Open Access

West Nile alternative open reading frame (N-NS4B/WARF4) is produced in infected West Nile Virus (WNV) cells and induces humoral response in WNV infected individuals

  • Giovanni Faggioni1Email author,
  • Alice Pomponi1,
  • Riccardo De Santis1,
  • Laura Masuelli2,
  • Andrea Ciammaruconi1,
  • Federica Monaco3,
  • Annapia Di Gennaro3,
  • Laura Marzocchella4,
  • Vittorio Sambri5,
  • Rossella Lelli3,
  • Giovanni Rezza6,
  • Roberto Bei4 and
  • Florigio Lista1
Virology Journal20129:283

DOI: 10.1186/1743-422X-9-283

Received: 1 September 2011

Accepted: 12 November 2012

Published: 22 November 2012

Abstract

Background

West Nile Virus (WNV) is a flavivirus that requires an efficient humoral and cellular host response for the control of neuroinvasive infection. We previously reported the existence of six alternative open reading frame proteins in WNV genome, one of which entitled WARF4 is exclusively restricted to the lineage I of the virus. WARF4 is able to elicit antibodies in WNV infected horses; however, there was no direct experimental proof of the existence of this novel protein. The purpose of this study was to demonstrate the in vitro production of WARF4 protein following WNV infection of cultured VERO cells and its immunity in WNV infected individuals.

Results

We produced a monoclonal antibody against WARF4 protein (MAb 3A12) which detected the novel protein in WNV lineage I-infected, cultured VERO cells while it did not react with WNV lineage II infected cells. MAb 3A12 specificity to WARF4 protein was confirmed by its reactivity to only one peptide among four analyzed that cover the full WARF4 amino acids sequence. In addition, WARF4 protein was expressed in the late phase of WNV lineage I infection. Western blotting and bioinformatics analyses strongly suggest that the protein could be translated by programmed −1 ribosomal frameshifting process. Since WARF4 is embedded in the NS4B gene, we rename this novel protein N-NS4B/WARF4. Furthermore, serological analysis shows that N-NS4B/WARF4 is able to elicit antibodies in WNV infected individuals.

Conclusions

N-NS4B/WARF4 is the second Alternative Reading Frame (ARF) protein that has been demonstrated to be produced following WNV infection and might represent a novel tool for a better characterization of immune response in WNV infected individuals. Further serological as well as functional studies are required to characterize the function of the N-NS4B/WARF4 protein. Since the virus might actually make an extensive use of ARFs, it appears important to investigate the novel six ARF putative proteins of WNV.

Keywords

West Nile Virus WNV WARF4 N-NS4B/WARF4 Alternative open reading frame

Background

West Nile virus (WNV) is an arthropod-borne virus maintained in a bird-mosquito transmission cycle. Birds are the natural reservoir hosts while humans and other mammals are dead-end hosts occasionally infected through mosquito bite [1]. The virus, which was identified in 1937 [2], has been the cause of sporadic cases and outbreaks of disease in Africa, Australasia, Europe, and Middle East [35]. Since 1996, WNV has gained growing importance in the western world, causing massive outbreaks and/or small clusters of encephalitis in Europe [68]. The virus was introduced for the first time in the North America in 1999 [9, 10], spreading to several countries [11], and becoming a major public health problem in the USA [12].

WNV is a small enveloped virus [13] belonging to the Flaviviriade family, genus Flavivirus[14]. Phylogenetic analysis reveals two distinct viral lineages, lineage I and lineage II [15]. Lineage I is involved in human and equine outbreaks while lineage II is not associated with clinical manifestations in humans [1517]. WNV genome is a positive single-stranded RNA of about 11 kb containing a single open reading frame flanked by two untranslated regions. RNA translation produces a long polyprotein processed by viral and cellular proteases in three structural (C, preM/M and E) and seven non structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5) [18]. Overall, the structural proteins are involved in virus binding and penetration of host cells [19], while the non-structural proteins are involved in the replicative cycle [20] and induce immunological evasion mainly through the inhibition of type I interferon signaling [21, 22]. Still, it has been suggested that other unidentified factors could play a role in the pathogenesis of WNV neuroinvasive disease [23]. Melian et al. demonstrated that the NS1’ protein, a known variant of the canonical NS1 protein, results from a ribosomal frame shifting process [24]. The variant protein appears to play a role in WNV neuroinvasiveness. We recently reported the existence of six alternative open reading frames (ARFs) in the WNV genome by in silico analysis. We also demonstrated a significant antibody response to one of this six novel putative proteins (WARF4) in the serum of horses testing positive for antibodies to WNV. However, there was no direct experimental proof of the in vivo existence of this novel protein [25]. The aim of this study was to demonstrate that WARF4 protein is synthesized following WNV infection of mammalian cultured cells. To address this objective, a monoclonal antibody against WARF4 protein was produced. In addition, sera of WNV infected individuals were analyzed in order to test the capacity of WARF4 to induce an immune response in humans as well.

Results

Generation of a mouse monoclonal antibody against WARF4 protein

In order to demonstrate the in vitro production of the WARF4 protein following WNV lineage I infection, a mouse monoclonal antibody to His-WARF4 fusion protein was generated. The selected MAb 3A12 recognized the His-WARF4 by western blotting while it did not show cross-reactivity with the crude lysate from E. coli transformed with the empty vector (Figure 1).
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig1_HTML.jpg
Figure 1

Reactivity of MAb 3A12 with WARF4 recombinant protein. Protein extracts from E. coli BL21 transformed with His-WARF4 and with the empty vector (pRSETC) were analyzed by western blotting. MAb 3A12 reacted with the recombinant His-WARF4 while it did not show reactivity with the crude lysate of E. coli.

In silico aminoacid alignment and identification of the N-NS4B/WARF4 region detected by MAb 3A12

WARF4 is an alternative gene overlapping the COOH-terminal region of the NS4B gene and a small NH2-terminal portion of the NS5 gene (Figure 2). The genomic organization of WNV implies that a −1 ribosomal frameshifting process translates WARF4, thus the novel protein has been renamed N-NS4B/WARF4. The amino acid composition of N-NS4B/WARF4 is completely different from the NS4B viral protein in the COOH-terminal region, this is shown by the amino acid alignment analysis in Figure 3A. To further support the bioinformatics evidence, a western blotting analysis of the recombinant His-WARF4 and His-NS4B proteins employing the polyclonal anti-NS4B antibody and MAb 3A12 was performed to demonstrate that His-WARF4 and NS4B proteins are dissimilar (Figure 3B). The commercial polyclonal antibody anti-NS4B was developed to a NS4B fragment from AA 126 to AA 145, which thus overlaps the first 14 AA of the N-NS4B/WARF4 COOH-terminal region. As shown in Figure 3B (lane 4), the anti-NS4B antibody did not recognized the recombinant His-WARF4 protein. The recombinant His-NS4B fragment encompasses the COOH-terminal portion of NS4B starting from AA 120 thus including the entire ARF coding sequence. In addition, MAb 3A12 did not recognize the His-NS4B fragment (lane 9).
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig2_HTML.jpg
Figure 2

