The complete and detailed Malaria vaccine review:

Note: This section was written by Dr. Stephanie James and Dr.Louis Miller for NIAID (a US govt. agency) and I totally disown any adverse effects resulting from reading this article.. (also read "Hyper-electrosis Cerebri" , posted on this fortnight's opening page... )

The brutal fact is that this article is too difficult to be completely understood by medical graduates. If someone was to talk to me about Gp-120, I might think for a while and might even realise that the particular person was talking about HIV / AIDS. However, PfEMP-1 makes about as much sense to me as my mother.. :) just kidding...

The thing is that we have been taught NOTHING regarding the antigenicity of the malarial parasite, and hence understanding the following article is quite difficult , if not impossible for most of us. I hardly understood around 30 % of it. But that is the way it is.. The less that humankind knows about something, the more complex it seems. (case in point : Amylodosis )

 You might want to read the table below and skip the rest of the section. If however, you are fascinated by the malarial parasite and are seriously considering marrying the damn bug, you might want to read the rest of it too.
On second thought even if you have talked to your mother about bringing the parasite home with you, you should visit this site instead of reading the part below the table :

Malaria : Opportunities and Obstacles (Published by the National Academy press)
This is a verrrrrry good online book no part of which, sadly, I am allowed to publish on this site.The link above takes you directly to the "vaccines" part of the book. You are allowed to take printouts of the pages from the site if you want to.

The Understandable Table  
(If you think that is bad English, read the small text above...)

Introduction:

More than 30 distinct antigens identified in various life cycle stages of the malaria parasite have been proposed, at some level, as potential vaccine candidates based on observations such as: surface expression of the antigen on one or more life cycle stages; in vitro inhibitory (e.g. invasion-blocking) effects of specific antibodies; or, in vivo experiments showing protective effects of either direct immunization or passive transfer of antibody in animal models.

Several early vaccine candidates, many based on the circumsporozoite (CS) protein, the dominant surface antigen of the sporozoite stage, progressed into Phase I/II clinical trials but were halted by problems of low immunogenicity and efficacy or, in some cases, by reactogenicity. Overall, those candidates that have proceeded to trials have generally had some form of corporate co-sponsorship. Only one candidate vaccine, SPf66, based on antigens from both merozoite and sporozoite stages, has undergone extensive field trials. Efficacy was reported in several early clinical trials in South America, and one in Africa, but results from subsequent trials in Africa and Southeast Asia were not as promising.

These results underscore the need not only to identify the right antigenic components for a vaccine, but also to find presentation and delivery methods that induce qualitatively and quantitatively appropriate immune responses.

Immunology :

Experimental observations indicate that protective immunity may involve multiple different immune responses, both humoral and cellular (Fig. 1). The focus has been largely on induction of antibodies in the case of blood-stage and transmission-blocking vaccines.

Pre-erythrocytic vaccine developers initially focused on induction of antibodies, then on CD8+ cytotoxic T lymphocytes (CTL), and now additionally on T cell derived cytokines. Interferon gamma (IFNg) and other cytokines appear to play a role in the elimination of liver stage parasites, possibly through induction of mediators such as nitric oxide that kill parasites within hepatocytes (Fig. 1).

Figure 1 — Possible Mechanisms of Host Defense Against Malaria

To date, no pattern of immune response fully predictive of protection has been identified or validated. Naturally occurring immunity wanes rapidly in the absence of ongoing parasite exposure, and protection has been similarly short-lived in those few subunit vaccine trials that have demonstrated measurable efficacy. Such a vaccine might be useful for travelers. Unless new technologies can be found to improve the longevity of vaccine-induced resistance, however, it is likely that a vaccine to be used in endemic areas will need to take advantage of natural boosting provided by ongoing parasite exposure in order to provide long-lived protection.

In natural infection, unregulated immune responses may contribute to the pathogenesis of disease. For example, an association of cerebral malaria with high plasma levels of pro-inflammatory cytokines has been reported. Thus, the potential for enhanced immunopathogenesis must also be taken into account in vaccine development efforts.

