Genetically modified Plasmodium highlights the potential of whole parasite vaccine strategies

Malaria

Malaria has had a greater impact on world history than any other infectious disease. More than 300 to 500 million individuals worldwide are infected with Plasmodium spp, and 1.5 to 2.7 million people a year, most of whom are children, die from the infection. Malaria is endemic in over 90 countries in which 2400 million people live; this represents 40% of the world's population. Approximately 90% of malaria deaths occur in Africa. Despite continuing efforts in vaccine development, malaria prevention is difficult, and no drug is universally effective. This article examines malaria caused by the 4 most common Plasmodium spp that infect humans, P vivax, P ovale, P malariae, and P falciparum, as well as mixed infections and the simian parasite P knowlesi. A comprehensive review of the microbiology, clinical presentation, pathogenesis, diagnosis, and treatment of these forms of malaria is given.

Genetically engineered, attenuated whole-cell vaccine approaches for malaria

Malaria remains one of the most significant infectious diseases affecting human populations in developing countries. The quest for an efficacious malaria vaccine has been ongoing for nearly a century with limited success. The identification of malaria parasite antigens focused efforts on the development of subunit vaccines but has so far yielded only one partially efficacious vaccine candidate, RTS/S. The lack of high vaccine efficacy observed to date with subunit vaccine candidates raises doubts that the development of a single antigen or even a multi-antigen malaria subunit vaccine is possible. Fortunately, it has been demonstrated in animal studies and experimental clinical studies that immunizations with live-attenuated sporozoite stages of the malaria parasite confer long lasting, sterile protection against infection, providing a benchmark for vaccine development. These early successful vaccinations with live-attenuated malaria parasites did not however, promote a developmental path forward for such a vaccine approach. The discovery of genetically engineered parasite strains that are fully attenuated during the early asymptomatic liver infection and confer complete sterile protection in animal malaria models support the development of a live attenuated sporozoite vaccine for Plasmodium falciparum and its accelerated safety and efficacy testing in malaria challenge models and in malaria endemic areas.

Advances and challenges in malaria vaccine development

Malaria remains one of the most devastating infectious diseases that threaten humankind. Human malaria is caused by five different species of Plasmodium parasites, each transmitted by the bite of female Anopheles mosquitoes. Plasmodia are eukaryotic protozoans with more than 5000 genes and a complex life cycle that takes place in the mosquito vector and the human host. The life cycle can be divided into pre-erythrocytic stages, erythrocytic stages and mosquito stages. Malaria vaccine research and development faces formidable obstacles because many vaccine candidates will probably only be effective in a specific species at a specific stage. In addition, Plasmodium actively subverts and escapes immune responses, possibly foiling vaccine-induced immunity. Although early successful vaccinations with irradiated, live-attenuated malaria parasites suggested that a vaccine is possible, until recently, most efforts have focused on subunit vaccine approaches. Blood-stage vaccines remain a primary research focus, but real progress is evident in the development of a partially efficacious recombinant pre-erythrocytic subunit vaccine and a live-attenuated sporozoite vaccine. It is unlikely that partially effective vaccines will eliminate malaria; however, they might prove useful in combination with existing control strategies. Elimination of malaria will probably ultimately depend on the development of highly effective vaccines.

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.

Proteomic profiling of Plasmodium sporozoite maturation identifies new proteins essential for parasite development and infectivity

Plasmodium falciparum sporozoites that develop and mature inside an Anopheles mosquito initiate a malaria infection in humans. Here we report the first proteomic comparison of different parasite stages from the mosquito—early and late oocysts containing midgut sporozoites, and the mature, infectious salivary gland sporozoites. Despite the morphological similarity between midgut and salivary gland sporozoites, their proteomes are markedly different, in agreement with their increase in hepatocyte infectivity. The different sporozoite proteomes contain a large number of stage specific proteins whose annotation suggest an involvement in sporozoite maturation, motility, infection of the human host and associated metabolic adjustments. Analyses of proteins identified in the P. falciparum sporozoite proteomes by orthologous gene disruption in the rodent malaria parasite, P. berghei, revealed three previously uncharacterized Plasmodium proteins that appear to be essential for sporozoite development at distinct points of maturation in the mosquito. This study sheds light on the development and maturation of the malaria parasite in an Anopheles mosquito and also identifies proteins that may be essential for sporozoite infectivity to humans.

