Mature liver stages of cloned Plasmodium falciparum share epitopes with proteins from sporozoites and asexual blood stages

Enhancing longevity of Plasmodium vivax and P. falciparum sporozoites after dissection from mosquito salivary glands

The pre-erythrocytic stages of Plasmodium vivax and Plasmodium falciparum remain challenging for experimental research in part due to limited access to sporozoites. An important factor limiting availability is the laboratory support required for producing infected mosquitoes and the ephemeral nature of isolated extracellular sporozoites. This study was undertaken to investigate methods to improve the availability of this limited resource by extending the longevity of the extracellular sporozoites after mosquito dissection. Our goal in this study was to determine whether buffer conditions more closely mimicking the insect microenvironment could prolong longevity of ex vivo P. vivax and P. falciparum sporozoites. The study compared the current standard dissection buffer RPMI1640 to Hank's Balanced Salt Solution with 1g/L glucose (HBSS-1) or 2g/L glucose (HBSS-2) and Grace's Insect Medium for ability to extend longevity of ex vivo P. vivax and P. falciparum sporozoites. The effect of each buffer on sporozoite viability was evaluated by measuring sporozoite gliding motility at 0, 4, 8, and 24h post-dissection from mosquito salivary glands. Comparisons of mean gliding percentages of ex vivo sporozoites in the different buffers and time points found that RPMI and Grace's both showed strong gliding at 0h. In contrast, by 4h post-dissection sporozoites in RPMI consistently had the lowest gliding activity, whereas sporozoites in Grace's had significantly more gliding compared to all other buffers at almost all time points. Our results indicate that P. vivax and P. falciparum sporozoites maintained in insect media rather than the standard dissection buffer RPMI and HBSS retain viability better over time.

Clinical trial in healthy malaria-naïve adults to evaluate the safety, tolerability, immunogenicity and efficacy of MuStDO5, a five-gene, sporozoite/hepatic stage Plasmodium falciparum DNA vaccine combined with escalating dose human GM-CSF DNA

When introduced in the 1990s, immunization with DNA plasmids was considered potentially revolutionary for vaccine development, particularly for vaccines intended to induce protective CD8 T cell responses against multiple antigens. We conducted, in 1997−1998, the first clinical trial in healthy humans of a DNA vaccine, a single plasmid encoding Plasmodium falciparum circumsporozoite protein (PfCSP), as an initial step toward developing a multi-antigen malaria vaccine targeting the liver stages of the parasite. As the next step, we conducted in 2000–2001 a clinical trial of a five-plasmid mixture called MuStDO5 encoding pre-erythrocytic antigens PfCSP, PfSSP2/TRAP, PfEXP1, PfLSA1 and PfLSA3. Thirty-two, malaria-naïve, adult volunteers were enrolled sequentially into four cohorts receiving a mixture of 500 μg of each plasmid plus escalating doses (0, 20, 100 or 500 μg) of a sixth plasmid encoding human granulocyte macrophage-colony stimulating factor (hGM-CSF). Three doses of each formulation were administered intramuscularly by needle-less jet injection at 0, 4 and 8 weeks, and each cohort had controlled human malaria infection administered by five mosquito bites 18 d later. The vaccine was safe and well-tolerated, inducing moderate antigen-specific, MHC-restricted T cell interferon-γ responses but no antibodies. Although no volunteers were protected, T cell responses were boosted post malaria challenge. This trial demonstrated the MuStDO5 DNA and hGM-CSF plasmids to be safe and modestly immunogenic for T cell responses. It also laid the foundation for priming with DNA plasmids and boosting with recombinant viruses, an approach known for nearly 15 y to enhance the immunogenicity and protective efficacy of DNA vaccines.

Superior antimalarial immunity after vaccination with late liver stage-arresting genetically attenuated parasites

While subunit vaccines have shown partial efficacy in clinical trials, radiation-attenuated sporozoites (RAS) remain the "gold standard" for sterilizing protection against Plasmodium infection in human vaccinees. The variability in immunogenicity and replication introduced by the extensive, random DNA damage necessary to generate RAS could be overcome by genetically attenuated parasites (GAP) designed via gene deletion to arrest at defined points during liver-stage development. Here, we demonstrate the principle that late liver stage-arresting GAP induce larger and broader CD8 T cell responses that provide superior protection in inbred and outbred mice compared to RAS or early-arresting GAP immunizations. Late liver stage-arresting GAP also engender high levels of cross-stage and cross-species protection and complete protection when administered by translationally relevant intradermal or subcutaneous routes. Collectively, our results underscore the potential utility of late liver stage-arresting GAP as broadly protective next-generation live-attenuated malaria vaccines and support their potential as a powerful model for identifying antigens to generate cross-stage protection.

