Immuno-Inflammatory Mechanisms in the Chronification of Pain

Abstract

Chronic pain affects 20% of the global population, with current treatments achieving meaningful relief in less than 30% of patients. Growing evidence indicates that immuno-inflammatory mechanisms critically mediate the transition from acute to chronic pain, extending beyond sustained nociceptive input. This narrative review synthesizes current understanding of cellular and molecular immuno-inflammatory processes underlying pain chronification, emphasizing therapeutic implications of immune–neural interactions. Peripheral tissue injury triggers coordinated immune responses involving pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, tumor necrosis factor alpha (TNF-α), and algesic mediators that sensitize nociceptors. Infiltrating macrophages, T lymphocytes, and mast cells perpetuate pro-nociceptive environments. Centrally, microglial and astrocytic activation induces persistent neuroinflammation, synaptic remodeling, and enhanced excitatory neurotransmission while impairing descending inhibition. The balance between pro-inflammatory T helper 1 and T helper 17 (Th1/Th17) and anti-inflammatory T helper 2 and regulatory T cell (Th2/Treg) responses determines pain outcomes. Critically, premature suppression of acute inflammation with nonsteroidal anti-inflammatory drugs (NSAIDs) or corticosteroids may paradoxically promote chronification by disrupting endogenous resolution pathways mediated by specialized pro-resolving mediators and regulatory immune cells. Local inflammation proves more relevant than systemic inflammation for pain persistence. The gut–brain–immune axis emerges as a novel therapeutic target, with microbiota composition influencing pain susceptibility through immunomodulation. Finally, chronic pain represents a failure of natural resolution mechanisms rather than prolonged nociceptive activation. Understanding temporal dynamics of immune responses, individual variability, and sex-specific mechanisms opens avenues for precision medicine approaches. Future strategies should restore homeostatic mechanisms rather than simply suppress symptoms, incorporating biomarker-guided treatment selection and multimodal interventions targeting the complex immuno-inflammatory cascade.

Key Summary Points

Immuno-inflammatory mechanisms are central to the transition from acute to chronic pain, involving coordinated interactions among immune cells, cytokines, and neural pathways in both peripheral tissues and the central nervous system.

Preclinical and translational studies demonstrate that premature suppression of acute inflammation—for example, with nonsteroidal anti-inflammatory drugs (NSAIDs) or corticosteroids—may interfere with endogenous resolution pathways; however, prospective human evidence remains limited and requires further validation.

Local inflammation appears more relevant than systemic inflammation in driving pain persistence, as supported predominantly by animal models and mechanistic studies; targeted local interventions show promise but are not yet established in clinical practice.

Pro-inflammatory T helper 1 (Th1) and Th17 responses, contrasted with anti-inflammatory Th2 and regulatory T cells (Tregs), shape the immune balance that determines whether pain resolves or progresses to chronicity.

Neuroimmune interactions involving microglia, astrocytes, dorsal root ganglia, cytokines, and neurotrophic factors contribute to central sensitization and pain amplification, with most mechanistic insights derived from preclinical research.

Emerging therapeutic strategies include supporting endogenous resolution mechanisms—such as specialized pro-resolving mediators—and integrating nutritional, psychosocial, and immune-based interventions, while maintaining awareness of the translational gap between animal models and human pain conditions.

Introduction

Chronic pain affects nearly 20% of the global population and remains one of the foremost causes of disability worldwide [1, 2]. Unlike acute pain, which functions as a protective alarm signal, chronic pain arises from complex and multifactorial processes that extend beyond sustained nociceptive input. Increasing evidence implicates immuno-inflammatory mechanisms as pivotal mediators of pain chronification [3–5]. Epidemiological findings highlight the dual role of inflammation, acting as both “foe and friend” depending on temporal and spatial context [3]. Rosenström et al. [4] demonstrated that specific cerebrospinal fluid (CSF) cytokine signatures correlate with pain severity in lumbar disc herniation, directly linking neuroimmune dysfunction to human pain conditions.

The transition from acute to chronic pain is driven not only by maladaptive neuronal plasticity but also by persistent activation of innate and adaptive immune responses, which remodel peripheral and central nociceptive circuits [6–8]. Peripheral injury or nerve trauma triggers immune activation with the release of pro-inflammatory cytokines, including interleukin (IL)-1β, tumor necrosis factor alpha (TNF-α), IL-6, and IL-17. These mediators, together with infiltrating macrophages, mast cells, and T lymphocytes, establish a pro-inflammatory microenvironment that sensitizes nociceptors and drives peripheral sensitization [9]. At the central level, microglia and astrocytes become activated within the dorsal horn, initiating neuroinflammation, synaptic remodeling, enhanced excitatory signaling, and loss of inhibitory control—hallmarks of central sensitization [8]. Comparable mechanisms contribute to overlapping chronic pain conditions [9].

Recent research emphasizes neuroimmune cross-talk in dorsal root ganglia (DRG) and peripheral nerves, where immune receptors on sensory neurons and immune complex-mediated activation of nociceptors directly link immune signaling to pain transmission [6, 10, 11]. Furthermore, epigenetic modifications, including DNA methylation and histone remodeling at pro-inflammatory gene loci, sustain nociceptor hyperexcitability and chronic pain phenotypes [12].

The persistent pain enigma reflects a convergence of molecular drivers orchestrating the transition to chronic states through gene expression, protein synthesis, and altered cellular phenotypes [5]. This review aims to synthesize current insights into immuno-inflammatory mechanisms, including cytokine signaling, immune cell activation, glial-neuronal cross-talk, neurogenic inflammation, and epigenetic regulation, and to outline emerging targets for mechanism-based interventions.

