Potential conflicts of interest: X.F. received an unrestricted grant support from Schering and Roche. M.G. is currently employed by Bitplane but his affiliation was the one stated above during the period of study design, laboratory work, and article preparation.
Article first published online: 3 FEB 2011
Copyright © 2011 American Association for the Study of Liver Diseases
Volume 53, Issue 5, pages 1436–1445, May 2011
How to Cite
Mensa, L., Crespo, G., Gastinger, M. J., Kabat, J., Pérez-del-Pulgar, S., Miquel, R., Emerson, S. U., Purcell, R. H. and Forns, X. (2011), Hepatitis C virus receptors claudin-1 and occludin after liver transplantation and influence on early viral kinetics. Hepatology, 53: 1436–1445. doi: 10.1002/hep.24110
X. Forns received support in part by a grant from Instituto de Salud Carlos III (PI080239), cofunded by the European Regional Development Fund (ERDF), and by the Spanish Association for the Study of Liver Diseases (AEEH, Beca Hernández-Guío). G. Crespo was supported by Hospital Clinic (Ajut a la Recerca Josep Font) and Fundación BBVA. L. Mensa was supported by Instituto de Salud Carlos III (PFIS). This study was supported in part by the Intramural Research Program of the National Institute of Allergy and Infectious Diseases, National Institutes of Health.
- Issue published online: 22 APR 2011
- Article first published online: 3 FEB 2011
- Accepted manuscript online: 14 DEC 2010 05:44PM EST
- Manuscript Accepted: 29 NOV 2010
- Manuscript Received: 15 JUN 2010
Liver transplantation (LT) is a unique model to study hepatitis C virus (HCV) entry into hepatocytes. Recent in vitro studies suggest significant changes in the expression of the HCV receptors claudin-1 and occludin after HCV infection. Our aims were: (1) to characterize claudin-1 and occludin expression in grafts from LT recipients and (2) to explore their potential influence on early HCV kinetics and their changes after HCV infection. We included 42 HCV-infected LT recipients and 19 uninfected controls. Claudin-1 and occludin were detected in paraffin-embedded liver biopsies obtained during reperfusion and 3 and 12 months after LT. HCV receptors were characterized by confocal immunofluorescence microscopy; quantification and colocalization studies were performed with dedicated software. Claudin-1 and occludin expression were restricted to the apical pole of hepatocytes. There was a significant correlation between the amount of scavenger receptor B1 at the time of reperfusion and the HCV-RNA decay during the first 24 hours following LT (r = 0.55, P = 0.007). Similarly, there was a significant correlation between the levels of claudin and occludin and the slope of HCV-RNA increase during the first week after LT (r = 0.63, P = 0.005). Occludin and claudin-1 levels increased significantly 12 months after LT (P = 0.03 and P = 0.007, respectively). The expression pattern of both proteins, however, remained unchanged, colocalizing strongly (60%-94%) at the apical membrane of hepatocytes. Conclusions. HCV receptor levels at the time of LT seem to modulate early HCV kinetics. Hepatitis C recurrence after LT was associated with increased levels of claudin-1 and occludin in the hepatocyte cell membrane, although it did not alter their localization within the tight junctions. (HEPATOLOGY 2011;.)
