Hepatitis D Virus

Hepatitis delta antigen (HDAg) was first described by Rizzetto et aL. (1977). Hepatitis D virus (HDV), also known as the 'delta agent', is a satellite agent of hepatitis B virus (HBV). HDV does not synthezise its own coat; it is enveloped by hepatitis B surface antigen (HBsAg), which occurs as large (L), middle (M) and smaII or major (S) surface proteins, differing in the length of the amino-terminal extension (Ueda et al., 1991). ln infectious hepatitis B virions, the ratio of the three HBsAgs L:M:S in theenvelope is 1:1:4 (Bruss & Ganem, 1991). ln comparison, the ratio L:M:S in HDV particles is 1:5:95 (Taylor, 1992). ln this respect, HDV envelopes are very similar to empty spherical HBsAg particles, suggesting that HDV uses the excess production of empty HBsAg particles. It has been shown in vitro that the smallest HBsAg (S) is sufficient to package the HDV genome (Wang et al., 1991). Interestingly, HDAg could also be packaged by woodchuck hepatitis virus (WHV) surface antigen, even in the absence of HDV RNA (Ryu et al., 1992). NaturaIIy occurring HDV particles have a diameter of about 36 nm (Wang et aL., 1986) but are somewhat heterogeneous in size. Inside the HBsAg particle is the HDAg and the viral RNA genome. HDV RNA and HDAg exist as a ribonucleoprotein complex (Taylor, 1992).