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Journal of Virology, March 2008, p. 3154-3160, Vol. 82, No. 6
0022-538X/08/$08.00+0     doi:10.1128/JVI.02474-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

High Level of PD-1 Expression on Hepatitis C Virus (HCV)-Specific CD8⁺ and CD4⁺ T Cells during Acute HCV Infection, Irrespective of Clinical Outcome

Victoria Kasprowicz,¹ Julian Schulze zur Wiesch,^1,2,10 Thomas Kuntzen,¹ Brian E. Nolan,¹ Steven Longworth,¹ Andrew Berical,¹ Jenna Blum,¹ Cory McMahon,¹ Laura L. Reyor,¹ Nahel Elias,^1,3,10 William W. Kwok,⁴ Barbara G. McGovern,⁵ Gordon Freeman,⁶ Raymond T. Chung,^7,10 Paul Klenerman,⁸ Lia Lewis-Ximenez,⁹ Bruce D. Walker,^1,10 Todd M. Allen,¹ Arthur Y. Kim,¹ and Georg M. Lauer^1*

Partners AIDS Research Center, Massachusetts General Hospital, Harvard Medical School, 149 13th Street, 6th Floor, Room 6001, Charlestown, Massachusetts 02129-2000,¹ Med. Klinik I, Universität Hamburg, Germany,² Transplantation Unit, Department of Surgery, Massachusetts General Hospital, Boston, Massachusetts 02114,³ Benaroya Research Institute at Virginia Mason, Seattle, Washington,⁴ Lemuel Shattuck Hospital and Tufts University School of Medicine, Boston, Massachusetts,⁵ Department of Medical Oncology, Dana-Farber Cancer Institute, Department of Medicine, Harvard Medical School, Boston, Massachusetts 02115,⁶ Gastrointestinal Unit, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts 02114,⁷ Nuffield Department of Medicine, Peter Medawar Building for Pathogen Research, University of Oxford, South Parks Road, Oxford OX1 3SY, United Kingdom,⁸ Departmento de Virologia, Instituto Oswaldo Cruz/Fiocruz, Rio de Janeiro, Brazil,⁹ Howard Hughes Medical Institute, Chevy Chase, Maryland,¹⁰

Received 16 November 2007/ Accepted 13 December 2007

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We monitored expression of PD-1 (a mediator of T-cell exhaustion and viral persistence) on hepatitis C virus (HCV)-specific CD8⁺ and CD4⁺ T cells from blood and liver during acute and chronic infections and after the resolved infection stage. PD-1 expression on HCV-specific T cells was high early in acute infection irrespective of clinical outcome, and most cells continued to express PD-1 in resolved and chronic stages of infection; intrahepatic expression levels were especially high. Our results suggest that an analysis of PD-1 expression alone is not sufficient to predict infection outcome or to determine T-cell functionality in HCV infection.

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The up-regulation of the inhibitory receptor programmed death 1 (PD-1; a member of the CD28 costimulatory family) on exhausted T cells has recently been described as a critical determinant of disease outcome in the mouse model of chronic lymphocytic choriomeningitis virus (LCMV) infection (1). For humans, PD-1 has been suggested as a marker of T-cell exhaustion in human immunodeficiency virus type 1 (HIV-1) infection and expression levels have been correlated with disease progression (2, 11, 13). As acute hepatitis C virus infection has two distinct natural outcomes, spontaneous resolution and viral persistence, it is a uniquely pertinent model to define how closely the findings of the role of PD-1 in mice translate into its role in human disease (6).

We performed a comprehensive assessment of PD-1 expression levels on hepatitis C virus (HCV)-specific CD4⁺ and CD8⁺ T-cell populations in 16 subjects with acute infections (9 with chronically evolving infections and 7 with self-limiting courses) and in 21 subjects with either chronic infections for more than 2 years or resolved infections. Clinical and virological data for these subjects are listed in Table 1. In addition, we studied intrahepatic lymphocytes from 35 individuals with persistent viremia. Subjects were recruited in Boston (Gastrointestinal Unit, Massachusetts General Hospital, Boston, MA; Lemuel Shattuck Hospital, Jamaica Plain, MA) and Brazil (Departmento de Virologia, Instituto Oswaldo Cruz/Fiocruz, Rio de Janeiro, Brazil). Informed written consent was obtained from each patient, and the study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki, as reflected in a priori approval from the local institutional review boards. We used 13 HCV-specific class I HLA multimeric complexes restricted by a wide range of alleles (HLA-A2, A1, B35, B7, B57, and A24) and 6 HCV class II HLA multimeric complexes, including novel DR1101-, DR0701-, and DR0404-restricted tetramers (Table 2). HLA multimeric complexes were obtained from ProImmune (Oxford, United Kingdom), Beckman Coulter (Fullerton, CA), and from William W. Kwok (Benaroya Research Institute at Virginia Mason, Seattle, WA).

