Selection of high-avidity CD8 T cells correlates with control of hepatitis C virus infection

Newborn and child-like molecular signatures in older adults stem from TCR shifts across human lifespan

CD8+ T cells provide robust antiviral immunity, but how epitope-specific T cells evolve across the human lifespan is unclear. Here we defined CD8+ T cell immunity directed at the prominent influenza epitope HLA-A*02:01-M158-66 (A2/M158) across four age groups at phenotypic, transcriptomic, clonal and functional levels. We identify a linear differentiation trajectory from newborns to children then adults, followed by divergence and a clonal reset in older adults. Gene profiles in older adults closely resemble those of newborns and children, despite being clonally distinct. Only child-derived and adult-derived A2/M158+CD8+ T cells had the potential to differentiate into highly cytotoxic epitope-specific CD8+ T cells, which was linked to highly functional public T cell receptor (TCR)αβ signatures. Suboptimal TCRαβ signatures in older adults led to less proliferation, polyfunctionality, avidity and recognition of peptide mutants, although displayed no signs of exhaustion. These data suggest that priming T cells at different stages of life might greatly affect CD8+ T cell responses toward viral infections.

Adoptive Cellular Therapy for Multiple Myeloma Using CAR- and TCR-Transgenic T Cells: Response and Resistance

Despite the substantial improvement of therapeutic approaches, multiple myeloma (MM) remains mostly incurable. However, immunotherapeutic and especially T cell-based approaches pioneered the therapeutic landscape for relapsed and refractory disease recently. Targeting B-cell maturation antigen (BCMA) on myeloma cells has been demonstrated to be highly effective not only by antibody-derived constructs but also by adoptive cellular therapies. Chimeric antigen receptor (CAR)-transgenic T cells lead to deep, albeit mostly not durable responses with manageable side-effects in intensively pretreated patients. The spectrum of adoptive T cell-transfer covers synthetic CARs with diverse specificities as well as currently less well-established T cell receptor (TCR)-based personalized strategies. In this review, we want to focus on treatment characteristics including efficacy and safety of CAR- and TCR-transgenic T cells in MM as well as the future potential these novel therapies may have. ACT with transgenic T cells has only entered clinical trials and various engineering strategies for optimization of T cell responses are necessary to overcome therapy resistance mechanisms. We want to outline the current success in engineering CAR- and TCR-T cells, but also discuss challenges including resistance mechanisms of MM for evading T cell therapy and point out possible novel strategies.

A minimal model of T cell avidity may identify subtherapeutic vaccine schedules

T cells protect the body from cancer by recognising tumour-associated antigens. Recognising these antigens depends on multiple factors, one of which is T cell avidity, i.e., the total interaction strength between a T cell and a cancer cell. While both high- and low-avidity T cells can kill cancer cells, durable anti-cancer immune responses require the selection of high-avidity T cells. Previous experimentation with anti-cancer vaccines, however, has shown that most vaccines elicit low-avidity T cells. Optimising vaccine schedules may remedy this by preferentially selecting high-avidity T cells. Here, we use mathematical modelling to develop a simple, phenomenological model of avidity selection that may identify vaccine schedules that disproportionately favour low-avidity T cells. We calibrate our model to our prior, more complex model, and then validate it against several experimental data sets. We find that the sensitivity of the model's parameters change with vaccine dosage, which allows us to use a patient's data and clinical history to screen for suitable vaccine strategies.

Role of CD4+ T Cells in the Control of Viral Infections: Recent Advances and Open Questions

CD4+ T cells orchestrate adaptive immune responses through their capacity to recruit and provide help to multiple immune effectors, in addition to exerting direct effector functions. CD4+ T cells are increasingly recognized as playing an essential role in the control of chronic viral infections. In this review, we present recent advances in understanding the nature of CD4+ T cell help provided to antiviral effectors. Drawing from our studies of natural human immunodeficiency virus (HIV) control, we then focus on the role of high-affinity T cell receptor (TCR) clonotypes in mediating antiviral CD4+ T cell responses. Last, we discuss the role of TCR affinity in determining CD4+ T cell differentiation, reviewing the at times divergent studies associating TCR signal strength to the choice of a T helper 1 (Th1) or a T follicular helper (Tfh) cell fate.

Chronic Viral Liver Diseases: Approaching the Liver Using T Cell Receptor-Mediated Gene Technologies

Chronic infection with viral hepatitis is a major risk factor for liver injury and hepatocellular carcinoma (HCC). One major contributing factor to the chronicity is the dysfunction of virus-specific T cell immunity. T cells engineered to express virus-specific T cell receptors (TCRs) may be a therapeutic option to improve host antiviral responses and have demonstrated clinical success against virus-associated tumours. This review aims to give an overview of TCRs identified from viral hepatitis research and discuss how translational lessons learned from cancer immunotherapy can be applied to the field. TCR isolation pipelines, liver homing signals, cell type options, as well as safety considerations will be discussed herein.

Peripheral PD-1+ T Cells Co-expressing Inhibitory Receptors Predict SVR With Ultra Short Duration DAA Therapy in HCV Infection

Direct acting antiviral (DAA) regimens of 12 weeks result in HCV clearance in vast majority of patients across genotypes. We previously demonstrated an ultra-short regimen of 4 weeks DAA cleared HCV in a subset of patients. Here, we hypothesized that individual level of antiviral immunity differentially influenced viral clearance and investigated biomarkers of a successful response. Cohorts of HCV patients treated for 4 weeks with DAA therapy who either achieved sustained virologic response (SVR) or relapsed were compared at baseline and at end of therapy (EOT) for immune cell phenotypes and HCV specific immunity. Higher levels of PD-1⁺ CD8⁺ and CD4⁺ T lymphocytes co-expressing inhibitory receptors (IR) were present at baseline and at EOT in HCV patients who eventually achieved SVR compared with those who relpased. HCV specific CD8⁺ T cells were predominantly contained within these IR expressing PD-1⁺ subsets. Patients in the SVR group had significantly higher CD8⁺ T cell degranulation in response to HCV peptides at baseline and higher levels of cytokine producing T cells at EOT time-point, relative to those who relapsed. In ex vivo cultures, PD-1⁺CD160⁺ CD8⁺ T cells had higher HCV specific degranulation and PD-1⁺2B4⁺ CD8⁺ T cells had higher cytokine expression (IFNγ⁺TNFα⁺ or IFNγ⁺CD107a⁺) compared with single or no IR expressing subsets, indicating higher virus specific functional capacity of these subsets. Receiver operating characteristics curve (ROC) for baseline circulating frequencies of PD-1⁺CD160⁺, PD-1⁺Tim-3⁺ CD8⁺ T cells and PD-1⁺CD160⁺, PD-1⁺Blimp-1⁺, PD-1^-CTLA4⁺ CD4⁺ T cells respectively, had associated C-statistics of 0.8214 and 0.9451 for discriminatin of patients who successfully cleared HCV with 4 weeks treatment. Thus, PD-1⁺ virus-specific CD8⁺ T cell subsets with cytotoxic capacity are present in a subset of chronic HCV infected individuals that associate with ability to achieve SVR, indicating role of immunity in DAA mediated viral clearance with short duration therapy.

