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

Does rapid oligomerization of hepatitis B envelope proteins play a role in resistance to proteasome degradation and enhance chronicity?

This review discusses the nature of hepatitis B and C chronicity from a virological perspective. Work described in the literature and our in vitro studies of HBV polypeptide morphogenesis lead us to speculate about a role for HBsAg complex formation in immune evasion that may be especially important during the initial period of infection. Briefly, although viral structural proteins do eventually provide epitopes recognized by the host, we suggest that these HBs Ag complexes, which may themselves be refractory to proteasomal degradation, are an important way by which the virus shields its epitopes and evades early recognition by the cellular immune system. This suggests a central strategy by which the virus has evolved, structurally, to enable the establishment of persistent infection of its host. The concept offers an explanation for the nearly unidirectional and rapid kinetics whereby HBV proteins form multimers and generate a surplus of viral structures that have not been thought to serve any useful structural purpose.

Detection of Wilms' tumor antigen--specific CTL in tumor-draining lymph nodes of patients with early breast cancer

PURPOSE

The Wilms' tumor antigen (WT1) is overexpressed in approximately 90% of breast tumors and, thus, is a potential target antigen for the immunotherapy of breast cancer. We have tested the working hypotheses that WT1 can be immunogenic in patients with breast cancer and can stimulate CTL of sufficient avidity to kill tumor cells.

EXPERIMENTAL DESIGN

Paired tumor-draining lymph node and peripheral blood samples were analyzed from five HLA-A2-positive patients with stage I/II breast cancer. Fluorescent HLA-A*0201/WT1 tetramers were used to quantify WT1-specific CTL and the functional capacity of the CTL was assessed using cytotoxicity assays and intracellular cytokine staining.

RESULTS

WT1 tetramer-binding T cells expanded from all lymph node samples but none of the corresponding peripheral blood samples. Functional assays were carried out on T cells from the patient who had yielded the highest frequency of HLA-A*0201/WT1 tetramer-positive cells. The cytotoxicity assays showed WT1 peptide--specific killing activity of the CTL, whereas intracellular cytokine staining confirmed that the tetramer--positive T cells produced IFN-gamma after stimulation with WT1 peptide. These WT1-specific T cells killed HLA-A2-positive breast cancer cell lines treated with IFN-gamma but no killing was observed with untreated tumor cells.

CONCLUSIONS

These results show that WT1-specific CTL can be expanded from the tumor-draining lymph nodes of breast cancer patients and that they can display peptide-specific effector function. However, the CTL only killed IFN-gamma-treated tumor targets expressing high levels of HLA-A2 and not tumor cells with low HLA expression. This suggests that induction of autologous WT1-specific CTL may offer only limited tumor protection and that strategies that allow a high level of peptide/MHC complex presentation and/or improve CTL avidity may be required.

Loss of CD8 and TCR binding to Class I MHC ligands following T cell activation

The capacity of T cells to bind peptide/MHC ligands changes with T cell development and differentiation. Here we study changes in peptide/MHC multimer binding following T cell activation. Surprisingly, T cell activation caused a marked reduction in specific peptide/MHC Class I multimer binding, which was distinct from transient TCR down-regulation, and was especially dramatic for engagement with low-affinity peptide/MHC ligands. Direct CD8-Class I interactions were also profoundly and rapidly impaired following T cell stimulation, even though surface CD8alpha and CD8beta levels were unchanged after activation, suggesting that decreased CD8 co-receptor binding contributes to this effect. Finally, we show that enzymatic desialylation restores much of the multimer binding on activated T cells, suggesting that altered glycosylation may inhibit TCR/CD8 binding to peptide/MHC ligands. These radical changes in activated T cells' ability to perceive peptide/MHC ligands may contribute to selective outgrowth of clones with high affinity for the stimulatory ligand.

Transient loss of MHC class I tetramer binding after CD8+ T cell activation reflects altered T cell effector function

Engagement of the Ag receptor on naive CD8+ T cells by specific peptide-MHC complex triggers their activation/expansion/differentiation into effector CTL. The frequency of Ag-specific CD8+ T cells can normally be determined by the binding of specific peptide-MHC tetramer complexes to TCR. In this study we demonstrate that, shortly after Ag activation, CD8+ T cells transiently lose the capacity to efficiently bind peptide-MHC tetramer complexes. This transient loss of tetramer binding, which occurs in response to naturally processed viral peptide during infection in vitro and in vivo, is associated with reduced signaling through the TCR and altered/diminished effector activity. This change in tetramer binding/effector response is likewise associated with a change in cell surface TCR organization. These and related results suggest that early during CD8+ T cell activation, there is a temporary alteration in both cell surface Ag receptor display and functional activity that is associated with a transient loss of cognate tetramer binding.

