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The human immunodeficiency virus type 1 long terminal repeat, HIV-1-LTR, contains binding sites for several cellular transcription factors which contribute to HIV-1 gene expression. Our previous studies on the function of the HIV-1-encoded Nef protein suggested that Nef may be an inhibitor HIV-1 transcription. To determine whether Nef affects the binding of cellular factors implicated in HIV-1 regulation, 32P-labeled oligonucleotides corresponding to the binding sites were incubated with nuclear extracts prepared from Nef-expressing T-cell lines that were not stimulated or were stimulated with T-cell mitogens. We found that Nef inhibited the recruitment of AP-1 DNA-binding activity in mitogen-stimulated human T-cells. Additionally, Nef expressing cells were transiently transfected with a plasmid in which HIV-1 AP-1 DNA recognition sequences were cloned downstream of the chloramphenicol acetyltransferase (CAT) gene. Mitogen-mediated transcriptional activation of the CAT gene in this construct was inhibited in Nef-expressing cells but not in control cells. These studies suggest that, by inhibiting AP-1 activation, Nef may play a role in regulating HIV-1 gene expression in infected T-cells.

Human immunodeficiency virus isolates express a Nef protein with either an alanine or a threonine at amino acid residue 15. The threonine residue is a site for phosphorylation by protein kinase C. Jurkat T cells constitutively expressing the alanine variant of Nef exhibit the ability to downregulate the induction of transcription factors NF-kB and AP-1. In contrast, Jurkat cells with the threonine variant of Nef are at least partially restored in their ability to recruit NF-kB and AP-1.

Nef transcripts were analyzed from peripheral blood mononuclear cells of 10 HIV-1-infected subjects with 9-822 CD4+ lymphocytes/cu mm, including 4 individuals with a probable common source infection. There was no relationship between the phylogenetic position of the various nef sequences and the disease state of the person from whom they were derived. The nef open reading frame was disrupted in all three clones from only 1 subject. Functional analyses of a representative clone from each of the remaining 9 subjects showed that all nef alleles were capable of CD4 cell surface down-regulation, but only three nef alleles suppressed the induction of IL-2 transcription.

HIV Nef protein downregulates the surface expression of CD4 on T-cells but its mechanism of activity is unknown. We have analyzed the relevance of different portions of the CD4 molecule with respect to the activity of Nef. Upon transfection of Jurkat T-cells that express Nef or do not express Nef with constructs that include different domains of the CD4 molecule, we demonstrated that Nef downregulation of CD4 requires the first 30 amino acids of the CD4 cytoplasmic tail and that the Ick-binding domain of CD4 was at least partially responsible for this effect. Furthermore, upon transfection of U-937 cells with an Ick-expressing plasmid we showed that CD4 downregulation by Nef is significantly more efficient when Ick is present. Finally, by measuring the rate of CD4 endocytosis by a novel flow cytometric method we showed that the effect of Nef on CD4 surface expression resulted from accelerated CD4 internalization.