HCV is a major public health problem, infecting more than 170 million people worldwide. Most cases of HCV infection become persistent and may eventually lead to chronic liver disease, cirrhosis, and hepatocellular carcinoma. HCV is an enveloped virus classified in the Flaviviridae family. The single-strand, positive-sense viral RNA genome encodes a single polyprotein precursor that is processed into three structural proteins (C, E1, E2) and seven nonstructural proteins (p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B) by host and viral proteases. The RNA genome of HCV has significant heterogeneity with six major genotypes and numerous subtypes. Until recently, chronic HCV infection has been treated with pegylated alpha interferon and ribavirin. Although the treatment outcome is variable among the six major HCV genotypes, only 50% of treated patients infected with genotype 1 respond to therapy. Since the past two years, new treatments for HCV infection have emerged and hold great promise for cure, but numerous challenges to establishing care and receiving therapy exist. The ability of HCV to establish persistent infection with great success in humans has been attributed, in part, to a variety of strategies to evade host immune and IFN-induced defenses. Epidemiological studies suggest that up to 20% of acutely infected HCV patients can resolve the infection without treatment, which implies that innate and/or adaptive immune responses are indeed capable of controlling the outcome of HCV infection. The molecular mechanisms that regulate innate intracellular antiviral responses may therefore serve as pivotal points of control, potentially limiting host permissiveness for HCV replication and favorably modulating subsequent adaptive immune responses. Although hepatocytes are the major site of viral replication, a broad clinical spectrum of extrahepatic complications and diseases are associated with chronic HCV infection, including mixed cryoglobulinemia, non-Hodgkins lymphoma, cutaneous vasculitis, glomerulonephritis, neuropathy, and lymphoproliferative disorders. The existence of extrahepatic reservoirs of HCV replication, particularly in PBMCs, remains highly controversial. It is unclear how B cells become dysregulated during the course of chronic HCV infection. In our previous study, we demonstrated that in 7 out of 7 chronic HCV patients, HCV is preferentially associated with CD19+ B cells in PBMC subpopulations. Using in vitro reconstitution experiments by mixing cell-free HCV particles from chronic HCV carriers with PBMCs from healthy blood donors, we found that HCV particles preferentially bind to CD19+ B cells. HCV particles produced from in vitro cell cultures have minimal binding capacity to CD19+ B cells. However, when HCV particles produced in cell culture were pre-incubated with serum samples from HCV recovered patients and then mixed with PBMCs from healthy blood donors, the viral particles become preferentially bound to CD19+ B cells. Initially we thought the serum components which promote the binding of HCV to B cells were induced by HCV infection, such as HCV antigen-specific antibodies. Later, we demonstrated that the same activity can also be found in serum from healthy blood donors without any evidence of previous exposure to HCV. Furthermore, we showed that the serum components which promote HCV binding to B cells are heat-labile. For our next phase of study, we will focus on the following projects: 1) To investigate the mechanism of preferential binding of hepatitis C virus to B cells in peripheral blood mononuclear cells from chronic HCV carriers; 2) To identify specific cell surface receptors involved in HCV binding to B cells; 3) To determine whether the virus is merely attached to the cell surface or becomes intracellular and replicates; and 4) To elucidate the possible biological responses and consequences of HCV binding to B cells.