Methods

This narrative review was conducted following the Scale for the Assessment of Narrative Review Articles (SANRA) guidelines to ensure methodological rigor and transparency in the synthesis of evidence [13]. The review aimed to critically appraise and integrate current knowledge on immuno-inflammatory mechanisms contributing to the chronification of pain, encompassing peripheral and central neuroimmune interactions, cytokine signaling, glial–neuronal cross-talk, neurogenic inflammation, and epigenetic regulation.

Literature Search Strategy

A comprehensive literature search was performed in PubMed, Scopus, Web of Science, and Embase databases to identify peer-reviewed articles published between January 2000 and June 2025. The search strategy combined controlled vocabulary [(Medical Subject Headings (MSH terms)] and free-text terms, including “chronic pain,” “immuno-inflammatory mechanisms,” “neuroinflammation,” “microglia,” “astrocytes,” “cytokines,” “glial activation,” “dorsal root ganglia,” “epigenetic regulation,” and “neuroimmune interactions.” Boolean operators (AND, OR) were used to optimize sensitivity and specificity. The search strategy was refined based on recent systematic analyses identifying key immunological pathways consistently implicated across different pain conditions. Reference lists of key reviews were also screened, and included articles were manually reviewed to identify additional relevant studies.

Eligibility Criteria

Studies were included if they (1) investigated immuno-inflammatory processes in chronic pain conditions (neuropathic, nociplastic, or mixed pain); (2) provided mechanistic insights into immune cell activation, cytokine signaling pathways, glial responses, or epigenetic modulation relevant to pain chronification; and (3) were original research articles, systematic reviews, or high-quality narrative reviews. Non-English articles, conference abstracts, and non-peer-reviewed sources were excluded.

Study Selection and Data Extraction

Titles and abstracts were independently screened by two reviewers (MAAM and MANE), followed by full-text assessment of potentially eligible articles. Discrepancies were resolved through consensus with a third reviewer (MANT). Extracted data included study design, pain condition, investigated immunological mediators or pathways, experimental models (animal or human), and main findings related to peripheral and central sensitization.

Data Synthesis

A qualitative synthesis approach was employed. Evidence was categorized as follows:

1. Neuroimmune priming by psychosocial and perioperative risk factors

2. Neuroimmune interactions in pain perception

3. Cellular mediators of pain

4. Pro-inflammatory versus anti-inflammatory responses

5. Influence of local versus systemic inflammation

6. Therapeutic implications and interventions

7. Gut microbiota and nutritional immunomodulation

Emerging therapeutic targets were identified based on preclinical and clinical findings demonstrating modulation of these pathways. Given the narrative nature of the review, no quantitative meta-analysis was performed.

Ethics Compliance

This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.

Narrative Results and Discussion

The following section presents an integrative thematic synthesis derived from the literature reviewed on immuno-inflammatory mechanisms in pain chronification. Although this work is a narrative review and does not follow systematic review protocols, the organization of content reflects recurring mechanistic domains identified across high-quality studies. These themes are grouped under psychosocial, cellular, molecular, and therapeutic dimensions that converge in the transition from acute to chronic pain. This narrative structure offers a coherent interpretation of complex neuroimmune interactions supported by experimental and clinical evidence.

Neuroimmune Priming by Psychosocial and Perioperative Risk Factors

Although comprehensive large-scale studies examining pain chronification remain limited, emerging evidence has identified multiple risk factors that predispose individuals to pain chronification, highlighting that the immune system operates within a broader neuroendocrine-immunological framework [14, 15]. In both pediatric and adult populations, psychological factors significantly influence pain outcomes. Analysis of existing data suggests that individuals may become predisposed to chronic pain through cognitive and emotional pathways, anticipatory anxiety, and fear-avoidance behaviors [16]. The experience of trauma, whether accompanied by pain or not, represents a significant risk factor for chronic pain development, with the social environment playing a crucial modulatory role [17]. In pediatric and adolescent populations, high levels of parental catastrophizing, anticipatory anxiety, and maladaptive coping have been linked to increased postoperative pain and delayed recovery [18–20]. These psychological stressors are known to modulate hypothalamic–pituitary–adrenal (HPA) axis activity, alter glucocorticoid sensitivity, and bias T cell polarization toward pro-inflammatory T helper (Th) 1 and Th17 profiles, thereby enhancing glial reactivity after injury [21]. Perioperative clinical factors—such as high baseline pain intensity, pre-existing functional disability, certain high-risk surgical procedures (e.g., spinal fusion, thoracotomy), and large surgical wounds—can increase local tissue inflammation and amplify the recruitment of neutrophils, macrophages, and T lymphocytes [22–25]. This can lead to neuroimmune priming of the dorsal horn and dorsal root ganglia (DRG), making central sensitization more likely. These risk factors do not operate alone but interact synergistically, creating vulnerability windows during critical developmental periods, particularly in adolescence when neural plasticity is heightened. The interplay between these psychosocial and perioperative factors and immune–neural mechanisms is summarized in Table 1, which links each risk factor to its potential pathway for promoting neuroimmune priming and pain chronification.

Table 1.

