Recombinant viruses expressing a human malaria antigen can elicit potentially protective immune CD8+ responses in mice

Enhancement of the HIV-1-Specific Immune Response Induced by an mRNA Vaccine through Boosting with a Poxvirus MVA Vector Expressing the Same Antigen

Development of a vaccine against HIV remains a major target goal in the field. The recent success of mRNA vaccines against the coronavirus SARS-CoV-2 is pointing out a new era of vaccine designs against pathogens. Here, we have generated two types of mRNA vaccine candidates against HIV-1; one based on unmodified vectors and the other on 1-methyl-3′-pseudouridylyl modified vectors expressing a T cell multiepitopic construct including protective conserved epitopes from HIV-1 Gag, Pol and Nef proteins (referred to as RNA-TMEP and RNA-TMEPmod, respectively) and defined their biological and immunological properties in cultured cells and in mice. In cultured cells, both mRNA vectors expressed the corresponding protein, with higher levels observed in the unmodified mRNA, leading to activated macrophages with differential induction of innate immune molecules. In mice, intranodal administration of the mRNAs induced the activation of specific T cell (CD4 and CD8) responses, and the levels were markedly enhanced after a booster immunization with the poxvirus vector MVA-TMEP expressing the same antigen. This immune activation was maintained even three months later. These findings revealed a potent combined immunization regimen able to enhance the HIV-1-specific immune responses induced by an mRNA vaccine that might be applicable to human vaccination programs with mRNA and MVA vectors.

An Influenza Virus Hemagglutinin-Based Vaccine Platform Enables the Generation of Epitope Specific Human Cytomegalovirus Antibodies

Human cytomegalovirus (CMV) is a highly prevalent pathogen with ~60%–90% seropositivity in adults. CMV can contribute to organ rejection in transplant recipients and is a major cause of birth defects in newborns. Currently, there are no approved vaccines against CMV. The epitope of a CMV neutralizing monoclonal antibody against a conserved region of the envelope protein gH provided the basis for a new CMV vaccine design. We exploited the influenza A virus as a vaccine platform due to the highly immunogenic head domain of its hemagglutinin envelope protein. Influenza A variants were engineered by reverse genetics to express the epitope of an anti-CMV gH neutralizing antibody that recognizes native gH into the hemagglutinin antigenic Sa site. We determined that the recombinant influenza variants expressing 7, 10, or 13 residues of the anti-gH neutralizing antibody epitope were recognized and neutralized by the anti-gH antibody 10C10. Mice vaccinated with the influenza/CMV chimeric viruses induced CMV-specific antibodies that recognized the native gH protein and inhibited virus infection. In fact, the influenza variants expressing 7–13 gH residues neutralized a CMV infection at ~60% following two immunizations with variants expressing the 13 residue gH peptide produced the highest levels of neutralization. Collectively, our study demonstrates that a variant influenza virus inserted with a gH peptide can generate a humoral response that limits a CMV infection.

Recombinant influenza A viruses as vaccine vectors

INTRODUCTION

Various viruses, including poxviruses, adenoviruses and vesicular stomatitis virus, have been considered as vaccine vectors for the delivery of antigens of interest in the development of vaccines against newly emerging pathogens.

AREAS COVERED

Here, we review results that have been obtained with influenza A viruses (IAV) as vaccine vectors. With the advent of reverse genetics technology, IAV-based recombinant vaccine candidates have been constructed that induce protective immunity to a variety of different pathogens of interest, including West Nile virus, Plasmodium falciparum and respiratory syncytial virus. The various cloning strategies to produce effective and attenuated, safe to use IAV-based viral vectors are discussed.

EXPERT COMMENTARY

It was concluded that IAV-based vector system has several advantages and holds promise for further development.

CD8+ T-cell mediated anti-malaria protection induced by malaria vaccines; assessment of hepatic CD8+ T cells by SCBC assay

Malaria is a severe infectious disease with relatively high mortality, thus having been a scourge of humanity. There are a few candidate malaria vaccines that have shown a protective efficacy in humans against malaria. One of the candidate human malaria vaccines, which is based on human malaria sporozoites and called PfSPZ Vaccine, has been shown to protect a significant proportion of vaccine recipients from getting malaria. PfSPZ Vaccine elicits a potent response of hepatic CD8+ T cells that are specific for malaria antigens in non-human primates. To further characterize hepatic CD8+ T cells induced by the sporozoite-based malaria vaccine in a mouse model, we have used a cutting-edge Single-cell Barcode (SCBC) assay, a recently emerged approach/method for investigating the nature of T-cells responses during infection or cancer. Using the SCBC technology, we have identified a population of hepatic CD8+ T cells that are polyfunctional at a single cell level only in a group of vaccinated mice upon malaria challenge. The cytokines/chemokines secreted by these polyfunctional CD8+ T-cell subsets include MIP-1α, RANTES, IFN-γ, and/or IL-17A, which have shown to be associated with protective T-cell responses against certain pathogens. Therefore, a successful induction of such polyfunctional hepatic CD8+ T cells may be a key to the development of effective human malaria vaccine. In addition, the SCBC technology could provide a new level of diagnostic that will allow for a more accurate determination of vaccine efficacy.

Modeling the effect of boost timing in murine irradiated sporozoite prime-boost vaccines

Vaccination with radiation-attenuated sporozoites has been shown to induce CD8+ T cell-mediated protection against pre-erythrocytic stages of malaria. Empirical evidence suggests that successive inoculations often improve the efficacy of this type of vaccines. An initial dose (prime) triggers a specific cellular response, and subsequent inoculations (boost) amplify this response to create a robust CD8+ T cell memory. In this work we propose a model to analyze the effect of T cell dynamics on the performance of prime-boost vaccines. This model suggests that boost doses and timings should be selected according to the T cell response elicited by priming. Specifically, boosting during late stages of clonal contraction would maximize T cell memory production for vaccines using lower doses of irradiated sporozoites. In contrast, single-dose inoculations would be indicated for higher vaccine doses. Experimental data have been obtained that support theoretical predictions of the model.

Conserved Binding Regions Provide the Clue for Peptide-Based Vaccine Development: A Chemical Perspective

Synthetic peptides have become invaluable biomedical research and medicinal chemistry tools for studying functional roles, i.e., binding or proteolytic activity, naturally-occurring regions’ immunogenicity in proteins and developing therapeutic agents and vaccines. Synthetic peptides can mimic protein sites; their structure and function can be easily modulated by specific amino acid replacement. They have major advantages, i.e., they are cheap, easily-produced and chemically stable, lack infectious and secondary adverse reactions and can induce immune responses via T- and B-cell epitopes. Our group has previously shown that using synthetic peptides and adopting a functional approach has led to identifying Plasmodium falciparumconserved regions binding to host cells. Conserved high activity binding peptides’ (cHABPs) physicochemical, structural and immunological characteristics have been taken into account for properly modifying and converting them into highly immunogenic, protection-inducing peptides (mHABPs) in the experimental Aotus monkey model. This article describes stereo–electron and topochemical characteristics regarding major histocompatibility complex (MHC)-mHABP-T-cell receptor (TCR) complex formation. Some mHABPs in this complex inducing long-lasting, protective immunity have been named immune protection-inducing protein structures (IMPIPS), forming the subunit components in chemically synthesized vaccines. This manuscript summarizes this particular field and adds our recent findings concerning intramolecular interactions (H-bonds or π-interactions) enabling proper IMPIPS structure as well as the peripheral flanking residues (PFR) to stabilize the MHCII-IMPIPS-TCR interaction, aimed at inducing long-lasting, protective immunological memory.

