Direct immunization of malaria DNA vaccine into the liver by gene gun protects against lethal challenge of Plasmodium berghei sporozoite

Malaria Vaccines: From the Past towards the mRNA Vaccine Era

Plasmodium spp. is the etiological agent of malaria, a life-threatening parasitic disease transmitted by infected mosquitoes. Malaria remains a major global health challenge, particularly in endemic regions. Over the years, various vaccine candidates targeting different stages of Plasmodium parasite life-cycle have been explored, including subunit vaccines, vectored vaccines, and whole organism vaccines with Mosquirix, a vaccine based on a recombinant protein, as the only currently approved vaccine for Plasmodium falciparum malaria. Despite the aforementioned notable progress, challenges such as antigenic diversity, limited efficacy, resistant parasites escaping protective immunity and the need for multiple doses have hindered the development of a highly efficacious malaria vaccine. The recent success of mRNA-based vaccines against SARS-CoV-2 has sparked renewed interest in mRNA vaccine platforms. The unique mRNA vaccine features, including their potential for rapid development, scalability, and flexibility in antigen design, make them a promising avenue for malaria vaccine development. This review provides an overview of the malaria vaccines' evolution from the past towards the mRNA vaccine era and highlights their advantages in overcoming the limitations of previous malaria vaccine candidates.

Viral infections of pigs used for medical education. A Japanese experience

Infectious viruses pose a threat to all living organisms, including humans, and can cause significant morbidity. Previous experience with pigs in medical education and research, rather than in domestic control settings, has led to a unique perspective on viral infections in swine. In this article, common porcine infectious diseases have been listed, based mainly on the authors' experience thus far. For example, young domestic pigs that were used in surgical training and infected with hepatitis E were subjected to quarantine and isolation treatment, and attempts were made to develop a DNA vaccine for swine influenza arising from swine-to-human transmission. More recent research has focused on preventing infection by the African swine virus, a current threat. We hope that this article of porcine infectious diseases identified at the School of Medicine will help develop a breakthrough with regard to coronavirus disease.

Delivery methods to increase cellular uptake and immunogenicity of DNA vaccines

DNA vaccines are ideal candidates for global vaccination purposes because they are inexpensive and easy to manufacture on a large scale such that even people living in low-income countries can benefit from vaccination. However, the potential of DNA vaccines has not been realized owing mainly to the poor cellular uptake of DNA in vivo resulting in the poor immunogenicity of DNA vaccines. In this review, we discuss the benefits and shortcomings of several promising and innovative non-biological methods of DNA delivery that can be used to increase cellular delivery and efficacy of DNA vaccines.

Rapid nuclear import of short nucleic acids

Exogenous short-chain nucleic acids undergo rapid import into the nucleus. Fluorescence-labeled dT1-13 DNA microinjected into the cytoplasm domain of a HeLa cell was rapidly imported into the nucleus domain within 1min. This is much more rapid than what has been observed for intracellular diffusion of small molecules. In contrast, import of longer nucleic acids with a length of over 30nt into the nucleus was suppressed.

Plasmonic nanobubble-enhanced endosomal escape processes for selective and guided intracellular delivery of chemotherapy to drug-resistant cancer cells

Cancer chemotherapies suffer from multi drug resistance, high non-specific toxicity and heterogeneity of tumors. We report a method of plasmonic nanobubble-enhanced endosomal escape (PNBEE) for the selective, fast and guided intracellular delivery of drugs through a self-assembly by cancer cells of separately targeted gold nanoparticles and encapsulated drug (Doxil). The co-localized with Doxil plasmonic nanobubbles optically generated in cancer cells released the drug into the cytoplasm thus increasing the therapeutic efficacy against these drug-resistant cells by 31-fold, reducing drug dose by 20-fold, the treatment time by 3-fold and the non-specific toxicity by 10-fold compared to standard treatment. Thus the PNBEE mechanism provided selective, safe and efficient intracellular drug delivery in heterogeneous environment opening new opportunities for drug therapies.

Novel prophylactic vaccine using a prime-boost method and hemagglutinating virus of Japan-envelope against tuberculosis

OBJECTIVE

Mycobacterium tuberculosis infection is a major global threat to human health. The only tuberculosis (TB) vaccine currently available is bacillus Calmette-Guérin (BCG), although it has no efficacy in adults. Therefore, the development of a novel vaccine against TB for adults is desired.