DNA Vaccination by Electroporation Amplifies Broadly Cross-Restricted Public TCR Clonotypes Shared with HIV Controllers

Rare patients who spontaneously control HIV replication provide a useful model to inform HIV vaccine development. HIV controllers develop particularly efficient antiviral CD4+ T cell responses mediated by shared high-affinity TCRs. To determine whether the candidate DNA vaccine ADVAX could induce similar responses, we analyzed Gag-specific primary CD4+ T cells from healthy volunteers who received ADVAX DNA by electroporation. Vaccinated volunteers had an immunodominant response to the Gag293 epitope with a functional avidity intermediate between that of controllers and treated patients. The TCR repertoire of Gag293-specific CD4+ T cells proved highly biased, with a predominant usage of the TCRβ variable gene 2 (TRBV2) in vaccinees as well as controllers. TCRα variable gene (TRAV) gene usage was more diverse, with the dominance of TRAV29 over TRAV24 genes in vaccinees, whereas TRAV24 predominated in controllers. Sequence analysis revealed an unexpected degree of overlap between the specific repertoires of vaccinees and controllers, with the sharing of TRAV24 and TRBV2 public motifs (>30%) and of public clonotypes characteristic of high-affinity TCRs. MHC class II tetramer binding revealed a broad HLA-DR cross-restriction, explaining how Gag293-specific public clonotypes could be selected in individuals with diverse genetic backgrounds. TRAV29 clonotypes also proved cross-restricted, but conferred responses of lower functional avidity upon TCR transfer. In conclusion, DNA vaccination by electroporation primed for TCR clonotypes that were associated with HIV control, highlighting the potential of this vaccine delivery method. To our knowledge, this study provides the first proof-of-concept that clonotypic analysis may be used as a tool to monitor the quality of vaccine-induced responses and modulate these toward "controller-like" responses.

Immune Evasion Strategies during Chronic Hepatitis B and C Virus Infection

Both hepatitis B virus (HBV) and hepatitis C virus (HCV) infections are a major global healthcare problem with more than 240 million and 70 million infected, respectively. Both viruses persist within the liver and result in progressive liver disease, resulting in liver fibrosis, cirrhosis and hepatocellular carcinoma. Strikingly, this pathogenesis is largely driven by immune responses, unable to clear an established infection, rather than by the viral pathogens themselves. Even though disease progression is very similar in both infections, HBV and HCV have evolved distinct mechanisms, by which they ensure persistence within the host. Whereas HCV utilizes a cloak-and-dagger approach, disguising itself as a lipid-like particle and immediately crippling essential pattern-recognition pathways, HBV has long been considered a "stealth" virus, due to the complete absence of innate immune responses during infection. Recent developments and access to improved model systems, however, revealed that even though it is among the smallest human-tropic viruses, HBV may, in addition to evading host responses, employ subtle immune evasion mechanisms directed at ensuring viral persistence in the absence of host responses. In this review, we compare the different strategies of both viruses to ensure viral persistence by actively interfering with viral recognition and innate immune responses.

TCR-ligand dissociation rate is a robust and stable biomarker of CD8+ T cell potency

Despite influencing many aspects of T cell biology, the kinetics of T cell receptor (TCR) binding to peptide-major histocompatibility molecules (pMHC) remain infrequently determined in patient monitoring or for adoptive T cell therapy. Using specifically designed reversible fluorescent pMHC multimeric complexes, we performed a comprehensive study of TCR-pMHC off-rates combined with various functional assays on large libraries of self/tumor- and virus-specific CD8+ T cell clones from melanoma patients and healthy donors. We demonstrate that monomeric TCR-pMHC dissociation rates accurately predict the extent of cytotoxicity, cytokine production, polyfunctionality, cell proliferation, activating/inhibitory receptor expression, and in vivo antitumor potency of naturally occurring antigen-specific CD8+ T cells. Our data also confirm the superior binding avidities of virus-specific T cells as compared with self/tumor-specific T cell clonotypes (n > 300). Importantly, the TCR-pMHC off-rate is a more stable and robust biomarker of CD8+ T cell potency than the frequently used functional assays/metrics that depend on the T cell's activation state, and therefore show major intra- and interexperimental variability. Taken together, our data show that the monomeric TCR-pMHC off-rate is highly useful for the ex vivo high-throughput functional assessment of antigen-specific CD8+ T cell responses and a strong candidate as a biomarker of T cell therapeutic efficacy.

Hepatitis C Virus-Specific T Cell Receptor mRNA-Engineered Human T Cells: Impact of Antigen Specificity on Functional Properties

Therapy with genetically modified autologous T cells has shown great promise in cancer therapy. For an efficient control of hepatitis C virus (HCV) infection, cytotoxic T cells (CTL) are pivotal, but persistence of activated T cells may lead to liver toxicity. Here, anti-HCV T cell receptors (TCRs) recognizing the HCV nonstructural (NS) NS3 or NS5 viral peptide target were examined by mRNA transfection of human peripheral blood lymphocytes (PBLs) derived from healthy donors as well as chronically infected HCV patients. Immunological analysis shows that while the CTLs expressing the NS5-specific TCR reduced HCV RNA replication by a noncytotoxic mechanism, the NS3-specific TCR-redirected CTLs were polyfunctional and inhibited HCV RNA replication through antigen-specific cytotoxicity. Transcriptome signatures from these two types of CTL responses revealed uniquely expressed gene clusters upon encountering hepatoma target cells presenting endogenously expressed HCV proteins. The NS3 TCR induced a rapid expression of apoptotic signaling pathways and formation of embryonic gene clusters, whereas the NS5A TCR activation induced extended proliferative and metabolic pathways as the HCV target cells survived. Our results provide detailed insights into basic HCV T cell immunology and have clinical relevance for redirecting T cells to target virally infected hepatoma cells.IMPORTANCE Due to the protective ability of HCV-specific T cells and the hepatotoxic potential that they possess, there is a great need for the understanding of the functional aspects of HCV-specific T cells. To circumvent the low level of precursor frequency in patients, we engineered primary CD8⁺ T cells by mRNA TCR vectors to confer HCV specificity to new T cells. HCV TCRs that differ in antigen specificity and polyfunctionality were examined. mRNA TCR engineering of peripheral blood lymphocytes from healthy donors or chronically infected HCV patients resulted in strikingly high levels of HCV TCR expression and HCV-specific responses. While a cytotoxicity response from a polyfunctional T cell activation caused hepatotoxicity and the rapid induction of apoptotic signaling pathways, the noncytotoxic T cell activation showed extended proliferative, metabolic pathways and persistence of HCV target cells. Our results provide detailed insights into basic HCV T cell immunology and have clinical relevance for immune protection of HCV-associated diseases.

Selective expansion of high functional avidity memory CD8 T cell clonotypes during hepatitis C virus reinfection and clearance

The dynamics of the memory CD8 T cell receptor (TCR) repertoire upon virus re-exposure and factors governing the selection of TCR clonotypes conferring protective immunity in real life settings are poorly understood. Here, we examined the dynamics and functionality of the virus-specific memory CD8 TCR repertoire before, during and after hepatitis C virus (HCV) reinfection in patients who spontaneously resolved two consecutive infections (SR/SR) and patients who resolved a primary but failed to clear a subsequent infection (SR/CI). The TCR repertoire was narrower prior to reinfection in the SR/SR group as compared to the SR/CI group and became more focused upon reinfection. CD8 T cell clonotypes expanding upon re-exposure and associated with protection from viral persistence were recruited from the memory T cell pool. Individual CD8 T cell lines generated from the SR/SR group exhibited higher functional avidity and polyfunctionality as compared to cell lines from the SR/CI group. Our results suggest that protection from viral persistence upon HCV reinfection is associated with focusing of the HCV-specific CD8 memory T cell repertoire from which established cell lines showed high functional avidity. These findings are applicable to vaccination strategies aiming at shaping the protective human T cell repertoire.

In this study we examined the diversity and dynamics of the repertoire of receptors of CD8 T cells that are selected and enriched upon real-life multiple exposures to viral infections. Using hepatitis C virus (HCV) infection in a cohort of high risk people who inject drugs, we demonstrate that protection upon two subsequent infections was associated with a narrow repertoire of virus-specific CD8 T cells and selective expansion of cells with high polyfunctionality (increased TNFα production and cytotoxic potential). Our results have important implications in vaccination programs aiming at shaping the CD8 T cell repertoire against viral infections and cancers.

