Abstract
The global control of Tuberculosis remains elusive, and Bacillus Calmette-Guérin (BCG) -the most widely used vaccine in history—has proven insufficient for reversing this epidemic. Several authors have suggested that the mass presence of vaccinated hosts might have affected the Mycobacterium tuberculosis (MTB) population structure, and this could in turn be reflected in a prevalence of strains with higher ability to circumvent BCG-induced immunity, such as the recent Beijing genotype. The effect of vaccination on vaccine-escape variants has been well-documented in several bacterial pathogens; however the effect of the interaction between MTB strains and vaccinated hosts has never been previously described. In this study we show for the first time the interaction between MTB Beijing-genotype strains and BCG-vaccinated hosts. Using a well-controlled murine model of progressive pulmonary tuberculosis, we vaccinated BALB/c mice with two different sub-strains of BCG (BCG-Phipps and BCG-Vietnam). Following vaccination, the mice were infected with either one of three selected MTB strains. Strains were selected based on lineage, and included two Beijing-family clinical isolates (strains 46 and 48) and a well-characterized laboratory strain (H37Rv). Two months after infection, mice were euthanized and the bacteria extracted from their lungs. We characterized the genomic composite of the bacteria before and after exposure to vaccinated hosts, and also characterized the local response to the bacteria by sequencing the lung transcriptome in animals during the infection. Results from this study show that the interaction within the lungs of the vaccinated hosts results in the selection of higher-virulence bacteria, specifically for the Beijing genotype strains 46 and 48. After exposure to the BCG-induced immune response, strains 46 and 48 acquire genomic mutations associated with several virulence factors. As a result, the bacteria collected from these vaccinated hosts have an increased ability for immune evasion, as shown in both the host transcriptome and the histopathology studies, and replicates far more efficiently compared to bacteria collected from unvaccinated hosts or to the original-stock strain. Further research is warranted to ascertain the pathways associated with the genomic alterations. However, our results highlight novel host-pathogen interactions induced by exposure of MTB to BCG vaccinated hosts.
Highlights
In 1921 the Bacillus Calmette-Guérin (BCG) vaccine, constituting of viable but attenuated Mycobacterium bovis bacteria, was introduced as the first and so far only tuberculosis (TB) preventive vaccine approved by the World Health Organization (WHO) [1]
Survival Rate In order to test whether immunization exerted a change in the virulence in Mycobacterium tuberculosis (MTB) bacilli exposed to vaccinated mice, groups of 50 naïve BALB/c mice were intratracheally infected with each of the following bacteria: original stock strain (n = 50), “P” strain (n = 50) isolated from the lungs of BCG Phipps vaccinated mice, “V” strain (n = 50) isolated from the lungs of BCG Vietnam vaccinated mice and control “S” strain (n = 50) isolated from the lungs of sham-vaccinated, control mice
Lungs from animals infected with EuroAmerican genotype laboratory strain 1 (H37Rv) and vaccineexposed strains 1P, 1V, and 1S showed no significant difference in terms of bacillary load on days 1, 3, 7, 21, 28, 60, and 120 postinfection (Supplementary Figure 1B), we observed a four times lower bacillary load in lungs collected from animals infected by strain 1 (4.5 × 106 Colony Forming Units (CFUs)/mL lung homogenate) compared with those infected by strain 1V (17.4 × 106 CFU/mL lung homogenate) on day 14 post-infection (p < 0.01)
Summary
In 1921 the Bacillus Calmette-Guérin (BCG) vaccine, constituting of viable but attenuated Mycobacterium bovis bacteria, was introduced as the first and so far only tuberculosis (TB) preventive vaccine approved by the World Health Organization (WHO) [1]. The vaccine presents considerable shortcomings in terms of preventing pulmonary tuberculosis, with a considerable variability in efficacy, ranging from 0 to 75% in different regions of the world [3, 4]. Hypotheses to explain this remarkable heterogeneity in efficacy include flaws in the design of longterm studies, genomic differences between the BCG daughter strains, and that BCG protection may differ by Mycobacterium tuberculosis (MTB) genotype [5,6,7]. There would be an ongoing selection of MTB genotypes with an increased ability to circumvent BCG-induced immunity, in high prevalence countries with high vaccination coverage [5]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
