Why Smokers Are More Susceptible to Tuberculosis

What does a see-through fish with a genetic mutation have in common with a smoker? If this sounds like the start of a very bad joke, just wait until you hear the punchline: tuberculosis.

We've long understood that smokers are at a higher risk of contracting tuberculosis—up to three times more likely than non-smokers. But exactly why that is has been a medical mystery. Researchers recently stumbled on a partial answer to this question while studying translucent, genetically-mutated zebrafish to watch how they contracted the "fish version" of TB. As it turns out, one genetic mutation caused a similar effect as smoking by clogging up and weighing down the cells that are the body's first line of defense against bacteria like the kind that causes tuberculosis—so much so that the cells can't even move, according to the new study published Thursday in Cell.

And the study also raises the possibility that there's an undetected genetic mutation hiding throughout the population that would explain why non-smokers can get TB too.

"We're not saying it's the full explanation, but it's a really cool explanation and a really simple one," said Lalita Ramakrishnan, a professor in the Department of Medicine at the University of Cambridge, who led the research.

The researchers weren't even trying to solve this particular puzzle about smokers and TB when they set out, Ramakrishnan explained. Their goal was really to develop a better understanding of the pathogenesis of TB: how exactly it develops in the body. To do this, they used an approach called forward genetics where, rather than specifically altering a gene and observing what happens, they would introduce a different, random genetic mutation to hundreds of different zebrafish, expose them to TB, and then investigate why the ones that got sick, got sick.

Zebrafish, Ramakrishnan explained, are particularly handy for this kind of research because they're translucent for the first few weeks of life, making it easy to watch what's going on inside of them. You can also inject color into different cells—like the TB bacteria—to make it even easier to observe what's happening in the fish. It's a technique that's been used to provide a window (literally) into lots of diseases, such as meningitis.

"So not only do you have visualization, but you have visualization in technicolor," Ramakrishnan said.

In this case, the researchers observed one mutation of zebrafish that was more susceptible to TB, so they started investigating what kind of mutation it had and why it might be causing this increased susceptibility. They found that the fish's macrophages—cells that "eat" and break down the body's garbage, like dead cells and invasive bacteria—were missing a particular enzyme that allow them to break down the matter they consume. Without this enzyme, the macrophages were just consuming matter, getting fat, and unable to move.

When the TB bacteria entered the system, the few new macrophages that hadn't eaten anything yet would come attack it, then carry it deeper into the body where the infected cells would form a mass called a tubercle. Without their buddies able to come in and help them fight off the bacteria, these infected macrophages eventually burst open and infection can set in.

"If the macrophage can't move, it can't get to the bacteria, and if it can't get to the bacteria, it obviously cannot kill them," Ramakrishnan said.

So what does all this have to do with smokers? The researchers took their findings to Joe Keane, a TB and lung disease researcher at Trinity College Dublin who had been observing macrophages in smokers, former smokers, and non-smokers. When the team compared the mutated zebrafish macrophages with the ones in the smokers' lungs, they saw they had the same problem: they were bloated, fat, and immobile. But unlike the zebrafish, this wasn't due to a genetic deficiency that caused the macrophages to be missing an important enzyme.

"In the smokers, there was no enzymatic deficiency. It's just that these were inorganic materials that our macrophages don't have ability to degrade," Ramakrishnan explained.

The good news? The macrophages in former smokers who had quit didn't have this problem, and looked almost identical to the ones in non-smokers' lungs, meaning smokers can greatly reduce their risk of TB by quitting.

And there's one last interesting finding that could help develop better treatments or even preventative medicine for TB. Genetic enzymatic deficiencies—where the enzymes in macrophages are missing—happen in humans, such as Gaucher disease, but they're pretty rare and severe. But Ramakrishnan explained that these deficiencies are different than what they found in the zebrafish because they usually cause all of the enzymes to be low, whereas the zebrafish mutation meant only one kind of enzyme—lysosome—was low.

This kind of mutation would be less obvious and not as easily detected, and could explain why some people (even non smokers) get sick when they encounter TB, while others are able to fend it off naturally. It's just a theory for now, but one that Ramakrishnan said she is excited to explore.

"There could be this hidden group of people with a very slight lysosomal deficiency. If we can figure out what it is that makes these people more susceptible, then we can come up with pharmacological agents to help treat or prevent tuberculosis," Ramakrishnan said. "But we never would have figured it out if we hadn't let the fish tell us what was going on."