Human malaria is caused by Plasmodium falciparum, a unicellular protozoan parasite that is transmitted by Anopheles mosquitoes. An infectious mosquito injects saliva containing sporozoite forms of the parasite and these then migrate from the skin to the liver, where they establish an infection. Many intervention strategies are currently focused on preventing the establishment of infection by sporozoites. Clearly, an understanding of the biology of the sporozoite is essential for developing new intervention strategies. Sporozoites are produced within the oocyst, located on the outside wall of the mosquito midgut, and migrate after release from the oocysts to the salivary glands where they are stored as mature infectious forms. Comparison of the proteomes of sporozoites derived from either the oocyst or from the salivary gland reveals remarkable differences in the protein content of these stages despite their similar morphology. The changes in protein content reflect the very specific preparations the sporozoites make in order to establish an infection of the liver. Analysis of the function of several previously uncharacterized, conserved proteins revealed proteins essential for sporozoite development at distinct points of their maturation.

Antibody-mediated growth inhibition of Plasmodium falciparum: relationship to age and protection from parasitemia in Kenyan children and adults

Background

Antibodies that impair Plasmodium falciparum merozoite invasion and intraerythrocytic development are one of several mechanisms that mediate naturally acquired immunity to malaria. Attempts to correlate anti-malaria antibodies with risk of infection and morbidity have yielded inconsistent results. Growth inhibition assays (GIA) offer a convenient method to quantify functional antibody activity against blood stage malaria.

Methods

A treatment-time-to-infection study was conducted over 12-weeks in a malaria holoendemic area of Kenya. Plasma collected from healthy individuals (98 children and 99 adults) before artemether-lumefantrine treatment was tested by GIA in three separate laboratories.

Results

Median GIA levels varied with P. falciparum line (D10, 8.8%; 3D7, 34.9%; FVO, 51.4% inhibition). The magnitude of growth inhibition decreased with age in all P. falciparum lines tested with the highest median levels among children <4 years compared to adults (e.g. 3D7, 45.4% vs. 30.0% respectively, p = 0.0003). Time-to-infection measured by weekly blood smears was significantly associated with level of GIA controlling for age. Upper quartile inhibition activity was associated with less risk of infection compared to individuals with lower levels (e.g. 3D7, hazard ratio = 1.535, 95% CI = 1.012–2.329; p = 0.0438). Various GIA methodologies had little effect on measured parasite growth inhibition.

Conclusion

Plasma antibody-mediated growth inhibition of blood stage P. falciparum decreases with age in residents of a malaria holoendemic area. Growth inhibition assay may be a useful surrogate of protection against infection when outcome is controlled for age.

What can transgenic parasites tell us about the development of Plasmodium-specific immune responses?

Malaria infects 500 million people and kills an estimated 2.7 million annually, representing one of the most significant diseases in the world. However, efforts to develop effective vaccines have met with limited success. One reason is our lack of basic knowledge of how and where the immune system responds to parasite antigens. This is important as the early events during induction of an immune response influence the acquisition of effector function and development of memory responses. Our knowledge of the interactions of Plasmodia with the host immune system has largely been derived through in vitro study. This is a significant issue as the component parts of the immune system do not work in isolation and their interactions occur in distinct and specialized micro- and macro-anatomical locations that can only be assessed in the physiological context, in vivo. In this context, the availability of transgenic malaria parasites over the last 10 years has greatly enhanced our ability to understand and evaluate factors involved in host-parasite interactions in vivo. In this article, we review the current status of this area and speculate on what parasite transgenesis approaches will tell us about the development of Plasmodium-specific immune responses in the future.

Preerythrocytic malaria vaccine development

PURPOSE OF REVIEW

This review examines the potential of current preerythrocytic stage malaria vaccine approaches to reduce the global burden of malaria.

RECENT FINDINGS

Radiation-attenuated parasite vaccines induce lasting sterile protection in all models tested. Inherent safety concerns in conjunction with challenges to produce and deliver a radiation-attenuated parasite vaccine have prevented its mass production and application. Recent advances in genetic engineering and initiatives in production process development of live attenuated malaria vaccines, however, will overcome roadblocks that currently prevent their large-scale application. Development of preerythrocytic subunit vaccines has focused on the circumsporozoite protein and the thrombospondin related anonymous protein, yet the most advanced circumsporozoite protein-based vaccine confers limited protection against infection in malaria endemic areas. Work in rodent malaria models demonstrated that circumsporozoite protein-based immunity is not required for to achieve sterile protection.

