Cytotoxic T lymphocyte response to a wild type hepatitis B virus epitope in patients chronically infected by variant viruses carrying substitutions within the epitope

Intrahepatic CD8 T-lymphocytes and HBV core expression in relation to response to antiviral therapy for chronic hepatitis B patients

Recognition of HBV-infected hepatocytes by CD8 T-lymphocytes is important for viral clearance. Expression of hepatitis B core antigen (HBcAg) in HBV-infected hepatocytes can trigger this antiviral T-cell response. The intrahepatic CD8 T-lymphocytes and HBcAg expression were investigated in relation to response to antiviral therapy. Forty chronic HBeAg-positive patients treated with either lamivudine (n = 20) or interferon-alpha (n = 20) were investigated. Ten patients from each treatment group exhibited a response. Liver biopsies were carried out before and after therapy. CD8 T-lymphocytes and HBcAg expression were detected by immunohistochemistry. The number of pretreatment intrahepatic CD8 T-lymphocytes was significantly higher in responders (P = 0.008). In responders baseline nuclear HBcAg expression tended to be lower (P = 0.09). Cytoplasmic expression was not significantly different between responders and non-responders (P = 0.46). The number of CD8 T-lymphocytes correlated with cytoplasmic HBcAg (r(s) = 0.31; P = 0.04); CD8 T-lymphocytes were situated in clusters of hepatocytes with cytoplasmic HBcAg. Longitudinal analysis showed a significant reduction of CD8 T-lymphocytes after treatment in responders (P < 0.001). Multivariate analysis revealed pretreatment CD8 T-lymphocytes and age as independent prognostic factors for response (n = 40). The number of pretreatment CD8 T-lymphocytes was the only independent prognostic indicator for response to interferon-alpha (P = 0.03); it was of borderline significance for lamivudine therapy (P = 0.06). It is concluded that the number of pretreatment intrahepatic CD8 T-lymphocytes is an important predictor of response to HBV therapy with either interferon-alpha or lamivudine. Response to therapy led to a significant reduction of intrahepatic CD8 T-lymphocytes. Co-localisation of CD8 T-lymphocytes and HBcAg-positive hepatocytes suggests antiviral activity predominantly at the site of maximum HBV replication.

Impaired effector function of hepatitis C virus-specific CD8+ T cells in chronic hepatitis C virus infection

The cellular immune response contributes to clearance of hepatitis C virus (HCV) and persists for decades after recovery from infection. The immunological basis for the inefficiency of the cellular immune response in chronically infected persons is not known. Here, we used four HLA-A2 tetramers, specific for two HCV core and two HCV NS3 epitopes, to investigate at the single-cell level effector function and phenotype of HCV-specific CD8+ T cells in 20 chronically infected and 12 long-term recovered patients. Overall, HCV-specific, tetramer+ T cells were more frequently found in PBMCs of chronically infected patients than in those of recovered patients. However, when compared with HCV-tetramer+ T cells of recovered patients, they displayed an impaired proliferative capacity. As a result of the impaired proliferative capacity, HCV-specific T cell lines derived from chronically infected patients displayed less peptide-specific cytotoxicity than those from recovered patients. In addition, proliferation and ex vivo IFN-gamma production of HCV-tetramer+ cells, but not influenza-virus-specific T cells, were defective in chronically infected patients and could not be restored by in vitro stimulation with peptide and IL-2. At least three distinct phenotypes of HCV-specific CD8+ T cells were identified and associated with certain functional characteristics. In addition, impairment of proliferative, cytokine, and cytotoxic effector functions of tetramer+ T cells in viremic patients was associated with weak ex vivo HCV-specific CD4+ T cell responses. Thus, the defective functions of HCV-specific CD8+ T cells might contribute to viral persistence in chronically infected patients, and knowledge on their reversibility may facilitate the development of immunotherapeutic vaccines.

Hepatitis B treatment: rational combination chemotherapy based on viral kinetic and animal model studies

Hepatitis B virus (HBV) causes a generally non-cytopathic infection in the liver. Even though HBV is a DNA virus, it replicates via reverse transcription which is coordinated within the viral nucleocapsid by the virus-specific polymerase. The major transcriptional template is the viral mimichromosome from which the viral DNA exists as a covalently closed circular (ccc) molcule. The virus infects hepatocytes but can also be found in non-hepatocyte reservoirs such as bile-duct epithelium, mesangial cells of the kidney, pancreatic islet cells and lymphoid cells. When patients infected with HBV are treated with either interferon alpha or lamivudine, responses are variable and unpredictable. Sophisticated mathematical models analysing the dynamics of viral clearance during antiviral therapy have recently been applied to chronic hepatitis B. Typically complex profiles, rather than the usual biphasic responses seen with other diseases have been observed, indicating that antiviral efficacy requires substantila improvement. This may be achieved with combination chemotherapy. However, chronic hepatitis B is a complex and heterogeneous disease entity, and the challenge for the future is to define measurable end-points of treatment and address key virological issues such as the role of cccDNA and extra-hepatocyte replication in treatment failure. Clearly, new therapies and effective combination therapy protocols are urgently required in order to improve the present poor response rates in patients undergoing treatment.

Escaping high viral load exhaustion: CD8 cells with altered tetramer binding in chronic hepatitis B virus infection

Deletion, anergy, and a spectrum of functional impairments can affect virus-specific CD8 cells in chronic viral infections. Here we characterize a low frequency population of CD8 cells present in chronic hepatitis B virus (HBV) infection which survive in the face of a high quantity of viral antigen. Although they do not appear to exert immunological pressure in vivo, these CD8 cells are not classically "tolerant" since they proliferate, lyse, and produce antiviral cytokines in vitro. They are characterized by altered HLA/peptide tetramer reactivity, which is not explained by TCR down-regulation or reduced functional avidity and which can be reversed with repetitive stimulation. CD8 cells with altered tetramer binding appear to have a specificity restricted to envelope antigen and not to other HBV antigens, suggesting that mechanisms of CD8 cell dysfunction are differentially regulated according to the antigenic form and presentation of individual viral antigens.

Escaping high viral load exhaustion: CD8 cells with altered tetramer binding in chronic hepatitis B virus infection

Deletion, anergy, and a spectrum of functional impairments can affect virus-specific CD8 cells in chronic viral infections. Here we characterize a low frequency population of CD8 cells present in chronic hepatitis B virus (HBV) infection which survive in the face of a high quantity of viral antigen. Although they do not appear to exert immunological pressure in vivo, these CD8 cells are not classically "tolerant" since they proliferate, lyse, and produce antiviral cytokines in vitro. They are characterized by altered HLA/peptide tetramer reactivity, which is not explained by TCR down-regulation or reduced functional avidity and which can be reversed with repetitive stimulation. CD8 cells with altered tetramer binding appear to have a specificity restricted to envelope antigen and not to other HBV antigens, suggesting that mechanisms of CD8 cell dysfunction are differentially regulated according to the antigenic form and presentation of individual viral antigens.

