Researchers at the Francis Crick Institute have uncovered a new mechanism that macrophages, a specialised type of immune cell, use to eliminate the bacteria causing tuberculosis (TB). This offers a new potential target for therapies against bacterial infections such as TB.
For the first time, in a paper published today in the Journal of Cell Biology, researchers have shown how human macrophages use highly reactive chemicals called ‘reactive oxygen species’ (ROS) from peroxisomes to restrict the replication of TB bacteria.
Peroxisomes are a type of small structure inside the cell called an organelle. They are vital for cell metabolism by making and breaking down fatty acids, alongside generating ROS such as hydrogen peroxide (H2O2).
More recently, they have been found to help immune cells, such as macrophages, fight bacterial infections. In healthy conditions, cells can use ROS to regulate oxidative stress – a balance of these chemicals – in the cell. During infection, macrophages produce ROS when they engulf microbes in vesicles, or sacs, called phagosomes. These phagosomes help to break down and kill the bacteria and are a key first-line defence for the immune system against infection.
Mycobacterium tuberculosis (Mtb), the bacteria responsible for TB, infects around 10 million people worldwide each year and kills around 1.6 million of those infected. One reason for this high infection rate may be the bacteria’s ability to damage the macrophage’s phagosome system, avoiding destruction and remaining in the water-soluble part of the cell, called the cytosol, for long periods of time. Until now, this specific mechanism of how macrophages fight back against this was not known.
The researchers at the Crick used a combination of human stem-cell-derived macrophage (iPSDM) cells with fluorescent reporters to investigate the impact of peroxisomes on TB infection and ROS production, particularly hydrogen peroxide.
They showed that infection with TB bacteria causes an increase in the number of peroxisomes in the cytosol of human macrophages, and more peroxisomes had an altered shape.
When the researchers used gene editing technology (known as CRISPR/Cas9) to delete genes responsible for peroxisome production, they saw an increase in TB bacteria replication. This observation suggests that peroxisomes are crucial in restricting bacterial replication happening within the cell’s cytosol.
In the infected cells, the researchers noticed that an enzyme linked to hydrogen peroxide degradation was increased, showing that hydrogen peroxide must be involved in controlling infection. They tagged hydrogen peroxide with a fluorescent marker and saw that its levels increased in peroxisomes in cells infected with TB bacteria, but not in uninfected cells or in cells where bacteria couldn’t enter the cytosol.
These results suggest that human macrophages take advantage of hydrogen peroxide produced by peroxisomes to control the number of bacteria in the cytosol, even after these bacteria have been successful in evading the phagosome system.
The team then used drugs which boost peroxisome synthesis or increase hydrogen peroxide production in peroxisomes, which were found to restrict TB bacteria replication in the cytosol. This is independent of phagosome-associated pathways in macrophages, highlighting a back-up plan for cells to battle infection.
Enrica Pellegrino, first author and post-doctoral researcher at the Crick, said: “There’s not a lot of research done with mammalian cells, so we aimed to use both human macrophages and gene editing technology to understand in a human-relevant system how these cells can fight infection when bacteria outsmart the phagosome degradation pathways. Understanding these pathways may help us develop host-targeted therapies against infection, not only against TB but also other bacteria that can access the cytosol of infected cells.”
Max Gutierrez, Group Leader of the Host-Pathogen Interactions in Tuberculosis Laboratory at the Crick, said: “Our study brings another layer of control in the development of an immune response against intracellular bacteria and highlight that our immune cells have multiple mechanisms to restrict the spreading of infections.”
The next steps for the research will focus on investigating if this new mechanism applies to infections with other types of bacteria, with the hope that this opens up new potential avenues for antimicrobial therapies.