Open access peer-reviewed chapter - ONLINE FIRST

Cytokine Profile as a Marker of Cell Damage and Immune Dysfunction after Spinal Cord Injury

By Georgii Telegin, Aleksandr Chernov, Alexey Belogurov, Irina Balmasova, Nikolai Konovalov and Aleksandr Gabibov

Submitted: September 23rd 2020Reviewed: December 22nd 2020Published: January 15th 2021

DOI: 10.5772/intechopen.95614

Downloaded: 115

Abstract

The study reviews findings of the recent experiments designed to investigate cytokine profile after a spinal cord injury. The role of key cytokines was assessed in the formation of cellular response to trauma. The specific immunopathogenic interaction of the nervous and immune systems in the immediate and chronic post-traumatic periods is summarized. The practicality of a step-by-step approach to assessing the cytokine profile in spinal cord injury is shown, the need to take into account the combination of pathogenetic and protective components in the implementation regulatory effects of individual cytokines, their integration into regenerative processes in the damaged spinal cord, which allows a rational approach to the organization of the treatment process and the development of new medicines.

Keywords

  • Spinal cord injury
  • glial scare
  • cytokines
  • cellular response

1. Introduction

Spinal cord injury (SCI) is a significant global public health issue and a common cause of permanent disability in patients [1, 2]. According to the WHO world population estimates, every year up to 500,000 people suffer a spinal cord injury [3], including young adults between the ages of 20 and 35 [4]. The annual incidence rate of traumatic SCI (TSCI) in developed countries is approx. 3 per 100,000 population [5], though these data could be inconsistent with the big picture, since 16%-30% of patients with spinal injuries die before being admitted to the hospital [6, 7]. Thus, functional recovery of the spinal cord with structural damages caused by trauma is recognized as one of the most challenging and socially essential topics of modern regenerative medicine [8].

Mortality from SCI depends mainly on the severity of spinal cord lesion, and at the pre-hospital phase, it reaches 37% [9]. In-hospital mortality rates are affected by the severity of spinal cord damage and the SCI-related early or late complications, as well as the timeliness of specialized health care provision. Mortality rates range between 8 and 58.3% in different medical settings, depending on their capacity [10, 11, 12]. High mortality rates (ranging between 16% and 18%) are reported for children. Frequently they are associated with a trauma of the cervical spine, especially its upper portion [13, 14, 15, 16]. The leading causes of death comprise respiratory problems, cardiovascular disorders, thromboembolic events, infectious complications, and suicides [17]. Disability rates after vertebral column and spinal cord injuries vary from 57.5 to 100%, and the data indicate a trend towards an annual increase of people with disabilities after SCI [18].

Prominent underlying SCI causes include road traffic injuries (36–43%), falls from height (24.2–63.2%), shallow water diving (3–32%), sports activities and accidents (22.5%) [19, 20, 21, 22], while criminal traumas account for 10-25% of the injuries [23]. The leading causes of injuries vary for different years and across geographic regions [24]. In this context, spinal cord injuries related to ocean waves are commonly reported in the coastal areas, among beachgoers, etc. [25].

Spinal cord injuries resulting from vertebral column trauma are reported for 36–72% of patients [10, 11, 26, 27]. Craniocerebral trauma is more commonly associated with cervical spine fractures (18–72%). Thoracic spine fractures are usually combined with multiple non-vertebral injuries, such as bone fractures (10.3–48%), traumas of the thoracic cavity and its internal organs (as high as 52%), and lumbar spine injuries – with broken limb bones (up to 27%) and pelvic bones (up to 15%), and damage of the abdominal organs (9.8–18.7%) [21, 27, 28, 29, 30, 31]. By type, SCIs are divided into open (penetrating) and closed (nonpenetrating) injuries to the spine. In peacetime, closed SCI account for 70.1–88.6% of cases [26, 32].

2. Factors that determine the course of spinal cord injury and its classification

The level and length of SCI, as well as the timeline of the treatment of spinal cord compression, affect the grade and severity of neurological problems and, ultimately, mobility and self-care of patients, as well as their prognosis and recovery, and return to normal life [33]. The cervical spine trauma is associated with spinal cord lesions in 12–70% of injured people and characterized by the predominance of severe damages (contusion, compression, haematomyelia) and high mortality rates (35–70%). Spinal cord lesions occur in 31–75% of thoracic and lumbar spine traumas [21, 26, 27, 29, 34]. In general, injuries of the cervical spine account for 17–61% of cases [30, 34], thoracic – 7.2–40% [26, 29, 34, 35], and lumbar spine – from 8.7 to 57.8% [26, 29, 31, 34].

