Vaccination Against Tuberculosis: Revamping BCG by Molecular Genetics Guided by Immunology

Abstract

Tuberculosis (TB) remains a major health threat. Although a vaccine has been available for almost 100 years termed Bacille Calmette-Guérin (BCG), it is insufficient and better vaccines are urgently needed. This treatise describes first the basic immunology and pathology of TB with an emphasis on the role of T lymphocytes. Better understanding of the immune response to Mycobacterium tuberculosis (Mtb) serves as blueprint for rational design of TB vaccines. Then, disease epidemiology and the benefits and failures of BCG vaccination will be presented. Next, types of novel vaccine candidates are being discussed. These include: (i) antigen/adjuvant subunit vaccines; (ii) viral vectored vaccines; and (III) whole cell mycobacterial vaccines which come as live recombinant vaccines or as dead whole cell or multi-component vaccines. Subsequently, the major endpoints of clinical trials as well as administration schemes are being described. Major endpoints for clinical trials are prevention of infection (PoI), prevention of disease (PoD), and prevention of recurrence (PoR). Vaccines can be administered either pre-exposure or post-exposure with Mtb. A central part of this treatise is the description of the viable BCG-based vaccine, VPM1002, currently undergoing phase III clinical trial assessment. Finally, new approaches which could facilitate design of refined next generation TB vaccines will be discussed.

Keywords: Bacille Calmette-Guérin; T lymphocyte; biomarker; clinical trial; macrophage; subunit; tuberculosis; vaccine.

FIGURE 1

Epidemiologic data for tuberculosis (TB).

FIGURE 2

Major mechanisms underlying induction of the host immune response by VPM1002 and M. tuberculosis (Mtb) (for further details see text). (A) VPM1002. VPM1002 (rBCGΔureC::Hly) expresses listeriolysin and lacks urease C activity. Following phagocytosis, VPM1002 ends up in a phagosome. Principally phagosomes become acidic after uptake of particles, but BCG and Mtb actively keep the phagosomal pH neutral. Due to the absence of ureaseC in VPM1002, acidification takes place. This facilitates perturbation of the phagosomal membrane by biologically active listeriolysin. (1) Membrane perturbation allows egress of antigens into the cytosol for processing through the MHC class I pathway. (2) Perturbation can lead to apoptosis. (3) Double-strand DNA released into the cytosol is sensed by absence in melanoma 2 (AIM2). (4) AIM2 activates the inflammasome to generate IL-1β and IL-18. (5) Cyclic GMP-AMP synthase (cGAS) is formed which is then transformed into cyclic guanosine monophosphate-adenosine monophosphate (cGAMP). (6) The latter molecule is sensed by stimulator of IFN genes (STING) which induces autophagy and type I IFN responses. (7) Antigen egress into the cytosol allows stimulation of CD8 T cells in addition to CD4 T cells. (8) Apoptosis promotes crosspriming. (9) Autophagy accelerates elimination of VPM1002 and improves antigen presentation and T cell stimulation. (10) IL-1β and IL-18 induce an inflammatory response. Through these mechanisms, VPM1002 induces an immune response with more depth and breadth than parental BCG (B) Mtb. The genome of Mtb comprises the region of difference 1 (RD-1) which encodes numerous virulence factors which are absent in BCG. Notably genes for Esx dependent mechanisms cause perturbation of phagosomal membranes, very similar to VPM1002. For further details see (A). Because the RD-1 encoded gene products are not degraded after their egress into the cytosol, pathologic consequences prevail. Moreover, RD-1 encoded gene products are not controlled by pH. Hence, inbuilt safety mechanisms of VPM1002 are absent from Mtb (see also Figure 3).

FIGURE 3

Safety mechanisms of listeriolysin render VPM1002 less virulent than parental BCG. Listeriolysin contains a PEST-like sequence which promotes its degradation. (1) Only at acidic pH, listeriolysin is biologically active and hence perturbates the phagosomal membrane. (2) In the cytosol, monomeric listeriolysin aggregates. (3) Aggregated listeriolysin is degraded by ubiquitin resulting in inactive peptides. (4) Multimeric listeriolysin complexes are formed at the plasma membrane. (5) These complexes are translocated into autophagosomes by ubiquitin. (6) These listeriolysin complexes are inactivated in the phagosome. PEST = Proline (P), Gutalate (E), Serine (S), and Threonine (T). Modified from (–90).

FIGURE 4

Fate of household contacts of a TB index case. Household contacts of a TB index case are either already latently TB infected (LTBI) or do not show evidence for immunity against Mtb infection. After sustained contact with a TB index case, the majority of naïve individuals will rapidly convert to LTBI because they mount an immune response against Mtb infection. Most of these early converters will remain LTBI and hence become sustained converters. A small proportion of early converters reverts to naïve, i.e. devoid of a measurable immune response to Mtb infection. Some naïve individuals will remain permanent non-converters, i.e. they do not change their status of absent immunity indicating absence of Mtb infection. Finally, some individuals with LTBI will revert to naïve, i.e. they lose their detectable immune response to Mtb indicating elimination of Mtb. The mechanisms underlying these conversions/reversions remain elusive. (A) Indicates response in TST/IGRA and (B) depicts resulting conclusions on conversion/reversion (for further details see text).

FIGURE 5

Possible scenarios of TB vaccine development given that adequate financial funding is provided for research & development (R&D). Upper, single step event; Lower, multistep event.