A new study led by researchers at Harvard Medical School (HMS) has detailed the step-by-step cascade that allows bacteria to break through the brain’s protective meninges layers and cause the dangerous, potentially fatal brain infection, meningitis.

The research, conducted in mice, demonstrated that bacteria exploit nerve cells in the meninges to suppress the immune response and allow the infection to spread into the brain. The experiments showed how bacteria activate pain receptors and initiate a signaling cascade that disables immune cells and culminates in bacterial invasion of the brain.

The work identified two central players—a chemical released by nerve cells and an immune cell receptor blocked by the chemical—in the molecular chain of events that leads to infection, and found that blocking either one can interrupt the cascade and thwart bacterial invasion. “We’ve identified a neuroimmune axis at the protective borders of the brain that is hijacked by bacteria to cause infection—a clever maneuver that ensures bacterial survival and leads to widespread disease,” said study senior author Isaac Chiu, PhD, associate professor of immunology in the Blavatnik Institute at Harvard Medical School.

The research provides much-needed understanding into a critical window during the early stages of bacterial invasion when intervention could potentially halt the spread of infection. If replicated through further research,

Researchers suggest new findings could lead to new meningitis therapies that target early stages of infection before bacteria can spread deep into the brain.

Chiu and colleagues reported on their results in Nature, in a paper titled “Bacteria hijack a meningeal neuroimmune axis to facilitate brain invasion.” In their paper they concluded, “Targeting this neuroimmune axis in the meninges can enhance host defences and potentially produce treatments for bacterial meningitis.”

The meninges consist of three membranes—dura, arachnoid, and pia—that protect the central nervous system (CNS) from injury and infection, the authors explained. However, pathogenic bacteria can invade the meninges and brain and cause severe, potentially life-threatening infections. U.S. Centers for Disease Control figures indicate that 1.2 million cases of bacterial meningitis occur globally each year. Untreated, the infection kills seven out of 10 people who contract it. Treatment can reduce mortality to three in 10, but among those who survive, one in five experience serious consequences, including hearing or vision loss, seizures, chronic headache, and other neurological problems. “Acute bacterial meningitis has mortality rates of up to 30%, and survivors often show post-infectious neurological sequelae,” the investigators stated.

The meninges are densely innervated by nociceptive sensory neurons that mediate pain and headache, the authors continued, but how pain and neuroimmune interactions impact meningeal antibacterial host defences is not clear. They noted, “A repertoire of innate and adaptive immune cells reside within the dural meninges that have roles in tissue repair, antigen sampling and host defence. However, the impact of neuronal signals on these meningeal immune cells in host defence has not been clearly defined.”

“The meninges are the final tissue barrier before pathogens enter the brain, so we have to focus our treatment efforts on what happens at this border tissue,” commented study first author Felipe Pinho-Ribeiro, PhD, a former post-doctoral researcher in the Chiu lab, now an assistant professor at Washington University in St. Louis.

Current therapies for meningitis include antibiotics that kill bacteria and steroids that tame infection-related inflammation. Even these treatments may still fail to ward off the worst consequences of the disease, particularly if therapy is initiated late due to delays in diagnosis. Inflammation-reducing steroids can also suppress immunity, weakening protection further and fueling infection spread. Thus, physicians must strike a precarious balance if they are to rein in brain-damaging inflammation with steroids, while also ensuring that these immunosuppressive drugs do not further disable the body’s defenses.

The need for new treatments is magnified by the lack of a universal meningitis vaccine. Many types of bacteria can cause meningitis, and designing a vaccine for all possible pathogens is impractical. “There is a need to better understand host responses to these bacterial pathogens and to define factors that protect the CNS against pathogen invasion,” the team continued.

Current vaccines are formulated to protect against only some of the more common bacteria known to cause meningitis. Vaccination is recommended only for certain populations deemed at high risk for bacterial meningitis. Additionally, vaccine protection wanes after several years.

Chiu and colleagues have long been interested in the interplay between bacteria and the nervous and immune systems and by how the crosstalk between nerve cells and immune cells may either precipitate or ward off disease. Previous research led by Chiu has shown that the interaction between neurons and immune cells plays a role in certain types of pneumonia and in flesh-destroying bacterial infections. For their newly reported research, Chiu and Pinho-Ribeiro turned their attention to meningitis—a condition in which they suspected the relationship between nervous and immune systems also plays a role.

The three meningeal membranes lie atop one another, wrapping the brain and spinal cord to shield the central nervous system from injury, damage, and infection. The outermost of the three, the dura mater, contains pain neurons that detect signals. Such signals could come in the form of mechanical pressure, blunt force from impact, or toxins that make their way into the central nervous system through the bloodstream. The researchers focused precisely on this outermost layer as the site of initial interaction between bacteria and protective border tissue.

Recent research has revealed that the dura mater also harbors a wealth of immune cells, and that immune cells and nerve cells reside right next to each other—a clue that captured Chiu’s and Pinho-Ribeiro’s attention.

“When it comes to meningitis, most of the research so far has focused on analyzing brain responses, but responses in the meninges—the barrier tissue where infection begins—have remained understudied,” Ribeiro said. So what exactly happens in the meninges when bacteria invade? How do they interact with the immune cells residing there? These questions remain poorly understood, the researchers said.

