We have analyzed the replication of Nef(+) and Nef(-) isogenic human immunodeficiency virus in CEM, HUT78, MT4 lymphoid, and U937 monocytic cell lines, At each passage of infected cells, we have assessed the relative infectivity of the virus particles released in culture media by measuring the number of infectious units per nanogram of p24 protein. Values appeared to be 3- to 10-fold higher for the Nef(+) virus than for the Nef(-) virus. The positive effect of Nef was observed regardless of the cell line, the multiplicity of infection, and the number of virus replication cycles achieved. We showed, by using cells expressing glycosylphosphatidylinositol-linked CD4, that the enhancement of virion infectivity could be dissociated from the down-regulation of cell surface CD4 also induced by Nef. The gp120-to-p24 ratio and the RNA content of virus particles produced in the presence or in the absence of Nef were equivalent. Virions bound to cell surface CD4 receptors with equal efficiencies. Equivalent reverse transcriptase activities were measured both on exogenous substrate and on particle genomic RNAs. In contrast, reverse transcription in infected cells generated 5- to 10-fold less DNA when the virions were produced in the absence of Nef, indicating that these particles performed reverse transcription in a suboptimal environment. These data suggest that the expression of Nef in virus-producing cells is required for efficient processing of the early stages of virus replication in target cells, including the internalization in an appropriate cell compartment and the uncoating of the particle.
