Identification of vaccinia virus epitope-specific HLA-A*0201-restricted T cells and comparative analysis of smallpox vaccines

Dissociation between epitope hierarchy and immunoprevalence in CD8 responses to vaccinia virus western reserve

Understanding immunity to vaccinia virus (VACV) is important for the development of safer vaccines for smallpox- and poxvirus-vectored recombinant vaccines. VACV is also emerging as an outstanding model for studying CD8(+) T cell immunodominance because of the large number of CD8(+) T cell epitopes known for this virus in both mice and humans. In this study, we characterize the CD8(+) T cell response in vaccinated BALB/c mice by a genome-wide mapping approach. Responses to each of 54 newly identified H-2(d)-restricted T cell epitopes could be detected after i.p. and dermal vaccination routes. Analysis of these new epitopes in the context of those already known for VACV in mice and humans revealed two important findings. First, CD8(+) T cell epitopes are not randomly distributed across the VACV proteome, with some proteins being poorly or nonimmunogenic, while others are immunoprevalent, being frequently recognized across diverse MHC haplotypes. Second, some proteins constituted the major targets of the immune response by a specific haplotype as they recruited the majority of the specific CD8(+) T cells but these proteins did not correspond to the immunoprevalent Ags. Thus, we found a dissociation between immunoprevalence and immunodominance, implying that different sets of rules govern these two phenomena. Together, these findings have clear implications for the design of CD8(+) T cell subunit vaccines and in particular raise the exciting prospect of being able to choose subunits without reference to MHC restriction.

Genomic differences of Vaccinia virus clones from Dryvax smallpox vaccine: the Dryvax-like ACAM2000 and the mouse neurovirulent Clone-3

Conventional vaccines used for smallpox eradication were often denoted one or another strain of Vaccinia virus (VACV), even though seed virus was sub-cultured multifariously, which rendered the virion population genetically heterogeneous. ACAM2000 cell culture vaccine, recently licensed in the U.S., consists of a biologically vaccine-like VACV homogeneous-sequence clone from the conventional smallpox vaccine Dryvax, which we verified from Dryvax sequence chromatograms is genetically heterogeneous. ACAM2000 VACV and CL3, a mouse-neurovirulent clone from Dryvax, differ by 572 single nucleotide polymorphisms and 53 insertions-deletions of varied size, including a 4.5-kbp deletion in ACAM2000 and a 6.2-kbp deletion in CL3. The sequence diversity between the two clones precludes precisely defining why CL3 is more pathogenic; however, four genes appear significantly dissimilar to account for virulence differences. CL3 encodes intact immunomodulators interferon-alpha/beta and tumor necrosis factor receptors, which are truncated in ACAM2000. CL3 specifies a Cowpox and Variola virus-like ankyrin-repeat protein that might be associated with proteolysis via ubiquitination. And, CL3 shows an elongated thymidylate kinase, similar to the enzyme of the mouse-neurovirulent VACV-WR, a derivative of the New York City Board of Health vaccine, the origin vaccine of Dryvax. Although ACAM2000 encodes most proteins associated with immunization protection, the cloning probably delimited the variant epitopes and other motifs produced by Dryvax due to its VACV genetic heterogeneity. The sequence information for ACAM2000 and CL3 could be significant for resolving the dynamics of their different proteomes and thereby aid development of safer, more effective vaccines.

Mass spectrometry and peptide-based vaccine development

The development of new vaccines against pathogens is an important part of infectious disease control. In the last decade, a variety of proteins giving rise to naturally processed pathogen-derived antigenic peptides, representing B-cell and T-cell epitopes, have been characterized. Numerous candidate vaccines consisting of synthetic peptides are being designed and evaluated, with encouraging results. In this context, the application of mass spectrometry based on the isolation and identification of pathogen-derived peptides from the human leukocyte antigen (HLA) molecules is a major focus of peptide-based vaccine development. Dramatic improvements have been made in mass spectrometer performance for peptide sequencing in terms of increased sensitivity, the ability to rapidly obtain data-directed tandem mass spectra, and the accuracy of mass measurement. This review focuses on the efforts to identify T-cell epitopes for viral and microbial pathogens for directed vaccine development.

Antibody profiling by proteome microarray reveals the immunogenicity of the attenuated smallpox vaccine modified vaccinia virus ankara is comparable to that of Dryvax

Modified vaccinia virus Ankara (MVA) is a highly attenuated vaccinia virus that is under consideration as an alternative to the conventional smallpox vaccine Dryvax. MVA was attenuated by extensive passage of vaccinia virus Ankara in chicken embryo fibroblasts. Several immunomodulatory genes and genes that influence host range are deleted or mutated, and replication is aborted in the late stage of infection in most nonavian cells. The effect of these mutations on immunogenicity is not well understood. Since the structural genes appear to be intact in MVA, it is hypothesized that critical targets for antibody neutralization have been retained. To test this, we probed microarrays of the Western Reserve (WR) proteome with sera from humans and macaques after MVA and Dryvax vaccination. As most protein sequences of MVA are 97 to 99% identical to those of other vaccinia virus strains, extensive binding cross-reactivity is expected, except for those deleted or truncated. Despite different hosts and immunization regimens, the MVA and Dryvax antibody profiles were broadly similar, with antibodies against membrane and core proteins being the best conserved. The responses to nonstructural proteins were less well conserved, although these are not expected to influence virus neutralization. The broadest antibody response was obtained for hyperimmune rabbits with WR, which is pathogenic in rabbits. These data indicate that, despite the mutations and deletions in MVA, its overall immunogenicity is broadly comparable to that of Dryvax, particularly at the level of antibodies to membrane proteins. The work supports other information suggesting that MVA may be a useful alternative to Dryvax.

Human cytotoxic CD4+ T cells recognize HLA-DR1-restricted epitopes on vaccinia virus proteins A24R and D1R conserved among poxviruses

We previously demonstrated that vaccinia virus (VV)-specific CD4(+) cytolytic T cells can persist for >50 years after immunization against smallpox in the absence of re-exposure to VV. Nevertheless, there have been few studies focusing on CD4(+) T cell responses to smallpox vaccination. To ensure successful vaccination, a candidate vaccine should contain immunodominant CD4(+) T cell epitopes as well as CD8(+) T and B cell epitopes. In the present study, we established cytotoxic CD4(+) T cell lines from VV-immune donors, which recognize epitopes in VV proteins D1R and A24R in association with HLA-DR1 Ags. Comparisons of sequences between different members of the poxvirus family show that both epitopes are completely conserved among VV, variola viruses, and most mammalian poxviruses, including monkeypox, cowpox, and ectromelia. The CD4(+) T cell lines lysed VV-infected, Ag- and peptide-pulsed targets, and the lysis was inhibited by concanamycin A. We also detected these peptide-specific cytolytic and IFN-gamma-producing CD4(+) T cells in short-term bulk cultures of PBMC from each of the three VV-immune donors tested. These are the first VV-specific CD4(+) T cell epitopes identified in humans restricted by one of the most common MHC class II molecules, HLA-DR1, and this information may be useful in analyzing CD4(+) T cell responses to pre-existing or new generation VV vaccines against smallpox.

Initiation of primary anti-vaccinia virus immunity in vivo

The primary focus of our work is the initiation of an antiviral immune response. While we employ many experimental systems to address this fundamental issue, much of our work revolves around the use of vaccinia virus. Concerns over the negative effects of vaccination have prevented the return of the smallpox immunization program to the general population and underscored the importance of understanding the primary immune response to vaccinia virus. This response is comprised of a complex symphony of immune system components employing a variety of different mechanisms. In this review, we will both highlight the roles of many of these components and touch on the applications of vaccinia virus in the laboratory and the clinic.

