Preventing reactivation is essential. But the location where the bacteria hide was not known

The hiding place of dormant tuberculosis bacteria in humans has been finally found. The bacteria are present in a nonreplicating, dormant but viable state, and can get reactivated to cause active TB in people who have been successively treated earlier. The remarkable study published today (January 31) in the Science Translational Medicine journal unequivocally demonstrates that a particular kind of bone marrow mesenchymal stem cells (CD271CD45) provide a safe shelter for the dormant TB bacteria.

Recurrence of TB after successful drug treatment remains a big challenge in controlling the disease. Recurrence could happen either due to reactivation of the dormant TB bacteria residing somewhere inside the body or through reinfection.

Several studies have shown that more than reinfection, it is the reactivation that causes disease recurrence. Hence management to prevent reactivation is essential. But the precise location where the bacteria hides was not known. But, no longer.

The first step adopted by Bikul Das from Departments of Medicine and Pathology, Stanford University, and the lead author of the paper, and others was to study bone marrow stem cells. There was a reason to target these cells — they provide a safe haven for the dormant TB bacteria to hide from the immune system; the bacteria are not exposed to TB drugs either. In fact, the stem cells have an efficient way of removing TB drugs that can kill the bacteria, thus providing the bacteria a perfect environment to remain viable for extended periods of time.

The germ of an idea to look for TB bacteria in bone marrow samples occurred to Dr. Das way back in the mid 1990s when he was doing biopsies of bone marrow samples. “In those biopsies I saw AFB [stained] positive bugs in bone marrow [samples], and occasionally inside progenitor cells. I was not sure if they were stem cells, as a biopsy cannot confirm those cells to be stem cells or progenitor cells,” noted Dr. Das in an email to this Correspondent (see box item). “An idea came to my mind that if TB bugs indeed reside inside bone marrow stem cells, it could explain why it is so difficult to treat TB.”

So they started off by studying different bone marrow stem cells from healthy humans to know if the bacteria could infect specific stem cells in vitro. They solved the first piece of the puzzle when they found that the bacteria could infect a few BM stem cells in vitro. Of them, the CD271/CD133 stem cells proved to be the “most permissive” for TB bacteria infection.

After infection, the bacteria multiplied just twofold to threefold within four days and then remained “unchanged.” Also the bacteria remained viable inside the cells for about two weeks. Differentiation of the stem cells was the only factor that could affect the long-term viability. The viability reduced fourfold when the stem cells differentiated to form specific cell types.

The stem cells contain both haematopoietic and mesenchymal stem cells. On further scrutiny, they found that the bacteria “preferentially infects” only the mesenchymal stem cells (CD271). Mesenchymal stem cells are capable of becoming any of the specialised cells in the body. They are essentially found in the bone marrow but can migrate to the lungs.

They then performed several experiments using mice. First, they demonstrated the ability of the TB bacteria to migrate to the mesenchymal stem cells (MSC) of mice once the animals were exposed to a low dose of aerosolised virulent TB bacteria. Four weeks after infection, the animals were killed and the presence of TB bacteria in MSC studied. They did find the bacteria in the bone marrow cells. By performing another kind of experiment, they found that TB bacteria “retained viability” in the mice mesenchymal stem cells.

Besides bone marrow mesenchymal stem cells, they found viable nonreplicating TB bacteria in lung mesenchymal stem cells too.

They progressed to the next stage of studying the infectious nature of the nonreplicating bacteria hiding in the stem cells. To do this, they retrieved the dormant bacteria from the lung mesenchymal stem cells of some mice and injected them into other healthy mice. The recipient mice did develop lung granulomas.

The final piece of the puzzle was solved by looking for the presence of dormant TB bacteria in humans who were successfully treated using anti-TB drugs. Nine individuals who had undergone complete treatment and a control group of six healthy people from non-endemic regions were chosen. While all the six healthy people in the control group did not “exhibit measurable” dormant TB bacteria, eight of the nine people who had successfully undergone treatment did have the bacteria in the CD271 BM mesenchymal stem cells.

“Our work is only a basic research and does not provide any therapy for patients with tuberculosis,” stated Dean W. Felsher of Stanford University in an email to this Correspondent. “We do hope that our work will provide potentially new scientific approaches that may lead to new treatments for TB.”

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