Limitations of predictive motifs revealed by cytotoxic T lymphocyte epitope mapping of the human papilloma virus E7 protein

Ex Vivo PD-L1/PD-1 Pathway Blockade Reverses Dysfunction of Circulating CEA-Specific T Cells in Pancreatic Cancer Patients

Purpose: Carcinoembryonic antigen (CEA) is a candidate target for cellular immunotherapy of pancreatic cancer. In this study, we have characterized the antigen-specific function of autologous cytotoxic T lymphocytes (CTL) specific for the HLA-A2-restricted peptide, pCEA691-699, isolated from the peripheral T-cell repertoire of pancreatic cancer patients and sought to determine if ex vivo PD-L1 and TIM-3 blockade could enhance CTL function.Experimental Design: CD8⁺ T-cell lines were generated from peripheral blood mononuclear cells of 18 HLA-A2⁺ patients with pancreatic cancer and from 15 healthy controls. In vitro peptide-specific responses were evaluated by flow cytometry after staining for intracellular cytokine production and carboxy fluorescein succinimydyl ester cytotoxicity assays using pancreatic cancer cell lines as targets.Results: Cytokine-secreting functional CEA691-specific CTL lines were successfully generated from 10 of 18 pancreatic cancer patients, with two CTL lines able to recognize and kill both CEA691 peptide-loaded T2 cells and CEA⁺ HLA-A2⁺ pancreatic cancer cell lines. In the presence of ex vivo PD-L1 blockade, functional CEA691-specific CD8⁺ T-cell responses, including IFNγ secretion and proliferation, were enhanced, and this effect was more pronounced on Ag-specific T cells isolated from tumor draining lymph nodes.Conclusions: These data demonstrate that CEA691-specific CTL can be readily expanded from the self-restricted T-cell repertoire of pancreatic cancer patients and that their function can be enhanced by PD-L1 blockade. Clin Cancer Res; 23(20); 6178-89. ©2017 AACR.

Steric recognition of T-cell receptor contact residues is required to map mutant epitopes by immunoinformatical programmes

MHC class I-restricted CD8 T-lymphocyte epitopes comprise anchor motifs, T-cell receptor (TCR) contact residues and the peptide backbone. Serial variant epitopes with substitution of amino acids at either anchor motifs or TCR contact residues have been synthesized for specific interferon-γ responses to clarify the TCR recognition mechanism as well as to assess the epitope prediction capacity of immunoinformatical programmes. CD8 T lymphocytes recognise the steric configuration of functional groups at the TCR contact side chain with a parallel observation that peptide backbones of various epitopes adapt to the conserved conformation upon binding to the same MHC class I molecule. Variant epitopes with amino acid substitutions at the TCR contact site are not recognised by specific CD8 T lymphocytes without compromising their binding capacity to MHC class I molecules, which demonstrates two discrete antigen presentation events for the binding of peptides to MHC class I molecules and for TCR recognition. The predicted outcome of immunoinformatical programmes is not consistent with the results of epitope identification by laboratory experiments in the absence of information on the interaction with TCR contact residues. Immunoinformatical programmes based on the binding affinity to MHC class I molecules are not sufficient for the accurate prediction of CD8 T-lymphocyte epitopes. The predictive capacity is further improved to distinguish mutant epitopes from the non-mutated epitopes if the peptide-TCR interface is integrated into the computing simulation programme.

TriVax-HPV: an improved peptide-based therapeutic vaccination strategy against human papillomavirus-induced cancers

BACKGROUND

Therapeutic vaccines for cancer are an attractive alternative to conventional therapies, since the later result in serious adverse effects and in most cases are not effective against advanced disease. Human papillomavirus (HPV) is responsible for several malignancies such as cervical carcinoma. Vaccines targeting oncogenic viral proteins like HPV16-E6 and HPV16-E7 are ideal candidates to elicit strong immune responses without generating autoimmunity because: (1) these products are not expressed in normal cells and (2) their expression is required to maintain the malignant phenotype. Our group has developed peptide vaccination strategy called TriVax, which is effective in generating vast numbers of antigen-specific T cells in mice capable of persisting for long time periods.

MATERIALS AND METHODS

We have used two HPV-induced mouse cancer models (TC-1 and C3.43) to evaluate the immunogenicity and therapeutic efficacy of TriVax prepared with the immunodominant CD8 T-cell epitope HPV16-E7(49-57), mixed with poly-IC adjuvant and costimulatory anti-CD40 antibodies.

RESULTS

TriVax using HPV16-E7(49-57) induced large and persistent T-cell responses that were therapeutically effective against established HPV16-E7 expressing tumors. In most cases, TriVax was successful in attaining complete rejections of 6-11-day established tumors. In addition, TriVax induced long-term immunological memory, which prevented tumor recurrences. The anti-tumor effects of TriVax were independent of NK and CD4 T cells and, surprisingly, did not rely to a great extent on type-I or type-II interferon.

CONCLUSIONS

These findings indicate that the TriVax strategy is an appealing immunotherapeutic approach for the treatment of established viral-induced tumors. We believe that these studies may help to launch more effective and less invasive therapeutic vaccines for HPV-mediated malignancies.

A novel self-assembled nanoparticle vaccine with HIV-1 Tat₄₉₋₅₇/HPV16 E7₄₉₋₅₇ fusion peptide and GM-CSF DNA elicits potent and prolonged CD8⁺ T cell-dependent anti-tumor immunity in mice

Peptide-based vaccines derived from the E7 protein of human papillomavirus (HPV) type 16 were developed to induce effective T cell responses against established cervical cancer, but have met with limited clinical success. It is necessary to develop novel peptide-based strategies to substantially improve the immune response against HPV16-related cancer. In this study, we aimed to design a novel peptide-based self-assembled nanoparticle HPV16 vaccine by combining the cell-penetrating peptide HIV-1 Tat(49-57) that was fused with the HPV16 E7(49-57) cytotoxic T lymphocyte (CTL) epitope and the granulocyte-macrophage colony stimulating factor (GM-CSF) gene, and to investigate how it improves the immune response and the therapeutic outcome ex vivo and in vivo. Nanoparticles were prepared and identified by transmission electron microscopy (TEM), gel retardation and DNase I protection assays. This type of vaccine formulation formed the 20-80 nm nanoparticles, and greatly improved epitope-specific immunity both ex vivo and in vivo. Importantly, this vaccine type was associated with decreased tumor growth and enhanced long-term survival in the prophylactic and therapeutic mouse models. The underlying mechanisms were determined to involve priming of enhanced frequency of CD8(+) memory T subtype cells. These results suggest that the nanoparticle Tat-E7/pGM-CSF represents a promising novel approach to enhance the potency of peptide-based cervical cancer vaccines, and this vaccine design strategy may act as a useful reference for research of virus-associated diseases and specific tumor immunotherapies.