Lack of Heterologous Cross-reactivity toward HLA-A*02:01 Restricted Viral Epitopes Is Underpinned by Distinct αβT Cell Receptor Signatures

αβT cell receptor (TCR) genetic diversity is outnumbered by the quantity of pathogenic epitopes to be recognized. To provide efficient protective anti-viral immunity, a single TCR ideally needs to cross-react with a multitude of pathogenic epitopes. However, the frequency, extent, and mechanisms of TCR cross-reactivity remain unclear, with conflicting results on anti-viral T cell cross-reactivity observed in humans. Namely, both the presence and lack of T cell cross-reactivity have been reported with HLA-A*02:01-restricted epitopes from the Epstein-Barr and influenza viruses (BMLF-1 and M1₅₈, respectively) or with the hepatitis C and influenza viruses (NS3₁₀₇₃ and NA₂₃₁, respectively). Given the high sequence similarity of these paired viral epitopes (56 and 88%, respectively), the ubiquitous nature of the three viruses, and the high frequency of the HLA-A*02:01 allele, we selected these epitopes to establish the extent of T cell cross-reactivity. We combined ex vivo and in vitro functional assays, single-cell αβTCR repertoire sequencing, and structural analysis of these four epitopes in complex with HLA-A*02:01 to determine whether they could lead to heterologous T cell cross-reactivity. Our data show that sequence similarity does not translate to structural mimicry of the paired epitopes in complexes with HLA-A*02:01, resulting in induction of distinct αβTCR repertoires. The differences in epitope architecture might be an obstacle for TCR recognition, explaining the lack of T cell cross-reactivity observed. In conclusion, sequence similarity does not necessarily result in structural mimicry, and despite the need for cross-reactivity, antigen-specific TCR repertoires can remain highly specific.

Antigen spreading-induced CD8+T cells confer protection against the lethal challenge of wild-type malignant mesothelioma by eliminating myeloid-derived suppressor cells

A key focus in cancer immunotherapy is to investigate the mechanism of efficacious vaccine responses. Using HIV-1 GAG-p24 in a model PD1-based DNA vaccine, we recently reported that vaccine-elicited CD8⁺ T cells conferred complete prevention and therapeutic cure of AB1-GAG malignant mesothelioma in immunocompetent BALB/c mice. Here, we further investigated the efficacy and correlation of protection on the model vaccine-mediated antigen spreading against wild-type AB1 (WT-AB1) mesothelioma. We found that this vaccine was able to protect mice completely from three consecutive lethal challenges of AB1-GAG mesothelioma. Through antigen spreading these animals also developed tumor-specific cytotoxic CD8⁺ T cells, but neither CD4⁺ T cells nor antibodies, rejecting WT-AB1 mesothelioma. A majority of these protected mice (90%) were also completely protected against the lethal WT-AB1 challenge. Adoptive cell transfer experiments further demonstrated that antigen spreading-induced CD8⁺ T cells conferred efficacious therapeutic effects against established WT-AB1 mesothelioma and prevented the increase of exhausted PD-1⁺ and Tim-3⁺ CD8⁺ T cells. A significant inverse correlation was found between the frequency of functional PD1⁻Tim3⁻ CD8⁺ T cells and that of MDSCs or tumor mass in vivo. Mechanistically, we found that WT-AB1 mesothelioma induced predominantly polymorphonuclear (PMN) MDSCs in vivo. In co-cultures with efficacious CD8⁺ T cells, a significant number of PMN-MDSCs underwent apoptosis in a dose-dependent way. Our findings indicate that efficacious CD8⁺ T cells capable of eliminating both tumor cells and MDSCs are likely necessary for fighting wild-type malignant mesothelioma.

Human influenza viruses and CD8(+) T cell responses

Influenza A viruses (IAVs) cause significant morbidity and mortality worldwide, despite new strain-specific vaccines being available annually. As IAV-specific CD8(+) T cells promote viral control in the absence of neutralizing antibodies, and can mediate cross-reactive immunity toward distinct IAVs to drive rapid recovery from both mild and severe influenza disease, there is great interest in developing a universal T cell vaccine. However, despite detailed studies in mouse models of influenza virus infection, there is still a paucity of data on human epitope-specific CD8(+) T cell responses to IAVs. This review focuses on our current understanding of human CD8(+) T cell immunity against distinct IAVs and discusses the possibility of achieving a CD8(+) T cell mediated-vaccine that protects against multiple, distinct IAV strains across diverse human populations. We also review the importance of CD8(+) T cell immunity in individuals highly susceptible to severe influenza infection, including those hospitalised with influenza, the elderly and Indigenous populations.

Frequency, Private Specificity, and Cross-Reactivity of Preexisting Hepatitis C Virus (HCV)-Specific CD8+ T Cells in HCV-Seronegative Individuals: Implications for Vaccine Responses

T cell responses play a critical role in controlling or clearing viruses. Therefore, strategies to prevent or treat infections include boosting T cell responses. T cells specific for various pathogens have been reported in unexposed individuals and an influence of such cells on the response toward vaccines is conceivable. However, little is known about their frequency, repertoire, and impact on vaccination. We performed a detailed characterization of CD8(+) T cells specific to a hepatitis C virus (HCV) epitope (NS3-1073) in 121 HCV-seronegative individuals. We show that in vitro HCV NS3-1073-specific CD8(+) T cell responses were rather abundantly detectable in one-third of HCV-seronegative individuals irrespective of risk factors for HCV exposure. Ex vivo, these NS3-1073-specific CD8(+) T cells were found to be both naive and memory cells. Importantly, recognition of various peptides derived from unrelated viruses by NS3-1073-specific CD8(+) T cells showed a considerable degree of T cell cross-reactivity, suggesting that they might in part originate from previous heterologous infections. Finally, we further provide evidence that preexisting NS3-1073-specific CD8(+) T cells can impact the T cell response toward peptide vaccination. Healthy, vaccinated individuals who showed an in vitro response toward NS3-1073 already before vaccination displayed a more vigorous and earlier response toward the vaccine.

IMPORTANCE

Preventive and therapeutic vaccines are being developed for many viral infections and often aim on inducing T cell responses. Despite effective antiviral drugs against HCV, there is still a need for a preventive vaccine. However, the responses to vaccines can be highly variable among different individuals. Preexisting T cells in unexposed individuals could be one reason that helps to explain the variable T cell responses to vaccines. Based on our findings, we suggest that HCV CD8(+) T cells are abundant in HCV-seronegative individuals but that their repertoire is highly diverse due to the involvement of both naive precursors and cross-reactive memory cells of different specificities, which can influence the response to vaccines. The data may emphasize the need to personalize immune-based therapies based on the individual's T cell repertoire that is present before the immune intervention.

Tetramer enrichment reveals the presence of phenotypically diverse hepatitis C virus-specific CD8+ T cells in chronic infection

Virus-specific CD8(+) T cells are rarely detectable ex vivo by conventional methods during chronic hepatitis C virus (HCV) infection. In this study, however, we were able to detect and characterize HCV-specific CD8(+) T cells in all chronically HCV genotype 1a-infected, HLA-A*02:01-positive patients analyzed by performing major histocompatibility complex (MHC) class I tetramer enrichment. Two-thirds of these enriched HCV-specific CD8(+) T-cell populations displayed an effector memory phenotype, whereas, surprisingly, one-third displayed a naive-like phenotype despite ongoing viral replication. CD8(+) T cells with an effector memory phenotype could not expand in vitro, suggesting exhaustion of these cells. Interestingly, some of the naive-like CD8(+) T cells proliferated vigorously upon in vitro priming, whereas others did not. These differences were linked to the corresponding viral sequences in the respective patients. Indeed, naive-like CD8(+) T cells from patients with the consensus sequence in the corresponding T-cell epitope did not expand in vitro. In contrast, in patients displaying sequence variations, we were able to induce HCV-specific CD8(+) T-cell proliferation, which may indicate infection with a variant virus. Collectively, these data reveal the presence of phenotypically and functionally diverse HCV-specific CD8(+) T cells at very low frequencies that are detectable in all chronically infected patients despite viral persistence.