Escaping high viral load exhaustion: CD8 cells with altered tetramer binding in chronic hepatitis B virus infection

Deletion, anergy, and a spectrum of functional impairments can affect virus-specific CD8 cells in chronic viral infections. Here we characterize a low frequency population of CD8 cells present in chronic hepatitis B virus (HBV) infection which survive in the face of a high quantity of viral antigen. Although they do not appear to exert immunological pressure in vivo, these CD8 cells are not classically "tolerant" since they proliferate, lyse, and produce antiviral cytokines in vitro. They are characterized by altered HLA/peptide tetramer reactivity, which is not explained by TCR down-regulation or reduced functional avidity and which can be reversed with repetitive stimulation. CD8 cells with altered tetramer binding appear to have a specificity restricted to envelope antigen and not to other HBV antigens, suggesting that mechanisms of CD8 cell dysfunction are differentially regulated according to the antigenic form and presentation of individual viral antigens.

Sequence variation in a newly identified HLA-B35-restricted epitope in the influenza A virus nucleoprotein associated with escape from cytotoxic T lymphocytes

Here, we describe a new HLA-B*3501-restricted cytotoxic T lymphocyte (CTL) epitope in the influenza A virus (H3N2) nucleoprotein, which was found to exhibit a high degree of variation at nonanchor residues. The influenza virus variants emerged in chronological order, and CTLs directed against old variants failed to recognize more recent strains of influenza A virus, indicating an escape from CTL immunity.

Immunology of hepatitis B infection

The immune response initiated by the T-cell response to viral antigens is thought to be fundamental for viral clearance and disease pathogenesis in hepatitis B virus (HBV) infection. The T-cell response during acute self-limited hepatitis B in people is characterised by a vigorous, polyclonal, and multispecific cytotoxic and helper-T-cell response. By contrast, the immune response in chronic carriers, not able to eliminate the virus, is weak or undetectable. Thus a dominant cause of viral persistence could be the existence of a weak antiviral immune response. Methodological progress in animal models allows more precise investigation of the mechanisms by which the immune system resolves viral infection or develops chronic infection. Although clearance of most virus infections is widely thought to indicate the killing of infected cells by virus-specific T cells, data suggest that non-cytolytic intracellular viral inactivation by cytokines released by virus-inactivated lymphomononuclear cells could have an important role in the clearance of this virus without killing the infected cell. Additional factors that could contribute to viral persistence, which have been partly proven in animal models, are viral inhibition of antigen processing or presentation, modulation of the response to cytotoxic mediators, immunological tolerance to viral antigens, viral mutations, and infection of immunologically privileged sites. In view of the central role of cellular immunity in disease pathogenesis, strategies have been proposed to manipulate this cellular immune response in favour of protection from disease.

Quantity and quality of virus-specific CD8 cell response: relevance to the design of a therapeutic vaccine for chronic HBV infection

Chronic hepatitis B virus (HBV) infection is a major cause of morbidity and mortality worldwide, yet currently available therapies fail to provide long-term control of viral replication in most patients. Strategies to boost the weak virus-specific T-cell response typically found in patients with chronic hepatitis B have been proposed as a means of terminating persistent HBV infection. The potential problems arising from the stimulation of virus-specific immunity in a disease caused by a non-cytopathic virus, where viral control and liver injury are mediated by the immune system, are discussed. Furthermore, the concept of augmenting the HBV-specific T-cell response, which has previously been focused solely on quantitative issues, is expanded in the light of new findings of qualitative differences in the HBV-specific CD8 cell response.

Direct functional analysis of epitope-specific CD8+ T cells in peripheral blood

The functional status of virus-specific CD8+ T cells is important for the outcome and the immunopathogenesis of viral infections. We have developed an assay for the direct functional analysis of antigen-specific CD8+ T cells, which does not require prolonged in vitro cultivation and amplification of T cells. Whole blood samples were incubated with peptide antigens for <5 h, followed by staining with peptide-MHC tetramers to identify epitope-specific T cells. The cells were also stained for the activation marker CD69 or for the production of cytokines such as interferon-gamma (IFNgamma) or tumor necrosis factor-alpha (TNFalpha). With the combined staining with tetramer and antibodies to CD69 or cytokines the number of antigen-specific CD8+ T cells as well as the functional response of each individual cell to the cognate antigen can be determined in a single experiment. Virus-specific CD8+ T cells that are nonfunctional, as well as those that are functional under the same stimulating conditions can be simultaneously detected with this assay, which is not possible by using other T-cell functional assays including cytotoxicity assay, intracellular cytokine staining, and enzyme-linked immunospot (ELISPOT) assay.

Immune escape by hepatitis B viruses

Hepatitis B viruses are DNA viruses characterized by their very small genome size and their unique replication via reverse transcription. The circular genome has been efficiently exploited, thereby limiting genome variation, and leaves no space for genes in addition to those essentially needed during the viral live cycle. Hepatitis B viruses are prototype non-cytopathic viruses causing persistent infection. Human hepatitis B virus (HBV), as well as the closely related animal viruses, most frequently are transmitted vertically from mothers to their offspring. Because infection usually persists for many years, if not lifelong, hepatitis B viruses need efficient mechanisms to hide from the immune response of the host. To escape the immune response, they exploit different strategies. Firstly, they use their structural and non-structural proteins multiplely. One of the purposes is to alter the immune response. Secondly, they replicate by establishing a pool of stable extrachromosomal transcription templates, which allow the virus to react sensitively to changes in its microenvironment by up- or downregulating gene expression. Thirdly, hepatitis B viruses replicate in the liver which is an immunopriviledged site.

