2. The main mechanism of macrophages to induce pain
Acquirement of M1 phenotype
Under conditions of nerve injury or
infection, activated sensory neurons in the DRG or spinal cord release
neuropeptides and cytokines such as TNF, promoting macrophage
programming to an M1 phenotype (7, 8). The neuropeptides Substance P
(SP) and calcitonin gene-related peptide (CGRP) have been shown to cause
a significant increase in IL-1β and TNF production by macrophages alone
or in coordination with each other, enhancing the macrophage-mediated
inflammatory response (9) (Figure 1a). TNF not only prevents the myelin
phagocytosis-mediated switch from M1 to M2 but also mediates the
increased IL-4-polarized M2 cells to M1, maintaining M1 polarization in
the injured spinal cord (10). Additionally, the central terminals of
sensory neurons communicate with microglia via the release of the
cytokine colony-stimulating factor 1 (CSF1). In the spinal cord, CSF1
activates the CSF1 receptor in microglia via the transmembrane protein
(DAP-12) (P2X4 independent pathway) and up-regulates genes critical to
the development of allodynia(11).
Additionally, miRNAs have been found to regulate macrophage polarization
(12). For example, miR-21-5p in DRG neurons has been shown to promote
macrophages to acquire a pro-inflammatory phenotype (13). Similarly,
miR-9-5p transferred from neurons to microglia has been shown to promote
M1 polarization in microglia, leading to an over-release of
proinflammatory cytokines, such as IL-1β, IL-6, and TNF-α, exacerbating
neurological damage (14).
Inflammatory cytokine
Macrophages use various mechanisms to induce pain. Typically, M1
macrophages release a multitude of inflammatory mediators, including
interleukin-1β (IL-1β), tumor necrosis factor-alpha (TNF-α)(15, 16),
interleukin-6 (IL-6) (17, 18), nerve growth factor (NGF) (19),
insulin-like growth factor 1 (IGF-1) (20), COX2 and PGE2 (21), which
stimulate their specific receptors expressed in the nociceptive neurons.
This activation leads to the generation of various neuronal cell
signals, such as Ca2+ and cAMP, PKA, PKC, PI3K, MAPKs (ERK, p38, and
JNK), NF-κB, JAK, STAT, and Src. These kinases cause hypersensitivity
and hyperexcitability of nociceptor neurons by enhancing the activity of
pro-nociceptive cation channels, such as TRPA1 and TRPV1, and sodium
channels NaV1.7, NaV1.8, and NaV1.9 (22, 23, 24, 25) (Figure 1b).
Moreover, macrophages release reactive oxygen species (ROS) that
maintain their infiltration into the injured nerve and send paracrine
signals to activate TRPA1 of Schwann cells surrounding nociceptors (26).
Schwann cell TRPA1 can release M-CSF to sustain peripheral nerve
resident macrophage expansion and generate oxidative stress that targets
neuronal TRPA1, thereby sustaining mechanical allodynia (27). Moreover,
heterodimerization between TLR4 and TLR2 or TLR4 and TLR6 in microglial
cells and macrophages trigger inflammatory responses via a
MyD88-dependent mechanism (28, 29).
The peripheral inflammation causes CNS hyperactivity, which leads to the
development of central sensitization, a state characterized by
hyperactivity and hyperexcitability of neurons in the spinal cord and
brain (30). Recent studies suggest that TRPV4 channels expressed in
spinal microglia play a crucial role in converting peripheral nerve
injury to spinal central sensitization and neuropathic pain. The
microglial TRPV4 channels mediate microglial activation and
proliferation and enhance the synaptic transmission and plasticity of
excitatory neurons by releasing LCN2 (31).
It is important to note that SNX25 expression in dermal macrophages, but
not DRG macrophages, has been shown to inhibit the ubiquitination and
proteasomal degradation of Nrf2, which is involved in maintaining the
production of NGF and contributes to the maintenance of pain
sensitivity. This SNX25-Nrf2 pathway in dermal macrophages may help to
optimize the concentration of NGF, which modulates neuronal responses to
mechanical stimuli under both normal and pain-inducing conditions(32).
Chemokine
Chemokines
play a crucial role in the communication between neurons and
macrophages, leading to the induction of peripheral and central
sensitization. Among these, CCL2 (monocyte chemoattractant protein 1,
MCP1) is produced by both macrophages and neurons, and its primary
receptor is CCR2. Upon binding to CCR2, CCL2 recruits peripheral
macrophages or spinal microglia to the site of nerve damage, including
the cell body and peripheral and central axons of neurons, through
proliferation, infiltration, or migration(3, 23, 33, 34, 35). CCL2/CCR2
signaling is involved in the development of peripheral sensitization by
enhancing the activity of tetrodotoxin-resistant (TTX-R) sodium channel
Nav1.8 and upregulating the expression and function of the
capsaicin-sensitive TRPV1 ion channel (36). Additionally, CCL2 can
regulate neuronal and synaptic plasticity underlying central
sensitization through CCR2 in the spinal cord (37). CX3CR1, the only
receptor for CX3CL1, is specifically expressed in microglia and is
commonly used as a marker for resident macrophages in tissues and
microglia in the spinal cord and brain. CX3CL1 is secreted by DRG
neurons and spinal neurons following nerve injury, and it binds to
CX3CR1, leading to the activation of p38 MAPK in spinal microglia, which
promotes chronic pain (23, 33)
(Figure 1c).