A Prime/Boost PfCS14KM/MVA-sPfCSM Vaccination Protocol Generates Robust CD8+ T Cell and Antibody Responses to Plasmodium falciparum Circumsporozoite Protein and Protects Mice against Malaria

Vaccines against the preerythrocytic stages of malaria are appealing because the parasite can be eliminated before disease onset and because they offer the unique possibility of targeting the parasite with both antibodies and T cells. Although the role of CD8⁺ T cells in preerythrocytic malaria stages is well documented, a highly effective T cell-inducing vaccine remains to be advanced. Here we report the development of a prime-boost immunization regimen with the Plasmodium falciparum circumsporozoite protein (PfCS) fused to the oligomer-forming vaccinia virus A27 protein and a modified vaccinia virus Ankara (MVA) vector expressing PfCS. This protocol induced polyfunctional CD8⁺ T cells with an effector memory phenotype and high PfCS antibody levels. These immune responses correlated with inhibition of liver-stage parasitemia in 80% and sterile protection in 40% of mice challenged with a transgenic P. berghei parasite line that expressed PfCS. Our findings underscore the potential of T and B cell immunization strategies for improving protective effectiveness against malaria.

Robust antibody and CD8+ T-cell responses induced by P. falciparum CSP adsorbed to cationic liposomal adjuvant CAF09 confer sterilizing immunity against experimental rodent malaria infection

Despite several decades of extensive research, the development of a highly efficacious malaria vaccine has yet to be accomplished. While the RTS,S malaria vaccine candidate shows the potential to prevent a substantial number of clinical malaria cases, significant improvements in protective efficacy are still needed. Multiple studies have shown that RTS,S induces protective antibody and CD4⁺ T-cell responses, but limited or negligible CD8⁺ T cells. In this study, we evaluated the immunogenicity and protective capacity of full-length recombinant Plasmodium falciparum circumsporozoite protein administered with the novel cationic liposomal adjuvant system CAF09. Using newly developed transgenic rodent malaria parasites expressing the full-length Plasmodium falciparum circumsporozoite protein, we demonstrate that this liposome-based protein-in-adjuvant formulation is capable of inducing robust antibody and CD8⁺ T-cell responses that strongly inhibit parasite infection and development of liver stages, conferring durable sterilizing immunity. These findings underscore the potential of liposome-based adjuvants for inducing robust humoral and CD8⁺ T-cell responses and warrant further studies toward the development of novel subunit vaccine formulations with this adjuvant system.

A vaccine consisting of parasitic proteins enveloped by fatty molecules provides comprehensive protection against malaria in a rodent model, Previous and current malaria vaccines concentrate on priming antibodies to recognize malarial infection, despite evidence that, by activating ‘killer’ CD8⁺ T cells, greater protection is conferred against the disease. Fidel Zavala, of the Johns Hopkins University, United States, and an international group of researchers developed their vaccine by encapsulating proteins from the malaria-causing parasite Plasmodium falciparum in fat-based carriers called liposomes. In past experiments, killer T cells recruited via this vaccine-type have effectively protected against other diseases. In this study, the vaccine induced both CD8⁺ T cell and antibody responses and provided significant immunity against P. falciparum-instigated malaria. As a highly efficacious vaccine against malaria is not yet available, this research will likely prove invaluable in guiding further studies.

Enhancing poxvirus vectors vaccine immunogenicity

Attenuated recombinant poxvirus vectors expressing heterologous antigens from pathogens are currently at various stages in clinical trials with the aim to establish their efficacy. This is because these vectors have shown excellent safety profiles, significant immunogenicity against foreign expressed antigens and are able to induce protective immune responses. In view of the limited efficacy triggered by some poxvirus strains used in clinical trials (i.e, ALVAC in the RV144 phase III clinical trial for HIV), and of the restrictive replication capacity of the highly attenuated vectors like MVA and NYVAC, there is a consensus that further improvements of these vectors should be pursuit. In this review we considered several strategies that are currently being implemented, as well as new approaches, to improve the immunogenicity of the poxvirus vectors. This includes heterologous prime/boost protocols, use of co-stimulatory molecules, deletion of viral immunomodulatory genes still present in the poxvirus genome, enhancing virus promoter strength, enhancing vector replication capacity, optimizing expression of foreign heterologous sequences, and the combined use of adjuvants. An optimized poxvirus vector triggering long-lasting immunity with a high protective efficacy against a selective disease should be sought.

Epitope-specific CD4+, but not CD8+, T-cell responses induced by recombinant influenza A viruses protect against Mycobacterium tuberculosis infection

Tuberculosis remains a global health problem, in part due to failure of the currently available vaccine, BCG, to protect adults against pulmonary forms of the disease. We explored the impact of pulmonary delivery of recombinant influenza A viruses (rIAVs) on the induction of Mycobacterium tuberculosis (M. tuberculosis)-specific CD4(+) and CD8(+) T-cell responses and the resultant protection against M. tuberculosis infection in C57BL/6 mice. Intranasal infection with rIAVs expressing a CD4(+) T-cell epitope from the Ag85B protein (PR8.p25) or CD8(+) T-cell epitope from the TB10.4 protein (PR8.TB10.4) generated strong T-cell responses to the M. tuberculosis-specific epitopes in the lung that persisted long after the rIAVs were cleared. Infection with PR8.p25 conferred protection against subsequent M. tuberculosis challenge in the lung, and this was associated with increased levels of poly-functional CD4(+) T cells at the time of challenge. By contrast, infection with PR8.TB10.4 did not induce protection despite the presence of IFN-γ-producing M. tuberculosis-specific CD8(+) T cells in the lung at the time of challenge and during infection. Therefore, the induction of pulmonary M. tuberculosis epitope-specific CD4(+), but not CD8(+) T cells, is essential for protection against acute M. tuberculosis infection in the lung.

RNA-based viral vectors

The advent of reverse genetic approaches to manipulate the genomes of both positive (+) and negative (-) sense RNA viruses allowed researchers to harness these genomes for basic research. Manipulation of positive sense RNA virus genomes occurred first largely because infectious RNA could be transcribed directly from cDNA versions of the RNA genomes. Manipulation of negative strand RNA virus genomes rapidly followed as more sophisticated approaches to provide RNA-dependent RNA polymerase complexes coupled with negative-strand RNA templates were developed. These advances have driven an explosion of RNA virus vaccine vector development. That is, development of approaches to exploit the basic replication and expression strategies of RNA viruses to produce vaccine antigens that have been engineered into their genomes. This study has led to significant preclinical testing of many RNA virus vectors against a wide range of pathogens as well as cancer targets. Multiple RNA virus vectors have advanced through preclinical testing to human clinical evaluation. This review will focus on RNA virus vectors designed to express heterologous genes that are packaged into viral particles and have progressed to clinical testing.

