**2.1 The central nervous system immunity**

*Parasitology and Microbiology Research*

of latent *Toxoplasma* cysts [4, 5]. Parasite multiplication in the CNS induces a local inflammatory reaction resulting in brain damage and a strong synthesis of neurotransmitters which are involved in necrotizing brain lesions and in neurological disorders. The relationship between *T. gondii* infection and the development of the bipolar disorder (BD) has long been investigated, Evidence studies suggest that this infection may be related to neuropsychiatric disorders, especially Schizophrenia. Immune response has a major role in the persistence of *T. gondii* cysts in the brain. The protective immune reaction against *Toxoplasma* is very complex. During primary infection, the components of the nonspecific immune response occur first. *T. gondii* tachyzoites reach the intestinal lumen, enter the intestinal cells, and multiply in the cells of the lamina propria [6]. At this stage, infected enterocytes activate an immune response to limit parasite multiplication [7]. This response mainly involves neutrophils, macrophages, monocytes and dendritic cells via the secretion of different chemoattractant proteins. Secondarily, cellular immune response is specifically activated against the parasite [8]. It is essentially an immune response Th1-type. This response is marked by interleukin (IL)-12 production by antigen presenting cells, and interferon-gamma (IFN-γ) production by CD4+ T cells, CD8+ T cells, and natural killer (NK) cells (**Figure 1**). The Th1-type reaction most often induces a local inflammatory reaction, hence the interest of activating a Th2-type immune reaction that inhibits the activation of the Th1 immune response. The Th2 response is mediated primarily by two interleukins IL-4 and IL-10 [6] (**Figure 1**). This immune regulation seems to be of great interest in reducing the inflammatory reactions responsible for most lesions, but it makes the soil favorable to parasite multiplication. Despite the immune responses against *T. gondii*, this parasite has the ability to escape and spread via the bloodstream to infect different organs. In the central nervous system, tachyzoites have the ability to cross the blood-brain barrier (BBB), infect different types of nerve cells, and become encysted bradyzoites that persist throughout the life of the host. Several studies have shown in the mouse model the importance of IFN-γ in this control [9]. Among the parasitic factors, the genotype also has a role in the influence of the evolution of *Toxoplasma* infection. Type I strains are associated with high virulence in mice, whereas type II or III strains are considered avirulent for mice. Type I tachyzoites have the ability to cross epithelial barriers and reach immune-privileged sites more rapidly than type II

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**Figure 1.**

*Diagram showing the cerebral immune response during a cerebral toxoplasmosis infection in mice.*

The central nervous system is closely linked to the immune system at several levels. The cerebral parenchyma is separated from the periphery by the BBB, the integrity of which is maintained by tight endothelial junctions. This barrier under normal conditions prevents the entry of mediators such as activated leukocytes, antibodies, complement factors, and cytokines. The myeloid cell line plays a crucial role in the development of immune responses at the central level, it includes two main subtypes: microglial cells, distributed in the cerebral parenchyma; perivascular macrophages located in the capillaries of the basal lamina brain and the choroid plexus. In addition, astrocytes, oligodendrocytes, endothelial cells, and neurons are also involved in the immune response in the CNS. By modulating synaptogenesis, microglial cells are more particularly involved in the restoration of neuronal connectivity following inflammation. These cells release immune mediators, such as cytokines, that modulate synaptic transmission and alter the morphology of dendritic spines during the inflammatory process after injury. Thus, the expression and release of immune mediators in the cerebral parenchyma are closely related to the plastic morphophysiological changes in the dendritic spines of neurons. Based on these data, it has been proposed that these immune mediators are also involved in the learning and memory processes. Microvasculature is a key element of brain damage. Endothelial cells are an important source of immune mediators such as a nitric oxide (NO), which are involved in the process of immune cell adhesion [12, 13]. A recent study shows that *T. gondii* invasion of neural tissue is mediated by epidermal growth factor receptor (EGFR), enhancing invasion likely by promoting survival of the parasite within endothelial cells [14]. Damage to the BBB can lead to increased permeability, which facilitates leucocyte access to the brain parenchyma. The release of mediators of endogenous inflammation and neurotoxins and the promotion of phagocytosis of cellular debris [6, 14]. Excessively activated microglial cells can cause major histocompatibility complex class II (MHCII) expression. This molecule facilitates the involvement of these cells in immune responses. Activation of microglial cells strongly influences the profile of cytokines released by two distinct mechanisms: recognition receptors and activation of the immune response [13, 15]. Activation of monocytes and macrophages is an important part of the innate immune response; it induces the production of proinflammatory cytokines and chemokines such as IL-8, monocyte chemoattractant protein1 (MCP-1), macrophage inflammatory proteins (MIP-1α and MIP-1β). Regulatory cytokines can also be activated within the CNS, such as IL-4 and IL-10 [8]. Expression of MHCII and adhesion molecules (CD11a, CD40, CD54, CD80, CD86) in activated microglial cells indicates that these cells can acquire antigen presenting activity and participate in the activation of T cells [16]. This suggests the existence of a network of complex interactions linking microglial cells, astrocytes and T cells; which creates a balance between the Th1/Th2 signals, which defines the immune response of the CNS. The role of astrocytes in the CNS is even more complex than that of microglial cells. Astrocytes are divided into two main subtypes: fibrous astrocytes located in the white matter; protoplasmic astrocytes located in the gray matter. These contribute to

