Characterization of in vivo primary and secondary CD8+ T cell responses induced by recombinant influenza and vaccinia viruses

Progress toward a malaria vaccine: efficient induction of protective anti-malaria immunity

Malaria can be a very severe disease, particularly in young children, pregnant women (mostly in primipara), and malaria naïve adults, and currently ranks among the most prevalent infections in tropical and subtropical areas throughout the world. The widespread occurrence and the increased incidence of malaria in many countries, caused by drug-resistant parasites (Plasmodium falciparum and P. vivax) and insecticide-resistant vectors (Anopheles mosquitoes), indicate the need to develop new methods of controlling this disease. Experimental vaccination with irradiated sporozoites can protect animals and humans against the disease, demonstrating the feasibility of developing an effective malaria vaccine. However, developing a universally effective, long lasting vaccine against this parasitic disease has been a difficult task, due to several problems. One difficulty stems from the complexity of the parasite's life cycle. During their life cycle, malaria parasites change their residence within the host, thus avoiding being re-exposed to the same immunological environment. These parasites also possess some distinct antigens, present at different life stages of the parasite, the so-called stage-specific antigens. While some of the stage-specific antigens can induce protective immune responses in the host, these responses are usually genetically restricted, this being another reason for delaying the development of a universally effective vaccine. The stage-specific antigens must be used as immunogens and introduced into the host by using a delivery system that should efficiently induce protective responses against the respective stages. Here we review several research approaches aimed at inducing protective anti-malaria immunity, overcoming the difficulties described above.

Prospects for an SV40 vaccine

The identification of SV40 as a possible cause of human cancer leads to the question of whether the unique properties of the virus can be exploited to treat patients with SV40-positive mesotheliomas, which are otherwise refractory to successful intervention. A modified SV40 T antigen, from which the transforming domains have been removed, has been cloned into a vaccinia virus vector and tested in animal tumor model systems. It has been shown to be effective against both subsequent tumor challenge and pre-existing tumors. Thus, the potential exists for use of such a vaccine in mesothelioma patients.

Protective CD8+ T cell responses against the pre-erythrocytic stages of malaria parasites: an overview

CD8+ T cells have been implicated as critical effector cells in protection against the pre-erythrocytic stage of malaria in mice and humans following irradiated sporozoite immunization. Immunization experiments in animal models by several investigators have suggested different strategies for vaccination against malaria and many of the targets from liver stage malaria antigens have been shown to be immunogenic and to protect mice from the sporozoite challenge. Several prime/boost protocols with replicating vectors, such as vaccinia/influenza, with non-replicating vectors, such as recombinant particles derived from yeast transposon (Ty-particles) and modified vaccinia virus Ankara, and DNA, significantly enhanced CD8+ T cell immunogenicity and also the protective efficacy against the circumsporosoite protein of Plasmodium berghei and P. yeti. Based on these experimental results the development of a CD8+ T cell inducing vaccine has moved forward from epitope identification to planning stages of safety and immunogenicity trials of candidate vaccines.

Nucleic acid vaccines: tasks and tactics

There are no adequate vaccines against some of the new or reemerged infectious scourges such as HIV and TB. They may require strong and enduring cell-mediated immunity to be elicited. This is quite a task, as the only known basis of protection by current commercial vaccines is antibody. As DNA or RNA vaccines may induce both cell-mediated and humoral immunity, great interest has been shown in them. However, doubt remains whether their efficacy will suffice for their clinical realization. We look at the various tactics to increase the potency of nucleic acid vaccines and divided them broadly under those affecting delivery and those affecting immune induction. For delivery, we have considered ways of improving uptake and the use of bacterial, replicon or viral vectors. For immune induction, we considered aspects of immunostimulatory CpG motifs, coinjection of cytokines or costimulators and alterations of the antigen, its cellular localization and its anatomical localization including the use of ligand-targeting to lymphoid tissue. We also thought that mucosal application of DNA deserved a separate section. In this review, we have taken the liberty to discuss these enhancement methods, whenever possible, in the context of the underlying mechanisms that might argue for or against these strategies.

