Cyclophilin B facilitates the replication of Orf virus

Background Viruses interact with host cellular factors to construct a more favourable environment for their efficient replication. Expression of cyclophilin B (CypB), a cellular peptidyl-prolyl cis-trans isomerase (PPIase), was found to be significantly up-regulated. Recently, a number of studies have shown that CypB is important in the replication of several viruses, including Japanese encephalitis virus (JEV), hepatitis C virus (HCV) and human papillomavirus type 16 (HPV 16). However, the function of cellular CypB in ORFV replication has not yet been explored. Methods Suppression subtractive hybridization (SSH) technique was applied to identify genes differentially expressed in the ORFV-infected MDBK cells at an early phase of infection. Cellular CypB was confirmed to be significantly up-regulated by quantitative reverse transcription-PCR (qRT-PCR) analysis and Western blotting. The role of CypB in ORFV infection was further determined using Cyclosporin A (CsA) and RNA interference (RNAi). Effect of CypB gene silencing on ORFV replication by 50% tissue culture infectious dose (TCID50) assay and qRT-PCR detection. Results In the present study, CypB was found to be significantly up-regulated in the ORFV-infected MDBK cells at an early phase of infection. Cyclosporin A (CsA) exhibited suppressive effects on ORFV replication through the inhibition of CypB. Silencing of CypB gene inhibited the replication of ORFV in MDBK cells. In conclusion, these data suggest that CypB is critical for the efficient replication of the ORFV genome. Conclusions Cellular CypB was confirmed to be significantly up-regulated in the ORFV-infected MDBK cells at an early phase of infection, which could effectively facilitate the replication of ORFV.


Background
Orf virus (ORFV) is the type species of the genus Parapoxvirus, which has a worldwide distribution and is the causative agent of Orf, a contagious debilitating skin disease of sheep and goats also known as contagious ecthyma, contagious pustular dermatitis, infectious labial dermatitis, scabby mouth or sore mouth [1]. Primary infections are usually resolved within 1-2 months, however repeated and persistent infections can occur in the presence of an intensive inflammatory host antivirus immune response [2,3]. The mechanisms that establish the repeated and persistent infections in vivo are almost completely unknown. Currently, the host immune response to ORFV has been extensively studied, yet many aspects of the complex host-virus interactions remain unclear.
Cyclophilins (Cyp) comprise a family of peptidyl-prolyl cis/trans isomerases, which are originally discovered as a cellular factor with high affinity for the immunosuppressant CsA [4,5]. CypB is one of the most abundant members among the Cyp family, is ubiquitously expressed in most cells, and predominantly resides in the endoplasmic reticulum (ER) through the ER retention signal sequence in the C-terminus [6,7]. CypB functions in various cellular processes, including transcriptional regulation, protein secretion, immune response and apoptosis [8][9][10]. In addition, some researchers have shown that many viruses such as Japanese encephalitis virus (JEV), hepatitis C virus (HCV) and human papillomavirus type 16 (HPV 16) require CypB for their replication [11][12][13]. However, there has been no report on the involvement of CypB in the replication of ORFV.
In the present study, CypB was found to be significantly up-regulated in the ORFV-infected MDBK cells at an early phase of infection. However, the function of cellular CypB in ORFV replication has not yet been explored. Sequently, we investigated the role of CypB in the replication of ORFV in MDBK cells. Cyclosporin A (CsA) exhibited suppressive effects on ORFV replication through the inhibition of CypB. Silencing of CypB gene inhibited the replication of ORFV in MDBK cells. In conclusion, these data suggest that CypB is critical for the efficient replication of the ORFV genome.

Cells and virus
The Madin-Darby bovine kidney (MDBK) cells were maintained in minimal essential medium (MEM) (GIBCO, Invitrogen), supplemented with 10% fetal bovine serum (FBS) and penicillin (100 U/mL); streptomycin (100 mg/ mL); and nystatin (20 mg/mL). The Orf virus used in this study (ORFV-Jilin) was isolated using MDBK cells from scab specimens collected from skin lesions of a 6-weekold small-tailed Han sheep afflicted with orf in November 2008 in the Jilin province of China [14].