IMPORTANCE

In this study, we analyzed CD8(+) T-cell responses specific for HLA-A*02:01-restricted epitopes in chronically HCV-infected patients, using MHC class I tetramer enrichment. Importantly, we could detect HCV-specific CD8(+) T-cell populations in all patients. To further characterize these HCV-specific CD8(+) T-cell populations that are not detectable using conventional techniques, we performed phenotypic, functional, and viral sequence analyses. These data revealed different mechanisms for CD8(+) T-cell failure in HCV infection, including T-cell exhaustion, viral escape, and functional impairment of naive-like HCV-specific CD8(+) T cells.

Dendritic cell-mediated, DNA-based vaccination against hepatitis C induces the multi-epitope-specific response of humanized, HLA transgenic mice

Hepatitis C virus (HCV) is the etiologic agent of chronic liver disease, hepatitis C. Spontaneous resolution of viral infection is associated with vigorous HLA class I- and class II-restricted T cell responses to multiple viral epitopes. Unfortunately, only 20% of patients clear infection spontaneously, most develop chronic disease and require therapy. The response to chemotherapy varies, however; therapeutic vaccination offers an additional treatment strategy. To date, therapeutic vaccines have demonstrated only limited success. Vector-mediated vaccination with multi-epitope-expressing DNA constructs alone or in combination with chemotherapy offers an additional treatment approach. Gene sequences encoding validated HLA-A2- and HLA-DRB1-restricted epitopes were synthesized and cloned into an expression vector. Dendritic cells (DCs) derived from humanized, HLA-A2/DRB1 transgenic (donor) mice were transfected with these multi-epitope-expressing DNA constructs. Recipient HLA-A2/DRB1 mice were vaccinated s.c. with transfected DCs; control mice received non-transfected DCs. Peptide-specific IFN-γ production by splenic T cells obtained at 5 weeks post-immunization was quantified by ELISpot assay; additionally, the production of IL-4, IL-10 and TNF-α were quantified by cytokine bead array. Splenocytes derived from vaccinated HLA-A2/DRB1 transgenic mice exhibited peptide-specific cytokine production to the vast majority of the vaccine-encoded HLA class I- and class II-restricted T cell epitopes. A multi-epitope-based HCV vaccine that targets DCs offers an effective approach to inducing a broad immune response and viral clearance in chronic, HCV-infected patients.

Peptide Dose and/or Structure in Vaccines as a Determinant of T Cell Responses

While T cells recognise the complex of peptide and major histocompatibility complex (MHC) at the cell surface, changes in the dose and/or structure of the peptide component can have profound effects on T cell activation and function. In addition, the repertoire of T cells capable of responding to any given peptide is variable, but broader than a single clone. Consequently, peptide parameters that affect the interaction between T cells and peptide/MHC have been shown to select particular T cell clones for expansion and this impacts on clearance of disease. T cells with high functional avidity are selected on low doses of peptide, while low avidity T cells are favoured in high peptide concentrations. Altering the structure of the peptide ligand can also influence the selection and function of peptide-specific T cell clones. In this review, we will explore the evidence that the choice of peptide dose or the structure of the peptide are critical parameters in an effective vaccine designed to activate T cells.

CD4+ T cell subset differentiation and avidity setpoint are dictated by the interplay of cytokine and antigen mediated signals

CD4(+) T cell differentiation has been shown to be regulated by the cytokine milieu present during activation as well as peptide MHC levels. However, the extent to which these two important regulatory signals work in concert to shape CD4(+) T cell function has not been investigated. Using a murine OT-II transgenic TCR model of in vitro differentiation, we demonstrate that the ability of CD4(+) T cells to commit to a distinct lineage, i.e. Th1 vs. Th2 vs. Th17, is restricted by the amount of peptide antigen present in the stimulating environment. In addition, whether cells succumb to inhibitory effects associated with high dose antigen is dependent on the array of cytokine signals encountered. Specifically, stimulation with high dose antigen in Th1 or Th17 conditions promoted efficient generation of functional cells, while Th2 polarizing conditions did not. Finally, we found that the peptide sensitivity of an effector cell was determined by the combined actions of cytokine and peptide level, with Th1 cells exhibiting the highest avidity, followed by Th17 and Th2 cells. Together, these data show that the interplay of antigen and cytokine signals shape both the differentiation fate and avidity setpoint of CD4(+) T cells.

Structural and functional correlates of enhanced antiviral immunity generated by heteroclitic CD8 T cell epitopes

Peptides that bind poorly to MHC class I molecules often elicit low-functional avidity T cell responses. Peptide modification by altering the anchor residue facilitates increased binding affinity and may elicit T cells with increased functional avidity toward the native epitope ("heteroclitic"). This augmented MHC binding is likely to increase the half-life and surface density of the heteroclitic complex, but precisely how this enhanced T cell response occurs in vivo is not known. Furthermore, the ideal heteroclitic epitope will elicit T cell responses that completely cross-react with the native epitope, maximizing protection and minimizing undesirable off-target effects. Such epitopes have been difficult to identify. In this study, using mice infected with a murine coronavirus that encodes epitopes that elicit high (S510, CSLWNGPHL)- and low (S598, RCQIFANI)-functional avidity responses, we show that increased expression of peptide S598 but not S510 generated T cells with enhanced functional avidity. Thus, immune responses can be augmented toward T cell epitopes with low functional avidity by increasing Ag density. We also identified a heteroclitic epitope (RCVIFANI) that elicited a T cell response with nearly complete cross-reactivity with native epitope and demonstrated increased MHC/peptide abundance compared with native S598. Structural and thermal melt analyses indicated that the Q600V substitution enhanced stability of the peptide/MHC complex without greatly altering the antigenic surface, resulting in highly cross-reactive T cell responses. Our data highlight that increased peptide/MHC complex display contributes to heteroclitic epitope efficacy and describe parameters for maximizing immune responses that cross-react with the native epitope.

A Novel Method Linking Antigen Presentation by Human Monocyte-Derived Macrophages to CD8(+) T Cell Polyfunctionality

To understand the interactions between innate and adaptive immunity, and specifically how virally infected macrophages impact T cell function, novel assays examining the ability of macrophages to present antigen to CD8⁺ T cells are needed. In the present study, we have developed a robust in vitro assay to measure how antigen presentation by human monocyte-derived macrophages (MDMs) affects the functional capacity of autologous CD8⁺ T cells. The assay is based on the polyfunctional characteristics of antigen-specific CD8⁺ T cells, and is thus called a Mac-CD8 Polyfunctionality Assay. Following purification of monocytes and their maturation to MDMs, MDMs were pulsed with an antigenic peptide to be presented to CD8⁺ T cells. Peptide-pulsed MDMs were then incubated with antigen-specific CD8⁺ T cells in order to assess the efficacy of antigen presentation to T cells. CD8⁺ T cell polyfunctionality was assessed by staining with mAbs to IFN-γ, TNF-α, and CD107a in a multi-color intracellular cytokine staining assay. To highlight the utility of the Mac-CD8 Polyfunctionality Assay, we assessed the effects of influenza infection on the ability of human macrophages to present antigen to CD8⁺ T cells. We found that influenza infection of human MDMs can alter the effector efficacy of MDMs to activate more CD8⁺ T cells with cytotoxic capacity. This has important implications for understanding how the virus-infected macrophages affect adaptive immunity at the site of infection.

