Binding of MACV GP1Δ variants to hTfR1 and the surface of MACV-susceptible cells.

<p>A, expression of wild-type MACV GP1Δ, mutants thereof, and Fc control. The numbers to the left of the blots indicate relative molecular mass in kDa. B, ability of wild-type MACV GP1Δ and variants thereof to co-immunoprecipitate hTfR1. Shown is a representative western blot from two independent experiments. C, ability of these proteins to bind to the surface of MACV-permissive (Vero) cells as analyzed by flow cytometry. This assay was performed twice in duplicates yielding similar results. Bars indicate average of duplicates in one representative experiment. Results were normalized by subtracting measurements with secondary antibody only. D, far-UV circular dichroism (CD) of wt GP1Δ and mutants D123A, D159A, and N178A.</p

MACV glycoprotein determinants for hTFR1 binding and cell entry.

<p>Top panel, structure of the MACV GP1-hTfR1 complex (as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021398#pone.0021398-Abraham2" target="_blank">[25]</a>) with the cell surface orientated to the bottom (PDB ID number: 3KAS). The TfR1 protease-like, helical, and apical domains are colored blue, gold, and magenta, respectively. MACV GP1 is colored in dark grey. Mutated residues are colored as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021398#pone-0021398-g001" target="_blank">Fig. 1</a>. Bottom panel, enlargement of the TfR1∶MACV GP1 contact sites. MACV residues important for TfR1 binding are labeled and colored as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021398#pone-0021398-g001" target="_blank">Fig. 1</a>. TfR1 residues are labeled and colored in magenta.</p

AMAV and TCRV GPs bind and use the TfR1 orthologs of their respective host species.

<p>(A) HEK293T cells were transfected with plasmids encoding human TfR1 (hTfR1), <i>N. spinosus</i> TfR1 (NsTfR1), or <i>A. jamaicensis</i> TfR1 (AjTfR1). Cells were incubated 48 hr later with an α-flag antibody or Ig-fusion proteins comprising the truncated GP1 subunits <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#ppat.1000358-Radoshitzky1" target="_blank">[32]</a> of AMAV and TCRV (AMAV GP1Δ-Ig and TCRV GP1Δ-Ig, respectively). The association of these proteins with cells was measured by flow cytometry. The data shown are representative of two independent experiments, duplicated in each assay, with similar results. Mean fluorescence values for TfR1 ortholog expression were 435, 380, and 458 for human, Ns, and AjTfR1, respectively. Mean fluorescence values for AMAV GP1Δ-Ig binding to transfected cells were 1.5, 597, and 1.6, and those for TCRV GPΔ-Ig binding were 3.7, 425, and 553 for human, Ns, and AjTfR1, respectively. (B) HEK293T cells were transfected with plasmids encoding human, ZbTfR1, NsTfR1, AjTfR1 orthologs or vector alone. Cell surface expression of the TfR1 orthologs was determined as in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#ppat-1000358-g003" target="_blank">Figure 3</a>. Aliquots of these cells were infected with AMAV, TCRV, or VSV pseudoviruses, and infection levels were assessed as in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#ppat-1000358-g002" target="_blank">Figure 2</a>. (C) An experiment similar to the one performed in (B), except CHO cells were used for transfection and infected with MACV, JUNV, GTOV, or VSV pseudoviruses. Expression levels of the various TfR1 orthologs were normalized to that of human TfR1 (α-flag, left panels). Infection levels were normalized to that of mock-transfected cells.</p

Cell transduction efficiency of eGFP-expressing retrovirus pseudotyped with wild-type MACV GPC, or variants thereof (grey bars, HeLa cells; black bars, Vero cells).

<p>This assay was performed twice in triplicates yielding similar results. Bars indicate average of triplicates in one representative experiment. A representative western blot analysis of the various MoMLV pseudotypes is shown using an antibody against MACV GP2. The numbers to the left of the blots indicate relative molecular mass in kDa.</p

Modest mutations convert human TfR1 into an efficient receptor for AMAV and TCRV.

<p>(A) The structure of the human TfR1 dimer is shown, oriented with the cell membrane at the bottom. The apical, protease-like, and helical domains are indicated in green, red, and yellow, respectively, on one monomer. The other monomer is shown in cyan. In the right panel, the TfR1 apical domain is enlarged; a loop comprising residues 202-212, implicated as a site of interaction with the GPs of NW clade B arenaviruses, is shown. The side chains of residues D204, K205, R208, V210, and Y211 are colored yellow. The image was rendered using PyMol <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#ppat.1000358-DeLano1" target="_blank">[59]</a>. (B) Sequence alignment of residues 195 through 216 of human TfR1 with analogous sequences of the TfR1 orthologs of <i>Z. brevicauda</i> (ZbTfR1), <i>A. jamaicensis</i> (AjTfR1), and <i>N. spinosus</i> (NsTfR1). Variants of human TfR1 containing sequence from <i>Z. brevicauda</i> (zh1 and zh2), <i>A. jamaicensis</i> (ah2 through ah5), and <i>N. spinosus</i> (nh2, nh4, nh5, nh7, and nh8) TfR1 were generated based on this sequence alignment. Zb, Aj, and NsTfR1 sequences are shown in green, blue, and yellow, respectively. The right panel summarizes the entry data. ND = not determined. (C–E) HEK293T cells were transfected with plasmids encoding human, Zb, Aj, or NsTfR1 along with the zh variants (C), the ah variants (D), or the nh variants (E). The expression level of each TfR1 variant was assessed as in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#ppat-1000358-g003" target="_blank">Figure 3</a>. In parallel, cells were infected with AMAV, TCRV, or VSV pseudoviruses. The expression levels of the various TfR1 orthologs were normalized to that of human TfR1 (α-flag, left panels), and infection levels were normalized to that of mock-transfected cells.</p

