Human CD4+ T cells induced by synthetic peptide malaria vaccine are comparable to cells elicited by attenuated Plasmodium falciparum sporozoites

Immunogenicity, Efficacy, and Safety of a Novel Synthetic Microparticle Pre-Erythrocytic Malaria Vaccine in Multiple Host Species

We previously reported a protective antibody response in mice immunized with synthetic microparticle vaccines made using layer-by-layer fabrication (LbL-MP) and containing the conserved T1BT* epitopes from the P. falciparum circumsporozoite protein. To further optimize the vaccine candidate, a benchtop tangential flow filtration method (LbL-by-TFF) was developed and utilized to produce vaccine candidates that differed in the status of base layer crosslinking, inclusion of a TLR2 ligand in the antigenic peptide, and substitution of serine or alanine for an unpaired cysteine residue in the T* epitope. Studies in mice revealed consistent superiority of the Pam3Cys-modified candidates and a modest benefit of base layer crosslinking, as evidenced by higher and more persistent antibody titers (up to 18 months post-immunization), a qualitative improvement of T-cell responses toward a Th1 phenotype, and greater protection from live parasite challenges compared to the unmodified prototype candidate. Immunogenicity was also tested in a non-human primate model, the rhesus macaque. Base layer-crosslinked LbL-MP loaded with T1BT* peptide with or without covalently linked Pam3Cys elicited T1B-specific antibody responses and T1BT*-specific T-cell responses dominated by IFNγ secretion with lower levels of IL-5 secretion. The Pam3Cys-modified construct was more potent, generating antibody responses that neutralized wild-type P. falciparum in an in vitro hepatocyte invasion assay. IgG purified from individual macaques immunized with Pam3Cys.T1BT* LbL-MP protected naïve mice from challenges with transgenic P. berghei sporozoites that expressed the full-length PfCS protein, with 50-88% of passively immunized mice parasite-free for ≥15 days. Substitution of serine for an unpaired cysteine in the T* region of the T1BT* subunit did not adversely impact immune potency in the mouse while simplifying the manufacture of the antigenic peptide. In a Good Laboratory Practices compliant rabbit toxicology study, the base layer-crosslinked, Pam3Cys-modified, serine-substituted candidate was shown to be safe and immunogenic, eliciting parasite-neutralizing antibody responses and establishing the dose/route/regimen for a clinical evaluation of this novel synthetic microparticle pre-erythrocytic malaria vaccine candidate.

An unstable Th epitope of P. falciparum fosters central memory T cells and anti-CS antibody responses

Malaria is transmitted by Plasmodium-infected anopheles mosquitoes. Widespread resistance of mosquitoes to insecticides and resistance of parasites to drugs highlight the urgent need for malaria vaccines. The most advanced malaria vaccines target sporozoites, the infective form of the parasite. A major target of the antibody response to sporozoites are the repeat epitopes of the circumsporozoite (CS) protein, which span almost one half of the protein. Antibodies to these repeats can neutralize sporozoite infectivity. Generation of protective antibody responses to the CS protein (anti-CS Ab) requires help by CD4 T cells. A CD4 T cell epitope from the CS protein designated T* was previously identified by screening T cells from volunteers immunized with irradiated P. falciparum sporozoites. The T* sequence spans twenty amino acids that contains multiple T cell epitopes restricted by various HLA alleles. Subunit malaria vaccines including T* are highly immunogenic in rodents, non-human primates and humans. In this study we characterized a highly conserved HLA-DRβ1*04:01 (DR4) restricted T cell epitope (QNT-5) located at the C-terminus of T*. We found that a peptide containing QNT-5 was able to elicit long-term anti-CS Ab responses and prime CD4 T cells in HLA-DR4 transgenic mice despite forming relatively unstable MHC-peptide complexes highly susceptible to HLA-DM editing. We attempted to improve the immunogenicity of QNT-5 by replacing the P1 anchor position with an optimal tyrosine residue. The modified peptide QNT-Y formed stable MHC-peptide complexes highly resistant to HLA-DM editing. Contrary to expectations, a linear peptide containing QNT-Y elicited almost 10-fold lower long-term antibody and IFN-γ responses compared to the linear peptide containing the wild type QNT-5 sequence. Some possibilities regarding why QNT-5 is more effective than QNT-Y in inducing long-term T cell and anti-CS Ab when used as vaccine are discussed.

