The human APOBEC3G protein restricts the replication of Vif-deficient HIV-1 by deaminating nascent viral cDNA cytosines to uracils, leading to viral genomic strand G-to-A hypermutations [1-4]. However, the HIV-1 Vif protein triggers APOBEC3G degradation, which helps to explain why this innate defense does not protect patients . The APOBEC3G-Vif interaction is a promising therapeutic target, but the benefit of the enabling of HIV-1 restriction in patients is unlikely to be known until Vif antagonists are developed. As a necessary prelude to such studies, cell-based HIV-1 evolution experiments were done to find out whether APOBEC3G can provide a long-term block to Vif-deficient virus replication and, if so, whether HIV-1 variants that resist restriction would emerge. APOBEC3G-expressing T cells were infected with Vif-deficient HIV-1. Virus infectivity was suppressed in 45/48 cultures for more than five weeks, but replication was eventually detected in three cultures. Virus-growth characteristics and sequencing demonstrated that these isolates were still Vif-deficient and that in fact, these viruses had acquired a promoter mutation and a Vpr null mutation. Resistance occurred by a novel tolerance mechanism in which the resistant viruses packaged less APOBEC3G and accumulated fewer hypermutations. These data support the development of antiretrovirals that antagonize Vif and thereby enable endogenous APOBEC3G to suppress HIV-1 replication.
Bibliographical noteFunding Information:
We thank P. Bieniasz, J. Coffin, S. Goff, A. Haase, P. Hackett, L. Mansky, V. Planelles, and P. Southern for valuable feedback; M. Malim, J. Lingappa, and M. Stevenson for key reagents; and M. Hertzberg and L. Mansky for the generous provision of facilities. These studies were supported by grants from the National Institutes of Health (AI064046), the Campbell Foundation, and the University of Minnesota (Grant-In-Aid and Academic Health Center Faculty Research Development Awards). G.H., J.S.A., and R.S.H., respectively, were supported in part by a Canadian Institutes of Health Research predoctoral studentship, a Medical Scientist Training Program grant (T32 GM008244, with supplemental support from the Mayo Foundation), and a Searle Scholarship. The University of Minnesota Advanced Genetic Analysis Center assisted with DNA sequencing, and the Minnesota Supercomputing Institute provided computational support.