Host-animal orthologs of TfR1 support the entry of infectious AMAV and TCRV.

<p>HEK293 cells were transfected with plasmids encoding human TfR1, AjTfR1, or NsTfR1, and infectious viruses added at 36 hr post-transfection. Incubation was continued for 12 hr, and the cells were washed, fixed, and stained as described in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#s4" target="_blank">Materials and Methods</a>.</p

AMAV and TCRV pseudovirus entry does not depend on human TfR1.

<p>HEK293T cells were infected with GTOV, AMAV, TCRV, or MACV pseudoviruses expressing GFP in the presence or absence of the indicated concentrations of an α-human TfR1 (BD Pharmingen) or a control (α-HLA) antibody (BD Pharmingen). Infection levels were assessed 48 hr later by measuring GFP expression by flow cytometry. Mean fluorescence values were normalized to that of cells infected in the absence of antibody. GFP mean fluorescence values for virus entry in the absence of antibody were 144.0, 89.7, 92.3, and 115.0 for GTOV, AMAV, TCRV, and MACV, respectively.</p

AMAV and TCRV pseudoviruses can use animal orthologs of TfR1.

<p>CHO cells ((A), left panel) were transfected with vector alone (mock) or plasmids encoding human, mouse, rat, feline, canine, <i>C. callosus</i> (Cc), <i>C. musculinus</i> (Cm), and <i>Z. brevicauda</i> (Zb) TfR1 orthologs. HEK293T cells ((B), right panel) were transfected with the same plasmids with the exception of the one encoding canine TfR1. Cell surface expression was determined 48 hr later by flow cytometry using an antibody directed against a flag tag present at the C-terminus of each ortholog. In parallel, aliquots of these cells were infected with AMAV, TCRV, or LCMV pseudoviruses. Infection levels were assessed as in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000358#ppat-1000358-g002" target="_blank">Figure 2</a>. Expression levels of various TfR1 were normalized to that of human TfR1 (α-flag, top panels). Infection levels were normalized to that of mock-transfected cells.</p

Expression of human CD26/DPP4 confers MERS-CoV susceptibility to otherwise resistant bat cells.

<p>(A) Viral yields from MERS-CoV-resistant PESU-B5L, R05T, R06E, and Tb1Lu bat cells. Cells were transfected with a plasmid expressing human CD26/DPP4 or empty control plasmid and exposed 48 h later to MERS-CoV/EMC at an MOI of 3. Supernatants were harvested at 24 h after virus exposure for quantification of virus yields by plaque assay. (B) Same experiment: representative immunofluorescence assay (IFA) images of cells stained with anti-MERS-CoV spike protein antibody (green, top) or anti-human CD26/DPP4 antibody (red, bottom). (C) Merged IFA images demonstrate colocalization of MERS-CoV spike protein and CD26/DPP4. (D). Viral yields from MERS-CoV-susceptible bat cells transfected with a plasmid expressing human CD26/DPP4 or empty control plasmid using procedures identical to resistant cells in (A) except that cells were exposed to virus 24 h after transfection. Error bars indicate the standard deviation of duplicate samples.</p

Persistent MERS-CoV infection of bat cells induces downregulation of bat cell CD26/DPP4 expression.

<p>Bat cell lines susceptible to infection were infected with MERS-CoV/EMC (A) or MERS-CoV/Jor (B) at an MOI of 1. After 7 days, supernatants were harvested for virus yield analysis by plaque assay, and the cells were subcultured at a 1∶10 dilution in new flasks. Subsequently, the persistently infected cells were passaged at a 1∶10 dilution weekly. Error bars indicate the standard deviation of duplicate samples. (C and D) Same experiment: immunofluorescence assay (IFA) images of bat cells persistently infected with MERS-CoV/EMC (C) or MERS-CoV/Jor (D) at day +33 stained with anti-MERS-CoV spike protein antibody (green). (E) Same experiment: TEM images of bat cells persistently infected with MERS-CoV/EMC at day 56. (F) Flow cytometry data of CD26/DPP4 surface expression (red line: anti-human CD26-/DPP4 antibody; black line: control antibody) in persistently infected cells. (G) CD26/DPP4 expression in persistently infected EidNi/41.3 cells (day 63) as detected by western blot.</p