Decreased frequency and function of circulating plasmocytoid dendritic cells (pDC) in hepatitis B virus infected humans

The Type 2 precursor plasmacytoid dendritic cells (pDC) represent the most important cell type in antiviral innate immunity. To understand the function of pDC during hepatitis B virus infection, the frequency and function of circulating pDC were analyzed by flow cytometric analysis, and IFN-alpha secretion of total PBMCs was determined by ELISA assay in 25 healthy subjects and 116 patients at various stages of chronic hepatitis B virus infection (CHB). The number of circulating pDC was found to be significantly lower in patients with CHB and associated liver cirrhosis (LC). The ability of PBMCs to secrete IFN-alpha also decreased significantly. There was a corresponding decrease of circulating NK cells and CD8+ T cells. We observed that lamuvidine antiviral therapy restored the number of circulating pDC and there was a reversal of pDC frequency with the control of HBV replication in chronic HBV patients, indicating these subjects are unlikely to be totally immunocompromised. The decrease of pDC was found to be related to nosocomial infections in LC patients. Our results suggest that CHB patients probably have a quantitative and qualitative impairment of circulating pDC or NK cells, which may be associated with HBV persistent infection as well as the nosocomial infections that arise in LC patients.

Development of a nasal vaccine for chronic hepatitis B infection that uses the ability of hepatitis B core antigen to stimulate a strong Th1 response against hepatitis B surface antigen

There are estimated to be 350 million chronic carriers of hepatitis B infection worldwide. Patients with chronic hepatitis B are at risk of liver cirrhosis with associated mortality because of hepatocellular carcinoma and other complications. An important goal, therefore, is the development of an effective therapeutic vaccine against chronic hepatitis B virus (HBV). A major barrier to the development of such a vaccine is the impaired immune response to HBV antigens observed in the T cells of affected patients. One strategy to overcome these barriers is to activate mucosal T cells through the use of nasal vaccination because this may overcome the systemic immune downregulation that results from HBV infection. In addition, it may be beneficial to present additional HBV epitopes beyond those contained in the traditional hepatitis B surface antigen (HbsAg) vaccine, for example, by using the hepatitis B core antigen (HBcAg). This is advantageous because HBcAg has a unique ability to act as a potent Th1 adjuvant to HbsAg, while also serving as an immunogenic target. In this study we describe the effect of coadministration of HBsAg and HBcAg as part of a strategy to develop a more potent and effective HBV therapeutic vaccine.

The functional avidity of virus-specific CD8+ T?cells is down-modulated in Borna disease virus-induced immunopathology of the central nervous system

Borna disease virus (BDV) infection of the central nervous system (CNS) leads to severe neurological symptoms in susceptible MRL mice. The disease is mainly mediated by CD8+ T cells specific for the immunodominant epitope TELEISSI in the BDV nucleoprotein. In this study, TELEISSI/MHC class I tetramers were used to directly visualize antigen-specific CD8+ T cells. We found that on average approximately 30% of the ex vivo analyzed CD8+ T cells in the CNS of diseased mice were specific for TELEISSI. Unexpectedly, the frequency of tetramer-reactive brain-derived CD8+ T cells doubled following overnight culture in the absence of antigen. The majority of CD8+ T cells showed enhanced tetramer binding without up-regulation of T cell receptor surface expression. The frequency of IFN-gamma-secreting CD8+ T cells after antigen-specific stimulation was higher in overnight cultures than in freshly isolated BDV-specific brain lymphocytes, and enhanced tetramer binding correlated with elevated sensitivity to lower levels of peptide antigen in cytotoxicity assays. These results indicate that the functional avidity of virus-specific CD8+ T cells was down-modulated in vivo. Thus, quantification of tissue-infiltrating CD8+ T cells by the tetramer technique must be interpreted with caution as it may underestimate the real frequency of antigen-specific CD8+ T cells.

High frequency of functional anti-YMDD and -mutant cytotoxic T lymphocytes after in vitro expansion correlates with successful response to lamivudine therapy for chronic hepatitis B

BACKGROUND

Many determinants for a sustained response to lamivudine therapy have been reported but the role of T cell responsiveness remains unclear. The finding that tyrosine-methionine-aspartate-aspartate (YMDD) motif of the reverse transcriptase domain of hepatitis B virus (HBV) DNA polymerase carries a HLA-A2 restricted cytotoxic T lymphocyte (CTL) epitope makes quantitative measurement of the numbers of peptide specific CTLs feasible using MHC tetramer-peptide complex staining.

AIM

To investigate the correlation between anti-YMDD motif CTL activity and the efficacy of lamivudine therapy in HLA-A2 positive patients with chronic hepatitis B (CH-B).

