**5.2 Immune cell-specific contribution to TBI**

*Neutrophils:* Neutrophils arrive at the lesion area in the early stages of injury these cells migrate to the area of injury and infiltrate damaged brain tissue within the first 24 hours postinjury [33]. These cells are recruited by the release of IL-8, a chemoattractant cytokine known to be generated in the early stage of TBI [35]. Numbers of circulating neutrophils rise significantly in the acute phase of TBI. One study found that neutrophils present following TBI appear to be less susceptible to apoptosis than neutrophils in uninjured patients, which may contribute to the increased numbers observed [36]. In contrast to the few studies implicating a positive role for neutrophils in TBI recovery [33, 37], numerous show deleterious effects. One study, using the CCI model, found that neutrophil depletion improved tissue recovery. Neutrophil-depleted mice in this study showed decreased cell death and tissue loss following TBI [38]. Another study assessed the effects of decreased immune cell infiltration following TBI via administration of anti-intercellular adhesion molecule 1 (ICAM1) antibody in a fluid percussion model of rat TBI. Rats given anti-ICAM1

#### **Figure 1.**

*Overview of major peripheral immune cell response to TBI.*

showed decreased neutrophil infiltration following injury 26 hours following TBI, which correlated with increased motor recovery [39]. Several mechanisms have been suggested to explain these negative effects. Some studies have indicated that neutrophils bind endothelial cells and platelets after TBI, decreasing blood flow and promoting ischemia [33]. As previously mentioned, neutrophils can also damage the BBB through release of MMPs and reactive oxygen and nitrogen species [33]. In addition, many of the cytokines generated by neutrophils following TBI have been

**21**

*Peripheral Immune Response Following Traumatic Brain Injury*

shown to have negative effects on neural recovery. These cytokines include IL-9, IL-12, CXCL5, and TNFα. IL-9 can increase the damage caused by excitotoxicity following TBI, and high levels of IL-12 have been correlated with poor postinjury outcome (**Figure 1**). CXCL5 contributes to BBB compromise, and TNFα plays a role in neurotoxicity [33]. However, other studies have found that infiltration of peripheral cells in the acute stage of injury has little effect on recovery, suggesting instead that infiltration of peripheral-derived monocytes in the later stages of injury (greater than 48 hours after injury) has the greatest influence on injury progression [7].

*Monocytes:* The role of monocyte/macrophages has been particularly well-studied

in regard to the effects of infiltrating peripheral-derived immune cells after TBI. Although a minority in terms of numbers of circulating immune cells, composing only 5–10% of the peripheral immune cell population, monocytes play an important role in TBI recovery [36]. Monocytes are the primary infiltrating immune cells observed at 3–5 days following injury [29]. While some studies have even argued that peripheral monocytes are the most prominent infiltrating immune cell at 24 hours postinjury as well [40]. Circulating monocytes can display pro- or anti-inflammatory properties. When monocytes migrate into affected tissue, they mature into macrophages with pro- or anti-inflammatory characteristics [36]. These cells can have a neuroprotective effect via phagocytosis of dead cell debris, release of growth factors, and production of anti-inflammatory cytokines. Monocyte/macrophages also release granulocyte-macrophage colony-stimulating factor (GM-CSF), which may have a neuroprotective effect through promotion of stem cell differentiation and suppression of apoptotic pathways [41]. However, monocyte/macrophages may have differing effects on TBI recovery depending on their inflammatory profile. While monocyte/macrophages may be beneficial in some aspects of TBI recovery, other studies have found that these cells may also negatively affect neural recovery through different mechanisms. One study assessed the influence of macrophages on TBI recovery using a chemokine CC ligand-2 (CCL2) knockout mouse model. This study found increased levels of CCL2 following TBI in both human patients and in a murine weight drop injury model. CCL2 knockout mice showed decreased macrophage accumulation and smaller lesion volumes at 2 and 4 weeks after injury [42]. One study showed that depletion of monocytes using clodronate liposomes decreased neutrophil infiltration and edema and resulted in improved neurobehavioral recovery [43]. Several mechanisms have been suggested by which macrophages could exert neurotoxic effects. Infiltrating macrophages may release reactive oxygen and nitrogen species, increase additional recruitment of neutrophils and monocytes, and generate multiple pro-inflammatory cytokines including TNF, IL-1β, and IL-6 (**Figure 1**) [41]. The apparent discrepancy between the neurodegenerative and pro-resolving effects of macrophages following TBI is most likely due to the release of both pro- and anti-inflammatory signals from these cells, with corresponding positive or negative effects [29]. As previously mentioned, monocytes are capable of maturing into macrophages with either pro- or anti-inflammatory characteristics [36]. These two populations are traditionally defined as M1 (pro-inflammatory) and M2 (anti-inflammatory) macrophages. Although the overall balance between these phenotypes is driven by injury processes [44], their differential characteristics and

the mechanisms underlying their fate choice remain under investigation.

