middle and right panels: Phagolysosomal fusion after the infection of host cells with recombinant BCG ΔureC:hly.