Proposed mechanism of N-NS4B/WARF4 synthesis. In the center the WNV 3 genomic organization is shown. WARF4 is dashed, the first and the last base of the alternative reading frame are pointed by stars. On the top, the synthesis of NS4B by canonical translation mechanism in 0 frame and maturation is shown. Below is displayed the proposed mechanism of N-NS4B/WARF4 synthesis through translation in −1 frame. The amino acids sequences of the two proteins are showed, the different COOH terminals of the two proteins are underlined.

https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig3_HTML.jpg
Figure 3

(A) Comparative aminoacid sequence analysis. Amino acids alignment of the NS4B and N-NS4B/WARF4. Identical residues are shown as dots. The arrow represents the 14 AA target of the anti-NS4B antibody. (B) Western blotting analysis of His-NS4B/His-WARF4 proteins employing MAb 3A12 and the anti-NS4B antibody. The analysis was carried out to demonstrate the different amino acids composition of N-NS4B/WARF4 protein relative to the terminal region of NS4B protein. The first three lanes show the reactivity of the anti-His antibody with the His-tagged proteins used in the same western blotting as positive controls. The anti-NS4B antibody did not react with the His-WARF4 protein (lane 4). Similarly, the MAb 3A12 did not react with His-NS4B protein (lane 9). The His-Envelope protein was used as negative control.

The N-NS4B/WARF4 region recognized by MAb 3A12 was identified by analyzing MAb 3A12 reactivity to four synthetic overlapping peptides (SP1, SP2, SP3, SP4) which cover the full N-NS4B/WARF4 COOH-terminal amino acids sequence (Figure 4). The four synthetic peptides, as well as recombinant His-WARF4, His-NS4B, Env proteins and Bovine Serum Albumin (BSA) were spotted in replicates on nitrocellulose membranes. Membranes (panels A, B, C) were then reacted with MAb 3A12, anti-NS4B antibody and anti-His antibody respectively. As shown in Figure 4, MAb 3A12 recognized only SP2, while lacking reactivity to His-NS4B, His-EnV and BSA. Conversely, anti-NS4B or anti-His antibody did not recognize any of the WARF4 peptides. These results corroborate the specificity of MAb 3A12 to N-NS4B/WARF4 protein and allow to recognize the epitope of MAb 3A12 between amino acids 165–212 of N-NS4B/WARF4 sequence.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig4_HTML.jpg
Figure 4

Identification of the N-NS4B/WARF4 COOH-terminal amino acids sequence detected by MAb 3A12. The amino acids sequence of the N-NS4B/WARF4 COOH-terminal region with the graphical representation of the four synthetic peptides (SP1, SP2, SP3, SP4) covering the full amino acids sequence codified by WARF4 is shown on the top. The amino acids sequence of the four synthetic peptides is showed in the center, the overlapping sequences among the contiguous peptides are underlined. Peptides (500 ng), Envelope protein and BSA (500 ng) and His-WARF4 and His-NS4B proteins (50 and 150 ng, respectively) were analyzed by dot blotting employing MAb 3A12 (panel A), anti-NS4B antibody (panel B) and MAb anti-His (panel C). The star shows the peptide (SP2) recognized by MAb 3A12.

Expression of N-NS4B/WARF4 protein in WNV lineage I infected cells

Expression of N-NS4B/WARF4 following WNV lineage I infection of VERO cells in vitro was demonstrated by immunofluorescence and western blotting analyses employing MAb 3A12. Figure 5 shows the reactivity of MAb 3A12 with uninfected and infected VERO cells. MAb 3A12 showed a strong cytoplasmic immunoreactivity in infected cells (panel b), while it did not react with uninfected cells (panel c). MOPC-21 was used as negative control (panel a and c).
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig5_HTML.jpg
Figure 5

Expression and intracellular localization of N-NS4B/WARF4 in VERO infected cells. Reactivity of MAb 3A12 with the cytoplasm of VERO WNV infected cells (panel b). No reactivity was observed with non infected cells (panel d). MOPC-21 was used as negative control with infected (panel a) and non infected cells (panel c).

In order to determine the apparent molecular weight of the in vitro produced N-NS4B/WARF4 protein, western blotting analysis was carried out. As shown in Figure 6, in infected VERO cell lysate MAb 3A12 detected a protein showing an apparent molecular weight of about 28 kDa (lane 3). Recombinant His-WARF4 (lane 1) was used as positive control. No reactivity of MAb 3A12 was observed with uninfected VERO cells (lane2). The commercially available anti-NS4B antibody was used as positive control to monitor the infection of VERO cells by WNV (lane 4) and to compare the apparent molecular weight of NS4B protein to N-NS4B/WARF4 protein. Our result shows that the electrophoretic mobility of N-NS4B/WARF4 is slightly lower than that of NS4B. The recombinant His-NS4B was used as positive control for anti-NS4B antibody (lane 6).
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig6_HTML.jpg
Figure 6

Reactivity of MAb 3A12 with VERO WNV lineage I infected cells by western blotting. MAb 3A12 detects a protein with an apparent molecular weight of about 28 kDa in WNV lineage I infected VERO cells (line 3), no reactivity was observed in uninfected VERO cells (lane 2). The recombinant His-WARF4 protein was used as positive control (lane 1). The commercial anti-NS4B antibody was used to monitor the infection of VERO cells and to compare the migration of NS4B protein (lane 4) with the novel protein. The electrophoretic mobility of N-NS4B/WARF4 protein (lane 3) resulted slightly less than NS4B protein (lane 4). The recombinant His-NS4B positive control was loaded with a delay of about twenty minutes (lane 6).

Next, the expression of N-NS4B/WARF4 was evaluated and compared to the expression of NS4B protein through a time-course infection (24–72 hours). Figure 7 shows the results of western blotting analysis performed with MAb 3A12 (panel A) and polyclonal anti-NS4B (panel B). N-NS4B/WARF4 and NS4B proteins show similar kinetics of expression after WNV lineage I infection, the highest level of expression of both in the late phase of infection.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig7_HTML.jpg
Figure 7

Time-course of VERO cells infection and western blot analysis. The expression of the N-NS4B/WARF4 protein (panel A) was evaluated by western blotting analysis and compared to the expression of the NS4B protein (panel B) at 24–72 hours post-infection. Both proteins show a similar behaviour with maximum expression in the late phase of infection. His-WARF4 (20 ng) and His-NS4B (50 ng) were used as positive controls.

N-NS4B/WARF4 expression is restricted to WNV lineage I

To corroborate bioinformatic analysis indicating that N-NS4B/WARF4 is restricted to WNV lineage I, the reactivity of MAb 3A12 was analyzed against WNV lineage II infected VERO cells (Figure 8). Infection of VERO cells was performed for 72 hours with WNV lineage I and lineage II. Western blotting analysis shows that MAb 3A12 identifies N-NS4B/WARF4 protein in the WNV lineage I infected VERO cells (panel A, line 3) but not in WNV lineage II infected VERO cells (panel A, line 4). To monitor infection of VERO cells by WNV lineage I and II, infected cells were also analyzed for the expression of matrix protein (M) employing the anti-M antibody (panel B). The anti-M antibody recognized the mature form of M protein showing a molecular weight of about 8 kD in both lineage I and II WNV infected cells (line 3 and 4). The antibody also detected an additional band representing the immature form of the M protein (preM/M). Recombinant His-WARF4 and His-PreM/M proteins were used as positive controls.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig8_HTML.jpg
Figure 8

N-NS4B/WARF4 expression is restricted to WNV lineage I. Panel A shows the reactivity of WNV lineage I with MAb3A12 (lane 3), no reactivity was observed in WNV lineage II (lane 4), VERO cells were used as negative control (lane 2). To monitor the infection of VERO cells with both lineage I and II, the membrane was reprobed with a commercial antibody anti-M (panel B). The M protein and other immature forms were detected in both the WNV lineage I and II (lanes 3 and 4). His-WARF4 and His-preM/M proteins were used as positive controls.