Vaccination with human papillomavirus type 16-derived peptides using a tattoo device

Tattooing has been shown to be very efficient at inducing immunity by vaccination with DNA vaccines. In this study, we examined the usability of tattooing for delivery of peptide vaccines. We compared tattooing with subcutaneous (s.c.) needle injection using peptides derived from human papillomavirus type 16 (HPV16) proteins. We observed that higher peptide-specific immune responses were elicited after vaccination with the simple peptides (E7(44-62) and E7(49-57)) and keyhole limpet hemocyanin-(KLH)-conjugated peptides (E7(49-57), L2(18-38) and L2(108-120)) with a tattoo device compared to s.c. inoculation. The administration of the synthetic oligonucleotide containing immunostimulatory CpG motifs (ODN1826) enhanced the immune responses developed after s.c. injection of some peptides (E7(44-62), KLH-conjugated L2(18-38) and L2(108-120)) to levels close to or even comparable to those after tattoo delivery of identical peptides with ODN1826. The highest efficacy of tattooing was observed in combination with ODN1826 for the vaccination with the less immunogenic E6(48-57) peptide and KLH-conjugated and non-conjugated E7(49-57) peptides which form the visible aggregates that could negatively influence the development of immune responses after s.c. injection but probably not after tattooing. In summary, we first evidenced that tattoo administration of peptide vaccines that might be useful in some cases efficiently induced both humoral and cell-mediated immune responses.

Mutation in the immunodominant epitope of the HPV16 E7 oncoprotein as a mechanism of tumor escape

Infection with high-risk types of human papillomavirus (HPV) can cause the development of malignant tumors. To study mechanisms responsible for immune escape of tumor cells infected with HPV16, we previously used mouse oncogenic TC-1 cells producing HPV16 E6 and E7 oncoproteins to derive TC-1 clones resistant to immunization against E7. We have found immunoresistance of the clones to correlate with the point mutation in the E7 oncogene, which resulted in the N53S substitution in the immunodominant epitope RAHYNIVTF (aa 49-57). Here, we have shown that this mutation reduced stabilization of H-2D(b) molecules on RMA-S cells and eliminated immunogenicity of E7. The resistance of TC-1 clones was E7-specific as immunization against E6 inhibited tumor growth. Transduction of the TC-1/F9 clone carrying the mutated epitope with the wild-type E7 gene restored susceptibility to immunization against E7. Our results suggest that mutagenesis of tumor antigens can lead to the escape of malignant cells and should be considered in the development and evaluation of cancer immunotherapy.

Mapping of cytotoxic T lymphocytes epitopes in E7 antigen of human papillomavirus type 11

One of the critical steps in the progression to condyloma acuminatum (CA) is the establishment of a persistent human papillomavirus (HPV) infection, majority of HPV type 6 and 11. Cytotoxic T lymphocytes (CTL), which can be induced by the epitope-based peptides in vitro, are thought to be able to recognize and destroy virus-infected cells. In order to screen and identify HLA-A*0201 restricted HPV-11E7 CTL epitopes, five epitope peptides and tetramers were selected including HPV-11E7 7-15 (TLKDIVLDL), 15-23 (LQPPDPVGL), 47-55 (PLTQHYQIL), 81-89 (DLLLGTLNI) and 82-90 (LLLGTLNIV). Human monocyte-derived dendritic cells (DCs) from HLA-A*0201 healthy individuals were pulsed with these peptides to assess the expression of CD83, CD86, HLA-DR and the secretion of IL-12. The ability of peptide-loaded mature DCs to activate autologous T cells was evaluated by analyzing the frequency of specific tetramer(+) CD8(+) T cells using flow cytometry, and the level of IFN-gamma secretion by ELISA. The ability of the epitope-specific CTLs to kill the target cells was also analysed. It was found that the immature DCs could be fully activated by all the five HPV-11E7 peptides and peptide-loaded mature DCs were able to stimulate the epitope-specific T cells in vitro. There was an increased frequency of CD8(+) T cells specific for the E7 7-15 epitope when compared to other four predicted epitopes of HPV-11E7 (P < 0.05). The epitope-specific CTLs for E7 7-15 induced the strongest cytotoxicity to HPV-11E7 expressing cell line at an E:T ratio of 50:1 (P < 0.05). Taken together, these findings demonstrate that E7 7-15 (TLKDIVLDL) is an HLA-A*0201-restricted CTL epitope of HPV type 11. We propose that this epitope could be more helpful in the characterization of HPV control mechanism and be useful for the development of immunotherapeutic approaches for low-risk HPV infectious diseases such as CA.

Strategies for studying mouse and human immune responses to human papillomavirus type 16

Cytotoxic T lymphocytes (CTL) are an important protective mechanism in viral infection and can be effective against tumours. We have investigated the tumour-associated E6 and E7 genes of human papillomavirus type 16 as CTL targets. In H-2b mice we have defined epitopes in E6 and E7 which can readily generate CTL in vivo and we have shown that HLA-A2.1 transgenic mice can generate an HLA-A2.1-restricted response. We have been unable to reveal a primed CTL response in humans. These paradoxical findings imply that human papillomavirus may fail to stimulate a systemic CTL response and/or employ strategies for evading or down-regulating such a response.

CD8alpha/alpha homodimers fail to function as co-receptor for a CD8-dependent TCR

In this study, we have started to dissect the molecular basis of CD8 dependence of a high and low avidity CTL clone specific for the same peptide epitope. Using anti-CD8alpha and anti-CD8beta antibodies, we found that cytotoxicity and IFN-gamma production by high but not by low avidity CTL was strongly CD8 dependent. We isolated the TCR genes of both types of CTL clones and used retroviral gene transfer to analyse the function of these TCR in primary T cells of wild-type and CD8beta-deficient mice. Both TCR triggered antigen-specific killing in wild-type T cells, and blocking experiments showed that CD8 dependence/independence co-transferred with the TCR into primary T cells, indicating that it was dictated by the TCR itself. Gene transfer experiments into CD8beta-deficient T cells revealed that only the TCR derived from the CD8-independent CTL clone elicited antigen-specific cytotoxicity, while the CD8-dependent TCR was non-functional in the absence of the CD8beta-chain. These data indicate a striking difference between CD8alpha/beta heterodimers and CD8alpha/alpha homodimers as only the former were able to provide co-receptor function for the CD8-dependent TCR.

A study of T cell tolerance to the tumor-associated antigen MDM2: cytokines can restore antigen responsiveness, but not high avidity T cell function

Background

Most tumor-associated antigens (TAA) currently used for immunotherapy of cancer are also expressed in normal tissues, which may induce tolerance and impair T cell-mediated immunity. However, there is limited information about how physiological expression in normal tissues alters the function of TAA-specific T cells.