Psychosocial and perioperative risk factors for chronic pain: neuroimmune implications

Domain	Risk factor	Neuroimmune mechanism
Psychological	Parental catastrophizing, anticipatory anxiety	↑ Hypothalamic–pituitary–adrenal (HPA) axis activation → altered cortisol signaling → T helper (Th) and Th17 polarization → microglial priming
	Fear of pain/reinjury, poor coping strategies, depressive symptoms	↑ Pro-inflammatory cytokine release of interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-α) via stress–immune pathways
Preoperative clinical	Age 12–18 years	Developmental window with heightened neuroimmune plasticity
	Pre-existing pain > 1 month, high visual analogue scale (VAS) score	Chronic low-grade inflammation → dorsal root ganglion (DRG) macrophage infiltration
	Higher body mass index (BMI)	↑ Adipokines and IL-6 → systemic low-grade inflammation
Intraoperative	Orthopedic/spinal fusion/thoracotomy	Greater tissue trauma → massive local immune cell recruitment
Postoperative	Acute severe postoperative pain	Sustained nociceptor activation → prolonged cytokine release
	Poor social support	Chronic psychosocial stress → impaired resolution-phase immune responses

Neuroimmune Interactions in Pain Perception

The subjective experience of pain emerges from complex neuroendocrine-immune interactions that can be conceptualized through three dimensions: sensory-discriminative (location and intensity), motivational-affective (emotional response), and cognitive-evaluative (interpretation and meaning) [26]. Within this framework, immune cells and their secreted proteins play crucial roles in modulating nociceptive processing at multiple levels of the nervous system.

Cellular Mediators of Pain

Pain and inflammation are intrinsically linked, particularly in experimental models where controlled tissue injury triggers predictable inflammatory cascades. These models consistently demonstrate elevated levels of prostaglandins, histamine, and platelet-activating factor (PAF), and a coordinated release of cytokines including interleukin-1 (IL-1), IL-6, TNF-α, and interferons [27, 28]. The persistent upregulation of these cytokines supports the presence of local inflammation due to tissue injury and has been shown to activate microglial cells. Once activated, microglia further contribute to the production of pro-inflammatory cytokines such as IL-1, IL-6, and TNF-α, thereby maintaining or amplifying pain states. However, cytokine release is not restricted to the immune system alone; cells of the nervous system also express cytokines. For instance, brain-derived neurotrophic factor (BDNF) is synthesized by neurons and can interact with other neurons within this pro-inflammatory environment, contributing to pain amplification [29]. BDNF triggers intracellular second messenger pathways with neuronal proliferative potential [30, 31].

Another molecule of interest is nerve growth factor (NGF), initially recognized for its role in embryogenesis [32]. In both the postnatal and adult periods, NGF sensitizes nociceptors (particularly those in the DRG) and promotes mast cell activation, leading to sustained pain [33]. Therapeutic inhibition of NGF using monoclonal antibodies, such as tanezumab, has shown promise in experimental models, particularly in chronic pain associated with osteoarthritis, highlighting its pivotal role in nociceptive modulation [34–36].

The bidirectional interactions between microglia, neurons, and immune mediators in sustaining or resolving pain are depicted in Fig. 1.

Fig. 1.

Microglial–neuronal interactions in pain modulation. Under homeostatic conditions, microglia support neuronal function through trophic factors, phagocytosis, and debris clearance. Inflammatory stimuli shift microglia toward a pro-inflammatory phenotype, releasing cytokines that promote neuronal degeneration and amplify pain. Neurons and mast cells contribute by releasing neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), further modulating microglial activity and sustaining inflammation. Conversely, anti-inflammatory microglia can counteract these effects, promoting tissue repair and pain resolution

It has been confirmed that local inflammation plays a more critical role than systemic inflammation in the persistence of chronic pain [14, 37]. Systemic suppression of TNF-α has not demonstrated consistent effectiveness in alleviating pain. However, localized interventions (such as intraspinal administration of anti–TNF-α agents) have shown the ability to modulate pain, particularly in models of post-osteoarthritic pain [14].

Several studies have investigated the therapeutic effect of eliminating pro-inflammatory cytokines involved in pain generation [14, 38–40]. Interestingly, while the rationale suggested that blocking these cytokines would alleviate pain, the outcomes were not as straightforward. In some cases, systemic cytokine inhibition failed to produce the anticipated analgesic effect, suggesting that local cytokine activity, rather than their systemic presence, may be more relevant in the pathophysiology of chronic pain [14]. These findings highlight the need to reevaluate the spatial and functional dynamics of cytokine signaling in nociceptive pathways.

Similarly, local release of calcitonin gene-related peptide (CGRP) is more closely associated with chronic migraine than its systemic presence [15]. For this reason, it is much more difficult to analyze localized inflammation in humans, and most studies are therefore conducted in rats, where serial biopsies can be performed to evaluate which substances increase more than others and at what stages in a given tissue or injured area [41]. All these cytokines released during inflammation act on second messengers such as nuclear factor kappa-light-chain-enhancer of activated b cells (NF-kB), mitogen-activated protein kinases (MAPKs), and Janus kinases (JAKs), all of which are activated to amplify the inflammatory signal and promote the release of more cytokines such as IL-1, IL-6, and TNF-α [42–44]. These, in turn, induce sensitization of neurons in the dorsal horn of the spinal cord and the dorsal root ganglion, increasing pain hypersensitivity. This initial inflammatory phase is driven by the innate immune system, including neutrophils, macrophages, and dendritic cells, among others [45].

From this initial mechanism, it has been found that localized inflammation activates T lymphocytes (T cells), specifically Th1 cells, which secrete pro-inflammatory interleukins such as IFN-γ and IL-2 [46]. These cytokines reactivate macrophages to release more cytokines and promote proliferation. Another T cell involved in pain is the Th17 lymphocyte, along with its IL-17A, which has been implicated in pain [47, 48]. It appears that there is a prior signaling event, as a specific class II major histocompatibility complex (MHC), haplotype DQB1*03:02, has been associated with a higher risk of chronic pain, in concordance with the presence of CD4⁺ T cells and Th17 cells in the dorsal root ganglion [49–51]. In cases of disc herniation with nerve root compression, radicular pain has shown a CD4⁺ T cell component, predominantly expressing IL-2 and IL-4, possibly activating a B cell-mediated humoral response [52].