Maintenance of CD8(+) T-cell memory following infection with recombinant sindbis and vaccinia viruses

CD8(+) T-cell memory is critical for protection against pathogens poorly controlled by humoral immunity. To characterize two distinct vaccine vectors, the acute and memory CD8(+) T-cell responses to an HIV-1 epitope (p18) expressed by recombinant vaccinia (vp18) and Sindbis (SINp18) viruses were compared. Whereas 9 to 13% of CD8(+) splenocytes were p18 specific during the acute response to vp18, 4% were induced by SINp18 as revealed by class I tetramer staining. Increased T-cell activation by vp18 was confirmed by higher numbers of both p18-specific IFN-gamma-secreting splenocytes and activated CD8(+) and CD4(+) T cells. Although higher frequencies of p18-specific CD8(+) T cells during primary responses correlated with higher frequencies during memory, the overall decline was only two- to threefold during the transition to memory, demonstrating equally efficient maintenance of memory in SINp18- as in vp18-immune mice. Despite modest in vivo activation, SINp18-induced CD4(+) T cells secreted substantial amounts of IFN-gamma and IL-2, potentially contributing to sustained CD8(+) memory. Collectively the data indicate that Sindbis virus recombinants provide effective vaccines for inducing protective memory CD8(+) T cells in the absence of the extensive inflammation and replication associated with vaccinia virus.

Differing roles of inflammation and antigen in T cell proliferation and memory generation

Recent studies have demonstrated that viral and bacterial infections can induce dramatic in vivo expansion of Ag-specific T lymphocytes. Although presentation of Ag is critical for activation of naive T cells, it is less clear how dependent subsequent in vivo T cell proliferation and memory generation are upon Ag. We investigated T cell expansion and memory generation in mice infected alternately with strains of Listeria monocytogenes that contained or lacked an immunodominant, MHC class I-restricted T cell epitope. We found substantial differences in the responses of effector and memory T cells to inflammatory stimuli. Although effector T cells undergo in vivo expansion in response to bacterial infection in the absence of Ag, memory T cells show no evidence for such bystander activation. However, Ag-independent expansion of effector T cells does not result in increased memory T cell frequencies, indicating that Ag presentation is critical for effective memory T cell generation. Early reinfection of mice with L. monocytogenes before the maximal primary T cell response induces typical memory expansion, suggesting that the capacity for a memory T cell response exists within the primary effector population. Our findings demonstrate that T cell effector proliferation and memory generation are temporally overlapping processes with differing requirements for Ag.

Immunogenicity of Ty-VLP bearing a CD8(+) T cell epitope of the CS protein of P. yoelii: enhanced memory response by boosting with recombinant vaccinia virus

We characterized the immunogenicity of the hybrid Ty-virus-like carrying the CD8(+) T cell epitope (SYVPSAEQI) of the circumsporozoite (CS) protein of Plasmodium yoelii (TyCS-VLP), a rodent malaria parasite. Balb/c mice were immunized with hybrid TyCS-VLP, and their CS-specific CD8(+) T cell response was quantitatively evaluated with the ELISPOT assay, based on the enumeration of epitope specific gamma-interferon secreting CD8(+) T cell. A single immunization with the TyCS-VLP by a variety of routes and doses indicated that the maximal response occurred in mice, which were immunized with 50 micrograms of these particles, administered via intramuscular. Combined immunization of mice with this TyCS-VLP followed by recombinant vaccinia virus expressing the entire P. yoelii CS protein (VacPyCS) or irradiated sporozoites, induced high levels of IFN-gamma-producing cells. The immunization regime, priming with TyCS-VLP and boosting with VacPyCS generated a potent protective immune response, which strongly inhibited P. yoelii liver stages development and protected 62% of the mice against a subsequent live P. yoelii sporozoite challenge.