In vivo selection of a lymphocytic choriomeningitis virus variant that affects recognition of the GP33-43 epitope by H-2Db but not H-2Kb

CD8 T cells drive the protective immune response to lymphocytic choriomeningitis virus (LCMV) infection and are thus a determining force in the selection of viral variants. To examine how escape mutations affect the presentation and recognition of overlapping T-cell epitopes, we isolated an LCMV variant that is not recognized by T-cell receptor (TCR)-transgenic H-2Db-restricted LCMV GP33-41-specific cytotoxic T lymphocytes (CTL). The variant virus carried a single-amino-acid substitution (valine to alanine) at position 35 of the viral glycoprotein. This region of the LCMV glycoprotein encodes both the Db-restricted GP33-43 epitope and a second epitope (GP34-42) presented by the Kb molecule. We determined that the V-to-A CTL escape mutant failed to induce a Db GP33-43-specific CTL response and that Db-restricted GP33-43-specific CTL induced by the wild-type LCMV strain were unable to kill target cells infected with the variant LCMV strain. In contrast, the Kb-restricted response was much less affected. We found that the V-to-A substitution severely impaired peptide binding to Db but not to Kb molecules. Strikingly, the V-to-A mutation did not change any of the anchor residues, and the dramatic effect on binding was therefore unexpected. The strong decrease in Db binding explains why the variant virus escapes the Db GP33-43-specific response but still elicits the Kb-restricted response. These findings also illustrate that mutations within regions encoding overlapping T-cell epitopes can differentially affect the presentation and recognition of individual epitopes.

Selection of and evasion from cytotoxic T cell responses in the central nervous system

Cytotoxic CD8 T lymphocytes (CTLs) are critical for the clearance of noncytopathic viruses from infected cells. This chapter discusses one mechanism used by viruses to persist—namely, the selection of a variant virus in which changes in the sequence of a CTL epitope abrogate recognition. The unique features of cytotoxic CD8 T cell function in the central nervous system (CNS) are discussed. The role of CTL escape mutants in the viral evasion of the immune system and subsequent disease progression in non-CNS infections are summarized. The immune response in the CNS is similar to the response in extraneural tissue, but several aspects of the activation of the immune response, cellular trafficking, and antigen presentation are unique to the CNS. Although the CNS has classically been considered a site of immune privilege, surveillance of the normal CNS by circulating, activated lymphocytes occurs, with a limited number of lymphocytes being present in the normal CNS at any given time. In mice infected with mouse hepatitis virus and in some humans persistently infected with human immunodeficiency virus type1, hepatitis B virus or hepatitis C virus, CTL escape mutants play an important role in virus amplification and disease progression.

A longitudinal analysis of cytotoxic T lymphocyte precursor frequencies to the hepatitis B virus in chronically infected patients

Individuals with acute hepatitis B virus (HBV) infection characteristically mount a strong, multispecific cytotoxic T lymphocyte (CTL) response that is effective in eradicating virus. In contrast, this response in chronic carriers is usually weak or undetectable. Since it is generally acknowledged that HBV pathogenesis is immune-mediated, the occurrence of episodes of active liver disease in many carriers suggests that these individuals can mount active CTL responses to HBV. To see whether the detection of circulating CTLs is related to these flare episodes, we have determined the CTL precursor (CTLp) frequencies to HLA-A2-restricted viral peptides in seven patients over a 12-24-month period of their disease. Limiting dilution analyses (LDA) were performed longitudinally to five epitopes comprising the viral capsid (HBc), envelope (HBs) and polymerase (pol) proteins. Assays were performed against a mixture of peptides, or against each individual peptide, to measure overall CTL activity and the multispecificity of the responses, respectively. Since two of the patients were treated with recombinant human interleukin-12 (rHuIL-12) at the time, with one individual achieving complete disease remission a year later after being treated with interferon-alpha, we were also able to examine the effects of these cytokines on HBV cytotoxicity. Our results indicate that weak but detectable CTL responses do occur in chronic carriers which are generally associated with disease flares, although CTLps were also seen occasionally during minimal disease activity. The range of specificities varied between individuals and within each individual during the course of the disease. Finally, we also provide evidence that CTL reactivity is stimulated following treatment with certain cytokines, but is dependent on the time of administration.

Cytotoxic T cell responses against hepatitis B virus polymerase induced by genetic immunization

BACKGROUND/AIMS

Individuals with chronic hepatitis B may benefit from genetic (DNA-based) immunization through induction of viral clearance by enhancement of suboptimal cellular immune responses. While marked cellular immune responses to hepatitis B virus (HBV) nucleocapsid and envelope proteins occur after genetic immunization in mice, it is unknown whether genetic immunization is capable of eliciting such responses to HBV polymerase. We wished to develop assays for the determination of HBV polymerase specific immune responses in mice and investigate whether genetic immunization may elicit humoral and cellular immune responses to HBV polymerase.

METHODS

BALB/c (H-2d) mice were injected with a DNA expression construct for HBV polymerase. Humoral immune responses to HBV polymerase were analyzed with a newly established ELISA. Cellular immune responses were determined using recombinant vaccinia virus infected target cells expressing HBV polymerase at high levels.

RESULTS

Assays for the detection of HBV polymerase-specific immune responses were developed. Immunized animals exhibited substantial polymerase-specific cytotoxic T lymphocyte responses. However, no humoral immune responses to HBV polymerase were detectable.

CONCLUSIONS

Our study demonstrates that DNA-based immunization will generate substantial CTL responses to HBV polymerase and may be an important component of an immunotherapeutic strategy to combat chronic HBV infection.

T cell receptor usage of virus-specific CD8 cells and recognition of viral mutations during acute and persistent hepatitis B virus infection

T cells specific for a single viral epitope, but using different T cell receptors, should have flexibility in their epitope recognition to protect the infected host against the emergence of viral escape mutants. Therefore, polyclonality of the hepatitis B virus (HBV)-specific cytotoxic T lymphocyte response has been hypothesized to be a major determinant in the control of infection. We analyzed the Vbeta chain composition of the core 18-27-specific CD8 cells in acute and persistently HBV-infected patients using HLA-A2 tetrameric complexes and a panel of Vbeta antibodies. Different T cell receptors were utilized by core 18-27-specific CD8 cells both in patients with acute and chronic infection. The functional ability of these epitope-specific T cells to respond to potential viral mutations was then tested. The polyclonal HBV-specific CD8 response present in patients with acute hepatitis displayed a limited efficiency to recognize mutations introduced within the epitope. The ability of core 18-27-specific CD8 to tolerate epitope mutations was found only during persistent HBV infection. The data suggest that although a clonally heterogeneous CD8 response can be largely inhibited by the occurrence of single epitope mutations in primary HBV infection, preferential selection of T cells able to counteract the emergence of viral mutations can occur during persistent infection.

How can the cellular immune response control hepatitis B virus replication?