Antibodies and reagents
The antibodies used in this study were anti-Cyclophilin B polyconal rabbit antibody (Abcam), PE-Cy5 conjugated rabbit anti-Cyclophilin B antibody (Bioss), anti-β-actin (Proteintech), Peroxidase-conjugated Affinipure Goat Anti-Mouse IgG (Proteintech) and Peroxidase-conjugated Affinipure Goat Anti-Rabbit IgG (Proteintech). CsA were purchased from Sigma. PolyATtract® mRNA Isolation Systems was purchased from Promega. PCR-Select™ cDNA Subtraction Kit and Advantage cDNA PCR Kit & Polymerase Mix were obtained from Clontech. X-treme GENE HP DNA Transfection Reagent and X-treme GENE siRNA Transfection Reagent were purchased from Roche.

Virus infection
MDBK cells have been reported to be least partially permissive for ORFV replication [14]. A confluent monolayer of MDBK cells cultured in 75-cm 2 flasks were adsorbed with ORFV-Jilin (multiplicity of infection [MOI] = 3) for 2 h at 37°C. After adsorption, unbound virus was removed by gentle washing with serum-free medium followed by the addition of fresh medium and further incubation at 37°C. At 2 h after further incubation, ORFV transcripts were detected in MDBK cells infected with ORFV-Jilin. Thus, the starting point of early phase of ORFV infection was determined for 4 h. The cultures of incubated cells were harvested at 4 h post infection, and cell lysates were collected. Normal cell controls were collected in a similar manner. All samples were frozen in liquid nitrogen and stored at −80°C until SSH library construction.

Construction and screening of Suppression subtractive hybridization (SSH) library
Total RNA was isolated from both mock-and ORFVinoculated cell lysates using RNAiso Plus (Takara, Dalian, China) according to the manufacturer's instructions. The quality of total RNA were determined by ultraviolet spectrophotometry (260 and 280 nm) and 1% agarose/ ethidium bromide (EtBr) gel electrophoresis. Messenger RNA (mRNA) was isolated from total RNA using Poly-ATtract® mRNA Isolation Systems (Promega) according to the manufacturer's instructions. The purified mRNA was treated with 1 mL of 75% ethanol and 0.1 volume of 3 mol/L NaOAc and used as the starting material to construct the SSH cDNA library. PCR select cDNA subtraction was conducted using PCR-Select™ cDNA Subtraction Kit (Clontech, USA) with 2 μg of mRNA for each sample as starting material according to the manufacturer's instructions. Subtraction hybridization was conducted to perform subtraction in both forward and reverse directions. After two rounds of hybridization, PCR was conducted to selectively amplify cDNAs differentially expressed between ORFV-inoculated and normal MDBK cells. Each PCR amplification described below was conducted using a thermal cycler MyCycler (Bio-Rad) in a total volume of 25 μL with 1 μL template DNA, 0.5 U of 50 × Advantage cDNA polymerase (Clontech, USA), 200 μM of each dNTP, and forward and reverse primers (1 μM each). The cDNA fragments for subtracted secondary PCR products were inserted into the pMD18-T vector (TaKaRa, Dalian, China) with T4 DNA ligase and transformed into DH5a Escherichia coli cells. The clones with recombinant plasmid were identified by the LB-ampicillin/IPTG/X-Gal medium colony screening. Recombinant white clones were selected randomly and amplified by PCR using the M13 primer to construct the corresponding SSH cDNA library. The clones that yielded a single PCR product were selected for the next analysis.

qRT-PCR analysis of CypB expression in ORFV-infected MDBK cells
Among the differentially expressed genes obtained by SSH technique, CypB was found to be significantly upregulated in the ORFV-infected MDBK cells at an early phase of infection. The qRT-PCR method based on the CypB gene was applied to determine the mRNA expression level of CypB in MDBK cells infected with ORFV-Jilin (MOI = 3) for 0 h, 2 h, 4 h, 8 h, 12 h, 18 h and 24 h. The primer sets used in the study were specific for CypB (forward primer 5′-GAGACGGCACTGGAGG-TAAG-3′; reverse primer 5′-TCGTGATGAAGAACT GGGAG-3′). Briefly, total RNA was isolated from both mock-and ORFV-inoculated cell lysates using RNAiso Plus (Takara, Dalian, China) according to the manufacturer's instructions. Then, 200 ng of each RNA sample was used in a Oligo (dT) cDNA synthesis. The qPCRs for CypB gene were performed in 20 μL of reaction mixture containing 10 μL SYBR Premix ExTaq (Takara, Dalian, China), 0.2 μM of each oligonucleotide primer, 100 ng of each cDNA sample, and 7.2 μL ddH 2 O. Each sample was repeated three times. Fluorescent signals were analyzed by an ABI PRISM 7000 (Applied Biosystems, Tokyo, Japan). Data were statistically analyzed by Student's t-test using GraphPad Prism 6 software.

