Germinal center formation, immunoglobulin production and hindlimb nociceptive sensitization after tibia fracture

Autoantibodies cause nociceptive sensitization in a mouse model of degenerative osteoarthritis

ABSTRACT

Previous preclinical and translational studies suggest that tissue trauma related to bony fracture and intervertebral disk disruption initiates the formation of pronociceptive antibodies that support chronic musculoskeletal pain conditions. This study tested this hypothesis in the monosodium iodoacetate (MIA) mouse model of osteoarthritis (OA) and extended the findings using OA patient samples. Monosodium iodoacetate was injected unilaterally into the knees of male and female wild-type (WT) and muMT mice (lacking B cells) to induce articular cartilage damage. Repeated nociceptive behavioral testing was performed, and serum was collected for antibody isolation and passive transfer experiments. Serum antibodies collected from patients with OA were tested in MIA-treated muMT mice. Biochemical analyses were performed on knee joint tissues. Monosodium iodoacetate-treated WT mice developed chronic ipsilateral hindlimb allodynia, hyperalgesia, and unweighting, but these pain behaviors were absent in MIA-treated muMT mice, indicating that cartilage injury-induced pain is B-cell dependent. IgM accumulation was observed in the knee tissues of MIA-treated mice, and intra-articular injection of IgM from MIA-treated mice into MIA-treated muMT mice caused nociceptive sensitization. Similarly, intra-articular injection of IgM from patients with OA was pronociceptive in muMT MIA mice and control subject IgM had no effect. Monosodium iodoacetate-injected joints demonstrate elevated levels of complement component 5a (C5a) and C5a receptor blockade using intra-articular PMX-53-reduced sensitization. These data suggest that MIA-treated mice and patients with OA generate pronociceptive antibodies, and further support the pronociceptive autoimmunity hypothesis for the transition from tissue injury to chronic musculoskeletal pain.

Effects of immunosuppression after limb fracture in mice on nociceptive, cognitive, and anxiety-related outcomes

Introduction

Chronic pain is a common and problematic consequence of injuries with few proven methods for prevention or treatment. In addition to pain, functional limitations and neuropsychiatric changes such as cognitive impairment and anxiety worsen outcomes.

Objectives

To determine whether inhibiting activation of the adaptive immune response after limb fracture would reduce pain, functional loss, memory changes, and anxiety.

Methods

These experiments used a murine tibial fracture/cast immobilization model that develops these adverse outcomes. Adaptive immunity was blocked using the immunosuppressant FK506 beginning at the time of fracture.

Results

The administration of FK506 reduced mechanical allodynia and hind limb unweighting for weeks after cast removal as well as nonevoked pain measures. Fracture was associated with working memory loss in the Y-maze assay in vehicle- but not FK506-treated mice. Object recognition memory was not improved with FK506 after fracture. Also, vehicle- but not FK506-treated mice developed an anxiety phenotype. Impaired running wheel performance after cast removal over the following 2 weeks was not improved with FK506 administration. In addition, FK506 treatment blocked Immunoglobulin M (IgM) accumulation in the skin of the fractured limbs, and hippocampal enhancement of matrix metalloproteinase-8 expression, a metalloproteinase associated with neuroplastic changes after injuries, was completely blocked.

Conclusion

Taken together, our results show that blocking the adaptive immune response after limb trauma reduces the severity of nociceptive and biological changes. The same treatment may reduce the adverse consequences of anxiety and memory deficits using some measures, but other measures of memory are not affected, and activity is not enhanced.

Osteoimmunology of Fracture Healing

PURPOSE OF REVIEW

The purpose of this review is to summarize what is known in the literature about the role inflammation plays during bone fracture healing. Bone fracture healing progresses through four distinct yet overlapping phases: formation of the hematoma, development of the cartilaginous callus, development of the bony callus, and finally remodeling of the fracture callus. Throughout this process, inflammation plays a critical role in robust bone fracture healing.