In this review we focus on aspects of the virus-specific cellular immune response, although we should point out that all the components of the innate and adaptive immune response are likely to play a role in successful control of hepatitis B virus (HBV) infection. We concentrate particularly on the relevance of the polyclonality and multispecificity of the HBV-specific cytotoxic T cell response to its antiviral activity. In this context, we discuss the possible role of viral escape mutations and highlight evidence from other models of the benefit of multispecificity in antiviral responses. We stress the contribution of CD4 help for effective CD8 responses and raise the possibility that HBV may produce factors inhibiting the antiviral response.

Antigenic drift in the influenza A virus (H3N2) nucleoprotein and escape from recognition by cytotoxic T lymphocytes

Viruses exploit different strategies to escape immune surveillance, including the introduction of mutations in cytotoxic T-lymphocyte (CTL) epitopes. The sequence of these epitopes is critical for their binding to major histocompatibility complex (MHC) class I molecules and recognition by specific CTLs, both of which interactions may be lost by mutation. Sequence analysis of the nucleoprotein gene of influenza A viruses (H3N2) isolated in The Netherlands from 1989 to 1999 revealed two independent amino acid mutations at the anchor residue of the HLA-B27-specific CTL epitope SRYWAIRTR (383 to 391). A R384K mutation was found in influenza A viruses isolated during the influenza season 1989-1990 but not in subsequent seasons. In the influenza season 1993-1994, a novel mutation in the same CTL epitope at the same position was introduced. This R384G mutation proved to be conserved in all influenza A viruses isolated from 1993 onwards. Both mutations R384K and R384G abrogated MHC class I presentation and allowed escape from recognition by specific CTLs.

Identification of a pre-S2 mutant in hepatocytes expressing a novel marginal pattern of surface antigen in advanced diseases of chronic hepatitis B virus infection

BACKGROUND AND AIMS

The expression of hepatitis B viral (HBV) antigens in liver tissue reflects the replicative status of chronic HBV infection. We have previously recognized a novel marginal pattern of hepatitis B surface antigen (HBsAg) in hepatocytes, which usually clusters in groups and emerges at the late non-replicative phase. This study was designed to investigate whether the marginal-type HBsAg represented the gene product of a specific HBV-surface mutant.

METHODS

Microdissection of cirrhotic nodules homogeneously expressing marginal HBsAg was performed on two of 12 resected livers from HBsAg-seropositive patients with hepatocellular carcinoma. The gene presumably encoding marginal HBsAg was polymerase chain reaction (PCR)-cloned, sequenced and analysed. In vitro transfection and expression of the cloned surface mutant plasmids were performed on the Huh7 cell line to illustrate intrahepatic HBsAg expression.

RESULTS

Immunohistochemical staining revealed that the marginal HBsAg was positive for pre-S1 and thus contained large surface proteins. The PCR cloning and sequencing of the genes presumably encoding marginal-type HBsAg in both cases revealed the same deletion at the 5' terminus (nt 2-55) of pre-S2. A point mutation on the small-surface (S) antigen was also found in one case. The pre-S2 deletion sequence and the mutation sites of the S gene coincide with human lymphocyte antigen-restricted T- and/or B-cell epitopes. In vitro transfection of the mutant plasmid revealed a blot-like retention or accumulation of HBsAg in the cytoplasm or at the periphery of hepatocytes, accompanied by a decreased secretion of HBsAg in the culture supernatant, mimicking intrahepatic expression.

CONCLUSION

A natural pre-S2 deletion mutant was identified in hepatocytes expressing a novel marginal pattern of HBsAg, which probably contains mutant, large, surface proteins. The biological significance of the pre-S2 deletion mutant should be interesting in view of the clustering proliferation of hepatocytes expressing marginal HBsAg.

The role of virus-specific CD8(+) cells in liver damage and viral control during persistent hepatitis B virus infection

Hepatitis B virus (HBV) is a noncytopathic virus, and the recognition of infected hepatocytes by HBV-specific CD8 cells has been assumed to be the central mechanism causing both liver damage and virus control. To understand the role of cytotoxic T cells in the pathogenesis of HBV infection, we used functional assays that require T cell expansion in vitro and human histocompatibility leukocyte antigen (HLA)-peptide tetramers that allow direct ex vivo quantification of circulating and liver-infiltrating HBV-specific CD8 cells. Two groups of patients with persistent HBV infection were studied: one without liver inflammation and HBV replication, the other with liver inflammation and a high level of HBV replication. Contrary to expectation, a high frequency of intrahepatic HBV-specific CD8 cells was found in the absence of hepatic immunopathology. In contrast, virus-specific T cells were more diluted among liver infiltrates in viremic patients, but their absolute number was similar because of the massive cellular infiltration. Furthermore, inhibition of HBV replication was associated with the presence of a circulating reservoir of CD8(+) cells able to expand after specific virus recognition that was not detectable in highly viremic patients with liver inflammation. These results show that in the presence of an effective HBV-specific CD8 response, inhibition of virus replication can be independent of liver damage. When the HBV-specific CD8 response is unable to control virus replication, it may contribute to liver pathology not only directly but by causing the recruitment of nonvirus-specific T cells.

The role of virus-specific CD8(+) cells in liver damage and viral control during persistent hepatitis B virus infection

Hepatitis B virus (HBV) is a noncytopathic virus, and the recognition of infected hepatocytes by HBV-specific CD8 cells has been assumed to be the central mechanism causing both liver damage and virus control. To understand the role of cytotoxic T cells in the pathogenesis of HBV infection, we used functional assays that require T cell expansion in vitro and human histocompatibility leukocyte antigen (HLA)-peptide tetramers that allow direct ex vivo quantification of circulating and liver-infiltrating HBV-specific CD8 cells. Two groups of patients with persistent HBV infection were studied: one without liver inflammation and HBV replication, the other with liver inflammation and a high level of HBV replication. Contrary to expectation, a high frequency of intrahepatic HBV-specific CD8 cells was found in the absence of hepatic immunopathology. In contrast, virus-specific T cells were more diluted among liver infiltrates in viremic patients, but their absolute number was similar because of the massive cellular infiltration. Furthermore, inhibition of HBV replication was associated with the presence of a circulating reservoir of CD8(+) cells able to expand after specific virus recognition that was not detectable in highly viremic patients with liver inflammation. These results show that in the presence of an effective HBV-specific CD8 response, inhibition of virus replication can be independent of liver damage. When the HBV-specific CD8 response is unable to control virus replication, it may contribute to liver pathology not only directly but by causing the recruitment of nonvirus-specific T cells.