Immune responses and protection of Aotus monkeys immunized with irradiated Plasmodium vivax sporozoites

A non-human primate model for the induction of protective immunity against the pre-erythrocytic stages of Plasmodium vivax malaria using radiation-attenuated P. vivax sporozoites may help to characterize protective immune mechanisms and identify novel malaria vaccine candidates. Immune responses and protective efficacy induced by vaccination with irradiated P. vivax sporozoites were evaluated in malaria-naive Aotus monkeys. Three groups of six monkeys received two, five, or ten intravenous inoculations, respectively, of 100,000 irradiated P. vivax sporozoites; control groups received either 10 doses of uninfected salivary gland extract or no inoculations. Immunization resulted in the production low levels of antibodies that specifically recognized P. vivax sporozoites and the circumsporozoite protein. Additionally, immunization induced low levels of antigen-specific IFN-γ responses. Intravenous challenge with viable sporozoites resulted in partial protection in a dose-dependent manner. These findings suggest that the Aotus monkey model may be able to play a role in preclinical development of P. vivax pre-erythrocytic stage vaccines.

Vaccination with live Plasmodium yoelii blood stage parasites under chloroquine cover induces cross-stage immunity against malaria liver stage

Immunity to malaria has long been thought to be stage-specific. In this study we show that immunization of BALB/c mice with live erythrocytes infected with nonlethal strains of Plasmodium yoelii under curative chloroquine cover conferred protection not only against challenge by blood stage parasites but also against sporozoite challenge. This cross-stage protection was dose-dependent and long lasting. CD4(+) and CD8(+) T cells inhibited malaria liver but not blood stage. Their effect was mediated partially by IFN-gamma, and was completely dependent of NO. Abs against both pre-erythrocytic and blood parasites were elicited and were essential for protection against blood stage and liver stage parasites. Our results suggest that Ags shared by liver and blood stage parasites can be the foundation for a malaria vaccine that would provide effective protection against both pre-erythrocytic and erythrocytic asexual parasites found in the mammalian host.

Reduced immunogenicity of DNA vaccine plasmids in mixtures

We measured the ability of nine DNA vaccine plasmids encoding candidate malaria vaccine antigens to induce antibodies and interferon-gamma responses when delivered alone or in a mixture containing all nine plasmids. We further examined the possible immunosuppressive effect of individual plasmids, by assessing a series of mixtures in which each of the nine vaccine plasmids was replaced with a control plasmid. Given alone, each of the vaccine plasmids induced significant antibody titers and, in the four cases for which appropriate assays were available, IFN-gamma responses. Significant suppression or complete abrogation of responses were seen when the plasmids were pooled in a nine-plasmid cocktail and injected in a single site. Removal of single genes from the mixture frequently reduced the observed suppression. Boosting with recombinant poxvirus increased the antibody response in animals primed with either a single gene or the mixture, but, even after boosting, responses were higher in animals primed with single plasmids than in those primed with the nine-plasmid mixture. Boosting did not overcome the suppressive effect of mixing for IFN-gamma responses. Interactions between components in a multiplasmid DNA vaccine may limit the ability to use plasmid pools alone to induce responses against multiple targets simultaneously.

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.