RECENT FINDINGS

At the onset of injury, vessel and matrix disruption lead to the generation of an inflammatory response: inflammatory cells are recruited to the injury site where they differentiate, activate, and/or polarize to secrete cytokines for the purposes of cell signaling and cell recruitment. This process is altered by age and by sex. Bone fracture healing is heavily influenced by the presence of inflammatory cells and cytokines within the healing tissue.

Alpha-7 Nicotinic Acetylcholine Receptor Activation Inhibits Trauma Induced Pronociceptive Autoimmune Responses

Both autonomic nervous system dysfunction and immune system activation are characteristic of chronic pain after limb injuries. Cholinergic agonists reduce immune system activation in many settings. We hypothesized, therefore, that alpha-7 nicotinic acetylcholine receptor (α7nAChR) agonist administration would reduce nociceptive and immune changes after tibia fracture and cast immobilization in mice. Fracture mice were treated with either vehicle, a low (.2 mg/kg) dose, or a high (1 mg/kg) dose of the selective α7nAChR agonist PNU-282987 for 4 weeks. We assessed hindpaw allodynia and weight bearing as behavioral outcomes. The assessment of adaptive immune responses included regional lymph node hypertrophy, germinal center formation, α7nAChR expression, and IgM deposition. Assessment of innate immune system activation focused on IL-1β and IL-6 generation in fractured hindlimb skin. We observed that mechanical allodynia and unweighting were alleviated by PNU-282987 treatment. Drug treatment also reduced popliteal lymph node hypertrophy and germinal center formation. Immunohistochemical studies localized α7nAChR to germinal center B lymphocytes, and this expression increased after fracture. Analysis of fracture limb hindpaw skin demonstrated increased inflammatory mediator (IL-1β and IL-6) levels and IgM deposition, which were abrogated by PNU-282987. Serum analyses demonstrated fracture-induced IgM reactivity against keratin 16, histone 3.2, GFAP, and NMDAR-2B. Administration of PNU-282987 reduced the enhancement of IgM reactivity. Collectively, these data suggest that the α7nAChR is involved in regulating posttraumatic innate and adaptive immune responses and the associated nociceptive sensitization. PERSPECTIVE: These studies evaluate the effects of a selective α7nAChR agonist in a tibial fracture/cast immobilization model of limb pain. Administration of the drug reduced nociceptive and functional changes 4 weeks after injury. These novel findings suggest that well-tolerated α7nAChR agonists may be viable analgesics for chronic pain after limb injuries.

Pronociceptive autoantibodies in the spinal cord mediate nociceptive sensitization, loss of function, and spontaneous pain in the lumbar disk puncture model of chronic back pain

ABSTRACT

Previously, we observed that B cells and autoantibodies mediated chronic nociceptive sensitization in the mouse tibia fracture model of complex regional pain syndrome and that complex regional pain syndrome patient antibodies were pronociceptive in fracture mice lacking mature B cells and antibodies (muMT). The current study used a lumbar spinal disk puncture (DP) model of low back pain in wild-type (WT) and muMT mice to evaluate pronociceptive adaptive immune responses. Spinal disks and cords were collected 3 weeks after DP for polymerase chain reaction and immunohistochemistry analyses. Wild-type DP mice developed 24 weeks of hindpaw mechanical allodynia and hyperalgesia, grip weakness, and a conditioned place preference response indicative of spontaneous pain, but pain responses were attenuated or absent in muMT DP mice. Spinal cord expression of inflammatory cytokines, immune cell markers, and complement components were increased in WT DP mice and in muMT DP mice. Dorsal horn immunostaining in WT DP mice demonstrated glial activation and increased complement 5a receptor expressionin spinal neurons. Serum collected from WT DP mice and injected into muMT DP mice caused nociceptive sensitization, as did intrathecal injection of IgM collected from WT DP mice, and IgM immune complexes were observed in lumbar spinal disks and cord of WT DP mice. Serum from WT tibia fracture mice was not pronociceptive in muMT DP mice and vice versa, evidence that each type of tissue trauma chronically generates its own unique antibodies and targeted antigens. These data further support the pronociceptive autoimmunity hypothesis for the transition from tissue injury to chronic musculoskeletal pain state.

