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Cracking the TB code
Studying how tuberculosis spreads and why patients cough
COVID-19 has drawn the world’s attention to the dangers of infectious diseases that can quickly kill thousands.
Dr. Michael Shiloh, Associate Professor of Internal Medicine and Microbiology in the Division of Infectious Diseases and Geographic Medicine, has spent nearly two decades studying just such a disease: tuberculosis, which kills about 1.5 million people a year, mostly in developing countries. Despite heightened focus on the current COVID-19 global pandemic, TB remains the No. 1 cause of death from an infectious disease.
Following a college internship at a pharmaceutical company, Dr. Shiloh decided to become a physician-scientist. He became aware of the extreme need to find better treatments and vaccines for TB, a widespread and often fatal disease, inspiring him to focus his research on the disease. As such, he has spent the majority of his career studying how the bacterium that causes tuberculosis, Mycobacterium tuberculosis, enters the body, establishes infection, and then spreads to infect other people.
“When I started out in science, it seemed like everyone was striving to make a big contribution to their fields, but most individual discoveries are small,” Dr. Shiloh says. “Tuberculosis is a serious global health issue, so I felt that even a small contribution to a massive problem would be meaningful.“
Recently, Dr. Shiloh and researchers in his lab have made two significant discoveries that could translate into new ways to protect people from the disease. The first finding is centered on how M. tuberculosis makes it past the body’s defenses in the lungs. The second relates to how the bacterium causes cough, a classic symptom of tuberculosis that spreads the disease.
“Tuberculosis is a serious global health issue, so I felt that even a small contribution to a massive problem would be meaningful.”
— Dr. Michael Shiloh
Hijacking immune defenses
Many bacteria harness the immune system’s own tools – the system designed to protect the body from illness – to make their way inside a host. For example, bacteria that cause infectious diarrhea, such as Salmonella and Shigella species, enter the body in the digestive tract through microfold cells, special types of cells that evolved as sentries to alert the immune system to potential dangers.
A type of microfold cell also exists in the lungs, which made Dr. Shiloh and his colleagues ask whether M. tuberculosis might be using these cells to gain entry into the body. In 2016, the team used cell culture and mouse models to show that M. tuberculosis indeed uses microfold cells in the lungs as a doorway to infection. Their findings were published in the journal Cell Reports.
In 2020, the team identified a receptor on the microfold cell surface used by M. tuberculosis to attach and facilitate entry into the cell. The investigators next partnered with Children’s Medical Center Dallas to obtain discarded adenoids from children who had these organs removed to treat sleep apnea. Using adenoids, they were able to show that a protein made by M. tuberculosis binds to receptors on the surface of human microfold cells, suggesting that this entry method can occur when a person inhales infectious bacteria. These results were published in eLife. Eventually, Dr. Shiloh says, researchers might be able to use these results to develop a vaccine that blocks M. tuberculosis from entering microfold cells.
A bacteria-neuron-cough connection
More recently, the team showed that M. tuberculosis facilitates spreading by producing a molecule that triggers cough, a finding published in Cell.
Coughing is a primary symptom of tuberculosis that enables the spread of disease from person to person; however, Dr. Shiloh explains, the cause of tuberculosis-related coughs had been unclear. The prevailing assumption was that lung irritation caused by the presence of the infection and its associated inflammation triggered coughing – but this idea had not been proved.
To test the idea, the team used guinea pigs, which have been used in tuberculosis research for more than a century. What was not clear was whether the disease causes the animal to cough. To answer this question, Dr. Shiloh and his colleagues placed guinea pigs infected with tuberculosis into special chambers that registered pressure and volume changes caused by coughing and found that tuberculosis-infected animals coughed significantly more than those that were tuberculosis-free.
To determine whether the bacteria themselves produce a substance that triggered coughing, the team prepared extracts from M. tuberculosis and tested whether different fractions made guinea pigs cough. They also asked whether the fractions stimulated nociceptive neurons, which are the cells responsible for stimulating coughing in the lungs.
“My long-term goal is to eradicate tuberculosis and put myself out of business…”
— Dr. Michael Shiloh
Honing in on SL-1
After a series of experiments, Dr. Shiloh’s team ultimately identified the mycobacterial cell-surface molecule known as sulfolipid-1 (SL-1) as the principal component that activated the cultured nociceptive neurons. This response also occurred in human nociceptive neurons, suggesting evolutionary conservation among different mammals. Importantly, when guinea pigs were exposed to purified SL-1, the animals coughed.
The researchers then used a mutant strain of M. tuberculosis that cannot produce SL-1 to show that this lipid was required to induce coughing during infection. When guinea pigs were infected with the mutant tuberculosis strain lacking SL-1, they developed all the typical symptoms of tuberculosis but did not cough, further supporting the conclusion that SL-1 is crucial for triggering coughing during infection.
Together, he says, “the findings suggest that tuberculosis-causing bacteria produce SL-1 primarily to stimulate a cough reflex that in turn facilitates spread of the organism and disease.”
The virus that causes COVID-19 – SARS-CoV-2 – shares similar features with tuberculosis, namely, being transmitted in part via cough and entering the body through the airway. As such, Dr. Shiloh is now adapting his research to study the mechanisms of cough and entry of SARS-CoV-2.
Ultimately, Dr. Shiloh believes, if research shows that suppressing cough is not harmful to infected individuals, scientists may be able develop a way to prevent disease transmission by administering a drug that counteracts SL-1 or prevents its production while simultaneously using antibiotics to treat the infection.
“My long-term goal is to eradicate tuberculosis and put myself out of business,” he says. “I could always find something else to work on.”
Dr. Shiloh holds the Professorship in Infectious Diseases, in honor of James P. Luby, M.D.