Background:  Membrane fusion events have been shown to contribute to HIV-1 cytopathic effects in vitro. The relationship between the level of fusion activity and disease progression is not clear. In this study, we characterized envelope mediated cell-cell fusion, and also evaluated the fusogenicity of viruses from recent and chronic infection patients.

Methods:  The gp160 env gene was amplified from patient HIV-1 plasma samples and cloned into an expression vector. Membrane fusion was measured using a cell-cell fusion assay. Co-receptor tropism and entry inhibitor susceptibility were determined by using a single-cycle HIV envelope-pseudotyping assay. Expression vectors containing chimeric envelope sequences (gp120 and gp41) were used to map fusion determinants. Fusion activity of 400 primary viruses was evaluated and included 150 viruses from recently infected patients in the AIEDRP cohort, 150 from chronically infected patients in SCOPE, and 70 peripheral blood mononuclear cell (PBMC)-derived isolates obtained from the ARRRP.

Results:  A broad range of membrane fusion activity was observed in patient HIV-1 isolates, which was independent of virus co-receptor tropism. PBMC-derived viruses were more fusogenic than primary isolates (p < 0.001). The determinants of fusion were mapped to gp120 using chimeric envelope constructs. Compared with low-fusion viruses, high-fusion viruses were more susceptible to CD4 blocking agents and less susceptible to anti-CD4 antibodies. To investigate the effect of membrane fusion on disease progression, viruses from recently and chronically infected patients were studied. R5-tropic viruses predominated in recent infection (R5 = 97%, X4/R5 = 3%) and X4/dual/mixed-tropic viruses were more common in chronic infection (R5 = 66%, X4/R5 and X4 = 32%). R5-tropic viruses from chronic infection were more fusogenic than R5-tropic viruses from recent infection (p < 0.001), but were not distinguishable from X4/dual-tropic viruses from chronic infection (p > 0.5).

Conclusions:  Patient viruses exhibit broad differences in env-mediated membrane fusion. The envelope gp120 surface protein is a strong determinant of membrane fusion activity. Highly fusogenic viruses, including PBMC-derived viruses, were associated with increased susceptibility to CD4 inhibition. A comparison of plasma derived viruses indicated that both X4- and R5-tropic viruses from chronic infection are more fusogenic than R5-tropic viruses from recent infection.

Keywords: Envelope; Fusion; Disease Stage