HYPERIMMEDIATE ENTRY OF HUMAN CYTOMEGALOVIRUS VIRIONS AND DENSE BODIES INTO HUMAN FIBROBLASTS

Previous ultrastructural and cytochemical examination of the HCMV inoc ulum as used in the laboratory enabled the distinction of 7 morphologi cally different types of structures including complete virions, other enveloped and non-enveloped particles and dense bodies (Topilko and Mi chelson, 1994). In the present study, electron microscopy was used to investigate the kinetics and modalities of the earliest interactions b etween these components of inoculum and human foreskin fibroblasts (FS F). Particles did not attach to cells during incubation at 4 degrees C . However, when FSF were adsorbed with virus for 30 seconds at 37 degr ees C, HCMV particles attached to the cell surface. Particle attachmen t was mediated by fine virus envelope-cell membrane bridges. Within 60 seconds, numerous virions had fused with cell membranes, and nucleoca psids had entered the cytoplasm. Enveloped particles with translucent cores, designated non-infectious enveloped particles (NIEP), were also seen to enter cells in the same way and with the same kinetics as com plete virions. Uptake pf dense bodies followed the same kinetics and m ode of penetration as complete virus particles. These findings reveal that in vitro, enveloped particles (virions and NIEP) and dense bodies enter the cytoplasm of the host cell simultaneously, immediately (< 6 0 seconds) after contact with the cell membrane. Our results suggest t hat activation of immediate early cellular responses may not simply be due to transmembrane signal transduction, but that hyperimmediate ent ry of these elements into cells may participate directly in host cell activation.