Characterization of recombinant influenza A virus as a vector expressing respiratory syncytial virus fusion protein epitopes

Respiratory syncytial virus (RSV) is the most common cause of respiratory infection in infants and the elderly, and no vaccine against this virus has yet been licensed. Here, we report a recombinant PR8 influenza virus with the RSV fusion (F) protein epitopes of the subgroup A gene inserted into the influenza virus non-structural (NS) gene (rFlu/RSV/F) that was generated as an RSV vaccine candidate. The rescued viruses were assessed by microscopy and Western blotting. The proper expression of NS1, the NS gene product, and the nuclear export protein (NEP) of rFlu/RSV/F was also investigated using an immunofluorescent assay. The rescued virus replicated well in the MDCK kidney cell line, A549 lung adenocarcinoma cell line and CNE-2Z nasopharyngeal carcinoma cell line. BALB/c mice immunized intranasally with rFlu/RSV/F had specific haemagglutination inhibition antibody responses against the PR8 influenza virus and RSV neutralization test proteins. Furthermore, intranasal immunization with rFlu/RSV/F elicited T helper type 1-dominant cytokine profiles against the RSV strain A2 virus. Taken together, our findings suggested that rFlu/RSV/F was immunogenic in vivo and warrants further development as a promising candidate vaccine.

Malaria vaccine R&D in the Decade of Vaccines: breakthroughs, challenges and opportunities

While recent progress has been made in reducing malaria mortality with other interventions, vaccines are still urgently needed to further reduce the incidence of clinical disease, including during pregnancy, and to provide "herd protection" by blocking parasite transmission. The most clinically advanced candidate, RTS,S, is presently undergoing Phase 3 evaluation in young African children across 13 clinical sites in eight African countries. In the 12-month period following vaccination, RTS,S conferred approximately 50% protection from clinical Plasmodium falciparum disease in children aged 5-17 months, and approximately 30% protection in children aged 6-12 weeks when administered in conjunction with Expanded Program for Immunization (EPI) vaccines. The development of more highly efficacious vaccines to prevent clinical disease caused by both P. falciparum and Plasmodium vivax, as well as vaccines to support elimination efforts by inducing immunity that blocks malaria parasite transmission, are priorities. Some key barriers to malaria vaccine development include: a paucity of well-characterized target immunogens and an absence of clear correlates of protection to enable vaccine development targeting all stages of the P. falciparum and P. vivax lifecycles; a limited number of safe and effective delivery systems, including adjuvants, that induce potent, long-lived protective immunity, be it by antibody, CD4+, and/or CD8+ T cell responses; and, for vaccines designed to provide "herd protection" by targeting sexual stage and/or mosquito antigens, the lack of a clear clinical and regulatory pathway to licensure using non-traditional endpoints. Recommendations to overcome these, and other key challenges, are suggested in this document.

Reactivity of autoantibodies against not only erythrocytes but also hepatocytes in sera of mice with malaria

In order to further examine the reactivity of autoantibodies, mice were infected with a non-lethal strain of Plasmodium yoelii. Parasitemia appeared between days 10 and 21. During this period, hyperglycemia and hypothermia were serially obeserved and this phenomenon resembled stress-associated responses. In parallel with parasitemia, autoantibodies appeared against nucleus and double-stranded DNA in the sera. To examine further the reactivity of autoantibodies against tissues, immunohistochemical staining using sera from mice with or without malaria was conducted. Autoantibodies contained reactivity to erythrocytes in the spleen, bone marrow and peripheral blood, especially against tissues obtained from mice with malaria. In the liver and intestine, autoantibodies reacted with hepatocytes and intestinal epithelial cells, respectively. These results suggested that the reactivity of autoantibodies against erythrocytes and hepatocytes might be associated with the modulation of the disease course in malaria.

Reflections on the early development of poxvirus vectors

Poxvirus expression vectors were described in 1982 and quickly became widely used for vaccine development as well as research in numerous fields. Advantages of the vectors include simple construction, ability to accommodate large amounts of foreign DNA and high expression levels. Numerous poxvirus-based veterinary vaccines are currently in use and many others are in human clinical trials. The early reports of poxvirus vectors paved the way for and stimulated the development of other viral vectors and recombinant DNA vaccines.

Actively induced antigen-specific CD8+ T cells by epitope-bearing parasite pre-infection but not prime/boost virus vector vaccination could ameliorate the course of Plasmodium yoelii blood-stage infection

The lack of MHC molecules on red blood cells (RBCs) has led to questions regarding the immunological function of CD8(+) T cells against malarial blood-stage (MBS). However, several recent reports contradicting with this concept have suggested that they play an important role in the course of MBS infection. The present study generated genetically engineered murine malaria, Plasmodium yoelii, which expresses a well-defined Trypanosoma cruzi-derived, H-2K(b)-restricted CD8(+) T cell epitope, ANYNFTLV. Prime/boost vaccination by the use of recombinant adenovirus and recombinant modified vaccinia virus Ankara (MVA), which induced an enhanced number of ANYNFTLV-specific CD8(+) T cells, failed to prevent a pathological outcome to occur upon ANYNFTLV-expressing murine MBS infection. This outcome did not change even with the combination of passive transfer of an appreciable number of in vitro-expanded ANYNFTLV-specific CD8(+) T cells. In contrast, the pre-infection of mice with T. cruzi, which intrinsically bears the same CD8(+) T cell epitope significantly improved the survival of ANYNFTLV-expressing malaria-infected mice but not that of control malaria-infected ones. This protective effect was abrogated by the use of a CD8(+) T cell-depleting monoclonal antibody. Although the protective effect was observed only in certain situations, the actively induced antigen-specific CD8(+) T cells could ameliorate the pathologies caused by the MBS. This is the first study to implicate that the active induction of antigen-specific CD8(+) T cells should be included in the development of a vaccine against MBS.

Induction of protective CD4+ T cell-mediated immunity by a Leishmania peptide delivered in recombinant influenza viruses

The available evidence suggests that protective immunity to Leishmania is achieved by priming the CD4⁺ Th1 response. Therefore, we utilised a reverse genetics strategy to generate influenza A viruses to deliver an immunogenic Leishmania peptide. The single, immunodominant Leishmania-specific LACK_158–173 CD4⁺ peptide was engineered into the neuraminidase stalk of H1N1 and H3N2 influenza A viruses. These recombinant viruses were used to vaccinate susceptible BALB/c mice to determine whether the resultant LACK_158–173-specific CD4⁺ T cell responses protected against live L. major infection. We show that vaccination with influenza-LACK_158–173 triggers LACK_158–173-specific Th1-biased CD4⁺ T cell responses within an appropriate cytokine milieu (IFN-γ, IL-12), essential for the magnitude and quality of the Th1 response. A single intraperitoneal exposure (non-replicative route of immunisation) to recombinant influenza delivers immunogenic peptides, leading to a marked reduction (2–4 log) in parasite burden, albeit without reduction in lesion size. This correlated with increased numbers of IFN-γ-producing CD4⁺ T cells in vaccinated mice compared to controls. Importantly, the subsequent prime-boost approach with a serologically distinct strain of influenza (H1N1->H3N2) expressing LACK_158–173 led to a marked reduction in both lesion size and parasite burdens in vaccination trials. This protection correlated with high levels of IFN-γ producing cells in the spleen, which were maintained for 6 weeks post-challenge indicating the longevity of this protective effector response. Thus, these experiments show that Leishmania-derived peptides delivered in the context of recombinant influenza viruses are immunogenic in vivo, and warrant investigation of similar vaccine strategies to generate parasite-specific immunity.