CD8+ T Cell Responses to Plasmodium and Intracellular Parasites

Parasitic protozoa are major threats to human health affecting millions of people around the world. Control of these infections by the host immune system relies on a myriad of immunological mechanisms that includes both humoral and cellular immunity. CD8⁺ T cells contribute to the control of these parasitic infections in both animals and humans. Here, we will focus on the CD8⁺ T cell response against a subset of these protozoa: Plasmodium, Toxoplasma gondii, Leishmania and Trypanosoma cruzi, with an emphasis on experimental rodent systems. It is evident a complex interaction occurs between CD8⁺ T cells and the invading protozoa. A detailed understanding of how CD8⁺ T cells mediate protection should provide the basis for the development of effective vaccines that prevent and control infections by these parasites.

Natural acquisition of immunity to Plasmodium vivax: epidemiological observations and potential targets

Population studies show that individuals acquire immunity to Plasmodium vivax more quickly than Plasmodium falciparum irrespective of overall transmission intensity, resulting in the peak burden of P. vivax malaria in younger age groups. Similarly, actively induced P. vivax infections in malaria therapy patients resulted in faster and generally more strain-transcending acquisition of immunity than P. falciparum infections. The mechanisms behind the more rapid acquisition of immunity to P. vivax are poorly understood. Natural acquired immune responses to P. vivax target both pre-erythrocytic and blood-stage antigens and include humoral and cellular components. To date, only a few studies have investigated the association of these immune responses with protection, with most studies focussing on a few merozoite antigens (such as the Pv Duffy binding protein (PvDBP), the Pv reticulocyte binding proteins (PvRBPs), or the Pv merozoite surface proteins (PvMSP1, 3 & 9)) or the circumsporozoite protein (PvCSP). Naturally acquired transmission-blocking (TB) immunity (TBI) was also found in several populations. Although limited, these data support the premise that developing a multi-stage P. vivax vaccine may be feasible and is worth pursuing.

T1BT* structural study of an anti-plasmodial peptide through NMR and molecular dynamics

Background

T1BT* is a peptide construct containing the T1 and B epitopes located in the 5’ minor repeat and the 3’ major repeat of the central repeat region of the Plasmodium falciparum circumsporozoite protein (CSP), respectively, and the universal T* epitope located in the C-terminus of the same protein. This peptide construct, with B = (NANP)₃, has been found to elicit antisporozoite antibodies and gamma-interferon-screening T-cell responses in inbred strains of mice and in outbred nonhuman primates. On the other hand, NMR and CD spectroscopies have identified the peptide B’ = (NPNA)₃ as the structural unit of the major repeat in the CSP, rather than the more commonly quoted NANP. With the goal of assessing the structural impact of the NPNA cadence on a proven anti-plasmodial peptide, the solution structures of T1BT* and T1B’T* were determined in this work.

Methods

NMR spectroscopy and molecular dynamics calculations were used to determine the solution structures of T1BT* and T1B’T*. These structures were compared to determine the main differences and similarities between them.

Results

Both peptides exhibit radically different structures, with the T1B’T* showing strong helical tendencies. NMR and CD data, in conjunction with molecular modelling, provide additional information about the topologies of T1BT* and T1B’T*. Knowing the peptide structures required to elicit the proper immunogenic response can help in the design of more effective, conformationally defined malaria vaccine candidates. If peptides derived from the CSP are required to have helical structures to interact efficiently with their corresponding antibodies, a vaccine based on the T1B’T* construct should show higher efficiency as a pre-erythrocyte vaccine that would prevent infection of hepatocytes by sporozoites.

