Severe Fever With Thrombocytopenia Syndrome Virus Infection Drives Macrophage Differentiation Skewed To M2 Phenotype Which Is Regulated By MiR-146

Severe fever with thrombocytopenia syndrome(SFTS)is an emerging hemorrhagic fever with a high mortality rate in humans,which is caused by SFTS virus(SFTSV),a novel phlebovirus in the Bunyaviridae family,is tick borne and is endemic in Eastern Asia.These emerging phleboviruses have raised new concerns and are posing serious threats to livestock and human public health due to its high fatality rate.Macrophages are known as an important component of the immune system,which is a critical first defense in innate immunity and modulates adaptive immune response to pathogens through antigen processing and presentation.Essential to such functions is classical activation(M1)and alternative activation(M2)of macrophages.The classical M1 macrophages are activated by “pro-inflammatory”cytokines(i.e.IL-6 and TNF-α)and act?to?kill?intracellular?pathogens,while the alternative M2 macrophages appear to be involved in immunosuppression.Previous studies have demonstrated that SFTS virus is capable of hijacking splenic macrophage for replication which was identified as primary target cells of SFTSV infection and the virus could harbor within splenic macrophages for long periods.However,the role of macrophages in virus replication and the potential pathogenic mechanisms of SFTSV in macrophage remain unclear.Our data demonstrated that SFTSV infection drives macrophage differentiation skewed to M2 phenotype which the expression of the CD206 increased.M2 macrophages showed significantly higher viral copy numbers in both extracellular and intracellular compartments than those in M1 macrophages,suggesting that SFTSV replicates more efficiently in M2 macrophages.Accumulating evidences have shown that micro RNAs could modulate macrophage differentiation,we investigated mi RNA expression profiles in FCM sorted M1 and M2 macrophage-like THP-1 cells infected by SFTSV using small RNA sequence techniques.Among these mi RNAs,we focused on mi R-146a/b that was maximally upregulated upon SFTSV infection and that it was previously reported to modulate macrophage differentiation by targeting various transcription factors and adaptor proteins.Consistent with database prediction and our results,we found that mi R-146 a and b significantly reduced luciferase activity of a reporter gene containing the STAT1 3’UTR,and the deletion of 6 nt in the 3’UTR abolished the inhibitory effect of mi R-146 a or b,indicating that SFTSV infection suppresses STAT1 expression by upregulating mi R-146a/b.The interplay of various cytokines plays a crucial role in the pathogenesis of SFTSV infection.We observed that IL-10,which is involved in modulation of mi R-146 expression,was elevated in SFTS patients and produced at robust levels in fatal cases.And notably,our initial study confirmed that IL-10 expression was upregulated during SFTSV infection of THP-1 cells.To further explore the mechanism of mi R-146 upregulation upon SFTSV infection,we investigated the non-structural protein(NSs),which was reported to be involved in viral replication and modulation of host response.These data suggest that endogenous mi R-146b-induced in the suppression of endogenous STAT1 expression is mediated through NSs protein.Together,in this study,we provided evidence that the SFTSV infection drove macrophage differentiation skewed to M2 phenotype,facilitated virus shedding and resulted in viral spread.We showed evidence that mi R-146 a and b were significantly upregulated in macrophages during the SFTSV infection,driving the differentiation of macrophages into M2 cells by targeting STAT1.Further analysis revealed that the elevated mi R-146 b but not mi R-146 a was responsible for IL-10 stimulation.We also found that SFTSV increased endogenous mi R-146b-induced differentiation of macrophages into M2 cells mediated by viral non-structural protein(NSs).The M2 skewed differentiation of macrophages may have important implication to the pathogenesis of SFTS.