Viruses: HIV entry and accessory genes

The cellular entry of human immunodeficiency virus 1 (HIV-1) involves the adhesion of the virus to CD4+ T cells, the fusion of the cell and viral membranes, and the subsequent delivery of the viral core into the cytoplasm. The initial binding of virions to the target cell is usually mediated by the HIV-1 envelope (Env) protein, a heavily glycosylated trimer of gp120–gp41 heterodimers. Its attachment to the target cell can be relatively nonspecific, with Env interacting with negatively charged cell-surface heparan sulfate proteoglycans, or it can result from more specific interactions. The second step of virus entry entails the binding of Env to its primary receptor, the host protein CD4. Entry also requires coreceptor binding, which is thought to be the trigger that activates the membrane fusion potential of Env. HIV-1 strains can be broadly classified based on their coreceptor usage, but all require binding to the CCR5 and/or CXR5 host cell coreceptor for entry.

Restriction factors are proteins on or in the host cell that can help inhibit cellular entry, as well as other stages of the viral life cycle, including replication, assembly, and release. To counteract these host defense mechanisms, HIV has evolved various accessory genes that encode proteins capable of antagonizing restriction factors.

Researchers evaluating the role of HIV-1 accessory genes in cell infectivity generated four HIV-1 packaging vectors with single mutations in the vif, vpr, vpu, or nef accessory genes and used them to transfect lymphoid CD4 and non-lymphoid CD4 cells. Cells were cotransfected using an inLuc-mR reporter plasmid, a plasmid containing a luciferase gene with expression that is dependent on HIV-1 replication. Scientists analyzed cell-coculture infection, in which the target cells are cultured together with other virus-infected cells, as well as cell-free infection, in which target cells are exposed to purified or concentrated virus particles (virions) that have been removed from infected cells. In both cases, the cells were lysed after three days, and luciferase activity was measured. The results are shown in Figure 1.

Figure 1 Relative luciferase activity in lymphoid and non-lymphoid cells following cell-coculture infection and cell-free infection; * and ** indicate significant differences from wt results at a p < 0.05 and p < 0.01, respectively