Cross-competition of CD8+ T cells shapes the immunodominance hierarchy during boost vaccination

CD8⁺ T cell responses directed against multiple pathogen-derived epitopes are characterized by defined immunodominance hierarchy patterns. A possible explanation for this phenomenon is that CD8⁺ T cells of different specificities compete for access to epitopes on antigen-presenting cells, and that the outcome of this so-called cross-competition reflects the number of induced T cells. In our study using a vaccinia virus infection model, we found that T cell cross-competition is highly relevant during boost vaccination, thereby shaping the immunodominance hierarchy in the recall. We demonstrate that competition was of no importance during priming and was unaffected by the applied route of immunization. It strongly depended on the timing of viral antigen expression in infected APCs, and it was characterized by poor proliferation of T cells recognizing epitopes derived from late viral proteins. To our knowledge, this is the first demonstration of the functional importance of T cell cross-competition during a viral infection. Our findings provide a basis for novel strategies for how boost vaccination to defined antigens can be selectively improved. They give important new insights into the design of more efficient poxviral vectors for immunotherapy.

Cross-priming of cytotoxic T cells dictates antigen requisites for modified vaccinia virus Ankara vector vaccines

Recombinant vaccines based on modified vaccinia virus Ankara (MVA) have an excellent record concerning safety and immunogenicity and are currently being evaluated in numerous clinical studies for immunotherapy of infectious diseases and cancer. However, knowledge about the biological properties of target antigens to efficiently induce MVA vaccine-mediated immunity in vivo is sparse. Here, we examined distinct antigen presentation pathways and different antigen formulations contained in MVA vaccines for their capability to induce cytotoxic CD8(+) T-cell (CTL) responses. Strikingly, we found that CTL responses against MVA-produced antigens were dominated by cross-priming in vivo, despite the ability of the virus to efficiently infect professional antigen-presenting cells such as dendritic cells. Moreover, stable mature protein was preferred to preprocessed antigen as the substrate for cross-priming. Our data are essential for improved MVA vaccine design, as they demonstrate the need for optimal adjustment of the target antigen properties to the intrinsic requirements of the delivering vector system.

A quantitative analysis of the variables affecting the repertoire of T cell specificities recognized after vaccinia virus infection

Many components contribute to immunodominance in the response to a complex virus, but their relative importance is unclear. This was addressed using vaccinia virus and HLA-A*0201 as the model system. A comprehensive analysis of 18 viral proteins recognized by CD8(+) T cell responses demonstrated that approximately one-fortieth of all possible 9- to 10-mer peptides were high-affinity HLA-A*0201 binders. Peptide immunization and T cell recognition data generated from 90 peptides indicated that about one-half of the binders were capable of eliciting T cell responses, and that one-seventh of immunogenic peptides are generated by natural processing. Based on these results, we estimate that vaccinia virus encodes approximately 150 dominant and subdominant epitopes restricted in by HLA-A*0201. However, of all these potential epitopes, only 15 are immunodominant and actually recognized in vivo during vaccinia virus infection of HLA-A*0201 transgenic mice. Neither peptide-binding affinity, nor complex stability, nor TCR avidity, nor amount of processed epitope appeared to strictly correlate with immunodominance status. Additional experiments suggested that vaccinia infection impairs the development of responses directed against subdominant epitopes. This suggested that additional factors, including immunoregulatory mechanisms, restrict the repertoire of T cell specificities after vaccinia infection by a factor of at least 10.

Real time PCR for the assessment of CD8+ T cellular immune response after prophylactic vaccinia vaccination

BACKGROUND

The magnitude of specific CD8+ T cell reactivity responsible for vaccine-induced protection against smallpox infection has not yet been fully elucidated. Among other techniques, RT-PCR for the monitoring of cytokine release in effector T cells against tumor and viral antigens has demonstrated a novel promising method.

OBJECTIVE

To determine the functional status of antigen specific CD8+ T cells in healthy participants before and 4 weeks after prophylactic vaccination (Lister strain) against smallpox using quantitative real-time PCR (qRT-PCR).

STUDY DESIGN

Changes of interferon-gamma (IFNgamma) mRNA expression levels on short term ex vivo peptide antigen stimulation were measured. The corresponding specific CD8+ T cell reactivity was then displayed as CD8-normalized IFN-gamma levels (IFN-gamma/CD8 ratio).

RESULTS

We found a 5-9 fold increase of CD8+ T cell reactivity in three out of four vaccinated individuals. The kinetics and strength determined in responders reveal a virus specific T cell effector repertoire pre-vaccination and a corresponding functional state after immunization comparable also to data obtained from tetramer- and ELISPOT analysis.

CONCLUSIONS

Apart from protective vaccinia-specific neutralizing antibodies, the presence of antigen-specific CD8+ T-cells has been demonstrated after vaccinia vaccination. In concordance with others, results from this PCR-based study indicate that this smallpox vaccine induces strong vaccinia virus-specific CD8+ and IFN-gamma producing T cell responses.

Vaccines for preventing smallpox

BACKGROUND

Smallpox was eradicated by 1980, but its possible use as a bioweapon has rekindled interest in the development of protective vaccines. Therefore, stockpiled calf lymph-derived vaccines and recently developed cell-cultured vaccines have been investigated to contribute information to smallpox emergency response plans, while newer (non-replication competent) vaccines are developed.

OBJECTIVES

To assess the effects of smallpox vaccines in preventing the disease, in inducing immunity, and in regard to adverse events.

SEARCH STRATEGY

In December 2006, we searched the Cochrane Infectious Diseases Group Specialized Register, CENTRAL (The Cochrane Library 2006, Issue 4), MEDLINE, EMBASE, LILACS, and Current Controlled Trials, and handsearched Index Medicus. We also searched three databases of vaccine safety in December 2005.

SELECTION CRITERIA

Randomized controlled trials of smallpox vaccines versus placebo, other smallpox or non-smallpox vaccine, no intervention, or different dose of the same vaccine in people receiving smallpox vaccination irrespective of age.

DATA COLLECTION AND ANALYSIS

Both authors independently assessed trial quality and extracted data. We combined dichotomous data using relative risk with a random-effects model.

MAIN RESULTS

Ten trials involving 2412 participants were included. The vaccines investigated were calf-lymph derived first-generation vaccines (Dryvax, APVS, Lancy-vaxina, Lister), and cell-cultured second-generation vaccines (ACAM, CCSV). Vaccines were investigated in different dilutions. All undiluted vaccines induced a reaction in 95% of people vaccinated in terms of pustule and immunogenicity. Also 1:10 dilutions were fully efficient when the starting concentration was defined. Serious adverse events were reported in 1% to 2% of the volunteers. Fever was observed in 11% to 22% of participants, and headache in roughly half of the participants. Fever was less frequent when new vaccines were administered, but rates of headache were similar in new and old vaccines.

AUTHORS' CONCLUSIONS

The evidence shows that stockpiled vaccines have maintained their immunogenicity and new cell-cultured vaccines are similar to stockpiled vaccines in terms of vaccination success rate and immunogenicity. First- and second-generation vaccines diluted to at least 1:10 are as effective as undiluted vaccine in terms of clinical success rate and immunogenicity. Dilution did not reduce the frequency of adverse events. Success rate and immunogenicity were similar in naive and previously vaccinated persons, but there were fewer adverse events in previously vaccinated persons. The rate of adverse events found in this review reveals the need for further development and improvement of smallpox vaccines.