Immune escape by hepatitis B viruses

Hepatitis B viruses are DNA viruses characterized by their very small genome size and their unique replication via reverse transcription. The circular genome has been efficiently exploited, thereby limiting genome variation, and leaves no space for genes in addition to those essentially needed during the viral live cycle. Hepatitis B viruses are prototype non-cytopathic viruses causing persistent infection. Human hepatitis B virus (HBV), as well as the closely related animal viruses, most frequently are transmitted vertically from mothers to their offspring. Because infection usually persists for many years, if not lifelong, hepatitis B viruses need efficient mechanisms to hide from the immune response of the host. To escape the immune response, they exploit different strategies. Firstly, they use their structural and non-structural proteins multiplely. One of the purposes is to alter the immune response. Secondly, they replicate by establishing a pool of stable extrachromosomal transcription templates, which allow the virus to react sensitively to changes in its microenvironment by up- or downregulating gene expression. Thirdly, hepatitis B viruses replicate in the liver which is an immunopriviledged site.

Direct ex vivo analysis of hepatitis B virus-specific CD8(+) T cells associated with the control of infection

BACKGROUND & AIMS

Cytotoxic T cells have been suggested to be responsible for lysis of hepatitis B virus (HBV)-infected hepatocytes and control of virus infection. The frequency, kinetics, phenotype, and capacity for clonal expansion of circulating HBV-specific CD8 cells were analyzed directly in patients with acute HBV infection to clarify their pathogenetic role.

METHODS

Three HLA-A2 peptide tetramers able to visualize HBV core, envelope, and polymerase epitope-specific cytotoxic T lymphocytes were synthesized and used for flow cytometric analysis of antigen-specific populations.

RESULTS

Tetramer-positive cells specific for the core 18-27 epitope were found at a higher frequency than those specific for polymerase 575-583 and envelope 335-343 epitopes in most patients with acute HBV. The number of HBV-specific CD8 cells was highest during the clinically acute stage of infection and decreased after recovery. These cells expressed an activated phenotype and had an impaired capacity to expand in vitro and to display cytolytic activity in response to peptide stimulation. Recovery of these functions was observed when the frequency of specific CD8 cells decreased, coincident with a progressive decrease in their expression of activation markers.

CONCLUSIONS

This study provides the first ex vivo evidence that the highest frequency of circulating HBV-specific CD8 cells coincides with the clinically acute phase of hepatitis B. These cells exhibit an activated phenotype with limited further proliferative capacity that is restored during recovery.

Mutation "hot spot" in HLA class I-restricted T cell epitope on hepatitis B surface antigen in chronic carriers and hepatocellular carcinoma

Mutations in the human hepatitis B virus (HBV) genome contribute to its escape from host immune surveillance and result in persistent infection. We report the identification of frequent mutations encompassing residues 29 to 53 of the HBV surface antigen in chronic HBV carriers as well as in patients with hepatocellular carcinoma. The location of these mutations, not found in patients with acute hepatitis and vaccinated infants, coincides with a human leukocyte antigen class I-restricted cytotoxic T lymphocyte epitope. Significantly, mutations occur at a higher frequency (83%) compared with those identified on the immunogenic "a" determinant (25%) of the corresponding patients. Our findings therefore suggest the potential importance of this novel mutation "hot spot" in the establishment of chronic HBV infection.

Retreatment of chronic hepatitis B e antigen-positive patients with recombinant interferon alfa-2a. The European Concerted Action on Viral Hepatitis (EUROHEP)

Fifty-seven patients with chronic hepatitis B, hepatitis B virus (HBV) e antigen (HBeAg) and HBV DNA positivity, and aminotransferase elevation despite a previous course of any type of adequate interferon alfa (IFN-alpha) therapy were included in a multicenter prospective randomized controlled trial. The objective of the study was to compare a second course of IFN-alpha therapy (9 million units [MU] of IFN-alpha-2a, Roferon-A, thrice weekly for 6 months) versus no therapy in terms of loss of HBV DNA and HBeAg. At the end of the study, a sustained clearance of HBV DNA and HBeAg was observed in 9 of the 27 (33.3%) patients who had received retreatment with IFN-alpha compared with 3/30 (10%) patients who spontaneously cleared these markers in the untreated control group (chi2 = 4.66, P =.031; odds ratio: 4.5, 95%; confidence interval: 1.1-18.9). None of the responders lost HBsAg. Patients retreated with IFN-alpha were more likely to have biochemical remission in association with HBV clearance (5/27, 18.5%) compared with untreated patients (1/30, 3. 3%; Fisher's exact test P =.09 ). Histological improvement in the liver necroinflammatory activity was observed among sustained responders to IFN-alpha retreatment, consisting of regression of the portal and periportal inflammation and of the piecemeal necrosis; there was no change in the degree of liver fibrosis. Side effects were similar to those previously reported during IFN-alpha treatment; these were mild and reversible on IFN-alpha discontinuation. None of the baseline features were associated with response by Cox's regression analysis. In summary, viremic patients with chronic HBeAg-positive hepatitis may experience disease remission following retreatment with IFN-alpha. Thus, retreatment with IFN-alpha may be considered a therapeutic option.

Natural and iatrogenic variation in hepatitis B virus

The existence of HBV as quasispecies is thought to be favoured by the infidelity of HBV RT, which would account for the emergence of the many natural mutants with point substitutions. RT infidelity may also underlie the hypermutation phenomenon. Indeed, the oft-reported point mutation in the preC gene that leads to failure of HBeAg synthesis may be driven by a hypermutation-related mechanism. The presence of mutants with deletions and insertions involving single nucleotides and oligonucleotides at specific positions in the genome, and of mutants with deletions of even longer stretches particularly in the C gene, suggests that other mutagenic mechanisms operate. Candidates include slippage during mispairing between template and progeny DNA strand, the action of cellular topoisomerase I, and gene splicing using alternative donor and acceptor sites. Natural substitutions, deletions or insertions involving the Cp/ENII locus in the X gene can significantly alter the extent of viral replicative activity. Similar mutations occurring at other locations of Cp/ENII, and at B-cell epitope sites of the S gene are associated with failure to detect serological markers of HBV infection. HBV variation can also arise from recombination between coinfecting strains. S gene mutations that become evident following HBIG administration and HBV vaccination are all point substitutions, as are mutations in functional RT domains of the P gene after treatment with viral RT-inhibitory drugs. Widespread and long-term use of prophylactic and therapeutic agents may potentially generate serologically occult HBV variants that might become difficult to eradicate.