On the other hand, T cells that predominantly secrete IL-10 and IL-4, such as CD4⁺ T cells or Th2 cells, appear to play a protective role against the development of chronic pain [53]. However, this duality of functions remains under debate.

Other cells involved alongside T lymphocytes are B cells, which act as perpetuators of pain [54]. Their elimination using anti-CD20 therapy did not eliminate pain but merely delayed it, and at the end of the inflammatory cycle, B cells perpetuated it [55, 56]. Even with a low number of CD19⁺ B cells, pain was re-induced. In mouse models of chronic pain, B cells are activated and produce IgG; however, no specific autoantibody, like those found in encephalitis, has been identified. This seems to be a case of polyclonal activation triggered by uncontrolled innate immune responses at a specific site, leading to local antibody production and high expression of immunoglobulin receptors (FcγRs), and thus the perpetuation of inflammation and chronic pain [56]. The differentiation of naïve CD4⁺ T cells into distinct helper T cell subsets and their interactions with macrophages and B cells in modulating inflammation and pain are summarized in Fig. 2.

Fig. 2.

Key cellular mediators of chronic pain. Peripheral immune cells (macrophages, mast cells, T and B lymphocytes) and central glial cells (microglia, astrocytes) interact through cytokines, neurotrophic factors, and chemokines to drive peripheral and central sensitization. The schematic illustrates the differentiation of naïve CD4⁺ T cells into T helper (Th) 1, Th2, and Th17 subsets under specific polarizing cytokines, each producing distinct mediators that modulate immune responses. Pro-inflammatory subsets (Th1 and Th17) promote cytokine release and pain amplification, whereas anti-inflammatory Th2 cells contribute to immune regulation and pain resolution

Emerging Role of B Cells and Autoantibodies in Pain Chronification

Recent evidence has substantially expanded our understanding of B cell-mediated mechanisms in chronic pain beyond the polyclonal activation described previously. Lacagnina et al. [56] demonstrated that B cell depletion with anti-CD20 monoclonal antibody at the time of nerve injury prevented allodynia development in mice. Nerve injury induced immunoglobulin G (IgG) accumulation in ipsilateral lumbar DRG and dorsal spinal cords, where IgG colocalized with sensory neurons, macrophages, and microglia. The pathogenic role of autoantibodies extends across diverse conditions. In fibromyalgia, anti-satellite glial cell IgG antibodies correlate with pain severity, and passive transfer of patient IgG to mice induces pain-like behavior [57]. Similar findings have been reported in complex regional pain syndrome (CRPS), chronic post-traumatic limb pain, and rheumatoid arthritis-associated pain [58].

Multiple specific autoantibody targets have been identified. Autoantibodies against contactin-associated protein-like 2 (CASPR2), particularly IgG4 subclass, cause neuropathic pain by disrupting voltage-gated potassium channel function and inducing DRG hyperexcitability [59]. Autoantibodies against fibroblast growth factor receptor 3 (FGFR3) activate p38 MAPK, ERK, and JNK signaling pathways in sensory neurons [60]. Additional targets include plexin D1, anti-CV2/CRMP5, and novel antigens such as MX1, DBNL, and KRT8 [61]. These autoantibodies drive pain through multiple mechanisms: (1) complement system activation promoting inflammation and tissue damage; (2) signaling through Fc gamma receptors (FcγRs) on sensory neurons, triggering calcium mobilization via Syk and TRPC3 channels; (3) direct disruption of ion channel function; and (4) modulation of satellite glia and macrophage activity in DRG [28]. These mechanisms contribute to both peripheral and central sensitization. However, the predominance of IgG4 autoantibodies poses challenges, as IVIg contains minimal IgG4 and lacks sufficient idiotypic antibodies for neutralization, potentially explaining variable treatment responses. The identification of pain-sensitizing autoantibodies represents a paradigm shift in understanding chronic pain etiology, particularly in “unexplained” chronic primary pain conditions. These findings suggest that a subset of chronic pain patients may have an autoimmune basis, opening avenues for diagnostic biomarker development and targeted immunotherapies.

Pro-inflammatory versus Anti-inflammatory Responses

CD8⁺ T lymphocytes also have dual roles: They can promote neuropathic pain by releasing pro-inflammatory cytokines, with cytotoxic CD8⁺ T cells primarily found in neuropathic pain associated with diabetes or HIV [62, 63]. However, they can also prevent the progression of neuropathic pain, as seen in a study of chemotherapy-induced neuropathic pain, where CD8⁺ T cells were observed to release IL-13, which in turn activates macrophages to secrete IL-10. This IL-10–rich microenvironment led to the resolution of mechanical allodynia [17].

During a certain phase of inflammation, everything is activation and pro-inflammatory signaling, with the release of cytokines. However, whether inflammation persists or resolves depends on the activation signals and the perpetuation of these signals in the dorsal horn of the spinal cord. For example, if the environment is rich in IL-12 and interferon-gamma (IFN-γ), a naïve T cell will differentiate into a Th1 cell; if the environment is rich in IL-6, IL-21, and transforming growth factor beta (TGF-β), it will become a Th17 cell; if the environment is rich in IL-4 and IL-2, it will become a Th2 cell; and if only TGF-β is present, it will differentiate into a regulatory T cell [48]. Each of these subtypes has different functions; the first two are pro-inflammatory, while the latter two are regulatory. Translated into pain mechanisms, if the DRG is infiltrated by Th1 and Th17 lymphocytes, macrophages, and neutrophils, this will lead to pain amplification and the onset of chronic pain [48]. Conversely, if the infiltrate consists of Th2 or regulatory T cells, the result will be the opposite: type 2 macrophages, pain resolution, and neural regeneration [48]. These represent two possible scenarios in the context of pain.