the formation of the BBB [6, 15]. Astrocytes also act as neuroprotectants by secreting neurotrophic and releasing potentially toxic pro-inflammatory molecules.

## **2.2 The mechanism of** *T. gondii* **invasion in the brain**

In the brain, dendritic cells and monocytes are the most permissive cells for initial *Toxoplasma* infection [17]. These cell populations probably play an important role in the spread of the parasite, including in the brain. The CD11c<sup>+</sup> CD11b<sup>−</sup> dendritic cells infected promote the diffusion of *T. gondii* of the lamina propria to the mesenteric lymph nodes. Monocytes CD11c<sup>+</sup> CD11b<sup>−</sup> are the main cell population that contains tachyzoites in the blood [17]. *T. gondii* was detected in mononuclear cells + of infected mice 1 day after receiving an intravenous injection of CD11b<sup>+</sup> cells [18, 19].

The transepithelial migration capacity of tachyzoites is implicated in the passage of the parasite across the BBB. This could be done through the interaction of the intercellular adhesion molecule 1 (ICAM-1) of the BBB cells with the parasite MIC2 protein [20]. This interaction is important for transmigration of tachyzoites, as demonstrated *in vitro* in monolayers of several different cell lines [20] (**Figure 1**).

Macrophages are also responsible for the spread of *T. gondii* in the brain [21, 22]. The migration of infected macrophages into the CNS is mediated by the uPA/uPAR pathway and the expression of metalloproteinases 9 (MMP9) [23]. In the mouse brain during the acute phase of infection, *T. gondii* tachyzoites are able to infect all brain cells, mainly microglial cells, astrocytes and neurons. However, microglial cells tend to react against the parasite. These cells inhibit the growth of *T. gondii* and can, therefore, function as important inhibitors of *T. gondii* propagation in the CNS by mechanisms independent of NO and IFN-γ; but this does not prevent *T. gondii* from being able to encyst in these cells [15, 24]. Astrocytes and rat neurons are also suitable to host cells for the intracerebral proliferation of the PLK (type II) *T. gondii* strain [24]. These cells can harbor tachyzoites and cysts during the two respective phases of *Toxoplasma* infection [25]. Astrocytes also have the ability to control the proliferation of tachyzoites during the acute phase of infection for the local control of this opportunistic pathogen [26]. In neurons, the presence of tachyzoites or bradyzoites alters the functioning of infected neurons. These modifications will favor the persistence of the parasite [27, 28].