Heterologous prime-boost immunotherapy of melanoma patients with Influenza virosomes, and recombinant Vaccinia virus encoding 5 melanoma epitopes and 3 co-stimulatory molecules. A multi-centre phase I/II open labeled clinical trial

To the exception of early stages of disease, the morbidity and mortality of melanoma is considerable, with no acknowledged therapeutic options beyond surgery. Immunotherapy of melanoma has achieved some success, but further refinements are urgently needed in order to realize its potential. This paper describes a multi-centre phase I/II open labeled, controlled clinical trial investigating 2 innovative immunotherapeutic reagents. Two successive groups of 20 resected AJCC stages IIb-IV melanoma patients will be treated, first with melanoma epitopes included into Influenza virosomes (group 1), and second with a heterologous prime-boost protocol priming with a recombinant Vaccinia virus, and boosting with Influenza virosomes (group 2). Five melanoma epitopes from three different melanoma differentiation antigens were included into Influenza virosomes, that cross-stimulate CD4+ T cells and are endowed with high adjuvant capacity in the generation of CTL. The same five melanoma epitopes, two co-stimulatory molecules CD80 and CD86, and the CD40 ligand, a marker known to play a crucial role in CTL generation and memory maintenance were encoded in a recombinant Vaccinia virus. GM-CSF will be administered as a supporting cytokine. Both Influenza virosomes and octo-recombinant Vaccinia virus are innovative and original constructs assessed for the first time in human. Immunotherapy foresees 12 weekly immunizations for each group. Toxicity and adverse events will be monitored clinically. Immunological efficacy will be assessed dynamically by ex-vivo multimer analysis, Elispot, and quantitative real-time PCR for up to 3 months following completion of immunotherapy schedule. Disease free survival will be assessed by 4-monthly serial clinic visits, including physical and FDG-PET examinations, for a follow-up time of 2 years. Quality of life will be assessed with a dedicated FACT-BRM 4 questionnaire.

Immunization of mice with the nef gene from Human Immunodeficiency Virus type 1: study of immunological memory and long-term toxicology

Background

The human immunodeficiency virus type 1 (HIV-1) regulatory protein, Nef, is an attractive vaccine target because it is involved in viral pathogenesis, is expressed early in the viral life cycle and harbors many T and B cell epitopes. Several clinical trials include gene-based vaccines encoding this protein. However, Nef has been shown to transform certain cell types in vitro. Based on these findings we performed a long-term toxicity and immunogenicity study of Nef, encoded either by Modified Vaccinia virus Ankara or by plasmid DNA. BALB/c mice were primed twice with either DNA or MVA encoding Nef and received a homologous or heterologous boost ten months later. In the meantime, the Nef-specific immune responses were monitored and at the time of sacrifice an extensive toxicological evaluation was performed, where presence of tumors and other pathological changes were assessed.

Results

The toxicological evaluation showed that immunization with MVAnef is safe and does not cause cellular transformation or other toxicity in somatic organs.

Both DNAnef and MVAnef immunized animals developed potent Nef-specific cellular responses that declined to undetectable levels over time, and could readily be boosted after almost one year. This is of particular interest since it shows that plasmid DNA vaccine can also be used as a potent late booster of primed immune responses. We observed qualitative differences between the T cell responses induced by the two different vectors: DNA-encoded nef induced long-lasting CD8⁺ T cell memory responses, whereas MVA-encoded nef induced CD4⁺ T cell memory responses. In terms of the humoral immune responses, we show that two injections of MVAnef induce significant anti-Nef titers, while repeated injections of DNAnef do not. A single boost with MVAnef could enhance the antibody response following DNAnef prime to the same level as that observed in animals immunized repeatedly with MVAnef. We also demonstrate the possibility to boost HIV-1 Nef-specific immune responses using the MVAnef construct despite the presence of potent anti-vector immunity.

Conclusion

This study shows that the nef gene vectored by MVA does not induce malignancies or other adverse effects in mice. Further, we show that when the nef gene is delivered by plasmid or by a viral vector, it elicits potent and long-lasting immune responses and that these responses can be directed towards a CD4⁺ or a CD8⁺ T cell response depending on the choice of vector.

Dominance and diversity in the primary human CD4 T cell response to replication-competent vaccinia virus

Vaccination with replication-competent vaccinia protects against heterologous orthopoxvirus challenge. CD4 T cells have essential roles helping functionally important Ab and CD8 antiviral responses, and contribute to the durability of vaccinia-specific memory. Little is known about the specificity, diversity, or dominance hierarchy of orthopoxvirus-specific CD4 T cell responses. We interrogated vaccinia-reactive CD4 in vitro T cell lines with vaccinia protein fragments expressed from an unbiased genomic library, and also with a panel of membrane proteins. CD4 T cells from three primary vaccinees reacted with 44 separate antigenic regions in 35 vaccinia proteins, recognizing 8 to 20 proteins per person. The integrated responses to the Ags that we defined accounted for 49 to 81% of the CD4 reactivity to whole vaccinia Ag. Individual dominant Ags drove up to 30% of the total response. The gene F11L-encoded protein was immunodominant in two of three subjects and is fragmented in a replication-incompetent vaccine candidate. The presence of protein in virions was strongly associated with CD4 antigenicity. These findings are consistent with models in which exogenous Ag drives CD4 immunodominance, and provides tools to investigate the relationship between Ab and CD4 T cell specificity for complex pathogens.

Long-term T cell memory to human leucocyte antigen-A2 supertype epitopes in humans vaccinated against smallpox

Identification of human leucocyte antigen (HLA) class I-restricted T cell epitopes is important to develop methods to track the evolution of T cell memory to new generation smallpox vaccines and allow comparison to older vaccinia virus preparations known to induce protection against smallpox. We evaluated the relative predictive values of four computational algorithms to identify candidate 9-mer HLA-A2 supertype epitopes that were confirmed to stimulate preferentially T cell interferon (IFN)-gamma responses by subjects last vaccinated with Dryvax 27-54 years previously. Six peptides encoded by I4L, G1L, A8R, I8R, D12L and H3L open reading frames that were identical for Vaccinia (Copenhagen), Variola major (Bangledesh 1975) and modified vaccinia Ankara strain preferentially stimulated IFN-gamma responses by healthy HLA-A2 supertype adults last given Dryvax 27-49 years earlier relative to remotely vaccinated non-HLA-A2 supertype and unvaccinated HLA-A2 supertype adults. Combining results from at least two computational algorithms that use different strategies to predict peptide binding to HLA-A2 supertype molecules was optimal for selection of candidate peptides that were confirmed to be epitopes by recall of T cell IFN-gamma responses. These data will facilitate evaluation of the immunogenicity of replication incompetent smallpox vaccines such as modified vaccinia Ankara and contribute to knowledge of poxvirus epitopes that are associated with long-lived T cell memory.

IgE anti-Borrelia burgdorferi components (p18, p31, p34, p41, p45, p60) and increased blood CD8+CD60+ T cells in children with Lyme disease

Immunoglobulin (Ig) E may provide immunity against Borrelia burgdorferi infection (Lyme disease) in children which lasts throughout adulthood. We investigated the presence and persistence of IgE anti-B. burgdorferi antibodies (Abs) in paediatric patients infected with Lyme disease over time. Serum immunoglobulin levels, presence of IgG and IgE anti-B. burgdorferi components, and distributions of blood T, B and natural killer lymphocyte subsets were studied in B. burgdorferi-infected and -uninfected children (nephelometry, UniCAP Total IgE Fluoroenzymeimmunoassay, Western blot, flow cytometry). Total serum IgM, IgG, IgE and IgA levels, and distributions of blood lymphocytes (CD4(+), CD8(+), CD19(+)) of both groups, excluding CD8(+)CD60(+) T cells, were within normal ranges. However, infected, but not uninfected children made IgG anti-B. burgdorferi proteins p18, p31, p34, p41, p45, but not IgG anti-p60, and IgE anti-B. burgdorferi proteins p31, p34, p41, p45, p60, but not IgE anti-p18. These proteins were also detected in an infected child 1 year post-infection. Interestingly, CD8(+)CD60(+) T-cell numbers were significantly increased (fourfold) in infected, compared with uninfected, patients (P=0.001). These results demonstrate that specific IgE anti-B. burgdorferi Abs are generated and persist in children with Lyme disease and that CD8(+)CD60(+) T cells may play an important role in these responses.