Hepatitis C virus (HCV) is the leading cause of chronic liver disease in many regions of the world. Chronic hepatitis C progresses to cirrhosis and endstage liver failure in a significant proportion of patients over the years and is the main indication for liver transplantation (LT) in the Western world and Japan.1
Major advances have been achieved in the last few years towards a better understanding of the HCV life cycle. The development of a retroviral pseudoparticle system (HCVpp)2 and the ability of an HCV strain (JFH-1)3 to replicate and release infectious particles in cell culture have been very relevant to the study of HCV entry into hepatocytes. Virus entry is commonly a complex event that requires sequential interactions between viral surface proteins and cellular factors.4-10 The exact mechanisms by which HCV reaches the cytoplasm of liver cells and initiates replication are not yet completely understood. The fact that HCV needs several receptors with different membrane distributions favors the hypothesis of a coordinated entry process, such as what occurs with other viruses (i.e., Coxackie virus B).7, 10, 11 Ploss et al.10 recently proposed that HCV may initially interact with the luminal (sinusoidal) surface of the hepatocyte by contact with scavenger receptor B1 (SR-B1) and CD81. Thereafter a virus-receptor complex might migrate to the biliary pole (apical membrane), where the virus-receptor complex reaches the tight junctions and uptake into the cytoplasm would occur. The recent discovery that the tight junction proteins claudin-1 and occludin are essential factors for HCV entry into cells suggests a role for these proteins in HCV cell-cell transmission, a route of spread that is still under investigation.7
Recent studies have analyzed the potential role of HCV infection in the regulation of its putative receptors, particularly those located in the tight junctions. In one study, HCV infection appeared to down-regulate claudin-1 and occludin in Huh7 cells.12 In another study, expression of HCV structural proteins in Huh7 cells was not associated with changes in claudin-1 and occludin levels, but these proteins accumulated in the endoplasmic reticulum and their altered localization disrupted the tight junction barrier function.13 These differences between studies reflect the complexity of the HCV entry mechanisms and the fact that current in vitro systems may not completely reproduce the virus life cycle in a human liver.14
Liver transplant patients undergo frequent liver biopsies, allowing in vivo assessment of the potential changes in the expression of such HCV receptors over time. The aim of this study was to evaluate the potential changes in tight junction proteins claudin-1 and occludin following HCV graft infection and to analyze if their expression could influence early HCV kinetics.
Patients and Methods
Forty-two HCV-infected patients undergoing LT from January 2000 to January 2008 were included in the study. Selection of patients was based on the type of hepatitis C recurrence and individuals at both extremes of the disease spectrum (mild and severe) were selected. Mild disease recurrence was defined as absent (F0) or mild (F1) fibrosis 1 year after transplantation, and a normal hepatic venous pressure gradient (HVPG). Severe disease recurrence was defined as the presence of advanced fibrosis (F ≥3) and/or clinically significant portal hypertension (HVPG ≥10 mmHg) 1 year after transplantation. Nineteen HCV-negative liver transplant recipients served as controls.15, 16
All patients were followed in our Liver Unit and underwent standard immunosuppression protocols.15 Induction immunosuppression consisted of cyclosporine A or tacrolimus and prednisone. After hospital discharge patients visited the outpatient clinic monthly for 3 months for complete recording of clinical and analytical data and every 2 or 3 months thereafter. Liver biopsies were obtained after graft reperfusion (revascularization of the graft during the surgical procedure) and at 3 and 12 months after LT in accordance with the standard protocol. Patients whose liver disease was likely caused by another reason (rejection, cytomegalovirus [CMV] infection) were excluded. The study was previously approved by the Investigation and Ethics Committee of the Hospital Clinic of Barcelona following the ethical guidelines of the 1975 Declaration of Helsinki. We obtained informed consent from all patients included in the study.
Percutaneous liver biopsies were performed by expert radiologists. HVPG measurements and transjugular liver biopsies were performed at the Hepatic Hemodynamics Laboratory as described.16 Liver samples were processed by the Pathology Department. All tissue samples were formalin-fixed and paraffin-embedded (FFPE). For diagnostic purposes, samples were stained with hematoxylin-eosin and Masson's trichromic. All histological samples were examined by an expert pathologist (R.M.). Fibrosis stage was scored using the Scheuer classification.
Representative 5-10 μm sections were cut from paraffin blocks and mounted on charged slides. Slides were heated overnight at 37°C and thereafter deparaffinized by treating the slides with xylene 3 times (10 minutes each) followed by ethanol rehydration.
