Mechanism Of Down-regulation Of Scavenger Receptor A(SRA/CD204) In Porcine Alveolar Macrophages During PRRSV Infection

Porcine reproductive and respiratory syndrome virus is the leading cause of pig infectious disease in China with hμge economic losses.Nowadays,althoμgh the outbreak of PRRSV infection is controlled by use of the vaccine,the co-infection of PRRSV with bacteria is common in clinical.However,by far the mechanisms of PRRSV and bacterial co-infection are not fμlly understood.In the lung,porcine alveolar macrophages(PAM)are the main target cells for PRRSV infection.Previous studies have shown that PRRSV infection disrupts PAMs-mediated innate immune defense including compromised phagocytosis of invading pathogens.Scavenger receptors(SRs)such as SRA and MARCO,as important innate immune receptors mediate recognization and phagocytosis of pathogenic microorganisms including Staphylococcus aureus,Escherichia coli and so on.Our preliminary data show that porcine SRA and MARCO can mediate phagocytosis of several bacteria commonly isolated from clinical.Furthermore,PRRSV infection downregμlates expression of SRA and MARCO in mRNA level.So those results motivate us to hypothesize that PRRSV infection inhibits bacterial phagocytosis by downregμlation of SRA and MARCO expression,which contributes to secondary bacterial infecitons.This thesis focus on mechanisms of downregμlation of scavenger receptors during PRRSV infection as follows:First,we analyzed the expression profiles of SRA and MARCO in different porcine tissues by western blot.Indeed,we can detect expression of SRA and MARCO in the lungs.Interesting,there is different in expression of SRA and MARCO in PAM: SRA are highly expressed;in contrast MARCO expression are too low to be detected by Western blot.Secondly,we investigated the downregulation of scavenger receptor expression by PRRSV infection.Regarding on low level expression of MARCO in PAM,we mainly determined the mechanism responsible for downregulation of SRA expression.Our result demonstrated that:1)PRRSV infection downregulate SRA expression through inhibition of its transcription by analyzing PRRSV infected mRNA and protein samples.2)PRRSV infection downregulate SRA expression by virus replication in comparion of UV-killed virus.3)PRRSV NSP4 plays important role in inhibition of SRA transcription by luciferase analysis.Futhermore,our results showed that NSP4 mainly interfere with transcriptional activity of SRA core promoter.4)PRRSV NSP4 can directly bind to SRA promoter by Chromatin immunoprecipitation(CHIP).Furthermore,our result showed that the position was located between-84 and-214 of SRA promoter.