Nociceptors and Macrophages in Bacterial Meningitis: Partners in Crime?

Bacterial meningitis is a serious and potentially life-threatening infection that affects the protective layers surrounding the brain and spinal cord, known as the meninges. Several types of bacteria, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae, can cause this condition. Certain populations, such as infants, children, immunocompromised adults, and the elderly, are particularly vulnerable to these pathogens. Symptoms of bacterial meningitis can manifest suddenly and may include high fever, severe and persistent headache, stiff neck, vomiting, sensitivity to light, and confusion or changes in mental state [1, 2]. Identifying these symptoms in infants can be more challenging and may lead to long-term disability or death [3]. Despite the effectiveness of vaccines, it is crucial to understand how bacteria invade the brain and how our innate immune system responds to infection (Fig. 1).

Fig. 1.

Nociceptors in the meninges and the gut have distinct roles in host defense against bacteria invasion.

Pinho-Ribeiro et al. [4] demonstrated that bacteria directly activate TRPV1+ nociceptors in the meninges, leading to the release of CGRP. This neuropeptide suppresses chemokine expression in meningeal macrophages through CGRP-RAMP1 signaling, thereby dampening the host defense and promoting bacterial meningitis. On the other hand, Zhang et al. [7] showed that TRPV1+ nociceptors release substance P (SP) to regulate the intestinal microbiota in a mouse model of colitis induced by DSS. This regulation helps protect the barrier tissue from inflammation

A recent study by Pinho-Ribeiro et al. demonstrated that bacterial infections have a direct impact on Nav1.8⁺ nociceptors in the meninges, resulting in the release of neuropeptide calcitonin gene-related peptide (CGRP), which dampens host defenses in the meninges and exacerbates bacterial meningitis [4]. The authors first induced bacterial meningitis in mice by injecting S. pneumoniae or Streptococcus agalactiae (S. agalactiae) intravenously, which replicates the spread of bacterial pathogens through the bloodstream in humans. Subsequently, the authors investigated the timeline of bacterial invasion. Notably, the bacteria first reached the dura mater, followed by the pia and arachnoid, and finally the brain. The authors demonstrated that the elimination of Nav1.8⁺ nociceptors in Nav1.8-Cre::DTA mice through genetic ablation or the use of resiniferatoxin (RTX), an extremely potent TRPV1 agonist, to chemically ablate TRPV1⁺ nociceptors resulted in reduced bacterial loads in the central nervous system (CNS). However, when bacteria were injected directly into the cistern bypassing the dura, the elimination of nociceptors no longer had an impact on the infection compared to the control group. This suggests that nociceptors regulate bacterial invasion, and this occurs prior to the bacteria entering the brain.

Headaches are a common manifestation of bacterial meningitis due to the innervation of the meninges by pain-inducing trigeminal ganglia (TG) nociceptors. The authors proposed a hypothesis that the activation of TG nociceptors in bacterial meningitis could lead to the release of CGRP from nerve terminals in the dura, thereby causing headaches [5]. The findings of the study support this hypothesis, as an increase in CGRP release was observed in meninges explants following injection of S. pneumoniae. Additionally, TG neurons released CGRP when they were incubated directly with S. pneumoniae. These results suggest a direct action of bacteria on TG nociceptors. This observation aligns with a previous study that reported elevated arterial levels of CGRP in patients with acute bacterial meningitis and sepsis [6].

Using live cell calcium imaging, the authors confirmed that bacteria can directly activate TG neurons. Furthermore, they found that ablation of CGRP⁺ nerves in the meninges resulted in a decrease in bacterial invasion into the CNS while administering exogenous CGRP led to an increase in invasion and a decrease in immune cell numbers in the meninges. These findings support the role of CGRP in promoting bacteria invasion. Additionally, the authors discovered that knockout of the CGRP receptor RAMP1 reduced bacterial invasion of the meninges and brain. Furthermore, inhibiting RAMP1 signaling with the antagonist BIBN4096 delayed the onset of clinical symptoms of S. pneumoniae infection in mice. This treatment also reduced the bacteria load in the meninges and brain but had no effect on other organs such as the lung, spleen, and skin.

