A merozoite vaccine effective against Plasmodium knowlesi malaria

Using infective mosquitoes to challenge monkeys with Plasmodium knowlesi in malaria vaccine studies

BACKGROUND

When rhesus monkeys (Macaca mulatta) are used to test malaria vaccines, animals are often challenged by the intravenous injection of sporozoites. However, natural exposure to malaria comes via mosquito bite, and antibodies can neutralize sporozoites as they traverse the skin. Thus, intravenous injection may not fairly assess humoral immunity from anti-sporozoite malaria vaccines. To better assess malaria vaccines in rhesus, a method to challenge large numbers of monkeys by mosquito bite was developed.

METHODS

Several species and strains of mosquitoes were tested for their ability to produce Plasmodium knowlesi sporozoites. Donor monkey parasitaemia effects on oocyst and sporozoite numbers and mosquito mortality were documented. Methylparaben added to mosquito feed was tested to improve mosquito survival. To determine the number of bites needed to infect a monkey, animals were exposed to various numbers of P. knowlesi-infected mosquitoes. Finally, P. knowlesi-infected mosquitoes were used to challenge 17 monkeys in a malaria vaccine trial, and the effect of number of infectious bites on monkey parasitaemia was documented.

RESULTS

Anopheles dirus, Anopheles crascens, and Anopheles dirus X (a cross between the two species) produced large numbers of P. knowlesi sporozoites. Mosquito survival to day 14, when sporozoites fill the salivary glands, averaged only 32% when donor monkeys had a parasitaemia above 2%. However, when donor monkey parasitaemia was below 2%, mosquitoes survived twice as well and contained ample sporozoites in their salivary glands. Adding methylparaben to sugar solutions did not improve survival of infected mosquitoes. Plasmodium knowlesi was very infectious, with all monkeys developing blood stage infections if one or more infected mosquitoes successfully fed. There was also a dose-response, with monkeys that received higher numbers of infected mosquito bites developing malaria sooner.

CONCLUSIONS

Anopheles dirus, An. crascens and a cross between these two species all were excellent vectors for P. knowlesi. High donor monkey parasitaemia was associated with poor mosquito survival. A single infected mosquito bite is likely sufficient to infect a monkey with P. knowlesi. It is possible to efficiently challenge large groups of monkeys by mosquito bite, which will be useful for P. knowlesi vaccine studies.

New approaches to studying Plasmodium falciparum merozoite invasion and insights into invasion biology

Merozoite invasion of human red blood cells by Plasmodium falciparum is essential for blood stage asexual replication and the development of malaria disease. Despite this, many of the processes involved in invasion are poorly understood. Recent advances have been made in methods to isolate viable merozoites for studies of invasion. The application of these approaches is providing new insights into the kinetics of invasion and merozoite survival, as well as proteins and interactions involved in invasion, and will facilitate the development and testing of anti-merozoite vaccines and the identification of invasion-inhibitory compounds with potential for drug development. This review discusses these recent advances and considers potential avenues for future research.

Plasmodium knowlesi: a malaria parasite of monkeys and humans

Plasmodium knowlesi is a malaria parasite of monkeys of Southeast Asia that is transmitted by mosquitoes of the Anopheles leucosphyrus group. Humans are frequently infected with this parasite and misdiagnosed as being infected with Plasmodium malariae. The parasite was a major monkey animal model for developing antimalarial vaccines and investigations of the biology of parasite invasion. P. knowlesi is the first monkey malaria parasite genome to be sequenced and annotated.

Parasitostatic effect of maslinic acid. II. Survival increase and immune protection in lethal Plasmodium yoelii-infected mice

Background

The anti-malarial activity of maslinic acid (MA), a natural triterpene which has been previously shown to exert a parasitostatic action on Plasmodium falciparum cultures, was analysed in vivo by using the Plasmodium yoelii 17XL murine model.