N-NS4B/WARF4 induces antibodies in WNV infected individuals

In order to determine whether human sera from WNV infected individuals were able to recognize N-NS4B/WARF4 protein, western blotting analysis was carried out. Reactivity of human sera to His-WARF4 protein was compared to that of 3 other recombinant WNV proteins, including the domain III of the envelope, a preM/M fragment and the NH2-terminal portion of NS5 (Figure 9A). Eight human sera, 4 of which positive for IgGs anti-WNV, were assayed. Sera from individuals testing negative for WNV by IFA and seroneutralization test were also negative for reactivity with recombinant WNV proteins. Conversely, sera from individuals testing positive for WNV by IFA and seroneutralization test showed different patterns of reactivity with the recombinant proteins analyzed (Figure 9B). Two sera were able to react with His-WARF4. Of these, one was able to recognize also the His-preM/M, His-NS5 and His-EnvIII proteins while the other recognized only the preM/M. In addition, one serum was able to detected EnvIII only, while another produced a weakly positive signal for the envelope and NS5 only after a long exposure (data not shown).
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig9_HTML.jpg
Figure 9

Immune recognition of His-WARF4 by WNV-positive human sera. Eight human sera, 4 of which positive for IgGs anti-WNV by IFA were analyzed for the presence of anti-N-NS4B/WARF4 antibodies by western blotting. The assay was performed by testing simultaneously the reactivity to 3 other recombinant WNV proteins: the domain III of the Envelope, a prem/M protein fragment and the NH2-terminal portion of NS5. Panel A shows the 4 recombinant proteins stained with the comassie-blue. Panel B shows the results of the western blotting analysis; 4 human sera testing negative for IgGs-anti WNV (1–4) showed no reactivity with the four antigens. The WNV-positive human sera (5–8) showed a different reactivity with the four recombinant antigens. Two sera (7, 8) reacted with His-WARF4 protein.

Bioinformatic results

Alignment and cluster analysis of 384 WNV strains assigned 368 samples to lineage I according to previous studies [15] (Figure 10, Table 1). WARF4 was found in 361 genomes out of 368 belonging to lineage I. Two different mutually exclusive slippery sequences within the NS4B gene were detected in the WARF4 group. The first slippery sequence (UUUUUUG), with a pseudknot structure of 80 nucleotides (ΔG −27.8 kcal/mol) [bps 7268, an. AF260967] positioned 6 nucleotides downstream, is shared by 91% of genomes. The second slippery sequence (CCCUUUG/T), with a downstream pseudknot structure of 40 nucleotides (ΔG −14.5 kcal/mol) [bps 7174, an. AF260968] positioned 8 nucleotides downstream, is shared by 5% of genomes. The WARF4 sub-group genomes lacking slippery sequences are 4%.
https://static-content.springer.com/image/art%3A10.1186%2F1743-422X-9-283/MediaObjects/12985_2011_Article_2022_Fig10_HTML.jpg
Figure 10

UPMGA analysis of WNV genomes. The image summarizes the results of the bioinformatics analysis. Some reference strains are shown. Sequence alignment identifies WARF4 in 98% of strain belonging to lineage I (black collapsed form). The WARF4 group (dashed) may be further separated in two groups, depending on the type of programmed −1 ribosomal frame shifting sequence detected. Group 1 carries the UUUUUUG slyppery sequence. Group 2 carries the CCCUUUG/T slippery sequence. The two pseudoknot sequences with the predicted base pairing are shown. The slippery sequences are in gray while the stop codon (AUG) and the first codon (GGC) of WARF4 are underlineed.

Table 1

WNV accession numbers

LINEAGE I

WARF4 Group 1

AY646354

DQ066423

HQ891010

DQ164186

JF730043

DQ164199

HM488224

HM488159

AY660002

HM756658

FJ411043

HQ891011

HM488183

DQ164198

HM488208

HM488172

AY289214

JF488087

HM488118

HQ671697

HQ891013

HM488142

DQ164205

HM488253

HQ671702

HM488194

HM488218

HM488119

HQ671714

JF730040

HM488147

HM488177

HM488133

HM488180

HM756660

HM488236

HM488123

HM488247

HQ671669

DQ164200

HM488251

HM488136

HM488189

HM488165

HM756678

HM488124

HM756661

HQ671673

HM488181

HM488140

HQ671719

DQ005530

HM488199

HM488206

JF488089

HM756663

HQ671677

HM488209

HM756672

HQ671713

HM488162

HM488243

HM488238

HM488237

HQ671711

HQ671687

HQ671722

HM488141

AF404755

HM488195

JF488097

HM488203

DQ666452

AF404756

HQ671688

HM756667

HQ705659

GQ379156

FJ527738

HM488205

GQ379157

HQ671715

EF657887

HQ671689

HQ671723

HM488114

DQ164202

HQ671701

HM488171

GQ379158

HM488200

EF530047

HQ671690

HM488137

HM488139

AY712947

HM488227

HM488230

HM488233

HQ671672

HM756662

HQ705670

HQ671699

HQ671724

DQ164189

HM488228

HQ671726

HM488234

HQ891012

HM756664

HQ705674

HM488138

HM756650

EU155484

HM488176

HM488201

HM488116

HQ671718

HM488130

HQ671682

HM488211

HQ705660

HM488231

HM488163

HM488207

JF488086

HQ671730

HM488249

HQ671683

HM756673

HM488214

HM488232

HM488164

HM488245

HM756657

HQ671728

HM488132

HQ671685

DQ164191

HM488216

HM488121

HM488161

HM488202

GQ379160

JF488096

HQ671708

HQ705675

HM756648

HM488145

HM488122

HM488246

HM488179

JF488088

HM488235

HQ671710

JF357959

AY795965

HM488146

HQ671703

HM488167

JF357960

HQ671671

HQ671721

AF404753

HQ705672

HM488156

HM488144

HM488155

HM488170

HQ705669

JF357958

HM488127

HQ671712

HQ671681

HM488158

HM488157

HQ671720

HM488168

HM756676

JF784158

HM488128

HQ671668

HM488248

HM488148

HM756671

DQ164206

HM488169

HQ671698

HQ671676

AF196835

HQ671680

DQ164188

HM488151

HQ671729

HM488175

HM488166

HM756669

DQ118127

HM488125

HQ671674

HM488134

HM488154

HM488225

HM756653

HM488174

HM488193

DQ374652

HM488126

HQ671675

HM488135

HM488150

HM488226

DQ164190

HM488204

HQ671700

DQ411029

AF260967

HQ671679

HQ671716

HM488153

HM488229

HQ671725

HM488254

HM488217

 