A review of malaria vaccine clinical projects based on the WHO rainbow table

Development and Phase 3 testing of the most advanced malaria vaccine, RTS,S/AS01, indicates that malaria vaccine R&D is moving into a new phase. Field trials of several research malaria vaccines have also confirmed that it is possible to impact the host-parasite relationship through vaccine-induced immune responses to multiple antigenic targets using different platforms. Other approaches have been appropriately tested but turned out to be disappointing after clinical evaluation.

As the malaria community considers the potential role of a first-generation malaria vaccine in malaria control efforts, it is an apposite time to carefully document terminated and ongoing malaria vaccine research projects so that lessons learned can be applied to increase the chances of success for second-generation malaria vaccines over the next 10 years.

The most comprehensive resource of malaria vaccine projects is a spreadsheet compiled by WHO thanks to the input from funding agencies, sponsors and investigators worldwide. This spreadsheet, available from WHO's website, is known as "the rainbow table". By summarizing the published and some unpublished information available for each project on the rainbow table, the most comprehensive review of malaria vaccine projects to be published in the last several years is provided below.

Leishmaniasis

Leishmaniasis is a disease that ranges in severity from skin lesions to serious disfigurement and fatal systemic infection. WHO has classified the disease as emerging and uncontrolled and estimates that the infection results in two million new cases a year. There are 12 million people currently infected worldwide, and leishmaniasis threatens 350 million people in 88 countries. Vaccination remains the best hope for control of all forms of the disease, and the development of a safe, effective and affordable antileishmanial vaccine is a critical global public-health priority. However, to date, no such vaccine is available despite substantial efforts by many laboratories. Main obstacle in vaccine design is the transition from the laboratory to the field and extrapolation of data from animal models to humans. This review discusses recent findings in the antileishmania vaccine field and current difficulties hampering vaccine implementation.

Quantitative and qualitative features of heterologous virus-vector-induced antigen-specific CD8+ T cells against Trypanosoma cruzi infection

We studied some aspects of the quantitative and qualitative features of heterologous recombinant (re) virus-vector-induced, antigen-specific CD8(+) T cells against Trypanosoma cruzi. We used three different, highly attenuated re-viruses, i.e., influenza virus, adenovirus and vaccinia virus, which all expressed a single, T. cruzi antigen-derived CD8(+) T-cell epitope. The use of two out of three vectors or the triple virus-vector vaccination regimen not only confirmed that the re-vaccinia virus, which was placed last in order for sequential immunisation, was an effective booster for the CD8(+) T-cell immunity in terms of the number of antigen-specific CD8(+) T cells, but also demonstrated that (i) the majority of cells exhibit the effector memory (T(EM)) phenotype, (ii) robustly secrete IFN-γ, (iii) express higher intensity of the CD122 molecule and (iv) present protective activity against T. cruzi infection. In contrast, placing the re-influenza virus last in sequential immunisation had a detrimental effect on the quantitative and qualitative features of CD8(+) T cells. The triple virus-vector vaccination was more effective at inducing a stronger CD8(+) T-cell immunity than using two re-viruses. The different quantitative and qualitative features of CD8(+) T cells induced by different immunisation regimens support the notion that the refinement of the best choice of multiple virus-vector combinations is indispensable for the induction of a maximum number of CD8(+) T cells of high quality.

MVA recombinants expressing the fusion and hemagglutinin genes of PPRV protects goats against virulent challenge

Peste des Petits Ruminants (PPR) is a highly contagious animal disease caused by the Peste des Petits Ruminants virus (PPRV) belonging to the genus morbillivirus and family Paramyxoviridae. The disease results in high morbidity and mortality in goats, sheep and in some small wild ruminants. The presence of large number of small ruminants reared in endemic areas makes PPR a notorious disease threatening the livelihood of poor farmers. Conventional vaccination using a live, attenuated vaccine gives adequate protection but cannot be used in case of eradication of the disease due to difficulty in differentiation of infected animals from the vaccinated ones.In the present study, we constructed two recombinant viruses using attenuated Modified Vaccinia virus Ankara virus (MVA) namely MVA-F and MVA-H expressing the full length PPRV fusion (F) and hemagglutinin (H) glycoproteins, respectively. Goats were vaccinated intramuscularly with 105 plaque forming units (PFU) each of the recombinant viruses and a live attenuated vaccine (RAKSHA PPR) and challenged 4 months later with PPRV challenge virus (10(3) goat LD(50)). All goats were completely protected from the clinical disease. This study gave an indication that mass vaccination of small ruminants with either of the above or both recombinant inexpensive virus vaccines could help in possible eradication of PPRV from endemic countries like India and subsequent seromonitoring of the disease for differentiation of infected animals from vaccinated ones.

Prime and boost immunization with influenza and adenovirus encoding the Toxoplasma gondii surface antigen 2 (SAG2) induces strong protective immunity

In this work, we explored an original vaccination protocol using recombinant influenza and adenovirus. We constructed recombinant influenza viruses harboring dicistronic NA segments containing the surface antigen 2 (SAG2) from Toxoplasma gondii under control of the duplicated 3' promoter. Recombinant influenza viruses were able to drive the expression of the foreign SAG2 sequence in cell culture and to replicate efficiently both in cell culture and in lungs of infected mice. In addition, mice primed with recombinant influenza virus and boosted with a recombinant adenovirus encoding SAG2 elicited both humoral and cellular immune responses specific for SAG2. Moreover, when immunized animals were challenged with the cystogenic P-Br strain of T. gondii, they displayed up to 85% of reduction in parasite burden. These results demonstrate the potential use of recombinant influenza vectors harboring the dicistronic segments in the development of vaccines against infectious diseases.

Prime-Boost Immunization of Codon Optimized HIV-1 CRF01_AE Gag in BCG with Recombinant Vaccinia Virus Elicits MHC Class I and II Immune Responses in Mice

The HIV-1 CRF01_AE gag gene was modified by codon restriction for Mycobacterium spp. and transformed into BCG; and it was designated as rBCG/codon optimized gagE. This produced 11 fold higher HIV-1 gag protein expression than the recombinant native gene rBCG/HIV-1gagE. In mice, CTL activity could be induced either by a single immunization of the codon optimized construct or by using it as a priming antigen in the prime-boost modality with recombinant Vaccinia virus expressing native HIV-1 gag. Specific secreted cytokine responses were also investigated. Only when rBCG gag was codon optimized did the prime-boost immunization produce significantly enhanced IFN-γ and IL-2 secretion indicating recognition via CD4+ and CD8+ T cells, and these responses seemed to be codon optimized immunogen dose-responsive. On contrary, the prime-boost vaccination using an equal amount of native rBCG/HIV-1gagE instead, or a single rBCG/codon optimized gagE immunization, had no similar effect on the cytokine secretion. These findings suggest that the use of recombinant codon BCG construct with recombinant Vaccinia virus encoding CRF01_AE gag as the prime-boost HIV vaccine candidate, will induce CD4+ Th1 and CD8+ T cell cytokine secretions in addition to enhancing CD8+ CTL response.

