TLR7 Deficiency Leads To TLR8 Compensative Regulation Of Immune Response Against JEV In Mice

Japanese encephalitis Virus (JEV) is a mosquito-borne flavivirus in East and South Asia, estimated to cause 60 000 human cases of Japanese encephalitis each year. Japa-nese encephalitis virus (JEV) is a highly fatal pathogen to human beings which causes Japanese encephalitis (JE). JE is a vector-harbored disease originated in Asia having potential to blowout into temperate regions across the whole world. As a neurotropic virus, JEV can evade the body immune defenses, cross the blood- brain barrier, enter the central nervous system, infect the neuron, activate microglia and astrocytes to re-lease large amounts of inflammatory factors, and trigger neuroinflammation. Various studies have shown that TLR7 is part of the first host defense against most ssRNA fla-viviruses. However, the underlying mechanism responsible for TLR7-mediated im-mune responses in JE is unclear. This study was conducted in TLR7 deficient mice(TLR7-/-) to investigate how TLR7 triggers immune responses. We divided this study into three parts in vitro study, in vivo study and immunological study to investigate the response of TLR7 expression post JEV infection. In in vitro study, the bone mar-row-derived DC (bmDC) cells were cultured to investigate the expression of TLR7,and some surface stimulatory and inhibitory molecules. The expression of TLR7 was significantly increased on bmDC at 48 hour as compare to control group and other time interval. The expression of CD80, CD86 and CD273 on bmDC cells were significantly increased at 48h and 72h in TLR7 deficient mice after JEV-infection compared with the control group. The in vitro results suggested that TLR7 is important in immune regulation. Based on in vitro study, we further investigated the function of TLR7 on murine models. Compared with TLR8 and TLR9, TLR7 was the most highly expressed receptor after JEV infection. However,there was no significant difference in the sur-vival and body weight of C57BL/6 and TLR7-/- mice following JEV infection. The vi-ral load was significantly increased in TLR7-/- mice compared with C57BL/6 mice. In-terestingly, TLR8 was also up-regulated in TLR7 deficient mice that survived JEV in-fection. The expression of TLR8 was significantly increased in TLR7 deficient mice as compare to C57BL/6 mice through immunofluorescence assay. However, expression of TLR8 was lower on brain tissue of TLR7 deficient mice with disease symptom than mice without disease symptom after JEV infection. This result indicates that TLR8 may have a compensatory function in the absence of TLR7, maybe activate the im-mune response, and play an antiviral effect. In immunology study, we investigated the response of T cell subsets, DC subsets, and pDC on spleen and blood samples in TLR7 deficient mice after JEV infection. The population of pDCs was increased in TLR7 de-ficient mice in the JEV infected group compared with the mock group; The ratio of the DC subset of cells, CD11c+ CD80+ and CD11c+CD86+, was also significantly in-creased in TLR7-/- mice post-JEV infection compared with control mice; TLR7 defi-ciency led to an increase in the ratio of the Treg subset of cells, CD4+CD25+Foxp3+,response in the JEV infected group compared with the control group. Our result con-cluded that, in TLR7-/- mice, the TLR8 signaling pathway is involved in the induction of immunity. Future research is needed to explore the underlying mechanism.