Restriction of foamy viruses by APOBEC cytidine deaminases - PubMed (original) (raw)

Comparative Study

Restriction of foamy viruses by APOBEC cytidine deaminases

Frédéric Delebecque et al. J Virol. 2006 Jan.

Abstract

Foamy viruses (FVs) are nonpathogenic retroviruses infecting many species of mammals, notably primates, cattle, and cats. We have examined whether members of the apolipoprotein B-editing catalytic polypeptide-like subunit (APOBEC) family of antiviral cytidine deaminases restrict replication of simian FV. We show that human APOBEC3G is a potent inhibitor of FV infectivity in cell culture experiments. This antiviral activity is associated with cytidine editing of the viral genome. Both molecular FV clones and primary uncloned viruses were susceptible to APOBEC3G, and viral infectivity was also inhibited by murine and simian APOBEC3G homologues, as well as by human APOBEC3F. Wild-type and bet-deleted viruses were similarly sensitive to this antiviral activity, suggesting that Bet does not significantly counteract APOBEC proteins. Moreover, we did not detect FV sequences that may have been targeted by APOBEC in naturally infected macaques, but we observed a few G-to-A substitutions in humans that have been accidentally contaminated by simian FV. In infected hosts, the persistence strategy employed by FV might be based on low levels of replication, as well as avoidance of cells expressing large amounts of active cytidine deaminases.

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Figures

FIG. 1.

FIG. 1.

Human APOBEC3G (hA3G) restricts FV infection. (A) hA3G does not affect the production of FV virions. hA3G and FV virions (clone 13 provirus) were produced in 293T cells by transfection of plasmids (at the ratio indicated). Expression of FV Gag and hA3G was assessed by Western blot analysis of cell lysates. FV proteins were detected with an anti-Gag serum and hA3G proteins were detected with an anti-V5 tag MAb. Apparent molecular masses (in kilodaltons) are indicated on the right. (B) hA3G inhibits infectivity of wild-type and Δbet FV. The infectivity of viral preparations described in the legend to panel A was tested on FAB indicator cells, which express β-Gal upon FV infection. β-Gal levels obtained with FV and Δbet produced without hA3G were set at 100%. (C) Comparison of the infectivity of FV and Δbet. Similar amounts of virions were used to infect indicator FAB cells, and infectivity of FV was set at 100%. Bet expression was detected in cells infected with FV and not with Δbet. A CMV-Bet expression plasmid was transfected as a control. (D) hA3G inhibits infectivity of the FV strain HSRV2. hA3G and FV virions (HSRV2 provirus) were produced, and viral infectivity was tested as described above. Data are means ± standard deviation (SD) of triplicates and are representative of two to five independent experiments.

FIG. 2.

FIG. 2.

Susceptibility of a primary FV strain to human APOBEC3G. (A) Viral replication and syncytium formation in HeLa and HeLa-hA3G cells. The indicated cells were infected at a high multiplicity of infection with T1FV, a primary viral strain isolated from a naturally infected macaque. At various days postinfection, cells were stained with Giemsa to visualize syncytium formation and virus-induced cytopathic effect. (B) Quantification of syncytium formation in HeLa and HeLa-hA3G cells. Cells were infected with T1FV (at either a high or low multiplicity of infection value) and cultured up to 16 days p.i. Numbers of syncytia per well (from six-well plates) were scored at the indicated time points. Data are representative of three independent experiments.

FIG. 3.

FIG. 3.

FV Bet does not rescue the HIV Δvif defect. (A) HIV Vif but not FV Bet decreases hA3G steady-state levels. 293T cells were transfected with HIV or Δvif proviruses and, when stated, with hA3G- and Bet-encoding plasmids. Expression of hA3G was assessed by Western blot analysis of cell lysates. As controls, levels of HIV Gag and FV Bet were also visualized. Apparent molecular masses (in kilodaltons) are indicated on the right. (B) hA3G-mediated restriction of HIV and Δ_vif_ viruses, with or without FV Bet. Viral supernatants from transfected 293T cells described in the legend to panel A were normalized for Gag p24 release and used to infect target P4 cells, which express β-Gal upon HIV infection. The infectivities of HIV and Δvif in the absence of hA3G were set at 100%. Data are means ± SD of two independent experiments.

FIG. 4.

FIG. 4.

hA3G triggers G-to-A mutations in the FV genome. HeLa and HeLa-hA3G were infected with two primary FV isolates, T1FV (top) and T2FV (bottom). At day 16 p.i., FV genomes present in cell extracts were amplified by PCR, cloned, and sequenced. For points from HeLa-hA3G cells, individual sequences from 10 to 12 different clones with a high number of G-to-A substitutions were selected and aligned with the consensus FV sequence (corresponding to the major sequence obtained from HeLa cells). A representative fragment of the integrase gene is shown. Similar results were observed at an earlier time point (day 6 p.i.; data not shown).

FIG. 5.

FIG. 5.

Effect of various APOBEC proteins on FV infectivity. 293T cells were transiently transfected with WT or Δbet FV (clone 13 strain), along with expression plasmids for the indicated APOBEC proteins. Viral stocks were prepared, and their infectivity was tested on FAB indicator cells. β-Gal levels obtained with FV and Δbet produced in the absence of any APOBEC proteins were set at 100%. The FV:APOBEC plasmid ratio was 1:0.2 or 1:0.5 (for CpzA3G). Similar behavior in WT and Δbet FV was observed at an FV:CpzA3G plasmid ratio of 1:0.05 (not shown). Data are means ± SD of triplicates and are representative of two to three independent experiments.

FIG. 6.

FIG. 6.

Analysis of naturally occurring viral sequences from FV-infected humans. Uncultured PBMCs from four humans (P1 to P4) accidentally infected by gorilla FVs were analyzed for the presence of FV sequences. A total of 16 to 20 different PCR-derived clones are depicted for each individual. Analysis was performed using either regular PCR (95°C denaturation temperature; top part of the sequences for each individual) or 3DPCR (85°C denaturation temperature; bottom part of the sequences). Sequences are aligned to the major sequence of each individual, which served as a reference. The frequency of occurrence of each unique sequence is depicted on the right. A fragment of the integrase gene is shown.

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