One of the main regulators of pain are regulatory T cells (CD4⁺CD25⁺), which are capable of secreting large amounts of IL-10, thereby suppressing the function of Th1 T lymphocytes. In this way, they help control inflammation and promote an anti-inflammatory state. This is one of the reasons that not all individuals develop chronic pain despite experiencing the same stimulus. The absence or failure to induce these regulatory cells can sometimes explain the persistence of chronic pain [26].

Influence of Local versus Systemic Inflammation

IL-10 has been demonstrated to exert antinociceptive effects when stimulated by other interleukins. For instance, intrathecal administration of IL-27 elicited IL-10-dependent antinociception in a peripheral neuropathic pain model. Likewise, intrathecal delivery of IL-35 reduced pain-related behaviors in experimental autoimmune encephalomyelitis and ameliorated diabetic neuropathic pain. Both interleukins enhance IL-10 production within inflamed environments [64, 65].

IL-10 expression occurs in monocytes, macrophages, dendritic cells, T cells, B cells, astrocytes, microglia, and oligodendrocytes, the very cells driving inflammation [66]. Insufficient IL-10 production has been linked to chronic pain, as IL-10 suppresses second messenger pathways that promote pro-inflammatory cytokine release [67]. Although its therapeutic application remains under debate, particularly concerning the optimal route of administration (local versus systemic) and dosage (low versus high), there is consensus that IL-10 is pivotal in regulating inflammation [68].

Inflammation is orchestrated by both adaptive and innate immunity. While T lymphocytes (T cells) are central to adaptive responses, innate cells, such as neutrophils and macrophages, are equally critical. These cells initiate and sustain inflammation but also contribute to its resolution and tissue repair, functions now recognized as integral to the inflammatory cascade. Once trauma subsides, mechanisms must actively suppress inflammation [69].

Peripheral monocytes infiltrate the DRG and differentiate into macrophages. Their function depends on the microenvironment, contributing initially to inflammation but later supporting resolution—an area still underexplored. Type 2 macrophages, induced by cytokines such as IL-4 and TGF-β, are central to neural repair and anti-inflammatory activity. They release endogenous opioids at injury sites, modulating local opioid signaling and pain perception. The absence of these cells may predispose individuals to chronic pain development [50].

Therapeutic Implications and Interventions

Polarization of macrophages toward the M2 phenotype and the consequent release of IL-10 have been shown to reduce inflammation and alleviate pain in murine models of osteoarthritis induced by platelet-rich plasma infiltration. In this context, reductions in IL-1 and IL-6 levels were accompanied by decreased expression of NGF, highlighting the close interplay between immune modulation and nociceptive signaling [70].

The immune system interacts extensively with both the central and peripheral nervous systems, primarily through microglia. These resident immune cells of the central nervous system share numerous surface markers with macrophages and monocytes. Neuropathic pain is accompanied by increased numbers and heightened activity of microglia; however, their inactivation after pain onset does not necessarily resolve symptoms [71]. Microglia secrete inflammatory mediators including colony-stimulating factor (CSF-1), neuregulin-1 (NRG-1), metalloproteases, and caspases, which activate Toll-like receptors (TLR2 and TLR4) on neutrophils. These neutrophils, in turn, amplify and perpetuate inflammation, releasing mediators that re-stimulate microglia and sustain a pathological feed-forward loop [72]. Notably, this process is sex-dependent: male mice recover more rapidly from neuropathic pain despite early surges of pro-inflammatory cytokines, likely reflecting more efficient nerve regeneration [73].

Although microglia were initially thought to induce chronic pain in concert with astrocytes [72, 74], subsequent studies demonstrated that their early inactivation or depletion, followed by repopulation, can paradoxically trigger the re-emergence of pain [9, 75]. Advances in transcriptomic profiling have shifted attention from cell numbers to functional subtypes, identifying a CD11c⁺ “type 2” microglial phenotype that promotes pain resolution. Artificial induction of these cells in animal models has demonstrated analgesic potential [76].

Parallel research has targeted microglial polarization. Attempts to engineer an IL-4, IL-10, and TGF-β–rich microenvironment through plasmid-based gene expression show promise, polarizing microglia toward an anti-inflammatory phenotype. In murine models, IL-4 plasmids conferred the most sustained analgesic benefit, compared with IL-10 or TGF-β plasmids [77].

Robust evidence demonstrates fundamental sex differences in immune–neural pathways underlying pain. In men, spinal microglia, purinergic receptors, and BDNF signaling predominantly drive neuropathic and inflammatory pain, whereas in women, adaptive immune T cell-mediated mechanisms are more prominent [78]. Testosterone plays a crucial role in this divergence, functioning as a switch between microglial- and T cell–driven pathways [79]. Peripheral immune responses also differ by sex. Female mice exhibit greater sensitivity to macrophage-derived mediators, and macrophage-conditioned media can induce mechanical hypersensitivity only in females. Male and female macrophages differ in polarization, motility, and cytokine secretion after TNF-α stimulation, and the X-linked gene Tlr7 supports adaptive pain resolution in both sexes through potentially distinct mechanisms [80]. Sex hormones strongly modulate these pathways: testosterone predominantly suppresses immune activation, enhancing Th1 and reducing Th2 responses, while estrogen boosts humoral immunity, Th2 skewing, and immune-cell homing [81]. Human data from transgender individuals receiving testosterone therapy confirm hormone-driven remodeling of the interferon–TNF axis [82]. Key pain mediators also display sex-dependent activity. CGRP produces periorbital hypersensitivity only in females in preclinical models [83], and prolactin, which is higher in females, selectively sensitizes female nociceptors and promotes migraine-like behaviors [84].