HQ596519

HQ671684

HQ671717

HM488115

HM488185

HM488191

HM488196

HQ671692

 

DQ211652

HQ671678

HQ671696

HM488117

HM488143

HM488198

HQ671727

HQ671707

 

NC_009942

HQ671686

DQ164194

HM488120

HM488221

HM488173

HM756675

HQ671709

 

AY842931

HQ671691

DQ164193

HM488149

DQ164196

HM756659

HM488244

HM488240

 

EF571854

HQ671732

HM488178

HM488152

DQ164197

HM488210

HQ671705

HM756666

 

AY848695

HQ671733

HQ671693

HM756656

AY712948

HM488197

HM488188

JF730042

 

AY848696

HQ671731

HQ705677

HM488213

HM488186

HM756652

HM488219

HM488239

 

HM488131

HQ705663

HQ671694

JF488091

HM488190

HM756654

AF481864

HQ671704

 

AF404754

HQ705671

HQ705678

HM488215

HM488222

DQ164204

HM488252

HM488160

 

HM488129

HQ705673

HQ671695

JF488094

HM756665

DQ176637

JF488093

HM756649

 

AF206518

HQ671670

HM488250

HM488220

HM756668

AY712945

JF488090

HM488242

 

HQ671706

HQ705676

HM488182

HM756670

HM488212

AY712946

JF488092

HM488241

 

AY848697

HQ891009

DQ164195

HM488184

HM756651

HM488223

DQ666450

JF488095

 
 

WARF4 Group 2

DQ411034

DQ411035

DQ374651

DQ377179

DQ377178

DQ377180

DQ411033

DQ411032

DQ374650

AY278441

DQ411030

DQ411031

AF404757

AF260968

EU081844

AM404308

EU249803

HM051416

AY490240

AY603654

       
  

HM488187

DQ164187

DQ164192

DQ164203

DQ164201

DQ666448

DQ666451

HM756677

 
 

DQ666449

GQ379159

AY277252

      

The table shows the accession numbers of WNV strains belonging to lineage I analyzed in silico in this work.

Discussion

The use of ARFs in viruses belonging to the Flaviviriade family was first reported for the hepatitis C viruses [26, 27]. Recently, it has been demonstrated that WNV uses a short ARF, termed foo, for the synthesis of NS1’ , a known variant of the canonical NS1 [24, 28]. We earlier reported the presence of other ARFs embedded in the coding frame of the WNV genome [25]. Our bioinformatic analysis detected six ARFs, one of which, designed WARF4, was the longest and restricted exclusively to lineage I of WNV. Since WARF4 is embedded in the NS4B gene, the novel protein has been renamed N-NS4B/WARF4. Our results suggested the production of N-NS4B/WARF4 protein in WNV infected horses because of their ability to mount a humoral immune response to N-NS4B/WARF4. However, there was no direct evidence proving the actual existence of the N-NS4B/WARF4 protein. In order to demonstrate the production of N-NS4B/WARF4 in vitro after WNV cells infection, we produced a monoclonal antibody to the N-NS4B/WARF4 COOH-terminal amino acid sequence (MAb 3A12). MAb 3A12 strongly reacted with VERO WNV infected cells by immunofluorescence and detected a ~ 28-kDa protein by western blotting. The predicted aminoacids of N-NS4B/WARF4 preclude the possibility that MAb 3A12 could react with epitopes shared with the NS4B protein (Figure 3A), however to support this prediction a western blotting analysis was performed. As shown in Figure 3B, MAb 3A12 did not recognize the recombinant COOH terminal portion of the recombinant his-tagged NS4B protein. In addition, the anti-NS4B did not recognize the recombinant His-tagged WARF4 protein. To definitely asses the specificity of MAb 3A12 against the alternative reading frame, four overlapping peptides covering the full N-NS4B/WARF4 COOH- terminal amino acid sequence were synthesized and analyzed for their reactivity to MAb 3A12. As shown in Figure 4 the results confirm the specificity of the monoclonal antibody and allow to recognize its epitope between the amino acids 165–212 of N-NS4B/WARF4 sequence. In addition we demonstrated that N-NS4B/WARF4 protein expression is restricted to WNV lineage I infection and that it is expressed at high level in the late phase of infection (Figures 7, 8). Overall, our results demonstrate that N-NS4B/WARF4 is a novel protein, different from NS4B, and that is expressed in WNV infected cells.

Furthermore, we indirectly demonstrated the “in vivo” production of N-NS4B/WARF4 by showing its immunoreactivity with human sera obtained from WNV infected patients (Figure 9). The heterogeneous reactivity to the recombinant WNV antigens displayed by sera testing positive for WNV reflects the complex humoral response elicited by WNV infection [29, 30]. In addition, it is known that the ARF proteins are expressed with both less and variable efficiency if compared to the canonical proteins [31].

To date, we have no experimental information on N-NS4B/WARF4 protein translation, but it appears reasonable to assume that a −1 ribosomal frame shifting mechanism produces the novel protein. Indeed, in Flaviviriade the translation process is implemented by a cap-dependent scanning process, which produces a single polyprotein [32]. The sequence encoding N-NS4B/WARF4 COOH-terminal is in −1 frame, moreover it is far from the 5 terminal end, lacks an AUG codon and no internal ribosomal entry site (IRES) is described for WNV. The translation by ribosomal frame shifting is the only realistic explanation for N-NS4B/WARF4 protein synthesis. Since the proposed model requires the presence of specific RNA structures such as slippery sequences associated with pseudknot [3335], a bioinformatics analysis was performed to predict these structures. All the complete genomes of WNV available on gene bank were aligned and assigned to the two main lineages (Figure 10, Table 1). The strains belonging to the lineage 1 were first analyzed to confirm the association with WARF4 and then a further analysis was carried out looking for slippery sequences and pseudknot structure within the NS4B coding region. WARF4 was detected in 361 out of 368 genomes belonging to lineage 1. Seven genomes lacked WARF4 because of a single nucleotide substitution that interrupts the alternative frame. In the WARF4 group, two different and mutually exclusive slippery sequences with downstream frameshift-stimulating pseudknot structures were predicted. The first UUUUUUG sequence is the most representative (91%). The pseudknot structure is 80 nucleotides long and includes the initial aminoacids codified by the −1 frame. This slippery sequence is associated with the American viral strains. The second CCCUUUG/T sequence is present in 5% of the genomes, it is positioned 129 nucleotide upstream of WARF4 and has a pseudoknot structure of 40 nucleotides. The second slippery sequence is associated with circulating viral strain in Mediteranean Bacin and Est-Europe. The strain analyzed in this work belongs to this second group [Egyptian strain, an. AF260968]. It should be noticed that both the structures must promote the suppression of a termination codon (UGA) located just before the first codon of −1 frame [36].