Plasmodium falciparum synthetic LbL microparticle vaccine elicits protective neutralizing antibody and parasite-specific cellular immune responses

Epitopes of the circumsporozoite (CS) protein of Plasmodium falciparum, the most pathogenic species of the malaria parasite, have been shown to elicit protective immunity in experimental animals and human volunteers. The mechanisms of immunity include parasite-neutralizing antibodies that can inhibit parasite motility in the skin at the site of infection and in the bloodstream during transit to the hepatocyte host cell and also block interaction with host cell receptors on hepatocytes. In addition, specific CD4+ and CD8+ cellular mechanisms target the intracellular hepatic forms, thus preventing release of erythrocytic stage parasites from the infected hepatocyte and the ensuing blood stage cycle responsible for clinical disease. An innovative method for producing particle vaccines, layer-by-layer (LbL) fabrication of polypeptide films on solid CaCO3 cores, was used to produce synthetic malaria vaccines containing a tri-epitope CS peptide T1BT comprising the antibody epitope of the CS repeat region (B) and two T-cell epitopes, the highly conserved T1 epitope and the universal epitope T. Mice immunized with microparticles loaded with T1BT peptide developed parasite-neutralizing antibodies and malaria-specific T-cell responses including cytotoxic effector T-cells. Protection from liver stage infection following challenge with live sporozoites from infected mosquitoes correlated with neutralizing antibody levels. Although some immunized mice with low or undetectable neutralizing antibodies were also protected, depletion of T-cells prior to challenge resulted in the majority of mice remaining resistant to challenge. In addition, mice immunized with microparticles bearing only T-cell epitopes were not protected, demonstrating that cellular immunity alone was not sufficient for protective immunity. Although the microparticles without adjuvant were immunogenic and protective, a simple modification with the lipopeptide TLR2 agonist Pam3Cys increased the potency and efficacy of the LbL vaccine candidate. This study demonstrates the potential of LbL particles as promising malaria vaccine candidates using the T1BT epitopes from the P. falciparum CS protein.

Linear and cyclic dipeptides with antimalarial activity

Several natural and synthetic polypeptides possess important antimalarial activity. Shorter peptides with potent antimalarial activity have also been described, among them linear di-, tri-, tetra- and pentapeptides and their cyclic analogs. In an attempt to find dipeptides with antimalarial activities we show that linear and cyclic dipeptides, the latter known as diketopiperazines, still retain the fundamental core to preserve antimalarial activity. Thirteen linear dipeptides and ten diketopiperazines were investigated. Eight linear dipeptides showed IC(50) values between 2.78 and 7.07 μM, while eight diketopiperazines were also active with IC(50) values between 2.26 and 4.26 μM on Plasmodium berghei schizont cultures.

Immunization with the RTS,S/AS malaria vaccine induces IFN-γ(+)CD4 T cells that recognize only discrete regions of the circumsporozoite protein and these specificities are maintained following booster immunizations and challenge

In a Phase 2a trial of the RTS,S/AS vaccine, we described significant association between protection against infection and vaccine-induced CD4 T cells. To determine whether processing of the circumsporozoite protein as a component of the RTS,S particulate antigen yields the same HLA-DR-restricted epitopes as those recognized by CD4 T cells from donors immunized by exposure to attenuated or infectious sporozoites we mapped the specificities of the RTS,S primed CD4 T cells by measuring IFN-γ cultured Elispot responses to pairs of overlapping 15 a.a. peptides that span the protein's C-terminus. Peptide pairs representing the previously described TH2R, T* and CS.T3 epitopes, were immunoprevalent and immunodominant. There was no response to the peptides corresponding to the human thrombospondin homology region. Responses to the CD4 T cell epitopes were restricted by multiple HLA-DR haplotypes. Of note, HLA-DR4 and HLA-DR11 restricted epitopes in the T* region and in the location on the CS protein defined by peptide pair 4, respectively. We conclude that processing of the CS protein derived from the RTS,S antigen leads to the generation of HLA-DR-restricted epitopes that are similar to those identified previously using CD4 T cells from subjects immunized with and protected by attenuated sporozoites or exposed to infectious sporozoites. This may in part account for the protective efficacy of the RTS,S/AS vaccine.