Characterization of the human CD4(+) T-cell repertoire specific for major histocompatibility class I-restricted antigens

While CD4(+) T lymphocytes usually recognize antigens in the context of major histocompatibility (MHC) class II alleles, occurrence of MHC class-I restricted CD4(+) T cells has been reported sporadically. Taking advantage of a highly sensitive MHC tetramer-based enrichment approach allowing detection and isolation of scarce Ag-specific T cells, we performed a systematic comparative analysis of HLA-A*0201-restricted CD4(+) and CD8(+) T-cell lines directed against several immunodominant viral or tumoral antigens. CD4(+) T cells directed against every peptide-MHC class I complexes tested were detected in all donors. These cells yielded strong cytotoxic and T helper 1 cytokine responses when incubated with HLA-A2(+) target cells carrying the relevant epitopes. HLA-A2-restricted CD4(+) T cells were seldom expanded in immune HLA-A2(+) donors, suggesting that they are not usually engaged in in vivo immune responses against the corresponding peptide-MHC class I complexes. However, these T cells expressed TCR of very high affinity and were expanded following ex vivo stimulation by relevant tumor cells. Therefore, we describe a versatile and efficient strategy for generation of MHC class-I restricted T helper cells and high affinity TCR that could be used for adoptive T-cell transfer- or TCR gene transfer-based immunotherapies.

In vivo modulation of avidity in highly sensitive CD8(+) effector T cells following viral infection

Numerous studies have demonstrated a critical role for T cell avidity in predicting in vivo efficacy. Even though the measurement of avidity is now a routine assessment for the analysis of effector and memory T cell populations, our understanding of how this property is controlled in vivo at both the population and individual cell levels is limited. Our previous studies have identified high avidity as a property of the initial effector population generated in mice following respiratory virus infection. As the response progresses, lower avidity cells appear in the effector pool. The studies described here investigate the mechanistic basis of this in vivo regulation of avidity. We present data supporting in vivo avidity modulation within the early high avidity responders that results in a population of lower avidity effector cells. Changes in avidity were correlated with decreased lck expression and increased sensitivity to lck inhibitors in effector cells present at late versus early times postinfection. The possibility of tuning within select individual effectors is a previously unappreciated mechanism for the control of avidity in vivo.

Bacterial lipopolysaccharide inhibits influenza virus infection of human macrophages and the consequent induction of CD8+ T cell immunity

It is well established that infection with influenza A virus (IAV) facilitates secondary bacterial disease. However, there is a growing body of evidence that the microbial context in which IAV infection occurs can affect both innate and adaptive responses to the virus. To date, these studies have been restricted to murine models of disease and the relevance of these findings in primary human cells remains to be elucidated. Here, we show that pre-stimulation of primary human monocyte-derived macrophages (MDMs) with the bacterial ligand lipopolysaccharide (LPS) reduces the ability of IAV to infect these cells. The inhibition of IAV infection was associated with a reduced transcription of viral RNA and the ability of LPS to induce an anti-viral/type I interferon response in human MDMs. We demonstrated that this reduced rate of viral infection is associated with a reduced ability to present a model antigen to autologous CD8+ T cells. Taken together, these data provide the first evidence that exposure to bacterial ligands like LPS can play an important role in modulating the immune response of primary human immune cells towards IAV infection, which may then have important consequences for the development of the host's adaptive immune response.

High, broad, polyfunctional, and durable T cell immune responses induced in mice by a novel hepatitis C virus (HCV) vaccine candidate (MVA-HCV) based on modified vaccinia virus Ankara expressing the nearly full-length HCV genome

A major goal in the control of hepatitis C infection is the development of a vaccine. Here, we have developed a novel HCV vaccine candidate based on the highly attenuated poxvirus vector MVA (referred to as MVA-HCV) expressing the nearly full-length (7.9-kbp) HCV sequence, with the aim to target almost all of the T and B cell determinants described for HCV. In infected cells, MVA-HCV produces a polyprotein that is subsequently processed into the structural and nonstructural HCV proteins, triggering the cytoplasmic accumulation of dense membrane aggregates. In both C57BL/6 and transgenic HLA-A2-vaccinated mice, MVA-HCV induced high, broad, polyfunctional, and long-lasting HCV-specific T cell immune responses. The vaccine-induced T cell response was mainly mediated by CD8 T cells; however, although lower in magnitude, the CD4(+) T cells were highly polyfunctional. In homologous protocol (MVA-HCV/MVA-HCV) the main CD8(+) T cell target was p7+NS2, whereas in heterologous combination (DNA-HCV/MVA-HCV) the main target was NS3. Antigenic responses were also detected against other HCV proteins (Core, E1-E2, and NS4), but the magnitude of the responses was dependent on the protocol used. The majority of the HCV-induced CD8(+) T cells were triple or quadruple cytokine producers. The MVA-HCV vaccine induced memory CD8(+) T cell responses with an effector memory phenotype. Overall, our data showed that MVA-HCV induced broad, highly polyfunctional, and durable T cell responses of a magnitude and quality that might be associated with protective immunity and open the path for future considerations of MVA-HCV as a prophylactic and/or therapeutic vaccine candidate against HCV.

Correlates and characteristics of hepatitis C virus-specific T-cell immunity in exposed uninfected high-risk prison inmates

Some hepatitis C (HCV)-uninfected, high-risk individuals have HCV-specific cellular immunity without viraemia or seroconversion. The characteristics of these responses and the risk behavioural associations were studied in 94 subjects in a prospective cohort of recently seronegative prisoners reporting injecting drug use (IDU). Detailed behavioural data were collected. HCV antibody and PCR testing were performed. ELISpot assays for HCV-induced interferon (IFN)-γ and interleukin (IL)-2 production by T lymphocytes, as well as multiplex in vitro cytokine production assays, were performed. Seventy-eight subjects remained antibody and PCR negative and 16 seroconverted. Of the seronegative group, 22 (28%) had IFN-γ ELISpot responses in comparison with 13 of the 16 seroconverters (82%). This seronegative immune status was associated positively with injecting anabolic steroids and negatively with a recent break from IDU. The IFN-γ ELISpot responses involved both CD4 and CD8 T lymphocytes and were comparable in magnitude, but narrower in specificity, in uninfected subjects than in seroconverters. A subset of seronegative subjects had HCV-induced cytokine production patterns comparable with the seroconverters with increased production of IFN-γ, IL-2 and tumour necrosis factor (TNF)-α and reduced IL-10 in response to nonstructural peptides. In conclusion, comparable patterns of HCV-specific cellular immunity are found in recently infected subjects and in a minority of high-risk, uninfected subjects. Further characterization of these responses and their protective efficacy will inform HCV vaccine development.

Virus-induced hepatocellular carcinomas cause antigen-specific local tolerance

T cell surveillance is often effective against virus-associated tumors because of their high immunogenicity. It is not clear why surveillance occasionally fails, particularly against hepatitis B virus- or hepatitis C virus-associated hepatocellular carcinoma (HCC). We established a transgenic murine model of virus-induced HCC by hepatocyte-specific adenovirus-induced activation of the oncogenic SV40 large T antigen (TAg). Adenovirus infection induced cytotoxic T lymphocytes (CTLs) targeted against the virus and TAg, leading to clearance of the infected cells. Despite the presence of functional, antigen-specific T cells, a few virus-infected cells escaped immune clearance and progressed to HCC. These cells expressed TAg at levels similar to HCC isolated from neonatal TAg-tolerant mice, suggesting that CTL clearance does not select for cells with low immunogenicity. Virus-infected mice revealed significantly greater T cell infiltration in early-stage HCC compared with that in late-stage HCC, demonstrating progressive local immune suppression through inefficient T cell infiltration. Programmed cell death protein-1 (PD-1) and its ligand PD-L1 were expressed in all TAg-specific CD8+ T cells and HCC, respectively, which contributed to local tumor-antigen-specific tolerance. Thus, we have developed a model of virus-induced HCC that may allow for a better understanding of human HCC.