4-1BB Ligand, a Member of the TNF Family, Is Important for the Generation of Antiviral CD8 T Cell Responses

4-1BB (CD137) is a costimulatory molecule expressed on activated T cells and interacts with 4-1BB ligand (4-1BBL) on APCs. To investigate the role of 4-1BB costimulation for the development of primary immune responses, 4-1BBL-deficient (4-1BBL−/−) mice were infected with lymphocytic choriomeningitis virus (LCMV). 4-1BBL−/− mice were able to generate CTL and eliminate acute LCMV infection with normal kinetics, but CD8 T cell expansion was 2- to 3-fold lower than in wild-type (+/+) mice. In the same mice, virus-specific CD4 Th and B cell responses were minimally affected, indicating that 4-1BB costimulation preferentially affects CD8 T cell responses. This result contrasts with our earlier work with CD40L-deficient (CD40L−/−) mice, in which the CD8 T cell response was unaffected and the CD4 T cell response was markedly impaired. When both 4-1BBL- and B7-dependent signals were absent, CD8 T cell expansion was further reduced, resulting in lower CTL activity and impairing their ability to clear LCMV. Altogether, these results indicate that T cells have distinct costimulatory requirements: optimal CD8 responses require 4-1BBL-dependent interactions, whereas CD4 responses are minimally affected by 4-1BB costimulation, but require CD40-CD40L and B7-dependent interactions.

The Induction of Virus-Specific CTL as a Function of Increasing Epitope Expression: Responses Rise Steadily Until Excessively High Levels of Epitope Are Attained

The role of epitope expression levels in CD8+ T cell priming has been controversial. Yet this parameter is of great importance in the design of rational approaches to optimize CTL responses to a variety of pathogens. In this paper we examine the influence of epitope production on CD8+ T cell priming by exploiting a system that allows a 200-fold range of cell surface epitope expression in vitro with a fixed dose of vaccinia virus. Our results demonstrate that, with the exception of a notable decline at the highest level of epitope, the magnitude of the responding CTL population generated in vivo following equivalent viral infections is essentially proportional to epitope density.

Induction of CD8+ T cells using heterologous prime-boost immunisation strategies

One of the current challenges in vaccine design is the development of antigen delivery systems or vaccination strategies that induce high protective levels of CD8+ T cells. These cells are crucial for protection against certain tumours and intracellular pathogens such as the liver-stage parasite of malaria. A liver-stage malaria vaccine should therefore include CD8+ T-cell-inducing components. This review provides an overview of prime-boost immunisation strategies that result in protective CD8+ T-cell responses against malaria with an emphasis on work from our laboratory. Possible mechanisms explaining why heterologous prime-boost strategies, in particular boosting with replication-impaired recombinant poxviruses, are so effective are discussed.

Immune effector mechanisms in malaria

Malaria, a disease responsible for immense human suffering, is caused by infection with Plasmodium spp. parasites, which have a very complex life cycle - antigenically unique stages infect different tissues of the body. This review details recent developments in our understanding of immunity both to pre-erythrocytic stage antigens and to erythrocytic stage antigens. The former is largely mediated via CD8(+) T cells and involves IFN-gamma, nitric oxide, IL-12 and natural killer cells; the latter varies (in different hosts and with different parasites) but is largely mediated by antibody, helper T cells, nitric oxide and gammadelta T cells. The recent progress towards clinical trials of vaccine candidates against both the pre-erythrocytic stage and erythrocytic stage is also summarized, in particular the use of heterologous prime/boost strategies for the former and the use of MSP1 as a candidate vaccine for the latter.