Cross-Recognition of Two Middle T Protein Epitopes by Immunodominant Polyoma Virus-Specific CTL

We recently identified the immunodominant epitope for polyoma virus-specific CTL as the Dk-associated peptide MT389–397 derived from the middle T (MT) viral oncoprotein. Another Dk-restricted peptide corresponding to residues 236–244 of MT was recognized by nearly all MT389–397-reactive CTL clones, but required concentrations at least 2 logs higher to sensitize syngeneic target cells for lysis. Except for identity at the three putative Dk-peptide anchor residues, MT236–244 shares no homology with MT389–397. Using a novel europium-based class I MHC-peptide binding immunoassay, we determined that MT236–244 bound Dk 2–3 logs less well than MT389–397. Infection with a mutant polyoma virus whose MT is truncated just before the MT389–397 epitope or immunization with MT389–397 or MT236–244 peptides elicited CTL that recognized both MT389–397 and MT236–244. Importantly, infection with a polyoma virus lacking MT389–397 and mutated in an MT236–244 Dk anchor position induced polyoma virus-specific CTL recognizing neither MT389–397 nor MT236–244 epitopes. Despite predominant usage of the Vβ6 gene segment, MT389–397/MT236–244 cross-reactive CTL clones possess diverse complementarity-determining region 3β domains; this is functionally reflected in their heterogeneous recognition patterns of alanine-monosubstituted MT389–397 peptides. Using Dk/MT389–397 tetramers, we directly visualized MT236–244 peptide-induced TCR down-modulation of virtually all MT389–397-specific CD8+ T cells freshly explanted from polyoma-infected mice, suggesting that a single TCR recognizes both Dk-restricted epitopes. The availability of immunodominant epitope-specific CTL capable of recognizing a second epitope in MT, a viral protein essential for tumorigenesis, may serve to amplify the CTL response to the immunodominant epitope and prevent the emergence of immunodominant epitope-loss viruses and virus-induced tumors.

Immune response to the immunodominant epitope of mouse hepatitis virus is polyclonal, but functionally monospecific in C57Bl/6 mice

Mutations in an immunodominant CD8 CTL epitope (S-510-518) are selected in mice persistently infected with the neurotropic JHM strain of mouse hepatitis virus. These mutations abrogate recognition by T cells harvested from the infected CNS in direct ex vivo cytotoxicity assays. Previous reports have suggested that, in general, an oligoclonal, monospecific T cell response contributes to the selection of CTL escape mutants. Herein, we show that, in MHV-JHM-infected mice, the CD8 T cell response after intraperitoneal infection is polyclonal and diverse. This diverse response was shown to include both polyclonal and oligoclonal components. The polyclonal data were shown to fit a logarithmic distribution. With regard to specificity, we used a panel of peptide analogues of epitope S-510-518 and spleen-derived CD8 T cell lines to determine why only a subset of possible mutations was selected in persistently infected mice. At a given position in the epitope, the mutations identified in in vivo isolates were among those that resulted in the greatest loss of recognition. However, not all such mutations were selected, suggesting that additional factors must contribute to selection in vivo. By extrapolation of these results to the persistently infected CNS, they suggest that the selection of CTL escape mutants requires the presence of a monospecific T cell response but also show that this response need not be oligoclonal.

Antiviral cell-mediated immune responses during hepatitis B and hepatitis C virus infections

Cell-mediated immune responses to hepatitis B (HBV) and hepatitis C virus (HCV) antigens are vigorous and multispecific in acute, self-limited infections. Moreover, the prevalent cytokine pattern of circulating virus-specific T cells from patients who recover spontaneously from acute hepatitis is Th1-like. Longitudinal analysis of the T cell response to HCV antigens from the early stages of HCV infection in patients who recover from hepatitis and those who do not indicates that weaker responses and a prevalent Th2 pattern of cytokine production is associated with viral persistence and chronic evolution of disease. Although similar sequential studies are missing in hepatitis B, the observation that HBV-specific T cell responses are very weak or totally undetectable in the peripheral blood of patients with long-lasting chronic hepatitis B suggests that strength and quality of virus-specific T cell responses at the early stages of infection may influence the final outcome of both hepatitis B and C. While T cell hyporesponsiveness seems to be an important determinant for HBV persistence once chronic hepatitis has developed, this mechanism appears to be less critical in chronic HCV infection, because the vigor and quality of HCV-specific T cell responses seem to improve as a function of the duration of infection. This is shown by the finding that HCV-specific CD4- and CD8-mediated responses are easily detectable in the peripheral blood of patients with long-lasting chronic hepatitis C and that production of Th1 cytokines predominates within their livers. HCV therefore seems to be able to persist even in the face of an active T cell response and to acquire the capacity to survive within a host environment apparently unfavorable to its persistence. The high variability of HCV may explain its efficiency in escaping immune surveillance.

Polyclonality and Multispecificity of the CTL Response to a Single Viral Epitope

The molecular anatomy of an immunodominant, Ld restricted CTL epitope located between residues 28–39 in hepatitis B surface Ag was defined to explore the immunologic constraints on mutational escape from the CTL response during a viral infection. Using a panel of hepatitis B surface Ag residue 28–39-specific CTL clones, the response to this epitope was found to be extremely diverse at the level of TCR fine specificity and β-chain usage. Although each clone recognized shared as well as unique residues within the epitope as TCR contact sites, even the shared residues were recognized differently by different TCRs. Despite these differences, all clones were comparably cytolytic following Ag stimulation and produced similar amounts of antiviral cytokines previously shown to inhibit HBV replication. These results demonstrate that the CTL response to individual viral epitopes can be markedly polyclonal and multispecific, such that mutational inactivation of a single TCR contact site will not usually lead to viral escape from all CTL clones of the same epitope specificity. Given these constraints and the fact that the CTL response is usually directed against several different epitopes during most viral infections, mutational inactivation of a single epitope is not likely to be sufficient to cause viral persistence.

Protection from Plasmodium berghei infection by priming and boosting T cells to a single class I-restricted epitope with recombinant carriers suitable for human use

The desirability of inducing cytotoxic T cell responses to defined epitopes in humans has led to the development of a variety of recombinant delivery systems. Recombinant protein particles derived from a yeast retrotransposon (Ty) and the modified Ankara vaccinia (MVA) virus can deliver large epitope strings or even whole proteins. Both have previously been administered safely in humans. Immunization with recombinant Ty and MVA containing a single Plasmodium berghei class I-binding epitope provided 95% sterile protection against malaria in mice. The sequence of immunization, Ty followed by MVA, was critical to elicit high levels of IFN-gamma-producing cells and protection. The reciprocal sequence (MVA/TY) or homologous boosting was not protective. Both constructs (Ty and MVA) contain the H-2Kd-restricted pb9 CTL epitope from the circumsporozoite protein of P. berghei among a string of 8-15 human P. falciparum-derived CTL epitopes restricted through 7 common HLA alleles as well as widely recognized CD4 T cell epitopes. Thus, the novel recombinant Ty/MVA prime/boost combination with these constructs provides a safe alternative for evaluation for human vaccination against P. falciparum malaria.