Protective immunity to pre-erythrocytic stage malaria

The development of a vaccine against malaria is a major research priority given the burden of disease, death and economic loss inflicted upon the tropical world by this parasite. Despite decades of effort, however, a vaccine remains elusive. The best candidate is a subunit vaccine termed RTS,S but this provides only partial protection against clinical disease. This review examines what is known about protective immunity against pre-erythrocytic stage malaria by considering the humoral and T cell-mediated immune responses that are induced by attenuated sporozoites and by the RTS,S vaccine. On the basis of these observations a set of research priorities are defined that are crucial for the development of a vaccine capable of inducing long-lasting and high-grade protection against malaria.

Why functional pre-erythrocytic and bloodstage malaria vaccines fail: a meta-analysis of fully protective immunizations and novel immunological model

BACKGROUND

Clinically protective malaria vaccines consistently fail to protect adults and children in endemic settings, and at best only partially protect infants.

METHODOLOGY/PRINCIPAL FINDINGS

We identify and evaluate 1916 immunization studies between 1965-February 2010, and exclude partially or nonprotective results to find 177 completely protective immunization experiments. Detailed reexamination reveals an unexpectedly mundane basis for selective vaccine failure: live malaria parasites in the skin inhibit vaccine function. We next show published molecular and cellular data support a testable, novel model where parasite-host interactions in the skin induce malaria-specific regulatory T cells, and subvert early antigen-specific immunity to parasite-specific immunotolerance. This ensures infection and tolerance to reinfection. Exposure to Plasmodium-infected mosquito bites therefore systematically triggers immunosuppression of endemic vaccine-elicited responses. The extensive vaccine trial data solidly substantiate this model experimentally.

CONCLUSIONS/SIGNIFICANCE

We conclude skinstage-initiated immunosuppression, unassociated with bloodstage parasites, systematically blocks vaccine function in the field. Our model exposes novel molecular and procedural strategies to significantly and quickly increase protective efficacy in both pipeline and currently ineffective malaria vaccines, and forces fundamental reassessment of central precepts determining vaccine development. This has major implications for accelerated local eliminations of malaria, and significantly increases potential for eradication.

Antimalarial activity of ultra-short peptides

Ultra-short peptides 1-9 were designed and synthesized with phenylalanine, ornithine and proline amino acid residues and their effect on antimalarial activity was analyzed. On the basis of the IC₅₀ data for these compounds, the effects of nature, polarity, and amino acid sequence on Plasmodium berghei schizont cultures were analyzed too. Tetrapeptides Phe-Orn-Phe-Orn (4) and Lys-Phe-Phe-Orn (5) showed a very important activity with IC₅₀ values of 3.31 and 2.57 μM, respectively. These two tetrapeptides are candidates for subsequent in vivo assays and SARS investigations.

A retrospective evaluation of the role of T cells in the development of malaria vaccine

Due to the fact that the life cycle of malaria parasites is complex, undergoing both an extracellular and intracellular phases in its host, the human immune system has to mobilize both the humoral and cellular arms of immune responses to fight against this parasitic infection. Whereas humoral immunity is directed toward the extracellular stages which include sporozoites and merozoites, cell-mediated immunity (CMI), in which T cells play a major role, targets hepatic stages - liver stages - of the parasites. In this review, the role of T cells in protective immunity against liver stages of the malaria infection is being re-evaluated. Furthermore, this review intends to address how to translate the findings regarding the role of T cells obtained in experimental systems to actual development of malaria vaccine for humans.