Types of spinal cord trauma include contusion, concussion, compression, crush, and disruption. The spinal cord’s compression is found in 20–26.7% of the injured persons, compression and contusion – in 40–50.5%, compression and crush – in 7–15.7%, and anatomical disruption – in 4.3–7.1% of patients [34]. The grade of the spinal cord injury is one of the principal prognostic factors. The distinction is made between “complete” and “incomplete” SCI, or its morphological disruption (anatomical or axonal). Complete SCI at the cervical level is reported for 33.7–52% of patients, thoracic level – 12.5-54%, and at the lumbar level – 15–21% [33].

In addition to SCI, non-traumatic spinal cord lesions may occur due to epidural abscess and hematoma, intradural tumor or other types of metastatic tumors, and complications after surgical treatment [7, 36]. The treatment of the acute phase in SCI cases takes more time than the treatment of spinal cord lesions of non-traumatic origin. Also, patients with SCI are more likely to have urinary tract infections and other complications [37].

Before early 1990s, a uniform or generally recognized classification system of SCI was not available. Physicians usually distinguished different levels of injury, complete and incomplete SCI.

Then, in 1992 the American Spinal Injury Association (ASIA) developed a classification system for identifying the severity of spinal cord injury based on descriptions of motor and sensory functions [38, 39].

  1. A = complete spinal cord injury: no motor or sensory function is preserved in the sacral segments S4-S5;

  2. В = incomplete injury: sensory function preserved but not motor function is preserved below the neurological level and includes the sacral segments S4-S5;

  3. С = incomplete injury: motor function is preserved below the neurological level, but less than half of key muscles below the neurological level have a muscle grade less than 3 (i.e., they are not strong enough to move against gravity);

  4. D = incomplete injury: motor function is preserved below the neurological level, and at least half of key muscles below the neurological level have a muscle grade of 3 or more (i.e., the joints can be moved against gravity);

  5. Е = normal: motor and sensory functions are normal.

Many researchers indicate that in addition to diagnostic implications the ASIA scale has tremendous prognostic value [38, 40, 41, 42, 43]. Later, the AO Subaxial Cervical Spine Injury Classification (SLIC) system was published, which includes morphological information in its scoring that helps determine the extent of the patient’s injury [44].

Practicing surgeons and radiologists use classification and scoring systems Subaxial Cervical Injury Classification and Severity (SLICS) and Thoracolumbar Injury Classification and Severity (TLICS), respectively, to evaluate the severity of cervical and thoracolumbar spine injuries [45]. Besides, the Spinal Cord Independence Measure (SCIM) has been developed to assess functional improvements of the spinal cord in the course of treatment and rehabilitation. Also, the SCIM has prognostic value [46].

SCI is classified chronologically into the following four phases:

  1. acute phase – less than first 48 hours after the injury; the clinical course comprises of spinal shock and, as a result, the symptoms and signs are similar for various grades of SCI;

  2. early (subacute) phase is defined to be 48 h – 14 days after the injury; likewise in the acute phase, clinical observations may include a syndrome of complete block of conduction in the spinal cord due to spinal shock, altered blood and cerebrospinal fluid flow; edema and swelling of the spinal cord;

  3. intermediate phase is defined to be 14 days – 3 months; spinal shock symptoms disappear, and the actual severity and range of spinal cord injury is determined;

  4. chronic phase – more than 3 months after the injury; the recovery of spinal cord functions occurs depending on the SCI grade; neurological status may deteriorate due to the scarring process, cyst formation, post-traumatic syringomyelia, etc. [23, 47, 48, 49, 50, 51].

Spinal cord injury triggers the development of a complex series of pathophysiological reactions, including primary and secondary damage of the nervous tissue [52, 53, 54]. The inflammatory response to the primary structural changes in the spinal cord is followed by the release of multiple regulatory peptides, including proinflammatory cytokines [55, 56].

3. A role of the immune system and cytokines in the acute phase of spinal cord injury

3.1 Cells of the nervous system in spinal cord injury as inductors, effectors and targets of inflammatory acute phase reactions

Two different phases are distinguished in the pathogenesis of the acute period of SCI; each of them is associated with a complex series of pathophysiological reactions in response to the nervous tissue damage [57, 58].

