The role of a toxin-antitoxin system in tuberculosis

Shalini Roy Choudhury

The tuberculosis bacterium is notorious for its ability to stay dormant for years within the human body, evading the immune system and always one step away from causing aggressive infection. Now, a study from researchers at Translational Health Science and Technology Institute (THSTI), Faridabad, and Institute of Microbial Technology (IMTech), Chandigarh, has investigated a novel molecular pathway that helps the bacteria avoid being noticed by the immune system.

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Transmission Electron Micrograph of Mycobacterium tuberculosis, the causative agent of tuberculosis. Obtained from the CDC Public Health Image Library.

Tuberculosis, caused by the bacteria Mycobacterium tuberculosis, is a leading health hazard responsible for a large number of deaths every year (1.4 million in 2019 alone). These sly bacteria can reside inside their hosts for years before causing any disease symptoms. Now, a team of researchers provide new insights into the role of VapBC22, a toxin-antitoxin system, in helping the bacteria evade the host immune response and establish infection.

Whether the bacteria remain dormant or establish infection in their host, the process is facilitated by a pair of linked genes called a toxin-antitoxin system. One of these genes produces a toxin’ that can disrupt bacterial cell growth and function under unfavourable conditions, while the other produces an antitoxin’ which can neutralize the toxin. These systems are present in all known bacteria and their functions are not yet completely understood. At least 90 such toxin-antitoxin systems are known so far in the tuberculosis bacteria.

The present study was led by Ramandeep Singh, Professor, Translational Health Science and Technology Institute (THSTI), Faridabad, and Krishan Gopal Thakur, senior scientist at Institute of Microbial Technology (IMTech), Chandigarh. The researchers used a multitude of approaches to probe the role of one such toxin-antitoxin system — VapBC22 — in bacterial pathogenesis. 

In the VapBC22 (Virulence-associated protein B and C 22) system, VapC22 is the toxin and VapB22 is the antitoxin. An earlier study indicated that VapB22 was essential for the growth of tuberculosis bacteria, which got Singh and his team interested in exploring its function. 

The researchers used genetic techniques to artificially change the levels of the toxin inside the bacteria and measured the bacteria’s growth rate. When the toxin levels were high, the bacteria did not grow. When the toxin was absent, the bacteria grew faster than usual. Bacteria which lacked the toxin also expressed high levels of the antitoxin and low levels of proteins that are essential for establishing infection, called virulence proteins. 

When mice or guinea pigs were infected with the mutant bacteria that lacked the VapC22 toxin, bacterial growth inside the host was attenuated. These bacteria were vulnerable to oxidative stress inside their host and their capacity to cause disease was reduced. The lungs and spleens of these animals, which are the main targets of tuberculosis infection, showed minimal symptoms of the disease. 

To understand how the host contributes to this state of reduced infection, the researchers examined gene expression in the lungs of mice infected with the mutant bacteria. To the best of our knowledge, this is the first study where we show the role of any toxin-antitoxin system in modulating host immune responses,” says Singh. They observed that the bacteria had a profound impact on the host immune system, stimulating the expression of an assortment of proteins which protects the host from infection and removes the bacteria from their system. These bacteria essentially die by a form of assisted suicide, by provoking a strong host immune response. Meanwhile, bacteria with normal levels of the toxin-antitoxin resist and evade the immune response and discreetly proliferate. 

The study suggests that a relatively balanced expression of the VapBC22 toxin-antitoxin system is essential for the tuberculosis bacteria to adapt to the environment inside their host, and to sneakily proliferate without being spotted by the host immune system. This is necessary for the bacteria to subsequently establish a successful tuberculosis infection.

Based on their observations, the researchers view VapBC22 as an important drug target for tuberculosis treatment. Our next objective is to design new small molecule inhibitors and test if they can be used as drugs in the future,” says Thakur. He hypothesizes that drugs against the highly conserved VapC22 toxin could also treat drug-resistant tuberculosis.

Amit Singh, Associate Professor at the Indian Institute of Science, Bengaluru, who was not associated with this research, says that despite there being multiple toxin-antitoxin systems, the dependency of tuberculosis bacteria on this single system for gene expression, growth, resistance to stress, and survival is quite intriguing. He adds that it would be helpful to know the specific roles of VapBC22 and any overlapping functions with other toxin-antitoxin systems. 

Singh’s team is presently exploring the molecular mechanisms behind their observations as part of a DBT/​Wellcome Trust India Alliance Senior Fellowship. According to Singh, this study has opened a new avenue to explore the role of toxin-antitoxin systems in host immune response, bacterial pathogenesis, and the regulation of other virulence proteins.