Avidity of influenza-specific memory CD8+ T-cell populations decays over time compromising antiviral immunity

Decline of cell-mediated immunity is often attributed to decaying T-cell numbers and their distribution in peripheral organs. This study examined the hypothesis that qualitative as well as quantitative changes contribute to the declining efficacy of CD8(+) T-cell memory. Using a model of influenza virus infection, where loss of protective CD8(+) T-cell immunity was observed 6 months postinfection, we found no decline in antigen-specific T-cell numbers or migration to the site of secondary infection. There was, however, a large reduction in antigen-specific CD8(+) T-cell degranulation, cytokine secretion, and polyfunctionality. A profound loss of high-avidity T cells over time indicated that failure to confer protective immunity resulted from the inferior functional capacity of remaining low avidity cells. These data imply that high-avidity central memory T cells wane with declining antigen levels, leaving lower avidity T cells with reduced functional capabilities.

TCR-redirected human T cells inhibit hepatitis C virus replication: hepatotoxic potential is linked to antigen specificity and functional avidity

Virus-specific CTL with high levels of functional avidity have been associated with viral clearance in hepatitis C virus (HCV) infection and with enhanced protective immunity. In chronic HCV infection, lack of antiviral CTL is frequently observed. In this study, we aim to investigate novel HCV TCRs that differ in Ag specificity. This involved isolating new HCV-specific murine TCRs that recognize a conserved HLA-A2-restricted CTL epitope within the nonstructural protein (NS) 5A viral protein and comparing them with TCRs recognizing another conserved CTL target in the NS3 viral protein. This was done by expressing the TCRs in human T cells and analyzing the function of the resulting TCR-transduced T cells. Our result indicates that these TCRs are efficiently assembled in transduced human T cells. They recognize peptide-loaded targets and demonstrate polyfunctional features such as IL-2, IFN-γ, and TNF-α secretion. However, in contrast to NS3-specific TCRs, the NS5A TCR-transduced T cells consist of a smaller proportion of polyfunctional T cells and require more peptide ligands to trigger the effector functions, including degranulation. Despite the differences, NS5A TCRs show effective inhibition of HCV replication in human hepatoma cells with persistent HCV RNA replication. Moreover, cellular injury demonstrated by aspartate aminotransferase release and cell death is less significant in the hepatoma cells following coincubation with NS5A TCR-transduced T cells, which is a property consistent with noncytotoxic antiviral CTLs. Our results suggest that HCV TCR-transduced T cells may be promising for the treatment of patients with chronic HCV infections.

Complete definition of immunological correlates of protection and clearance of hepatitis C virus infection: a relevant pending task for vaccine development

Hepatitis C virus (HCV) infects approximately 3% of global population. This pathogen is one of the main causes of chronic viral hepatitis, cirrhosis, and liver cancer, as well as the principal reason for liver transplant in Western countries. Therapy against HCV infection is effective in only half of treated patients. There is no vaccine available against HCV. Some vaccine candidates have reached the clinical trials but several factors, including the incomplete definition of immunological correlates of protection and treatment-related clearance have slowed down vaccine development. Precisely, the present review discusses the state of the art in the establishment of parameters related with immunity against HCV. Validity and limitations of the information accumulated from chimpanzees and other animal models, analysis of studies in humans infected with HCV, and relevance of aspects like type, strength, duration, and specificity of immune response related to successful outcome are evaluated in detail. Moreover, the immune responses induced in some clinical trials with vaccine candidates resemble the theoretical immunological correlates, raising questions about the validity of those correlates. When all facts are taken together, complete definition of immunological correlates for protection or treatment-related clearance is an urgent priority. A limited or wrong criterion with respect to this relevant matter might cause incorrect vaccine design and selection of immunization strategies or erroneous clinical evaluation.

Changes in functional but not structural avidity during differentiation of CD8+ effector cells in vivo after virus infection

By the peak of the CD8(+) T cell response, the effector cell pool consists of a heterogeneous population of cells that includes both those with an increased propensity to become long-lived memory cells (memory precursor effector cells; MPEC) and those that are terminally differentiated cells (short-lived effector cells; SLEC). Numerous studies have established the critical role that functional avidity plays in determining the in vivo efficacy of CD8(+) effector cells. Currently, how functional avidity differs in MPEC versus SLEC and the evolution of this property within these two populations during the expansion and contraction of the response are unknown. The data presented in this study show that at the peak of the effector response generated after poxvirus infection, SLEC were of higher functional avidity than their MPEC counterpart. Over time, however, SLEC exhibited a decrease in peptide sensitivity. This is in contrast to MPEC, which showed a modest increase in peptide sensitivity as the response reached equilibrium. The decrease in functional avidity in SLEC was independent of CD8 modulation or the amount of Ag receptor expressed by the T cell. Instead, the loss in sensitivity was correlated with decreased expression and activation of ZAP70 and Lck, critical components of TCR membrane proximal signaling. These results highlight the potential contribution of avidity in the differentiation and evolution of the T cell effector response after viral infection.

Enhanced cytotoxicity and decreased CD8 dependence of human cancer-specific cytotoxic T lymphocytes after vaccination with low peptide dose

In mice, vaccination with high peptide doses generates higher frequencies of specific CD8+ T cells, but with lower avidity compared to vaccination with lower peptide doses. To investigate the impact of peptide dose on CD8+ T cell responses in humans, melanoma patients were vaccinated with 0.1 or 0.5 mg Melan-A/MART-1 peptide, mixed with CpG 7909 and Incomplete Freund's adjuvant. Neither the kinetics nor the amplitude of the Melan-A-specific CD8+ T cell responses differed between the two vaccination groups. Also, CD8+ T cell differentiation and cytokine production ex vivo were similar in the two groups. Interestingly, after low peptide dose vaccination, Melan-A-specific CD8+ T cells showed enhanced degranulation upon peptide stimulation, as assessed by CD107a upregulation and perforin release ex vivo. In accordance, CD8+ T cell clones derived from low peptide dose-vaccinated patients showed significantly increased degranulation and stronger cytotoxicity. In parallel, Melan-A-specific CD8+ T cells and clones from low peptide dose-vaccinated patients expressed lower CD8 levels, despite similar or even stronger binding to tetramers. Furthermore, CD8+ T cell clones from low peptide dose-vaccinated patients bound CD8 binding-deficient tetramers more efficiently, suggesting that they may express higher affinity TCRs. We conclude that low peptide dose vaccination generated CD8+ T cell responses with stronger cytotoxicity and lower CD8 dependence.

Polyfunctional CD8(+) T cells are associated with the vaccination-induced control of a novel recombinant influenza virus expressing an HCV epitope

In hepatitis C virus (HCV) infection, CD8(+) T cell responses have been shown to be important in viral clearance. Examining the efficacy of CD8(+) T cell vaccines against HCV has been limited by the lack of an HCV infectious model in mice and the differences between MHC restriction in humans and mice. Using HLA-A2 transgenic HHD mice, we demonstrate that intranasally delivered Pam2Cys-based lipopeptides containing HLA-A2-restricted HCV epitopes can induce polyfunctional CD8(+) T cell responses in several organs including the liver. To examine the activity of these responses in an infectious context, we developed a recombinant influenza virus that expresses the NS5B(2594-2602) epitope from non-structural protein 5B of hepatitis C virus (PR8-HCV(NS5B)). We showed that mice inoculated with a lipopeptide containing the NS5B epitope had reduced viral loads following challenge with the PR8-HCV(NS5B) virus. This reduction was associated with the induction of NS5B(2594-2602)-specific IFN-γ and TNF-α co-producing CD8(+) T cells. The T cell receptor usage in the NS5B(2594-2602) response was found to exhibit a Vβ8.1/8.2 bias that was characterized by a narrow repertoire and a common CDR3β motif. This work has identified CD8(+) T cell functions induced by lipopeptides that are associated with viral control and demonstrate the potential of lipopeptide-based vaccines as candidates for treatment of HCV infection.