Methods

ICR mice were infected with P. yoelii and treated with a single dose of MA by a intraperitoneal injection of MA (40 mg kg^-1 day^-1) followed by identical dose administration for the following three days. Parasitaemia and accumulation of intraerythrocytic stages was monitored microscopically. To assess protective immunity, cured mice were challenged with the same dose of parasites 40 days after recovery from the primary infection and parasitaemia was further monitored for 30 days. Humoral response was tested by ELISA and visualization of specific anti-P. yoelii antibodies was performed by Western-blotting.

Results

ICR mice treated with MA increased the survival rate from 20% to 80%, showing an arrest of parasite maturation from day 3 to 7 after infection and leading to synchronization of the intraerythrocytic cycle and accumulation of schizonts by day 6, proving that MA also behaves as a parasitostatic agent in vivo. Mice which survived the primary infection displayed lower rates of parasitic growth, showing a decline of parasitaemia after day 15, and complete clearance at day 20. These mice remained immunoprotected, showing not malaria symptoms or detectable parasitaemia after rechallenge with the same lethal strain. The analysis of specific antibodies against P. yoelii, present in mice which survived the infection, showed a significant increase in the number and intensity of immunoreactive proteins, suggesting that the protected mice may trigger a strong humoral response.

Conclusion

The survival increase observed in MA-treated mice can be explained considering that the parasitostatic effect exerted by this compound during the first days of infection increases the chances to develop effective innate and/or acquired immune responses. MA may represent a new class of anti-malarial compounds which, as a consequence of its parasitostatic action, favours the development of more effective sterilizing immune responses.

Acquired immunity to malaria

Naturally acquired immunity to falciparum malaria protects millions of people routinely exposed to Plasmodium falciparum infection from severe disease and death. There is no clear concept about how this protection works. There is no general agreement about the rate of onset of acquired immunity or what constitutes the key determinants of protection; much less is there a consensus regarding the mechanism(s) of protection. This review summarizes what is understood about naturally acquired and experimentally induced immunity against malaria with the help of evolving insights provided by biotechnology and places these insights in the context of historical, clinical, and epidemiological observations. We advocate that naturally acquired immunity should be appreciated as being virtually 100% effective against severe disease and death among heavily exposed adults. Even the immunity that occurs in exposed infants may exceed 90% effectiveness. The induction of an adult-like immune status among high-risk infants in sub-Saharan Africa would greatly diminish disease and death caused by P. falciparum. The mechanism of naturally acquired immunity that occurs among adults living in areas of hyper- to holoendemicity should be understood with a view toward duplicating such protection in infants and young children in areas of endemicity.

Development of malaria blood-stage vaccines: learning from mosquitoes

If current methods of vaccine development for malaria continue to result in vaccines with only relatively limited degrees of protection, what is the alternative? Here, a totally different approach to blood-stage vaccine research is suggested, focusing on malarial immunity as it develops in macaque monkeys, but using methodology already well established in mosquito research.

Chimeric epitopes delivered by polymeric synthetic linear peptides induce protective immunity to malaria

Polymeric linear peptide chimeras (LPCs) that incorporate Plasmodium vivax promiscuous T cell epitopes and the P. falciparum circumsporozoite protein B cell epitope have been shown to induce a high level of immunogenicity and overcome genetic restriction when tested as vaccine immunogens in BALB/c mice. The present study evaluates the biological relevance of several LPCs using a well characterized rodent malaria model. Polymeric peptide constructs based on P. berghei and P. yoelii sequences, and orthologous to the human malaria sequences included in the original LPCs, were designed and tested for immunogenicity in mice of different H-2 haplotypes. We demonstrate that robust immune responses are induced and that peptides containing the orthologous rodent Plasmodium sequences exhibited similar immunogenic capabilities. Unique to this report, we show that LPCs can also prime MHC class I-restricted cytotoxic T lymphocytes (CTLs) and, most relevantly, that a peptide construct prototype incorporating single B, T and CTL epitopes induced protection against an experimental challenge with P. berghei or P. yoelii sporozoites. Collectively, these results suggest that polymeric polypeptide chimeras can be used as a platform to deliver subunit vaccines.