A novel peptide mimotope identified as a potential immunosuppressive vaccine for organ transplantation

We reported that anti-histone H1 autoantibody is one of the main immunosuppressive factors in serum that is induced after orthotopic liver transplantation in a rat tolerogenic model. We generated a novel anti-histone H1 IgM mAb produced by hybridoma 16G9 (16G9 mAb) that shows MLR-inhibitory activity. Identification of a functional epitope responsible for the immunosuppressive activity of 16G9 mAb may lead to the establishment of a novel therapeutic strategy. We used a combinatorial phage display peptide library to screen for peptides that bind to 16G9 mAb. Consequently, two peptides that bind to 16G9 mAb, SSV and LPQ, were selected from the library. The binding of 16G9 mAb to histone H1 was inhibited by SSV. SSV was recognized by rat tolerogenic post-orthotopic liver transplantation serum and the binding to SSV was inhibited by histone H1. Mice were immunized with keyhole limpet hemocyanin-conjugated SSV and LPQ. Abs induced by SSV immunization inhibited Con A-stimulated splenocyte proliferation, and the inhibition was neutralized by preincubation with SSV. Splenocytes stimulated by anti-CD3 Ab were inhibited by SSV-induced Abs using CFSE labeling. SSV immunization in rats before heterotopic heart transplantation resulted in significant prolonged allograft survival. These findings suggested that SSV is a functional histone H1-binding epitope for 16G9 mAb. SSV is capable of determining serum immunoreactivity against histone H1 as an index marker for tolerance. The inhibitory activity of SSV-induced Abs on blast cell proliferation and the prolonged graft survival that results from SSV immunization imply a potential for the development of an immunosuppressive vaccine.

Enhanced immunogenicity of Plasmodium falciparum peptide vaccines using a topical adjuvant containing a potent synthetic Toll-like receptor 7 agonist, imiquimod

Plasmodium sporozoites injected into the skin by malaria-infected mosquitoes can be effectively targeted by antibodies that block parasite invasion of host hepatocytes and thus prevent the subsequent development of blood stage infections responsible for clinical disease. Malaria subunit vaccines require potent adjuvants, as they lack known pathogen-associated molecular patterns found in attenuated viral or bacterial vaccines that function as Toll-like receptor (TLR) agonists to stimulate dendritic cells and initiate strong adaptive immune responses. A synthetic TLR7 agonist, imiquimod, which is FDA approved for topical treatment of various skin conditions, can function as a potent adjuvant for eliciting T-cell responses to intracellular pathogens and model protein antigens. In the current studies, the topical application of imiquimod at the site of subcutaneously injected Plasmodium falciparum circumsporozoite (CS) peptides elicited strong parasite-specific humoral immunity that protected against challenge with transgenic rodent parasites that express P. falciparum CS repeats. In addition, injection of a simple linear peptide followed by topical imiquimod elicited strong Th1 CD4(+) T-cell responses, as well as high antibody titers. The correlation of high anti-repeat antibody titers with resistance to sporozoite challenge in vivo and in vitro supports use of this topical TLR7 agonist adjuvant to elicit protective humoral immunity. The safety, simplicity, and economic advantages of a topical synthetic TLR7 agonist adjuvant also apply to other vaccines requiring high antibody titers, such as malaria asexual or sexual blood stage antigens to prevent red blood cell invasion and block transmission to the mosquito vector, and to vaccines to other extracellular pathogens.

Imaging effector functions of human cytotoxic CD4+ T cells specific for Plasmodium falciparum circumsporozoite protein