Three multigene families in Plasmodium parasites: facts and questions

Multigene families optimise fitness by providing a set of related genes with possibly different temporal and/or topological expression patterns. We analyse here the structural organisation and sequence diversity of the rDNA, sera and var C Plasmodium falciparum families, and discuss their consequences for parasite biology. The low rDNA copy number, which reduces reshuffling, is probably the corollary of the need for functionally distinct rRNAs in the insect and in the vertebrate host. The unusual intra-genome and population rDNA sequence diversity results in cells equipped with mosaic ribosome sets. The functional constraints are such that ribosome compatibility could influence parasite fitness and contribute to population structuring. Unlike the dispersed rDNA units, the sera family is arranged as a tandem gene cluster, with seven contiguous similar genes, and one more distantly related paralog. We address the question of the inclusion criteria in family definition. We discuss the results concerning the SERA proteins expression and function in the context of the long overlooked multigene family. The var C module is shared by var genes, 'orphan' var C and var C pseudogenes. Analysis of 125 var C deduced protein sequences highlights a well-conserved framework, including putative phosphorylation sites, consistent with the proposed function of mediating interaction with cytoskeletal proteins. The 5' and 3' flanking sequences of the var C pseudogenes are heterogeneous. In contrast, the flanking sequences of the uninterrupted var C modules show remarkable conservation. This is interesting in view of the silencing activity of the var intronic sequence on var expression. The 5' flanking sequence dichotomy reported for internal and sub-telomeric var genes extends to the 3' flanking sequences. This has profound implications for transcription regulation and generation of diversity. The var C family suggests a role for pseudogenes as a diversity reservoir and in genome dynamics by promoting ectopic recombination.

Simultaneous induction of multiple antigen-specific cytotoxic T lymphocytes in nonhuman primates by immunization with a mixture of four Plasmodium falciparum DNA plasmids

CD8(+) T cells have been implicated as critical effector cells in protective immunity against malaria parasites developing within hepatocytes. A vaccine that protects against malaria by inducing CD8(+) T cells will probably have to include multiple epitopes on the same protein or different proteins, because of parasite polymorphism and genetic restriction of T-cell responses. To determine if CD8(+) T-cell responses against multiple P. falciparum proteins can be induced in primates by immunization with plasmid DNA, rhesus monkeys were immunized intramuscularly with a mixture of DNA plasmids encoding four P. falciparum proteins or with individual plasmids. All six monkeys immunized with PfCSP DNA, seven of nine immunized with PfSSP2 DNA, and five of six immunized with PfExp-1 or PfLSA-1 DNA had detectable antigen-specific cytotoxic T lymphocytes (CTL) after in vitro restimulation of peripheral blood mononuclear cells. CTL activity was genetically restricted and dependent on CD8(+) T cells. By providing the first evidence for primates that immunization with a mixture of DNA plasmids induces CD8(+) T-cell responses against all the components of the mixture, these studies provide the foundation for multigene immunization of humans.

Enhanced detection of Plasmodium vivax liver stages by cytocentrifugation

Malaria parasites circulating in the blood or developing in the mosquito host can easily be seen and studied. We know much less about malaria parasites in the liver not only because of their location, but also because there are so few of them. Hepatic parasites are most often grown within hepatoma cells in culture, stained and visualized on slides under direct microscopy. In this report, Chitraporn Karnasuta and George Watt investigate the potential of cytocentrifugation as a tool for improving the detection of liver-stage malaria parasites.

NYVAC-Pf7: a poxvirus-vectored, multiantigen, multistage vaccine candidate for Plasmodium falciparum malaria

The highly attenuated NYVAC vaccinia virus strain has been utilized to develop a multiantigen, multistage vaccine candidate for malaria, a disease that remains a serious global health problem and for which no highly effective vaccine exists. Genes encoding seven Plasmodium falciparum antigens derived from the sporozoite (circumsporozoite protein and sporozoite surface protein 2), liver (liver stage antigen 1), blood (merozoite surface protein 1, serine repeat antigen, and apical membrane antigen 1), and sexual (25-kDa sexual-stage antigen) stages of the parasite life cycle were inserted into a single NYVAC genome to generate NYVAC-Pf7. Each of the seven antigens was expressed in NYVAC-Pf7-infected culture cells, and the genotypic and phenotypic stability of the recombinant virus was demonstrated. When inoculated into rhesus monkeys, NYVAC-Pf7 was safe and well tolerated. Antibodies that recognize sporozoites, liver, blood, and sexual stages of P. falciparum were elicited. Specific antibody responses against four of the P.falciparum antigens (circumsporozoite protein, sporozoite surface protein 2, merozoite surface protein 1, and 25-kDa sexual-stage antigen) were characterized. The results demonstrate that NYVAC-Pf7 is an appropriate candidate vaccine for further evaluation in human clinical trials.