Antigen retrieval was performed by treating the tissue with heat using a pressure cooker (Pascal, Dako, Carpinteria, CA). When the temperature reached 80°C the sections were placed on metal racks and submerged in a 1 mM EDTA solution (pH 8). The pressure cooker was set to reach 125°C (21 psi) for 2 minutes. Thereafter, the sections were blocked for 30 minutes with phosphate-buffered saline (PBS) / 10% goat serum (Jackson ImmunoResearch, West Grove, PA), followed by incubation for 2 hours at room temperature with the primary antibody: 2.5 μg/mL rabbit polyclonal antihuman claudin-1 (Zymed, Invitrogen, Carlsbad, CA), 5 μg/mL mouse monoclonal antihuman occludin (Zymed, Invitrogen), 2.5 μg/mL mouse monoclonal antihuman SR-B1 (BD Transduction Laboratories, San Jose, CA), or 2 μg/mL rat antihuman CD10 (Santa Cruz Biotechnology, Santa Cruz, CA). After three washes with PBS (10 minutes each), sections were incubated with the secondary antibody for 1 hour at room temperature. Secondary antibodies were: Alexa Fluor 488 goat antirabbit immunoglobulin G (IgG), Alexa Fluor 568 F(ab')2 fragment of goat antimouse IgG (H+L), and Alexa Fluor 647 goat antirat IgG (Invitrogen). Slides were washed three times with PBS (10 minutes each); after the second wash slides were incubated for 2 minutes with DAPI (Sigma Aldrich, St. Louis, MO). Slides were then mounted with Hard Mounting Media (Vector Laboratories, Burlingame, CA) and kept overnight at room temperature in the dark.
Analysis of Receptor Expression by Confocal Microscopy.
Images were acquired on a Leica SP5 confocal microscope (Leica Microsystems, Exton, PA) using 488 nm, 561 nm, or 633 nm laser lines. Hoechst dye was excited using a 364 nm Enterprise II UV laser (Coherent, Santa Clara, CA). Sequential frame averaged scans were set up for each fluorophore to eliminate emission crosstalk. All images were saved in a 12 bit TIFF format at 512 × 512 or 1024 × 1024 pixels. Surface rendering and colocalization analysis were performed using Imaris (v. 6.2.2, Bitplane, Zurich, Switzerland). For each channel an individual threshold was selected and maintained for all processed samples. Sum of intensities (representing the sum of the intensities obtained in each voxel above the threshold) and number of positive voxels (those with intensity above the established threshold) were calculated for each individual sample. In order to correctly sample the entire liver biopsy, 10 different image acquisitions were obtained for each liver section. The sum of intensities and the number of positive voxels for each biopsy was calculated as the geometrical mean of the individual values obtained in the different acquisitions.
A detailed colocalization study for claudin-1 and occludin was performed in 20 selected specimens. For this purpose, triple staining was carried out with rabbit anti-claudin-1, mouse anti-occludin, and rat anti-CD10 (a commonly used marker of the biliary canalicula). Sequential sections of stained samples were acquired with the 63×-oil immersion objective (NA 1.4) at a zoom of 5 to 7 with a Z-step of 0.20-0.25 μm through the entire volume of the paraffin section (≈7-10 μm section thickness). All collected images for 3D analyses were deconvolved by Huygens Essential software (v. 3.4, Scientific Volume Imaging, Hilversum, The Netherlands). A 3D image volume was reconstructed from sequential z-sections and colocalization analyses were performed in Imaris software. Surface rendering and channel masking was used in conjunction with manual thresholding to calculate protein colocalization statistics in a 3D environment. The same level of thresholding was applied to each dataset; unlabeled regions were not included in this analysis (masking). The level of colocalization in the 3D volume was measured as percent of volume of the channel above threshold colocalized (the total number of colocalized voxels divided by the total number of voxels in each channel that are above the threshold). A second measure of the intensity of colocalization between claudin-1 and occludin was obtained by calculating the correlation between the intensities of the colocalized voxels (Pearson correlation).