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.

Extracellular development of Plasmodium knowlesi erythrocytic stages in an artificial intracellular medium

The development of erythrocytic stages of Plasmodium knowlesi separated from their host cells has been determined in terms of the capacity of the isolated organisms to carry out the synthesis and secretion of proteins. P. knowlesi trophozoites and schizonts were released from host cells by nitrogen decompression and cultivated in a medium consisting of 20 mM Na+; 120 mM K+; 1 mM Mg2+; no Ca2+; 100 mM Cl-; 20 mM HCO3-; 5 mM Hepes [pH 6.73], glucose, vitamins, amino acids and 10% fetal calf serum. The yield was about 97% intact parasites, judging by their ability to maintain a membrane potential, and these parasites had more than 80% the capacity of infected cells for nuclear replication and macromolecule biosynthesis. Pulse and pulse-chase labeling studies with [35S]methionine show that parasite-synthesized proteins with Mr 160 000, 140 000, 100 000 and 58 000 are exported from the parasite in soluble form. Proteins with Mr 140 000, 100 000, 58 000-60 000, 40 000 were recovered in a particulate fraction isolated from the parasite culture fluid. An Mr 62 000 protein synthesized in large amounts by isolated parasites during the last 2h of the developmental cycle, could not be detected in infected erythrocytes, and a minor early Mr 74 000 protein becomes prominent in free parasites but not infected cells toward the end of the developmental cycle. Parasite-synthesized proteins with Mr 230 000, 160 000, 140 000, 62 000, 58 000 and 45 000 were labeled by incubation with radioactive N-acetylglucosamine during short term incubation in vitro. About 80% of label incorporation occurred via N-glycosylation supported by dolichol derived from the blood, and about 20% via glycolytic intermediates.

Biosynthesis of a putative protective Plasmodium knowlesi merozoite antigen

A putative protective Plasmodium knowlesi antigen, recognized by two monoclonal antibodies which prevent invasion of erythrocytes by merozoites in vitro, is synthesised only by schizonts with 7 or more nuclei, during the last 1.5-2 h of the 24 h erythrocytic cycle of parasite development. The 66 000 MW antigen, which constitutes a minor parasite protein, is processed at the time of schizont rupture and merozoite release, to give rise to two smaller molecules of 44 000 and 42 000 MW. The 44 000 and 42 000 MW antigens and traces of the 66 000 MW antigen are present on the surface of isolated merozoites and the smaller antigens are readily shed. The antigens which are shed are soluble in culture medium and are not readily degraded further. None of the molecules can be detected in newly invaded ring stage parasites, indicating that they are either excluded when the merozoite invades, or if internalised, they rapidly undergo further structural alteration since newly parasitised red cells no longer contain epitopes which react with the inhibitory monoclonal antibodies. This antigen is distinct from other putative protective antigens described in bloodstage malaria parasites.

A comparison of saponin with other adjuvants for the potentiation of protective immunity by a killed Plasmodium yoelii vaccine in the mouse

The protective immunity conferred by subcutaneous injection of outbred CD-1 mice with a killed Plasmodium yoelii (YM strain) vaccine was strongly potentiated by saponin. By adjusting the dose of antigen, the number of immunizations and the number of living parasites in the challenge infection, conditions were defined where antigen alone was non-protective but 100% protection was obtained by the addition of saponin. Inbred BALB/c, CBA/CA and C57 B1 mice were much less responsive than the CD-1 mice. The following adjuvants were compared with saponin: mineral oil emulsions (Freund's incomplete and complete adjuvants); A1(OH)3(Alhydrogel); bacteria and synthetic bacterial derivatives (Bordetella pertussis, Corynebacterium parvum and muramyl dipeptide); surface active materials (digitonin, vitamin A, Arquad 18, dimethyldioctadecyl ammonium bromide, and the polyene antibiotics, Nystatin and Amphotericin B). None of these adjuvants were as effective as saponin, although FCA, A1(OH)3 and C. parvum augmented immunity considerably. The possible reasons for the efficacy of saponin as an adjuvant for protozoal vaccines are discussed. The P. yoelli/mouse system provides a sensitive and rapid screening assay for comparison of potential adjuvants suitable for use with a malaria vaccine.