### **2.3 The regulation of the cerebral immune mechanism during** *T. gondii* **infection**

After *T. gondii* has entered the CNS through the BBB, a cerebral immune response is triggered against the parasite. Experimental data on animal models show that the Th1-type immune response is activated against *T. gondii* to control the replication of the parasite in the brain. In the murine model, this immune response leads to the production of IFN-γ after infection with the ME49 (type II) strain [29–31]. This cytokine is the main mediator of parasite resistance in the murine model. IFN-γ produced by brain-resident cells is crucial for facilitating both the protective innate and T cell-mediated immune responses to control cerebral infection with *T. gondii*. Mice deficient for the IFN-γ receptor or those that are neutralized by an anti-IFN-γ antibody, are unable to control acute *Toxoplasma* infection [32]. During this host response, other cytokines and chemokines are produced. These may promote the infiltration of immune cells to the site of infection [33]. T cells and IFN-γ are essential for maintaining the latency of chronic infection in the brain and the prevention of reactivation of latent infection. The protective activity of T lymphocytes (CD4+, CD8+) results in the production of IFN-γ (**Figure 1**). The protective activity of these cells has

**121**

Toxoplasma *Immunomodulation Related to Neuropsychiatric Diseases*

been demonstrated in a transfer of immune cells T, which conferred protection against the reactivation of cerebral toxoplasmosis in an IFN-γ deficient mouse [34]. However, the presence of TCD4 and TCD8 cells appears to be critical for the long-term maintenance of the latency of chronic *Toxoplasma* infection in the brain. In addition, mRNA encoding IFN-γ was also detected in the brains of *T. gondii*-infected and NK-cell deficient mice. This suggests that IFN-γ could be produced by non-T cells and non-NK cells. This production is important for the maintenance of chronic toxoplasmosis in the brains of mice [35]. Microglial cells and macrophages are identified as the major non-T and non-NK cells that express IFN-γ in the brains of *T. gondii*-infected mice [36]. Therefore, it is possible that the production of IFN-γ by these cells plays an important role in the prevention of cerebral toxoplasmosis. Microglial cells play an important role on the one hand in the innate defense system that limits parasite proliferation and on the other hand by regulating the production of chemokines that facilitate the accumulation of T lymphocytes at the parasite multiplication site. Murine astrocytes have also been implicated in inhibiting the growth of type II *T. gondii* strain *in vitro*. Astrocytes infected with ME49 (type II) *Toxoplasma* strain, produce IL-1, IL-6 and Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) [37]. During the innate immune response, dendritic cells, macrophages, and neutrophils produce IL-12 in response to *T. gondii* infection. This cytokine is essential for the production of IFN-γ. Neutralization of IL-12 with antibodies to this cytokine resulted in 100% mortality in mice infected with an avirulent strain of *T. gondii*, mortality was associated with decreased production of IFN-γ [38]. In addition, IL-12 is also important for maintaining IFN-γ production by T cells during chronic infection. The production of IL-12 is regulated by Lipoxin A4 (LXA4), in order to avoid pathogenic inflammatory reactions in the brain during the chronic phase of *T. gondii* infection. This has been demonstrated by the high production of LXA4 in the serum of mice during chronic *Toxoplasma* infection 5-Lipoxygenase (5-LO) is an essential enzyme in the production of LXA4, 5-LO deficient mice succumbed to infection during the chronic phase, thus presenting a cerebral inflammatory reaction [39]. LXA4 is important for the downregulation of pro-inflammatory responses during the chronic phase of *T. gondii* infection. Studies have shown that lipoxins activate two receptors (AhR and LXAR) in dendritic cells. This activation triggers the expression of the suppressor of cytokine signaling (SOCS)-2. SOCS-2 deficient mice succumb to chronic *T. gondii* infection. This is accompanied by a strong production of IL-12, IFN-γ, and reduction of cerebral cysts [40]. Although Th1 immune responses play a critical role in resistance to *T. gondii* infection, Th2-like immune responses are also implicated in protective immunity. IL-4 plays a major role in the development of the cellular immune response and in the differentiation of T cells into Th2 cells. During the chronic phase of *Toxoplasma* infection, IL-4 inhibits the production of proinflammatory mediators to prevent the development of local inflammations, promoting the persistence of cysts in the brain. Mice deficient in IL-4 die during the late phase of infection [41]. In these mice, a histological study reveals local areas of acute inflammation associated with the multiplication of tachyzoites in the brain. These results indicate that IL-4 is protective against the development of toxoplasmic encephalitis by preventing the formation of cysts and the proliferation of tachyzoites in the brain. The action of IL-4 during the chronic phase of infection is enhanced by the production of IL-10 by T cells [41]. IL-10 also exerts an immunomodulatory role in regulating the Th1-type inflammatory immune response [42]. Mice deficient in IL-10 and treated with sulfadiazine develop fatal inflammatory responses in the brain during the late stage of infection [43]. IL-10 is important for the survival of mice during the acute and chronic phases