Anti-ORFV activity of CsA in vitro detected by MTT assay
MTT assay used for measuring cell survival and proliferation was performed in order to determine the effect of CsA on the inhibitory ratio of ORFV. Briefly, a confluent monolayer of MDBK cells grown on 96-well plates were inocubated with ORFV-Jilin (MOI = 0.1) and incubated in a CO 2 incubator with a 5% CO 2 atmosphere. After 2 h of adsorption, the maintenance medium containing various concentrations of CsA (0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 μg/ mL) was added after removing the virus inoculum. At 24 h after drug action, 100 μL MTT (tetrazolium salt 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) were added to each well and kept in a dark environment for 4 h at 37°C. Then, MTT was aspirated and 100 μL well −1 of DMSO (Merck, Darmstad, Germany) was added to each well. Subsequently, the absorbance value at a wavelength of 570 nm was measured using a UV-visible spectrophotometer (LPB Pharmacia, Bromma, Sweden) for the calculating the inhibitory rate of CsA on ORFV proliferation. The untreated cell cultures were cultured under the same conditions. Data were statistically analyzed by one-way ANOVA followed by Fisher's least significant difference (LSD) test using GraphPad Prism 6 software.

CypB gene silencing by siRNA
Based on the sequences of CypB, two pairs of siRNAs, CypB-specific siRNA and negative siRNA control (scrambled siRNA) were designed and synthesized by Shanghai GenePharma Co., Ltd. The sequences are shown in Table 1. The cells were grown on 12-well plates and incubated in a CO 2 incubator with a 5% CO 2 atmosphere. Approximately 50-60% confluent, the cells were transfected with 35 nM siRNA by using X-tremeGENE siRNA transfection reagent (Roche, USA) according to the manufacturer's protocol. The transfected cells were then washed with MEM without serum and incubated in MEM supplemented with 2% fetal bovine serum. Untransfected MDBK cells were used as a control. Typical siRNA transfection efficiency was found to be 83% for MDBK cells as monitored by fluoresceinlabeled control siRNA duplex. CypB knockdown was confirmed 48 h post siRNA transfection by qRT-PCR and Western blotting (data not shown).

Immunofluorescence assay after siRNA knockdown of CypB
The MDBK cells were mock-transfected or transfected with CypB siRNAs or scrambled siRNAs for 48 h and subjected to immunofluorescence assay. Cells cultured on glass slides were washed gently in phosphate buffered saline (PBS), and fixed with 4% paraformaldehyde for 30 min, PBS washed (three times, each 5 min); 0.2% Triton X-100 enhanced the permeability of cell membranes for 10 min at room temperature and PBS washed; 5% skim milk incubated 1 h, PBS washed. Then cells were incubated with PE-Cy5 conjugated rabbit anti-Cyclophilin B antibody (1:500) at 4°C over the night. Cell nuclei were stained blue with DAPI for 15 min. Finally, the samples were washed three times with PBS and observed under immunofluorescent microscope. The qPCR assay was performed as described previously [15]. Furthermore, viral yields were quantitated in the siRNA-treated cells by the 50% tissue culture infectious dose (TCID 50 ) method. Briefly, the siRNA-treated cells were infected with 100 TCID 50 (10 6.5 ) of ORFV, and the infection was allowed to proceed for the indicated time periods. Untransfected MDBK cells were used as a control. Data were statistically analyzed by one-way ANOVA followed by Fisher's LSD test using GraphPad Prism 6 software.

