Study Of Microglia Activation-mediated Neuronal Injury In Toxoplasma Encephalitis

Background:Toxoplasma gondii is an obligate intracellular protozoan parasite that is capable ofinfecting all warm-blooded vertebrates. Infection in immunocompetent individualsusually is asymptomatic or is a benign self-limiting illness and results in chronicinfection. Immunosuppression in the chronically infected individuals may result inlife-threatening toxoplasmic encephalitis(TE).TE is a major cause of morbidity andmortality in immunocompromised patients. The complex process of Toxoplsamsgondii-induced pathology in the central nervous system(CNS) has many aspects as yetto be clarified. Microglia are the resident immune cells in the CNS that are activated inresponse to injury, inflammation or the presence of pathogens. Activated microgliasecrete inflammatory cytokines, which can induce neuronal apoptosis. Many lines ofevidence suggest that activated microglia play a critical role in pathogenesis of manyCNS diseases. However, whether activated microglia is involved in the pathogenesis ofTE has not yet been investigated. We hypothesized that the inflammatory environment generated during exposure of microglia to tachyzoites may result in neurons damage ofTE.Objective:To investigate the role of microglia in the immunopathogenesis of TE. To providetheoretical and experimental evidences for treatment of TE.Meterials:In the present study， we generated a murine model of Toxoplasmicencephalitis.Microglia phenotype was assessed by Immunohistochemistry，inflammatory cytokines such as interleukin-1β（IL-1β）, interleukin-（6IL-6）and tumornecrosis factor-α（TNF-α）were investigated by RT-PCR and ELISA analysis. Theexpression of inducible nitric oxide synthase(iNOS) was determined withImmunofluorescence and Western blotting., Neuronal apoptosis was detected terminaldeoxynucleotidyl transferase-mediated dUTP end-label-ing (TUNEL)staining,Western blotting and Immunofluorescence.Results:(1) In the TE of wild-type mice, we found that microglial cells were dramaticallyactivated in cortex and hippocampus. The expression of IL-1β, IL-6, TNF-α and iNOSare up-regulated. In parallel, neuronal apoptosis was significantly increased in TE.(2) In vitro studies result further confirmed that T.gondii tachyzoites inducedmicroglia activation and increased the production of IL-1β, IL-6,TNF-α and iNOS.When mouse neuroblastoma Neuro2a (N2a) were co-cultured with Mouse microglialcell line BV-2and tachyzoites, apoptosis of N2a cells significantly increasedcompared with controls.(3) Minocycline (microglial activation inhibitor) treatment significantly reduced boththe microglial activation and neuronal apoptosis in vivo and in vitro.Conclusions：Our results first demonstrated that T.gondii tachyzoites induced microgliaactivation,which upregulate proinflammatory cytokines expression, such immunopathological changes were accompanied with neuronal apoptosis. Reatmentwith minocycline not only inhibited microglia activation and the expression ofproinflammatory cytokines, but attenuated neuronal apoptosis. Taken together, ourfindings from this research suggest that microglia activation may playan important role in the pathogenesis of TE. Thus, inhibiting microglia activation mayrepresent a novel therapeutic strategy for toxoplasmic encephalitis. This is a novelapproach for the involvement of microgia-activated pathogenesis in encephalitiscaused by T. gondii.