Discovery of novel targets in a complex regional pain syndrome mouse model by transcriptomics: TNF and JAK-STAT pathways

Complex Regional Pain Syndrome (CRPS) represents severe chronic pain, hypersensitivity, and inflammation induced by sensory-immune-vascular interactions after a small injury. Since the therapy is unsatisfactory, there is a great need to identify novel drug targets. Unbiased transcriptomic analysis of the dorsal root ganglia (DRG) was performed in a passive transfer-trauma mouse model, and the predicted pathways were confirmed by pharmacological interventions. In the unilateral L3-5 DRGs 125 genes were differentially expressed in response to plantar incision and injecting IgG of CRPS patients. These are related to inflammatory and immune responses, cytokines, chemokines and neuropeptides. Pathway analysis revealed the involvement of Tumor Necrosis Factor (TNF) and Janus kinase (JAK-STAT) signaling. The relevance of these pathways was proven by abolished CRPS IgG-induced hyperalgesia and reduced microglia and astrocyte markers in pain-associated central nervous system regions after treatment with the soluble TNF alpha receptor etanercept or JAK inhibitor tofacitinib. These results provide the first evidence for CRPS-related neuroinflammation and abnormal cytokine signaling at the level of the primary sensory neurons in a translational mouse model and suggest that etanercept and tofacitinib might have drug repositioning potentials for CRPS-related pain.

Autonomic Regulation of Nociceptive and Immunologic Changes in a Mouse Model of Complex Regional Pain Syndrome

Chronic pain frequently develops after limb injuries, and its pathogenesis is poorly understood. We explored the hypothesis that the autonomic nervous system regulates adaptive immune system activation and nociceptive sensitization in a mouse model of chronic post-traumatic pain with features of complex regional pain syndrome (CRPS). In studies sympathetic signaling was reduced using 6-hydroxydopamine (6-OHDA) or lofexidine, while parasympathetic signaling was augmented by nicotine administration. Hindpaw allodynia, unweighting, skin temperature, and edema were measured at 3 and 7 weeks after fracture. Hypertrophy of regional lymph nodes and IgM deposition in the skin of injured limbs were followed as indices of adaptive immune system activation. Passive transfer of serum from fracture mice to recipient B cell deficient (muMT) mice was used to assess the formation of pain-related autoantibodies. We observed that 6-OHDA or lofexidine reduced fracture-induced hindpaw nociceptive sensitization and unweighting. Nicotine had similar effects. These treatments also prevented IgM deposition, hypertrophy of popliteal lymph nodes, and the development of pronociceptive serum transfer effects. We conclude that inhibiting sympathetic or augmenting parasympathetic signaling inhibits pro-nociceptive immunological changes accompanying limb fracture. These translational results support the use of similar approaches in trials potentially alleviating persistent post-traumatic pain and, possibly, CRPS. PERSPECTIVE: Selective treatments aimed at autonomic nervous system modulation reduce fracture-related nociceptive and functional sequelae. The same treatment strategies limit pain-supporting immune system activation and the production of pro-nociceptive antibodies. Thus, the therapeutic regulation of autonomic activity after limb injury may reduce the incidence of chronic pain.

Animal Models of Complex Regional Pain Syndrome Type I

Complex regional pain syndrome (CRPS) is a chronic pain disorder characterized by spontaneous or evoked regionally-confined pain which is out of proportion to the initial trauma event. The disease can seriously affect the quality of the patients' life, increase the psychological burden, and cause various degrees of disability. Despite the awareness of CRPS among medical practitioners for over a century, its pathogenesis remains unclear, and the available treatment is still unsatisfactory. Effective animal models are the foundation of disease research, which is helpful in understanding the pathogenesis and an in-depth exploration of the appropriate therapeutic approaches. Currently, researchers have established a series of animal models of the disease. There are four main CRPSI animal models: chronic post-ischemic pain (CPIP) model, tibial fracture/cast immobilization model, passive transfer-trauma model, and the needlestick-nerve-injury (NNI) model. The modeling methods of these models are constantly improving over time. In preclinical studies, the interpretation of experimental results and the horizontal comparison between similar studies may be affected by the nature of the experimental animal breeds, sex, diet, and psychology. There is need to facilitate the choice of appropriate animal models and avoid the interference of the factors influencing animal models on the interpretation of research results. The review will provide a basic overview of the influencing factors, modeling methods, and the characteristics of CRPSI animal models.