Monocyte/macrophages display different phenotypes depending on the cellular

microenvironment. Classical macrophages, called M1 macrophages, specialize in promoting inflammation and phagocytosing pathogens. The second class of

**6. The M1/M2 continuum in TBI**

*DOI: http://dx.doi.org/10.5772/intechopen.93597*

#### *Peripheral Immune Response Following Traumatic Brain Injury DOI: http://dx.doi.org/10.5772/intechopen.93597*

shown to have negative effects on neural recovery. These cytokines include IL-9, IL-12, CXCL5, and TNFα. IL-9 can increase the damage caused by excitotoxicity following TBI, and high levels of IL-12 have been correlated with poor postinjury outcome (**Figure 1**). CXCL5 contributes to BBB compromise, and TNFα plays a role in neurotoxicity [33]. However, other studies have found that infiltration of peripheral cells in the acute stage of injury has little effect on recovery, suggesting instead that infiltration of peripheral-derived monocytes in the later stages of injury (greater than 48 hours after injury) has the greatest influence on injury progression [7].

*Monocytes:* The role of monocyte/macrophages has been particularly well-studied in regard to the effects of infiltrating peripheral-derived immune cells after TBI. Although a minority in terms of numbers of circulating immune cells, composing only 5–10% of the peripheral immune cell population, monocytes play an important role in TBI recovery [36]. Monocytes are the primary infiltrating immune cells observed at 3–5 days following injury [29]. While some studies have even argued that peripheral monocytes are the most prominent infiltrating immune cell at 24 hours postinjury as well [40]. Circulating monocytes can display pro- or anti-inflammatory properties. When monocytes migrate into affected tissue, they mature into macrophages with pro- or anti-inflammatory characteristics [36]. These cells can have a neuroprotective effect via phagocytosis of dead cell debris, release of growth factors, and production of anti-inflammatory cytokines. Monocyte/macrophages also release granulocyte-macrophage colony-stimulating factor (GM-CSF), which may have a neuroprotective effect through promotion of stem cell differentiation and suppression of apoptotic pathways [41]. However, monocyte/macrophages may have differing effects on TBI recovery depending on their inflammatory profile.

While monocyte/macrophages may be beneficial in some aspects of TBI recovery, other studies have found that these cells may also negatively affect neural recovery through different mechanisms. One study assessed the influence of macrophages on TBI recovery using a chemokine CC ligand-2 (CCL2) knockout mouse model. This study found increased levels of CCL2 following TBI in both human patients and in a murine weight drop injury model. CCL2 knockout mice showed decreased macrophage accumulation and smaller lesion volumes at 2 and 4 weeks after injury [42]. One study showed that depletion of monocytes using clodronate liposomes decreased neutrophil infiltration and edema and resulted in improved neurobehavioral recovery [43]. Several mechanisms have been suggested by which macrophages could exert neurotoxic effects. Infiltrating macrophages may release reactive oxygen and nitrogen species, increase additional recruitment of neutrophils and monocytes, and generate multiple pro-inflammatory cytokines including TNF, IL-1β, and IL-6 (**Figure 1**) [41]. The apparent discrepancy between the neurodegenerative and pro-resolving effects of macrophages following TBI is most likely due to the release of both pro- and anti-inflammatory signals from these cells, with corresponding positive or negative effects [29]. As previously mentioned, monocytes are capable of maturing into macrophages with either pro- or anti-inflammatory characteristics [36]. These two populations are traditionally defined as M1 (pro-inflammatory) and M2 (anti-inflammatory) macrophages. Although the overall balance between these phenotypes is driven by injury processes [44], their differential characteristics and the mechanisms underlying their fate choice remain under investigation.