The ribosomal frameshifting model also explains the discrepancy between the predicted and the observed molecular weight of the alternative N-NS4B/WARF4. The predicted alternative reading frame protein consists of 148 AA that account for a molecular weight of 16,7 kDa. However, the protein detected by MAb 3A12 in WNV infected cells migrates with an apparent molecular weight of about 28–30 kDa. Although this discrepancy could be due to a post-translation modification, the proposed model appears the most reasonable explanation for the observed molecular weight of N-NS4B/WARF4 protein. The novel protein would exist as COOH-terminal variant of the NS4B protein, indeed, the ribosomal shift in −1 frame would give rise to a NS4B variant protein where the last 123 AA should be replaced by a longer amino acid tail of 148 A. Thus, the variant protein should exhibit a molecular weight of about 30 kDa, consistent with our results (Figure 6, lane 3). The proposed ribosomal frameshifting model implies that the expression kinetic of N-NS4B/WARF4 should be like that of NS4B protein even if the amount of the novel protein should be less than that of NS4B. Figure 7 shows the expression of N-NS4B/WARF4 and NS4B in time-corse infection, the two proteins exhibit a similar kinetics. N-NS4B/WARF4 is clearly detected in the late phase of infection, such as the NS4B protein. The level of NS4B and N-NS4B/WARF4 proteins expression based on a densitometric analysis (data not shown) indicates a ratio of about 25 to 1 respectively. It should be highlighted that this ratio is estimated for the strain Eg101 [an. AF260968] that our biofinformatic analysis associates with the strain circulating in the Mediterranea area (Figure 10). The predicted pseudoknot associated with the American strain is thermodinamically more stable. It should be intersting to estimate this ratio in the American viral strain.

Conclusions

Overall, our results show for the first time that the novel ARF protein, N-NS4B/WARF4, is produced during the late stage of WNV lineage I infection and that N-NS4B/WARF4 is able to elicit antibodies in WNV infected individuals. To date, the biological function of N-NS4B/WARF4 and the role of anti- NS4B/WARF4 antibodies are unknown; however, it is suggestive that N-NS4B/WARF4 is restricted exclusively to the lineage I of WNV, which is known to be associated with the more severe clinical manifestations of WNV disease. This protein might represent a novel tool for a better understanding of WNV biology and for an improved characterization of immune response in WNV infected individuals.

Methods

Cell culture and virus strain

The VERO E6 cell line was cultured in Eagle’s minimal essential medium (MEM) with 10% Fetal Calf Serum (FCS), 100 U/mL penicillin, 200 μg/mL streptomycin in the presence of 5% CO2. WNV lineage I [strain Eg101, an. AF260968] and WNV lineage II [strain B956, an. AY532665] were propagated by infecting VERO E6 cells monolayers in MEM with 2% FCS and titrated according to the Reed and Muench formula.

Additional WNV genomic RNA [strain NY 1999, an. AF260967] was obtained from European Network for Diagnostics of Imported Viral Disease (http://www.enivd.de/ENIVD_P.HTM) during the External Quality Assurance (EQA) for molecular detection of West Nile virus. The nucleic acids were extracted by NucleoSpin RNA Virus kit according to manufacturer’s instructions (Macherey-Nagel, Düren, Germany).

Expression of WARF4, NS4, envelope, preM/M and NS5 recombinant proteins

Since the previously described recombinant His-WARF4 protein [25] does not comprise the full alternative reading frame, we cloned a novel fragment which includes the entire ARF. The 444 bps fragment spanning the WARF4 position 7311–7754 [accession number AF260967] was amplified in a single step by Superscript III One step RT-PCR, (Invitrogen, California). The fragment was cloned in pRSET vector and expressed in the BL21 Star (DE3)pLysS competent cells (Invitrogen, California). Other WNV recombinant proteins including the domain III of the Envelope (Env III), a preM/M and a NS5 fragment were produced as described above using primers and vectors reported in Table 2. The NS4B fragment was expressed by Rapid Translation System 100 E.coli HY (http://www.5PRIME.com). The His-tagged recombinant proteins were purified by Ni-NTA affinity chromatography kit according to manufacturer’s instructions (QIAGEN, Hilden, Germany). All the oligonucleotides were synthesized by Eurofins MWG Operon (http://www.eurofinsdna.com).
Table 2

Primers and vectors used to generate WNV recombinant proteins

Name

Position AF260967

Primer forward

Primer reverse

Vector

WARF4

7310-7757

5-atggcggatcc aggcaatgcgctcagcccagcg-3 BamHI

5- tctttgaagctt ctagtgaactcttcttttgtc-3 HindIII

pRSET B

ENV

1852-2198

5- ggaaaggatcc agttgaagggaacaacc -3 BamHI

5-ctcctgaattc gtggttgtaaaggctttgcc-3 EcoRI

pRSET C

preM/M

464-968

5-cagcgggatcc cagttaccctctctaacttcc-3 BamHI

5-caagggaattc aagctgtaagctggggccacc-3 EcoRI

pRSET C

Partial NS5

9015-9645

5-gaggcggatcc gcggggggaatgtcacac-3 BamHI

5-ttccgaattc tcaaacagccaggtcctg-3EcoRI

pRSET B

Partial NS4B

7276-7680

5-tcgatggatcc gacactatgcctacatggttcc-3BamHI

5-ggatcaagctt ttatctttttagtcctggtttttc-3HindIII

pRSET C

The table shows the primers and vectors used for molecular cloning. The restriction sites are underlined. Because of BamHI restriction site generated by a T/C transition in position 850 [AF260967], the preM/M cloned fragment encompassed the positions 464–850.

Production of a monoclonal antibody (MAb) recognizing the WARF4 recombinant protein

Four week-old BALB/mice were immunized twice by intraperitonal injection with 25 μg of purified His-WARF4 protein emulsified in RIBI adjuvant (RBI Immununochemical Research). Mice were then given a booster immunization intravenously with 10 μg of the immunogen, and immune splenocytes were removed 3 days later. Somatic cell hybrids were prepared with NS-1 mouse non secreting myeloma cells as previously described [3739]. Hybridoma supernatants were screened for differential immunoreactivity to His-WARF4 and His-purified control proteins by enzyme linked immunosorbent assay [40, 41]. Positive hybridoma cell lines were cloned twice by limiting dilution. One MAb was selected and designed 3A12.

Peptide scanning analysis

Four synthetic peptides named SP1, SP2, SP3 and SP4, which cover the full N-NS4B/WARF4 COOH-terminal amino acid sequence and which have amino acids overlapping sequences were designed (Figure 4).

SP1 (H-GNALSPAADSGWNHEKRCSGWHRGHGRPRIRAHHTHHAEESWANHA), SP2 (H-THHAEESWANHADLGVSSCSSSEPVCEDSARSRNSDHGSSGDTLGEW), SP3 (H-SDHGSSGDTLGEWSKLCLECNNCHRTLPHHAWGLVVMLIHNMDTHKE) and SP4 (HVVMLIHNMDTHKEHGKTRTKKRWGKGTHLGRGLERKTQPDDKRRVH) were synthesized by PEPSCAN (http://www.pepscan.com). Five hundred ng of peptide as well as Env and BSA proteins, 50 ng of His-WARF4 and 200 ng of His-NS4B were spotted in replicates on nitrocellulose membranes. Membranes were blocked for 4 h at RT in 5% non-fat dry milk-PBS-0.1% Tween-20 and incubated 1 hour at RT with MAb 3A12, anti-NS4B antibody (Abcam plc, Cambridge, UK) and MAb anti-Histidine (Invitrogen). The membranes were washed four times and then incubated with goat anti-mouse or rabbit IgG peroxidase-conjugated antibodies (Sigma, MI, IT) and developed by a chemiluminescent kit (Sigma, MI, IT) as previously described [42, 43].

