The hepatitis B virus (HBV) core antigen (HBcAg) has a unique ability to bind a high frequency of naive human and murine B cells. The role of HBcAg-binding naive B cells in the immunogenicity of HBcAg is not clear. The HBcAg-binding properties of naive B cells were characterized using HBcAg particles with mutated spike region (residues 76-85) sequences. Deletion of residues 76-85 (HBcDelta76-85) destroyed naive B cell binding, whereas deletion of residues 79-85 did not. HBcAg particles with an Ile instead of the natural Ala at position 80 did not bind naive B cells, whereas reversion of Ile80-->Ala restored B cell binding. Destroying the B cell-binding ability of HBcAg had a marginal effect on the overall B cell immunogenicity of the different particles, suggesting that they were equally efficient in priming T helper cells. Therefore, the importance of HBcAg-binding B cells is studied with relation to the priming of HBcAg-specific cytotoxic T cells (CTLs). The role of HBcAg-binding B cells in the priming of HBcAg-specific CTLs was evaluated by immunization with endogenous HBcAg (DNA immunization) and exogenous recombinant HBcAg particles. Endogenous HBcAg primed HBcAg-specific CTLs in wild-type and B cell-deficient mice, whereas exogenous HBcAg primed HBcAg-specific CTLs only in wild-type mice. Importantly, HBcDelta76-85 did not prime CTLs despite the presence of B cells. Thus, the ability of exogenous HBcAg particles to prime specific CTLs is B cell dependent, suggesting a possible role for HBcAg-binding B cells in HBV infections.