T-Cell epitope discovery for variola and vaccinia viruses

Variola major, the causative agent of smallpox, afflicted mankind throughout history until the worldwide World Health Organisation WHO vaccination campaign successfully eradicated the disease. Unfortunately, recent concerns about bioterrorism have renewed scientific interest in this virus. One essential component of our biodefense and preparedness efforts is an understanding of poxvirus immunity. To this end a number of laboratories have sought to discover T- and B-Cell epitopes from select agents such as variola virus. This review focuses on the efforts to identify CD8(+) T-Cell epitopes from poxviruses as a means to develop new vaccines and therapeutics. A wide variety of techniques have been employed by several research groups to provide complementary information regarding cellular immune responses to poxviruses. In the last several years well over 100 T-Cell epitopes have been identified and the work rapidly continues. The information gleaned from these studies will not only give us a greater understanding of immunity to variola virus and other viruses, but also provide a foundation for next generation vaccines and additional tools with which to study host-pathogen interactions.

Rapid identification of an HLA-B*1501-restricted vaccinia peptide antigen

We identified a modified vaccinia virus Ankara (MVA)-peptide recognized by T cells of a vaccinated patient, applying a modified expression-based procedure. CD8(+) T cells reactive with MVA were expanded from peripheral blood of a patient after MVA-vaccination. The restricting HLA-elements and the antigen were determined applying stably HLA-class I-transfected K562 cells that were either infected wit MVA or transfected with a panel of MVA open reading frames (ORF). The T cells recognized the ORF 28R (K7R) gene product in association with HLA-B*1501 and the peptide-coding region was localized applying truncated in vitro transcribed mRNA. The 9-mer peptide encoded (ORF 28(25-33), SIIDLIDEY) was identified that was also recognized by ex vivo CD8(+) T cells from the patient and further healthy individuals.

Adaptive immunity to vaccinia virus: revisiting an old friend

The study of immunity to vaccinia virus has had a renaissance in recent years, largely owing to a desire to produce a safe and efficacious vaccine against smallpox in the context of biodefense concerns. This review focuses on three areas that have seen recent advances: the mapping of epitopes, data from the clinic (including efforts to determine the longevity of responses in humans and recent smallpox vaccine trials) and the use of animal models to determine the requisites for successful immunity and the effect of vaccinia virus immunomodulators on adaptive immune responses. This is intended to be a snapshot of areas generating current interest rather than a comprehensive review, and each area is discussed in terms of current achievements and challenges for the future.

Enhancement of CD8+ T cell immunity in the lung by CpG oligodeoxynucleotides increases protective efficacy of a modified vaccinia Ankara vaccine against lethal poxvirus infection even in a CD4-deficient host

Immunostimulatory CpG oligodeoxynucleotides (ODN) have proven effective as adjuvants for protein-based vaccines, but their impact on immune responses induced by live viral vectors is not known. We found that addition of CpG ODN to modified vaccinia Ankara (MVA) markedly improved the induction of longer-lasting adaptive protective immunity in BALB/c mice against intranasal pathogenic vaccinia virus (Western Reserve; WR). Protection was mediated primarily by CD8(+) T cells in the lung, as determined by CD8-depletion studies, protection in B cell-deficient mice, and greater protection correlating with CD8(+) IFN-gamma-producing cells in the lung but not with those in the spleen. Intranasal immunization was more effective at inducing CD8(+) T cell immunity in the lung, and protection, than i.m. immunization. Addition of CpG ODN increased the CD8(+) response but not the Ab response. Depletion of CD4 T cells before vaccination with MVA significantly diminished protection against pathogenic WR virus. However, CpG ODN delivered with MVA was able to substitute for CD4 help and protected CD4-depleted mice against WR vaccinia challenge. This study demonstrates for the first time a protective adjuvant effect of CpG ODN for a live viral vector vaccine that may overcome CD4 deficiency in the induction of protective CD8(+) T cell-mediated immunity.

Characterization of highly frequent epitope-specific CD45RA+/CCR7+/- T lymphocyte responses against p53-binding domains of the human polyomavirus BK large tumor antigen in HLA-A*0201+ BKV-seropositive donors

Human polyomavirus BK (BKV) has been implicated in oncogenic transformation. Its ability to replicate is determined by the binding of its large tumor antigen (LTag) to products of tumor-suppressor genes regulating cell cycle, as specifically p53. We investigated CD8+ T immune responses to BKV LTag portions involved in p53 binding in HLA-A*0201+ BKV LTag experienced individuals. Peptides selected from either p53-binding region (LTag351-450 and LTag533-626) by current algorithms and capacity to bind HLA-A*0201 molecule were used to stimulate CD8+ T responses, as assessed by IFN-gamma gene expression ex vivo and detected by cytotoxicity assays following in vitro culture. We observed epitope-specific immune responses in all HLA-A*0201+ BKV LTag experienced individuals tested. At least one epitope, LTag579-587; LLLIWFRPV, was naturally processed in non professional antigen presenting cells and induced cytotoxic responses with CTL precursor frequencies in the order of 1/20'000. Antigen specific CD8+ T cells were only detectable in the CD45RA+ subset, in both CCR7+ and CCR7- subpopulations. These data indicate that widespread cellular immune responses against epitopes within BKV LTag-p53 binding regions exist and question their roles in immunosurveillance against tumors possibly associated with BKV infection.

Immunity and immunological memory following smallpox vaccination

The smallpox vaccine consists of live vaccinia virus and is generally considered the gold standard of vaccines, since it is the only one that has led to the complete eradication of an infectious disease from the human population. Renewed fears that smallpox might be deliberately released in an act of bioterrorism have led to resurgence in the study of immunity and immunological memory to vaccinia virus and other poxviruses. Here we review our current understanding of memory T-cell, memory B-cell, and antibody responses to vaccinia and related poxviruses, both in animal models and human subjects. Of particular interest are recent advances in understanding protective immunity to poxviruses, quantifying immunological memory to the smallpox vaccine in humans, and identifying major vaccinia-specific T-cell and B-cell epitopes. In addition, potential mechanisms for maintenance of immunological memory are discussed.

Human T-cell responses to vaccinia virus envelope proteins

One approach for a safer smallpox vaccine is to utilize recombinant subunits rather than live vaccinia virus (VACV). The products of the VACV envelope genes A27L, L1R, B5R, and A33R induce protective antibodies in animal models. We propose that proteins that elicit T-cell responses, as well as neutralizing antibodies, will be important to include in a molecular vaccine. To evaluate VACV-specific memory T-cell responses, peripheral blood mononuclear cells (PBMC) from four VACV vaccinees were tested against whole VACV and the individual envelope proteins A27, B5, L1, and A33, using gamma interferon enzyme-linked immunospot and cytokine flow cytometry assays. PBMC were stimulated with autologous dendritic cells infected with VACV or electroporated with individual VACV protein mRNAs. T-cell lines from all donors, vaccinated from 1 month to over 20 years ago, recognized all four VACV envelope proteins. Both CD4(+) and CD8(+) T-cell responses to each protein were detected. Further analysis focused on representative proteins B5 and A27. PBMC from a recent vaccinee exhibited high frequencies of CD4(+) and CD8(+) T-cell precursors to both B5 (19.8 and 20%, respectively) and A27 (6.8 and 3.7%). In comparison, B5- and A27-specific T-cell frequencies ranged from 0.4 to 1.3% in a donor vaccinated 3 years ago. Multiple CD4(+) and CD8(+) T-cell epitopes were identified from both A27 and B5, using overlapping 15-mer peptides. These data suggest that all four VACV envelope proteins may contribute to protective immunity, not only by inducing antibody responses, but also by eliciting T-cell responses.