Indirect immunofluorescence assay (IFA)

The VERO E6 cell line was grown in eight wells Chamber slides™ (Nunc, USA). 200 μl of a viral suspension (104 xTCID50/ml) were used to infect VERO cells monolayers (40-50% confluent). WNV was subsequently allowed to adsorb for 1 hour at 37°C. MEM medium with 2% FCS, was then added to the infected cells monolayer. After 36 hours, the cells monolayer was washed 2 times in PBS 1X and fixed with 4% paraformaldehyde for 20 min at room temperature, followed by treatment with 0.1 M glycine for 20 min at 25°C and with 0.1% Triton X-100 for an additional 5 min at 25°C to allow permeabilization. Cells were incubated for 30 min at room temperature with MAb 3A12 or the antibody MOPC-21 used as negative control as previously described [44, 45]. Nuclei were stained with Hoechst 33342 (blue).

Western blotting analysis

The VERO cells were infected with a multiplicity of infection (MOI) of 0.1 for 24 to 72 hours or for 96 hours at 37°C in 5% CO2. Cells were scraped and harvested by centrifugation at 1,500 X g at 4°C and rinsed in ice-cold phosphate-buffered saline (PBS). Cell lysates were prepared in Staph A buffer (10 mM sodium phosphate pH 7.4, 100 mM NaCl, 5 mM EGTA, 1% Triton, 0.1% SDS, 0.5% deoxycholate) containing 1 mM PMSF [46, 47] and a protease inhibitor cocktail (complete Mini EDTA-free, Roche). Proteins from uninfected and infected VERO cells lysate (50 μg/well) or purified recombinant WARF4 (10–100 ng/well), Env III (10–100 ng/well), preM (100 ng/well), NS5 (100 ng/well), NS4B (10 ng/well) protein fragments were separated on a NuPAGE 4-12% or 12% Bis-Tris gel and transferred onto nitrocellulose membranes (Invitrogen) [48]. Membranes were blocked for 6 h at 4°C in 5% nonfat dry milk-PBS-0.1% Tween-20 and incubated overnight at 4°C with MAb 3A12, anti-NS4b antibody, anti-M (Abcam plc, Cambridge, UK) or human sera (dilution 1:100). The membranes were washed four times and then incubated with goat anti-mouse IgG, anti-rabbit or anti-human IgG peroxidase-conjugated antibodies (Sigma, MI, IT) and developed by a chemiluminescent kit (Sigma, MI, IT) as previously described [42, 49].

Human sera

Human serum samples, obtained from convalescent patients suffering from neuro-invasive WNV infection, testing positive for IgGs anti-West Nile by IFA and confirmed by Micro-Neutralization Test Assay - MNTA [50] were kindly provided by Dr. Vittorio Sambri (St. Orsola-Malpighi University Hospital, University of Bologna, Bologna, Italy). Human sera from healthy donors testing negative for IgGs anti-West Nile were used as negative controls.

Bioinformatic analysis

384 full genomes of WNV were retrived from gene bank and aligned with BioNumerics software package (version 6.5 Applied-Maths, Belgium). Genomes were further analized by KnotInFrame software (http://bibiserv.techfak.uni-bielefeld.de/knotinframe/) to predict ribosomal −1 frameshift sites with a pseudoknot strucuture.

Human sera and experimental animals

Human sera were collected after informed consent and sent to the Regional Reference Centre for Microbiological Emergencies (CRREM, Prof. Vittorio Sambri, St. Orsola-Malpighi University Hospital, University of Bologna, Bologna, Italy). The protocol was approved under the regional plan of surveillance and control for west nile disease in Emilia Romagna (5 June 2009, prot. PG/2009/128190).

Mice were sacrificed by cervical dislocation and later the spleens were removed by surgical resection. The procedures were carried out in the presence of a veterinarian who monitored proper compliance with the decree law 116/92. The protocol was approved by the ethics committee of the Station Technology for Animal (STA) (http://www.sta.uniroma2.it/) of the University of Rome “Tor Vergata” on 06/12/2010.

Abbreviations

WARF4: 

West Nile Alternative open Reading Frame 4

WNV: 

West Nile Virus

ARF: 

Alternative Reading Frame.

Declarations

Authors’ Affiliations

(1)
Histology and Molecular Biology Section, Army Medical and Veterinary Research Center Via Santo Stefano Rotondo
(2)
Department of Experimental Medicine, “Sapienza” University of Rome
(3)
Istituto Zooprofilattico Sperimentale dell, “G. Caporale” Abruzzo e del Molise
(4)
Department of Clinical Sciences and Translational Medicine, University of Rome “Tor Vergata”
(5)
Regional Reference Centre for Microbiological Emergencies (CRREM), Microbiology Unit, Azienda Ospedaliero-Universitaria di Bologna
(6)
Department of Infectious Diseases, Istituto Superiore di Sanità