A Novel Approach to Visualize Polyclonal Virus-Specific CD8 T Cells In Vivo

Recent technical breakthroughs in generating soluble MHC class I-peptide tetramers now allow the direct visualization of virus-specific CD8 T cells after infection in vivo. However, this technique requires the knowledge of the immunodominant viral epitopes recognized by T cells. Here, we describe an alternative approach to visualize polyclonal virus-specific CD8 T cells in vivo using a simple adoptive transfer system. In our approach, C57BL/6 (Thy1.2) mice were infected with lymphocytic choriomeningitis virus, vesicular stomatitis virus, or vaccinia virus to induce virus-specific memory T cells. Tracer T cells (2 × 106) from these virus-immune mice were adoptively transferred into nonirradiated (C57BL/6 × B6.PL-Thy-1a)F1 mice. After infection of the F1-recipient mice with the appropriate virus, the transferred cells expanded vigorously, and on day 8 postinfection 60–80% of total CD8 T cells were of donor T cell origin. Under the same conditions memory CD4 T cells gave rise to at least 10 times less cell numbers than memory CD8 T cells. The transfer system described here not only allows to visualize effector and memory CD8 T cells in vivo but also to isolate them for further in vitro characterization without knowing the epitopes recognized by these Ag-specific CD8 T cells.

Enhanced CD8+ T cell response to HIV-1 env by combined immunization with influenza and vaccinia virus recombinants

With the aim to determine if immunization with two different live recombinant viral vectors could lead to an enhancement of the cellular immune response to HIV-1 antigens, we have characterized the CD8+ T cell response elicited against the V3 loop epitope from HIV-1 env protein in Balb/c mice immunized with either: a recombinant influenza virus (Flu-Env) expressing the V3 loop epitope from HIV-1 strain IIIB, a vaccinia virus recombinant (VV-Env) expressing the complete HIV-1-IIIB env protein, or a combination of both. The CD8+ T cell response, measured by the ELISPOT assay, in animals primed with Flu-Env and boosted with VV-Env was 5 to 6 times higher than in animals inoculated with either Flu-Env or VV-Env alone. Similar results were obtained with recombinant viruses expressing the V3 loop epitope or the complete env protein, respectively, from the MN strain of HIV-1. Our results indicate that the use of two different live vectors for priming and boosting has a synergistic effect on the immune response against HIV-1, and could represent a novel vaccination strategy against AIDS.

Induction of CD8+ T cell-mediated protective immunity against Trypanosoma cruzi

Trypanosoma cruzi was transformed with the Plasmodium yoelii gene encoding the circum-sporozoite (CS) protein, which contains the well-characterized CD8+ T cell epitope, SYVPSAEQI. In vivo and in vitro assays indicated that cells infected with the transformed T. cruzi could process and present this malaria parasite-derived class I MHC-restricted epitope. Immunization of mice with recombinant influenza and vaccinia viruses expressing the SYVPSAEQI epitope induced a large number of specific CD8+ T cells that strongly suppressed parasitemia and conferred complete protection against the acute T. cruzi lethal infection. CD8+ T cells mediated this immunity as indicated by the unrelenting parasitemia and high mortality observed in immunized mice treated with anti-CD8 antibody. This study demonstrated, for the first time, that vaccination of mice with vectors designed to induce CD8+ T cells is effective against T. cruzi infection.

Existing antilisterial immunity does not inhibit the development of a Listeria monocytogenes-specific primary cytotoxic T-lymphocyte response

Infection of BALB/c mice with Listeria monocytogenes stimulates an antilisterial immune response evident by the appearance of H2-Kd-restricted CD8(+) cytotoxic T lymphocytes (CTLs) specific for the nanomer peptides amino acids (aa) 91 to 99 of listeriolysin O (LLO 91-99) and aa 217 to 225 of the p60 molecule (p60 217-225). We have introduced point mutations at anchor residues within LLO 91-99 (92F) or p60 217-225 (218F), and BALB/c mice infected with L. monocytogenes strains containing these point mutations do not develop CTLs specific for LLO 91-99 or p60 217-225, respectively. We have used these strains to test whether primary CTL responses against L. monocytogenes-derived determinants can be stimulated within an environment of existing antilisterial immunity. We found that the development of a primary L. monocytogenes-specific CTL response is not altered by existing immunity to L. monocytogenes. For example, primary immunization with the p60 218F strain of L. monocytogenes followed by a secondary immunization with wild-type L. monocytogenes results in stimulation of p60 217-225-specific CTLs at primary response levels and LLO 91-99-specific effectors at levels consistent with a memory CTL response. Similarly, primary immunization with the 92F strain of L. monocytogenes followed by a secondary immunization with wild-type L. monocytogenes results in stimulation of LLO 91-99-specific CTLs at primary response levels and p60 217-225-specific effectors at levels consistent with a memory CTL response. These results provide additional support for the use of L. monocytogenes as a recombinant vaccine vector and show that antivector immunity does not inhibit the development of a primary CTL response when the epitope is delivered by L. monocytogenes as the vaccine strain.