The first phase of the injury, which starts on the first day, immediately after mechanical trauma, involves mechanisms of the injury and disorders associated with these mechanisms. Neurons, astrocytes, oligodendrocytes, as well as other components of nerve signal transmission, are physically affected, and these events are accompanied by disorders of vascular components, including the blood–brain barrier (BBB) [56, 57, 58, 59, 60], which results in the tissue infiltration by inflammatory cells [61, 62, 63].

The inflammatory response to primary structural changes in the SC is associated with the release of multiple regulatory peptides, including proinflammatory ones, and cytokines [64, 65]. Cytokines are synthesized by the activated macro- and microglia, damaged vascular endothelium, as well as the immune system cells mobilized from the circulation system and transported to the site of injury and the adjacent areas due to a change in the BBB permeability [66].

It has been established that several important molecular components of the immune system, including tumor necrosis factor (TNF-α), inducible nitric oxide synthase (iNOS), nuclear factor (NF)-kB, interleukin (IL)-1β, and/or a factor of apoptosis Fas ligand (FasL), are activated as early as within a few minutes after SCI [67, 68, 69]. The activation of these molecules further results in inflammation and other kinds of significant neurological disorders [70].

The second phase comprises of endogenously induced degradation of the nervous tissue and associated consequences [70]. Increased glutamate concentration in the damaged spinal cord (SC) tissue induces neuronal excitotoxicity (a pathological process causing the neurotransmitter-mediated damage and death of nerve cells) due to the excess of intracellular Ca2+. This process promotes the accumulation of reactive oxygen species [71, 72, 73], which, in turn, affect such cellular components as nucleic acids, proteins, and phospholipids and cause considerable cell losses and subsequent neurological dysfunction [74, 75].

It is important to highlight that the endogenous cells (neurons and glial cells) of human SC (but not the blood leukocytes) contribute to the early production of IL-1β, IL-6, and TNF-α during the post-traumatic inflammatory response [76, 77, 78].

Activated astrocytes represent the primary source of all damaging factors: they account for about 30% of the cellular composition; and the overexpression of the microRNA miR-136-5p in these cells during SCI is considered as one of the inducers of proinflammatory factors and chemokines (primarily TNF-α and IL-1β) [79, 80, 81]. This process triggers an inflammatory immune response involving type 17 T-helpers [82]. Angiogenesis mediated by microRNA (miR-210) is another SCI-related event [83, 84].

3.2 Distinct role of the innate immune cells and its cytokine release in acute phase reactions to spinal cord injury

However, the role of the immune cells secreting proinflammatory cytokines in SCI should not be underestimated. This process is stimulated by hemorrhages in the SC tissue after its damage [85, 86]. They contribute to the infiltration of affected regions by neutrophils, monocytes/macrophages, and T lymphocytes [87, 88, 89, 90], i.e. cells releasing the same factors, i.e. TNF-α, IL-1α, IL-1β, and IL-6 [91, 92].

Typically, these cytokines reach their peak level 6–12 hours after the injury; they also induce an inflammatory response in acute and subacute phases and contribute to the lesion extension in rostral and caudal directions [93, 94, 95]. It was shown that activated microglia and macrophages infiltrating the SC are responsible for the subsequent necrosis and apoptosis of neurons, astrocytes, and oligodendrocytes located closely to the site of the lesion [96, 97], thus worsening the neurological outcome [98, 99]. The modulation of proinflammatory and immune effects in the SC tissue during its injury involves interferons through the increase in the number of stimulators of interferon genes (STING) in the tissue [100, 101].

Within the first 24 hours after SCI, an additional immunological effect takes place: the number of natural killer (NK) cells with an activated phenotype increases significantly, which is manifested by the overexpression of CD69, HLA-DR, NKG2D, and NKp30 on their membrane as well as the enhanced cytotoxic activity [102]. Furthermore, an increased level of the brain-derived neurotrophic factor (BDNF) that can be produced by vascular endothelial cells was found in the patients’ plasma samples. At this phase of SCI, it strongly correlated with the percentage of NK cells and the expression of CD69 and NKp30 activating molecules on their surface [103].

Early interventions for reducing inflammation and preventing apoptosis have become a common strategy in the targeted medical care provided to SCI patients. However, the latest updates in this field suggest that the inflammatory process has apparent protective aspects that should not be ignored during treatment [104].

