**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

**11**

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

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

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

**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

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

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 colonystimulating 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

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

cells [141].

of neuropathic pain [143].

similar neurotoxic properties.

during the chronic phase [144].

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

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 colonystimulating 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

*Connectivity and Functional Specialization in the Brain*

**spinal cord injury**

**of spinal cord injury**

migration to the affected organ.

persistent chronic inflammation [88, 135].

which eventually leads to lethal infection [136].

**4. Role of the immune system and cytokines in the chronic phase of** 

**4.1 The importance of immunosuppressive manifestations in the chronic 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

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

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

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,

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

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

reactions also maintain their significant role in pathogenesis.

The functions of the immune system change dramatically as the SCI acute phase

Systemic changes at the level of cell populations and lymphocyte subpopulations

**10**

after SCI [51].

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.
