Summary:

In humans, sterile immunity against malaria has only been obtained after exposure to irradiated sporozoites inoculated by mosquitoes. The full repertoire of parasite antigens that underlie this protection is not known. Recently, antigens by invasion blood stage parasites have been shown to be expressed by sporozoites, which lead to the hypothesis that they might also be involved in sporozoite invasion of hepatocytes. Using the availability of suitable model systems, this project proposes to study the expression of these invasion-associated sporozoite proteins and host immunity. This information will then be used to test a suitable approach for a new malaria vaccine formulation.

Background:

The full repertoire of pre-erythrocytic antigens that underlie the sterile protection induced by irradiated sporozoites is not actually known. The three antigens investigated to date may not be responsible for induction of optimal protective responses. This would account for the difficulties encountered in reproducing this sterile long-lasting immunity by current sub-unit vaccines. It would clearly be desirable to investigate other pre-erythrocytic antigens. Such efforts to expand targets of pre-erythrocytic stage immunity are justified because the infection is at its most vulnerable between the time when sporozoites are injected and when hepatic merozoites emerge into the blood stream. During this period, there are two distinct stages, the sporozoite and infected hepatocyte, (whose numbers rarely exceed 100 in the human body) that can be attacked. The protective responses could comprise not only humoral but also cellular effector mechanisms, since this is the only time during the infection when the parasite can be found for 5 to 14 days inside a major histocompatibility complex, MHC-expressing cell. Even if sub-optimal, the appropriate immune response could fully abrogate the infection, thus preventing both pathology and transmission to another host.

Aim:

1. To obtain a list of all potential genes within the ebl and rh families in P. falciparum, P. berghei and P. yoelii. Real time polymerase chain reaction (RT-PCR) will then be performed for each gene on ribonucleic acid (RNA) purified from all the stages of these parasites and the expression profile in sporozoites determined.
2. To obtain recombinant proteins, peptides and immunological reagents specific to each gene product expressed in sporozoites.
3. To characterise the proteins expressed in the sporozoite using reagents obtained in (2).
4. To obtain transgenic parasite lines in which sporozoite-expressed genes identified in (1) are individually disrupted or knocked-out, and purify the corresponding sporozoites.
5. To employ the reagents from (2) to determine the functional role of each protein in sporozoite invasion and development in the hepatocyte.
6. To employ the sporozoites obtained in (4) to assess the functional role of each protein during the life cycle.
7. To employ the reagents from (2) and (4) to conduct immunisation studies, so as to quantify the level of protection obtained.
8. To conduct a detailed analysis of immune responses resulting from (7), and derive surrogate markers of protection.

Expected results:

It is expected to characterise the new invasion-associated sporozoite antigens from three species of Plasmodium, to assess their role in hepatocyte invasion, to develop immunogens for vaccination studies and assess their efficacy against a sporozoite challenge.

Potential applications:

The results of this project will be useful in the design and development of malaria vaccines based on these new antigens.

Coordinator:

Partners:

Nº	Principal Scientific Participants	Official Address	Other Information
2	Dominique Mazier	INSERM U 511 Immunobiologie Cellulaire et Moléculaire des Infections Parasitaires Hôpital Pitié-Salpêtrière 91 Bld de l’Hôpital FR-75013 Paris France	Tel: +33 1 42 82 28 59 Fax: +33 1 45 83 88 58 E-mail: mazier@ext.jussieu.fr; Website:www.inserm-u511.jussieu
3	Robert Sauerwein	Department of Medical Microbiology University Medical Center P.O. Box 9101 NL-6500 HB Nijmegen The Netherlands	Tel: +31 24 36 10 557 Fax: +31 24 36 14666 E-mail: R.Sauerwein@mmb.umcn.nl
4	Maria M. Mota	Malaria Cell Biology Group Instituto Gulbenkian de Ciencia Rua da Apart. 14, Quinta Grande 6 PT-2781-901 Oeiras Portugal	Tel: +351 21 446 45 17 Fax: +351 21 440 7970 E-mail: mmota@igc.gulbenkian.pt; Website: http://www.igc.gulbenkian.pt
5	Alan Cowman	Division of Infection and Immunity The Walter and Eliza Hall Institute (WEHI) of Medical Research 1G Royal Parade Melbourne 3050 Australia	Fax: +613 9347 0852 E-mail: cowman@wehi.edu.au Website: http://www.wehi.edu.au