Background:  The loss of fitness observed in HIV drug-resistant variants is usually attenuated by secondary/compensatory mutations. Resistance to enfuvirtide (ENF) is associated with mutations in gp41 sequence. The effect of these mutations on replicative capacity is controversial and may depend of genetic envelope (env) background. The aim of this study was to analyze the effect of ENF-resistance mutations on HIV replication and to identify possible compensatory mutations in the complete env region.

Methods:  Env sequences were obtained from sequential plasma samples of a patient initiating ENF treatment. A total of 38 recombinant viruses were constructed containing gp41 sequences from baseline, weeks 4, 8, and 24 of ENF therapy. Plasma-derived gp120 from week 24 was cloned onto selected variants with the 43D genotype. All recombinant viruses were sequenced and their growth rates were measured after transfection of MT-4 cells.

Results:  As previously observed, the 3 main mutations implicated in resistance did not co-exist in the same viral genome and the patient-derived gp41 recombinant viruses obtained harbored only one mutation at positions 36, 38, or 43. In replication capacity assays, the 38A single mutants had a growth rate comparable to the baseline recombinant viruses. In contrast, changes at positions 36 and 43 induced a profound impairment in replication. To explore the relative contribution of the ENF-resistance mutations in gp41 and the gp120 to the HIV replication we constructed gp41-43D-gp120 recombinant viruses cloning the patient-derived gp120 from week 24 onto 2 recombinant viruses with the 43D genotype. We observed that several (4 of 6) gp41-gp120 recombinant viruses containing the 43D mutation recovered baseline replicative capacity, suggesting a role for gp120 modulating the growth rate of gp41 mutants. Sequence analysis from these recovered recombinant viruses revealed that the gp120 of these viruses was different (12 and 3 changes in V3 and C4 regions, respectively) from those recombinant viruses in which the replication was still impaired.

Conclusions:  ENF-selected mutation in gp41 had a high negative effect on the HIV replication capacity. However, fitness loss induced by primary ENF-resistance mutations could be compensated by changes in the gp120 protein. These results point to a key role of gp120 in the acquisition of ENF resistance, suggesting that for fusion inhibitors the whole Env genetic context may condition viral evolution.