Malaria vaccines, comprised of irradiated Plasmodium falciparum sporozoites or a synthetic peptide containing T and B cell epitopes of the circumsporozoite protein (CSP), elicit multifunctional cytotoxic and non-cytotoxic CD4(+) T cells in immunised volunteers. Both lytic and non-lytic CD4(+)T cell clones recognised a series of overlapping epitopes within a 'universal' T cell epitope EYLNKIQNSLSTEWSPCSVT of CSP (NF54 isolate) that was presented in the context of multiple DR molecules. Lytic activity directly correlated with T cell receptor (TCR) functional avidity as measured by stimulation indices and recognition of naturally occurring variant peptides. CD4(+) T cell-mediated cytotoxicity was contact-dependent and did not require de novo synthesis of cytotoxic mediators, suggesting a granule-mediated mechanism. Live cell imaging of the interaction of effector and target cells demonstrated that CD4(+) cytotoxic T cells recognise target cells with their leading edge, reorient their cytotoxic granules towards the zone of contact, and form a stable immunological synapse. CTL attacks induced chromatin condensation, nuclear fragmentation and formation of apoptotic bodies in target cells. Together, these findings suggest that CD4(+) CTLs trigger target cell apoptosis via classical perforin/granzyme-mediated cytotoxicity, similar to CD8(+) CTLs, and these multifunctional sporozoite- and peptide-induced CD4(+) T cells have the potential to play a direct role as effector cells in targeting the exoerythrocytic forms within the liver.

Cellular effector mechanisms against Plasmodium liver stages

Advances in our understanding of the molecular and cell biology of the malaria parasite have led to new vaccine development efforts resulting in a pipeline of over 40 candidates undergoing clinical phase I-III trials. Vaccine-induced CD4+ and CD8+ T cells specific for pre-erythrocytic stage antigens have been found to express cytolytic and multi-cytokine effector functions that support a key role for these T cells within the hepatic environment. However, little is known of the cellular interactions that occur during the effector phase in which the intracellular hepatic stage of the parasite is targeted and destroyed. This review focuses on cell biological aspects of the interaction between malaria-specific effector cells and the various antigen-presenting cells that are known to exist within the liver, including hepatocytes, dendritic cells, Kupffer cells, stellate cells and sinusoidal endothelia. Considering the unique immune properties of the liver, it is conceivable that these different hepatic antigen-presenting cells fulfil distinct but complementary roles during the effector phase against Plasmodium liver stages.

Plasmodium falciparum malaria vaccines in development

The development and implementation of a malaria vaccine would constitute a major breakthrough for global health. Recently, numerous new candidates have entered clinical testing, following strategies that are as diverse as the malaria cycle is complex. While promising results have been obtained, some candidate vaccines have not fulfilled expectations. The challenges are not merely scientific; further progresses will require the development of competent investigator networks, partnerships between academics, industry and funding agencies, and continuous political commitment. In this review, we present the developmental status of all malaria vaccine candidates that are currently in human clinical testing against Plasmodium falciparum, as well as selected malaria vaccine candidates at preclinical development stage, and discuss the main challenges facing the field of malaria vaccine development.

Class II-restricted protective immunity induced by malaria sporozoites

The irradiated-sporozoite vaccine elicits sterile immunity against Plasmodium parasites in experimental rodent hosts and human volunteers. Based on rodent malaria models, it has been proposed that CD8+ T cells are the key protective effector mechanism required in sporozoite-induced immunity. To investigate the role of class II-restricted immunity in protective immunity, we immunized beta2-microglobulin knockout (beta2M-/-) mice with irradiated Plasmodium yoelii or P. berghei sporozoites. Sterile immunity was obtained in the CD8+-T-cell-deficient mice immunized with either P. berghei or P. yoelii sporozoites. beta2M-/- mice with the BALB/c (H-2d) genetic background as well as those with the C57BL (H-2b) genetic background were protected. Effector mechanisms included CD4+ T cells, mediated in part through the production of gamma interferon, and neutralizing antibodies that targeted the extracellular sporozoites. We conclude that in the absence of class I-restricted CD8+ T cells, sporozoite-induced protective immunity can be effectively mediated by class II-restricted immune effector mechanisms. These results support efforts to develop subunit vaccines that effectively elicit high levels of antibody and CD4+ T cells to target Plasmodium pre-erythrocytic stages.