Methodology/Principal Findings

We used a T cell receptor transgenic model to study how MDM2 expression in normal tissues affects the function of T cells specific for this TAA that is found at high levels in many different types of tumors. We found that some MDM2-specific T cells escaped thymic deletion and persisted in the peripheral T cell pool. When stimulated with antigen, these T cells readily initiated cell division but failed to proliferate and expand, which was associated with a high rate of apoptosis. Both IL-2 and IL-15 efficiently rescued T cell survival and antigen-specific T cell proliferation, while IL-7 and IL-21 were ineffective. Antigen-stimulated T cells showed impaired expression of the effector molecules CD43, granzyme-B and IFN-γ, a defect that was completely restored when T cells were stimulated in the presence of IL-2. In contrast, IL-15 and IL-21 only restored the expression of CD43 and granzyme-B, but not IFN-γ production. Finally, peptide titration experiments with IL-2 rescued T cells indicated that they were of lower avidity than non-tolerant control T cells expressing the same TCR.

Conclusions/Significance

These data indicate that cytokines can rescue the antigen-specific proliferation and effector function of MDM2-specific T cells, although this does not lead to the recovery of high avidity T cell function. This study sheds light on possible limitations of immunotherapy approaches that target widely expressed TAA, such as MDM2.

HLA class I binding promiscuity of the CD8 T-cell epitopes of human papillomavirus type 16 E6 protein

One of the critical steps in the progression to cervical cancer appears to be the establishment of persistent human papillomavirus (HPV) infection. We have demonstrated that the lack of cytotoxic T-lymphocyte response to HPV type 16 (HPV 16) E6 protein was associated with persistence and that the potential presence of dominant CD8 T-cell epitopes was most frequently found (n = 4 of 23) in the E6 16-40 region by examining the pattern of CD8 T-cell epitopes within the E6 protein in women who had cleared their HPV 16 infections. The goal of this study was to define the minimal/optimal amino acid sequences and the HLA restricting molecules of these dominant CD8 T-cell epitopes as well as those of subdominant ones if present. Three dominant epitopes, E6 29-38 (TIHDIILECV; restricted by the HLA-A0201 molecule), E6 29-37 (TIHDIILEC; restricted by B48), and E6 31-38 (HDIILECV; restricted by B4002), and one subdominant epitope, E6 52-61 (FAFRDLCIVY; restricted by B35) were characterized. Taken together with a previously described dominant epitope, E6 52-61 (FAFRDLCIVY; restricted by B57), the CD8 T-cell epitopes demonstrated striking HLA class I binding promiscuity. All of these epitopes were endogenously processed, but the presence of only two of the five epitopes could have been predicted based on the known binding motifs. The HLA class I promiscuity which has been described for human immunodeficiency virus may be more common than previously recognized.

Definition of an HPV18/45 cross-reactive human T-cell epitope after DNA immunisation of HLA-A2/KB transgenic mice

Although human papillomavirus (HPV) types 16 and 18 are the most common types associated with cervical cancer worldwide, other related HPV types such as HPV 35, 45 and 58 have significant prevalence in geographically distinct populations. For development of global prophylactic and therapeutic vaccine strategies, it is important to study immune responses against these viruses and to define the degree of cross-reactivity between related HPV types. To investigate the potential for T cell cross-reactivity after vaccination, HLA-A2/Kb transgenic mice were immunised with DNA plasmid constructs containing HPV18 and 45 E6 and E7. Splenocytes from immunised mice were tested in direct ELIspot assays against overlapping pools of HPV 18 peptides. Immunisation with either HPV18 or HPV45 E6 DNA produced dominant T cell responses against an epitope (KCIDFYSRI) that was shared between HPV18 and HPV45. This peptide was shown to bind to HLA-A*0201 but not Db or Kb molecules on the cell surface. Furthermore this peptide was shown to be immunogenic in vitro to human T cells from 2 out of 3 HLA-A2+ healthy donors. Collectively, these results demonstrate that HPV 18 and 45 E6 DNA vaccines are immunogenic in mice and demonstrate that cross-reactive T cell responses against closely related HPV types can be induced in vivo. The use of the HLA-A2/Kb transgenic mice allowed definition of an HLA-A*0201 binding peptide epitope that would have been rejected on the basis of predicted major histocompatibility complex binding affinity.

Activation and inactivation of antiviral CD8 T cell responses during murine pneumovirus infection

Pneumonia virus of mice (PVM) is a natural pathogen of mice and has been proposed as a tractable model for the replication of a pneumovirus in its natural host, which mimics human infection with human respiratory syncytial virus (RSV). PVM infection in mice is highly productive in terms of virus production compared with the situation seen with RSV in mice. Because RSV suppresses CD8 T cell effector function in the lungs of infected mice, we have investigated the nature of PVM-induced CD8 T cell responses to study pneumovirus-induced T cell responses in a natural virus-host setting. PVM infection was associated with a massive influx of activated CD8 T cells into the lungs. After identification of three PVM-specific CD8 T cell epitopes, pulmonary CD8 T cell responses were enumerated. The combined frequency of cytokine-secreting CD8 T cells specific for the three epitopes was much smaller than the total number of activated CD8 T cells. Furthermore, quantitation of the CD8 T cell response against one of these epitopes (residues 261-270 from the phosphoprotein) by MHC class I pentamer staining and by in vitro stimulation followed by intracellular IFN-gamma and TNF-alpha staining indicated that the majority of pulmonary CD8 specific for the P261 epitope were deficient in cytokine production. This deficient phenotype was retained up to 96 days postinfection, similar to the situation in the lungs of human RSV-infected mice. The data suggest that PVM suppresses T cell effector functions in the lungs.

Induction of Unresponsiveness Limits Tumor Protection by Adoptively Transferred MDM2-Specific Cytotoxic T Lymphocytes

There is evidence showing that high avidity CTLs can be more effective than low avidity CTLs for adoptive tumor immunotherapy. Because many T cell-recognized tumor antigens are nonmutated self-proteins, tolerance mechanisms are likely to render high avidity T cells unresponsive or cause T cell elimination by clonal deletion. We recently used the allo-restricted strategy to circumvent immunologic tolerance to a ubiquitously expressed tumor-associated protein, MDM2, and raised high avidity CTLs in humans and in mice. In this study, we investigated whether high avidity MDM2-specific CTLs can mediate tumor protection without causing damage to normal tissues in mice. Although the CTLs prolonged survival of tumor-bearing mice without causing damage to normal tissues, tumor protection was incomplete. We show that tumor growth occurred despite the continued presence of MDM2-specific CTLs and the continued susceptibility of tumor cells to CTL killing. However, analysis of the CTLs revealed that they had been rendered unresponsive in vivo because they did not produce interferon gamma in response to antigen-specific stimulation. These experiments suggest that induction of unresponsiveness may be an important mechanism limiting the efficacy of adoptive CTL therapy.