Transfectant influenza A viruses are effective recombinant immunogens in the treatment of experimental cancer

Using reverse genetics methods, we constructed three different transfectant influenza A viruses encoding an Ld-restricted, nine amino-acid-long fragment, corresponding to amino-acid residues 876-884, of beta-galactosidase (beta-gal). Sequences encoding this epitope were nested within the hemagglutinin (HA) or neuraminidase (NA) open reading frames. Alternatively, an independent beta-gal mini-gene, preceded by an endoplasmic reticulum insertion signal sequence, was placed in a bicistronic arrangement in the NA RNA segment of the virus. All three transfectants mediated the presentation of the epitope to a beta-gal-specific CTL clone. Furthermore, each of the three transfectant viruses expressing the beta-gal fragment elicited specific cytolytic responses in vivo. Most importantly, these H1N1 transfectants mediated the regression of established murine pulmonary metastases. Tumor regression in mice was also achieved in the presence of preexisting immunity against an H3N2 influenza A virus serotype. Nononcogenic and nonintegrating, transfectant influenza A viruses are attractive candidates for development as antitumor vaccines.

Recombinant Sindbis viruses expressing a cytotoxic T-lymphocyte epitope of a malaria parasite or of influenza virus elicit protection against the corresponding pathogen in mice

Subcutaneous administration in mice of recombinant Sindbis viruses expressing a class I major histocompatibility complex-restricted 9-mer epitope of the Plasmodium yoelii circumsporozoite protein or the nucleoprotein of influenza virus induces a large epitope-specific CD8(+) T-cell response. This immunization also elicits a high degree of protection against infection with malaria or influenza A virus.

Negative-strand RNA viruses: applications to biotechnology

The establishment of reverse-genetics techniques to manipulate the genome of negative-strand RNA viruses has contributed enormously to a better understanding of the replication mechanisms and pathogenicity of this group of viruses. The generation of recombinant viruses bearing specific mutations in the coding and noncoding regions of their genomic RNAs now allows the functions in the replicative cycle of specific RNA regions and protein domains of these viruses to be studied. In addition, recombinant negative-strand RNA viruses can now be designed to have specific properties that make them attractive biotechnological tools.

Cytotoxic T lymphocyte response in chickens immunized with a recombinant fowlpox virus expressing Marek's disease herpesvirus glycoprotein B

Previously, we demonstrated that cytotoxic T lymphocytes (CTLs) from MHC: B19B19 and MHC: B21B21 chickens inoculated with a non-oncogenic Marek's disease virus (MDV) vaccine strain, SB-1/12 can lyse syngeneic reticuloendotheliosis virus (REV)-transformed cell lines expressing MDV pp38 or gB genes. In this study, we report the characterization of MDV gB-specific CTLs in chickens immunized with recombinant fowlpox virus expressing MDV gB gene (rFPV-gB). Spleen cells from rFPV-gB inoculated chickens (MHC: B19B19), depleted for CD4+, CD8+, TCR gamma delta+, TCR alpha beta 1+ or TCR alpha beta 2+ cells were used as effector cells in chromium release assays. Effector cells depleted of CD8+ or TCR alpha beta 1+, but not CD4+, TCR gamma delta+ or TCR alpha beta 2+ markedly reduced the percentage of specific release (%SR). Compared to the %SR caused by the SB-1/12-sensitized CTLs, the %SR caused by rFPV-gB-sensitized CTLs was low, but statistically significant. This is a first report on the induction of MDV gB-specific CD8+ CTLs in chickens immunized with rFPV-gB vaccine.