Protection against vaccinia virus challenge by CD8 memory T cells resolved by molecular mimicry

Live vaccinia virus (VV) vaccination has been highly successful in eradicating smallpox. However, the mechanisms of immunity involved in mediating this protective effect are still poorly understood, and the roles of CD8 T-cell responses in primary and secondary VV infections are not clearly identified. By applying the concept of molecular mimicry to identify potential CD8 T-cell epitopes that stimulate cross-reactive T cells specific to lymphocytic choriomeningitis virus (LCMV) and VV, we identified after screening only 115 peptides two VV-specific immunogenic epitopes that mediated protective immunity against VV. An immunodominant epitope, VV-e7r130, did not generate cross-reactive T-cell responses to LCMV, and a subdominant epitope, VV-a11r198, did generate cross-reactive responses to LCMV. Infection with VV induced strong epitope-specific responses which were stable into long-term memory and peaked at the time virus was cleared, consistent with CD8 T cells assisting in the control of VV. Two different approaches, direct adoptive transfer of VV-e7r-specific CD8 T cells and prior immunization with a VV-e7r-expressing ubiquitinated minigene, demonstrated that memory CD8 T cells alone could play a significant role in protective immunity against VV. These studies suggest that exploiting cross-reactive responses between viruses may be a useful tool to complement existing technology in predicting immunogenic epitopes to large viruses, such as VV, leading to a better understanding of the role CD8 T cells play during these viral infections.

Short-term, but not post-exposure, protection against lethal orthopoxvirus challenge after immunization with modified vaccinia virus Ankara

Safety-tested vaccinia virus (VACV) MVA serves as a candidate third-generation vaccine against smallpox. Here, MVA immunization of mice shortly before or after lethal respiratory challenge with VACV Western Reserve was investigated. Whilst post-exposure treatment failed to protect animals, immunizations on day 2 prior to challenge were fully protective. On the day of challenge, MVA inoculation may prevent death, but not onset of severe respiratory disease. After intranasal MVA application, massive influx of leukocytes (such as neutrophils, macrophages, natural killer cells and T cells) was found in the lungs of the animals, indicating the contribution of innate responses to protection. Correspondingly, in RAG-1-/- mice, MVA inoculation delayed onset of disease significantly, but did not prevent fatal infection. Thus, short-term protection required a tight interplay of both innate and adaptive antiviral immunity. These data suggest that, in addition to conventional vaccination, MVA may serve for potent emergency prophylaxis against orthopoxvirus infection.

The potential role of fowlpox virus in rational vaccine design

The design of optimal vaccines requires detailed knowledge of how protective immune responses are generated in vivo under normal circumstances. This approach to vaccine development, where the immune correlates of protection are defined and vaccines are designed to elicit the same response, is called rational vaccine design. Poxviruses are attractive candidates for inclusion in such design strategies owing to their large genome, which allows for the inclusion of multiple heterologous genes, including those encoding antigens, co-stimulatory molecules and cytokines. Fowlpox virus, the prototypical member of the Avipoxvirus genus, is particularly suitable, as it is also incapable of replicating in mammalian cells. The potential of recombinant fowlpox virus as a safe vaccine vector is being evaluated currently in a number of clinical trials for diseases, including HIV, malaria and various types of cancer. Despite their promise, intricate details regarding how fowlpox virus interacts with the host immune system have not been resolved. In this review, the issues surrounding the use of fowlpox virus as a vaccine vector and possible strategies for enhancing its efficacy are discussed.

An update on smallpox vaccine candidates and their role in bioterrorism related vaccination strategies

The threat of using variola virus in a bioterrorist attack urged different countries to renew the production of traditional vaccines and develop new generations of smallpox vaccines. Manufacturers try to combine smallpox vaccine past experience with technological advances in vaccine development to achieve protection similar to that of the traditional vaccines with a higher level of safety and fewer contraindications. In light of the reported immunogenicity and reactogenicity of the stockpiled smallpox vaccines employed in the last immunization campaigns of "first responders", we review recently accumulated data on the assessment of new smallpox vaccine candidates and discuss their role in possible vaccination policies.

Comparative efficacy of modified vaccinia Ankara (MVA) as a potential replacement smallpox vaccine

International concern over the potential consequences of a Bioterrorist or Biowarfare associated release of variola virus have prompted renewed interest in the vaccines for smallpox. The traditional live, replicating vaccine strains are subject to novel safety concerns associated with historical production methods in domesticated ruminants and the additional hazards that vaccinia virus poses for people with immune system abnormalities or a history of eczematous skin conditions. In this study we have examined the longevity and efficacy of immunity induced by a non-replicating smallpox vaccine candidate, modified vaccinia Ankara (MVA) in a murine model using intranasal and aerosol routes of infection. Two-step vaccinations of MVA followed by traditional Lister vaccine are compared with either Lister alone or MVA alone, and the longevity of the protection induced by MVA is assessed. MVA is found to be broadly similar to Lister. Although protection is shown to decay with time, when administered at a standard human dose the longevity of protection induced by MVA is comparable to that induced by Lister.

Identification of vaccinia CD8+ T-cell epitopes conserved among vaccinia and variola viruses restricted by common MHC class I molecules, HLA-A2 or HLA-B7

Immunization with vaccinia virus results in long-lasting protection against smallpox and is an approach that has been successfully used to eliminate natural smallpox infections worldwide. Today, vaccinia virus is very important not only as a vaccine virus to protect human against smallpox, but also as an expression vector for immunization against other infectious diseases, such as HIV and cancer. In this article, we identify three new vaccinia human CD8+ T-cell epitopes conserved among vaccinia and variola viruses restricted by HLA-A2, HLA-B7, or HLA-B*3502, which belongs to the HLA-B7 supertype. Identification of these CD8+ T-cell epitopes restricted by common HLA alleles will help to quantitate human CD8+ T-cell responses to licensed and experimental smallpox vaccines and to vaccinia virus vectors. CD8+ T-cell responses specific to these epitopes can also be used to quantitate cellular immune responses, especially with new smallpox vaccines that do not induce a "take," such as the modified vaccinia virus Ankara strain. Combined with previous reports by us and others, these results show that there are some vaccinia viral proteins containing multiple epitopes restricted by different MHC molecules of humans and mice.

The smallpox vaccine

BACKGROUND

A heightened awareness of the potential for bioterrorist attacks in the United States has led to the expansion of the nation's supply of smallpox vaccine and the institution of procedures to distribute this vaccine in the unlikely event of a release of this potentially deadly agent.

METHODS

The authors conducted a review of the relevant smallpox literature through a MEDLINE search. They also reviewed the Web site of the Centers for Disease Control and Prevention and numerous other Web sites.

RESULTS

The authors considered for inclusion more than 100 articles discussing smallpox, the smallpox vaccine and the role of the dental professional in a bioterrorist attack.

CONCLUSIONS

Dentists may detect the initial signs of a smallpox infection, provide information concerning the disease to the public and potentially assist in the administration of smallpox vaccine.

CLINICAL IMPLICATIONS

Should an intentional release of smallpox occur, the dental professional may play an important role in its treatment and prevention.