Cytotoxic-T-lymphocyte human papillomavirus type 16 E5 peptide with CpG-oligodeoxynucleotide can eliminate tumor growth in C57BL/6 mice

Previously, we identified human papillomavirus type 16 (HPV-16) E5 as a tumor rejection antigen that can induce cytotoxic T lymphocytes (CTLs) to protect against tumor growth (D. W. Liu et al., J. Virol. 74:9083-9089, 2000). In the present study, we further mapped the CTL epitope of E5 protein by analyzing E5-specific CD8(+) gamma interferon-positive (IFN-gamma(+)) double-positive cells in C57BL/6 mice with flow cytometry. The results showed the region spanning amino acids 25 to 33 (VCLLIRPLL) contained the potential D(b)-restricted CTL epitope. Subsequently, to determine whether peptide E5 25-33-based vaccination could induce E5-specific CTL activity, syngeneic animals received E5 25-33 emulsified with either CpG oligodeoxynucleotide (CpG ODN 1826) or Freund's adjuvant, and the growth of the tumors was monitored. The results showed that although both adjuvants induced E5-specific CD8(+) IFN-gamma(+) T cells and eradicated E5-containing tumor growth, CpG ODN was found to stimulate stronger CTL response than Freund's adjuvant. We also compared the immune response of the effector/memory/recall phase induced by E5 25-33 peptide or by E5 protein that was synthesized in vivo by adenovirus-based E5 gene delivery. E5 25-33 peptide plus CpG ODN was shown to be a superior vaccine compared to the adenovirus-based E5 gene. Interestingly, their chronological patterns of immune response were similar, suggesting that E5 25-33 is a major CTL peptide of E5 protein.

Yellow fever virus 17D envelope and NS3 proteins are major targets of the antiviral T cell response in mice

The yellow fever virus 17D vaccine strain is one of the most effective and safe vaccines available. The immune response after immunization is characterized by long-lasting high titers of neutralizing antibodies. Here, we have initiated a characterization of YFV-17D-specific cellular immune responses. This study makes three points. First, we have identified two CD8 T cell epitopes and one CD4 T cell epitope. An H-2Kb-restricted dominant epitope was mapped in the NS3 protein, whereas the viral envelope protein harbored an H-2Db-restricted subdominant epitope and the I-Ab-restricted CD4 T cell epitope. Second, illustrating the concept of immunodomination, we found that after abrogation of the dominant response in H-2Kb knockout mice, the frequencies of T cells recognizing the subdominant Db-restricted epitope increased dramatically. Finally, the H-2Db-restricted epitope lacks the canonical Asn anchor residue at position 5, indicating that epitopes may be missed by strict application of the H-2Db-binding motif. Identification of these T cell epitopes will facilitate studies on the cellular immunity against YFV-based expression or immunization vectors.

Visualization and characterization of respiratory syncytial virus F-specific CD8(+) T cells during experimental virus infection

CTL play a major role in the clearance of respiratory syncytial virus (RSV) during experimental pulmonary infection. The fusion (F) glycoprotein of RSV is a protective Ag that elicits CTL and Ab response against RSV infection in BALB/c mice. We used the strategy of screening a panel of overlapping synthetic peptides corresponding to the RSV F protein and identified an immunodominant H-2K(d)-restricted epitope (F(85-93); KYKNAVTEL) recognized by CD8(+) T cells from BALB/c mice. We enumerated the F-specific CD8(+) T cell response in the lungs of infected mice by flow cytometry using tetramer staining and intracellular cytokine synthesis. During primary infection, F(85-93)-specific effector CD8(+) T cells constitute approximately 4.8% of pulmonary CD8(+) T cells at the peak of the primary response (day 8), whereas matrix 2-specific CD8(+) T cells constituted approximately 50% of the responding CD8(+) T cell population in the lungs. When RSV F-immune mice undergo a challenge RSV infection, the F-specific CD8(+) T cell response is accelerated and dominates, whereas the primary response to the matrix 2 epitope in the lungs is reduced by approximately 20-fold. In addition, we found that activated F-specific effector CD8(+) T cells isolated from the lungs of RSV-infected mice exhibited a lower than expected frequency of IFN-gamma-producing CD8(+) T cells and were significantly impaired in ex vivo cytolytic activity compared with competent F-specific effector CD8(+) T cells generated in vitro. The significance of these results for the regulation of the CD8(+) T cell response to RSV is discussed.

Modified HPV16 E7 Genes as DNA Vaccine against E7-Containing Oncogenic Cells

Therapeutic vaccines against tumors associated with human papillomaviruses (HPV) should elicit cellular immune responses against early HPV antigens, primarily the oncoproteins E7 and E6. Because of safety concerns, the direct use of an unmodified oncogene is impossible in human DNA vaccination. Therefore, we introduced three point mutations into the pRb-binding site of HPV16 E7 oncogene to eliminate its transformation potential. The resultant gene was denoted E7GGG. The rates of expression and the cellular localization of E7 and E7GGG proteins were comparable. In immunization-challenge experiments, the efficacy of plasmids containing the E7, E7GGG, or fusion genes of HPV16 E7, viz. L1DeltaCE7(1-60) (M. Muller et al., 1997, Virology 234, 93-111), and Sig/E7/LAMP-1 (T. C. Wu et al., 1995, Proc. Natl. Acad. Sci. USA 92, 11671-11675), was compared. While tumors developed in all animals immunized with the wild-type E7 gene, a significant proportion of mice remained tumor-free after vaccination with the E7GGG gene. The fusion gene L1DeltaCE7(1-60) induced negligible protection, but Sig/E7/LAMP-1 conferred the highest protection. Intradermal immunization by gene gun proved superior to i.m. inoculation. In "therapeutic" experiments, a 1-day delay between inoculation of oncogenic cells and the start of DNA immunization resulted in partial therapeutic effect, but a 3-day delay produced a substantially lower immunization effect. A combination of Sig/E7/LAMP-1 and E7GGG genes did not enhance the immune response. These results demonstrate a significant enhancement of HPV16 E7 immunogenicity after mutagenesis of the pRb-binding site, but the mutated E7 gene did not excel the Sig/E7/LAMP-1 fusion gene.

Dissimilar immunogenicities of human papillomavirus E7 and adenovirus E1A proteins influence primary tumor development

Although human papillomaviruses (HPV) and adenoviruses (Ad) both transform cells by expressing functionally related oncogenes (Ad-E1A/E1B; HPV-E7/E6), only HPV are oncogenic in humans. Prior studies have shown that HPV-transformed cells are resistant to NK cell lysis and E7- and E6-specific CTL are inefficiently generated in women with HPV-induced cervical cancer. Therefore, we postulated that the dissimilar oncogenicities of Ad and HPV may be caused by a protective NK and T cell response that is triggered by transformed cells expressing E1A, but not by E7. To test this hypothesis, mice that were either immunologically intact, lacked T cells, or lacked both NK and T cells were challenged with Ad serotype 5 (Ad5)-E1A- or HPV16-E7-transfected tumor cells. E7-expressing tumor cells were resistant to NK cell lysis in vitro and failed to elicit a measurable anti-tumor NK or T cell response in vivo. The concomitant expression of E6 did not change this phenotype. In contrast, E1A-expressing tumor cells were sensitive to NK lysis in vitro and triggered a protective NK and T cell immune response in vivo. These data suggest differences in the capacities of E1A or E7 oncoproteins to trigger protective anti-tumor immune responses may contribute to the dissimilar oncogenicities of Ad and HPV in humans.

