REVIEWS TOWARDS A BLOOD-STAGE VACCINE FOR MALARIA: ARE WE FOLLOWING ALL THE LEADS? Michael F. Good Although the malaria parasite was discovered more than 120 years ago, it is only during the past 20 years, following the cloning of malaria genes, that we have been able to think rationally about vaccine design and development. Effective vaccines for malaria could interrupt the life cycle of the parasite at different stages in the human host or in the mosquito. The purpose of this review is to outline the challenges we face in developing a vaccine that will limit growth of the parasite during the stage within red blood cells — the stage responsible for all the symptoms and pathology of malaria. More than 15 vaccine trials have either been completed or are in progress, and many more are planned. Success in current trials could lead to a vaccine capable of saving more than 2 million lives per year. The malaria parasite remains a scourge on human civi- the Nussenzweig’s group in New York); and now early- lization and in recent years the incidence of the disease phase vaccine trials (BOX 1). Although there are dozens has been increasing. It is estimated that 1.5–2.5 million of species of malaria parasites, those that infect humans people die each year from malaria — mostly young are limited to Plasmodium falciparum, P. vivax, P. ovale children and pregnant women. Although most of these and P. malariae. P. falciparum and P. vivax are the deaths occur in sub-Saharan Africa, no country is with- most common, and P. falciparum is responsible for out malaria — either through endemic transmission or most of the malaria deaths. Multiple strains of each through importation of cases from endemic regions of species exist, differing at crucial antigenic determi- the world. nants. Although some degree of immunity can devel- It is now more than 120 years since the French op between strains, there is generally thought to be no physician Charles Louis Alphonse Laveran first inter-species immunity. observed malaria parasites under the microscope, and more than 100 years since Ronald Ross and Giovanni Malaria vaccine development Grassi identified the vector of malaria transmission. Vaccination is a successful method of disease control Malaria is not, therefore, a newly described disease. and there have been numerous success stories — the Since then, there have been many significant develop- most notable being the eradication of smallpox and Cooperative Research ments in malaria research, which have included: the virtual elimination of polio. Many factors conspire Centre for Vaccine unravelling the complex life cycle of the parasite; the to make the development of a malaria vaccine an Technology, The Queensland Institute development of anti-malarial drugs and insecticides; incredibly difficult challenge. An important factor of Medical Research, the discovery of ‘malaria therapy’ for tertiary syphilis impeding vaccine development is the complex biolo- The Bancroft Centre, (in which the fever associated with malaria killed the gy of the life cycle of the parasite, which exists in dif- 300 Herston Road, heat-sensitive Treponema pallidum organism responsi- ferent forms (and each form with a different pattern Herston QLD 4006, Australia. ble for syphilis); the development of drug resistance; of antigen expression) in different tissues of the body e-mail: the cloning of malaria genes (by Kemp and Anders and the mosquito (FIG. 1). In these various forms the [email protected] and colleagues in Melbourne, and by Ellis, Godson and parasite is susceptible to immune attack, although NATURE REVIEWS | IMMUNOLOGY VOLUME 1 | NOVEMBER 2001 | 117 © 2001 Macmillan Magazines Ltd REVIEWS Box 1 | Dates in malaria research Most of the vaccines that have been developed for various diseases have relied on the ‘simple’ approach of • 1880 Discovery of the malaria parasite by Charles Louis Alphonse Laveran108. presenting the entire antigenic compartment of the • 1897–1898 Discovery of the malaria insect vector and life cycle by Ronald Ross109 organism in the form of killed or living, but attenuated, and Giovanni Grassi110. organisms5. Such an approach is not possible at present • 1922 Malaria therapy for syphilis by Julius Wagner von Jauregg111. for malaria as the organisms grow within red blood cells • 1939 DDT (1,1,1-trichloro-2,2-bis-(p-chlorophenyl)-ethane) discovered as an (RBCs) in the mammalian host (FIG. 1), and although insecticide by Paul Muller112. in vitro culture is possible (for example, for P. falciparum, the most virulent of the human plasmodia), a source of • 1943 Chloroquine discovered. RBCs is required. However, it is simply impractical and 113 • 1947 Exoerythrocytic stage of parasite life cycle described by Fairley . potentially unsafe to