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The natural ligands for the CCR5 chemokine receptor, macrophage inflammatory protein 1alpha (MIP-1alpha), MIP-1beta, and RANTES (regulated on T-cell activation, normal T-cell expressed and secreted), are known to inhibit human immunodeficiency virus (HIV) entry, and N-terminally modified RANTES analogues are more potent than native RANTES in blocking infection. However, potent CCR5 blocking agents may select for HIV-1 variants that use alternative coreceptors at less than fully inhibitory concentrations. In this study, two N-terminal chemical modifications of RANTES produced by total synthesis, aminooxypentane (AOP)-RANTES[2-68] and N-nonanoyl (NNY)-RANTES[2-68], were tested for their ability to prevent HIV-1 infection and to select for coreceptor switch variants in the human peripheral blood lymphocyte-SCID mouse model. Mice were infected with a CCR5-using HIV-1 isolate that requires only one or two amino acid substitutions to use CXCR4 as a coreceptor. Even though it achieved lower circulating concentrations than AOP-RANTES (75 to 96 pM as opposed to 460 pM under our experimental conditions), NNY-RANTES was more effective in preventing HIV-1 infection. However, in a subset of treated mice, these levels of NNY-RANTES rapidly selected viruses with mutations in the V3 loop of envelope that altered coreceptor usage. These results reinforce the case for using agents that block all significant HIV-1 coreceptors for effective therapy.

Most individuals infected with human immunodeficiency virus type 1 (HIV-1) initially harbor macrophage-tropic, non-syncytium-inducing (M-tropic, NSI) viruses that may evolve into T-cell-tropic, syncytium-inducing viruses (T-tropic, SI) after several years. The reasons for the more efficient transmission of M-tropic, NSI viruses and the slow evolution ofT-tropic, SI viruses remain unclear, although they may be linked to expression of appropriate chemokine coreceptors for virus entry. We have examined plasma viral RNA levels and the extent of CD4+ T-cell depletion in SCID mice reconstituted with human peripheral blood leukocytes following infection with M-tropic, dual-tropic, or T-tropic HIV-1 isolates. The cell tropism was found to determine the course of viremia, with M-tropic viruses producing sustained high viral RNA levels and sparing some CD4+ T cells, dual-tropic viruses producing a transient and lower viral RNA spike and extremely rapid depletion of CD4+ T cells, and T-tropic viruses causing similarly lower viral RNA levels and rapid-intermediate rates of CD4+ T-cell depletion. A single amino acid change in the V3 region of gp120 was sufficient to cause one isolate to switch from M-tropic to dual-tropic and acquire the ability to rapidly deplete all CD4+ T cells.

Individuals homozygous for a 32-bp deletion (delta 32) in the CCR5 gene encoding the coreceptor for macrophage-tropic human immunodeficiency virus type 1 (HIV-1) are resistant to virus infection, and heterozygous individuals show some slowing of disease progression. The impact of the CCR5 genotype on HIV-1 infection was assessed in vitro and in the human PBL-SCID (hu-PBL-SCID) model. Cells and hu-PBL-SCID mice from CCR5 delta 32/delta 32 donors were resistant to infection with macrophage-tropic HIV-1 and showed slower replication of dual-tropic HIV-1. hu-PBL-SCID mice derived from CCR5 delta 32/+ heterozygotes showed delayed replication of macrophage-tropic HIV-1 despite a small and variable effect of heterozygosity on viral replication in vitro. The level of CCR5 expression appears to limit replication of macrophage-tropic and dual-tropic HIV-1 strains in vivo.

The pathogenicity of four human immunodeficiency virus type 1 (HIV-1) isolates with nef deleted for SCID mice repopulated with human peripheral blood leukocytes (hu-PBL-SCID mice) was studied. Deletion of nef led to a substantial reduction in CD4-positive T-cell depletion and delayed kinetics of plasma viremia in infected hu-PBL-SCID mice. Deletion of the nef gene impacts both the efficiency of primary infection and the cytopathicity of virus for infected CD4-positive T cells in this animal model of HIV-1 infection.

We have used envelope recombinant viruses generated between two molecular clones of human immunodeficiency virus type 1 (HIV-1), T-cell-tropic HIV-1SF2 and macrophage-tropic HIV-1SF162, to assess pathogenic potential in the human peripheral blood leukocyte-reconstituted severe combined immune deficiency mouse model. Recombinant HIV-1SF2 viruses expressing the envelope gp120 gene of HIV-ISF162 caused as rapid a CD4+ T-cell depletion as did HIV-1SF162. The reciprocal HIV-1SF162 recombinant virus with the HIV-1SF2 envelope caused slower CD4+ T-cell loss. Although changing the V3 loop sequence of HIV-1SF162 to that of HIV-1SF2 did not change the rate of CD4+ T-cell depletion, replacing the V3 of HIV-1SF2 with the sequence of HIV-1SF162 resulted in virus that was poorly infectious in vivo but not in vitro. These studies suggest that the envelope gene determines properties important for pathogenesis in vivo as well as for cell tropism in vitro. HIV-1 infection in vivo may have more stringent requirements for envelope conformation.