We addressed a presumption that hepatitis A virus (HAV) particles in the blood circulation might be lipid-associated, and have significantly lower density and larger size than so far described in textbook. Sera from patients with acute hepatitis A served as materials. In CsCl density gradient ultracentrifugation, the serum-derived HAV particles banded initially at a density of 1.17 g/m
l, but shifted to a heavier density of 1.31 g/m
l when treated with protease (Pronase) plus lipid solvent (chloroform) or detergent (NP40). In parallel, the sizes of HAV particles also shifted from 50-80 nm to 30-50 nm in diameter by these treatments. The heavier HAV particles (1.31 g/m
l) could bind to anti-HAV antibodies, while those with a low density (1.17 g/m
l) could not. Conversely, the low density (1.17 g/m
l) HAV particles could bind to anti-CD59 antibodies as well as some lectins, but the high density (1.31 g/m
l) ones could not. These results suggested that HAV, despite taxonomically classified as a "nonenveloped" virus, exists in the blood of infected hosts as though being an "enveloped" virus, possessing a lipid membrane-like coat, which masks viral antigens from circulating antibodies.
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