*DOI: http://dx.doi.org/10.5772/intechopen.86695*

#### Toxoplasma *Immunomodulation Related to Neuropsychiatric Diseases DOI: http://dx.doi.org/10.5772/intechopen.86695*

*Parasitology and Microbiology Research*

the formation of the BBB [6, 15]. Astrocytes also act as neuroprotectants by secreting

In the brain, dendritic cells and monocytes are the most permissive cells for initial *Toxoplasma* infection [17]. These cell populations probably play an important

dritic cells infected promote the diffusion of *T. gondii* of the lamina propria to the

that contains tachyzoites in the blood [17]. *T. gondii* was detected in mononuclear cells + of infected mice 1 day after receiving an intravenous injection of CD11b<sup>+</sup>

The transepithelial migration capacity of tachyzoites is implicated in the passage of the parasite across the BBB. This could be done through the interaction of the intercellular adhesion molecule 1 (ICAM-1) of the BBB cells with the parasite MIC2 protein [20]. This interaction is important for transmigration of tachyzoites, as demonstrated *in vitro* in monolayers of several different cell lines [20] (**Figure 1**). Macrophages are also responsible for the spread of *T. gondii* in the brain [21, 22]. The migration of infected macrophages into the CNS is mediated by the uPA/uPAR pathway and the expression of metalloproteinases 9 (MMP9) [23]. In the mouse brain during the acute phase of infection, *T. gondii* tachyzoites are able to infect all brain cells, mainly microglial cells, astrocytes and neurons. However, microglial cells tend to react against the parasite. These cells inhibit the growth of *T. gondii* and can, therefore, function as important inhibitors of *T. gondii* propagation in the CNS by mechanisms independent of NO and IFN-γ; but this does not prevent *T. gondii* from being able to encyst in these cells [15, 24]. Astrocytes and rat neurons are also suitable to host cells for the intracerebral proliferation of the PLK (type II) *T. gondii* strain [24]. These cells can harbor tachyzoites and cysts during the two respective phases of *Toxoplasma* infection [25]. Astrocytes also have the ability to control the proliferation of tachyzoites during the acute phase of infection for the local control of this opportunistic pathogen [26]. In neurons, the presence of tachyzoites or bradyzoites alters the functioning of infected neurons. These modifications will

**2.3 The regulation of the cerebral immune mechanism during** *T. gondii* **infection**

After *T. gondii* has entered the CNS through the BBB, a cerebral immune response is triggered against the parasite. Experimental data on animal models show that the Th1-type immune response is activated against *T. gondii* to control the replication of the parasite in the brain. In the murine model, this immune response leads to the production of IFN-γ after infection with the ME49 (type II) strain [29–31]. This cytokine is the main mediator of parasite resistance in the murine model. IFN-γ produced by brain-resident cells is crucial for facilitating both the protective innate and T cell-mediated immune responses to control cerebral infection with *T. gondii*. Mice deficient for the IFN-γ receptor or those that are neutralized by an anti-IFN-γ antibody, are unable to control acute *Toxoplasma* infection [32]. During this host response, other cytokines and chemokines are produced. These may promote the infiltration of immune cells to the site of infection [33]. T cells and IFN-γ are essential for maintaining the latency of chronic infection in the brain and the prevention of reactivation of latent infection. The protective activity of T lymphocytes (CD4+, CD8+) results in the production of IFN-γ (**Figure 1**). The protective activity of these cells has

CD11b<sup>−</sup> den-

CD11b<sup>−</sup> are the main cell population

neurotrophic and releasing potentially toxic pro-inflammatory molecules.

role in the spread of the parasite, including in the brain. The CD11c<sup>+</sup>

**2.2 The mechanism of** *T. gondii* **invasion in the brain**

mesenteric lymph nodes. Monocytes CD11c<sup>+</sup>

favor the persistence of the parasite [27, 28].

cells [18, 19].