METHODS

The function and phenotype of peptide and interleukin 2 expanded peripheral blood mononuclear cells were quantified by cell lytic assay and immunocytochemical analysis by staining with HLA-A2-peptide tetramer complexes.

RESULTS

After in vitro expansion, sustained responders had more potent CTL responses against YMDD, YVDD, and YIDD, as well as other epitopes on HBV antigens than non-responders. The frequency of YMDD/YVDD/YIDD motif specific CTLs increased significantly with an effective cell lytic function during and after therapy in sustained responders but not in non-responders. YMDD specific CTLs cross reacted with YIDD and YVDD mutant epitopes, and shared T cell receptor gene usages with YIDD and YVDD specific CTLs.

CONCLUSIONS

Sustained responders, at least HLA-A2 patients, elicited a more potent CTL immunity against YMDD and its mutants. YMDD specific CTLs are cross reactive with YVDD and YIDD mutant epitopes, which may further contribute to immune clearance of the mutant viruses and a successful response to lamivudine therapy in CH-B patients.

Pathogen-host standoff: immunity to polyomavirus infection and neoplasia

Polyomaviruses establish persistent infection in a variety of hosts, including humans, where they pose an oncogenic threat under conditions of depressed immune function. Control of persistent infection by these DNA tumor viruses requires continuous immunosurveillance by functionally competent antiviral CD8+ T cells. Repetitive antigen encounter by these T cells, however, often leads to their deletion or inactivation. Elucidation of the in vivo mechanisms that sustain antigen-specific CD8+ T-cell effector activity in the face of persistent antigen is essential for devising immunotherapeutic strategies against viral oncogenesis.

Effector and regulatory T?cells derived from the same T?cell clone differ in MHC class II-peptide multimer binding

MHC class II-peptide multimers are a valuable tool for antigen-specific detection of CD4(+) T cells. However, it has been proposed that T cells in a hypo-responsive state can have diminished binding of such multimers. In the present study, we investigated this phenomenon at the clonal level. We found that anergic CD4(+) T cells had a reduced capacity to bind MHC class II-peptide multimers compared to their non-anergic counterparts. Increasing the incubation temperature, time, or MHC-peptide valency could not equalize multimer binding by anergic and non-anergic T cells. Neither anergic T cells nor non-anergic T cells internalized the MHC class II-peptide dimers efficiently, and in both cases the dimers bound to the plasma membrane at locations containing a low amount of raft-associated lipids. Disruption of lipid rafts, however, led to decreased dimer binding by non-anergic T cells and to a lesser extent by anergic T cells. Finally, we show that the depth of the anergic state of the T cell, which determines its ability to regulate other T cell responses, correlates with the reduced dimer binding. We here demonstrate for the first time differential MHC class II-peptide multimer binding by regulatory (anergic) and effector T cells with identical TCR.

Cytolytic CD8+ T cells recognizing CFP10 are recruited to the lung after Mycobacterium tuberculosis infection

Optimum immunity against Mycobacterium tuberculosis requires both CD4⁺ and CD8⁺ T cells. In contrast with CD4⁺ T cells, few antigens are known that elicit CD8⁺ T cells during infection. CD8⁺ T cells specific for culture filtrate protein-10 (CFP10) are found in purified protein derivative positive donors, suggesting that CFP10 primes CD8⁺ T cells in vivo. Using T cells from M. tuberculosis–infected mice, we identified CFP10 epitopes recognized by CD8⁺ T cells and CD4⁺ T cells. CFP10-specific T cells were detected as early as week 3 after infection and at their peak accounted for up to 30% of CD8⁺ T cells in the lung. IFNγ-producing CD8⁺ and CD4⁺ T cells recognizing CFP10 epitopes were preferentially recruited to the lungs of M. tuberculosis–infected mice. In vivo cytolytic activity of CD8⁺ T cells specific for CFP10 and TB10.3/10.4 proteins was detected in the spleen, pulmonary lymph nodes, and lungs of infected mice. The cytolytic activity persisted long term and could be detected 260 d after infection. This paper highlights the cytolytic function of antigen-specific CD8⁺ T cells elicited by M. tuberculosis infection and demonstrates that large numbers of CFP10-specific cytolytic CD8⁺ T cells are recruited to the lung after M. tuberculosis infection.