Human papillomavirus type 16 E7 DNA vaccine: mutation in the open reading frame of E7 enhances specific cytotoxic T-lymphocyte induction and antitumor activity

A human papillomavirus type 16 E7 DNA vaccine with the open reading frame encoding mutations in two zinc-binding motifs expressed a rapidly degraded E7 protein. This vaccine induced a significantly stronger E7-specific cytotoxic T-lymphocyte response and better tumor protection in mice than did a wild-type E7 DNA vaccine expressing a stable E7 protein.

The MHC Class I-Restricted Immune Response to HIV-gag in BALB/c Mice Selects a Single Epitope That Does Not Have a Predictable MHC-Binding Motif and Binds to Kd Through Interactions Between a Glutamine at P3 and Pocket D

Using a strain of Listeria monocytogenes that stably expresses and secretes HIV gag to deliver this Ag to the MHC class I pathway of Ag processing, we have identified the immunodominant CTL epitope to gag in the BALB/c mouse and shown that it is Kd restricted. The specific motif for the peptides that bind the MHC class I molecule H-2 Kd is believed to be a nonamer with residues tyrosine or phenylalanine in the second amino acid position and leucine or isoleucine in the carboxyl-terminal or ninth amino acid position as dominant anchoring positions. Surprisingly, the identified gag peptide, AMQMLKETI, does not contain an anchoring aromatic residue in position two although competition assays with other Kd-restricted epitopes indicated that it binds to Kd with comparable affinity. Using a theoretical molecular dynamics approach to probe the stability of peptide binding to MHC class I molecules, we show that the absence of an appropriate anchor residue at P2 in AMQMLKETI is compensated by favorable interactions of the glutamine at P3 with pocket D of Kd. These findings were verified experimentally, demonstrating the predictive power of this theoretical approach in analyzing MHC class I/peptide interactions. These studies also indicate that CTL epitope prediction that relies on dominant peptide motifs may not always identify the correct epitope.

Priming of cytotoxic T lymphocytes by five heat-aggregated antigens in vivo: conditions, efficiency, and relation to antibody responses

Mice were immunized i.p. with soluble or heat-denatured protein antigens [ovalbumin, beta-galactosidase, or recombinant E7 protein of human papilloma virus type 16 (HBV)]. Heat-denatured (100 degrees C) preparations of these proteins were able to induce cytotoxic T lymphocytes (CTL) that recognize cells expressing the respective genes, whereas native protein was either inefficient or required up to 30-fold higher doses. If the heat-treated proteins were separated into aggregated and soluble fractions by ultracentrifugation, only the aggregated fractions were able to induce specific CTL; this is probably because of the easier access to one of the major histocompatibility complex class I loading pathways for exogenous antigen. Addition of the adjuvant aluminium hydroxide (alum) to aggregated proteins abolished their ability to induce CTL; thus, a condition leading to a strong antibody response appeared to inhibit CTL induction. Interestingly, immunization with heat-denatured ovalbumin plus alum increased the IgM/IgG1 ratio compared to immunization with native ovalbumin and alum. Immunization of B6 mice transgenic for an HLA-A2/H-2K(b) hybrid gene with heat-denatured, recombinant HPV 16-E7 protein induced D(b)-restricted CTL specific for the peptide 49-57 of E7, indicating that this epitope is immunodominant over any A2-restricted E7 epitope in these mice. A whole influenza virus preparation heated to 100 degrees C or even autoclaved was still able to induce virus-specific CTL and BALB/c spleen cells heated to 100 degrees C could still cross-prime minor H-specific CTL in B6 mice, although with lower efficiency than fresh spleen cells. Thus, aggregated proteins can be considered as components for future vaccines.

Cellular immunotherapy and autologous transplantation for hematologic malignancy

The success of allogeneic transplantation is in part due to the immunotherapeutic effect mediated by the graft. Autologous transplantation is hampered by the absence of this effect, leading to a higher relapse rate. We have conducted a series of studies designed to augment the immunologic activity of the graft after autologous transplant with a view towards introducing an autologous graft-versus-tumor effect that could decrease the rate of relapse after autologous transplant. These studies have included IL-2 activation of marrow followed by post-transplant infusional IL-2, the development of a novel protocol for the generation of highly efficient cytotoxic effector cells, termed cytokine-induced killer (CIK) cells, with broad and potent antitumor activity. In order to determine the potential for generating peptide-specific cytolytic T cells, studies have been conducted upon transducing antigen-presenting cells (APC) with AAV vector-mediated gene transfer, a vector capable of transducing non-proliferating target cells. Transduction of human monocytes and macrophages resulted in high expression of the transduced gene. This latter study forms the basis for determining whether genetic modification of APC can potentiate specific immune responses to tumor-specific gene products. Taken together, these strategies will hopefully increase the therapeutic efficacy of autologous transplantation.

Characteristics of the intrahepatic cytotoxic T lymphocyte response in chronic hepatitis C virus infection

Based on our CTL studies of over 44 persons with chronic HCV infection, we are able to arrive at a number of conclusions. Clearly this cellular immune response is heterogeneous among infected persons. We have not identified any specific HCV protein which appears to be immunodominant for CTL responses, but rather we have detected diverse responses to both structural and non-structural proteins. Using an identical stimulation strategy for all persons studied, we have been able to detect responses in only approximately one third of persons with chronic infection. Among these persons, the responses among liver-infiltrating lymphocytes are greater than those detected in fresh peripheral blood, suggesting that the CTL are homing to the site of maximal viral burden in these persons. Some viral proteins contain overlapping epitopes presented by more than one HLA class I molecule, and we have also found cases where peptides in the same HLA superfamily, such as the HLA A3 superfamily which contains A11, for which the same peptide can be presented by both alleles (manuscript in preparation). Although sequence variation between the infecting strain and the vaccinia constructs used to test for responses may lead to non-recognition of some variants, even the highly conserved core protein appears to be an inconsistent and actually infrequent target for detectable CTL responses. The magnitude of the CTL response appears to vary greatly, from being undetectable to being so vigorous that it an be detected in stimulated peripheral blood. The breadth of the response also varies widely, ranging from the detection of a response to a single epitope in some persons, to the simultaneous recognition of up to five different epitopes in others. Even in persons of the same HLA type, we have not seen consistent targeting of the same epitopes except in rare cases. Despite the detection of over 20 epitopes and their restricting class I alleles using CTR derived from liver-infiltrating lymphocytes, we have identified only one epitope that has been shown to be targeted by more than one person of the same HLA type. These findings lead us to speculate that the CTL response may be submaximal in the majority of infected persons. The reasons for this are presently obscure, but could relate to a number of factors. The epitopes targeted are found within variable regions of the virus, such that immune escape from established CTL responses has to be considered a real possibility. Sequence variation may also lead to antagonism of CTL responses, as has been demonstrated for both HIV and HBV infections. Furthermore, sequence variation either within or adjacent to regions containing CTL epitopes can lead to altered antigen processing, either due to alteration of proteolytic processing of the viral peptides in the cytoplasm or to altered transport and altered association with class I molecules. A number of issues regarding the CTL response in HCV infection still require substantial attention. The apparent inability of CTL to clear this virus needs to be addressed, as does the potential role for viral immunomodulatory molecules in HCV persistence. Although we and others have shown CTL responses to be present in persons with chronic infection, the role of CTL in acute HCV infection needs to be determined. The best studied chronic human viral infection is HIV infection, in which expanding data indicate that the early events following primary infection predict the subsequent course of illness. Viral load in the first 1-2 years after infection is highly predictive of the subsequent disease course in HIV infection, and recent experimental data in humans suggest that early immune responses may be predictive of subsequent disease course. Such studies in HCV infection have been difficult to achieve, since primary HCV infection is often asymptomatic, and transfusion-related cases are now rare. (ABSTRACT TRUNCATED)

