Pandemic HIV-1 Vpu overcomes intrinsic herd immunity mediated by tetherin

Abstract

Among the four groups of HIV-1 (M, N, O, and P), HIV-1M alone is pandemic and has rapidly expanded across the world. However, why HIV-1M has caused a devastating pandemic while the other groups remain contained is unclear. Interestingly, only HIV-1M Vpu, a viral protein, can robustly counteract human tetherin, which tethers budding virions. Therefore, we hypothesize that this property of HIV-1M Vpu facilitates human-to-human viral transmission. Adopting a multilayered experimental-mathematical approach, we demonstrate that HIV-1M Vpu confers a 2.38-fold increase in the prevalence of HIV-1 transmission. When Vpu activity is lost, protected human populations emerge (i.e., intrinsic herd immunity develops) through the anti-viral effect of tetherin. We also reveal that all Vpus of transmitted/founder HIV-1M viruses maintain anti-tetherin activity. These findings indicate that tetherin plays the role of a host restriction factor, providing 'intrinsic herd immunity', whereas Vpu has evolved in HIV-1M as a tetherin antagonist.

Figure 1. Construct of the mathematical model.

The variables S(t, V) and I(t, V) denote the number of susceptible and infected individuals, respectively, and N(t) is the total number of host individuals. All variables are specified at time t. The parameters b(V) and d denote the birth and removal rates of susceptible individuals respectively, and μ(V) is the death rate of infected individuals. The force of infection is assumed as where c and β(V) denote the partner exchange rate and the transmission probability, respectively. Therefore, the rate of change in the number of susceptible and infected individuals at time t (i.e., the de novo transmission) is . Figure 1 was drawn by KS using Illustrator CS5.1 (Adobe).

Figure 2. Prediction of tetherin-mediated intrinsic herd immunity.

(A) (left) The distribution of set-point viral load in untreated individuals in the Zambian Transmission Study (black bars). The black curve is the distribution predicted by the structured epidemiological model. (right) The predicted distribution of set-point viral load in untreated individuals in the Zambian Transmission Study, assuming non-functional Vpu (orange bars). The bar at “0” shows the fraction of populations protected from vpu-deficient HIV-1 infection. (B) Simulated spread of wild-type and vpu-deficient HIV-1 among a population of 1 million individuals. The black and orange curves describe the number of individuals infected by wild-type and mutant virus, respectively. (C) Prevalence of wild-type (black curve) and vpu-deficient (orange curve) HIV-1 infection in (B).

Figure 3. Conservation of Vpu motifs and the ability of T/F virus Vpus.

(A) Conservation of the motifs in HIV-1M Vpus that counteract tetherin and down-regulate CD4 at each stage of viral infection. (left) Logo plot of the Vpu amino acid sequences at Fiebig stages III (n = 183), IV (n = 29), and V (n = 119). The crucial Vpu motifs for tetherin counteraction (AxxxAxxxA motif) and CD4 down-regulation (SxxxS motif) are indicated in black. (right) The percentage conservation of these motifs at each Fiebig stage. (B–E) Infection ability of the Vpus T/F viruses. The HA-tagged Vpu-expressing plasmids were co-transfected with pNL4-3ΔvpuΔnef (B, D, E) or without pNL4-3ΔvpuΔnef (C) into HeLa cells (B, D, E) or TZM-bl cells (C). Labels denote: –, empty vector; +, NL4-3 Vpu; 1-10, T/F virus Vpus: 1, THRO_TF1 (Genbank accession no. JN944930); 2, SUMA_TF1, (JN944928); 3, RHPA_TF1 (JN944917); 4, TRJO_TF1 (JN944936); 5, CH040_TF1 (JN944905); 6, REJO_TF1 (JN944911); 7, CH077_TF1 (JN944909); 8, CH058_TF1 (JN944907); 9, WITO_TF1 (JN944938); 10, CH106_TF1 (JN944897). (B) Representative result of Western blotting (kDa = kilodalton). (C) Surface expression levels of tetherin (top) and CD4 (bottom) on cells transfected with Vpu-expressing plasmids. (D) The infectivity of released virus was determined by TZM-bl assay. (E) The correlation between the level of surface tetherin (shown in panel c, top) and virus infectivity (shown in panel d). The data are expressed as averages and the standard deviations of triplicate assays. In Panels C and D, statistical significance was determined by Student’s t test (P < 0.05). In Panel E, the correlation between the level of surface tetherin (x-axis) and virus infectivity (y-axis) was determined by Spearman’s rank test.