As for the cytokine release signals, they can enter the cells through the Toll-like receptors (TLRs) of the SC [105, 106]. TLRs are best known as the structures for pathogen recognition and initiation of the innate immune response [107, 108]. However, they can also detect tissue damage and trigger sterile inflammation by binding to endogenous ligands typical for stressed or damaged cells. In addition to the cells associated with the immune system, TLRs have also been identified in neurons of the central nervous system (CNS) and glial components, including microglia, astrocytes, and oligodendrocytes [109, 110]. To this end, Toll-like receptors may play both direct and indirect roles in SCI [111]. Indirect effects are most likely mediated by microglia or immune system cells penetrating the damaged CNS tissue [112]. It is also established that restorative responses in SCI-related ischemic disorders are taking place with the predominant participation of Toll-like receptor 3 and subsequent regulation by TLR4 [113].

One of the mechanisms of innate immune defense during SCI-related inflammatory response is associated with the unique role of mast cells [114]. Mast cells are abundant in the CNS and play an intricate role in the progression of neuroinflammatory disorders. In particular, it was shown that the experimental mast-cell deficient mice had increased astrogliosis and T-cell infiltration, while their functional recovery after SCI was significantly reduced [115]. Moreover, these mice have significantly increased levels of the cytokines MCP-1, NFα, IL-10, and IL-13 in the SC. The available data demonstrate the relationship between these findings and the fact that, if the same number and functional activity of mast cells are maintained, their chymases cleave MCP-1, IL-6, and IL-13. This suggests a protective role of the above cellular elements in the development of inflammatory changes in the nervous tissue in SCI cases [116]. It should be noted that, in addition to astrocytes and microglia, IL-10 is also produced by macrophages, B cells, and Th2 cells [117, 118]. Being an immunomodulator, IL-10 stimulates the generation of regulatory T cells while suppressing the activity of Th1 and NK cells [119].

The cytokine and hormone secretion pattern after spinal cord injury largely depends not only on the mechanisms of induction and immune response but also on the level of injury. Thus, the experiments in the rat model clearly demonstrated similar differences in the production of vascular endothelial growth factor (VEGF), leptin, interferon-γ-induced chemokine IP-10, IL-10, IL-18, granulocyte colony-stimulating factor (G-CSF), and chemokine fractalkine in animals’ plasma. In contrast to the thoracic spine trauma, injury to the cervical spine is associated with a reduced expression of these mediators. A potential mechanism underlying this finding is sympathetic dysregulation caused by a higher location of the spine injury [120, 121]. Experiments in mice have also demonstrated that cytokines (e.g. interleukins IL-3, IL-6, IL-10, IL-13, and G-CSF) impacted the systemic changes after spinal cord injury in the lower thoracic region (Th910). In parallel, the activation of T lymphocytes and neutrophils was determined during the acute phase of the reported changes [122]. Thus, the immunopathogenic mechanisms primarily linked to innate immune cells and proinflammatory cytokines have a central role in the SCI acute phase.

Damaged neurons and neuroglial cells after spinal cord injury become a source of chemokines (fractalkine, MCP-1, and IP-10) [120, 122] targeting monocytes/macrophages and lymphocytes and promoting their entry into the lesion site. Mast cells represent one of the first cells of the innate immune system that exert their effect in the injury site. As already mentioned, mast cells can regulate chemokine secretion; however, their role is far from being clear. On the one hand, these cells can be a source of cytokines and other mediators promoting inflammation [123]. On the other hand, chymases released from mast cells during their activation and subsequent degranulation can destroy chemokines and proinflammatory cytokines, limiting the intensity of inflammatory response [116].

Most chemokines produced by cells of the injured spinal cord promote the recruitment of monocytes/macrophages [124], which eliminate cell debris, while chemokine IP-10 also recruits NK cells [125]. The involvement of NK cells in the innate immune response is also facilitated by the fact that after SCI the spinal cord cells express injury patterns, particularly stress-induced molecules (MICA, MICB), that are considered as ligands for NKG2D receptors [126]. In turn, their high level of expression by NK cells was demonstrated for spinal cord injury [101]. At first glance, manifestations of NK cells’ cytotoxic activity against the nervous tissue in spinal cord injury significantly aggravate the destructive processes during trauma [101]. However, the involvement of NK cells in the elimination of exclusively the cells carrying injury patterns contributes to a more rapid suppression of destructive processes at the site of spinal cord lesion.