METHOD

A novel TB vaccine expressing mycobacterial heat shock protein 65 (HSP65) and interleukin-12 (IL-12) delivered by the hemagglutinating virus of Japan- (HVJ)- envelope was evaluated against TB infection in mice. Bacterial load reductions and histopathological assessments were used to determine efficacy.

RESULTS

Vaccination by BCG prime with IgHSP65+murine IL-12/HVJ-envelope boost resulted in significant protective efficacy (>10, 000-fold versus BCG alone) against TB infection in the lungs of mice. In addition to bacterial loads, significant protective efficacy was demonstrated by histopathological analysis of the lungs. Furthermore, the vaccine increased the number of T cells secreting IFN-γ.

CONCLUSION

This vaccine showed extremely significant protection against TB in a mouse model, consistent with results from a similar paper on cynomolgus monkeys. The results suggest that further development of the vaccine for eventual testing in clinical trials may be warranted.

Selective gene transfection of individual cells in vitro with plasmonic nanobubbles

Gene delivery and transfection of eukaryotic cells are widely used for research and for developing gene cell therapy. However, the existing methods lack selectivity, efficacy and safety when heterogeneous cell systems must be treated. We report a new method that employs plasmonic nanobubbles (PNBs) for delivery and transfection. A PNB is a novel, tunable cellular agent with a dual mechanical and optical action due to the formation of the vapor nanobubble around a transiently heated gold nanoparticle upon its exposure to a laser pulse. PNBs enabled the mechanical injection of the extracellular cDNA plasmid into the cytoplasm of individual target living cells, cultured leukemia cells and human CD34+ CD117+ stem cells and expression of a green fluorescent protein (GFP) in those cells. PNB generation and lifetime correlated with the expression of green fluorescent protein in PNB-treated cells. Optical scattering by PNBs additionally provided the detection of the target cells and the guidance of cDNA injection at single cell level. In both cell models PNBs demonstrated a gene transfection effect in a single pulse treatment with high selectivity, efficacy and safety. Thus, PNBs provided targeted gene delivery at the single cell level in a single pulse procedure that can be used for safe and effective gene therapy.

Gene doping

Gene doping abuses the legitimate approach of gene therapy. While gene therapy aims to correct genetic disorders by introducing a foreign gene to replace an existing faulty one or by manipulating existing gene(s) to achieve a therapeutic benefit, gene doping employs the same concepts to bestow performance advantages on athletes over their competitors. Recent developments in genetic engineering have contributed significantly to the progress of gene therapy research and currently numerous clinical trials are underway. Some athletes and their staff are probably watching this progress closely. Any gene that plays a role in muscle development, oxygen delivery to tissues, neuromuscular coordination, or even pain control is considered a candidate for gene dopers. Unfortunately, detecting gene doping is technically very difficult because the transgenic proteins expressed by the introduced genes are similar to their endogenous counterparts. Researchers today are racing the clock because assuring the continued integrity of sports competition depends on their ability to develop effective detection strategies in preparation for the 2012 Olympics, which may mark the appearance of genetically modified athletes.

A baculovirus dual expression system-based malaria vaccine induces strong protection against Plasmodium berghei sporozoite challenge in mice

We have previously shown that a recombinant baculovirus that displays Plasmodium berghei circumsporozoite protein (PbCSP), a homolog of the leading human malaria vaccine candidate, on the viral envelope protected 60% of mice against P. berghei infection. Here, we describe a second-generation baculovirus vaccine based on the "baculovirus dual expression system," which drives PbCSP expression by a dual promoter that consists of tandemly arranged baculovirus-derived polyhedrin and mammal-derived cytomegalovirus promoters. The baculovirus-based PbCSP vaccine not only displayed PbCSP on the viral envelope but also expressed PbCSP upon transduction of mammalian cells. Immunization with the baculovirus-based PbCSP vaccine elicited high PbCSP-specific antibody titers (predominantly immunoglobulin G1 [IgG1] and IgG2a) and PbCSP-specific CD8(+) T-cell responses without extraneous immunological adjuvants in mice, indicating that there was induction of both Th1 and Th2 responses (a mixed Th1/Th2 response). Importantly, upon intramuscular inoculation, the baculovirus-based PbCSP vaccine conferred complete protection against sporozoite challenge. Thus, the baculovirus-based PbCSP vaccine induced strong protective immune responses against preerythrocytic parasites. These results introduce a novel concept for the baculovirus dual expression system that functions as both a subunit vaccine and a DNA vaccine and offer a promising new alternative to current human vaccine delivery platforms.