Macrophages in PNS and microglia in CNS
Peripheral nerve injury triggers a considerable increase in the number
of DRG macrophages, alongside a pronounced proliferation and activation
of microglia. Both DRG macrophages and microglia (the tissue-resident
macrophages of CNS) are strongly implicated in the development of
neuropathic pain(38, 39).
In most tissues, two classes of macrophages can be distinguished:
infiltrating and tissue-resident macrophages (22, 34). In conditions
such as osteoarthritis (OA) pain and radiculopathy, the increase of DRG
macrophages is dominated by CC2R+ macrophages, suggesting that the
accumulation of macrophages is at least partially due to infiltration of
circulating monocytes into the DRG(8, 40). However, studies have shown
that after peripheral spinal nerve injury, 99% of myeloid cells in the
spinal cord are resident microglia, indicating that persistent pain is
primarily driven by the proliferation of resident microglia (31, 41).
The main mechanism of macrophages to resolve pain
M1 macrophages are pro-inflammatory,
while M2 macrophages are anti-inflammatory and have potent phagocytosis
capacity, scavenging debris and apoptotic cells, promoting tissue repair
and wound healing, which can contribute to pain resolution (6). As we
mentioned above, neuropeptides SP and CGRP not only can promote the M1
phenotype to activate inflammation but also produce anti-inflammatory
actions by facilitating the M2 switch of macrophages (42) (Figure 1a).
SP can directly induce macrophages to become M2-like macrophages through
the activation of the PI3K/Akt/mTOR/S6kinase pathway and the induction
of Arginase-1, CD163, and CD206 (43). Similarly, CGRP can inhibit
inflammation and promote the transformation through the PI3K/AKT
signaling pathway (44). Under various pathological conditions,
macrophages can rapidly switch from one phenotype to the other. For
example, activation of NF-κB or IRF family members in macrophages by
TLR4 or other TLRs can drive macrophage polarization towards either M1
or M2 phenotype in response to surrounding microenvironment (45).
M2 macrophages secrete
anti-inflammatory cytokines, such as IL-10, TGF-β, and GPR37, which play
a key role in inhibiting neuropathic pain. IL-10 is a potent
anti-inflammatory cytokine that can downregulate TTX-S and Nav1.8 sodium
channels and counteract the effects of TNF-α on sodium channel
regulation in DRG neurons (46).
TGF-β inhibits inflammatory cytokine
production through cross-talk between MAPKs, specifically ERK-dependent
inhibition of p38 MAPK caused by up-regulation of MAPK phosphatase-1
(47). TGF-β appears to promote the expression of endogenous opioids and
inhibit the neuroimmune responses of glial cells and neurons in the
spinal cord following peripheral injuries (48). Furthermore, TGF-β
downregulates CCL4 expression through ERK1/2 signaling activation to
inhibit inflammatory responses in the DRG and prevent pain development
(49). Neuroprotectin D1 (NPD1), one of the specialized pro-resolving
mediators (SPMs), has potent anti-nociceptive effects on different pain
pathologies. GPR37, expressed by macrophages but not microglia, can
increase the NPD1-evoked iCa2+ via Gi-coupled signaling, thereby
triggering macrophage phagocytosis via signaling through G protein
subunit Gi/o, ERK, and PI3K/AKT (50, 51) (Figure 1d). Both IL-10 and
GPR37 can regulate macrophage phenotypes(52, 53), and the combination of
IL-10 with its receptor (IL-10R) results in activating transcription
factor STAT3 and promotes M2 phenotype (45).
In addition, M2 macrophages induced by IL-4 may release opioid peptides
such as Met-enkephalin, dynorphin, and β-endorphin that bind to opioid
receptors on nociceptors and attenuate neuropathic pain (3, 25) (Figure
1d). A recent study has revealed a novel mechanism for the active
resolution of inflammatory pain, demonstrating that CD206+ M2-like
macrophages accumulate in the DRG and transfer mitochondria to sensory
neurons, which recovers the oxidative phosphorylation in sensory neurons
to resolve inflammatory pain (54). Furthermore, tissue-resident
ganglionic macrophages exhibit an M2 phenotype after nerve injury and
proliferate rapidly. They enter between satellite glial cells and
neurons and directly contact neurons, leading to tissue repair (55).
Conclusion and perspective
It is commonly accepted that M1 macrophages promote pain by releasing
pro-inflammatory mediators, whereas M2 macrophages alleviate pain by
producing anti-inflammatory mediators. However, resolving inflammation
alone is not sufficient to completely address pain. Nonsteroidal
anti-inflammatory drugs (NSAIDs) such as COX-2 inhibitors are known to
suppress inflammation to manage acute pain, but they may also prolong
inflammation, potentially leading to chronic pain (56). Therefore,
maintaining a dynamic balance of inflammation related to macrophage
polarization is critical in preventing acute pain from developing into
chronic pain, which may represent a novel therapeutic target for
neuropathic pain in the future.
Author Contributions: Xiaoye Zhu and Xiaoyan Zhu designed the
structure of this article and wrote the manuscript. Zhigang Cheng and
Qulian Guo reviewed the manuscript. Yuqi Zhang and Yunchuan Xiong made
substantial and intellectual contributions to the work. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by Xiaoyan Zhu, Natural
Science Foundation of Hunan Province (2021JJ41060), Changsha Municipal
Natural Science Foundation (kq2014280), Research Program Project of
Hunan Health Commission(B202304119634), Yunchuan Xiong, Natural Science
Foundation of Hunan Province (2021JJ31113).
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of
interest.