Discussion

Naegleria fowleri is the causal agent of PAM in experimental animals and humans. Naegleria spp. are widely found in soil and water (Schuster and Visvesvara, 2004). N. fowleri usually propagates in warm, stagnant bodies of freshwater (typically during the summer season), and opportunistically infects the central nervous system via insufflation of water conjugated with the amoeba (CDC, 2008). The amoeba attaches to the olfactory nerve, migrates through the cribriform plate and then reaches to the olfactory bulbs of the forebrain (Cervantes-Sandoval et al., 2008), where it actively multiplies by feeding the nerve tissue. During this stage, usually occurring approximately 3–7 days post-infection, the infected person showed typical symptoms such as parosmia, rapidly progressing anosmia (with resultant ageusia) as the nerve cells of the olfactory bulbs are disrupted and replaced with necrotic lesions.

Microglial cells are the resident macrophages of the brain and spinal cord, and play as first-like immune defense cells in the CNS. The primary role of microglia, involves the engulfing of various materials including cellular debris, lipid apoptotic cell death in the non-inflamed state, and invading virus, bacteria, or other foreign materials in the inflamed state. In addition, microglia also play various roles in the CNS including cytotoxicity for infectious organisms, antigen presenting, synaptic stripping, promotion of repair and extracellular signaling and homeostasis in

non-29

infected region and promoting inflammation infected or damaged tissue (Gehrmann et al., 1995). Extracellular signaling is complicated connected with other microglia, astrocytes, nerves, T-cells and progenitor cells. Microglia activation is induced by IFN-γ, and this activation increases IFN-γ release into the extracellular environment.

The activated microglia induces expressions of several cytokines, and they rapidly activate nearby microglia. Microglia-induced TNF-α causes neural tissue to undergo apoptosis and induces inflammation. Another cytokine IL-1 inhibits IL-10 and TGF-β, which down-regulate antigen presentation and pro-inflammatory signaling. The pro-inflammatory cytokines IL-1α, IL-1β and TNF-α induced by microglia stimulation of CNS, which play a potential role in neurodegeneration when microglia remain in a sustained activated state (Wood, 1998; Aloisi, 2001).

Elucidation of pathogenicity-related factors in PAM is important for understanding the mechanism of N. fowleri-host interactions. Amoebic pathogenicity may consist of complex processes that include both contact-dependent and contact-incontact-dependent mechanisms that lead to host cell death. In contact-dependent mechanism, Nfa1 protein, HSP 70s and Nf-actin, which are closely related with phagocytic food-cup formation, seem to play critical roles (Shin et al., 2001; Song et al., 2006; Sohn et al., 2010). Meanwhile, in a contact-independent mechanism, Nf-ESPs, which consisted of peroxiredoxins, proteases and thrombin receptor, may play a role in host cell invasion and lytic activity for host immunoglobulin and immune evasion (Kim et al., 2009).

30

IL1-β and IL-6 are produced in primary astrocytes stimulated with N. fowleri lysate (Kim et al., 2013). It is also reported that IL1-β, IL-6 and TNF-α were produced in primary microglia co-cultured with N. fowleri trophozoite. IL-18 and IFN-γ were also induced in microglial cell line by N. fowleri lysate (Oh et al., 2005). However, no studies have been on inflammatory responses against Nf-ESPs., In this study, the changes and immune responses in microglia cell line, BV2 cells, treated with Nf-ESPs were analyzed. BV2 cells treatment with Nf-ESPs markedly reduced cell viability in dose dependent manners, which supporting that Nf-ESPs is a significance effector factor for cell damages observed in PAM. The BV2 cells secreted various cytokines and chemokines in response to Nf-ESPs.

Especially, expressions of pro-inflammatory cytokines IL-1α and TNF-α were highly increased. To confirm mRNA expressions of IL-1α and TNF-α, RT-PCR was performed. Both IL-1α and TNF-α expressions was increased in BV2 cells in a time dependent manner by treatment of Nf-ESPs.

