Background Hepatitis B virus (HBV) is a major health problem with almost 300 million people affected worldwide. Long-term infection may be established in the liver, especially if exposure occurs early in life, and this increases the risk of scar tissue formation, cancer and liver failure. Current treatments (such as tenofovir, entecavir) are well tolerated, available in tablet form and highly effective at suppressing viral replication; however, traces of the virus persist and life-threatening inflammation may occur if treatment is stopped prematurely. As new treatments become available over the next few years, it is increasingly important to develop blood tests that closely reflect the burden of virus in the liver and activity of the immune population. Ultimately, this will allow individuals to be matched with the most appropriate treatments and thus minimise side effects. Aim(s) of the research Our research project will explore the distribution of hepatitis B virus in the liver and identify markers in the bloodstream that reflect the burden of infection (as well as the immune response). Design and methods used The research will be conducted across three sites, namely King's College Hospital (London), Francis Crick Institute (London) and the Nuffield Department of Medicine (Oxford). In the first part of the project, individuals with chronic hepatitis B infection who have tissue samples stored in our research biobank, will be identified. Using techniques called "in-situ hybridisation" and "multiplex antibody staining", we will map the distribution of viral activity (genetic material, proteins) within the liver and the layout of local immune cells. In parallel, we will measure a panel of molecules in the bloodstream and explore how closely they reflect viral activity within the liver. In the second part of the study, a cohort of patients (n=100) will be approached at King's College Hospital and extra blood tests will be taken at the time of clinical review. After isolating immune cells from the bloodstream, a series of experiments will be conducted to assess their function and ability to replicate; the extent to which these cells are "exhausted" and unable to carry out their normal protective functions will be correlated with molecules in the serum (including Tim3 and sPD-1). Potential applications and benefits The research project has the potential to fundamentally change our understanding of HBV replication in the liver. From a clinical perspective, blood tests that closely reflect viral activity will help clinicians to decide which patients should be started on treatment, the duration of therapy, and suitability for withdrawal. In addition, they may help to identify those at greatest risk of developing liver cancer, in whom a more stringent surveillance programme should be adopted. Communication plan We envisage that our results will have a significant impact on the way patients with chronic hepatitis B infection are managed in the future, and we want to maximise visibility amongst other experts in the field. To achieve this, the data will be presented at international conferences and published in medical journals. In addition, we recognise the importance of updating patients on the progress of the research, and this will be achieved through educational programmes that we run regularly at King's College Hospital, as well as newsletters, websites and local networks.