Efficacy of murine malaria sporozoite vaccines: implications for human vaccine development

Immunization of cattle with a 36-kilodalton surface protein induces protection against homologous and heterologous Anaplasma marginale challenge

Immunization of cattle with a purified Anaplasma marginale major surface protein, AmF36, induced protection against homologous challenge with the Florida isolate. Similarly, immunized cattle were protected from challenge with the antigenically and structurally distinct Washington-O isolate of A. marginale. The degree of protection in AmF36-immunized cattle varied from complete prevention of rickettsemia to significant delay in the onset of rickettsemia compared with control immunized cattle. A single AmF36 vaccinate was not protected against homologous challenge despite development of a strong antibody response. Immunoprecipitation of A. marginale proteins with a monoclonal antibody to AmF36 identified minor molecular size heterogeneity in this protein from different isolates, including the Florida and Washington-O isolates. The apparent molecular size of this surface protein in the Florida isolate was 36 kilodaltons, whereas the analogous proteins in Washington-O and four other isolates of A. marginale from the United States had molecular masses of 33 to 34 kilodaltons. Significantly, the surface-exposed peptides of these proteins appear to be conserved among the different isolates. These results demonstrate the potential of AmF36 as a subunit immunogen for bovine anaplasmosis and indicate a structural basis for its cross-protective ability.

Characterization of antibodies to sporozoites in Plasmodium falciparum malaria and correlation with protection

The antibody response to sporozoites of Plasmodium falciparum and the role of these antibodies in protection against malaria have not been systematically investigated. An understanding of antisporozoite antibodies in natural infection is, however, important to the development of a human malaria vaccine. In a prospective study in Thailand, an antibody response to sporozoites was observed only in individuals who developed parasitemia. Antibodies were detected against an epitope in the repeat region of the circumsporozoite (CS) protein. Current candidate sporozoite vaccines are based on CS repeat antigens. The CS antibody response was of low magnitude, peaked after detection of parasitemia, and had a serum half-life of less than 1 month. CS antibody boosting occurred in only 6% of reinfected individuals. These observations suggest that antisporozoite antibody is poorly developed under natural conditions and appears not to protect against development of malaria.

Oral Salmonella typhimurium vaccine expressing circumsporozoite protein protects against malaria

Immunization with radiation-attenuated malaria sporozoites induces potent cellular immune responses, but the target antigens are unknown and have not previously been elicited by subunit vaccines prepared from the circumsporozoite (CS) protein. A method is described here for inducing protective cell-mediated immunity to sporozoites by immunization with attenuated Salmonella typhimurium transformed with the Plasmodium berghei CS gene. These transformants constitutively express CS antigens and, when used to immunize mice orally, colonize the liver, induce antigen-specific cell-mediated immunity, and protect mice against sporozoite challenge in the absence of antisporozoite antibodies. These data indicate that the CS protein contains T cell epitopes capable of inducing protective cell-mediated immunity, and emphasize the importance of proper antigen presentation in generating this response. Analogous, orally administered vaccines against human malaria might be feasible.

Features of T-cell recognition and antigen structure useful in the design of vaccines to elicit T-cell immunity

In contrast to antibodies, T lymphocytes tend to recognize a limited number of immunodominant antigenic sites on a protein antigen. Therefore, the effective design of synthetic or recombinant fragment vaccines would be better approached if these sites could be identified. From studies of the processing and genetically restricted presentation of the model protein antigen myoglobin to murine T cells, certain general principles have emerged which should be useful in this endeavour. The immunodominant sites for helper T cells in model proteins tend to be regions which can fold as alpha-helices and, in particular, helices which are amphipathic, i.e. which have a hydrophobic side and a hydrophilic side separated in space. These two sides may serve to interact with the major histocompatibility molecule on the antigen-presenting cell and with the T-cell receptor, respectively. A computer algorithm has been developed to search for such sites in protein sequences and it has been applied to two proteins of interest for vaccine development, the circumsporozoite protein of Plasmodium falciparum malaria and the envelope protein of the AIDS virus (HIV). Corresponding peptides were synthesized and shown to induce helper and/or proliferative T-cell immunity.