Positive (strongly positive samples) and negative controls (samples stained with an irrelevant primary antibody) were included in each experiment. In order to ensure that differences in the expression of receptors were not due to methodological issues, 20 random liver biopsies were processed in triplicate on different days following the same protocols. Sum of intensities for SR-B1 and claudin-1 as well as the number of positive voxels for each channel were compared for each independent experiment. Samples were always processed blindly. This applied both to the immunofluorescence protocol and for image processing. Coding of slides allowed the staining of samples belonging to the same patient in the same experiment.
Quantitation of Claudin-1 and Occludin Messenger RNA (mRNA) by Real-Time Polymerase Chain Reaction (PCR).
Total RNA was extracted from 5 μm FFPE liver sections (five sections for each sample) using the RNeasy FFPE Kit (Qiagen, Hilden, Germany) and then stored at −80°C in 66 available samples. Reverse transcription was performed with the Archive High Capacity complementary DNA (cDNA) Synthesis Kit (Applied Biosystems, Foster City, CA). Levels of claudin-1 and occludin were measured with TaqMan Gene Expression Assays (Applied Biosystems). Ribosomal protein L13a (RPL13a) was chosen as an endogenous control for mRNA normalization. Relative quantitation was carried out using the standard curve method. Data are expressed as the fold change relative to an arbitrary calibrator.
To study early HCV kinetics, serum samples were obtained immediately before LT and daily during the first week following LT. Thereafter, samples were collected weekly during the first month and at months 3, 6, and 12. Viral load in serum specimens was determined by real-time PCR (m2000rt, Abbott, with a detection limit of 30 IU/mL), as reported.15 Samples belonging to the same patient were assayed in the same run.
Quantitative variables are expressed as medians (range) and depicted in the figures as boxplots. Differences between qualitative variables were assessed with the Fisher exact test. Differences between quantitative variables were analyzed with a nonparametric test (Mann-Whitney or Kruskal-Wallis for unpaired samples, Wilcoxon for paired samples). Correlations between quantitative variables were expressed by the Pearson coefficient. The software used for statistical analysis was SPSS 16.0 (Chicago IL).
Forty-two HCV-infected patients and 19 HCV-negative controls were included in the study. The baseline characteristics of the patients are summarized in Table 1. Hepatitis C recurrence was mild in 23 individuals and severe in 19. A liver biopsy obtained at time of liver reperfusion and 12 months after LT was available for all 42 patients; a 3-month biopsy was available in 36. For the 19 HCV-negative controls, liver biopsies were available for all individuals at the three timepoints. The indication for LT in the controls was alcoholic cirrhosis (14), hepatitis B (1), primary sclerosing cholangitis (1), NASH (2), and familiar amyloidotic polyneuropathy (1).