References

  1. Monath TP: Flaviviruses. In Virology. Edited by: Fields BN, Knipe DM. New York: Raven Press; 1990:763-814.
  2. Smithburn KC, Hughes TP, Burke AW, Paul JH: A Neurotropic Virus Isolated from the Blood of a Native of Uganda. Am J Trop Med 1940, 4: 471-492.
  3. Calistri P, Giovannini A, Hubalek Z, Ionescu A, Monaco F, Savini G, Lelli R: Epidemiology of west nile in europe and in the mediterranean basin. Open Virol J 2010, 4: 29-37.PubMedPubMed Central
  4. Hall RA, Broom AK, Smith DW, Mackenzie JS: The ecology and epidemiology of Kunjin virus. Curr Top Microbiol Immunol 2002, 267: 253-269. 10.1007/978-3-642-59403-8_13PubMed
  5. Murgue B, Zeller H, Deubel V: The ecology and epidemiology of West Nile virus in Africa, Europe and Asia. Curr Top Microbiol Immunol 2002, 267: 195-221. 10.1007/978-3-642-59403-8_10PubMed
  6. Tsai TF, Popovici F, Cernescu C, Campbell GL, Nedelcu NI: West Nile encephalitis epidemic in southeastern Romania. Lancet 1998, 352: 767-771. 10.1016/S0140-6736(98)03538-7PubMedView Article
  7. Angelini P, Tamba M, Finarelli AC, Bellini R, Albieri A, Bonilauri P, Cavrini F, Dottori M, Gaibani P, Martini E, Mattivi A, Pierro AM, Rugna G, Sambri V, Squintani G, Macini P: West Nile virus circulation in Emilia-Romagna, Italy: the integrated surveillance system 2009. Euro Surveill 2010, 15: pii: 19547.
  8. Papa A, Danis K, Baka A, Bakas A, Dougas G, Lytras T, Theocharopoulos G, Chrysagis D, Vassiliadou E, Kamaria F, Liona A, Mellou K, Saroglou G, Panagiotopoulos T: Ongoing outbreak of West Nile virus infections in humans in Greece, July-August 2010. Euro Surveill 2010, 15: pii: 19644.
  9. Anderson JF, Andreadis TG, Vossbrinck CR, Tirrell S, Wakem EM, French RA, Garmendia AE, Van Kruiningen HJ: Isolation of West Nile virus from mosquitoes, crows, and a Cooper’s hawk in Connecticut. Science 1999, 286: 2331-2333. 10.1126/science.286.5448.2331PubMedView Article
  10. Lanciotti RS, Roehrig JT, Deubel V, Smith J, Parker M, Steele K, Crise B, Volpe KE, Crabtree MB, Scherret JH, Hall RA, MacKenzie JS, Cropp CB, Panigrahy B, Ostlund E, Schmitt B, Malkinson M, Banet C, Weissman J, Komar N, Savage HM, Stone W, McNamara T, Gubler DJ: Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States. Science 1999, 286: 2333-2337. 10.1126/science.286.5448.2333PubMedView Article
  11. Gould LH, Fikrig E: West Nile virus: a growing concern? J Clin Invest 2004, 113: 1102-1107.PubMedPubMed CentralView Article
  12. Kramer LD, Li J, Shi PY: West Nile virus. Lancet Neurol 2007, 6: 171-181. 10.1016/S1474-4422(07)70030-3PubMedView Article
  13. Mukhopadhyay S, Kim BS, Chipman PR, Rossmann MG, Kuhn RJ: Structure of West Nile virus. Science 2003, 302: 248. 10.1126/science.1089316PubMedView Article
  14. Murphy FA, Fauquet CM, Bishop DHL, Ghabrial SA, Jarvis AW, Martelli GP, Summers MD, M.A.M: Virus taxonomy, classification and nomenclature of viruses. Arch Virol 1995,10(Suppl.)):1-586.
  15. Lanciotti RS, Ebel GD, Deubel V, Kerst AJ, Murri S, Meyer R, Bowen M, McKinney N, Morrill WE, Crabtree MB, Kramer LD, Roehrig JT: Complete genome sequences and phylogenetic analysis of West Nile virus strains isolated from the United States, Europe, and the Middle East. Virology 2002, 298: 96-105. 10.1006/viro.2002.1449PubMedView Article
  16. Beasley DW, Li L, Suderman MT, Barrett AD: Mouse neuroinvasive phenotype of West Nile virus strains varies depending upon virus genotype. Virology 2002, 296: 17-23. 10.1006/viro.2002.1372PubMedView Article
  17. Platonov AE, Shipulin GA, Shipulina OY, Tyutyunnik EN, Frolochkina TI, Lanciotti RS, Yazyshina S, Platonova OV, Obukhov IL, Zhukov AN, Vengerov YY, Pokrovskii VI: Outbreak of West Nile virus infection, Volgograd Region, Russia, 1999. Emerg Infect Dis 2001, 7: 128-132. 10.3201/eid0701.010118PubMedPubMed CentralView Article
  18. Coia G, Parker MD, Speight G, Byrne ME, Westaway EG: Nucleotide and complete amino acid sequences of Kunjin virus: definitive gene order and characteristics of the virus-specified proteins. J Gen Virol 1988,69(Pt 1):1-21.PubMedView Article
  19. Allison SL, Schalich J, Stiasny K, Mandl CW, Kunz C, Heinz FX: Oligomeric rearrangement of tick-borne encephalitis virus envelope proteins induced by an acidic pH. J Virol 1995, 69: 695-700.PubMedPubMed Central
  20. Westaway EG, Mackenzie JM, Khromykh AA: Replication and gene function in Kunjin virus. Curr Top Microbiol Immunol 2002, 267: 323-351. 10.1007/978-3-642-59403-8_16PubMed
  21. Guo JT, Hayashi J, Seeger C: West Nile virus inhibits the signal transduction pathway of alpha interferon. J Virol 2005, 79: 1343-1350. 10.1128/JVI.79.3.1343-1350.2005PubMedPubMed CentralView Article
  22. Samuel MA, Diamond MS: Pathogenesis of West Nile Virus infection: a balance between virulence, innate and adaptive immunity, and viral evasion. J Virol 2006, 80: 9349-9360. 10.1128/JVI.01122-06PubMedPubMed CentralView Article
  23. Evans JD, Seeger C: Differential effects of mutations in NS4B on West Nile virus replication and inhibition of interferon signaling. J Virol 2007, 81: 11809-11816. 10.1128/JVI.00791-07PubMedPubMed CentralView Article
  24. Melian EB, Hinzman E, Nagasaki T, Firth AE, Wills NM, Nouwens AS, Blitvich BJ, Leung J, Funk A, Atkins JF, Hall R, Khromykh AA: NS1’ of flaviviruses in the Japanese encephalitis virus serogroup is a product of ribosomal frameshifting and plays a role in viral neuroinvasiveness. J Virol 2010, 84: 1641-1647. 10.1128/JVI.01979-09PubMedPubMed CentralView Article
  25. Faggioni G, Ciammaruconi A, De Santis R, Pomponi A, Scicluna MT, Barbaro K, Masuelli L, Autorino G, Bei R, Lista F: Evidence of a humoral response to a novel protein WARF4 embedded in the West Nile virus NS4B gene encoded by an alternative open reading frame. Int J Mol Med 2009, 23: 509-512.PubMed
  26. Walewski JL, Keller TR, Stump DD, Branch AD: Evidence for a new hepatitis C virus antigen encoded in an overlapping reading frame. RNA 2001, 7: 710-721. 10.1017/S1355838201010111PubMedPubMed CentralView Article
  27. Bain C, Parroche P, Lavergne JP, Duverger B, Vieux C, Dubois V, Komurian-Pradel F, Trépo C, Gebuhrer L, Paranhos-Baccala G, Penin F, Inchauspé G: Memory T-cell-mediated immune responses specific to an alternative core protein in hepatitis C virus infection. J Virol 2004, 78: 10460-10469. 10.1128/JVI.78.19.10460-10469.2004PubMedPubMed CentralView Article
  28. Firth AE, Atkins JF: A conserved predicted pseudoknot in the NS2A-encoding sequence of West Nile and Japanese encephalitis flaviviruses suggests NS1’ may derive from ribosomal frameshifting. Virol J 2009, 6: 14. 10.1186/1743-422X-6-14PubMedPubMed CentralView Article
