Smoking increases an individual’s risk of developing tuberculosis (TB) – and makes the infection worse – because it causes vital immune cells to become clogged up, slowing their movement and impeding their ability to fight infection, according to new research published in the journal Cell.
TB is an infectious disease caused by Mycobacterium tuberculosis that primarily infects the lungs, but can also infect other organs. It is transmitted from person to person through the air. The disease can cause breathlessness, wasting, and eventual death. While treatments do exist, the drug regimen is one of the longest for any curable disease: a patient will typically need to take medication for six months.
For people exposed to TB, the biggest risk factor for infection is exposure to smoke, including active and passive cigarette smoking and smoke from burning fuels. This risk is even greater than co-infection with HIV. However, until now it has not been clear why smoke should increase this risk.
When TB enters the body, the first line of defence it encounters is a specialist immune cell known as a macrophage (Greek for ‘big eater’). This cell engulfs the bacterium and tries to break it down. In many cases, the macrophage is successful and kills the bacterium, preventing TB infection, but in some cases TB manages not just to avoid destruction, but to use macrophages as ‘taxi cabs’ and get deep into the host, spreading the infection. TB’s next step is to cause infected macrophages to form tightly-organised clusters known as tubercles, or granulomas. Once again here, the macrophages and bacteria fight a battle – if the macrophages lose, the bacteria use their advantage to spread from cell to cell within this structure.
Macrophage image. Left: normal macrophages (green) Right: dysfunctional macrophages whose lysosomes (red) are clogged with cell debris
Image Credit: Steven Levitte/Lalita Ramakrishnan