LncRNA HAGLR absorbing miR-214-3p promotes BMP2 expression and improves tibial fractures

OBJECTIVE

To determine whether long-chain non-coding RNA (lncRNA) HAGLR can regulate BMP2 by absorbing microRNA-214-3p (miR-214-3p), and to explore its role and mechanism in tibial fracture (TF) healing.

METHODS

The HAGLR, miR-214-3p, and BMP2 expression levels in TF and in adjacent normal tissues were measured using quantitative real-time polymerase chain reaction (qRT-PCR). MC3T3-E1 osteoblasts were used to construct the in vitro model. HAGLR was localized subcellularly through RNA-fluorescence in situ hybridization (FISH). A dual-luciferase report experiment confirmed that miR-214-3p has a targeted relationship with HAGLR and BMP2. It was then divided into a HAGLR over-expression group, an miR-214-3p mimic group, a HAGLR+miR-214-3p mimic group, an sh-HAGLR group, a BMP over-expression group, an sh-HAGLR+over-expression BMP2 group, and a negative control group. The proliferation and apoptosis of the MC3T3-E1 osteoblasts were examined using MTT assays and flow cytometry. A TF model was established in male C57BL/6J mice. The serum alkaline phosphatase (ALP) and osteoprotegerin (OPG) levels in the sham group, the TF group, and the TF group that were injected with HAGLR were compared using ELISA. Hematoxylin-eosin (HE) staining was used to confirm the fracture healing in the mouse model.

RESULTS

Compared with the adjacent normal tissues in the TF patients, the HAGLR and BMP2 expressions decreased but the miR-214-3p expressions increased in the TF tissues (P<0.05). HAGLR, an endogenous sponge, absorbed the miR-214-3p, and the BMP2 expression was directly regulated by miR-214-3p. HAGLR increased the proliferative activity of the osteoblasts and decreased the apoptosis rate. The over-expression of miR-214-3p partly reversed the effect of HAGLR on the cells, decreased the proliferative activity, and increased the apoptosis rate (all P<0.05). The sh-HAGLR decreased the proliferative activity and increased the apoptosis rate. But after the over-expression of BMP2, the proliferative activity of the cells was higher, and the apoptosis rate was lower than it was in the sh-HAGLR group (all P<0.05). The over-expression of HAGLR can up-regulate the ALP and OPG levels in mouse models (P<0.05).

CONCLUSION

lncRNA HAGLR can regulate BMP2 to play a protective role in TF by absorbing miR-214-3p, and it is related to promoting the osteoblast proliferation, inhibiting apoptosis, and up-regulating the serum ALP and OPG levels to accelerate bone healing.

IL-6 signaling mediates the germinal center response, IgM production and nociceptive sensitization in male mice after tibia fracture

BACKGROUND

Up-regulated interleukin 6 (IL-6) signaling, immune system activation, and pronociceptive autoantibodies are characteristic of complex regional pain syndrome (CRPS). IL-6 is known to promote B cell differentiation, thus we hypothesized that IL-6 signaling plays a crucial role in the development of adaptive immune responses and nociceptive sensitization in a murine tibia fracture model of CRPS.

METHODS

Mice deficient in IL-6 expression (IL-6^-/-) or B cell deficient (muMT) underwent tibia fracture and 3 weeks of cast immobilization or sham injury. The deposition of IgM in fractured limbs was followed using Western blotting, and passive serum transfer to muMT fracture mice was used to detect nociception-supporting autoantibodies. Lymph nodes were assessed for hypertrophy, IL-6 expression was measured using qPCR and ELISA, and germinal center formation was evaluated using FACS and immunohistochemistry. The therapeutic effects of exogenous neutralizing anti-IL-6 antibodies were also evaluated in the CRPS fracture model.