|HCV-Infected Patients n=42|
|Controls n=19||HCV-Infected Patients n=42||P||Mild Recurrence n=23||Severe Recurrence n=19||P|
|Age (years)||58 (42-68)||57 (36-69)||0.51||55 (39-66)||62 (36-69)||0.22|
|Sex, males (%)||15 (79)||22 (52)||0.09||13 (57)||9 (47)||0.8|
|Donor age (years)||47 (25-76)||45 (13-77)||0.59||34 (13-76)||53 (18-77)||0.005|
|Ischemia time (min)||458 (275-900)||342 (60-720)||0.007||322 (60-720)||362 (85-485)||0.5|
|1||37 (88)||20 (87)||17 (90)||0.3|
|2/3||3 (7)||2 (9)||1 (5)|
|Unknown||2 (5)||1 (4)||1 (5)|
|Pre-LT viral load (log10IU/mL)||5 (2-6.6)||4.5 (2-6.6)||5.32 (2.9-6.56)||0.47|
|Viral load 1 month (log10IU/mL)||6.4 (4-7.03)||5.9 (4.9-7.8)||6.8 (4.7-9)||0.03|
|Viral load 3 month (log10IU/mL)||6.6 (4.7-8.5)||6.2 (4.7-7.9)||6.7 (5.8-8.5)||0.02|
|Viral load 12 month (log10IU/mL)*||6.3 (3.9-7.5)||6.3 (4-7)||6.3 (3.9-7.5)||0.32†|
|Type of transplantation DDLT vs. LDLT||15 (79)/4 (21)||37 (88)/5 (12)||0.44||21 (91)/2 (9)||16 (84)/ 3 (16)||0.6|
|CyA||6 (32)||23 (55)||0.4||13 (57)||10 (53)||1|
|FK||13 (68)||19 (45)||10 (43)||9 (47)|
|Acute rejection||8 (42)||11 (26)||0.24||7 (30)||4 (21)||0.49|
|Corticosteroid bolus||3 (16)||7 (17)||1||5 (28)||2 (11)||0.33|
|CMV disease‡||0||5 (12)||0.14||2 (9)||3 (16)||0.48|
|Biliary complications||3 (16)||14 (33)||0.22||7 (30)||7 (37)||0.75|
|Fibrosis stage (1y)|
|F0||16 (38)||16 (70)||—|
|F1||7 (17)||7 (30)||—|
|F3||13 (31)||—||13 (68)|
|F4||5 (12)||—||5 (26)|
|HVPG (1 y)§|
|<6 mmHg||21 (52)||21||—|
|≥10 mmHg||19 (48)||—||19|
Quantitative Expression of HCV Receptors: Reproducibility.
Twenty random liver biopsies were stained for claudin-1 and SR-B1 in three independent experiments using slices from the same biopsy. For claudin-1 the correlation coefficients between the sum of intensities obtained in the three independent experiments ranged from 0.72 to 0.75 (P < 0.01 in all cases). For SR-B1 the comparable values ranged from 0.89 to 0.91 (P < 0.01 in all cases). These data support the excellent reproducibility of receptor quantification using our methodology.
Localization of HCV Receptors Claudin-1 and Occludin in Hepatocytes.
Immunostaining of claudin-1 and occludin in liver biopsies demonstrated the expression of both tight junction proteins in the apical membrane of the hepatocytes, whereas SR-B1 was expressed in the sinusoidal pole of liver cells (Fig. 1). To confirm that expression of claudin-1 and occludin was restricted to the apical pole of hepatocytes, we performed a triple staining, including CD10 in 20 representative samples. CD10, also known as common ALLantigen (CALLA) is a cell membrane metallopeptidase that is expressed in the canaliculi of normal or neoplastic liver. As shown in Fig. 2A, claudin-1, occludin, and CD10 were localized in the apical pole of hepatocytes. We were unable to detect significant amounts of claudin-1 and occludin in the basolateral/sinusoidal membrane of liver cells in any of the studied samples. High-resolution 3D images were obtained in the 20 selected liver biopsies. As shown in Fig. 2B, CD10 was localized within the biliary canalicula, which is most likely explained by its distribution along the surface of the microvilli of the liver cells.17 Claudin-1 and occludin distribution followed the apical membrane of adjacent hepatocytes, corresponding to proteins localized in tight junctions.
We also used the high-resolution images to study the colocalization pattern of the two HCV receptors. Our results indicate that overall, claudin-1 and occludin colocalized strongly in all studied samples: 60% to 94% of claudin-1 volume colocalized with occludin. The coefficients of correlation between colocalized voxels, however, varied significantly from sample to sample and ranged from 0.20 to 0.86 and correlated strongly with the amount of expressed claudin-1 (r = 0.8, P < 0.001).
Viral Kinetics and Receptor Expression.