Malaria vaccines: immunogen selection and epitope mapping

In recent years major efforts have been made to characterize parasite antigens thought to be suitable candidates for malaria vaccines. Many of the relevant plasmodial antigens have been found to contain extensive areas of short amino acid sequences organized in tandem repeats. These are usually strongly antigenic, forming linear epitopes seen by antibodies of the infected host. Several such epitopes have been identified and subunit vaccines are being designed in which synthetic peptides or gene constructs serve as immunogens. However, as an efficient malaria vaccine should give rise to anamnestic T-dependent antibody responses following reinfection after vaccination as well as to antibody independent cell-mediated immunity, efforts are now also being made to identify T-cell epitopes on the vaccine candidate antigens. In this paper the current Plasmodium falciparum sporozoite vaccines and the merozoite antigen Pf155/RESA, a possible candidate for a P. falciparum blood stage vaccine, serve as examples to illustrate recent advances made in this area as well as some of the problems remaining to be resolved.

The main obstacle to a malaria vaccine: the malaria parasite

The challenge to development of a malaria vaccine will be the variability of the sequences of target antigens. We discuss the variability in the T cell epitopes on the circumsporozoite protein of Plasmodium falciparum and the rapid selection of mutants after immunization with a protein from the asexual erythrocytic stage of P. knowlesi.

Lacteal immunity to enteric cryptosporidiosis in mice: immune dams do not protect their suckling pups

The susceptibilities of passively immunized principal and nonimmunized control suckling mice to orogastric challenge with Cryptosporidium parvum oocysts were compared. Principals were suckled by dams that had recovered from C. parvum infection. Controls were suckled by dams reared free of C. parvum infection. Principals and controls were equally susceptible to challenge. Principals were susceptible even when their dams were hyperimmunized by oral and parenteral booster inoculations with C. parvum oocysts. Immune dams produced serum antibody against C. parvum, while nonimmune dams did not. Anti-cryptosporidia immunoglobulin G (IgG) and IgA were demonstrated in whey extracted from the stomachs of principals that had suckled immune dams but not in whey extracted from the stomachs of controls. It was concluded that passive lacteal immunity is not an efficient means of protection against cryptosporidiosis in mice. As in other coccidian infections, protective immunity against cryptosporidiosis may depend more on immune cells than on antibody.

Liposomes, lipid A, and aluminum hydroxide enhance the immune response to a synthetic malaria sporozoite antigen

A liposome-encapsulated cloned protein (R32tet32) containing sequences from the tetrapeptide repeat region of the circumsporozoite protein of Plasmodium falciparum sporozoites was examined for immunogenicity with rabbits and monkeys. Effects of adjuvants were tested by encapsulation of the antigen in liposomes either lacking or containing lipid A and adsorption with aluminum hydroxide (ALUM). When rabbits were immunized with R32tet32 alone, a primary antibody response was not seen and a secondary response did not appear until 32 to 36 weeks after boosting. Immunization with ALUM-adsorbed R32tet32 resulted in a minimal primary antibody response. A moderate secondary antibody response was detected within 2 weeks after boosting, but antibody levels decreased to preimmunization levels 8 weeks after boosting. When R32tet32 was encapsulated in liposomes containing lipid A, strong primary and secondary antibody responses were observed. Strong primary and secondary responses also were obtained when R32tet32 was encapsulated in liposomes either containing or lacking lipid A and the liposomes were adsorbed with ALUM. The strongest antibody response was obtained by immunization with ALUM-adsorbed liposomes containing lipid A and R32tet32, suggesting that the adjuvant effects of liposomes, lipid A, and ALUM were additive or synergistic.

Variation among circumsporozoite protein genes from rodent malarias

We investigated the effect of long term passage of parasites in naive animals on the circumsporozoite protein (CSP) gene of Plasmodium yoelii. The CSP gene sequence was determined from a non-lethal cloned line of P. yoelii and compared to the CSP gene sequence from a lethal strain of P. yoelii. The two parasite lines were originally derived from the same isolate, but were separated 17 years ago followed by continued passage. The sequence of the CSP gene and its surroundings from the non-lethal line remains identical to that from the lethal isolate except for a deletion within the repeated central domain. This result contrasts with the results obtained by sequencing a number of clones from different geographical field isolates of Plasmodium falciparum where there appears to be rapid accumulation of sense mutations within putative functional domains. These observations are consistent with the suggestion that strong biological pressure in a field environment results in selection of parasite types on the basis of different CSP gene sequences.

Human T-cell recognition of the circumsporozoite protein of Plasmodium falciparum: immunodominant T-cell domains map to the polymorphic regions of the molecule

The definition of human T-cell antigenic sites is important for subunit vaccine development of a peptide immunogen if the goal is to allow antibody boosting during infection or to stimulate antibody-independent T-cell immunity. To identify such sites on the circumsporozoite (CS) protein of Plasmodium falciparum, 29 overlapping synthetic peptides spanning the entire CS protein were made and tested for their ability to stimulate peripheral blood lymphocytes from 35 adults living in a P. falciparum malaria-endemic region of West Africa. Three immunodominant domains were located outside the repetitive region. These domains, however, occurred in the polymorphic regions of the molecule, suggesting that parasite mutation and selection has occurred in response to immune pressure from T cells. Such polymorphism may impose an obstacle for vaccine development.