St ate of the Science

To date, most of the effort on vaccine development has focused on P. falciparum for several reasons:
1) high mortality from infection;
2) capability for experimental challenge infection; and,
3) relative ease of in vitro studies and availability of animal models for in vivo studies.

While P. vivax has a wider geographic distribution than P. falciparum, including in emerging economies such as Southeast Asia, India and Brazil, work on vaccine development has been impeded by several technical obstacles, such as the difficulty of culturing the parasite in vitro.

Because it is impossible to be all-encompassing in the review of current research on malaria vaccines within the limitations of this document, the summary below aims only to illustrate some of the newest approaches to vaccine development.

1)Recombinant vaccines

A. The Walter Reed Army institute vaccine:
The Walter Reed Army Institute of Research (WRAIR) and SmithKline Beecham Biologicals (SBBio), through a partnership extending uninterruptedly over the past 17 years, are developing a multi-antigen, multi-stage malaria vaccine based upon recombinant protein antigens.

 This collaboration has led to development of the CS-based RTS,S vaccine, which in combination with SBAS2 adjuvant repeatedly induced protection of volunteers in a Phase IIa trial. Subsequent re-challenge of volunteers revealed that protection waned substantially by 6 months after the last immunization. The first field trial of RTS,S/SBAS2, conducted by the Medical Research Council in The Gambia, reported 65% efficacy in adult males in a regions of intense transmission where both homologous and heterologous P. falciparum strains are prevalent. Efficacy persisted for 2 months and diminished afterward.

 In partnership with SBBio, USAID, the Naval Medical Research Center (NMRC) and others, WRAIR is conducting preclinical, clinical, and field trials to:
1) optimize RTS,S/SBAS2 vaccine regimens;
2) evaluate RTS,S with improved adjuvants;
3) develop the blood-stage antigen MSP-1 as a potential component of a multi-stage, multi-antigen vaccine; and
4) explore prime/boost strategies.

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B)The NIAID Malaria Vaccine Development Unit (MVDU)
The MVDU is focusing on recombinant proteins derived from blood stages and sexual stages of parasite development.

The MVDU has facilities for protein expression in a variety of recombinant systems as well as subsequent process development. Once produced and purified, blood-stage antigens are being tested in Aotus monkeys to identify the most promising candidates.

 Blood-stage vaccines currently under development in the NIAID MVDU include the C-terminus of MSP1, and recent protection studies in the Aotus model system have shown promising results with a 42 kd MSP-1 protein produced in baculovirus in collaboration with Novavax. A similar approach is also being tested by investigators at the University of Hawaii. A second candidate -PfEMP1 ( The reason why I included that article on PfEMP1 - Oncogen ) - is expressed on the surface of infected red cells and is thus available to the immune system. A region of this variant parasite antigen that mediates binding of the infected cell to CD36 on vascular endothelial cells has shown promising results in an Aotus trial.

 Transmission-blocking vaccines under development at the MVDU include Pfs25 and Pvs25, sexual stage (ookinete) antigens expressed by P. falciparum and P. vivax, respectively. Clinical grade Pfs25 has been produced and a plan is underway for evaluating these antigens for safety and immunogenicity in clinical trials later this year. Clinical grade Pvs25 is also being prepared for Phase I testing.

C. New York University:
Investigators are investigating the use of CS-based multiple antigenic peptides (MAP) for induction of anti-sporozoite immunity. A synthetic MAP vaccine containing minimal T and B cell epitopes from the repeat region of the P. falciparum CS protein with alum and QS21 elicited high levels of parasite-specific antibodies in a recent clinical trial, but immunogenicity was HLA-restricted. Newer methods for MAP synthesis and inclusion of universal epitopes are currently being explored.