Inducing protective immune responses against the sporozoite and liver stages of Plasmodium

Work on vaccines against the pre-erythrocytic stages of the Plasmodium life cycle is based on the observation that immunization with irradiated sporozoites (IRR SPZ) is protective. Antibodies against several SPZ surface proteins can prevent SPZ from effectively invading hepatocytes; antibodies and cytolytic-T lymphocytes directed against at least 3 parasite proteins expressed in infected hepatocytes can kill infected hepatocytes; and cytokines can activate infected hepatocytes to kill the intracellular parasite. Work is in progress to identify additional pre-erythrocytic parasite targets and to develop methods for optimally inducing protective immunity against SPZ and infected hepatocytes. The goal is to construct a vaccine that protects by inducing antibody and cellular immune responses against multiple parasite proteins.

Immunogenicity of the Plasmodium falciparum serine repeat antigen (p126) expressed by vaccinia virus

cDNA encoding the serine repeat antigen (SERA) (also called p126) of Plasmodium falciparum has been isolated from the FCR3 strain and inserted into a recombinant vaccinia virus designated vP870. Expression analysis of vP870-infected Vero cells by immunoprecipitation has demonstrated several intracellular forms of SERA and a single secreted SERA peptide. Endoglycosidase digestion of these immunoprecipitated SERA peptides indicated that the intracellular SERA peptides contain simple, high-mannose N-linked oligosaccharides and that the secreted SERA peptide contains complex N-linked oligosaccharides. Pulse-chase experiments indicate that the multiple intracellular SERA peptides in infected Vero cells represent a trafficking pathway whereby the smallest SERA peptide is converted into larger peptides by co- and posttranslational modifications, including glycosylation, and eventually secreted from the cell with complex N-linked oligosaccharides. To study the immunogenicity of vaccinia virus-expressed SERA, rabbits were immunized with vP870 and their sera were analyzed for reactivity with authentic, parasite-derived SERA protein. The anti-vP870 rabbit sera reacted with P. falciparum-infected erythrocytes by immunofluorescence analysis, recognized authentic SERA from schizonts by both immunoprecipitation and Western blot (immunoblot) analyses, and recognized proteolytically processed fragments of SERA secreted into the culture medium by Western blot analysis. These results indicate that when expressed by vaccinia virus, SERA is glycosylated and secreted from infected cells and that in immunized rabbits, vaccinia virus-expressed SERA can stimulate a humoral immune response against SERA derived from blood-stage parasites.

Vaccination with SPf66, a chemically synthesised vaccine, against Plasmodium falciparum malaria in Colombia

Preclinical and clinical studies have established the safety and immunogenicity of the chemically synthesised SPf66 malaria vaccine. The present study is a phase III randomised, double-blind, placebo-controlled, efficacy trial completed in La Tola, Colombia. 1548 volunteers over one year of age received three doses of either the vaccine (n = 738) or placebo (n = 810). Active and passive case detection methods were used to document clinical episodes of malaria among the study population. The follow-up period began one month after the third dose and lasted for one year. 168 and 297 episodes of Plasmodium falciparum malaria were documented in the SPf66 group and the placebo group, respectively; this corresponds to a crude protective efficacy of 38.8%. Incidence rates for first or only P falciparum malarial episodes were 22.3% per annum among the vaccinee group and 33.5% among the placebo group (RR = 1.5; 95% Cl 1.23, 1.84). Therefore, the protective efficacy of SPf66 against first or only episodes was 33.6% (95% Cl 18.8, 45.7), being highest in children aged 1-4 years (77%) and adults older than 45 years (67%). The estimated protective efficacy against second episodes was 50.5% (95% Cl 12.9-71.9). Our study shows that the chemically synthesised SPf66 malaria vaccine is safe, immunogenic, and protective against P falciparum malaria in semi-immune populations subject to natural challenge.

Mouse model for exoerythrocytic stages of Plasmodium falciparum malaria parasite

Research on the exoerythrocytic (EE) stages of human malaria parasites has been hindered because of the lack of an easily available suitable animal model. We report here an approach to produce mature EE-stage Plasmodium falciparum parasites by using severe combined immunodeficient (scid) mice with transplanted human hepatocytes. Transplantation of human hepatocytes into scid mice (scid hu-hep), their subsequent intravenous infection with P. falciparum sporozoites, and the development of mature liver-stage merozoites was achieved. Immunofluorescent staining of scid hu-hep kidney tissue sections demonstrated the presence of circumsporozoite protein (early during infection), merozoite surface antigen 1, and liver schizont antigen 1. The scid hu-hep model can serve as a source of human malaria liver-stage parasites, decreasing the need for nonhuman primates. Use of this model will facilitate characterization of EE-stage antigens and the assessment of stage-specific chemotherapeutic agents and candidate vaccines.