**120**

been demonstrated in a transfer of immune cells T, which conferred protection against the reactivation of cerebral toxoplasmosis in an IFN-γ deficient mouse [34]. However, the presence of TCD4 and TCD8 cells appears to be critical for the long-term maintenance of the latency of chronic *Toxoplasma* infection in the brain. In addition, mRNA encoding IFN-γ was also detected in the brains of *T. gondii*-infected and NK-cell deficient mice. This suggests that IFN-γ could be produced by non-T cells and non-NK cells. This production is important for the maintenance of chronic toxoplasmosis in the brains of mice [35]. Microglial cells and macrophages are identified as the major non-T and non-NK cells that express IFN-γ in the brains of *T. gondii*-infected mice [36]. Therefore, it is possible that the production of IFN-γ by these cells plays an important role in the prevention of cerebral toxoplasmosis. Microglial cells play an important role on the one hand in the innate defense system that limits parasite proliferation and on the other hand by regulating the production of chemokines that facilitate the accumulation of T lymphocytes at the parasite multiplication site. Murine astrocytes have also been implicated in inhibiting the growth of type II *T. gondii* strain *in vitro*. Astrocytes infected with ME49 (type II) *Toxoplasma* strain, produce IL-1, IL-6 and Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) [37]. During the innate immune response, dendritic cells, macrophages, and neutrophils produce IL-12 in response to *T. gondii* infection. This cytokine is essential for the production of IFN-γ. Neutralization of IL-12 with antibodies to this cytokine resulted in 100% mortality in mice infected with an avirulent strain of *T. gondii*, mortality was associated with decreased production of IFN-γ [38]. In addition, IL-12 is also important for maintaining IFN-γ production by T cells during chronic infection. The production of IL-12 is regulated by Lipoxin A4 (LXA4), in order to avoid pathogenic inflammatory reactions in the brain during the chronic phase of *T. gondii* infection. This has been demonstrated by the high production of LXA4 in the serum of mice during chronic *Toxoplasma* infection 5-Lipoxygenase (5-LO) is an essential enzyme in the production of LXA4, 5-LO deficient mice succumbed to infection during the chronic phase, thus presenting a cerebral inflammatory reaction [39]. LXA4 is important for the downregulation of pro-inflammatory responses during the chronic phase of *T. gondii* infection. Studies have shown that lipoxins activate two receptors (AhR and LXAR) in dendritic cells. This activation triggers the expression of the suppressor of cytokine signaling (SOCS)-2. SOCS-2 deficient mice succumb to chronic *T. gondii* infection. This is accompanied by a strong production of IL-12, IFN-γ, and reduction of cerebral cysts [40]. Although Th1 immune responses play a critical role in resistance to *T. gondii* infection, Th2-like immune responses are also implicated in protective immunity. IL-4 plays a major role in the development of the cellular immune response and in the differentiation of T cells into Th2 cells. During the chronic phase of *Toxoplasma* infection, IL-4 inhibits the production of proinflammatory mediators to prevent the development of local inflammations, promoting the persistence of cysts in the brain. Mice deficient in IL-4 die during the late phase of infection [41]. In these mice, a histological study reveals local areas of acute inflammation associated with the multiplication of tachyzoites in the brain. These results indicate that IL-4 is protective against the development of toxoplasmic encephalitis by preventing the formation of cysts and the proliferation of tachyzoites in the brain. The action of IL-4 during the chronic phase of infection is enhanced by the production of IL-10 by T cells [41]. IL-10 also exerts an immunomodulatory role in regulating the Th1-type inflammatory immune response [42]. Mice deficient in IL-10 and treated with sulfadiazine develop fatal inflammatory responses in the brain during the late stage of infection [43]. IL-10 is important for the survival of mice during the acute and chronic phases

of infection. This is confirmed by the neutralization of this cytokine in *T. gondii*infected mice [44] and following vaccination of the mice with *T. gondii* antigens (E/SA) [45].