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TABLE 1. Patient clinical informationa

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TABLE 2. HLA multimeric complex information

PD-1 expression levels on HCV-specific CD8⁺ and CD4⁺ T cells do not predict clinical outcomes and remain high in both resolved and chronic infections. We observed high levels of PD-1 expression (60 to 100%) on all HCV-specific CD8⁺ and CD4⁺ T cells during the early phase of acute infection, regardless of clinical outcome or viral load (Fig. 1A and B and data not shown). By analyzing on the basis of percent positive PD-1 expression, we observed a decline in the proportion of positive cells after the acute phase of infection (more than 6 months) that reached significance in individuals with spontaneous resolution of infection (P = 0.0313 [Mann-Whitney U test]) but not in subjects with chronically evolving courses (data not shown). However, the majority of HCV-specific T cells remained PD-1 positive and, in contrast to results from the LCMV model, we observed no normalization of PD-1 levels (i.e., relative to bulk T-cell expression levels).

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FIG. 1. High PD-1 expression levels in the acute and chronic phases of HCV infection. (A) PD-1 expression levels on HCV-specific CD8+ T-cell populations during the acute stage of HCV infection. For clarity, only data from the first available time point are displayed on the graph. Open symbols indicate individuals with chronically evolving acute infections, while filled symbols indicate individuals with self-limiting infections. (B) PD-1 expression levels on HCV-specific CD4+ T-cell populations during the acute stage of infection. The graph shows data for six HCV-specific CD4+ T-cell responses from three individuals with chronically evolving HCV infections and four HCV-specific CD4+ T-cell responses from three individuals with self-limiting courses of infection. (C) High PD-1 expression levels can be seen on HCV-specific cells from both long-term chronically infected individuals and individuals with documented resolved infections. The left panel shows data as analyzed by percent positive PD-1 expression, and the right panel represents the data as analyzed by MFI. (D) PD-1 expression levels on HCV-specific CD4+ T cells from individuals with resolved infections. (E) Increased proliferation of HCV-specific CD8+ T cells in the presence of anti PD-L1. Individual 01-49 targets two epitopes (NS3 1406-1415 and NS5A 1987-1996). The addition of anti-PDL1 alone resulted in a 14-fold increase in NS3 1406-1415-specific T-cell proliferation and a 60-fold increase in NS5A 1987-1996-specific T-cell proliferation.

In order to establish whether distinct differences in PD-1 expression levels in different disease outcomes would be revealed after a longer period of time after acute infection, we analyzed peripheral blood mononuclear cells (PBMC) from subjects that either had resolved HCV infections or had displayed persistent viremia for many years (at least 2 years). An analysis of HCV-specific CD4⁺ T cells was limited to resolved infection due to the lack of detectable virus-specific CD4⁺ T cells in the chronic phase (3, 7). We observed elevated PD-1 expression (by both percent positive expression and mean fluorescent intensity [MFI]) on almost all HCV-specific cells relative to the bulk T-cell population, irrespective of the clinical status of the patient (Fig. 1c and d). We conclude that PD-1 expression levels on HCV-specific T cells do not predict the outcome of infection and that continued antigenic stimulation is not required for PD-1 expression. This is clearly different from the original description of PD-1 expression in the LCMV model, where high PD-1 expression during the earliest phase of infection is followed by a rapid decline of PD-1 to normal levels only in mice resolving viremia (1).

Our data are also partially different from the results of a previous HCV study by Urbani et al. that reported lower levels of PD-1 expression in subjects with HCV resolved infections (12). Factors that contributed to this discrepancy may be the use of a more limited number of subjects in the original study than in this one and/or the use of a different PD-1 antibody. The study by Urbani et al. also found significant differences in PD-1 expression on bulk CD8⁺ T cells during the early phase of infection, whereas we observed similar bulk CD8⁺ (and CD4⁺) T-cell expression levels regardless of the stage of infection (data not shown) (12). While these bulk data contrast with what has been observed for HIV infection (2, 9, 11, 13), we find our observation unsurprising as, typically, the cumulative HCV-specific T-cell response is of a magnitude 10-fold lower than that targeting HIV and, thus, is less likely to significantly influence the bulk population (5).