Hormonal mechanisms are further shaped by developmental exposure: testosterone confers protection via androgen receptors and aromatization to estradiol activating estrogen receptor-α supraspinally [85], while estrogen effects vary by pain model and menstrual cycle phase, with low-estrogen states promoting CGRP release [86]. These mechanisms have meaningful therapeutic consequences. Rituximab is more effective in inducing remission in women, and microglial depletion reduces mechanical hypersensitivity after nerve injury in male mice but not female mice, except under specific conditions [87].

Despite preclinical evidence, translation of cytokine- and specialized pro-resolving mediator (SPMs)-targeted therapies into effective clinical pain treatments remains limited due to key mechanistic and clinical challenges. Systemic cytokine blockade has shown inconsistent analgesic benefit, particularly for TNF-α inhibitors [37]. Several biological constraints limit systemic cytokine therapies: pleiotropy, redundancy, and impaired host defense, such as increased tuberculosis reactivation with TNF-α inhibitors—especially monoclonal antibodies compared with soluble receptors [88]. Although gene therapy approaches (e.g., IL-10 viral vectors, HSV-TNFR constructs) provide targeted local expression in rodent models [38], none have advanced to late-phase clinical trials due to regulatory, safety, and delivery challenges [89]. SPMs such as resolvins, protectins, and maresins exert potent analgesic effects in preclinical inflammatory, neuropathic, and cancer pain models, where their roles include reduced neutrophil infiltration, enhanced macrophage efferocytosis, M2 polarization, and direct inhibition of nociceptor activity [90]. However, clinical evidence remains sparse, with human studies showing inconsistent analgesic benefit [91]. Paradoxically, early anti-inflammatory intervention may impede natural resolution, as nonsteroidal anti-inflammatory drugs (NSAIDs) and corticosteroids can block COX-dependent SPM biosynthesis and disrupt endogenous pathways involving neutrophil-derived alarmins (S100A8/A9) [92]. These observations challenge traditional anti-inflammatory strategies and emphasize the importance of optimal therapeutic timing. Major translational barriers include species differences in immune networks, limited relevance of animal pain models to human chronic pain, a focus on acute pain endpoints in preclinical studies, and absence of psychosocial and environmental modifiers in animal research.

Importantly, the persistence of chronic pain cannot be attributed to inflammation alone, since not all patients with acute pain progress to chronic states. This raises a key clinical question: why does pain chronification occur only in certain individuals? Conventional management of acute inflammatory pain often relies on NSAIDs, which inhibit cyclooxygenase (COX) activity, or corticosteroids, which provide broader immunosuppression. While these agents suppress inflammation, their long-term impact on pain trajectories remains uncertain. Indeed, animal studies indicate that blocking inflammation does not reverse chronic pain, and clinical trials of epidural corticosteroids for spinal pain have yielded inconsistent outcomes [93, 94].

Transcriptomic analyses provide further insight. Parisien et al. [92] demonstrated that disrupting the natural progression of inflammation may paradoxically foster pain persistence by preventing activation of endogenous resolution pathways. Neutrophils play a central role in this resolution phase, particularly through alarmins such as S100A8 and S100A9. Suppression of their expression, by NSAIDs such as diclofenac or by corticosteroids like dexamethasone, can predispose individuals to chronic pain [95]. This raises critical concerns about whether commonly used anti-inflammatory therapies may inadvertently perpetuate pain.

One mechanistic explanation involves specialized pro-resolving mediators (SPMs), particularly resolvins derived from omega-3 fatty acids. Resolvins, synthesized from eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) by macrophages, eosinophils, neutrophils, and endothelial cells, exert strong anti-inflammatory and pro-resolving effects. Aspirin-acetylated COX-2 can also trigger resolvin production. Resolvins attenuate neuroinflammation by reducing IFN-γ levels, downregulating IL-1 and IL-18, and promoting macrophage polarization from M1 to M2 phenotypes [96, 97]. Importantly, COX-1 inhibitors may disrupt this pathway, potentially explaining the paradoxical effects of NSAIDs.

Preclinical models support the analgesic actions of resolvins. Administration of resolvins has reduced pain following thoracotomy and tibial fracture [98]. Intrathecal delivery of resolvin D2 alleviated thermal and mechanical hypersensitivity and improved depressive-like behavior in murine fibromyalgia models [99]. These findings underscore the need to design pain therapies that preserve or enhance endogenous resolution pathways.

Another layer of complexity involves endogenous opioids. Microglia express kappa-opioid receptors (KOR), which interact with Met-enkephalin to suppress astrocytic proliferation. Met-enkephalin and related endorphins, released by T lymphocytes under inflammatory conditions, promote repair and analgesia. T cells and macrophages also express KOR and µ-opioid receptors (MOR), facilitating their regulatory functions. However, systemic opioid administration may cause global immunosuppression, rendering T cells anergic and unresponsive to tolerogenic signals such as IL-10 and TGF-β [100]. Thus, optimal timing, location, and delivery of opioids remain unresolved.

Electroacupuncture has been shown to induce macrophage-mediated opioid release, reducing chronic pain, particularly within IL-10-enriched anti-inflammatory environments supported by type 2 microglia [101]. Interestingly, morphine demonstrates dose-dependent immunomodulatory effects: low concentrations (50 nM) activate pro-inflammatory pathways in macrophages, an effect reversed by naloxone [102], whereas higher concentrations exert anti-inflammatory effects [103]. These findings emphasize the importance of distinguishing systemic from local opioid effects.