Hepatitis C virus nonstructural protein specific T cells are associated with virological responses to combination therapy in chronic HCV patients

BACKGROUND

Virus-specific T-cell responses play a major role in antiviral immune response. However, the effect of hepatitis C virus (HCV)-specific T-cell responses on combination therapy still remains controversial.

AIMS

To identify the association between HCV-specific T cell responses and efficiency of combination therapy.

METHODS

To address this issue, a longitudinal analysis of HCV-specific T-cell responses to overlapping peptides covering HCV-nonstructural protein (NS) was performed using ELISpot assay in 48 chronically infected HCV-1b patients during combination treatment with peginterferon-alfa and ribavirin.

RESULTS

Fifty-two percent of chronic HCV patients showed detectable HCV-NS3, NS4 or NS5A specific T-cell responses before therapy, with NS3 appearing to be the most immunodominant protein followed by NS5A and NS4. In addition, the percentage of patients responding to peptide stimulation was higher in patients with sustained virological response (SVR) when compared with those without SVR. Dynamics of HCV-NS-specific T-cell responses were further analysed; we found that HCV-specific T-cell responses maintained higher levels at 12 weeks into treatment in patients with SVR. In contrast, HCV-specific T-cell responses in patients without SVR declined significantly at 4 weeks into treatment and maintained low levels at 12 weeks.

CONCLUSION

We found that the HCV-specific T-cell responses were associated with good viral control in patients with combination therapy.

Chronic hepatitis C infection blocks the ability of dendritic cells to secrete IFN-α and stimulate T-cell proliferation

Dendritic cells (DCs) are likely to play a key role in the compromised T-cell function associated with hepatitis C Virus (HCV) infection. However, studies of DC function in HCV-infected patients to date have yielded conflicting findings possibly because of patient and virus heterogeneity. Here, we report the characterization of monocyte-derived DCs obtained from a homogenous cohort of women who were infected with HCV genotype 1b following exposure to contaminated anti-D immunoglobulin from a single donor source. Patients included in the study had not received anti-viral therapy and all had mild liver disease. We show that phenotypically normal monocyte-derived dendritic cells (MDDCs) (CD11c(+) HLA(-) DR(+) CD1a(+) CD14(lo) ) can be obtained from these patients. These cells respond to both Poly(I:C) and LPS, by up-regulating expression of CD86. They secrete high levels of IL-8 and CCL5 in response to LPS, an indication that the MyD88-dependent and MyD88-independent signalling pathways downstream of TLR4 ligation are functioning normally. However, these cells are poor stimulators of T-cell proliferation in allogeneic mixed lymphocyte reactions. Furthermore, patient MDDCs fail to secrete IFN-α in response to poly(I:C) or IFN-β stimulation. Altered DC function may contribute to impaired cellular immune responses and chronicity of disease following HCV infection in this cohort. An effective therapeutic vaccine for chronic HCV infection will most likely need to target DCs to elicit an appropriate cellular response; therefore, it is important to resolve how the DCs of different patient cohorts respond to stimulation via TLRs.

Generation of T-cell receptors targeting a genetically stable and immunodominant cytotoxic T-lymphocyte epitope within hepatitis C virus non-structural protein 3

Hepatitis C virus (HCV) is a major cause of severe liver disease, and one major contributing factor is thought to involve a dysfunction of virus-specific T-cells. T-cell receptor (TCR) gene therapy with HCV-specific TCRs would increase the number of effector T-cells to promote virus clearance. We therefore took advantage of HLA-A2 transgenic mice to generate multiple TCR candidates against HCV using DNA vaccination followed by generation of stable T-cell–BW (T-BW) tumour hybrid cells. Using this approach, large numbers of non-structural protein 3 (NS3)-specific functional T-BW hybrids can be generated efficiently. These predominantly target the genetically stable HCV genotype 1 NS3_1073–1081 CTL epitope, frequently associated with clearance of HCV in humans. These T-BW hybrid clones recognized the NS3₁₀₇₃ peptide with a high avidity. The hybridoma effectively recognized virus variants and targeted cells with low HLA-A2 expression, which has not been reported previously. Importantly, high-avidity murine TCRs effectively redirected human non-HCV-specific T-lymphocytes to recognize human hepatoma cells with HCV RNA replication driven by a subgenomic HCV replicon. Taken together, TCR candidates with a range of functional avidities, which can be used to study immune recognition of HCV-positive targets, have been generated. This has implications for TCR-related immunotherapy against HCV.

Progress in the development of preventive and therapeutic vaccines for hepatitis C virus

Hepatitis C virus (HCV) is a blood borne disease estimated to chronically infect 3% of the worlds' population causing significant morbidity and mortality. Current medical therapy is curative in approximately 50% of patients. While recent treatment advances of genotype 1 infection using directly acting antiviral agents (DAAs) are encouraging, there is still a need to develop vaccine strategies capable of preventing infection. Moreover, vaccines may also be used in future in combination with DAAs enabling interferon-free treatment regimens. Viral and host specific factors contribute to viral evasion and present important impediments to vaccine development. Both, innate and adaptive immune responses are of major importance for the control of HCV infection. However, HCV has evolved ways of evading the host's immune response in order to establish persistent infection. For example, HCV inhibits intracellular interferon signalling pathways, impairs the activation of dendritic cells, CD8(+) and CD4(+) T cell responses, induces a state of T-cell exhaustion and selects escape variants with mutations CD8(+) T cell epitopes. An effective vaccine will need to produce strong and broadly cross-reactive CD4(+), CD8(+) T cell and neutralising antibody (NAb) responses to be successful in preventing or clearing HCV. Vaccines in clinical trials now include recombinant proteins, synthetic peptides, virosome based vaccines, tarmogens, modified vaccinia Ankara based vaccines, and DNA based vaccines. Several preclinical vaccine strategies are also under development and include recombinant adenoviral vaccines, virus like particles, and synthetic peptide vaccines. This paper will review the vaccines strategies employed, their success to date and future directions of vaccine design.

Increased sensitivity to antigen in high avidity CD8(+) T cells results from augmented membrane proximal T-cell receptor signal transduction

The functional avidity of a cytotoxic T lymphocyte (CTL) is known to be a critical determinant of the efficacy with which it clears pathogens. High avidity cells, which are by definition highly sensitive to peptide antigen, are superior for elimination of viruses and tumours. Our studies have established the ability of T cells to undergo avidity modulation as a result of antigen encounter. High and low avidity cells established in this manner exhibit significant differences in the amount of peptide required to elicit effector function. However, how signalling is regulated in these cells as it relates to the control of peptide sensitivity remains to be defined. To address this question, we compared T-cell receptor (TCR) signal transduction events in high and low avidity CTL generated from OT-I(rag2-) TCR transgenic mice. Our data suggest that divergent signalling is initiated at the TCR-associated CD3ζ, with low avidity CTL requiring higher amounts of pMHC to achieve threshold levels of phosphorylated CD3ζ compared with high avidity CTL. Further, this difference is transduced further downstream to mitogen-activated protein kinase and Ca(2+) signalling pathways. These results suggest that regulated control of the initiation of TCR signalling in high versus low avidity cells determines the amount of peptide required for T-cell activation.

Immune mechanisms of HIV control

HIV-1 can be contained by the immune system, as demonstrated by the existence of rare individuals who spontaneously control HIV-1 replication in the absence of antiretroviral therapy. Emerging evidence points to the importance of a very active cellular immune response in mediating HIV-1 control. The rapid induction of interferon-dependent HIV restriction factors, the presence of protective MHC class I alleles, and the development of a high avidity T-cell response may all cooperate in limiting HIV replication at an early stage. This review will focus on recent advances in understanding the immune mechanisms of HIV control, and on the lessons that may be drawn for the development of candidate HIV vaccines.