While we could not determine a clear distinction in PD-1 expression between T cells associated with controlled or uncontrolled infection, we do have data supporting that the PD-1 pathway is relevant in HCV-specific T cells. Blocking the PD-1/PD-L1 inhibitory pathway by an anti-PD-L1-specific antibody results in an increased HCV-specific T-cell proliferation (4, 8, 12). However, we demonstrated this increase even in individuals with resolved infections. For example, in subject 01-49 with a resolved infection, the addition of anti-PDL1 resulted in significant increases of both the NS3 1406-1415-specific and the NS5A 1987-1996-specific T-cell proliferation (Fig. 1e). Interestingly, baseline PD-1 expression levels on these T-cell populations were strikingly different (19% and 76%, respectively) (data not shown). These results highlight the powerful effect of the PD-1 pathway blockade of T-cell proliferation, but they also indicate the additional relevance of this regulatory pathway in the homeostatic control of effective T-cell responses.

PD-1 expression on individual T-cell populations responds to changes in viral load in the acute phase of infection. With almost all HCV-specific T cells expressing PD-1, we could not detect an association between PD-1 expression levels and viral load (cross-sectionally) in our cohort (data not shown). However, when characterizing individual HCV-specific T-cell responses, we saw a close association between viral load and the MFI of PD-1 expression on both CD4⁺ and CD8⁺ T-cell responses (Fig. 2). This effect was observed for 10 different CD4⁺ and CD8⁺ T-cell populations in six subjects with acute infections, with rapid declines and increases in viremia during spontaneous- or therapy-induced viral resolution or with viral relapse/reinfection after control of viremia. We also observed that the kinetics of PD-1 expression were similar to those of the established activation marker CD38 (data not shown). A similar link between PD-1 upregulation and CD38 can be seen in chronic HIV infection (10), indicating that while the general induction of PD-1 expression is not dependent on the continuous presence of viral antigen, there is an additional component of PD-1 expression that responds to antigen load and resembles a marker of T-cell activation.

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FIG. 2. PD-1 expression on individual T-cell populations is partially associated with viral load in the acute phase of infection. Longitudinal PD-1 expression levels on the DR0401-124 CD4+ T-cell population from individual 05H (left panel). The clinical course of the acute infection is displayed in the right panel. Subject 05H cleared acute HCV infection following the administration of antiviral therapy. Two HCV-specific T-cell populations were tracked in this individual: NS5B 2841-2849 CD8+ T cells and DR0401-124 CD4+ T cells.

Increased PD-1 expression levels on liver-infiltrating lymphocytes. In order to investigate whether PD-1 expression on T cells from the liver, the primary site of HCV infection, was altered in comparison to that on PBMC, we obtained biopsy and explant tissue from 35 chronically infected individuals (with matched blood samples available from 24 of these individuals). Bulk PD-1 expression levels found on liver-infiltrating CD8⁺ and CD4⁺ lymphocytes were significantly higher than peripheral blood levels (P values were <0.0001 in both cases) (Fig. 3a and b). The mean PD-1 expression level on bulk CD8⁺ liver-residing lymphocytes was 71%, whereas the mean expression on CD8⁺ T cells in blood from the same subjects was 33% (Fig. 3a). For bulk liver-derived CD4⁺ T lymphocytes, the mean PD-1 expression level was 53%, while the value for cells in the periphery was only 25% (Fig. 3b). A significantly lower level of expression of PD-1 on the CD4⁺ T-cell population compared to the level on the CD8⁺ T-cell population was observed in the liver (P = <0.0001 [Mann-Whitney U test]) but not the periphery (P = 0.1313 [Mann-Whitney U test]).

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FIG. 3. PD-1 expression levels on bulk and liver-infiltrating CD8+ and CD4+ T cells. PD-1 expression levels on peripheral and liver-infiltrating CD8+ (A) and CD4+ (B) T cells from chronically infected individuals. Significantly higher levels were found on liver-infiltrating CD8+ and CD4+ lymphocytes compared to peripheral blood levels (P < 0.0001 and P < 0.0001, respectively). (C) PD-1 expression levels on 10 HCV-specific CD8+ liver-infiltrating T-cell populations from seven chronically infected individuals. Also shown are the PD-1 expression levels on two HBV-specific CD8+ T cell populations from one chronically HBV-infected individual and one Epstein-Barr virus-specific population. (D) PD-1 expression levels are displayed for liver, lymph node, and PBMC samples from individual 00-26. PD-1 expression levels were increased in liver compared to that in periphery on both the HCV-specific population and bulk CD8+ T-cell population.

