In the advent of the promise for HIV-1 entry inhibitors to treat patients with drug resistance to available antiretroviral therapy, there has been a surge in studies related to HIV-1 entry. It is clear that the intrinsic susceptibility of primary virus isolates to entry inhibitors varies, which indicates a greater probability for intrinsic resistance to this class of drugs. The studies presented here provide tools and insights into the impact of the envelope (env) gene on HIV-1 replicative capacity, how that capacity influences the emergence of drug resistance, and how easily drug resistance is selected in vivo to a microbicide entry inhibitor.
The fitness of a virus is a marker of its replication capability in a given environment. The specific impact of HIV-1 entry on the overall fitness of a virus was tested by cloning a region of the env gene into a common backbone and comparing the resultant recombinant fitness to that of the wildtype virus. The env gene was sufficient to determine the overall fitness of these viruses, which also correlated with sensitivity to entry inhibitors. Specifically, the binding avidity of a virus to the host cell coreceptor contributed the most to fitness.
PSC-RANTES is an entry inhibitor that acts on the coreceptor CCR5 to block HIV-1 binding and down-regulate coreceptor expression. When used as a microbicide in a rhesus macaque model, PSC-RANTES failed to block transmission of SHIV¬¬¬SF162 at high doses in some animals. Two mutations were identified in isolates from one of these animals and were cloned into a common HIV-1 backbone to create a chimeric virus. The chimeric virus exhibited resistance to PSC-RANTES and an increase in fitness over that of the wildtype virus used to infect the rhesus macaques. This study showed for the first time the selection of drug resistant viruses to a microbicide in an HIV-1 animal model system.
Lastly, a cloning strategy was developed to quickly create replication-competent, fully infectious HIV-1 chimeric viruses. This system eliminates the need for common endonuclease sites among diverse primary isolates of HIV-1 and can be used to clone any gene region from any HIV-1 isolate.