Gene delivery into hepatocytes with the preS/liposome/DNA system

Gene delivery into human hepatocytes remains a critical issue for the development of liver-directed gene therapy. Gene delivery based on non-viral vectors is an attractive approach relative to viral vectors. In this report, novel delivery system of preS/liposome/DNA virus-like particle (VLP) was developed for gene transfection into hepatocytes in vivo and in vitro. Plasmid pCMVbeta, expressing beta-galactosidase, was encapsulated with cationic liposome, and then the histidine-tagged preS domain of hepatitis B virus was coated on the surface of liposome/DNA to form preS/liposome/ DNA VLP. Transfection efficiencies of preS/liposome/DNA, liposome/DNA, naked DNA and preS were analyzed using several different human cell lines. The highest transfection efficiency was found using preS/liposome/DNA VLP as the transfection reagent in human hepatocyte (HH) cell line. Results show that preS domain of hepatitis B virus coated on liposome/DNA can be used for highly efficient gene transfection into human hepatocytes. Moreover, the target characteristic of preS/liposome/DNA was analyzed in vivo. After preS/liposome/DNA VLP was injected into immunocompromised (Nude) mice via the tail vein, most of beta-galactosidase was expressed in the liver; however, no significant target expression was found with the injection of liposome/ DNA or naked DNA. Our results show that preS/liposome/DNA VLP can be used as a novel liver-specific gene delivery system.

Genetic vaccination approaches against malaria based on the circumsporozoite protein

Malaria is the world's major parasitic disease, for which effective control measures are urgently needed. Despite considerable efforts, no successful vaccine against malaria has been developed so far. The method of DNA-based immunization offers the possibility to induce both antibody- and cell-mediated immune responses to a variety of antigens. The flexibility of the DNA vaccine technology permits the combination of several antigens from different developmental stages of the parasite's complicated life cycle. This review covers the development of DNA-based immunization against malaria from initial experiments in small animals to recently conducted clinical studies. Focusing on one of the best characterized malaria vaccine candidate antigens, the circumsporozoite protein, an overview of strategies to enhance vaccine efficacy is provided. Advanced application methods such as the gene gun technology or the needle-less jet injection device are described. As DNA vaccination represents a relatively new methodology, safety concerns associated with planned clinical applications are discussed. In summary, this novel type of vaccine has to be considered as a promising tool for future malaria vaccination strategies.

Antitumor Activity of IFN-λ in Murine Tumor Models

IFN-lambda 1, -lambda 2 and -lambda 3 have been discovered as the latest members of the class II cytokine family and shown to possess antiviral activity. Murine B16 melanoma and Colon26 cancer cells were transduced with mouse IFN-lambda to determine whether IFN-lambda possesses antitumor activity. Overexpression of IFN-lambda induced cell surface MHC class I expression and Fas/CD95 Ag, induced significant caspase-3/7 activity, and increased p21(Waf1/Cip1) and dephosphorylated Rb (Ser(780)) in B16 cells in vitro. IFN-lambda expression in tumor cell lines markedly inhibited s.c. and metastatic tumor formation in vivo compared with mock transfections (p < 0.05). Moreover, IFN-lambda expression induced lymphocytic infiltrates, and an Ab-mediated immune cell depletion assay showed that NK cells were critical to IFN-lambda-mediated tumor growth inhibition. Hydrodynamic injection of IFN-lambda cDNA successfully targeted liver metastatic foci of Colon26 cells, and moderately decreased the mortality of mice with tumors. IFN-lambda overexpression in the liver increased NK/NKT cells and enhanced their tumor-killing activity, and suggested the activation of innate immune responses. Thus, IFN-lambda induced both tumor apoptosis and NK cell-mediated immunological tumor destruction through innate immune responses. These findings suggested that local delivery of IFN-lambda might prove a useful adjunctive strategy in the clinical treatment of human malignancies.