We wished to determine if SR-B1 and tight junction proteins claudin-1 and occludin (which most likely represent the final step in HCV entry into hepatocytes) influenced early HCV kinetics. For this purpose, we divided early viral kinetics into two different components: (1) the initial viral load decay, which occurs during the first 24 hours following graft reperfusion and (2) the viral load increase the first week following LT (Fig. 3A).18 The first viral load decline may represent massive viral uptake by the liver, whereas the viral load increase during the first week indicates HCV replication in the newly infected liver. There was a significant correlation between the viral load decay and the levels of SR-B1 in the graft at the time of reperfusion (r = 0.55, P = 0.007) (Fig. 3B). Interestingly, there was a significant relationship between the levels of occludin and claudin-1 in the graft at the time of reperfusion and the slope of HCV-RNA increase during the first week after LT (r = 0.63; P = 0.005) (Fig. 3C), suggesting a potential role of these receptors in regulating early HCV kinetics.
Changes of Claudin-1 and Occludin Expression Levels and Pattern Following HCV Infection.
We analyzed if the expression pattern of these proteins changed following HCV infection after LT. For this purpose we compared the patterns of claudin-1 and occludin expression in liver samples obtained during graft reperfusion (before HCV replication starts in the liver) and at 3 and 12 months after LT. Localization of claudin-1 and occludin was limited to the apical pole of the hepatocyte membrane at all timepoints, independently of the severity of hepatitis C recurrence (Fig. 4A,B). Reconstruction of 3D images in xz sections supported the absence of significant amounts of these proteins in the basolateral/sinusoidal membrane of the hepatocytes. Moreover, we did not observe cytoplasmic retention of claudin-1 or occludin after HCV infection, as described in vitro.19 We observed a significant increase in the levels of occludin and claudin-1 1 year after LT (P = 0.03 and P = 0.007, respectively), both in patients with mild and severe disease recurrence (Supporting Table 1 and Supporting Fig. 1). In the HCV-negative controls the levels of claudin-1 and occludin remained unchanged after transplantation (Supporting Fig. 1).
The amount of claudin-1 was higher in liver samples from patients with severe hepatitis C recurrence (particularly 12 months after LT) compared to those with mild recurrence, but the differences did not reach statistical significance (Supporting Table 1). Interestingly, in the subgroup of patients with severe cholestatic hepatitis (n = 12) the amount of claudin-1 12 months after LT was significantly higher compared to the remaining patients (P = 0.005) (Fig. 5). Claudin-1 levels did not correlate with any of the biochemical markers of cholestasis (gamma glutamyl transpeptidase, alkaline phosphatase, bilirubin) or HCV-RNA concentration obtained at the same timepoints.
With regard to mRNA quantification, we found no correlation between mRNA and protein abundance levels (as quantified by confocal microscopy) either for claudin-1 (r = 0.2, not significant [ns]) or for occludin (r = 0.1, ns). Indeed, claudin-1 mRNA levels remained stable over time in HCV-infected patients; occludin mRNA levels increased, although the difference did not reach statistical significance (Supporting Fig. 1). Similarly, we did not detect significant differences in the mRNA levels of these two proteins in individuals with mild or severe disease recurrence.
HCV entry is a complex process involving several receptors. It is believed that HCV particles are consecutively bound by a complex formed by SR-B1 and CD81. Virus associated with CD81 would then be transferred into tight junctions, where HCV would interact with claudin-1 and occludin to enter the cell by clathrin-dependent endocytosis.4-10 Another hypothesis suggests that internalization of HCV is not limited to tight junctions and that the virus might use claudin-CD81 complexes in the basolateral surface of hepatocytes to enter into the cell.20 Tight junctions are multiprotein complexes that seal the space between adjacent cells. In fact, hepatocyte plasma membranes are separated by tight junctions into sinusoidal-basolateral and apical domains.21 These two domains are very important for hepatocytes to perform diverse functions, such as canalicular bile secretion and simultaneous sinusoidal secretion of serum proteins into blood.
