The Effect Of Interferon Regulatory Factor 7 On The Pathogenicity Of Rabies Virus

Rabies is a zoonosis caused by rabies virus?RABV?.It is a neurotropic virus,which mainly causes fatal encephalitis.The mortality is almost 100%once the clinical symptoms appear and there is no effective treatment for this disease so far.Human rabies has been eliminated by vaccination in most developed countries,but it remains a serious threat to developing countries with poor health care.Due to increased prevention and control of rabies,the number of rabies cases in China has dropped from 3,302 in 2007 to 516 in 2017which has fallen by 84.37%.But given the current situation,the goal of eliminating rabies remains difficult.At present,there are still many blind spots in the pathogenesis of RABV.Further improvement is conducive to the development of rabies treatment programs so as to eliminate the threat to humans and animals as soon as possible.Interferon regulatory factor 7?IRF7?is known as a master transcriptional regulator of type I IFN.It involved in innate and adaptive immunity through regulating cytokines.IRF7has been reported to play a significant antiviral role in the case of West Nile virus?WNV?infection,but there is no studies have shown whether IRF7 plays a role in RABV infection.Therefore,IRF7^-/-mice were used in this study to explore the effect of IRF7 on RABV pathogenicity.In the experiment of pathogenicity under the condition of attack on hind limb muscle,IRF7^-/-mice showed earlier disease course,higher mortality and increased pathogenesis of RABV compared with WT mice,indicating that IRF7 was involved in activation of peripheral immune response induced by RABV infection.RT-q PCR was used to detect the viral load in the infected site,spinal cord and brain tissues and it was found that the viral load in all tissues of IRF7^-/-mice was higher than WT mice.Notably,there was a difference in the viral load in the infected site at the early stage of infection,indicating that IRF7 was involved in inhibiting the replication of RABV in hind limb muscle.In order to explore the mechanism of IRF7 inhibiting RABV replication,on the one hand,we detected the expression levels of various cytokines in draining lymph nodes,among the expression levels of MCP-1,MCP-1?and MIP-1?were decreased in IRF7^-/-mice compared to WT mice.It is known that MCP-1,MIP-1?and MIP-1?belong to the CC family.They are mainly responsible for monocyte movement.Thus,we speculated IRF7 may be involved in recruitment of immune cells such as p DC,c DC,and macrophages in lymph nodes by inducing chemokine production.By day 4 and day6 after RABV infection,we observed fewer p DC,c DC and macrophages in the draining lymph node of RABV-infected IRF7^-/-mice compared to WT mice.It was consistent with our prediction.On the other hand,in order to explore the way IRF7 works when p DC,c DC and macrophages are activated by RABV in vitro,we directly infected these cells with RABV respectively and measured the viral titers in the cell supernatant,as well as the expression levels of intracellular viral RNA,IFN?and ISG.The results showed that IRF7 had antiviral effect,as well as participated in IFN?and ISG expression in p DC and macrophages.However,IRF7 is not particularly useful in the c DC.The difference may be due to the constitutive expression of IRF7 in p DC and macrophages.After intracranial infection,IRF7^-/-mice showed earlier disease course,higher mortality and higher viral load in each brain region than WT mice,indicating that IRF7could inhibit the replication of RABV in the CNS.We measured the levels of cytokines in the CNS of RABV-infected IRF7^-/-and WT mice.The level of IFN?was lower and the levels of inflammatory factors were higher in RABV-infected IRF7^-/-mice than WT mice.Brain sections of IRF7^-/-mice showed more severe inflammation and lesions in the brain than those in WT mice.In summary,IRF7 is involved in antiviral immune response during RABV infection in peripheral and the CNS.In peripheral,IRF7 promotes recruitment of p DC,c DC and macrophages,as well as involved in inducing the expression of IFN?and ISG in p DC,c DC and macrophages.In the CNS,IRF7 is crucial for induction of IFN?.