Influenza epitope-specific CD8+ T cell avidity, but not cytokine polyfunctionality, can be determined by TCRβ clonotype

Cytokine polyfunctionality has recently emerged as a correlate of effective CTL immunity to viruses and tumors. Although the determinants of polyfunctionality remain unclear, there are published instances of a link between the production of multiple effector molecules and the peptide plus MHC class I molecule avidity of T cell populations. Influenza A virus infection of C57BL/6J mice induces CTL populations specific for multiple viral epitopes, each with varying proportions of monofunctional (IFN-γ(+) only) or polyfunctional (IFN-γ(+)TNF-α(+)IL-2(+)) CTLs. In this study, we probe the link between TCR avidity and polyfunctionality for two dominant influenza epitopes (D(b)NP(366) and D(b)PA(224)) by sequencing the TCR CDR3β regions of influenza-specific IFN-γ(+) versus IFN-γ(+)IL-2(+) cells, or total tetramer(+) versus high-avidity CTLs (as defined by the peptide plus MHC class I molecule-TCR dissociation rate). Preferential selection for particular clonotypes was evident for the high-avidity D(b)PA(224)-specific set but not for any of the other subsets examined. These data suggest that factors other than TCRβ sequence influence cytokine profiles and demonstrate no link between differential avidity and polyfunctionality.

PD-1/PD-L1 pathway and T-cell exhaustion in chronic hepatitis virus infection

Dysfunctional virus-specific T cells are a hallmark of many chronic viral infections. Recent studies have implicated the inhibitory PD-1/PD-L1 pathway with the functional impairment of T cells. In this respect, we will review the latest research on PD-1/PD-L1 pathway and T-cell exhaustion in the context of human chronic hepatitis B and C virus infections. We will also discuss the therapeutic potential of PD-1 blockade and how it may be enhanced through the modulation of other co-stimulatory/inhibitory pathways.

Impact of TCR reactivity and HLA phenotype on naive CD8 T cell frequency in humans

The impact of MHC phenotype on the shaping of the peripheral naive T cell repertoire in humans remains unknown. To address this, we compared the frequency and antigenic avidity of naive T cells specific for immunodominant self-, viral, and tumor Ags presented by a human MHC class I allele (HLA-A*02, referred to as A2) in individuals expressing or not this allele. Naive T cell frequencies varied from one Ag specificity to another but were restrained for a given specificity. Although A2-restricted T cells showed similar repertoire features and antigenic avidities in A2+ and A2- donors, A2 expression had either a positive, neutral, or negative impact on the frequency of A2-restricted naive CD8 T cells, depending on their fine specificity. We also identified in all donors CD4 T cells specific for A2/peptide complexes, whose frequencies were not affected by MHC class I expression, but nevertheless correlated with those of their naive CD8 T cell counterparts. Therefore, both selection by self-MHC and inherent TCR reactivity regulate the frequency of human naive T cell precursors. Moreover this study also suggests that T cell repertoire shaping by a given self-MHC allele is dispensable for generation of immunodominant T cell responses restricted by this particular allele.

Public TCR use by herpes simplex virus-2-specific human CD8 CTLs

Recombination of germline TCR alpha and beta genes generates polypeptide receptors for MHC peptide. Ag exposure during long-term herpes simplex infections may shape the T cell repertoire over time. We investigated the CD8 T cell response to HSV-2 in chronically infected individuals by sequencing the hypervariable regions encoding TCR alpha and beta polypeptides from T cell clones recognizing virion protein 22 aa 49-57, an immunodominant epitope. The most commonly detected TCRBV gene segment, found in four of five subjects and in 12 of 50 independently derived T cell clones, was TCRBV12-4. Nineteen to seventy-two percent of tetramer-binding cells in PBMCs were stained ex vivo with a TCRBV12 mAb. Three alpha-chain and three beta-chain public TCR sequences were shared between individuals. Public heterodimers were also detected. Promiscuous pairing of a specific TCRVA1-1 sequence with several different TCRB polypeptides was observed, implying a dominant structural role for the TCRA chain for these clonotypes. Functional avidity for cytotoxicity and IFN-gamma release was relatively invariant, except for one subject with both high avidity and unique TCR sequences and lower HSV-2 shedding. These data indicate that the CD8 response to a dominant alpha-herpesvirus epitope converges on preferred TCR sequences with relatively constant functional avidity.

HIV controller CD4+ T cells respond to minimal amounts of Gag antigen due to high TCR avidity

HIV controllers are rare individuals who spontaneously control HIV replication in the absence of antiretroviral treatment. Emerging evidence indicates that HIV control is mediated through very active cellular immune responses, though how such responses can persist over time without immune exhaustion is not yet understood. To investigate the nature of memory CD4+ T cells responsible for long-term anti-HIV responses, we characterized the growth kinetics, Vbeta repertoire, and avidity for antigen of patient-derived primary CD4+ T cell lines. Specific cell lines were obtained at a high rate for both HIV controllers (16/17) and efficiently treated patients (19/20) in response to the immunodominant Gag293 peptide. However, lines from controllers showed faster growth kinetics than those of treated patients. After normalizing for growth rates, IFN-gamma responses directed against the immunodominant Gag293 peptide showed higher functional avidity in HIV controllers, indicating differentiation into highly efficient effector cells. In contrast, responses to Gag161, Gag263, or CMV peptides did not differ between groups. Gag293-specific CD4+ T cells were characterized by a diverse Vbeta repertoire, suggesting that multiple clones contributed to the high avidity CD4+ T cell population in controllers. The high functional avidity of the Gag293-specific response could be explained by a high avidity interaction between the TCR and the peptide-MHC complex, as demonstrated by MHC class II tetramer binding. Thus, HIV controllers harbor a pool of memory CD4+ T cells with the intrinsic ability to recognize minimal amounts of Gag antigen, which may explain how they maintain an active antiviral response in the face of very low viremia.

Using 3 TLR ligands as a combination adjuvant induces qualitative changes in T cell responses needed for antiviral protection in mice

TLR ligands are promising candidates for the development of novel vaccine adjuvants that can elicit protective immunity against emerging infectious diseases. Adjuvants have been used most frequently to increase the quantity of an immune response. However, the quality of a T cell response can be more important than its quantity. Stimulating certain pairs of TLRs induces a synergistic response in terms of activating dendritic cells and eliciting/enhancing T cell responses through clonal expansion, which increases the number of responding T cells. Here, we have found that utilizing ligands for 3 TLRs (TLR2/6, TLR3, and TLR9) greatly increased the protective efficacy of vaccination with an HIV envelope peptide in mice when compared with using ligands for only any 2 of these TLRs; surprisingly, increased protection was induced without a marked increase in the number of peptide-specific T cells. Rather, the combination of these 3 TLR ligands augmented the quality of the T cell responses primarily by amplifying their functional avidity for the antigen, which was necessary for clearance of virus. The triple combination increased production of DC IL-15 along with its receptor, IL-15Ralpha, which contributed to high avidity, and decreased expression of programmed death-ligand 1 and induction of Tregs. Therefore, selective TLR ligand combinations can increase protective efficacy by increasing the quality rather than the quantity of T cell responses.

T cell sensitivity and the outcome of viral infection

The importance of CD8(+) T cells in the control of viral infections is well established. However, what differentiates CD8(+) T cell responses in individuals who control infection and those who do not is not well understood. 'Functional sensitivity' describes an important quality of the T cell response and is determined in part by the affinity of the T cell receptor for antigen. A more sensitive T cell response is generally believed to be more efficient and associated with better control of viral infection, yet may also drive viral mutation and immune escape. Various in vitro techniques have been used to measure T cell sensitivity; however, rapid ex vivo analysis of this has been made possible by the application of the 'magic' tetramer technology. Such tools have potentially important applications in the design and evaluation of vaccines.