SUMMARY

We conclude that preerythrocytic malaria vaccine efforts should focus on two major areas: development of a safe live attenuated sporozoite vaccine with its accelerated testing in malaria endemic areas and identification of as yet unknown antigens that reproduce sterilizing immune responses induced by vaccination with whole parasites. The sporozoite challenge model provides a unique opportunity to rapidly test preerythrocytic vaccine candidates.

Genetically attenuated Plasmodium berghei liver stages persist and elicit sterile protection primarily via CD8 T cells

Live-attenuated Plasmodium liver stages remain the only experimental model that confers complete sterile protection against malaria. Irradiation-attenuated Plasmodium parasites mediate protection primarily by CD8 T cells. In contrast, it is unknown how genetically attenuated liver stage parasites provide protection. Here, we show that immunization with uis3(-) sporozoites does not cause breakthrough infection in T and B-cell-deficient rag1(-/-) and IFN-gamma(-/-) mice. However, protection was abolished in these animals, suggesting a crucial role for adaptive immune responses and interferon-gamma. Although uis3(-) immunization induced Plasmodium-specific antibodies, B- cell-deficient mice immunized with uis3(-) sporozoites were completely protected against wild-type sporozoite challenge infection. T-cell depletion experiments before parasite challenge showed that protection is primarily mediated by CD8 T cells. In good agreement, adoptive transfer of total spleen cells and enriched CD8 T cells from immunized animals conferred sterile protection against malaria transmission to recipient mice, whereas adoptive transfer of CD4 T cells was less protective. Importantly, primaquine treatment completely abolished the uis3(-)-mediated protection, indicating that persistence of uis3(-)-attenuated liver stages is crucial for their protective action. These findings establish the basic immune mechanisms underlying protection induced by genetically attenuated Plasmodium parasites and substantiate their use as vaccines against malaria.

A clash to conquer: the malaria parasite liver infection

All mammalian malaria parasite species have an initial tissue stage in liver cells. The liver stage produces new parasite forms that can enter and live inside red blood cells. Accordingly, the first place of residence provides parasites with a radically different cellular and molecular environment from their subsequent red blood cell home. Liver stages have remained refractory to reveal their secrets, yet the last few years have seen several advances in elucidating their biology. This review looks at the more recent findings concerning the liver stage-host hepatocyte association, some of which may become powerful weapons in the prevention of malaria infection. We also outline areas of liver stage research and technological development that provide promising foci to accelerate a better understanding of this most elusive of the parasites many life cycle stages.

The chequered history of malaria control: are new and better tools the ultimate answer?

Following Ronald Ross' demonstration in 1897 that mosquitoes transmit malarial parasites, efforts to control malaria were naturally focussed on attacking the mosquito vector by various measures, mainly directed against the aquatic stages. Although the results were spectacular in some areas, there was a growing realisation that effective control of malaria depended on other factors, including the availability of better drugs than quinine and a greater understanding of the epidemiology of the disease under various environmental conditions. With the discovery of DDT, an all-out effort was made to eradicate malaria by attacking adult mosquitoes. Eradication was not achieved in many countries, mainly because of inadequate health infrastructures. With the emergence of chloroquine-resistant parasites, the search for more effective drug regimens intensified, various drugs and drug combinations were assessed, and methods were developed to monitor and assess degrees of resistance. Since resistance to drugs can develop very quickly, the use of drug combinations, especially those containing artemisinin derivatives, is now recommended. Insecticide-impregnated bednets have become the preferred method of vector control. Although the search for better tools must continue, the events of the past century emphasise the need to strengthen health systems to ensure that they are capable of delivering effective interventions and of assessing their effectiveness in controlling malaria.

Manipulation of host innate immune responses by the malaria parasite

It has long been known that malaria infection causes host immune modulation by various mechanisms. However, the role of Toll-like receptors (TLRs) in mediating innate immune responses to parasite-derived components during the blood stages of malaria has only recently been described. TLRs might have an important role in pathogenesis during malaria infection, as supported by genetic analyses in mice and humans. Moreover, recent findings revealed that sporozoites can partially differentiate in lymph nodes and that liver stages induce the formation of previously unknown parasite-filled vesicles (merosomes) that could function as immune escape machinery. Elucidation of the mechanisms by which the host innate immune system responds to, and/or is manipulated by, Plasmodium infection will hopefully lead to discoveries of potential targets that will ultimately prevent and/or intervene in malaria infection.

