Toll like receptors (TLRs) signaling plays a crucial role in the eradication of invading viruses.In parallel,viruses have developed various strategies to disrupt the TLR cascade.For hepatitis B virus (HBV),we have previously reported that the level of plasma Hepatitis B surface antigen (HBsAg) correlates closely with an impaired TLR response in peripheral blood mononuclear cells (PBMCs) from chronic hepatitis B (CHB) patients.Here we examined the suppression mechanism of HBsAg on TLR signaling in monocytic cell lines.Pretreatment cells with HBsAg suppressed,in a dose-dependent manner,LPS-induced proinflammatory cytokines expression and NF-κB,c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) activation.Luciferase reporter assay showed that HBsAg interferes with NF-κB promoter activity driven by ectopic expression of myeloid differentiation factor 88 (MyD88) and TAK1,while not affecting the responses induced by p65,implying a block upstream in the TLR signaling.LPS-stimulated polyubiquitination of tumor necrosis factor receptor-associated factor 6 (TRAF6) and formation of TRAF6-TAB2 complex were further found to be inhibited in HBsAg-pretreated cells.Moreover,a strong up-regulation of A20,which is known as an ubiquitin-editing enzyme that can negatively regulate TLR signaling,was observed upon addition of HBsAg.Knockdown of A20 by siRNA restored LPSmediated cytokines induction,reflecting a role of A20 in HBsAgmediated inhibition of TLR signaling.These results demonstrate a novel anti-TLR function of HBsAg which involves up-regulation of A20 and provide a possible mechanism by which HBV escapes immunity.