Tuberculosis (TB) is a very slow-progressing disease in humans, taking months for a person to develop symptoms after being infected with the bacteria Mycobacterium tuberculosis. For Max Gutierrez, who leads the Host-Pathogen Interactions in Tuberculosis Laboratory, this highlights an increasing need to understand what’s happening in the early stages, when TB bacteria first encounter the body’s defences in the air sacs in the lungs. 

The air sacs in the lungs are a critical first barrier against infections in humans, but we’ve traditionally looked at them in animals like mice,” says Max. “These studies are fundamental for our understanding, but animals and humans have differences in the makeup of immune cells and disease progression, sparking interest in alternative technologies.

One such alternative technology is an ‘organ-on-a-chip’, a recreated human organ system on a plastic chip that contains tiny channels and compartments. These mini organ simulations were highlighted as a promising new approach in the recent Government roadmap to phase out animal testing.  

‘Lung-on-chip’ systems already exist, but with many limitations. “Until now, lung-on-chip devices have been made of a mixture of patient-derived and commercially available cells,” says Max. “This means that they can’t fully recreate the lung function or disease progression of a single individual, as each type of cell is genetically different.” 

An air sac simulation  

To plug this gap, Max and Jakson Luk in the team developed a new lung-on-chip model that contains only genetically identical cells derived from a single human stem cell. Using a protocol developed by Max’s team, the alveolar cells are grown on a very thin membrane in a device manufactured by biotechnology company AlveoliX to recreate an air sac barrier. 

“We used human induced pluripotent stem cells, which can virtually become any cell in the body, to produce type I and II alveolar epithelial cells,” explains Jakson. “These are grown on the top of the membrane. Using the same stem cells, we also produced vascular endothelial cells that are grown on the bottom of the membrane.” Faithfully recreating an air sac barrier, both layers of cells developed on their own. “And the technology goes even further to simulate the human lung,” adds Max. “AlveoliX has designed specialised machines to impose rhythmic three-dimensional stretching forces on the recreated air sac barrier, mimicking the motion of breathing. This stimulates the formation of microvilli, a key feature of alveolar epithelial cells to increase surface area for lung functions.” 

The black-box period in TB 

The most exciting part for Jakson was that the new chip system enabled them to look at what he calls the ‘black-box’ period in TB, the time between infection and symptom onset.

“We wanted to look for hallmarks of disease that have been reported in patients from the clinic and animal studies,” he says.  

Jakson added immune cells called macrophages into the chip, the first responders to an infection, before exposing the chip to TB bacteria. In the chips infected by TB, he saw large macrophage clusters containing ‘necrotic cores’, a group of dead macrophages in the centre, surrounded by live macrophages.  

“Eventually, five days after infection, the endothelial and epithelial cell barriers collapsed, showing that the air sac function had broken down,” Jakson concluded.  

A future tool to test TB treatments 

The team are excited about the potential applications for the novel chip system.

“We could now build chips from people with particular genetic mutations to understand how infections like TB will impact them and test the effectiveness of treatments like antibiotics,” says Jakson.  

Jakson and Max made some headway in understanding how genetic differences impact the lungs’ response to infections.

“We removed the ATG14 gene, which is involved in a natural process for degrading damaged cells and foreign materials,” explains Jakson. “Macrophages lacking this gene were more susceptible to cell death in resting conditions, and tried to engulf more TB bacteria when infected, confirming the gene’s role in keeping our immune defences intact.” 

And beyond TB, a genetically identical lung model holds promise for a huge range of research into other respiratory infections or lung cancer.

“The chip supports the big push into personalised medicine,” considers Max. “It could help us understand the impact of genetics on whether a treatment is effective or not.” 

Jakson also received the 3Rs Crick Annual Award last year for developing the innovative lung-on-chip technology as a step towards the replacement of a subset of animal experiments.