Deletion of gene A41L enhances vaccinia virus immunogenicity and vaccine efficacy

Vaccinia virus (VACV) is the vaccine that was used to eradicate smallpox and is being developed as a recombinant vaccine for other pathogens. Removal of genes encoding immunomodulatory proteins expressed by VACV may enhance virus immunogenicity and improve its potential as a vaccine. Protein A41 is a candidate for removal, having sequence similarity to the VACV chemokine-binding protein, vCKBP, and an association with reduced inflammation during dermal infection. Here, it is shown that, at low doses, VACV strain Western Reserve (WR) lacking A41L (vDeltaA41L) was slightly more virulent than wild-type and revertant controls after intranasal infection of BALB/c mice. The primary immune response to vDeltaA41L was marked by an increase in the percentage of VACV-specific gamma interferon-producing CD8(+) T cells and enhancement of cytotoxic T-cell responses in the spleen. However, this augmentation of cellular response was not seen in lung infiltrates. Splenic CD8(+) T-cell responses were also enhanced when VACV strain modified vaccinia virus Ankara (MVA) lacking A41L was used to immunize mice. Lastly, immunization with VACV MVA lacking A41L provided better protection than control viruses to subsequent challenge with a 300 LD(50) dose of VACV WR. This study provides insight into the immunomodulatory role of A41 and suggests that MVA lacking A41 may represent a more efficacious vaccine.

Class I T-cell epitope prediction: improvements using a combination of proteasome cleavage, TAP affinity, and MHC binding

Cleavage by the proteasome is responsible for generating the C terminus of T-cell epitopes. Modeling the process of proteasome cleavage as part of a multi-step algorithm for T-cell epitope prediction will reduce the number of non-binders and increase the overall accuracy of the predictive algorithm. Quantitative matrix-based models for prediction of the proteasome cleavage sites in a protein were developed using a training set of 489 naturally processed T-cell epitopes (nonamer peptides) associated with HLA-A and HLA-B molecules. The models were validated using an external test set of 227 T-cell epitopes. The performance of the models was good, identifying 76% of the C-termini correctly. The best model of proteasome cleavage was incorporated as the first step in a three-step algorithm for T-cell epitope prediction, where subsequent steps predicted TAP affinity and MHC binding using previously derived models.

Diversity in the acute CD8 T cell response to vaccinia virus in humans

Orthopoxviruses have complex proteomes. Infection provokes a brisk CD8 response, which is required in some systems for recovery from primary infection. Little is known concerning the Ags and epitopes recognized by CD8 T cells. We examined the fine specificity of cloned and bulk human vaccinia-specific CD8 CTL by expressing polypeptide fragments from a library of vaccinia genomic DNA. This epitope discovery method emphasizes virus-specific biological activity, as the responder cells are all reactive with whole vaccinia virus. Sixteen novel epitopes, restricted by several HLA A and B alleles, were defined to the nomamer peptide level in diverse vaccinia open reading frames. An additional seven epitope were mapped to short regions of vaccinia proteins. Targets of the CD8 response included proteins assigned to structural, enzymatic, transcription factor, and immune evasion functions, and included members of all viral kinetic classes. Most epitopes were conserved in other orthopoxviruses. Responses to at least 18 epitopes were detected within a single blood sample, revealing a surprising degree of diversity. These epitopes will be useful in natural history studies of CD8 responses to vaccinia, a nonpersisting virus with long-term memory, and in the design and evaluation of attenuated and replication-incompetent vaccinia strains being tested for variola and monkeypox prevention and for the delivery of heterologous Ags.

HLA-A*0201, HLA-A*1101, and HLA-B*0702 transgenic mice recognize numerous poxvirus determinants from a wide variety of viral gene products

In virus models explored in detail in mice, CTL typically focus on a few immunodominant determinants. In this study we use a multipronged approach to understand the diversity of CTL responses to vaccinia virus, a prototypic poxvirus with a genome approximately 20-fold larger than that of the model RNA viruses typically studied in mice. Based on predictive computational algorithms for peptide binding to HLA supertypes, we synthesized a panel of 2889 peptides to begin to create an immunomic map of human CTL responses to poxviruses. Using this panel in conjunction with CTLs from vaccinia virus-infected HLA transgenic mice, we identified 14 HLA-A*0201-, 4 HLA-A*1101-, and 3 HLA-B*0702-restricted CD8(+) T cell determinants distributed over 20 distinct proteins. These peptides were capable of binding one or multiple A2, A3, and B7 supertype molecules with affinities typical of viral determinants. Surprisingly, many of the viral proteins recognized are predicted to be late gene products, in addition to the early intermediate gene products expected. Nearly all of the determinants identified have identical counterparts encoded by modified vaccinia virus Ankara as well as variola virus, the agent of smallpox. These findings have implications for the design of new smallpox vaccines and the understanding of immune responses to large DNA viruses in general.

Immunodominance of Poxviral-Specific CTL in a Human Trial of Recombinant-Modified Vaccinia Ankara

Many recombinant poxviral vaccines are currently in clinical trials for cancer and infectious diseases. However, these agents have failed to generate T cell responses specific for recombinant gene products at levels comparable with T cell responses associated with natural viral infections. The recent identification of vaccinia-encoded CTL epitopes, including a new epitope described in this study, allows the simultaneous comparison of CTL responses specific for poxviral and recombinant epitopes. We performed detailed kinetic analyses of CTL responses in HLA-A*0201 patients receiving repeated injections of recombinant modified vaccinia Ankara encoding a string of melanoma tumor Ag epitopes. The vaccine-driven CTL hierarchy was dominated by modified vaccinia Ankara epitope-specific responses, even in patients who had not received previous smallpox vaccination. The only recombinant epitope that was able to impact on the CTL hierarchy was the melan-A26-35 analog epitope, whereas responses specific for the weaker affinity epitope NY-ESO-1(157-165) failed to be expanded above the level detected in prevaccination samples. Our results demonstrate that immunodominant vaccinia-specific CTL responses limit the effectiveness of poxviruses in recombinant vaccination strategies and that more powerful priming strategies are required to overcome immunodominance of poxvirus-specific T cell responses.

A Toll-like receptor-independent antiviral response induced by double-stranded B-form DNA

The innate immune system recognizes nucleic acids during infection or tissue damage; however, the mechanisms of intracellular recognition of DNA have not been fully elucidated. Here we show that intracellular administration of double-stranded B-form DNA (B-DNA) triggered antiviral responses including production of type I interferons and chemokines independently of Toll-like receptors or the helicase RIG-I. B-DNA activated transcription factor IRF3 and the promoter of the gene encoding interferon-beta through a signaling pathway that required the kinases TBK1 and IKKi, whereas there was substantial activation of transcription factor NF-kappaB independent of both TBK and IKKi. IPS-1, an adaptor molecule linking RIG-I and TBK1, was involved in B-DNA-induced activation of interferon-beta and NF-kappaB. B-DNA signaling by this pathway conferred resistance to viral infection in a way dependent on both TBK1 and IKKi. These results suggest that both TBK1 and IKKi are required for innate immune activation by B-DNA, which might be important in antiviral innate immunity and other DNA-associated immune disorders.

Biodistribution and persistence of an MVA-vectored candidate HIV vaccine in SIV-infected rhesus macaques and SCID mice

Recombinant modified vaccinia virus Ankara (MVA) is together with a few other attenuated viral vectors on the forefront of human immunodeficiency virus type 1 (HIV-1) vaccine development. As such, MVA-vectored vaccines are likely to be administered into immunocompromized individuals. Here, we demonstrated in a good laboratory practice study safety and biological clearance of candidate HIV-1 vaccine MVA.HIVA in simian immunodeficiency virus (SIV)-infected rhesus macaques and mice with a severe combined immunodeficiency (SCID) following an intradermal vaccine administration. In SIV-infected macaques, MVA.HIVA DNA was undetectable by nested PCR 6 weeks after dosing. In SCID mice, the MVA.HIVA vaccine was well tolerated and a positive PCR signal was only observed at the site of injection 49 days after dosing in four out of six mice, but even these sites were negative by day 81 post-injection. Therefore, the MVA.HIVA vaccine is considered safe for application in phase I clinical trials in HIV-1-infected human subjects. These results also contribute to the confidence of using MVA as a smallpox vaccine.