Crossreactive antigens between life cycle stages ofplasmodium faiciparum

The high mortality and morbidity induced by falciparum malaria has motivated research to find an efficient antimalarial vaccine. The parasite has a complex life cycle, both in the mosquito and human hosts, and presents a number of potential targets for vaccine-induced immune attack. Here, Inge Moelans and John Schoenmakers discuss how the search for protective antigens has been complicated by the discovery of multiple crossreactivities between different parasite proteins.

Primary structure and localization of a conserved immunogenic Plasmodium falciparum glutamate rich protein (GLURP) expressed in both the preerythrocytic and erythrocytic stages of the vertebrate life cycle

A gene coding for a 220-kDa glutamate rich protein (GLURP), an exoantigen of Plasmodium falciparum, was isolated and its nucleotide sequence was determined. The deduced amino acid sequence contains 2 repeat regions. The sequence of one of these was shown to be conserved among geographically dispersed isolates, and a fusion protein containing that sequence was able to stimulate B- and T-cells. Antibodies against GLURP stained erythrocytic stages of the parasite as well as the hepatic stage as detected by electron microscopy.

Fine specificities of monoclonal antibodies against the Plasmodium falciparum circumsporozoite protein: recognition of both repetitive and non-repetitive regions

The fine specificities of 6 monoclonal antibodies (MoAbs) raised against the circumsporozoite (CS) protein of the human malaria parasite, Plasmodium falciparum, were defined by their binding to a series of overlapping octapeptides corresponding to the 7G8 variant of the CS protein. The precise specificities of the MoAbs to the immunodominant NANP repeat region were elucidated by their binding to all possible 4, 5, 6, 7 and 8 amino acid peptides in this region. All 6 MoAbs recognized the NANP repeats. In addition all MoAb bound to nonrepetitive sites with 4 of the 6 MoAbs recognizing known functional sites outside the repeat region including sites required for T cell recognition and hepatocyte invasion. Antibody pressure may therefore be responsible for generating the epitope variation observed at T cell sites. The multiple specificities for all the MoAbs suggests that the repeat region may act as an internal immunological 'smokescreen' by competing more effectively for antibody binding compared to single epitope copy functional sites located outside the repeat region.

Plasmodium cynomolgi: immunization of a rhesus monkey with exoerythrocytic stages cultured in autologous hepatocytes

To investigate the immune response to exoerythrocytic stages of malaria parasites, a rhesus monkey was immunized with autologous primary hepatocyte cultures infected with 7-day-old liver stage parasites of Plasmodium cynomolgi. A primary antibody response against EE stage antigens was obtained, and boosted after injection of homologous viable sporozoites. Antibodies directed against sporozoites and blood stages were also detected. The polyvalent immune response observed demonstrates the antigenicity of the liver stages and suggests their involvement in the general immune response against malaria.

Immunity to the liver stage of malaria

The search for subunit vaccines against malaria has concentrated on asexual and sexual blood stage and sporozoite antigens. In recent years the search for the basis of the protection against sporozoite challenge obtained in mice immunized with irradiated sporozoites has focused attention on the liver or exoerythrocytic (EE) stage of the malaria life cycle. Here, Andreas Suhrbier looks at the various immune responses that appear to be active against this stage, which was once thought to be immunologically insignificant. The liver stage of malaria has thus emerged as a legitimate target for vaccine development.

Survival and antigenic profile of irradiated malarial sporozoites in infected liver cells

Exoerythrocytic (EE) stages of Plasmodium berghei derived from irradiated sporozoites were cultured in vitro in HepG2 cells. They synthesized several antigens, predominantly but not exclusively those expressed by normal early erythrocytic schizonts. After invasion, over half the intracellular sporozoites, both normal and irradiated, appeared to die. After 24 h, in marked contrast to the normal parasites, EE parasites derived from irradiated sporozoites continued to break open, shedding their antigens into the cytoplasm of the infected host cells. Increasing radiation dosage, which has previously been shown to reduce the ability of irradiated sporozoites to protect animals, correlated with reduced de novo antigen synthesis by EE parasites derived from irradiated sporozoites.