For the work described in Nature the researchers focused on the two leading bacterial causes of human meningitis,Streptococcus pneumoniae and Streptococcus agalactiae. S. pneumoniae is a major cause of bacterial meningitis in children, and in immunocompromised adults and the elderly. S. agalactiae is a key cause of meningitis in newborns.

Through a series of experiments, the team found that when bacteria reach the meninges, the pathogens trigger a chain of events that culminates in disseminated infection. First, researchers found that bacteria release a toxin, called pneumolysin, that activates pain neurons in the meninges. This activation of pain neurons by bacterial toxins, the team suggested, could explain the severe, intense headache that is a hallmark of meningitis. Next, the activated neurons release a signaling chemical, CGRP, which attaches to an immune-cell receptor RAMP1. This receptor is particularly abundant on the surface of immune macrophages. “Macrophages are the most abundant immune cell type in the meninges and therefore may be the first responders to bacteria,” the authors noted.

Under normal conditions, as soon as macrophages detect the presence of bacteria, they spring into action to attack, destroy, and engulf them. Macrophages also send distress signals to other immune cells to provide a second line of defense. However, the team’s studies showed that CRGP engagement of the RAMP1 receptor on macrophages effectively disables these immune cells. The results found that when CGRP released from the activated neurons attaches to the RAMP1 receptor on macrophages, it prevented these immune cells from recruiting help from fellow immune cells. These data, the a authors suggested, “… indicate that the CGRP–RAMP1 axis induces a transcriptional programme in macrophages that blunts cytokine expression.” As a result, the bacteria proliferated and caused widespread infection.

To confirm that the bacterially induced activation of pain neurons was the critical first step in disabling the brain’s defenses, the researchers carried out experiments to see what would happen to infected mice lacking pain neurons. They found that mice without pain neurons developed less severe brain infections when exposed to the two types of bacteria known to cause meningitis. The meninges of these mice demonstrated high levels of immune cells to combat the bacteria. By contrast, the meninges of mice with intact pain neurons showed weaker immune responses and far fewer activated immune cells, demonstrating that neurons get hijacked by bacteria to subvert immune protection.

To confirm that CGRP was the activating signal, the researchers compared the levels of CGRP in the meningeal tissue of infected mice with intact pain neurons and the meningeal tissue of mice lacking pain neurons. The results showed that the brain cells of mice lacking pain neurons had barely detectable levels of CGRP and few signs of bacterial presence. “Nociceptor ablation resulted in reduced brain invasion and pathology, and was associated with an increased number of meningeal immune cells,” the researchers added. By contrast, meningeal cells of infected mice with intact pain neurons showed markedly elevated levels of both CGRP and more bacteria.

In another experiment, the investigators used a chemical to block the RAMP1 receptor, preventing it from communicating with CGRP, the chemical released by activated pain neurons. The RAMP1 blocker worked both as preventive treatment before infection and as a treatment once infection had occurred. Mice pretreated with RAMP1 blockers showed reduced bacterial presence in the meninges. Likewise, mice that received RAMP1 blockers several hours after infection and regularly thereafter had milder symptoms and were more capable of clearing bacteria, compared with untreated animals. “Macrophage-specific RAMP1 deficiency or pharmacological blockade of RAMP1 enhanced immune responses and bacterial clearance in the meninges and brain,” the team wrote. The experiments suggest treatment using drugs that block either CGRP or RAMP1 could allow immune cells to do their job properly and increase the brain’s border defences.

Compounds that block CGRP and RAMP1 are found in widely used drugs to treat migraine, a condition believed to originate in the top dura mater meningeal layer. Could these compounds become the basis for new medicines to treat meningitis? It’s a question the researchers say merits further investigation. “RAMP1 antagonists and antibodies against CGRP are currently used for the prevention and treatment of migraine,” the investigators noted. “Our observation that treatment with a RAMP1 antagonist ameliorated bacterial meningitis in mice holds potential for therapeutic translation.”

One line of future research could examine whether CGRP and RAMP1 blockers could be used in conjunction with antibiotics to treat meningitis and augment protection. “Anything we find that could impact treatment of meningitis during the earliest stages of infection before the disease escalates and spreads could be helpful either to decrease mortality or minimize the subsequent damage,” Pinho-Ribeiro said.

More broadly, the direct physical contact between immune cells and nerve cells in the meninges offers tantalizing new avenues for research. “… our study suggests that nociceptors could affect the function of the CNS and play parts beyond bacterial meningitis,” the scientists wrote. “Future research could lead to the development of treatments for infections and other CNS diseases by targeting the somatosensory nervous system.”

In summary, they commented, “Overall, our data uncover a nociceptive neuroimmune axis in the meninges and reveal how bacterial pathogens exploit this signalling pathway to evade antimicrobial immunity … we showed that S. pneumoniae and S. agalactiae, two bacterial pathogens, activate nociceptors to promote meningeal and brain invasion, thereby linking neurons to the pathogenesis of bacterial meningitis.”

Chiu added, “There has to be an evolutionary reason why macrophages and pain neurons reside so closely together. With our study, we’ve gleaned what happens in the setting of bacterial infection, but beyond that, how do they interact during viral infection, in the presence of tumor cells, or the setting of brain injury? These are all important and fascinating future questions.”

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