At the intracellular level, inflammation activates nuclear factor κB (NF-κB), driving the expression of pro-inflammatory cytokines but also inducing regulatory proteins such as A20 (TNFAIP3). A20 modulates β-arrestin 2, a key regulator of MOR signaling. Chronic opioid exposure suppresses A20 expression via overactivation of inhibitory microRNAs, particularly miR-873a-5p, leading to opioid tolerance [104]. Defining optimal dosing regimens that balance analgesia with preservation of endogenous regulatory mechanisms remains a significant challenge. Gene expression and proteomic studies in diverse pain states may ultimately guide personalized opioid therapies [105].

The anti-inflammatory activity mediated by IL-10 and TGF-β secreted by regulatory T cells and macrophages can be further stimulated by IL-35. This sex-dependent cytokine promotes a phenotypic shift in microglia from a pro-inflammatory state, characterized by IL-9 and monocyte chemoattractant protein-1 (MCP-1) expression, to an anti-inflammatory profile dominated by IL-10 production. Interestingly, IL-35 demonstrates greater efficacy in males than in females, though its therapeutic potential and optimal dosing require further clarification [106].

In summary, chronic pain emerges not merely from unresolved inflammation but from disrupted resolution processes. The interplay among macrophages, microglia, neutrophils, resolvins, and opioids highlights the need for therapeutic approaches that restore balance rather than indiscriminately suppress immune activity. A deeper understanding of cellular phenotypes, gene expression signatures, and endogenous pro-resolving pathways may enable the development of personalized interventions that prevent or reverse chronic pain.

Gut Microbiota and Nutritional Immunomodulation

The gut–brain–immune axis has emerged as a promising therapeutic target in chronic pain. Short-chain fatty acids (SCFAs), particularly butyrate produced by specific gut microbiota, can polarize macrophages toward the M2 phenotype and reduce inflammation, including within the dorsal root ganglion. This was demonstrated in both obese and non-obese mice in which intestinal microbiota transfer alone was sufficient to confer analgesic and immunomodulatory effects [107]. These findings emphasize the role of diet in modulating pain by shifting the immune system from pro- to anti-inflammatory states. Similarly, supplementation with the probiotic Lactobacillus paracasei in a murine model of lumbar disc herniation reduced mechanical and thermal hypersensitivity. This effect was associated with decreased Th1, Th2, and Th17 lymphocytes, alongside increased regulatory T cells (Tregs) and upregulation of Foxp3, TGF-β, and IL-10 [108]. Collectively, these results highlight the importance of gut microbial composition, dietary intake, and cautious use of antibiotics. Antibiotics may eradicate beneficial microbiota, thereby facilitating colonization by antibiotic-resistant and pro-inflammatory strains.

The mechanisms through which gut microbiota and diet shape systemic and neuroimmune responses relevant to chronic pain are illustrated in Fig. 3. Probiotics, dietary fiber, and microbial diversity enhance SCFA production, M2 macrophage polarization, and Treg activation, fostering anti-inflammatory cytokine release (IL-10, IL-35). Conversely, dysbiosis and excessive antibiotic use impair mucosal barriers, recruit neutrophils, and enhance pro-inflammatory cytokine production (TNF-α, IL-23, IL-6), sustaining neuroinflammation.

Fig. 3.

Gut–brain–immune axis modulation in chronic pain. Probiotics, microbial diversity, and dietary fiber enhance beneficial microbial adherence, antigen presentation, and short-chain fatty acid (SCFA) production, promoting M2 macrophage polarization, Treg activation, and anti-inflammatory cytokine release [interleukin (IL) IL-10, IL-35]. Conversely, dysbiosis and antibiotic overuse reduce mucus barrier integrity, impair Paneth cell function, promote neutrophil recruitment, and enhance pro-inflammatory cytokine production of tumor necrosis factor alpha (TNF-α, IL-23, IL-6), contributing to neuroinflammation and pain persistence

A recent comprehensive review reinforces the concept of a bidirectional gut–brain–immune axis that regulates neuroinflammation, glial activation, and peripheral nerve regeneration. Preclinical data across chemotherapy-induced, diabetic, and trauma-related neuropathies support targeted interventions, including probiotics, fecal microbiota transplantation, dietary modulation, and vitamin D supplementation, as strategies to improve pain outcomes through immunometabolic pathways [109]. These findings strengthen the rationale for personalized immuno-nutritional approaches in chronic pain management.

Clinical Translation and Future Directions

Mechanistic discoveries must ultimately inform precision medicine strategies, with interventions tailored to each patient’s immunological profile and shaped by variability in genetics, age, sex, comorbidities, and microbiome composition. A central priority is the identification of immune signatures that predict the risk of pain chronification, enabling proactive interventions before maladaptive neuroimmune alterations become established [110]. Biomarker panels integrating cytokine expression, immune cell phenotypes, genetic polymorphisms, epigenetic modifications, and microbiome-derived features could generate comprehensive “immunological fingerprints” to guide individualized therapy.

Equally important is the temporal dimension of inflammation. Evidence indicates that therapeutic windows are narrower than previously assumed; some interventions may be effective during acute inflammatory phases but deleterious if administered during resolution [111]. This underscores that timing can be as critical as the therapeutic modality itself.

Such insights support the development of synergistic, multi-target approaches capable of modulating several nodes of the immuno-inflammatory cascade while safeguarding endogenous resolution mechanisms, always keeping in mind the problem of translational use of experimental data [112]. Promising strategies include combining specialized pro-resolving mediators (SPMs) with selective immunomodulation, interventions supporting the gut–brain–immune axis, and approaches addressing psychosocial determinants of pain.

The ultimate goal is to stratify patients according to their immune profiles, identifying subgroups most likely to benefit from specific interventions. With the emergence of point-of-care immune profiling, clinicians may soon assess inflammatory status in real time and dynamically adapt treatments. This would enable a paradigm shift from static, diagnosis-based algorithms toward real-time, mechanism-driven therapy, treating pain as an individualized immunological entity. Achieving this vision will require advances in technology, trial methodology, regulatory frameworks, and healthcare delivery.