  29. Diamond MS, Pierson TC, Fremont DH: The structural immunology of antibody protection against West Nile virus. Immunol Rev 2008, 225: 212-225. 10.1111/j.1600-065X.2008.00676.xPubMedPubMed CentralView Article
  30. Nelson S, Jost CA, Xu Q, Ess J, Martin JE, Oliphant T, Whitehead SS, Durbin AP, Graham BS, Diamond MS, Pierson TC: Maturation of West Nile virus modulates sensitivity to antibody-mediated neutralization. PLoS Pathog 2008, 4: e1000060. 10.1371/journal.ppat.1000060PubMedPubMed CentralView Article
  31. Baril M, Dulude D, Gendron K, Lemay G, Brakier-Gingras L: Efficiency of a programmed −1 ribosomal frameshift in the different subtypes of the human immunodeficiency virus type 1 group M. RNA 2003, 9: 1246-1253. 10.1261/rna.5113603PubMedPubMed CentralView Article
  32. Egloff MP, Benarroch D, Selisko B, Romette JL, Canard B: An RNA cap (nucleoside-2'-O-)-methyltransferase in the flavivirus RNA polymerase NS5: crystal structure and functional characterization. EMBO J 2002, 21: 2757-2768. 10.1093/emboj/21.11.2757PubMedPubMed CentralView Article
  33. Farabaugh PJ: Programmed translational frameshifting. Microbiol Rev 1996, 60: 103-134.PubMedPubMed Central
  34. Giedroc DP, Theimer CA, Nixon PL: Structure, stability and function of RNA pseudoknots involved in stimulating ribosomal frameshifting. J Mol Biol 2000, 298: 167-185. 10.1006/jmbi.2000.3668PubMedView Article
  35. Giedroc DP, Cornish PV: Frameshifting RNA pseudoknots: structure and mechanism. Virus Res 2009, 139: 193-208. 10.1016/j.virusres.2008.06.008PubMedPubMed CentralView Article
  36. Howard MT, Aggarwal G, Anderson CB, Khatri S, Flanigan KM, Atkins JF: Recoding elements located adjacent to a subset of eukaryal selenocysteine-specifying UGA codons. EMBO J 2005, 24: 1596-1607. 10.1038/sj.emboj.7600642PubMedPubMed CentralView Article
  37. Bei R, Moretti A, Visco V, Defilippi R, Tsang K, Frati L, Muraro R: Cell mediated cytotoxicity of human colon carcinoma cells by a monoclonal antibody (R4) recognizing the carcinoembryonic antigen (CEA) and CEA-related molecules. Int J Oncol 1996, 8: 1127-1135.PubMed
  38. Bei R, Masuelli L, Trono P, Orvietani PL, Losito S, Marzocchella L, Vitolo D, Albonici L, Mrozek MA, Di Gennaro E, Lista F, Faggioni G, Ionna F, Binaglia L, Manzari V, Budillon A, Modesti A: The ribosomal P0 protein induces a spontaneous immune response in patients with head and neck advanced stage carcinoma that is not dependent on its overexpression in carcinomas. Int J Oncol 2007, 31: 1301-1308.PubMed
  39. De Smaele E, Di Marcotullio L, Moretti M, Pelloni M, Occhione MA, Infante P, Cucchi D, Greco A, Pietrosanti L, Todorovic J, Coni S, Canettieri G, Ferretti E, Bei R, Maroder M, Screpanti I, Gulino A: Identification and characterization of KCASH2 and KCASH3, 2 novel Cullin3 adaptors suppressing histone deacetylase and Hedgehog activity in medulloblastoma. Neoplasia 2011, 13: 374-385.PubMedPubMed CentralView Article
  40. Masuelli L, Focaccetti C, Cereda V, Lista F, Vitolo D, Trono P, Gallo P, Amici A, Monaci P, Mattei M, Modesti M, Forni G, Kraus MH, Muraro R, Modesti A, Bei R: Gene-specific inhibition of breast carcinoma in BALB-neuT mice by active immunization with rat Neu or human ErbB receptors. Int J Oncol 2007, 30: 381-392.PubMed
  41. Masuelli L, Marzocchella L, Focaccetti C, Lista F, Nardi A, Scardino A, Mattei M, Turriziani M, Modesti M, Forni G, Schlom J, Modesti A, Bei R: Local delivery of recombinant vaccinia virus encoding for neu counteracts growth of mammary tumors more efficiently than systemic delivery in neu transgenic mice. Cancer Immunol Immunother 2010, 59: 1247-1258. 10.1007/s00262-010-0850-0PubMedPubMed CentralView Article
  42. Bei R, Budillon A, Masuelli L, Cereda V, Vitolo D, Di Gennaro E, Ripavecchia V, Palumbo C, Ionna F, Losito S, Modesti A, Kraus MH, Muraro R: Frequent overexpression of multiple ErbB receptors by head and neck squamous cell carcinoma contrasts with rare antibody immunity in patients. J Pathol 2004, 204: 317-325. 10.1002/path.1642PubMedView Article
  43. Masuelli L, Budillon A, Marzocchella L, Mrozek MA, Vitolo D, Di Gennaro E, Losito S, Sale P, Longo F, Ionna F, Lista F, Muraro R, Modesti A, Bei R: Caveolin-1 overexpression is associated with simultaneous abnormal expression of the E-cadherin/α-β catenins complex and multiple ErbB receptors and with lymph nodes metastasis in head and neck squamous cell carcinomas. J Cell Physiol 2012, 227: 3344-3353. 10.1002/jcp.24034PubMedView Article
  44. Bei R, Mentuccia D, Trono P, Masuelli L, Cereda V, Palumbo C, Marzocchella L, Mrozek MA, Pallotta P, Di Lella G, Modesti M, Cerilli M, Frajese GV, Frajese G, Zambruno G, Modesti A: Immunity to extracellular matrix antigens is associated with ultrastructural alterations of the stroma and stratified epithelium basement membrane in the skin of Hashimotos thyroiditis patients. Int J Immunopathol Pharmacol 2006, 19: 661-674.PubMed
  45. Masuelli L, Marzocchella L, Quaranta A, Palumbo C, Pompa G, Izzi V, Canini A, Modesti A, Galvano F, Bei R: Apigenin induces apoptosis and impairs head and neck carcinomas EGFR/ErbB2 signaling. Front Biosci 2011, 16: 1060-1068. 10.2741/3735View Article
  46. Masuelli L, Bei R, Sacchetti P, Scappaticci I, Francalanci P, Albonici L, Coletti A, Palumbo C, Minieri M, Fiaccavento R, Carotenuto F, Fantini C, Carosella L, Modesti A, Di Nardo P: Beta-catenin accumulates in intercalated disks of hypertrophic cardiomyopathic hearts. Cardiovasc Res 2003, 60: 376-387. 10.1016/j.cardiores.2003.08.005PubMedView Article
  47. Visco V, Bei R, Moriconi E, Gianni W, Kraus MH, Muraro R: ErbB2 immune response in breast cancer patients with soluble receptor ectodomain. Am J Pathol 2000, 156: 1417-1424. 10.1016/S0002-9440(10)65010-9PubMedPubMed CentralView Article
  48. Bei R, Masuelli L, Moriconi E, Visco V, Moretti A, Kraus MH, Muraro R: Immune responses to all ErbB family receptors detectable in serum of cancer patients. Oncogene 1999, 18: 1267-1275. 10.1038/sj.onc.1202442PubMedView Article
  49. Masuelli L, Marzocchella L, Focaccetti C, Tresoldi I, Palumbo C, Izzi V, Benvenuto M, Fantini M, Lista F, Tarantino U, Modesti A, Galvano F, Bei R: Resveratrol and diallyl disulfide enhance curcumin-induced sarcoma cell apoptosis. Front Biosci 2012, 17: 498-508. 10.2741/3940View Article
  50. Capobianchi MR, Sambri V, Castilletti C, Pierro AM, Rossini G, Gaibani P, Cavrini F, Selleri M, Meschi S, Lapa D, Di Caro A, Grossi P, De Cillia C, Venettoni S, Landini MP, Ippolito G, Nanni Costa A: Italian Transplant Network: Retrospective screening of solid organ donors in Italy, 2009, reveals unpredicted circulation of West Nile virus. Euro Surveill 2010, 15: pii: 19648.

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