Respiratory Bacteria “Talk” Immune System into Tolerating Infection

Researchers at the University of Queensland (UQ) have identified how a common bacterium is able to manipulate the human immune system during respiratory infections and cause persistent illness. The research, led by professor Ulrike Kappler, PhD, from UQ’s School of Chemical and Molecular Biosciences, studied the virulence mechanisms of Haemophilus influenzae, a bacterium that plays a significant role in worsening respiratory tract infections. The team’s in vitro studies in cultured human respiratory epithelia showed how infections with H. influenzae can induce tolerance and dampen immune responses.

Kappler said the bacterium had a unique ability to “talk” to and deactivate the immune system, convincing it there was no threat. “These bacteria are especially damaging to vulnerable groups, such as those with cystic fibrosis, asthma, the elderly, and Indigenous communities,” Kappler said. “In some conditions, such as asthma and chronic obstructive pulmonary disease, they can drastically worsen symptoms. Our research shows the bacterium persists by essentially turning off the body’s immune responses, inducing a state of tolerance in human respiratory tissues.”

The researchers reported on their studies, and results, in PLOS Pathogens, in a paper titled “Tolerance to Haemophilus influenzae infection in human epithelial cells: Insights from a primary cell-based model.”

Respiratory tract infections are highly debilitating, and Haemophilus influenzae is a bacterial pathogen that is associated with persistent acute and chronic respiratory tract infections, particularly among vulnerable individuals, the authors explained. “Haemophilus influenzae is a human-adapted pathobiont that inhabits the nasopharynx as a commensal but causes disease in other parts of the respiratory tract.” What are classed as nontypeable strains of H. influenzae (NTHi) are the most common type of clinical isolate, the team continued. In addition to causing acute diseases such as pneumonia, these strains are a major cause of exacerbations of chronic lung diseases, including in patients recovering from COVID-19. Interactions between the bacteria and the respiratory epithelia represent a key factor in NTHi virulence,” the team continued. “Despite this, insights into the molecular interactions that allow NTHi to persist in contact with human epithelia are lacking, but likely hold the key to uncovering both bacterial and host processes that are crucial for infection.”

For their newly reported studies the team generated primary normal human nasal epithelia (NHNE), derived from cells from five healthy donors, and monitored the effects on tissue gene expression of NTHi infection. They first prepared the human nasal tissue in the lab, growing it to resemble the surfaces of the human respiratory tract. They then monitored post infection (p.i.) gene expression changes over a 14-day period of “infection” with the NTHi. “Persistent infections rely on close molecular interactions between the human respiratory cells and the bacterial pathogen,” the team noted, “… and here we have investigated changes in host and bacterial cells during persistent, long-term infections with H. influenzae.”

Their found very limited production of inflammation molecules over time, which normally would be produced within hours of bacteria infecting human cells. “We then applied both live and dead Haemophilus influenzae, showing the dead bacteria caused a fast production of the inflammation makers, while live bacteria prevented this,” professor Kappler said.  “This proved that the bacteria can actively reduce the human immune response.”

In their paper the authors wrote, “… Physiological assays combined with dualRNAseq revealed that NHNE from five healthy donors all responded to H. influenzae infection with an initial, ‘unproductive’ inflammatory response that included a strong hypoxia signature but did not produce pro-inflammatory cytokines. Subsequently, an apparent tolerance to large extracellular and intraepithelial burdens of H. influenzae developed, with NHNE transcriptional profiles resembling the pre-infection state.”

This is the first time that large-scale, persistence-promoting immunomodulatory effects of H. influenzae during infection have been observed, they stated. “In addition to providing first molecular insights into mechanisms enabling persistence of H. influenzae in the host, our data further indicate the presence of infection stage-specific gene expression modules, highlighting fundamental similarities between immune responses in NHNE and canonical immune cells, which merit further investigation.”

Co-author and pediatric respiratory physician emeritus professor Peter Sly, MD, at UQ’s Faculty of Medicine, said the results show how Haemophilus influenzae can cause chronic infections, essentially living in the cells that form the surface of the respiratory tract.

“This is a rare behavior that many other bacteria don’t possess,” professor Sly said.
“If local immunity drops, for example during a viral infection, the bacteria may be able to ‘take over’ and cause a more severe infection.” The findings will lead to future work towards new treatments to prevent these infections by helping the immune system to recognize and kill these bacteria. “We’ll look at ways of developing treatments that enhance the immune system’s ability to detect and eliminate the pathogen before it can cause further damage,” Kappler added.

In their paper the authors concluded, “… our data provide first evidence that NTHi infections can delay strong inflammatory responses in human epithelia and induce an apparent tolerance of NTHi infection that had not been previously observed, but could be a driver of NTHi persistence in the human respiratory tract. This state of ‘peaceful’ coexistence of NHNE and NTHi required infection with live NTHi, which indicates an active immunomodulatory role for NTHi.”

They pointed out that further research is needed to investigate whether bacterial effector proteins or metabolites are involved in triggering NHNE tolerance of NTHi infection, and what mechanisms within human epithelia cause differences in tolerance of NTHi infections.