Characterization of recombinant influenza A virus as a vector for HIV-1 p17Gag

We have generated a recombinant influenza A virus with the HIV-1 p17(Gag) (rFlu-p17) gene inserted into the influenza virus neuraminidase (NA) gene. Expression of HIV-1 p17 protein was detected by conventional Western blot analysis and also by liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis of rFlu-p17 infected cells. The latter method does not depend on protein-specific antibody preparations and proved to be a powerful tool to detect proteins of interest. Next, antigen presentation of p17 expressed after infection of antigen-presenting cells was determined. Cloned p17-specific CD8+ T-cells were co-cultured with rFlu-p17 infected B-cells and produced IFN-gamma upon stimulation. Furthermore, we showed that immunization with rFlu-p17 elicited a humoral immune response in mice. This study shows that replication-deficient rFlu-p17 is antigenic in vitro and immunogenic in vivo and warrants further development as a candidate vaccine vector.

Recombinant Newcastle disease virus expressing human interleukin-2 serves as a potential candidate for tumor therapy

A recombinant Newcastle disease virus (NDV) containing human interleukin-2 (IL-2) gene was generated by applying reverse genetics technique and further evaluated for its suitability to express and deliver IL-2 for cancer therapy. We have further analyzed the ability of rNDV/IL2 to express IL-2 in several human tumor cell lines, including the human breast carcinoma cell line MCF-7, the human colon-adenocarcinoma cell line HT29, and human Jurkat cell line. IL-2 expressed by tumor cells infected with rNDV/IL-2 was stable up till 16 days, at body temperature, and with biological activity. Expression kinetics indicated that the expression level of IL-2 was already high at 24 h after infection and reached the highest level at 48 h after infection. As NDV was proposed as a very promising oncolytic agent in a new age of therapeutic viruses, our data strongly support the application of recombinant NDV/IL-2 virus as an anti-tumor agent.

Trypanosoma cruzi infection from the view of CD8+ T cell immunity--an infection model for developing T cell vaccine

Chagas' disease is caused by Trypanosoma cruzi (T. cruzi) which was once prevalent in Central and South America. Although the recent success in Triatoma vector control has made the disease being possibly "extinct" in the near future, the development of effective preventive and therapeutic vaccines is still necessary to prevent the resurgence of the neglected infection. In addition to the importance for containing the disease, T. cruzi infection presents unique features for elucidating hosts' immune responses against intracellular infectious agents. Due to its biological capacity for invading into principally any types of cells and for causing systemic infection which damages particularly muscle and neural cells, T cell immunity is critical for resolving its infection. Although T cell-mediated immune responses have been, so far, extensively investigated in viral and bacterial infections, parasitic infection such as malaria has presented epoch-making discovery in T cell immunity. Recent advances in the analyses of T cell-mediated immune responses against T. cruzi infection now make this infectious disease potentially more suitable for detecting subtle immunological changes in hosts' immune defense upon modifying immune system. The current review focuses on the usefulness of T. cruzi infection as a model for developing effective CD8(+) T cell-mediated vaccine against intracellular infectious agents.

Recombinant influenza virus vectors

Newly optimized reverse genetics techniques have allowed influenza researchers to generate recombinant influenza viruses expressing mutant viral proteins, as well as foreign proteins. Approaches include the insertion of noninfluenza epitopes and polypeptides into viral glycoproteins, foreign open reading frames as additional segments, and the fusion of independent proteins into viral genes encoding glycoproteins or the nonstructural protein 1. These genetically engineered viruses have been demonstrated to be good viral vectors for mounting B- and T-cell responses and are attractive candidates for vaccine development. As the molecular biology of influenza viral infection is more fully understood, influenza vectors can be concurrently manipulated to produce designed chimeric viruses, unveiling the possibility of a prosperous future with cheap, effective and safe vaccines against different human diseases.

Antigen-specific and non-specific CD4+ T cell recruitment and proliferation during influenza infection

To track epitope-specific CD4(+) T cells at a single-cell level during influenza infection, the MHC class II-restricted OVA(323-339) epitope was engineered into the neuraminidase stalk of influenza/A/WSN, creating a surrogate viral antigen. The recombinant virus, influenza A/WSN/OVA(II), replicated well, was cleared normally, and stimulated both wild-type and DO11.10 or OT-II TCR transgenic OVA-specific CD4(+) T cells. OVA-specific CD4 T cells proliferated during infection only when the OVA epitope was present. However, previously primed (but not naive) transgenic CD4(+) T cells were recruited to the infected lung both in the presence and absence of the OVA(323-339) epitope. These data show that, when primed, CD4(+) T cells may traffic to the lung in the absence of antigen, but do not proliferate. These results also document a useful tool for the study of CD4 T cells in influenza infection.

Malaria vaccine developments

Large gains in the reduction of malaria mortality in the early 20th century were lost in subsequent decades. Malaria now kills 2-3 million people yearly. Implementation of malaria control technologies such as insecticide-treated bednets and chemotherapy could reduce mortality substantially, but an effective malaria vaccine is also needed. Advances in vaccine technology and immunology are being used to develop malaria subunit vaccines. Novel approaches that might yield effective vaccines for other diseases are being evaluated first in malaria. We describe progress in malaria vaccine development in the past 5 years: reasons for cautious optimism, the type of vaccine that might realistically be expected, and how the process could be hastened. Although exact predictions are not possible, if sufficient funding were mobilised, a deployable, effective malaria vaccine is a realistic medium-term to long-term goal.

Toward novel vaccines and therapies based on negative-strand RNA viruses

The study of negative-strand RNA viruses has suggested new strategies to produce more attenuated viruses. Reverse genetics has allowed the implementation of the strategies, and new or improved monovalent vaccines are being developed. In addition, recombinant viruses expressing foreign proteins or epitopes have been produced with the aim of developing multivalent vaccines capable of stimulating humoral and cellular immune responses against more than one pathogen. Finally, recombinant viruses that selectively enter cells expressing tumor markers or the HIV envelope protein have been engineered and shown to lyse target cells. Preclinical and clinical trials of improved and multivalent vaccines and therapeutic (oncolytic) viruses are ongoing.

Induction of protective immunity against malaria by priming-boosting immunization with recombinant cold-adapted influenza and modified vaccinia Ankara viruses expressing a CD8+-T-cell epitope derived from the circumsporozoite protein of Plasmodium yoelii

We immunized mice with an attenuated (cold-adapted) influenza virus followed by an attenuated vaccinia virus (modified vaccinia virus Ankara), both expressing a CD8(+)-T-cell epitope derived from malaria sporozoites. This vaccination regimen elicited high levels of protection against malaria. This is the first time that the vaccine efficacy of a recombinant cold-adapted influenza virus vector expressing a foreign antigen has been evaluated.

