Reactivity of human coronavirus OC43 and neonatal calf diarrhoea coronavirus membrane-associated antigens

Human embryonic lung fibroblast cultures and Vero cell cultures infected with cell culture-adapted strains of human coronavirus (HCV) OC 43 or neonatal calf diarrhoea coronavirus (NCDCV) were shown to possess highly cross-reactive membrane-associated antigens (MMA) by the indirect fluorescent antibody technique (IFAMA). MAA appeared 3 h post-infection, concurrently with the appearance of cytoplasmic antigens. Electron microscopic observations of cell cultures infected with either coronavirus strain and labelled with the immunoperoxidase antibody (IPA) technique for MAA detection showed that MAA consisted mainly of a strongly labelled, discontinuous, brush-like layer of amorphous material, strictly associated with the infected cell membrane. By light microscopy, reactivity of MAA with homologous and heterologous immune serum was similar to that of antigens detected by IPA in ethanol-fixed infected cells. IPA and IFAMA, but not haemagglutination-inhibiting (HI) and neutralizing (Nt) antibody, were strongly decreased by absorption of immune sera with trypsin-treated glutaraldehyde-fixed cell cultures infected with homologous virus. MAA IgG antibodies were detected by IFAMA in both human and animal sera. Sera from infants showing an HI and Nt, but not an IPA, antibody response to HCV OC43 were also free of detectable IFAMA antibody to HCV OC43.