Persistent viral diseases such as hepatitis B virus (HBV), hepatitis C virus (HCV) and HIV affect hundreds of millions of people worldwide. Antiviral therapy is only accessible to a minority of those in need, and preventive vaccines for HCV and HIV remain unavailable. Neonatal exposure to HBV accounts for a majority of the 240 million HBV carriers and typically results in antiviral tolerance. Unlike for CD8 T cells, however, the mechanisms governing antibody responsiveness to neonatal infection remain poorly defined. Vectored immunoglobulin gene delivery (VIGD) represents an innovative approach to circumvent immune exhaustion and tolerance in the treatment of persistent viral diseases. Whether VIGD can help restoring the host’s endogenous immune response, to complement the antiviral effects of VIGD and resolve chronic infection, remains yet to be investigated.Working hypothesis: I) Humoral tolerance to congenital viral infection is incomplete, both at the level of B cells and antiviral CD4 T cells. Some non-classical T cells and/or T follicular helper cells provide help to antiviral B cells upon escape from central and peripheral tolerance mechanisms. The resulting antibody response exerts constant selection pressure on the persisting virus. II) The antiviral efficacy of VIGD relies to a significant extent on its synergy with the host’s cellular and humoral immune defense mechanisms. These are augmented and functionally restored upon VIGD.Experimental models and methods: We will exploit reverse genetic techniques for lymphocytic choriomeningitis virus (LCMV) in conjunction with innovative gene-targeted mouse models. Antiviral immune defense, immune exhaustion and tolerance will be investigated in the context of persistent infection and its treatment by VIGD. Recombinant adeno-associated viral (AAV) vectors, genome-wide transcriptome analyses, TaqMan RT-PCR technology, immunohistochemistry and flow cytometry will be combined with state-of-the-art immunological and virological techniques.Specific aims: In addressing fundamental questions centered around immunity and tolerance to systemic persistent viral infection we will aim to:1.phenotypically, functionally and molecularly define humoral immune responsiveness to congenitally acquired, persistent viral infection2.determine the impact of vectored antibody gene delivery on the host’s endogenous immune defense, and to define resulting synergies in the resolution of persistent infection.Significance: We will combine an array of cutting-edge molecular tools and will apply them to relevant animal models of viral infection. The project thus has potential for a quantum leap in our understanding of immunological tolerance and dysfunction in persistent viral infection, and how to restore immune control by passive antibody delivery strategies. Fundamental novel concepts in these areas are urgently needed to combat persisting viral diseases of global health impact.