CD8+ T cells (cytotoxic/suppressors) are required for protection in mice immunized with malaria sporozoites

In recent malaria sporozoite vaccine trials in humans and mice, antibodies to the sporozoite coat protein have given only modest protection against sporozoite challenge. In contrast, irradiated sporozoites can protect mice against massive sporozoite infections. Evidence suggests that immunity in these mice is mediated by T cells. To identify the mechanism of immunity, we used monoclonal antibodies specific for either the CD4 or CD8 molecule to selectively deplete sporozoite-immunized mice of T-cell subsets. Though in vivo depletion of CD4+ T cells did not reduce immunity, depletion of CD8+ T cells abolished protection. Monoclonal antibody treatment did not affect anti-sporozoite antibody levels. Our data indicate that cytotoxic T cells are critical for immunity to large numbers of sporozoites and suggest that vaccine development should be reoriented toward stimulating cellular as well as humoral immunity.

Progress in the development of vaccines against parasitic diseases

Vaccines for parasitic diseases have lagged behind the development of vaccines for important viral and bacterial diseases. Among the reasons for this is the extreme antigenic complexity of parasites and also the adaptive ability of parasites to co-exist with their host, often through modulation of the host's immune response. Advances in technology including in vitro cultivation, hybridoma and recombinant DNA methods have provided valuable tools for the study of parasite immunology and have resulted in major strides toward vaccines for several protozoan and helminth parasites.

In vivo testing of subunit vaccines against malaria sporozoites using a rodent system

To test the putative in vivo protective effects of antibodies to circumsporozoite (CS) protein repeats against malarial infection, different strains of mice were immunized against various repetitive regions of the Plasmodium yoelii CS protein in the form of synthetic peptides conjugated to keyhole limpet hemocyanin. Complete Freund's adjuvant or saponin was used as adjuvant. When vaccinated mice were challenged with 500 sporozoites almost all animals became infected. There were no significant protective effects in vaccinated versus unvaccinated mice. Furthermore, there was no correlation between the antibody titer to the CS repeats and infection. The parasites from infected animals were shown to encode a CS protein containing the same repeats as those used for immunization, indicating that the infections were not due to selection for variant parasites. These experiments demonstrate that antibodies to the CS repeats, as derived in vivo with peptides, despite being surface reactive, do not provide protection against sporozoite challenge in vivo. This conclusion is in contrast to previous conclusions based on studies showing protection by way of in vitro sporozoite neutralization procedures and passive transfer of monoclonal antibody.

Synthetic peptide vaccine confers protection against murine malaria

A synthetic peptide, (DPPPPNPN)2D, representing a subunit of the repeat domain of the Plasmodium berghei circumsporozoite protein, was conjugated to tetanus toxoid using bisdiazobenzidine. Immunization of mice and rats with the conjugate induced high serum titers of antibodies to the parasite, and most of the animals were completely protected from malaria infection when challenged with sporozoites.

Antigen presentation and the response to parasitic infection

Antigen presentation to T-cells is central to the induction and maintenance of the cell-mediated immune response. Many approaches have been used to define a complex accessory cell compartment, which performs the variety of functional roles encompassed by this term. Resulting from this, antigen presentation has become the cornerstone behind many theories of disease susceptibility and is an important consideration for vaccine design. In this article, Paul Kaye reviews aspects of antigen presentation by accessory cells with particular emphasis on how parasites and their antigens interact with this heterogeneous group of cells.

Cell-mediated immunity in protection and pathology of malaria

The stimulation of protective immunity against malaria is the goal of many research groups. But trials with antigens that stimulate antibodies have yet to fulfil these expectations, and it is increasingly recognized that non-antibody-mediated immunity is also important in immunity to malaria - especially through mediators such as gamma interferon, tumour necrosis factor and reactive forms of oxygen. However, the host can suffer if this type of immune response is too exuberant, and in this review, Ian Clark argues that much of what is recognized as clinical malaria is caused in this way. He suggests that only when discussed in these terms can malaria illness and pathology be seen as a coherent, predictable entity instead of a sea of unconnected surprises. Moreover, these ideas have important implications for vaccine development that, although requiring more basic work, must not be neglected.