consider growing a vaccine in • 1957 Chloroquine resistance first developed. human RBCs for a disease for which 40% of the world’s • 1983 Malaria antigens cloned by Kemp et al.114 and Nussenzweig and population is potentially at risk. Malaria vaccine devel- co-workers115. opment has therefore focused on the development of a • 1986 The first human vaccine trials underway. subunit vaccine. The purpose of this review is to discuss the differ- • 2001 Sequence of malaria genome (near completion). ent strategies for developing a vaccine to the blood stage of malaria. Why is it important to target the the type of immune response required is very different blood stage of the parasite? There are different phases for each form. Several vaccine strategies, therefore, in the life cycle of the parasite (FIG. 1): the stage inside need to be used. the Anopheline mosquito vector, the ‘pre-erythrocytic A second factor that impedes vaccine development stage’ and the erythrocytic or blood stage. The pre- is the ability of the malaria parasite to alter itself. ANTI- erythrocytic stage, during which the sporozoites travel GENIC VARIATION and ALLELIC POLYMORPHISM are important in the blood after inoculation, and then invade and obstacles to SUBUNIT VACCINE development, especially develop within hepatocytes, is perhaps the best under- given that many of the sequence alterations in malaria stood in terms of immunity6,7, and vaccine development proteins occur in regions that are crucial to immunity. is further advanced for this stage than others. The Other factors impeding malaria vaccine development mechanism of immunity required to block parasite include: immunological non-responsiveness of cer- transmission by the mosquito is also well understood — tain individuals (depending on their human leukocyte antibodies taken up by the mosquito during its blood antigen and other antigens) to proteins that might meal can prevent gamete fertilization or zygote devel- comprise a vaccine1; CLONAL IMPRINTING or ORIGINAL ANTI- opment8. This type of vaccine protects only at the pop- GENIC SIN influenced by stochastic events and prior ulation level and does not protect the vaccinee from exposure to other, perhaps crossreactive antigens2–4; malaria, and it will be suitable for only certain popula- the difficulties encountered in properly folding tions. The blood stage is the stage for which immunity recombinant subunit vaccines so as to maintain their is least well understood and which arguably presents immunogenic properties; the lack of suitably potent the greatest challenges in terms of vaccine develop- adjuvants necessary to induce high-titre antibody ment. It is also the stage that is responsible for all the responses; and the lack of animal/parasite systems symptoms and pathology of malaria, the most serious which adequately model the situation with humans of which are anaemia and cerebral malaria9, and is and malaria parasites in terms of disease pathogenesis therefore an important target for vaccine develop- and immunological responses. ment. In spite of much effort, a blood-stage malaria ANTIGENIC VARIATION The antigenic changes that Table 1 | Blood-stage malaria vaccines trialled and in development can occur within a parasite clone through switching Name Type Status the expression of different SPf66 Synthetic peptide Multiple trials completed, not progressing ‘variant’ genes. NYVAC-Pf7 antigen Viral-vector-encoding parasite Not progressing ALLELIC POLYMORPHISM CSP.MSP2 Recombinant protein Not progressing Multiple forms of a gene at a single genetic locus. MSP1/MSP2/RESA Multicomponent recombinant protein Phase I and II trials in Australia and PNG adults and children (continuing) SUBUNIT VACCINES AMA1 Recombinant protein Phase I complete (continuing) Vaccines comprising only a small part of the entire MuStDO15 DNA, multi-gene Under development organism, typically a FALVAC-1,-2 Recombinant protein, multi-epitope Under development recombinant protein. MSP119 Recombinant protein Under development CLONAL IMPRINITNG/ORIGINAL MSP142 Recombinant protein Under development ANTIGENIC SIN MSP3–5 Recombinant protein Under development Prior exposure to one strain diverts the antibody response RAP2 Recombinant protein Under development following exposure to a second AMA1, apical membrane antigen 1; MSP, merozoite surface-protein; PNG, Papua New Guinea; RAP2, rhoptry-associated protein 2; strain to shared epitopes. RESA, ring-infected erythrocyte surface antigen. See REFS 33,75,76. 118 | NOVEMBER 2001 | VOLUME 1 www.nature.com/reviews/immunol © 2001 Macmillan Magazines Ltd REVIEWS Figure 1 | Life cycle of the malaria parasite. The life cycle in the mammalian host commences with the inoculation of sporozoites by an infected anopheline mosquito that travel by