Baruch Blumberg’s 1965 discovery of HBsAg as a specific virological marker led to the realization that HBV causes a chronic form of hepatitis of worldwide prevalence (reviewed in ref. 8). Notably, despite decades of infection, a small percentage (~1% annually) of all patients spontaneously lose HBsAg and develop anti-HBs antibodies. Loss of HBsAg with or without emergence of anti-HBs (defined “functional” cure) is the goal of current treatment efforts, because it reduces the risk of liver cirrhosis, cancer, and liver-related death (9). Because of the unique life cycle of HBV, functional cure requires silencing or elimination of both cccDNA and integrated HBV DNA (9). While neither can be readily achieved by a finite treatment course with the currently available reverse transcription inhibitors, there is evidence that immune responses can be harnessed to contribute to functional cure. HBV, while not inducing innate cytokines such as type I IFN, TNF-α, and IL-6, is sensitive to them (10). This sensitivity explains why HBV/HCV-coinfected patients, in whom HCV induces an IFN-mediated activation of IFN-stimulated genes and natural killer cells, experience HBV reactivation when the IFN response is diminished after HCV is cleared (11). In vitro, high doses of IFN-α have been shown to affect different stages of the HBV life cycle, including the induction of epigenetic changes in cccDNA-bound histones (12), inhibition of HBV replication and transcription, and even degradation of cccDNA (reviewed in ref. 3). Therapy with pegylated IFN-α activates intrahepatic innate immune responses via induction of IFN-stimulated genes and stimulation natural killer cells (13), yet it results in HBsAg clearance in only a small percentage of patients.

With current efforts aiming at developing and combining novel treatments that target HBV replication, cccDNA, and HBsAg levels and/or to reinvigorate or substitute via adoptive transfer immune responses (14), there are still important virological and immunological aspects of chronic HBV infection that are not yet sufficiently understood. Translational research in well-defined, ideally prospectively followed patient cohorts may help to obtain insights into immune control that can be exploited to prevent the severe long-term effects of chronic HBV infection and to identify biomarkers for immune monitoring in the course of natural infection and during novel treatments. The following are some of the open questions:

How does HBV establish chronic infection in early life? Several studies in mice have identified potential factors that may contribute to the age-dependent differences in immune responses against HBV. While mice are not permissive for HBV due to the absence of the HBV entry receptor, it is possible to launch HBV replication from a transgene or an adenoviral vector. Adoptive transfer of naive immune cells into these mice during “adulthood” results in T and B cell priming and seroconversion from HBsAg+ to anti-HBs+ whereas adoptive transfer of immune cells into young transgenic mice does not (15). Factors that may contribute to this differential response include age-dependent expression of costimulatory molecules on antigen-presenting cells in the liver, decreased expression of chemokines that are required for optimal induction of virus-specific B cells, reduced T follicular helper responses, and age-dependent changes in gut microbiota (reviewed in ref. 16). Early exposure to secreted HBeAg during vertical transmission has also been implicated in this response (17). However, to date, there are almost no translational studies in humans. A notable exception is the demonstration of enhanced innate immune cell maturation and Th1 cell differentiation in cord blood from neonates of HBV-infected and uninfected mothers (18). This immunological state was associated with an enhanced ability of cord blood immune cells to respond in in vitro assays and may represent a state of trained immunity (18), consistent with the idea that HBV may be a symbiont that confers advantages to the host.

Which immunological mechanisms prevent immune activation despite high levels of HBV DNA in the first decades after infection? An important phase is the HBeAg+ noninflammatory phase of chronic HBV infection because HBV DNA integration and clonal hepatocyte expansion have been shown to start at this early stage (19) and shown to increase the risk of developing hepatocellular carcinoma (20). Patients at this stage have exceedingly high HBV DNA levels yet no/minimal evidence of liver inflammation. Even after cessation of antiviral therapy, this group does not experience the disease flares that are common in other patients with HBV. Intriguingly, functional HBV-specific T cell responses are detected in the blood of these patients (21), raising the question of how the immune response is regulated at this stage.

Which factors drive the progression through the different phases of chronic HBV infection? Host, viral, and environmental factors that drive the transition from noninflammatory to inflammatory phases and from HBeAg+ to anti-HBe+ status are still poorly understood. Due to the unpredictable timing of the transition between disease phases, there is a lack of prospective, immunological studies. Cross-sectional studies identified a decrease and ultimate loss of HBs-specific T cell responses with increasing age (22), a heterogeneous and complex profile of HBV-specific CD4+ and CD8+ T cells with increased expression of exhaustion markers (reviewed in ref. 11), altered energy metabolism, and mitochondrial dysfunction (23). However, the transcriptional profile of HBV-specific T cells is distinct from that of exhausted T cells in other viral infections (24), opening avenues for immunotherapy and restoration of T cell function. At the same time, studies on HBcAg and HBsAg-specific B cells have just recently started. Beyond antibody production, B cells can serve as antigen-presenting cells and exert regulatory function (Breg), and a dysfunctional phenotype has been reported (25). As such, there is a continuing need to perform studies on liver tissue to delineate the differential roles of HBV-specific immune cells and nonspecific, secondarily recruited bystander cells, the contribution of tissue-resident immune cells to HBV control versus disease, and the interactions of immune cells with each other and with parenchymal cells in the liver during the course of infection and novel interventions. New techniques, such as spatial transcriptomics and single-cell analysis will aid in this effort.