Antigenic diversity in the human malaria parasite Plasmodium falciparum

Monoclonal antibodies against blood forms of Plasmodium falciparum were used to demonstrate considerable antigenic diversity in this species. Different isolates were distinguished by their ability to react with certain antibodies, and most of the antibodies reacted specifically with merozoites, schizonts, or both. The distribution of different antigenic types appeared not to be related to geographic origin. Serological typing with monoclonal antibodies extends the range of methods for identification of different strains of this malaria parasite.

Rhesus monkeys protected against Plasmodium knowlesi malaria produce antibodies against a 65,000-MrP. knowlesi glycoprotein at the surface of infected erythrocytes

Sera from 27 rhesus monkeys immunized in various ways against the H strain of Plasmodium knowlesi were analyzed by quantitative crossed immunoelectrophoresis. The reaction of the sera was compared with a reference immune serum only reactive with P. knowlesi-specific 65,000-Mr glycoprotein-immune component 13 (gp65/ic13) in membranes of infected rhesus monkey erythrocytes. Triton X-100-solubilized, 125I-labeled membranes of schizont-infected erythrocytes were used as an antigen. Sera from 9 or 10 monkeys immunized by repeated infections with P. knowlesi reacted with gp65/ic13. In 6 of 10 sera, anti-gp65/ic13 was the only antibody reacting with host cell membrane proteins. In contrast, vaccination of 15 monkeys with predominantly sexual stages or trophozoites of P. knowlesi in Freund complete adjuvant resulted in protection against blood challenges in 7 monkeys, only 2 of which contained precipitating antibody against gp65/ic13. None of the sera from monkeys not protected by infections or vaccinations contained detectable levels of precipitating antibodies against gp65/ic13. Our data indicate that gp65/ic13 acts as a prominent immunogen in vivo during natural p. knowlesi infections of rhesus monkeys. There is a positive correlation suggested between anti-gp65/ic13 antibody and protection in the monkeys analyzed. This correlation does not apply to monkeys protected against P. knowlesi malaria by vaccination, pointing to other effective immune defense mechanisms.

Plasmodium knowlesi variant antigens are found on schizont-infected erythrocytes but not on merozoites

In this report, we demonstrate that the variable antigens present on the surface of Plasmodium knowlesi-infected erythrocytes could not be found on the surface of merozoites. A number of technical problems had to be solved to make such a comparative study possible, including the purification of merozoites by affinity columns and gradient centrifugation, the use of hyperimmune rabbit sera instead of monkey sera, and the use of immunocytological methods (indirect immunofluorescence antibody test and electron microscopy with ferritin-labeled antibodies) instead of the schizont-infected cell agglutination test. To ensure that these new techniques were valid for variant-specific serotyping, we compared two well-characterized variant populations with both the standard and the new methods. The removal of variant antigens from the surface of infected erythrocytes by proteolytic enzymes provides further information on the biochemical nature of these antigens.

Monoclonal antibodies to stage-specific, species-specific, and cross-reactive antigens of the rodent malarial parasite, Plasmodium yoelii

Eighteen hybridoma cell lines were used to study species-specific, stage-specific, and serological cross-reactive antigens of the rodent malarial parasite, Plasmodium yoelii. Specificity and location of plasmodial antigens were determined by indirect fluorescent-antibody analysis. Results showed that a minimum of 12 distinct plasmodial antigens could be distinguished by the 18 hybridomas. Antigens were found on the surface or within the cytoplasm of the parasite, but not on the surface of erythrocytes from infected animals. The majority (11 of 12) of antigens were present in all erythrocytic stages of the parasite, but one was stage-specific for merozoites. Additional studies showed that 6 of 18 of the monoclonal antibodies identified species-specific antigens, 2 of 18 recognized antigens confined to related rodent malarial parasites (Plasmodium berghei, Plasmodium vinckei, and Plasmodium chabaudi), whereas 8 of 18 detected cross-reactive antigens common to rodent, primate (Plasmodium knowlesi, Plasmodium falciparum), and avian (Plasmodium gallinaceum) malarias.

