T-20 is a synthetic peptide that potently inhibits replication of human immunodeficiency virus type 1 by interfering with the transition of the transmembrane protein, gp41, to a fusion active state following interactions of the surface glycoprotein, gp120, with CD4 and coreceptor molecules displayed on the target cell surface. Although T-20 is postulated to interact with an N-terminal heptad repeat within gp41 in a transdominant manner, we show here that sensitivity to T-20 is strongly influenced by coreceptor specificity. When 14 T-20-naive primary isolates were analyzed for sensitivity to T-20, the mean 50% inhibitory concentration (IC 50 ) for isolates that utilize CCR5 for entry (R5 viruses) was 0.8 log 10 higher than the mean IC 50 for CXCR4 (X4) isolates (P ‫؍‬ 0.0055). Using NL4.3-based envelope chimeras that contain combinations of envelope sequences derived from R5 and X4 viruses, we found that determinants of coreceptor specificity contained within the gp120 V3 loop modulate this sensitivity to T-20. The IC 50 for all chimeric envelope viruses containing R5 V3 sequences was 0.6 to 0.8 log 10 higher than that for viruses containing X4 V3 sequences. In addition, we confirmed that the N-terminal heptad repeat of gp41 determines the baseline sensitivity to T-20 and that the IC 50 for viruses containing GIV at amino acid residues 36 to 38 was 1.0 log 10 lower than the IC 50 for viruses containing a G-to-D substitution. The results of this study show that gp120-coreceptor interactions and the gp41 N-terminal heptad repeat independently contribute to sensitivity to T-20. These results have important implications for the therapeutic uses of T-20 as well as for unraveling the complex mechanisms of virus fusion and entry.

Cells of the mononuclear phagocyte system are the predominant cell producing HIV-1 in most tissues including the central nervous system (CNS), spinal cord, lung and skin; infection is associated with dementia, neuropathy, pneumonitis, and dermatitis respectively. Different HIV-1 isolates vary markedly in their ability to infect mononuclear phagocytes productively. Here we describe molecular clones of a CNS-derived isolate, HIV-1(JR-FL), which can replicate efficiently in mononuclear phagocytes. Analysis by polymerase chain reaction of early events after infection indicates that the early phase of viral replication before reverse transcription determines tropism. Genetic mapping of the macrophage-tropic phenotype by construction of recombinant viruses indicates that mononuclear phagocyte infectivity can be determined by a 157-amino-acid region of the gp 120 glycoprotein of HIV-1(JR-FL). Significantly, this region is upstream from the previously defined CD4-binding domain. We propose that at least one determinant for mononuclear phagocyte tropism involves target cell interactions with regions of gp120 distinct from the CD4-binding domain.

In mouse embryos, germ cells arise during gastrulation and migrate to the early gonad. First, they emerge from the primitive streak into the region of the endoderm that forms the hindgut. Later in development, a second phase of migration takes place in which they migrate out of the gut to the genital ridges. There, they co-assemble with somatic cells to form the gonad. In vitro studies in the mouse, and genetic studies in other organisms, suggest that at least part of this process is in response to secreted signals from other tissues. Recent genetic evidence in zebrafish has shown that the interaction between stromal cell-derived factor 1 (SDF1) and its G-protein-coupled receptor CXCR4, already known to control many types of normal and pathological cell migrations, is also required for the normal migration of primordial germ cells. We show that in the mouse, germ cell migration and survival requires the SDF1/CXCR4 interaction. First, migrating germ cells express CXCR4, whilst the body wall mesenchyme and genital ridges express the ligand SDF1. Second,the addition of exogenous SDF1 to living embryo cultures causes aberrant germ cell migration from the gut. Third, germ cells in embryos carrying targeted mutations in CXCR4 do not colonize the gonad normally. However, at earlier stages in the hindgut, germ cells are unaffected in CXCR4-/-embryos. Germ cell counts at different stages suggest that SDF1/CXCR4 interaction also mediates germ cell survival. These results show that the SDF1/CXCR4 interaction is specifically required for the colonization of the gonads by primordial germ cells, but not for earlier stages in germ cell migration. This demonstrates a high degree of evolutionary conservation of part of the mechanism, but also an area of evolutionary divergence.