Since the discovery of the Australia antigen, now known as the hepatitis B surface antigen (HBsAg), significant research has been conducted to elucidate its physical, chemical, structural, and functional properties. Subviral particles (SVPs) containing HBsAg are highly immunogenic, non-infectious entities that have not only revolutionized vaccine development but also provided critical insights into HBV immune evasion and viral assembly. Recent advances in cryo-electron microscopy (cryo-EM) have uncovered the heterogeneity and dynamic nature of spherical HBV SVPs, emphasizing the essential role of lipid–protein interactions in maintaining particle stability. In this review, recent progress in understanding the molecular architecture of HBV SVPs is consolidated, focusing on their symmetry, lipid organization, and disassembly–reassembly dynamics. High-resolution structural models reveal unique lipid arrangements that stabilize hydrophobic residues, preserve antigenicity, and contribute to SVP functionality. These findings highlight the significance of hydrophobic interactions and lipid–protein dynamics in HBV SVP assembly and stability, offering valuable perspectives for optimizing SVP-based vaccine platforms and therapeutic strategies.

Hepatitis B virus (HBV) subviral particles (SVPs) are primarily stabilized by protein–protein interactions, with discrete lipid patches playing a supporting role, rather than forming a conventional lipid bilayer, challenging classical assumptions about viral membrane architecture.

How changes in the quality of anti-viral antibody (Ab) responses due to pre-existing or acquired CD4 T cell insufficiency affect virus evolution during persistent infection are unknown. Using mouse polyomavirus (MuPyV), we found that CD4 T cell depletion before infection results in short-lived plasma cells secreting low-avidity antiviral IgG with limited BCR diversity and weak virus-neutralizing ability. CD4 T cell deficiency during persistent infection incurs a shift from a T-dependent (TD) to T-independent (TI) Ab response, resembling the pre-existing TI Ab response. CD4 T cell loss before infection or during persistent infection is conducive for emergence of Ab-escape variants. Cryo-EM reconstruction of complexes of MuPyV virions with polyclonal IgG directly from infected mice with pre-existing or acquired CD4 T cell deficiency enabled visualization of shortfalls in TI IgG binding. By debilitating the antiviral IgG response, CD4 T cell deficiency sets the stage for outgrowth of variant viruses resistant to neutralization. ONE SENTENCE SUMMARY Pre-existing and acquired CD4 T cell deficiency facilitates outgrowth of Ab-escape viral variants during persistent infection.