The study focused on another crucial player, macrophages, under the conditions of tissue damage has demonstrated that there are two stages of their activation [127]. During the first stage, these cells acquire an inflammatory (M1) phenotype mediated by endogenous molecules released during cellular damage. When reparative processes are triggered in response to damage at later stages, activated macrophages are polarized into the resident (M2) phenotype [127]. Thus, it is suggested that M1 macrophages are predominantly produced during the SCI acute phase. Their induction after SCI is also stimulated by interferons [128] that accumulate (as mentioned above) in the damaged tissues [100]. The macrophages secrete IL-12, IL-10, IL-1β, IL-6, IL-23, IL-21, TNF-α, and iNOS specific for this phenotype; high levels of these factors have been reported for the described pathological condition [120, 122, 127].

The cytokines have different functions: IL-12 further triggers adaptive cellular responses; IL-10 has an immunosuppressive effect and is involved in the induction of regulatory T cells; IL-1β, IL-6, IL-21, IL-23, and TNF-α exert a proinflammatory effect; TNF-α and iNOS provoke cellular damage reactions [128, 129].

The predominant cytokine profile, as well as the presence of M1 macrophage-producing cells in combination with the effect of autoantigens of the damaged spinal cord, suggests that the population of T lymphocytes involved in the immune response at the initial stage includes Th17 cells whose functional role has already been proven in the SCI acute phase. The functional role of this subpopulation is closely related to achieving the balance T-helper-17/regulatory T-cells (Th17/Treg). Q. Fu et al. [82] described these processes as follows. The Th17/Treg cell balance is regulated by molecules RORγT and FoxP3, while FoxP3 expression can be inhibited by RORγT expression. As mentioned above, SCI is accompanied by the migration of M1 macrophages to the injury site and the release of proinflammatory cytokines, including IL-6 and IL-21. As a result, T-helpers (CD4+ T lymphocytes) are able to differentiate into CD4+IL17A+ Th17, which contribute to the inflammatory response by recruiting neutrophilic granulocytes. In combination with proinflammatory cytokines produced at the injury site by macrophages, neurons, and neuroglia cells, the products of Th17 and neutrophils considerably enhance the inflammation process. Researchers consider the latter as a harmful component of the pathogenesis of post-traumatic changes in the spinal cord.

It is worth noting that Th17 induction during the initial phase requires one more cytokine, the transforming growth factor β (TGFβ), which is mainly secreted by Treg cells. The formation of these cells playing an important role in the Th17/Treg balance is mediated primarily by IL-10, which is also secreted by M1 macrophages in relatively small amounts during the initial phase of tissue damage. Like TGFβ, IL-10 has an immunosuppressive effect, limiting an excessive autoimmune inflammatory process after spinal cord injury [127, 130].

Thus, innate immune responses and T cell-mediated responses prevailing during the SCI acute phase could be assessed controversially. On the one hand, they aim to destroy cells in the damaged spinal cord tissue through their apoptosis or cytolysis and to induce inflammatory response enhancing neurological dysfunction. On the other hand, these reactions contribute to the elimination of destroyed cell elements along with their intrinsic autoantigens, injury patterns, and inflammation mediators, and also involve the inflammatory response regulation mechanisms. Based on these conclusions, a simplified approach cannot be used for assessing the role of immune processes in spinal cord injury. These processes are also important for selecting a treatment strategy during the SCI acute phase. It is necessary to evaluate the balance between the immune mechanisms prevailing in each particular case and exhibiting either a protective or pathogenic effect, instead of relying on individual indicators.

Already during the acute phase, spinal cord injury induces a strong inflammatory response [131] and a robust immune response both within and beyond the injury site [132]; these responses do not tend to resolve. In this case, the interaction takes place between the CNS and the immune system (i.e., the two central systems maintaining homeostasis in the entire body). That is why the process is not limited by the immune response in the site of spinal cord injury but also affects the whole immune system [133].

4. Role of the immune system and cytokines in the chronic phase of spinal cord injury

4.1 The importance of immunosuppressive manifestations in the chronic phase of spinal cord injury

The functions of the immune system change dramatically as the SCI acute phase progresses to the chronic stage. The failure or insufficient activity of vegetative innervation in the lymphatic and endocrine tissues disturb immune function for a long time after the initial trauma [134]. The main manifestations of such disorders are immune depression and the autoimmune process [133], although inflammatory reactions also maintain their significant role in pathogenesis.

