Putative site for the acquisition of human herpesvirus 6 virion tegument

The virion of human herpesvirus 6 (HVV-6) contains a very distinct tegument layer, occupying the space between the nucleocapsid and the virion envelope. Ultrastructural analyses of thymocytes infected with HHV-6 revealed the presence of intrauclear spherical compartments, approximately 1.5 μm in diameter, in which tegumentation seems to take place. These compartments,termed tegusomes, were bounded by two membranes and contained ribosomes, consistent with their derivation by eytoplasmic incagination into the nucleus. Capsids located within the nucleus outside the tegusomes were all naked, while those located in the cytoplasm were uniformly tegumented. In constrast, capside present inside the tegusomes contained teguments of variable thicknesses. In addition, nucleocapsode were documented in the process of budding into the tegusomes. We thus suggest that the tegusome represent a cellular site in which HHV-6 virions acquire their tegument.