Recovery of functional cytotoxic T lymphocytes during lamivudine therapy by acquiring multi-specificity

To characterize cytotoxic T lymphocytes (CTLs) that appeared in circulation during lamivudine therapy, we analyzed HBV-specific CTLs using HLA-A24 tetramer and HBcAg-specific Th1 cells in patients receiving lamivudine therapy. Six patients (HLA-A24(+)) with chronic hepatitis B, six patients (HLA-A24(-)) with chronic hepatitis B, and six patients (HLA-A24(+)) with chronic hepatitis C were studied. In addition to known CTL epitopes (C117 and P756), three epitopes were confirmed as CTL epitopes (C23, S89, S226) by chromium release assay and by staining intracellular perforin. CTLs specific for P756 were most frequently found at pre-treatment. During lamivudine therapy, increase in the frequencies of HLA-tetramer(+) cells was found for C117, S89, and S226. Recovery of CTLs was observed earlier in patients with HBeAg(-)/anti-HBe(+) compared with those with HBeAg(+)/anti-HBe(-). HBcAg-specific Th1 cells did not increase significantly up to 8 weeks. T cell lines from patients with chronic hepatitis B had a lower level of proliferation (0- to 24.9-fold expansion by in vitro stimulation) and a higher ability to produce interferon-gamma (0-84% except for S89), while perforin-positive cells showed low frequencies (0-50% except for S89). In conclusion, these results suggest that lamivudine therapy induces mainly CTLs that were less frequent before the therapy. Since recovered CTLs maintained the ability to produce interferon-gamma in response to peptides, these CTLs apparently contribute to the efficacy of lamivudine therapy in patients with hepatitis B.

Induction or expansion of T-cell responses by a hepatitis B DNA vaccine administered to chronic HBV carriers

Despite the availability of effective hepatitis B vaccines for many years, over 370 million people remain persistently infected with hepatitis B virus (HBV). Viral persistence is thought to be related to poor HBV-specific T-cell responses. A phase I clinical trial was performed in chronic HBV carriers to investigate whether HBV DNA vaccination could restore T-cell responsiveness. Ten patients with chronic active hepatitis B nonresponder to approved treatments for HBV infection were given 4 intramuscular injections of 1 mg of a DNA vaccine encoding HBV envelope proteins. HBV-specific T-cell responses were assessed by proliferation, ELISpot assays, and tetramer staining. Secondary end points included safety and the monitoring of HBV viraemia and serological markers. Proliferative responses to hepatitis B surface antigen were detected in two patients after DNA injections. Few HBV-specific interferon gamma-secreting T cells were detectable before immunization, but the frequency of such responses was significantly increased by 3 DNA injections. Immunization was well tolerated. Serum HBV DNA levels decreased in 5 patients after 3 vaccine injections, and complete clearance was observed in 1 patient. In conclusion, this study provides evidence that HBV DNA vaccination is safe and immunologically effective. We demonstrate that DNA vaccination can specifically but transiently activate T-cell responses in some chronic HBV carriers who do not respond to current antiviral therapies.

Novel peptide-based vaccines efficiently prime murine "help"-independent CD8+ T cell responses in the liver

Vaccines for the prophylactic and/or therapeutic immunization against hepatotropic pathogens (e.g., hepatitis B and hepatitis C virus) should establish long-lasting, specific antiviral effector/memory CD8+ T cell immunity in the liver. We describe a novel peptide-based vaccine in which antigenic major histocompatibility complex Class I-binding peptides are fused to a cationic (e.g., human immunodeficiency virus tat-derived) domain and complexed to immune-stimulating oligonucleotides. This vaccine formulation efficiently primes liver-homing, Class I-restricted CD8+ effector/memory T cell responses. In different antigen systems, this formulation was more potent in priming liver-homing CD8+ T cell responses than DNA-based vaccines delivering the same epitopes. CD8+ T cell priming was independent of CD4+ T cell "help" but submitted to regulatory control by CD25+ CD4+ T cells. The vaccine efficiently primed memory/effector CD8+ T cells detectable in the liver for more than 3 months after a single injection. With increasing time after priming, the phenotype of these specific memory CD8+ T cells shifted from an effector memory to a central memory type. The vaccine could override T cell tolerance in mice expressing the relevant antigen from a transgene in the liver. The CD8+ T cell immunity in the liver primed by this peptide formulation could be boosted by challenge injections. In conclusion, we describe a simple and potent vaccine formulation that has the potential to generate or reconstitute specific CD8+ T cell immunity to hepatotropic pathogens in the liver.

