Tuberculosis hides in bone marrow stem cells

The bacteria that cause tuberculosis (TB) evade immune cells and drug treatments by hiding in bone marrow stem cells, according to research.

In the first part of their
study, scientists isolated live bacteria from bone marrow stem cells of mice
with a 'latent' form of TB where the infection is not accompanied by disease symptoms.
Then, in the clinical part of the study, nine people who had been successfully
treated for TB and showed no signs of the disease were shown to have bacterial DNA in
their stem cells. Viable TB bacteria were recovered from two of these patients.

This hiding tactic, which the
researchers call the 'wolf in stem cell clothing', may help to explain why it is so
difficult to treat latent TB. Between five and ten percent of patients with latent TB
will go on to develop the active form of the disease.

Professor Dean Felsher of
Stanford University School of Medicine, a senior author of the study, said: 'Self-renewing
stem cells like these in the bone marrow have properties - such as natural drug
resistance, infrequent division and a privileged immune status - that make them
immune to many types of treatment. Now it turns out that this ancient organism,
Mycobacterium tuberculosis, figured out a long time ago that, for the same
reasons, these cells are ideal hosts to invade and in which to hide'.

Latent TB affects over two billion
people worldwide, although most will be unaware that they have been
infected. The active form of TB, which causes devastating lung disease, led to
an estimated 1.4 million deaths in 2011. Drugs effective against TB have been
available for 50 years but TB often recurs years after the initial treatment.

The President of the Australian
Medical Association Dr Steve Hambleton, who was not part of the study, told ABC
News that this was the first time that living TB had been found in patients
after six months of treatment. He said: 'It may actually help us in working out
why recurrences occur. We may actually be able to stop them recurring. It is a
huge global health problem'.

The stem cells most affected
were a sub-population known as mesenchymal stem cells, which are present in
bone marrow and can migrate to the lungs where TB thrives. Professor Felsher noted
that these cells had 'never been implicated as a host for
tuberculosis'.

Mesenchymal stem cells may yet become
a target for drug therapies against TB but any commercially available treatment
would likely be decades away.

The research was published in the journal Science
Translational Medicine.