Controlled delivery of antigens and adjuvants in vaccine development

Advances in the understanding of the pathogenesis of infectious diseases and cancer immunology have inspired many new approaches to vaccine development. Many subunit antigens and peptides that are effective for vaccination have been discovered. These subunit antigens in tum stimulate synthesis of effective adjuvants to enhance their immunogenicity. Controlled-release technology offers the potential of further improving the efficacy of conventional vaccine formulations by optimizing the temporal and spatial presentation of the-antigens and adjuvants to the immune system. The combination of sustained release and depot effect may also reduce the amount of antigens or adjuvants needed and eliminate the booster shots that are necessary for the success of many vaccinations. This review examines the contribution controlled release technology can make in various areas of vaccination, with an emphasis on tumor vaccines.

Peptide-specific cytotoxic T lymphocytes restricted by nonself major histocompatibility complex class I molecules: reagents for tumor immunotherapy

Studies in melanoma patients have revealed that self proteins can function as targets for tumor-reactive cytotoxic T lymphocytes (CTL). One group of self proteins MAGE, BAGE, and GAGE are normally only expressed in testis and placenta, whilst another group of CTL recognized proteins are melanocyte-specific differentiation antigens. In this study we have investigated whether CTL can be raised against a ubiquitously expressed self protein, mdm-2, which is frequently overexpressed in tumors. The observation that T-cell tolerance is self major histocompatibility complex-restricted was exploited to generate CTL specific for an mdm-2 derived peptide presented by nonself major histocompatibility complex class I molecules. Thus, the allo-restricted T-cell repertoire of H-2d mice was used to isolate CTL specific for the mdm100 peptide presented by allogeneic H-2Kb class I molecules. In vitro, these CTL discriminated between transformed and normal cells, killing specifically Kb-positive melanoma and lymphoma tumors but not Kb-expressing dendritic cells. In vivo, the CTL showed antitumor activity and delayed the growth of melanoma as well as lymphoma tumors in H-2b recipient mice. These experiments show that it is possible to circumvent T-cell tolerance to ubiquitously expressed self antigens, and to target CTL responses against tumors expressing elevated levels of structurally unaltered proteins.

Construction and characterisation of a recombinant vaccinia virus expressing human papillomavirus proteins for immunotherapy of cervical cancer

The presence and consistent expression of the genes encoding the human papillomavirus (HPV) E6 and E7 proteins in the great majority of cervical tumours presents the opportunity for an immunotherapeutic approach for control of the disease. This report describes the construction and characterisation of a recombinant vaccinia virus designed to express modified forms of the E6 and E7 proteins from HPV16 and HPV18, the viruses most commonly associated with cervical cancer. The recombinant virus (designated TA-HPV) was based on the Wyeth vaccine strain of vaccinia, and was shown to express the desired gene products. Studies in mice indicated that the recombinant virus was less neurovirulent than the parental virus and was capable of inducing an HPV-specific CTL response. This pre-clinical evaluation has provided a basis for the initiation of human trials in cervical cancer patients.

Immunotherapy of cancer by peptide-based vaccines for the induction of tumor-specific T cell immunity

Recent progress in defining the molecular nature of antigens and in finding ways to manipulate T cell-mediated immune responses may provide new modalities for cancer treatment. In this report, we review preclinical studies as well as the first clinical trials with vaccination strategies aiming at the induction of anti-tumor immunity. In particular, we focus on the development of a vaccine against human papillomavirus-induced cervical carcinoma.

Induction of rat CD4+ proliferative and mouse CD8+ cytotoxic T-cell lines specific for human papillomavirus type 16 antigens

Infection by human papillomavirus type 16 (HPV16) has been associated with cervical dysplasia and carcinoma. To detect a possible cellular immune response against HPV16, we investigated the induction of T cells specific for antigens encoded by the virus. Two CD4+ T-cell lines with a specific proliferative response were isolated from the spleens of rats vaccinated using vaccinia virus vectors expressing E6 or E7 and challenged with syngeneic HPV-transformed cells. Two CD8+ T-cell lines with a specific cytotoxic activity were obtained from mice immunized using a syngeneic squamous cell tumour cell line transfected with the full-length HPV16 DNA. These results demonstrate that both CD8-mediated cytotoxic responses and CD4-mediated delayed-type hypersensitivity responses are involved in immunologic reactions to HPV antigens.

Use of combinatorial peptide libraries to construct functional mimics of tumor epitopes recognized by MHC class I-restricted cytolytic T lymphocytes

Identification of cytolytic T lymphocyte (CTL) epitopes presented by major histocompatibility complex (MHC) class I molecules on tumor cells is critical for the design of active immunotherapy. We describe the use of combinatorial peptide libraries with defined amino acids in two MHC anchor positions to search for epitopes that are recognized by H-2Db- and Kb-restricted CTL specific for the mouse lymphoma EL4. An iterative strategy was used for screening libraries in which 16 amino acids were divided into 3 groups and 3 subgroups: alpha (AL, VT, FY); beta (GS, P, DE); gamma (KR, H, NQ). The proportions of each group and subgroup at individual peptide positions were changed in the library synthesis, and the effect of these changes on CTL activity was measured in a sensitive RMA-S cell assay. A single H-2Db epitope mimic was deduced from the original library that contained > 2 x 10(8) potential peptides and was at least 9 logs more potent than the original library. Immunization of syngeneic mice with this peptide elicited CTL that lysed EL4 cells as well as RMA-S cells pulsed with peptides isolated from Db molecules of EL4 cells, indicating functional similarity between the mimicking peptide and the naturally processed CTL epitope. Furthermore, adoptive transfer of such a CTL line had a therapeutic effect in mice with EL4 established as an ascites tumor. Two H-2Kb-restricted epitope mimics of the same tumor were also identified. Our method represents a novel approach for the construction of MHC class I-restricted targets that can serve as immunogens for active immunotherapy of cancer.