DNA vaccine using hemagglutinating virus of Japan-liposome encapsulating combination encoding mycobacterial heat shock protein 65 and interleukin-12 confers protection against Mycobacterium tuberculosis by T cell activation

We investigated the immunogenicity and protective efficacy of DNA vaccine combinations expressing mycobacterial heat shock protein 65 (Hsp65) and interleukin-12 (IL-12) using gene gun bombardment and the hemagglutinating virus of Japan (HVJ)-liposome method. A mouse IL-12 expression vector (mIL-12 DNA) encoding single-chain IL-12 proteins comprised of p40 and p35 subunits were constructed. In a mouse model, a single gene gun vaccination with the combination of Hsp65 DNA and mIL-12 DNA provided a remarkably high degree of protection against challenge with virulent Mycobacterium tuberculosis; bacterial numbers were 100-fold lower in the lungs compared to BCG-vaccinated mice. To explore the clinical use of the DNA vaccines, we evaluated HVJ-liposome encapsulated Hsp65 DNA and mIL-12DNA (Hsp65 + mIL-12/HVJ). The HVJ-liposome method improved the protective efficacy of the Hsp65 DNA vaccine compared to gene gun vaccination. Hsp65 + mIL-12/HVJ induced CD8+ cytotoxic T lymphocyte activity against Hsp65 antigen. Most importantly, Hsp65+mIL-12/HVJ vaccination resulted in a greater degree of protection than that evoked by BCG. This protective efficacy was associated with the emergence of IFN-gamma-secreting T cells and activation of proliferative T cells and cytokines (IFN-gamma and IL-2) production upon stimulation with Hsp65 and antigens from M. tuberculosis. These results suggest that Hsp65 + IL-12/HVJ could be a promising candidate for a new tuberculosis DNA vaccine, which is superior to BCG vaccine.

Molecular adjuvants for mucosal immunity

Mucosal surfaces represent the entry route of a multitude of viral pathogens. For many of these viruses, such as the herpes simplex viruses and human immunodeficiency virus, no effective vaccine exists. Hence, it is important that prospective vaccines engender maximal immunity at these susceptible sites. Genetic vaccines encoding adjuvant molecules represent one approach to optimize mucosal as well as systemic immunity. Promising candidates include various inflammatory cytokines and chemokines that might be used to enhance the primary response to a level sufficient for protection. Encouraging studies involving cytokines such as granulocyte/macrophage colony-stimulating factor, interleukin-2 (IL-2), IL-12, IL-18, and many others are examined. Notable chemokines that may offer hope in such efforts include IL-8, RANTES, CCL19, CCL21, and a few others. Combinatorial approaches utilizing several cytokines and chemokines will most likely yield the greatest success. In addition, as more is discovered regarding the requirements for memory development of T cells, boosters involving key cytokines such as IL-15 and IL-23 may prove beneficial to long-term maintenance of the memory pool. This review summarizes the progress in the use of genetic vaccines to achieve mucosal immunity and discusses the needed strategies to maximize long-term prospective immunity at this vulnerable entry site.

Gene therapy and CVD: how near are we?

Critical for success of any gene therapy approach is the efficient packaging, effective cell specific delivery and nuclear translocation of the nucleic acid with minimal toxicity. Delivery systems utilizing a wide variety of viral vectors have traditionally been used to modify genomic DNA. However, drawbacks to the viral vectors include difficulties in large-scale production, potential contamination by wild-type viral particles and immunogenicity. Thus, efficient non-viral delivery of both plasmids for transgene expression and short oligonucleotides for modulating cellular functions has been developed. Gene therapy is now a consideration in the treatment of certain inherited and acquired genetic disorders associated with cardiovascular disease (CVD). Furthermore, many other cardiovascular conditions are potential targets for gene therapy, and advances in knowledge will increase the ability to link specific genes to a disease, resulting in the identification of further targets. With improvements in delivery and targeting, gene therapy is likely to substantially augment established and emerging therapies in reducing the global burden of cardiovascular disease.

Enhanced CD8+T Cell Immune Responses and Protection Elicited againstPlasmodium bergheiMalaria by Prime Boost Immunization Regimens Using a Novel Attenuated Fowlpox Virus

Sterile immunity can be provided against the pre-erythrocytic stages of malaria by IFN-gamma-secreting CD8(+) T cells that recognize parasite-infected hepatocytes. In this study, we have investigated the use of attenuated fowlpox virus (FPV) strains as recombinant vaccine vectors for eliciting CD8(+) T cells against Plasmodium berghei. The gene encoding the P. berghei circumsporozoite (PbCS) protein was inserted into an FPV vaccine strain licensed for use in chickens, Webster's FPV, and the novel FPV vaccine strain FP9 by homologous recombination. The novel FP9 strain proved more potent as a vaccine for eliciting CD8(+) T cell responses against the PbCS Ag. Sequential immunization with rFP9 and recombinant modified vaccinia virus Anakara (MVA) encoding the PbCS protein, administered by clinically acceptable routes, elicited potent CD8(+) T cell responses against the PbCS protein. This immunization regimen elicited substantial protection against a stringent liver-stage challenge with P. berghei and was more immunogenic and protective than DNA/MVA prime/boost immunization. However, further improvement was not achieved by sequential (triple) immunization with a DNA vaccine, FP9, and MVA.

