Tuberculosis microbes invading human immune cells carry a cargo that increases TB virulence by inducing the cells to act less like sentinels and more like bystanders, tests in mice show. In a report in the June 11 Nature, a team hypothesizes that this initial infection strategy lays the groundwork for TB’s uncanny ability to lie dormant in an infected person for years.

Even though TB has been studied for hundreds of years, it still guards many secrets — including precisely how it undercuts immune cells.

“Understanding the mechanisms by which the bacteria are having their way with our host cells will be very helpful in coming up with targets that we might hit,” says Kathleen McDonough, a microbiologist at the State University of New York at Albany and the Wadsworth Center of the New York State Department of Health, also in Albany.

Mycobacterium tuberculosis gets engulfed by cells called macrophages, the shock troops of the immune system. Scientists knew that the bacteria had evolved mechanisms to proliferate in these cells, but exactly how was unclear, says coauthor William Bishai, an infectious disease physician at Johns Hopkins University in Baltimore.

Bishai and his colleagues had noted that both TB bacteria and the macrophages they had entered had excess amounts of cyclic AMP, a compound that serves as a master regulator of many cell functions. So the researchers investigated an enzyme needed to make cyclic AMP. Curiously, M. tuberculosis has 17 genes that code for versions of this enzyme.

To test the effects of these 17 genes, the researchers infected mice with TB using aerosol sprays containing various combinations of the genes. Microbes harboring the enzyme encoded by a gene called Rv0386 out-competed the other microbes for survival in the mice and caused more severe lung disease. That suggests that the gene, along with the excess cyclic AMP production it induces, helps the microbe to sabotage the macrophage’s defensive abilities, thus making the TB more virulent.

In lab tests on human macrophages, the researchers showed that M. tuberculosis gins up cyclic AMP production upon entering a macrophage and unleashes this cargo once inside. That sets off a chain reaction that steers the cells to overproduce TNF alpha, a protein that causes inflammation, which contributes to tissue damage. Mice infected with real TB made 10 times as much TNF alpha in their lungs as did mice infected with TB lacking the Rv0386 gene.

This study is the first to pin down that M. tuberculosis is actively trafficking cyclic AMP into the cells and that “this really does affect the outcome of infections,” says McDonough.This reliance on cyclic AMP and the enzyme that makes it could prove useful to drug makers in the long run, she says.

Meanwhile, excess inflammation spurred by TNF alpha may be linked to the formation of lesions called granulomas, a hallmark of TB, Bishai says. TB granulomas have a central core of dead tissue surrounded by fatty debris and an outer rim of fibrous tissue and immune cells. Granuloma formation corrals the microbe but also enables it to lie latent in a person for decades. “We hypothesize that TB has a program that leads to granuloma formation, which gives the organism sanctuary,” Bishai says.

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