Direct binding of hepatitis C virus core to gC1qR on CD4+ and CD8+ T cells leads to impaired activation of Lck and Akt

Complement plays a pivotal role in the regulation of innate and adaptive immunity. It has been shown that the binding of C1q, a natural ligand of gC1qR, on T cells inhibits their proliferation. Here, we demonstrate that direct binding of the hepatitis C virus (HCV) core to gC1qR on T cells leads to impaired Lck/Akt activation and T-cell function. The HCV core associates with the surface of T cells specifically via gC1qR, as this binding is inhibited by the addition of either anti-gC1qR antibody or soluble gC1qR. The binding affinity constant of core protein for gC1qR, as determined by BIAcore analysis, is 3.8 x 10(-7) M. The specificity of the HCV core-gC1qR interaction is confirmed by reduced core binding on Molt-4 T cells treated with gC1qR-silencing small interfering RNA and enhanced core binding on GPC-16 guinea pig cells transfected with human gC1qR. Interestingly, gC1qR is expressed at higher levels on CD8(+) than on CD4(+) T cells, resulting in more severe core-induced suppression of the CD8(+)-T-cell population. Importantly, T-cell receptor-mediated activation of the Src kinases Lck and ZAP-70 but not Fyn and the phosphorylation of Akt are impaired by the HCV core, suggesting that it inhibits the very early events of T-cell activation.

Phase I/II clinical trial of a nonreplicative vaccinia virus expressing multiple HLA-A0201-restricted tumor-associated epitopes and costimulatory molecules in metastatic melanoma patients

We performed a phase I/II clinical trial in metastatic melanoma patients with an ultraviolet (UV)-inactivated nonreplicating recombinant vaccinia virus enabling the expression, from a single construct, of endoplasmic reticulum-targeted HLA-A0201-restricted Melan-A/MART-1(27-35), gp100(280-288), and tyrosinase(1-9) epitopes, together with CD80 and CD86 costimulatory proteins. Corresponding soluble peptides were used to boost responses and granulocyte-macrophage colony-stimulating factor was used as systemic adjuvant. Safety and immunogenicity, as monitored with in vitro-restimulated peripheral blood mononuclear cells by cytotoxic T lymphocyte precursor (CTLp) frequency analysis and tetramer staining, were specifically addressed. Of 20 patients entering the protocol, 2 had to withdraw because of rapidly progressing disease. Immune responses were evaluated in 18 patients (stage III, n = 5; stage IV, n = 13) and increases in specific CTLp frequencies were observed in 15. In 16 patients responsiveness against all 3 antigens could be analyzed: 7 (43%), including all stage III cases, showed evidence of induction of CTLs specific for the three epitopes, and 2 (12%) and 4 (25%), respectively, showed reactivity against two or one tumor-associated antigen. In three stage IV patients no specific CTL reactivity could be induced. Increases in CTLp frequency were detected mostly after viral vaccine injections. However, in a majority of patients final CTLp levels were comparable to initial levels. Tetramer characterization of Melan-A/MART-1(27-35)-specific CTLs during the protocol also suggested preferential expansion after recombinant virus administration. Vector-specific humoral responses, frequently undetectable in stage IV patients, did not appear to prevent tumor-associated antigen-specific CTL induction. Aside from a single occurrence of transient grade 3 leukopenia, no major clinical toxicity was reported. Seventeen of 18 patients completed the 3-month trial (one patient died before the last delayed-type hypersensitivity test). Three displayed regression of individual metastases, seven had stable disease, and progressive disease was observed in seven patients. This is the first report on the administration of a UV-inactivated recombinant vaccinia virus coexpressing five transgenes in cancer patients. The results described here, in terms of safety and immunogenicity, support the use of this reagent in active specific immunotherapy.

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Longitudinal analysis of CD8+ T cells specific for structural and nonstructural hepatitis B virus proteins in patients with chronic hepatitis B: implications for immunotherapy

The cytotoxic T-cell response in chronic hepatitis B virus (HBV) infection has been described as weak and mono- or oligospecific in comparison to the more robust virus-specific T-cell response present in resolved infection. However, chronic hepatitis B is a heterogeneous disease with markedly variable levels of virus replication and liver disease activity. Here we analyzed (both directly ex vivo and after in vitro stimulation) the HBV-specific CD8 T-cell responses against structural and nonstructural HBV proteins longitudinally in patients with different patterns of chronic infections. We found that the profiles of virus-specific CD8(+)-T-cell responses during chronic infections are highly heterogeneous and influenced more by the level of HBV replication than by the activity of liver disease. An HBV DNA load of <10(7) copies/ml appears to be the threshold below which circulating multispecific HBV-specific CD8(+) T cells are consistently detected. Furthermore, CD8(+) T cells with different specificities are differentially regulated during chronic infections. HBV core-specific CD8(+) T cells are associated with viral control, while CD8(+) T cells specific for envelope and polymerase epitopes can occasionally be found in the setting of high levels (>10(7) copies) of HBV replication. These findings have implications for the design of immunotherapy for chronic HBV infections.

