Viral Antigens

HBV is a partially double-stranded DNA virus with a diameter of approximately 42 nm . The virus consists of an outer lipid envelope and an icosahedral nucleocapsid core containing the viral DNA and a DNA polymerase . HBV primarily infects hepatocytes (liver cells), where it replicates and persists. The virus expresses several proteins, including the surface antigen (HBsAg), core antigen (HBcAg), and the X protein (HBx), which play crucial roles in its life cycle and pathogenicity .

The primary function of HBV is to replicate within hepatocytes and produce new virions. HBV infection triggers immune responses, including the activation of cytotoxic T cells that target infected hepatocytes . The virus also modulates the host’s immune system to evade detection and establish chronic infection . HBx protein, in particular, is involved in regulating viral gene expression and interacting with host cellular pathways .

HBV enters hepatocytes through the sodium taurocholate co-transporting polypeptide (NTCP) receptor . Once inside the cell, the virus’s relaxed circular DNA (rcDNA) is converted into covalently closed circular DNA (cccDNA), which serves as a template for viral transcription . The viral proteins interact with various host factors to facilitate replication and evade immune responses. For example, HBx protein binds to host proteins such as DDB1 and CUL4 to modulate transcription and degrade cellular restriction factors .

The expression and activity of HBV are tightly regulated at multiple levels. Transcriptional regulation involves the recruitment of host transcription factors and epigenetic modifications of cccDNA . HBx protein plays a pivotal role in this process by recruiting acetyltransferases and inhibiting histone deacetylases . Post-translational modifications, such as phosphorylation and ubiquitination, also influence the stability and function of viral proteins .

HBV research has led to significant advancements in biomedical science. The development of the hepatitis B vaccine has been a major public health success, drastically reducing the incidence of HBV infection and related liver diseases . Diagnostic tools, such as serological assays for HBsAg and HBV DNA quantification, are essential for detecting and monitoring infection . Therapeutic strategies include antiviral drugs like nucleoside analogs and interferons, which suppress viral replication and improve liver function .

HBV plays a critical role throughout its life cycle, from initial infection to chronic disease progression. During acute infection, the virus replicates rapidly, leading to liver inflammation and immune activation . In chronic infection, HBV persists in hepatocytes, causing ongoing liver damage and increasing the risk of cirrhosis and hepatocellular carcinoma . The virus’s ability to establish a stable cccDNA reservoir in the nucleus of infected cells is a key factor in its persistence and pathogenicity .