To examine PD-1 levels on antigen-specific CD8⁺ T-cell populations, we assessed expression on 10 liver-infiltrating, HCV-specific populations, 2 HBV-specific populations, and 1 Epstein-Barr virus-specific population. Figure 3c shows high PD-1 expression levels on all the liver-infiltrating, virus-specific, CD8⁺ T-cell populations (93 to 100%). Figure 3d displays increased PD-1 expression levels in liver-residing T cells from individual 00-26 compared to the levels in both a perihepatic lymph node and PBMC for the NS3 1406-1415-specific HCV-specific population and bulk CD8⁺ T-cell population. Clearly, tissue-dependent differences exist in PD-1 expression (9a) and their significance needs to be further evaluated.

In conclusion, we demonstrate the pervasive and long-term induction of PD-1 expression on the vast majority of HCV-specific CD8⁺ and CD4⁺ T cells, irrespective of infection outcome. More subtle effects of viral load fluctuation on individual PD-1 MFI accompany the universal induction of PD-1 expression, suggesting that PD-1 is also partially an activation marker. Finally, PD-1 expression is also dependent on the tissue environment where the T cells exercise their antiviral functions. Our data highlight the fact that the expression pattern of PD-1 in human infection differs from what has been described for the LCMV model and that analysis of PD-1 expression on its own is insufficient to explain different clinical outcomes and distinct T-cell functionality for the HCV model. Additional studies on the expression patterns of different splice variants of PD-1 and its ligands, and receptor-ligand interactions in infected tissue, will be important benchmarks for providing a more comprehensive assessment of the level of inhibitory signaling and its impact on the outcome of human infection.

 
We thank the patients who participated in this study. We also thank John Wherry and David Kaufmann for critical reading of the manuscript.

This study was supported by the Deutsche Forschungsgemeinschaft grant KU-2250/1-1 (T.K.), National Institutes of Health grants U19-AI066345-02 (T.M.A., G.L., L.L.-X., and B.D.W.), RO1-AI067926-01 (T.M.A.), RO1-AI031563-13 (B.D.W.), and K23-AI054379-05 (A.Y.K.); the Wellcome Trust (P.K.); the American Liver Foundation (G.M.L.); the Howard Hughes Medical Institute (B.D.W. and J.S.Z.W.); and the Forschungsförderungsfond Medizin, Universitätsklinikum Hamburg Eppendorf (J.S.Z.W.), the Werner Otto Stiftung (J.S.Z.W.), and the MSD Stipendium HIV (J.S.Z.W.).

 
* Corresponding author. Mailing address: Partners AIDS Research Center, Massachusetts General Hospital, Harvard Medical School, 149 13th Street, Charlestown, MA 02129. Phone: (617) 724-7515. Fax: (617) 726-5411. E-mail: glauer{at}partners.org

Published ahead of print on 26 December 2007.

