**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) [56–60], which results in the tissue infiltration by inflammatory cells [61–63].

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 [67–69]. 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 [71–73], 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 [76–78].

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) [79–81]. This process triggers an inflammatory immune response involving type 17 T-helpers [82]. Angiogenesis mediated by microRNA (miR-210) is another SCIrelated event [83, 84].

**7**

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

**3.2 Distinct role of the innate immune cells and its cytokine release in acute** 

cells releasing the same factors, i.e. TNF-α, IL-1α, IL-1β, and IL-6 [91, 92].

of stimulators of interferon genes (STING) in the tissue [100, 101].

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 [87–90], i.e.

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 [93–95]. 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

Within the first 24 hours after SCI, an additional immunological effect takes

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

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 mastcell 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

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

*DOI: http://dx.doi.org/10.5772/intechopen.95614*

molecules on their surface [103].

and subsequent regulation by TLR4 [113].

**phase reactions to spinal cord injury**

*Cytokine Profile as a Marker of Cell Damage and Immune Dysfunction after Spinal Cord Injury DOI: http://dx.doi.org/10.5772/intechopen.95614*

## **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 [87–90], 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 [93–95]. 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 mastcell 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

*Connectivity and Functional Specialization in the Brain*

**targets of inflammatory acute phase reactions**

reactions in response to the nervous tissue damage [57, 58].

adjacent areas due to a change in the BBB permeability [66].

other kinds of significant neurological disorders [70].

subsequent neurological dysfunction [74, 75].

matory cytokines [55, 56].

**cord injury**

cells [61–63].

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

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

**3.1 Cells of the nervous system in spinal cord injury as inductors, effectors and** 

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

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) [56–60], which results in the tissue infiltration by inflammatory

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

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 [67–69]. The activation of these molecules further results in inflammation and

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 [71–73], which, in turn, affect such cellular components as nucleic acids, proteins, and phospholipids and cause considerable cell losses and

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 [76–78]. 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) [79–81]. This process triggers an inflammatory immune response involving type 17 T-helpers [82]. Angiogenesis mediated by microRNA (miR-210) is another SCI-

**6**

related event [83, 84].

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 colonystimulating 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

**9**

indicators.

immune system [133].

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

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 patho-

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

The predominant cytokine profile, as well as the presence of M1 macrophageproducing 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

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

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 inflam-

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

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

Th17, which contribute to the inflammatory response

effect; TNF-α and iNOS provoke cellular damage reactions [128, 129].

17A+

of post-traumatic changes in the spinal cord.

matory process after spinal cord injury [127, 130].

*DOI: http://dx.doi.org/10.5772/intechopen.95614*

logical condition [120, 122, 127].

differentiate into CD4+IL−

#### *Cytokine Profile as a Marker of Cell Damage and Immune Dysfunction after Spinal Cord Injury DOI: http://dx.doi.org/10.5772/intechopen.95614*

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 macrophageproducing 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+IL− 17A+ 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].

*Connectivity and Functional Specialization in the Brain*

cells [119].

acute phase.

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

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 colonystimulating 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

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,

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

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

limiting the intensity of inflammatory response [116].

processes at the site of spinal cord lesion.

**8**