Plasmodium yoelii sporozoites with simultaneous deletion of P52 and P36 are completely attenuated and confer sterile immunity against infection

Malaria infection starts when sporozoites are transmitted to the mammalian host during a mosquito bite. Sporozoites enter the blood circulation, reach the liver, and infect hepatocytes. The formation of a parasitophorous vacuole (PV) establishes their intracellular niche. Recently, two members of the 6-Cys domain protein family, P52 and P36, were each shown to play an important albeit nonessential role in Plasmodium berghei sporozoite infectivity for the rodent host. Here, we generated p52/p36-deficient Plasmodium yoelii parasites by the simultaneous deletion of both genes using a single genetic manipulation. p52/p36-deficient parasites exhibited normal progression through the life cycle during blood-stage infection, transmission to mosquitoes, mosquito-stage development, and sporozoite infection of the salivary glands. p52/p36-deficient sporozoites also showed normal motility and cell traversal activity. However, immunofluorescence analysis and electron microscopic observations revealed that p52/p36-deficient parasites did not form a PV within hepatocytes in vitro and in vivo. The p52/p36-deficient parasites localized as free entities in the host cell cytoplasm or the host cell nucleoplasm and did not develop as liver stages. Consequently, they did not cause blood-stage infections even at high sporozoite inoculation doses. Mice immunized with p52/p36-deficient sporozoites were completely protected against infectious sporozoite challenge. Our results demonstrate for the first time the generation of two-locus gene deletion-attenuated parasites that infect the liver but do not progress to blood-stage infection. The study will critically guide the design of Plasmodium falciparum live attenuated malaria vaccines.

High road, low road? Choices and challenges on the pathway to a malaria vaccine

Malaria causes much physical and economic hardship in endemic countries with billions of people at risk. A vaccine would clearly benefit these countries, reducing the requirement for hospital care and the economic impact of infection. Successful immunization with irradiated sporozoites and the fact that repeated exposure to malaria induces partial immunity to infection and high levels of protection against the clinical manifestations, suggest that a vaccine is feasible. Numerous candidate antigens have been identified but the vaccine, which has been promised to be 'just round the corner' for many years, remains elusive. The factors contributing to this frustratingly slow progress are discussed including gaps in the knowledge of host/parasite biology, methods to induce potent cell-mediated immune responses, the difficulties associated with defining immune correlates of protection and antigen production and delivery. Finally, the use of attenuated organism vaccines is discussed.

Cellular interactions of Plasmodium liver stage with its host mammalian cell

The Plasmodium liver forms are bridgehead stages between the mosquito sporozoite stages and mammalian blood stages that instigate the malaria disease. In hepatocytes, Plasmodium achieves one of the fastest growth rates among eukaryotic cells. However, nothing is known about host hepatic cell interactions, e.g. nutrient scavenging and/or subversion of cellular functions necessary for Plasmodium development and replication. Plasmodium usually invades hepatocytes by establishing a parasitophorous vacuole wherein it undergoes multiple nuclear division cycles. We show that Plasmodium preferentially develops in the host juxtanuclear region. By comparison with the parasitophorous vacuole of other apicomplexan parasites which associate with diverse host organelles, the Plasmodium parasitophorous vacuole only forms an association with the host endoplasmic reticulum. Intrahepatic Plasmodium actively modifies the permeability of its vacuole to allow the transfer of a large variety of molecules from the host cytosol to the vacuolar space through open channels. In contrast with malaria blood stages, the pores within the parasitophorous vacuole membrane of the liver stage display a smaller size as they restrict the passage of solutes to less than 855Da. These pores are stably maintained during parasite karyokinesis until complete cellularisation. Host-derived cholesterol accumulated at the parasitophorous vacuole membrane may modulate the channel activity. These observations define the parasitophorous vacuole of the Plasmodium liver stage as a dynamic and highly permeable compartment that can ensure the sustained supply of host molecules to support parasite growth in the nutrient-rich environment of liver cells.

Parasite vaccines – recent progress and problems associated with their development

The treatment and prevention of parasitism in both humans and livestock continues to rely almost exclusively on the use of antiparasitic drugs – an approach which has limitations, particularly as reinfection, which occurs rapidly in endemic regions, is not prevented. In addition, the widespread appearance of drug-resistant parasites of animals (Kaplan, 2004;) together with emerging evidence of resistance problems in human parasites (Fallonet al. 1995; Ismailet al. 1996; De Clerqet al. 1997; East African Network for Monitoring Antimalarial Treatment, 2003), emphasise the importance of developing alternative methods of control, with anti-parasite vaccines a prime target.