Importance of CD8 T cell-mediated immune response during intracellular parasitic infections and its implications for the development of effective vaccines

Obligatory intracellular parasites such as Plasmodium sp, Trypanosoma cruzi, Toxoplasma gondii and Leishmania sp are responsible for the infection of hundreds of millions of individuals every year. These parasites can deliver antigens to the host cell cytoplasm that are presented through MHC class I molecules to protective CD8 T cells. The in vivo priming conditions of specific CD8 T cells during natural infection are largely unknown and remain as an area that has been poorly explored. The antiparasitic mechanisms mediated by CD8 T cells include both interferon-gamma-dependent and -independent pathways. The fact that CD8 T cells are potent inhibitors of parasitic development prompted many investigators to explore whether induction of these T cells can be a feasible strategy for the development of effective subunit vaccines against these parasitic diseases. Studies performed on experimental models supported the hypothesis that CD8 T cells induced by recombinant viral vectors or DNA vaccines could serve as the basis for human vaccination. Regimens of immunization consisting of two different vectors (heterologous prime-boost) are much more efficient in terms of expansion of protective CD8 T lymphocytes than immunization with a single vector. The results obtained using experimental models have led to clinical vaccination trials that are currently underway.

Activation of tumor antigen-specific cytotoxic T lymphocytes (CTLs) by human dendritic cells infected with an attenuated influenza A virus expressing a CTL epitope derived from the HER-2/neu proto-oncogene

The development of cancer vaccines requires approaches to induce expansion and functional differentiation of tumor antigen-specific cytotoxic T lymphocyte (CTL) effectors which posses cytolytic capability and produce cytokines. Efficient induction of such cells is hindered by the poor immunogenicity of tumor antigens and by the poor transduction efficiency of dendritic cells (DCs) with current nonreplicating vectors. We have investigated the use of influenza A virus, a potent viral inducer of CTLs, as a vector expressing the immunodominant HER-2 CTL epitope KIF (E75). For this purpose, an attenuated influenza A/PR8/34 virus with a truncated nonstructural (NS1) gene was generated containing the E75 epitope in its neuraminidase protein (KIF-NS virus). Stimulation of peripheral blood mononuclear cells from healthy donors and of tumor-associated lymphocytes from ovarian and breast cancer patients with DCs infected with KIF-NS virus (KIF-NS DC) induced CTLs that specifically recognized the peptide KIF and HER-2-expressing tumors in cytotoxicity assays and secreted gamma interferon (IFN-gamma) and interleukin-2 at recall with peptide. Priming with KIF-NS DCs increased the number of E75(+) CD45RO(+) cells by more than 10-fold compared to nonstimulated cells. In addition, KIF-NS virus induced high levels of IFN-alpha in DCs. This is the first report demonstrating induction of human epitope-specific CTLs against a tumor-associated antigen with a live attenuated recombinant influenza virus vector. Such vectors may provide a novel approach for tumor antigen delivery, lymphocyte activation, and differentiation in human cancer vaccine development.

Critical contribution of CD28-CD80/CD86 costimulatory pathway to protection from Trypanosoma cruzi infection

The CD28-CD80/CD86-mediated T-cell costimulatory pathway has been variably implicated in infectious immunity. In this study, we investigated the role of this costimulatory pathway in resistance to Trypanosoma cruzi infection by using CD28-deficient mice and blocking antibodies against CD80 and CD86. CD28-deficient mice exhibited markedly exacerbated T. cruzi infection, as evidenced by unrelenting parasitemia and 100% mortality after infection with doses that are nonlethal in wild-type mice. The blockade of both CD80 and CD86 by administering specific monoclonal antibodies also exacerbated T. cruzi infection in wild-type mice. Splenocytes from T. cruzi-infected, CD28-deficient mice exhibited greatly impaired gamma interferon production in response to T. cruzi antigen stimulation in vitro compared to those from infected wild-type mice. The induction of T. cruzi antigen-specific CD8(+) T cells was also impaired in T. cruzi-infected, CD28-deficient mice. In addition to these defects in natural protection against T. cruzi infection, CD28-deficient mice were also defective in the induction of CD8(+)-T-cell-mediated protective immunity against T. cruzi infection by DNA vaccination. These results demonstrate, for the first time, a critical contribution of the CD28-CD80/CD86 costimulatory pathway not only to natural protection against primary T. cruzi infection but also to DNA vaccine-induced protective immunity to Chagas' disease.

Activation of natural killer T cells by alpha-galactosylceramide impairs DNA vaccine-induced protective immunity against Trypanosoma cruzi

Innate immunity as a first defense is indispensable for host survival against infectious agents. We examined the roles of natural killer (NK) T cells in defense against Trypanosoma cruzi infection. The T. cruzi parasitemia and survival of CD1d-deficient mice exhibited no differences compared to wild-type littermates. NK T-cell activation induced by administering alpha-galactosylceramide (alpha-GalCer) to T. cruzi-infected mice significantly changed the parasitemia only in the late phase of infection and slightly improved survival when mice were infected intraperitoneally. The combined usage of alpha-GalCer and benznidazole, a commercially available drug for Chagas' disease, did not enhance the therapeutic efficacy of benznidazole. These results suggest that NK T cells do not play a pivotal role in resistance to T. cruzi infection. In addition, we found that the coadministration of alpha-GalCer with DNA vaccine impaired the induction of epitope-specific CD8(+) T cells and undermined the DNA vaccine-induced protective immunity against T. cruzi. Our results, in contrast to previous reports demonstrating the protective roles of NK T cells against other infectious agents, suggest that these cells might even exhibit adverse effects on vaccine-mediated protective immunity.

Coadministration of an interleukin-12 gene and a Trypanosoma cruzi gene improves vaccine efficacy

We tested the immunogenicity of two Trypanosoma cruzi antigens injected into mice in the form of DNA vaccine. Immunization with DNA encoding dihydroorotate dehydrogenase did not confer protective immunity in all mouse strains tested. Immunization with DNA encoding trans-sialidase surface antigen (TSSA) protected C57BL/6 (H-2(b)) mice but not BALB/c (H-2(d)) or C3H/Hej (H-2(k)) mice against lethal T. cruzi infection. In vivo depletion of CD4(+) or CD8(+) T cells abolished the protective immunity elicited by TSSA gene in C57BL/6 mice. Enzyme-linked immunospot assay with splenocytes from T. cruzi-infected mice or TSSA gene-vaccinated mice identified an H-2K(b)-restricted antigenic peptide, ANYNFTLV. The CD8(+)-T-cell line specific for this peptide could recognize T. cruzi-infected cells in vitro and could protect naive mice from lethal infection when adoptively transferred. Coadministration of the interleukin-12 (IL-12) gene with the TSSA gene facilitated the induction of ANYNFTLV-specific CD8(+) T cells and improved the vaccine efficacy against lethal T. cruzi infection. These results reinforced the utility of immunomodulatory adjuvants such as IL-12 gene for eliciting protective immunity against intracellular parasites by DNA vaccination.

