Negative-stain electron microscopy (EM) of self-assembled astro virus and calicivirus capsids

Abstract
Astroviruses (AstV) and caliciviruses (CacV) are members of a group of 20-40nm infectious gastroenteric viruses. These viruses often have few distictive morphologic features and occur only in small numbers, making them difficult to detect by EM or serology. Specific antisera in animals or production of monoclonal antibodies has only recently been achieved for a few serotypes. The genomes of AstV (AstV genotype 2) and CacV (Toronto virus genotype) have recently been characterized. The complete genome of each virus consists of three open reading frames (ORF). ORF-2 of each virus codes for a capsid precursor. We have expressed capsid protein of each virus in Autographica califonica nuclear polyhedrosis virus (AcNPV) infected Spodoptera fruigiperda (Sf9) cells. Capsid protein precursors expressed in AcNPV-infected insect cells self-assembled into virus-like particles (VLPs) that were antigenic by immunofluorescence and enzyme-linked immunosorbent assays as well as immunogenic in laboratory animals. Purification of the VLPs was monitored by negative stain direct EM (2 % phosphotungstic acid pH 6.5 or 0.5% uranyl acetate) or by immune EM. AcNPV expressed preparations were examined by using monoclonal antibodies, anti sera raised in animals, or from humans previously infected with the respective AstV or CacV.