Transient alteration of T cell fine specificity by a strong primary stimulus correlates with T cell receptor down-regulation

P14 mice expressing a transgenic TCR specific for the lymphocytic choriomeningitis virus glycoprotein p33 epitope were used to study the induction of CTL effector activity by a variety of ligands. Surprisingly, p33 variants which are weaker agonists for the P14 TCR than the wild-type p33 peptide were able to induce more potent effectors with a broader range of cytolytic specificity. Similarly, low concentrations of p33 were more effective than higher concentrations. These results correlated with no or only moderate TCR down-regulation by variants of p33 and low p33 concentrations. This phenotype observed after 18 h of culture was transient as progressive restoration of reactivity was observed at 42 or 66 h in the cultures stimulated with high p33 concentrations and this correlated with recovery of TCR surface levels. TCR down-regulation was blocked by src family kinase inhibitors. These findings indicate that the specificity of a T cell can be fine-tuned by the nature of the primary stimulus correlating with surface TCR level and imply an important role for src family kinases in the differential regulation of surface TCR levels upon TCR engagement by different ligand/MHC complexes.

Cytotoxic T-lymphocyte escape viral variants: how important are they in viral evasion of immune clearance in vivo?

Although viral variants which are not recognized by epitope-specific cytotoxic T lymphocytes (CTL) have been shown to arise during a number of persistent virus infections, in many cases their significance remains controversial: it has been argued that the immune response is sufficiently plastic to contain their replication. In this review, we describe the mechanisms by which amino acid changes in viral proteins may affect epitope recognition by virus-specific CTL, and discuss the viral and immunological basis for the emergence of viral variants bearing such amino acid changes during infection. We then consider the impact that viral variation may have on the host CTL response and its ability to contain virus replication. We argue that the emergence of a viral variant demonstrates that it must have an in vivo replicative advantage, and that as such, the variant must tip the balance between virus replication and immune control somewhat in favor of the virus. Further, we suggest that although the immune response can evolve to recognize new viral epitopes, the CTL generated following such evolution frequently have a reduced ability to contain virus replication. We conclude that this escape mechanism likely does make a significant contribution to persistence/pathogenesis during a number of different virus infections.

Original antigenic sin impairs cytotoxic T lymphocyte responses to viruses bearing variant epitopes

Some viruses, including human immunodeficiency virus (HIV) and hepatitis B virus (HBV) in humans, and lymphocytic choriomeningitis virus (LCMV) in mice, are initially controlled by cytotoxic T lymphocytes (CTLs), but may subsequently escape through mutation of the relevant T-cell epitope. Some of these mutations preserve the normal binding to major histocompatibility complex class I molecules, but present an altered surface to the T-cell antigen receptor. The exact role of these so-called altered peptide ligands in vivo is not clear. Here we report that mice primed with LCMV-WE strain respond to a subsequent infection by WE-derived CTL epitope variants with a CTL response directed against the initial epitope rather than against the new variant epitope. This phenomenon of 'original antigenic sin' was initially described in influenza and is an asymmetric pattern of protective antibody crossreactivity determined by exposure to previously existing strains, which may therefore extend to some CTL responses. Original antigenic sin by CTL leads to impaired clearance of variant viruses infecting the same individual and so may enhance the immune escape of mutant viruses evolving in an individual host.

Infection with cytotoxic T-lymphocyte escape mutants results in increased mortality and growth retardation in mice infected with a neurotropic coronavirus

C57BL/6 mice infected with mouse hepatitis virus strain JHM (MHV-JHM) develop a chronic demyelinating encephalomyelitis several weeks after inoculation. Previously, we showed that mutations in the immunodominant CD8 T-cell epitope (S-510-518) could be detected in nearly all samples of RNA and virus isolated from these mice. These mutations abrogated recognition by T cells harvested from the central nervous systems of infected mice in direct ex vivo cytotoxicity assays. These results suggested that cytotoxic T-lymphocyte (CTL) escape mutants contributed to virus amplification and the development of clinical disease in mice infected with wild-type virus. In the present study, the importance of these mutations was further evaluated by infecting naive mice with MHV-JHM variants isolated from infected mice and in which epitope S-510-518 was mutated. Compared to mice infected with wild-type virus, variant virus-infected animals showed higher mortality and morbidity manifested by decreased weight gain and neurological signs. Although a delay in the kinetics of virus clearance has been demonstrated in previous studies of CTL escape mutants, this is the first illustration of significant changes in clinical disease resulting from infection with viruses able to evade the CD8 T-cell immune response.

Lack of strong immune selection pressure by the immunodominant, HLA-A*0201-restricted cytotoxic T lymphocyte response in chronic human immunodeficiency virus-1 infection

Despite detailed analysis of the HIV-1-specific cytotoxic T lymphocyte response by various groups, its relation to viral load and viral sequence variation remains controversial. We analyzed HLA-A*0201 restricted cytotoxic T lymphocyte responses in 17 HIV-1-infected individuals with viral loads ranging from < 400 to 221,000 HIV RNA molecules per milliliter of plasma. In 13 out of 17 infected subjects, CTL responses against the SLYNTVATL epitope (p17 Gag; aa 77-85) were detectable, whereas two other HLA-A*0201 restricted epitopes (ILKEPVHGV, IV9; and VIYQYMDDL, VL9) were only recognized by six and five individuals out of 17 individuals tested, respectively. Naturally occurring variants of the SL9 epitope were tested for binding to HLA-A*0201 and for recognition by specific T cell clones generated from five individuals. Although these variants were widely recognized, they differed by up to 10,000-fold in terms of variant peptide concentrations required for lysis of target cells. A comparison of viral sequences derived from 10 HLA-A*0201-positive individuals to sequences obtained from 11 HLA-A*0201-negative individuals demonstrated only weak evidence for immune selective pressure and thus question the in vivo efficacy of immunodominant CTL responses present during chronic HIV-1 infection.