RESULTS

Functional IL-6 signaling was required for the post fracture development of nociceptive sensitization, vascular changes, and IgM immune complex deposition in the skin of injured limbs. Passive transfer of sera from wild-type, but not IL-6^-/- fracture mice into muMT fracture mice caused enhanced allodynia and postural unweighting. IL-6^-/- fracture mice displayed reduced popliteal lymphadenopathy after fracture. Germinal center responses were detected in the popliteal lymph nodes of wild-type, but not in IL-6^-/- fracture mice. We observed that IL-6 expression was dramatically enhanced in popliteal lymph node tissue after fracture. Conversely, administration of anti-IL-6 antibodies reduced nociceptive and vascular changes after fracture and inhibited lymphadenopathy.

CONCLUSIONS

Collectively, these data support the hypothesis that IL-6 signaling in the fracture limb of mice is required for germinal center formation, IgM autoantibody production and nociceptive sensitization. Anti-IL-6 therapies might, therefore, reduce pain after limb fracture or in the setting of CRPS.

Review of complex regional pain syndrome and the role of the neuroimmune axis

Background

Complex regional pain syndrome (CRPS) is a progressive and painful disease of the extremities that is characterized by continuous pain inconsistent with the initial trauma. CRPS is caused by a multi-mechanism process that involves both the peripheral and central nervous system, with a prominent role of inflammation in CRPS pathophysiology. This review examines what is currently known about the CRPS inflammatory and pain mechanisms, as well as the possible impact of neurostimulation therapies on the neuroimmune axis of CRPS.

Study design

A narrative review of preclinical and clinical studies provided an overview of the pain and inflammatory mechanisms in CRPS and addressed the effect of neurostimulation on immunomodulation.

Methods

A systematic literature search was conducted based on the PRISMA guidelines between September 2015 to September 2020. Data sources included relevant literature identified through searches of PubMed, Embase and the Cochrane Database of Systematic Reviews.

Results

Sixteen preclinical and eight clinical studies were reviewed. Preclinical studies identified different mechanisms of pain development in the acute and chronic CRPS phases. Several preclinical and clinical studies investigating inflammatory mechanisms, autoimmunity, and genetic profiles in CRPS, supported a role of neuroinflammation in the pathophysiology of CRPS. The immunomodulatory effects of neurostimulation therapy is still unclear, despite clinical improvement in the CRPS patients.

Conclusions

Increasing evidence supports a role for inflammation and neuroinflammation in CRPS pathophysiology. Preliminary neurostimulation findings, together with the role of (neuro)inflammation in CRPS, seems to provide a compelling rationale for its use in CRPS pain treatment. The possible immunomodulatory effects of neurostimulation opens new therapeutic possibilities, however further research is needed to gain a better understanding of the working mechanisms.

Autoimmune regulation of chronic pain

Chronic pain is an unpleasant and debilitating condition that is often poorly managed by existing therapeutics. Reciprocal interactions between the nervous system and the immune system have been recognized as playing an essential role in the initiation and maintenance of pain. In this review, we discuss how neuroimmune signaling can contribute to peripheral and central sensitization and promote chronic pain through various autoimmune mechanisms. These pathogenic autoimmune mechanisms involve the production and release of autoreactive antibodies from B cells. Autoantibodies-ie, antibodies that recognize self-antigens-have been identified as potential molecules that can modulate the function of nociceptive neurons and thereby induce persistent pain. Autoantibodies can influence neuronal excitability by activating the complement pathway; by directly signaling at sensory neurons expressing Fc gamma receptors, the receptors for the Fc fragment of immunoglobulin G immune complexes; or by binding and disrupting ion channels expressed by nociceptors. Using examples primarily from rheumatoid arthritis, complex regional pain syndrome, and channelopathies from potassium channel complex autoimmunity, we suggest that autoantibody signaling at the central nervous system has therapeutic implications for designing novel disease-modifying treatments for chronic pain.