Stem cells shield TB from onslaught of immune cells

By positioning themselves between tuberculosis bacteria and white blood cells, stem cells actually shield the invaders from the barrage of our bodies’ defenses, a new study shows.

TB kills about 2 million a year worldwide. About a third of the world carry the bacteria in their bodies, but only about 10% of those infected will develop the disease.

In these people, the TB bacterium (Mycobacterium tuberculosis) becomes surrounded by a variety of cells attempting to wall off the foreign threat by forming a tight sphere around them. These spherical compartments are called granulomas (pictured).

By hiding inside granulomas in infected lung tissues, the bacteria shirk off the immune system’s attacks. Unable to fully eradicate the bacteria, the immune system reaches a stalemate with the quarantined pathogens—resulting in chronic infection.

A team led by Gobardhan Das from the International Center for Genetic Engineering and Biotechnology in New Delhi decided to examine TB’s successful evasion techniques.

Using spleens and lungs from 40 female FoxP3 knock-in mice, they found that the bacteria recruit stem cells to infection sites.

In particular, mesenchymal stem cells (MSC), which are normally found in bone marrow and become important parts like cartilage, bone, muscles cells, and even nerve cells.

The stem cells fortify the granulomas, sheltering bacteria from the onslaught of immune cells. “What happens is these stem cells make a barrier,” Das says, “between the T-cells that are supposed to kill the bacteria, and the bacteria.”

Then the stem cells secrete nitric oxide, which actually suppresses the immune response and helps keep the Mycobacterium tuberculosis numbers at a sustainable level inside their granuloma fallout shelters.

“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,” Das says. “If you can target MSC, you can kill the nest.”

This protective layer of stem cells could be the next TB drug target.

The study appeared in Proceedings of the National Academy of Sciences yesterday.

The Global Fund to Fight AIDS, Tuberculosis and Malaria is one of the biggest sources of funding for TB control, and just today, 15 people in Mali were accused of embezzling $4 million in grant money meant to help the country battle TB.

Image: Granuloma by Mutleysmith via Wikimedia Commons

This post was originally published on Smartplanet.com