Comprehensive analysis of the quantity of epitope-specific cytotoxic T lymphocytes in chronic viral hepatitis B infection

AIM: To investigate the function state of epitope-specific cytotoxic T lymphocytes (CTLs) in chronic hepatitis B infection

METHODS: The study was performed to quantify the HBV specific CTL directly in vitro by HLA-A2 tetrameric complexes for core 18-27 (Tc 18-27), envelope 183-191 (Te 183-191), envelope 335-343 (Te 335-343), and polymerase 575-583 (Tp 575-583) in active chronic hepatitis patients, and then the correlation of HBV epitope-specific CTL between serum HBV DNA loads or alanine aminotransmerase (ALT) levels were analyzed by multiple regression analysis.

RESULTS: It was found that there were multiple CTLs responses in active chronic hepatitis patients. The frequency of Tc18-27 response was higher than the other three epitope-specific CTLs. No significant correlation was found either between the frequency of HBV specific CD8⁺ T cells and the viral load, or the frequency of HBV specific CD8⁺ T cells and the levels of alanine transaminase.

CONCLUSION: The frequencies of HBV-specific T cells are not determinant of immune-mediated protection in HBV infection and the existence of epitope-specific HBV CTLs is not directly correlated to hepatocytic injury.

Differential tissue-specific regulation of antiviral CD8+ T-cell immune responses during chronic viral infection

The hallmarks of the immune response to viral infections are the expansion of antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) after they encounter antigen-presenting cells in the lymphoid tissues and their subsequent redistribution to nonlymphoid tissues to deal with the pathogen. Control mechanisms exist within CTL activation pathways to prevent inappropriate CTL responses against disseminating infections with a broad distribution of pathogen in host tissues. This is demonstrated during overwhelming infection with the noncytolytic murine lymphocytic choriomeningitis virus, in which clonal exhaustion (anergy and/or deletion) of CTLs prevents immune-mediated pathology but allows persistence of the virus. The mechanism by which the immune system determines whether or not to mount a full response to such infections is unknown. Here we present data showing that the initial encounter of specific CTLs with infected cells in lymphoid tissues is critical for this decision. Whether the course of the viral infection is acute or persistent for life primarily depends on the degree and kinetics of CTL exhaustion in infected lymphoid tissues. Virus-driven CTL expansion in lymphoid tissues resulted in the migration of large quantities of CTLs to nonlymphoid tissues, where they persisted at stable levels. Surprisingly, although virus-specific CTLs were rapidly clonally exhausted in lymphoid tissues under conditions of chronic infection, a substantial number of them migrated to nonlymphoid tissues, where they retained an effector phenotype for a long time. However, these cells were unable to control the infection and progressively lost their antiviral capacities (cytotoxicity and cytokine secretion) in a hierarchical manner before their eventual physical elimination. These results illustrate the differential tissue-specific regulation of antiviral T-cell responses during chronic infections and may help us to understand the dynamic relationship between antigen and T-cell populations in many persistent infections in humans.

Analysis of CD8 T-cell response by IFNgamma ELISPOT and H-2L(d)/pRL1a tetramer assays in pRL1a multiple antigen peptide-immunized and RL male 1-bearing BALB/c and (BALB/c x C57BL/6) F(1) mice

We previously identified an H-2L(d)-binding peptide pRL1a (IPGLPLSL) on RL male 1 that is predominantly recognized by cytotoxic T-lymphocytes (CTLs). MAP is a multibranched lysine core with antigenic peptides. Immunization of BALB/c mice with pRL1a MAP effectively induced pRL1a CTLs. Here, we demonstrate the presence of pRL1a-recognizing CD8(+) T-cells in pRL1a MAP-immunized and RL male 1-bearing BALB/c and (BALB/c x C57BL/6)F(1) mice by using IFNgamma ELISPOT and H-2L(d)/pRL1a tetramer assays. A few IFNgamma ELISPOTs and no tetramer-positive cells were detected ex vivo in spleen cells from BALB/c mice immunized with pRL1a MAP. After a single in vitro stimulation with RL male 1, 432 and 741 IFNgamma ELISPOTs/10(5) cells were detected and tetramer-positive CD8(+) T-cells occurred at relative frequencies of 5.7% and 30.8% in splenic CD8(+) T-cells from mice that had been doubly and triply immunized, respectively, against pRL1a MAP. Tetramer-positive cells displayed two distinct cell populations, CD62L(low) and CD62L(high). Secondary in vitro stimulation expanded CD62L(high) cells more efficiently than CD62L(low) cells. Furthermore, a higher frequency of IFNgamma-producing and tetramer-positive CD8(+) T-cells was detected ex vivo in RL male 1-bearing semi-allogeneic (BALB/c x C57BL/6)F(1) than in BALB/c mice on day 14 after tumor inoculation.