Because the tight-junction proteins claudin-1 and occludin are thought to be a relevant part of HCV entry into hepatocytes, our goal was to characterize (1) the expression pattern of these proteins in liver tissue of patients undergoing LT, (2) their influence on early HCV kinetics following recurrent hepatitis C and their potential changes following HCV infection of the graft.
We used the LT model for several reasons: (1) tissue samples can be obtained before and after HCV infection; (2) infection can be monitored from the beginning and thus, it is possible to obtain data on early HCV kinetics; (3) hepatitis C recurrence after LT is a rapidly progressive disease and patients with mild or very severe hepatitis recurrence can be well characterized; (4) finally, expression of HCV receptors has not been characterized in this model so far.
Several reports based on in vitro experiments have suggested major changes in the expression of these proteins after HCV infection of liver cells. Using the replicon system Benedicto et al.13 explored the effect of HCV on tight junction organization, demonstrating that in Huh7 cells containing a genomic replicon, occludin and claudin-1 accumulated in the cytoplasm of the cells as dot-like structures (and were not detected in the tight junction). Colocalization studies suggested that the envelope protein E2 could play a role in the mislocalization of tight junction-associated proteins. Our results show that, in vivo, HCV infection is not associated with retention of claudin-1 and occludin in the cytoplasm of hepatocytes. We found that claudin-1 and occludin remained in the apical pole of hepatocytes even in cases with severe cholestatic hepatitis. In the latter cases, the only structural change observed was a slight dilation of the biliary canaliculi. The absence of mislocalized claudin-1 and occludin was verified by using additional antibodies directed to distinct protein epitopes (data not shown). A potential limitation of our findings is the possibility that only a small proportion of hepatocytes are infected with HCV and, thus, that morphological changes are restricted to areas of infected cells.22 Nevertheless, we analyzed a large number of liver cells per biopsy (>3,000). Moreover, changes in tight junction proteins affecting a very small proportion of hepatocytes would not explain the significant clinical expression (cholestasis) found in hepatitis C recurrence. Because tight junctions are multiprotein complexes highly regulated by cytokines and interleukins,23, 24 we cannot exclude that alterations in permeability or function may be explained by changes in protein composition during a strong inflammatory event such as hepatitis C.
Despite the absence of structural changes in the tight junctions, we observed an increased expression of claudin-1 and occludin over time in HCV-infected patients. The increase in claudin-1 was particularly significant in individuals with cholestatic hepatitis. Enhanced apical expression of claudin-1 and occludin after HCV infection could represent a mechanism favoring cell-to cell transmission of HCV within the liver.7 We did not find a correlation between claudin-1 and occludin mRNA and protein levels, although the association between levels of RNA and protein products can vary greatly.25, 26 What our results may indicate is that HCV proteins influence claudin-1 and occludin expression either by affecting them at a posttranscriptional level or by altering the complex membrane traffic of tight-junction proteins. These proteins are recycled by way of various mechanisms such as by clathrin-mediated and caveolae-mediated endocytosis, as well as by macropinocytosis; these pathways may be altered by HCV infection, particularly in cases with high HCV replication. Our data, however, are based on a small number of samples and, more important, do not allow for a functional analysis of tight junctions. Thus, we must be cautious with our conclusions.
Up to a certain extent, our findings are in agreement with Reynolds et al.,27 who reported a significant increase in claudin-1 expression after infecting Huh7 cells with HCVcc. The latter was also observed in tissue from HCV-infected patients as compared to samples from uninfected livers, with focal regions of basolaterally expressed claudin-1. The increase in both HCV receptors found in our study was not attributable, however, to the presence of of claudin-1 or occludin in the basolateral/sinusoidal membrane, but rather to an increased presence of these proteins in the apical membrane of hepatocytes. We showed that claudin-1 and occludin localization followed a similar pattern to that of CD10 and confirmed the findings in high resolution images. The discrepancies between our results and those by Reynolds et al. may be explained by the different methodology (we used imaging software that allowed precise and reproducible quantification of these proteins) and the different patient population (they used livers from patients with end-stage cirrhosis).
