New, Safer, TB Vaccine with BCG “Kill Switch” Protects Mice and Monkeys

Researchers headed by a team at Pitt University, in collaboration with scientists at Cornell University, have developed a self-destructing, intravenous (i.v.) Bacillus Calmette-Guérin (BCG) vaccine that in preclinical tests provided additional safety and protection against tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) in macaque monkeys.

The in-built safety mechanism—a BCG “kill switch”—in the live, attenuated Mycobacterium bovis strain circumvents the possibility of accidental self-infection with the weakened mycobacteria, which the developers say offers a safe and effective way to combat TB.

“Although the idea of intravenous vaccination with a live vaccine may sound scary, it was very effective in our previous studies in non-human primates,” stated JoAnne Flynn, PhD, distinguished professor and chair of microbiology and molecular genetics at Pitt. “Here, we focused on the safety aspect of IV vaccination and used a strain of mycobacteria that kills itself once administered to the animals. To our surprise, it was equal or slightly better than the regular TB vaccine in protecting monkeys against infection, providing sterilizing immunity in almost all animals.”

Flynn is senior author of the team’s published paper in Nature Microbiology, titled, “A BCG kill switch strain protects against Mycobacterium tuberculosis in mice and non-human primates with improved safety and immunogenicity.” In their paper, the researchers concluded: “Using a self-killing BCG strain may increase the safety of i.v. BCG vaccination strategies while maintaining remarkable protective efficacy.”

A 2024 World Health Organization report suggested that TB represents the world’s leading infectious disease killer, but despite the ongoing global public health burden of the disease, safe and effective protection strategies against the infection are lacking, the authors suggested. BCG vaccine—named for its developers—contains inactivated mycobacteria that infect cattle and remains the only vaccination strategy against the infection in humans. Injected into the skin, it provides only partial protection against TB in young children and no protection in adults.

“BCG has probably been administered to more humans than any other vaccine and is generally safe,” the team wrote. “BCG protects children against disseminated TB and TB meningitis but is only partially protective against pulmonary TB.” And while rare, complications from BCG vaccination is “… a common cause of death in immunocompromised children.”

To develop a more universally effective vaccination approach, Flynn partnered with colleagues at Cornell University. Flynn’s earlier work with collaborators at the National Institutes of Health had demonstrated a 100,000-fold reduction in bacterial burden in the lungs of non-human primate (NHP) macaque monkeys given the BCG vaccine intravenously, compared with the standard intradermal route. In that study, nine out of 10 animals showed no inflammation in their lungs. “Typically, BCG is delivered by intradermal injection, but studies in non-human primates (NHPs) revealed that high-dose intravenous (i.v.) administration or endobronchial instillation result in excellent protection against M. tuberculosis (Mtb),” the team continued.

To improve the safety of iv. BCG delivery, in the newly reported study the researchers engineered two built-in mechanisms that instruct the BCG particles to lyse either upon exposure to the antibiotic doxycycline, or when chronic doxycycline treatment is stopped. “We sought to construct a BCG strain that can be killed by addition or removal of a small molecule, such as a tetracycline, and would be safer than BCG so that it could be used with non-conventional delivery approaches and increased dosage,” they wrote.

Mouse experiments showed that the BCG vaccine containing this dual safety switch protected the animals against TB comparable to a standard (wild type; WT) BCG vaccination, but had the added benefit of faster elimination and safety, even for mice that were immunocompromised. “Modified BCG elicited similar immune responses and provided similar protection against Mtb challenge as wild-type BCG in mice,” the investigators wrote.

In macaque monkeys, the updated self-destructing BCG vaccine caused an even stronger immune response and better protection against TB than a standard intravenously administered BCG injection. None of the monkeys that received the modified BCG vaccine had any detectable level of lung inflammation eight weeks after being infected with live Mycobacterium tuberculosis. In addition, six out of eight monkeys had no traces of recoverable live M. tuberculosis compared to two out of eight monkeys that received the standard BCG intravenously. “Our data indicate that this ‘kill-switch’ BCG strain induces greater CD4 T-cell responses in lungs and may provide even more robust protection than WT BCG in macaques,” the investigators further noted. Sterilizing immunity occurred in 6 of 8 macaques compared with 2 of 8 wild-type BCG-immunized macaques. Thus, a ‘kill-switch’ BCG strain provides additional safety and robust protection against Mtb infection.”

Flynn added, “The live-attenuated form of the mycobacteria does not need to be alive for very long to provide outstanding protection and with this strain, there is essentially no chance for a vaccine-derived infection, even in an immunocompromised host.”

Despite the additional challenges of clinical testing required for expanding the use of the updated BCG vaccine in humans, researchers are optimistic.

“We hope that this ‘kill switch’ BCG strain could limit safety concerns over intravenous vaccine administration and provide an option for a safer and more effective vaccination route for individuals who are immunocompromised,” Flynn said.