Researchers at the Crick have shown how the bacteria causing tuberculosis (TB) directly cross the brain’s protective barrier, causing meningitis, a very severe form of the disease that occurs in 1-10% of cases. The team also investigated how HIV co-infection impacts TB bacteria entering and infecting cells in the brain.

Tuberculosis (TB) is an infectious disease caused by a bacteria called Mycobacterium tuberculosis which mainly affects the lungs. In a subset of people with TB, the bacteria can infect the brain and spinal cord linings, known as meningitis, which is hard to treat and deadly in many cases.  

People who are HIV positive are more likely to develop a severe form of TB, such as meningitis. Understanding co-infection is critical given HIV and TB are often prevalent in the same areas of the world. 

In a new study published in the Journal of Neuroinflammation, researchers at the Crick aimed to explore how TB bacteria cross the thin membrane of the blood-brain barrier by seeing how the bacteria interact with brain cells in the lab. 

Infecting brain cells in a dish

The researchers first introduced TB bacteria to different types of brain cells separately, including astrocytes, pericytes, microglia and endothelial cells, finding that the bacteria effectively entered and grew in each cell type. 

When cells were incubated with HIV before TB exposure, the researchers observed increased entry of TB bacteria into astrocytes, pericytes and microglia, but not endothelial cells. 

After two days, TB bacteria successfully grew in all cell types and HIV co-infection increased growth in both pericytes and endothelial cells. 

Interestingly, they also observed that TB bacteria most commonly infected cells that weren’t directly infected by HIV but had been exposed to the virus, suggesting that HIV might increase entry through a bystander effect. 

Replicating the blood-brain barrier

The team then created a 3D replica of the blood-brain barrier, by mixing pericytes and astrocytes on each side of a porous membrane. Immune cells called microglia were also added to the bottom of the dish to mimic the ‘brain side’. 

By measuring how well molecules crossed the barrier with and without TB infection, the team confirmed that TB bacteria increased the permeability or ‘leakiness’ of the barrier after just two days of infection and even more so after six days. However, co-infection with HIV did not have an impact on the leakiness of the barrier.  

The impact of TB on the brain and its barrier 

By monitoring a single layer of astrocytes, pericytes and endothelial cells infected with TB for 80 hours, the scientists observed that all the cell types in the layer failed to maintain their integrity. This shows that TB bacteria weaken the barrier between the blood and the brain. 

TB infection also led to a build-up of a chemical messenger called glutamate and produced reactive oxygen species, a sign of cell damage. The bacteria also caused astrocytes to become toxic to nerve cells and produced inflammatory molecules. 

The researchers concluded that TB bacteria can enter the brain directly by damaging the integrity of the blood-brain barrier. This offers an alternative mechanism to the hypothesis that that the bacteria hijack immune cells to cross the barrier.

Although HIV co-infection didn’t make the barrier more leaky, the research team suggest it may help TB bacteria to cross by facilitating them to enter, grow and weaken the brain cells at the border. 

Alize Proust, Senior Laboratory Research Scientist, previously in the Tuberculosis Laboratory and now in the Worldwide Influenza Centre at the Crick, said:

“Meningitis is a not uncommon and a serious outcome from a TB infection, so it’s critical to understand how TB bacteria enter the brain. For treatment, we either need to stop TB bacteria entering the brain or develop drugs that can cross the blood-brain barrier, giving the right amount to avoid side effects. We’ve shown that TB bacteria do enter all brain cells and that it disrupts the leakiness of the barrier – maybe we can target this weakness by administering drugs to cross the barrier while it’s more open.”

Robert Wilkinson, Principal Group Leader of the Tuberculosis Laboratory at the Crick, said:

“People who are HIV positive are at higher risk of developing more severe TB, and our paper helps to unravel why. Through a bystander effect, HIV can impact all the cells in the brain even if they are not directly infected, helping TB bacteria to enter and grow. The question now is how to treat people with co-infection, as this interaction between the two diseases is happening in the brain, a part of the body that is notoriously hard to treat.”