Systemic changes at the level of cell populations and lymphocyte subpopulations during the SCI chronic phase are mainly related to the T cell-mediated adaptive immunity. Thus, it has been demonstrated that the total count of T cells (CD3+) and T helper cell subpopulation (CD3+ CD4+) in the blood is declining, although the count of activated CD4+ T cells (HLA-DR + CD4+) remains elevated [135]. This situation may occur if the count of T helper cells in the blood decreases due to their migration to the affected organ.

Regulatory T cells (Tregs) exhibiting suppressive properties are of special interest in this scenario. These cells have a CD3 + CD4 + CD25 + CD127lo phenotype with the predominance of activated CCR4 + НLA-Dr + fraction. The level of transforming growth factor β (TGFβ), the major cytokine of these cells, is considerably increased in SCI cases, which largely explains the observed immune dysfunction and its consequences, such as the impaired defense against infections and/or persistent chronic inflammation [88, 135].

The deficiency of T-cell-mediated immunity at the systemic level is also accompanied by a significant decrease in NK cell count during the chronic phase of SCI, which eventually leads to lethal infection [136].

Thus, starting on day 7 after the spinal cord injury, signs of regeneration of the myelin sheath of neurons associated with biochemically detectable activity of oligodendrocytes and production of proinflammatory cytokines TNF-α, IL-1β, and IL-6 were found [137]. Meanwhile, it was noted that a higher level of proinflammatory cytokines during the chronic phase correlated with a faster remission after SCI [51].

In fact, the proinflammatory cytokines trigger activation of astrocytes in the spinal cord glial tissue [138]. Astrocytes undergo proliferation and acquire one of the two phenotypes. Astrocytes having one phenotype actively secrete a glial fibrillary acidic protein (GFAP), which contributes to neuroregeneration. Contrariwise, astrocytes of the other phenotype secrete glutamine synthetase, which participates in glutamate accumulation and slows down neuronal regeneration in the injured spinal cord region. The balance between astrocytes of these two phenotypes determines the efficiency of repair processes in the neuronal tissue [139]. Neurons secrete neuregulin-1 (Nrg-1), which stimulates cell regeneration, contributes to the preservation of the spinal cord white matter, and positively regulates the functions of macrophages, T cells, and B cells. Today, it is even recommended as a medication for patients with spinal cord injury [140]. Although this positive regulation may occur, it is necessary to remember that all the described processes occur in the CNS; therefore, they can have both local and systemic manifestations.

Speaking about one of the key mechanisms of induction of the observed changes, a reference should be made to the data published by C.J. Ferrante and S.J. Leibovich [127]. They reported that after the acute phase of tissue damage, the macrophage phenotype switched abruptly from M1 to M2, which differs much from the typical M2 cells in terms of cytokine secretion. This variety was called the angiogenic M2d phenotype. The main products of M2d macrophage secretion included vascular endothelial growth factor (VEGF) and IL-10, inducing the formation of regulatory T cells. That is why the angiogenic and immunosuppressive effects are predominant. Similar transformations were also found for macrophage microglial cells [141].

Special focus is placed on the role of tumor necrosis factor α during the SCI chronic phase. During this phase, the level of brain-derived neurotrophic factor (BDNF) is decreasing in the hippocampus, and at the same time, it is rising in the lateral part of the spinal cord. A deletion within the gene encoding TNF-α receptor blocks this effect, but the presence of this cytokine restores the effect. These findings suggest that various structural synaptic changes in the spinal cord and hippocampal neurons are mediated by the overproduction of TNF-α in activated microglial cells, which can be associated with the development of chronic neuropathic pain and memory deficit after spinal cord injury [142]. IL-1β reducing the efficiency of calcium pump function in neurons also contributes to the development of neuropathic pain [143].

4.2 Autoimmune component of the chronic phase in spinal cord injury

Particular attention should be paid to the autoimmune processes associated with spinal cord injury. D.P. Ankeny et al. [144] demonstrated that spinal cord injury and related immunodepression cause profound long-lasting changes in the functions of B cells of the peripheral lymphoid tissue (the bone marrow and spleen) and the injured spinal cord. In particular, after their differentiation, the activated B cells are able to secrete autoantibodies that bind CNS proteins and nuclear antigens, including DNA and RNA. In patients with systemic lupus erythematosus, anti-DNA antibodies cross-reactively interact with glutamate receptors, causing excitotoxicity [145]. The same effect is reported for SCI-related autoantibodies, which exhibit similar neurotoxic properties.