Suppression of HCV-specific T cells without differential hierarchy demonstrated ex vivo in persistent HCV infection

Hepatitis C virus (HCV) has a high propensity for persistence. To better define the immunologic determinants of HCV clearance and persistence, we examined the circulating HCV-specific T-cell frequency, repertoire, and cytokine phenotype ex vivo in 24 HCV seropositive subjects (12 chronic, 12 recovered), using 361 overlapping peptides in 36 antigenic pools that span the entire HCV core, NS3-NS5. Consistent with T-cell-mediated control of HCV, the overall HCV-specific type-1 T-cell response was significantly greater in average frequency (0.24% vs. 0.04% circulating lymphocytes, P =.001) and scope (14/36 vs. 4/36 pools, P =.002) among the recovered than the chronic subjects, and the T-cell response correlated inversely with HCV titer among the chronic subjects (R = -0.51, P =.049). Although highly antigenic regions were identified throughout the HCV genome, there was no apparent difference in the overall HCV-specific T-cell repertoire or type-1/type-2 cytokine profile relative to outcome. Notably, HCV persistence was associated with a reversible CD4-mediated suppression of HCV-specific CD8 T cells and with higher frequency of CD4(+)CD25(+) regulatory T cells (7.3% chronic vs. 2.5% recovered, P =.002) that could directly suppress HCV-specific type-1 CD8 T cells ex vivo. In conclusion, we found that HCV persistence is associated with a global quantitative and functional suppression of HCV-specific T cells but not differential antigenic hierarchy or cytokine phenotype relative to HCV clearance. The high frequency of CD4(+)CD25(+) regulatory T cells and their suppression of HCV-specific CD8 T cells ex vivo suggests a novel role for regulatory T cells in HCV persistence.

A novel MHCp binding prediction model

Many statistical and molecular mechanics models have been developed and tested for major histocompatibility complex peptide (MHCp) binding predictions during the last decade. The statistical model prediction using pooled peptide sequence data and three-dimensional modeling prediction by molecular mechanics calculations have been assessed for efficiency and human leukocyte antigen diversity coverage. We describe a novel predictive model using information gleaned from 29 human MHCp crystal structures. The validation for the new model is performed using four different sets of data: (1) MHCp crystal structures, (2) peptides with known IC(50) binding values, (3) peptides tested positive by tetramer staining, (4) peptides with known binding information at the MHCBN database. The model produces high prediction efficiencies (average 60 %) with good sensitivity (approximately 50%-73%) and specificity (52%-58%) values. The average positive predictive value of the model is 89%, while the average negative predictive value is only 18%. The efficiency is very high in predicting binders and very low in predicting nonbinders. This model is superior to many existing methods because of its potential application to any given MHC allele whose sequence is clearly defined.

Expression of HLA-I on HepG2 cells by hepatitis B virus nucleocapsid mutants

AIM

To study the expression of HLA-I/antigen peptide complex on HepG2 cells transfected with HBV (adr) wild type and nucleocapsid protein mutants.

METHODS

The site-directed mutation was performed to introduce nucleocapsid protein point mutations V60 and L97 into 1.2 copies of HBV genome plasmid p3.8 Ⅱ. After identification of DNA sequence and biological activities, the plasmid p3.8Ⅱ and mutant plasmid constructs were subcloned respectively into EB virus based vector EBO-plpp for stable expression. The vector constructed EBO-wild type, EBO-V60, and EBO-L97 were analyzed by restriction enzyme digestion and DNA sequenceing, then transfected into HepG2 cells via the liposome technique, respectively. HBV antigen in their culture supernatants was quantified by Abbott kits. The cells were stained with murine monoclonal antibody anti-HLA-ABC conjugated directly to FITC, and expression of HLA-I on their membrane was analyzed by flow-cytometry.

RESULTS

Restriction enzyme digestion of 3 vector constructs showed two bands similar to HBV 1.2 copies genome and EBO vector, respectively. Analysis of DNA sequence confirmed the mutated nucleotides of EBO-V60 and EBO-L97 (i.e nt2078 C→G, nt2189 A→C). The expression of HBeAg S/CO in culture supernatant of EBO-wild type was much higher than that of mutant EBO-V60 and EBO-L97, while the expression of HBsAg S/N in three constructs had similar level, indicating similar transfecting rate in this experiment. The expression of HLA-I on HepG2 cells transfected with EBO empty vector was at low level. Fluorescence intensity of HLA-I expression of transfected cells was elevated by EBO-wild type (18.2), while that of L97 was increased to 34.5 and V60 declined to 3.4.