Naturally acquired antibodies to sporozoites do not prevent malaria: vaccine development implications

The first human vaccines against the malaria parasite have been designed to elicit antibodies to the circumsporozoite protein of Plasmodium falciparum. However, it is not known whether any level of naturally acquired antibodies to the circumsporozoite protein can predict resistance to Plasmodium falciparum malaria. In this study, 83 adults in a malaria-endemic region of Kenya were tested for circumsporozoite antibodies and then treated for malaria. They were monitored for the development of new malaria infections for 98 days. Antibody levels, as determined by four assays in vitro, were indistinguishable between the 60 individuals who did and the 23 who did not develop parasitemia during follow-up, and there was no apparent relation between day of onset of parasitemia and level of antibodies to circumsporozoite protein. Unless immunization with sporozoite vaccines induces antibodies that are quantitatively or qualitatively superior to the circumsporozoite antibodies in these adults, it is unlikely that such antibodies will prevent infection in areas with as intense malaria transmission as western Kenya.

Protein antigenic structures recognized by T cells: potential applications to vaccine design

In summary, our results using the model protein antigen myoglobin indicated, in concordance with others, that helper T lymphocytes recognize a limited number of immunodominant antigenic sites of any given protein. Such immunodominant sites are the focus of a polyclonal response of a number of different T cells specific for distinct but overlapping epitopes. Therefore, the immunodominance does not depend on the fine specificity of any given clone of T cells, but rather on other factors, either intrinsic or extrinsic to the structure of the antigen. A major extrinsic factor is the MHC of the responding individual, probably due to a requirement for the immunodominant peptides to bind to the MHC of presenting cells in that individual. In looking for intrinsic factors, we noted that both immunodominant sites of myoglobin were amphipathic helices, i.e., helices having hydrophilic and hydrophobic residues on opposite sides. Studies with synthetic peptides indicated that residues on the hydrophilic side were necessary for T-cell recognition. However, unfolding of the native protein was shown to be the apparent goal of processing of antigen, presumably to expose something not already exposed on the native molecule, such as the hydrophobic sides of these helices. We propose that such exposure is necessary to interact with something on the presenting cell, such as MHC or membrane, where we have demonstrated the presence of antigenic peptides by blocking of presentation of biotinylated peptide with avidin. The membrane may serve as a short-term memory of peptides from antigens encountered by the presenting cell, for dynamic sampling by MHC molecules to be available for presentation to T cells. These ideas, together with the knowledge that T-cell recognition required only short peptides and therefore had to be based only on primary or secondary structure, not tertiary folding of the native protein, led us to propose that T-cell immunodominant epitopes may tend to be amphipathic structures. An algorithm to search for potential amphipathic helices from sequence information identified 18 of 23 known immunodominant T-cell epitopes from 12 proteins (p less than 0.001). Another statistical approach confirmed the importance of amphipathicity and also supported the importance of helical structure that had been proposed by others. It suggested that peptides able to form a stable secondary structure, especially a helix, more commonly formed immunodominant epitopes. We used this approach to predict potential immunodominant epitopes for induction of T-cell immunity in proteins of clinical relevance, such as the malarial circumsporozoite protein and the AIDS viral envelope.(ABSTRACT TRUNCATED AT 400 WORDS)

Safety and efficacy of a recombinant DNA Plasmodium falciparum sporozoite vaccine

A recombinant DNA Plasmodium falciparum sporozoite vaccine produced in Escherichia coli (FSV-1) was tested in doses of 10 micrograms to 800 micrograms protein in fifteen volunteers. No serious adverse reactions occurred. Antibodies that reacted with P falciparum sporozoite antigens by enzyme-linked immunoassay developed in twelve of the volunteers. The highest antibody titres induced were similar to those resulting from lifelong natural exposure to sporozoite-infected mosquitoes. Postimmunization serum samples from a majority of volunteers mediated the circumsporozoite (CS) precipitation reaction and inhibited sporozoite invasion of hepatoma cells in vitro. Serum from the three volunteers who received 800 micrograms doses reacted with the surface of sporozoites in an immunofluorescence assay. Six immunised volunteers receiving a fourth dose of FSV-1 and two non-immunised controls were challenged by bites of mosquitoes infected from cultured P falciparum gametocytes. Parasitaemia did not develop in the volunteer with the highest titre of CS antibodies, and parasitaemia was delayed in two other immunised volunteers. This study confirms that human beings can be protected by CS protein subunit vaccines and provides a framework for the further development and testing of more immunogenic sporozoite vaccines.