The authors conducted single-cell RNAseq analysis on CD45⁺ meningeal T cells and found that RAMP1 was highly expressed in immune cell populations in the meninges. They further demonstrated that RAMP1 signaling in myeloid immune cells hinders host defenses against bacterial meningitis by using specific Cre lines for immune cells along with Ramp1 flox mice. Further examination of the single-cell RNAseq data revealed that macrophages, monocytes, and neutrophils among the myeloid immune cell clusters exhibited the most significant differential gene expression in response to S. pneumoniae infection. Lastly, the authors identified macrophages as the primary immune cells responsible for the RAMP1-mediated immune inhibition by depleting meningeal macrophages expressing mannose receptor C-type 1 (MRC1) chemically. This finding was further supported by Gene Ontology and biological processes analysis of macrophages following in vivo CGRP treatment, confirming the suppression of meningeal macrophage function by CGRP.

The meninges, a set of three membranes known as the dura mater, arachnoid mater, and pia mater, play a vital role in protecting the brain. The authors demonstrated that when exposed to S. pneumoniae macrophages exhibited increased levels of chemokines which recruit other immune cells to combat the infection. However, the study also revealed that the protective function of the immune cells was hindered by CGRP-RAMP1 signaling in macrophages, which was initiated by the release of CGRP from activated TRVP1⁺ nociceptors in response to bacterial infection. This resulted in a decrease in chemokine production. The findings of Pinho-Ribeiro's study establish a connection between nociceptor activation following pathogen infections and immune modulation through CGRP-RAMP1 signaling in macrophages. This unique interaction between the nervous and immune systems allows nociceptors to suppress the host immune response, facilitating bacterial invasion and the development of meningitis.

In contrast to their role in the meninges, where primary nociceptors play a pathogenic role in host defense, primary nociceptors of the gut serve a protective function during inflammatory bowel disease. Zhang et al. showed that both AAV-mediated silencing of colon-innervating TRPV1⁺ visceral nociceptors and chemical-induced ablation of TRPV1⁺ nociceptors led to more severe inflammation and altered intestinal microbiota in a mouse model of colitis induced by dextran sodium sulfate (DSS) which is characterized by compromised gastrointestinal barrier integrity and inflammation [7]. This protective role of TRPV1⁺ nociceptors is mediated by the release of nociceptor-derived substance P (SP), which helps maintain a balance between protective and pathogenic microbes.

Although colonic SP and CGRP levels were both decreased in the DSS-induced colitis in RTX-treated mice, only the SP level was decreased in the mouse line that is more susceptible to DSS-induced colitis. Furthermore, the administration of SP was effective in reducing the severity of the disease and reversing the tissue damage caused by RTX treatment. These findings suggest that TRPV1⁺ nociceptors may serve as sources for both SP and CGRP in the gut, but SP is specifically necessary for protection against DSS-induced colitis, while CGRP does not play a protective role. A previous study also demonstrated that SP, but not CGRP, contributes to pial arteriolar vasodilation during the early phase of pneumococcal meningitis in rats [8]. It is worth noting that CGRP released from TRPV1⁺ nociceptor terminals in the skin can suppress host defense mechanisms, leading to reduced neutrophil recruitment and bactericidal activity in cases of flesh-eating bacterial infections [9]. Additionally, CGRP has been reported to have a protective role in the brain during ischemia, a non-infectious disease [10, 11].

Together, these studies highlight the versatile roles of nociceptors in host defense and uncover a direct response of the sensory nociceptors to bacterial infections. The unique interaction between the sensory nociceptors and immune cells in both the meninges and the skin significantly contributes to the pathogenesis of infectious diseases, whereas in the gastrointestinal tract, the interaction between the visceral nociceptors and immune cells is essential for preserving "microbial homeostasis" and safeguarding tissues against infection.