Statistical analysis
Statistical analysis was carried out using GraphPad Prism v6.0 (GraphPad Software, La Jolla, CA). Student's t-test and one-way ANOVA followed by Fisher's LSD test were used for comparisons of two and multiple groups, respectively. Data were expressed as the means with error bars depicting ± SEM, and differences were considered statistically significant at * P < 0.05, ** P < 0.01, *** P < 0.001.

Results
CypB was found to be significantly up-regulated in the ORFV-infected MDBK cells by SSH technique In order to explore host genes inovolved in the process of ORFV invasion, the differentially expressed genes from ORFV-infected MDBK cells at an early phase of infection were identified using SSH technique. A total of 81 expressed sequence tags (ESTs) were obtained from the SSH libraries by sequencing and alignment. Among of all the ESTs, 60 ESTs came from the forward SSH library and 21 ESTs came from the reverse SSH library. BLAST analysis against the Genbank database revealed that 79 ESTs showed significantly homology to known genes. Of those, all the ESTs showed significantly homology with Bos Taurus related sequences (Tables 2 and 3). CypB as an important member of cyclophilins family, was found to be significantly up-regulated in the ORFVinfected MDBK cells at an early phase of infection.

Verification of the mRNA expression levels of CypB gene by qRT-PCR
We further verified the mRNA expression levels of CypB gene by qRT-PCR (Fig. 1a). CypB was confirmed by qRT-PCR as differentially expressed. The transcript levels of CypB was overexpressed in MDBK cells infected with ORFV-Jilin compared with mock-infected MDBK cells ( Fig. 1b), with CypB mRNA levels 10 6 -fold higher in ORFV-infected MDBK cells at 4 h post infection than in the corresponding mock-infected MDBK cells.

Verification of the protein expression levels of CypB by Western blotting
The expression of CypB gene was confirmed by Western blotting using β-actin as an internal control. Western blot analysis showed increased expression in the ORFVinfected MDBK cells, especially at 4 h post infection (Fig. 2). The results were consistent with those obtained from qRT-PCR. These findings suggested that changes in the expression of CypB gene and protein are associated with ORFV early infection.

Inhibition effect of CsA on ORFV proliferation in MDBK cells
The inhibition of CsA on ORFV proliferation in vitro was evaluated by MTT assay. The MDBK cells infected with ORFV were exposed to increasing concentrations of CsA from 0 to 3.2 μg/mL and maximal ethanol concentration used to solubilize and dilute CsA as vehicle.
As demonstrated in Fig. 3, the inhibition rate was increased in different concentrations of CsA (0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 μg/mL) in comparision with the control (no treatment) and this increase was significant in 0.2 μg/mL (*** P < 0.001). Our results showed that treatment with 0.2 μg/mL CsA could effectively inhibit the cytotoxicity induced by ORFV.

Examination of siRNA effect by Immunofluorescence assay
The immunofluorescence assay was performed in order to determine the inhibitory effect of CypB specific siR-NAs. As shown in Fig. 4, the CypB-specific red fluorescence signals distributed in the cytoplasm of MDBK cells was found to be attenuated at 48 h post transfection. The cells transfected with scrambled siRNAs exhibited relative apparent fluorescent signals, which was similar to that in mock-transfected cells. However, no red fluorescence signals were observed in the cells transfected with CypB specific siRNAs. Nuclei were stained blue with DAPI.

Examination of siRNA effect by infectious virus assay
To further investigate the inhibitory effect of siRNAs, the microtiter method was used for titration of ORFV at 5 days postinfection. The results (Fig. 5b) showed that viral titers in untransfected MDBK cells was higher than  Table 3 Similarity between expressed sequence tags in the rverse suppression subtractive hybridization library and genes in GenBank