Analysis of hepatitis B virus populations in an interferon-alpha-treated patient reveals predominant mutations in the C-gene and changing e-antigenicity

It is largely unknown whether hepatitis B virus (HBV) sequence variation during chronic infection hampers HBV immune recognition or the antiviral effect of cytokines on HBV production. Here we have analyzed which region of the HBV genome changes most drastically during an interferon-alpha (IFNalpha)-stimulated immune response. In addition, we have investigated whether the mutations affect viral replication, gene expression, and immune recognition of the mutant viral proteins. The study was performed with full-length HBV genomes taken longitudinally from a patient who transiently cleared HBV and seroconverted to anti-HBe during a long-term IFNalpha treatment. We found a replacement of the predominant virus population during IFNalpha therapy The virus populations differed mainly by a cluster of nucleotide changes in the C-gene and a pre-S2 deletion. Most of the newly emerging mutations localized within core/HBe B-cell epitopes, changed HBe antigenicity toward mono- and polyclonal antibodies, and also influenced the reactivity of the anti-HBc/e antibodies of the patient. All genomes tested expressed less HBeAg than wild-type HBV, while replication and IFNalpha susceptibility were similar. These data indicate that IFNalpha therapy can lead to the emergence of HBV variants with mutations mainly affecting recognition of the core/HBe proteins by antibodies. Taken together, the type of core/HBe-specific B-cell immune response, the sequence of the corresponding epitopes, and the HBe expression level appear to contribute to the decision on viral clearance or persistence.

Distinct patterns of T cell receptor distribution of peripheral blood CD8+ cells during different stages of chronic infection with hepatitis B virus

We compared the Vbeta TCR repertoires of CD8+ peripheral blood lymphocytes between 21 patients with chronic hepatitis B (CHB) and 9 healthy individuals, using RT-PCR analysis. Several differences were seen between CHB patients and controls, including a marked increase in the expression of two to five Vbeta families in the CHB patients. There was no evidence for a superantigen effect, although an increase in Vbeta7 was seen in 64% of patients. A significant under-expression of Vbeta families were also detected, particularly in patients with active liver disease in which under-expression of Vbeta14 and Vbeta15 was associated with acute exacerbations of liver disease. We also did a longitudinal analysis of the TCR repertoire in two patients over a period of 6 months, from the initiation of a disease flare to its resolution. One patient continued to experience spontaneous flares following the completion of this study, while the other patient underwent spontaneous remission with long-term (> 12 months) loss of HBeAg following resolution of the flare. The TCR repertoires of both patients were altered during the flare, and there was a higher degree of TCR variability in the patient who went into remission.

Mechanisms of viral clearance and persistence

Using examples predominantly drawn from study of the lymphocytic choriomeningitis virus (LCMV) model system, this review describes the mechanisms involved in control of virus infections by the cell-mediated immune response, and some of the different strategies viruses have evolved to evade such immune clearance so that they can persist in their hosts. The important role played by the CD8+ cytotoxic lymphocyte (CTL) response in clearance of many systemic virus infections is discussed; and it is emphasized that although CD8+ CTL are classically thought of as lymphocytes which mediate lysis of virus-infected target cells, the principal mechanism by which CD8+ T cells effect clearance of persistent and many acute virus infections via production of antiviral cytokines such as tumour necrosis factor-alpha and interferon-gamma, not via destruction of virus-producing cells. To avoid immune-mediated clearance, viruses frequently use a combination of several different strategies. These can be grouped into mechanisms for avoiding recognition by the immune response (such as establishing latent infections, replicating in immune-privileged sites, down-regulating the expression of immune recognition signals on the surface of infected cells, or undergoing antigenic variation); and mechanisms for suppressing the immune response. The latter include generalized immune suppression mechanisms, and strategies for more precisely disabling the specific immune response such as inducing tolerance or exhaustion of virus-specific CTL. The value of understanding both immune clearance mechanisms and viral evasion strategies in the rational design of immune-based therapies to combat persistent virus infections is discussed.

Immunological significance of cytotoxic T lymphocyte epitope variants in patients chronically infected by the hepatitis C virus

This study was performed to test the hypothesis that cytotoxic T lymphocyte (CTL) selection of hepatitis C virus (HCV) escape variants plays a role in HCV persistence. The peripheral blood CTL responsiveness of patients with well-established chronic hepatitis C to a panel of 10 prototype HCV peptides (genotype 1a) was compared with the corresponding sequences encoded by the infecting viruses in each patient. Variant viral peptide sequences were threefold more frequent in the presence of a CTL response than in its absence, and CTL responses were detected nearly twice as often in association with variant rather than with prototype viral peptide sequences. Furthermore, over half of the patients were infected with potential CTL escape variants that contained nonimmunogenic and noncross-reactive variant peptides many of which displayed reduced HLA-binding affinity. Surprisingly, follow up analysis over a period of up to 46 mo revealed that, in contrast to the relatively high frequency of escape variants initially observed, the subsequent emergence rate of CTL escape variants was very low. Interestingly, the one escape variant that was detected proved to be a CTL antagonist. Collectively, these observations suggest that CTL selection of epitope variants may have occurred in these patients before their entrance into the study and that it may have played a role in HCV persistence. The low apparent rate of ongoing CTL selection in chronically infected patients, however, suggests that if CTL escape occurs during HCV infection it is probably an early event.

Cytotoxic T-cell-resistant variants arise at early times after infection in C57BL/6 but not in SCID mice infected with a neurotropic coronavirus

Under certain conditions, C57BL/6 mice persistently infected with mouse hepatitis virus strain JHM (MHV-JHM) develop clinical disease and histological evidence of demyelination several weeks after inoculation with virus. In a previous report, we showed that mutations in the RNA encoding an immunodominant CD8 T-cell epitope within the surface glycoprotein (epitope S-510-518) were present in all persistently infected animals and that these mutations abrogated recognition by virus-specific cytotoxic T cells (CTLs) in direct ex vivo cytotoxicity assays. To obtain further evidence that these mutations were necessary for the development of clinical disease, the temporal course of their appearance was determined. Mutations in the epitope were identified by 10 to 12 days after inoculation, and in some mice, virus containing mutated epitope was the dominant species detected by 15 days. In addition, most mice that remain asymptomatic at 80 days after inoculation, a time after which clinical disease almost never develops, were infected with only wild-type virus. Finally, analysis of virus isolated from mice with severe combined immunodeficiency (SCID) revealed the presence only of wild-type epitope S-510-518. These results, by showing that mutations are not selected in SCID mice and occur at early times after inoculation in C57BL/6 mice, support the view that they result from immune pressure and contribute to virus persistence and demyelination in mice infected persistently with MHV-JHM.