Accurate mass precursor ion data and tandem mass spectrometry identify a class I human leukocyte antigen A*0201-presented peptide originating from vaccinia virus

We have used accurate mass precursor ion data generated on a hybrid linear-ion trap-Fourier transform ion cyclotron resonance mass spectrometer to augment tandem mass spectrometry (MS/MS) data generated on two different instrument types. Results from these experiments have allowed us for the first time to identify a naturally processed peptide presented by a class I human leukocyte antigen allele (HLA-A*0201) that was isolated from B cells infected by live vaccinia, the viral agent of the smallpox vaccine. The accurate mass data, in conjunction with MS/MS data, was able to identify the sequence IVIEAIHTV (aa 187-195) from the protein thymidylate kinase of vaccinia, distinguishing it from a similar sequence IVLEAIAEH: a "self-peptide" from the human protein phospholipase Cbeta3. Accurate mass data for the doubly charged species from the naturally processed and presented peptide was 497.8006, which was within 0.8 ppm of the calculated m/z of 497.8002, while being -37.3 ppm from the calculated m/z (497.7820) of the second-ranked peptide sequence IVLEAIAEH. Accurate mass data ranged from less than 0.1 to 1.2 ppm for other peptides identified in this sample. A BLAST search shows this sequence, IVIEAIHTV, is conserved in the same protein of a number of other orthopoxviruses, including the variola (smallpox) virus. Additionally, accurate mass data were able to uncover a false positive search result that was not distinguished by scoring of the match to the MS/MS data.

An attenuated LC16m8 smallpox vaccine: analysis of full-genome sequence and induction of immune protection

The potential threat of smallpox bioterrorism has made urgent the development of lower-virulence vaccinia virus vaccines. An attenuated LC16m8 (m8) vaccine was developed in 1975 from the Lister strain used in the World Health Organization smallpox eradication program but was not used against endemic smallpox. Today, no vaccines can be tested with variola virus for efficacy in humans, and the mechanisms of immune protection against the major intracellular mature virion (IMV) and minor extracellular enveloped virion (EEV) populations of poxviruses are poorly understood. Here, we determined the full-genome sequences of the m8, parental LC16mO (mO), and grandparental Lister (LO) strains and analyzed their evolutionary relationships. Sequence data and PCR analysis indicated that m8 was a progeny of LO and that m8 preserved almost all of the open reading frames of vaccinia virus except for the disrupted EEV envelope gene B5R. In accordance with this genomic background, m8 induced 100% protection against a highly pathogenic vaccinia WR virus in mice by a single vaccination, despite the lack of anti-B5R and anti-EEV antibodies. The immunogenicity and priming efficacy with the m8 vaccine consisting mainly of IMV were as high as those with the intact-EEV parental mO and grandparental LO vaccines. Thus, mice vaccinated with 10(7) PFU of m8 produced low levels of anti-B5R antibodies after WR challenge, probably because of quick clearance of B5R-expressing WR EEV by strong immunity induced by the vaccination. These results suggest that priming with m8 IMV provides efficient protection despite undetectable levels of immunity against EEV.

The nonreplicating smallpox candidate vaccines defective vaccinia Lister (dVV-L) and modified vaccinia Ankara (MVA) elicit robust long-term protection

Current smallpox vaccines are live vaccinia viruses that replicate in the vaccinee inducing immunity against the deadly disease smallpox. Replication resulting in virus spread within the host, however, is the major cause of severe postvaccinal adverse events. Therefore, attenuated strains such as modified vaccinia Ankara (MVA) or LC16m8 are candidates as next generation vaccines. These strains are usually grown in primary cells in which mass production is difficult and have an unknown protective potential in humans. Proven vaccine strains of defined origin and modern production techniques are therefore desirable. In this study, defective vaccinia virus (dVV) lacking a gene essential for replication (derived from the Lister vaccine in a complementing cell line) was compared with the Wyeth smallpox vaccine strain and with MVA in mouse animal models using cowpox and ectromelia virus challenge. Similar to MVA, prime-boost immunizations with defective vaccinia induced robust long-term immunity, suggesting it as a promising next generation smallpox vaccine.

HLA class I-restricted responses to vaccinia recognize a broad array of proteins mainly involved in virulence and viral gene regulation

We have analyzed by ex vivo ELISPOT the anti-vaccinia cytotoxic T lymphocyte responses of peripheral blood mononuclear cells from humans vaccinated with Dryvax vaccine. More than 6,000 peptides from 258 putative vaccinia ORFs predicted to bind the common molecules of the HLA A1, A2, A3, A24, B7, and B44 supertypes were screened with peripheral blood mononuclear cells of 31 vaccinees. A total of 48 epitopes derived from 35 different vaccinia antigens were identified, some of which (B8R, D1R, D5R, C10L, C19L, C7L, F12, and O1L) were recognized by multiple donors and contain multiple epitopes recognized in the context of different HLA types. The antigens recognized tend to be >100 residues in length and are expressed predominantly in the early phases of infection, although some late antigens were also recognized. Viral genome regulation and virulence factor were recognized most frequently, whereas few structural proteins were immunogenic. Finally, most epitopes were highly conserved among vaccinia virus Western Reserve, variola major and modified vaccinia Ankara, supporting their potential use in vaccine and diagnostic applications.

Novel conjugates of epitope fusion peptides with CpG-ODN display enhanced immunogenicity and HIV recognition

Vaccination strategies remain elusive that are effective against viral disease pathogens yet remain gentle enough for widespread human use. We developed a model system that relies on the recognition of specific T-cell epitopes from immunodominant antigens of HIV to explore single-stranded CpG-oligodeoxynucleotides (ODN) (CpG) as an adjuvant. We improved upon current strategies of utilizing CpG in combination with peptide vaccines by covalently modifying epitope fusion peptides with CpG motifs. Characterization of the immune recognition of DNA-peptide conjugates was carried out in a murine model of human HLA A2. Immunogenicity of DNA-peptide conjugates was superior in sensitivity to non-covalently linked mixtures of the same functional molecules as measured by peptide-mediated cytotoxicity and IFN-gamma release, as well as protection against viral infection. Enhancement of sensitivity of immune recognition by covalent attachment of DNA to epitope peptides should be further evaluated as a novel prophylactic vaccine strategy for HIV infection and other infectious diseases.

Identification and preliminary characterization of vaccinia virus (Dryvax) antigens recognized by vaccinia immune globulin

Using vaccinia immune globulin (VIG), a high-titer antibody preparation from immunized subjects, we demonstrate that the humoral immune response in humans is directed against numerous antigens in the Dryvax vaccine strain. Western blot and immunoprecipitation analyses revealed highly antigenic proteins associated with both the extracellular enveloped virus and intracellular mature virus forms. The modified vaccinia virus Ankara (MVA), a new generation smallpox vaccine that is attenuated for replication in humans, expresses most, but not all, of the major vaccinia antigens recognized by antibodies in VIG, lacking the highly antigenic protein corresponding to the A-type inclusion body protein. Since new-generation smallpox vaccines such as MVA will require extensive comparison to traditional smallpox vaccines in animal models of immunogenicity and protection, we compared the vaccinia virus antigens recognized by VIG to those recognized by sera from Dryvax and MVA immunized mice. The humoral immune response in immunized mice is qualitatively similar to that in humans.

Modified vaccinia virus Ankara as antigen delivery system: how can we best use its potential?