Discussion
Viruses are strict intracellular pathogens that require the cellular environment to complete a successful infection. During the infection process, viruses could affect several major cellular responses by altering the expression of the key host genes. Considering viral infection strongly relies on several factors from the host, the study of virus-host interactions is essential to the understanding of the infection process of disease and the mechanism of the virus persistence in the host, and to help with the development of effective vaccines and perhaps the cure of viral infections. ORFV is a highly epitheliotropic virus, that infects damaged or scarified skin and replicates in regenerating epidermal keratinocytes. An important characteristic of the virus is its ability to cause repeated and persistent infections [16] even in the presence of an intensive inflammatory and multiple host immune response [3,17]. At present, little is known about the mechanisms of ORFV infection of host, although several studies have  demonstrated that ORFV encoded protein might interact with a host protein involved in the pathway and/or might mimic the structurally similar host protein to invade/manipulate the cellular pathway [18,19].
With the purpose of identify genes involved in ORFV early invasion, we constructed a cDNA library and identified the differently expressed genes from ORFV-infected MDBK cells at an early phase of infection in the study. By using SSH technique, a total of 81 EST sequences with differing functions were identified, including 60 differently up-regulated genes from the forward library and 21 differently down-regulated genes from the reverse library. CypB as an important member of cyclophilins family, was found to be significantly up-regulated in the ORFV-infected MDBK cells at an early phase of infection. In the present study, the upregulation of CypB in host cells was further validated by qRT-PCR and Western blotting. The results showed that CypB was overexpressed in MDBK cells infected with ORFV-Jilin at 4 h post infection, which were consistent with those obtained by SSH technique.
Recently, increasing studies have demonstrated that cellular CypB are closely related with viral infections such as JEV [11], HCV [12] and HPV 16 [13]. However, the role of CypB in ORFV proliferation have remained unknown. By using CsA, we determined that it could effectively inhibit the cytotoxicity induced by ORFV, which suggested that CypB was required for the efficient proliferation of ORFV in vitro. To further ascertain the Fig. 4 Examination of siRNA effect by Immunofluorescence assay. The MDBK cells were mock-transfected or transfected with CypB siRNAs or scrambled siRNAs for 48 h and subjected to immunofluorescence assay. As shown in Fig. 4, the CypB-specific red fluorescence signals distributed in the cytoplasm of MDBK cells were found in the cells transfected with scrambled siRNAs, which was similar to that in mock-transfected cells. However, no red fluorescence signals were observed in the cells transfected with CypB specific siRNAs. Nuclei were stained blue with DAPI The MDBK cells infected with ORFV (MOI = 0.1) were exposed to increasing concentrations of CsA (0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 μg/ mL) and maximal ethanol concentration used to solubilize and dilute CsA as vehicle. The anti-ORFV activity of CsA in vitro was detected by MTT assay. Subsequently, the absorbance value at a wavelength of 570 nm was measured for the calculating the inhibitory rate of CsA on ORFV proliferation. The untreated cells were cultured under the same conditions. Data were statistically analyzed by oneway ANOVA followed by Fisher's LSD test using GraphPad Prism 6 software. Values are the means ± SEM. (*** P < 0.001). The results showed that treatment with 0.2 μg/mL CsA could effectively inhibit the cytotoxicity induced by ORFV regulatory role of CypB for ORFV replication, CypB specific siRNAs were synthesized and then transfected. The efficiency of inhibition of viral replication was evaluated by immunofluorescence assay, qPCR and infectious virus assay. The results demonstrated that silencing of CypB gene by RNAi could efficiently inhibit ORFV genome replication and infectious virus production. On the basis of the above studies, CypB has been confirmed to facilitate ORFV replication.
Moreover, CypB has been demonstrated to involve in inflammatory events [20]. The overexpressed CypB could activate the extracellular signal-regulated kinase intracellular signaling pathway such as ERK signaling through binding with CD147, which has been shown to function as a signalling receptor for CypB to mediate chemotactic activity towards a variety of immune cells [21][22][23]. Currently, the upregulation of CypB has been confirmed to facilitate ORFV replication, presumably through interactions with CD147 or other some agents to activate the associated immunological and inflammatory signaling pathways. Further investigations on the cooperation between these molecules might provide new insights on the mechanisms controlling ORFV infection within target cells and lead to the development of new therapeutic tools to block the virus infection.

Conclusions
The results of the present study demonstrated that CypB could effectively facilitate the replication of ORFV, suggesting CypB might be involved in the process of ORFV infection. However, the detailed interaction between CypB and ORFV is still unknown. Further studies will be focused on the interplay between ORFV and cellular CypB, which may contribute to clarify virus pathogenesis and the development of antiviral vaccine candidates and novel inhibitors.