Recognition of HBV antigens and HBV DNA by dendritic cells

Hepatitis B virus (HBV) is a hepatotropic, noncytopathic, DNA virus which can cause acute and chronic infection. Viral persistence is associated with a weak or absent specific immune responses to HBV, particularly the cellular immune response. Dendritic cells (DCs) are professional antigen-presenting cells with a unique T cell stimulatory aptitude that play a crucial role in the instruction of adaptive immune responses upon infection. An impaired function of DCs was suggested by recent studies to account for the T and B cell hyporesponsiveness in chronic HBV infection. This review summarizes recent insights into the recognition of HBV antigens by DCs.

Studies were identified by searching MEDLINE and/or PubMed for articles using the key words "hepatitis B virus (HBV)", "dendritic cells", "C-type lectins", "mannose receptor", "toll-like receptor", and "dendritic cell-specific intercellular-adhesion-molecule-3 grabbing nonintegrin (DC-SIGN)" up to December 2009. Additional papers were identified by a manual search of the references from the key articles.

DCs play an important role in the progress of hepatitis B, especially in the recognition of HBV. There are three main ways of recognition of HBV antigens by DCs. First, HBV DNA can be recognized by DCs through toll-like receptor 9 (TLR9) which activates the NF-κB signal pathway and p38 MAPK to up-regulate the expression of interferon (IFN) regulatory factor 7 (IRF-7) in a manner independent of type I IFN signaling, resulting in secretion of type I IFN and inflammatory cytokines, and induction of DC maturation and the adaptive immune response. Second, HBc/HBeAg cannot be recognized by DCs, but DNA or ssRNA encapsulated within HBcAg can be internalized by DCs through TLRs. Third, HBsAg can be internalized by DCs through the mannose receptor, which lacks the ability to induce DC maturation without the assistance of DC-SIGN. Meanwhile, there is some cross-talk among the three mechanisms, which induces an effective anti-viral response or HBV persistence.

On the basis of these recognition processes, methods have been used to enhance the efficacy of DC-based vaccine against HBV and have been useful in the clinical application of HBV vaccine therapy. But the interactions between HBV antigens/HBV DNA and DCs are not clear, and cross-talk between TLRs and various ligands makes HBV antigen recognition by DCs more complicated. More efforts should be made to define the mechanisms and develop effective vaccines and therapies.