Pro-inflammatory actions of cytokines in the central nervous system

In CNS injury, resident cells function as the first line of defence prior to the infiltration of inflammatory cells (Fig. 5; Rothwell and Hopkins, 1995; Gröne et al., 2000; Markus et al., 2002; Wang and Shuaib, 2002; Beineke et al., 2008). They are capable of producing and responding to pro-inflammatory cytokines that can contribute to further tissue damage, e.g. via induction of inducible nitric oxide synthase and initiation of apoptotic cell death (Wang and Shuaib, 2002; Hehlgans and Pfeffer, 2005). Cultured murine astrocytes and microglia for example secrete IL-1 following stimulation with lipopolysaccharides (LPS) and TNF (Giulian et al., 1986). Vice versa, TNF secretion can be induced in astrocytes and microglia upon stimulation with LPS, interferon (IFN)-γ or IL-1 (Lieberman et al., 1989; Chung and Benveniste, 1990). IL-1 activates microglia, induces nitric oxide synthesis, activates complement and stimulates leukocyte invasion (Wang and Shuaib, 2002; Gibson et al., 2004). Further, IL-1 and TNF induce proliferation of astrocytes and contribute to astrogliosis (Giulian and

Lachman, 1985; Barna et al., 1990). IL-1, and similarly TNF, are potent contributors to neuronal cell death and are often associated with demyelination and auto-aggressive changes in the CNS (Rothwell and Hopkins, 1995; Ledeen and Chakraborty, 1998; Wang and Shuaib, 2002; Gibson et al., 2004). IL-6, inducible by TNF and IL-1β (Hopkins and Rothwell, 1995), promotes an enhanced inflammatory response of microglia (Klusman and Schwab, 1997). Consequently, antibody-based blockade of IL-6 signalling reduces the extension of inflammation, decreases astrogliosis, and enhances tissue sparing following SCI in mice (Mukaino et al., 2010). Another pro-inflammatory mediator, IL-12, can be induced in microglia in vitro by TNF and endotoxins, subsequently promoting a T helper (Th)1-shifted T cell response potentially leading to a deleterious delayed type hypersensitivity reaction (Adorini et al., 1997; Olson et al., 2001). Interferons are a distinct class of cytokines with numerous different members. Their name derives from their well-known antiviral properties. IFN-γ is additionally a powerful immune activator enhancing MHC class I expression on T cells and stimulating a Th1 shifted immune response. In addition, IFN-γ is a potent activator of macrophages and microglia as it induces MHC class II-expression on APCs (Allan and Rothwell, 2001). During various CNS diseases infiltrating Th1 cells are thus able to contribute to a pro-inflammatory microenvironment, e.g. by the secretion of IL-2, and IFN-γ. Apart from the classical Th cell subsets recent investigations have highlighted the pathogenetic importance a novel class of Th cells. IL-17 secreting T cells - consecutively termed Th17 cells - play a substantial role in autoimmune responses, e.g. in MS (Fletcher et al., 2010).

IL-23 is a heterodimeric protein produced by APCs and shifts the T cell response towards Th17 domination (Kikly et al., 2006). Further, IL-23 appears to be required for the survival of Th17 cells (Kikly et al., 2006). IL-17 acts on a receptor that is ubiquitously expressed and initiates the production of pro-inflammatory cytokines and chemokines causing enhanced tissue damage (Kikly et al., 2006).

IL-8, now termed CXCL8, functions as a potent pro-inflammatory mediator and represents the prototype of chemokines, a family of small cytokines that is mainly involved in migration and chemoattraction of both neutrophilic granulocytes and T cells (Tipold et al., 1999; Gangur et al., 2002; Kuhle et al., 2009; Kwon et al., 2010b).

Locally produced IL-8 in the CNS thus contributes to an early attraction of infiltrating immune cells from the periphery (Tipold et al., 1999). CXCL7, another member of the chemokine family, has recently been cloned from canine tissue and has proven to be highly attractant to neutrophils using a neutrophil migration assay (Wang et al., 2009). In addition, cultured canine dendritic cells express high levels of CXCL7 when treated with pro-inflammatory cytokines such as IL-1β, TNF and IL-6 (Wang et al., 2009).

However, the classical view of pro-inflammatory cytokines as the “bad guys” in the CNS would be an oversimplification as several lines of evidence also indicate a beneficial role of these molecules in various CNS pathologies. For instance, IL-6 deficient mice show increased neurodegeneration and oxidative stress following brain injury suggestive of a neuroprotective role of IL-6 (Penkowa et al., 2000).

Further, IL-1β deficient mice show deficits in proper remyelination following cuprizone-induced demyelination (Mason et al., 2001). Dependent on the dosage, TNF may also initiate neuroprotective effects (Pan et al., 1997). For example, up-regulation of TNF receptor 2 leads to the secretion of neuroprotective cytokines including IL-10 in cultured mouse microglia, suggestive of TNF induced counter-inflammatory and neuroprotective pathways (Veroni et al., 2009).

Besides their local production, cytokines are able to cross the blood brain barrier (BBB) by passing via circumventricular organs or through specific transport mechanisms found in endothelial cells (Gutierrez et al., 1993). In addition, cytokines produced by resident glial and endothelial cells have direct effects on the BBB, as they lead to an increased permeability of the BBB by causing a reorganization of the actin cytoskeleton (Deli et al., 1995; Webb and Muir, 2000). Moreover, IL-1, IL-6, and TNF induce the expression of adhesion molecules in brain endothelial cells, representing a prerequisite for the transmigration of immune cells through the BBB (Tuttolomondo et al., 2008).

Regarding SCI, there are many reports of an early up-regulation of TNF-α, IL-6 and IL-1β with a delayed or absent up-regulation of anti-inflammatory cytokines such as IL-10 and TGF-β in experimentally induced rodent SCI (Wang et al., 1996; Bartholdi and Schwab, 1997; Wang et al., 1997; Pan et al., 2002; Tyor et al., 2002). Kwon et

al. (2010b) demonstrated a severity-dependent up-regulation of several signalling factors including IL-6 and IL-8 in the cerebrospinal fluid (CSF) of human patients with naturally occurring SCI. In rat thoracic contusion injury, IL-8, IL-6, IL-1α, IL-1β, IL-13, MCP-1, MIP1a, RANTES, and TNF are elevated within the spinal cord, however, compared to human SCI the up-regulation of IL-6, IL-8, and MCP-1 is earlier in rodents (Stammers et al., 2012). Similar to humans, IL-6-, IL-8- and MCP-1-expression levels correlate with injury severity in experimental rodent SCI (Stammers et al., 2012). Recently, treatment of spinal injured mice with an anti-IL-6 receptor antibody has shown to promote the alternative macrophage activation pathway (M2) with subsequent spared myelin, enhanced axonal regeneration, and improved locomotor function (Guerrero et al., 2012). This demonstrates a crucial role of IL-6 in damage during secondary injury, possibly due to promotion of a classically activated M1 phenotype of macrophages (Guerrero et al., 2012).