Specificity of the H-2 L(d)-restricted cytotoxic T-lymphocyte response to the mouse hepatitis virus nucleocapsid protein

Cytotoxic T lymphocytes provide protection against persistent infection of the central nervous system by the JHM strain of mouse hepatitis virus. In BALB/c (H-2d) mice, the dominant response is directed against an Ld-restricted peptide in the nucleocapsid protein (APTAGAFFF). Characterization of the fine specificity of this response revealed that the predicted anchor residues at positions 2 and 9 were the most critical for class I binding. Amino acids at positions 7 and 8 were identified as T-cell receptor contact residues. Virus-induced cytotoxic T lymphocytes to other Ld motif-containing nucleocapsid peptides were not detected, despite the identification of two epitopes with reduced Ld affinity. These data suggest that mutations within four residues of the dominant epitope could contribute to the persistence of the JHM strain of mouse hepatitis virus.

Amino acid preferences in the octapeptide subunit of the major histocompatibility complex class I heterotrimer H-2Ld

Major histocompatibility complex class I (MHC-I) molecules are heterotrimers composed of polymorphic alpha-chains, monomorphic beta-chains, and peptides of eight or nine amino acids. The peptides are derived from various intracellularly occurring proteins and are very heterogeneous. They are essential for a stable conformation of the MHC-I protein at physiological temperature. This study presents results from stabilization experiments that were designed to determine the impact of the amino acids in every sequence position of octapeptides on the thermal stability of the mouse MHC-I molecule H2-Ld. OX7 octapeptide libraries with one defined and seven randomized positions were employed as they allow the effects of individual amino acids to be determined. The results confirm the importance of the motif amino acids proline and leucine for positions 2 and 8, respectively, of octapeptides. They are among the most efficient amino acids for these positions. However, with a few exceptions, all amino acids are permitted in all eight sequence positions. Hydrophobic amino acids are generally favored. Charged amino acids, especially aspartic acid and glutamic acid, are disfavored. Stabilization indices were defined as measures for the MHC stabilization power of the amino acids. These indices can serve to predict the efficiency of peptide binding to H-2Ld and can guide the design of T-cell epitopes.

Priming in vivo and quantification in vitro of class I MHC-restricted cytotoxic T cells to human papilloma virus type 11 early proteins (E6 and E7) using immunostimulating complexes (ISCOMs)

Immunostimulating complexes (ISCOMs) efficiently deliver soluble antigen into both the cytosolic (endogenous) and endosomal (exogenous) pathways of antigen processing. Cytosolic delivery to antigen-presenting cells (APCs) may therefore be useful for the stimulation and assay of class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL) in vitro. In this study, mice were immunized with ISCOMs containing fusion proteins of the E6 or E7 early proteins of human papilloma virus type 11 (HPV 11) to elicit CTL. These CTL were then restimulated in vitro using APCs pulsed with the same ISCOMs, prior to cytotoxicity assay using syngeneic target cells infected with recombinant vaccinia viruses. In this way, antigen-specific, MHC-restricted lysis by CD8+ cells was detected. However, this was dependent on the use of low density splenocytes as APCs for restimulation in vitro. Limiting dilution analyses showed a direct correlation between the CTL responder frequency and the number of times the animals were immunized in vivo. We conclude that in lieu of infectious virus, the use of ISCOMs to mediate antigen delivery to APCs in vitro can be used to quantitate CTL activity. This may have applications in monitoring vaccine efficacy, particularly to viruses such as HPV, which cannot be presently obtained as infectious virus in sufficient quantity for CTL propagation and assay.

T-cell epitope determination

Synthetic approaches to T-cell epitope determination have recently been developed that complement the search for natural T-cell epitopes and the investigation of the preferences of the different MHC alleles for particular motifs in cognate peptide sequences. The combination of these different strategies opens new possibilities for basic, as well as for applied, immunology. The outlines of the strategies for determination of natural T-cell epitopes are well established. These strategies have contributed substantially to our understanding of the nature of T-cell epitopes and of many diseases. Positional scanning approaches with random synthetic peptide libraries allow comprehensive surveys of the sequence requirements for peptide selection by MHC molecules and for induction of T-cell responses. Synthetic T-cell epitopes can be determined independently of the knowledge of the natural T-cell antigen. This opens new perspectives for the development of synthetic vaccines, TCR antagonists and MHC blockers.

Both immunization with protein and recombinant vaccinia virus can stimulate CTL specific for the E7 protein of human papilloma virus 16 in H-2d mice

The transforming protein E7 of human papilloma virus type 16 can stimulate cytotoxic T lymphocytes (CTL) which can protect experimental animals against growth of E7 expressing tumour cells. In this study we compared CTL responses in mice immunized with either E7 protein in MF59 adjuvant or with recombinant vaccinia virus expressing E7 (Vac-E7). We have chosen H-2d mice because no E7-specific CTL responses have been described in this MHC haplotype. Immunization of these mice with Vac-E7 generated CTL which lysed target cells infected with Vac-E7 or transfected with the E7 gene. CTL from mice immunized with E7 protein in MF59 adjuvant showed specificity for the same target cells. Antibody blocking experiments revealed that both immunization with Vac-E7 and E7 protein stimulated CD8+ effector CTL. The find specificity of CTL induced by the two immunization protocols was similar. A major CTL epitope was mapped to the carboxyl terminal amino acids 48-98 of the E7 protein. Peptide isolation from E7 expressing cells followed by HPLC separation indicated that CTL induced by immunization with protein and Vac-E7 recognized the same HPLC purified peptide fractions. Together, the study suggests that vaccines based on protein can activate CTL with similar fine specificity to CTL induced by vaccines based on recombinant vaccinia virus.

Cytotoxic T lymphocytes raised against a subdominant epitope offered as a synthetic peptide eradicate human papillomavirus type 16-induced tumors

Previously, we have shown that immunization with human papillomavirus (HPV) type 16-derived cytotoxic T lymphocyte (CTL) epitope E7 49-57 (RAHYNIVTF) renders C57BL/6 mice insensitive to tumors formed by HPV16-transformed cells. In this study, we provide evidence that E7 49-57 is expressed as a subdominant CTL epitope on HPV16-transformed C57BL/6 cells. Using acid peptide elution, it is shown that HPV16-transformed cells express another CTL epitope, besides E7 49-57, which appears to be dominant. We demonstrate that a CTL line raised against the subdominant CTL epitope, offered as synthetic peptide E7 49-57, eradicates established HPV16-induced tumors in mice. Our data show that synthetic peptide-induced CTL can be applied successfully in vivo against (virus-induced) tumor, and emphasize that subdominant CTL epitopes are useful targets for immunotherapy. Furthermore, it is illustrated for the first time that HPV16-specific CTL interfere directly with HPV16-induced tumors.

