Certain stem cells protect tuberculosis (TB) bacteria from being destroyed, which explains why TB can lie dormant for years or even decades in the human body, researchers said on Tuesday.
The human immune system produces T-cells that can kill TB, but the body also deploys mesenchymal stem cells (MSC) to sites of TB infection, researchers said in a paper published in the Proceedings of the National Academy of Sciences on Tuesday.
"MSC are recruited to the sites of infection. What happens is these stem cells make a barrier ... between the T-cells that are supposed to kill the bacteria, and the bacteria," said Gobardhan Das, staff scientist at the International Center for Genetic Engineering and Biology in New Delhi.
MSC are master cells produced in the bone marrow that can develop into a variety of cells and tissues in the body, such as bone and cartilage.
Das and colleagues infected mice with TB and found MSC at all the sites where they also found TB, such as the lungs and spleen. They also extracted lymph node tissues from human TB patients where they found MSC.
"MSC produce nitric oxide that kill bacteria, but the amount is not sufficient to kill the TB bacteria, it just keeps them in check," Das said.
"Nitric oxide also inactivates T-cells. So the presence of MSC inhibits the proliferation of the bacteria and T-cells, and there is an equilibrium established. That's why the incubation is very very long and can maintain for an entire lifetime."
More than 2 billion people, or a third of the world's population, are infected with Mycobacterium tuberculosis, the bacteria that causes tuberculosis.
Nearly all TB infections are latent, with carriers showing no symptoms and they are not infectious. One in 10 will become sick with active TB in their lifetime, primarily as a result of a weakened immune system.
Das and colleagues appealed for more research to be aimed at fighting this function of MSC.
"MSC create a suppressive microenvironment... they are almost like a nest. If they (TB bacteria) don't have a nest, they will be exposed to our regular immune system and will be killed by our immune system," Das added.
"If you can target MSC, you can kill the nest."
TB killed 1.8 million people worldwide last year, up from 1.77 million in 2007.