Hepatitis B virus surface antigen (HBsAg) antigenic loop (HBsAg antigenic loop)

The Hepatitis B virus surface antigen (HBsAg) antigenic loop, specifically the 'a' determinant (residues 124–147), is a highly conserved region within the Major Hydrophilic Region (MHR) of the HBV envelope protein (UniProt: P03141). This loop is the principal target for neutralizing antibodies (anti-HBs) produced during natural infection or following vaccination (PubMed: 22230479). Biologically, the loop is essential for viral attachment and entry into hepatocytes, interacting with host cell receptors like sodium taurocholate cotransporting polypeptide (NTCP) to initiate infection (NCBI: NBK6211). In chronic hepatitis B, HBsAg is produced in vast excess, serving as an immune decoy to subvert the host's immune system and maintain chronic infection (PubMed: 25232063). Therapeutic interventions like VIR-3434 and Hepatitis B Immune Globulin (HBIG) target this loop to neutralize virions and promote the clearance of HBsAg from the blood (ClinicalTrials.gov: NCT04507269). However, mutations within this antigenic loop can result in 'immune escape,' allowing the virus to evade vaccine-induced immunity and complicating diagnostic detection (PubMed: 15352179).

Other names

'a' determinantMajor Hydrophilic RegionMHRHBsAg hydrophilic loopHBsAg S-domain loop

Neutralization of viral infectivity by blocking attachment to host cells (specifically the NTCP receptor) and promoting the clearance of circulating viral particles and subviral particles through antibody-dependent mechanisms (PubMed: 22230479, PubMed: 33035203).