After spinal cord injury, the autoimmunity can also promote CNS regeneration and/or neuroprotection, though a tendency towards neurotoxicity manifestations can still be present. Myelin-reactive T cells exhibit a similar neuroprotective effect in the rat model of SCI [146]. The data on the role played by autoantibodies are inconsistent. After all, the antibodies specific to CNS proteins can promote axonal regeneration and remyelination [147], as well as demyelination, because anti-myelin antibodies can participate in building a “bridge” between myelin of nerve fibers and oligodendrocytes [148]. In any case, despite the ambiguity of the effects and their interpretations, it has been verified that B cells infiltrate the injured spinal cord during the chronic phase [144].

The presented review demonstrates that the interpretation of the results is challenging because it is difficult to distinguish local and systemic effects after spinal cord injury. In this regard, the feasibility of differentiating between the local and systemic manifestations of immune response opens up certain prospects. For example, significant changes in the cytokine profile after SCI, especially during the chronic phase, were found not only in blood. Changes in the cytokine profile in CSF were even more informative. Thus, A.R. Taylor et al. [149] determined the levels of IL-2, IL-6, IL-7, IL-8, IL-10, IL-15, IL-18, granulocyte-macrophage colony-stimulating factor (GM-CSF), interferon-γ (IFNγ), keratinocyte chemoattractant (KC-like protein), IFNγ-inducible protein 10 (IP-10), monocyte chemotactic protein-1 (MCP-1), and tumor necrosis factor α (TNF-α) in cerebrospinal fluid as criteria characterizing the intensity of chronic inflammation. The concentrations of most cytokines and chemokines in CSF of animals after SCI correlated with the injury duration and trauma severity at sampling and the long-term neurological outcome. Thus, after spinal cord injury, the IL-8 level was significantly higher than in the control group of healthy animals but showed a negative correlation with the injury duration. At the same time, the levels of colony-stimulating factors and MCP-1 negatively correlated with the long-term positive outcome.

5. Conclusions

The review of publications focused on the problem of SCI-related immune (including cytokine) processes demonstrates that the available data are inconsistent and difficult to interpret.

Both the nervous and the immune systems have essential regulatory functions in the body and are tightly interrelated, while their interaction mechanisms are very diverse. Both local and systemic effects are associated with the neurological and immune changes occurring after spinal cord injury.

Along with these general aspects, the SCI-related local and systemic changes in the central nervous system and immune processes should be assessed on a stage-by-stage basis [150, 151]. Each phase is characterized by specific prevailing pathogenesis, which is initially linked to the response to injury and targeted at eliminating the damaged cells; then focus moves towards the inflammatory response with the aim of containing the affected area. Finally, a transition from local reactions to systemic processes occurs during later stages; the outcome of the pathological process depends on the efficiency of these phases. Each phase is associated with a specific category of immune response. In this respect, various cell subpopulations characterizing the innate and adaptive immunity or cytokines, the products secreted by these cells, can serve as markers of these immune responses [152, 153].

A specific feature of cytokines as markers of pathological changes after spinal cord injury is that they are secreted not only by immune cells but also by cells of the damaged spinal cord. The interaction between the nervous and immune systems can be characterized using the cytokine profile model. It has both theoretical research implications and diagnostic value and provides an opportunity to highlight the critical therapeutic targets.

Thus, cytokines contribute significantly to the pathogenesis of SCI-related traumatic disease and are responsible for its various manifestations. The cytokines can be secreted by immune cells; however, neurons of the damaged spinal cord are the main source of these biologically active substances. Therefore, the SCI-related cytokine pattern characterizes both the immune and neurological status and has a tremendous diagnostic and prognostic value.

Abbreviations

SCspinal cord
SCIspinal cord injury
CNScentral nervous system
BBBblood–brain barrier

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Georgii Telegin, Aleksandr Chernov, Alexey Belogurov, Irina Balmasova, Nikolai Konovalov and Aleksandr Gabibov (January 15th 2021). Cytokine Profile as a Marker of Cell Damage and Immune Dysfunction after Spinal Cord Injury [Online First], IntechOpen, DOI: 10.5772/intechopen.95614. Available from:

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