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Due to increased and broadened screening efforts, the last decade has seen a rapid expansion in the number of viral species classified into the Hepacivirus genus. Conserved genetic features of hepaciviruses suggest that they have undergone specific adaptation and have evolved to hijack similar host proteins for efficient propagation in the liver. Here, we developed pseudotyped viruses to elucidate the entry factors of GB virus B (GBV-B), the first hepacivirus described in an animal after hepatitis C virus (HCV). GBV-B-pseudotyped viral particles (GBVBpp) were shown to be uniquely sensitive to the sera of tamarins infected with GBV-B, validating their usefulness as a surrogate for GBV-B entry studies. We screened GBVBpp infection of human hepatoma cell lines that were CRISPR/Cas9 engineered to ablate the expression of individual HCV receptors/entry factors and found that claudin-1 is essential for GBV-B infection, indicating the GBV-B and HCV share an entry factor. Our data suggest that claudin-1 facilitates HCV and GBV-B entry through distinct mechanisms since the former requires the first extracellular loop and the latter is reliant on a C-terminal region containing the second extracellular loop. The observation that claudin- 1 is an entry factor shared between these two hepaciviruses suggests that the tight junction protein is of fundamental mechanistic importance during cell entry. [ABSTRACT FROM AUTHOR]

The major mechanism for determination of HCV infection outcomes has not been fully described, particularly in the early phase of the "window-period" of infection. Based on two groups of marmosets infected with HCV-CE1E2p7/GBV-B chimeric virus (HCV chimera) or GBV-B, the immune mechanism correlating with the different outcomes of virus infections was explored in this study. HCV chimera containing the entire HCV core and envelope proteins (CE1E2p7) and GBV-B RNA were intrahepatically injected into four marmosets in each group, respectively. Blood samples were taken from individual animals in an interval of 2 weeks. Viral load and specific T cell responses were detected in two groups of HCV chimera- and GBV-B-infected marmosets. HCV chimera-infected marmosets appeared to have a virally persistent infection over 6 months post inoculation of the virus. Of these, the specific IFN-γ-secretion T cell response slowly developed over 13 to 19 weeks and was maintained at a relatively low level with 40-70 SFC/10 6 PBMCs, while the specific Treg cell response was rapidly activated over 3 weeks and was maintained at a high level around 5% among lymphocytes. In contrast, GBV-B-infected marmosets presented spontaneous viral clearance within 6 months; the specific IFN-γ-secretion T cell response was quickly established over 5 to 7 weeks and was maintained at a high level with 50-130 SFC/10 6 PBMCs, while the specific Treg cell response was inactivated and maintained at a baseline below 3% among lymphocytes. In conclusion, the HCV structural proteins inducing immune suppression in the early phase of HCV infection contributed to the viral persistence, of which the activation of Treg cells might play an important role in the inhibition of an effective T cell antiviral response.