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1
1. Barber, D. L., E. J. Wherry, D. Masopust, B. Zhu, J. P. Allison, A. H. Sharpe, G. J. Freeman, and R. Ahmed. 2006. Restoring function in exhausted CD8 T cells during chronic viral infection. Nature 439:682-687.[CrossRef][Medline]
2
2. Day, C. L., D. E. Kaufmann, P. Kiepiela, J. A. Brown, E. S. Moodley, S. Reddy, E. W. Mackey, J. D. Miller, A. J. Leslie, C. Depierres, Z. Mncube, J. Duraiswamy, B. Zhu, Q. Eichbaum, M. Altfeld, E. J. Wherry, H. M. Coovadia, P. J. Goulder, P. Klenerman, R. Ahmed, G. J. Freeman, and B. D. Walker. 2006. PD-1 expression on HIV-specific T cells is associated with T-cell exhaustion and disease progression. Nature 443:350-354.[CrossRef][Medline]
3
3. Day, C. L., N. P. Seth, M. Lucas, H. Appel, L. Gauthier, G. M. Lauer, G. K. Robbins, Z. M. Szczepiorkowski, D. R. Casson, R. T. Chung, S. Bell, G. Harcourt, B. D. Walker, P. Klenerman, and K. W. Wucherpfennig. 2003. Ex vivo analysis of human memory CD4 T cells specific for hepatitis C virus using MHC class II tetramers. J. Clin. Investig. 112:831-842.[CrossRef][Medline]
4
4. Golden-Mason, L., B. Palmer, J. Klarquist, J. A. Mengshol, N. Castelblanco, and H. R. Rosen. 2007. Upregulation of PD-1 expression on circulating and intrahepatic hepatitis C virus-specific CD8⁺ T cells associated with reversible immune dysfunction. J. Virol. 81:9249-9258.[Abstract/Free Full Text]
5
5. Kim, A. Y., G. M. Lauer, K. Ouchi, M. M. Addo, M. Lucas, J. Schulze Zur Wiesch, J. Timm, M. Boczanowski, J. E. Duncan, A. G. Wurcel, D. Casson, R. T. Chung, R. Draenert, P. Klenerman, and B. D. Walker. 2005. The magnitude and breadth of hepatitis C virus-specific CD8+ T cells depend on absolute CD4+ T-cell count in individuals coinfected with HIV-1. Blood 105:1170-1178.[Abstract/Free Full Text]
6
6. Lauer, G. M., and B. D. Walker. 2001. Hepatitis C virus infection. N. Engl. J. Med. 345:41-52.[Free Full Text]
7
7. Lucas, M., A. Ulsenheimer, K. Pfafferot, M. H. Heeg, S. Gaudieri, N. Gruner, A. Rauch, J. T. Gerlach, M. C. Jung, R. Zachoval, G. R. Pape, W. Schraut, T. Santantonio, H. Nitschko, M. Obermeier, R. Phillips, T. J. Scriba, N. Semmo, C. Day, J. N. Weber, S. Fidler, R. Thimme, A. Haberstroh, T. F. Baumert, P. Klenerman, and H. M. Diepolder. 2007. Tracking virus-specific CD4+ T cells during and after acute hepatitis C virus infection. PLoS One 2:e649.[CrossRef]
8
8. Penna, A., M. Pilli, A. Zerbini, A. Orlandini, S. Mezzadri, L. Sacchelli, G. Missale, and C. Ferrari. 2007. Dysfunction and functional restoration of HCV-specific CD8 responses in chronic hepatitis C virus infection. Hepatology 45:588-601.[CrossRef][Medline]
9
9. Petrovas, C., J. P. Casazza, J. M. Brenchley, D. A. Price, E. Gostick, W. C. Adams, M. L. Precopio, T. Schacker, M. Roederer, D. C. Douek, and R. A. Koup. 2006. PD-1 is a regulator of virus-specific CD8+ T cell survival in HIV infection. J. Exp. Med. 203:2281-2292.[Abstract/Free Full Text]
9
10. Radziewicz, H., C. C. Ibegbu, M. L. Fernandez, K. A. Workowski, K. Obideen, M. Wehbi, H. L. Hanson, J. P. Steinberg, D. Masopust, E. J. Wherry, J. D. Altman, B. T. Rouse, G. J. Freeman, R. Ahmed, and A. Grakoui. 2007. Liver-infiltrating lymphocytes in chronic human hepatitis C virus infection display and exhausted phenotype with high levels of PD-1 and low levels of CD127 expression. J. Virol. 81:2545-2553.[Abstract/Free Full Text]
10
11. Sauce, D., J. R. Almeida, M. Larsen, L. Haro, B. Autran, G. J. Freeman, and V. Appay. 2007. PD-1 expression on human CD8 T cells depends on both state of differentiation and activation status. AIDS 21:2005-2013.[Medline]
11
12. Trautmann, L., L. Janbazian, N. Chomont, E. A. Said, G. Wang, S. Gimmig, B. Bessette, M. R. Boulassel, E. Delwart, H. Sepulveda, R. S. Balderas, J. P. Routy, E. K. Haddad, and R. P. Sekaly. 2006. Upregulation of PD-1 expression on HIV-specific CD8+ T cells leads to reversible immune dysfunction. Nat. Med. 12:1198-1202.[CrossRef][Medline]
12
13. Urbani, S., B. Amadei, D. Tola, M. Massari, S. Schivazappa, G. Missale, and C. Ferrari. 2006. PD-1 expression in acute hepatitis C virus (HCV) infection is associated with HCV-specific CD8 exhaustion. J. Virol. 80:11398-11403.[Abstract/Free Full Text]
13
14. Zhang, J. Y., Z. Zhang, X. Wang, J. L. Fu, J. Yao, Y. Jiao, L. Chen, H. Zhang, J. Wei, L. Jin, M. Shi, G. F. Gao, H. Wu, and F. S. Wang. 2007. PD-1 upregulation is correlated with HIV-specific memory CD8+ T-cell exhaustion in typical progressors, but not in long-term non-progressors. Blood 109:4671-4678.[Abstract/Free Full Text]

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Journal of Virology, March 2008, p. 3154-3160, Vol. 82, No. 6
0022-538X/08/$08.00+0     doi:10.1128/JVI.02474-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kasprowicz, V. Articles by Lauer, G. M. Search for Related Content PubMed Articles by Kasprowicz, V. Articles by Lauer, G. M.