Mice deficient in interleukin-4 (IL-4) or IL-4 receptor alpha have higher resistance to sporozoite infection with Plasmodium berghei (ANKA) than do naive wild-type mice

BALB/c interleukin-4 (IL-4(-/-)) or IL-4 receptor-alpha (IL-4ralpha(-/-)) knockout (KO) mice were used to assess the roles of the IL-4 and IL-13 pathways during infections with the blood or liver stages of plasmodium in murine malaria. Intraperitoneal infection with the blood-stage erythrocytes of Plasmodium berghei (ANKA) resulted in 100% mortality within 24 days in BALB/c mice, as well as in the mutant mouse strains. However, when infected intravenously with the sporozoite liver stage, 60 to 80% of IL-4(-/-) and IL-4ralpha(-/-) mice survived, whereas all BALB/c mice succumbed with high parasitemia. Compared to infected BALB/c controls, the surviving KO mice showed increased NK cell numbers and expression of inducible nitric oxide synthase (iNOS) in the liver and were able to eliminate parasites early during infection. In vivo blockade of NO resulted in 100% mortality of sporozoite-infected KO mice. In vivo depletion of NK cells also resulted in 80 to 100% mortality, with a significant reduction in gamma interferon (IFN-gamma) production in the liver. These results suggest that IFN-gamma-producing NK cells are critical in host resistance against the sporozoite liver stage by inducing NO production, an effective killing effector molecule against Plasmodium. The absence of IL-4-mediated functions increases the protective innate immune mechanism identified above, which results in immunity against P. berghei infection in these mice, with no major role for IL-13.

DNA immunization of the grafted liver by particle-mediated gene gun1

BACKGROUND

Direct DNA vaccination of liver allografts before transplantation may provide an effective strategy for inducing protective immunity to infection and malignancy.

METHODS

In this study, the authors examined the feasibility of gene gun-mediated vaccination of liver grafts. Using plasmids expressing luciferase and green fluorescent proteins, their expression was tested in a graft liver.

RESULTS

Protein expression was observed in the graft liver and significantly enhanced in hepatectomized rats. A short course of tacrolimus (FK506) also evoked the expression of these proteins. Effects of primary immunization to the liver on the humoral response were then tested using an expression plasmid encoding hepatitis B virus surface (HBs) antigen and were compared to that of skin immunization alone. The results showed that local immunization to the liver strongly induced antibody formation. Furthermore, the combination of an immunized partial liver graft with tacrolimus significantly enhanced antibody production against HBs antigen.

CONCLUSIONS

A DNA vaccine to the liver may be one strategy for preventing infectious disease associated with liver transplantation under tacrolimus treatment.

Baculovirus virions displaying Plasmodium berghei circumsporozoite protein protect mice against malaria sporozoite infection

The display of foreign proteins on the surface of baculovirus virions has provided a tool for the analysis of protein-protein interactions and for cell-specific targeting in gene transfer applications. To evaluate the baculovirus display system as a vaccine vehicle, we have generated a recombinant baculovirus (AcNPV-CSPsurf) that displays rodent malaria Plasmodium berghei circumsporozoite protein (PbCSP) on the virion surface as a fusion protein with the major baculovirus envelope glycoprotein gp64. The PbCSP-gp64 fusion protein was incorporated and oligomerized on the virion surface and led to a 12-fold increase in the binding activity of AcNPV-CSPsurf virions to HepG2 cells. Immunization with adjuvant-free AcNPV-CSPsurf virions induced high levels of antibodies and gamma interferon-secreting cells against PbCSP and protected 60% of mice against sporozoite challenge. These data demonstrate that AcNPV-CSPsurf displays sporozoite-like PbCSP on the virion surface and possesses dual potentials as a malaria vaccine candidate and a liver-directed gene delivery vehicle.

Predominant cell-mediated immunity in the oral mucosa: gene gun-based vaccination against infectious diseases

BACKGROUND

Direct immunization via epithelial surfaces has been considered for many vaccine approaches, including DNA vaccines. It remains to be determined, however, which body site is suitable for genetic vaccination.

