Ultrastructure and protein A-gold immunolabelling of HRT-18 cells infected with turkey enteric coronavirus

The Minnesota strain of turkey enteric coronavirus (TCV) was propagated in HRT-18 cells, a cell line derived from human rectum adenocarcinoma. A productive non-cytopathic infection was established, without a previous adaptation, in these cells as shown by the specific hemagglutinating activity in cell culture supernatants. A post-embedding immunochemical technique, using specific antiserum directed against the original egg-adapted virus and colloidal-gold-labelled protein A as the electron-dense marker, was used for the identification of the virus and related antigens in the cells by electron microscopy. Budding of typical coronavirus particles, through intracytoplasmic membranes and accumulation of complete virus within cytoplasmic vesicles or the lumen of rough endoplasmic reticulum, were the main features of the viral morphogenesis. Late in infection, numerous progeny viral particles were shown at the outer surface of infected cells, but budding could not be demonstrated at this level. Two different types of surface projections were observed on the extracellular particles of this avian coronavirus. These morphological characteristics have been thus far described only for mammalian hemagglutinating coronaviruses.