Human Longitudinal Studies and Immune Biomarker Profiling in Chronic Pain

Translation of preclinical immune mechanisms into clinical practice depends on validated human biomarkers. CSF fluid analysis provides direct insight into central nervous system immune activity, with a comprehensive review identifying 49 proteins studied across chronic pain conditions [113]. Among these, 21 proteins showed consistent upregulation, with IL-8 specifically elevated in nociceptive pain states [4]. Critically, 31 proteins demonstrated positive correlation with pain intensity in at least one study, establishing their potential as severity biomarkers. Condition-specific CSF signatures have emerged. Fibromyalgia studies show elevated CSF fractalkine (CX3CL1) and confirm increased IL-8 in both CSF and plasma, supporting neuroinflammatory involvement [114]. In CRPS, CSF levels of IL-6, IL-8, TNF-α, and glial fibrillary acidic protein indicate combined immune and glial activation [115]. Paradoxically, some chronic pain conditions—such as painful polyneuropathy—show reduced CSF cytokine levels across 23 of 40 mediators (including TNF-α, IL-2, IL-6, IL-10), suggesting immune exhaustion or compartmentalized inflammation rather than global hyperactivation [116]. Multi-omic platforms now enable broad biomarker profiling. Proteomic analyses identify immune signatures predicting postsurgical pain resolution, while lipidomic metabolites track persistent postsurgical pain [117]. However, interpretation requires attention to methodological constraints. Biomarker correlations differ according to blood–brain barrier integrity [118], while most studies use single time points rather than serial trajectories, and immunoassay platforms lack standardization, producing inconsistent quantitative values. Future progress depends on prospective cohorts with repeated biomarker sampling, integration with imaging, genetics, and psychosocial variables, and application of machine learning for patient stratification and treatment selection [117]. Ultimately, validated biomarker panels may enable early chronification risk prediction, guide mechanism-targeted therapies, and provide objective endpoints for clinical trials.

Limitations

Several limitations should be considered when reading this review. As a narrative review, our synthesis lacks the systematic methodology and quantitative analysis of a systematic review or meta-analysis, potentially introducing selection bias, even if the use of SANRA guidelines may provide a better reliability. The heavy reliance on preclinical rodent models raises questions about translational validity, given significant species differences in immune and pain processing systems. Most human studies provide cross-sectional rather than longitudinal data, limiting our understanding of temporal dynamics during pain chronification. Publication bias toward positive findings may overestimate the importance of certain immune pathways while neglecting others. The heterogeneity of chronic pain conditions and patient populations makes it challenging to draw universal conclusions about immune mechanisms. Finally, the complex interactions between immunological, genetic, epigenetic, psychological, and social factors in pain chronification could not be fully addressed within the scope of this review. These limitations highlight the need for prospective human studies with standardized immune profiling, careful patient phenotyping, and integration of multidimensional data to advance our understanding of immune contributions to chronic pain.

Furthermore, several mechanistic aspects discussed in this review—such as the paradoxical effects of early anti-inflammatory suppression, the differential impact of systemic versus local TNF-α modulation, the immunoregulatory role of T cells, and microglial depletion or repopulation—are predominantly supported by preclinical or translational animal data. These findings provide valuable mechanistic hypotheses but should not be interpreted as established clinical phenomena. Additional prospective human studies are necessary to validate these processes and determine their clinical significance in chronic pain chronification.

Conclusion

The chronification of pain involves a complex interaction between immune, nervous, and endocrine systems. Central to this process is the regulation of inflammation, primarily through immune cells such as T cells, B cells, macrophages, and microglia. The presence or absence of regulatory cytokines like IL-10 and TGF-β appears to be a key determinant in whether pain resolves or persists. Modifiable factors including medications, diet, and microbiota also play important roles. Continued research on localized inflammation, immune cell phenotypes, and resolution pathways is essential for developing more targeted, personalized treatments for chronic pain.

Author Contributions

Marcelo Abel Alcon Moncada: Contributed to the conception and design of the review, literature selection, critical interpretation of immunological mechanisms, and revision of the manuscript for important intellectual content. Marco Antonio Narvaez Tamayo: Contributed to the conceptual framework, clinical interpretation of neuroimmune pathways, and critical revision of the manuscript for accuracy and clinical relevance. Miguel Andres Narvaez Encinas: responsible for overall conceptualization, literature review, integration of mechanistic evidence, drafting the initial and revised versions of the manuscript, addressing reviewer comments, and coordinating the final approval of the submitted version. Matteo Luigi Giuseppe Leoni: Contributed to the development and preparation of the figures, verified the scientific accuracy of visual and textual content, and ensured medical relevance throughout the manuscript. Giustino Varrassi: Provided senior scientific oversight, contributed to the conceptual design, critically reviewed the manuscript for intellectual content, ensured scientific rigor, and approved the final version. All authors reviewed and approved the final manuscript and agree to be accountable for all aspects of the work.

Funding

No funding or sponsorship was received for this study or publication of this article.

Data Availability

Data sharing is not applicable to this article as no new datasets were generated or analyzed during the current study.

Declarations

Conflicts of Interest

Marcelo Abel Alcon Moncada has nothing to disclose. Marco Antonio Narvaez Tamayo has nothing to disclose. Miguel Andres Narvaez Encinas has nothing to disclose. Matteo Luigi Giuseppe Leoni has nothing to disclose. Giustino Varrassi is an Editor-in-Chief of Pain and Therapy. Prof. Varrassi was not involved in the selection of peer reviewers for this manuscript nor in any of the subsequent editorial decisions. None of the authors has had a change in affiliation since completion of this work.

Ethical Approval

This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.