Identification of H-2Kb binding and immunogenic peptides from human papilloma virus tumour antigens E6 and E7

Peptides can be used to induce MHC class I restricted cytotoxic T cells (CTL) through in vivo immunization. This approach may enable the development of peptide vaccination schemes for immunization against viral infection in humans. Human papillomavirus (HPV) is one of a few viruses associated with human cancer and the development of an anti-cancer vaccine seems possible. As a model approach, we searched the E6 and E7 proteins of the human papillomavirus type 16 for possible murine MHC class I restricted peptide epitopes. We utilized the mouse H2-Kb peptide binding motif which consists of phenylalanine or tyrosine at position five and leucine at the carboxy-terminus with the modification that leucine could be replaced by other aliphatic but non-aromatic amino acids. Four peptide sequences from E6 and two from E7 were selected. These peptides were tested for their ability to bind and stabilize Kb and for their immunogenicity in vivo. It was shown that one peptide from E6, E6.1 (50-57), bound Kb, but was not able to prime mice in vivo. In contrast, the two selected E7 peptides E7.1 (21-28) and E7.2 (48-55) bound Kb and were immunogenic in vivo. The peptide induced CTL lysed syngeneic EL-4 cells transfected with the open reading frame of E7 but not vector only transfectants. This implies that both peptides were naturally processed and presented by Kb on the surface of target cells. MHC class I peptide binding motifs therefore appear to be an effective and useful tool to predict peptide epitopes of proteins associated with cancer.

Competition inhibition of cytotoxic T-lymphocyte (CTL) lysis, a more sensitive method to identify candidate CTL epitopes than induction of antibody-detected MHC class I stabilization

We compared the efficiency of two commonly used cellular major histocompatibility complex (MHC) class I peptide-binding assays to identify a cytotoxic T lymphocyte (CTL) epitope-containing peptide among length variants derived from the human papilloma virus type 16 (HPV 16) oncoprotein E7. Although both assays identified the same sequence (E7 49-57) as the most efficient Db-binding peptide, the efficiency by which they did so differed markedly. In a peptide competition cytotoxicity (PCC) assay, based on inhibition of CTL lysis by competition for binding to MHC class-I molecules between a known CTL epitope-containing peptide and peptide of interest, E7 49-57 bound 45-fold more efficiently to Db than the second Db-binding peptide in line. In the widely used RMA-S MHC class I peptide-binding assay, based on peptide-induced stabilization of 'empty' MHC class-I molecules at the surface of antigen-processing defective RMA-S cells, this difference was only 3 fold. Similar differences were observed when other Db-restricted CTL clones and CTL epitope-containing peptides were used in the PCC assay. The same phenomenon was observed when peptide binding affinities for H-2Kb were analyzed in both assays. We conclude that the PCC assay discriminates more efficiently between high- and low-affinity MHC class I binding peptides than the RMA-S assay. This observation is ascribed to the fact that peptide-MHC class I dissociation is an important parameter in the PCC but not the RMA-S assay.

Decrypting the structure of major histocompatibility complex class I-restricted cytotoxic T lymphocyte epitopes with complex peptide libraries

Complex synthetic peptide libraries with defined amino acids in one or more positions of the H-2Kb-restricted cytotoxic T lymphocyte (CTL) epitopes SIINFEKL and RGYVYQGL and mixtures of 19 amino acids in the remaining positions were used to analyze the structural requirements of peptide binding to MHC class I molecules and antigen recognition by CTLs. This approach provides means to assess semiquantitatively the contribution of every amino acid to the binding of peptides to major histocompatibility complex (MHC) molecules without biases introduced by naturally processed peptides. Primary and secondary anchor residues were defined for their major contribution to the binding efficiency of the peptides. In contrast to primary anchors, secondary anchor amino acids vary greatly in their side chains and position in the sequences. All amino acids in the octapeptide sequences were found to exhibit positive or negative influences on binding to the MHC molecules and on recognition of the resulting complexes by CTLs. Strong interdependence of the effects of the individual residues in the epitope sequences was demonstrated. CTL responses to peptide libraries were suppressed when residues were introduced; however, they were augmented when the critical residues for T cell recognition were fixed, suggesting a potential use of the peptide libraries for defining epitope sequences in general.

The association of an HPV16 oncogene variant with HLA-B7 has implications for vaccine design in cervical cancer

HLA-restricted cytotoxic T-lymphocyte (CTL) recognition of human papillomavirus (HPV) oncogene products may be important in the control of the HPV infections associated with the development of cervical cancer. We have identified, in HLA-B7 individuals, a consistent variation in the HPV16 E6 oncoprotein sequence, which alters an HLA-B7 peptide binding epitope in a way likely to influence immune recognition by CTLs. These results illustrate a biologically relevant mechanism for escape from immune surveillance of HPV16 in HLA-B7 individuals. Thus, both HLA type and HPV16 strain variation need to be considered in the screening of at-risk individuals and for the rational design of anti-HPV vaccines.

Analysis of the fine specificity of rat, mouse and human TAP peptide transporters

Prior to their association with major histocompatibility complex (MHC) class I molecules, peptides generated from cytosolic antigens need to be translocated by the MHC-encoded peptide transporter (TAP) into the lumen of the endoplasmic reticulum (ER). While class I molecules possess well-known binding characteristics for peptides, the fine specificity of TAP for its peptide substrates has not been analyzed in detail. Previously, we have studied the effect of amino acid variations at the N-terminal, the C-terminal, and the penultimate residue on the efficiency of peptide translocation. Using permeabilized cells, we have shown that TAP pre-selects peptides in an allele- and species-specific manner, for which only the C-terminal residue is crucial. This finding is confirmed in the present study by using microsomes containing different TAP. The influence of amino acid substitutions at positions 2 to 7 of 9-residue model peptides on TAP-dependent peptide translocation is systematically examined. Only a few amino acid substitutions at these positions affect the efficiency of peptide translocation significantly, e.g. Pro at position 2 or 3 negatively influences transport whereas Glu at positions 6 and 7 enhances transport. The differences in translocation by the rat TAP alleles a or u, mouse TAP and human TAP are, however, minor for the peptide with internal substitutions used in this study. These results show that the C-terminal residue essentially governs the species-specific substrate specificity of TAP.

Contribution of proteasome-mediated proteolysis to the hierarchy of epitopes presented by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL) recognize peptide epitopes of protein antigens in a hierarchical fashion. We investigated whether proteolytic cleavage, in particular by proteasomes, is important in determining epitope hierarchy. Using highly purified 20S proteasomes, we find preferred cleavage sites directly adjacent to the N- and C-terminal ends of the immunodominant epitope of chicken ovalbumin, Ova257-264, while most of the subdominant epitope, Ova55-62, is destroyed by a major cleavage site located within this epitope. Moreover, we show that variations in amino acid sequences flanking these epitopes influence proteasomal cleavage patterns in parallel with the efficacy of their presentation. The results suggest that proteasomal cleavage within and adjacent to class I-restricted epitopes contributes to their level of presentation.