We studied early HCV kinetics by assessing daily HCV-RNA concentrations in a subgroup of patients. Because SR-B1 may be the first putative HCV receptor which contacts the virus, we explored if its levels of expression at the time of LT influenced the initial viral decay immediately following graft reperfusion. In vitro, SR-B1 surface expression has been reported to affect HCV infection: SR-B1 overexpression enhances HCV internalization whereas SR-B1 silencing reduces infectivity of cell culture-produced HCV (HCVcc) and HCVpp.28-30 We found a significant correlation between the levels of expression of SR-B1 in the graft (at the time of LT) and the magnitude of the viral decrease (during the first 24 hours following transplantation). This supports a massive uptake of HCV by the liver immediately after graft reperfusion. It is obvious that other variables may play a role in early viral decay, such as the amount of blood loss or transfusion requirements during the surgical procedure.18 We were particularly interested in exploring the potential effect of claudin-1 and occludin expression in early HCV kinetics after graft reperfusion. We observed that the viral load increase slope during the first 7 days following graft reperfusion was significantly greater in the patients with high claudin-1 and occludin levels, showing a significant correlation between their expression in the graft and the slope of viral increase. Timpe et al.31 recently suggested that HCV can be transmitted directly between cells, most likely using the HCV receptors found in tight junctions. Data obtained after visualization of HCV antigens in human livers by two-photon microscopy indicate that infected hepatocytes are found in clusters, further supporting the spread of HCV from cell to cell.22 Our results would also suggest a role of cell-to-cell transmission during the first days following graft infection: the presence of high levels of claudin-1 and occludin might facilitate HCV spread within the liver, resulting in a faster increase in HCV-RNA concentrations. It is clear that other variables not analyzed in this study (such as HCV fitness, quasispecies evolution) may also play a role in early HCV kinetics.
Our study has some limitations. First, the study is retrospective in its design and preservation of liver samples may not have been completely homogeneous across the study period. Second, liver tissue obtained before HCV infection (reperfusion liver biopsies) cannot be considered normal, because samples are obtained from the liver of a deceased donor after treatment of the organ with a preservation solution. Finally, patients undergoing LT are treated with immunosuppression drugs, which may influence the expression of HCV receptors.
In summary, hepatitis C recurrence after LT is associated with increased levels of claudin-1 and occludin in hepatocyte membranes, although this does not alter their localization or expression pattern within the tight junctions. HCV receptor levels at the time of LT seem to modulate early HCV kinetics, which may be relevant when designing strategies to prevent HCV infection in the graft.
Additional supporting information may be found in the online version of this article.
|HEP_24110_sm_suppinfofig.tif||123K||Supporting Figure 1. Sum of intensities (SI) and mRNA (as determined by real-time PCR) box plots for claudin-1 and occludin over time. In HCV-infected patients (A,D), there was a significant increase in the levels of claudin-1 and occludin 12 months after LT (p= 0.007 and p= 0.03, respectively). In HCV-negative patients, the levels of both proteins remained unchanged over time (B,E). In contrast, mRNA claudin-1 levels remained unchanged (C); occludin mRNA levels increased overtime; however, the difference was not statistically significant (F).|
|HEP_24110_sm_suppinfotable.doc||27K||Supporting Table 1. Quantification of the levels of claudin-1 and occludin over time. The numbers represent the sum of intensities of claudin-1 and occludin at each time point. Values are depicted as median (ranges) in a logarithmic scale. Pre-LT: biopsies obtained at time of graft reperfusion; 3M: 3 months after liver transplantation; 12M: 12 months after liver transplantation.|
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