OBJECTIVE

To characterize the effects of the oral mucosa-mediated genetic vaccination, we compared antigen-specific immune responses of the oral mucosal DNA vaccine to the flank skin vaccination against influenza virus and malaria parasite.

METHODS

DNA vaccines against the influenza A/WSN/33 (H1N1) hemagglutinin and the malaria Plasmodium berghei circumsporozoite protein were administered respectively three times at 3-week intervals into the oral mucosa, skin, or liver of hamsters. The effects of their vaccine were evaluated by antigen-specific antibody production and cell-mediated killing activity. Furthermore, the in vivo malaria challenge test was also performed after the vaccination.

RESULTS

Significant specific antibody production was not observed in each case, but interferon-gamma production and cell-mediated killing activity were strongly induced in splenic lymphocytes from hamsters with the oral vaccination. The in vivo malaria challenge after the oral mucosal vaccination significantly delayed the blood-appearance day of the parasites in comparison with other immunization sites (P<0.05).

CONCLUSION

These results suggest that gene immunization via the oral mucosa may induce cell-mediated immunity more efficiently than via the skin or liver, and that the oral mucosa may be one of the most suitable tissues for gene gun-based DNA vaccination against infectious diseases.

Gene therapy as an alternative to liver transplantation

Liver transplantation has become a well-recognized therapy for hepatic failure resulting from acute or chronic liver disease. It also plays a role in the treatment of certain inborn errors of metabolism that do not directly injure the liver. In fact, the liver maintains a central role in many inherited and acquired genetic disorders. There has been a considerable effort to develop new and more effective gene therapy approaches, in part, to overcome the need for transplantation as well as the shortage of donor livers. Traditional gene therapy involves the delivery of a piece of DNA to replace the faulty gene. More recently, there has been a growing interest in the use of gene repair to correct certain genetic defects. In fact, targeted gene repair has many advantages over conventional replacement strategies. In this review, we will describe a variety of viral and nonviral strategies that are now available to the liver. The ever-growing list includes viral vectors, antisense and ribozyme technology, and the Sleeping Beauty transposon system. In addition, targeted gene repair with RNA/DNA oligonucleotides, small-fragment homologous replacement, and triplex-forming and single-stranded oligonucleotides is a long-awaited and potentially exciting approach. Although each method uses different mechanisms for gene repair and therapy, they all share a basic requirement for the efficient delivery of DNA.

Gene therapy progress and prospects: nonviral vectors

The success of gene therapy is largely dependent on the development of the gene delivery vector. Recently, gene transfection into target cells using naked DNA, which is a simple and safe approach, has been improved by combining several physical techniques, for example, electroporation, gene gun, ultrasound and hydrodynamic pressure. Chemical approaches have been utilized to improve the efficiency and cell specificity of gene transfer. Novel gene carrier molecules, which facilitate DNA escape from the endosome into the cytosol, have been developed. Several functional polymers, which enable controlled release of DNA in response to an environmental change, have also been reported. Plasmids with reduced number of CpG motifs, the use of PCR fragments and the sequential injection method have been established for the reduction of immune response triggered by plasmid DNA. Construction of a long-lasting gene expression system is also an important theme for nonviral gene therapy. To date, tissue-specific expression, self-replicating and integrating plasmid systems have been reported. Improvement of delivery methods together with intelligent design of the DNA itself has brought about large degrees of enhancement in the efficiency, specificity and temporal control of nonviral vectors.

Exoerythrocytic malaria vaccine development: understanding host-parasite immunobiology underscores strategic success

Malaria imposes an enormous health burden on people living in the tropics and effective control measures are urgently needed. The vast majority of deaths in humans from malaria are caused by one species of the protozoan, Plasmodium falciparum. An efficacious and cost-effective vaccine against this parasite is considered a holy grail of modern molecular medicine. A vaccine that targets liver-stage parasites would prevent infection from reaching the blood and causing clinical disease. Among around 40 known Plasmodium falciparum antigens, only a few are expressed exclusively by mosquito-transmitted sporozoites or infected hepatocytes. Studies in humans have consistently related immune responses to these antigens with resistance to infection or disease, providing a powerful rationale for the development of pre-erythrocytic vaccines. By dissecting the mechanism(s) of immunity to these antigens, we can best evaluate in different delivery systems epitopes associated with protection as components of a focused and coordinated multiantigen malaria vaccine strategy.