Natural killer cells and malaria

Malaria, caused by the infection with parasites of the germs Plasmodium, is one of the three most important infectious diseases worldwide, along with tuberculosis and infection with human immunodeficiency virus. Natural killer (NK) cells are lymphocytes classically involved in the early defense against viral infections and intracytoplasmic bacterial infections and are also implicated during the course of tumor development and allogeneic transplantation. These cells display important cytotoxic activity and produce high levels of proinflammatory cytokines. In both mouse and human models of malaria, NK cells appear to be a major source of interferon-gamma during the early phase of infection. In humans, indirect signaling through monocytes/macrophages required to optimally stimulate NK cell activity. However, the in vivo functions of NK cells during malaria are still enigmatic, and many issues remain to be dissected, such as the molecular basis of the direct recognition of iRBCs by NK cells.

A novel chimeric Plasmodium vivax circumsporozoite protein induces biologically functional antibodies that recognize both VK210 and VK247 sporozoites

A successful vaccine against Plasmodium vivax malaria would significantly improve the health and quality of the lives of more than 1 billion people around the world. A subunit vaccine is the only option in the absence of long-term culture of P. vivax parasites. The circumsporozoite protein that covers the surface of Plasmodium sporozoites is one of the best-studied malarial antigens and the most promising vaccine in clinical trials. We report here the development of a novel "immunologically optimal" recombinant vaccine expressed in Escherichia coli that encodes a chimeric CS protein encompassing repeats from the two major alleles, VK210 and VK247. This molecule is widely recognized by sera from patients naturally exposed to P. vivax infection and induces a highly potent immune response in genetically disparate strains of mice. Antibodies from immunized animals recognize both VK210 and VK247 sporozoites. Furthermore, these antibodies appear to be protective in nature since they cause the agglutination of live sporozoites, an in vitro surrogate of sporozoite infectivity. These results strongly suggest that recombinant CS is biologically active and highly immunogenic across major histocompatibility complex strains and raises the prospect that in humans this vaccine may induce protective immune responses.

[Malaria vaccines: prospects and reality]

The development of a malaria vaccine has been accelerating in the last ten years. The number of clinical trials has increased and some malaria antigens have been tested in endemic areas. No potential vaccine has yet shown sufficient and lasting efficacy to justify its inclusion in a public health program. However, trials have unambiguously shown that some level of anti-malaria clinical immunity can be achieved by vaccination, both in experimental and in field conditions. Advances in malaria vaccine development are presented.

Immunization with a circumsporozoite epitope fused to Bordetella pertussis adenylate cyclase in conjunction with cytotoxic T-lymphocyte-associated antigen 4 blockade confers protection against Plasmodium berghei liver-stage malaria

The adenylate cyclase toxoid (ACT) of Bordetella pertussis is capable of delivering its N-terminal catalytic domain into the cytosol of CD11b-expressing professional antigen-presenting cells such as myeloid dendritic cells. This allows delivery of CD8+ T-cell epitopes to the major histocompatibility complex (MHC) class I presentation pathway. Recombinant detoxified ACT containing an epitope of the Plasmodium berghei circumsporozoite protein (CSP), indeed, induced a specific CD8+ T-cell response in immunized mice after a single application, as detected by MHC multimer staining and gamma interferon (IFN-gamma) ELISPOT assay. This CSP-specific response could be significantly enhanced by prime-boost immunization with recombinant ACT in combination with anti-CTLA-4 during the boost immunization. This increased response was accompanied by complete protection in a number of mice after a challenge with P. berghei sporozoites. Transient blockade of CTLA-4 may overcome negative regulation and hence provide a strategy to enhance the efficacy of a vaccine by amplifying the number of responding T cells.

Quantitative imaging of Plasmodium transmission from mosquito to mammal

Plasmodium, the parasite that causes malaria, is transmitted by a mosquito into the dermis and must reach the liver before infecting erythrocytes and causing disease. We present here a quantitative, real-time analysis of the fate of parasites transmitted in a rodent system. We show that only a proportion of the parasites enter blood capillaries, whereas others are drained by lymphatics. Lymph sporozoites stop at the proximal lymph node, where most are degraded inside dendritic leucocytes, but some can partially differentiate into exoerythrocytic stages. This previously unrecognized step of the parasite life cycle could influence the immune response of the host, and may have implications for vaccination strategies against the preerythrocytic stages of the parasite.