Essential role of extrathymic T cells in protection against malaria

Athymic nude mice carry neither conventional T cells nor NKT cells of thymic origin. However, NK1.1(-)TCR(int) cells are present in the liver and other immune organs of athymic mice, because these lymphocyte subsets are truly of extrathymic origin. In this study, we examined whether extrathymic T cells had the capability to protect mice from malarial infection. Although B6-nu/nu mice were more sensitive to malaria than control B6 mice, these athymic mice were able to survive malaria when a reduced number of parasitized erythrocytes (5 x 10(3) per mouse) were injected. At the fulminant stage, lymphocytosis occurred in the liver and the major expanding lymphocytes were NK1.1(-)TCR(int) cells (IL-2Rbeta(+)TCRalphabeta(+)). Unconventional CD8(+) NKT cells (V(alpha)14(-)) also appeared. Similar to the case of B6 mice, autoantibodies (IgM type) against denatured DNA appeared during malarial infection. Immune lymphocytes isolated from the liver of athymic mice which had recovered from malaria were capable of protecting irradiated euthymic and athymic mice from malaria when cell transfer experiments were conducted. In conjunction with the previous results in euthymic mice, the present results in athymic mice suggest that the major lymphocyte subsets associated with protection against malaria might be extrathymic T cells.

Extrathymic T cells in malaria protection, including evidence for the onset of erythropoiesis in the liver during infection

This review proposes the possibility that malarial protection might not be achieved through the process of acquired immunity in which the constituents are conventional T and B (B-2) cells. On the other hand, malarial protection might be achieved by the process of innate immunity in which the constituents are extrathymic T cells and autoantibody-producing B-1 cells. Accordingly, mice infected with malaria exhibited severe thymic atrophy, and the expansion of IL-2Rbeta+ CD3int cells and its subset of NK1.1-CD3int cells were simultaneously induced. In parallel with the expansion of extrathymic T cells in the liver, extramedullary erythropoiesis was found to begin in the liver of these mice. Interestingly, malarial protozoa were primarily seen in only these nucleated erythrocytes in the liver at the early stage of infection. These results suggest that malaria immunology falls into a new field of immunology, namely, innate immunity. The similarity of the immune states among malaria, aging, and autoimmune diseases also suggest that the immunosuppression of a conventional, acquired immune system is more likely the common mechanism underlying these diseases or physiological responses.

Intranasal inoculation of a recombinant influenza virus containing exogenous nucleotides in the NS segment induces mucosal immune response against the exogenous gene product in mice

To establish a mucosal vaccine system, we evaluated the immunogenicity of a recombinant influenza virus, designated NS2Acat, in which the chloramphenicol acetyltransferase (CAT) reporter gene is fused in frame to the NS1 gene in the NS gene segment. The NS2Acat replicated slightly within the lungs in BALB/c mice after intranasal administration, accompanying expression of the CAT and the viral HA mRNA. NS2Acat induced delayed-type hypersensitivity (DTH) and sensitized the CAT-specific T lymphocytes in the regional lymph nodes, which proliferated and synthesized several cytokines in vitro upon antigenic challenge. These results suggest that NS2Acat is capable of eliciting a respiratory immune response against the fused gene product.

Surface expression of an immunodominant malaria protein B cell epitope by yellow fever virus

The yellow fever 17D virus (YF17D) has several characteristics that are desirable for the development of new, live attenuated vaccines. We approached its development as a vector for heterologous antigens by studying the expression of a humoral epitope at the surface of the E protein based on the results of modelling its three-dimensional structure. This model indicated that the most promising insertion site is between beta-strands f and g, a site that is exposed at the external surface of the virus. The large deletion of six residues from the fg loop of the E protein from yellow fever virus, compared to tick-born encephalitis virus, leaves space at the dimer interface for a large insertion without creating steric hindrance. We have tested this hypothesis by inserting a model humoral epitope from the circumsporozoite protein of Plasmodium falciparum consisting of triple NANP repeats. Recombinant virus (17D/8) expressing this insertion flanked by two glycine residues at each end, is specifically neutralized by a monoclonal antibody to the model epitope. Furthermore, mouse antibodies raised to the recombinant virus recognize the parasite protein in an ELISA assay. Serial passage analysis confirmed the genetic stability of the insertion made in the viral genome and the resulting 17D/8 virus is significantly more attenuated in mouse neurovirulence tests than the 17DD vaccine. The fg loop belongs to the dimerization domain of the E protein and lies at the interface between monomers. This domain undergoes a low pH transition, which is related to the fusion of the viral envelope to the endosome membrane. It is conceivable that a slower rate of fusion, resulting from the insertion close to the dimer interface, may delay the onset of virus production and thereby lead to a milder infection of the host. This would account for the more attenuated phenotype of the recombinant virus in the mouse model and lower extent of replication in cultured cells. The vectorial capacity of the yellow fever virus is being further explored for the expression and presentation of other epitopes, including those mediating T-cell responses.

A heterologous prime-boost regime using DNA and recombinant vaccinia virus expressing the Leishmania infantum P36/LACK antigen protects BALB/c mice from cutaneous leishmaniasis

A heterologous prime-boost vaccination with DNA vectors and vaccinia virus recombinants (VVr) has been shown to enhance specific cellular immune responses and to elicit significant protection against pathogens in animal models. In this study, we have analyzed, in the leishmaniasis cutaneous murine model, the effectiveness of this prime-boost strategy by immunizing with a DNA vector followed by boost with a VVr expressing the same Leishmania infantum P36/LACK antigen. After DNA priming and VVr boost, we challenged susceptible BALB/c mice with live L. major promastigotes, and examined the increase in footpad lesion size and parasite load in draining lymph nodes. Compared to controls, we observed reduction of up to 70% in lesion size and 1000-fold in parasite load. DNA prime-VVr boost before challenge elicited a Th1 type immune response in spleen cells from immunized animals. This DNA/VVr vaccination approach could be of utility in the prophylaxis against leishmaniasis.

Immunogenicity and protective efficacy of replication-incompetent influenza virus-like particles

Despite the success of influenza virus vaccines in reducing severe illness, their efficacy is suboptimal. We describe here the immunogenicity and protective capacity of replication-incompetent influenza virus-like particles (VLPs) which were generated entirely from cDNAs and lacked either the entire NS gene (encoding both the NS1 and NS2 protein) or only the NS2 gene. In mammalian cells infected with NS gene-deficient VLPs, the nucleoprotein, but not other viral proteins including hemagglutinin (HA) and neuraminidase (NA), was detected. In contrast, cells infected with VLPs expressing NS1 but not NS2 (NS2 knockout) expressed multiple viral proteins, including HA and NA. When challenged with lethal doses of an antigenically homologous mouse-adapted influenza virus, 94% of mice vaccinated with the NS2-knockout VLPs survived, compared with less than 10% of those given the NS-deficient VLPs. These results demonstrate the potential of replication-incompetent NS2-knockout VLPs as novel influenza vaccines and perhaps also as vectors to express genes from entirely unrelated pathogens.