CONCLUSION

HBV might enhance the expression of HLA-I/antigen peptide complex on HepG2 cells. Hot-spot mutations of HBV nucleocapsid protein L97 and V60 could influence the expression level of HLA-I on host cells.

Kinetics of the immune response during HBV and HCV infection

The innate immune system has a role not only in protecting the host during the initial period of virus infection, but also in shaping the nature of the adaptive immune response. In this review, we follow the kinetics of the virologic and immunologic events occurring from the time of hepatitis B virus (HBV) and hepatitis C virus (HCV) infection. We primarily discuss how the early events after infection might influence the development of the adaptive immune response in these 2 important viral infections and how new strategies for more efficient preventive and therapeutic vaccines can be derived from this knowledge.

T-cell responses of vaccinated cancer patients

Following vaccination with defined tumor antigens that are recognized by T cells, a small proportion of cancer patients display tumor regressions. Several reports describe anti-vaccine T-cell responses, evaluated with a variety of methods, for example, by assessing T-cell function or expression of specific TCR. However, a correlation between these T-cell responses and the tumor regressions has not yet been established. It appears that some patients display tumor regression with an unexpectedly low frequency of anti-vaccine T cells.

Lack of effector cell function and altered tetramer binding of tumor-infiltrating lymphocytes

Tumor-specific CD8 T cell responses to MCA102 fibrosarcoma cells expressing the cytotoxic T cell epitope gp33 from lymphocytic choriomeningitis virus were studied. MCA102(gp33) tumors grew progressively in C57BL/6 mice, despite induction of peripheral gp33-tetramer(+) T cells that were capable of mediating antiviral protection, specific cell rejection, and concomitant tumor immunity. MCA102(gp33) tumors were infiltrated with a high number ( approximately 20%) of CD11b(+)CD11c(-) macrophage-phenotype cells that were able to cross-present the gp33 epitope to T cells. Tumor-infiltrating CD8 T cells exhibited a highly activated phenotype but lacked effector cell function. Strikingly, a significant portion of tumor-infiltrating lymphocytes expressed TCRs specific for gp33 but bound MHC tetramers only after cell purification and a 24-h resting period in vitro. The phenomenon of "tetramer-negative T cells" was not restricted to tumor-infiltrating lymphocytes from MCA102(gp33) tumors, but was also observed when Ag-specific T cells derived from an environment with high Ag load were analyzed ex vivo. Thus, using a novel tumor model, allowing us to trace tumor-specific T cells at the single cell level in vivo, we demonstrate that the tumor microenvironment is able to alter the functional activity of T cells infiltrating the tumor mass.

Labeling antigen-specific CD4(+) T cells with class II MHC oligomers

Class I MHC-peptide oligomers (MHC tetramers) have become popular reagents for the detection and characterization of antigen-specific CD8(+) T cells. Class II MHC proteins can be produced by expression in Escherichia coli followed by in vitro folding, or by native expression in insect cells; biotin can be introduced by site-specific chemical modification of cysteine, or by enzymatic modification of a peptide tag; and a variety of fluorescent streptavidin preparations can be used for oligomerization. Here we review methodologies for production of fluorescent oligomers of soluble class II MHC proteins and discuss their use in analysis of antigen-specific CD4(+) T cells. We explore the experimental conditions necessary for efficient staining of CD4(+) T cells using oligomers of class II MHC proteins, and we establish a standard protocol. Finally, we consider complications and challenges associated with these reagents, discuss the interpretation of staining results, and suggest future directions for investigation, in particular the use of MHC oligomers for the study of T cell avidity modulation.

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.

Differential kinetics of antigen-specific CD4+ and CD8+ T cell responses in the regression of retrovirus-induced sarcomas

Despite the accepted role for CD4+ T cells in immune control, little is known about the development of Ag-specific CD4+ T cell immunity upon primary infection. Here we use MHC class II tetramer technology to directly visualize the Ag-specific CD4+ T cell response upon infection of mice with Moloney murine sarcoma and leukemia virus complex (MoMSV). Significant numbers of Ag-specific CD4+ T cells are detected both in lymphoid organs and in retrovirus-induced lesions early during infection, and they express the 1B11-reactive activation-induced isoform of CD43 that was recently shown to define effector CD8+ T cell populations. Comparison of the kinetics of the MoMSV-specific CD4+ and CD8+ T cell responses reveals a pronounced shift toward CD8+ T cell immunity at the site of MoMSV infection during progression of the immune response. Consistent with an important early role of Ag-specific CD4+ T cell immunity during MoMSV infection, CD4+ T cells contribute to the generation of virus-specific CD8+ T cell immunity within the lymphoid organs and are required to promote an inflammatory environment within the virus-infected tissue.