January 20, 2020Infectious Disease
Tuberculosis (TB), an ancient disease, is the leading infectious cause of death globally, yet the world's only licensed TB vaccine, Bacille Calmette-Guerin (BCG), was developed a century ago. Given to infants via a needle placed just under the skin, BCG protects babies from a form of the disease called disseminated TB but is far less effective at preventing pulmonary TB, the major cause of illness and deaths, in teens or adults.
Now, according to a study published in Nature (2 January 2020), it has been shown that simply changing the dose and route of administration from intradermal (ID) to intravenous (IV) greatly increases the vaccine's ability to protect rhesus macaques from infection following exposure to "Mycobacterium tuberculosis (Mtb)", the bacterium that causes TB. The findings provide a new understanding of the mechanisms of BCG-elicited protection against tuberculosis infection and disease. In addition, the findings support investigation of IV BCG administration in clinical trials to determine whether this route improves its effectiveness in teens and adults.
To control "Mtb" infection and prevent clinical disease, a TB vaccine must elicit strong, sustained responses from the immune system's T cells, specifically those in the lungs. However, the standard, ID, route of BCG administration may not generate enough of these critical cells in the lungs. The authors therefore hypothesized that administration of BCG by IV or aerosol (AE) routes would overcome this hurdle and thus confer substantially better protection from infection and/or disease in rhesus macaques following challenge with virulent "Mtb".
For the study, groups of animals received the BGC vaccine by ID, AE or IV routes. The authors then assessed immune responses in blood and in fluid drawn from the lungs for a 24-week period following vaccination. Results showed that IV BCG vaccination had the highest durable levels of T cells in the blood and lungs. Six months after vaccination, the authors exposed groups of vaccinated rhesus macaques (immunized via ID, AE or IV routes) and a group of unvaccinated macaques to a virulent strain of Mtb by introducing the bacteria directly into the animals' lungs. They then tracked the infection and disease development over three months. Nine out of 10 animals vaccinated with IV BCG were highly protected; six showed no detectable infection in any tissue tested and three had only very low counts of "Mtb" bacteria in lung tissue. All unvaccinated animals and those immunized via ID or AE routes showed signs of significantly greater infection.
The authors concluded that IV BCG conferred an unprecedented degree of protection in an animal model of severe TB and "represents a major step forward in the field of TB vaccine research."