Vaccination of chicks against Plasmodium gallinaceum by erythrocytic and exoerythrocytic parasites attenuated by gamma irradiation

Plasmodium gallinaceum-infected blood which received up to 24 krad during exposure to gamma-rays from a cobalt-60 source produced infections of normal course and duration when injected into chickens. The prepatent period advanced with increasing exposure of infected blood to radiation, suggesting some degree of attenuation. At 26, 28 and 30 krad, the infections were transient and the parasites were morphologically abnormal. It is thought that the amount of radiation required to render the parasites non-viable is about 45 krad for an inoculum of 10(6) parasites. There is evidence that exoerythrocytic stages may be more susceptible to gamma-rays than are blood parasites. Chickens were inoculated three times, over a period of four weeks, with vaccines prepared from gamma-irradiated infected blood and brain tissue. Half the birds which had been inoculated with attenuated parasitized blood exhibited mild infections during vaccination, and they were the only birds to show at challenge immunity to both homologous blood and exoerythrocytic parasites.

Recent advances in applied malaria immunology

Our present knowledge of cellular and humoral factors which are involved in immunity to plasmodial infections are discussed. Immunization against plasmodial infection has been achieved in birds, rodents, simians, and humans. Avian hosts have been immunized against gametocytes which resulted in inhibition of gametocytes within the mosquito vector. Immunization of humans against plasmodial gametocytes would indirectly protect them against malaria by blocking mosquito transmission to other susceptible individuals. Immunization by sporozoites provides short-lived protection against sporozoite challenge, but gives no protection against erythrocytic forms. Some success has been obtained in immunizing avian and mammalian hosts with exoerythrocytic forms obtained from cultured avian cells. The most significant advances have occurred in immunizing simian hosts against simian or human malaria by vaccinating with fresh erythrocytic merozoites or a nonviable lyophilized antigen obtained from intraerythrocytic forms. The development of an antigen preparation suitable for use as a human malaria vaccine is dependent upon prior development of an in vitro system which would provide adequate amounts of parasite material. Efforts to cultivate the sporogonic, exoerythrocytic, and erythrocytic, and erythrocytic phases of plasmodia as well as the feasibility of using these forms for vaccination are discussed.

An effective immunization of experimental monkeys against a human malaria parasite, Plasmodium falciparum

This is the first report of successful immunization of experimental monkeys against a human malaria parasite, Plasmodium falciparum. Of the five owl monkeys (Aotus trivirgatus) used in this pilot study, two served as controls and the other three were immunized with P. falciparum antigen consisting primarily of mature segmenters containing fully developed merozoites. Two injections of antigen emulsified with Freund's complete adjuvant were administered intramuscularly 3 weeks apart. Three weeks after the second vaccination, all monkeys were challenged with the homologous strain of P. falciparum. The control monkeys died with high levels of parasitemia within 2 weeks of challenge. The three immunized monkeys survived and showed strong protection against P. falciparum. These results are encouraging for the possible future development of an effective vaccine against human malaria.

In vitro isolation of Plasmodium knowlesi merozoites using polycarbonate sieves

A culture chamber fitted with a polycarbonate sieve has been used to isolate Plasmodium knowlesi merozoites as they are released from schizonts. A 3 mum pore-size sieve allows passage of normal erythrocytes and red cells containing rings and trophozoites and can be used to concentrate schizonts from a mixed cell population. A 2 mum pore-size sieve retains normal and parasitized cells and provides uncontaminated merozoites in high yield (5 x 10(10) merozoites per ml schizonts). Merozoite viability diminishes rapidly during 30 min after isolation. These preparations should prove valuable for studies of the biochemical, physiological and antigenic properties of this transient phase of the malaria parasite.