Safety-tested modified vaccinia virus Ankara (MVA) has been established as a potent vector system for the development of candidate recombinant vaccines. The versatility of the vector system was recently demonstrated by the rapid production of experimental MVA vaccines for immunization against severe acute respiratory syndrome associated coronavirus. Promising results were also obtained in the delivery of Epstein-Barr virus or human cytomegalovirus antigens and from the clinical testing of MVA vectors for vaccination against immunodeficiency virus, papilloma virus, Plasmodium falciparum or melanoma. Moreover, MVA is considered to be a prime candidate vaccine for safer protection against orthopoxvirus infections. Thus, vector development to challenge dilemmas in vaccinology or immunization against poxvirus biothreat seems possible, yet the right choice should be made for a most beneficial use.

Genetically stable and fully effective smallpox vaccine strain constructed from highly attenuated vaccinia LC16m8

A highly attenuated LC16m8 (m8) smallpox vaccine has been licensed in Japan because of its extremely low neurovirulence profile, which is comparable to that of replication incompetent strains of vaccinia virus. From 1973 to 1975, m8 was administrated to >100,000 infants where it induced levels of immunity similar to that of the originating Lister strain, without any serious side effects. Recently, we observed that m8 reverts spontaneously to large plaque forming clones that possess virulence equivalent to that of LC16mO, a parental virus strain of m8. Here, we report that the B5R gene is responsible for the reversion, and that we could construct a more genetically stable virus by deleting B5R from m8. The protective immunogenicity of the vaccine candidate proved to be equivalent to that of the U.S.-licensed product Dryvax, and much superior to modified vaccinia Ankara in a mouse model. Furthermore, the vaccine strain never elicited any symptoms in severe combined immunodeficiency disease mice, even at a dose 1,000-fold greater than that used in the immune protection experiments, which is in contrast to the lethal pathogenicity induced by Dryvax inoculation of severe combined immunodeficiency disease mice. Our results suggest that this vaccine strain is a good candidate as a suitable smallpox vaccine and a vector virus, and that B5R is not essential for protective immunity against smallpox.

The Future of Smallpox Vaccination: is MVA the key?

Eradication of the smallpox virus through extensive global vaccination efforts has resulted in one of the most important breakthroughs in medical history, saving countless lives from the severe morbidity and mortality that is associated with this disease. Although smallpox is now extinct in nature, laboratory stocks of this virus still remain and the subject of smallpox vaccination has gained renewed attention due to the potential risk that smallpox may be used as a biological weapon by terrorists or rogue states. Despite having the longest history of any modern vaccine, there is still much to be learned about smallpox vaccination and the correlates of protection remain to be formally defined. This Commentary will discuss the strengths and weaknesses of traditional smallpox vaccination in comparison with immunization using modified vaccinia virus Ankura (MVA), a non-replicating virus with a strong safety record but weakened immunogenicity.

Identification of poxvirus CD8+ T cell determinants to enable rational design and characterization of smallpox vaccines

The large size of poxvirus genomes has stymied attempts to identify determinants recognized by CD8⁺ T cells and greatly impeded development of mouse smallpox vaccination models. Here, we use a vaccinia virus (VACV) expression library containing each of the predicted 258 open reading frames to identify five peptide determinants that account for approximately half of the VACV-specific CD8⁺ T cell response in C57BL/6 mice. We show that the primary immunodominance hierarchy is greatly affected by the route of VACV infection and the poxvirus strain used. Modified vaccinia virus ankara (MVA), a candidate replacement smallpox vaccine, failed to induce responses to two of the defined determinants. This could not be predicted by genomic comparison of viruses and is not due strictly to limited MVA replication in mice. Several determinants are immunogenic in cowpox and ectromelia (mousepox) virus infections, and immunization with the immunodominant determinant provided significant protection against lethal mousepox. These findings have important implications for understanding poxvirus immunity in animal models and bench-marking immune responses to poxvirus vaccines in humans.

Modified vaccinia virus Ankara immunization protects against lethal challenge with recombinant vaccinia virus expressing murine interleukin-4

Recent events have raised concern over the use of pathogens, including variola virus, as biological weapons. Vaccination with Dryvax is associated with serious side effects and is contraindicated for many people, and the development of a safer effective smallpox vaccine is necessary. We evaluated an attenuated vaccinia virus, modified vaccinia virus Ankara (MVA), by use of a murine model to determine its efficacy against an intradermal (i.d.) or intranasal (i.n.) challenge with vaccinia virus (vSC8) or a recombinant vaccinia virus expressing murine interleukin-4 that exhibits enhanced virulence (vSC8-mIL4). After an i.d. challenge, 15 of 16 mice who were inoculated with phosphate-buffered saline developed lesions, one dose of intramuscularly administered MVA was partially protective (3 of 16 mice developed lesions), and the administration of two or three doses of MVA was completely protective (0 of 16 mice developed lesions). In unimmunized mice, an i.n. challenge with vSC8 caused a significant but self-limited illness, while vSC8-mIL4 resulted in lethal infections. Immunization with one or two doses of MVA prevented illness and reduced virus titers in mice who were challenged with either vSC8 or vSC8-mIL4. MVA induced a dose-related neutralizing antibody and vaccinia virus-specific CD8+-T-cell response. Mice immunized with MVA were fully protected from a low-dose vSC8-mIL4 challenge despite a depletion of CD4+ cells, CD8+ cells, or both T-cell subsets or an antibody deficiency. CD4+- or CD8+-T-cell depletion reduced the protection against a high-dose vSC8-mIL4 challenge, and the depletion of both T-cell subsets was associated with severe illness and higher vaccinia virus titers. Thus, MVA induces broad humoral and cellular immune responses that can independently protect against a molecularly modified lethal poxvirus challenge in mice. These data support the continued development of MVA as an alternative candidate vaccine for smallpox.

Smallpox in the 21st century

The viral disease, smallpox, was well known through the end of the 20th Century. Because it has been eradicated from natural populations, the present clinical experience with managing the disease is limited. Similarly, research in the pathophysiology, treatment, and prevention of the disease has recently become a priority. Concerns regarding smallpox as a weapon of bioterrorism have led to the implementation of a new prophylactic vaccine program, a renewal in variola vaccine research, and treatment regimens against variola infection.

Hepatitis C virus mutation affects proteasomal epitope processing

The high incidence of hepatitis C virus (HCV) persistence raises the question of how HCV interferes with host immune responses. Studying a single-source HCV outbreak, we identified an HCV mutation that impaired correct carboxyterminal cleavage of an immunodominant HLA-A2-restricted CD8 cell epitope that is frequently recognized by recovered patients. The mutation, a conservative HCV nonstructural protein 3 (NS3) tyrosine to phenylalanine substitution, was absent in 54 clones of the infectious source, but present in 15/21 (71%) HLA-A2-positive and in 11/24 (46%) HLA-A2-negative patients with chronic hepatitis C. In order to analyze whether the mutation affected the processing of the HLA-A2-restricted CD8 cell epitope, mutant and wild-type NS3 polypeptides were digested in vitro with 20S constitutive proteasomes and with immunoproteasomes. The presence of the mutation resulted in impaired carboxyterminal cleavage of the epitope. In order to analyze whether impaired epitope processing affected T cell priming in vivo, HLA-A2-transgenic mice were infected with vaccinia viruses encoding either wild-type or mutant HCV NS3. The mutant induced fewer epitope-specific, IFN-gamma;-producing and fewer tetramer(+) cells than the wild type. These data demonstrate how a conservative mutation in the flanking region of an HCV epitope impairs the induction of epitope-specific CD8(+) T cells and reveal a mechanism that may contribute to viral sequence evolution in infected patients.