INTERFERENCE TO HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 INFECTION IN THE ABSENCE OF DOWN-MODULATION OF THE PRINCIPAL VIRUS RECEPTOR, CD4

It is thought that interference during human immunodeficiency virus ty pe 1 (HIV-1) infection is established by downmodulation of the princip al virus receptor, CD4. Here we present evidence to the contrary. At v arious times after primary infection, we superinfected T cells in vitr o by exposure to a genetically distinct viral clone or to a virus carr ying the chloramphenicol acetyltransferase gene. Replication of each v irus strain was determined by restriction enzyme analysis of total cel lular DNA, by PCR amplification of viral DNA, or by assay of cell extr acts for chloramphenicol acetyltransferase activity. We found that eff icient viral interference is established within 24 h of infection at a multiplicity of infection of 1. At that time, expression of viral str uctural proteins was low and infected cells displayed undiminished lev els of surface CD4 and were fully susceptible to virus binding and fus ion. Superinfection by either cell-free HIV-I or cocultivation was blo cked. Cells resistant to superinfection by HIV-1 remained susceptible to Moloney murine leukemia and vaccinia viruses, No interference was o bserved 4 h after primary infection or in cells infected with either W -inactivated HIV-1 or a mutant virus defective in virus-cell fusion ac tivity, indicating that binding of primary virus to CD4 is insufficien t to prevent superinfection. The minimum viral requirements for this i nterference are that HIV-1 must be able to enter cells and synthesize viral DNA; Tat-mediated transcription is dispensable. Our results supp ort the existence of a novel pathway to interference to HIV-1 infectio n, which we term postentry interference, which blocks superinfection d uring intracellular phases of the virus life cycle.