Malaria protection in beta 2-microglobulin-deficient mice lacking major histocompatibility complex class I antigens: essential role of innate immunity, including gammadelta T cells

It is still controversial whether malaria protection is mediated by conventional immunity associated with T and B cells or by innate immunity associated with extrathymic T cells and autoantibody-producing B cells. Given this situation, it is important to examine the mechanism of malaria protection in beta(2)-microglobulin-deficient (beta(2)m(-/-)) mice. These mice lack major histocompatibility complex class I and CD1d antigens, which results in the absence of CD8(+) T cells and natural killer T (NKT) cells. When C57BL/6 and beta(2)m(-/-) mice were injected with parasitized (Plasmodium yoelii 17XNL) erythrocytes, both survived from the infection and showed a similar level of parasitaemia. The major expanding T cells were NK1.1(-) alphabeta T-cell receptor(int) cells in both mice. The difference was a compensatory expansion of NK and gammadelta T cells in beta(2)m(-/-) mice, and an elimination experiment showed that these lymphocytes were critical for protection in these mice. These results suggest that malaria protection might be events of the innate immunity associated with multiple subsets with autoreactivity. CD8(+) T and NKT cells may be partially related to this protection.

Viral peptide immunogens: current challenges and opportunities

Synthetic peptide vaccines have potential to control viral infections. Successful experimental models using this approach include the protection of mice against the lethal Sendai virus infection by MHC class I binding CTL peptide epitope. The main benefit of vaccination with peptide epitopes is the ability to minimize the amount and complexity of a well-defined antigen. An appropriate peptide immunogen would also decrease the chance of stimulating a response against self-antigens, thereby providing a safer vaccine by avoiding autoimmunity. In general, the peptide vaccine strategy needs to dissect the specificity of antigen processing, the presence of B-and T-cell epitopes and the MHC restriction of the T-cell responses. This article briefly reviews the implications in the design of peptide vaccines and discusses the various approaches that are applied to improve their immunogenicity.

A linear peptide containing minimal T- and B-cell epitopes of Plasmodium falciparum circumsporozoite protein elicits protection against transgenic sporozoite challenge

An effective malaria vaccine is needed to address the public health tragedy resulting from the high levels of morbidity and mortality caused by Plasmodium parasites. The first protective immune mechanism identified in the irradiated sporozoite vaccine, the "gold standard" for malaria preerythrocytic vaccines, was sporozoite-neutralizing antibody specific for the repeat region of the surface circumsporozoite (CS) protein. Previous phase I studies demonstrated that a branched peptide containing minimal T- and B-cell epitopes of Plasmodium falciparum CS protein elicited antirepeat antibody and CD4(+)-T-cell responses comparable to those observed in volunteers immunized with irradiated P. falciparum sporozoites. The current study compares the immunogenicity of linear versus tetrabranched peptides containing the same minimal T- and B-cell epitopes, T1BT*, comprised of a CS-derived universal Th epitope (T*) synthesized in tandem with the T1 and B repeats of P. falciparum CS protein. A simple 48-mer linear synthetic peptide was found to elicit antisporozoite antibody and gamma interferon-secreting T-cell responses comparable to the more complex tetrabranched peptides in inbred strains of mice. The linear peptide was also immunogenic in outbred nonhuman primates (Aotus nancymaae), eliciting antibody titers equivalent to those induced by tetrabranched peptides. Importantly, the 48-mer linear peptide administered in adjuvants suitable for human use elicited antibody-mediated protection against challenge with rodent malaria transgenic sporozoites expressing P. falciparum CS repeats. These findings support further evaluation of linear peptides as economical, safe, and readily produced malaria vaccines for the one-third of the world's population at risk of malaria infection.

