FIG. 7.

CsCl gradient analysis of HBV-positive cell line culture medium. One milliliter of purified HBV-positive cell line (2.2.15) medium was centrifuged across a CsCl gradient(see Materials and Methods). The resulting gradient was fractionated, and density, HBsAg titer, and HBV DNA level were determined. HBV DNA density was determined by refractometry. The HBsAg titer was determined for a 1:40-diluted sample. Two HBV DNA peaks are found, one at the density expected to contain the HBV Dane particle population (1.28 g/ml) and one at 1.33 g/ml, where nude core particles representing the majority of HBV DNA-containing particles are expected (33). The peak of HBsAg at lower densities (around 1.20 g/ml) represents empty HBV envelope particles, which are plentiful in both cell line material and cell line medium. The HBV DNA detected by the real-time HBV DNA NASBA was found only in association with a particle.