A quarter of the global population is estimated to have been infected with Mycobacterium tuberculosis, yet only 5-10% of those infected go on to develop active tuberculosis (TB).

“The big question has always been what distinguishes people who control the infection from those who don’t,” says Anne O’Garra, whose lab at the Crick studies how the immune system responds to infection.

For Anne and her team, understanding the tipping point toward active disease has been the subject of long-running investigations. In 2010, they helped change how researchers thought about TB. Their study, published in Nature, found a previously unexpected immune signature in the blood of people with active TB: showing that active disease was not simply marked by a weak immune response, but by a particular kind of inflammatory response.

“That was a real and unexpected shift,” recalls Anne. “We showed that TB was associated with this early immune alarm signal called type I interferon which was then well known for its anti-viral function.”

The team also identified a blood transcriptional signature that distinguished active TB from latent infection and other inflammatory or infectious diseases.

But blood can only tell part of the story. TB begins in the lung, and the earliest immune decisions are likely to be made at the site where the bacteria first take hold.

To see what was happening in the lung itself and gain an early snapshot of the body’s response to infection, they collaborated with clinicians at University Hospitals of Leicester NHS Trust, to analyse bronchoalveolar lavage (BAL) samples – fluid collected from the lower airways – from recent household contacts of people with pulmonary TB.

These samples were fast-tracked down the M1 by specialist couriers to the Crick labs where first author Will Branchett used bulk RNA sequencing, single-cell RNA sequencing, and flow cytometry, to build a detailed picture of which immune cells were present and what those cells were doing.

What he and the team found was two very different immune responses to infection.

In their new study, published in Nature Immunology, the researchers observed that in people who developed active TB, the airways were dominated by immune cells called neutrophils. When they tracked which genes were active in the neutrophils, they found that half these immune cells had switched on a set of genes linked to type I interferon signalling. All neutrophils appeared to make high levels of CXCL8, a molecule that can bring more neutrophils into the lung, revealing potential for a vicious cycle of inflammation.

Single cell RNA sequencing data from bronchoalveolar lavage (BAL) samples, taken from household contacts of people with pulmonary TB or from active pulmonary TB patients. Plots on the right show relative increases in these cell types in non-progressor contacts (left), contacts progressing to TB (centre) and patients with established active TB (right). Increased neutrophils were observed in the airways with progression to TB. – William Branchett

The team then examined this early response to disease in more detail and found that T-cells in the lungs of patients with high numbers of neutrophils appeared to be under strain, preventing them from controlling the infection.

“We saw evidence of exhaustion and cell death in the T cells,” explains Will. “They look like they’ve been pushed too hard.”

The balance was markedly different in people who controlled the infection.

“There, the T cells weren’t highly activated, but they weren’t exhausted either,” adds Will. “Instead, these cells expressed genes linked to regulation and a more ‘stem-like’ state, suggesting they can persist and respond over a longer period and thus remain ready to fight the infection.”

The team identified the same patterns in data which they analysed from published studies of non-human primates with TB, as well as previous studies from Anne’s lab involving mice, where similar immune cell responses were linked to either protection or active disease.

Together, the results suggest that the balance of inflammatory neutrophils and T cells in the airways may be a key determinant of whether an infected individual progresses to TB or remains healthy.

“It’s about getting the balance right,” concludes Will. “The immune system must control the bacterial infection without causing excessive inflammation and damage to the lungs. And that balance likely determines who stays well. In another recent study from the lab, we showed that type I interferon signalling drives neutrophil swarming in the lungs, limiting the interactions between T cells and macrophages that are needed to control TB.”

The team’s work points towards a way to address a critical gap in TB treatment.

“TB diagnosis is challenging since the bacteria can be hard to detect and current tests cannot determine whether an infected person is likely to become ill with TB,” explains Anne. “If we can find a way to identify these immune signatures early in people who have been infected, we might be able to predict who is at risk.”

“More targeted interventions might look like vaccines designed to steer the immune system towards a stem-like and durable state, or therapies that dampen harmful inflammation to favour protective T cell responses and control infection. For example, drugs like CXCR2 inhibitors that target CXCL8 function to limit neutrophil movement into and within tissues are already in trials for other lung conditions.”

Will now plans to set up his own independent research group to further these mechanistic studies of TB infection to identify host-directed therapies.  Anne will continue to identify and validate potential targets for new treatments and new biomarkers for detection of early TB and for stratification of patients for treatment.

Source – The Francis Crick Institute