Differential evidence of natural selection on two leading sporozoite stage malaria vaccine candidate antigens

Experimental malaria vaccines based on two sporozoite stage candidate antigens of Plasmodium falciparum, the circumsporozoite protein (CSP) and thrombospondin-related adhesive protein (TRAP), have undergone clinical trials of efficacy. The relevance of naturally existing polymorphism in these molecules remains unknown. Sequence polymorphism in the genes encoding these antigens was studied in a Gambian population (sample of 48 trap and 44 csp gene sequences) to test for signatures of selection that would result from naturally acquired immunity. Allele frequency distributions were analyzed and compared with data from another population (in Thailand). Patterns of non-synonymous and synonymous polymorphism in P. falciparum and in Plasmodium vivax were compared with divergence from related species. Results indicate that polymorphism in TRAP is under strong selection for amino acid sequence diversity and that allele frequencies are under balancing selection within the Gambian P. falciparum population. There was no such evidence for CSP, calling into question the idea that most polymorphisms in this gene are under immune selection. There was a weak trend for regions known to encode T cell epitopes to have slightly higher indices suggesting balancing selection. Overall, the results predict more allele-specific immunity to TRAP than to CSP and should be considered in design and efficacy testing of vaccine candidates based on these antigens.

Vaccine development against malaria

Malaria vaccine development aims to significantly reduce mortality and morbidity in the two high-risk groups: young children and pregnant women in sub-Saharan Africa. A pre-erythrocytic subunit vaccine, RTS,S, was recently evaluated in a first Phase IIb clinical trial in young children that live in a mesoendemic area of malaria transmission. The identification of a major parasite-encoded ligand that was found to be involved in pregnancy-associated malaria allows for the rational development of a tailor-made subunit vaccine for young women. The generation of transgenic yellow fever viruses and defined attenuated vaccine lines by reverse genetics are complementary approaches that might further boost the development of safe vaccines that elicit protective immune responses in this population.

Advances in vaccine development against the pre-erythrocytic stage of Plasmodium falciparum malaria

With approximately 2.4 billion people at risk, Plasmodium falciparum malaria infection caused by an infectious bite of an Anopheles mosquito continues to be a major cause of mortality and morbidity, mainly in the tropics and subtropics. Measures to control the mosquito vector on a broad scale are expensive and need to be maintained continuously. The rapid emergence of parasite strains that are resistant to affordable drugs highlights the urgent need for a cheap and effective vaccine. Candidate vaccines that have been developed to date target different stages of the parasite life cycle. This review describes the recent advances in the development of a vaccine that aims to terminate the infection at its first stage in the liver. The candidate vaccines that are currently under clinical evaluation are introduced and the results from recent trials discussed. The review aims to explain the immunologic challenges a successful vaccine has to meet, as well as the different strategies that are currently employed in an attempt to induce a protective immune response. Furthermore, an outline of available options to be tested in the near future will be presented.

Major histocompatibility complex and T cell interactions of a universal T cell epitope from Plasmodium falciparum circumsporozoite protein

A 20-residue sequence from the C-terminal region of the circumsporozoite protein of the malaria parasite Plasmodium falciparum is considered a universal helper T cell epitope because it is immunogenic in individuals of many major histocompatibility complex (MHC) haplotypes. Subunit vaccines containing T* and the major B cell epitope of the circumsporozoite protein induce high antibody titers to the malaria parasite and significant T cell responses in humans. In this study we have evaluated the specificity of the T* sequence with regard to its binding to the human class II MHC protein DR4 (HLA-DRB1*0401), its interactions with antigen receptors on T cells, and the effect of natural variants of this sequence on its immunogenicity. Computational approaches identified multiple potential DR4-binding epitopes within T*, and experimental binding studies confirmed the following two tight binding epitopes: one located toward the N terminus (the T*-1 epitope) and one at the C terminus (the T*-5 epitope). Immunization of a human DR4 volunteer with a peptide-based vaccine containing the T* sequence elicited CD4+ T cells that recognize each of these epitopes. Here we present an analysis of the immunodominant N-terminal epitope T*-1. T*-1 residues important for interaction with DR4 and with antigen receptors on T*-specific T cells were mapped. MHC tetramers carrying DR4/T*-1 MHC-peptide complexes stained and efficiently stimulated these cells in vitro. T*-1 overlaps a region of the protein that has been described as highly polymorphic; however, the particular T*-1 residues required for anchoring to DR4 were highly conserved in Plasmodium sequences described to date.