Malaria vaccine: roadblocks and possible solutions

Malaria remains the most prevalent and devastating parasitic disease worldwide. Vaccination is considered to be an approach that will complement other strategies for prevention and control of the disease in the future. In the last 10 years, intense studies aimed at the development of a malaria vaccine have provided important knowledge of the nature of the host immunological mechanisms of protection and their respective target antigens. It became well established that protective immune responses can be generated against the distinct stages of Plasmodium. However, in general, protective immune responses are directed at stage-specific antigens. The elucidation of the primary structure of these antigens made possible the generation of synthetic and recombinant proteins that are being extensively used in experimental immunizations against the infection. Today, several epitopes of limited polymorphism have been described and protective immunity can be generated by immunization with them. These epitopes are being tested as primary candidates for a subunit vaccine against malaria. Here we critically review the major roadblocks for the development of a malaria vaccine and provide some insight on how these problems are being solved.

Counting antigen-specific CD8 T cells: a reevaluation of bystander activation during viral infection

Viral infections induce extensive T cell proliferation in vivo, but the specificity of the majority of the responding T cells has not been defined. To address this issue we used tetramers of MHC class I molecules containing viral peptides to directly visualize antigen-specific CD8 T cells during acute LCMV infection of mice. Based on tetramer binding and two sensitive assays measuring interferon-gamma production at the single-cell level, we found that 50%-70% of the activated CD8 T cells were LCMV specific [2 x 10(7) virus-specific cells/spleen]. Following viral clearance, antigen-specific CD8 T cell numbers dropped to 10(6) per spleen and were maintained at this level for the life of the mouse. Upon rechallenge with LCMV, there was rapid expansion of memory T cells, but after infection with the heterologous vaccinia virus there was no detectable change in the numbers of LCMV-specific memory CTL. Therefore, much of the CD8 T cell expansion seen during viral infection represents antigen-specific cells and warrants a revision of our current thinking on the size of the antiviral response.

Enhancing efficacy of recombinant anticancer vaccines with prime/boost regimens that use two different vectors

BACKGROUND

The identification of tumor-associated antigens and the cloning of DNA sequences encoding them have enabled the development of anticancer vaccines. Such vaccines target tumors by stimulating an immune response against the antigens. One method of vaccination involves the delivery of antigen-encoding DNA sequences, and a number of recombinant vectors have been used for this purpose. To optimize the efficacy of recombinant vaccines, we compared primary and booster treatment regimens that used a single vector (i.e., homologous boosting) with regimens that used two different vectors (i.e., heterologous boosting).

METHODS

Pulmonary tumors (experimental metastases) were induced in BALB/c mice inoculated with CT26.CL25 murine colon carcinoma cells, which express recombinant bacterial beta-galactosidase (the model antigen). Protocols for subsequent vaccination used three vectors that encoded beta-galactosidase--vaccinia (cowpox) virus, fowlpox virus, naked bacterial plasmid DNA. Mouse survival was evaluated in conjunction with antibody and cytotoxic T-lymphocyte responses to beta-galactosidase.

RESULTS

Heterologous boosting resulted in significantly longer mouse survival than homologous boosting (all P<.0001, two-sided). Potent antigen-specific cytotoxic T lymphocytes were generated following heterologous boosting with poxvirus vectors. This response was not observed with any of the homologous boosting regimens. Mice primed with recombinant poxvirus vectors generated highly specific antibodies against viral proteins.

CONCLUSIONS

The poor efficacy of homologous boosting regimens with viral vectors was probably a consequence of the induction of a strong antiviral antibody response. Heterologous boosting augmented antitumor immunity by generating a strong antigen-specific cytotoxic T-lymphocyte response. These data suggest that heterologous boosting strategies may be useful in increasing the efficacy of recombinant DNA anticancer vaccines that have now entered clinical trials.