An antigen specific to the liver stage of rodent malaria recognized by a monoclonal antibody

Vaccines currently being evaluated against malaria are based on proteins derived from the blood, sporozoite and sexual stages. Antigens from the liver stage, which is now recognized as the major target of protective sporozoite induced immunity, have received comparatively little attention. This paper describes the generation of a monoclonal antibody (MoAb), which recognizes an antigen specific to the liver stage of the rodent malaria Plasmodium berghei. The antigen is expressed throughout liver stage development and appears to be localized to the parasitophorous vacuole membrane. The MoAb did not affect the growth of liver stages cultured in vitro nor could protection be demonstrated in vivo following passive transfer of the antibody.

Irradiated sporozoite vaccine induces cytotoxic T lymphocytes that recognize malaria antigens on the surface of infected hepatocytes

The observation that protective immunity induced by immunization with radiation attenuated Plasmodium berghei and Plasmodium yoelii sporozoites is dependent on CD8+ T lymphocytes in some strains of mice led us to speculate that immunization with sporozoites induces cytotoxic T lymphocytes (CTL) that recognize malaria antigens on the surface of malaria-infected hepatocytes. In this report we summarize a series of experiments that confirm this hypothesis. We first showed that when immune mice are challenged with live sporozoites they develop malaria-specific, CD8+ T cell-dependent infiltrates in their livers. Next we demonstrated that spleen cells from immune mice eliminate malaria infected hepatocytes from in vitro culture in an antigen specific and genetically restricted manner, indicating that these immune cells recognize malaria antigens on the surface of infected hepatocytes. Finally we defined a CTL epitope of the P. yoelii CS protein, and demonstrated that CTL against this 16-amino-acid peptide (PYCTL1) eliminate infected hepatocytes from culture in an antigenic specific, and MHC restricted manner, indicating that this 16-amino-acid peptide from the CS protein is present on the surface of the infected hepatocytes. We are currently working on constructing vaccines that induce protective CTL against PYCTL1, and identifying additional pre-erythrocytic stage targets of CTL mediated protective immunity.

Plasmodium falciparum: studies on mature exoerythrocytic forms in the liver of the chimpanzee, Pan troglodytes

Mature exoerythrocytic forms (EEF) of Plasmodium falciparum from the chimpanzee were examined by light- and transmission electron microscopy from a liver biopsy taken on Day 6 after sporozoite inoculation. Infectivity of the sporozoites obtained from whole mosquitoes which were membrane fed on cultured gametocytes was about 4-6%. In comparison, salivary gland sporozoites added to human hepatocytes in vitro had only a developmental percentage of 0.02 to 0.05% at Day 5. The EEF found in the liver biopsy were not all at the same stage of development. Immature compact parasites were seen simultaneously with stages with fully formed merozoites, indicating a rapid final maturation or asynchrony. At Day 7.5, large numbers of rings were already seen in the peripheral blood, indicating a duration of the liver development of P. falciparum in the chimpanzee of about 5.5-6 days. The process of merogony at the fine structural level was comparable to that described for rodent and other primate parasites in vivo. Compared to the fine structure of EEF in vitro in cultured human hepatocytes, the parasites described here were much more advanced in development. There appeared to be some cell infiltration with collagen deposition around the intracellular parasite; however, no marked degeneration of EEF was observed.

Sporozoite vaccine induces genetically restricted T cell elimination of malaria from hepatocytes

The target of the CD8+ T cell-dependent immunity that protects mice immunized with irradiation-attenuated malaria sporozoites has not been established. Immune BALB/c mice were shown to develop malaria-specific, CD8+ T cell-dependent inflammatory infiltrates in their livers after challenge with Plasmodium berghei sporozoites. Spleen cells from immune BALB/c and C57BL/6 mice eliminated hepatocytes infected with the liver stage of P. berghei in vitro. The activity against infected hepatocytes is not inhibited by antibodies to interferon-gamma and is not present in culture supernatants. It is genetically restricted, an indication that malaria antigens on the hepatocyte surface are recognized by immune T effector cells. Subunit vaccine development will require identification of the antigens recognized by these T cells and a method of immunization that induces such immunity.