D. The CDC malaria vaccine program is focused on development of multivalent, multistage vaccine formulations that contain a series of antigenic domains from all of the developmental stages of the parasite. The multivalent, multistage malaria vaccine development strategy, which is aimed at inducing "multiple layers" of long-lasting, effective immunity, takes into consideration the immunogenicity and genetic diversity of antigenic fragments contained in stage-specific proteins.
Two P. falciparum candidate vaccines are under investigation:
 One, an ~41 kd protein called FALVAC-1, contains 21 B-and T-cell epitopes from a variety of pre-erythrocytic, erythrocytic and sexual stages: CS, LSA1, MSP1, SSP2, MSP2, AMA1, RAP1, EBA-175, and Pfg27. FALVAC-1 has been expressed in a baculovirus expression system in collaboration with National Institute of Immunology, New Delhi, India, and Protein Sciences Corporation, Connecticut. Mouse, rabbit, and monkey immunization studies of FALVAC-1 with various adjuvants demonstrated induction of immune responses that recognizes different stages of the parasite.
 A second candidate, FALVAC-2, containing the 19 kd fragment of MSP1, the third epidermal growth factor domain of Pfs25, Region II of EBA-175, as well as 30 B-cell epitopes and 25 T cell epitopes from a total of 13 stage-specific antigens, is under development. Similar approaches are underway for the development of multivalent, multistage P. vivax vaccines. CDC has entered into a Collaborative Research and Development Agreement (CRADA) with the Bharat Biotech. International Limited (BBIL), Hyderabad, India, for production of GMP-grade candidate vaccine antigens. The goal for the next 5 years is to test multivalent, multistage P. falciparum and P. vivax recombinant vaccines in non-immune persons and individuals living in malaria endemic areas.

2) DNA vaccine and prime-boost approaches

The Naval Medical Research Center (NMRC), in collaboration with Vical, Inc., USAID, Aventis Pasteur, Entremed, Inc., and multiple investigators around the world, has initiated a DNA-based malaria vaccine development effort called the "Multi-Stage DNA Vaccine Operation" or "MuStDO"

Researchers at Oxford are assessing the safety, immunogenicity and efficacy of a DNA prime-MVA (modified vaccinia virus Ankara) boost regime in healthy volunteers. The insert, which is the same in the DNA and the MVA components, is a polyepitope string fused to the entire TRAP antigen (Thrombospondin-Related Anonymous Protein, also known as Sporozoite Surface Protein 2, SSP2, which is expressed primarily by sporozoites and liver stage parasites). The delivery of DNA by intramuscular route and by a needleless delivery device into the skin is being compared. MVA is delivered intradermally. Initial studies of both DNA and MVA vaccines established adequate safety and immunogenicity. The first prime-boost studies of volunteers were initiated in December, 1999, and are ongoing, with challenge studies anticipated later this year. Plans are underway to test both DNA and MVA vaccines in The Gambia in late 2000.

3) Transgenic vaccines

Genzyme Transgenics Corp. and NIAID have established a collaboration for preclinical development of an MSP-1-based vaccine in transgenic animals.

The 42 kd fragment of MSP1 has been produced in milk of transgenic mice, and a purification process is now under development. If the purified product can be shown to protect Aotus monkeys, it will provide supporting data supporting the development of other transgenic animals such as goats.

4) Genomic and proteomic approaches :

In 1996, a collaborative international effort was undertaken to sequence the complete genome of P. falciparum.

To date, two of the parasite's 14 chromosomes have been completely sequenced and the sequences of many of the remaining chromosomes are nearing completion. Although there have been significant technical hurdles in sequencing this A-T rich organism, it is now estimated that essentially all of the parasites approximately 6000 genes are available in existing databases. Thus, the malaria community and vaccine developers have access to virtually all of the genes encoding the antigens and proteins expressed by this organism. Although not validated, computer algorithms are being conceived to identify genes whose expressed products are potential candidate vaccine antigens by virtue of their predicted cell surface localization, stage specific expression, or structural features that may interact with components of the human host immune system to initiate a protective response. Accompanying the development of such computational methods are high throughput methods (DNA chips and microarrays, and proteomics) of differential gene and protein expression to identify and characterize antigens that may be appropriately exposed to the immune system. A challenge for malaria investigators is the limiting amounts of material that are available for certain stages of the parasite's life cycle, especially the liver stages.

Investigators are developing strategies to identify genes whose products are essential to parasite survival and/or contribute to disease manifestations.

Interfering with the function of these gene products with targeted immune responses is another approach to malaria vaccine development. New tools of molecular genetics, e.g. efficient and global gene knockouts, are needed to make this approach truly feasible and economically viable.