IL-1α is the prototypic pro-inflammatory cytokine and effective nearly every cell type, often in concert with another pro-inflammatory cytokine, TNF-α (Dinarello, 1997). Previous study of Leshmaniasis, IL-1 production was induced in monocytes (Crawford et al., 1985). Leshmania mexicana lipophosphoglican (LPG) induced the production of TNF-α, IL-1β, IL-12P70 and IL-10 when human macrophage was stimulated with L. mexicana LPG (Rojas-Bernabe et al., 2014).

Another study showed that IL-1α and TNF-α production increased in human oral

31

and vaginal epithelial cells during Candida albicans infection (Steele and Fidel, 2002). Pro-inflammatory cytokine such as IL-1α, IL-1β and TNF-α mRNA expression increased in primary rat-microglia co-cultured with Acanthamoeba castellanii (Marciano-carbral et al., 2000). In experiment of chlamydia trachomatis, IL-1α was increased in both apical and basolaterial of C. trachomatis infected polarized, immotalized, endocervical epithelial cell model (polA2EN) (Buckner et al., 2013). Some case report of carmeroonians infected by Onchocerca volvulus, observed that IL-1α, IL-6, IL-10 and IL-13 were detected from blood sample by Enzyme-linked immunoabsorbant assay (ELISA) (Nmorsi et al., 2012). In addition, some reports have described that proinflammatory cytokines IL-1, IL-6 and TNF-α level were increased mainly in the aged hippocampus but also in cortical regions (Murray et al., 1997; Katafuchi et al., 2003; Maher et al., 2004; Sierra et al., 2007).

Moreover, the peripheral blood mononuclear cells (PBMC) from parturient and non-pregnant woman was exposed with live tarchyzoites of Toxoplasmma gondii, pro-inflammatory cytokines such as TNF-α and IL-2 was produced significantly higher level (Rezende-Oliveira et al., 2012). Moreover, release of IL-1α, IL-6 and GM-CSF activity were increased in the astroglial cells infected by T. gondii (Fischer et al., 1997). So I suggest that pro-inflammatory cytokine IL-1α and TNF-α may play as an inducer of inflammatory response during the N. fowleri infection.

MAPK are serine/threonine/tyrosine-specific protein kinase belonging to the CDK, MAPK, GSK and CLK kinase group (Manning et al., 2002). MAPKs

32

signaling pathway is essential in regulating diverse cellular processes including inflammatory response, cell differentiation, proliferation and death. MAPKs signaling modulated which mediate extracellular signals into the nucleus to turn on the responsive genes in mammalians cells, including P38, JNK and ERK kinase.

MAPKs phosphorylate a variety of intracellular targets including transcription factors, nuclear pore proteins, membrane transporters, cytoskeletal elements, and other protein kinases. Especially, MAPKs activation was related with pro-inflammatory response in astrocyte (Kim et al., 2013). Activation of MAPKs is also crucial for regulating cytokine expressions. Treatment of Nf-ESPs induced high level expressions of several cytokines, especially IL-1α and TNF-α.

IL-1 is responsible for inflammation as well as the promotion of fever and sepsis. Especially, IL-1α is mainly produce by activated macrophages, neutrophils, epithelial cells and endothelial cells. It plays one of the central roles in the regulation of immune response and possesses metabolic, physiological, haematopoietic activities (Bankers-Fulbright et al., 1996; Dinarello, 1997) and it is on the pathway that activates TNF-α. The role of TNF-α is regulation of immune cells, being an endogenous pyrogen, can induce fever, apoptotic cell death, inflammation and inhibit tumorgenesis. In addition, TNF-α activation closely relate with various human disease including Alzheimer’s disease (Locksley et al., 2001), cancer (Dowlati et al., 2010), major depression (Bryuskov et al., 2002) and inflammatory bowel disease (IBD) (Mikocka-walus et al., 2007). In conclusion,

33

Nf-ESPs induced inflammatory responses in BV2 microglial cells. Increased levels of several cytokines and chemokines were identified and IL-1α and TNF-α were the most predominant cytokines effected by Nf-ESPs. The expression of these cytokines is likely to be regulated by MAPKs signal pathway, but more detailed study on interaction of transcription factors such as AP-1 and NF-κB would be necessary to fully understand the precise mechanism for up-regulation of cytokine production. The result obtained, in this study suggest that Nf-ESPs may play an important role in contact-independent pathogenicity of N. fowleri in PAM.

34