Enhanced CD8+ T cell immune response against a V3 loop multi-epitope polypeptide (TAB13) of HIV-1 Env after priming with purified fusion protein and booster with modified vaccinia virus Ankara (MVA-TAB) recombinant: a comparison of humoral and cellular immune responses with the vaccinia virus Western Reserve (WR) vector

The humoral and cytotoxic T-lymphocyte (CTL) responses have been shown to be determinant in the clearance of many viral infections and because of those characteristics, vaccine candidates against AIDS are designed to enhance both arms of the immune system. While a protocol of immunization able to confer protection in humans against HIV will have to await the results of current clinical trials, it remains important to identify protocols of immunization in animals that achieve significant levels of humoral and cellular immune responses to HIV. In this study we have carried out a comparative analysis of the immune responses elicited in mice immunized with recombinants based on the modified vaccinia virus Ankara strain (rMVA) versus the Western Reserve strain (WR) of vaccinia virus (rVV), both expressing a V3 loop multi-epitopic protein from eight different HIV isolates (TAB13). We found that during priming, rMVA elicited a two- to three-fold higher specific CD8+ T cell response than rVV. Similar enhancement was observed during priming with purified protein TAB13 followed by a booster with rMVA. The epitopes LR150, MN and IIIB, located at the ends and in the middle of the chimeric protein, were able to induce a specific CD8+ T cell response, both after priming or prime/booster with the recombinant viruses but not after prime/booster with TAB13. By examining the cytokine pattern, the immune response triggered by these vectors was of Th-1 type. Humoral immune responses were higher in animals immunized with TAB13/TAB13 or TAB13/rVV than in animals immunized with TAB13/rMVA. These findings demonstrate that during priming or in a prime/booster immunizations, rMVA is superior to rVV in the ability to enhance specific cellular responses to an HIV-1 protein, and that both humoral and cellular immune responses to theV3 loop epitope of HIV-1 Env can be obtained by priming with TAB13 followed by a booster with viral vectors.

Peptide-based subunit vaccines against pre-erythrocytic stages of malaria parasites

Malaria currently ranks among the most prevalent infections in tropical and sub-tropical areas throughout the world with relatively high morbidity and mortality particularly in young children. 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 radiation-attenuated sporozoites can protect animals and humans against the disease, demonstrating the feasibility of developing an effective malaria vaccine. However, vaccines based on radiation-attenuated sporozoites are not feasible for large scale application due to lack of in vitro culture system. Therefore, the development of peptide-based subunit vaccines has been undertaken as an alternative approach. Synthetic peptides containing defined B- and T-cell epitopes of different antigens expressed in sporozoites and/or liver stages have been used as subunit vaccines in experimental animal models. They have been shown to be highly immunogenic and capable of inducing protective immunity mediated by antibodies, as well as CD4+ and CD8+ T-cells.

Hyperattenuated recombinant influenza A virus nonstructural-protein-encoding vectors induce human immunodeficiency virus type 1 Nef-specific systemic and mucosal immune responses in mice

We have generated recombinant influenza A viruses belonging to the H1N1 and H3N2 virus subtypes containing an insertion of the 137 C-terminal amino acid residues of the human immunodeficiency virus type 1 (HIV-1) Nef protein into the influenza A virus nonstructural-protein (NS1) reading frame. These viral vectors were found to be genetically stable and capable of growing efficiently in embryonated chicken eggs and tissue culture cells but did not replicate in the murine respiratory tract. Despite the hyperattenuated phenotype of influenza/NS-Nef viruses, a Nef and influenza virus (nucleoprotein)-specific CD8(+)-T-cell response was detected in spleens and the lymph nodes draining the respiratory tract after a single intranasal immunization of mice. Compared to the primary response, a marked enhancement of the CD8(+)-T-cell response was detected in the systemic and mucosal compartments, including mouse urogenital tracts, if mice were primed with the H1N1 subtype vector and subsequently boosted with the H3N2 subtype vector. In addition, Nef-specific serum IgG was detected in mice which were immunized twice with the recombinant H1N1 and then boosted with the recombinant H3N2 subtype virus. These findings may contribute to the development of alternative immunization strategies utilizing hyperattenuated live recombinant influenza virus vectors to prevent or control infectious diseases, e.g., HIV-1 infection.

Protective immune response against cutaneous leishmaniasis by prime/booster immunization regimens with vaccinia virus recombinants expressing Leishmania infantum p36/LACK and IL-12 in combination with purified p36

In susceptible mice Leishmania infection triggers a CD4(+) Th2 response that has been correlated with evasion of the host immune system. To develop approaches that might trigger a Th1 response leading to protection against Leishmania we generated vaccinia virus recombinants (VVr) expressing the relevant p36/LACK protein of Leishmania infantum (VVp36) or co-expressing p36/LACK and interleukin-12 (VVp36IL12). Susceptible BALB/c mice were immunized with the VVr in various prime/booster protocols that included purified p36/LACK protein, followed 3 weeks later by a challenge with live L. major promastigotes. The course of the infection was monitored by measuring lesion development, parasite load and immunological parameters (IFN-gamma and IL-10 secretion by in vitro-stimulated lymphocytes, and specific IgG isotypes), before and after challenge. We found protocols of prime/booster immunization (VVp36/VVp36; VVp36IL12/p36; p36/VVp36IL12) that elicited different levels of protection in infected animals. The protocol of priming with purified p36 followed by a booster with VVp36IL12 induced 52% reduction in lesion size and a two-log unit reduction in parasite load. This partial protection correlated with activation of a specific Th1 type of immune response. These protocols could be of interest in the prophylaxis against Leishmania spp. and other parasitic diseases.

ELISPOT assay to measure antigen-specific murine CD8(+) T cell responses

The enzyme-linked immunospot technique (ELISPOT) relies on the visualization of cytokine secretion by individual T cells following in vitro stimulation with antigen. This assay has been developed and standardized for the quantitative detection of antigen-specific CD8(+) T cells in mice subjected to different immunization protocols [J. Immunol. Methods 181 (1995) 45]. We have identified important variables that affect the efficacy of the ELISPOT assay and in this protocol we describe this methodology in detail. As a model, we used the production of interferon-gamma by CD8(+) T cells from peripheral blood, spleen and liver of mice immunized with malaria sporozoites expressing the H-2K(d)-restricted SYVPSAEQI. This protocol has also been used successfully to detect Th1 and Th2 epitope specific CD4(+) T cells.

DNA-based vaccines against malaria: status and promise of the Multi-Stage Malaria DNA Vaccine Operation

The introduction of DNA vaccine technology has facilitated an unprecedented multi-antigen approach to developing an effective vaccine against complex pathogens such as the Plasmodium spp. parasites that cause malaria. We have established the capacity of DNA vaccines encoding Plasmodium antigens to induce CD8(+) cytotoxic T lymphocyte and interferon-gamma responses in mice, monkeys and humans. However, like others, we have found that the first or second generation DNA vaccines on their own are not optimal, and have demonstrated the potential of heterologous prime/boost immunisation strategies involving priming with DNA and boosting with poxvirus or recombinant protein in adjuvant. In this review, we summarise the current status and promise of our programmatic efforts to develop a DNA-based vaccine against malaria, our Multi-Stage Malaria DNA Vaccine Operation, and illustrate the transition of promising developments in the laboratory to clinical assessment in humans.