It is thought that interference during human immunodeficiency virus type 1 (HIV-1)
infection is established by downmodulation of the principal
virus receptor, CD4. Here we present evidence to the contrary. At various times after primary
infection, we superinfected T cells in vitro by exposure to a genetically distinct viral clone or to a virus carrying the
chloramphenicol acetyltransferase gene. Replication of each virus strain was determined by restriction
enzyme analysis of total cellular
DNA, by PCR amplification of
viral DNA, or by assay of
cell extracts for
chloramphenicol acetyltransferase activity. We found that efficient viral interference is established within 24 h of
infection at a multiplicity of
infection of 1. At that time, expression of
viral structural proteins was low and infected cells displayed undiminished levels of surface CD4 and were fully susceptible to virus binding and fusion.
Superinfection by either cell-free HIV-1 or cocultivation was blocked. Cells resistant to
superinfection by HIV-1 remained susceptible to Moloney murine
leukemia and vaccinia viruses. No interference was observed 4 h after primary
infection or in cells infected with either UV-inactivated HIV-1 or a mutant virus defective in virus-cell fusion activity, indicating that binding of primary virus to CD4 is insufficient to prevent
superinfection. The minimum viral requirements for this interference are that HIV-1 must be able to enter cells and synthesize
viral DNA; Tat-mediated transcription is dispensable. Our results support the existence of a novel pathway to interference to HIV-1
infection, which we term postentry interference, which blocks
superinfection during intracellular phases of the virus life cycle.